APPLIED DIGITAL OPTICS
APPLIED DIGITAL OPTICS FROM MICRO-OPTICS TO NANOPHOTONICS
Bernard C. Kress
Photonics Systems Laboratory, Universite de Strasbourg, France
Patrick Meyrueis
Photonics Systems Laboratory, Universite de Strasbourg, France This edition first published 2009 Ó 2009 John Wiley & Sons, Ltd
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Library of Congress Cataloguing-in-Publication Data Kress, B. Applied digital optics : from micro-optics to nanophotonics / Bernard C. Kress, Patrick Meyrueis. p. cm. Includes bibliographical references and index. ISBN 978-0-470-02263-4 (cloth) 1. Optical MEMS. 2. Nanophotonics. 3. Integrated optics. 4. Signal processing–Digital techniques. 5. Diffraction gratings. I. Meyrueis, Patrick. II. Title. TK8360.O68.K74 2009 621.36–dc22 2009004108 A catalogue record for this book is available from the British Library.
ISBN: 978-0-470-02263-4
Set in 9/11pt, Times by Thomson Digital, Noida, India. Printed in Great Britain by CPI Antony Rowe, Chippenham, Wiltshire. To my lovely wife Mei-Mei, whose unconditional love and support made this book possible. I even learned to appreciate her constant nagging, which drove me up the wall but helped me finish this project. Bernard
I would like to dedicate this book to all my university colleagues, students, Photonics Systems Laboratory staff, my assistant Anne and members of institutions and companies all over the world that allowed us, by contributing to or supporting our microphotonics and nanophotonics activities in research and education, to gather the information that made this book possible. Patrick
Contents
About the Authors xi Foreword by Professor Joseph Goodman xiii Foreword by Professor Trevor Hall xv Acknowledgments xvii Acronyms xix
Introduction 1 Why a Book on Digital Optics? 1 Digital versus Analog 2 What are Digital Optics? 2 The Realm of Digital Optics 3 Supplementary Material 4
1 From Refraction to Diffraction 5 1.1 Refraction and Diffraction Phenomena 5 1.2 Understanding the Diffraction Phenomenon 5 1.3 No More Parasitic Effects 8 1.4 From Refractive Optics to Diffractive Optics 9 1.5 From Diffractive Optics to Digital Optics 11 1.6 Are Diffractives and Refractives Interchangeable Elements? 13
2 Classification of Digital Optics 15 2.1 Early Digital Optics 15 2.2 Guided-wave Digital Optics 16 2.3 Free-space Digital Optics 17 2.4 Hybrid Digital Optics 19
3 Guided-wave Digital Optics 21 3.1 From Optical Fibers to Planar Lightwave Circuits (PLCs) 21 3.2 Light Propagation in Waveguides 22 3.3 The Optical Fiber 25 3.4 The Dielectric Slab Waveguide 27 3.5 Channel Waveguides 28 3.6 PLC In- and Out-coupling 30 3.7 Functionality Integration 36 viii Contents
4 Refractive Micro-optics 47 4.1 Micro-optics in Nature 47 4.2 GRIN Lenses 49 4.3 Surface-relief Micro-optics 55 4.4 Micro-optics Arrays 58
5 Digital Diffractive Optics: Analytic Type 71 5.1 Analytic and Numeric Digital Diffractives 73 5.2 The Notion of Diffraction Orders 73 5.3 Diffraction Gratings 76 5.4 Diffractive Optical Elements 90 5.5 Diffractive Interferogram Lenses 106
6 Digital Diffractive Optics: Numeric Type 111 6.1 Computer-generated Holograms 111 6.2 Designing CGHs 115 6.3 Multiplexing CGHs 149 6.4 Various CGH Functionality Implementations 151
7 Hybrid Digital Optics 157 7.1 Why Combine Different Optical Elements? 157 7.2 Analysis of Lens Aberrations 157 7.3 Improvement of Optical Functionality 163 7.4 The Generation of Novel Optical Functionality 166 7.5 Waveguide-based Hybrid Optics 169 7.6 Reducing Weight, Size and Cost 171 7.7 Specifying Hybrid Optics in Optical CAD/CAM 173 7.8 A Parametric Design Example of Hybrid Optics via Ray-tracing Techniques 175
8 Digital Holographic Optics 181 8.1 Conventional Holography 181 8.2 Different Types of Holograms 185 8.3 Unique Features of Holograms 188 8.4 Modeling the Behavior of Volume Holograms 192 8.5 HOE Lenses 199 8.6 HOE Design Tools 203 8.7 Holographic Origination Techniques 203 8.8 Holographic Materials for HOEs 207 8.9 Other Holographic Techniques 212
9 Dynamic Digital Optics 217 9.1 An Introduction to Dynamic Digital Optics 217 9.2 Switchable Digital Optics 223 9.3 Tunable Digital Optics 235 9.4 Reconfigurable Digital Optics 244 9.5 Digital Software Lenses: Wavefront Coding 250
10 Digital Nano-optics 253 10.1 The Concept of ‘Nano’ in Optics 253 10.2 Sub-wavelength Gratings 253 Contents ix
10.3 Modeling Sub-wavelength Gratings 255 10.4 Engineering Effective Medium Optical Elements 267 10.5 Form Birefringence Materials 272 10.6 Guided Mode Resonance Gratings 275 10.7 Surface Plasmonics 277 10.8 Photonic Crystals 279 10.9 Optical Metamaterials 288
11 Digital Optics Modeling Techniques 295 11.1 Tools Based on Ray Tracing 295 11.2 Scalar Diffraction Based Propagators 298 11.3 Beam Propagation Modeling (BPM) Methods 321 11.4 Nonparaxial Diffraction Regime Issues 323 11.5 Rigorous Electromagnetic Modeling Techniques 326 11.6 Digital Optics Design and Modeling Tools Available Today 327 11.7 Practical Paraxial Numeric Modeling Examples 330
12 Digital Optics Fabrication Techniques 339 12.1 Holographic Origination 340 12.2 Diamond Tool Machining 342 12.3 Photo-reduction 346 12.4 Microlithographic Fabrication of Digital Optics 347 12.5 Micro-refractive Element Fabrication Techniques 385 12.6 Direct Writing Techniques 388 12.7 Gray-scale Optical Lithography 394 12.8 Front/Back Side Wafer Alignments and Wafer Stacks 406 12.9 A Summary of Fabrication Techniques 408
13 Design for Manufacturing 413 13.1 The Lithographic Challenge 413 13.2 Software Solutions: Reticle Enhancement Techniques 418 13.3 Hardware Solutions 445 13.4 Process Solutions 449
14 Replication Techniques for Digital Optics 453 14.1 The LIGA Process 453 14.2 Mold Generation Techniques 455 14.3 Embossing Techniques 459 14.4 The UV Casting Process 464 14.5 Injection Molding Techniques 464 14.6 The Sol-Gel Process 471 14.7 The Nano-replication Process 472 14.8 A Summary of Replication Technologies 475
15 Specifying and Testing Digital Optics 479 15.1 Fabless Lithographic Fabrication Management 479 15.2 Specifying the Fabrication Process 480 15.3 Fabrication Evaluation 494 15.4 Optical Functionality Evaluation 510 x Contents
16 Digital Optics Application Pools 521 16.1 Heavy Industry 522 16.2 Defense, Security and Space 532 16.3 Clean Energy 539 16.4 Factory Automation 541 16.5 Optical Telecoms 544 16.6 Biomedical Applications 548 16.7 Entertainment and Marketing 553 16.8 Consumer Electronics 554 16.9 Summary 574 16.10 The Future of Digital Optics 574
Conclusion 581
Appendix A: Rigorous Theory of Diffraction 583 A.1 Maxwell’s Equations 583 A.2 Wave Propagation and the Wave Equation 583 A.3 Towards a Scalar Field Representation 584
Appendix B: The Scalar Theory of Diffraction 587 B.1 Full Scalar Theory 587 B.2 Scalar Diffraction Models for Digital Optics 594 B.3 Extended Scalar Models 595
Appendix C: FFTs and DFTs in Optics 597 C.1 The Fourier Transform in Optics Today 597 C.2 Conditions for the Existence of the Fourier Transform 600 C.3 The Complex Fourier Transform 600 C.4 The Discrete Fourier Transform 601 C.5 The Properties of the Fourier Transform and Examples in Optics 604 C.6 Other Transforms 606
Index 611 About the Authors
Bernard Kress has been involved in the field of digital optics since the late 1980s. He is an associate professor at the University of Strasbourg, France, teaching digital optics. For the last 15 years Dr Kress has been developing technologies and products related to digital optics. He has been working with established industries around the world and with start-ups in the Silicon Valley, California, with applications ranging from optical data storage, optical telecom, military and homeland security applications, LED and laser displays, industrial and medical sensors, biotechnology systems, optical security devices, high power laser material processing, to consumer electronics. He is on the advisory boards of various photonics companies in the US and has also been advising venture capital firms in the Silicon Valleyfor due diligence reviews in photonics, especially in micro- and nano-optics. He holds more than 25 patents based on digital optics technology and applications, and is the author of more than 100 papers on this subject. He has taught several short courses given at SPIE conferences. His first book on digital optics, Digital Diffractive Optics (2000), was published by John Wiley & Sons, Ltd and has been translated into Japanese in 2005 (published by Wiley-Maruzen). He is also the author of a chapter in the best seller Optical System Design (2007), edited by R. Fisher and published by McGraw-Hill. Bernard Kress can be contacted at [email protected]. Patrick Meyrueis is full professor at the University of Strasbourg since 1986 (formerly Louis Pasteur University). He is the founder of the Photonics Systems Laboratory which is now one of the most advanced labs in the field of planar digital optics. He is the author of more than 200 publications and was the chairman of more than 20 international conferences in photonics. He was the representative of the Rhenaphotonics cluster and one of the founders of the CNOP in 2001 (national French committee of optics and photonics). He is now acting as the scientific director of the Photonics Systems Lab and the head of the PhD and undergraduate program in the ENSPS National School of Physics in Strasbourg.
