Femtosecond Laser Direct-Write of Optofluidic Lab in Fiber Through Polymer-Coated Optical Fibers

Femtosecond Laser Direct-Write of Optofluidic Lab in Fiber through Polymer-Coated Optical Fibers

Kevin A. J. Joseph

A thesis submitted in conformity with the requirements for the degree of Master of Applied Science Graduate Department of The Edward S. Rogers Sr. Department of Electrical and Computer Engineering University of Toronto

Femtosecond Laser Direct-Write of Optoﬂuidic Lab in Fiber through Polymer-Coated

Optical Fibers

Kevin A. J. Joseph

Master of Applied Science

Graduate Department of The Edward S. Rogers Sr. Department of Electrical and

Computer Engineering

University of Toronto

2017

Three-dimensional femtosecond laser processing of lab in fiber, the combination of integrated photonics and microfluidics inside optical fiber, was demonstrated in silica glass fibers coated with polymer buffer. This enables the laser fabrication of lab-on- chip functionalities onto optical fiber without the time-consuming and mechanically- compromising process of buffer removal. In this thesis, an assessment of laser-induced damage in buffer-coated fiber is reported, along with methods to avoid and mitigate this damage. Further, selective buffer machining is studied enabling in-tandem processing of the glass fiber core and cladding with the polymer buffer. Structuring of the fiber core, cladding photonic circuits, and microfluidics were demonstrated in buffer-coated fiber without removal of or damage to the polymer buffer. The methods and processes here make the lab in fiber platform to be more industry viable and opens new opportunities for device architectures spanning across the fiber core, cladding, and buffer, representing a significant technological advancement.

ii Acknowledgements

I once had a friend who studied geology tell me that when you look at the world on a geological time-scale, all of our human affairs seem fleeting and insignificant. I have chosen to study femtosecond laser processing, harnessing time-scales of millionths of billionths of seconds. It is not surprising, therefore, that I have never thought of life (or at least of graduate school) of being too short. It is in that spirit of recognition that I would like to take a moment to acknowledge those who have helped to guide and support me throughout the completion of this thesis.

Since starting my studies at the University of Toronto, I have been fortunate to receive guidance and mentorship from my predecessors. I would like to thank Dr. Kenneth Lee and Dr. Jason Grenier for their training and advice, as well as for their pioneering work on cladding photonics. Dr. Stephen Ho has always been willing to take time to talk through a problem and offer invaluable insight and training, particularly in matters concerning chemical etching and microfluidics. Dr. Jianzho Li’s indomitability and infinite repository of knowledge have been both a life-saver as well as an inspiration. Dr. Moez Haque is a forerunner of lab in fiber research and his training, insights, and accomplishments were critical to my achievements. I also acknowledge his help in preparing the LaTex code used to write this thesis. He has been a role model both in terms of academic excellence and professionalism. Dr. Erden Ertorer’s patience and perspective are qualities which I strive to emulate. Erden approaches every situation with the mindset of a scientist, an engineer, and a technician, and has taught me countless practical techniques in the lab.

I thank my colleagues for all of their helpful advice, useful conversations, and our mutual commiserations. Zeinab Mohammadi, thank you for helping to show me the ropes around the laboratory and helping to train me on the IMRA America laser system. I could always count on David Roper to pick through a tough problem and oﬀer unique perspectives, or just talk to endlessly about nothing. Ehsan Alimohammadian has been a friend and adviser, upon whom I can always depend for sincere and thoughtful guidance.

Hydroﬂuoric acid etching work discussed in this thesis was conducted in the laboratory of Professor Nazir Kherani, whom I gratefully acknowledge and thank.

iii Potassium Hydroxide etching was done with help from Kevin Yang of Prof. Ted Sargent’s research group, both of whom I also aknowledge and thank.

I would like to acknowledge the support and encouragement of my family and friends. Thank you for your patience, your tolerance of my tardiness and last-minute cancellations, and your support. I dare not try listing everyone at the risk of omission, but in particular, I must thank my selﬂess and unconditionally supportive parents, Lalitha and Anthony, my always-encouraging siblings, Shobani, Sean, and Daniel, my charismatic nephew Alec (AKA “Li’l Al”), all of my cousin-friends, and of course, my best friend, Brittany De Pompa.

I would like to thank my MASc. proposal committee, Prof. Ofer Levi and Prof. Li Qian. Thank you both for your valuable advice and suggestions, which helped to shape the direction of this research. I also thank my MASc. defence committee, Prof. Ofer Levi, Prof. Mo Mojahedi, and Prof. Konstantinos Plataniotis. Your questions and suggestions are recoginzed in strengthening this thesis.

I must of course acknowledge the support, guidance, and mentorship of my supervisors, Prof. J. Stewart Aitchison and Prof. Peter Herman. Prof. Aitchison has always reminded me to not get bogged down in day-to-day problems, and to think about what I want out of life and how to make decisions that would put me in that direction. Prof. Hermans door has always been open. He has helped guide me through countless challenges and shaped my thinking. I am forever grateful to both of you.

Lastly, I would like to gratefully acknowledge that partial funding for this work was provided through grants from the National Sciences and Engineering Research Council of Canada (NSERC).

iv Contents

Abstract ii

Acknowledgements iii

List of Conference Proceedings viii

1 Introduction1

1.1 Chapter-by-Chapter Outline...... 6

2 Background8

2.1 Laser Material Interactions in

Transparent Solids...... 9

2.1.1 Laser Processing Optical Fiber...... 10

2.1.2 Laser Processing of Glasses...... 11

2.1.3 Laser Processing of Polymers...... 12

2.2 Laser Processing of Lab in Fiber...... 13

2.2.1 Laser Processing in / near the Fiber Core...... 13

2.2.2 Cladding Photonics...... 15

2.2.3 Microﬂuidics in Fiber...... 17

2.3 Optical Fiber Buﬀers...... 20

3 Methods 23

v 3.1 Femtosecond Laser Processing System...... 23

3.2 Fiber Preparation, Handling, and Alignment...... 29

3.3 Laser Processing of Fibers...... 32

3.3.1 Core Modiﬁcations...... 33

3.3.2 Cladding Photonics...... 34

3.3.3 Microﬂuidics in Fiber...... 36

3.3.4 Buﬀer Ablation...... 37

3.4 Optical Characterization...... 38

4 Laser Modiﬁcations to the Polymer Buﬀer 40

4.1 Buﬀer Damage Zones from Laser-Focusing in Fiber Cladding...... 41

4.2 Sources of Damage in the Polymer Buﬀer...... 46

4.2.1 Intensity-Driven Damage in the Buﬀer...... 46

4.2.2 Thermal Damage in the Buﬀer...... 49

4.3 Laser Ablation of Polymer Buﬀer...... 54

4.4 Chemical Etching Silica Fiber Cladding through Polymer Buﬀer..... 62

4.4.1 Chemical Resistance of Acrylate Buﬀer to KOH and HF..... 62

4.4.2 Enabling Fluid Flow through Ablated Buﬀer...... 66

4.4.3 Mitigating Interfacial Damage after HF Etching...... 72

4.5 Chapter 4 Summary...... 78

5 Lab in Fiber Devices in Buﬀered Fiber 81

5.1 Structures Written in the Core of Buﬀered

Optical Fiber...... 82

5.1.1 Structures Written in the Core of Acrylate-Coated

Fiber...... 82

5.1.2 Structures Written in the Core of Polyimide-Coated

Optical Fiber...... 86

vi 5.2 Cladding Photonic Structures Written in the

Cladding of Acrylate-Coated Optical Fiber...... 90

5.2.1 Optical Components: Cross-Couplers, Cladding

Waveguides, and Bragg Grating Waveguides...... 90

5.2.2 Bend Proﬁler in Acrylate-Coated Fiber...... 94

5.3 Microﬂuidics Written in Acrylate-Coated Fiber...... 99

5.3.1 FLICE in Acrylate-Coated Fiber...... 99

5.3.2 Fluid Flow in Acrylate-Coated Fiber...... 101

5.3.3 Particle Flow in Acrylate-Coated Fiber...... 107

5.4 Chapter 5 Summary...... 108

6 Discussion and Future Work 112

7 Conclusion 118

Bibliography 122

vii List of Conference Proceedings

1. K. A. J. Joseph, M. Haque, S. Ho, J. S. Aitchison, P. R. Herman, “Femtosecond Laser Direct-Write of Optoﬂuidics in Polymer-Coated Optical Fiber”, Frontiers in Ultrafast Optics: Biomedical, Scientiﬁc, and Industrial Applications XVII, SPIE Photonics West (2017) (oral).

• 1st Place − Best Student Paper Awards for Frontiers in Ultrafast Optics: Biomedical, Scientiﬁc, and Industrial Applications XVII ($1000 USD)

2. K. A. J. Joseph, M. Haque, J. S. Aitchison, P. R. Herman, “Femtosecond Laser Direct Write of Optoﬂuidic Lab-in-Fiber through Polymer-Coated Optical Fiber”, Advanced Fabrication Technologies for Micro/Nano Optics and Photonics IX, SPIE Photonics West (2016) (oral).

3. M. Haque, S. Ho, E. Ertorer, K. A. J. Joseph, J. Li, P. R. Herman, “Packaging and micro-structuring for enabling multi-functional ﬁber cladding photonics and lab-in-ﬁber”, Advanced Fabrication Technologies for Micro/Nano Optics and Photonics IX, Photonics West (2016) (oral).

4. K. A. J. Joseph, M. Haque, J. R. Grenier, Ho, J. S. Aitchison, P. R. Herman, “Direct write of optofluidic lab-in-fiber sensors through polymer-coated optical fiber”, Photonics North (2015) (oral).

viii List of Tables

4.1 Incidence of Cladding / Buﬀer Interface Damage Before and After 2 hours of HF Etching and of Cladding Blind-Hole Etching over 6 Trials in Figure 4.16 ...... 76

5.1 Comparison of optimized writing parameters for 4.5 mm FBGs in Stripped and in Acrylate-Coated Fiber ...... 85 5.2 Comparison of optimized writing parameters for FBGs in Stripped and in Polyimide-Coated Fiber ...... 88 5.3 Incidence of Cladding / Buﬀer Interface Damage and Blind-Hole Etching before and after 2 hours of HF etching and of Cladding Blind-Hole Etching over 26 Ablation Holes/Access Ports (see Figure 5.11) ...... 105

ix List of Figures

1.1 Schematic of buﬀered single mode optical ﬁber...... 3

2.1 Schematic of diﬀerent cladding waveguide taps. Reproduced, with permission, from Figure 4.5 of Grenier et al. [20] c 2015 Springer..... 16 2.2 Distributed Bragg grating waveguide 3D shape sensor. Reproduced, with permission, from Figure 1 of Lee et al. [16] c 2013 OSA...... 17 2.3 Polarization sensitive nanogratings formed in bulk fused silica. Reproduced, with permission, from Figure 4 of Hnatovsky et al. [27] c 2005 OSA...... 18 2.4 Multiplexed Lab in Fiber Device. Reproduced, with permission, from Figure 1 of Haque et al. [15] c 2014 The Royal Society of Chemistry... 20

3.1 Modular schematic of femtosecond laser system and optical delivery pathway. 25 3.2 Images of femtosecond laser system and optical delivery pathway..... 26 3.3 Schematic of acousto-optic gating mechanism...... 27 3.4 Schematic of coordinate system...... 29 3.5 Fiber preparation tools...... 30 3.6 Fiber holder consisting of fiber clamps, a goniometer, and a rotating stage. 31 3.7 Schematic of cross-coupler Bragg grating waveguide...... 35 3.8 Different approaches examined for laser buffer ablation: (a) percussion, (b) rastering, and (c) trepanning...... 37

4.1 Micrographs of damage at the cladding / buffer interface...... 42 4.2 Empirically determined damage-free radial focusing distances...... 44 4.3 Direct intensity-driven model of interfacial buffer damage...... 47 4.4 Accumulated, thermally-driven model of interfacial buffer damage..... 50

x 4.5 Comparison of intensity- and thermal-driven models of interfacial buffer damage with empirical data...... 53 4.6 Micrographs of buffer percussion ablation ports at the -z and the ±y cladding / buffer interface...... 56 4.7 Effects of varying repetition rate, pulse energy, and number of applied pulses on percussion ablation ports in the buffer...... 58 4.8 Morphological assessment of percussion ablation ports...... 60 4.9 The effect of aqueous potassium hydroxide on ablated buffer ports.... 64 4.10 The effect of hydrofluoric acid on ablated buffer ports...... 66 4.11 Raster-scanned ablated buffer ports and cladding nanograting tracks before and after hydrofluoric acid etching...... 67 4.12 Air bubbles trapped in linearly rastered buffer ablation ports...... 69 4.13 Schematic of spiral-scanned and trepanned conical buffer ports...... 69 4.14 Spiral-scanned and trepanned conical and cylindrical buffer ports..... 71 4.15 Radial cladding blind holes etched through conically trepanned buffer ports. 72 4.16 Mitigating interfacial buffer damage when etching radial cladding blind holes through conically trepanned buffer ports...... 74

5.1 Micrographs of an FBG fabricated in acrylate-coated fiber...... 83 5.2 Transmission spectra for FBGs fabricated in acrylate-coated and in buffer- stripped fibers...... 86 5.3 Micrographs of FBGs fabricated in polyimide-coated fibers...... 87 5.4 Transmission spectra for FBGs fabricated through polyimide-coated and buffer-stripped optical fiber...... 89 5.5 Reflection spectra for Bragg grating waveguides fabricated in the xz plane crossing the core waveguides at varying crossing angles...... 91 5.6 Peak reflection for Bragg grating waveguides crossing the core waveguide at varying crossing angles...... 92 5.7 Peak reflection for Bragg grating waveguides crossing the core waveguide fabricated using varying pulse energies...... 93 5.8 Reflection spectra of Bragg grating waveguides in acryate-coated and in buffer-stripped fiber...... 95 5.9 Bend profiler fabricated in acrylate-coated fiber...... 97 5.10 Microfluidic through holes fabricated in buffer-stripped fiber...... 100

xi 5.11 Micrographs of axial microfluidic channel fabricated in acrylate-coated fiber.103 5.12 Micrographs of a helical microfluidic channel fabricated in acrylate-coated fiber...... 106 5.13 Polystyrene bead passing through a helical microfluidic channel fabricated in acrylate-coated fiber...... 108

xii Glossary

3D Three-dimensional. 3, 4, 8, 12, 19, 81, 90, 94, 98, 109, 120

AOM Acousto-optic modulation. 23, 82

BGW Bragg grating waveguide. 24, 34, 35, 81, 91, 93, 96, 109, 120

FBG Fiber Bragg grating. 13, 14, 17, 21, 24, 32, 34, 35, 82, 83, 84, 85, 87, 88, 109, 114, 116, 120

FLICE Femtosecond laser irradiation followed by chemical etching. 4, 12, 13, 17, 19, 36, 40, 62, 65, 66, 81, 99, 113, 115, 121

HF Hydroﬂuoric acid. 17, 36, 65, 66, 70, 72, 73, 75, 77, 79, 99, 101, 104, 105, 110, 119

KOH Potassium hydroxide. 36, 62, 65, 115

LBO Lithium triobate crystal. 27

LIF Lab in ﬁber. 3, 5, 6, 8, 13, 17, 19, 22, 31, 32, 38, 40, 41, 45, 46, 54, 79, 81, 89, 99, 107, 108, 110, 112, 113, 114, 116, 118, 120, 121

LOC Lab on chip. 1, 2, 4, 113

LOF Lab on ﬁber. 2, 3, 14

PMMA Poly (methyl methacrylate). 1, 12, 20, 46, 49, 51, 55

RMSE Root mean square error. 52

xiii Chapter 1

Introduction

For nearly forty years, the promises of “lab on chip” (LOC) technology have been pursued by academia and industry alike [1]. The production of LOC deices uses established micro- and nanofabrication technologies to create complex miniaturized laboratory components on chip [2,3]. This way reams of electrical, mechanical, chemical, and / or biological information can probed, processed, and delivered using a single chip platform. Although initially developed in silicon, LOC technology has migrated to other substrates including polymers such as poly(methyl methacrylate) (PMMA) [4] and polydimethylsiloxane (PDMS) [5] and glasses such as fused silica for favourable properties including rapid prototyping, biocompatibility, gas diﬀusivity, and optical transparency from infrared to ultraviolet radiation [3].

LOC technology is based on the use of microfluidics, the engineering of fluidic structures capable of transporting and reacting small volumes of gases, liquids, and dissolved solids. As fabrication processes have become more precise and reliable, integrated photonics, the engineering of micro- and nanoscale structures capable of manipulating light, have been made possible. Optofluidics, the combination of microfluidics and integrated photonics, has proven to be a promising route for LOC technology, as extremely small volumes of fluids can be probed and analyzed with the high sensitivity of optical instruments [6]. In addition to high sensitivity, integrated photonics offer further benefits including the ability to bypass molecular tagging processes for rapid, in-situ sensing and characterization. While LOC technology has the technological capacity to rapidly monitor very small samples, there are significant

1 Chapter 1. Introduction 2 practical challenges in deploying chip-based technologies to certain environments, for example, those which are tightly conﬁned and those which are broadly distributed.

While glasses including fused silica have been used in a number of optical / optofluidic LOC applications [3], fused silica has had another application which is far more mature, widely adopted, and lucrative: laying the foundation for international telecommunication networks in the form of optical fiber. Silica optical fiber consists of three concentric layers: a guiding core, a cladding, and a protective buffer over-layer (also referred to as a coating, jacket, or encasement) (Figure 1.1). While the core and cladding may guide light along the fiber by a number of means (e.g. total internal reflection due to a step index contrast, graded index profile, photonic bandgap), a step-index contrast is the most common type of fiber employed by industry. The fiber typically has an outermost buffer layer consisting of other non-optical materials, often polymers, which do not directly contribute to the optical guidance mechanism of the fiber. Instead, the buffer is selected to impart a combination of physical, mechanical, optical, electrical, thermal, biological, and / or chemical properties which serve to protect the underlying silica fiber. The ubiquity of silica optical fiber in global telecommunications is due in part to its abundance, machinability, and extremely low transmission loss (0.2 dB/km) [7]. With silica optical fiber as an established means of transporting optical information over thousands of kilometers, and fused silica as an attractive material for LOC applications, there is clear utility in building optofluidic LOC functionality onto and into fused silica optical fiber. Such a translation presents opportunities to package more information onto existing optical communication infrastructure, further developing sophisticated sensing networks and the internet of things. LOC-enabled fibers have the potential to collect environmental information across distributed networks and transmit such information over vast distances, easily integrating with existing optical infrastructure. In addition to application over distributed networks, LOC-enabled fibers are also deployable over sinuous pathways including the human body for biological applications, or hazardous environments in which regular sampling is challenging.

One way to achieve LOC-enabled ﬁber-optic sensors is through the principle of lab on ﬁber (LOF). In this approach, sensing architecture is built on the outer surface of optical Chapter 1. Introduction 3

Figure 1.1: Schematic of buﬀered single mode optical ﬁber

fiber, and information is then drawn into the centrally guiding fiber core by various invasive means of fiber modification [8,9]. Although this thesis is concerned with laser- processing, LOF sensors are not exclusively laser fabricated and some of these devices are, nonetheless, worth discussing to appreciate the current state of this technology. One such strategy, employed by Pevec et al., is to selectively etch a region of the fiber cladding region sensitized by phosphorous pentoxide doping. The selectively etched fiber can be used to build a fiber microresonator [10]. Another approach, demonstrted by Consales et al., is to build gold-capped nano-pillars on the end-facet of an optical fiber to harness localized surface plasmon resonance shifts from different chemical and biological stimuli to perturb the measured reflection spectrum of the fiber [9]. Even the magnetic field of a magnetic fluid can be detected by a fiber sensor, as demonstrated by Chen et al., by splicing a portion of multimode coreless fiber between two single-mode fiber arms [11]. Laser-fabricated devices have also been demonstrated for LOF applications, using grating technologies such as tilted fiber Bragg gratings [12] and long period gratings which couple light out of the guiding core and into the cladding to enable optical sensing at the cladding surface [13,14].

An alternative approach to developing LOF devices, which is explored in this dissertation, is lab in fiber (LIF), in which the outside environment is drawn into the fiber core waveguide through subtractive fabrication techniques and sensing is carried out therein [15]. While such LIF devices suffer from compromised mechanical strength over LOF counterparts, 3D microfluidic design integrated with cladding photonic Chapter 1. Introduction 4 circuits offer increased degrees of flexibility and control in design and function. LIF sensors can be much further enabled by 3D direct write to create cladding photonic waveguides. By interlacing such cladding optofluidic microsystems with the fiber core waveguide, one can create elaborate highly functional sensor systems than can be deployed over long flexible distances.

Cladding photonics refers to the development of optical circuits inside the bulk of fused silica fiber cladding which can favourably tap light into and out of the pre-existing fiber core waveguide [16, 17]. Cladding photonic technology is enabled by the discovery of Davis et al. that focused femtosecond laser pulses are capable of inducing positive refractive index changes in fused silica glass, confined within the focal volume of the focused laser [18]. This enabled the development of 3D optical circuits in bulk silica glass [19] and was the foundation upon which optical circuitry was then developed in silica fiber cladding [20]. Three-dimensional cladding photonic circuits can be used to obtain interferometric and / or differential information from light travelling through the core and through non-central cladding waveguides. For example, 3D shape sensing has been enabled by monitoring differential Bragg reflection shifts in perpendicular cladding arms [16] and torsion sensing by monitoring differential Bragg reflections between helical Bragg grating waveguides wrapping around the fiber core with opposite chirality [21].

The opening up of optical fibers to introduce microfluidic quantities of gases and liquids internally into the fiber has been demonstrated using direct laser-machining approaches [22–24], albeit with limited precision and channel smoothness. An attractive alternative approach to fabricating microfluidics and microcavities in fused silica glass involves a first femtosecond laser irradiation step followed by a selective chemical etching step (FLICE). In the FLICE method, polarization-sensitive parallel nanograting structures are formed as a product of femtosecond laser exposure [25, 26]. Nanograting tracks written in fused silica then guide chemical etchant (e.g. hydrofluoric acid [27] or potassium hydroxide [28]) to selectively open the glass along laser modification tracks. FLICE has been demonstrated in silica fiber cladding as a means of guiding fluid to the central guiding core waveguide for sensing applications [29, 30] as well as to probe laser-formed cladding photonic circuits [15]. By integrating laser-formed integrated photonic and microfluidic components simultaneously in optical fiber, Haque Chapter 1. Introduction 5 demonstrated a translation of multifunctional LOC principles into optical fiber in ways which were previously not anticipated [15].

To date the majority of laser-fabricated optical fiber sensors are processing after the fiber has been stripped of its protective polymer buffer (see Figure 1.1). Buffer stripping enables laser processing to be conducted on a nearly optically homogenous system and without concern over opacity, absorption, incubation, or damaging / debris effects from the polymer buffer. Despite these advantages in processing simplicity, buffer-stripping and subsequent recoating increases processing time and compromises device strength [31]. Further, microfluidic elements developed in fiber would be blocked by efforts to recoat the buffer after processing. Starodubov et al. first demonstrated through-buffer processing of grating structures in fiber using mid-UV writing processing of hydrogen-loaded fibers [32]. Mihailov et al. demonstrated through-buffer writing of non-sensitized fiber cores using femtosecond laser exposure through a phase mask in fibers coated with acrylate [33, 34] and with polyimide coatings [35]. Martinez et al. demonstrated point-by-point direct processing of grating structures through acrylate-buffered fiber using single pulse exposure methods whereby laser repetition rate and scan speed define grating pitch [31, 36], as opposed to burst-train grating writing employed in this thesis wherein high-repetition rate firing produces high-index voxels and pitch is defined by external acousto-optic gating [37]. Cursory work has also been demonstrated by Waltermann et al. in developing through-buffer cladding photonic waveguides in acrylate coated fiber, reporting minimal damage to the acrylate buffer [38]. To date there has not yet been any reported research on laser-processing of microfluidic devices in buffer-coated fiber. The physical phenomena behind damage incurred in the fiber buffer has also not been assessed when laser processing the core and cladding of buffer-coated fiber.

The motivation of this thesis is to study the limits of laser processing through buffer-coated fiber and develop a full suite of lab in fiber capabilities core structures, cladding photonics, and microfluidics in fiber without any damage to the fiber buffer. By understanding the nature of such buffer damage one can then develop damage free laser processing of new direction for LIF. Chapter 1. Introduction 6

1.1 Chapter-by-Chapter Outline

To achieve the objectives of this thesis, a broad range of topics encompassing the pursuit of damage-free optofluidic lab in fiber through polymer-coated optical fiber are discussed, which are divided as follows:

In Chapter2, Background, the information necessary to understanding the work presented in this thesis, as well as the context into which it fits, is laid out. This begins with the broad scope of integrated optofluidics, and how lasers have proven to be such useful tools in this field, in particular femtosecond-pulse laser systems. The nature of femtosecond laser processing and ultrafast laser interactions with transparent solids are presented to an extent as is necessary to follow the subsequent thesis work. The difference between processing glasses and polymers is discussed briefly, followed by the unique challenges and benefits of ultrafast laser processing optical fibers. This idea is explored more explicitly in the realms of (1) processing within the fiber’s waveguiding core, (2) making modifications in the fiber’s outer cladding layer, and (3) the fabrication of fluidic channels within the fiber core and cladding. This chapter closes with a discussion of the fiber’s surrounding protective buffer, the motivations behind this thesis seeking to fabricate devices with the buffer intact, and why this objective as to-date has remained largely elusive.

Chapter3 discusses the preparation, fabrication, and characterization technologies employed in this thesis. The femtosecond laser processing system used in this research is described (including acousto-optic gating, second harmonic generation, and optical delivery systems). The unique challenges of fiber handling are addressed in terms of fiber preparation, handling, and alignment. The technicalities of structuring the fiber core, cladding, and developing microfluidics are described, including laser ablation of the fiber buffer as is necessary for microfluidics in buffered fiber. Finally, optical characterization tools and methods are described briefly.

Before any devices are presented, Chapter4 discusses the practical limitations in laser fabrication of LIF in polymer-coated optical fiber, and wherefrom these limitations arise. Empirical study into damage-free processing zones that leave both glass fiber and Chapter 1. Introduction 7 buffer intact are presented to establish radial and azimuthal boundaries for focusing at varying degrees of laser energy. Direct intensity-driven absorption and accumulated, thermal damage models are presented, compared, and evaluated to explain empirical observations. Methods of selective buffer ablation are further discussed, first in terms of structuring the buffer and then in terms of enabling the flow of etchants to open underlying cladding nanograting tracks without damaging the buffer in the process.

Equipped with an understanding of damage mechanisms and limitations, as well as of buffer structuring and chemical etching ascertained in Chapter4, Chapter5 discusses the fabrication of various optical and optofluidic devices in coated fiber. This chapter first explores fiber Bragg grating formation in the core of acrylate-coated and of polyimide- coated fiber. Next, cladding photonics are developed in acrylate-coated fiber without incurring any damage to the acrylate buffer. Finally, microfluidic structures capable of guiding liquids and particles are presented in buffer-coated fiber for optical sensing by the fiber core waveguide.

The signiﬁcance and future directions of this work are presented in Chapter6, with new opportunities revealed and enabled as a direct consequence of this work discussed. Finally, Chapter7 will conclude the thesis with emphasis on the key ﬁndings and advancements. Chapter 2

Background

As introduced in Chapter 1, there are clear packaging and deployment benefits to translating the principles of lab on chip (LOC) into silica optical fiber. While significant advancements have been made into achieving lab on fiber (LOF) technology, the vast majority of devices have been fabricated in devices stripped of their protective polymer buffer (also referred to as a polymer coating, jacket, or encasement), which is a hindrance to making this technology industrially viable. Existing work into developing through-buffer processes has been mostly limited to forming structures in the fiber core. Further, there is an absence of methodological study into the causes and nature of damage to the polymer buffer when laser processing in the fiber core or cladding, as well as into integrating the full suite of 3D optofluidic LIF principles into buffer-coated fibers, which is the motivation of this thesis.

In this chapter, the background information necessary to reaching the research goal of damage-free through-buffer processing of LIF is outlined. The focus is mainly on the necessary physical insight for laser processing inside transparent media, given in the context of important foundational work done by predecessors in the field of fiber optic devices. In Section 2.1, laser material interactions within transparent solids are described briefly, discussing processes adapted for glasses as well as transparent polymers. In Section 2.2, background literature reviewing laser-formed fiber devices is reviewed, discussing devices built by structuring fiber core waveguide, by structuring the fiber cladding, and by opening up microfluidic networks in fiber cladding. Finally, in Section 2.3, the types and applications of polymer buffers are discussed, and the current state

8 Chapter 2. Background 9 of through-buﬀer processing is reviewed. The new opportunities for laser fabrication of LIF are then highlighted as the main objectives of this thesis.

2.1 Laser Material Interactions in Transparent Solids

Transparent materials offer unique benefits for laser processing as laser energy can penetrate through the surface of the material and interact within the transparent bulk using methods including sensitization [39], interference effects [40], and nonlinear processes [41]. The nature of the modifications induced by laser exposure depend both on the properties of the laser as well as of the substrate. In this thesis, polymer-coated glass fibers are processed, and it is important to recognize the differences between laser processing of transparent glasses and transparent polymers, and appreciate both the challenges posed and opportunities presented by this concentric two-material system.

