POLYMER OPTICAL FIBER
BRAGG GRATINGS
By Huiyong Liu
A thesis submitted in fulfilment of the requirement for
the degree of Doctor of Philosophy
School of Electrical Engineering and Telecommunications
The University of New South Wales
February 2003 U N S W 1 7 JUL 2003
LIHRARY Acknowledgment
Acknowledgment
First, I would like to thank my supervisors, A/Professor Gangding Peng and
Professor Pak Lim Chu, for their close supervision over the years. They have given
me invaluable guidance in the production of this work. I am particular graceful for
A/Professor Gangding Peng in the formulation of the topic, in the development of the thesis theme, in the implementation of the detailed experimental work, and in writing
up of this thesis. He has been helpful, encouraging and patient and I am indebt to him for his help.
Second, I would like to give my special thanks to the colleagues and friends in
Photonics and Optical Communication Group at the School of Electrical Engineering and Telecommunications for their support and encouragement. I like to thank Mr
Trevor Whitbread for his valuable assistance and discussions on laboratory experiments. I also like to thank Dr Skinner for spending hours in discussing about my thesis writing and giving me helpful feedbacks. I must also give my sincere gratitude to Mr Hongbo Liu, Mr Xiaogang Li, Mr Philip Ji, Ms Margaret Lu, Ms
Emily Lee, Dr Bin Wu, Mr Ruofei Peng and Dr Liguo Luo for their help discussions and support. Their friendships will be everlasting precious gifts to the author. I also want to thank Mr Yannic Aubree for his hard work in proof-reading my draft. Also thank Ms Stella Chau and Ms Vicky Hui for their helps.
Last, I would like to thank my parents, Shigui Liu and Hongbin Su, my sister,
Huiyun Liu, and my brother, Huiming Liu, for their support and encouragement throughout these years of my study to bring this thesis to a conclusion. Abstract
Abstract
This thesis presents a first systematic study on the fabrication, formation mechanism
and characterization of polymer optical fiber (POF) Bragg gratings.
Firstly, the investigation on the growth dynamics of PMMA-based POF Bragg
gratings has been carried out. In this investigation, we found that there exist two
distinctive stages in POF Bragg gratings formation. We also revealed that the growth
behaviour of POF Bragg gratings bears remarkable resemblance to that of silica fiber
Bragg gratings. Based on these findings, we named the two different types, i.e. Type I
and Type II, of POF Bragg gratings, following the nomenclatures of silica fiber Bragg gratings.
Based on the insights gained in the growth dynamics investigation, we produced
POF Bragg gratings with the best result ever reported: the reflectivity of 0.999 and the line width of less than 0.5 nm, by optimizing the UV exposure fluence.
We also tested the thermal and strain characteristics of Type I and Type II PMMA based POF Bragg gratings. The test showed that POF Bragg gratings have excellent thermal tunability: large tuning range, absence of thermal hysteresis and large temperature sensitivity (10 times larger than that of silica fiber Bragg gratings). The test also demonstrated that, by simply applying tensile stress, large tuning range of 32 nm in
Bragg wavelength shift and large strain sensitivity (higher than that of silica fiber Bragg gratings) have been achieved.
Apart from the work on PMMA-based POF, photosensitivity in the novel and low loss perfluropolymer (CYTOP) fiber material has also been examined. Significant photosensitivity has been observed and gratings have been successfully written in the Abstract
CYTOP fiber material for the first time. This is an exciting start showing the possibility of writing fiber Bragg gratings in the low loss CYTOP fiber.
In addition, we proposed a new scheme of the hybrid silica and polymer fiber Bragg grating sensor for simultaneous strain and temperature measurement. Detailed analysis predicted that the proposed scheme will achieve high accuracy and large perturbation tolerance.
Dynamic dispersion compensation applications of POF Bragg gratings have been studied as well. Analysis and numerical simulations have been performed and the results demonstrated that the linearly chirped POF Bragg gratings fabricated by the tapered
POF method can be used to achieve a dynamic dispersion compensation range from
1534 ps/nm to 66 ps/nm. Publications
Publications
Journal paper:
1. H.Y. Liu, G.D.Peng, P.L. Chu, "Photosensitivity in low-loss perfluoropolymer
(CYTOP) fiber material", Electronics Letters, vol. 37, No. 6(2001), p. 347.
2. H.Y. Liu, G.D.Peng, P.L. Chu, "Thermal tuning of polymer optical Bragg
gratings", IEEE Photonics Technology Letters, vol. 13, No. 8 (2001), p. 824.
3. H.Y. Liu, G.D.Peng, P.L. Chu, “Thermal characteristics of Bragg gratings in
CYTOP fiber material,” Optics Communications, vol. 24, No. 4 (2002), p.151.
4. H. Y. Liu, G. D. Peng and P. L. Chu, “Polymer Fiber Bragg Gratings with 28dB
Transmission Rejection”, IEEE Photonics Technology Letters, vol. 14, No. 7
(2002), p. 935.
5. H. Y. Liu, G. D. Peng and P. L. Chu, “Observation of Type I and Type II Bragg
Grating behaviour Polymer Optical Fibre”, Optics Communications, vol. 220, No.
4-6 (2003), p. 337.
6. H. B. Liu, H. Y. Liu, and G. D. Peng, “Different Types of Polymer Fiber Bragg
Gratings (FBGs) and Their Strain/Thermal Properties”, to be appeared in Optical
Memory and Neural Networks special issue on "Holographic Memory and
Applications”.
7. H. B. Liu, H. Y. Liu, and G. D. Peng, “Strain and Temperature Sensor using a
combination of Polymer and Silica Fibre Bragg Gratings”, Optics Communications,
vol. 219 (2003), p. 139.
Conference paper:
1. H.Y. Liu, G.D.Peng, G.J. Destura, B.Wu and P.L. Chu, "Highly thermal tunable
polymer optical fiber gratings", 9th International POF conference 2000, July,
2000, Boston, USA. Publications
2. H. Y. Liu, G.D. Peng, and P.L. Chu, "Photosensitivity and Bragg gratings in novel
low-loss fluoropolymer (CYTOP) for polymer optical fiber", The 25th Australian
Conference on Optical Fibre Technology (ACOFT'2000), Canberra, 16-18, June
2000.
3. H.Y. Liu, G.D.Peng, P.L. Chu, " Thermal Stability of Gratings in PMMA and
CYTOP Fibres ", 10th International POF conference 2001, July, 2001,
Amsterdam, Netherland.
4. H.Y. Liu, G.D.Peng, P.L. Chu, “High reflective polymer fiber gratings and the
growth dynamics,” OFC 2002, March, 2002, Anaheim, USA.
5. H. Y. Liu, G. D. Peng and P. L. Chu, “Research on the Mechanism of Polymer
Fiber Bragg Grating Formation”, 27th Australian Conference on Optical Fiber
Technology, Sydney, Australia, July, 2002.
6. G. D. Peng, H. Y. Liu, and P. L. Chu, "Dynamics and Threshold Behaviour in
Polymer Fibre Bragg Grating Creation", invited paper, presented at the the 47th
SPIE conference on Photorefractive Fibre and Crystal Devices, Materials, Optical
Properties and Applications VII, Seattle, WA, USA, July 2002.
7. H. Y. Liu, G. D. Peng, and P. L. Chu, “Thermal characterization of Type I and
Type II Optical Fiber Bragg gratings”, Second Asia-Pacific Polymer Fiber Optics
Workshop, January, 2003.
8. H. B. Liu, H. Y. Liu, R. Wang and G. D. Peng, “Strain and temperature response
of polymer fiber Bragg gratings”, First international conference on Optical
Communications and Networks (ICOCN 2002), Shangri-La Hotel, Singapore,
November 11-14, 2002. Contents
Contents
Acknowledgment i
Abstract ii
Publications iv
Contents vi
Chapter 1: Introduction 1
1.1 Novelties of the thesis 1
1.2 Introduction to fiber Bragg gratings 2
1.3 Status of the research on POF Bragg gratings 10
1.4 Thesis outline 12
1.5 References 13
Chapter 2: Growth Dynamics of Polymer Optical Fiber (POF) Bragg
Gratings 16
2.1 Introduction 16
2.2 Basic relations of optical fiber gratings 20
2.3 POF fabrication 25
2.4 POF Bragg gratings fabrication 27
2.5 Growth dynamics of POF Bragg gratings 30 Contents
2.6 Type I POF Bragg gratings 38
2.7 Type II POF Bragg gratings 39
2.8 Conclusions 42
2.9 References 43
Chapter 3: Thermal Characteristics of Polymer Optical Fiber (POF)
Bragg gratings 48
3.1 Introduction 48
3.2 Set-up for thermal characterization measurement 52
3.3 Thermal tuning of Type I POF Bragg gratings 53
3.4 Thermal sensitivity of Type I POF Bragg gratings 57
3.5 Thermal tuning and thermal sensitivity of Type II POF Bragg gratings 58
3.6 Thermal decay of Type I & II POF Bragg gratings 61
3.7 Stabilization of Type I POF Bragg gratings 67
3.8 Discussion on the mechanism of Type I POF Bragg gratings decay 68
3.9 Conclusions 69
3.10 References 70
Chapter 4: Strain Characteristics of Polymer Optical Fiber (POF)
Bragg Gratings 73
4.1 Introduction 73
4.2 Experimental method for strain tuning characterization 76
4.3 Strain sensitivity of Type I POF Bragg gratings 76
vii Contents
4.4 Strain tunability of Type I POF Bragg gratings 78
4.5 Strain characterization of Type II POF Bragg gratings 83
4.5 Simultaneous strain and temperature measurement by using POF Bragg
gratings 84
4.7 Conclusions 89
4.8 References 90
Chapter 5: Simulation of Dynamic Dispersion Compensation by
Linearly Chirped Polymer Optical Fiber (POF) Bragg
gratings 92
5.1 Introduction 92
5.2 Scheme for linearly chirped POF Bragg gratings fabrication 98
5.3 Dynamic dispersion compensation by linearly chirped POF Bragg gratings 103
5.4 Conclusions 115
5.5 References 116
Chapter 6: Photosensitivity Study of Low-loss Perfluoropolymer
(CYTOP) Fiber Material 121
6.1 Introductions 121
6.2 CYTOP material and CYTOP optical fiber 121
6.3 Experimental methods 124
6.4 Photosensitivity of CYTOP fiber material 127
6.5 Thermal test of CYTOP gratings 130
viii Contents
6.6 Conclusions 134
6.7 References 134
Chapter 7: Conclusions and Further Work 137
7.1 Conclusions 137
7.2 Suggestions on further work 139
IX Chapter 1: Introduction
Chapter 1: Introduction
1.1 Original contributions of the thesis
Since photosensitivity in silica fiber was first revealed by Hill et al in 1978 [1],
silica fiber Bragg grating has been developed for many important applications in optical
communication systems. However, the tunability of silica fiber grating is still a big
concern, which disfavours its application in some fields. In recent years polymer optical
fiber (POF) has shown great potential for short-distance communication systems
because of its ease of handling and possible low cost [2]. The tunability for polymer
fiber gratings is expected to be much better because of its much higher elasticity
(equivalently much smaller Young’s modulus). This thesis is the first systematic study on POF Bragg gratings about its fabrication, formation mechanism and characterization.
There are several original contributions in this thesis.
The growth dynamics of polymethylmethacrylate (PMMA) based POF Bragg grating is investigated for the first time. Two types of POF gratings, named Type I and
Type II POF Bragg gratings, are clearly identified and reported in Chapter Two. With the knowledge on dynamic growth of POF gratings, we optimise the UV exposure fluence and produce a POF Bragg grating with the best results ever reported: the reflectivity of 99.9% and less than 0.5 nm line width. The basic optical spectrum characteristics and formation mechanism of the gratings are also examined.
The thermal and strain effects on Type I and Type II POF Bragg gratings are characterized as well. Excellent tunability by temperature and strain variation is found in the POF Bragg gratings, as reported in Chapter Three and Chapter Four, respectively.
1 Chapter 1: Introduction
Thermal decay behaviours of Type I and Type II POF Bragg gratings are also
investigated in Chapter Three.
In addition, photosensitivity has been shown and gratings have been written in the
low-loss perfluoropolymer (CYTOP) fiber material for the first time, as reported in
Chapter Six.
Although the thesis mainly emphasizes on the fabrication, mechanism study and characterization of POF Bragg gratings, some further applications of POF Bragg gratings have also been discussed. A new scheme of the hybrid silica and polymer fiber
Bragg grating sensor for simultaneous strain and temperature measurement is proposed in Chapter Four. Analysis and numerical simulation on dynamic dispersion compensation by linearly chirped POF Bragg gratings is also included in Chapter Five.
1.2 Introduction to fiber Bragg gratings
The idea of using glass fiber to transmit optical signal originated with Alexander
Graham Bell as early as 1880 [3]. However, a renewed interest in optical communication was stimulated in the early 1960s with the invention of laser. After the first proposal for optical communication via dielectric waveguides or optical fiber fabricated from glass by Kao and Hockham in 1966 and the demonstration of this proposal at Coming Glass Works in 1970 [4], the low loss optical fiber has rapidly evolved. Then the development of optical fibers and devices began and continues strongly today. Communications using an optical carrier wave guided along optical fiber has a number of extremely attractive features, such as large bandwidth, small size and weight, low transmission loss and immunity from external electromagnetic interference.
2 Chapter 1: Introduction
1.2.1 Historical Background
Since the photosensitivity in silica optical fiber was discovered by Hill et al in 1978
[1], the new type of in fiber component, called fiber Bragg gratings, has been developed.
Now fiber Bragg gratings has become the key components in optical communication systems and been widely used in many applications, such as fiber laser, dispersion compensator, add-drop WDM multiplexer, mode coupler, fiber amplifier gain controller and optical sensor etc [5-7]. In the most basic form, fiber Bragg grating consists of a periodic modulation of refractive index along the fiber core, shown in Fig. 1.1. For a conventional fiber Bragg grating the periodicity of the index modulation has a physical spacing that is one half of the wavelength of light propagating in the waveguide; it is the phase matching between the grating planes and incident light that results in coherent back reflection. The grating period and length, together with the strength of the modulation of the refractive index, determine whether the grating has a high or low reflectivity over a wide or narrow range of wavelengths. Therefore, fiber Bragg gratings are suitable for telecommunications where they are used to reflect, filter or disperse light. Furthermore, fiber Bragg gratings also take the advantages offered by optical fiber, such as low transmission loss, immunity to electromagnetic interference and light weight, making the intra-core grating an ideal candidate device for telecommunications and sensing.
Xb refl. X - Xa trails. / Z
•i Him z z
spacing - XB/2niJll X inc.
Figure 1.1 A schematic graph of a fiber Bragg grating [8].
3 Chapter 1: Introduction
Fiber Bragg gratings are produced by exposing an optical fiber to a spatially varying
pattern of ultraviolet (UV) intensity. The formation of gratings into the fiber core was
first demonstrated by Hill et al in 1978 [1] at the Communication Research Center in
Canada. In their experiment, they launched the 488 nm argon ion laser light into the core
of the fiber. Under prolonged exposure, an increase in the attenuation of the fiber was
observed and it was also determined that the intensity of light reflected from the fiber
increased significantly with time during exposure. This behaviour was the result of a
grating formed in the core of the fiber with the strength of the grating increasing as time
progressed. This grating recorded the pattern formed by the interference of the light
transmitted through the fiber and the Fresnel reflected beam (4% reflection from the
cleaved end of the fiber). Thus, the interference pattern only formed within the length
limited by the coherence length of the writing beam. However, the grating fabricated by
this technique is very weak with the estimated index modulation to be of order of 10"6.
Furthermore, the Bragg reflection wavelength is the same as the writing wavelength.
Research on optical fiber gratings remained dormant for several years afterwards due
to the limitation of the “Hill grating writing technique” until Meltz et al [9] demonstrated
the side writing technique. They showed that gratings that would reflect any wavelength
could be fabricated by illuminating the fiber through the side of the cladding with two
intersecting beams of UV light. Thus the period of the interference maxima and the index
change was set by the angle between the beams and the UV wavelength. Moreover, the
grating formation by 244 nm UV light was found to be orders of magnitude more efficient.
Since then, research has been carried out on the fiber gratings inscription and significant breakthroughs have been achieved. Now high quality silica fiber Bragg gratings can be fabricated using various techniques such as interferometric, phase mask, and point-by- point exposure to UV laser light. Gratings with very large range of Bragg wavelength,
4 Chapter 1: Introduction
bandwith and reflectivity can be formed on time scales ranging from a few nanoseconds to
a few minutes. Now, fiber Bragg gratings technologies have moved from laboratory
interests and curiosities to the implementation in optical communications and sensor
systems.
1.2.1 Fiber Bragg gratings fabrication techniques
Followings will be some brief descriptions about these efficient techniques for
inscribing fiber Bragg gratings.
interference pattern.-mini ~"-n V..__ \ MifOf >
W laser Beam
Beam
Optical spectrum arvatyze*
Figure 1.2 Set-up of interferometric writing technique for fiber Bragg grating fabrication [10].
The interferometric or holographic technique is the first external written method invented by Meltz et al [11]. Fig. 1.2 illustrates the interferometric writing technique. In this method, a coherent UV source is first split into two beams by a beam splitter. Then the two beams are combined and intersected to form an interference pattern. The fringe pattern is used to expose to the fiber and thus induces the periodic refractive index modulation in the fiber core. The Bragg grating period, A, which is identical to the period of the interference fringe pattern, depends on both the irradiation wavelength A,w, and the half-
5 Chapter 1: Introduction
angle between the intersecting UV beam, (p, as shown in Fig. 1.2. The period of the grating
is given by,
A = ——— (1.1) 2 sin (p
Following the Bragg condition, which is XB=2nA, the Bragg wavelength, XB, can be
presented in terms of the UV writing wavelength and the half angle between the
intersecting UV beams as,
*B= — ■ (1.2) sin (p
From the above equation, it is easy to find that the Bragg wavelength can be varied
either by changing Xw and/or (p. Since the choice for Aw is limited to photosensitivity
region of the fiber, it is flexible to change the angle (p to get the designed Bragg
wavelength.
As shown above, the most important advantage of this method is the ability to fabricate Bragg gratings at any wavelength by simply changing the intersecting angle.
Also it is flexible to write gratings with various lengths. Furthermore, some unique grating pattern, such as linearly chirped gratings, can be produced by using curved reflecting surfaces in the beam delivery path. But this technique is susceptible to the mechanical vibrations, which is its main disadvantage. Moreover, the air currents will have effects and may cause problems in forming a stable fringe pattern. In addition, a laser source with good spatial and temporal coherence is required for this technique.
One of the most popular and effective techniques for fiber Bragg grating fabrication is the phase mask method [11]. In this method, a diffractive optical component (phase mask) is employed to spatially modulate the UV writing beam. The phase mask is made from high quality flat slab of silica glass, which is transparent to UV light. On one of the
6 Chapter 1: Introduction
flat surfaces, a one dimensional periodic surface relief structure is fabricated using
photolithographic technique. The set-up for writing fiber gratings by this method is very
simple—just placing the optical fiber almost in contact with the corrugations of the phase mask, as shown in Fig. 1.3. The UV light, which is incident normal to the phase mask, passes through and is diffracted by the corrugations of the phase mask. The profile of the phase mask is designed so that the zero order of the diffractive beam is suppressed to several percent (typically less than 5%) of the transmitted power by controlling the depth of the corrugations in the phase mask. Therefore, the diffractive +1 and -1 order will be maximized. Thus, the two ±1 diffractive order beams interfere to form the fringe pattern that photoimprints a refractive index modulation in the fiber core.
The period of photoimprinted gratings in the fiber core is half of the period of the phase mask gratings. Therefore, the period is independent of the irradiating UV wavelength.
Incident urfrvxtfe?
Sfco Goss Prose Mas*:
XOiftsuefeU Byymi
Z&C QKi& (<3% a? tfuoughpu!)
Figure 1.3 Fabrication of fiber Bragg gratings by phase mask technique [10].
The phase mask technique greatly simplifies the fiber gratings fabrication process and it has also been extended to the fabrication of apodization, tilted and chirped fiber gratings. Its simplicity provides a robust and an inherently stable method for fiber Chapter 1: Introduction
grating fabrication. Thus it makes mass production for fiber gratings with high
performance possible. In addition, because the fiber is quite close to the phase mask, the
coherence requirement on the UV laser source is much lower. Since the period of the
fiber grating is half of that of phase mask, the main drawback for phase mask technique
is that a separate phase mask is required for each different designed Bragg wavelength.
ii IWI.i
Pfibw ivtuiuiiiitju f ibef hdder Trarefc'jltonal 3k Jyn
Figure 1.4 Set-up for point-by-point fiber Bragg grating inscription technique [10].
Another approach for fiber grating fabrication is the point-by-point technique [12].
In this method, Bragg grating is accomplished by inducing a change in the refractive index change point-by-point along the fiber core. The set-up for point-by-point technique is shown in Fig. 1.4. Each “point” grating is produced separately by a focused single UV pulse. A single pulse of UV light passes through a mask containing a slit and a focus lens images the slit onto the core of the optical fiber from the side. The refractive index of the exposed section increases locally. The fiber is then translated through a distance, A, corresponding to the pitch of the gratings and the process is repeated to form the grating structure in the fiber core. From the above description, it is shown that a very stable and precise submicron translation stage is essential for this technique.
Because it is a step-by-step method, it requires a relatively long process time, thus a
8 Chapter 1: Introduction tedious process. Furthermore, errors in the grating space due to thermal effects and strain variation in fiber core will occur. This limits the length of the gratings fabricated.
However, it is really flexible to alter the parameters of Bragg gratings. Because the grating is inscribed point-by-point, variations in grating length, grating pitch, and spectral response can be easily incorporated. Chirped gratings can be very accurately fabricated by this method.
1.2.3 Properties of Fiber Bragg gratings
Because of the periodic index perturbation in the fiber core, fiber grating acts as a stop band filter. A narrow band of incident optical light within the fiber is reflected by successive, coherent scattering from the index variation. The strongest interaction or mode coupling occurs at a specific wavelength - Bragg wavelength Xq (Fig. 1.5). The basic introduction on the spectra for fiber Bragg gratings can be found in Chapter Five.
Bragg grating CJcticflng Core
index modulation Wavelength
Figure 1.5 A schematic illustration of a fiber Bragg grating [13].
Furthermore, changes in fiber characteristics, such as strain, temperature, or polarization, which varies the modal index or grating pitch, will change the Bragg wavelength. Therefore, fiber grating is an intrinsic sensor, which changes the spectrum of an incident signal by coupling energy to other fiber modes. Among all these
9 Chapter 1: Introduction characteristics, thermal and strain effects on fiber gratings are the two fundamental ones.
Thermal and strain effects on silica fiber gratings will be reviewed in Chapter Three and
Chapter Four, respectively.
1.2.4 Mechanism of the photosensitivity in silica fiber
Though Bragg gratings have been written and developed in many types of silica fibers using various methods, the mechanism of the index change in silica fiber is still not fully understood. There are several models proposed for the photo-induced refractive changes in silica fiber, such as color center model [14], permanent dipole model [14], compaction model [15] and stress relief model [16] etc. The only common element in these models is that the germanium-oxygen vacancy defects, Ge-Si or Ge-Ge (the so- called “wrong bonds”) are responsible for the photo-induced index change. The detailed description for all these models won’t be given here.
1.3 Status of the research on POF gratings
Compared with the enormous researches on silica fiber gratings, there are few reports on POF gratings.
Although silica glass fiber remains unsurpassed in current optical fiber networks,
POF is very promising for some certain applications in the future. The competitiveness of POF against silica optical fiber comes from its ease of handling, flexibility and potential low cost. Therefore, POF could offer significant advantages in short-distance data communications, such as local area networks (LANs) and those found in aircraft and automobiles, where ease of handling and installation are very important.
The commercial development of POFs started in 1970s. In 1974, Mitsubishi set the patent covering the melt-spinning of a simple light-guiding core-cladding structures
[17]. Poly-methylmethacrylate (PMMA) or polystyrene (PS) was employed as the core
10 Chapter 1: Introduction material and various fluorinated polymers as cladding. The minimum attenuation was over 3500dB/km at an unspecified wavelength. Later they patented another process for
high-purity acrylics [18] which has since led to the development of Eska™. This is a
PMMA-core, poly(fluoroalkylmethacrylate)-cladding fiber which now dominates the markets for POFs. The current, premium grade Eska has a minimum loss of 125 dB/km at 567 nm. Since the early 1980s, the further developments in the fields of POFs have mainly been the work of Kaino and co-workers at NTT’s Ibaraki laboratories. Their work has greatly reduced the losses of PMMA-core fibers to 55 dB/km at 567 nm and those of PMMA-D8 core fibers to 20 dB/km at 680 nm [19].
So far little work has been carried out in POF Bragg gratings, but polymer optical waveguide devices based on polymer waveguide gratings have been widely studied. The grating structures have been fabricated on polymer waveguide by using photolithography [20], e-beam direct writing [21] and reactive ion etching (RIE) [21].