Foreword by Professor Joseph Goodman
The field of digital optics is relatively new, especially when compared with the centuries-long life of the more general field of optics. While it would perhaps have been possible to imagine this field a century or more ago, the concept would not have been of great interest, due to the lack of suitable sources, computing power and fabrication tools. But digital optics has now come of age, aided by the extraordinary advances in lasers, processor speed and the remarkable development of tools for fabricating such optics, driven in part by the tools of the semiconductor industry. It was perhaps in the seminal work of Lohmann on computer-generated holograms that interest in the field of digital optics was launched. Lohmann based his experimental work on the use of binary plotters and photo-reduction, but today the plotting tools have reached a level of sophistication not even imagined at the time of Lohmann’s invention, allowing elements with even sub-wavelength structure to be directly fabricated on a broad range of materials. Applied Digital Optics is a remarkable compendium of concepts, techniques and applications of digital optics. The book includes in-depth discussions of guided-wave optics, refractive optics, diffractive optics and hybrid (diffractive/refractive) optics. Also included is the important area of ‘dynamic optics’, which covers devices with diffractive properties that can be changed at will. The optics of sub-wavelength structures is also covered, adding an especially timely subject to the book. Most interesting to me is the extremely detailed discussion of fabrication and replication techniques, which are of great importance in bringing diffractive optics to the commercial marketplace. Finally, the wide-ranging discussion of applications of digital optics is almost breathtaking in its range and coverage. Professors Kress and Meyrueis provide therefore a comprehensive overview of the current state of research in the field of digital optics, as well as an excellent analysis of how this technology is implemented today in industry, and how it might evolve in the next decade, especially in consumer electronics applications. In summary, this book will surely set the standard for a complete treatment of the subject of digital optics, and will hopefully inspire even more innovation and progress in this important field.
Professor Joseph W. Goodman William Ayer Professor, Emeritus Department of Electrical Engineering, Stanford University Stanford, CA, USA
Foreword by Professor Trevor Hall
It was my privilege to host Bernard Kress at an early stage in his career. I was very impressed by his creativity, determination and tireless energy. I knew then that he would become a champion in his field of diffractive optics. Applied Digital Optics is the second book written by Bernard and Professor Patrick Meyrueis from the Photonics Systems Laboratory (LSP) at Universit e de Strasbourg (UdS) in France. While their first book, Digital Diffractive Optics, was solely dedicated to diffractive optics, this one covers a much wider range of fields associated with digital optics, namely: waveguide optics, refractive micro-optics, hybrid optics, optical MEMS and switchable optics, holographic and diffractive optics, photonic crystals, plasmonics and metamaterials. Thus, the book’s subtitle, From Micro-optics to Nanophotonics, is indeed a faithful description of its broad contents. After reviewing these optical elements throughout the first chapters, emphasis is set on the numerical modeling techniques used in industry and research to design and model such elements. The last chapters describe in detail the state of the art in micro-fabrication techniques and technologies, and review an impressive list of applications using such optics in industry today. Professors Kress and Meyrueis have been investigating the field of digital optics at LSP since the late 1980s, when photonics was still struggling to become a fully recognized field, like electronics or mechanics. The LSP has been very active since its creation, not only by promoting education in photonics but also by promoting national and international university/industry relations, which has yielded a number of impressive results: publications, patents, books, industrial applications and products as well as university spin-offs both in Europe and the USA. This experience fueled also several European projects, such as the Eureka FOTA project (Flat Optical Technologies and Applications), which coordinated 27 industrial and academic partners, or more recently the European NEMO network (Network in Excellence in Micro-Optics). The LSP has thus become today one of the premier laboratories in photonics and digital optics, through education, research and product development, and this book serves as a testimonial to this continuous endeavor.
Professor Trevor Hall Director, Centre for Research in Photonics University of Ottawa, School of Information Technology and Engineering Ottawa, Canada
Acknowledgments
We wish to acknowledge and express many thanks to the following individuals who helped directly or indirectly in the production of the material presented within this book:
Prof. Pierre Ambs (ESSAIM, Mulhouse, France) Prof. Stephan Bernet (Innsbruck Medical University, Austria) Mr Ken Caple (HTA Enterprises Inc., San Jose, USA) Dr Chris Chang (Arcus Technology Inc., Livermore, USA) Prof. Pierre Chavel (IOTA, Paris, France) Mrs Rosie (Conners Photronics Corp., Milpitas, USA) Mr Tom Credelle (Holox Inc., Belmont, USA) Dr Walter Daschner (Philips Lumileds, San Jose, USA) Mr Gilbert Dudkiewicz (Telmat Industrie S.A., Soultz, France) Mrs Judy Erkanat (Tessera Corp. San Jose, USA) Dr Robert Fisher (Optics 1 Corp., Los Angeles, USA) Prof. Jo€el Fontaine (INSA, Strasbourg, France) Prof. Joseph Ford (UCSD, San Jose, USA) Dr Keiji Fuse (SEI Ltd, Osaka, Japan) Prof. Joseph Goodman (Stanford University, Stanford, USA) Prof. Michel Grossman (UdS, Strasbourg, France) Prof. Trevor J. Hall (University of Ottawa, Canada) Mrs Kiomi Hamada (Photosciences Inc., Torrance, USA) Dr Phil Harvey (Wavefront Technologies Inc., Long Beach, USA) Mr Vic Hejmadi (USI Inc., San Jose, USA) Dr Martin Hermatschweiler (Nanoscribe GmbH, Germany) Dr Alex Kazemi (Boeing Corp., Pasadena, USA) Prof. Ernst-Bernhart Kley (FSU, Jena, Germany) Prof. Sing H. Lee (UCSD, San Diego, USA) Mr Ken Mahdi (Rokwell Collins Inc., Santa Clara, USA) Prof. Jan Masajada (Wroclaw Institute of Technology, Wroclaw, Poland) Dr Nicolas Mauduit (Vision int egr ee, Paris, France) Prof. Juergen Mohr (Forschungszentrum Karlsruhe, Germany) Mr Paul Moran (American Precision Dicing Inc., San Jose, USA) Prof. Guy Ourisson (ULP, Strasbourg, France) Prof. Olivier Parriaux (Universit e St. Etienne, France) Prof. Pierre Pfeiffer (UdS, Strasbourg, France) Dr Milan Popovitch (SBG Labs Inc., Sunnyvale, USA) Dr Steve Sagan (BAE Corp., Boston, USA) xviii Acknowledgments
Prof. Pierre Saint-Hilaire (Optical Science Center, University of Arizona, USA) Dr Edouard Schmidtlin (JPL/NASA, Pasadena, USA) Mr Michael Sears (Flextronics Inc., San Jose, USA) Prof. Bruno Serio (UdS, Strasbourg, France) Dr Michel Sirieix (Sagem SA, Paris, France) Dr Ron Smith (Digilens Inc., Sunnyvale, USA) Dr Suning Tang (Crystal Research Inc., Fremont, USA) Dr Tony Telesca (New York, USA) Prof. Hugo Thiepont (Vrije Universiteit Brussel, Belgium) Dr Jim Thomas (UCSD, San Diego, USA) Prof. Patrice Twardowsky (UdS, Strasbourg, France) Dr Jonathan Waldern (SBG Labs Inc., Sunnyvale, USA) Dr Paul Wehrenberg (Apple Corp., Cupertino, USA) Prof. Ming Wu (UCLA, Los Angeles, USA) Prof. Frank Wyrowsky (LightTrans GmbH, Jena, Germany) Dr Zhou Zhou (UCSD, San Diego, USA)
We also wish to express our gratitude to all our friends and family, who contributed to the completion of the book (Janelle, Sandy, Erik, Kevin, Dan, H el ene, Sabine, Christine, Claire, etc.), and a special thank you to Geoff Palmer, who did a terrific job in copy editing this book. Acronyms
Optical Design Acronyms
BPM Beam Propagation Method CGH Computer-Generated Hologram DBS Direct Binary Search DFT Discrete Fourier Transform DOE Diffractive Optical Element DOF Depth Of Focus EMT Effective Medium Theory FDTD Finite Difference Time Domain FFT Fast Fourier Transform FZP Fresnel Zone Plate HOE Holographic Optical Element IFTA Iterative Fourier Transform Algorithm M-DOE Moir e DOE MTF Modulation Transfer Function NA Numeric Aperture PSF Point Spread Function RCWA Rigorous Coupled Wave Analysis SBWP Space Bandwidth Product