Davis et al. first demonstrated that ultrafast laser pulses could be used to induce positive refractive index changes in fused silica glass [18]. This was further developed by researchers to produce optical waveguide circuits in bulk silica via direct write procedures [42]. In direct-write processes, modification zones confined to the focal volume of the writing laser are traced by scanning the position of the lasers focus relative to the substrate, with methods used in this thesis further discussed in Section 3.3. As no sensitization or preloading of the substrate is required, arbitrary three-dimensional patterned structures are possible. There is a wealth of literature spanning decades regarding the physics of laser material interactions in transparent solids, including ultrafast laser processing, and how these physics can be exploited in LOC and fiber-optic applications. However, given that the scope of this thesis does not advance the understanding of such physics, the discussion presented here is limited to what is necessary for the readers comprehension of the fabrication methods and advancements explored in this dissertation. A more comprehensive overview of the underlying physical phenomena behind ultrafast laser modification of fused silica can be found in a referenced MRS review by K. Itoh et al. [43]. Chapter 2. Background 10

2.1.1 Laser Processing Optical Fiber

While there are a number of ways in which optical ﬁbers can be processed to develop increased functionality and sensing capabilities, lasers in particular pose a number of beneﬁts. Lasers can be used to precisely modify materials through linear and nonlinear interactions based on a number of laser properties including wavelength, polarization state, pulse duration, repetition rate, pulse energy, spatial and temporal shape, focusing conditions, and scan speed. Laser energy can be delivered in an uninterrupted stream of light in the case of continuous wave (CW) lasers, or in single or burst trains of pulses of light. In this thesis, femtosecond-pulse lasers were used to access high-intensity physical phenomena via ultrafast laser writing.

Ultrafast laser processing is particularly attractive for structuring transparent solids. With ultrafast (also known as ultrashort-pulse) lasers, the light of the laser is confined temporally to pulses on the order of pico- (10−12) to femtoseconds (10−15). This produces very high intensity exposures with relatively low energies such that nonlinear processes (e.g. multiphoton / tunneling ionization, avalanche ionization, Kerr effects, microplasma formation) can be driven in the laser’s focal volume while avoiding linear modifications outside this volume [18,19,43]. A drawback of this writing process is that the focal area to which the pulse energy is confined will have a near-ellipsoidal profile as the lasers depth of focus, 2zR, exceeds the transverse spot diameter, 2ω0, by a factor of approximately

2z n R = , (2.1) 2ω0 NA where n is the index of refraction and NA is the numerical aperture of the focusing objective lens. It is possible to mitigate this asymmetry using techniques including high numerical aperture focusing which limits the depth at which devices can be fabricated [44], or through beam shaping techniques including specialty lenses [45] and spatial light modulation [46] which increase the complexity of the optical delivery pathway. Section 3.1 further discusses the beam delivery techniques used in this thesis.

By spacing pulses farther apart (low repetition rate (νRR)) and using short pulse durations, cold modifications are possible in which dissipated laser energy has diffused and the material has relaxed to thermal equilibrium in the time in between successive Chapter 2. Background 11 laser pulses. This way, subsequent dissipated energy does not compound and accumulate to create a heat-affected zone. Alternatively, higher repetition rates and longer pulses can be intentionally utilized to create a precise heat affected zone for localized thermal modification.

The nature of laser-induced modifications depend on both the writing parameters of the laser, as well as the material properties of the target. In the case of optical fiber, modifications vary depending on whether the laser is targeting the glass core and / or cladding, or the protective polymer buffer.

2.1.2 Laser Processing of Glasses

The majority of work done in ultrafast laser processing of silica optical fiber has used fibers which have been stripped of their polymer buffer, allowing access to work within an optically homogenous system. This leaves behind the glass fiber core and cladding.

Strong linear absorption of laser light can drive high-energy thermal processes [47], ablation [48], scribing [49], or welding [50]. Weak linear absorption enables deeper penetration and enables annealing [51] and colour marking [52]. Ultrafast lasers are particularly suited for nonlinear processing of materials including direct ablation [43], filamentation (a balancing of Kerr focusing and plasma defocusing) [53], surface rippling [54], colour center formation [55], Type I and Type II refractive index modifications with positive and / or negative refractive index changes [43], and nanograting formation [46]. In particular, refractive index changes and nanograting formation in glass through ultrafast laser processing can be broadly categorized into three regimes of modification. At relatively low pulse energies (≈ 100 nJ for NA = 0.6, λ = 800 nm, ∆t = 100 fs), isotropic index change occurs as glass melts and rapidly quenches into a denser state. Higher pulse energies (≈ 150 - 500 nJ for NA = 0.6, λ = 800 nm, ∆t = 100 fs) can create a birefringent index modification in the form of linear nanogratings perpendicular to the electric field of the writing laser, due in part to interference effects between the plasma wave and laser field. At relatively higher pulse energies (> 500 nJ for NA = 0.6, λ = 800 nm, ∆t = 100 fs) plasma shockwaves are formed as electrons impart energy to ions which radially propagate from the writing Chapter 2. Background 12 lasers focal volume; these shockwaves create a void encased by an encasement of higher index material by virtue of mass conservation [43].

The aforementioned formation of longitudinal nanogratings in glass can and has be exploited in a process known as femtosecond laser irradiance followed by chemical etching (FLICE), in which the presence of gratings is used to facilitate the flow of etchant (e.g. hydrofluoric acid, potassium hydroxide) to preferentially etch a 3D laser-written pathway [56–58]. Nanogratings written with a laser E~ -field perpendicular to scan direction yield longitudinal nanograting tracks which are useful to form smooth capillary sidewalls by facilitating etchant flow. Circular polarized light can be used to enable etching with less directional dependency at similar fast etch rates, but with rougher sidewalls [56]. Perpendicular nanograting tracks written with an E~ -field parallel to scan direction can be used as etch stops to form smooth cavity sidewalls.

2.1.3 Laser Processing of Polymers

In principle, many of the laser modifications outlined in Section 2.1.2 are possible in polymers as well as glasses, but the material properties of polymers dictate alternate physical interactions by the laser. In this thesis, fused silica glass fiber overcoated with polyimide and urethane acrylate polymers are processed and focused through. Poly (methyl methacrylate) (PMMA) is an acrylate which is similar to the urethane acrylate coating found on SMF-28, the industry standard for fused silica optical fiber. The exact composition of this urethane acrylate is a trade secret as it is a proprietary material [59, 60]. Polyimide is another popular buffer material for silica fiber, due in part to a high thermal stability [35].

Polymers are susceptible to laser incubation eﬀects, an accumulated material damage in weakly absorbing materials. In incubation, ablation does not occur at threshold pulse energy, but rather after an accumulation of material defects over many sub-threshold pulses [61]. In a scanning exposure procedure, the number of pulses which are absorbed can be determined by how many pulses are dissipated within a focal volume during a scan, determined from dwell times by the repetition rate, scanning speed, and focusing Chapter 2. Background 13 conditions. The size of a heat-aﬀected zone, can be modelled using Equation 2.2,

∂T (~r, t) − K ∇2T (~r, t) + ρC = (1 − R)I(~r, t)α (~r, t), (2.2) T p ∂t eff where KT represents thermal conductivity, T (~r, t) represents temperature profile, rho represents material density, Cp represents specific heat capacity, R represents reflectance, I(~r, t) represents intensity profile, and αeff represents the effective absorption coefficient [37, 42]. The first term of left-hand side of the equation describes the redistribution of heat within the solid, while the second term describes heat absorbed by the solid. The right-hand side of the equation describes laser energy dissipation within the material. If the size of the heat effected zone is significantly smaller than the laser penetration depth and larger than the writing beam waist, then a three-dimensional radial heat flow model can be assumed. If the penetration depth is smaller than the heat accumulation radius (for instance, because of limited physical dimensions), then a two-dimensional cylindrical heat flow model can be assumed.

2.2 Laser Processing of Lab in Fiber

Lasers open a host of possibilities for finely machined structures in and around fibers. In this thesis, the principle laser-induced technologies which lay the foundation for LIF technology are broadly identified around structures that can be fabricated in / near the fiber core (Section 2.2.1), cladding photonics (Section 2.2.2), and FLICE-enabled microfluidics in fiber (Section 2.2.3). In this section, the fundamental principles and a brief review of these three categories of fiber devices are reviewed.

2.2.1 Laser Processing in / near the Fiber Core

An early example of laser-fabricated fiber optic sensors is the fiber Bragg grating (FBG), first invented by Hill et al. [40]. FBGs are fibers in which a linear periodic refractive index modulation is induced in the core waveguide which reflects a narrow spectrum of light. The multiple reflections at periodic index modulations result in a strong, sharp, reflection band centered at the Bragg resonance (λB)[40]. A desired peak reflection can be designed by controlling the periodicity of the grating (Λ) using Equation Chapter 2. Background 14

2.3: mλ Λ = B (2.3) 2neff FBGs can be fabricated by inducing thermally driven, reversible Type I index modifications using phase mask delivery systems with UV light [40], or by inducing damage-driven, irreversible, Type II index modifications using pulsed laser light delivered either through a phase mask [62] or with direct point-by-point writing procedures [63]. Variations of FBGs types include apodized [64], chirped [65], and phase-shifted [66] FBGs in which the reflection spectra of the device are side-lobe supressed, broadened, or interrupted by an even sharper reflection dip nested inside the spike (like a band-pass filter), respectively. This approach can be extended to produce grating structures, such as tilted FBGs [67] and long period gratings (LPGs) [68], which are fabricated by similar means, but function by coupling light out of the fiber core into cladding modes that can couple back into the core. This is attractive in sensor applications at the cladding surface.

There are numerous LOF sensors which use gratings as a tool for collecting and conveying information about the outer-fiber environment. For example, Shevchenko et al. used UV-phase mask processes to fabricate tilted FBGs in SMF-28 which were then coated with gold and submerged in a medium where bacteria adhered to the fiber. The tilted FBG coupled light into cladding modes which interacted with the thin coating of gold on the outer fiber. The surface plasmon resonances drew light out of the fiber, resulting in a transmission dips in light collected back into the tilted FBG corresponding to stimuli which trigger detachment, serum uptake, or metabolic inhibition [12]. In another LOF device developed by Nemova and Kashyap, a FBG was fabricated in a large-core (multimode) fiber with a relatively thin cladding. Light which leaked out of the core interacted with the outer-fiber environment to act as an evanescent sensor [69]. UV-written LPGs have also been developed for LOF applications, including by Koba et al. who developed an LPG fiber coated with adhesive proteins which promoted selective binding to bacteria or bacterial toxins, drawing cladding modes out of the fiber [13]. In another device, designed by Wang et al., the LPG fiber was coated with alternating layers of poly(acrylic acid) (PAA) and poly(allyamine hydrochloride) (PAH). The refractive index of the PAA / PAH structure shifted in the presence of amine compounds (e.g. ammonia), changing the LPG Chapter 2. Background 15 transmission signal to work as a fiber-optic ammonium sensor [14]. LOF technology is not limited to passive devices. Chen et al. have demonstrated that light can be tapped out of FBGs using ion implantation, with the optical energy then used to activate transducers (i.e. heating a metal film), for an active fiber device capable of adjusting sensing parameters (i.e. temperature) [70].

Gratings are not the only type of laser-written structures positioned in and about the fiber core in order to add increased levels of functionality to the platform. It has been demonstrated that creating laser-induced stresses adjacent to the fiber core can tune the birefringent properties of light guiding through the core for polarization-based analyses [71]. If parallel modifications are written close enough to the fiber core it is possible to couple light into supermodes over the pre-existing waveguide core and laser- formed waveguide region, which may then be used as a multimode interferometer with periodic spectral modulations used to monitor the fibers environment [72].

2.2.2 Cladding Photonics

While the waveguide core of optical fiber is where light is confined in standard optical communications applications, the fiber cladding accounts for 99% of the platform by volume. It has been demonstrated that light confined to the core can be tapped out by writing waveguides in the fiber cladding. Cladding waveguides fabricated using the femtosecond laser direct-write procedures are lossier than pre-existing core waveguides. Cladding waveguide losses are reported as ≥ 0.5 dB/cm [20], as compared to 0.2 dB/km for SMF-28 core waveguides [59]. High losses for cladding waveguides make cladding photonic circuits in SMF-28 best suited for short-range (millimeter - centimeter) application. Cladding taps can be fabricated using cross-couplers, directional couplers, and S bend couplers [16, 17] as shown schematically in Figure 2.1. Light is guided out of the fiber core into one or more cladding waveguides extending into different azimuthal directions such that information can be processed across numerous independent channels, and then recombined into the core for differential analysis. Within these cladding waveguides, modulations in laser energy application can create Bragg grating structures known as Bragg grating waveguides [16]. Examples of such devices include a Mach-Zehnder interferometer in which light is tapped out of Chapter 2. Background 16

Figure 2.1: Schematic demonstrating diﬀerent types of cladding waveguide taps, (a) cross coupler, (b) S-bend coupler and (c) directional coupler. Image is reproduced, with permission, from Figure 4.5 of Grenier et al. [20] c 2015 Springer.

the pre-existing waveguide into a spur waveguide written in the fiber cladding and then tapped back into the fiber core. Although the physical path lengths of these two waveguides are similar, the beta mismatch between the core and spur waveguides augments the optical path length difference [20]. Another example of a cladding photonic device is a three-dimensional shape sensor, first demonstrated by Lee et al [16]. In this device light was coupled out of the fiber core into two perpendicular waveguides written at 90◦ to one another cross-sectionally. At three axial positions, three Bragg grating structures were written into the core and two cladding waveguide arms (nine total) at different Bragg resonances. This nine-grating structure was then repeated at different wavelengths further along the fiber at a greater radial distance from the fiber core. By simultaneously monitoring the reflections in these gratings coupled into the fiber core and how they shift, it was possible to determine the strain applied to each grating, and then using known constants determine the three-dimensional bend-profile of the fiber in real-time [16] (device schematic shown in Figure 2.2). In addition to knowing a fiber’s bend profile in real time, cladding photonics can be used to determine a fiber’s torsion. Fernandes et al. demonstrated that by fabricating two helical cladding Bragg grating waveguides twisting around the Chapter 2. Background 17

Figure 2.2: Distributed Bragg grating waveguide 3D shape sensor: (a) device schematic showing distributed BGW network, (b) cross-sectional microscope image showing coupling region into perpendicular waveguide, and (c) cross-sectional microscope image showing perpendicular BGW sensing region. Image is reproduced, with permission, from Figure 1 of Lee et al. [16] c 2013 OSA.

fiber core with opposite chirality it was possible to know the degree and orientation of torsion applied to the fiber. This is because when the fiber was twisted, one Bragg grating would straighten out while the other would elongate, resulting in different shifts in their Bragg resonance [21].

2.2.3 Microﬂuidics in Fiber

The third element of accomplishing sensing capabilities in fiber is to draw analyte into the glass fiber through microfluidic ports, channels, and reservoirs. The simplest approach is to directly ablate the glass using direct laser energy, but this is a very volatile process and lacks the finesse necessary to create more sophisticated microfluidic networks [73]. Alternatively, glasses can be sensitized to etchants using laser exposure, such that when the glass is submerged in a bath of etchant, the laser-written tracks will Chapter 2. Background 18

Figure 2.3: Polarization-sensitive nanogratings formed in bulk fused silica by scanning ultrafast laser pulses, with writing laser polarization state (a) perpendicular to scan direction, yielding longitudinal nanograting structures, and (b) parallel to scan direction, yielding lateral nanograting structures. Image is reproduced, with permission, from Figure 4 of Hnatovsky et al. [27] c 2005 OSA. etch preferentially. One such example is photo-structuring of glass ceramics (PSGCs) (i.e. Foturan), in which UV light followed by thermal processing ceramizes the exposed regions of glass, making them dissolve in dilute hydrofluoric acid (HF) etchant [74]. Another method, which is explored in this thesis, is the use of FLICE. As discussed in Section 2.2.2, exposing fused silica to ultrafast laser pulses can create polarization sensitive linear nanogratings, as demonstrated by Taylor et al. [25], and depicted in Figure 2.3. Others, including Osellame [75], Mihailov [56], and Ho [76] further demonstrated the use of such nanograting structures to facilitate the flow of etchant (e.g. hydrofluoric acid [27], aqueous potassium hydroxide [28]) such that relatively smooth channels could be etched out in bulk fused silica. Laser-formed nanograting track etching enables complex fluidic geometries to interact with light passing through the fiber core or cladding waveguides, but it is a challenge to balance the complexity and density of desired fluidic networks against the structural integrity of the fiber. Fluidic fiber devices have been explored using both ablative and sensitized etching processes. Simple laser ablation has been used by Wang et al. to open microcavities which partially cross the fiber core. When the fiber was immersed, fluid filled the cavity such that light passing through the core was split at the cavity, with some passing through the filled cavity and some through the remaining core. When the light recombined after the cavity a Mach-Zehnder interference effect could be analyzed to monitor the refractive index of the fluid [73]. A similar device was demonstrated by Sun et al. in which FLICE with HF etching was used instead of direct Chapter 2. Background 19 laser ablation. This imparts less damage onto the fiber and results in a more sensitive LIF sensor [77]. The finesse of FLICE has been used to build well-defined microchannels leading to reservoirs which completely overlap with a portion of the fiber core. Yuan et al. demonstrated such a reservoir formation in which sidewalls are smooth enough to elicit a Fabry-Perot response inside of the cavity to monitor refractive index [78]. Zhou et al. demonstrate a similar Fabry-Perot LIF device in which FBGs flank the reservoir to act as high-reflectance mirrors [79].

Combining laser-written waveguides and FLICE-formed microfluidics enables 3D freeform optofluidic applications which have previously been demonstrated in bulk fused silica. For example, Maselli et al. demonstrated evanescent probing of a FLICE-formed microstructure with a Bragg grating waveguide [80]. The integrability of microfluidics with cladding photonics and core structures in fiber have perhaps best been demonstrated in the work of Haque et al. wherein a multiplexed LIF device was demonstrated in which microfluidic channels and optically smooth (12 nm rms) reservoirs were probed by the core waveguide as well as cladding photonics to enable simultaneous monitoring of fluorescence, temperature, strain (bend), and refractive index [15] (Figure 2.4).

In this section, the principles and foundations of LIF were discussed exclusively within the context of silica glass optical fiber (coreless, single-mode, and multi-mode). Fiber optic sensors have also been demonstrated in fiber waveguides in which the core and cladding are composed of other types of glasses (e.g. fluorinated glass, chalcogenides, sapphire) and polymers (e.g. PMMA, polystyrene and polycarbonate) [81]. Polymer optical fibers (POFs) are sometimes preferred to glass fibers for physical properties including elasticity, toughness, and thermo-optic coefficient [81], and have been used in applications including optogenetic brain probing [82]. POFs are most commonly multi-mode fiber waveguides, with a graded index profile achieved through doping [81]. As in glass fibers, precise index modifications have been demonstrated to yield FBGs in POFs using UV exposure of cores doped to increase photosensitivity [83]. Although femtosecond laser irradiation cannot be used to produce longitudinal nanograting tracks in polymer fibers, CO2-laser ablated surface microchannels have been demonstrated in PMMA fiber for on-fiber fluorescence sensing [84]. Buried microchannels have be fabricated in bulk Chapter 2. Background 20

Figure 2.4: Multiplexed LIF device integrating laser-formed structures in the fiber core, cladding photonics, and FLICE-opened microfluidic channels and resonators capable of simultaneously monitoring fluorescence, temperature, strain, and refractive index. Image is reproduced, with permission, from Figure 1 of Haque et al. [15] c 2014 The Royal Society of Chemistry. photosensitive polymer such as SU-8 [85], and could be applied to POFs. The principles of LIF are applicable to POFs, and POFs present unique opportunities for LIF applications. However, the nature of laser-induced modifications in POFs is distinct from modifications in silica optical fiber. As such, further discussion of non-silica optical fiber is beyond the scope of the present work.

2.3 Optical Fiber Buﬀers

As discussed in Section 2.1, optical fibers are typically packaged in a protective buffer coating to impart a number of desirable properties onto the fiber including mechanical / thermal sturdiness, biochemical inertness / compatibility, and desirable optical characteristics. Commonly used materials include polyacrylates (e.g. PMMA Chapter 2. Background 21

[86]), polyimides (e.g. Kapton [35]), polyethylenes (e.g. Tefzel [87], Teflon [88]), polyamides (e.g. nylon [87]), and silicone [88]. The fiber is typically coated with buffer during the drawing process, with the buffer broadly uniform over the length of the fiber.

To date, the majority of laser-fabricated optical fiber sensors, including those discussed in Section 2.2, are processed after the fiber has been stripped of its protective polymer buffer. One of the desirable properties of urethane acrylate, the buffer material used in industry-standard SMF-28, is easy-stripping [31]. Buffer stripping allows for laser processing to be done in a nearly optically homogenous system, especially if index-matching oil is applied between the focusing objective and the fiber surface, as the index mismatch between glass and polymer is avoided. This bypasses many challenges in alignment and focusing distortion, as well as issues with opacity, absorption, and incubation in the polymer. If damage is done to the polymer buffer while laser-processing the glass fiber, polymer ablation damage debris can interfere with the deeper beam propagation either by introducing additional optical inhomogeneities and / or by damaging the optics. Furthermore, the polymer buffer is not designed as an optical system, as so it is subject to a higher tolerance of non-unformities (± 5 µm [59]) than the underlying optical fiber.

For these reasons, it is considerably simpler to strip a fiber of its buffer, and then laser structure the bare optical fiber (core and / or cladding). This is often followed with recoating of a polymer buffer. This approach, while convenient, increases processing time and compromises the mechanical integrity of the device [31], which are particularly salient concerns for the eventual mass-production of sensors. In the case of microfluidics in fiber, recoating is not a viable option as the recoated buffer would block access channels, inhibiting the flow of analyte into the fiber sensor.

In spite of these challenges, there has been limited work demonstrating laser fabrication of sensing elements in the core of optical fiber through a protective polymer buffer (i.e. without stripping). This was first demonstrated by Starodubov et al. using UV exposure of a hydrogen-loaded core through a phase mask to produce FBGs. The conventional mid-UV writing wavelength (242 nm) absorbs strongly in the acrylate coating of SMF-28, so near-UV exposure was necessary (334 nm) [32]. Phase-mask based methods have the benefit of being able to mass-produce many identical Chapter 2. Background 22 structures in a relatively short span of time, but are fundamentally limited to the waveguide core (i.e. incompatible with cladding photonics) and relatively inflexible as compared to direct write processes. Mihailov et al. later demonstrated that FBGs could be fabricated using infrared femtosecond laser exposure through both acrylate-coated [33, 34] as well as polyimide-coated [35] fibers. While this avoids having to use near-UV exposure, it still suffers from the rigidity of phase-mask based processes. Martinez et al. were able to first demonstrate point-by-point direct write of FBGs through acrylate-coated fiber, but this is done by matching the scan-speed of the writing laser its repetition rate such that each pulse fired produced one grating pitch [31, 36]. This process works for producing FBGs through coated fiber, and can be modified to produce structures such as apodized or chirped FBGs, but is less suitable for integration with cladding photonics than burst-train methods (see Section 3.1) employed in this thesis [37].

More recently, there has also been limited work demonstrating laser fabricating cladding photonics in optical fiber through a protective polymer buffer reported by Schade et al. In this work, light was tapped out of a single mode fiber core into perpendicular Bragg grating waveguides such that the relative grating shifts in different arms could be monitored to keep track of the fiber’s three-dimensional bend profile [38]. Further, although cladding photonics without significant material damage was reported, demonstration and discussion of damage-free processing was not presented.

There has yet, to our knowledge, not been any reported research on developing microfluidics in coated optical fibers, nor any substantive discussion on the nature and limitations of polymer damage limits when focusing the laser into the core and cladding of optical fiber. This thesis seeks to fill these gaps and develop a more thorough account of the possibilities of LIF through polymer-coated optical fibers through modifications in the core and cladding of coated optical fibers as well as the development of microfluidics in coated fibers. Chapter 3

Methods

In this chapter, experimental set-up and methods are discussed, illuminating additional contributions made throughout the course of the present work. In Section 3.1, the laser and optical delivery systems used in this thesis are described. In Section 3.2, techniques for preparing and handling optical fibers are presented, with emphasis on the challenges and methods developed for buffer-coated fiber. Section 3.3 briefly describes processes for creating femtosecond laser induced modifications in buffer-stripped optical fiber developed by predecessors, with contributions made in this thesis described in Chapters 4 and 5. Section 3.3.4 describes the process of buffer ablation from a technical standpoint, with research into ablation optimization described in Chapter4. Lastly, in Section 3.4, methods used to probe and characterize results and devices are discussed briefly.

3.1 Femtosecond Laser Processing System

The fiber-amplified femtosecond laser and optical beam delivery system employed is described here briefly, with necessary information pertaining to use and control of the system as is relevant to the following chapters of this thesis. More thorough descriptions can be found in the PhD theses of Dr. Shane Eaton [44] and Dr. Moez Haque [89]. The laser processing system is divided into four sections, as depicted schematically in Figure 3.1 and through a series of pictures in Figure 3.2: the fiber laser and pulse energy attenuation mechanism (Figures 3.1(a) and 3.2(a)), the acousto-optic modulator (AOM) gating pathway (Figures 3.1(b) and 3.2(b)), the second-harmonic generation

23 Chapter 3. Methods 24

(SHG) pathway (Figures 3.1(c) and 3.2(c)), and the fabrication / ﬁber alignment pathway (Figures 3.1(d) and 3.2(d)).

Chirped femtosecond pulses of 1045 nm light were produced by an IMRA America Jewel D-400-VR Yb-fiber-amplified laser (“laser head” in Figure 3.1(a)). Pulses were spectrally chirped to enable pulse amplification without incurring undue nonlinear effects and / or damage. The mode-locked repetition rate (νRR) of these pulses could be tuned using the laser control module, ranging between 100 kHz and 2 MHz. The work presented in this thesis was all conducted using a mode-locked repetition rate of 500 kHz. The spectrally chirped pulse was compressed by a chirped-pulse amplification (CPA) system (“CPA compressor” in Figure 3.1(a)) using bulk gating compression to spectrally tune optical path lengths such that the chirp induced prior to amplification was undone, emitting spectrally compressed 300 fs pulses. The emitted light was then linearly polarized by a rotating half-waveplate (Thorlabs WPH05M -1053) affixed to a rotational stage (Aerotech ART310 rotational stage) (“rotating waveplate” in Figure 3.1(a)), with the angle of polarization tuned using an Aerotech A3200 controller. The linearly polarized light was then passed through a horizontal polarizer (CVI PS1047-050) (“polarizer” in Figure 3.1(a)), such that the angle of polarization relative to the horizontal laser polarizer attenuated the energy of the pulses. The angle of the rotational stage thereby acted as a means of pulse energy (EP ) attenuation. The polarizer was aligned to ensure that the beam was linearly polarized, with E~ -field parallel to the surface of the optical table.

The linearly polarized light was then passed through an AOM (Neos 23080-3-1.06) (“AOM gate” in Figure 3.1(b)) powered by a digitally-modulated radio-frequency (RF) driver (Neos 21080-2DS) (modulation represented by “RF signal to AOM” in Figure 3.1(b)). Zeroth order light was sent to a beam blocking element (i.e. absorbed) while the first order diffracted path lead to the SHG track and eventually the substrate. The position of the beam entering the AOM was optimized for the first order beam. The AOM enabled rapid modulation of beam delivery (depicted in Figure 3.3) by periodically triggering the AOM in order to (1) block and restart the beam such that uniform exposure tracks and patterns could be formed in the fiber during scanning, (2) to direct-write periodic grating structures such as fiber Bragg gratings (FBG) in the fiber core or Bragg grating waveguides (BGWs) in fiber cladding [37], and (3) to down count the repetition Chapter 3. Methods 25

Figure 3.1: Modular schematic of the femtosecond laser system and optical delivery pathway used to fabricate lab in fiber devices. Not depicted are turning mirrors and focusing lenses used to align the beam-path and focus/collimate the beam as needed. (a) Laser and output power attenuation, (b) AOM pathway used to gate the beam, (c) SHG pathway used to convert the fundamental infrared frequency of the laser output (1045 nm) to visible light (522 nm), and (d) the fabrication/fiber alignment pathway used to focus laser energy into optical fiber and/or monitor reflections of the output laser light from the fiber in real-time. Chapter 3. Methods 26

Figure 3.2: Images of the femtosecond laser system and optical delivery pathway used to fabricate lab in fiber devices. (a) Laser and output power attenuation, (b) AOM pathway used to gate the beam, (c) SHG pathway used to convert the fundamental infrared frequency of the laser output (1045 nm) to visible light (522 nm), and (d) the fabrication/fiber alignment pathway used to focus laser energy into optical fiber and/or monitor reflections of the output laser light from the fiber in real-time. Chapter 3. Methods 27

Figure 3.3: Schematic depiction of how a pulsed laser output (depicted in green) can be gated by an RF signal to an AOM (depicted in red) to produce a modulated output (depicted in blue), either (a) to create pulse trains spaced out to create grating structures of deﬁned periodicity, grating [37], or (b) to down count the mode-locked 0 repetition rate of the laser pulses from νRR to νRR

rate (i.e. νRR < 100 kHz) to avoid over exposure and reduce heated aﬀected zones during fabrication. G-Code scripts were written to enable the third application of the AOM, external repetition rate down counting. These scripts enabled rapid switching between high and low repetition rates during a fabrication routine without otherwise putting the laser system on standby, as was necessary when laser-exposing glass cladding and acrylate buﬀer in tandem.