Several classes of polymers have been suggested and tested for polymer waveguide grating investigation. Among them the most striking results was achieved in research using the AlliedSignal optical polymers [22]. These materials are formed from the combination of multifunctional acrylate monomers and oligomers together with various additives. Polymer waveguide grating based on AlliedSignal polymers with 99.997% reflection were produced [23]. Multichannel optical add/drop multiplexers (OADM’s)
[24] and arrayed-waveguide gratings (AWGs) [25] were fabricated by using
AlliedSignal waveguide gratings.
The research on POF Bragg gratings was started in the University of New South
Wales in recent years. PMMA based POFs have been shown to have useful photosensitivity and Bragg gratings can be written on them [26]. Further study demonstrated that the POF Bragg gratings have very high tunability in comparison with
11 Chapter 1: Introduction silica fiber Bragg gratings [13]. However, these study are just a simple start for POF
Bragg gratings and a lot of work still needs carrying out. First, the maximum reflectivity of the POF Bragg gratings achieved is only 80%, which is much weaker than that achieved in the conventional silica fiber gratings. Optimization in POF Bragg grating fabrication technique is worth doing in order to get POF Bragg gratings with better performance. Second, the understanding and knowledge of the mechanism of photosensitivity and the Bragg grating formation in POFs so far have been very limited.
In addition, the POF gratings have not been fully characterized, such as strain effects and thermal effects. This thesis endeavours on discovering the POF grating formation mechanism, fabricating high quality POF Bragg gratings, and characterizing the POF
Bragg gratings.
1.4 Thesis outline
This thesis consists of seven chapters, including this introduction. Chapter Two gives the results of the systematic investigation on the growth dynamics of POF Bragg gratings. Examination of the optical spectra characteristics and formation mechanism of
POF Bragg gratings are also included in this chapter.
Chapter Three details the study of thermal effects on POF Bragg gratings, which includes the thermal sensitivity and thermal decay characteristics.
Results of strain effects on POF Bragg gratings are presented in Chapter Four.
Based on the thermal and strain characteristics of POF Bragg gratings, a scheme for instantaneous strain and temperature measurement by using the hybrid sensor of a POF
Bragg grating and a silica fiber Bragg grating is proposed in this chapter as well.
12 Chapter 1: Introduction
Chapter Five is the simulation of dynamic dispersion compensation by linearly
chirped POF Bragg gratings. This offers an example of further application for POF
Bragg gratings.
Chapter Six covers the photosensitivity study in the novel and low loss
perfluoropolymer (CYTOP) fiber material. An introduction to the CYTOP material and fiber is also given in this chapter.
Concluding summaries for the whole thesis and suggestions for future work are given in Chapter Seven.
1.5 References
1. K.O. Hill, Y. Fuji, D.C. Johnson, and B.S. Kawasaki, Photosensitivity in optical
fiber waveguides: Application to reflection filter fabrication. Applied Physics
Letters, 1978. 32: p. 647-649.
2. Y. Kioke, Progress in GI-POF —status of high speed plastic optical fiber and its
future prospect, in The international POF technical conference. 2000.
Massachusetts, USA.
3. J.M. Senior, Optical fiber communications. 1992, New York: Prentice Hall.
4. H.J.R. Dutton, Understanding optical communications. 1998, Sydney: Prentice
Hall.
5. L. Dong, P. Hua, T.A. Reekie, and P.S. Russell, Novel add/drop filters for
wavelength division multiplexing optical fiber system using a Bragg grating
assisted mismatched coupler. IEEE Photonics Technology Letters, 1996. 8: p.
1656-1658.
6. C.R. Giles, T. Strasser, K. Dryer, and C. Doerr, Concatenated fiber grating
optical monitor. IEEE Photonics Technology Letters, 1998. 10: p. 1452-1454.
13 Chapter 1: Introduction
7. H. Jones-Bey, Tunable fiber laser yields llnm range. Laser Focus World, 1999.
35: p. 20-25.
8. K.O. Hill and G. Meltz, Fiber Bragg grating technology fundamentals and
overview. Journal of Lightwave Technology, 1997. 15: p. 1263-1276.
9. G. Meltz, W.W. Morey, and G.H. Glenn, Formation of Bragg gratings in optical
fibers by transverse holographic method. Optics Letters, 1989. 14: p. 823-825.
10. A. Othonos, Fiber Bragg gratings. Review of Scientific Instrument, 1997. 68: p.
4309-4341.
11. K.O. Hill, B. Malo, F. Bilodeau, D.C. Johnson, and J. Albert, Bragg gratings
fabricated in monomode photosensitive optical fiber by UV exposure through a
phase mask. Applied Physics Letters, 1993. 62: p. 1035-1037.
12. B. Malo, K.O. Hill, F. Bilodeau, D.C. Johnson, and J. Albert, Point-by-point
fabrication of micro-Bragg gratings in photosensitive fiber using single excimer
pulse refractive index modification techniques. Electronics Letters, 1993. 29: p.
1668-1669.
13. Z. Xiong, G. Peng, B. Wu, and P.L. Chu, Highly tunable Bragg gratings in
single mode polymer optical fibers. IEEE Photonics Technology Letters, 1999.
11: p. 352-354.
14. D.P. Hand and P.S.J. Rusell, Photoinduced refractive index changes in
germanosilicate fibers. Optics Letters, 1990: p. 102-104.
15. C. Fiori and R.A.B. Devine, Evidence for a wide continuum of polymorphs in a-
SiC>2. Physics Review B, 1986. 33: p. 2972-2974.
16. D. Wong, S.B. Poole, and M.G. Sceats, Stress-birefringence reduction in
elliptical-core fibers under ultraviolet irradiation. Optics Letters, 1992. 17: p.
1346-1348.
14 Chapter 1: Introduction
17. M.R. Company, U. K. Patent 1431 157. 1974.
18. M.R. Company, U. K. Patent 1449 950. 1974.
19. C. Emslie, Review polymer optical fibers. Journal of Material Science, 1988. 23:
p. 2281-2293.
20. S. Aramaki, G. Assanto, G.I. Stegeman, and M. Marciniak, Realization of
integrated reflectors in DANS-polymer waveguides. Journal of Lightwave
Technology, 1993. 11: p. 1189-1195.
21. M. Oh, S. Ura, T. Suhara, and H. Nishihara, Integrated-optic focal-spot intensity
modulator using electrooptic polymer waveguide. Journal of Lightwave
Technology, 1994. 12: p. 1569-1576.
22. L. Eldada and L.W. Shachkette, Advances in polymer integrated optics. IEEE
Journal of Selected Topics in Quantum Electronics, 2000. 6: p. 54-68.
23. L. Eldada, R. Blomquist, A. Maxfield, D. Panti, G. Boudoughian, C. Poga, and
R.A. Norwood, Thermooptic plannar polymer Bragg grating OADM's with
broad tuning range. IEEE Photonics Technology Letters, 1999. 11: p. 448-450.
24. L. Eldada, S. Yin, C. Poga, C. Glass, R. Blomquist, and R.A. Norwood,
Integrated multichannel OADM's using polymer Bragg grating MZI's. IEEE
Photonics Technology Letters, 1998. 10: p. 1416-1418.
25. C.L. Callender, J.-F. Viens, J.P. Noad, and L. Eldada, Compact low-cost tunable
acrylate polymer arrayed-waveguide grating multiplexer. Electronics Letters,
1999. 35: p. 1839-1840.
26. G.D. Peng, Z. Xiong, and P.L. Chu, Photosensitivity and grating in dye-doped
polymer optical fibers. Optical Fiber Technology, 1999. 5: p. 242-251.
15 Chapter 2: Growth Dynamics of Polymer Optical Fiber (POF) Bragg Gratings
Chapter 2: Growth Dynamics of Polymer Optical
Fiber (POF) Bragg Gratings
Because of the significant and practical importance, photosensitivity in silica
optical fiber, which has resulted in the fiber Bragg gratings, has been extensively
explored. Photosensitivity has been observed and Bragg gratings have been created also
in polymethylmethacrylate (PMMA) based polymer optical fiber (POF) in recent years
[1, 2]. Nevertheless the mechanisms of Bragg grating formation in POF are yet to be
fully investigated and understood. We carried out the first experimental investigations
on the growth dynamics of POF Bragg grating under various exposure conditions. With
the better knowledge of a grating creation process from this investigation, PMMA-based
POF Bragg gratings with the best results ever reported: a reflectivity of 0.999 and a line
width less than 0.5 nm, is produced. This work provides the significant start for the
further development of POF Bragg gratings.
2.1 Introduction
Though the polymer fiber is in different material system from the silica fiber, some of the results achieved in silica fiber Bragg gratings can be used and referenced for POF
Bragg grating study. Photosensitivity in optical fiber refers to a permanent change in the refractive index of fiber core while irradiated to ultraviolet light with characteristic wavelength and intensity [3]. The wavelength and intensity of UV light depend on the core material. For many years, a lot of research work on the photosensitivity of silica fiber has been carried out, but some questions still remain unanswered. Initially,
16 Chapter 2: Growth Dynamics of Polymer Optical Fiber (POF) Bragg Gratings
photosensitivity in silica fiber was thought to be a phenomenon only associated with
optical fibers having a large concentration of germanium in the core. Then there are
numerous reports on the existence of photosensitivity in a wide range of silica fibers,
many of which do not contain germanium as a dopant. Fibers doped with europium [4],
cerium [5], and erbium:germanium [6] show varying degrees of sensitivity in a silica
host optical fiber, but none as sensitive as germanium. With the demonstration of
photosensitivity in most types of fiber, it is clear that photosensitivity is a function of
various mechanisms (photochemical, photomechanical and thermochemical) and the
relative contribution will be fiber dependent, as well as intensity and wavelength
dependent. Several models [7-9], that were proposed to describe the photo-induced
refractive index changes in germanium-doped fiber, share the common elements of the
germanium oxygen vacancy defects as precursors responsible for the photo-induced
index change.
Growth dynamics of fiber Bragg gratings is another important issue relevant to
fiber photosensitivity. From the studies on growth dynamics for silica fiber Bragg
gratings, one may distinguish several dynamic regimes. The first regime is called Type I
Bragg gratings, where the refractive index modulation increases almost linearly with
UV exposure energy density [10]. It is often observed in most photosensitive fibers
under either continuous wave or pulsed UV irradiation. The resulting refractive index change induced by Type I grating is positive. Protracted UV exposure of Type I grating
in some instances results in complete or partial grating erasure, followed by the formation of another type grating, which is called Type HA or Type ID gratings [11].
Type HA differs from Type I grating in refractive index modulation value. The refractive index change induced by Type DA grating is negative. Type IIA gratings are most often observed in high GeCU-doped fibers. Recently, a new type of grating named
17 Chapter 2: Growth Dynamics of Polymer Optical Fiber (POF) Bragg Gratings
Type IA grating was discovered in hydrogenated B/Ge-doped silica fiber [12]. Type IA
grating is also formed after the erasure of Type I grating and grating occurs with the
increasing exposure energy. Different from Type IIA grating, the Bragg grating in this
case shifts continuously to longer wavelength during the inscription. Therefore,
refractive index change induced by Type IA grating is positive. Furthermore, the
magnitude of Type IA Bragg wavelength red-shift is significantly larger than the blue-
shift value in Type IIA grating formation. Another dynamic regime corresponds to a
very high UV irradiation level, resulting in very large refractive index changes in small
localized regions at the core/cladding boundaries. This kind of grating is classified as
Type II gratings [13] and is a result of physical damage that is limited to the fiber core,
producing very large refractive index modulation high up even to 10 “.
Due to the vastly different material systems, the mechanism and behavior for POF
Bragg gratings might be quite different from those of silica fiber gratings.
Photosensitivity of polymethylmethacrylate (PMMA) has been studied for about 30 year
if not longer. The earliest report we found on the photosensitivity of PMMA is the work
of Tomlinson et al [14]. In their work, they found that properly prepared PMMA
(through oxidation of monomer) exhibited a substantial increase in refractive index after
irradiation with UV light at 325 nm (He-Cd+ Laser) or 365 nm (Hg arc.). Then there
were some other researches on the photosensitivity of PMMA bulk material induced by
UV light [15-18]. Researches showed that there is an increase in density of the
irradiation region and the density increase leads to the refractive index change. But there are some arguments on the origin of the density increase. Some believed that it is attributed to the crosslinkings of the residual monomer in PMMA [14]. The crosslinkings is envisaged as drawing the chains closer, thus increase the density. On
18 Chapter 2: Growth Dynamics of Polymer Optical Fiber (POF) Bragg Gratings
the other hand, others agreed that the density increase results from the polymerization of
the residual monomer [16, 17].
In the recent research carried out at the University of New South Wales,
photosensitivities of various PMMA-based POF were investigated [1, 2]. These include
PMMA POFs made from undoped, dye-doped or oxidated preforms under various
irradiation wavelength, intensity and time. In reference 2, the transmission rejection
value for the POF Bragg grating is only about 6 dB, corresponding a grating length of 1
cm and the index modulation of 6.6x10 5. This is far below the performances achieved
■j in silica fiber Bragg grating, for which the UV induced index change exceeds 10 . In
addition, the understanding and knowledge to the mechanism of photosensitivity and
grating formation in POFs so far have been very limited.
Our efforts are aimed at disclosing the mechanism of grating formation. Based on
the mechanism knowledge, we can improve our technique on POF Bragg gratings
inscription to achieve POF Bragg gratings with much better performance. Observation on the growth dynamics of POF Bragg gratings as they are exposed to UV irradiation will give very important insights into the photosensitivity of POFs. Therefore, a systematic investigation on the growth dynamics of POF Bragg gratings is earned out.
For the first time, we report the observation of a threshold in UV exposure or fluence in the Bragg grating creation process. The threshold distinguishes two types of fiber Bragg gratings that have quite different properties and performances. With the better knowledge of a grating creation process from this investigation, PMMA-based POF
Bragg gratings with the best results ever achieved: a reflectivity of 0.999 and a line width less than 0.5 nm, is produced.
19 Chapter 2: Growth Dynamics of Polymer Optical Fiber (POF) Bragg Gratings
2.1 Basic relations of optical fiber Bragg gratings
In the growth dynamics investigation, we have to work out the refractive index change or photosensitivity, induced into polymer fiber in the grating fabrication process; therefore, some basic relations about the optical spectra and refractive index change of fiber Bragg gratings will be given. The schematic diagram of a fiber Bragg grating is shown in Fig. 2.1. Here an incident light launched into the fiber section written with a
Bragg grating in the core is highly reflected within a narrow band around a Bragg wavelength. The Bragg wavelength is determined by the constructive interference, i.e. the in-phase condition, of partial reflections from each part of the grating.
fiber Bragg gratings
0 IUII 1 1 1 1 1 1 1 L ly * * W core cladding
incident reflected transmitted light light light
Figure 2.1 Schematic diagram of a fiber Bragg grating.
Within a simple model of periodic index change, the in-phase condition can be directly linked to the condition giving the Bragg reflection of a grating. This is the famous Bragg condition widely used and it can be obtained from the usual energy and momentum conservation laws involving light and matter interaction [19].
20 Chapter 2: Growth Dynamics of Polymer Optical Fiber (POF) Bragg Gratings
The momentum conservation related to the grating reflection can be explicitly expressed as,
K + ki = kr (2.1) where K, ki and kr are the wave-vectors of the grating, the incident light and the reflected light respectively. The vector K has a magnitude given by
where A is the grating period. For optical fiber gratings, the magnitudes of ki and kr, are usually referred to as propagation constants, and are respectively given by
(2.3)
where neff is the effective refractive index of the optical fiber mode (usually the fundamental mode). The incident and reflected waves are both along the fiber axis and in opposite directions to each other. To satisfy the momentum conservation condition, the direction of K is also along the fiber axis and in the direction of the reflected wave- vector. Under the energy conservation law, the incident wave and reflected wave should have the same energy, i.e.
/zo)j =h(aT (2.4) for the same wavelength and we assume there is no energy transfer from one wavelength to another and no radiation loss; h=2nh where h is the Planck’s constant. Hence we can write for frequency
/, = /,= f. (2.5) and for wavelength
21 Chapter 2: Growth Dynamics of Polymer Optical Fiber (POF) Bragg Gratings
(2.6)
with
(2.7)
where c is the light velocity and Xq denotes the Bragg wavelength. Thus from both the
energy and momentum conservation conditions, we can arrive at the Bragg condition,
(2.8)
The Bragg condition provides the simple and yet very important relation between the
Bragg wavelength and the grating parameter —the period A of the index variation.
The reflection and transmission in a Bragg grating of an optical fiber are described
by the Coupled Mode Theory (see for example [18]). Following the simple analysis of
[19], we consider a single fiber with a Bragg grating of length L and the light
propagating along z axis (shown in Fig. 2.1). We eliminate the propagation constant at
Bragg wavelength (say PB) and only consider the difference 8 between the actual propagation constant P and the propagation constant at Bragg wavelength Pb. We let A and B represent the complex amplitudes of the forward and backward propagating modes and then set
A{z) = R{z) exp(;&) (2.9) and
B(z) = S(z) exp(-jSz) (2.10)
This leads to
R'(z)~ jS R{z) = jK S(z) (2.11) and 22 Chapter 2: Growth Dynamics of Polymer Optical Fiber (POF) Bragg Gratings
S'(z) + jSS(z) = -j* R(z) (2.12)
The modulation An in the refractive index determines the Bragg (forward-to- backward) coupling coefficient. For a single-mode Bragg reflection grating and sinusoidal index change along the fiber axis, the coupling coefficient is given by
ntsn k - ~Y (2.13) where A, is the signal light wavelength. The frequency offset is S = j3 - j3B.
The range of frequencies where the light is reflected, i.e. the band-gap, has a width proportional to k. Within the band-gap, i.e. for frequency offsets 5 satisfying 82 < k2, almost all the light is reflected, but outside this range, i.e. 82 > k2, the fraction reflected is small. Physically, this means that each element of power decreased in the forward propagating mode is added to the reflected mode. We have the boundary conditions
R(0)=1 and S(L)=0. It can be shown that inside the band-gap, both |R| and |S| decrease monotonically as z increases and the curves are similar, as almost all the light is reflected. However, outside the band-gap, |R|“and |S|“ are oscillating functions of z, since standing waves are set up in the grating. In this range, |R| remains high for all values of z, while |S| is always fairly small. This means that the constant value of
(|R(z)|~-|S(z)|“) is now larger than it was when the parameters were within the band- gap.
Within the band-gap (5^ < k^), we let q2 = K2 -52 > 0. The exact solution of R(z) to the above system can be easily found [16]. We write R(z) in the form
R = Rr+jR,. (2.14)
The exact solution can be expressed as Rr and Rj 23 Chapter 2: Growth Dynamics of Polymer Optical Fiber (POF) Bragg Gratings
Rr = H{k1 cosh[g(z - 2L)] + (q2 - 82)cos(qz)} (2.15)
Rt = -2qSH sinh(gz) (2.16) where the constant H is
(2.17) 2[q2 cosh2 (qL) + 82 sinh2 (qL))'
The exact solution of S(z) to the above system within the band-gap (§2 < k2) can also be found, in the form
S = Sr + St (2.18) with
Sr =-2/cqH cosh(qL)sinh[q(z~ L)] (2.19) and
St =-2rcSHsin(qL)sir\h[q(z-L)]. (2.20)
The amplitude and power reflection coefficients p = S(0)/R(0) and r = |/?|“ from a fiber Bragg grating can be worked out as follows,
- k • sinh(gL) P = (2.21) 8 • sinh(gL) + iq ■ cosh(gL) and
5(0) K~ sinh2 {qL) r — (2.22) R( 0) S2 sinh2 (qL) + q2 cosh2 (qL)
As a simple example, we consider the case when the frequency offset is 5=0, i.e. the incident wavelength is at the center of the band-gap, where the maximum reflectivity rmax happens. A simple expression for rmax is obtained as 24 Chapter 2; Growth Dynamics of Polymer Optical Fiber (POF) Bragg Gratings
rmax = tanh2 (kL) . (2.23)
It is obvious that the reflectivity is increased with the increase of both the refractive
index change An (since K=7tAn/A,) and grating length L. Also, by re-writing equation
(2.23), the refractive index change induced by gratings can be estimated if the maximum
reflectivity is known and is expressed as,
(2.24)
where X is the wavelength at the peak reflectance and L is the grating length.
2.2 Polymer optical fiber fabrication [20]
The POF for gratings fabrication in our experiment is PMMA-based because methyl methacrylate is good in optical quality and compatible with most of the organic to be used as dopants. The refractive index of methyl methacrylate is about 1.41. When polymerised, the index of its polymer will increase up to 1.48-1.49, due to the volume reduction during liquid-to-solid phase transition. These indices are close to those of silica fibers. In making the polymer fiber preform, the tuning and controlling of the index are very important. The difference of the refractive index between the core and cladding is controlled by either adding trifluro-ethyl methacrylate to the cladding or adding benzyl methacrylate to the core. In our experiment, the core of the POF is
PMMA doped with BzMA; while the cladding is PMMA. Optical, mechanical and thermal properties of polymer can also be modified, according to specific application by the choice of the initiators and chain transfer agents, as well as other methacrylate or monomers. We used lauryl peroxide or benzoyl peroxide as initiator. The merit of peroxides is that there is no gas such as nitrogen released during polymerisation and this
25 Chapter 2: Growth Dynamics of Polymer Optical Fiber (POF) Bragg Gratings will prevent air bubble formation in the fiber. We used n-butyl mercaptan as transfer agent.
The POF preforms were prepared in “Teflon Technique” [21, 22]. In this technique, a Teflon string is properly placed in the center of a glass tube. One end of the tube is sealed and it is then filled with the initiated monomers for the cladding. Then the thermal polymerisation of the filled tube is carried out in an oil bath where its temperature is controlled in an appropriate pattern. After the monomers are fully polymerised and heat-treated, the Teflon string is removed and we obtain a polymer tube (polymer rod with a small hole in its center). The bottom side of the core is sealed and the hole is then filled with the initiated monomer for the core.
temperature control
preform furnace
drawing motor
bearing
feeding motor
drivers & electronics PC control
Figure 2.2 Schematic diagram of the fabrication rig for polymer optical fiber [20].
After the POF preforms are prepared, our POF can be fabricated by using the optical fiber drawing rig shown in Fig. 2.2. The polymer preform is slowly fed into the furnace under computer control, at a typical speed of 1 mm/min. The furnace is set at a temperature between 280°C and 290°C. The furnace has a heating length of 20 mm and
26 Chapter 2: Growth Dynamics of Polymer Optical Fiber (POF) Bragg Gratings
a hole of 18 mm in diameter. The temperature experienced by the preform depends
directly on the preform feeding speed (Vi) and the fiber drawing speed (V2). The
highest temperature in the preform usually occurs near its tip and it reaches a value
between 200°C and 250°C. The polymer fiber quality depends on the drawing
temperature and drawing tension. Fig. 2.3 shows the near-field pattern of the POF for
gratings fabrication. The POF has an outer diameter of 133 pm and a core diameter of 6
pm. The difference in the refractive index between the core and the cladding is
measured to be 0.0086, by employing a transverse field fringe method using an
interferometric microscope. Thus the fiber is single-mode in the 1550 nm window.
Figure 2.3 Near-field pattern for POF.
2.4 POF Bragg gratings fabrication
Because of the C-H bond in its molecular structure, PMMA shows serious intrinsic absorption loss from visible to near infrared region. The attenuation of PMMA POF is high up to 104 or even 105 dB/km near 1550 nm regions [23]. Due to this effect, we only use a very short section (around 5 cm) of PMMA POF for grating inscription. Whenever monitoring the grating growth during grating fabrication or testing afterwards, we use single-mode silica fiber to launch the monitoring light into the short POF sample. In
27 Chapter 2: Growth Dynamics of Polymer Optical Fiber (POF) Bragg Gratings
order to decrease the joint loss of the silica fiber and POF, a smooth and flat POF cross-
section has to be achieved. Since there are no commercial methods available to cleave
POF, we use AI2O3 powder to polish the cross-section of POF. There are several steps
for the preparation of POF samples for gratings fabrication. First, the small section of
POF is held above a piece of glass plate with dimension of 10x50 mm. Second, the two
ends of the POF is glued onto the glass plate surface by putting some epoxy on the two
ends of the glass plate. The epoxy we use is a two-part resin and a very good bonding is
achieved 24 hours after it is applied. Then we can start to polish the cross-section of the
sample. Several grades of AI2O3 grinding powder are adopted to polish the POF samples
step by step. During the polishing process, we use optical microscope to check if the
POF cross-section is smooth enough.
UV beam 3F under UV phase mask! Dosure
silica optical fiber
beam dumper- silica prism
ASE source
beam dumper
Figure 2.4 Set-up for POF Bragg gratings fabrication.
After the POF sample is ready, it can be put into set-up for fiber Bragg grating fabrication shown in Fig. 2.4. The POF sample is butt joint to the silica fiber for testing.