Computer Design Acronyms
CAD/CAM Computer-Aided Design/Computer-Aided Manufacturing CIF Caltech Intermediate Format DFM Design For Manufacturing DRC Design Rule Check EDA Electronic Design Automation EPE E-beam Proximity Effect GDSII Graphical Data Structure Interface OPC Optical Proximity Correction OPE Optical Proximity Effect RET Reticle Enhancement Techniques xx Acronyms
Fabrication-related Acronyms
AFM Atomic Force Microscope AOM Acousto-Optical Modulator ARS Anti-Reflection Surface CAIBE Chemically Aided Ion-Beam Etching DCG DiChromated Gelatin GRIN GRaded INdex HEBS High-Energy Beam-Sensitive Glass H-PDLC Holographic-Polymer Dispersed Liquid Crystal HTPS High-Temperature PolySilicon IC Integrated Circuit LBW Laser Beam Writer LC Liquid Crystal LCD Liquid Crystal Display LCoS Liquid Crystal on Silicon LIGA LIthography/GAlvanoforming MEMS Micro-Electro-Mechanical System MOEMS Micro-Opto-Electro-Mechanical System OCT Optical Coherence Tomography OE Opto-Electronic PLC Planar Lightwave Circuit PSM Phase Shift Mask RIBE Reactive Ion-Beam Etching SLM Spatial Light Modulator VLSI Very Large Scale Integration
Application-related Acronyms
BD Blu-ray Disk CATV CAble TV CD Compact Disk CWDM Coarse Wavelength Division Multiplexing DVD Digital Versatile Disk DWDM Dense Wavelength Division Multiplexing HMD Helmet-Mounted Display HUD Head-Up Display LED Light-Emitting Diode MCM Multi-Chip Module OPU Optical Pick-up Unit OVID Optically Variable Imaging Device VCSEL Vertical Cavity Surface-Emitting Laser VIPA Virtual Image Plane Array (grating) VOA Variable Optical Attenuator Introduction
Why a Book on Digital Optics?
When a new technology is integrated into consumer electronic devices and sold worldwide in super- markets and consumer electronic stores, it is usually understood that this technology has then entered the realm of mainstream technology. However, such progress does not come cheaply, and has a double-edge sword effect: first, it becomes widely available and thus massively developed in various applications, but then it also becomes a commodity, and thus there is tremendous pressure to minimize the production and integration costs while not sacrificing any aspects of performance. The field of digital optics is about to enter such a stage, which is why this book provides a timely insight into this technology, for the following prospective groups of readers:
. for the research world (academia, government agencies and R&D centers) to have a broad but condensed overview of the state of the art; . for foundries (optical design houses, optical foundries and final product integrators) to have a broad knowledge of the various design and production tools used today; . for prospective industries – ‘How can I use digital optics in my products to make them smaller, better and cheaper?’; and . for the mainstream public – ‘Where are they used, and how do they work?’
This book is articulated around four main topics:
1. The state of the art and a classification of the different physical implementations of digital optics (ranging from waveguide optics to diffractive optics, holographics, switchable optics, photonic crystals and metamaterials). 2. The modeling tools used to design digital optics. 3. The fabrication and replication tools used to produce digital optics. 4. A review of the main applications, including digital optics in industry today.
This introductory chapter will define what the term digital optics means today in industry, before we start to review the various digital optics implementation schemes in the early chapters.
Applied Digital Optics: From Micro-optics to Nanophotonics Bernard C. Kress and Patrick Meyrueis Ó 2009 John Wiley & Sons, Ltd 2 Applied Digital Optics
0000000000000000 0111111100000111 1000100011111000 1011011010001011
(a) Analog form (b) Sampled analog form (c) Digital form
Figure 1 Analog systems versus digital systems
Digital versus Analog
In attempting to define the term ‘digital’ as introduced in the title of this book, one has to consider its counterpart term ‘analog’. The ‘digital’ versus ‘analog’ concept can also be understood when considering the term ‘continuous’ versus ‘discrete’ (see Figure 1). History has proved that the move from analog systems to digital systems in technology (especially in electronics) has brought about a large number of improvements, for example:
. added flexibility (easy to program) and faster, more precise, computers; . new functionalities (built-in error detection and correction algorithms etc.); . ease of miniaturization (very large scale integration, VLSI); and . ease of mass replication (microlithographic fabrication techniques).
What are Digital Optics?
As far as optics are concerned, the move from analog (conventional lenses, mirrors and fiber optics) to digital (planar optical elements composed of microscopic structures) has been mainly focused on the last two points: miniaturization and mass replication. This said, new or improved optical functionalities have also been discovered and investigated, especially through the introduction of digital diffractive optics and digital waveguide optics, and their hybrid combination, as will be discussed in detail in the chapters to come. Miniaturization and mass-production have begun to lead the optical industry toward the same trend as in the micro-electronics industry in the 1970s, namely to the integration of densely packed planar systems in various fields of application (optical telecoms, optical data storage, optical information processing, sensors, biophotonics, displays and consumer electronics). At first sight, the term ‘digital optics’ could lead one to think that such elements might be either digital in their functionality (in much the same way that digital electronics provide digital signal processing) or digital in their form (much like digital – or binary – microscopic shapes rather than smooth shapes). Well, it actually takes none of these forms. The adjective ‘digital’ in ‘digital optics’ refers much more simply to the way they are designed and fabricated (both in a digital – or binary – way). The design tool is usually a digital computer and the fabrication tool is usually a digital (or binary) technology (e.g. by using binary microlithographic fabrication techniques borrowed from the Integrated Circuit, or IC, manufacturing industry). Figure 2 details the similarities between the electronic and optic realms, in both analog and digital versions. In the 1970s, digital fabrication technology (binary microlithography) helped electronics move from single-element fabrication to mass production in a planar way through very large scale integration (VLSI). Similarly, identical microlithographic techniques would prove effective in helping the optics industry to move from single-element fabrication (standard lenses or mirrors) down to planar integration Introduction 3
Electronic realm Optical realm . . Macroscopic . Singular, 3D elements Small-scale integration Analog electronics Analog optics . Analog functionality
. Microscopic . Planar, lithographically printed elements . Large-scale integration Digital electronics Digital optics . Digital/analog functionality
Figure 2 Analogies between the electronics and optics realms
with similar VSLI features. The door to planar optics mass production has thus been opened, exactly as it was for the IC industry 30 years earlier, with the noticeable difference that there was no need to invent a new fabrication technology, since this had already been developed for digital electronics. However, it is important to understand that although the fabrication technique used may be a binary microfabrication process, the resulting elements are not necessarily binary in their shape or nature, but can have quasi-analog surface reliefs, analog index modulations, gray-scale shades or even a combination thereof. Also, their final functionality might not be digital – or binary – as a digital IC chip would be, but could instead have parallel and/or analog processing capabilities (information processing or wavefront processing). This is especially true for free-space digital optics, and not so much for guided-wave digital optics. It is therefore inaccurate to draw a quick comparison between analog electronics versus digital electronics and analog (refractive) optics versus digital (diffractive or integrated) optics, since both optical elements (analog or digital) can yield analog or digital physical shapes and/or processing capabilities.