The first-order AOM-gated beam was then focused into a lithium triobate crystal (Newlight Photonics LBO1663) (“LBO SHG Crystal” in Figure 3.1(c)), wherein the fundamental 1045 nm wavelength was frequency-doubled to 522 nm, via second-harmonic generation. SHG efficiency was optimized using temperature, with a heating element in the crystal housing set to 170 ◦C, and phase matching through manual alignment. The polarization of the frequency-doubled beam was linearly rotated by 90◦ as compared to the polarization of the fundamental wavelength, aligning perpendicular to the optical table. A hot mirror (Thorlabs FM01) (“hot mirror” in Figure 3.1(c)) was used to reflect back remaining unconverted infrared light to a beam Chapter 3. Methods 28

blocker and ﬁlter through 522 nm pulses used for fabrication. Under some circumstances, infrared light was used for fabrication. For these experiments, the LBO crystal, hot mirror, and SHG focusing elements were removed from the optical table, bypassing the SHG pathway (Figure 3.1(c)). To compensate for the shift in polarization, a half-waveplate was added to the beam path to rotate the linear polarization by 90◦ in these experiments.

In the final section of the optical delivery system, pulses were focused through an objective lens (“objective lens (z-translation stage)” in Figure 3.1(d)). The experiments carried out in this thesis employed a 1.25 numerical aperture (NA) oil-immersion objective lens. The objective was mounted to an objective holder (Newport LP-1A) equipped with limited translation and rotation control to permit alignment of the beam through the center of the lens. The objective holder was affixed to a z-axis translation stage (Aerotech ALS130), which enabled the movement of the beam focus along the z-axis of the fiber. Throughout this thesis, references to the positive and negative z-hemispheres of the fiber refer the fiber hemisphere on the same side of and opposite the objective lens, respectively (see Figure 3.4). Procedures were designed to direct focusing light in the negative over the positive z-hemisphere of the fiber and thus prevent high focusing intensities from overheating the index-matching oil in the near vicinity of the objective lens, that would otherwise damage the lens. Light passing through the objective lens was focused in the fiber substrate, held in place by a precision fiber holder (“fiber holder (xy-translation stage)” in Figure 3.1(d)). The fiber holder was affixed to x- and y-axis linear motion stages (Aerotech ABL1000) to enable precise movement of the laser focus along the x- and y-axes, crossing axially and transversely through the fiber. Throughout this thesis, references to x- and y- axes refer to the long axis of the fiber and the radial fiber axis perpendicular to the axis of the laser focus, respectively, as depicted in Figure 3.4.A flip-mirror was placed in the path of light reflected from the top surface of the fiber was used to direct reflected beams to a charged-coupled device (CCD) camera (Sony XCD- X710) (“CCD Camera” in Figure 3.1(d)) for live monitoring, and when flipped-down the reflected beam would reach a beam stop (depicted by “flip mirror in path” element in Figure 3.1(d)). The fiber holder consisted of a goniometer to control fiber pitch and a rotating stage to control fiber yaw. Adjusting these components while monitoring reflections off of the fiber surface on the CCD camera enabled precise fiber alignment control. Chapter 3. Methods 29

Figure 3.4: Motion and rotation axes as deﬁned by Aerotech and manual translation stages relative to the ﬁber substrate and the focusing objective lens, in Cartesian (black) and cylindrical (red) coordinates. Rotational directions are depicted in blue, with roll, pitch, and yaw referring to rotations about the x, y, and z axes, respectively.

3.2 Fiber Preparation, Handling, and Alignment

Prior to laser exposure, all fibers were cleaned and prepared. This included stripping, cleaving, and splicing using apparati shown in Figure 3.5. Even though the fibers discussed in this thesis were fabricated with their polymer buffers intact, stripping was often necessary to permit direct laser expose to the fiber cladding for control experiments, as well as for cleaving and splicing. Acrylate buffers were either mechanically stripped from the fiber cladding using a standard fiber stripping apparatus (Miller CFS-2), or chemically dissolved using a toluene-based solvent. Stripped fiber surfaces were subsequently wiped with acetone and lens tissue to remove any acrylate debris remaining on the silica fiber cladding. Fibers chemically stripped with toluene took more time to prepare, but were more mechanically robust. Fibers coated with polyimide were found to be more challenging to strip, and required Chapter 3. Methods 30

Figure 3.5: Fiber preparation tools: (a) Fitel S324 diamond cleaver and (b) Fitel S182K fusion splicer

open-flame stripping in which the buffer was briefly exposed to an open flame, resulting in pyrolysis of the polyimide, which could then be wiped away using acetone and lens tissue. A high-temperature (110 ◦C) bath of sulphuric acid was another method of stripping polyimide buffer considered [90], but not conducted in this thesis due to additional safety procedures.

Stripped portions of fiber were cleaved using a diamond cleaver (Fitel S324) (Figure 3.5(a)) to produce clean end-facets, useful for fusion splicing and end-facet imaging. The cleaving apparatus used had limited precision in the location of the cleave-plane (relying on human-eye alignment), and so processes were developed to fabricate cleaving structures in fiber using femtosecond laser direct writing. A plane of nanograting modification tracks were fabricated perpendicular to the long axis axis of the fiber, extending 10 m in the z-axis from the bottom (-z) surface of the fiber cladding. These planes were opened by 5% hydrofluoric acid etching and manual force was applied perpendicular to the etched plane (i.e. by pulling the fiber taught), cleaving the fiber precisely at the laser mark. Although this process enabled a greater Chapter 3. Methods 31

Figure 3.6: Fiber holder consisting of ﬁber clamps, a goniometer (Newport GON65- L), and a rotating stage (Newport RS65) for control over tension, pitch, and yaw, respectively deal of precision in the position of the cleave relative to structures fabricated in the ﬁber, roughness from the etching process was evident in the end facets produced.

Splicing, the process of fusing together two fibers, was done using a fusion splicer (Fitel S182K) (Figure 3.5(b)). Splicing was used to connect segments of fiber in which LIF elements were fabricated to other pieces of fiber, for example patch-cords with FC / APC connectors that could be connected to sources or detectors. As fusion is a high-temperature process, erasure of laser-induced modifications in fiber segments was observed by Dr. Jason Grenier [72]. For this reason, fusion splicing was conducted prior to laser-fabrication of elements in fiber. After preparation, fibers were delicately loaded into a fiber holder depicted in Figure 3.6. The fiber holder consisted of a rotating stage (Newport RS65) to control fiber yaw (see Figure 3.4), a goniometer (Newport GON65-L) to control fiber pitch (see Figure 3.4), and an adjustable clamping mechanism to keep the fiber taught [91]. The fiber holder did not possess control over the roll rotational axis (see Figure 3.4), as the single-mode fibers used in this thesis were azimuthally symmetric. Prior to conducting any experiment, the fiber substrate had to be aligned. Alignment consisted of adjusting the fiber along the rotational axes and calibrating the position coordinates of the Aerotech motion stages. The pitch and yaw of the fiber were aligned to the movement axes of the motion stage such that the Chapter 3. Methods 32

long axis of the fiber was parallel to the x-axis of the motion stages (i.e. possessing no pitch or yaw). Position calibration consisted of identifying the y- and z- positions of the fiber interfaces (buffer / air, cladding / buffer, core / cladding) in terms of the motion stage coordinates. This was achieved by a two-step process in which the fiber was first roughly aligned using visual inspection of laser beams refraction through the cylindrical fiber and then more finely aligned by observing the position of laser beams directed by Fresnel reflections off of the fiber interfaces (described in more detail by Haque [89]). Precise alignment was particularly challenging for fibers coated in buffer with a thickness uniformity of only ± 5 µm (compared to ± 0.7 µm cladding diameter uniformity) [59]. The process was divided between monitoring reflections off of the buffer / air interface with only air between the objective lens and fiber, and then more precisely monitoring reflections off of the cladding / buffer interface after applying index matching oil matched to the polymer buffer (Cargille Series A Refractive Index Liquid, n = 1.4900). This process was more challenging when using infrared exposure as compared to visible light as visual inspection required the use of an infrared card and camera.

The Aerotech motion stage position for observing the smallest beam focus relative to the fiber surface shifts after the application of index matching oil by a constant value for a given oil and wavelength. This shift was defined in terms of the distance from the measured cladding surface in air compared to the fiber center in oil as z-offset (zoffset). The z-offset distance was determined empirically by fabricating FBGs at varying positions in the z-axis, and monitoring for a Bragg reflection for strongest response. Bragg gratings require a high degree of precise alignment to the fiber core for efficient reflection, this procedure was therefore a reliable means of alignment calibration.

3.3 Laser Processing of Fibers

Femtosecond laser machining of LIF devices in silica fibers stripped of their polymer buffer has been conducted and described in the PhD theses of Dr. Moez Haque [89], Dr. Kenneth Lee [91], and Dr. Jason Grenier [72]. In this section, processes described in these theses are described briefly with emphasis on laser and focusing parameters used. Writing wavelength (λ) refers to the wavelength of light striking / interacting with the Chapter 3. Methods 33

fiber substrate which was either the fundamental wavelength of 1045 nm or the second harmonic (Figure 3.1(c)) at 522 nm. Although most devices described in this thesis were fabricated using λ = 522 nm, fibers coated with a polyimide buffer (see Section 5.1.2) were preferably structured with λ = 1045 nm (i.e. bypassing the SHG pathway). Pulse energy was controlled using the rotating waveplate and linear polarizer described in Section 3.1. Waveplate rotation angles were calibrated with measured average power (Pavg) of the beam passing through the objective lens. Pulse energy was determined from average power using Equation 3.1.

Pavg EP = , (3.1) νRR

The repetition rate (νRR) of the laser was controlled using the laser module or using an acousto-optic modulator (see Section 3.1). The scan speed of the laser focus (vscan) refers to the scalar magnitude of the velocity at which the translation stages holding the fiber substrate (x- and y-translation stages) and objective lens (z-translation stage) move, which determined the speed at which the laser focus movies relative to the fiber. All results discussed in this thesis were conducted using an objective lens with a numerical aperture (NA) of 1.25 (oil immersion). This high-NA focusing enabled tight focusing to allow high intensities at the laser focus within the silica fiber core/cladding while maintaining relatively low intensities within the fiber buffer, preventing buffer damage.

Three-dimensional translation of the laser focus within the fiber was controlled by Aerotech stage motion control programmed using G-code scripts read by NView HMI v2.21 with a manufacturer-specified resolution of 2 nm. Power attenuation and AOM gating (see Section 3.1) was also controlled with the NView program, by the rotating polarizing waveplate controlled via a UU-axis and a digital signal triggering an RF signal to the AOM. Synchronization between the spatial axes motion, laser power attenuation, and the AOM enabled precise exposure and scanning control for three-dimensional structuring anywhere in the fiber core, cladding or buffer with localized variation in applied pulse energy, repetition rate, and gating.

3.3.1 Core Modiﬁcations

Methods for laser-exposure of the core waveguide of polymer-buﬀered SMF-28 explored in this thesis were based on procedures for fabricating grating structures in Chapter 3. Methods 34

buﬀer-stripped ﬁber described in the theses of Dr. Moez Haque [89] and Dr. Kenneth Lee [91]. Devices were fabricated by scanning the laser focus at a constant speed and

modulating RF signal to the AOM gate at frequency fAOM , assuming a 50% duty cycle. The modulating regions of fiber could then be exposed or blocked to create a modulating AC index profile in the fiber core to form a Bragg resonance (Equation 3.2) with 2n v f = eff scan . (3.2) AOM Λ Typical device lengths ranged between 4.5 mm and 1 cm. Adjacent FBGs were formed with 0.5 mm gaps to prevent spectral anomalies from the formation of resonance cavities. Typical pulse energies used to induce modifications in the fiber core using this laser system with a repetition rate of 500 kHz ranged from 20 nJ to 40 nJ. Scan speeds were explored

around vscan = 0.1 mm/s. The transmission and/or reﬂection spectra of FBGs were real-time recorded during fabrication (see Section 3.4).

3.3.2 Cladding Photonics

Cladding photonic elements fabricated in this thesis used methods based on the work described in the PhD theses of Dr. Jason Grenier [72], Dr. Kenneth Lee [91], and Dr. Moez Haque [89]. Grenier characterized a number of methods for coupling light out of and into pre-existing fiber cores using ultrafast laser direct-written cladding waveguides, based on S-bends, directional couplers, and cross couplers [72]. In this thesis, for simplicity, only cross-couplers were fabricated in acrylate-coated fibers. Cross-coupler devices examined here were based on devices designed by Dr. Moez Haque in which a linear cladding waveguide coupled light out of the fiber core, a circular bending waveguide reoriented the cladding waveguide to be parallel to the fiber core, and a linear cladding waveguide segment lead to a Bragg grating waveguide in the fiber cladding, shown schematically in Figure 3.7. The laser focus was programmed to precisely scan along the track illustrated in Figure 3.7 with specified geometry for the cross-coupler spur BGW. In this device, the axial offset distance of the linear coupling region (red in Figure 3.7) on the opposite side

of the grating (d1), the angle at which the linear coupling region crosses the ﬁber core

(θX ), the radius of curvature of the bending region (orange in Figure 3.7)(RC,XC ), the

radial distance from the center of the ﬁber to the grating structure (doffset), the axial distance from the coupling region to the start of the grating structure (green in Figure Chapter 3. Methods 35

Figure 3.7: Schematic of cross-coupler leading to cladding Bragg grating waveguide in single mode ﬁber divided between a linear coupling region crossing the x − ρ0 plane (depicted in red), a circularly bending curved region (depicted in orange), a linear spur waveguide parallel to the ﬁber core (depicted in green) and a Bragg grating waveguide (depicted in blue). Axial (x-axis) lengths of respective waveguide 0 components are denoted by li and radial (ρ -axis) lengths by di, with loffset and

doffset (depicted in purple) denoting the axial and radial distances, respectively, between the start of the grating region and the coupling position.

3.7)(loffset), the length of the grating structure (lFBG), and grating properties discussed in Section 3.3.1 were input into prepared G-code. From these inputs, the radial length of the linear coupling region on the same side as the grating (d2), the axial length of the linear coupling region (ll + l2) and the axial distance between the end bending region of the waveguide and the start of the grating region (l4) were calculated using the geometry illustrated in Equations 3.3, 3.4, and 3.5:

π π d = d − d = d − R × sin − sin − θ , (3.3) 2 offset 3 offset X,XC 2 2 X

d1 + d2 l1 + l2 = , (3.4) tan(θX )

d2 π π l4 = loffset − l2 − l3 = loffset − − RX,XC × cos − θX − cos . (3.5) tan(θX ) 2 2 The azimuthal alignment of the spur waveguide could be arbitrarily rotated in the yz plane. Cross-coupling angles were varied between 1.5◦ - 2.1◦ to obtain varying coupling efficiencies. The bending radius of curvature was fixed at 40 mm based on the work in stripped fiber of Grenier [72], Haque [89], and Lee [91]. High laser pulse energies are required for cladding index modifications as compared to modifications to fiber core Chapter 3. Methods 36

because the presence of Germanium dopant in ﬁber core facilitates index modiﬁcation.

In this thesis, cladding modiﬁcations were explored using pulse energies ranging from EP

= 50 nJ - 80 nJ. Scan speeds were explored ranging from vscan = 0.05 mm/s - 1 mm/s. The design of the cross-coupler spur BGW devices limited FBG analysis to probing of reﬂection spectra, which were real-time monitored during fabrication (see Section 3.4).

3.3.3 Microﬂuidics in Fiber

Microfluidic elements fabricated in fused silica fiber discussed in this thesis follow the method of femtosecond laser irradiation followed by selective chemical etching (FLICE) [56]. Methods for FLICE were based on those described in the PhD thesis of Dr. Stephen Ho using 5% hydrofluoric acid (HF) or 10 M aqueous potassium hydroxide (KOH), followed by dilution in deionized water and air drying [76]. Methods for etching fiber substrates followed those in the PhD thesis of Dr. Moez Haque [89]. Necessary safety precautions were undertaken when handling HF and KOH. Nanograting tracks were fabricated in fiber using a scanning laser focus with linear polarization perpendicular to the direction of fiber etching to align nanograting planes parallel to the etched channel. A half-waveplate was inserted in the beam path to switch the polarization orientation as necessary. More complex microfluidic channels that rotated in three-dimensions were opened more flexibly with circularly polarized light, which was generated by a quarter-waveplate inserted in the beam path. FLICE structures fabricated in this thesis include through holes, single scanned radial straight lines which cross the fiber diameter to reach the outer-cladding radius at two opposite radial ends. Blind holes were formed like through-holes but needed to reach the outer-cladding radius at only one end to enable chemical etchant access. Linear axial microfluidic channels were also fabricated which extended along the axial fiber axis, or helical channels could be formed to spiral in a radial motion in the yz plane while scanning in the x-axis. Axial channels exceeding hundreds of microns from primary inlets and outlets required periodic formation of access ports, consisting of radial line scans extending from the outer-cladding radius to the axial fluidic channel. Access ports were required to replenish etchant during the etching process, or else etchant drawn into long channels via capillary forces could be expended before reaching the end of the laser-written nanograting tracks and reduce etching efficiency [76]. In this thesis, Chapter 3. Methods 37

nanograting tracks were fabricated with pulse energies ranging from EP = 20 nJ to 60 nJ, based on previous work in fibers stripped of their polymer buffer. Here, lower pulse energies resulted in the formation of smoother channel walls but higher pulse energies were favourable in faster etch rates that offered less tapering around access ports [89].

Scan speeds were explored ranging from vscan = 0.1 mm/s to 1 mm/s.

3.3.4 Buﬀer Ablation

In order for chemical etchants to reach into the microfluidic access ports in silica cladding, the encasing polymer buffer had to be selectively machined. Buffer holes therefore were laser-fabricated by testing laser ablation in percussion, rastering, and trepanning procedures [92] as summarized in Figure 3.8. In percussion ablation (Figure

Figure 3.8: Diﬀerent approaches examined for laser buﬀer ablation to open access ports by (a) percussion, (b) rastering, and (c) trepanning. The top row illustrates the motion of the scanning laser, the middle row illustrates the beam path, and the third row illustrates the resulting structure.

3.8(a)), the substrate was stationary as the laser focus was linearly scanned radially through the buﬀer coating. While percussive drilling was a relatively simple and rapid Chapter 3. Methods 38

process, the small spot size of the writing laser focus (ω0 = 0.24 µm) was limiting in creating hole diameters. Raster scanning (Figure 3.8(b)) consisted of scanning the laser focus linearly along an array or matrix to open up rectangular structures. Raster scans had the advantage of thoroughly machining arbitrarily large spaces, but was relatively time-consuming, and was limited to openings with hard, 90◦ edges. With trepanning (Figure 3.8(c)), the laser focus spirals around a programmed surface area to carve out an ablated shell of material. This process is faster than rastering but leaves behind the interior unexposed conical buﬀer material that required additional eﬀort for removal.

In the context of this thesis, “damage-free” LIF refers to fiber devices in which unintended, or collateral, damage is absent from the fiber buffer. Intentional selective buffer machining for the purpose of enabling fluid flow to the fiber cladding is exempt in the characterization of “damage”. Selective buffer machining for precise microfluidics in buffered silica fibers had not been demonstrated prior to this thesis, and processes developed to accomplish this are discussed in Chapter4.

3.4 Optical Characterization

Cladding waveguides and fiber Bragg gratings were probed using a broadband laser source (Agilent 83437A) with a spectral range of 1260 nm - 1680 nm. Light travelling in the fiber core redirected by laser-formed modifications were monitored with an optical spectrum analyzer (OSA) (ANDO AQ6317B) using a spectral range of 600 nm - 1750 nm. Fiber samples were cleaved and spliced to patch cords to minimize coupling losses. Transmission spectra were monitored by probing light from the source into the fiber at one end of the fiber, and measuring light emitted at the other end of the fiber as read by an OSA. All spectra were normalized against a background spectra of the fiber and diagnostic arrangement prior to laser fabrication. Reflection spectra were monitored with an optical circulator (Thorlabs 6015-3-FC) in which a spliced patch chord connected one end of the fiber (e.g. the left end of Figure 3.7) to the circulator circuit which was connected to both the source and OSA. The circulator passed light from the source to the fiber substrate, and light reflected from the fiber substrate to the OSA. Qualitative characterization was assessed using white-light optical microscopy (Olympus BX51) with 10×, 20×, 40×, 50×, and 100× dry objective lenses. Fibers were monitored under the Chapter 3. Methods 39 microscope, both dry, and immersed in water or isopropanol underneath a microscope cover slip to improve on refraction distortions of the cylindrical geometry of the fiber. Fiber Orientation and scale were aided by laser-formed guide-marks. Chapter 4

Laser Modiﬁcations to the Polymer Buﬀer

This chapter examines the limitations of damage-free laser processing in acrylate coated silica optical fiber, both in terms of pulse energy as well as beam focusing position within the fiber. When processing with sufficiently high fluence to modify the cladding glass at positions near the cladding / buffer interface, the laser energy applied in the polymer in the form of directly applied fluence as well as through heat affected zones can cause damage or ablation within the buffer material. This is owing to the lower damage threshold of polymers as compared with glasses. While various groups [31–36, 38] have explored the laser-write process of optical elements in polymer-coated fiber, to date there has not been a study into laser damage to the fiber buffer while focusing inside the fiber. As noted in Chapter1, the goal of this thesis is to study and demonstrate the effectiveness and limitations of ultrafast laser writing of LIF devices in polymer-coated fibers. The first step before discussing the fabrication of devices is to establish the restrictions brought about by focusing light through a polymer coating and the means by which to work around these restrictions to fabricate functional LIF devices. In this chapter, these limitations are studied such that LIF devices in fiber can be designed and engineered with the foreknowledge of what can and cannot be achieved without the removal of the buffer coating.

In Section 4.1 empirical study is used to determine laser damage zones, mapping out radial and azimuthal boundaries of focusing varying degrees of laser energy inside

40 Chapter 4. Laser Modifications to the Polymer Buffer 41 silica fiber without causing damage to the surrounding acrylate buffer. In Section 4.2, underlying physical phenomena responsible for the damage discussed in Section 4.1 are compared and evaluated, including the consequences thereof. In Section 4.3, methods are discussed for selectively ablating the buffer without inducing unintended collateral damage. In Section 4.4, the effects of submerging coated fiber in silica etchants are explored, assessing the viability of creating microfluidic channels or other open structures by FLICE processing through the polymer-coated fiber. Finally, Section 4.5 summarizes the findings and significance of this chapter.

4.1 Buﬀer Damage Zones from Laser-Focusing in Fiber Cladding

Polymers have typically lower damage thresholds than glasses when laser processing, and so when focusing in the inner glass fiber, the susceptibility of the outer polymer buffer must be considered (see geometry outlined in Figure 3.4). During initial trials for laser-writing waveguides in acrylate coated fiber, focusing in silica glass fiber surrounded by buffer was found to yield significantly limiting exposure thresholds for buffer. This is demonstrated by the microscope images in Figure 4.1, where damage was found at the cladding / buffer interface when applying 70 nJ pulses at radial offset positions 40 µm from the fiber center (typical exposure conditions for cladding photonics (see Section 3.3)). In Figure 4.1(a), microscope images show a straight laser track formed at an angle of 2◦ with respect to the fiber center in the xz plane (see Figure 3.4) pictured approaching the -z cladding / buffer interface in the xz plane to a focal position at a distance of ∆z ≈ 22.5 µm from the cladding / buffer interface. The focus was thereupon scanned parallel to the x-axis. At this focal offset position of ∆z = 22.5 µm, laser damage was observed at the cladding / buffer interface. The damage appeared to be a blackening of the polymer at the interface, perhaps as a result of pyrolysis. The sources of laser damage to the acrylate depends on a number of laser focusing and writing parameters discussed in Section 4.2, but first, to account for all laser, writing, and material parameters involved in created LIF through acrylate-coated fibers, an empirical study was conducted to identify just how far from the fiber core varying laser pulse energies could be applied without imparting damage (as presented Chapter 4. Laser Modifications to the Polymer Buffer 42

Figure 4.1: Microscope images of acrylate-coated SMF-28 fiber with a laser track crossing the xz plane at an angle of 2deg, showing the onset of interfacial damage when at a radial distance of 40 µm from the fiber center (∆z = 22.5 µm from the cladding / buffer interface), with interface damage viewed in (a) the xz and (b) the xy planes; (c) Schematic of NA = 1.25 beam focusing in fiber cross-section depicting subthreshold fluence of 11.3 mJ/cm2 applied at the cladding / buffer interface when the laser is focused at a distance ∆z = 22.5 µm from the cladding / buffer interface

(λ = 522 nm, νRR = 500 kHz, vscan = 0.1 mm/s, EP = 70 nJ, NA = 1.25) Chapter 4. Laser Modifications to the Polymer Buffer 43

in Figure 4.1(a)) onto the ﬁber buﬀer. Standard fabrication parameters, as outlined in

Section 3.3, were used (λ = 522 nm, νRR = 500 kHz, vscan = 0.1 mm/s, NA = 1.25, EP = 20 nJ to 80 nJ). Axially spaced circular tracks were written clockwise in the cross section of an acrylate-coated ﬁber with radii ranging from 5 µm to 60 µm in intervals of 5 µm. The results of this study are summarized in Figure 4.2. Figure 4.2(a) depicts empirical damage-free of radial focusing distances (rdf), azimuthal, and of pulse energy. In Figures 4.2(a) - (c) the gradual decrease in damage free writing distances with increasing pulse energy are apparent throughout the ﬁber. Pulse energies between 20

nJ and 40 nJ could be focused at radial distances of at least rdf = 60 µm (the maximum distance measured), corresponding to an offset distance from the cladding / buffer interface of 2.5 µm. Buffer damage was first observed when applying 50 nJ pulses, with maximum damage-free radial focusing distances between rdf = 35 µm and rdf = 55 µm. Damage-free radial focusing distance was observed to decline from 50 nJ to 80 nJ, with damage observed when focusing 80 nJ pulses at radial distances as small as rdf = 25 µm. These trends are consistent with the understanding that buffer damage occurs as more energy is absorbed by the buffer, the mechanisms of which discussed in Section 4.2. Also apparent in Figure 4.2 are asymmetries in damage-free radial focusing distances (∆rdf ) azimuthally in the fiber, seen explicitly in Figures 4.2(b) and (c). Larger damage-free radial focusing distances were observed when focusing along the y-axis (Figure 4.2(c)) than along the z-axis (Figure 4.2(b)) 1 (∆rdf,avg = 8 ΣEP |(rdf,z+ + rdf,z−) − (rdf,y+ + rdf,y−)| = 10µm). Large asymmetries were also observed when focusing in the +z and +y hemispheres as compared to the -z and 1 -y hemispheres, respectively (∆rdf,avg,z = 4 ΣEP |rdf,z+ − rdf,z−| = 17.5µm, 1 ∆rdf,avg,y = 4 ΣEP |rdf,y+ − rdf,y−| = 10µm). Greater degrees of asymmetry were observed in the z-axis than in the y-axis, being most pronounced at higher pulse energies. For a given pulse energy, asymmetries as large as ∆rdf = 30 µm were observed (EP = 80 nJ, between the -z and +y hemispheres). Asymmetry observed between the y- and z-axes is consistent with damage arising from an ellipsoidal focal volume and Gaussian intensity profile (see Section 4.2.1), but cannot be predicted with a radially symmetric heat accumulation model (see Section 4.2.2). Another explanation for the greater damage-free radial focusing distances observed along the y-axis than z-axis is that the laser is focused through more polymer at larger radial offset distances along the y-axis, which induces aberration and defocusing effects, which could effectively reduce the intensity of the pulse at focus. Although index matching was employed (see Section Chapter 4. Laser Modifications to the Polymer Buffer 44

Figure 4.2: Empirically determined buffer damage limitations for laser-writing in acrylate-coated silica fiber (a) for all azimuthal positions depicted in a schematic of the fiber cross section wherein coloured regions indicate the maximum allowable pulse energy in that location, (b) damage-free focusing radial distances along the ± z-axis, and (c) damage-free focusing radial distances along the ± y-axis. The colour-gradient bar in the bottom right of the figure applies to Figure 4.1(a)-(c) with different pulse energies required for different LIF procedures denoted by colour. Chapter 4. Laser Modifications to the Polymer Buffer 45

3.3), the index mismatch between buffer and glass (ncladding = 1.446, nbuffer ≈ 1.49) enabled a degree of aberration, distortion, and Kerr nonlinear effects (see Section 2.1). The propagation distance of the laser through the buffer increases with oblique angle as

the oﬀset distance, yfocus, increases, according to Equation 4.1. s s 2 2 rbuff rclad tz(yfocus) = yfocus × − 1 − − 1 , (4.1) yfocus yfocus

Here, tz is the thickness of buﬀer through which the laser is focused along the z-axis, rclad

is the outer radius of the glass fiber, rbuff is the outer radius of the buffered fiber, and yfocus is the radial offset distance of the focus along the y-axis. Equation 4.1 indicates

that as yfocus approaches rclad the second term vanishes, increasing tz. The greater damage-free radial focusing range above the fiber core (+z) than below the fiber core (-z) may arise from a dependence of self-focusing effects on the rotational scan direction (clockwise in Figure 4.2(a)). Azimuthal asymmetry observed may also be influenced by pulse-front tilt, buffer-aberrations, and damage-zone propagation. Consequentially, Figure 4.2 presents practical boundaries for damage-free processing of LIF in acrylate- coated fiber. Pulse energies typically required for core modifications (20 nJ to 40 nJ) could be focused in fused silica at distances at least up to rdf = 60 µm from the fiber center at all azimuthal angles without producing damage in the buffer (Figure 4.2(a)). In single mode fiber, the fiber core only extends to a radial distance of 4 µm from the fiber center, but other fiber types, including multimode and multicore fiber, core regions can be found at greater distances. Pulse energies typically required for cladding photonics

(EP = 50 nJ to 80 nJ) were found to yield damage-free radial focusing distances between rdf = 25 µm (Figure 4.2(b)) and 55 µm (Figure 4.2(c)), depending on pulse energy and azimuthal angle. Pulse energies used to produce nanograting tracks (20 nJ to 60 nJ) may be applied without buffer damage at radial focusing distances up to at least 60 µm at all azimuthal angles (Figure 4.2(a)) for lower energy modifications (20 nJ to 40 nJ), but nanograting tracks written using higher pulse energies (50 nJ to 60 nJ) would be limited to as small a radius of rdf = 35 µm (Figure 4.2(b)). This suggests a limitation to damage-free microfluidics in acrylate-coated fiber to nanogratings written with pulse energies lower than 50 nJ, as access ports must be written the outer-cladding interface the enable flow of etchant (see Section 3.3.3). Chapter 4. Laser Modifications to the Polymer Buffer 46

4.2 Sources of Damage in the Polymer Buﬀer

Section 4.1 established limitations to how much pulse energy could be applied to fiber cladding and where without causing damage to the acrylate buffer, while remaining agnostic regarding the underlying physical phenomena responsible for this observed damage. In this section, two potential sources of this damage are evaluated and compared: direct, intensity-driven, absorption (Section 4.2.1), and indirect, accumulated, thermally-driven damage from heat-affected zones (Section 4.2.2).