This set-up uses the techniques of both phase mask and Sagnac interferometer. The POF
28 Chapter 2: Growth Dynamics of Polymer Optical Fiber (POF) Bragg Gratings
sample is not put directly behind the phase mask, but above the phase mask. A UV laser
beam with suitable wavelength is needed to induce a certain photosensitive process in
the POF.
The previous research [1] in our group indicated that the gratings created in
PMMA-based POF preform at 248 nm UV light was a surface grating formed by the
removal of the polymer, while the grating created at 325 nm UV light was a bulk
grating. Thus we use 325 nm UV light for the PMMA-based POF Bragg gratings
fabrication. The UV writing beam is from a frequency-doubled MOPO pumped by a
frequency-tripled Nd:YAG laser. The UV laser beam is not focused and has an effective
spot size of 3 mm with pulse duration of 5 ns at 10 Flz repetition rate.
The period of the phase mask used in our POF Bragg grating fabrication is 1.06pm,
which is originally designed for direct grating writing using 248 pm wavelength. Since
325 nm is not the designed wavelength of the phase mask, the zero diffraction order
from the phase mask is rather high and this zeroth order is detrimental to the formation
of Bragg gratings [24], Therefore we use three prisms to construct a modified Sagnac
interferometer where the two first-order diffraction beams form the required
interference pattern for the gratings, while the zeroth order is blocked. This configuration is an adaptation of the transverse method developed by Meltz, Morey and
Glenn [25] with the introduction of a static ring interferometer based on the patent invented by Ouellettee [26]. In the course of gratings inscription, we use ANDO
AQ63128 optical spectrum analyser (OSA) to on-line monitor the transmission and reflection spectra of the POF Bragg gratings at the same time. We employ a broadband
ASE source for the in-situ monitoring and characterisation of grating fabrication process. The optical spectra shown below are the transmission, or reflection power of
29 Chapter 2: Growth Dynamics of Polymer Optical Fiber (POF) Bragg Gratings
POF Bragg gratings normalized by the original transmission power of POFs before the
gratings are formed.
2.5 Dynamic growth of POF Bragg gratings
2.5.1 Optical spectra of POF Bragg gratings growth
Using the experimental set-up described above, we investigated the growth
dynamics of POF Bragg gratings. Fig. 2.5 shows the dynamic growth process of POF
Bragg gratings, where the transmission and reflection spectra at different UV exposure
time are demonstrated. In this experiment, the writing UV pulses have the energy
density of 6 mJ/cm at 325 nm.
In the transmission spectra (Fig. 2.5(a)), an obvious change in transmission at
around 1574 nm can be observed after 12 minutes’ exposure indicating an onset. Then
the first stage of grating formation started that is characterized by the maximum
rejection level increasing slowly up to about 62 minutes of exposure. After that, the
second stage of gratings formation began that is characterized by the rejection level
increasing significantly with further exposure. At the 67th minute, the rejection level is roughly doubled from that at the 62nd minute. At the exposure time of 85 minutes, the rejection level is increased so high that the transmission at the Bragg wavelength goes below to the noise level. In the last 23 minutes, the rejection level is increased from approximately 4 dB at the 62nd minute to about 30 dB at the 85th minute.
30 Chapter reflection(dB) transmission(dB) -20 -10 Figure
1567
- - 2:
Growth
2.5
The
Dynamics
transmission 1569
of different
Polymer
(a)
62nd 67th 76th 85th and 1571 wavelength(nm) wavelength(nm) 76th 67th 62nd 85
UV Fig. Fig.
th
minuter- minute minute Optical minute reflection
minute 31 minute minute
minute exposure
2.5 2.5 — —
_
(a) (b)
Fiber
(b)
time. 1573
spectra (POF)
of Bragg
POF ^ '27th
,12th 12th 27th 43rd 43
Gratings 1575
Bragg rd
minute
minute minute minute minute mi
mute
gratings
at 1577 Chapter -20 -15 reflectjon(dB) transmission (dB) -5 Figure 0 1563 1568
2:
2.6 Growth 1565
Transmission
Dynamics 1567 1570
(a) of 1569 exposed
and Polymer
reflection 1571
1572 after wavelength(nm) wavelength(nm) Fig. Fig.
Optical
32
the
(b) 2.6 2.6 1573
85th
spectra
(b) (a)
Fiber
minute. 1575 1574
of
(POF)
POF
1577
Bragg Bragg
gratings
Gratings 561578 1576 1579
when 1581
over 1583 Chapter 2: Growth Dynamics of Polymer Optical Fiber (POF) Bragg Gratings
From the reflection spectra in Fig. 2.5(b), it is obvious that there are significant side
lobes. Moreover, as the reflection level at Bragg wavelength increases, the amplitude of
side lobes also increases. The side lobes are due to the resonance of multiple reflections
to and from opposite ends of the grating regions. In order to suppress the side lobes, an
apodized refractive index profile has to be introduced [27]. However, it is not our main
concern in this work and no experiment has been done to confirm this yet.
The transmission and reflection spectra of the POF Bragg gratings beyond 85
minutes’ exposure are also recorded and displayed in Fig. 2.6. Due to the limitation of
the measurement system, the transmission rejection level for all the POF Bragg gratings
after the 85th minute are under noise level. However, it is clear from the transmission
spectra in Fig. 2.6(a) that the transmission is rapidly deteriorated by excessive exposure
- both the loss at wavelengths shorter than the Bragg wavelength and the grating
bandwidth increase remarkably. Furthermore, the transmission level at the wavelengths
larger than the Bragg wavelength decreases with further exposure. Thus indicates that
insertion loss of the POF Bragg gratings becomes higher as well. Therefore, too high
UV fluence can result in catastrophic failure of the polymer fiber. At the same time, from the reflection spectra in Fig. 2.6(b) it is shown that the peak reflection level is almost the same with more UV irradiation, while the line width of the grating becomes extraordinarily larger.
From the experimental results above, it appears that the strength of POF Bragg gratings becomes stronger with the increase of UV exposure. But with too much irradiation, i.e. exposure time over 85 minutes, POF Bragg gratings will be deteriorated with large insertion loss and may cause failure of the POF Bragg gratings. Therefore, we can optimize the UV exposure level to POFs, corresponding to 85 minutes’ of irradiation, to achieve POF Bragg gratings with large rejection value and narrow
33 Chapter 2: Growth Dynamics of Polymer Optical Fiber (POF) Bragg Gratings
bandwidth. The more accurate measurement for transmission spectrum of a POF Bragg
grating fabricated with 85 minutes’ exposure is shown in Fig. 2.7. The spectrum
indicates almost 30 dB (limited by the resolution of the measurement system) rejection
is achieved at the Bragg wavelength, corresponding to only 0.1% of the light is
transmitted. In this case, the line width of the grating is less than 0.5 nm.
-15 -
-20
-25 -
-30 -
wavelength (nm)
Figure 2.7 Transmission spectrum of the POF Bragg grating at optimised UV exposure time
with 30 dB transmission rejection and a line width of less than 0.5 nm.
2.5.2 Refractive index modulation of POF Bragg gratings growth
Refractive index modulation An of POF Bragg gratings induced by UV at different exposure time were estimated from the experimental reflection spectra. The corresponding data are summarized in Fig. 2.8. As we discussed in Section 2.2, by using the basic relations equation (2.13) and equation (2.23), the refractive index change An can be expressed in related to the grating peak reflection rmax in equation (2.24) as,
A« = 4'tanh-'(V^7) (2.24) 7lL
34 Chapter 2: Growth Dynamics of Polymer Optical Fiber (POF) Bragg Gratings
where X is the wavelength at the peak reflectance and L is the grating length. But for
high reflectance grating, the reflectance doesn’t give the sensitive measure of An, as the
case for gratings with UV exposure after the 85th minute. However, the FWHM
bandwidth of the reflectance AX is approximately linearly proportional to the index
modulation An [28]. Therefore, the index modulation An for gratings with UV exposure
after the 85th minute is estimated from the grating bandwidth AX by using,
where Ano and AXo are the index modulation and bandwidth of the grating with UV
exposure at the 85th minute, respectively. Therefore, the index modulation An for the
POF Bragg gratings with UV exposure before the 85th minute, where the grating at the
85th minute is also included, can be calculated by following equation (2.24). While for
the gratings with UV exposure after the 85th minute, equation (2.25) will be more accurate for index modulation An estimation.
From the index modulation versus exposure curve, it is apparent that a ‘threshold’ time occurs at around the 62nd minute separating the two stages of the grating formation - stage I represented by o and the dotted line, and stage II represented by A and the solid line. This threshold time clearly separates the two stages of grating formation as mentioned above. In stage I, An is small and increases slowly, almost linearly, with time. While in stage D, An increases rapidly with time. At the 85th minute, the An value is about four times greater than that of the 62nd minute and an index modulation as high as up to 10' is achieved. While at the 113th minute, the refractive index modulation almost reaches to 2x10°.
35 Figure refractive index modulati Chapter Figure 1.00E-05 1.00E-04 1.00E-03 1.00E-02 scales. r? «
2.9 0.2 0.4 0.6 1.4 1.6 1.8
2:
2.8
Two Refractive
------Growth
Refractive
different
Dynamics index
index slopes
modulation
modulation
in o-o of
the
Polymer exponential
graph
of
POF of
indicate Optical
POF time 36 time
Bragg
function.
Bragg
t(min) (min)
modulation Fiber
gratings
gratings
(POF)
versus
index
at
Bragg
different
exposure
curve
Gratings
100 doesn
exposure
time
’t 110
on follow
time. log-log 120
Chapter 2: Growth Dynamics of Polymer Optical Fiber (POF) Bragg Gratings
The index modulation curve in Fig. 2.8 somewhat looks like exponential function.
In order to check if the index modulation curve follows exponential function, we plot
the index modulation and exposure time graph on log-log scales, as shown in Fig. 2.9.
Apparently, the slopes of the curve for the two stages are quite different and the slope
for stage II is much larger. Therefore, it is clearly shown that index modulation versus
time curve doesn’t follow exponential function, which further confirms that there exist
two different stages in the POF Bragg gratings growth and in the second stage the index
modulation grows much faster with the UV irradiation than the first one.
Comparing our experimental results on the growth dynamics of POF Bragg gratings
with that of Ge-doped silica fiber Bragg gratings under UV pulse irradiation [13]
previously reported, it is interesting to reveal that their An versus UV exposure
behaviours are very much similar. In the process of silica fiber Bragg grating
fabrication, it is well known that there is a threshold of exposure, below which the index
modulation grows linearly. When above the threshold point, the induced index
modulation increases dramatically. Therefore, there are two stages with the low index
modulation and high modulation index in the silica fiber Bragg grating growth process.
Because of this similar behaviour of index modulation versus exposure energy between
the them, for convenience, we follow the nomenclatures used for Ge-doped silica fiber
gratings [13] to categorize the POF Bragg gratings with low (dotted line area) and high
index modulation (solid line area) as Type I and Type II POF Bragg gratings, respectively.
As different characteristics have been investigated and revealed for Type I and
Type II silica fiber Bragg gratings, questions about the characteristics for Type I and
Type II POF Bragg gratings will be raised as well. How are they compared with those of silica fiber Bragg grating? These are quite relevant questions to be answered. Hence
37 Chapter 2: Growth Dynamics of Polymer Optical Fiber (POF) Bragg Gratings
we carried out more investigation on the characteristics of Type I and Type II POF
Bragg gratings below.
2.6 Type I POF Bragg gratings
We first have a close examination of the transmission and reflection spectra for
Type I POF Bragg gratings from the above experiment. As shown in Fig. 2.10, the
reflection spectra of the guiding modes are in complimentary to the transmission ones.
This means that there are no significant excess losses due to absorption or coupling into
the claddings. This is quite consistent with the fundamental characteristics of Type I
silica fiber Bragg gratings which are characterized by the UV-induced refractive index
change in the fiber core resulted from the material photosensitivity. A genuine UV-
induced refractive index change by the material photosensitivity usually produces low
excess loss. Hence, in terms of transmission and reflection spectra characteristics, the
Type I POF Bragg grating resembles the Type I silica fiber Bragg grating.
Another noteworthy finding on the Type I POF Bragg grating is that the central
wavelength shifts to the blue part of the spectrum in the process of grating growth. If the
Type I fiber Bragg gratings is formed mainly due to the refractive index change in the
core, the central wavelength A,max, where the maximum reflection happens, will be
determined by
'Ll = (! + 1 neft (2.26)
where Aneff is the mean change in effective refractive index induced by Bragg gratings,
neff is the effective refractive index of the core and XBo is the nominal Bragg wavelength
when the index modulation is not considered. From equation (2.23), it is clear that Ab
increases with Aneff. Only if Aneff is negative, i.e. the refractive index change induced by the POF Bragg gratings is negative, XB will decreases with large value of Aneff. Though 38 Chapter 2: Growth Dynamics of Polymer Optical Fiber (POF) Bragg Gratings
several mechanisms have been put forward for the formation of the periodic structures
in bulk polymer material[29-32], our experiment clearly indicates that the refractive
index change induced by Type I POF Bragg gratings is negative. Determining the
formation mechanism of Type I POF Bragg gratings will be a complex work, which
might be linked to the joint of photochemical, photomechanical and thermochemical
research areas. Further study still needs carrying out to determine the photoreactions
that are responsible for Type I POF Bragg gratings formation.
0.8 - - 0.8
---- 12min - o.6 a 27min
- - 43min - 0.4 62min
- 0.2
1571 1573 wavelength(nm)
Figure 2.10 Transmission and reflection spectra of Type I POF Bragg gratings at different
irradiation time.
2.7 Type II POF Bragg gratings
We now examine the transmission and reflection spectra of the Type II POF Bragg gratings. Fig. 2.11 shows the spectra of the Type II POF Bragg gratings with 76 and 85 minutes’ UV exposure, respectively. Compared with Type I POF Bragg gratings, it is
39 Chapter 2: Growth Dynamics of Polymer Optical Fiber (POF) Bragg Gratings
obvious that the bandwidth of Type II POF Bragg gratings becomes broader and the loss
at the short wavelengths becomes significant. We also observed that, with the UV
exposure increase further, the bandwidth becomes even broader and the loss at the short
wavelengths increases as well. These features are again very similar to those of the
Type II silica fiber Bragg grating -with fairly large bandwidth and high losses
associated with the coupling power of short wavelengths into the claddings [33].
0.8 - - 0.8
o.6 a -----76min
■ ■ 85min
0.2 - - 0.2
wavelength(nm) Figure 2.11 Transmission and reflection spectra of Type II POF Bragg gratings.
Furthermore, we carried out the optical morphology test of the Type II POF Bragg gratings using optical microscope. The morphology photo is displayed in Fig. 2.12.
From the morphology study (Fig. 2.12(a)), we can see the existence of a damaged pattern at the core-cladding interface. The damage can explain the large index modulation as well as the strong coupling of the short wavelength into claddings induced by Type II POF Bragg gratings. Due to the uniqueness of this damage pattern in
Type II POF Bragg gratings, this observation strongly suggests that the damage is responsible for its formation. The damage has been identified as the formation
40 Chapter 2: Growth Dynamics of Polymer Optical Fiber (POF) Bragg Gratings mechanism of the Type II silica fiber Bragg gratings [13]. This provides clear evidence that, regardless of their respective formation process and mechanism, the Type II POF
Bragg grating and silica fibre Bragg grating have similar characteristics and that their characteristics are both linked to the damage grating formation.
Figure 2.12(a) Damaged pattern at core/ cladding interface in Type II POF Bragg grating.
core
damages in the claddings
ii wi *** -m
Figure 2.12(b) Damages in the claddings in Type II POF Bragg grating.
Figure 12 Optical morphology of Type II POF Bragg gratings.
41 Chapter 2: Growth Dynamics of Polymer Optical Fiber (POF) Bragg Gratings
Our study in Type II POF Bragg gratings also shows that there are also damages in
the cladding, but no clear periodic pattern is found, as shown in Fig. 2.12(b). However,
the whole picture of how the damage pattern forms at the core/cladding boundaries and
how the damages in the cladding relate to the periodic damage pattern at the interface is
still not clear. More efforts should be made to find out the damage formation
mechanism of Type II POF Bragg gratings.
Though lots of similarities exist between Type II POF Bragg gratings and Type II
silica fiber Bragg gratings, the resemblance may be only superficial. Noticeable
differences between them are present as well. First, typical Type II silica fiber Bragg
gratings are formed by several pulses, even single UV pulse, irradiation with UV
fluence of hundreds mJ/cm . While in our Type II POF Bragg gratings fabrication, the
gratings are formed by exposure of tens of thousands UV pulses with very low
irradiation power. Therefore, the formation mechanism for those two gratings might be
different though they are all linked to damage gratings formation. Second, the same
periodic damage patterns are found in the cladding in Type II silica fiber Bragg gratings
[34], but it is not the same case in Type II POF Bragg gratings. This may be also due to
the different formation mechanisms for the two kinds of gratings.
2.8 Conclusions
In summary, we carried out a detailed investigation on the growth dynamics of POF
Bragg gratings. From this work we observed for the first time that the growth dynamics, formation and resulting characteristics of POF Bragg grating bear remarkable similarities to those of silica fiber Bragg gratings. Of course the similarities could be merely superficial since the underlying mechanism or process could be vastly different because of totally different material systems. In particular, we observed that there is a
42 Chapter 2: Growth Dynamics of Polymer Optical Fiber (POF) Bragg Gratings distinctive threshold in UV exposure. The threshold distinguishes two different grating formation stages. In the first stage when the exposure is under the threshold, the index modulation grows slowly and linearly. Whereas in the second stage when exposure is above the threshold, the index modulation increases rapidly and dramatically. We demonstrated that the characteristics and formation behaviors of POF Bragg gratings clearly resembles that of silica fiber grating. Hence we can follow the nomenclatures of silica fiber grating by referring POF Bragg gratings formed in low and high index modulation regimes as Type I and Type II POF Bragg gratings, respectively. In particular, we show that Type I POF Bragg grating do have typical features of Type I silica fiber Bragg grating - relatively small index modulation and no significant excess loss with the transmission and reflection spectra complimentary to each other. In addition, we determined that the index modulation in POF grating is negative. We also showed that Type II POF Bragg gratings do have typical features of Type II silica fiber
Bragg grating - large losses at short wavelengths, broad bandwidth and distinctive damage at the core and cladding interface. Based on the knowledge of POF Bragg gratings growth, we can optimize the UV exposure fluence to produce the POF Bragg gratings with the best results ever reported: a reflectivity better than 0.999 and a line width less than 0.5 nm.
2.9 References 1. G.D. Peng, Z. Xiong, and P.L. Chu, Photosensitivity and grating in dye-doped
polymer optical fibers. Optical Fiber Technology, 1999. 5: p. 242-251.
2. Z. Xiong, G.D. Peng, B. Wu, and P.L. Chu, Highly tunable Bragg gratings in
single mode polymer optical fibers. IEEE Photonics Technology Letters, 1999.
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43 Chapter 2: Growth Dynamics of Polymer Optical Fiber (POF) Bragg Gratings
3. A. Othonos and K. Kalli, Fiber Bragg gratings: Fundamentals and applications
in telecommunications and sensing. 1999, Boston: Artech House.
4. K.O. Hill, B. Malo, F. Bilodeau, D.C. Johnson, T.F. Morse, A. Kilian, L.
Reinhart, and O. Kyunghwan. Photosensitivity in Eu2+.AI2O3 doped core fiber:
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Optical Fiber Communications. 1991. Washington DC, US: Optical Society of
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5. M.M. Broer, R.L. Cone, and J.R. Simpson, Ultraviolet-induced distributed-
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6. F. Bilodeau, D.C. Johnson, B. Malo, K.A. Vineberg, K.O. Hill, T.F. Morse, A.
Kilian, and L. Reinhart, Ultraviolet-light photosensitivity in Er3+-Ge-doped
optical fiber. Optics Letters, 1990. 15: p. 1138-1140.
7. D.P. Hand and P.S.J. Rusell, Photoinduced refractive index changes in
germanosilicate fibers. Optics Letters, 1990: p. 102-104.
8. C. Fiori and R.A.B. Devine, Evidence for a wide continuum of polymorphs in a-
Si02. Physics Review B, 1986. 33: p. 2972-2974.
9. D. Wong, S.B. Poole, and M.G. Sceats, Stress-birefringence reduction in
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10. H. Patrick and S.L. Gilbert, Growth of Bragg gratings produced by continuous-
wave ultraviolet light in optical fiber. Optics Letters, 1993. 18: p. 1484-1486.
11. P. Niay, P. Bemage, S. Legoubin, M. Douay, W.X. Xie, J.F. Bayon, T. Georges,
M. Monerie, and B. Poumellec, Behaviour of spectral transmissions of Bragg
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gratings written in germania-doped fibres: writing and erasing experiments
using pulsed or cw uv exposure. Optics Communications, 1994. 113: p. 176-192.
12. X. Shu., Y. Liu, D. Zhao, B. Gwandu, F. Floreani, L. Zhang, and I. Bennion.
Fiber grating type dependence of temperature and strain coefficient and
application to simultaneous temperature and strain measurement, in 15th
Optical Fiber Sensors Conference. 2002. Portland, OR, USA.
13. J.L. Archambault, L. Reekie, and P.S.J. Russell, 100% reflectivity Bragg
reflectors produced in optical fibers by single excimer laser pulses. Electronics
Letters, 1993. 29: p. 453-455.
14. W.J. Tomlinson, I.P. Kaminow, E.A. Chanderross, R.L. Fork, and W.T. Silfvast,
Photoinduced refractive index increase in poly(methyl methacrylate) and its
application. Applied Physics Letters, 1970. 16: p. 486-488.
15. I.P. Kaminow, H.P. Webster, and E.A. Chandross, Poly(methylmethacrylate)
dye laser with internal diffraction grating resonator. Applied Physics Letters,
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16. M.J. Bowden, E.A. Chandross, and I.P. Kaminow, Mechanism of photo-induced
refractive index increase in Polymethyl Methacrylate. Applied Optics, 1974. 13:
p. 112-117.
17. M.J. Kopietz, M.D. Lechner, D.G. Steinmeier, J. Marotz, H. Frank, and E.
Kratzig, Light-induced refractive index change in polymethylmethacrylate
(PMMA) blocks, in Polymer phototchemisty. 1984, Elsevier Applied Science
Publishers Ltd.: England, p. 109-120.
18. A. Yariv, Optical Electronics in Modern Communications. 1997, New York:
Oxford University Press. Chapter 13.
45 Chapter 2: Growth Dynamics of Polymer Optical Fiber (POF) Bragg Gratings
19. A. Ankiewicz, Z. Wang, and G.D. Peng, Analysis of narrow bandpass filter
using coupler with Bragg grating in transmission. Optics Communications,
1998. 156: p. 27-31.
20. G.D. Peng, P.L. Chu, Z. Xiong, T.W. Whitbread, and R.P. Chaplin, Dye-doped
step-index polymer optical fiber for broadband optical amplification. Journal of
Lightwave Technology, 1996. 14: p. 2215-2223.
21. G.D. Peng, A. Latif, P.L. Chu, and R.A. Chaplin. Polymeric guest-host system
for nonlinear optical fiber, in IEEE Nonlinear Optics: Materials, Fundamentals
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22. G.D. Peng, P.L. Chu, X. Lou, and R.A. Chaplin, Fabrication and
characterization of polymer optical fibers. Journal of Electronics and Electron
Engineering, 1995. 15: p. 289-296.
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p. 2281-2293.
24. Z. Xiong, G.D. Peng, B. Wu, and P.L. Chu, Effects of the zeroth-order
diffraction of a phase mask on Bragg gratings. Journal of Lightwave
Technology, 1999. 17: p. 2361-2365.
25. G. Meltz, W.W. Morey, and W.H. Glenn, Formation of Bragg gratings in
optical fibers by a transverse holographic method. Optics Letters, 1989. 14: p.
283-285.
26. F. Ouellette, in University of Sydney Patent Cooperation Treaty AL/96/00782.
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27. T. Erdogan, Fiber grating spectra. Journal of Lightwave Technology, 1997. 15:
p. 1277-1294.
46 Chapter 2: Growth Dynamics of Polymer Optical Fiber (POF) Bragg Gratings
28. P. Yeh, Optical Waves in Layered Media. 1988, New York: A Wiley-
Interscience Publication.
29. M. Bolle, S. Lazare, M.L. Blanc, and A. Wilmes, Submircon periodic structures
produced on polymer surfaces with polarized excimer laser ultraviolet
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30. S. Lazare, M. Bolle, A. Cros, and L. Bellard, Periodic structure of polymer
surface with excimer laser radiation. Nuclear Instrumental Method Physics
Review B, 1995. 105: p. 159-163.
31. D.Y. Kim, L. Li, X.L. Jiang, V. Shivshankar, J. Kumar, and S.K. Tripathy,
Polarized induced holographic surface relief gratings on polymer films.