The Realm of Digital Optics
Now that we have defined the term ‘digital optics’ in the previous section, the various types of digital optical elements will be described. The realm of digital optics (also referred to as ‘micro-optics’ or ‘binary optics’) comprises two main groups, the first relying on free-space wave propagation and the second relying on guided-wave propagation (see Figure 3). The various optical elements defining these two groups (free-space and guided-wave digital optics) are designed by a computer and fabricated by means similar to those found in IC foundries (microlithography). Figure 3 shows, on the free-space optics side, three main subdivisions, which are, in chronological order of appearance, refractive micro-optical elements, diffractive and holographic optical elements, and nano- optics (photonic crystals). On the guided-wave optics side, there are also three main subdivisions, which are, again in chronological order of appearance, fiber optics, integrated waveguide optics and nano-optics. It is worth noting that nano-optics (or photonic crystals) can actually be considered as guided-wave optics or free-space optics, depending on how they are implemented (as 1D, 2D or 3D structures). This book focuses on the analysis of free-space digital optics rather than on guided-wave optics. Guided-wave micro-optics, or integrated optics, are well described in numerous books, published over 4 Applied Digital Optics
Digital optics
Free-space digital optics Guided-wave digital optics
Micro-refractives Fiber optics
Integrated wave optics Diffractive/holographic optics (PLCs)
Nano-optics
Figure 3 The realm of digital optics more than three decades, and dedicated books on ‘guided-wave’ photonic crystals have been available for more than five years now. However, the combination of free-space digital optics and guided-wave digital optics is a very important and growing field, sometimes also referred to as ‘planar optics’, and that is what will be described in this book.
Supplementary Material
Supplementary book material is available at www.applieddigitaloptics.com including information about workshops and short courses provided by the authors. The design and modeling programs used in the book can be downloaded from the website. 1
From Refraction to Diffraction
1.1 Refraction and Diffraction Phenomena
In order to predict the behavior of light as it is affected when it propagates through digital optics, we have to consider the various phenomena that can take place (refraction, reflection, diffraction and diffusion). Thus, we have to introduce the dual nature of light, which can be understood and studied as a corpuscle and/or an electromagnetic wave [1]. The corpuscular nature of light, materialized by the photon, is the basis of ray tracing and the classical optical design of lenses and mirrors. The wave nature of light, considered as an electromagnetic wave, is the basis of physical optics used to model diffractive optics and other micro- or nano-optical elements, such as integrated waveguides, and photonic crystals (see Chapters 3–10). In the simple knife-edge example presented in Figure 1.1, the corpuscular nature of light (through ray tracing) accounts for the geometrical optics, whereas the wave nature of light (physical optics) accounts not only for the light present in the optical path, but also for the light appearing inside the geometrical shadow (the Gibbs phenomenon). According to geometrical optics, no light should appear in the geometrical shadow. However, physical optics can predict accurately where light will appear within the geometrical shadow region, and how much light will fall in particular locations. In this case, the laws of reflection and refraction are inadequate to describe the propagation of light; diffraction theory has to be introduced.
1.2 Understanding the Diffraction Phenomenon
Diffraction comes from the limitation of the lateral extent of a wave. Put in simple terms, diffraction arises when a wave of a certain wavelength collides with obstacles (amplitude or phase obstacles) that are either singular or abrupt (the knife-edge test, Young’s holes experiment) smooth but repetitive (the sinusoidal grating), or even abrupt and repetitive (binary gratings). The smaller the obstacles are, the larger the diffraction effects become (and also the larger the diffraction angles become). Today, when harnessing diffraction to be used in industrial applications, the obstacles are usually designed and fabricated as pure phase obstacles, either in reflection or in transmission [2–4]. Fine-tuning of the obstacle’s parameters through adequate modeling of the diffraction phenomenon can yield very specific diffraction effects with a maximum intensity (or diffraction efficiency).
Applied Digital Optics: From Micro-optics to Nanophotonics Bernard C. Kress and Patrick Meyrueis 2009 John Wiley & Sons, Ltd 6 Applied Digital Optics
Spherical wavefront Plane wavefront
Isophase wavefront lines Rays
Rays
Diffracted field Isophase wavefront lines Isophase wavefront Geometrical shadow
Aperture stop (knife edge)
Figure 1.1 The dual nature of light: geometrical and physical optics
1.2.1 Chronological Stages in Understanding Diffraction Phenomena The diffraction phenomenon was demonstrated for the first time by Leonardo da Vinci (1452–1519) in a very rudimentary way. The first accurate description of diffraction was introduced by Francesco Maria Grimaldi (1618–1663) in his book published in 1665, two years after his death. In those times, corpuscular theory, which was widely believed accurately to describe the propagation of light, had failed to explain the diffraction phenomenon. In 1678, Christian Huygens (1629–1695) proposed a wave theory for the propagation of light that described diffraction as a source of secondary spherical disturbance (see Appendix B). Sir Isaac Newton (1642–1727) had been a strong advocate of the corpuscular theory since 1704. His strong influence over contemporary scientists had halted progress in understanding diffraction during the 18th century. In 1804, Thomas Young (1773–1829) introduced the concept of interference, which directly proceeds from the wave nature of light. Augustin Jean Fresnel (1788–1827) brought together the ideas of Huygens and Young in his famous memoir. In 1860, James Clerk Maxwell (1831–1879) identified light as an electromagnetic wave (see Appendix A). Gustav Kirchhoff (1824–1887) gave a more mathematical form to Fresnel’s expression of diffraction. His work basically relied on two assumptions concerning the field at the diffraction aperture. Although those assumptions were quite empirical, his formulation provided a good approximation of the real diffracted field. In 1884, Arnold J.W. Sommerfeld (1868–1951) refined Kirchhoff’s theory. Thanks to Green’s theorem, he suppressed one of the two assumptions that Kirchhoff had made earlier, to derive the so-called Rayleigh–Sommerfeld diffraction theory. Table 1.1 summarizes, in a chronological way, the understanding of optics as both a corpuscular phenomenon and an electromagnetic field. When studying the propagation of light in a homogeneous or nonhomogeneous medium – such as a lens, a waveguide, a hologram or a diffractive element (through refraction, diffraction or diffusion) – the refractive index is one of the most important parameters. Light travels through a transparent medium (transparent to its specific wavelength) of index n at a speed vn that is lower than its speed c in a vacuum. The index of refraction, n, in a transparent medium is defined as the ratio between the speed of light in a rmRfato oDiffraction to Refraction From Table 1.1 Chronological events in the understanding of optics
… 130 Claudius Ptolemaeus tabulates angles of refraction for several media 1305 Dietrich von Freiberg uses water filled flasks to study the reflection/refraction in raindrops that leads to rainbows 1604 Johannes Kepler describes how the eye focuses light 1611 Marko Dominis discusses the rainbow in De Radiis Visus et Lucis 1611 Johannes Kepler discovers total internal reflection, a small-angle refraction law and thin lens optics 1621 1621 Willebrord Snell states his law of refraction 1637 René Descartes quantitatively derives the angles at which rainbows are seen with respect to the the Sun’s elevation 1678 1678 Christian Huygens states his principle of wavefront sources 1704 Isaac Newton publishes Opticks 1728 James Bradley discovers the aberration of starlight and uses it to determine the speed of light 1752 Benjamin Franklin shows that lightning is electricity 1785 Charles Coulomb introduces the inverse-square law of electrostatics Refraction/reflection 1800 William Herschel discovers infrared radiation from the Sun 1801 Johann Ritter discovers ultraviolet radiation from the Sun 1801 1801 Thomas Young demonstrates the wave nature of light and the principle of interference 1809 Etienne Malus publishes the law of Malus, which predicts the light intensity transmitted by two polarizing sheets 1811 François Arago discovers that some quartz crystals will continuously rotate the electric vector of light 1816 David Brewster discovers stress birefringence 1818 Siméon Poisson predicts the Poisson bright spot at the center of the shadow of a circular opaque obstacle 1818 François Arago verifies the existence of the Poisson bright spot 1825 Augustin Fresnel phenomenologically explains optical activity by introducing circular birefringence 1831 Michael Faraday states his law of induction 1845 Michael Faraday discovers that light propagation in a material can be influenced by external magnetic fields Diffraction 1849 Armand Fizeau and Jean-Bernard Foucault measure the speed of light to be about 298 000 km/s 1852 George Stokes defines the Stokes parameters of polarization 1864 James Clerk Maxwell publishes his papers on a dynamical theory of the electromagnetic field 1871 Lord Rayleigh discusses the blue sky law and sunsets 1873 1873 James Clerk Maxwell states that light is an electromagnetic phenomenon 1875 John Kerr discovers the electrically induced birefringence of some liquids 1895 Wilhelm Röntgen discovers X-rays 1896 Arnold Sommerfeld solves the half-plane diffraction problem … EM wave 7 8 Applied Digital Optics
Table 1.2 Refractive indices for conventional (natural) and nonconventional materials Media Refractive index Type Examples Conventional materials Vacuum 1 exactly Natural — Air (actual) 1.0003 Natural — Air (accepted) 1.00 — — Ice 1.309 Natural — Water 1.33 Natural Liquid lenses Oil 1.46 Natural/Synthetic Immersion lithography Glass (typical) 1.50 Natural BK7 lenses Polystyrene plastic 1.59 Natural/Synthetic Molded lenses Diamond 2.42 Natural TIR in jewelry Silicon 3.50 Natural Photonic crystals Germanium (IR) 4.10 Natural IR lenses
Media Refractive index Type Examples
Nonconventional materials Metamaterials Negative indices Synthetic, active High-resolution lens, materials (plasmon) Harry Potter’s invisibility cloak Bose–Einstein n 1, validated at Synthetic, T ¼ 0 K Low-consumption chips, condensate n > 1 000 000 000! (v < 1 mph) telecom ?0< n < 1.0 Improbable (v > c) Telecom, time machine,... vacuum (c) and the speed of light in the medium. This index can also be defined as the square root of the product of the permittivity and permeability of the material considered for the specific wavelength of interest (for most media, m ¼ 1): 8 c < n ¼ vn ð : Þ : pffiffiffiffiffiffiffi 1 1 n ¼ e:m At this point, one could ask whether there would be a medium with indices that are positive but lower than 1 (which would mean that light would travel faster than the speed of light in a vacuum). This is largely improbable: however, there are media in which the phase velocity of light is greater than c, but cannot be used to send energy or signals at a speed in excess of c. It is worth noting that the range of refractive indices in nature is much higher than one would imagine (from air ¼ 1.0 to glass ¼ 1.5). For example, silicon (Si) has a quite high index of 3.5 for infrared (IR) wavelengths, which enables the fabrication of photonic crystals in which the index change has to be the highest possible in order to achieve full photonic band gaps (see Chapter 10). Table 1.2 lists the refractive indices for some common materials. Interestingly, the range of refractive indices found in nature can be extrapolated by the fabrication of synthetic materials known as metamaterials (see also Chapter 10), and even materials with negative indices can be produced.