4.2.1 Intensity-Driven Damage in the Buﬀer

Prior studies of femtosecond ablation of poly(methyl methacrylate), an acrylate material similar to the buffer coating of SMF-28, consistently report threshold fluences 2 2 (Fth) for single-pulse and for incubated machining at around 2.6 J/cm and 0.6 J/cm , respectively [93], [94]. To assess the potential for damage in our present fiber, a simple Gaussian beam analysis (see Equation 4.2) was applied to assess the fluence delivered to the nearest cladding / buffer interface when focusing in an offset position in the silica fiber for a range of pulse energies that would typically be used in LIF laser processing of fiber cladding (Section 3.3), yielding the graphs of interface fluence versus radial distance in Figure 4.3. Applied fluence at the nearest cladding / buffer interface was plotted as a function of radial distance from the fiber center along the z-axis (Figure

4.3(a)) and along the y-axis (Figure 4.3(b)) for a range of pulse energies, EP = 20 nJ to 80 nJ, which are typically used to fabricate LIF devices. The interfacial ﬂuence calculation assumes a Gaussian beam proﬁle (see Section 2.1) with typical laser

exposure conditions discussed in Section 3.3( λ = 522 nm, νRR = 500 kHz, vscan = 0.1 mm/s, NA = 1.25). The threshold fluence for incubated pulse machining of PMMA, 2 Fth = 0.6 J/cm [93] is represented with a black dashed line. Due to the high numerical aperture of the lens through which the writing laser is focused, fluence at the cladding / buffer interface falls well below the ablation threshold reported in literature over a wide range of radial distances [93]. This is especially prominent for radial displacement along the y-axis (Figure 4.3(b)). The asymmetry between focal displacement along the y- (Figure 4.3(b)) and z-axes (Figure 4.3(a)) evident in Figure 4.3 arises from the asymmetry of Gaussian beam profile as depicted by the conical Chapter 4. Laser Modifications to the Polymer Buffer 47

Figure 4.3: Fluence applied to the cladding / buffer interface as a function of focus position centered at varying radial distances from the fiber center for various laser pulse energies: (a) distance in the z-axis, and (b) distance in the y-axis; the dashed black line denotes 0.6 J/cm2, the expected threshold fluence for incubated pulse damage for PMMA [93]. Graphical depictions of interfacial fluence when scanning

along z- and y-axes are shown to the right of respective plots λ = 522 nm, νRR =

500 kHz, vscan = 0.1 mm/s, NA = 1.25). Chapter 4. Laser Modifications to the Polymer Buffer 48 shadow in the diagrams on the right of the graphs in Figure 4.3. The laser energy is delivered along the z-axis and focused to a minimum spot size, ω0 = 0.24 µm, the xy plane, as expressed in Equation 4.2. s 2 2 λz 2 ω(z) = ω0 M + M ( 2 ) , (4.2) πω0 As the laser is diffracted beyond the focus, the applied fluence decreases at a rate inversely related to the lasers area in the xy plane (πω2(z)). The relationship between radial distance in the y-axis and offset distance from the nearest top/bottom cladding / buffer p 2 2 interface (through which the laser is focused) is circular (z(y) = rclad − y ), which is why the interfacial fluence plotted in Figure 4.3(b) rapidly increases as radial focusing distance in the y-axis approaches rclad.

For pulse energies of EP = 20 - 40 nJ, typically used to induce modifications in the fiber core (see Section 3.3), one finds exposure above the incubated threshold fluence at the cladding / buffer interface when focused at radial distances between 61.07 µm and 60.48 µm along the z-axis (Figure 4.3(a)) and between 62.49 µm and 62.47 µm along the y-axis (Figure 4.3(b)). Hence, laser modifications of the core is expected to be far below threshold for buffer damage (rcore = 8 µm in SMF-28). Higher pulse energies typically required to fabricate cladding photonics, in the range of EP = 50 nJ - 80 nJ, only slightly narrowed these distances for threshold fluence at the cladding / buffer interface, yielding 60.24 µm to 59.64 µm in the z-axis (Figure 4.3(a)) and 62.46 µm to 62.43 µm in the y- axis (Figure 4.3(b)). In principle, these damage-free radial focusing distances would not pose a significant limitation for writing cladding photonics, where cladding waveguides can be written with pulse energies ≤80 nJ to radial distances within 5 µm of the cladding / buffer interface, making 99.4% of the cladding accessible. However, the data in Figure 4.3 show that intensity-driven buffer damage cannot be avoided for the formation of microfluidics in fiber where access ports for chemical etching are required to reach to the outer-cladding interface. Here, pulse energies ranging from below 20 nJ to over 80 nJ 2 (see Section 3.3.3) are found to produce interfacial fluence exceeding Fth = 0.6 J/cm at radial focusing distances of ≥ 59.64 µm from the fiber center (i.e. ≤ 2.9 µm from the cladding / buffer interface) in Figure 4.3.

The empirical studies discussed in Section 4.1, however, found that focusing in silica glass fiber surrounded by thermally-insulating buffer was found to yield substantially Chapter 4. Laser Modifications to the Polymer Buffer 49 lower exposure thresholds for buffer damage than predicted by the intensity-driven damage model presented in Figure 4.3. For example, in Figure 4.1, the onset of cladding / buffer interface damage at an offset distance of ∆z = 22.5 µm is associated with an interfacial fluence of 11.3 mJ/cm2 according to Gaussian beam analysis shown in Figure 4.3. This value is significantly lower than the incubated pulse threshold fluence of 0.6 J/cm2 reported in literature [93]. Further, the damage-free radial focusing distances predicted by the direct-intensity model in Figure 4.3 fails to account for the empirically observed limitations over the range of pulse energies EP = 50 nJ to 80 nJ presented in Figure 4.2. Hence, interfacial damage cannot be the sole result of local, intensity-driven ablation or incubation effects.

4.2.2 Thermal Damage in the Buﬀer

Though comprehensive study has not been carried out to identify the precise nature and composition of the type of damage presented in Figure 4.1 and studied in Figure 4.2, the damage is visually consistent with accounts of sub-ablative damage found in literature for acrylate [93]. Such damage would be plausible in this system, as the buﬀer encases fused silica, serving as a thermal barrier for heat generated in the ﬁber cladding to dissipate. While glass can withstand relatively high temperatures, heated glass can in turn induce thermal damage in lower threshold polymer materials when in direct contact.

To assess such thermal damage, a heat accumulation model written by Dr. Haibin Zhang [95] (Figure 4.4) was applied following Section 2.1.2. This program modeled a radial temperature profile as a function of the number of laser pulses applied in a laser dwell time. The model gives radius of the heat accumulation (rHA) for which a critical temperature is reached, assuming the heat affected zone is significantly larger that the focal volume of the writing laser, and smaller than the physical boundaries of the material in which the laser is focused, such that a spherical three-dimensional heat flow can be assumed. A static exposure assumption was applied, such that the heat affected zone for the scanning laser process was calculated assuming the laser applied a number of pulses within a focal volume corresponding to the laser’s dwell time. This assumption was empirically validated by Eaton et al. in previous work [42]. During the effective time in which laser pulses are applied, a strong increase in temperature occurs within the Chapter 4. Laser Modifications to the Polymer Buffer 50

Figure 4.4: (a) Heat accumulation radius (rHA) calculated at critical temperature ◦ (Tg = 108 C), for PMMA plotted as function of number of applied pulses when J kg focusing 20 nJ to 80 nJ energy in fused silica (CP = 770 kg•K , ρd = 2200 m3 , DT = −7 2 7.5 ×10 m /s, 78% absorption). The dashed black line represents NP = 767, the number of applied pulses when using standard fabrication parameters (λ = 522 nm,

NA = 1.25, νRR = 500 kHz); (b) Schematic of fiber cross section with when focusing the laser at an offset distance of ∆z = 22.5 µm from the cladding / buffer interface

(Figure 4.1(c)), with the predicted rHA = 17 µm depicted in red and a predicted damage-free writing ellipse following the intensity-driven damage model from Figure 4.3 depicted in magenta.

laser’s focal volume, building on top of a slowly rising Gaussian heat distribution. It was understood, based on the work of Eaton et al., that after the application of pulses ceases, thermal diffusion effects dominate over heat accumulation effects such that dissipated thermal energy does not further increase heat affected zones (i.e. the Gaussian heat Chapter 4. Laser Modifications to the Polymer Buffer 51

distribution spreads but not grow) [42]. The modeled laser exposure conditions were

based on standard parameters outlined in Section 3.3( λ = 522 nm, νRR = 500 kHz, NA

= 1.25, EP = 20 nJ to 80 nJ), and NP = 767 pulses applied at scan speed (vscan) of 0.1 mm/s (NA = 1.25, λ = 522 nm). The model was used to simulate heat accumulation focusing in fused silica, with fused silica thermal properties defined as heat capacity, CP J 3 = 770 kg•K [96], material density, ρd = 2200 kg/m [96], thermal diffusivity, DT = −7 2 7.5 ×10 m /s [96], and 78% absorption [44]). The critical damage radius, rHA was ◦ determined for a critical temperature of Tg = 108 C, the glass transmission temperature for the acrylate buffer. Figure 4.4(a) presents radial offset limitations to laser processing in the cladding of polymer coated optical fiber due to heat accumulation effects. Within the range of pulse energies of 20 nJ to 40 nJ, typically used to induce core modifications, ◦ the radius of heat accumulation reaching 108 C was found to be between rHA = 6.05

µm and rHA = 11 µm, which translates to a damage-free radial focusing distance (rdf ) of 56.46 µm to 51.5 µm from the fiber center. Higher pulse energies of 50 nJ to 80 nJ, typically used for cladding photonics, were found to correspond to heat affected ◦ radii for Tg = 108 C of rHA = 11 µm to 18.71 µm, which predict damage free radial focusing distances of rdf = 49.34 µm to 43.79 µm. Figure 4.4(b) shows the cross-sectional schematic for 70 nJ pulses focused with NA = 1.25 at a distance ∆z = 22.5 µm from the cladding / buffer interface first shown in Figure 4.1(b). In Figure 4.4(b), the radius of ◦ heat accumulation reaching Tg = 108 C, rHA = 17 µm, is shown in red surrounding the

laser focus. A magenta ellipse of vertical radius rdf,z = 2.67 µm representing the region 2 in which threshold fluence Fth = 0.6 J/cm is applied is also shown surrounding the laser focus, based on the intensity-driven damage model in Figure 4.3. Although neither the thermal model nor the intensity model predict the damage at an offset distance of ∆z = 22.5 µm from the cladding / buffer interface (as seen in Figure 4.1(a)), the thermal

model is closer by an order of magnitude (∆z = 22.5 µm > rHA = 17 µm > rdf,z = 2.67 µm).

Laser energy absorbed in the cladding and accumulated in form of heat affected zones account for discrepancies between damage-free radial distance and sub-incubated- threshold interfacial fluences applied. Although the heat accumulation model presented in Figure 4.4 is clearly more representative of the experimental observations in Figure 4.1 than the interfacial fluence model presented in Figure 4.3, there are a number of simplifications made in the specific program model that limit the precision in predicting Chapter 4. Laser Modifications to the Polymer Buffer 52

damage-free focusing zones in acrylate-coated fiber. For example, this model was designed for a homogenous system of bulk glass, and does not account for additional absorption or defocusing effects caused by focusing through a layer of polymer, or for the boundary effects of the fibers cylindrical geometry. Hence, the estimated absorption of 78%, which was based on previous empirical study in bulk fused silica, may not accurately represent the laser heating conditions. Further, while the glass transition temperature of PMMA was used as a critical temperature for modelling a heat affected zone, the microscope images of the damage suggest a thermal damage effect by pyrolysis. Raman spectroscopy could be used to analyze the bonds present in the blackened polymer to determine its composition, and whether long polymer chains had been severed or rearranged. This analysis is left as a subject of future work. The glass transition temperature of the acrylate buffer may differ from PMMA and is undocumented (Section 2.3). Finally, the model neglects the effects of physical boundaries, precisely where the interaction of the heat-affected zone with a physical boundary is being studied. When the heat- affected zone reaches the cladding / buffer interface, the different thermal properties of

the polymer buffer (e.g. CP , DT ) are expected to increase heat accumulation effects, modifying the size and physical profile of the heat-affected zone such that the present spherical model becomes imprecise.

Nonetheless, it still provides valuable physical insight into the nature of buffer damage when focusing high-repetition rate laser pulses in silica cladding, which is quantified in Figure 4.5, where empirically determined process limitations presented in Figure 4.2 (green triangles and blue circles for positive and negative hemispheres, respectively) are directly compared to the intensity-driven (Figure 4.3) (dashed magenta line) and thermally-driven damage (Figure 4.4) (solid red line), with root mean square errors (RMSE) reported for discrepancies between values from experiments and the two models. Asymmetries observed in Figure 4.2 were addressed with different markers for rdf in the positive (green triangles) and negative (blue circles) hemispheres of the z- (Figure 4.5(a)) and y-axes (Figure 4.5(b)). The graphs show that the intensity damage model has a large discrepancy with the observed data (RMSE > 10 µm) and fails to follow the observed fall-off of damage-free offset radius with increasing pulse energy, especially along the y-axis (Figure 4.5(b)). Chapter 4. Laser Modifications to the Polymer Buffer 53

Figure 4.5: Comparison of damage-free radial focusing distance in acrylate-coated ﬁber calculated from the intensity-driven damage model (dashed magenta line) (Figure 4.3) and thermally-driven damage model (solid red line) (Figure 4.4) with empirical results (light blue triangles (positive hemisphere) and dark blue circles (negative hemisphere)) (Figure 4.2) for varying pulse energies when focusing along

(a) the ± z-axis and (b) the ± y-axis of the ﬁber (λ = 522 nm, νRR = 500 kHz, vscan = 0.1 mm/s).

The thermal damage model does much better to follow the observed radial fall-off with increasing pulse energy for processing along both the z- (Figure 4.5(a)) and y- axes (Figure 4.5(b)) (RMSE < 10 µm). As the thermal model is spherically symmetric, there is no discrepancy between predicted damage-free limits in the y- and z-axes. This difference in models is quantitatively reflected in the root mean square error, with RMSE values of 17 µm and 9 µm for the intensity and thermal damage models in the z-axis, respectively, and RMSE values of 11 µm and 7 µm for the intensity and thermal damage models in the y-axis, respectively. In both axes, the thermal model was more accurate Chapter 4. Laser Modifications to the Polymer Buffer 54

than the intensity model. The RMSE for the thermal model along the +z hemisphere in particular offered a good RMSE value of 5 µm, in contrast with a 12 µm RMSE value for the thermal model in the -z hemisphere. Given that the radial step size in the empirical studies was 5 µm (Figure 4.2), the RMSEs for the ±y and +z regions of the fiber of 7 µm and 5 µm, respectively, are reasonably within the experimental resolution. The high RMSE value in the -z region of the fiber, of 12 µm, was significantly lower than 22 µm, the RMSE value for the intensity model in the -z region of the fiber. Reasons for such discrepancy in the -z region may be attributed in part to scan-direction and self-focusing effects, as discussed above. Though interfacial damage is due to a complex combination of factors, Figure 4.5 supports the conclusion that heat accumulation in the glass cladding is the primary cause of damage observed at the cladding / buffer interface when focusing a laser in the glass cladding of an acrylate-coated fiber. This insight was of particular value when developing microfluidics in acrylate-coated fiber, discussed in Section 4.4.

4.3 Laser Ablation of Polymer Buﬀer

As discussed in Section 2.2.3, fabricating microfluidic structures in optical fibers introduces additional challenges to fabricating core modifications and cladding photonics. Buffer ablation is required for chemical etchant to reach the silica fiber where nanograting tracks have been written to the outer cladding surface. Further, the buffer must not be damaged when submerged into etchant to selectively open regions of silica cladding (discussed further in Section 4.4).

Laser exposure parameters were set around previous work by Snelling et al. in buffer-ablation with ultrafast laser pulses for the purpose of fiber stripping [60], discussed in Section 2.2. While prior work used a 5 kHz laser, the femtosecond laser system used in the present work has a minimum programmable repetition rate of 100 kHz (Section 3.1). Low repetition rates are necessary when ultrafast processing in materials with low thermal conductivity and high heat capacity as more time is necessary to dissipate heat between successive laser pulses. In contrast, core modifications, cladding photonics, and nanogratings could be written with a high repetition rate of 500 kHz, owing to the higher thermal conductivity of silica, as Chapter 4. Laser Modifications to the Polymer Buffer 55

discussed in Section 3.3. LIF devices which must combine these components with selective buﬀer ablation then requires rapid switching between high and low repetition rates, respectively. However, changing the lasers repetition rate requires warm-up time and beam realignment, which is prohibitively time consuming. For this reason, ablation was conducted with the lasers repetition rate set to 500 kHz while an acousto-optic modulator used to select pulses to yield a lower repetition rate for buﬀer ablation, as discussed in Section 3.1.

Ablation of access holes in buffer was carried out on the bottom surface (-z) of the fiber which is, as discussed in Section 3.3, the preferred position in which the laser is focused. This is because ablation near the top surface (+z) produces bubbles in the oil which disrupts the beam propagation and potentially harms the lens due to ablation debris. Ablating at the sides of the fiber (±y) also distorts the beam propagation, which was found to result in inconsistent, ellipsoidal-shaped vias. An assessment of these effects are presented in Figure 4.6, where linear, percussion ablation (see Section 3.3.4) was conducted. Synchronous scanning on the laser focus from the outer-buffer interface to the cladding / buffer interface was also preferred at the -z, +y, and -y interfaces (see

Figure 3.4). The radius of the ablated hole at the buffer / air interface, rperc, was observed and measured along the x-axis. Pulse energies of 10 nJ, 20 nJ and 40 nJ, corresponding to fluences of 22.0 J/cm2, 43.9 J/cm2, and 87.8 J/cm2 at focus, respectively. These 2 exposures exceed the incubated threshold fluence for PMMA of 0.6 J/cm (EP = 10 nJ at νRR = 500 kHz), which is near the noise-level for the power-meter presently used for calibration in this experiment (more sensitive detectors may be used in future work). The size of ablation sites on the y-axis increase from 7 µm and 8 µm to 17 µm and 19 µm, respectively, at the outer-buffer interface as pulse energy was increased. No significant increase in hole size was observed in the z-axis for similar exposure change.

The size of the ablation holes in the z-axis (rperc ≈ 30 µm) were all substantially larger than the writing lasers radius at focus (ω0 = 0.24 µm), indicating that excessive total exposure was applied, over-machining the polymer. Though relatively symmetric ablation ports were observed (Figure 4.6(a)) along the z-axis, ports formed along the y-axis were reproducibly distorted (Figure 4.6 c,d) into ellipsoidal shapes and tapered cross sections. The distortion appeared to be worse for the -y side (third column in Figure 4.6) than the +y side (fourth column in Figure 4.6) of the ﬁber for as of yet unknown reason. Interfacial damage was also observed when writing ports with 10 nJ for both the z- and Chapter 4. Laser Modifications to the Polymer Buffer 56

Figure 4.6: Microscope images of buffer ports ablated by percussion at different pulse energies observed at face (a, c, d) and cross-section (b,e): (a,b) bottom (-z interface) of the fiber, and (c-e) the two sides (± y interfaces) of the fiber, relative

to the position of the writing laser (λ = 522 nm, νRR = 1 kHz, vscan = 0.1 mm/s). y-axis, but only in the y-axis with highly exposure of 20 nJ and 40 nJ. The distortion of the ablation observed in the y-axis were attributed to the ellipsoidal profile of the focal volume ( ω0 = 0.865), and the relatively large and curved shape of buffer through zR which the laser must propagate, giving more opportunity for defocusing and aberration effects (see Equation 4.1). The observed interfacial damage at lower ablation energies (especially visible in Figure 4.6(a) and (e)) is attributed to thermal ventilation. Section 4.2 establishes that heat accumulation in the fused silica appears to induce interfacial damage in the buffer. Figures 4.6(a) and (b) present interfacial-damage-free fibers in which 50 nJ pulses were focused to the cladding / buffer interfacial (radial focusing distance of 62.5 µm) with parameters (λ = 522 nm, νRR = 1 kHz, vscan = 0.1 mm/s, NA = 1.25) identical to those used in the thermal damage model shown in Figure 4.4. The empirical study presented in Figure 4.2 reported interfacial damage when focusing 50 nJ pulses at radial focusing distances exceeding 35 µm in the -z hemisphere. One reason for this apparent contradiction is that in Figures 4.6(a) and (b), ablation was carried out in the buffer prior to applying a focused scan within the silica. Buffer which has been removed cannot be subject to damage once the laser enters the glass fiber. Further, Chapter 4. Laser Modifications to the Polymer Buffer 57 sufficiently large ablation ports can potentially reduce heat accumulation damage effects by reducing the heat affected zone with improved thermal ventilation into oil than into acrylate. For this to be the case, the thermal diffusivity of the index matching oil used during laser fabrication would have to be higher than that of the acrylate buffer. The thermal diffusivity of the Cargille index matching oil used is not available from the manufacturer, but the thermal conductivity is reported to be 0.12 W/m/K [97], which is lower than the thermal conductivity of PMMA, 0.18 W/m/K [98]. This belies, but does not necessarily invalidate the thermal ventilation explanation as the unreported specific heat capacity of Cargille index matching oil should be sufficiently lower than

κT that of PMMA (1466 J/kg/K [98]) to render the thermal diffusivity (DT = ) of the CP index matching oil higher (i.e. if the specific heat capacity of the index matching oil is ≤ 977 J/kg/K). A comprehensive study on the effects of ablation ports on reducing heat accumulation and subsequent polymer damage would be a subject of a further work that is beyond this thesis.

Further study was conducted towards optimizing laser exposure parameters for polymer access port formation in the -z position as outlined in Section 3.1, for the number of pulses delivered per focal volume (1 ≤ NP ≤ 10), the energy delivered per pulse (10 nJ ≤ EP ≤ 60 nJ), and the repetition rate of the laser pulses (100 Hz ≤ νRR ≤ 100 kHz). For this study, a single radial linear track was written into the polymer buffer, with the laser scanning from the outer radius of the buffer toward the inner radius (+z scan direction). The effects of varying these parameters are presented with the optical microscope images in Figure 4.7. Trends in hole size and shape arising from varying repetition rate, pulse energy, and number of pulses are apparent in Figures 4.7(a), (b), and (c), respectively. In Figure 4.7(a), repetition rate was increased from 100 Hz to 100 kHz, while the pulse energy and number of applied pulses were held constant at 40 nJ and 1, respectively. As repetition rate was increased the size of ablated buffer holes at the buffer / air interface (rperc) did not significant vary for repetition rates above 100 Hz (bottom row of Figure 4.7(a)), but reflow of melted buffer led to resealing at the cladding / buffer interface for repetition rates above 1 kHz (top row of Figure 4.7(a)). In Figure 4.7(b), percussed ablation holes in the buffer are shown for an increasing pulse energy from 10 nJ to 60 nJ (with νRR = 1 kHz, NP = 5 fixed). As pulse energy was increased, the ablated buffer holes were observed to become rougher and darker (top row of Figure 4.7(b)) and the ablated buffer hole size increased Chapter 4. Laser Modifications to the Polymer Buffer 58

Figure 4.7: Microscope images of ablation holes fabricated in acrylate buﬀer by

percussion with hole diameter at the air/buﬀer interface rperc measured: (a) varying

repetition rate (100 Hz ≥ νRR ≥ 100 kHz), (b) varying pulse energy (10 nJ ≥ EP ≥

60 nJ), and (c) varying number of pulses (1 ≥ NP ≥ 10) Chapter 4. Laser Modifications to the Polymer Buffer 59

monotonically from rperc = 2 µm to 13 µm (bottom row of Figure 4.7(b)). In Figure 4.7(c), the number of applied pulses was varied between 1, 5, and 10 pulses with 100 Hz repetition rate and 60 nJ pulse energy held constant. As more pulses were applied, the buffer hole diameter at the buffer / air interface increased from 9 µm to 14 µm (bottom row of Figure 4.7(c)and the buffer hole side-wall roughness increased noticeably (top row of Figure 4.7(c)). These results show that while low repetition rates increase processing time, higher repetition rates ((νRRR≥ 5 kHz for EP = 40 nJ, NP = 1) increase the prominence of heat accumulation effects which leads the opened ports to melt and reseal (Figure 4.7(a), νRR≥ 5 kHz). Further, pulse energies and numbers of pulses must be sufficiently high to machine the polymer, but excessive pulse energies

(EP ≥ 60 nJ for νRR= 1 kHz, NP = 5) and high numbers of pulses (NP ≥ 10 for νRR=

100 kHz, EP = 60 nJ) increase the level of collateral damage in the surrounding buﬀer (Figure 4.7(a) and (c), respectively).

The reported values for hole diameter in the bottom rows of Figure 4.7 track with expectations, but exceed the writing lasers spot size at focus (ω0 = 0.24 µm) by 30× to 60× because the applied fluences at focus strongly exceed the ablation threshold, and because of heat accumulation and incubation effects. Furthermore, the top rows of Figure 4.7(a) and (c) show qualitative limitations in melt resealing and wall roughness, respectively, not reflected by the buffer hole radius data.

In order to assess ideal ablation parameters balancing the three main exposure parameters (νRR, EP , NP ), more comprehensive study was carried out, varying repetition rate between 100 Hz and 100 kHz, pulse energy between 10 nJ and 60 nJ, and the number of applied pulses between 1 and 10 pulses (λ = 522 nm in all cases). Representative data was shown in Figure 4.7 Linear ablation tracks were written from the outer surface of the buffer toward the cladding / buffer interface at the -z side of the fiber. The ablation tracks were visually inspected using optical microscopy and assessed qualitatively to be either underexposed, well machined (good exposure), overexposed resulting in roughness, or subject to resealing as a result of heat accumulation. Qualitative assessment was consistent with the results shown in Figure 4.7, providing the more comprehensive assessment in Figure 4.8 that goes beyond the buffer hole size data. Chapter 4. Laser Modifications to the Polymer Buffer 60

Figure 4.8: Morphology assessment of radially ablated ports in acrylate-coated SMF-28 based on visual inspection, varying number of pulses per focal volume, pulse energy, and repetition rate for percussion drilling; the coloured-in polygons denote zones of diﬀerent types of modiﬁcation: underexposed (blue), good exposure (green), damaged (red), and resealed (navy). Yellow, cyan, and purple coloured hollow boxes

correspond to EP = 40 nJ and NP = 1 (Figure 4.7(a)); νRR = 1 kHz and NP = 5

(Figure 4.7(b)); and EP = 60 nJ and νRR = 100 Hz (Figure 4.7(c)), respectively. Chapter 4. Laser Modifications to the Polymer Buffer 61

Figure 4.8 graphically depicts a qualitative assessment of percussion radial ablation of ninety different exposure conditions, including the thirteen presented in Figure 4.7. Exposure varied as follows: pulse energy between 10 nJ and 60 nJ, repetition rate between 100 Hz and 100 kHz, and number of applied pulses between 1 pulse and 10 pulses. Each ablation port was visually inspected and classified to be either underexposed, well exposed, overexposed with excessive damage, or fused shut and resealed due to heat accumulation and reflow effects. Overarching trends were noted, with data zones labelled according to ablation port qualities denoted with colour coded polygons. Underexposure was observed when applying low numbers of pulses (NP ≤ 5), higher repetition rates

(νRR ≥ 10 kHz), and lower pulse energies (EP ≤ 20 nJ), depicted by blue shaded areas in Figure 4.8. Resealed holes are depicted by navy shaded areas in Figure 4.8 and observed

when applying high repetition rates (νRR ≥ 5 kHz) and high pulse energies (EP ≥ 40 nJ).