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33. B. Malo, D.C. Johnson, F. Bilodeau, J. Alber, and K.O. Hill, Single-pulse
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34. S.J. Mihailov and G.M. C., Periodic cladding surface structure induced when
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Applied Physics Letters, 1994. 65: p. 2639-2641.
47 Chapter 3: Thermal Characteristics of Polymer Optical Fiber (POF) Bragg gratings
Chapter 3: Thermal Characteristics of Polymer Optical
Fiber (POF) Bragg gratings
The Bragg wavelength of fiber gratings varies with temperature, so the thermal effect on fiber gratings is one of the most important characteristics of fiber Bragg grating description. Furthermore, as two distinctive types of gratings - Type I and Type II POF
Bragg gratings - have been clearly identified (see in Chapter Two), thermal behaviour study for those two gratings becomes interesting and noteworthy. Our thermal characterization investigation for Type I and Type II POF Bragg gratings includes thermal tuning, thermal sensitivity and thermal stability.
3.1 Introduction
Since thermal characterization for silica fiber Bragg gratings has been thoroughly studied, some of the results are also useful and can be the reference for the thermal investigation of POF Bragg gratings. First, a brief review on the research work about thermal characterization of silica fiber Bragg gratings will be given.
A change in the temperature of the fiber produces a shift in the Bragg wavelength due to the thermal expansion which results in the variation of grating period, and a change in the refractive index by the thermo-optic effect. The shift in Bragg wavelength,
AAb, due to temperature changes, AT, is given by [1]
3A dnoff A/L = 2(n « ——t A ——)AT (3.1) B ,IdTdT
48 Chapter 3: Thermal Characteristics of Polymer Optical Fiber (POF) Bragg gratings
where A is the grating period and neff is the refractive index of fiber core. The first term
represents the thermal expansion effect and second one represents the thermo-optic
effect. From equation (3.1) we can get the fractional Bragg wavelength change as,
Ai5//Ls =(a + ^)AT (3.2)
where a is the thermal expansion coefficient of the fiber material and E, is the thermo
optic coefficient of the fiber core. For silica, a is approximately 0.55xl0 6/°C. For
germania-doped silica fiber, the quantity for £, is approximately equal to 8.6xlO'6/°C [2].
Obviously, the index change by thermo-optic effect is the dominant factor over the thermal expansion effect for silica fiber gratings. Thus, the temperature coefficient for germania-doped silica fiber grating is about 9.15x10 6/°C. Recently, fiber grating type dependence of temperature coefficient was discovered in B/Ge doped fiber gratings [3].
(The detailed description of silica fiber Bragg grating type classification can be found in
Chapter Two.) In the hydrogen-free fiber, Type IIA fiber gratings was formed and the temperature coefficient for Type HA grating was 6.45x10 6/°C. While the temperature coefficient for Type IA gratings, which was formed in the hydrogenated fiber, was
4.74x10 6/°C. But they didn’t give out the underlying mechanisms for the difference.
Since silica fiber grating formation revolves from UV induced excitation of glass into a metastable state, the fiber grating inscription will result in long-time instability for silica fiber Bragg grating [4], which is also proved by the further studies. As a result, some decay of the silica fiber gratings occurs over time at elevated temperature. The thermal decay characteristics for silica fiber gratings are very complicated. The decay extent depends on the fiber and the grating type.
In 1995, Patrick et al [5] first reported the comparison of the thermal stability of
Type I Bragg gratings written in hydrogen-loaded and unloaded germanium-doped optical fiber, using either continuous-wave or pulsed UV laser source. All these gratings
49 Chapter 3: Thermal Characteristics of Polymer Optical Fiber (POF) Bragg gratings
exhibited the similar annealing curve with the same normalized index modulation trend,
which was a short period of rapid change followed by an extended period in which little change was observed. It was also found that the unloaded fiber gratings were more
thermally robust than hydrogen-loaded fiber gratings. For example the UV induced
index modulation in hydrogen-loaded fiber gratings was reduced by 40% after 10 hours
at 176°C, whereas it was only reduced by 4% in the unloaded fiber gratings under the
same condition. Furthermore, the decay behaviour of the fiber gratings written by pulse
UV was identical to that of gratings written by continuous source. Their annealing tests showed that the species responsible for the index change in the hydrogen-loaded fiber dissociated at lower temperatures than those in the unloaded fiber. In addition, they also revealed that the annealing of the UV-induced OH absorption in the hydrogen-loaded fiber was not correlated to the fiber grating decay.
Thermal decay for Type IIA silica fiber Bragg gratings was also examined. As we know, Type IIA grating forms after the erasure of Type I grating, and it has never been observed in hydrogenated fiber. One paper [6] compared the thermal decay results of
Type I and Type IIA gratings, written at 193 nm UV source in boron-codoped germanosilicate fiber. Type I grating decayed rapidly under heating, while Type IIA grating is stable to 300°C. Therefore, in this case Type HA grating was more stable and the Type HA grating was found to be able to operate at 300°C for more than 25 years without significant degradation.
Results of the thermal stability for Type II silica fiber grating can be found in the paper where Type II silica fiber grating was first reported [7]. In this work, Type I and
Type II gratings were fabricated in the germania-doped silica fiber at 248 nm UV pulses.
Type II grating showed much higher thermal stability. Below 800°C, no significant decay was observed to Type II gratings for a period of 24 hours. Only up to 900°C, Type
50 Chapter 3: Thermal Characteristics of Polymer Optical Fiber (POF) Bragg gratings
II gratings decayed slowly. While for Type I gratings written at energy level below the threshold were erased at 450°C. The superior thermal stability for Type II fiber gratings
makes it useful in fiber systems operating in hostile environment.
Because of the significant importance of fiber grating thermal decay characterization for its life-time prediction, two models have been proposed for analysing and predicting silica fiber Bragg grating decay. One is the “power law” model and the other is the “log time” model.
“Power law” model was first proposed by Erdogan et al [8] to predict the thermal degradation characteristics of fiber gratings written in germanium-doped silica fiber.
The model showed that the decay of the UV-induced index change could be described by a “power law” function of time with a small exponent («1). This is consistent with the rapid initial decay followed by a substantially decreasing rate of decay. The decay mechanism was one in which carriers excited during UV writing are trapped in a broad distribution of trap states and the rate of thermal depopulation was an activated function of the trap depth. Subsequently, some researches on thermal decay of silica fiber gratings have been carried out [5, 9-12]. “Power law” model seems to be able to explain the observed decay behaviour with different fitting coefficients for different fibers.
Though consensus has been given to “power law” model for fiber grating decay description, some study shows that it doesn’t apply for hydrogenated fiber gratings. For hydrogen loaded germanium-doped fiber, a new model named “log time” was proposed by Baker et al [5, 13]. This model demonstrates that for hydrogenated fiber gratings the rate of change in normalized index modulation with time is linear after some specific time and the gradient is equal for all temperatures. The “log time” model supports the argument that hydrogenated fiber shows a broadening trap population with an
51 Chapter 3: Thermal Characteristics of Polymer Optical Fiber (POF) Bragg gratings
approximate top hat shape. Baker also stated that hydrogenation reduces grating thermal
stability and thermal stabilization is required.
Despite the fact that the extensive work has been carried out in silica fiber grating
stability, some problems still remain unsolved. For example, the mechanism for grating
decay is yet unclear because of the large varieties of fiber properties and the UV laser
writing conditions. Moreover, there are also some conflicting reports. Some study
illustrates that hydrogen loaded fiber are less stable than those written in nonhydrogen
loaded fiber [5], whereas, Engan et al [10] states that this is not the case.
3.2 Set-up for thermal characterization measurement
The experimental set-up for POF Bragg grating thermal tuning and decay measurement is shown in Fig. 3.1. A small furnace was used as a heating source whose temperature was controlled by the PID method. The furnace was actually an aluminum block with the heating element and temperature sensor inserted in it. The two ends of the
POF grating samples for thermal characterization were first pigtailed to silica fibers for the convenience of measurement. Then the silica-fiber-pigtailed POF Bragg grating sample was put into the furnace for thermal test. The transmission spectra of the POF
Bragg gratings were recorded by an optical spectrum analyzer (OSA) when the thermal test was carried out. An ASE source was adopted for the POF Bragg grating characterization. The accuracy of the temperature measurement was ±0.5°C, and it took about 3 minutes for the heating system to reach the desired temperature.
For the thermal tuning characterization, the POF Bragg grating was heated from ambient temperature to 75°C with the heating step of 5°C. When the grating was heated to the desired temperature, the transmission spectra of the grating were recorded. In order to test the reversibility of the thermal tuning, the POF Bragg grating was also
52 Chapter 3: Thermal Characteristics of Polymer Optical Fiber (POF) Bragg gratings
characterized when it was cooled down from 75°C to ambient temperature. The cooling
down step was also 5°C. For the thermal decay measurement, the furnace was set at a
fixed temperature and the optical spectra of the POF Bragg grating were monitored with
time. Thus the degradation curve of the POF Bragg grating is obtained.
furnace with PID control
/ POF with gratings silica fiber
______\
Figure 3.1 Thermal characterization set-up for POF Bragg gratings.
3.3 Thermal tuning of Type I POF Bragg gratings
One key difference for polymeric material from silica glass is that its refractive index varies more rapidly with temperature. Therefore, POF Bragg gratings are expected to have much better thermal tunability and can be leveraged to produce efficient thermo- optically active optical components, such as switches, tunable lasers, or temperature sensors [14]. Since Type I and Type II POF Bragg gratings have been clearly categorized, thermal tunability for both these two kinds of POF Bragg gratings are tested and compared.
53 Chapter 3: Thermal Characteristics of Polymer Optical Fiber (POF) Bragg gratings
1571 wavelength (nm)
Figure 3.2 Transmission spectrum of Type I POF Bragg gratings for thermal tuning test.
-0.5 -
.2 -2
b -3.5 -
-4.5 - ~55ce 50°e 45a -
wavelength (nm)
Figure 3.3 Transmission spectra of Type 1 POF Bragg gratings thermally tuned at different
temperature.
The transmission spectrum of Type I POF Bragg gratings used for thermal tuning test is displayed in Fig. 3.2. The maximum rejection level in the transmission dip is
54 Chapter 3: Thermal Characteristics of Polymer Optical Fiber (POF) Bragg gratings about 4.3 dB, which corresponds to the reflectivity of 63% and the index modulation of
1.8x10 4 for the POF Bragg grating. Fig. 3.3 shows the transmission output of the POF
Bragg gratings tuned between the ambient temperature and 75°C. The reason for 75°C as the highest test temperature is that the appropriate operating temperature for PMMA
POF is below 80°C and higher temperature will cause the failure of the material. From
Fig. 3.3 we can find out that the grating strength for Type I POF Bragg grating doesn’t change much at the low temperature range, say less than 40°C. While noticeable decrease in the grating strength can be observed at high temperature, and the grating becomes weaker as the temperature increases. The reflectivity of the POF grating is decreased from 63.5% at 18°C to 56.3% at 75°C. This implies that the thermal stability for Type I POF Bragg gratings is a big issue and is worth investigating further.
1572 o Heating-up 1571 + Cooling-down |1570 - - linear regression ^ 1569 a £1 |fs 1568 r*; ai % 1567 | 1566 A o* 1565 a £ 1564 A. '6 1563 1562 10 20 30 40 50 60 70 80 Temperature (°C)
Figure 3.4 Bragg wavelength of Type I POF Bragg gratings as function of temperature,
revealing a highly linear dependence and no thermal hysteresis effect.
55 Chapter 3: Thermal Characteristics of Polymer Optical Fiber (POF) Bragg gratings
Fig. 3.4 shows the Bragg wavelength of Type I POF Bragg gratings as the function
of temperature. There are two sets of data in the figure. One is the measured Bragg
wavelength when the grating was heated up from room temperature to 75°C, while the
other is obtained when the grating was cooled down. The dashed line is the linear
regression according to the heating-up data. It can be seen that almost 10 nm tuning
range can be achieved only with the temperature variation of 55°C, which is larger than the few nanometers achieved in silica fiber grating by the several hundred degree temperature variation [15]. This can also indicate that POF Bragg gratings have higher thermal sensitivity than silica fiber Bragg gratings.
Furthermore, the two sets of data in Fig. 3.4 seem to coincide with each other quite well, which reveals that there is no hysteresis effect for the thermal tuning of Type I
POF Bragg gratings. This shows that thermal means can be used to tune the Bragg wavelength of Type I POF Bragg gratings in practice. The linear regression line in the figure indicates the high linearity of the Bragg wavelength dependence on the temperature within the whole tuning range, indicating that the control for this tuning will be very simple. While for silica fiber gratings, though it can be thermally tuned within hundreds of degree, the temperature response of the Bragg wavelength will become slightly nonlinear when the temperature is higher than 150°C [15, 16].
The repeatability of the thermal tuning is also tested. Ten cycles of repeatable heating-up and cooling-down process for the Type I POF Bragg gratings were carried out. The Bragg wavelength at different tuning temperature seems to coincide quite well each time. Consequently, the thermal tuning for Type I POF Bragg gratings shows sound repeatability as well.
56 Chapter 3: Thermal Characteristics of Polymer Optical Fiber (POF) Bragg gratings
Therefore, Type I POF Bragg grating shows excellent tunability with large tuning
range, no thermal hysteresis and high linearity of temperature dependence of the Bragg
wavelength.
3.4 Thermal sensitivity of Type I POF Bragg gratings
Form the thermal tuning experiment, we can tell that Type I POF Bragg gratings
have higher thermal sensitivity than silica fiber Bragg gratings. In the followings, we
will determine the thermal sensitivity of Type I POF Bragg gratings by the data in Fig.
3.4.
As mentioned in Section 3.1, the shift in fiber Bragg wavelength by the temperature
variation is due to the thermal expansion and thermo-optic induced change in the refractive index. The Bragg wavelength shift can be expressed in equation (3.2). In order to increase the accuracy, we make linear regression according to both the heating-up and cooling-down data in Fig. 3.4 to determine the temperature coefficient of Type I POF
Bragg gratings. The linear relation is expressed as,
Ab = 1573.6-0.1452*7 (3.3) where ?lb is the Bragg wavelength at tuning temperature of T(°C). Consequently, the
Bragg wavelength change ratio is,
A/ls /ig„ =-9.23x10_5A7’ . (3.4)
Thus, the temperature coefficient of Type I POF Bragg gratings is -9.23xlO’5/°C, which is more than 10 times larger than that of silica fiber Bragg gratings. As a result, the thermal sensitivity of Type I POF Bragg gratings is 10 times of silica fiber Bragg gratings.
The other noteworthy phenomenon is that Bragg wavelength for POF Bragg gratings blue shifts when the temperature of the POF increases. Therefore, its
57 Chapter 3: Thermal Characteristics of Polymer Optical Fiber (POF) Bragg gratings
temperature coefficient is negative. Quite different from silica, refractive index for
PMMA decreases more rapidly with the increase of temperature. As expressed in
equation (3.2), the temperature coefficient is the combination effects of thermal
expansion and thermal index change. Hence, the thermo-optic effect is also the dominant
effect for POF Bragg gratings. For pure bulk PMMA material, the thermal coefficient a
is about 5xlO'5/°C, while thermal index change rate dn/dT is -I.lxl0'4/°C [17]. By
using these data, the calculated temperature coefficient of the PMMA POF Bragg gratings is -2.5xlO‘5/°C, which is the same order as our experimental data. There are two factors that might be responsible for the difference between the experimental and calculated temperature coefficient values. First, due to the dopant in the fiber core, its thermal expansion and thermal index change rate of the PMMA-based POF in our experiment will be different from the pure PMMA bulk material. Second, even with the same composition, some characteristics, i.e. thermal expansion or thermal index change rate, for the POF will be different from those of bulk material. But by far, no accurate data of thermal expansion coefficient and thermal index change rate of PMMA POF are available.
The accuracy for the temperature measurement in the thermal tuning test is ±0.5°C and the resolution for Bragg wavelength measurement is 0.1 nm, which will have little effects in the temperature coefficient determination. Therefore, the error bars for temperature and wavelength weren’t shown in the above figures.
3.5 Thermal tuning and thermal sensitivity of Type II POF Bragg gratings
Apart from the thermal tuning characterization for Type I POF Bragg grating, Type
II POF Bragg grating is also tested and compared. Fig. 3.5 shows the transmission
58 Chapter 3: Thermal Characteristics of Polymer Optical Fiber (POF) Bragg gratings
spectrum of the Type II POF Bragg gratings for thermal tuning investigation. The
maximum transmission dip is about 13 dB, denoting the reflectivity of 95% and the
index modulation of 3.6x10‘4 for the POF grating. In the transmission spectrum,
noticeable transmission losses at the wavelengths shorter than Bragg wavelength can be
observed, which is the characteristics of Type II POF Bragg gratings discussed in
Chapter Two. Fig.3.6 illustrates the transmission spectra of Type II POF Bragg gratings
tuned at different temperatures. Different from Type I POF grating, no obvious
reduction in the grating strength is observed when Type II POF grating is submitted to
high temperature. This indicates that Type II POF Bragg grating is more thermally
stable.
-12 -
1573 1575 wavelength (nm)
Figure 3.5 Transmission spectrum of Type II POF grating for thermal tuning test.
Fig. 3.7 displays the Bragg wavelength shift for Type II POF Bragg grating as the function of the tuning temperature, similar to Fig. 3.4 for Type I POF Bragg grating.
Rather good thermal tunability is found in Type II POF Bragg grating as well: large tuning range, no thermal hysteresis effect and high linearity. Ten cycles of heating-up
59 Chapter 3: Thermal Characteristics of Polymer Optical Fiber (POF) Bragg gratings
and cooling-down test were also carried out on Type II POF Bragg gratings, and it is
shown that the repeatability of the thermal tuning for Type II POF Bragg grating is as good as Type I POF Bragg grating.
75°C 70°C65°C 60°c 55°c 50°C 45° 40oC35oC30oC 25°c 18.5°C
1569 1571 wavelength (nm)
Figure 3.6 Transmission spectra of Type II POF Bragg gratings thermally tuned at different
temperature.
1576 o o Heating-up
1574 + Cooling-down Cs 9s - - Linear regression
OX) 1572 c 9s
> 1570 'Qs £ ' s6 OX) OX) uCO CQ 1568 ¥
y = -0.1562x + 1578 1566 10 20 30 40 50 60 70 80 Temperature (°C)
Figure 3.7 Bragg wavelength of Type II POF Bragg gratings as a function of temperature.
60 Chapter 3: Thermal Characteristics of Polymer Optical Fiber (POF) Bragg gratings
In order to compare its temperature sensitivity to that of Type I POF Bragg grating,
the temperature coefficient for Type II POF Bragg grating is also determined from the
linear regression of both the heating-up and cooling-down data in Fig. 3.7. The linear
expression for the regression is given as,
Ab =1577.9-0.14777. (3.5)
Then, the temperature coefficient for Type II POF Bragg grating is -9.36xl0"5/°C,
which is the about same as Type I POF Bragg grating. As a result, the temperature
sensitivity for Type II POF Bragg grating is the same as that of Type I POF Bragg grating, which is about ten times higher than that of silica fiber Bragg gratings.
3.6 Thermal decay of Type I & II POF Bragg gratings
From the above thermal tuning test, some hints were found to demonstrate that the thermal stability for Type I POF Bragg grating is different from Type II POF Bragg gratings. In this section, the thermal decay characterization was accomplished for both
Type I and Type II POF Bragg grating. The thermal degradation behaviour of POF
Bragg gratings was monitored at 45°C, 55°C and 65°C. In the thermal decay experiment, when the spectrum analyser swept through the point of minimum transmission, the time and the minimum transmission T^n were recorded. The starting time, t=0, was taken as the instant when the grating was placed into the furnace. Type I
POF Bragg gratings for thermal decay examination had an initial transmission dip of about 4 dB (60% reflectivity and 1.73x10 4 index modulation). The initial transmission dip for Type II POF Bragg grating for the test was about 15 dB (96.8% reflectivity and
4xl0~4 index modulation).
Fig. 3.8 shows the peak reflectivity degradation of Type I POF Bragg gratings against time. The peak reflectivity is calculated from the Tmin recorded. From the graph,
61 Chapter 3: Thermal Characteristics of Polymer Optical Fiber (POF) Bragg gratings
it can be seen that a rapid decay of the peak reflectivity is followed by a substantially
decreasing rate of decay, which resembles the decay behaviour of Type I hydrogen free
silica fiber Bragg gratings.
©■
Time (hrs)
Figure 3.8 Reflectivity decay of Type I POF Bragg gratings at 55°C.
Due to the similarities, we also use the integrated coupling constant (ICC) as a measure of grating strength instead of using the maximum reflectivity or minimum transmission, which has been adopted for Type I silica fiber gratings description [18].
ICC is defined as,
ICC = tanh _1 (1 - 7^ )1/2. (3.6) where Tmjn is the minimum transmission of the fiber Bragg grating. As discussed in
Chapter Two, the index modulation An and the peak reflectivity rmin of a single mode uniform grating can be expressed as equation (2.24). Therefore, it is clearly proved that
ICC value is proportional to the index modulation of the fiber Bragg gratings, even for the case of a nonuniform gratings [19]. For UV-induced Bragg gratings, equation (3.6) is
62 Chapter 3: Thermal Characteristics of Polymer Optical Fiber (POF) Bragg gratings
not rigorous because of the inherent effective chirp resulting from the non-zero,
nonmodulated portion of the UV-induced index change, but it is an excellent
approximation for the grating strength [8]. For convenience, the quantity T| is defined to
be equal to the ICC normalized to its initial value at t=0. Because ICC is proportional to
grating induced index change, r\ can also be thought of as the normalized index change.
0.6 — .22 0.5 -
0.1 -
time (min)
Figure 3.9 Normalized integrated coupling coefficient versus decay time for Type I POF
Bragg gratings at 45°C, 55°C and 65°C.
Fig. 3.9 illustrates the thermal decay for Type I POF Bragg gratings by using normalized ICC at 45°C, 55°C and 65°C. It is apparent that Type I POF Bragg gratings exhibits the same decay behaviour at 55°C and 65°C as that at 45°C shown in Fig. 3.8.
Therefore, the thermal decay behaviour of Type I POF Bragg gratings is similar to Type
I nonhydrogenated silica fiber Bragg grating: an initial rapid decrease in normalized index change followed by a progressively diminishing but nonzero rate of change. This similar behaviour indicates that Type I POF Bragg grating may also follow the “power-
63 Chapter 3: Thermal Characteristics of Polymer Optical Fiber (POF) Bragg gratings
law” dependence on time as silica fiber grating does. But the data in Fig. 3.9 doesn’t fit
the same function in the “power law “ model for Type I unloaded silica fiber Bragg
gratings [8], which is expressed as,
_1___ (3.7) 1 + Ata
where the factor A and exponent a are both temperature dependant. In the end, the
regression function for the decay data in Fig. 3.9 is determined as,
1 (3.8) Ata
where the factor A and exponent a both depend on temperature as well. A and a are
dimensionless, and t is the time value normalized to 1 minute to keep dimensions
consistent. The regression curve for each temperature is also plotted in Fig. 3.9 to
compare with the experimental data. The mean error of the fit to the data and the
variance of the error are shown in Table 3.1. As T] is the normalized index modulation
change, the mean error is defined as the difference the experimental data and the fitted
value multiplied by 100. It is seen that the regression errors are very small; therefore, the
regression function fits well with the experimental data.
Table 3.1 Mean error and error variance of the regression.
45°C 55°C 65°C
Mean error (%) 0.005 0.028 0.019
Error variance (%) 0.46 1.25 2.28
It is noticed that the data at t=0, i.e. the original data before thermal decay starts, do not agree with the equation (3.8) function. Thus the function can only be used to describe Type I POF Bragg grating decay behavior after the decay process has progressed for some time. In our experiment, the first decay data were obtained at t=5,
64 Chapter 3: Thermal Characteristics of Polymer Optical Fiber (POF) Bragg gratings i.e. 5 minutes after the decay test started. The regression function for the different temperature are expressed as ij = , at 45°C, 77 =------at 55°C 1.0357 1.0137 0.0071
1 and/; = at 65°C, respectively. 0.5657 0.2408
320 325 330 335 340 temperature (K) Figure 3.10 Temperature dependence of parameter A.
320 325 330 335 340 temperature (K)
Figure 3.11 Temperature dependence of parameter a.
65 Chapter 3: Thermal Characteristics of Polymer Optical Fiber (POF) Bragg gratings
The attempt to determine the temperature dependence function for A and a was
made. It was found out that they don’t follow the “power law” model for
nonhydrogenated Type I silica fiber Bragg gratings. As shown in Fig 3.10 and Fig 3.11,
parameter A plotted on a logarithmic scale doesn’t vary linearly with temperature and a
doesn’t vary linearly with temperature. Neither does the decay curve for Type I POF
Bragg gratings fit the “log time” model for hydrogenated silica fiber Bragg gratings.