1.3 No More Parasitic Effects
History shows us that optical engineering has usually considered diffraction effects to be negative and parasitic. These effects usually manifest when the imaging resolution limit is approached. They are From Refraction to Diffraction 9 considered detrimental to the proper operation of optical instruments. It is only recently that such effects have been considered as being advantageous, and have been included in the optical engineer’s standard design toolbox. The catalyst has been mainly new microfabrication techniques, and their availability to optical engineers as borrowed from the IC industry (see Chapters 12–14). Many diffractive elements have counterparts in the classical realm of optical elements. However, the similarity is only superficial, since their behavior under various operating configurations can be very different. Furthermore, we will see in Chapter 7 that, in many cases, diffractives are actually best used in addition to refractive or reflective optics, in order to provide new and/or extended optical functionality (such as hybrid achromat or athermal singlets). So, a negative effect has been transformed into an advantage thanks to recent developments in both modeling and fabrication techniques and technologies [5].
1.4 From Refractive Optics to Diffractive Optics
Let us consider a simple example. At first sight, a linear grating and a prism may seem to bend an incoming laser beam in the same direction, but the similarity stops there, because different effects appear quickly as soon as one deviates from this particular operating configuration (e.g. by using an incoherent light source, varying the depth of the grooves, launching light at different angles, changing the wavelength or the polarization etc.). The same thing happens when one attempts to compare a refractive lens and its counterpart, the diffractive Fresnel lens, where chromatic dispersion appears in opposite directions, since the signs of the lenses’ respective Abbe V numbers are opposite, even though their focusing power and phase profiles might be exactly the same. Figure 1.2 shows both Snell’s law (from geometrical optics) and the grating equation (from physical optics), which accounts for the amount of light bending, for a small prism and a linear blazed grating. As one gets closer to a blazed grating structure, one can consider the various periods of this grating as many individual refractive micro-prisms, and therefore not only apply the grating equation to the entire blazed grating (array of micro-prisms) [6], but also apply Snell’s law of refraction to each individual micro-prism, as depicted in Figure 1.3. It is interesting to note that the light bending angle a predicted by refraction through the local micro- prism structures and the light bending angle b predicted by diffraction through the blazed grating are not necessarily the same. In effect, they are equal only in one very specific case: when the geometry of the
Figure 1.2 Snell’s law of refraction and the grating equation, both of which rule the amount of bending of light 10 Applied Digital Optics
Figure 1.3 The blazed grating and its micro-prism array structure micro-prism is carefully chosen (its height, length and refractive index carefully optimized), as shown in Figure 1.4. Maximum diffraction efficiency is then reached for the blazed grating (which can theoretically reach 100% efficiency when both previously described effects are yielding the same bending angle). Snell’s law predicts the amount of refraction (bending of light) at a given optical interface between a medium of refractive index n1 and a medium of refractive index n2, and thus also gives the expression for the angle of the refracted beam through each of the micro-prisms [1]:
n1sinða1Þ¼n2sinða2Þ, sinða þ gÞ¼n1sinðaÞð1:2Þ Physical optics, or the grating equation, predicts the diffraction angle of an electromagnetic wave at a similar interface (refractive indices n1 and n2), but this time constituted by a linear array of
Figure 1.4 The local micro-prism effect and the global grating effect From Refraction to Diffraction 11 micro-prisms [1]: l ðbÞ¼ ð : Þ sin m L 1 3 Intuitively, the maximum efficiency will thus occur when a ¼ b: l l a ¼ b Y h ¼ rffiffiffiffiffiffiffiffiffiffiffiffi Y h ¼ ð1:4Þ l n 1 1 n1 1 L Therefore, by using the same concepts to increase the light bending efficiency (a ¼ b), and by carefully shaping the overall grating geometry (the grating period, groove height, groove angles and refractive index), one can design any type of diffractive grating or diffractive lens to yield a specific optical functionality (aspheric lenses, circular gratings, etc. – see Chapters 5 and 6).
1.5 From Diffractive Optics to Digital Optics
Opposite to the previous section, an attempt will be made here to move upwards from the diffractive microstructures to the refractive macroscopic structures (see Figure 1.5). Figure 1.6 shows the similarities between the previous blazed grating and prism, and between the diffractive Fresnel lens and the refractive lens. One could then argue that diffractives are actually arrays of small refractives, in much the same way that the blazed grating is an array of small refractive prisms. In order to prove that this is wrong, Figure 1.7 shows optical similarities between an amplitude diffractive grating and a prism, and between an amplitude Fresnel lens (Fresnel Zone Plate) and a refractive lens. There are no similarities in the shape and form of the two elements, but the optical functionalities are the same (or at least the geometrical considerations – the energetic considerations are vastly different). A grating can diffract light in a given direction, just as a prism would, but it can actually take any physical configuration and form (as long as efficiency is of no concern). Such a grating can be built using any periodical perturbation (phase, amplitude or a combination thereof). The diffraction angle, as the opposite case to the prism, is not a function of the index of the materials, but only a function of the perturbation period. In order to increase the efficiency, and push more light in the desired direction (which
Analog Digital
Macroscopic ?
Microscopic
Figure 1.5 The macroscopic and microscopic realms for analog and digital optics 12 Applied Digital Optics
Glass material
Diffractive grating
Refractive prism
Refractive lens Diffractive lens
For the same optical function, diffractives look a lot like arrays of small refractives
Figure 1.6 The similarities between the blazed grating and prism and between the diffractive Fresnel lens and the refractive lens
Glass material
Amplitude Refractive prism diffractive grating Chrome on glass Refractive lens Amplitude diffractive lens
Here, for the same optical functionality, there are no similarities in shape as for blazed gratings
Figure 1.7 The differences between an amplitude grating and prism, and a Fresnel Zone Plate and a refractive lens From Refraction to Diffraction 13
we will call the ‘diffraction order’ in the chapter dedicated to digital diffractive optics), we will move rather to phase perturbations, and carefully optimize the phase perturbation so as to have the maximum efficiency (e.g. by applying the technique described in Equation (1.3)).