When only one pulse was applied by focal volume (NP = 1), the resealing was observed for repetition rates as low as 5 kHz, but as more pulses were applied, higher repetition

rates were observed to damage instead of reseal the holes for exposures of νRR ≤ 5 kHz

for NP = 5 and νRR ≤ 100 kHz for NP = 10. When 10 pulses were applied, resealing was

observed with pulse energies as low as 20 nJ (νRR= 100 kHz). Damage from overexposure

(depicted by red shaded areas in Figure 4.8) was observed at higher pulse energies (EP ≥ 40 nJ). As the number of applied pulses was increased, the incidence of overexposure

damage at high pulse energies (EP ≥ 40 nJ) extended to higher repetition rates. When

NP = 1, damage was observed for νRR ≤ 1 kHz; when NP = 5, damage was observed

for νRR ≤ 10 kHz; and when NP = 10, damage was observed for νRR ≤ 100 kHz. Ideal ablation (depicted by green shaded areas in Figure 4.8) was observed in the remaining

process windows, namely, low repetition ratesνRR (≤ 10 kHz) and low pulse energies (EP ≤ 40 nJ). As the number of applied pulses increased, higher repetition rates were observed

to be necessary for clean ablation at EP = 40 nJ. Consistently, ideal ablation ports were observed for pulse energies between 10 nJ and 20 nJ, repetition rates between 100 Hz and 5 kHz, and when applying between 1 and 10 pulses. These ﬁndings are consistent with the previously discussed understanding of ablation and heat accumulation eﬀects

(see Section 2.1). Larger deposits of energy dissipation (high EP ) applied with less time

for heat to dissipate (high νRR) results in greater heat accumulation and larger zones of temperatures high enough to induce polymer melt. At lower repetition rates, high pulse energies and numbers of pulses result in excessive ablation zones, as incubation effects and threshold fluences extend into collateral regions of buffer. The processes described Chapter 4. Laser Modifications to the Polymer Buffer 62

in Figure 4.8 wherein which ablation was well-contained (2 µm ≤ rperc ≤ 7 µm) were eﬃciently ablated, with smooth morphology and no resealing. These optimum conditions were were tested further for reproducibility and ability to guide etchant (discussed further in Section 4.4). Processes with higher repetition rates and lower numbers of pulses (e.g.

νRR = 1 kHz, NP = 1, 10 nJ ≤ EP ≤ 40 nJ) were given preference to favour reduced laser fabrication time.

4.4 Chemical Etching Silica Fiber Cladding through Polymer Buﬀer

In this section various laser exposure conditions and scanning approaches (see Section 3.3) for buffer ablation are assessed in terms of their compatibility with the FLICE process (i.e. the ability of ablated buffer ports to enable etchant to reach laser-structured glass fiber) described in Section 3.3.3. The nanograting tracks formed in the glass cladding were mostly limited to single laser scanning of blind-holes, with more complex microfluidic structures explored later in Section 5.3.

4.4.1 Chemical Resistance of Acrylate Buﬀer to KOH and HF

Two common chemical etchants for opening nanograting tracks laser-written in fused silica are aqueous potassium hydroxide (KOH) [28] and hydrofluoric acid (HF) [27]. These etchants were each tested on acrylate-coated fibers in which access ports were laser-machined in the buffer following methods described in Section 3.3.4 to meet with laser-written nanograting tracks in the glass fiber cladding. No interface or buffer damage was observed after laser buffer ablation (following exposure conditions presented in Figure 4.8), using processes described further in Section 4.4. This way, any subsequent observed damage to the buffer could only be attributed to exposure to the chemical etchant. Fibers were also mechanically stripped (see Section 3.2) to serve as a control in assessing the combined effects of laser buffer ablation and its chemical resistance to the etchants. Chapter 4. Laser Modifications to the Polymer Buffer 63

Results of KOH etching are presented first. KOH immersion was conducted on a fiber where conically trepanned buffer ablation was applied to form holes with variable radii ◦ using λ = 522 nm, νRR = 1 kHz, NP = 1, θt = 30 , rinterface = 5 µm, xsep = 200 µm,

Eabl= 16 nJ (see Section 4.4.2). A helical nanograting track was written next in the silica cladding, using λ = 522 nm, νRR,helix = 500 kHz, vscan,helix = 0.1 mm/s, EP,helix = 60 nJ, rhelix = 20 µm,Λhelix = 200 µm, νRR,ablation = 1 kHz, NP , ablation = 1, Eabl= 16 nJ, xsep ◦ = 200 µm, and θt = 30 , with procedures discussed further in Section 5.3.1. The fiber was submerged in 10 M KOH heated to 70 ◦C for 8 hours, following methods outlined in Section 3.3.3. Smaller samples subjected to the same aforementioned laser exposures were mechanically stripped and exposed to the same KOH etching step as a non-laser- structured control. Observations of these regions before and after KOH immersion are presented in Figure 4.9. Figure 4.9(a) presents the laser-machined fiber prior to KOH immersion, where trepanned holes are visible in the acrylate buffer (discussed further in Section 4.4.2) and a helical nanograting track is visible in the glass fiber cladding (discussed further in Section 5.3.1). No damage was observed in the fiber buffer or cladding / buffer interface. This same region of the fiber shown in Figure 4.9(a) is shown after 8 hours of 70 ◦C 10 M KOH etching in Figure 4.9(b). The nanograting structures have been etched out by KOH and the acrylate buffer has been completely removed by the etchant, leaving the silica fiber bare. Figure 4.9(c) shows the fiber just beyond the laser- modified zone (approximately 1.8 mm), to the left (top image of Figure 4.9(c)) and to the right (bottom image of Figure 4.9(c)). In the area closest to the laser modification, there was clear physical buffer damage in the form of total dissolution, with partial dissolution observed farther from the laser-modified region (visible to the right of the top image of Figure 4.9(c) and to the left of the bottom image of Figure 4.9(c)). Yellow-green discolouration in the buffer was observed just beyond the zone of partial dissolution (visible in the middle of the images in Figure 4.9(c)). Beyond the discolouration, no damage or further discolouration was observed in the fiber buffer (visible to the left of the top image of Figure 4.9(c) and to the right of the bottom image of Figure 4.9(c)). Figure 4.9(d) shows the fiber imaged to the left of where the fiber was mechanically stripped, where a similar pattern was observed as in the case around the laser-ablated buffer ports. Total and partial dissolution of the buffer is seen nearest the buffer stripping zone (visible in the right of Figure 4.9(d)), yellow-green discolouration exists beyond the dissolution (seen in the middle of Figure 4.9(d)), and no discolouration or damage is observed beyond the discolouration (seen in the left of Figure 4.9(d)). The physical Chapter 4. Laser Modifications to the Polymer Buffer 64

Figure 4.9: Optical microscope images of acrylate-coated ﬁber in which conically

trepanned holes were ablated in the acrylate buﬀer (λ = 522 nm, νRR = 1 kHz, NP = ◦ 1, θt = 30 , rinterface = 5 µm, xsep = 200 µm, Eabl = 16 nJ) (see Section 4.4.2) and

helical nanograting tracks were written in the ﬁber cladding (λ = 522 nm, νRR,helix

= 500 kHz, vscan,helix = 0.1 mm/s, EP,helix = 60 nJ, rhelix = 20 µm,Λhelix = 200

µm, νRR,ablation = 1 kHz, NP,ablation = 1, Eablation = 16 nJ, xsep = 200 µm, θt = 30) (see Section 5.3.2): (a) before and (b - d) after 8 hours of 70 ◦C 10 M KOH etching, (b) in the vicinity of trepanned buffer holes after KOH etching (i.e. (a) after etching), (c) to the left (above the dashed line) and right (below the dashed line) of trepanned buffer holes after KOH etching, and (d) to the left of mechanical buffer stripping after KOH etching Chapter 4. Laser Modifications to the Polymer Buffer 65 damage apparent in Figure 4.9(c) and (d) indicates a separation of buffer completely from the glass in the vicinity of both the ablation port holes (Figure 4.9(c)) and the mechanically stripped buffer zone (Figure 4.9(d)). A loose, partially dissolved, sleeve of buffer formed adjacent to these zones by the etchant. This suggests that buffer dissolution from the glass interface to the outside surface, originating from the area in which the cladding / buffer interface was exposed to KOH, is taking place. The observation that similar dissolution and discolouration was observed in regions where the fiber buffer was laser-ablated (Figure 4.9(c)) and where the buffer was mechanically stripped (Figure 4.9(d)) indicates that dissolution was not a laser-induced effect. Dissolution was assisted by inherent stresses resulting from the buffer-coating process, or the manner in which the polymer was bonded to the glass (e.g. hydrogen bonding, van der Waals forces). The observed discolouration suggests a chemical reaction. The nature of this dissolution was not further investigated in this thesis, but KOH was determined not to be a viable glass etchant for microfluidics in acrylate-coated fiber as it caused significant collateral damage to the buffer when exposed to the cladding / buffer interface.

HF immersion was conducted on a ﬁber where a rectangular hole shape was laser-ablated by 1-D linear raster scaning over 50 µm length using λ = 522 nm, νRR =

1 kHz, NP = 10, Eabl= 20 nJ, and draster,x = 100 µm (see Section 4.4.2). No laser processing (i.e. nanograting track formation) was conducted in the silica cladding of the fiber. Fiber samples were also mechanically-stripped of buffer in an area away from the laser processing. The fiber samples were submerged in 5% HF for 2 hours, following methods outlined in Section 3.3.3, with both the laser-machined and mechanically-stripped portions of the fiber exposed to HF. Figure 4.10(a) depicts the buffered fiber after laser exposure but before HF etching, forming a rectangular hole of 100 µm by 15 µm area by linear raster ablation (formed by the procedure in Section 4.4.2) without damage to or distortion of the surrounding acrylate buffer or to the cladding / buffer interface. Figure 4.10(b) depicts the same fiber after 2 hours of 5% HF etching, wherein damage at the cladding / buffer interface was observed to the right perimeter of the hole, appearing as delamination of the buffer at the glass interface. No similar damage was apparent to the buffer or the interface near the vicinity of mechanically stripped buffer (not pictured), nor in the regions where the cladding / buffer interface was not laser-exposed. The dissolution and discolouration of buffer observed after KOH immersion (depicted in Figure 4.9) were not observed with HF Chapter 4. Laser Modifications to the Polymer Buffer 66

Figure 4.10: Optical microscope images of acrylate-coated ﬁber in which linear raster ablation formed rectangular access ports in the acrylate buﬀer (λ = 522 nm,

νRR = 1 kHz, NP = 10, Eabl = 20 nJ, draster,x = 100 µm) (see Section 4.4.2): (a) before and (b) after 2 hours of 5% HF immersion. immersion. The relatively minor type of interfacial damage observed in Figure 4.10(b) was found to be intermittent in formation and sensitive to laser exposure as discussed further in Section 4.4.3, together with potentially exacerbating factors including heat accumulation weakening of the buffer-cladding bonds and propagation effects of surface damage along the length of the fiber. HF was therefore determined to be a viable etchant for FLICE in acrylate-coated fibers, but with limitations in terms of laser-processing conditions.

4.4.2 Enabling Fluid Flow through Ablated Buﬀer

As described in Section 3.3.3, the FLICE process requires chemical etchant to reach laser-written nanograting tracks at the glass cladding surface with a sufficient refresh- rate (cycling used acid with fresh acid). In this section, laser buffer ablation is optimized for enabling the etching of nanograting tracks in silica fiber through laser-ablated buffer ports. Ablation ports fabricated by percussion, rastering, and trepanning procedures (see Section 3.3.4) were evaluated and compared. Chapter 4. Laser Modifications to the Polymer Buffer 67

Figure 4.11: Top and sideview optical microscope images of acrylate-coated ﬁbers

in which the ﬁber buﬀer was ablated via linear raster scans (λ = 522 nm, νRR = 1

kHz, NP = 1, Eabl = 8 nJ) and single-scan radial nanograting tracks (λ = 522 nm,

νRR = 500 kHz, vscan = 0.1 mm/s, EBH = 40 nJ, dBH = 50 µm) were laser-written in the glass cladding leading to the buﬀer-ablation-exposed cladding / air interface, shown before and after 2 hours of 5% HF exposure to open blind holes in the glass

ﬁber: (a) 1-D scanning (draster,x = 50 µm, dseparation = 2 µm), (b) 2-D scanning

(draster,x = 50 µm, draster,y = 16 µm, dseparation = 2 µm).

Percussively ablated linear ports were found in Section 4.3 to be incapable of consistently etching blind-holes, even at optimized blind-hole writing pulse energies

(EBH ) as demonstrated earlier in Figure 4.6. Raster scanned ports were made with linear scans along the x-axis at defined length, draster,x, exposed in an array of tracks along the z-axis (1D linear raster) separated by dseparation = 2 µm, as well as in a matrix of lines along the y- and z-axes (2D linear raster) separated in both axes by dseparation = 2 µm, with the width of the ablation in y-axis defined as draster,y. Various repetition rates, numbers of pulses, and laser pulse energies were tested based on the optimal results as presented in Figure 4.8, and further evaluated here. Typical results of linear raster scans are shown in Figure 4.11 for fibers in which buffer the was ablated using νRR = 1 kHz, NP = 1, and Eabl= 8 nJ. Single-scan blind holes were written in the glass cladding along the -z axis using EBH = 40 nJ, νRR = 500 kHz, and vscan = 0.1 mm/s, and followed with the fiber submerged in 5% aqueous HF for 2 hours. The optical microscope images in Figure 4.11 demonstrate that rigid ablation ports without delamination at the cladding-buffer interface could be opened by ablation in acrylate buffer by scanning in a one-dimensional array along the z-axis in 50 µm long scans along the x-axis of the fiber, separated by 2 µm step sizes in the z-axis (Figure Chapter 4. Laser Modifications to the Polymer Buffer 68

4.11(a)). Buffer raster ablation ports were also fabricated by scanning in two-dimensional arrays along the y- and z-axes with 50 µm long scans along the x-axis of the fiber, separated by 2 µm step sizes in the y- and z-axes (Figure 4.11(b)). Under preferred ablation conditions (λ = 522 nm, νRR = 1 kHz, NP = 1, Eabl= 8 nJ), these 1-D and 2-D rastered ablation ports in the buffer did not result in damage or delamination at the cladding / buffer interface, following the laser buffer ablation (Before etch columns in Figure 4.11) as well as after 2 hours of 5% HF acid immersion (After etch columns in of Figure 4.11). Exposure of 2D-rastered ablation buffer holes was found to consistently avoid damage and delamination effects, as seen in the 1D-rastered ablation buffer hole in Figure 4.10. The larger rectangular ablation structures appear to sufficiently mitigate the heat accumulation effects discussed in Section 4.2, as evidenced in Figure 4.10(b). However, 2D-rastered ablation buffer holes were found to not reliably enable the etching of single-scan radial blind nanograting laser tracks written in the glass cladding (λ = 522 nm, νRR = 500 kHz, vscan = 0.1 mm/s, EBH = 40 nJ, dBH = 50 µm) to the cladding / air interface that was exposed by opened buffer ports (After etch columns in of Figure 4.11). The linearly rastered buffer ablation holes were determined incapable of reliably enabling the HF etching of blind holes in the underlying glass due to that the hard edges and shape of the buffer ablation zones which trap air bubbles that impeded the flow of chemical etchant to contact the glass surface. Such air bubbles are clearly visible in Figure 4.12, in which 2-D linear raster ablation zones were machined to form open ports in the acrylate buffer

(λ = 522 nm, νRR = 1 kHz, NP = 100, Eabl= 20 nJ, draster,x = 50 µm, draster,y = 16 µm) of fiber, and immersed in water. The presence of similar air bubbles in HF etchant would impede the flow of etchant to contact the glass cladding surface and thus inhibit etching of laser nanograting tracks situated inside such buffer port shapes. It was therefore determined that linear buffer ablation structures with hard, 90◦ edges were unsuitable for forming microfluidic structures in acrylate-coated fiber.

To assist with fluid flow and filling of ablated ports in acrylate buffer, circular tapered ablation ports were made with spiral scans traced in circular patterns in the xy plane, concentrically written with smaller radii (in steps of dseparation = 2 µm) to a reach a center-circle radius equal to dseparation. Such spirals were arrayed along the z-axis from the buffer / air interface to the cladding / buffer interface in steps of dseparation = 2 µm. The outer (maximum) radius of each spiral in the xy plane was defined by a minimum Chapter 4. Laser Modifications to the Polymer Buffer 69

Figure 4.12: Optical microscope image of air bubbles trapped in 2-D linear raster scanned buﬀer ablation ports in acrylate buﬀer, when immersed in water (λ = 522

nm, νRR = 1 kHz, NP = 100, Eabl = 20 nJ, draster,x = 50 µm, draster,y = 16 µm).

outer radius at the cladding / buffer interface, rinterface, and a taper angle in the z-ρ plane p 2 2 ◦ ◦ (ρ = x + y ), θt, where θt = 0 defines a uniform cylindrical ablation port, and 0 < θt < 90◦ defines a truncated conical buffer ablation structure. Trepanned conical ablation structures were also evaluated, in which just the outer radii of each circle in the xy plane along the z-axis were traced in the fiber buffer (tracing out the surface area of the spirally scanned structures). These ablation geometries are depicted in Figure 4.13. Trepanned

Figure 4.13: Schematics comparing laser path (solid green lines) for (a) spiral- scanned and (b) trepanned conical buffer ablation hole geometries. The dashed green line depicts taper angle of the resultant buffer hole. buffer ablation zones were ultrasonicated for 3 minutes in deionized water to facilitate the removal of unablated buffer inside the ablation zone. These two types conical buffer ablation structures are directly compared in Figure 4.14. The laser exposure parameters used were νRR = 1 kHz, NP = 2, and Eabl= 20 nJ. Rounded ablation holes were achieved Chapter 4. Laser Modifications to the Polymer Buffer 70 in acrylate buffer using both spiral scanning (Figures 4.14(a) and (b)) and trepanning

(Figures 4.14(c) and (d)) techniques (see Figure 4.13), and in a conical geometry with θt ◦ ◦ = 30 (Figures 4.14(a) and (c)) as well as in a cylindrical geometry with θt = 0 (Figures 4.14(b) and (d)). Ablation zones in buffer created using spiral scanning appeared darker and rougher compared with zones exposed similarly but by trepanning (i.e. comparing Figures 4.14(a) and (c)), suggesting rougher side walls and more optical scattering in the former case. Otherwise, there was no significant observable difference between the quality of buffer holes which were ablated by means of spiral scanning (Figures 4.14(a) and (b)) and trepanning (Figures 4.14(c) and (d)).

The fabrication time for spiral ablation holes was considerably longer than for trepanned holes, taking approximately 40 minutes per spirally scanned ablation with θt = 30◦ (Figures 4.14(a)), as compared to approximately 2 minutes per trepanned ◦ ablation hole with θt = 30 (Figures 4.14 (c)). This 20-fold discrepancy in fabrication time is a result of avoiding concentric circular laser scans that filled in the volume of the conical structure in the buffer when spiral scanning (see Figure 4.13). The significant difference in fabrication time and comparable quality of ablation zones which were spirally scanned and trepanned led to the conclusion that trepanning was the preferable process for ablating circular conical ports in buffer. Similarly, structures ◦ fabricated with a taper angle (e.g. θt = 30 in Figures 4.14(a) and (c)) eliminated the 90◦ edges in the z-ρ plane without introducing any apparent problems in terms of buffer ablation quality and reproducibility. It was therefore determined that trepanned and tapered ablation holes were optimal for structuring acrylate buffer by ablation.

The reproducibility of trepanned, tapered ablation zones to enable the ﬂow of etchant was further evaluated over 61 ablation holes spaced 200 µm apart, written with parameters rinterface = 5 µm, λ = 522 nm, νRR = 1 kHz, NP = 1, and Eabl= 20 nJ. Single-scan radial blind nanograting tracks were written in the glass cladding extending to the cladding / air interface, and opened through buﬀer port ablation using laser exposure parameters λ = 522 nm, νRR = 500 kHz, vscan = 0.1 mm/s, EBH = 60 nJ, and

EBH = 50 µm. Figure 4.15 depicts 4 out of 61 trepanned conical buffer ablation holes which were written in acrylate buffer and single-scan radial nanograting tracks were written in the glass cladding toward the cladding / air interface, exposed by the trepanned buffer holes. The fiber is shown both before (Figure 4.15(a)) and after Chapter 4. Laser Modifications to the Polymer Buffer 71

Figure 4.14: Microscope images of acrylate-coated SMF-28 comparing (a) spirally- ◦ scanned (Figure 4.13(a)) conical buﬀer ablation hole with θt = 30 and (b) cylindrical ◦ ◦ hole (θt = 0 ), and (c) trepanned buﬀer ablation hole 4.13(b) with θt = 30 and (d) ◦ cylindrical hole (θt = 0 )(rinterface = 10 µm, λ = 522 nm, νRR = 1 kHz, NP = 2,

Eabl = 20 nJ).

(Figure 4.15(b)) 2 hours of exposure to 5% HF acid. No damage was observed in the acrylate buffer or at the cladding / buffer interface immediately after trepanning conical ablation holes in the buffer (Figure 4.15(a)) in any of the 61 holes. After two hours of 5% HF immersion, all 61 of the nanograting tracks written in the glass cladding were successfully etched out, depicted in the bottom row of Figure 4.15(b). However, interfacial damage was observed surrounding 76% of the trepanned buffer ablation holes after acid immersion, even though this damage did not impede the etching of blind-hole laser tracks in the glass cladding (visible in the second and fourth area of the left in Figure 4.15(b)). The consistent etching of all 61 nanograting tracks in glass cladding through trepanned conical buffer ablation holes in acrylate buffer confirms that this buffer ablation process enables fluid filling of and flow through holes in acrylate as HF etchant was able to consistently open laser tracks written in the underlying glass cladding into blind holes. The 76% occurrence of interfacial damage after etching, however, demonstrates the need for further optimization of the laser exposure conditions and damage of buffer hole ports. As discussed in Section 4.4.1, this damage may be related to heat accumulation in the glass processing step that propagates and thermally damages the buffer at the cladding / buffer interface. Chapter 4. Laser Modifications to the Polymer Buffer 72

Figure 4.15: Microscope images of SMF-28 in which conical buﬀer ablation holes

were trepanned in the ﬁber acrylate buﬀer (λ = 522 nm, νRR = 1 kHz, NP = 1, Eabl ◦ = 20 nJ, rinterface = 5 µm, θt = 30 ) and single-scan radial blind holes were written

in the ﬁber glass cladding (λ = 522 nm, νRR = 500 kHz, vscan = 0.1 mm/s, EBH

= 60 nJ, dBH = 50 µm) toward the buﬀer ablation holes (a) before and (b) after 2 hours of 5% HF etching.

Approaches to mitigate interfacial damage after HF etching by reducing heat accumulation are explored in Section 4.4.3, including reducing the pulse energy of buﬀer ablation (Eabl), reducing the pulse energy of blind hole writing in glass cladding

(EBH ), increasing the spacing between adjacent ablated buffer holes (xsep), increasing the diameters of ablated holes in the buffer (rinterface), and reducing the repetition rate used during buffer ablation (νRR).

4.4.3 Mitigating Interfacial Damage after HF Etching

Figures 4.6, 4.10, and 4.15 presented images of laser-structured fiber in which interfacial damage was observed either from focusing laser energy in the buffer and/or cladding (Figure 4.6), or after immersing laser-exposed fibers in HF (Figures 4.10 and 4.15). A cursory examination into the nature of this type of damage has pointed in part to heat accumulation effects under the cladding / buffer interface leading to delamination. In Figure 4.6, this form of damage was the result of the laser-writing Chapter 4. Laser Modifications to the Polymer Buffer 73

process. Such buffer damage could be avoided for the case of focusing in the silica fiber by positioning the focal volume of the writing laser within a maximum radial distance from the fiber center, with value dependent on the pulse energy and azimuthal alignment, as outlined in Figure 4.2. The interfacial damage seen in Figures 4.10 and 4.15 is not solely the result of focusing the laser inside the silica fiber nor of selectively ablating the polymer buffer, and arose only after HF etching. The reproducibility and severity of the interfacial damage after HF etching was found to depend in part on the laser writing and chemical etching parameters. In order to identify appropriate exposure parameters which consistently exclude interfacial damage, buffer ablation hole

diameter (rinterface), center-to-center buffler ablation hole spacing (xsep), buffer ablation pulse energy, and glass cladding blind-hole writing pulse energy were further varied with results presented in Figure 4.16. Exposure parameters were identified that could mitigate heat accumulation either by increasing the size/volume of ablated buffer

(rinterface, xsep) or by reducing the amount of energy applied in the vicinity of the cladding / buﬀer interface (Eabl, EBH ) (see Sections 4.2 and 4.3).

As a reference, laser exposure parameters were varied with respect to those presented in Figure 4.15 which were capable of generating nanograting tracks that could be etched into blind-holes in the glass cladding and formed through ablation holes in the acrylate buffer. In this reference case, only 24% of ablation holes did not exhibit interfacial damage surrounding the buffer hole after acid etching. Six process sites were produced for each exposure condition, with three depicted in each subfigure of Figure 4.16 (except for Figure 4.16(c), which depicts all six sites). The reference exposure condition (Figure

4.16(a)) used trepanned ablation to form buﬀer hole ports with λ = 522 nm, νRR = 1 ◦ kHz, NP = 1, θt = 30 , Eabl= 20 nJ, rinterface = 5 µm, and xsep = 200 µm, while cladding nanograting tracks were written with λ = 522 nm, νRR = 500 kHz, vscan = 0.1 mm/s,

EBH = 50 µm, EBH = 60 nJ, and xsep = 200 µm. Consistent with the case shown in Figure 4.15, these exposure parameters yielded 6 out of 6 (100%) samples forming blind nangrating tracks in the glass cladding at the conical shaped buffer holes without any damage to the cladding / buffer interface (left box of Figure 4.16(a)). However, only 1 out of 6 (17%) samples showed no interfacial damage after 2 hours of 5% HF exposure (right two boxes of Figure 4.16(a)), while 6 out of 6 (100%) blind holes were successfully formed in the glass cladding etch (right box of Figure 4.16(a)). Figures 4.16(b) and (c) present variations in buffer ablation exposure conditions that aimed to reduce heat accumulation Chapter 4. Laser Modifications to the Polymer Buffer 74

Figure 4.16: Microscope images of SMF-28 ﬁber in which trepanned conical ablation

holes were written in the fiber acrylate buffer (λ = 522 nm, νRR = 1 kHz, NP = 1, ◦ θt = 30 ) and single-scan radial nanograting tracks were written in the fiber glass

cladding (λ = 522 nm, νRR = 500 kHz, vscan = 0.1 mm/s, dBH = 50 µm) toward the buﬀer ablation holes, shown before and after 2 hours of 5% HF etching. Varying

the amount of buﬀer removed via (a,b) hole diameter (rinterface), and (a,c) spacing

between adjacent holes (xsep), and varying the amount of energy focused (a,d,f) in

the acrylate buﬀer (Eabl), and (a,e,f) in the glass cladding (EBH ). Chapter 4. Laser Modifications to the Polymer Buffer 75

damage eﬀects by increasing the acrylate buﬀer hole sizes and frequency, via rinterface

and xsep, respectively. Increasing the trepanned buffer hole size from rinterface = 5 µm (Figure 4.16(a)) to 10 µm (Figure 4.16(b)) was found to result in 6 out of 6 (100%) blind holes forming without interfacial damage immediately after the laser exposure of both the acrylate buffer and the glass cladding (left box of Figure 4.16(b)). However, only 1 out of 6 (17%) ablation sites were without interfacial damage after 2 hours of 5% HF exposure (right two boxes of Figure 4.16(b)), and 4 out of 6 (67%) blind holes were opened in the glass cladding (right box of Figure 4.16(b)). Decreasing the spacing between adjacent trepanned buffer hole size from xsep = 200 µm (Figure 4.16(a)) to 100 µm (Figure 4.16(c)) was found to result in 6 out of 6 (100%) instances without interfacial damage after laser exposure in both the acrylate buffer and the glass cladding (left box of Figure 4.16(c)), and 6 out of 6 (100%) instances of no interfacial damage after 2 hours of 5% HF exposure (right two boxes of Figure 4.16(c)). In this case, 6 out of 6 (100%) blind holes were opened in the glass cladding (right box of Figure 4.16(c)).