More work needs to be carried out in order to determine A and a temperature function
in the decay curve function of Type I POF Bragg gratings. When the relation for A
versus T and a versus T is established, it is then possible to predict the stability of the
Type I POF grating for any given operating temperature and the life-time for the Type I
POF Bragg gratings can be estimated.
time (hrs)
Figure 3.12 Decay curve for Type II POF Bragg gratings at 65°C.
The thermal decay behavior for Type II POF Bragg gratings was also studied, and it is found to be much more stable than Type I POF Bragg gratings. Fig. 3.12 shows the decay curve of Type II POF Bragg gratings at 65°C. It is striking to notice that no
66 Chapter 3: Thermal Characteristics of Polymer Optical Fiber (POF) Bragg gratings
noticeable decay is observed for Type II POF Bragg gratings at 65°C. The reason for its
sound thermal stability is that the index change for Type II POF grating is caused by the
periodic damages pattern at the core/cladding boundaries. Temperature below 80°C will
have little effect on this periodic structure. The excellent stability for Type II POF Bragg
gratings makes it attractive in some high temperature environment applications.
3.7 Stabilization of Type I POF Bragg gratings
Since Type I POF Bragg gratings decay with time, the stabilization process
becomes necessary. As we found the decay value for Type I POF Bragg gratings is not
linear with time; therefore, a pre-annealing at higher temperature can be adopted to
remove the less stable index change so that the stability of the Type I POF Bragg grating
will be increased for subsequent use. The appropriate stabilization conditions are also determined by operating conditions and system tolerance. It is noted in Fig. 3.9, at 65 °C most of the decay of the gratings happens in the first three hours and doesn’t offer substantial changes beyond that time.
1.1 ___ with stabilizatin 1 IE0a©O©^)-o-Q -zq ; : Zq O annealing 0.9 • • • 1 without staBilization y o.7 ■§N 0.6 ^ 0.5 e 6 0.4
0.2 0.1 0 0 5 10 15 20 25 30 time (hrs)
Figure 3.13 Decay of Type I POF Brag gratings with and without stabilization annealing.
67 Chapter 3: Thermal Characteristics of Polymer Optical Fiber (POF) Bragg gratings
As a result, we pre-anneal the grating at 65 °C for three hours in order to stabilize the
grating. Its decay characterization at 55 °C was carried out and the decay curve is shown
in Fig. 3.13. In Fig. 3.13 the decay curve for grating without stabilization process is also
shown for comparison. It is apparent that the pre-annealing treatment increases the
stability of Type I POF Bragg grating. For example, the grating with stabilization annealing will only decay to 95% of its as written value after 25 hr, while the one without treatment will decay to 83%. In addition, no rapid decay behavior is observed for the Type I POF Bragg gratings with stabilization annealing treatment.
3.8 Discussion on the mechanism of Type I POF Bragg gratings decay
As the formation mechanism for Type I POF Bragg gratings is still not clear, it makes it even harder to uncover the mechanism for its decay behavior. But the author thinks that the thermal decay for Type I POF Bragg gratings might be linked to the internal stress relaxation in POF by annealing. Internal stress induced by POF grating fabrication can result in index change and the stress relaxation by annealing afterwards may reduce the index change. The index change with stress for x-polarized light is characterized by the stress-optic coefficients ci and C2 as [20],
nx =n0 -cx(7x -c2(oy +crz) (3.9) where no is the index of the unstressed material and ox, ay and gz are the three orthogonal principle stresses along the respective axes. Substitution of the stresses into equation (3.10) gives the magnitude of index change due to the thermo-elastic stress relaxation in optical fibers. The measurement of stress modifications in silica fiber
Bragg gratings showed that the tension in the core of single-mode germanosilicate fibers is greatly increased during Bragg grating formation [21]. The investigation on the stress modification in POF induced by Bragg grating formation and the subsequent annealing
68 Chapter 3: Thermal Characteristics of Polymer Optical Fiber (POF) Bragg gratings
should also be carried out. Philip Ji did some study on the transverse birefringence in
POF [22]. It is pointed out that the internal stress in POF contributes to the transverse
birefringence. His research about the annealing effects on the birefringence shows that
the birefringence decreases significantly during the first 2.5 hrs with diminishing effects
with further annealing period, which is similar to the thermal decay behavior of Type I
POF Bragg grating. This similarity can be an evidence to support the inference that the
thermal decay for Type I POF Bragg grating is relevant to internal stress relaxation. As
we pointed out, the work on the internal stress change in POF by Type I Bragg grating formation and the subsequent annealing needs future work. This will help for the decay mechanism study for Type I POF Bragg grating.
3.9 Conclusions
From the thermal tuning results, POF Bragg gratings have shown much better tunability than silica fiber Bragg gratings. Almost 10 nm tuning range has been achieved with only 55°C temperature variation. Furthermore, the high linearity and the absence of thermal hysteresis make the thermal tuning of POF Bragg gratings easy to be implemented and controlled. Moreover, the temperature sensitivity for POF Bragg gratings is much higher than that of silica fiber Bragg gratings. Type I and Type II POF
Bragg gratings are found to be equally thermal sensitive and the temperature coefficient value for POF Bragg gratings is 10 times larger than that of silica fiber gratings. Further analysis indicates that the thermo-optic effect plays the dominant role for the thermal tuning of POF Bragg gratings. Since the refractive index decreases with temperature increase for polymer material, Bragg wavelength of POF Bragg gratings blue shifts as the temperature increases.
69 Chapter 3: Thermal Characteristics of Polymer Optical Fiber (POF) Bragg gratings
Thermal decay study for POF Bragg gratings indicates that Type II POF Bragg gratings is much more stable than Type I POF Bragg gratings. Type II POF Bragg gratings is very stable and no noticeable decay can be observed at 65°C. While for Type
I POF Bragg gratings, a rapid decay followed by a substantially decreasing rate of decay is found. Therefore, the stabilization process for Type I POF Bragg gratings is necessary. It is shown that pre-annealing at escalating temperature will increase the stability of Type I POF Bragg gratings for subsequent use. However, the decay behaviour for Type I POF Bragg gratings doesn’t follow either “power law” model for nonhydrogenated silica fiber Bragg gratings or the “log time” model for hydrogenated silica fiber gratings. Further work on modelling and mechanism study of decay behaviour for Type I POF Bragg gratings needs to be done.
3.10 References
1. A. Othonos, Fiber Bragg gratings. Review of scientific instruments, 1997. 68: p.
4309-4341.
2. W.W. Morey, G. Meltz, and W.H. Glenn, Fiber optic Bragg grating sensors.
SPIE Fiber Optic and Laser Sensors, 1989. 1169: p. 89-107.
3. X. Shu, Y. Liu, D. Zhao, B. Gwandu, F. Floreani, L. Zhang, and I. Bennion.
Fiber grating type dependence of temperature and strain coefficients and
application to simultaneous temperature and strain measurement, in 15th
Optical Fiber Sensors Conference. 2002. Portland, OR, USA.
4. S. Kannan, J.Z.Y. Gao, and P.J. Lemaire, Thermal stability analysis of UV-
induced fiber Bragg gratings. Journal of Lightwave Technology, 1997. 15: p.
1478-1483.
70 Chapter 3: Thermal Characteristics of Polymer Optical Fiber (POF) Bragg gratings
5. H. Patrick and S.L. Gilbert, Annealing of Bragg gratings in hydrogen-loaded
optical fiber. Journal of Applied Physics, 1995. 78: p. 2940-2945.
6. L. Dong and W.F. Liu, Thermal decay of fiber Bragg gratings of positive and
negative index change fanned at 193 nm in a boron-codoped germanosilicate
fiber. Applied Optics, 1997. 36: p. 8222-8226.
7. J.L. Archambault, L. Reekie, and P.S. Russell, 100% reflectivity Bragg reflectors
produced in optical fibers by single excimer laser pulses. Electronics Letters,
1993. 29: p. 453-455.
8. T. Erdogan, V. Mizrahi, P.J. Lemaire, and D. Monroe, Decay of ultraviolet-
induced fiber Bragg gratings. Journal of Applied Physics, 1994. 76: p. 73-80.
9. D.L. Williams and R.P. Smith, Accelerated lifetime tests on UV written intra
core gratings in boron germania codoped silica fiber. Electronics Letters, 1995.
31: p. 2120-2121.
10. R.J. Egan, H.G. Inglis, P. Hill, P.A. Krung, and F. Ouellette. Effects of hydrogen
loading and grating strength on the thermal stability of fiber Bragg gratings, in
Optical Fiber Communications'96. 1996.
11. S. Kannan and P.J. Lemaire. Thermal reliability of Bragg gratings written in
hydro gen-sensitized fibers, in Optical Fiber Communications'96. 1996.
12. I. Riant, S. Borne, and O. Sansonetti. Dependence of fiber Bragg grating thermal
stability on grating fabrication process, in Optical Fiber Communications'96.
1996.
13. S.R. Baker, H.N. Rourke, V. Baker, and D. Goodchild, Thermal decay of fiber
Bragg gratings written in boron and germanium codoped silica fiber. Journal of
Lightwave Technology, 1997. 15: p. 1470-1477.
71 Chapter 3: Thermal Characteristics of Polymer Optical Fiber (POF) Bragg gratings
14. L. Eldada and L.W. Shachlette, Advances in polymer integrated optics. IEEE
Journal of Selected Topics in Quantum Electronics, 2000. 6: p. 54-68.
15. K.O. Hill and G. Meltz, Fiber Bragg grating technology fundamentals and
overxnew. Journal of Lightwave Technology, 1997. 15: p. 1263-1276.
16. G. Meltz and W.W. Morey, Bragg grating formation and germanosilicate fiber
photosensitivity. SPLE International Workshop on Photoinduced Self-
Organization Effects in Optical Fiber, 1991. 1516: p. 185-199.
17. J. Brandup, E.H. Immergut, E.A. Grulke, A. Abe, and D.R. Bloch, Polymer
Handbook. 1999, New York: Wiley.
18. T. Erdogan, V. Mizrahi, P.J. Leaire, and D. Monroe. Decay of UV-induced fiber
Bragg gratings, in Optical Fibre Communications'94. 1994. San Jose, US.
19. H. Kogelnik, Filter response of nonuniform almost-periodic structures. The Bell
System Technical Journal, 1976. 55: p. 109-127.
20. A. Kuske and G. Robertson, Photoelastic stress analysis. 1974, London: John
Wiley & Sons.
21. P.Y. Fonjallaz, H.G. Limberger, R.P. Salathe, F. Cochet, and B. Leuenberger,
Tension increase correlated to refractive index change in fibers containing UV-
written Bragg gratings. Optics Letters, 1995. 20: p. 1346-1348.
22. P. Ji, Transverse birefringence in polymer optical fiber due to fiber drawing, ME
thesis. 2002, University of New South Wales: Sydney, Australia.
72 Chapter 4: Strain Characteristics of Polymer Optical Fiber (POF) Bragg gratings
Chapter 4: Strain Characteristics of Polymer Optical Fiber
(POF) Bragg Gratings
Bragg wavelength shifts with the external strain and the mechanical stress becomes
one of the most popular methods for the tuning of fiber Bragg gratings. Because of the
promising application in optical communication systems and strain sensing fields,
characterization of the strain effects on fibre Bragg grating turns out to be very
important and attractive. The systematic investigation on the strain characterization of
POF Bragg gratings, such as strain sensitivity and tuning tunability, will be reported in the following part. The research on the strain characterization of POF Bragg gratings is
very significant for its further applications.
4.1 Introduction
When the fiber is strained, the Bragg wavelength varies due to the change in the grating spacing and the photoelastic induced change in the refractive index. The shift in the Bragg grating wavelength due to the strain change is given by [1],
dn~ 3 a Aie=2(A-f- + «eJ— )&L, (4.1) oL oL where A is the grating period, neff is the effective refractive index of the fiber core, L is the length of stressed length, and AL is the axial displacement. The first term in the above equation represents the elastic stress induced index change (the photoelastic effect) and the second term is about the change in grating period. By using the transversal strain (st) and axial strain (ez), the above strain induced Bragg wavelength shift may be expressed as [2],
73 Chapter 4: Strain Characteristics of Polymer Optical Fiber (POF) Bragg gratings
AAb Ab {£z [p\ j • £t + pl2 ' (£t + £z)]} (4.2)
where pij are the photoelastic constants of the strain optic tensor. The axial strain £z is
defined as,
ALZ £. - (4.3)
While the transversal strain 8t is expressed by Poisson ratio v as,
£t=~V£z. (4.4)
By further simplifying of equation (4.2), the Bragg wavelength shift ratio can be found
as [3],
A A, a-p.ye. (4.4)
where pe is an effective photoelastic constant defined as,
neff Pc = —[P12 ~v(Pn +Pn)h (4.5)
For a typical germanosilicate optical fiber, pn=0.113, pi2=0.252, v=0.16, and nefF=1.482
[1], the effective photoelastic constant is calculated to be 0.22. Therefore, the strain
coefficient for silica fiber Bragg gratings is 0.78. Thus, the expected strain sensitivity at
1550 nm for the silica fiber Bragg grating is 1.2 pm/ps. The simplest technique for fibre
grating strain tuning is stretching the fiber Bragg gratings. Bragg wavelength tuning by
tensile stress is limited by the fiber strength. For silica fiber, Young’s modulus is 7xl010
Pa and it is proof tested at 690xl010 Pa; therefore, a maximum strain of roughly 1% can be applied without degrading the fiber strength and eventually breaking the fiber [4].
Consequently, silica fiber Bragg gratings can only be tuned within several nanometers by tensile stress.
74 Chapter 4: Strain Characteristics of Polymer Optical Fiber (POF) Bragg gratings
The low tuning range for silica fiber Bragg gratings by means of tensile stress is due
to the fiber strength limitation associated with tensile stress. It can be relieved when compressive stress is implemented, because silica is 23 times stronger under compression than under tension [4], Ball et al first proposed to use the uniform compression to tune silica fiber gratings and a tuning range of 32 nm Bragg wavelength
shift was reported [4]. The tuning speed was limited up to a few seconds. Several years
later, Iocco et al [5, 6] improved Ball’s work by adding a second piezoelectric actuator, a maximum Bragg wavelength shift of 45 nm was achieved. Furthermore, the tuning speed was increased up to 21 nm/ms. Though a broad tuning range and a high tuning speed can be obtained by compressive tuning on silica fiber Bragg gratings, the reproducibility and reversibility of the tuning is very low [7]. In addition, these compressive tuning demonstrations typically have required both complicated and bulky components to perform the tuning action.
In comparison with silica optical fiber, polymer optical fiber (POF) is advantageous with larger breakdown strain and high stress sensitivity due to its much smaller Young’s modulus [8]. Therefore, the strain tunability for POF Bragg gratings is expected to be better than that of silica fiber Bragg gratings. The previous study on the strain tuning of
POF Bragg gratings from our group demonstrated that POF Bragg gratings can be tuned over a much larger range than that of silica fiber Bragg gratings by the tensile strain [9], but this was just a simple start. In the following section, the strain characterization of
POF Bragg gratings, by simple tension method is carried out. These include the strain sensitivity, the maximum tuning range, the reversibility and reproducibility of the tuning.
75 Chapter 4: Strain Characteristics of Polymer Optical Fiber (POF) Bragg gratings
4.2 Experimental method for strain tuning characterization
The strain tuning of POF Bragg gratings was investigated by the mechanical
stretching, whose basic concept is shown in Fig. 4.1. As we mentioned the preparation
of POF sample for Bragg gratings fabrication in Chapter Two, before the grating was
fabricated, the POF was mounted on a glass slide with both ends glued to it. The slide
was then cut to break apart in the middle using a diamond saw after the grating was
created. Subsequently, the two parts were glued and fixed on tow steel of blocks, which
were connected to two micropositioners. One of the micropositioners was fixed and the
other one can be moved longitudinally so that an axial strain can be applied to the POF
Bragg grating sample. The axial strain values were estimated according to equation (4.3)
by dividing the fiber longitudinal extension displacement, AL*, by the total length of
POF, Lz, shown in Fig. 4.1. The longitudinal displacement accuracy of the moving
micropositioner was 0.01 mm. The applied strain was applied by manually moving the
micropositioner; therefore, the loading speed was only about 1 me/min. The reflection
spectra of the POF Bragg gratings were recorded as the strain was applied.
Bragg gratings POf
H-HH-HH-l-m-
vh\\\\\\\\\
ALz i «—h U
Figure 4.1 Scheme of mechanical tensile tuning on POF Bragg gratings.
4.3 Strain sensitivity of Type I POF Bragg gratings
The reflectivity of the Type I POF Bragg gratings used for strain characterization is about 40% and the reflection output of the POF Bragg gratings under different tensile
76 Chapter 4: Strain Characteristics of Polymer Optical Fiber (POF) Bragg gratings
strain is shown in Fig. 4.2. The first spectrum corresponds to the original reflection
before any tension is applied, with the Bragg wavelength at about 1536.4 nm. The
following ones are the reflection spectra of Type I POF Bragg gratings under different
strain values. In Fig. 4.2 the maximum strain is 3.61% and corresponding Bragg
wavelength is 1589.1 nm. This means the POF Bragg gratings can be easily tuned over
50 nm by simple tension. It can be also seen from Fig. 4.2 that there is no obvious
changes in reflection level and the spectrum shape when the strain value is less than
2.22%. When strain is greater than 2.22%, the reflection level becomes smaller and the
line width of the spectrum also becomes wider.
6959681
1530 1540 1550 1560 1570 1580 1590 1600 wavelength (nm)
Figure 4.2 Reflection spectra of POF Bragg gratings by mechanical tensile stress tuning.
Fig. 4.3 displays the Bragg wavelengths of Type I POF Bragg gratings under different tensile strain. The circle symbol is the experimentally measured data and the dash line is the linear regression result. A very high linearity of the Bragg wavelength
77 Chapter 4: Strain Characteristics of Polymer Optical Fiber (POF) Bragg gratings
shift over the whole strain tuning range, indicated by the regression R-squared value of
0.9995, has been achieved. The regression line in Fig. 4.3 is expressed as,
Ab = 1535.6 + 1.464 * £(milli - strain) (4.6)
where Xb is the Bragg wavelength of POF Bragg gratings under strain value of 8.
Therefore, the Bragg wavelength change ratio is,
AAb/ab = 0.953 *£. (4.7)
Therefore, the strain coefficient of POF Bragg gratings is 0.95, which is larger than the
value for silica fiber Bragg gratings. The photoelastic constant for Type I POF Bragg
gratings is deduced to be 0.05. This indicates that the photoelastic effect in POF Bragg
gratings is insignificant on the Bragg wavelength shift in strain tuning. As a result, the
strain sensitivity for POF Bragg gratings should be type independent.
0 0.5 1 1.5 2 2.5 3 3.5 4 strain (%)
Fig. 4.3 Bragg wavelength of POF Bragg gratings under different tensile strain.
4.4 Strain tunability of Type I POF Bragg gratings
Reproducibility and reversibility are the two important prospects to measure the strain tunability of fiber Bragg gratings. Reproducibility is to check the grating strength
78 Chapter 4: Strain Characteristics of Polymer Optical Fiber (POF) Bragg gratings
variation with the different applied strain, while the reversibility is to examine the Bragg
wavelength shift during the loading-unloading process. In the loading-unloading
experiment, the tensile strain was gradually applied to the POF Bragg gratings up to
some value (loading process) and then gradually released to the zero stress state
(unloading process). During the course, the reflection spectra of the POF Bragg gratings
at each step were monitored and the Bragg wavelengths and the reflection peak level
were recorded.
0 -1
9 -aS "z
« -3 >■ J QJ £6 -4 a a> a -5 c .2 . a>w "6 C 7 ua» -/ -8 -9 0 0.5 1 1.5 2 2.5 3 3.5 4 strain (%)
Figure 4.4 Reflection peak level of POF Bragg gratings during tensile strain tuning.
The variation of the reflection peak level for Type I POF Bragg gratings at different tensile strain is recorded and is shown in Fig. 4.4. Below the strain value of 2.22%, which corresponds to Bragg wavelength shift of 32 nm, the reflection peak level only changes by less than 0.3 dB. With further increase of the strain value, the reflection will decrease continuously. At the tuning strain of 3.61%, corresponding to Bragg wavelength shift of 52 nm, the reflection level decreases by about 4 dB. Further cyclic loading-unloading test with the maximum strain of 2.22% shows that the reflection peak
79 Chapter 4: Strain Characteristics of Polymer Optical Fiber (POF) Bragg gratings level variation of Type I POF Bragg gratings is less than 0.5 dB. Therefore, Type I POF
Bragg gratings have excellent reproducibility when the tuning strain is less than 2.22%.
The reversibility of the strain tuning with maximum strain up to 2.22% was also examined. The Bragg wavelength variation at both loading and unloading process is shown in Fig. 4.5. The circle data in the figure are the Bragg wavelengths obtained during the loading process, while the cross symbols represent the Bragg wavelength obtained in the unloading process. From the graph, it is obvious that the Bragg wavelength variation curve for the loading process is coincident excellently with that of unloading process, thus indicating the good reversibility for the tuning. The linear regression is also made to the data, showing a very high linear relation for the Bragg wavelength and the strain. The regression R-squared value almost reaches to 1. From the data in Fig. 4.5, the strain coefficient is also calculated, which is 0.94. This is about the same as what we got in Section 4.3.
1570
1565 O loading -,o X unloading ,BT S' 1560 - * linear regression a 5 1555 ja OX ,8' sat ■§> 1550 ,8 co gr ox«1545 CO ,S5 £ 1540 gr 1535 55' 1530 0 0.5 1 1.5 2 2.5 strain (%)
Figure 4.5 Bragg wavelengths under reversible tensile strain tuning test, where the maximum
strain is 2.22%.
80 Chapter 4: Strain Characteristics of Polymer Optical Fiber (POF) Bragg gratings
The reversibility of the strain tuning with the maximum strain up to 3.61% and
2.78% is tested as well. The Bragg wavelength of Type I POF Bragg gratings recorded
during the reversibility experiment are displayed in Fig. 4.6 and Fig 4.7, which are
similar to Fig. 4.5. Corresponding to Fig. 4.6, the maximum strain value is 3.61%, where
52 nm Bragg wavelength shift is achieved. While for the case in Fig. 4.7, the maximum
strain value is about 2.78%, where 40 nm Bragg wavelength shift is achieved.
It can be found for both cases, though the highly linear strain dependence is present
for the loading curve, the loading curve is not coincident completely with the unloading
curve. This indicates that there is hysteresis, though not much, in the strain tuning when
the tuning range is over 40 nm. Therefore, the strain tuning for Type I POF Bragg
gratings with tuning range over 40 nm is not practicable for those applications that
require high accuracy of Bragg wavelength control.
As a result, POF Bragg gratings show sound reproducibility and reversibility in the strain tuning with the Bragg wavelength tuning range of 32 nm.
1600
O loading 1590 X unloading 8'' - - linear regression | 1580 JS jS If 1570 QJ 73 X, X, , © | 1560 X. - ou -o OX) 2 1550 QQ 1540 _____
1530 I r 0 0.5 1 1.5 2 2.5 3 3.5 4 strain (%)
Figure 4.6 Bragg wavelength of POF Bragg gratings under reversible strain tuning test, where
the maximum strain value is 3.61%.
81 Chapter 4: Strain Characteristics of Polymer Optical Fiber (POF) Bragg gratings
1580 o loading » P X unloading O 1570 c - - linear regression ■S'-* —
JS X wd 1560 X*------s X — x- & % 1550 ■ X WO WO 2 « 1540
65'
1530 0 0.5 1 1.5 2 2.5 3 strain (%)
Figure 4.7 Bragg wavelength of POF Bragg gratings under reversible strain tuning test, where
the maximum strain value is 2.78%.
Since the yielding strain for our POF is about 6% [9], it still doesn’t go beyond the elastic limit when the POF Bragg gratings was strain tuned up to 52 nm Bragg wavelength shift. We did some further examination and it was observed that the Bragg wavelength of POF Bragg gratings in the unloading process is time dependant. For the case when the maximum tensile strain is 3.61%, after the POF Bragg gratings sample was released to zero stress state, the Bragg wavelength is still about 2 nm different from the original value, as shown in Fig. 4.6. But as time went on, the Bragg wavelength recovered gradually close to the original value. In our observation, the Bragg wavelength recovered completely back to the original value in 20 minutes after the POF
Bragg gratings sample was released to the zero stress state. From this observation, it is clear that the hysteresis in the tensile strain tuning of POF Bragg gratings is due to the time-dependant elastic effects, which is called viscoelastic effect [10]. But as far as we know, there are no published results on the viscoelastic characteristics of PMMA
82 Chapter 4: Strain Characteristics of Polymer Optical Fiber (POF) Bragg gratings
material available. Further research on the viscoelastic properties of the PMMA based
POF will help to understand the strain effects on POF Bragg gratings.
As we found out, viscoelastic effect will play roles in the strain tuning of POF
Bragg gratings. The loading speed must have effects on reversibility of the strain tuning.