1.6 Are Diffractives and Refractives Interchangeable Elements?
We have seen in the previous section that it is theoretically possible (if a little more challenging in practice) to move smoothly from refraction through refractive optics (the prism) down to diffraction through diffractive optics (the blazed grating as a micro-prism array). This is, however, not the case when we consider structural binary or digital optics, and attempt to move in the opposite direction, towards the equivalent refractive optics. Digital optics have no direct refractive counterparts. The replacement of conventional refractive optics (i.e. a refractive lens) by similar digital diffractive optics (i.e. a planar diffractive Fresnel lens), which provides obvious gains in terms of the footprint, weight and perhaps price, usually results – after some difficulties – in a return to refractives, since the job was tailored for refractives in the first place. In a general way, if an optical system has been designed for refractive optics, it is very difficult simply to replace refractives by diffractive counterparts without dramatically altering the functionality of the system. However, if the optical system has been designed to be used with diffractives, the final system can potentially be smaller, lighter and cheaper, and can integrate more complex functionalities than a system that had been designed with refractive optics constraints in mind. This is a typical error that many optical engineers (and especially savvy marketing managers in high tech optical firms) tend to make. Although diffractives can be a very useful addition to refractive/reflective optics, the simple swap between refractives and their diffractive counterparts does not work in most cases, owing to the very different nature of diffractives, which are mainly as follows:
. many orders of diffraction can be produced; . a zero order may be present; . efficiency may vary strongly with wavelength; . efficiency may vary with the incoming angle; . strong spectral dispersion may appear (and with an opposite sign from refractives); and . strong thermal effects may appear (and with an opposite sign from refractives).
Many of the effects and specifications listed here should not be considered as negative, and can actually be used efficiently to refine the functionality of a train of refractive optics (for a simple example, again see Figure 1.7), or to implement novel optical functionalities that cannot be implemented by refractive/ reflective optics. However, these effects might be considered as negative effects if one attempts to simply swap elements in the hope of rapid gains in price or size/weight.
References
[1] M. Born and E. Wolf, ‘Principles of Optics’, 6th edn, Pergamon Press, London, 1980. [2] K. Miyamoto, ‘The phase Fresnel lens’, Journal of the Optical Society of America, 17, 1961, 17–21. [3] K. Iga, Y. Kokubun and M. Oikawa, ‘Fundamentals of Micro-optics’, Academic Press, Tokyo, 1984. [4] H. Nishihara and T. Suhara, ‘Micro Fresnel lenses’, in ‘Progress in Optics, XXIV’, E. Wolf (ed.), North Holland, Amsterdam, 1987, 3–37. [5] H.-P. Herzig, ‘Micro-optics: Elements, Systems and Application’, Taylor and Francis, London, 1997. [6] S. Sinzinger and M. Testorf, ‘Transition between diffractive and refractive micro-optical components’, Applied Optics, 34(26), 1995, 5970–5976.
2
Classification of Digital Optics
As defined in Chapter 1, the adjective ‘digital’ in digital optics does not refer, as in the case of digital electronics, to the digital functionality of the element (digital signal processing) but, rather, to the digital way in which the optics are designed (by a digital computer) and fabricated (by a digital or binary microlithography technology – i.e. successive binary photomasks or reticles). Chapter 1 has shown that the realm of digital optics can be split into two distinctive groups, free-space digital optics and guided-wave digital optics. The emphasis in this book will be on free-space digital optics, since guided-wave digital optics are a special subdivision of digital optics, and are better described in numerous books dedicated to fiber optics and integrated waveguide optics. However, Chapter 3 will briefly describe guided-wave digital optics and related technologies.
2.1 Early Digital Optics
Mother Nature had developed an infinite variety of high-end refractives, diffractives, waveguides and hybrid optics, and even photonic crystals, long before the first humans were able to carve out the stones with which to smash the skulls of animals that looked reasonably edible. Such examples range from micro-optics to nanophotonics. Figure 2.1 shows a replica of one of the first diffraction gratings (the feather) on the left and one of the first sub-wavelength gratings (or photonic crystals) on the right (morpho- butterfly wings). While the feather diffracts sunlight into a faint color spectrum (owing to the large periods – which are many times the wavelength – and the amplitude nature of the grating), the butterfly wing produces many more colorful effects, owing to the very small (sub-wavelength) periods and the reflective nature of the gratings (phase gratings). One of the first supposed attempts by man to produce a diffractive element was to scratch small, densely packed lines onto a shiny material such as metal or glassy lava rocks. Such early diffractives can be called ‘scratch-o-grams’, and are still today an efficient and playful way to teach children (and adults) the beauty of diffractive optics and holograms. ‘Scratch-o-grams’, or hand-drawn holograms, were first popularized by William J. Beaty in 1995. The Alsacian engraver Eugene Lacaque (1914–2005) has actually been recorded in the Guinness Book of Records in 1999 for having etched 78 lines per millimeter by hand. Note that such curved gratings with various chirps are actually intermediate elements between gratings and diffractive lenses. Figure 2.2 shows such a ‘scratch-o-gram’ and the method for producing them. Basically, a ‘scratch-o-gram’ is a spatial multiplexing of several circular scratches with the same radius of curvature, and various center locations and orientations. A circular fringe can be scratched on a planar
Applied Digital Optics: From Micro-optics to Nanophotonics Bernard C. Kress and Patrick Meyrueis Ó 2009 John Wiley & Sons, Ltd 16 Applied Digital Optics
Figure 2.1 The feather and butterfly wings as early micro- and nanostructured optics
Figure 2.2 ‘Scratch-o-grams’ or curved surface by fixing the center of a compass at one point of a real object drawn on the surface, and scratching a circular fringe. Repeating this process for all the points constituting the (sampled) object generates a spatial multiplexing of various circular fringes on the same surface. When looking at such ‘scratch-o-grams’ in sunlight (provided that the surface is clear, free of dust and debris, and makes the sunlight ‘shine’ through the clean grooves), one can see either the orthoscopic image of the object floating behind the surface or the pseudoscopic image floating in front of the surface. People have certainly been intrigued by the beauty of natural diffractives such as butterfly wings, and have tried to reproduce such beauty with various painting techniquesin caves and later on paper. People have also been intrigued by the mystical nature of such optical phenomena, especially the reproduction of selective angular effects, as well as the generation of a three-dimensional (3D) representation out of a planar surface. There have been reports that such ‘scratch-o-grams’ have been discovered in ancient Buddhist temples, with pictures of the Buddha floating in space behind carefully scratched glassy stones.
2.2 Guided-wave Digital Optics
Guided-wave optics is an important field of digital optics, and is gaining increasing attention from applications pools such as telecommunications, biotechnology and other sensor-related industries. Guided-wave optics rely on both refractive and diffractive effects within a microstructured optical material, which is usually fabricated by microlithographic techniques. Here, the electromagnetic wave is Classification of Digital Optics 17
Planar Lightwave Circuits (PLCs)
Planar slab waveguides Bragg grating waveguides
Channel-based waveguides Photonic crystal waveguides
Arrayed waveguides
Figure 2.3 Planar Lightwave Circuits
completely guided within the material by Total Internal Reflection (TIR). However, in many cases, the guided wave can be leaking light into free space, or can be coupled into free space and then re-coupled into guided waves, as in many Planar Lightwave Circuits (or PLCs). Similarly to free-space digital optics, guided-wave optics can be implemented on various technological platforms; however, they are mostly used in a phase medium, where the index of refraction or the surface relief of the phase material is modulated (as in buried, ridge or diffused waveguides). One main difference is that free-space digital optics can be implemented as an amplitude element, which is not the case for guided-wave optics. As we will see in Chapter 3, guided-wave optics can be used in numerous applications, ranging from optical telecommunications to integrated sensors, with a potentially high level of functionality in the form of PLCs. Figure 2.3 shows the different types of guided-wave digital optical elements (or PLCs) available today. In many cases, guided-wave digital optics are best used in conjunction with free-space digital optics, as we will see in Chapters 3 and 7.
2.3 Free-space Digital Optics
Free-space digital optics can be implemented in a number of different physical ways, and can be based on a number of physical phenomena. Throughout this book, we will review refractive micro-optics, diffractive optics, holographic optics and other sub-wavelength elements operating in free space. Two of the most important aspects of digital optics are diffractive optical elements and holographic optical elements. For an overwhelming number of people (including optical engineers), the field of holography is defined and limited to:
. 3D imaging (display holograms); . diffractive security tags (used on credit cards, bank notes, compact disks, clothes, etc.); and . Christmas wrapping paper (roll-embossed holographic gratings).