Figure 4.16(d) - (f) present variations in buffer ablation and cladding blind-hole nanograting track pulse energies (Eabl, EBH , and Eabland EBH simultaneously, respectively) aimed at applying less energy to the cladding / buffer interface to reduce heat accumulation. Decreasing the laser energy energy applied to the acrylate buffer from Eabl= 20 nJ (Figure 4.16(a)) to 16 nJ (Figure 4.16(d)) was found to result in 6 out of 6 (100%) instances without interfacial damage immediately after laser exposure in the acrylate buffer and glass cladding (left box of Figure 4.16(d)), and 6 out of 6 (100%) instances without interfacial damage after 2 hours of 5% HF exposure (right two boxes of Figure 4.16(d)). This condition provided 6 out of 6 (100%) blind holes opened in the glass cladding without interfacial damage (right box of Figure 4.16(d)).

Decreasing the energy focused in the glass cladding from EBH = 60 nJ (Figure 4.16(a)) to 50 nJ (Figure 4.16(e)) was found to result in 6 out of 6 (100%) instances without interfacial damage after laser exposure in the acrylate buﬀer and glass cladding (left box of Figure 4.16(e)), and 6 out of 6 (100%) instances of no interfacial damage after 2 hours of 5% HF exposure (right two boxes of Figure 4.16(e). Here, however, only 1 out of 6 (17%) blind holes formed in the glass cladding (right box of Figure 4.16(e)).

Decreasing both of the laser energy exposures in the acrylate buﬀer from Eabl= 20 nJ to

16 nJ and the glass cladding from EBH = 60 nJ to 50 nJ (Figure 4.16(f)) was found to result in 6 out of 6 (100%) instances without interfacial damage after acrylate buﬀer Chapter 4. Laser Modifications to the Polymer Buffer 76 and glass cladding exposure steps (left box of Figure 4.16(f)), and 6 out of 6 (100%) instances without interfacial damage after 2 hours of 5% HF exposure (right two boxes of Figure 4.16(f)). In this case, 3 out of 6 (50%) blind holes were opened in the glass cladding (right box of Figure 4.16(f)). The incidences of damage-free processing before and after HF etching as well as the success of cladding blind hole formation are summarized in Table 4.1. An increase in the buﬀer hole radius at the interface from

Table 4.1: Incidence of Cladding / Buﬀer Interface Damage Before and After 2 hours of HF Etching and of Cladding Blind-Hole Etching over 6 Trials in Figure 4.16

Before Etch After Etch Incidence of no Incidence of no Incidence of interfacial interfacial blind-hole damage damage etching

rinterface = 5µm, xsep = 200µm, Reference Eabl = 20nJ, 100% 17% 100% EBH = 60nJ, (Figure 4.16(a)) Increased r = 10µm, interface 100% 17% 67% hole size (Figure 4.16(b)) Reduced hole x = 100µm, sep 100% 100% 100% separation (Figure 4.16(c)) E , abl 100% 100% 100% (Figure 4.16(d)) Reduced E , BH 100% 100% 17% Laser Pulse (Figure 4.16(d))

Energy Eabl, EBH , 100% 100% 50% (Figure 4.16(f))

rinterface = 5 µm to 10 µm (Figure 4.16(b)) did not affect the incidence of interfacial damage after HF etching (83%) but reduced the incidence of blind-hole etching in the cladding from 100% to 67%. Alternatively, lowering the separation distance between buffer ablation holes (Figure 4.16(c)) had the significant benefit of reducing the incidence of interfacial damage after HF etching from 83% to 0%, while maintaining the cladding blind-hole etching incidence at 100%. Although the latter result is consistent with the heat accumulation damage model presented in Section 4.2, the fact that increasing buffer hole size did not mitigate damage is not. Perhaps the accumulated Chapter 4. Laser Modifications to the Polymer Buffer 77 effect of tracing larger and more concentric circular ablation tracks at the interface weakens the buffer, negating the benefit when writing nanograting tracks in the glass cladding. This would be consistent with the observed effect of ablation pulse energy in Figures 4.16(d) and (f).

Reducing the energy applied in the vicinity of the cladding / buffer interface, either by reducing the buffer ablation pulse energy and/or reducing the pulse energy applied to the glass cladding while writing blind-holes was also found (Table 4.1) to reduce the incidence of interfacial damage after HF etching. Lowering the buffer ablation hole pulse energy from 20 nJ to 16 nJ (Figure 4.16(d)), reducing the cladding blind-hole pulse energy from 60 nJ to 50 nJ (Figure 4.16(e)), and simultaneously reducing both the buffer ablation and cladding blind-hole writing energies by the aforementioned degrees (Figure 4.16(f)) were all found to offer 100% incidence of no interfacial damage after laser exposure at 100% and all had the effect of increasing the incidence of no interfacial damage after 2 hours of 5% HF etching from 17% to 100%. However, the incidence of successful blind-hole etching differed. 100%, 17% and 50% of blind holes were etched when buffer ablation pulse energy was reduced, when cladding blind-hole energy was reduced, and when both buffer ablation and cladding blind-hole energies were simultaneously reduced, respectively. Hence a 60 nJ exposure appears to be a minimum required exposure for nanograting tracks that can open in acrylate-buffered fiber cladding. Overall, reducing buffer hole spacing and decreasing the applied laser energy in the vicinity of the cladding / buffer interface were found to effectively mitigate interfacial damage, though increasing buffer hole size was not.

Optimized laser writing procedures for etching blind holes in acrylate-coated fused silica ﬁber were determined in this section to be 2 hours of 5% HF etching of ﬁbers in which conical structures were ablated via trepanning with λ = 522 nm, νRR = 1 kHz, NP ◦ = 1, θt = 30 , rinterface = 5 µm, xsep = 100 - 200 µm, and Eabl= 16 - 20 nJ, while etching of nanograting tracks in silica cladding (axial channel and access ports) was enabled with laser exposure of λ = 522 nm, νRR = 500 kHz, vscan = 0.1 mm/s, EBH = 50 µm, and

EBH = 60 nJ. Chapter 4. Laser Modifications to the Polymer Buffer 78

4.5 Chapter 4 Summary

This chapter studied the effects of focusing femtosecond laser pulses in glass fiber cladding coated with acrylate buffer as well as focusing in acrylate buffer and following with chemical etching for modifying the glass core/cladding, to selectively machine and open (ablate) the polymer buffer, and/or to produce axial and radial microfluidic channels in the glass cladding of acrylate-coated fiber.

Laser damage was observed and studied at the cladding / buffer interface while laser focusing in the glass fiber cladding through polymer buffer (Section 4.1). Laser damage thresholds were empirically evaluated as a function of radial distance of the focal volume from the fiber center and azimuthal angle relative to the orientation of the writing laser. It was determined in Figure 4.2 that pulse energies between 20 nJ and 40 nJ, typically used to induce refractive index modifications to the doped fiber core, could be applied at all evaluated cladding positions radially and azimuthally without causing observable buffer damage. Pulse energies between 60 nJ and 80 nJ, typically used to induce refractive index modifications in the fiber cladding, could be applied with limitations ranging from maximum radial distances of 35 µm to 55 µm from the fiber center without causing any observable damage to the buffer. There was a small variance in radius depending on the writing pulse energy and azimuthal position. Pulse energies between 20 nJ and 60 nJ, typically used to form nanograting tracks in the fiber cladding for chemical etching, were assessed to be possible at maximum radial distances ranging from 35 µm to ≥ 60 µm from the fiber center, also depending on writing pulse energy and azimuthal position. In order to write nanograting tracks to the cladding / buffer interface, buffer damage could not be avoided. The physical phenomena behind these observed limitations were explored in Section 4.2. Intensity-driven damage directly from incubation effects (Section 4.2.1) and thermally-induced damage accuulated from heat effects in the fiber cladding (Section 4.2.2) were assessed separately as potential causes of damage at the interface. Heat-accumulation better reflected empirical data and was determined to be the primary cause of damage at the cladding / buffer interface when focusing in the glass cladding of coated fiber. Chapter 4. Laser Modifications to the Polymer Buffer 79

In Section 4.3, selective machining of the polymer buffer was studied to determine laser parameters from which clean and efficient ablation of the buffer jacket could be achieved to open a hole to the outer-glass cladding interface without causing collateral damage. Results discussed in Section 4.3 showed that ablation resulted in the least buffer damage when applied the bottom (-z) interface of the fiber buffer, as compared to other azimuthal positions. Repetition rate, pulse energy, and number of applied pulses were optimized for ablation, balancing trade-offs between process speed with heat accumulation and between underexposure with overexposure, minimizing damage. Analysis of data showed that processes with lower repetition rates (100 Hz to 10 kHz), lower pulse energies (10 nJ to 20 nJ), and lower numbers of pulses (1 to 5) could reliably ablate buffer material with minimal damage.

Chemical etching through ablated ports in buffer was studied in Section 4.4 to optimize ablation parameters for reliable transport of etchant through the buffer to nanograting tracks within glass without collateral damage. Hydrofluoric acid etching (5%, ambient temperature, ≤ 2 hours) was found to not severely or categorically induce damage in the buffer material. Potassium hydroxide etching (10 M, 70 ◦C, 8 hours) was, however, found to dissolve the buffer in areas where the cladding / buffer interface was exposed to etchant, either modification by laser ablation or mechanical stripping (Section 4.4.1). Conical ablation ports were found to be more reliable than rectangular ports and linearly percussion ports for reliably guiding etchant to the glass cladding to etch out single-scan linear radial nanograting tracks into blind holes. Trepanning was found to be a more efficient (i.e. less time-consuming) process for producing conical ports than rastering (Section 4.4.2). The best writing parameters for such ablation ports that both enabled consistent cladding etching and did not result in interfacial damage after etching, were trepanned conical ablation ports with a taper angle of 30◦, and exposed by 1 kHz repetition rate, 1 pulse per focal volume, 16 nJ ablation pulse energy, and 200 µm center-to-center spacing between ablation zones. Here, single-scan linear blind-holes were opened from nanograting tracks fabricated with laser exposure conditions of 0.1 mm/s scan speed, 500 kHz repetition rate, and 60 nJ pulse energy (Section 4.4.3).

The research presented in this chapter present a groundwork for developing lab-in- fiber in acrylate-coated silica fiber. This is accomplished by avoiding undue damage to Chapter 4. Laser Modifications to the Polymer Buffer 80 buffer / cladding interface and to the buffer while focusing laser energy in the fused silica core / cladding, and by selectively ablating acrylate buffer in such a way that HF acid can selectively etch laser-written nanograting tracks without damaging the fiber buffer in the process. Optimized processes are further applied in Chapter5, where the findings of Chapter4 are used to design and fabricate LIF devices in acrylate-coated fiber without incurring damage to the fiber buffer. Chapter 5

Lab in Fiber Devices in Buﬀered Fiber

Having established precise methods for femtosecond laser alignment through acrylate buffer into underlying glass fiber (see Chapter3) and assessed damage-zones for laser exposure within fibers (see Chapter4), it is now possible to explore the laser-writing of LIF devices in acrylate-coated fibers. As discussed in Section 2.2, the foundations of LIF can be broadly divided between (1) making modifications in and around the core waveguide of optical fiber, (2) writing cladding photonic circuits, and (3) creating nanograting tracks to facilitate chemical etching of microfluidic channels, ports, and reservoirs. Each of these are addressed in this chapter, assessing the additional challenges and limitations brought about by propagating laser light through a polymer buffer. In principle, these elements together offer a cohesive picture of processing windows and limitations for femtosecond laser structuring polymer-coated optical fiber, upon which different types of devices can be practically designed and fabricated to develop integrated devices beyond the basic devices discussed here.

In Section 5.1, structures written within the core of polymer-coated optical fiber are evaluated. While relatively explored territory [31–36], unique laser exposure strategies are established that enable additional degrees of freedom and control. Fibers coated with conventional urethane acrylate (Section 5.1.1), as well as with thermally-stable polyimide (Section 5.1.2) are examined. In Section 5.2, formation of fiber cladding photonics with the additional challenges for writing through the polymer buffer are explored. These include cross-couplers (also called x-couplers), cladding waveguides,

81 Chapter 5. Lab in Fiber Devices in Buffered Fiber 82 and Bragg grating waveguides (BGWs) (Section 5.2.1) which are then combined to create a cladding photonic 3D shape sensor (Section 5.2.2). Finally, in Section 5.3, the limits of forming microfluidics in polymer-coated fiber are discussed, including enabling the FLICE process in coated fibers (Section 5.3.1), demonstrating complex fluidic structures including a helical microfluidic channel (Section 5.3.2) and particle flow as a precursor to cytometry in coated fibers (Section 5.3.3).

5.1 Structures Written in the Core of Buﬀered Optical Fiber

As discussed in Section 2.2.1, although the core waveguide of SMF-28 represents only 0.4% of the silica fiber by volume, the core is the region of the fiber in which almost all light is guided. It is therefore not surprising that this is where the majority of all laser-written sensor technologies induce modifications to perturb light. This is true for fibers that have been stripped of their protective buffer as well as devices currently written through polymer buffers [31–36]. To date, grating structures which have been laser-written in the core of coated fibers rely on tuning the lasers scan speed to its repetition rate such that periodic burst firing generates an appropriate periodicity of designed grating structures, the limitations of which are discussed in Section 2.1.1. In this section, laser modification of core waveguides were applied at a repetition rate of

νRR = 500 kHz and pulse train bursts selected by an acousto-optic modulator (AOM) as an arbitrary gating mechanism (see Section 3.1) to write fiber Bragg gratings through fibers coated with conventional acrylate (Section 5.1.1) as well as with thermally-stable polyimide (Section 5.1.2) buffers.

5.1.1 Structures Written in the Core of Acrylate-Coated Fiber

Corning SMF-28 ﬁber is coated in a proprietary urethane acrylate coating which was designed for desirable mechanical properties including easy stripping (See Section 2.3), but not for optical applications. As discussed in Section 3.2, the buﬀer introduces Chapter 5. Lab in Fiber Devices in Buffered Fiber 83

Figure 5.1: Microscope images of FBG produced in acrylate coated fiber: (a) coated side-view perspective without evidence of buffer damage, (b) stripped cross-sectional image showing FBG alignment to core, (c) with magnified view showing accuracy of

FBG alignment (λ = 522 nm, νRR = 500 kHz, vscan = 0.1 mm/s, NA = 1.25, EP =

25 nJ, duty = 0.6, zoffset = 55 µm).

challenges in optical alignment and focusing which can be monitored by the FBG spectrum due to the FBGs high sensitivity to confocal position in the core waveguide.

Section 4.2 established that laser pulses with pulse energy of EP ≤ 80 nJ (80 nJ being the maximum EP tested) could be focused in buffered SMF-28 to radial distances up to at least 35 µm (greater than the requisite 4 µm radius of the SMF-28 core waveguide) without introducing any damage to the acrylate buffer or cladding / buffer interface.

Speciﬁcally, laser pulse energies typical for producing core modiﬁcations (EP ≤ 40 nJ) could be focused to laser radial distances of at least 60 µm (60 µm being the maximum radial distance tested) (see Figure 4.2).

Figure 5.1 shows microscope images of a Bragg grating structure over 4.5 mm (lFBG

= 4.5 mm) in acrylate-coated fiber exposed with pulse energy EP = 25 nJ, 0.6 duty cycle, 1.25 NA oil immersion focusing and λ=522 nm wavelength. An axial view of the buffered fiber (Figure 5.1(a)) and cross sectional views of the fiber stripped of its buffer (Figures 5.1(b) - (c)) are shown after laser-processing. In Figure 5.1(a), the fiber is shown after laser-formation of FBG with its buffer undamaged. As expected, no apparent buffer damage is present after scanning focused pulses of EP = 25 nJ inside the fiber core. Alignment and focusing procedures discussed in Section 3.2 were precise Chapter 5. Lab in Fiber Devices in Buffered Fiber 84

enough to position grating tracing to ±0.5 µm precision with respect to the center core

over an axial length of lFBG = 4.5 mm. Figures 5.1(b) and (c) further illustrate the precision of alignment from a cross-sectional microscope view, in low (cladding-scale) and high (core-scale) magnification views, respectively. In these images, the buffer was mechanically stripped from the cladding to enable efficient cleaving (see Section 3.2). Laser modification centered in the fiber core is apparent (Figure 5.1(b)), with bright zones of positive-refractive index change clearly filling the vertical cross-section of the fiber core (Figure 5.1(c)). The dark zone observed above the bright zone is the a result of the negative index change induced by direct femtosecond laser exposure. The bright zone is formed as a result of material conservation [43]. Figure 5.1 validates the alignment and focusing methods outlined in Section 3.2 (Figures 5.1(b) - (c)) and the damage-free radial focusing distances first presented in Section 4.2 (Figure 5.1(a)).

Laser exposure parameters for fabricating FBGs were based on previous work

discussed in Sections 2.2.1 and 3.3.1. Scan speed (vscan), duty cycle, pulse energy (EP ),

and oﬀset distance from the top-interface of the silica ﬁber (zoffset) (see Section 3.2) were optimized by evaluating Bragg gratings based on transmission sharpness

(quantified by 3 dB power loss spectral width (∆λ3dB) and strength (quantified by baseline-peak offset). Optimization of transmission spectra was conducted for both through-buffer and bare (i.e. stripped) fibers, with ideal alignment and focusing parameters summarized in Table 5.1. Scan speed of vscan = 0.1 mm/s was found to be suitable for both stripped fiber and acrylate fiber, in agreement with previous works discussed in Section 3.3.1[89]. Optimal duty cycle, pulse energy, and z-offset distance (the distance in the z-axis between the top interface of the fiber, as measured by reflection in air, and the center of the fiber when immersed in index-matching oil (see Section 3.3)) were all found to be higher by 20%, 25%, and 3.6%, respectively, for FBGs fabricated in acrylate-coated fibers as compared to buffer-stripped fibers. A higher duty cycle was understood as a result of the larger and weaker laser focus spreading energy out more into the unmodified region of the FBG, requiring compensation by a shorter exposure region. Similarly, higher pulse energies required to create gratings in buffered fiber was the result of aberrated pulses applying lower intensity within the focal volume, requiring higher pulse energies to induce sufficient index modification. The higher z-offset distance was attributed to a Gaussian focusing shift by the higher index Chapter 5. Lab in Fiber Devices in Buffered Fiber 85 at the cylindrical buffer interface, a phenomenon discussed in Section 4.2 to help explain asymmetries in the xy plane observed in damage-free radial focusing distances.

Table 5.1: Comparison of optimized writing parameters for 4.5 mm FBGs in Stripped and in Acrylate-Coated Fiber

Stripped Fiber Acrylate-Coated Fiber

vscan 0.1 mm/s 0.1 mm/s Duty cycle 0.5 0.6

EP 20 nJ 25 nJ zoffset 53 µm 55 µm

Representative transmission spectra for 1 cm long (lFBG = 1 cm) FBGs written in stripped ﬁber and in acrylate-coated ﬁber are presented in Figure 5.2(a) and (b), respectively, for exposure of λ = 522 nm, νRR = 500 kHz, vscan = 0.1 mm/s, duty =

0.6, and EP = 20 nJ. The FBG fabricated in stripped fiber (Figure 5.2(a)) exhibits a transmission response of 26.7 dB with a ∆λ3dB peak linewidth of 0.28 nm. Similarly, the FBG fabricated in acrylate-coated fiber (Figure 5.2(b)) exhibits a transmission response of 25.1 dB with a ∆λ3dB peak linewidth of 0.27 nm, nearly identical to the grating fabricated in stripped fiber. FBGs written through acrylate coatings (e.g. Figure 5.2(b)) were found to typically have moderately weaker transmission responses as compared to devices written in stripped fiber (e.g. Figure 5.2(a)), with a reduction of 1.6 dB. FBGs fabricated in coated fibers were generally slightly broader by an order of magnitude of 10 pm on 270 pm linewidth (not observed in Figure 5.2). These small observable discrepancies between FBGs fabricated through stripped fiber and through acrylate-coated fiber were attributed to optical aberrations introduced by the acrylate buffer layer. Weaker, aberrated laser focusing results in lower and less confined intensities within the lasers focal volume, resulting in less efficient index modification. While higher pulse energy may compensate for such effects, higher exposure also raises the DC baseline in the refractive index modification across the entire grating and lowers the AC index contrast, compromising the FBG sharpness as larger laser exposure zones blur the induced periodic modification. Chapter 5. Lab in Fiber Devices in Buffered Fiber 86

Figure 5.2: Transmission spectra for FBGs fabricated in (a) stripped and in (b)

acrylate-coated ﬁbers using otherwise identical process parameters (λ = 522 nm, νRR

= 500 kHz, vscan = 0.1 mm/s, duty = 0.6, EP = 20 nJ, lFBG = 1 cm).

Overall, the anticipated aberrations, losses, and imhomogeneities introduced by the buffer are seen to only minimally limit the strength of FBGs fabricated in the core of fibers coated with urethane acrylate. The optimized FBG spectra were nonetheless very similar to that of devices written in stripped fiber, demonstrating that acrylate buffer is not a fundamental barrier to fabrication of grating structures in the core of acrylate- coated fiber.

5.1.2 Structures Written in the Core of Polyimide-Coated Optical Fiber

Laser modification of silica fibers coated with thermoplastic polyimide is of particular interest for through-jacket processing due to the limitations imposed by buffer-stripping as outlined in Section 3.2. Figure 5.3(a) depicts a fiber in which a 4.5 mm long Bragg grating was fabricated through polyimide-coated fiber. For this grating to exhibit a strong transmission peak, relatively high pulse energies (EP = 120 nJ) were required, but resulted in apparent cracking damage to the buffer layer as visible in Figure 5.3(a). This damage could be avoided by using lower pulse energies, as depicted in Figure 5.3(b) Chapter 5. Lab in Fiber Devices in Buffered Fiber 87

Figure 5.3: Microscope images of FBGs written in polyimide-coated ﬁber: (a) high

magniﬁcation view for lFBG = 4.5 mm, EP = 120 nJ, resulting in polymer damage,

and (b) low magniﬁcation view for lFBG = 14.5 mm, EP = 35 nJ, resulting in no

polymer damage (λ = 1045 nm, νRR = 500 kHz, vscan = 0.1 mm/s, duty = 0.6). wherein 35 nJ pulses were applied. Lower pulse energies predictably resulted in a weaker transmission response for a given device length, and longer devices (lFBG = 14.5 mm) were therefore necessary to compensate for the lower index modulation. Despite the challenges to align and focus 1064 nm light as outlined in Section 3.2, Figure 5.3(b) shows that precise alignment was possible through the polymer jacket without inducing any damage at the polymer/buﬀer interface.

Exposure optimization was achieved by evaluating the effects of scan speed, duty cycle, pulse energy, z-offset, and, as discussed, device length, as given in Section 5.1.1 for acrylate-coated fiber. Laser exposure parameters were optimized for grating strength, sharpness, and damage-free modification of the polyimide buffer in FBGs fabricated using 1045 nm infrared light. These are compared directly in Table 5.2. FBGs of 4.5 mm length were fabricated through polyimide (column 2 in Table 5.2) with a strong response, but they exhibited buffer damage as depicted in Figure 5.3(a). Lower pulse energies produced damage-free grating structures (Figure 5.3(b)), but had to be written over 14.5 mm (column 3 in Table 5.2) to yield a response comparable to a 4.5 mm grating fabricated in buffer-stripped fiber (column 1 in Table 5.2). Damage-free FBGs were optimized at 35 nJ pulse energy for polyimide-coated fibers. A suitable scan speed of vscan = 0.1 mm/s was found to be suitable for both stripped fiber and polyimide fiber, in further agreement with previous works discussed in Section 2.2.1[72, 89, 91]. As in the case of acrylate-coated fibers, optimal duty cycle was found to be higher in the case of coated fiber, moreso in the case of 4.5 mm gratings fabricated with 120 nJ pulse energy (40% increase) as compared Chapter 5. Lab in Fiber Devices in Buffered Fiber 88

to 14.5 mm gratings fabricated with 35 nJ pulse energy (20% increase). The increase in duty cycle is attributed to grating blurring eﬀects as discussed in Section 5.1.1, where the higher energy exposure raises the DC refractive index modiﬁcation to reduce the

AC modulation. The zoffset distance was found to be significantly lower when aligning 1045 nm light in polyimide-coated fibers (45.5 µm) as compared to stripped fiber (55 µm), a discrepancy of 17% compared to 3.6% for acrylate-coated fiber. The significant discrepancies in pulse energy and zoffset distance required to write comparable FBGs of similar length in polyimide-coated and stripped fiber suggests that the polyimide buffer significantly aberrates the focused beam. This may be attributed to small differences in the divergence of the incoming 1045 nm versus 522 nm beams. Strong absorption in the buffer would account for the pulse energy discrepancy and the nature of damage observed as cracking versus interfacial damage discussed in Section 4.2.

Table 5.2: Comparison of optimized writing parameters for FBGs in Stripped and in Polyimide-Coated Fiber

Stripped Fiber Polyimide-Coated Fiber Polyimide-Coated Fiber

lFBG = 4.5 mm lFBG = 4.5 mm lFBG = 14.5 mm vscan 0.1 mm/s 0.1 mm/s 0.1 mm/s Duty cycle 0.5 0.7 0.6

EP 40 nJ 120 nJ 35 nJ zoffset 55 µm 45.5 µm 45.5 µm

Figure 5.4 depicts representative FBG spectra for devices fabricated using λ = 1045 nm exposure in buffer-stripped (Figure 5.4(a)) as well as in polyimide-coated fiber (Figure 5.4(b)). The figure quality is compromised due to the unavailability of better data- capturing options at the time these experiments were conducted. For reasons stated above, the FBG fabricated in polyimide-coated fiber (Figure 5.4(b)) was written over a length 3 times longer than the FBG fabricated in polyimide-stripped fiber (Figure 5.4(a)). Under these conditions, a transmission response of 20.8 dB was observed for the grating fabricated through a polyimide buffer (Figure 5.4(b)), as compared to 27.4 dB response by the grating fabricated in buffer-stripped fiber (Figure 5.4(a)), representing a difference of 6.6 dB which amounts to little difference in actual signal when already 20 dB below baseline. The transmission response of the polyimide-coated FBG did, however, possess a narrower 3dB width of 0.24 nm for coated fiber compared to 0.78 nm for Chapter 5. Lab in Fiber Devices in Buffered Fiber 89

Figure 5.4: Transmission spectra for FBGs fabricated in (a) polyimide-stripped

ﬁber using lFBG = 4.5 mm, EP = 40 nJ, and in (b) polyimide-coated ﬁber using

lFBG = 14.5 mm, EP = 35 nJ (λ = 1045 nm, νRR = 500 kHz, vscan = 0.1 mm/s, duty = 0.6).

∆ν stripped fiber, a 69% improvement). This corresponds to a Q-factor ( ν ) decrease from 6.6 × 103 in polyimide-coated fiber to 2.0 × 103 in buffer-stripped fiber. The discrepancy is attributed to the lower pulse energy and longer grating length used to fabricated the device in buffer-coated fiber.

Polyimide-coated fiber has thus been demonstrated to be a viable platform for through-buffer laser-modification of the fiber core for 1045 nm light. Despite promising results and inherent benefits in high-temperature applications, the challenges posed by working with polyimide buffers in terms of fiber preparation and alignment (discussed in Section 3.2) were found to be significantly onerous. As such, acrylate-coated fibers were prioritized for cladding photonics (see Section 5.2) and microfluidics (see Section 5.3) for development in this thesis study. Further development of LIF in polyimide-coated fibers has been left as future work. Chapter 5. Lab in Fiber Devices in Buffered Fiber 90

5.2 Cladding Photonic Structures Written in the Cladding of Acrylate-Coated Optical Fiber

As discussed in Section 3.3.2, modifications within buffer-coated fibers for cladding photonics poses a twofold challenge over core-only modifications: (1) higher pulse energies

(EP ≥ 50 nJ) are required to create index modifications capable of tapping and guiding light with waveguides formed in the fiber cladding, and (2) as the focus of the writing laser is moved radially from the core toward the cladding / buffer interface, higher intensities and heat-affected zones within the buffer make damage-free writing more difficult. These challenges were addressed in Section 4.2, with damage-free radial focusing distances for varying pulse energies presented in Figure 4.2. In this section, these damage-free focusing conditions are used as guides for fabricating microphotonic elements in acrylate-coated fiber including taps, waveguides, and Bragg grating waveguides (Section 5.2.1). These components are then used to build a working device in acrylate-coated fiber: a 3D shape sensor (Section 5.2.2).

5.2.1 Optical Components: Cross-Couplers, Cladding Waveguides, and Bragg Grating Waveguides

As discussed in Section 2.2.2, taps and cladding waveguides are fundamental building blocks to producing cladding photonic circuits. In this section, taps, cladding waveguides, and cladding Bragg gratings were fabricated in acrylate-coated fibers, drawing on procedures developed for stripped fibers by Dr. Jason Grenier [72], Dr. Kenneth Lee [91], and Dr. Moez Haque [89]. These components are evaluated through cross-couplers leading to Bragg grating waveguides, with crossing angle, writing power, and duty cycle optimized for through-buffer writing. Devices were written using a maximum writing power of 70 nJ at a maximum radial distance of 35 µm from the fiber core in accordance with damage-free focusing distances outlined in Figure 4.2.