In our experiment, the tensile strain was applied manually and the loading speed was
about 1 me/min, which is rather slow. The reversibility of strain tuning for POF Bragg
gratings at higher loading speed needs examining.
4.5 Strain characterization of Type II POF Bragg gratings
As shown above, the photoelastic effect in POF Bragg gratings is insignificant on
the Bragg wavelength shift in strain tuning and the Bragg wavelength shift is mainly due
to the changes in the grating period. Therefore, the strain sensitivities for Type I and
Type II POF Bragg gratings are expected to be the same. Since all the strain tuning
results reported above are achieved in Type I POF Bragg gratings, some simple tests on
Type II POF Bragg gratings have also been carried out to further prove this inference.
1565
1560 o loading « & x unloading ja _ .. _ _ gfv. _ ___ | 1555 — Linear regression 'w' S5 H' I, 1550 .58 SL ss | 1545 ~er gr BT jg' * 1540 & WO .» WD JS -Bt- 2 1535 sr CQ gr H' 48T 1530 55' 1525 0 0.5 1 1.5 2 2.5 strain (%) Figure 4.8 Bragg wavelength shift with the applied strain for Type II POF Bragg gratings.
83 Chapter 4: Strain Characteristics of Polymer Optical Fiber (POF) Bragg gratings
Fig. 4.8 shows the Bragg wavelength shift of Type II POF Bragg gratings when the
applied strain is up to 2.2%. The reproducibility and reversibility of the tuning was also
tested. It is found that the tunability for Type II POF Bragg gratings is as good as Type I
POF Bragg gratings: high linear relationship between the Bragg wavelength and the
applied strain, excellent reproducibility and reversibility with the tuning range of 32 nm
Bragg wavelength shift. From the linear regression line in Fig. 4.8, the strain coefficient
for Type II POF Bragg gratings is found to be 0.95, which is the same value as that of
Type I POF Bragg gratings. As a result, Type II POF Bragg gratings have the same
strain sensitivity as Type I POF Bragg gratings and both of them have excellent strain
tunability.
The repeatability tests of the strain tuning for POF Bragg gratings, both for Type I and Type II POF Bragg gratings, were also examined. Twenty cycles of repeatable loading-unloading experiment with the maximum strain of 2.2% was carried out. The
Bragg wavelength at different tuning strain seems to coincide quite well with each different cycle and no obvious decay of the grating strength is observed, indicating an excellent repeatability. As a result, POF Bragg gratings can be tuned within 32 nm of
Bragg wavelength shift with excellent reproducibility and reversibility. Such outstanding strain tunability makes POF Bragg gratings as potential devices in standard WDM telecommunication for wavelength multiplexing and routing or as wavelength selective filter.
4.6 Simultaneous strain and temperature measurement by using POF
Bragg gratings
From the thermal and strain characterization shown above, it is clear that POF
Bragg gratings have very high strain and temperature sensitivity with extraordinarily
84 Chapter 4: Strain Characteristics of Polymer Optical Fiber (POF) Bragg gratings broad tuning range. Therefore, POF Bragg gratings are quite promising in fiber sensing
systems. Furthermore, due to its highly linear relationship between the Bragg wavelength and the measurand, the sensing system with POF Bragg gratings could be very simple with high accuracy. In this part, the scheme for simultaneous measurement of strain and temperature by using POF Bragg gratings is proposed, which is one possibility for POF Bragg gratings in fiber sensing application.
One of the very popular topics in fiber sensing field nowadays is the discrimination measurement of the strain and temperature. Since the absolute Bragg wavelength is dependent both on strain and temperature effects on the fibre gratings sensors, a single measurement of Bragg wavelength shift cannot distinguish between the effects of temperature and strain. But in most of the circumstances, the temperature and strain can change simultaneously and independently. Thus the discriminating technique involving fiber Bragg grating sensors is of great importance. Several proposals have been put forward, and among them the simplest one is to use two silica fiber Bragg gratings with different Bragg wavelength J^and A2to form the sensing head [11]. The simple scheme is shown as follows:
FBG1 FBG2 isolator coupler
Matching oil
Figure 4.9. Discrimination sensing system for temperature and strain measurement.
85 Chapter 4: Strain Characteristics of Polymer Optical Fiber (POF) Bragg gratings
The shift of Bragg wavelength AX as the reason of combined strain and temperature
changes, As and AT, is given by 1 1 i 1 ______< i — k TAT'atI TAT'at- = K (4.8) ax2 As A£ to to O) ____ i 1
where KlT and K2T are the temperature sensitivities for the two fiber Bragg gratings,
while Kle and K2£ are the strain sensitivities. Matrix K in equation (4.8) is often called
as transfer matrix. By calculating the inverse matrix of K, the information of T and e can
be recovered,
Tat'I 1i r K,k. c -k T'AX,'atI (4.9) As ^lT^2e ~ ^2T^\s rKir K„ _ AA,
However, there is one weakness in this method, which can even make it invalid.
Because of the similar strain and temperature coefficients for the two silica fiber Bragg
gratings, large errors might be lead into the resulted inverse transfer matrix. For worse cases, the transfer matrix K could be singular; hence the inversion operation of the transfer matrix may be a failure.
In order to overcome this difficulty, a number of other techniques have been proposed for the discrimination measurement of temperature and strain sensing. These include hybrid fiber Bragg grating/long period gratings [12], dual-diameter fiber Bragg gratings [13], fiber Bragg gratings superimposed with polarisation-rocking filter [14], fibre Bragg gratings combined with EDFAs [15], long period grating method [16], and fiber Bragg gratings written at splice joint [17]. The long period grating method utilises the multiple resonance bands in a single long period grating to measure strain and temperature simultaneously. However, it is difficult to accurately measure small shifts in the resonant wavelength in long period grating by using broad bandwidth spectrum. The splicing joint method complicates the sensor system and might limit the maximum
86 Chapter 4: Strain Characteristics of Polymer Optical Fiber (POF) Bragg gratings
strain, which can be applied to sensor head. All of these methods involve complex
gratings or other optical elements, making a more complicated and costly system.
Our proposed scheme for sensing discrimination is to use hybrid POF Bragg
gratings and silica fiber Bragg gratings as the sensor head, which is only a simple
modification over the set-up in Fig. 4.9. As demonstrated previously, strain and
temperature sensitivity of POF Bragg gratings is quite different from that of silica fiber
Bragg gratings. Furthermore, the temperature coefficient for POF Bragg gratings is
negative, which is a totally different response compared with that of silica fiber Bragg
gratings. Thus, if we combine one POF Bragg grating (FBG1) and one silica fiber Bragg grating (FBG2) together as a sensor head to improve the proposal in Fig. 4.9, the transfer matrix K in equation (4.8) is definitely non-singular. Hence, it is easy to work out the inverse transfer matrix in equation (4.9) and the temperature and strain components can be determined.
From our previous results, the temperature and strain coefficient for POF Bragg gratings are-9.3x10 5/°C and 0.95, respectively. While for silica fiber Bragg gratings, the temperature and strain coefficient are 9.15x10 VC and 0.78 [12]. Assuming that the operating wavelength for POF grating and silica fiber Bragg gratings are the same, say
A,i=A,2= 1550 nm, the transfer matrix K in equation (4.8) is found to be,
-0.144 1.473 K = (4.10) 0.0142 1.209
Therefore, the temperature and strain changes, AT (°C) and As (milli-strain) can be easily figured out as below when the Bragg wavelength shift is known,
AT~ -6.299 7.553 A/l, (4.11) Af 0.073 0.738
87 Chapter 4: Strain Characteristics of Polymer Optical Fiber (POF) Bragg gratings
As a result, simultaneous measurement of strain and temperature by using hybrid POF
Bragg grating and silica fiber Bragg grating sensor head can be easily accomplished.
Compared with other strain and temperature discrimination scheme also by using
two fiber Bragg gratings as sensor head, such as the use of two silica fiber Bragg
gratings of different core diameters [13] and a single silica fiber Bragg gratings
straddling over the splicing junction of two fibers [17], our scheme is advantageous with
higher accuracy and smaller error perturbation.
For the scheme in reference [13] and [17], since the two gratings are both silica
Bragg gratings, the strain and temperature sensitivity values of the two gratings are all positive. Therefore, when following equation (4.9) for the calculation of the numerator, the number of the significant digit will be less. While in our scheme, because the temperature sensitivity value of POF Bragg gratings is negative, all the significant digits will be kept when the numerator in equation (4.9) is calculated. Thus, a higher accuracy is expected from our scheme.
Furthermore, according to the matrix perturbation analysis, our scheme is less error perturbing. Since some small measurement error in Bragg wavelength might occur, some errors will be resulted in determining temperature and strain by using equation
(4.8). This is called error perturbation. If the result error is small, the matrix is called small error perturbation. The condition number [18] of the transfer matrix is always used to check its error perturbation. The condition number of a matrix is defined as the ratio of the largest singular value of the matrix to the smallest. The smaller condition number is, the less error perturbation. The condition number of our scheme here is 110. While for the case of the use of two silica fiber Bragg gratings, such as fiber Bragg gratings with different core diameters and a single fiber Bragg gratings over the splicing junction
88 Chapter 4: Strain Characteristics of Polymer Optical Fiber (POF) Bragg gratings
of two fibers, the condition numbers are 143 and 146, respectively. Apparently, our
scheme will be effective with high accuracy and small error perturbation.
4.7 Conclusions
The strain tunability and sensitivity of POF Bragg gratings are characterized. It is
found out that POF Bragg gratings has much better tunability than that of silica fiber
Bragg gratings. A Bragg wavelength shift of over 50 nm has been achieved in POF
Bragg gratings by means of simple tension. Further investigation indicates within 32 nm
of Bragg wavelength shift, the reproducibility and reversibility of the strain tuning is
very good. In addition, the relation between the tensile strain and Bragg wavelength shift
is linear. The strain coefficient of POF Bragg grating is about 0.95, which is higher than that of silica fiber Bragg gratings. This reveals the higher strain sensitivity for POF
Bragg gratings. Due to the dominant role of grating period change effect on the Bragg wavelength shift of POF Bragg gratings, Type II POF Bragg gratings have the same strain sensitivity as Type I POF Bragg gratings and both of them have excellent strain tunability. With high strain sensitivity and the excellent strain tunability, POF Bragg gratings can be very capable both in WDM telecommunication systems and fiber sensing system. A simple and effective scheme for simultaneous measurement of strain and temperature by using a hybrid POF Bragg grating and silica fiber Bragg grating as sensor head is also proposed as an example for POF Bragg grating future applications in fiber sensing fields. Moreover, our proposed method will be effective with high accuracy and small error perturbation.
89 Chapter 4: Strain Characteristics of Polymer Optical Fiber (POF) Bragg gratings
4.8 References
1. A. Othonos, Fiber Bragg gratings. Review of Scientific Instruments, 1997. 68:
p. 4309-4341.
2. S.M. Melle, K. Liu, and R.M. Measures, Practical fiber-optic Bragg grating
strain gauge system. Applied Optics, 1993. 32: p. 3601-3609.
3. K.O. Hill and G. Meltz, Fiber Bragg grating technology fundamental and
overview. Journal of Lightwave Technology, 1997. 15: p. 1263-1279.
4. G.A. Ball and W.W. Morey, Compression-tuned single-frequency Bragg
gratings fiber laser. Optics Letters, 1994. 19: p. 1979-1981.
5. A. Iocco, H.G. Limberger, and R.P. Salathe, Bragg grating fast tunable filter.
Electronics Letters, 1997. 33: p. 2147-2148.
6. A. Iocco, H.G. Limberger, R.P. Salathe, L.A. Everall, and K.E. Chrisholm,
Bragg grating fast tunable filter for wavelength division multiplexing. Journal of
Lightwave Technology, 1999. 17: p. 1217-1221.
7. I. Abe, O. Frazao, P.S. Andre, J.C.C.D. Silva, and H.J. Kalinowski. Fiber Bragg
grating filters for optical communications, in 3rd conference on
telecommunication. 2001. Figueira da Foz, Portugal.
8. J. Brandup, E.H. Immergut, E.A. Grulke, A. Abe, and D.R. Bloch, Polymer
Handbook. 1999, New York: Wiley.
9. Z. Xiong, G.D. Peng, B. Wu, and P.L. Chu, Highly tunable Bragg gratings in
single-mode polymer optical fiber. IEEE Photonics Technology Letters, 1999.
11: p. 352-353.
10. G.H. Edward, J.R. Griffiths, P.L. Rossiter, and P.R. Munore, Mechanical
properties of engineering materials. 1996, Melbourne: Monash University.
90 Chapter 4: Strain Characteristics of Polymer Optical Fiber (POF) Bragg gratings
11. M.G. Xu, J.-L. Archambault, L. Reekie, and J.P. Dakin, Discrimination between
strain and temperature effects using dual-wavelength fiber grating sensors.
Electronics Letters, 1994. 30: p. 1085-1087.
12. H.J. Patrick, G.M. Williams, A.D. Kersey, J.R. Pedrazzani, and A.M.
Vengsarkar, Hybrid fiber Bragg grating/long period fiber grating sensor for
strain/temperature discrimination. IEEE Photonics Technology Letters, 1996.
32: p. 1223-1225.
13. S.W. James, M.L. Docknhy, and R.P. Tatam, Simultaneous independent
temperature and strain measurement using in-fiber Bragg grating sensor.
Electronics Letters, 1996. 32: p. 1133-1134.
14. S.E. Kanellopoulos, V.A. Handerek, and A.J. Rogers, Simultaneous strain and
temperature sensing with photogenerated in-fiber gratings. Optics Letters, 1995.
20: p. 333-335.
15. J. Jung, H. Nam, J.H. Lee, N. Park, and B. Lee, Simultaneous measurement of
strain and temperature by using of a single-fiber Bragg grating and an erbium-
doped fiber amplifier. Applied Optics, 1999. 38: p. 2749-2751.
16. V. Bhatia, D. Campbell, R.O. Claus, and A.M. Vengsarkar, Simultaneous strain
and temperature measurement with long period gratings. Optics Letters, 1997.
22: p. 648-650.
17. B.O. Guan, H.Y. Tam, S.L. Ho, W.H. Chung, and X.Y. Dong, Simultaneous
strain and temperature measurement using a single fiber Bragg grating.
Electronics Letters, 2000. 36: p. 1018-1019.
18. R. Bhatia, Matrix Analysis. 1997, New York: Springer-Verlag.
91 Chapter 5: Simulation of Dynamic Dispersion Compensation by Linearly Chirped Polymer Optical Fiber (POF) Bragg Gratings
Chapter 5: Simulation of Dynamic Dispersion Compensation
by Linearly Chirped Polymer Optical Fiber (POF) Bragg
Gratings
As previously shown, POF Bragg gratings have sound tunability both by applying tensile strain and temperature variation. Apart from its promising applications in fiber sensing, POF Bragg gratings are also capable candidates in optical fiber communication systems. For example, we propose POF Bragg gratings for dynamic dispersion compensation application in fiber communication systems. In this chapter, the simulation on dynamic dispersion compensation by linearly chirped POF Bragg gratings are examined.
5.1 Introduction
5.1.1 Historical background on dispersion compensation research
Much of the installed standard optical silica fiber for telecommunication system has a low dispersion at the wavelengths of 1300 nm region. Due to the low loss of the silica fiber at 1550 nm region, together with the ready availability of erbium doped amplifier, the 1550 nm window becomes an attractive operation region. However, the chromatic dispersion of these fibers is relatively large within this window. The chromatic dispersion results in different wavelength components of a data pulse traveling at different phase velocities. This causes the broadening of the signal pulse and increasing bit error rates. Therefore, the accumulated dispersion in the span of the fiber severely
92 Chapter 5: Simulation of Dynamic Dispersion Compensation by Linearly Chirped Polymer Optical Fiber (POF) Bragg Gratings limits the transmission distance unless the dispersion compensation is used. With the speeding development of high rate transmission systems, such as wavelength-division multiplexing (WDM) systems, the dispersion management turns out to be a paramount issue. At higher bit rate, dispersion tolerances are small enough that variations in dispersion, which are negligible in slower systems, can severely degrade the system performance. The dispersion compensation can be accomplished by using an optical element that negated the accumulated dispersions of the system.
Two major techniques have emerged in recent years for the chromatic dispersion compensation. One is dispersion compensation fiber (DCF) [1], which through their fiber design can provide the desired negative dispersion over a broad wavelength range.
The other is dispersion compensating fiber gratings [2, 3], which via their chirped nature can delay one spectral component relative to another. For dispersion compensation fibers technique, the manufacture of the special designed fiber and the insertion loss are the dominant drawbacks, while fiber Bragg grating compensation is more attractive due to its compactness, potentially low cost and fiber compatibility. Therefore, extensive research on fiber Bragg grating compensation has been carried out.
5.1.2 Dispersion of the uniformed fiber Bragg gratings
Fiber Bragg grating consists of a periodic modulation of refractive index along the fiber core. Generally, the refractive index change in the core 8n(x,y,z) can be expressed by,
2 n Sncore (z) = Sneff (z){l + vcos[—-z + cp{z)]} (5.1) A
93 Chapter 5: Simulation of Dynamic Dispersion Compensation by Linearly Chirped Polymer Optical Fiber (POF) Bragg Gratings where Sneff is the “dc” index change spatially averaged over a grating period, v is the fringe visibility of the index change, A is the grating period, and (f){z) describes grating chirp, v • &ieff is the “ac” refractive index change, which is also called index modulation
An. For uniformed gratings, (f>(z) = 0. As shown in Chapter Two, couple mode theory
(see in example [4]) can be used to describe the optical spectra of fiber Bragg gratings.
The reflectivity of uniformed fiber Bragg gratings can be expressed as equation (2.22).
Fig. 5.1 shows the calculated reflection spectrum of the uniformed grating with “ac” index modulation index of 1.5x10 4 and grating length of 1 cm. In the calculation, the
“dc” index change is neglected. The Bragg wavelength of the fiber Bragg grating is
1550 nm. The sidelobes of the resonance are due to multiple reflections to and from opposite ends of the grating region.
0.8 -
.ti 0.6 -
0.4 --
1549.5 1550 1550.5 wavelength (nm)
Figure 5.1 Calculated reflection spectrum of a uniform fiber Bragg grating.
94 Chapter 5: Simulation of Dynamic Dispersion Compensation by Linearly Chirped Polymer Optical Fiber (POF) Bragg Gratings
The group delay and dispersion spectra of the reflected light of uniformed Bragg
gratings can be determined from the phase of the amplitude reflection coefficient p in
equation (2.21). If the phase is denoted Q - phase(p), at a local frequency a)Q, 6 p can
be expanded in a Taylor series about co0. Since the first derivative dOp Ida) is directly
proportional to the frequency a), this quantity can be identified as a time delay. Hence,
the group delay Tp for light reflected off of a grating is [5]
dep_ (5.2) da) 7.71c dA
where r is usually given in units of picoseconds. The dispersion d with units of
picoseconds per nanometer is the change rate of delay with wavelength. Thus, d can be
expressed as [5],
dr^_ dA
2rp /l2 d%
A 2tic dA2
2 TIC d 26, (5.3) A2 da)2
Fig. 5.2 displays the calculated group delay and dispersion spectra of the uniform
fiber Bragg grating with the same parameters as the one in Fig, 5.1. It is obviously found that group delay is symmetric to the Bragg wavelength. The dispersion is zero near
Bragg wavelength, and only becomes noticeable near the band edges and sidelobes of the reflection spectrum where it tends to vary rapidly with wavelength. In addition, the dispersion changes sign when detuning from one side of the bandgap to the other. A
95 Chapter 5: Simulation of Dynamic Dispersion Compensation by Linearly Chirped Polymer Optical Fiber (POF) Bragg Gratings useful qualitative understanding of the delay and dispersion behavior is provided by the effective medium approach developed by Sipe et al [6]. For wavelength outside the bandgap, the boundaries of the uniform grating act as abrupt interfaces, resulting in the formation of a Fabry-Perot-like cavity. Analogous to Fabry-Perot resonances, nulls are observed in the reflection spectrum at frequencies light is trapped inside the cavity for many round trips, thus experiencing enhanced delay.
---- group delay 100 — - 1000 -----dispersion
a 80 - -- 500 %
- -500 2
— -1000
-1500 1549.4 1549.6 1549.8 1550.2 1550.4 1550.6 wavelength (nm)
Figure 5.2 Calculated group delay and dispersion of a uniform fiber Bragg grating.
Although large values of group velocity dispersion are obtainable from the uniform fiber Bragg gratings, and these Bragg gratings have been proposed for dispersion compensation [7], the dispersion changes rapidly within the bandwidth of short optical pulses making uniform fiber Bragg gratings unsuitable for this application. Chirped fiber
Bragg gratings induce different time delays for different wavelengths and it has been
96 Chapter 5: Simulation of Dynamic Dispersion Compensation by Linearly Chirped Polymer Optical Fiber (POF) Bragg Gratings
proved that chirped fiber Bragg gratings could be exploited in telecommunications for
dispersion compensation in high bit rate transmission systems.
5.1.3 Dispersion compensation by chirped fiber Bragg gratings
Chirped gratings have a non-uniformed period along their length. Chirped gratings
can have lots of forms. The period may vary symmetrically, either increasing or
decreasing in period around a pitch in the middle of a grating. The chirp may be linear,
may be quadratic or even have jumps in the period. The application of using linearly
chirped fiber Bragg gratings for dispersion compensation was first proposed by
Ouellette [2]. Since linearly chirped fiber Bragg gratings can offer a large, constant
dispersion over bandwidths, lots of demonstration of using linearly chirped fiber Bragg
gratings to compensate chromatic dispersion for high transmission rate systems has been
given [8-10].
Nowadays, dynamic dispersion compensation scheme is becoming critical to
counteract the signal degradation found in the next generation optical communication
systems, i.e 40 Gbit/s dense wavelength-division multiplexing (DWDM) systems. The
necessity for dynamic dispersion compensation is that the accumulated dispersion at the
receiving end may vary in time due to several impairments [11]. The changes in the
operation conditions of the transmission systems, i.e. small variation in optical power, or
laser or modulator chirp, can result in additional phase shift, thus the optimal dispersion
compensation value for the whole system is modified [12-14]. Furthermore, dynamic
reconfigurations of the network, such as the channel path changes by add-drop
operations, can change the accumulated dispersion. Moreover, the environmental changes, such as the changes in ambient temperature, will also change the dispersion
97 Chapter 5: Simulation of Dynamic Dispersion Compensation by Linearly Chirped Polymer Optical Fiber (POF) Bragg Gratings
value [15]. For example, at communication wavelength, the temperature dependence of
the chromatic dispersion of a 2000-km span of nonzero-dispersion shifted fiber
(NZDSF) is approximately 5 ps/nm/°C. Therefore, given the dispersion tolerance of ~50
ps/nm at 40 Gb/s, systems impairments are expected to occur with temperature changes
as low as 10-20 °C [13]. As a result, tunable compensator is required to dynamically
negate the residual dispersion in the high bit rate transmission systems.
Since uniformly stretching the normal linearly chirped fiber Bragg gratings only
produces a shift in the resonant wavelength rang of the reflected band but does not
change the induced dispersion [16], non-linearly chirped fiber gratings were proposed in
some of the dynamic dispersion compensation studies already published [11, 13, 16].
But for the non-linearly chirped fiber Bragg gratings, the dispersion value is not constant
over the bandwidth.
We propose to use the linearly chirped fiber Bragg gratings fabricated by the
tapered POF method for dynamic dispersion compensation. Follows will be simulation
results for the dispersion characteristics of linearly chirped POF Bragg gratings
fabricated by the tapered POF method. It is shown that by optimizing the taper profile,
the dynamic dispersion compensation with a large tunable range could be accomplished
by this linearly chirped POF Bragg grating.
5.2 Scheme for linearly chirped POF Bragg gratings fabrication
5.2.1 Introduction
Several techniques have been proposed for the fabrication of chirped fiber Bragg gratings. The most straightforward method is the phase mask technique in direct
98 Chapter 5: Simulation of Dynamic Dispersion Compensation by Linearly Chirped Polymer Optical Fiber (POF) Bragg Gratings
holographic replication of the chirped pitch phase mask into the core of the fiber [17].
This method is simple and accurate, but a specifically fabricated phase mask is required
for different designed chirped gratings. The electron-beam lithography technique is
often used for direct production of the subsections one by one. Precise control is needed
during the manufacture of the phase mask, otherwise large stitching errors might occur.
Large stitching errors in the phase mask can result in ghost peak or asymmetry in the
fiber grating spectra [18]. Moreover, for chirped fiber Bragg gratings, stitching errors
can play significant roles.
Another method for chirped fiber grating fabrication is called moving fiber/phase
mask technique [19]. This technique is based on moving fiber with respect to phase
mask. The change of the Bragg wavelength is the function of the relative moving
velocity of the phase mask and the fiber. By controlling the moving speed of the fiber to
the phase mask, the designed chirped grating can be made. For this method, very
accurate and stable translation stage is needed. Any mechanical vibration may result in
large phase shift in the fiber gratings.