Similarly, for an overwhelming number of people (including optical engineers), the field of diffractive optics is defined and limited to:
. spectroscopic gratings (used in spectrometers, wavelength demultiplexers, etc.); and . laser pointer pattern generators. 18 Applied Digital Optics
Figure 2.4 The many names for digital free-space optics
One of the aims of this book is to broaden these established lists. Marketing engineers and sales managers, venture capitalists, engineers and academics, as well as technical writers, have started to give numerous names to digital optics. Some of the names most commonly used to refer to these elements are binary optics [1], Diffractive Optical Elements (DOEs), Computer Generated Holograms (CGHs), kinoforms [2], zone plates and so on. Figure 2.4 shows a compilation of these various names. There are roughly five different groups of digital free-space optics that have been reported in the literature since 1967 (when Professor Adolph Lohmann first introduced the concept of the ‘Synthetic Hologram’ [3]), which are categorized not so much according to their optical functionalities but, rather, with reference to the design techniques and the physical implementations used to manufacture them [1]. From that time onwards, Fourier optics has become an important part of modern optics technology, mainly due to the early works of Professor Joseph Goodman at Stanford [4]. Many different techniques can be used to design a diffractive element that produces the same optical functionality. Figure 2.5 summarizes these various types of free-space digital optics.
. Type 1 – Holographic Optical Elements (HOEs) – refers to the traditional optical holographic recording of volume-phase holograms (in phase modulation materials) or surface-relief holograms (in photoresist materials). These elements can be either thin or thick holograms [5], and are described in detail in Chapter 8. . Type 2 – analytic-type diffractives – refers mostly to elements that can be designed or optimized by means of analytic methods [6] such as ray tracing (as is done in most optical CAD tools), or by solving an analytic equation (as is done for Fresnel lenses or gratings). These are the most common diffractives. . Type 3 – numeric-type diffractives – refers mostly to elements that cannot be designed or optimized by analytic methods [6–9], and that require stochastic iterative optimization procedures and algorithms. Classification of Digital Optics 19
Figure 2.5 The main different types of free-space digital optical elements
These elements can implement more complex optical functions than analytic-type diffractives, but have their limitations (the amount of CPU power required, the need to rasterize the element in the design process, etc.). They are increasingly used in industry. . Type 4 – sub-wavelength diffractive elements (or Sub-Wavelength Gratings, SWGs) – refers to elements the basic structures of which are smaller than the reconstruction wavelength: they are thus highly polarization sensitive and they act very differently from the previous two diffractive types. Nano-optical or photonic crystals (photonic lattices) are included in these types, and are described in Chapter 10 as digital nano-optics. . Type 5 – dynamic diffractives – refers to all the technologies used to implement reconfigurable, tunable or switchable optical functionalities. Note that these elements can actually incorporate any of the four previous elements. This last type of diffractive has recently gained much attention in the emerging optical market and applications (especially in telecom and laser displays – see Chapter 16).
Chapter 5 will discuss these various types of elements in great detail (especially Types 2, 3, 4 and 5). Two additional chapters in this book are dedicated to Holographic Optical Elements (Type 1 – see Chapter 8) and Digital Nano-optics (part of Type 4 – see Chapter 10), since these elements differ in many ways from traditional Digital Diffractive elements.
2.4 Hybrid Digital Optics
Hybrid optics consists of mixing different types of optical elements in a single system, in order to improve existing optical systems or to generate new optical functionalities. Hybrid optics are used where a single type of optical element cannot address the optical functionality under a set of constraints, which can include the following:
. footprint, weight and packaging issues; . efficiency issues; 20 Applied Digital Optics
. budgeting issues; and . mass-replication issues.
For example, achromatic doublets are well-known pure refractive elements, but hybrid achromatic singlets are smaller, simpler and cheaper to mass-produce (see Chapter 7). Hybrid waveguide gratings are also good examples of hybrid optics, where new functionalities are generated that could not be integrated solely by waveguide optics (see, e.g., the AWG-based PLCs in Chapter 3). The chapters to follow will review the various elements discussed in this chapter in more detail.
References
[1] W.B. Weldkamp and T.J. McHugh, ‘Binary optics’, Scientific American, 266(5), 1992, 50–55. [2] L.B. Lesem, P.M. Hirsch and J. Jordan, ‘The kinoform: a new wavefront reconstruction device’, IBM Journal of Research and Development, 13, 1969, 150–155. [3] J.W. Goodman, ‘Introduction to Fourier Optics’, McGraw-Hill, New York, 1968. [4] A.W. Lohman and D.P. Paris, ‘Binary Fraunhofer holograms generated by computer’, Applied Optics, 6, 1967, 1739–1748. [5] T.K. Gaylor and M.G. Moharam, ‘Thin and thick gratings: terminology clarification’, Applied Optics, 20, 1981, 3271–3273. [6] H.-P. Herzig, ‘Micro-optics: Elements, Systems and Application’, Taylor and Francis, London, 1997. [7] B. Kress and P. Meyrueis, ‘Digital Diffractive Optics’, John Wiley & Sons, Ltd, Chichester, 1999. [8] B. Kress, ‘Diffractive Optics Technology for Product Development In Transportation, Display, Security, Telecom, Laser Machining and Biomedical Markets’, Short course, SPIE SC787, 2008. [9] S. Sinzinger and J. Jahns, ‘MicroOptics’, VCH, Weinheim, 1999. 3
Guided-wave Digital Optics
As pointed out in the previous chapter, guided-wave optics is an important field of digital optics. Guided- wave digital optics refers more to integrated waveguides rather than fiber optics (systems that are fabricated through digital lithographic means rather than the conventional fiber perform drawing process). However, integrated waveguide technology is closely related to fiber optics technology, for applications such as sensors, telecom modulators, DWDM devices and so on (especially to provide input and output interfaces for the digital waveguide device). Such systems are also known in industry as Planar Lightwave Circuits (PLCs) or planar integrated optics. As it is an intrinsic part of the realm of digital optics, this chapter will introduce the concept of guided- wave optics, define the various modes that can in optical waveguides, and explain the fundamentals of optical couplers and optical modulators. It will also show how free-space planar optics can be used in PLCs in order to integrate novel and complex optical functions.
3.1 From Optical Fibers to Planar Lightwave Circuits (PLCs)
As early as 1870, John Tyndall in the United Kingdom demonstrated light guiding in a thin water jet. Ten years later, Alexander Graham introduced for the first time the notion of an optical waveguide, and in the early 1930s the first patents on ‘optical tubing’ appeared. As early as 1950, a patent for a two-layer glass waveguide (two different indices of refraction) had been applied, and in 1960 the laser was used for the first time as a waveguide light source. Industry had to wait until 1965 [1] to find out how to take advantage of the low spectral absorption regions (see Figure 3.1), which would lead in the 1980s to the first optical fiber technology backbone of long-distance telephone networks [2,3]. Figure 3.1 shows that there are two low absorption levels around 1.3 mm and 1.5 mm. Today, industry standard values for propagation losses in telecom-grade single-mode optical fibers are 0.4 dB/km @ 1.3 mm and 0.2 dB/km @ 1.55 mm, which are the two most used wavelength regions (for CATV and DWDM/CWDM applications, respectively) [4]. The 10 Gb/s Ethernet application uses a shorter wavelength of 850 nm (VCSEL lasers or laser arrays), but over very small distances, since the absorption level is much greater in that wavelength range. For the 10 Gb/s Ethernet, multimode graded-index or even plastic fibers can be used. It is worth noting that Erbium Doped Fiber Amplifiers (EDFAs) are actually best suited to work over the DWDM -C and/or -L bands (1.53–1.65 mm). The C band is mostly used in the United States and Europe, and the L band in Japan for historical reasons. These spectral regions are located outside the water absorption peak, which is a good natural match for the telecom industry. Therefore, wavelengths within
Applied Digital Optics: From Micro-optics to Nanophotonics Bernard C. Kress and Patrick Meyrueis 2009 John Wiley & Sons, Ltd 22 Applied Digital Optics
6
5
4 Rayleigh scattering and ultraviolet ‘Water peak’ absorption 3 Peaks caused by OH– ions Infrared Loss (dB/km) 2 absorption
1
Wavelength 0 (μm) 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7
Figure 3.1 Propagation losses occurring in glass optical fibers as a function of wavelength the C band (1525–1565 nm) are commonly used today in conjunction with such EDFA amplifiers for submarine backbone networks or very long haul networks. It is worth noting that EDFA amplifiers are also integrated within PLC substrates. As we will see, optical fibers have been the first industrial development of optical waveguides. The fabrication of optical fibers doesnot require anymicrolithography equipment,although the core dimension might be on the order of several microns, and the core dimension has tolerances well within the realm of sub-micro lithography. Optical fibers are drawn from macroscopic glass preforms, which scale down the sizeofthe core byanverylargefactor– forexample,atypical fused silicapreform manufactured byMCVD may be 1 inch in diameter, and the drawn fiber only 250 mm in diameter [5]! Planar Lightwave Circuits (PLCs) are basically optical fibers fabricated within a planar substrate by lithographic tools [6,7]. PLCs can implement very complex optical functionalities, which are difficult to implement in fibers. Therefore, optical fibers are mainly used to transport lightwave signals over long distances, and are pigtailed to PLCs when it comes to processing the light in complex ways. As we will see below, coupling from fibers to PLCs and back to fibers is not a trivial task. The main advantages of PLCs over optical fibers are as follows:
. PLCs can implement complex optical functionality (more complex than fibers); . PLCs can be mass-produced by lithographic methods (plastic embossing or molding); . hybridization to electronic and mechanic circuits (planar geometry) for planar OE, MEMS or MOEMS devices is easy; and . PLCs are thin, planar and stackable, and can be built up into 3D systems.