The crossing angle (θt) in a cross-coupler refers to the angle at which a linear index modiﬁcation crosses the ﬁber core, tested here in either the xy or the xz plane (see Figure 3.4), for tapping light between the pre-existing core waveguide and the laser- Chapter 5. Lab in Fiber Devices in Buffered Fiber 91

Figure 5.5: Reﬂection spectra for couplers in the xz plane leading to cladding Bragg ◦ ◦ grating waveguides, varying crossing angle, θX between 1.5 and 3.1 (λ = 522 nm,

νRR = 500 kHz, EP = 60 nJ, vscan = 0.1 mm/s, RC,XC = 40 µm, doffset =35 µm,

LFBG = 4 mm, duty = 0.6).

written cladding waveguide. Crossing angle θt was optimized by monitoring the reflection spectrum of cross-couplers leading to a strong Bragg grating waveguide, with coupling efficiency determined based on peak reflection strength. Linear index modifications were created by scanning the laser from 10 µm above (in the positive z hemisphere of the xz plane) the fiber center to a radial distance, doffset, of 35 µm below the fiber center (in

the negative z hemisphere of the xz plane) at a crossing angle of θt, transitioning from a linear diagonal scan to a scan parallel to the x-axis by circularly bending the laser track

with a radius of curvature, RC,XC = 40 µm (see schematic in Figure 3.7). Pulse energy and duty cycle were held constant at 60 nJ and 0.6, respectively.

Figure 5.5 presents typical BGW reﬂection spectra recorded at diﬀerent crossing

angles (θX ) for cladding waveguide arms formed in the xz plane (see Figure 3.4) over a range of 1.5◦ - 3.1◦. The strongest peak reﬂection was observed for a device with a crossing angle of 2.1◦, with -17.1 dB resonance loss denoted in Figure 5.5 by the light green spectrum. The minimum peak reﬂection observed was -28.5 dB at a crossing Chapter 5. Lab in Fiber Devices in Buffered Fiber 92

Figure 5.6: Peak reﬂection of cross-couplers leading to BGWs plotted as a function of crossing angle in the xy (red circles) and xz (blue diamonds) planes (λ = 522 nm,

νRR = 500 kHz, vscan = 0.1 mm/s, RC,XC = 40 µm, doffset =35 µm, lFBG = 4 mm, duty = 0.6).

◦ angle of θX = 1.5 (denoted in Figure 5.5 by the pink spectrum). Assuming that the highest peak reflection of -17.1 dB arises from a 50% coupling efficiency (the highest round trip reflection), an additional -11.1 dB loss is still observed. This was attributed to a number of factors including modal mismatch loss, bend and propagation loss in cladding waveguides, and scattering by the Bragg grating. A thorough analysis of sources of loss in cladding waveguides fabricated through acrylate-coated fiber is not presented in this thesis, and is a subject of future work.

Crossing angles in the xy plane (see Figure 3.4) were optimized in addition to the xz plane, with typical peak reﬂectance values observed at varying angles for such perpendicular cross-coupler devices presented in Figure 5.6. For both sets of couplers, laser parameters were identical (λ = 522 nm, νRR = 500 kHz, vscan = 0.1 mm/s, RC,XC

= 40 µm, doffset =35 µm, lFBG = 4 mm, and duty = 0.6), except for pulse energy,

which was set to EP = 50 nJ for cross-couplers fabricated in the xy plane and EP = 60 nJ for cross-couplers fabricated in the xz plane due to points discussed further in Figure 5.7. Figure 5.6 shows a peak reflectance at a crossing angles of 1.9◦ in the xy plane (red circles in Figure 5.6) and 2.1◦ in the xz plane (blue diamonds in Figure 5.6) producing similar resonance strengths of -17.2 dB and -16.7 dB, respectively. The discrepancy in optimum laser pulse energy (50 nJ for the xy plane versus 50 nJ for the xz plane) arises from independent optimization as presented in Figure 5.7. Peak reflection is observed to drop off notably faster in the xy plane than in the xz plane. Chapter 5. Lab in Fiber Devices in Buffered Fiber 93

Figure 5.7: Peak reﬂection of cross-couplers in the xy (blue diamonds) and xz (red circles) planes leading to BGWs plotted as a function of varying pulse energy, (λ = ◦ 522 nm, νRR = 500 kHz, vscan = 0.1 mm/s, θX = 1.9 , RC,XC = 40 µm, doffset =35

µm, lFBG = 4 mm, duty = 0.6).

This asymmetry is largely a consequence of beam profile effects of the cross-coupler; the beam profile is asymmetrical in terms of its ellipsoidal focal volume ( ω0 = 0.865) as well zR as zones of positive and negative index modification (see light and dark zones in Figure 5.1(c)). Along the z-axis of the fiber, the exposed silica yields zones of positive index change beneath zones of negative index change. As this index profile is scanned in the xz plane, the positive index zones are partially overwritten by negative index zones.

Laser exposure power was also optimized for cladding BGWs formed in the xy and xz planes, tested over a range of 40 nJ to 70 nJ (see Section 4.2), using a crossing angle of θX ◦ = 1.9 (λ = 522 nm, νRR = 500 kHz, vscan = 0.1 mm/s, RC,XC = 40 µm, doffset =35 µm, lFBG = 4 mm, duty = 0.6), presented in Figure 5.7. Typical peak Bragg reflection values, as reported in Figure 5.7, were obtained using pulse energies of EP = 60 nJ and EP = 50 nJ for cross-couplers fabricated in xz (red circles in) and xy planes (blue diamonds), respectively. Peak reflectance values were comparable at -15.3 dB in the xz plane and - 15.1 dB in the xy plane. Asymmetry, was however, observed in the drop-off in reflectance at non-peak values, with cross-couplers in the xy plane experiencing more steep change than cross-couplers in the xz plane. This asymmetry was due to the asymmetry of the focusing beam and induced modifications, as discussed above in regards to Figure 5.6.

To evaluate the hindrance to fabricating cladding photonic BGWs through an undamaged acrylate buﬀer, cross-coupler devices were fabricated in the xz plane for Chapter 5. Lab in Fiber Devices in Buffered Fiber 94

buffer-coated and buffer-stripped fiber under identical exposure conditions (λ = 522 ◦ nm, νRR = 500 kHz, vscan = 0.1 mm/s, EP = 60 nJ, θX = 1.9 , RC,XC = 40 µm, doffset

=35 µm, lFBG = 4 mm, and duty = 0.6) in stripped fiber, with typical reflection spectra presented in Figure 5.8. The peak reflectances and baseline-peak strengths between devices fabricated in stripped fiber (Figure 5.8(a)) and in acrylate-coated (Figure 5.8(b)) fibers were consistently found to be comparable, with typical discrepancies of only 0.06 dB and 0.4 dB for peak reflectance and baseline-peak reflection strength, respectively. The comparable peak reflectance and reflectance strength in acrylate-coated and stripped fibers indicate that focusing through the acrylate buffer poses no significant hindrance to disturbing the coupling and cladding-waveguiding efficiency. The bandwidth of the reflection peaks (as quantified

by ∆λ3dB) were wider for devices fabricated in acrylate coated ﬁber at ∆λ3dB = 0.39 nm in Figure 5.8(b) as compared to the bandwidth of devices fabricated in stripped

fiber at ∆λ3dB = 0.27 nm in Figure 5.8(a), an increase of 44%. The broader Bragg reflection spectrum in devices fabricated through an acrylate buffer suggests a broadening and shifting of the beam focus size and position which can be understood to blur the sharp grating features as discussed in Section 5.1.1. Figure 5.8(c) shows micrographs of the fiber cladding circuit, confirming that no damage to the acrylate buffer was observed after 60 nJ pulses were exposed at a radial distance up to 35 µm from the fiber center, consistent with the damage assessment discussed in Section 4.2.

5.2.2 Bend Proﬁler in Acrylate-Coated Fiber

The cross-coupler grating waveguides presented in Section 5.2.1 were further developed with a 3D shape sensor, similar to devices demonstrated by Lee et al. [16] and Waltermann et al. [38], discussed in Section 2.2.2. A total of nine Bragg gratings were written, as shown in Figure 5.9(a): three axially along the core of fiber, three in a cladding waveguide coupling to the core with a cross-coupler in xz plane, and three in a cladding waveguide coupling to the core with a cross-coupler in the xy plane. Cross-couplers were written using the optimized pulse energies shown in Figure 5.7 and crossing angles tuned for comparable relative reflection peaks from cladding Bragg gratings coupling in both the xz and xy planes, based on the results presented in Figure 5.6. Light reflected from the first set of Bragg gratings (coupling in the xz Chapter 5. Lab in Fiber Devices in Buffered Fiber 95

Figure 5.8: Reflection spectra comparison of cross-couplers in the xz plane leading to cladding Bragg gratings fabricated (a) in stripped fiber and (b) in acrylate-coated fiber; (c) microscope image of device (b), demonstrating damage-free processing; (λ ◦ = 522 nm, νRR = 500 kHz, vscan = 0.1 mm/s, EP = 60 nJ, θX = 1.9 , RC,XC = 40

µm, doffset =35 µm, lFBG = 4 mm, and duty = 0.6). Chapter 5. Lab in Fiber Devices in Buffered Fiber 96 plane) experienced loss at the first coupler twice (reflection spectra representing a round-trip), expressed in Equation 5.1 where Rxz represents net reflection signal of the cross-coupler tap fabricated in the xz plane with ηxz single-pass efficiency and RBGW representing Bragg rating reflectance.

2 Rxz = ηxzRBGW , (5.1)

Light reflected from the second coupler (coupling in the xy plane) experienced double- pass loss at the first coupler twice (i.e. the remainder of light which is not coupled into the first coupler), and double pass loss at the second coupler yielding net reflectance,

Rxy, expressed in Equation 5.1 where ηxy represent single-pass transmission eﬃciency of the cross-coupler tap in the xy plane.

2 Rxz = (1 − ηxz)ηxy(1 − ηxz)RBGW , (5.2)

Ideally, a device coupling light into and out of both of these planes should be balanced to reﬂect light with similar intensity. This is achieved when Rxy is forced to equal Rxy, leading to the desired coupling relation expressed in Equation 5.1,

ηxz ηxy = , (5.3) 1 − ηxz

1 1 forcing values of ηxz = 4 and ηxy = 3 . Referring to Figure 5.6, this was approximately ◦ ◦ accomplished for coupling angles of 2.5 in the xz plane (ηxz = 0.257) and 2.1 in the xy plane (ηxy = 0.366), with efficiencies determined under the assumption that peak reflectance represents an optimal 50% coupling efficiency (-6 dB round-trip loss) [72]. However, additional sources of bend, propagation, and radiation losses were observed and assumed to be consistent over all tested devices, at -11.1 dB. Using these assumptions and Equations 5.1 and 5.2, it was predicted that gratings written in the xz plane would have a peak reflectance of Rxy = -22.47 dB, and in the xy plane of Rxy = -22.51 dB.

Recorded spectra are presented in Figure 5.9 for a grating distributed shape sensor fabricated using λ = 522 nm, νRR = 500 kHz, vscan = 0.1 mm/s, EP = 60 nJ, θX = ◦ 1.9 , RC,XC = 40 µm, doffset =35 µm, lFBG = 4 mm, duty = 0.6. Figure 5.9(a) depicts a graphic of the distributed fiber shape-sensing device, in which light is coupled into both the xz and xy planes leading to Bragg gratings which reflect light back into the core. The second coupler was designed to have a higher coupling efficiency to account for Chapter 5. Lab in Fiber Devices in Buffered Fiber 97

Figure 5.9: Bend profiler fabricated in acrylate-coated fiber: (a) Device schematic of three FBGs and two orthogonal sets of three BGWs in the cladding at 35 µm radial distance; (b) spectra obtained for a straight (blue spectrum) and a uniformly bent (blue spectrum) fiber. Blue, purple and orange boxes separate Bragg grating triplets in the core, in the cladding crossing in the xz plane and in the cladding crossing in the xy plane, respectively. The dashed red line denotes a reflection of -22.5 dB, the grating reflection predicted by Equations 5.1 and 5.2; (c) Bend radius and azimuthal bend angle calculated using Equations 5.5 and 5.6 from green spectrum in Figure 5.9(b), shown relative to waveguide positions and overlaid on cross-sectional damage-

free processing zone for 60 nJ pulses from Figure 4.2,(λ = 522 nm, νRR = 500 kHz,

vscan = 0.1 mm/s, RC,XC = 40 µm, doffset =35 µm, lFBG = 4 mm, duty = 0.6, ◦ ◦ EP,xz = 60 nJ, EP,xy = 50 nJ, θxz = 2.5 , θxy = 2.3 ). Chapter 5. Lab in Fiber Devices in Buffered Fiber 98 additional light lost to the first coupler, as discussed above. Figure 5.9(b) presents the recorded reflection spectrum of the device before and after applying a uniform bend. The dashed red line denotes -22.5 dB, the reflectance value of the cladding Bragg gratings predicted by Equations 5.1 to 5.3. The first Bragg resonance from each cross-couplers Bragg grating triplet closely matched this predicted reflectance. For each triplet, the gratings uniformly decreased in reflectance as Bragg resonant wavelength increases, as higher wavelength gratings were positioned further from the tap and therefore experienced greater propagation losses. Because of the 0.5 mm spacing between adjacent gratings, the uniform slope of the cladding BGW losses provide estimated cladding propagation losses of 2.6 dB/cm and 3.5 dB/cm for cladding waveguides written in the xz and xy planes, respectively.

After applying a uniform bend, 6.17 ×10−4 ± 2.05 fractional peak shifts ( ∆λ ) were λ0 observed in the xz plane, 2.06 ×10−4 ± 2.23 ×10−5 fractional shifts were observed in the xy plane, and no shifts were observed in the core gratings, which is consistent with a uniform fiber bend. Figure 5.9(c) presents a graphical depiction of the device cross section, with grating positions overlaid on non-damage process zones for 60 nJ pulses from Figure 4.2. The bend profile of the fiber was calculated following methods outlined by Lee et al. [16], using Equations 5.4- 5.6: ∆λ = (1 − pe)ε, (5.4) λ0 d ρ = offset , (5.5) c ε ε ϕ = arcsin xy , (5.6) εxz

Here, pe is the strain-optic coeﬃcient (pe = 0.204 for fused silica [99]), ε is the strain on the cladding waveguide arm, ρc is the radius of curvature in the cladding waveguide arm, and ϕ is the azimuthal angle of the applied bend. The fractional shifts presented in −4 Figure 5.9(b) correspond to strains in the xy and xz planes equal to εxy = 2.59 ×10 −4 and εxz = 7.75 ×10 , respectively. Trigonometric analysis was applied to determine an ◦ overall bend of radius ρc = 42.8 mm and bending axis on azimuthal angle ϕ = 109.5 , as depicted in Figure 5.9(c). This principle can be further extended to live 3D monitoring, as demonstrated by Lee et al. [16] and Waltermann et al. [38], but is beyond the scope of this thesis. Chapter 5. Lab in Fiber Devices in Buffered Fiber 99

Results presented in this section demonstrate the ability to fabricate cladding photonics in acrylate-coated silica fiber without inducing damage to the buffer coating, provided that laser focusing and exposure parameters were confined within previously determined process zones (see Section 4.2). Taps, cladding waveguides, and cladding Bragg gratings were demonstrated with crossing angle and writing conditions optimized for cladding photonics extending into the xz and xy planes of the fiber. A 3D shape sensing device was further designed and fabricated, capable of monitoring bend profile via differential fractional Bragg shifts within fibers processed with acrylate buffer intact.

5.3 Microﬂuidics Written in Acrylate-Coated Fiber

The third primary component of LIF in acrylate-coated fibers is microfluidics via the FLICE method, discussed in Section 3.3.3. In this two-step process, nanograting tracks were first laser-written in silica fiber cladding and subsequently submerged in an etchant that selectively opened laser-exposed tracks. Section 4.4 explored this topic as it pertains to selective acrylate machining via ablation. In Section 4.4, it was established that potassium hydroxide was an ineffective etchant for acrylate-coated fibers, as any exposure to the cladding / buffer interface (either due to mechanical stripping or to laser ablation) resulted in dissolution of the acrylate buffer (shown in Figure 4.9). Hydrofluoric acid was determined to be a viable etchant, under appropriate conditions. Conically trepanned ablation was found to most reliably guide etchant to nanograting tracks written in fiber in the form of blind holes, as compared to percussion and raster ablation. Ablation parameters were optimized to νRR = 1 kHz, NP = 1, Eabl= 16 nJ - 20 nJ, xsep = 100 um ◦ - 200 µm, and θt = 30 , with nanograting tracks exposed using νRR = 500 kHz, vscan =

0.1 mm/s, and EP = 60 nJ to the bottom (-z) interface of the ﬁber (see Table 4.1).

5.3.1 FLICE in Acrylate-Coated Fiber

Previous work conducted by Haque et al. report etching of nanograting tracks written

in stripped ﬁber using pulse energies as low as 6 nJ (λ = 522 nm, νRR = 1 MHz, vscan = 0.3 mm/s) [89]. Figure 5.10 presents microscope images of etched through-holes following nanograting tracks written in stripped SMF-28 ﬁber using pulse energies ranging from 30 Chapter 5. Lab in Fiber Devices in Buffered Fiber 100

Figure 5.10: Microscope images of through-holes in stripped ﬁber, fabricated by

creating nanograting tracks via scanned laser focusing (λ = 522 nm, νRR = 500 kHz,

vscan = 0.3 mm/s) with varying pulse energy and subsequent chemical etching (5% HF, 15 minutes) (a) crossing the y-axis of the ﬁber, (b) crossing the z-axis of the ﬁber

nJ to 100 nJ (λ = 522 nm, νRR = 500 kHz, vscan = 0.3 mm/s, 15 minutes 5% HF etch). The holes were crossing the fiber in the y- (Figure 5.10(a)) and z-axes (Figure 5.10(b)). All pulse energies presented in Figure 5.10 resulted in etching of open holes through the fibers with diameters of 2.1 µm (30 nJ exposure in Figure 5.10(a)) to 5.7 µm (80 nJ exposure in Figure 5.10(b)), defining high aspect ratio geometries. For 100 nJ pulse formation of nanograting tracks in the z-axis (Figure 5.10(b)), fibers were susceptible to cleaving, possibly owing to higher stresses induced at such exposures during HF etching. Microfluidic through-holes were reliably etched for pulse energies as low as 30 nJ crossing both the y- and z-axes. It was observed that for through-holes written in both the y- and z-axes, low EP = 30 nJ exposure resulted in thinner through-holes of 2.1 µm which appeared fainter in transmission microscopy. The cross-sectional geometry of through- Chapter 5. Lab in Fiber Devices in Buffered Fiber 101 holes etched in the y-axis were observed to be symmetric, but the diameters of through- holes written in the z-axis were observed to taper, for example from 5.7 µm diameter at +z cladding surface to 2.5 µm diameter at the -z cladding surface for nanogratings tracks exposed with 80 nJ pulses (last box of Figure 5.10(b)). Tapering was observed to increase steadily with increasing pulse energy. The observed increase in darkness in transmission optical microscopy with increasing pulse energy (e.g. comparing 30 nJ with 100 nJ exposure in Figure 5.10(a)) can be attributed to the formation of rougher side-walls that increase scattering. The taper observed in the z-axis is attributed to overheating of the index-matching oil during laser-scanning at the +z interface, leading to the formation of bubbles and aberrations in the beam propagation further into the glass.

As discussed in Section 4.2, pulse energies lower than 50 nJ were found to not etch reliably when exposed to HF acid through an ablated buffer port. Hence, 60 nJ pulses were required for consistent, reliable etching. The discrepancy between this threshold in acrylate-coated fiber and 30 nJ in stripped fiber is significantly greater than that observed for refractive index modifications (see Sections 5.1 and 5.2). The difference in discrepancies is attributed to the acrylate buffer introducing beam aberrations which compromise the formation of parallel nanogratings, a process more sensitive to perturbations induced in the wavefront than seen for index modification. Though laser-written nanograting tracks capable of guiding etchant were impeded by the acrylate buffer, sufficiently high pulse energy pulses were nevertheless found capable of enabling microfluidics in acrylate-coated fibers without causing damage to the cladding / buffer interface.

5.3.2 Fluid Flow in Acrylate-Coated Fiber

Section 4.4.2 explored the formation of microfluidic channels in acrylate-coated fiber, with laser exposure conditions for reliable blind-hole etching optimized to avoid cladding / buffer interface damage as presented in Table 4.1. Acrylate buffer ablation and silica cladding nanograting laser processes were further optimized for developing microfluidic structures in silica fiber cladding, extending beyond blind-holes to evaluate limitations introduced when writing microfluidic channels running parallel along with the fiber. Such Chapter 5. Lab in Fiber Devices in Buffered Fiber 102

a structure involves greater heat accumulation in the glass cladding as the laser focus scans along the x-axis of the fiber cladding. In addition, periodic access ports were required along the -z-axis of the fiber cladding to facilitate connecting of the microfluidic channel with the cladding/air interface to enable fresh HF to cycle through and fully etch long channel lengths (see Section 3.3.3). Periodic access ports in the fiber cladding were enabled to interact with the outside environment via laser-ablated buffer ports. Laser exposure parameters were selected based on successful processes presented in Figures

4.16(c) and (d): trepanned buﬀer ablation was applied with λ = 522 nm, νRR = 1 kHz, ◦ NP = 1, θt = 30 , Eabl= 16 nJ - 20 nJ, rinterface = 5 µm, and xsep = 100 - 200 µm, and

cladding blind-holes were formed with λ = 522 nm, νRR = 500 kHz, vscan = 0.1 mm/s,

EBH = 50 µm, Echannel = 60 nJ, xsep = 100 - 200 µm. The channel was formed with the

additional parameters of lchannel = 5 mm when xsep = 200 µm and lchannel = 2.5 mm when xsep = 100 µm to provide a total of 26 buffer ablation holes and cladding access ports. Straight microfluidic tracks written with these parameters and their variations with exposure conditions are presented in Figure 5.11 and summarized in Table 5.3. The microfluidic channels etched in silica cladding through trepanned buffer holes shown in Figure 5.11 present further limitations in laser exposure conditions as compared to the blind holes etched in silica cladding (Figure 4.16), as interfacial damage was observed in between adjacent buffer ablation holes. In Figure 5.11(a), ablation pulse energy was set to 16 nJ, based on the results presented in Figure 4.16(d). In this trial, there was no interfacial damage observed in the vicinity of 26 out of 26 (100%) attempts to form identical ablation holes both after laser ablation (left box in Figure 5.11(a)) and after 2 hours of 5% HF etching (right two boxes in Figure 5.11(a)). All 26 access ports in the silica cladding were found to etch successfully and open the entirety of the 5 mm axial channel (right box in Figure 5.11(a)).

A similar nanograting track was formed (Figure 5.11(b)) in which the spacing between adjacent buffer ablation holes and cladding access ports was reduced from xsep = 200 µm to xsep = 100 µm while maintaining ablation pulse energy at Eabl= 20 nJ, as based on the results presented in Figure 4.16(c). The result in the left box of Figure 5.11(b) shows the fiber after laser buffer ablation and cladding modification where no interfacial damage was observed in the vicinity of all 26 ablation sites for a 100% pre-etching damage-free success rate. However, after acid etching (right two boxes of Figure 5.11(b), interfacial damage was observed in the vicinity of all but 5/26 ablation holes for only a 19% post-etching Chapter 5. Lab in Fiber Devices in Buffered Fiber 103

Figure 5.11: Microscope images of SMF-28 in which trepanned conical ablation

holes were written in the fiber acrylate buffer (λ = 522 nm, νRR = 1 kHz, NP = 1, ◦ θt = 30 , rinterface = 5 µm) and a single-scan axial microfluidic channel with radial access ports connecting the axial channel to the trepanned buffer holes was written

in the ﬁber glass cladding (λ = 522 nm, νRR = 500 kHz, vscan = 0.1 mm/s, dBH = 50 µm, Echannel = 60 nJ), before and after 2 hours of 5% HF etching: varying the

pulse energy applied to the acrylate buﬀer between (a,c) Eabl= 16 nJ and (b) Eabl= 20 nJ, and varying the distance between adjacent buﬀer ablation holes and cladding

etched access ports between (a) xsep = 200 µm and (b,c) xsep = 100 µm, based on results presented in Table 4.16. Chapter 5. Lab in Fiber Devices in Buffered Fiber 104

damage-free success rate. Etching of the cladding nanograting tracks was observed in 22 out of 26 (85%) cladding access ports, with the axial channel interrupted where access ports etching was incomplete (right box of Figure 5.11(b)). These unetched cladding regions showed no buﬀer damage as depicted in Figure 5.11(b).

Figure 5.11(c) presents an axial channel in which successful laser exposure strategies from Figures 4.16(c) and (d) were simultaneously adopted by reducing the spacing

between adjacent buﬀer ablation holes from xsep = 200 µm to 100 µm and by reducing

buffer ablation pulse energy from Eabl= 20 nJ to 16 nJ. This trial eliminated cladding / buffer interface damage for 26 out of 26 (100%) buffer ablation holes (left box of Figure 5.11(c)). However, after 2 hours of 5% HF etching, only 2 out of 26 (8%) ablation zones were collateral-damage-free (right two boxes of Figure 5.11(c)). Further, 24 out of 26 (92%) cladding access ports were successfully opened via HF etching, and the axial microfluidic channel was interrupted where access ports failed to etch (right box of Figure 5.11(c)).

The process for creating blind holes in glass cladding through trepanned buffer holes presented in Figure 4.16(d) was found to extend to writing microfluidic tracks parallel to the fibre core at a radial distance of 12.5 µm from the fiber center, as shown in Figure

5.11(a) (xsep = 200 µm, Eabl= 16 nJ). Hence, laser ablation exposure of 16 nJ yielded sufficiently small heat affected zones, while the additional energy focused in the silica fiber (60 nJ) to write the axial microfluidic track were sufficiently far from the cladding (40 µm) to not result in interfacial damage after HF etching. When the spacing between

adjacent buﬀer holes and access ports was set to xsep = 100 m (Figures 5.11(b) and (c)), no interfacial damage was observed after laser machining of the buﬀer and cladding, as shown in Figure 4.16(c), but unlike in Figure 4.16(c), interfacial damage was observed in

82% (when Eabl= 20 nJ in Figure 5.11(b)) to 92% (when Eabl= 16 nJ in Figure 5.11(c)) of the ablation holes after 2 hours of 5% HF etching. In these trials, successful HF etching in the cladding was associated with localized interfacial damage. These results indicate that close spacing of cladding blind holes can mitigate heat accumulation effects (Figure 4.16(c)) but the nanograting formation step in the center of the fiber (i.e. between adjacent buffer ports) was sufficient to raise the buffer/cladding interface sensitivity to HF damage. Comparing Figures 5.11(a) and (c), the total thermal energy applied to the Chapter 5. Lab in Fiber Devices in Buffered Fiber 105

buﬀer is higher when access ports are spaced more closely together, passing a threshold not met when a focused axial channel was absent as in Figure 4.16(c).

Table 5.3: Incidence of Cladding / Buﬀer Interface Damage and Blind-Hole Etching before and after 2 hours of HF etching and of Cladding Blind-Hole Etching over 26 Ablation Holes/Access Ports (see Figure 5.11)

Before Etch After Etch Incidence of no Incidence of no Incidence of interfacial interfacial microﬂuidic channel damage damage etching in cladding

xsep = 200 µm Eabl = 16 nJ 100% 100% 100% (Figure 5.11(a))

xsep = 100 µm Eabl = 20 nJ 100% 19% 85% (Figure 5.11(b))

xsep = 100 µm Eabl = 16 nJ 100% 8% 92% (Figure 5.11(c))

Significant hindrances to writing three-dimensional microfluidic channels throughout the fiber cladding were encountered due to accumulated heat effects. Nonetheless, the optimized process demonstrated in Figure 4.16(a) (first row of Table 5.3) identifies a processing space where laser-processing of nanogratings in the fiber cladding, laser-ablation of the acrylate buffer, and the opening of cladding nanograting tracks into microfluidic channels were all simultaneously possible without incurring damage to the acrylate buffer. The laser writing procedures for etching an axial microfluidic channel in acrylate-coated fused silica fiber were determined to be 2 hours of 5% HF etching of fibers in which conical structures were ablated via trepanning with exposure ◦ of λ = 522 nm, νRR = 1 kHz, NP = 1, θt = 30 , rinterface = 5 µm, xsep = 200 µm, and

Eabl= 16 nJ, and microﬂuidic elements in silica cladding (axial channel and access ports) were written with λ = 522 nm, νRR = 500 kHz, vscan = 0.1 mm/s, EBH = 50 µm, and Echannel = 60 nJ. Next, the ﬂexibility of these process parameters are evaluated, both in terms of channel size and complexity. Chapter 5. Lab in Fiber Devices in Buffered Fiber 106

Figure 5.12: Microscope images of a segment of a helical microfluidic channel vertically crossing the fiber core written in acrylate-coated fiber: (a) devoid of fluid,

(b) ﬁlled with water (λ = 522 nm, RR,helix = 500 kHz, vscan,helix = 0.1 mm/s, EP,helix

= 60 nJ, rhelix = 20 µm,Λhelix = 200 µm, νRR,ablation = 1 kHz, N)P, ablation = 1, ◦ Eabl = 16 nJ, xsep = 200 µm, θt = 30 ).