Chirped fiber gratings can also be fabricated by the exposure to the interference
pattern formed by two beams with dissimilar phase fronts [20]. Though large chirp value
grating can be obtained by this method, the strong curvature of the fringe pattern
inscribed in the fiber will result in coupling of light to the radiation modes.
Another very simple method is to bend the fiber in a uniformed fringe pattern so
that a continuously changing period is projected on it [21]. With large bending curvature, the fiber grating can be substantially chirped. But bending has a similar effect of imparting a blaze, which result in undesirable radiation loss. Applying a temperature
99 Chapter 5: Simulation of Dynamic Dispersion Compensation by Linearly Chirped Polymer Optical Fiber (POF) Bragg Gratings
gradient along the length of the fiber during the writing of fiber grating can also result in
chirped fiber gratings [22]. But only small chirp values can be achieved by this method.
5.2.2 The tapered POF method
Apart from the above-mentioned methods, the technique based on applying gradient
strain on uniform fiber gratings is another choice for chirped fiber gratings fabrication.
Several schemes have been proposed to create the non-uniformed strain profile [23-25],
and applying longitudinal force to tapered fiber[26], called tapered fiber method, is very
simple and effective. Furthermore, local heating and stretching to fabricate tapered fiber
is very easy to implement for polymer fiber. Therefore, we propose to use tapered POF
method to fabricate chirped POF Bragg gratings.
For the tapered POF, it has a non-uniform diameter; therefore the application of a
longitudinal stress leads directly to strain that is non-uniformly distributed along its
length and consequently results in a non-uniformed Bragg wavelength along the fiber
grating. There are two ways to fabricate chirped gratings by this technique [26]. One is
to write a uniform grating in tension-free tapered fiber, causing a chirp when the fiber is
subject to tension. The other method is to write grating in the tapered fiber under
tension, causing a strong chirp when the tension is released. Though the total chirp in the
second case is larger than the first one when the same tension is applied, for the first
case the chirp can be changed and controlled with various applied tensions afterwards.
As a result, the first method is advantageous for the dynamic dispersion compensation
application.
The chirp profile of the gratings is determined by the taper fiber diameter profile. In the following, the theoretical analysis on the relation between the two is given.
100 Chapter 5: Simulation of Dynamic Dispersion Compensation by Linearly Chirped Polymer Optical Fiber (POF) Bragg Gratings r A
C/D 3 2 0 L z
Figure 5.3 Coordinate for tapered fiber profile analysis.
Consider a uniform grating is written in a section of tapered fiber with radius of r and grating length of L. The corresponding coordination is shown in Fig. 5.3. The tapered fiber radius profile is r(z). If the chirp function of the designed gratings is represented as f(z), the Bragg wavelength distribution /ls(z)for this chirped grating is given as,
Ab (z) - AB (0) + AAb • /(z) (5.3) where AB (0) is the Bragg wavelength at z=0 and AAB is the total chirp value. Thus for linear chirp grating, /(z) = z / L and AB (z) = AB (0) + AAg (z / L); for quadratic chirp grating, /(z) = (z/L)2 and AB(z) = AB(0) + AAB(z/L)2.
When tension F is applied to the fiber, a Bragg wavelength shift A/tB(z) at any position z is caused due to the strain effects shown in Chapter Four and can be given as
[27],
&AB(z) = (l-pe)- (5.4)
101 Chapter 5: Simulation of Dynamic Dispersion Compensation by Linearly Chirped Polymer Optical Fiber (POF) Bragg Gratings
where pe is the effective photoelastic coefficient for POF Bragg grating, E is the
Young’s modulus of POF and /lBOis the Bragg wavelength of the original uniform
grating. Therefore, the Bragg wavelengths at the beginning of the taper, z=0, and at the
end of the taper, z=L, are given as follows,
4,(0) = (l-p.)~^-r/lso+4o (5.5) Ettt{ 0)
and
AB(L) = (l-pe) (5.6) Em-(L) 2 ^B0 + ^BO •
Then, the total chirp value AAB can be found as,
FA 1 1 AAb=Ab(L)-Ab(0) = (1-pJ-^(--—-——). (5.7) En r(L) r(0)~
By substituting equation (5.4), (5.6) and (5.7) into equation (5.3), the relation between
the tapered fiber radius function, r(z), and the grating chirp function, f(z), is given as
[27],
r(Q)2r(L)2-r(L)2r(z)2 (5.8) [r(0)2-r(L)2]r(z)2
Consequently, for the designed chirp function f(z) the corresponding tapered fiber
profile r(z) is,
r(PML) r{z) = (5.9) r(L)2]f(z) + r(L)2
As a result, by the precise control of the tapered fiber radius profile r(z) of POF, it is easy to achieve the designed chirp grating profile when the tension is applied afterwards.
As it is seen in equation (5.8), the chirp profile is only determined by the tapered fiber
102 Chapter 5: Simulation of Dynamic Dispersion Compensation by Linearly Chirped Polymer Optical Fiber (POF) Bragg Gratings radius profile r(z) and has nothing to do with the tension. The tension will only change the total chirp value, as shown in equation (5.7). The higher the tension is, the larger chirp value is obtained.
5.3 Dynamic dispersion compensation by linearly chirped POF Bragg gratings
Next is the simulation analysis of the dispersion characteristics for linearly chirped
POF gratings fabricated by the tapered POF method and by using our novel design on the tapered POF radius, a large tunable dispersion compensation range at the same operating wavelength is achieved in the linearly chirped POF Bragg gratings. Therefore, the real dynamic dispersion compensation could be accomplished.
5.3.1 Reflection and group delay spectra of linearly chirped POF Bragg gratings
Chirped gratings can be exactly mathematically modeled using the direct integration technique in the couple mode theory, but the piecewise-uniform approach, also known as transfer matrix method [28], is more suitable to the chirped gratings. In this method, the grating is broken up into smaller sections with uniform period. By identifying 2x2 matrices for each uniform section of the grating, then the single 2x2 matrix that can describe the whole grating can be obtained by multiplying all of those matrices together.
In what follows, the transfer matrix method will be used to evaluate the dispersion characteristics of the linearly chirped POF Bragg gratings.
The chirped grating can be divided into M sections and each segment has a different period and has its own transfer matrix. Assume the total length of the grating is L. For section i, the field amplitudes /? and S., given as equation (2.14) and (2.18) for the
103 Chapter 5: Simulation of Dynamic Dispersion Compensation by Linearly Chirped Polymer Optical Fiber (POF) Bragg Gratings couple mode theory description, can be defined. Then we can start the calculation with
R0=R(L) = 1 and S0=S(L) = 0, and calculate RM = R(0) and SM=S(0). The propagation through each uniform section i is described by a matrix F, defined as,
F. (5.10) St
The Fi matrix is given as [5],
cosh(gAz) - / —sinh(gAz) - i — sinh(gAz) q q F: = c (5.11) i—sinh(gAz) cosh(gAz) + i—sinh(gAz) q q where Azis the length of the ith uniform section, the frequency offset 8 and coupling coefficient K are the local value in the ith section, and q the parameter as defined in
Chapter Two, which is given as,
q = . (5.12)
Compared with the uniform gratings, the non-zero z-dependent phase term (p{z) will have an effect on the frequency offset value S for the chirped fiber gratings. The frequency offset 8 for the chirp fiber grating is found to be,
S = k-kB--—. (5.13) 8 2 dz
For the linearly chirped fiber grating,
1 d(p _ 4mieff z dAB (5.14) 2 dz Ab2 dz
Once all of the matrices for all the individual sections are known, the output of the amplitude for the whole gratings is given as follows,
104 Chapter 5: Simulation of Dynamic Dispersion Compensation by Linearly Chirped Polymer Optical Fiber (POF) Bragg Gratings
= F f = fm : (5.15)
The number of sections needed for the piecewise-uniform calculation is determined by
the required accuracy. For most of the apodized and chirped gratings, M-100 is
sufficient. But M can’t be arbitrarily large, since the couple mode theory approximation
is not valid when the uniform gating section is only a few grating periods length.
Therefore, it is required that Az » A, which leads to below relation [5],
2 n~L M « lefT (5.16)
Consider a tapered POF with the radius r(0) of 62.5 /xm at the beginning of the taper
and the radius r(L) of 55 /xm at the taper end and a uniform grating is then written on the
tapered POF. In order to get the linearly chirped fiber gratings, the tapered profile
follows equation (5.9) with f(z) = z/L . When a strain value of 0.04% is applied to the
tapered POF, the reflection spectrum is displayed in Fig. 5.4. For comparison, the
spectrum of the tension-free grating (the uniform Bragg grating) is also shown in Fig.
5.4. The parameters for the uniform fiber Bragg grating in the simulation are as follows:
Bragg wavelength ABO=l550nm, grating length L=6 cm, index modulation
An = 1 x 10 4 and the effective refractive index n a = 1 48 • eff
It is clearly shown that a chirped grating is achieved when a strain value of 0.04% is
applied. It is also shown that for the chirped gratings achieved the reflection band shifts
to the longer wavelength, the peak reflection decreases, and there are ripples in the reflection band. However, the reflection ripples can be eliminated by adopting optimized
apodization profile in the grating design [29]. But exercises won’t be carried out here.
105 Chapter 5: Simulation of Dynamic Dispersion Compensation by Linearly Chirped Polymer Optical Fiber (POF) Bragg Gratings______
The achieved chirp grating is due to the faster changes in Bragg wavelength at the narrow part of the taper. For example, at 0.04% strain, Bragg wavelength at the beginning of the taper shifts to 1550.59 nm; while at the taper end it shifts to 1550.76 nm.
0.8 -
0.6 -
0.4 -
0.2 -
1549.5 1550.5 1551.5 wavelength (nm)
Figure 5.4 Reflection spectra of the fiber Bragg gratings for the unstrained and strained
tapered POF, (a) tension-free; (b) strain 0.04%.
Assuming the light is launched from the large end of the taper, then the group delay of the reflected light corresponding to Fig. 5.4 is shown in Fig. 5.5. It is clearly demonstrated that the group delay linearly increases and the average increasing slope is roughly constant within the reflection band for the chirp grating achieved. Therefore, the linearly chirped fiber gratings are obtained with the applied strained to the tapered POF.
The group delay value oscillates round zero at the shorter wavelength edge of the
106 Chapter 5: Simulation of Dynamic Dispersion Compensation by Linearly Chirped Polymer Optical Fiber (POF) Bragg Gratings reflection band; and the group delay value oscillates around 600 ps at the longer wavelength edge of the reflection band. Significant group delay ripples are present within the reflection band. The ripple can be substantially diminished by properly apodizing the grating profile [29].
1500 i
1000 -
500 -
1549.5 1550.5 1551.5 wavelength (nm)
Figure 5.5 Group delay of the grating for unstrained and strained tapered POF, (a) tension-
free; (b) strain 0.04%
We use the above-mentioned matrix transfer method for chirped fiber grating simulation. For this method, the number of the uniformed sections M should be determined. Several M values have been chosen to check the validity of the simulation program for chirp grating, and reflection and group delay spectra of the chirp POF gratings with M=10 and 40 are compared in Fig. 5.6. Some differences can be observed.
The further comparison indicates that calculation with M greater than 40 will give approximately the same results. We use M=100 for all the calculations.
107 Chapter 5: Simulation of Dynamic Dispersion Compensation by Linearly Chirped Polymer Optical Fiber (POF) Bragg Gratings
M=10.
0.8 -
0.6 -
0.4 -
0.2 -
1549.5 1550 1550.5 1551 1551.5 wavelength (nm)
Fig. 5.6 (a)
1000 -
500 -
1550 1550.2 1550.4 1550.6 1550.8 1551 1551.2 1551.4 wavelength (nm)
Fig. 5.6 (b)
Figure 5.6 Reflection (a) and group delay (b) spectra of the linearly chirped gratings with
different M values.
108 Chapter 5: Simulation of Dynamic Dispersion Compensation by Linearly Chirped Polymer Optical Fiber (POF) Bragg Gratings
Reflection spectra and the corresponding group delay of the linearly chirped POF
Bragg gratings fabricated at different applied strain are assembled in Fig. 5.7 and Fig.
5.8 respectively.
In Fig. 5.7, it is clearly shown that with the increase of the strain value the reflection
band shifts to longer wavelength, the reflection band becomes wider and the peak
reflection becomes smaller. Wider reflection band and smaller peak reflection are due to
the increased chirp value of the gratings with the larger applied strain. Ripples in the
reflection band can only be observed when the applied strain is comparatively large.
(a) (b) (c) (d)
0.8 -
0.6 -
0.4 -
0.2 -
wavelength (nm)
Figure 5.7 Reflection spectra of the linearly chirped POF gratings at different applied strain,
(a) original uniform gratings, tension-free; (b) strain 0.01%; (c) strain 0.02%; (d) strain 0.04%;
(e) strain 0.08%; (f) strain 0.1%.
In the group delay spectra shown in Fig. 5.8, similar group delay curves are found for all cases: group delay oscillates around zero at the shorter wavelength edge of the
109 Chapter 5: Simulation of Dynamic Dispersion Compensation by Linearly Chirped Polymer Optical Fiber (POF) Bragg Gratings
reflection band, it oscillates around 600 ps at the longer wavelength edge of the band,
and it increases linearly within the reflection band. However, with larger strain values,
the reflection band becomes wider and average slope of the group delay curves becomes
smaller. For example, for the chirp grating at applied strain 0.02%, the average slope of
the group delay curve is about 2360 ps/nm; while at strain 0.1%, the average slope is
only 500 ps/nm. The slope can be easily tuned by adjusting the applied strain value.
800 —
1549.5 1550 1550.5 1551 1551.5 1552 1552.5 1553 wavelength (nm)
Figure 5.8 Group delay of the linearly chirped POF gratings at different applied strain, (a)
original uniform gratings, tension-free; (b) strain 0.01%; (c) strain 0.02%; (d) strain 0.04%; (e)
strain 0.08%; (f) strain 0.1%.
Though the group delay slope can be tuned in the above case, it still can’t be
applicable to dynamic dispersion compensation. As shown above, when the applied
strain increases, both the central wavelength and the spectrum bandwidth of the linearly chirped grating will increase. Due to the larger value of central wavelength shift than the increase of the spectrum bandwidth versus the increase of the applied strain, there are
110 Chapter 5: Simulation of Dynamic Dispersion Compensation by Linearly Chirped Polymer Optical Fiber (POF) Bragg Gratings______
small overlapping areas of the reflection band between the chirped gratings only when
the variation of the applied strain is very small. For example, there is small overlapping
in the reflection band between the chirped gratings when the strain is varied from 0.02%
to 0.04%, while there is no overlapping between the chirp gratings formed at strain
0.02% and 0.06%. Therefore, at certain operating wavelength, only a very small
dynamic range can be achieved for the dispersion compensation.
5.3.2 Dynamic dispersion compensation by linearly chirped POF Bragg gratings
From the above discussion about the tapered fiber radius profile r(z) in Section
5.2.2, it can be seen that the central wavelength and the chirp value of the chirped
gratings are dependent not only on the applied strain value but also on the radius value
of r(0) and r(L). r(0) is the radius of the beginning of the tapered POF, which is the
original radius of the fiber. r(L) is the radius of the taper end. Hence, r(0) is normally
fixed, say 62.5 pm. Thus, we will carry out some studies on the effect of r(L), the radius
of the tapered POF end, on the central wavelength shift and total chirp value of the
linearly chirped gratings.
The central wavelength of the linearly chirped fiber Bragg gratings is the Bragg
wavelength at the central point (z=L/2) of the tapered POF. For linearly chirped
gratings, the tapered fiber radius profile r(z) should follow equation (5.9), where
f(z) = z/L. Consequently, the radius at the central point of the tapered POF, r(L/2), can be found as,
r(L/2)= , r(°)r(L) . (5.17) V|(K0)2+r(L)2)
Then the central wavelength shift AAB (L/ 2) of the linearly chirped gratings is given as,
111 Chapter 5: Simulation of Dynamic Dispersion Compensation by Linearly Chirped Polymer Optical Fiber (POF) Bragg Gratings
AAB(L/2) = (l-pe)eAm r<0)' (5.18) r(L/2) where £is the applied strain and /lgois the Bragg wavelength of the original uniform gratings. By rewriting equation (5.7), the total chirp value of the linearly chirped gratings, AAg, is expressed as,
AAB=(l-p,)eAm[^j-l]. (5.19) r(L)
3.5 t T 3.5
S 3 -
OJD 2 -
— 1.5 C
s 0.5 - - 0.5
radius of small end of the tapered POF (pm)
Figure 5.9 Effects of the radius of the small end of the tapered POF on the central
wavelength shift and total chirp value of the linearly chirped POF Bragg gratings.
Assuming r(0) = 62.5 jum, £ = 0.04%, and ABQ = 1550/im, the central wavelength shift AAB(L/2) and the total chirp value £JiBof the linearly chirped POF Bragg gratings versus the radius of the small end of the tapered POF, r(L), are plotted in Fig.
5.9. It is shown that both A/lB(L/2) and A/lg will increase when r(L) is getting smaller.
112 Chapter 5: Simulation of Dynamic Dispersion Compensation by Linearly Chirped Polymer Optical Fiber (POF) Bragg Gratings
It is also found out that when r(L) is greater than a certain value, say 35 (am, AAB will be smaller thanA/lfl(L/2). While when r(L/2)is smaller than 35 jam, AAB will be larger than AAB(L/2). Since the reflection bandwidth of the linearly chirped grating is proportional to its total chirp value, Fig. 5.9 might give us some idea why the central wavelength shift rate is faster than the bandwidth increase rate for the linearly chirped fiber Bragg gratings formed in the previous case, when r(L) is 55 pm.
As a result, it can also be inferred from the results shown by Fig. 5.9, if r(L) is small enough, for the linearly chirped fiber Bragg gratings formed by the tapered POF method, the bandwidth increase rate of the chirped gratings might be faster than the central wavelength shift rate. Thus, there will be overlapping in the reflection band areas between the chirped gratings formed at a large range of strain variation. As a result, large tunable dispersion compensation range can be achieved.
Then we set r(L)=30 pm, and the simulation results for the reflection and group delay spectra for the linearly chirped fiber Bragg gratings formed at different applied strain are shown in Fig. 5.10 and Fig. 5.11, respectively. Some similar results have been found between Fig. 5.10 and Fig. 5.7: the reflection band shifts to longer wavelength, the reflection band becomes wider and the peak reflection becomes smaller with the increase of the strain value. However, there is one significant difference: there is overlapping in the reflection band areas between all the chirp gratings when the applied strain is varied from 0.003% up to 0.06%. This is due to the faster increase of the bandwidth than the central wavelength shift when the strain is applied to the tapered
POF.
113 Chapter 5: Simulation of Dynamic Dispersion Compensation by Linearly Chirped Polymer Optical Fiber (POF) Bragg Gratings
0.8 -
0.4 -
1545 1547 1549 1551 1553 1555 1557 1559 wavelength (nm)
Figure 5.10 Reflection spectra of linearly chirped fiber gratings with r(L)= 30pm at different
applied strain, (a) tension-free; (b) strain 0.003%; (c) strain 0.01%; (d) strain 0.02%; (e) strain
0.03%; (f) strain 0.04%; (g) strain 0.06%. 1600 T 1400 -
1200 -
fi.1000 -
« 800 -
a 600 - S 400 -
200 -
wavelength (nm)
Figure 5.11 Group delay spectra of the linearly chirped fiber Bragg gratings with r(L)=30 pm at different applied strain, (a) tension-free; (b) strain 0.003%; (c) strain 0.01%; (d) strain 0.02%; (e)
strain 0.03%; (f) strain 0.04%; (g) strain 0.06%.
114 Chapter 5: Simulation of Dynamic Dispersion Compensation by Linearly Chirped Polymer Optical Fiber (POF) Bragg Gratings
Hence, by tapering r(L) small enough, a rather large dispersion compensation tuning
range can be achieved with the variation of the applied strain as shown in Fig. 5.11. For
example, at some specific operating wavelength, i.e. Xo in Fig. 5.11, the dispersion value
for the chirped gratings formed at strain 0.003% is 1534 ps/nm; and the average group
delay slope for the chirped gratings formed at strain 0.06% is 66 ps/nm. Therefore, we
can use the linearly chirped POF Bragg gratings formed by the tapered POF method to
accomplish the real dynamic dispersion compensation with a very large tuning range at
the same operating wavelength.
5.4 Conclusions
In this chapter, the simulation on the linearly chirped POF Bragg grating fabricated
by the tapered POF method has been carried out and its application on dynamic
dispersion compensation is proposed.
Following the theoretical analysis on the tapered fiber method already published, it
is indicated that, by carefully controlling the tapered POF radius profile, linearly chirped
POF Bragg gratings can be formed when the external strain is applied. Then, our
simulation results on the reflection and group delay spectra for the linearly chirped POF
Bragg gratings formed at different applied strained are given. Results on the group delay
simulation show that though the average slope can be tuned by adjusting the applied
strain, the central wavelength shifts fast with the applied strain and the tunable
dispersion compensation can’t be accomplished at the same operating wavelength.
We did some further investigation on the theoretical analysis of tapered fiber method and found out by tapering the POF taper end small enough, the bandwidth of the
115 Chapter 5: Simulation of Dynamic Dispersion Compensation by Linearly Chirped Polymer Optical Fiber (POF) Bragg Gratings linearly chirped POF Bragg grating will increase faster than its central wavelength shift when the external strain is applied. Therefore, a very large tunable dispersion compensation range at the same operating wavelength can be achieved. In our simulation results, a dispersion range from 1534 ps/nm to 66 ps/nm at a certain operating wavelength is achieved. Accordingly, by using the linearly chirped POF Bragg grating fabricated by our novel design, the real dynamic dispersion compensation could be accomplished.
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118 Chapter 5: Simulation of Dynamic Dispersion Compensation by Linearly Chirped Polymer Optical Fiber (POF) Bragg Gratings
19. H. Storoy, H.E. Engan, B. Sahlgren, and R. Stbbe, Position weighting of fiber
Bragg gratings for bandpass filtering. Optics Letters, 1997. 22: p. 784-786.
20. M.C. Farries, K. Sugden, D.C.J. Reid, I. Bennion, A. Molony, and M.J.
Goodwin, Very broad reflection bandwith (44nm) chirped fiber gratings with
narrow-bandpass filters produced by the use of an amplitude mask. Electronics
Letters, 1994. 30: p. 891-892.
21. K. Sugden, I. Bennion, A. Molony, and N.J. Cooper, Chirped grating produced
in photosensitive optical fibers by fiber deformation during exposure. Electronics
Letters, 1994. 30: p. 440-442.
22. J. Martin, J. Lauzon, S. Thibault, and F. Ouellette. Novel writing technique of
long highly reflective in-fiber Bragg gratings and investigation of linearly
chirped component, in Optical Fiber Communication Conference. 1994.
23. K.C. Byron, K. Sugden, T. Bircheno, and I. Bennion, Fabrication of chirped
Bragg gratings in photosensitive fiber. Electronics Letters, 1993. 29: p. 1659-
1660.
24. M.A. Putnam, G.M. Williams, and E.J. Friebele, Fabrication of tapered, strain
gradient chirped fiber Bragg gratings. Electronics Letters, 1995. 31: p. 309-310.
25. M.M. Ohn, A.T. Alavie, R. Maaskant, M.G. Xu, F. Bilodeau, and K.O. Hill,
Dispersion variable fiber Bragg grating using a piezoelectric stack. Electronics
Letters, 1996. 32: p. 2000-2001.
26. L. Dong, J.L. Cruz, L. Reekie, and J.A. Tucknott, Fabrication of chirped fiber
gratings using etched tapers. Electronics Letters, 1995. 31: p. 908-909.
119 Chapter 5: Simulation of Dynamic Dispersion Compensation by Linearly Chirped Polymer Optical Fiber (POF) Bragg Gratings
27. J.L. Cruz, L. Dong, S. Barcelos, and L. Reekie, Fiber Bragg gratings with
various chirp profiles made in etched tapers. Applied Optics, 1996. 34: p. 6781-
6787.
28. M. Yamada and K. Sakuda, Analysis on almost-periodic distributed feedback
slab waveguide via a fundamental matrix approach. Applied Optics, 1987. 26: p.
3474-3478.
29. R. Kashyap and M. de L. Rocha, On the group delay characteristics of chirped
fiber Bragg gratings. Optics Communications, 1998. 153: p. 19-22.
120 Chapter 6: Photosensitivity of Low-loss Perfluoropolymer (CYTOP) Fiber Materials
Chapter 6: Photosensitivity of Low-loss Perfluoropolymer
(CYTOP) Fiber Material
6.1 Introduction
As shown in the previous chapters, PMMA-based POFs have shown useful
photosensitivity and Bragg gratings can be written on them. Furthermore, it is
demonstrated that POF Bragg gratings have very high tunability, both by mechanical
tensile and thermal means, in comparison with silica fiber Bragg gratings. Therefore,
POF Bragg gratings can be very promising in optical communication and fiber sensing
systems. However, the big barrier of PMMA-based POF Bragg gratings for its further
application is the very high loss at the 1550 nm window, 104 or even 105 dB/km [1].