3.2 Light Propagation in Waveguides
The basis for electromagnetic wave propagation in a dielectric waveguide relies on Total Internal Reflection (TIR) via rapid or smooth refractive index variations. Usually, the core of an optical fiber (of refractive index n2) or an optical waveguide is composed of a higher-index material than the cladding (of refractive index n1). The cladding is the medium surrounding the core, which enables the basic TIR effect, as depicted in Figure 3.2. Guided-wave Digital Optics 23
Figure 3.2 The principle of Total Internal Reflection (TIR)
Total internal reflection occurs when the propagation angle is larger than the Brewster angle aB. This angle is also called the critical angle: n2 ac ¼ arcsin ð3:1Þ n1 However, the TIR effect does not occur as a rapid transition between transmission (refraction) and reflection effects at the index interface, but rather as a gradual transition as the incoming angle increases, as is described in the following equation: 8 8 ða Þ ða Þ ða Þ > ¼ 2n1cos i > ¼ n2 cos i n1 cos t < TII ða Þþ ða Þ UII < RII ða Þþ ða Þ UII n2 cos i n1cos t n2 cos i n1 cos t ð : Þ > > 3 2 > 2n1cos ðaiÞ > n1 cos ðaiÞ n2 cos ðatÞ : T? ¼ U? : R? ¼ U? n1 cos ðaiÞþn2 cos ðatÞ n1 cos ðaiÞþn2 cos ðatÞ where the subscripts II and ? indicate, respectively, the parallel and orthogonal polarizations of the wave under consideration (U, incoming; T, transmitted; R, reflected), see also Figure 3.3.
Figure 3.3 Transmission and reflection at a planar interface 24 Applied Digital Optics
Figure 3.4 The modern optical fiber structure
Any waveguiding principle is based on the TIR angle for mode confinement in the core. Any waveguide device (an optical fiber, a channel-based waveguide or a planar slab waveguide) is composed of a core, a cladding (and a cladding jacket for the fiber), as depicted in Figure 3.4. However, it is worth noting that the material is not necessarily glass, and can also be plastic, air or even – as will be seen later, in Chapter 10 – a nanostructured Photonic Crystal (PC waveguide) producing an effective refractive index. The acceptance cone angle for an optical fiber is the largest angle that can be launched in the fiber for which there is still propagation along the core (that is, below the critical angle ac). The numeric aperture (NA) of an optical waveguide is simply the sine of that maximum launch angle. See Figure 3.5, in which a step-index optical waveguide structure is depicted as an example. The NA of conventional telecom-grade optical fibers (for a single-mode fiber, or SMF) is approximately 0.13 (e.g. the Corning SMF28 fiber). As any TIR is not 100% effective, the common definition of the waveguiding effect is based on a maximum allowed loss of 10 dB at the core/cladding interface.
Figure 3.5 The numeric aperture of an optical waveguide Guided-wave Digital Optics 25
3.3 The Optical Fiber
There are three main optical waveguide structures that are used today in industry:
. the step-index multimode optical waveguide; . the graded-index multimode optical waveguide; and . the single-mode optical waveguide.
Figure 3.6 shows the internal structures of the three different waveguide architectures. In a step-index waveguide (both in multimode and single-mode configuration), the interface between core and cladding is a rapid index step, whereas in graded-index waveguides, the transition from the core index to the cladding index is very smooth and continuous. TIR can occur in both cases. A graded-index fiber is usually a multimode fiber. Refraction through the graded index bends the rays continuously and produces a quasi-sinusoidal ray path. In some cases, it is interesting to use asymmetric core sections in order to produce polarization- maintaining fibers (in order to lower Polarization-Dependent Loss – PDL – in otherwise high-PDL PLCs by launching only one polarization state). Such sections are described in Figure 3.7. The multicore optical fiber depicted in Figure 3.7 is not a polarization-maintaining fiber, but can serve many purposes in sensors and telecom applications, by introducing coupling functionalities between each core. Multicore optical fibers with up to 32 cores have been fabricated. Typically, a multicore optical fiber is fabricated by fusing together several preforms on which part of the cladding has been shaved down – ground – in order to have a core that is very close to the surface. The multicore optical fiber is then drawn as a standard fiber.
Multimode fiber Cladding Core
Cladding
Graded-index multimode fiber Cladding Core
Cladding
Single-mode fiber
Cladding
Core
Cladding
Figure 3.6 The main optical waveguide structures 26 Applied Digital Optics
Polarization-maintaining fibers
Elliptical core Bow-tie fiber Circular stress applying Elliptically stressed Multicore fiber part (SPA) fiber cladding
Figure 3.7 Polarization-maintaining fiber core structures
Table 3.1 summarizes the key parameters of step-index or graded-index optical fibers (where a is the radius of the core). The complex amplitude of low- and high-order modes that can travel within an optical fiber is shown in Figure 3.8. The mode size of the fundamental mode is also described. The higher the mode order, the more energy will be traveling within the cladding. Similarly, the greater the wavelength, the more energy will be propagating into the cladding. Figure 3.9 shows the two-dimensional mode profiles for some propagating modes in an optical fiber. The light intensity is highest at the center of the fiber. Depending on the size and geometry of the core, there can be a multitude of modes circulating in the optical fiber (see Table 3.1). In Chapter 16, an example is given of a doughnut mode in a multimode graded-index plastic fiber, which can be matched with a digital diffractive vortex lens in order to minimize coupling losses. As seen previously (see Figure 3.2), propagation in any waveguide (optical fiber or PLC) has to fulfill the condition of TIR. In Section 3.4, the cut-off frequency, which rules the propagation of the various modes within a planar dielectric slab waveguide, is derived.
Table 3.1 Key parameters of optical fibers Parameter Step-index fiber Graded-index fiber pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ; a n1 r a n1 1 2Dðr=aÞ ; r a Refractive index, n ; > n2 r a n2 : r > a pffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 ð Þ2 2; Numerical aperture, NA n1 n2 n r n2 r a p p 2 a 2 a Normalized frequency, (V) l NA l NA pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Cut-off frequency 2:405 2:405 1 þ 2=a V2a Number of modes V2=2 2ða þ 2Þ Guided-wave Digital Optics 27
m = 0 m = 1 m = 2 r2 w2 = 0 E(r) E0.e
2w0
TE TE1 TE2
Figure 3.8 Low and higher modes in an optical fiber
Fundamental mode LP 01 Mode LP11 Mode LP21
Cladding Core
Figure 3.9 Mode profiles in a circular waveguide
3.4 The Dielectric Slab Waveguide
The dielectric slab waveguide is a one-dimensional optical waveguide. The mode confinement is therefore active only in one direction, whereas in the other direction the beam can diverge as it would do in free space. The following section will discuss dielectric channel waveguides that have two-dimensional mode confinement. In a planar dielectric optical waveguide (slab), the lower cladding index is not necessarily the same as the upper cladding index, as it is for an optical fiber. Here, the upper cladding can even be air, while the lower cladding is usually a low-index glass (see Figure 3.10). Through TIR, the waves may bounce between the guide walls, as they would for an optical fiber waveguide. Let us consider the scalar wave equation for TE polarization along the y-axis (Equation (3.3)) (see also Appendices A and B): r2 ð ; Þþ 2 2 ð ; Þ¼ ¼ ; ; ð : Þ Ey x z ni k0Ey x z 0 for i 1 2 3 3 3 Appendix B shows that solutions can be written in the form