Direct laser writing, as discussed in Section 3.3, enables the formation of arbitrarily complex three-dimensional fluidic systems in much the same way as cladding photonic circuits. The microfluidic elements presented in Figure 5.11 were fabricated by single- scan processes. Channel diameter can be controlled by writing adjacent tracks [100] in a hexagonal cross-sectional pattern which then etch together to form super channels. A √ hexagonal close packing (2D hcp) arrangement with 2 µm or 5 µm center-to-center separation between adjacent nanograting tracks was found to enable the merging of adjacent overlapping channels to form a larger super-channel after HF etching. This cross section could be traced to form channels along the long (x) axis of the fiber with arbitrary bends and turns. This was extended to include a helical channel, presented in Figure 5.12, based on a similar helical microfluidic channel demonstrated by Dr. Moez Haque in stripped fiber [101]. Figure 5.12 shows an acrylate-coated fiber in which a helical microfluidic port wraps around the fiber over an axial length of 6 mm in the

x-axis. Nanograting tracks were written using vscan = 0.1 mm/s, EP = 60 nJ, and

νRR = 500 Hz based on resulted presented in Figure 5.11(a). The pitch of the helix

(Λhelix) was set to 200 µm to align with the xsep determined to enable ﬂuidic etching

without cladding / buﬀer interface damage, helical radius (rhelix) to 20 µm, and channel diameter to 10 µm (hexagonal cross section consisting of 5 adjacent nanograting tracks along longest dimensions), which corresponds to a maximum radial oﬀset distance of Chapter 5. Lab in Fiber Devices in Buffered Fiber 107

25 µm, within damage-free focusing limitations discussed in Section 4.2. In the center of the helical channel along the x-axis, the channel vertically crossed the fiber core to enable interaction between light guided by the fiber core and fluid guided along the fluidic channel. The helical channel was etched in 5% HF over 2 hours via periodic access ports (Λ = xsep = 200µm) leading to trepanned conical ablation ports, fabricated using optimized parameters described in Section 4.4( νRR = 1 kHz, NP = 1, Eabl= 16 nJ, ◦ xsep = 200 µm, θt = 30 ). This fiber demonstrated no damage to the cladding / buffer interface, further demonstrating the versatility of laser focusing and exposure presented in Chapter4.

5.3.3 Particle Flow in Acrylate-Coated Fiber

As a cursory demonstration of promise and utility for laser-writing of LIF devices in acrylate-coated fiber, the methods described heretofore were further extended to demonstrate a preliminary demonstration of particle flow through helical microchannels fabricated in buffered fiber similar to those shown in Figure 5.12. This is demonstrated in Figure 5.13, where 5 µm polystyrene microbeads (Thermo Scientific FM5CR06B) are shown passing through a buffered helical microchannel. A polystyrene bead is seen over a time-lapse of images passing from the right (Figure 5.13(a)) to the left (Figure 5.13(d)) of the axial device section shown. This device is not yet at a stage where meaningful flow rate and particle counting can be quantified, but it is an initial step toward in-line flow cytometry entirely in buffered fiber. In principle, flow rate and turbulence could be modulated by helical channel properties (e.g. through variations in bend curvature and diameter), and structures could be laser-structured in the buffer itself using buffer ablation as described in Section 4.3. Particles could then be probed by light passing through the pre-existing fiber core waveguide at specific points in the fiber. Chapter 5. Lab in Fiber Devices in Buffered Fiber 108

Figure 5.13: Time-lapsed microscope images of a segment of a helical microﬂuidic channel fabricated in acrylated-coated ﬁber similar to that shown in Figure 5.12 wherein polystyrene microbeads suspended in water were passed (a-d denotes

chronological order in which pictures were taken) (λ = 522 nm, νRR,helix = 500

kHz, vscan,helix = 0.1 mm/s, EP,helix = 60 nJ, rhelix = 20 µm,Λhelix = 200 µm, ◦ νRR,ablation = 1 kHz, NP,ablation = 1, Eabl= 16 nJ, xsep = 200 µm, θt = 30 ).

5.4 Chapter 5 Summary

In this chapter, devices were fabricated in polymer-buffered fiber, using methods and processes developed in Chapters3 and4. Functional grating structures in the fiber core, cladding photonics, and HF-etched microfluidics have been developed in polymer- coated fiber without incurring undue buffer damage. Together, the work presented here demonstrate for the first that the principles of LIF can all be developed in buffered fiber through simultaneous laser processing the silica fiber cladding and polymer buffer overlayer.

In Section 5.1, Bragg grating structures were demonstrated in the core waveguide of single-mode fibers buffered by acrylate (Section 5.1.1) and by polyimide (Section 5.1.2) coatings. FBGs fabricated in acrylate-coated fibers with exposure parameters λ = 522 nm, νRR = 500 kHz, vscan = 0.1 mm/s, duty = 0.6, and EP = 20 nJ yielded no buffer damage during fabrication (Figure 5.1(a)), were well aligned to the fiber core (Figure 5.1(b) and (c)), and exhibited performance virtually identical to counterparts fabricated Chapter 5. Lab in Fiber Devices in Buffered Fiber 109 in buffer-stripped fibers (Figure 5.2). Beam aberrations induced by optical inhomogeneities in the acrylate buffer necessitated minimal changes in optimized fabrication parameters as compared to working in stripped fiber (summarized in Table 5.1). FBGs were also demonstrates in polyimide-coated fibers, with additional challenges. Focusing through polyimide required 1045 nm exposure (i.e. without frequency doubling) instead of visible 522 nm exposure, making alignment and focusing more challenging. Nonetheless processes were developed to achieve strong alignment to the core of polyimide-coated fiber and structure FBGs without polymer damage (Figure 5.3b). Relatively low pulse energies (35 nJ) and long grating lengths (15 mm) were required to yield strong Bragg resonances while avoiding damage to the polyimide buffer (optimized parameters summarized in Table 5.2). FBGs in polyimide-coated fibers comparable device performance to structures produced using 1045 nm exposure in stripped fiber, as shown in Figure 5.4. Despite the high-temperature application benefits of polyimide-coated fiber devices, and appeal of developing through-buffer fabrication methods for polyimide owing to the challenges of stripping polyimide coatings (discussed in Section 3.2), the challenges of working with polyimide-coated fiber lead to the decision to focus further efforts in this thesis on acrylate-coated fibers, leaving cladding photonics and microfluidics through polyimide-coated fibers a subject of future work. Although not discussed in this thesis, other grating structures, including chirped, apodized, and π-phase shifted gratings are all compatible with direct-write procedures and are interesting areas of further study for through-buffer processing.

In Section 5.2, laser processing of acrylate-coated fibers was extended beyond the core and into the cladding. Although cladding photonics necessitate higher laser pulse energy and focusing closer to the cladding / buffer interface which make damage-free processing more cladding, processes developed in Section 4.2 were invoked to mitigate heat accumulation effects. In Section 5.2.1, cross-coupler taps, cladding waveguides, and Bragg grating waveguides were developed and optimized in acrylate-coated fiber, within the bounds established in Figure 4.2. Crossing angle (Figures 5.5 and 5.5) and pulse energy (Figure 5.7) were optimized for cladding waveguides coupling light out of the core and into the xy and xz planes. Asymmetries between cladding photonics in these orthogonal planes were observed and discussed with regards to the asymmetrical beam focal volume and index modification profile. The performance of BGWs fabricated in acrylate-coated and buffer-stripped fibers were compared in Figure 5.8, with comparable Chapter 5. Lab in Fiber Devices in Buffered Fiber 110 reflection strength and compromised linewidth observed. Cross couplers were further developed to demonstrate a 3D shape sensor based on the work of Lee et al. [91] in Section 5.2.2. By designing couplers in the xy and xz planes that yielded nearly equal net reflection, a device was developed that coupled light into BGWs fabricated in orthogonal planes, and the differential fractional Bragg shifts upon applied bend were measured to determine bend radius and azimuthal bend angle (Figure 5.9). Although not explored in this thesis, as mentioned with regards to core FBGs, BGWs could be fabricated through acrylate coatings with less uniform grating structure. In particular, -phase shifted gratings would yield sharper linewidths to mitigate the broadening effects observed in Figure 5.8 to yield higher-resolution devices, including shape sensors (see Equation 5.4).

Finally, microfluidics were demonstrated for the first time in acrylate-coated fiber in Section 5.3, enabled by selective buffer machining conducted in tandem with cladding-processing using processes developed in Section 4.4. Although buffered fibers required higher pulse energies to produce nanograting tracks which etch from 5% HF exposure as compared to nanograting tracks written in stripped fiber (Figure 5.10), nanogratings written through acrylate-buffer with pulse energies of 60 nJ consistently opened upon HF etching. This discrepancy was discussed with regards to the sensitivity of nanograting formation to beam aberration effects in the polymer buffer. In Section 5.3.2, processes developed to open blind holes in acrylate-coated fibers (Section 4.4.3) were further developed to open axial microfluidic channels. Laser exposure conditions ◦ were narrowed to λ = 522 nm, νRR = 1 kHz, NP = 1, θt = 30 , rinterface = 5 µm, xsep =

200 µm, and Eabl= 16 nJ for selective buffer ablation, and λ = 522 nm, νRR = 500 kHz, vscan = 0.1 mm/s, EBH = 50 µm, and Echannel = 60 nJ for nanograting structures in the silica cladding to be opened up by 2 hours of 5% HF etching (Figure 5.11). Helical channels developed by scanning multiple adjacent nanograting tracks through the fiber cladding were demonstrated in acrylate-coated fiber capable of guiding not only fluid (Figure 5.12), but also particles (Figure 5.13), demonstrating preliminary steps toward in-line flow cytometry entirely in acrylate-coated fiber developed by femtosecond-laser processing of the fiber core, cladding, and buffer in tandem.

The research presented in this chapter represent an adaptation of LIF principles into acrylate-coated fiber. This is accomplished by structuring the fiber core, cladding, and Chapter 5. Lab in Fiber Devices in Buffered Fiber 111 opening microfluidic channels through HF-etched nanograting tracks without incurring damage to the polymer buffer. Not only can these devices be fabricated faster and more robustly than buffer-stripped counterparts, but tandem processing of the fiber cladding and buffer opens new opportunities for bilayer architectures and devices which take advantage of the unique properties of each material. Chapter 6

Discussion and Future Work

The work presented in this thesis cumulatively represent a translation of photonic circuits and optofluidic lab in fiber to polymer-coated optical fibers. In this chapter, the significance of this work within the broader scope of LIF device fabrication is discussed. Further, in the process of developing this body of work, new technological and scientific opportunities have been revealed, which are also discussed here.

The selective buffer machining processes described in Sections 3.3.4 and 4.3 expand beyond work done by Snelling et al. [60], whose demonstration of femtosecond laser stripping of acrylate polymer buffer in part motivated this thesiss research into selective buffer machining. Snelling et al. were able to strip large portions of the fiber buffer using femtosecond laser ablation while our work demonstrated an advancement toward highly selective structuring of more functional geometries, with buffer structures precisely aligned to interface with structures developed in the fiber cladding.

Prior works discussed in Section 2.3, by Starodubov et al., Mihailov et al., and Martinez et al. demonstrated through-buffer processing of grating structures by UV exposure of hydrogen-loaded fibers [32], femtosecond laser exposure through a phase mask [33,34,36], and femtosecond laser direct write processes [31,36], respectively. The goal of this thesis was to demonstrate a comprehensive expansion of forming LIF devices through buffer-coated fibers, structuring and extending further beyond the fiber core. This thesis demonstrated burst-writing of grating structures having high-index voxels of externally modulated pitch with little distortion or disadvantage by focusing by through-buffer focusing for the first time. The high repetition rate process could

112 Chapter 6. Discussion and Future Work 113 seamlessly enable integration of cladding photonics and nanograting-enabled microfluidics as scan speed and repetition rate were not locked to control grating pitch. Acousto-optic modulation and flexible power switching on the fly (see Section 3.1) enabled rapid switching between formation of grating, waveguide, and nanograting structures, which enabled the integrated multifunctional element of LIF, a key feature of this technology over other fiber sensor fabrication processes.

In relatively recent work, Waltermann et al. demonstrated a cladding photonic distributed Bragg grating shape sensor similar to the device presented in Section 5.2.2[38] . Their cladding waveguide structures were fabricated through acrylate-coated fiber for the first time. Further, they reported to not generate significant material damage. However, the methodology for assessing damage against exposure levels were not disclosed. Their paper indicated the first demonstration of cladding photonic shape sensing in single-mode fiber in contrast with earlier work by Lee et al. [16] which presented a similar shape sensor based in coreless fiber and spliced to single mode fiber. In the present thesis, fiber cladding photonics was introduced for the first time within the context of damage-free processing through the polymer buffer. Moreover, limitations to the laser-processing window caused by heat accumulation effects were elaborated upon both experimentally and theoretically. The cladding photonic devices discussed in Section 5.2 were presented as establishing a practical foundation upon which to design more advanced LIF devices in buffered fiber, recognizing wide latitude for 3D-structuring with only modest limits imposed by the buffer layer and heat accumulation effects in the silica cladding.

Section 5.3 rounds out the suite of LIF capabilities by demonstrating, for the first time, FLICE-enabled microfluidics in acrylate-coated fiber by introducing periodically structured ablation ports in the fiber buffer. The result underpins the primary motivation behind this thesis building on the first combination of core waveguide structures, cladding photonics, and microfluidics in silica optical fiber by Haque et al. to demonstrate integrated multifunctional LIF [15]. Integrated multifunctional sensing is the foundation of the claim that LIF represents a true translation of LOC principles into optical fiber. This thesis presents a significant further advancement over Haque et al.s work by further enabling LIF principles in the a more robust and practical polymer-buffered fiber platform. Taking a technology from an exciting and novel Chapter 6. Discussion and Future Work 114 demonstration and making it into a practical, cost-effective, and deployable reality involves a number of translational innovations. Structural fortification of the buffer around the 125 µm diameter glass fiber is a valuable and important step towards viability of the weakened fiber when it is shaped into a complex optofluidic network.

Advancing LIF to through-buffer fabrication was in part motivated by the promise of developing more mechanically robust devices as compared to devices fabricated in stripped fiber. Though further characterization is warranted to quantify this improved mechanical strength of polymer coated FBGs and LIF devices. Further, mechanical characterization of acrylate-coated fiber devices, including cladding photonics and fibers with microfluidics opened in them, would offer valuable validation and quantify one of the key merits of this work.

Optical inhomogeneity and astigmatism introduced by the cylindrically-shaped polymer buffer were largely mitigated by employing index-matching oil between the objective lens and fiber substrate. Currently, our group is introducing a spatial light modulator (SLM) into the laser processing system as discussed in Section 3.1. SLM could be used to actively compensate for buffer inhomogeneity using an adaptive optic process, by assessing Fresnel reflection off of the fiber core to guide the laser focus position. Conkey et al. have demonstrated how feedback from photoacoustic waves generated from thermal expansion can be used for active wavefront correction using SLM. Since the resultant pressure wave is proportional to fluence, it can be used as a measure of local intensity. The speckle field resulting from the scattering medium (i.e. the polymer buffer) can be normalized over the photoacoustic transducer to provide feedback for wavefront shaping [102]. Adopting such a system would have the added benefit of real-time monitoring of heat effects in the fiber cladding. Improved focusing with SLM correction may close the gap in quality of fabricating in buffer-coated and buffer-stripped fibers, including reducing the discrepancy in minimum pulse energy required for forming nanograting tracks that etch open with hydrofluoric acid etching, as discussed in Section 5.3. Additionally, SLM beam-shaping could be used to control refractive index shape, and hence the optical mode profile, thereby providing improved control of loss and coupling between central core and laser-written cladding waveguides. Chapter 6. Discussion and Future Work 115

In Section 4.2, it was determined that heat accumulation was the limiting factor in damage-free laser processing of the fiber cladding. Exposure conditions evaluated in this thesis were based on prior work in buffer-stripped fiber [72, 89, 91] wherein heat accumulation effects were not a limiting factor for structuring the fused silica fiber core and cladding. Damage to the buffer was mitigated by setting maximum pulse energy exposure for radial focusing distance and azimuthal position, which was sufficient to permit the fabrication of a host of LIF devices in buffered fiber, as demonstrated in Chapter5. Heat accumulation may also be restrained by laser scan speed and repetition rate. Scan speeds were practically limited by motion stages with limits imposed by preventing vibrations in the fiber. These limits could be avoided by adopting a scanning galvanometer to move the laser focus within the fiber. Although high repetition rates are desirable for burst train processing, external down counting (as discussed in Section 3.1) could also be employed to reduce repetition rates when processing nearer the cladding / buffer interface. As discussed in Section 4.4.3, selective buffer machining was shown in cursory study to mitigate heat accumulation effects and enable the laser-writing of nanograting tracks beyond the damage-free process zones observed in Section 4.1 in fibers coated with an unstructured buffer. Similar ablated structures may be formed prior to cladding exposure to further develop cladding photonics with nanograting tracks forming without damage beyond the limits observed in Section 4.1. It may be possible to seal shut the damage-preventing buffer microstructres by using high repetition rate laser scans to melt and reseal the buffer as observed in Section 4.3. These strategies to further extend the boundaries of damage-free processing in polymer-buffered fibers are guided by known physics and observed phenomena and warrant further study.

In Section 4.4, aqueous potassium hydroxide was eliminated as an appropriate fluidic etchant through laser-formed nanograting tracks for the case of acrylate buffer coated fibers. KOH was found to severely damage the buffer layer. Although the discussion of KOH was terminated at this point in the thesis, there are known benefits (and disadvantages) of using KOH over aqueous HF as a FLICE etchant. For example, KOH etching can etch high aspect ratio channels reaching over centimeters without the need for periodic access ports [28] on 200 µm spacing as used in the present work. However, KOH etching requires approximately 10× longer etching times than HF. Fabrication of longer LIF structures in buffered fibers, including structures for which periodic access ports and buffer ablation are incompatible, would benefit from KOH Chapter 6. Discussion and Future Work 116 etching. This would require fibers coated in a different buffer material which is inert to KOH.

Polyimide-coated fibers are a promising direction for further development beyond the core-only FBGs as discussed in Section 5.1.2. Through-buffer processing is attractive for polyimide-coated fibers because of the difficulty involved in stripping away the buffer (involving open flame or hot sulphuric acid [90]), as well as because of the suitability of polyimide-coated fibers in high-temperature applications. Formation of cladding photonics and microfluidics in polyimide coated fiber would require carrying out similar damage and ablation studies as conducted in Chapter4 for acrylate-coated fibers. The methods and insights gained here would be transferable to this future study. The high thermal stability of polyimide suggests that it would not be as sensitive to heat accumulation effects as in the case of acrylate buffers, expanding the exposure window for damage-free processing.

Section 5.1 presented the development of fiber Bragg gratings in the core of buffered fiber as a demonstration of through-buffer core modifications. As discussed in Chapter 2, there are many varieties of grating structures which are, in principle, compatible with through-buffer fabrication methods, including phase-shifted, chirped, and apodized FBGs, as well as structures which couple light into the fiber cladding including titled FBGs and LPGs. Further, core-adjacent non-grating structures described in Section 2.2.1 can, in principle, be fabricated through buffer coatings to expand the catalogue of damage-free through-buffer fabrication processes.

In Section 5.2, it was established that cladding photonic waveguides could be fabricated in acrylate-coated optical fiber, as validated by the fabrication of cross couplers and Bragg grating waveguides. Bragg grating waveguides exposed through acrylate buffer were demonstrated to have a similar Bragg response to devices exposed in buffer-stripped fiber. More thorough characterization and comparison of cladding waveguides fabricated in buffer-coated and buffer-stripped fiber (e.g. waveguide loss, index profile, mode profile) would provide valuable insight into the nature of degradation brought about by buffer inhomogeneity and other distortion effects in the laser propagation and alignment. Chapter 6. Discussion and Future Work 117

The present thesis extends beyond the translation of LIF laser fabrication from fibers stripped of their polymer buffer to buffer-coated fiber. In-tandem processing of the glass fiber core / cladding and polymer fiber buffer leads to significant opportunities for entirely new technologies. In Section 4.3, laser ablation processes were developed in the acrylate buffer in order to open buffer ports and thus enable processing in the silica glass core and cladding without buffer damage. In future work, ablated structures may be used for more than just microfluidic ports and heat accumulation solutions. Opened structures may transport and guide fluids along the glass fiber in parallel with the silica fiber cladding optofluidics. Delamination effects were observed in the cladding / buffer interface throughout the thesis results sections. These may serve as a new means of introducing buried channels and openings between the polymer buffer and silica cladding. Some of the benefits of simultaneously structuring the polymer buffer and silica core / cladding include creating multi-tiered microfluidic systems extending between the fiber and the buffer, creating unique packaging and alignment capabilities in the buffer to better connect devices made in fibers (i.e. lock and key structures), and creating modifications in the encasement which manipulate light or fluids entering the fiber (e.g. using buffer-structures as refractive elements for light coupling into the fiber). Different buffer materials may be explored which have inherent material benefits including thermal stability, adhesive properties, chemical inertness, chemical sensitivity, mechanical strength, rigidity, flexibility, biocompatibility, analyte binding, polarity, and / or absorption characteristics. Heterogeneous bilayer device architectures could take full advantage of the progress made in this thesis, by recognizing that the buffer is not merely a hindrance to laser-structuring of LIF devices, but a potential opportunity for fabricating new devices by laser processing of both the acrylate buffer and glass cladding in tandem. Chapter 7

Conclusion

In summary, this thesis demonstrated the viability of damage-free femtosecond laser LIF processing through buffer-coated single mode optical fiber. Previous generations of devices have demonstrated the utility and capacity of novel devices formed through laser-structuring of the fiber core, through the development of cladding photonics, and through the opening up of microfluidics in fibers, which were most frequently stripped of their protective polymer buffer. Stripping increased process time, mechanically weakened the fiber, and forbade microfluidic devices in fiber from being coated with a protective polymer buffer, seriously hindering the practical viability of these prior technologies. Previously, fiber cores were processed through polymer buffers [31–34, 36], and cladding photonics had been formed in buffer-coated fiber reporting minimal damage [38]. Prior to this thesis, a comprehensive analysis of processing buffer-coated LIF had been absent. In this work, the causes of, limitations from, and processes around buffer damage induced while applying high repetition rate ultrafast laser pulses to the core / cladding of buffer coated fiber were studied. As a result of this analysis, damage-free processing of core waveguides, of cladding photonics, and of microfluidics in fiber were all demonstrated in buffer-coated optical fiber.

In Chapter4, laser damage arising in the buffer of acrylate coated fiber was empirically studied, and the limitations of damage-free processing mapped out throughout the fiber cladding as a function of pulse energy, of radial distance of the focal volume from the fiber center, and of azimuthal angle relative to the axis of laser focusing. It was determined that for 500 kHz repetition rate, 0.1 mm/s scan speed, and 522 nm exposure wavelength, pulse energies typically used to induce modifications within germanium doped single

118 Chapter 7. Conclusion 119 mode fiber core, 20 nJ to 40 nJ, could be applied to all evaluated cladding positions without incurring observable buffer damage. Pulse energies typically used to fabricate cladding waveguides, 50 nJ to 80 nJ, could be applied to radial distances from 35 µm to 55 µm without any observable buffer damage. Finally, pulse energies typically used to fabricate nanograting tracks which can be opened by chemical etchants, 20 nJ to 60 nJ could be focused to radial distances between 35 µm and at least 60 µm without buffer damage. Although pulse energies required to fabricate nanograting tracks could not be focused all the way to the outer cladding interface without generating buffer damage (as is necessary to interface with etchant), selective buffer removal processes by laser ablation were found capable of mitigating heat accumulation zones and subsequent buffer damage, as discussed in Section 4.4. The physical phenomena behind the outlined damage-free processing limits were studied, comparing a direct-damage intensity-driven damage model with an accumulated-damage thermally-driven heat accumulation model. The heat accumulation model was shown to better reflect the empirical data, and predict the observed trends in damage thresholds which the intensity model could not do.

In order to develop microfluidics in acrylate coated fiber, it was necessary to establish methods to selectively machine the polymer buffer to enable chemical etchant to interface with nanograting tracks fabricated in the fiber cladding. To achieve this with the same ultrafast laser processing system as used to fabricate structures in the fiber core / cladding, study was conducted into polymer ablation varying the number of applied pulses, pulse energy, and repetition rate. An external down-counting of the high repetition rate was necessary here using an acousto-optic modulator. From this analysis, laser processing zones were identified which did not over machine the polymer or lead to polymer melt that would reseal holes, while maintaining time efficiency.

The effects of hydrofluoric acid and of aqueous potassium hydroxide, etchants commonly used to open up nanograting tracks in fiber silica cladding, were tested on acrylate fiber buffer. Although the buffer dissolved in potassium hydroxide, it was found to be satisfactorily chemically inert in hydrofluoric acid. However, buffer subjected to heat affected zones when laser processing in the nearby silica cladding was found to delaminate after HF exposure. Percussion ablation scans were found to not remove enough buffer in such zones to effectively mitigate heat accumulation damage in the polymer buffer arising from writing nanograting tracks to the outer cladding Chapter 7. Conclusion 120 interface to interact with etchant. Rectangular raster-scanned structures were found to trap air bubbles because of their 90◦ corners, impeding wetting and the flow of etchant. Conically trepanned buffer ablation ports were found to reliably guide etchant to open up nanograting tracks in the fiber cladding. The best trepanning exposure parameters that both enabled consistent cladding etching and did not result in interfacial damage after etching were with conical ablation ports with a taper angle of 30◦, and exposed by 1 kHz repetition rate, 1 pulse per focal volume (zero pulse overlap), 16 nJ ablation pulse energy, and 200 µm center-to-center spacing between ablation zones.

Chapter5 drew upon the results of Chapter4 to demonstrate, for the first time, the full suite of LIF capabilities in buffer-coated optical fiber: structuring the fiber core (Section 5.1), fabricating cladding photonics (Section 5.2), and opening nanograting tracks to produce microfluidic structures (Section 5.3). Efficient FBGs were direct written using burst-write processes in the waveguide core of acrylate-coated (Section 5.1.2) and polyimide-coated single mode optical fiber. Through device optimization, the Bragg response of FBGs fabricated in acrylate-buffered fiber was virtually identical to a similar device fabricated in buffer-stripped fiber. FBGs were also demonstrated in polyimide-coated fiber, using 1045 nm writing wavelength, to avoid strong absorption in the polyimide buffer. Despite the alignment and focusing challenges associated with working with infrared versus visible light, high-strength, damage-free first-order FBGs were demonstrated in polyimide-coated optical fiber, though long device lengths ( 3 times longer than buffer-stripped counterparts) were necessary at relatively low pulse energies to prevent buffer damage. Further development of damage-free through-buffer LIF in polyimide-coated fibers was left as a subject of future work due to the practical challenges of working with polyimide-coated fibers, discussed in Section 3.2.

Damage-free cladding photonics were further developed in acrylate-coated optical fiber, drawing upon information developed earlier in4 (see Figure 4.2) about damage- free process limits imposed by heat accumulation within the silica cladding. Cross-coupler taps, cladding waveguides, and Bragg grating waveguides were demonstrated in acrylate coated fiber, with crossing angle and pulse energy optimized in azimuthally perpendicular fiber planes using Bragg grating waveguide response. Asymmetries between cladding photonic couplers written parallel and perpendicular to the plane of the writing laser were discussed, with discrepancies understood as a result of geometric asymmetries within the Chapter 7. Conclusion 121 writing beams focal volume and in the induced refractive index profile. Though very similar Bragg reflection responses were recorded in BGWs fabricated in buffer-coated fiber as compared to BGWs in buffer-stripped fiber, a moderate increase in linewidth was observed. BGW couplers in orthogonal fiber planes were fabricated and 3D bend profile was measured by monitoring differential fractional Bragg resonance shifts (5.2.2).

Finally, for the first time, FLICE-enabled microfluidics were demonstrated in acrylate- coated optical fiber by periodically ablating conical buffer ports which enable etchant to interface with and open up nanograting tracks fabricated in the silica fiber cladding. Although pulse energies required to yield refractive index modifications in the fiber core and cladding in acrylate-coated fiber were found to be similar to pulse energies required in the exposure of buffer-stripped fiber, significant discrepancies were observed for writing nanograting tracks. This discrepancy suggests that inhomogeneities in the fiber buffer were more hindering to nanograting formation. Nonetheless, 60 nJ pulses were capable of producing nanograting tracks which were consistently opened up in acrylate-coated fiber. Helical microfluidic channels wrapping around the fiber core were demonstrated in acrylate-coated fiber, with etchant interfacing with the silica cladding through conically trepanned buffer ports. Both fluid and particle flow were demonstrated through helical microchannels in acrylate-coated fiber, laying the groundwork for in-line flow cytometry processed damage-free in acrylate-coated fiber. That underpins highly promising future direction for this research work.

The work presented in this thesis make the LIF platform significantly more industry viable, with processing time reduced by bypassing the buffer-stripping and recoating steps. Mechanical integrity of the LIF devices has been improved and buffer-coated microfluidics have been demonstrated. Further, in-tandem processing of both the polymer buffer and the silica core / cladding (Section 5.3.1) opens opportunities to further expand LIF capabilities into two-tier heterogeneous optofluidic systems, with functional structures fabricated in both layers. Bibliography

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