The development of the low loss perfluorinated polymer (CYTOP) material and fiber by Asahi Glass Co. and Keio University breaks the bottleneck and makes POF more advantageous in data communication networks [2]. CYTOP POF may also find a niche in fiber gratings applications, if there is significant photosensitivity in it. Here we investigate for the first time the photosensitivity of CYTOP material and the possibility of writing gratings in CYTOP fiber.
6.2 CYTOP material and CYTOP optical fiber
As it has been shown, optical loss in POFs can be categorized as either intrinsic or extrinsic loss [3]. Intrinsic loss includes vibrational overtones of the polymer materials, electronic absorption, and Rayleigh scattering. Extrinsic loss includes absorption caused by impurities, scattering from dust and micro-voids, and imperfections in fiber
121 Chapter 6: Photosensitivity of Low-loss Perfluoropolymer (CYTOP) Fiber Materials
parameters. Intrinsic loss is material related and cannot be reduced without drastic
changes in material composition. While extrinsic loss is related to material processing
and fiber fabrication and thus can be reduced by perfection of each procedure.
Table 6.1 Wavelength of fundamental stretching vibration of several atomic bonds [4].
Atomic bond ?to(Frn)
Si-O 9.0-10.0
C-Cl 11.7-18.2
C-C 7.6-10
C-H 3.3-3.5
C-D 4.5
c=o 5.3-6.5
C-F 7.6-10
C-O 7.9-10
O-H 2.8
The intrinsic loss of POF in near infra-red region is mainly brought by the molecular vibration and electron transition, which are all caused by atomic bonds and functional groups in polymer molecule [5]. Therefore, the wavelength of fundamental stretching vibration of the atomic bonds, Ao, has a determined effect on the absorption loss. Table 6.1 [4] shows the wavelength of fundamental stretching vibration for several atomic bonds. Since larger Xq will lead to a low absorption of its overtone in the
122 Chapter 6: Photosensitivity of Low-loss Perfluoropolymer (CYTOP) Fiber Materials
wavelength range for the light source of 650 nm-1550 nm, polymer consisted of C-C,
C-F, C-Cl and C-0 bonds are preferable.
The innovation polymer, CYTOP, was invented in 1990s. Its molecular structure is
shown in Fig. 6.1 [4]. As shown in Fig. 6.1, CYTOP consists of C-C, C-F and C-O
bonds. The shortest Ao in CYTOP is for C-C bonds with the value of 7.6 pm-10 pm,
while the shortest one in PMMA is for C-H bonds with the value of 3.3 pm-3.5 pm.
CYTOP has a more than double Ao compared with PMMA, which makes it lower transmission loss.
-(CF2-CF-CF-CF2)
Figure 6.1 Molecular structure of CYTOP [4].
In view of the fact that the serious intrinsic absorption loss due to the carbon hydrogen vibration that exists in PMMA-based POF is completely eliminated in
CYTOP based fiber, the attenuation of CYTOP POF is rather low at visible as well as infrared wavelengths [4]. By optimal design of index profile, researchers in Keio
University managed to fabricate CYTOP based graded-index optical fiber with the loss down to 40dB/km even in the near infra-red region [6]. It is also clarified that the theoretical attenuation limit of CYTOP based fiber is much comparable to that of the silica based fiber (0.12dB/km at 1171 nm [7]). Furthermore, it also found that the material dispersion for CYTOP is very low, and a higher bandwidth than PMMA-based
123 Chapter 6: Photosensitivity of Low-loss Perfluoropolymer (CYTOP) Fiber Materials
graded-index fiber and Ge02-Si02 based multimode fiber can be expected [8]. Apart
from the above advantages, high temperature and humidity stability, excellent acid,
alkali and organic solvent resistance, and low bending loss [9] [10] are also shown in
the CYTOP fiber. As a result, the CYTOP graded-index fiber is very promising for
high-speed data communication and telecommunication.
6.3 Experimental methods
Since the main market for CYTOP fiber is in short distance communication, the commercial available CYTOP fibers are multi-mode. As there is no single-mode
CYTOP fiber available, we worked on thin slabs made from a CYTOP graded-index multimode optical fiber by Asahi Glass Company instead. The cross section of the multi-mode CYTOP fiber is shown in Fig. 6.2.
Figure 6.2 Cross section of the graded index CYTOP fiber.
This fiber has a CYTOP core of about 200 jam in diameter. There are two claddings in this fiber: the inner one is CYTOP cladding, and the outside one is PMMA cladding. The outer diameter of this CYTOP fiber is about 900 pm. The reason for using PMMA as the outside cladding is that PMMA is much cheaper than the CYTOP.
124 Chapter 6: Photosensitivity of Low-loss Perfluoropolymer (CYTOP) Fiber Materials
By using this two cladding configuration, the fiber is less costly, while meeting the requirement of outer diameter.
To examine the photosensitivity in CYTOP material, the PMMA cladding is removed and the remaining CYTOP fiber was squeezed into the fiber slab between two heated clean metal plates. The thickness of the slabs is normally about 100 pm. Bragg gratings are then written on these slabs using the simple phase mask technique, as shown in Fig.6.3. The fiber slabs were put quite close to the phase mask. In order to protect the phase mask, a thin glass slide was placed between the slabs and the phase mask. The phase mask has a period of 1.061 pm designed for operation at a wavelength of 248 nm. The UV writing beam of up to 500 mJ/cm at 355 nm wavelength was obtained from a frequency-tripled Nd:YAG laser. The beam diameter of the output UV pulse is about 1 cm with pulse duration of 5 ns at 10 Hz repetition rate.
UV laser
Phase mask
thin slab made from CYTOP
Figure 6.3 Experimental gratings fabrication plan.
125 Chapter 6: Photosensitivity of Low-loss Perfluoropolymer (CYTOP) Fiber Materials
A diffraction grating is created after the slab was exposed to UV light after a
certain time. The diffraction pattern can be directly observed using a He-Ne laser,
arranged as in Fig. 6.4.
diffraction pattern
CYTOP slab gratingf
He-Ne laser
Figure 6.4 Grating testing arrangement and diffraction pattern of a CYTOP grating. The inset at
the top is a diffraction pattern photo from the CYTOP grating.
The thermal tuning characteristics of the CYTOP slab grating is also tested with the experimental set-up shown in Fig. 6.5. The CYTOP slab grating is immersed in a transparent bath of water and the bath is placed in a glass container, which also has a heater. The water bath also acts as the matching oil to remove the surface grating effects on the CYTOP slab grating. (The refractive index of water is about 1.33, which is almost the same as that of CYTOP fiber material.) A He-Ne laser beam is normal incident to the slab grating. The diffraction pattern is shown on the front screen. The angle of the first order diffraction is detected at different water temperature, which is controlled by the heater. The distance from the screen to slab grating is about 3 m;
126 Chapter 6: Photosensitivity of Low-loss Perfluoropolymer (CYTOP) Fiber Materials
therefore the thickness of the water medium and the glass container is comparatively
quite small and can be neglected.
-1 ohler 0 order Screen
Diffraction N Heater
Glass container filled with water
CYTOP slab with gratings
He-Ne Laser
Figure 6.5 Thermal tuning test of CYTOP slab grating (dimension not in scale).
6.4 Photosensitivity of CYTOP fiber material
The diffraction pattern of the CYTOP slab gratings is also shown in Fig. 6.4. The diffraction angle of the first order was measured to be 36° in our case. According to the
Bragg diffraction relation,
A = /lsin 0 (6.1) where A is the period of Bragg gratings, A, is the incident wavelength and 0 is the diffraction angle of the first order diffraction. The period of the CYTOP slab grating is then calculated to be 1.06 pm, which is the same as the phase mask period. This is because the phase mask is not designed for the wavelength of the writing UV beam and
127 Chapter 6: Photosensitivity of Low-loss Perfluoropolymer (CYTOP) Fiber Materials the zero order of the diffracted UV beam is strong enough to form diffraction pattern.
By measuring the powers of the first and zero order fringes of the CYTOP slab gratings, the diffraction efficiency can be determined. Since the higher orders of diffraction fringes are negligible, the diffraction efficiency is simply expressed as
V = (Pt> + P.1)/Pl„,a, (6.2) where P_i and P+i are the powers of the first orders and Ptotai is the total power including the zero order as well. The diffraction efficiencies of a grating with various UV exposure durations were measured and Fig. 6.6 shows the results. In this experiment, the frequency-tripled YAG laser output is 280 mJ at 355 nm.
1.6 - ^ 1.4
5 0.4 -
0.2 -
UV exposure time (min) Figure 6.6 Diffraction efficiency of CYTOP slab gratings versus UV exposure time to a frequency-tripled YAG laser. The solid-line is from the bare grating. The dot-line is the result
from the same grating with the index matching oil on the surface.
It is obvious that the efficiency increases as exposure time increases up to about 20 minutes, achieving a maximum diffraction efficiency of 1.6%. Further increase in
128 Chapter 6: Photosensitivity of Low-loss Perfluoropolymer (CYTOP) Fiber Materials
exposure time reduces the diffraction efficiency. From the diffraction efficiency, we
can use Kogelink formula [11] to estimate the index modulation. The formula can be
represented as,
rj = sin2 (mAn / 2Ad cos 9d) (6.3)
where r| is the diffraction efficiency, t is the grating thickness, An is the index
modulation, A,d is the diffraction wavelength and 0d is the diffraction angle. The index
modulation corresponding for the maximum diffraction efficiency for CYTOP slab
grating is estimated to be 4x1 O'4.
In order to ascertain whether it is surface grating or bulk grating for the CYTOP
slab grating fabricated above, we added refractive index matching oil on the slab surface and measured the diffraction efficiency again. If it were a surface-relief type of grating, the diffraction pattern would disappear with the index matching oil on the surface. The experiment shows that the diffraction efficiency changes little as shown in
Fig. 6.6: where the solid-line is the diffraction efficiency from the grating without the matching oil and the dot-line is that with the index matching oil. Hence we confirm that the grating is bulk grating. Furthermore, the CYTOP slab grating is the result of the change of refractive index, not due to material ablation.
In the above CYTOP grating fabrication experiment, it is found that the grating can be inscribed only with a very high energy level. The UV output is 280 mJ at 355 nm for CYTOP grating fabrication. The maximum index modulation is 4xl04 at around 20 minutes’ irradiation. The index modulation will decrease with further exposure. The decrease in the index modulation may be due to the increase of the average refractive index caused by the further irradiation. In comparison with the
129 Chapter 6: Photosensitivity of Low-loss Perfluoropolymer (CYTOP) Fiber Materials
previous research from our group on the photosensitivity of PMMA preform, where
significant gratings can be fabricated in the PMMA preform by very low energy level
[12]. There are some researches on photosensitivity of PMMA bulk material induced
by UV light [11, 13-15]. Research shows that there is an increase in density of the
irradiation region and the density increase leads to the refractive index change, but
there are some arguments on the origin of the density increase. Because both PMMA
and CYTOP are acrylate-type polymer, it is reasonable to infer that this density
increase mechanism holds for CYTOP as well. Due to the higher initial density of
CYTOP (2.03g/cm3) [16] than that of PMMA (1.19g/cm3), higher UV irradiation energy is needed to form further density increase in CYTOP. Thus by considering the original density of the material, we can roughly explain why higher energy level is needed to form gratings in CYTOP.
6.5 Thermal test of CYTOP gratings
Before we report on the thermal test results of CYTOP slab gratings, we will have a close examination of the set-up for the test. The refraction sketch of the He-Ne light in the different media as indicated in the set-up diagram (Fig.6.5) is shown in Fig.6.7.
0i is the diffraction angle caused by the CYTOP gratings and we can only measure the output angle 64. From the refraction law, it is found,
nx • sin 6X = n2 ■ sin 02 - n3 • sin #3 = nA- sin 6X (6.5) where 11 is the refractive index of the different media and 0 is the refraction angle in the corresponding media shown in Fig. 6.7. Therefore, the diffraction angle can be found as,
130 Chapter 6: Photosensitivity of Low-loss Perfluoropolymer (CYTOP) Fiber Materials
<9, = sin"1 (sin1nx) (6.6)
From the above analysis, it is clear that the measured refraction angle 64 only depends
on the diffraction angle 0i. The changes in the refractive index of water (112) by the
variation of temperature will have no effects on the measurement. Therefore the
changes in the refractive angle 84 with the change of temperature are due to the
diffraction angle change of CYTOP gratings at different temperature.
Figure 6.7 Light refraction sketch of the thermal tuning test set-up (dimension not in scale).
By measuring the distance d and h shown in Fig. 6.5, 84 can be simply calcualted as atan(d/h). The measurement accuracy of the distance is 1 mm. Since both d and h are up to several meters, the accuracy for 84 is good enough. Fig. 6.8 shows the measured refraction angle 84 and the calculated diffraction angle of CYTOP gratings 81 at different temperature. The heating-up and cooling-down process was also carried out to check the reversibility of the thermal tuning. The refraction angle 84 is measured at both the heating-up process and the cooling-down duration. In Fig. 6.8, the heating
131 Chapter 6: Photosensitivity of Low-loss Perfluoropolymer (CYTOP) Fiber Materials
curve seems to be quite coincident with the cooling curve, thus indicating the excellent
reversibility of the tuning.
26.9 36.75
26.85 -f o Heating-up 36.7 A Cooling-down 2 26.8 OJj
3 26.75 + 36.6 o* fH 3 © 85 « 26.7 36.55 3 ’So QTQ 3 n 26.65 - Aq~ 36.5 c 'O AO 26.6 - 36.45 a A o V n c8 CTQ £ 26.55 36.4 3 -5 26.45 36.3 40 50 60 80 temperature(°C) Figure 6.8 The measured refraction angle 04 and the calculated CYTOP grating diffraction angle 0i at different tuning temperature. Following the Bragg diffraction equation (6.1), the changes in the CYTOP grating period A with temperature T changes are obtained and displayed in Fig.6.9. Around 7 nm change in the grating period is observed with the temperature variation of 60°C. Linear regression for the data was also carried out and the regression line is also shown in the figure. The mathematical expression for the line is expressed as, A = 1057.4 + 0.089*T = 1057.4(1 + 8.5xl0~5 *T). (6.7) The changes in the grating period are due to the thermal expansion. The linear expansion coefficient of CYTOP material is 7.4xlO°/°C [16], which is quite close to 132 Chapter 6: Photosensitivity of Low-loss Perfluoropolymer (CYTOP) Fiber Materials the gradient value in equation (6.7). Since the thermo-optic coefficient for CYTOP is not available, the thermal sensitivity and the tuning range for CYTOP fiber Bragg grating can’t be estimated yet. 1068 o Heating-up 1067 a Cooling-down 1066 - - linear regression o' 1065 S A?- 5 1064 'O •2 1063 e'A a 4 o' 1062 1061 4°' 1060 1059 20 30 40 50 60 70 80 temperature (°C) Figure 6.9 CYTOP grating period changes with thermal tuning temperature. The above results are just simple start for the photosensitivity study of CYTOP fiber. Lots of appealing work still needs doing. For the reason of the unavailable single mode CYTOP fiber and the existence of thick PMMA cladding, the CYTOP fiber Bragg grating fabrication can’t be put into action at the present stage. The interesting thermal characterization on the CYTOP fiber Bragg gratings can be carried out in the future research after the CYTOP fiber Bragg grating fabrication is successful. As also shown in the results, rather high UV energy level is needed for CYTOP grating 133 Chapter 6: Photosensitivity of Low-loss Perfluoropolymer (CYTOP) Fiber Materials fabrication; therefore, the attempt on the increase of its photosensitivity is worth making. 6.6 Conclusions The novel perfluropolymer (CYTOP) graded-index fiber is very promising for high-speed data communication and telecommunication because of its low-loss and high bandwith. The photosensitivity in CYTOP material is investigated for the first time. Significant diffraction has been observed from the slab gratings made from the CYTOP fiber material. A maximum diffraction efficiency of 1.6% has been achieved, corresponding to an index change approximately 4x10 4. This study demonstrates for the first time the existence of significant photosensitivity and the feasibility of writing Bragg grating in CYTOP fibers. In addition, thermal tuning of the CYTOP slab grating was also tested. The thermal tuning test on CYTOP fiber Bragg gratings needs carrying out in the future work after the attempt on the CYTOP fiber Bragg grating fabrication is successful. 6.7 References 1. C. Emslie, Review Polymer optical fibers. Journal of Material Science, 1988. 23: p. 2281-2293. 2. Y. Koike. Progress of plastic optical fiber technology, in European Conference on Optical Communications. 1996. Oslo, Norway. 3. C. Koeppen, R.F. Shi, W.D. Chen, and A.F. Garito, Properties of plastic optical fibers. Journal of Optical Society of America B, 1998. 15: p. 727-739. 134 Chapter 6: Photosensitivity of Low-loss Perfluoropolymer (CYTOP) Fiber Materials 4. N. Yoshihara. Performance of perflurinated POF. in International Polymer Optical Fiber Conference,. 1997. Hawaii, USA. 5. N. Tanio and Y. Koike. What is the most transparent polymer, in International Polymer Optical Fiber Conference,. 1997. Hawaii, USA. 6. Y. Koike. Progress of low-loss GI polymer optical fibre from visible to 1.5-pm wavelength, in European Conference on Optical Communications,. 1997. Edinburgh, Scotland. 7. Y. Koike, T. Ishigure, and E. Nihei, High-bandwith graded-index polymer optical fiber. Journal of Lightwave Technology, 1995. 13: p. 1475-1489. 8. Y. Koike and T. Ishigure, Recent progress of polymer optical fiber for high speed data communication. SPIE, 1999. 3799: p. 290-300. 9. K. Koganezawa and T. Onishi. Progress in perfluorinated GI-POF, LUCINA. in International Polymer Optical Fiber Conference. 1997. Hawaii, USA. 10. M. Sato, M. Hirari, T. Ishigure, and Y. Koike. Novel GI POF with high thermal stability, in International Polymer Optical Fiber Conference. 1997. Hawaii, USA. 11. M. Kopietz, M.D. Lechner, D.G. Steinmeier, J. Marotz, and H. Franke, Light- induced refractive index changes in polymethylmethacrylate (PMMA) blocks, in Polymer Photochemistry. 1984, Elsevier Applied Science Publishers Ltd.: England. 12. G.D. Peng, Z. Xiong, and P.L. Chu, Photosensitivity and gratings in dye-doped polymer optical fibers. Optical Fiber Technology, 1999. 5: p. 242-251. 135 Chapter 6: Photosensitivity of Low-loss Perfluoropolymer (CYTOP) Fiber Materials 13. W.J. Tomlinson, I.P. Kaminow, E.A. Chandross, R.L. Cork, and W.T. Silfvast, Photo-induced refractive index increase in poly (methylmethacrylate) and its application. Applied Physics Letters, 1970. 16: p. 486-489. 14. I.P. Kaminow, H.P. Weber, and E.A. Chandross, Poly (methylmethacrylate) dye laser with internal diffraction grating resonator. Applied Physics Letters, 1971. 18: p. 497-499. 15. M.J. Bowden, E.A. Chandross, and I.P. Kaminow, Mechanism of photo-induced refractive index increase in Polymethl Methacrylate. Applied Optics, 1974. 13: p. 112-117. 16. Product information, CYTOP-A high innovated fluoropolymer. 2000, Asahi Glass Company. 136 Chapter 7: Conclusions and Further Work Chapter 7: Conclusions and Further Work 7.1 Conclusions A systematic study on POF Bragg gratings is presented for the first time in this dissertation. Several novel conclusions on the fabrication, formation mechanism and characterization of POF Bragg gratings have been reached. The investigation on the growth dynamics of PMMA-based POF Bragg gratings clearly shows a distinctive threshold in UV exposure, separating two different stages in POF Bragg grating formation, which bears remarkable similarities to that of silica fiber Bragg gratings. By following the same nomenclatures of silica fiber Bragg gratings, the POF Bragg gratings formed in low and high index modulation regimes are named Type I and Type II POF Bragg gratings, respectively. Type I POF Bragg gratings have typical features of relatively small index modulation and no significant transmission losses, while Type II POF Bragg gratings have the features of large losses at short wavelengths, broad bandwidth and distinctive damages at the core and cladding interface. Based on the knowledge of POF Bragg gratings growth, we optimized the UV exposure fluence to produce the POF Bragg gratings with the best results ever achieved: a reflectivity of 0.999 and less than 0.5 nm line width. As Type I and Type II POF Bragg gratings are clearly identified, the two most important characteristics, the strain and temperature properties, are characterized. The thermal tuning research indicates that POF Bragg gratings have excellent thermal tunability with large tuning range, the absence of thermal hysteresis and highly linear temperature dependence of the Bragg wavelength. Moreover, the temperature sensitivity for POF Bragg gratings is much higher than that of silica fiber Bragg 137 Chapter 7: Conclusions and Further Work gratings. The temperature sensitivities for Type I and for Type II POF Bragg gratings are found to be equable and they are 10 times larger than those of silica fiber Bragg gratings. Thermal decay study of POF Bragg gratings indicate that Type II POF Bragg gratings is much more stable than Type I POF Bragg gratings. Type II POF Bragg gratings is very stable and no noticeable decay can be observed at 65°C. While for Type I POF Bragg gratings, a rapid decay followed by a substantially decreasing rate is found. Data analysis demonstrates that the decay behavior for Type I POF Bragg gratings doesn’t follow either “power law” model for nonhydrogenated Type I silica fiber gratings or the “log time” model for hydrogenated silica fiber gratings. Further work needs to be done on the modelling and mechanism of decay behaviour for Type I POF Bragg gratings. Strain characterization demonstrates that just by simple tensile strain POF Bragg gratings can achieve much better tunability than that of silica fiber gratings. Over 30 nm Bragg wavelength tuning range with no changes in the reflection level and excellent reversibility has been achieved. In addition, the relation between the tensile strain and Bragg wavelength shift is linear. Furthermore, the strain sensitivities for Type I and Type II POF Bragg gratings are proved to be the same and they are higher than those of silica fiber Bragg gratings. Due to the high strain and temperature sensitivity, POF Bragg gratings are very promising in the fiber sensing field. A simple and effective scheme for simultaneous measurement of strain and temperature by using a hybrid POF Bragg grating and silica fiber Bragg grating as sensor head is proposed as an example of future applications for POF Bragg gratings. Theoretical analysis proves that our proposed method is effective and simple with high accuracy and small error perturbation. 138 Chapter 7: Conclusions and Further Work Apart from fiber sensing applications, POF Bragg gratings are also capable in some optical communication systems. The simulation on linearly chirped POF Bragg gratings fabrication and its application in dynamic dispersion compensation has also been carried out. Results show that linearly chirped POF Bragg gratings can be easily fabricated by the tapered POF method. By controlling the tapered profile, a dynamic dispersion range from 1534 ps/nm to 66 ps/nm is achieved by the linearly chirped POF Bragg gratings. Besides the research on PMMA-based POF Bragg gratings, photosensitivity study is also carried out on the low loss perfluropolymer (CYTOP) fiber materials for the first time. Diffraction gratings have been successfully written on the CYTOP slab and a considerable diffraction efficiency is observed. This study demonstrates for the first time the existence of significant photosensitivity and the feasibility of writing Bragg grating in CYTOP fibers. 7.2 Suggestions on further work Some important discoveries on POF Bragg gratings have been achieved in this thesis, but it is just a starting point for this field and several topics with great interest need further exploring. Though Type I and Type II POF Bragg gratings have been clearly identified in POF Bragg gratings classification, the author believe it is still not the complete picture of the dynamics growth for POF Bragg gratings. As we all know the cases in silica fiber Bragg gratings, different kinds of gratings were found in fibers with different dopant, different UV exposure fluence and different types of UV laser (continuous or pulsed). In our study, we only examined the growth dynamics of PMMA-based POF Bragg gratings under pulsed UV exposure. Other tests on POF Bragg gratings growth such as using 139 Chapter 7: Conclusions and Further Work both pulsed and continuous UV source, and different power level would give us a more comprehensive picture about POF Bragg gratings formation. Further study of the mechanism on the formation of different types of POF Bragg gratings is also worth carrying out. Further work on the mechanism and modelling of thermal decay behaviour of Type I POF Bragg gratings still needs to be carried out. As we already pointed out, the thermal stress relaxation in POF Bragg gratings might be linked to its decay behaviour. Thus, the stress distribution of POF after UV exposure and thermal annealing is worth testing. Experiments on the linearly chirped POF Bragg gratings fabricated by tapered POF method can be carried out. The reflection and group delay spectra can be measured and compared with the simulation results given in Chapter Five. Due to its low loss, Bragg gratings on the CYTOP fiber are very attractive. The fabrication is worth trying when the single mode CYTOP fiber is available. Then, the mechanism study and characterization of the CYTOP fiber gratings can be done. 140