Fiber Bragg Gratings

Fiber Bragg Gratings

Raman Kashyap
Published in: Academic Press
Release Year: 1999
ISBN: 0-12-400560-8
Pages: 478
Edition: First Edition
File Size: 33 MB
File Type: pdf
Language: English

Description of Fiber Bragg Gratings

The field of fiber Bragg gratings is almost exactly twenty years old, dating back to its discovery by Ken Hill and co-workers in Canada. It grew slowly at first, but an important technological advance by Gerry Meltz and co-workers 10 years later, renewed worldwide interest in the subject. I was instrumental in setting up the first International Symposium on the photosensitivity of optical fibers, jointly with Francois Ouellette in 1991, a meeting with 22 presentations and attended by approximately 50 researchers.
Since, we have seen three further international conferences solely devoted to fiber Bragg gratings, the last of which was attended by approximately 300 researchers. As the applications of Bragg gratings are numerous, publications appear in widely differing conferences and journals. Surprisingly, apart from several review articles covering the most elementary aspects, no monograph is available on the subject and the quantity of available literature is spread across a number of specialist journals and proceedings of conferences. Thus, progress and the current state of the art are difficult to track, despite the approaching maturity of the field. 
More recently, the poling of glass optical fibers has resulted in an electrooptic coefficient almost rivaling that of lithium niobate. Germanium, the core dopant of low loss, fused silica optical fiber, is a rich defect former; ultraviolet radiation can strongly modify the nature of the defects causing large changes in the local refractive index. The mechanisms contributing to photosensitivity are complicated and still being debated. They depend on the types of defects present, dopants, and the presence of hydrogen whether in the molecular or in the ionic state. The lack of a thorough understanding has not, however, prevented the
exploitation of the effect in a large number of applications. The very large index changes reported to date (~0.03) allow, for the first time, the fabrication of ultra-short (~100 m long) broadband, high-reflectivity
Bragg gratings in optical fibers. The maximum index change may be an order of magnitude larger still, leading to many more exciting possibilities.
There are a number of methods of the holographic inscription of Bragg gratings, with the phase-mask technique holding a prominent position. Fiber Bragg Gratings' book was born as a result of growing demands for yet more
review articles on the subject. It aims to fill the gap by bringing together the fundamentals of fiber gratings, their specific characteristics, and many of the applications. The book covers much of the fundamental material on gratings and should be of interest to beginners, advanced researchers, as well as those interested in the fabrication of many types of gratings. 
It is impossible to cover the massive advances made in this field in a book of this size, a field that continues to grow at an enormous rate despite recent commercialization. A large reference list is provided, to allow the interested reader to seek out specific topics in more detail. The purpose of Fiber Bragg Gratings' book is, therefore, to introduce the reader to the extremely rich area of the technology of fiber Bragg, with a view to providing insight into some of the exciting prospects. It begins with the principles of fiber Bragg gratings, photosensitization of optical fibers, Bragg grating fabrication, theory, properties of gratings, and

Content of Fiber Bragg Gratings

Chapter 1 Introduction 1
1.1 Historical perspective 2
1.2 Materials for glass fibers 4
1.3 Origins of the refractive index of glass 6
1.4 Overview of chapters 8
References 10
Chapter 2 Photosensitivity and Photosensitization of

Optical Fibers 13
2.1 Photorefractivity and photosensitivity 14
2.2 Defects in glass 16
2.3 Detection of defects 19
2.4 Photosensitization techniques 20
2.4.1 Germanium-doped silica fibers 21
2.4.2 Germanium-boron codoped silicate fibers 27
2.4.3 Tin-germanium codoped fibers 29
2.4.4 Cold, high-pressure hydrogenation 29
2.4.5 rare-earth-doped fibers 34
2.5 Densification and stress in fibers 35
2.6 Summary of photosensitive mechanisms in
Germano silicate fibers 36
2.7 Summary of routes to photosensitization 38
2.7.1 Summary of optically induced effects 42
References 44
Chapter 3 Fabrication of Bragg Gratings 55
3.1 Methods for fiber Bragg grating fabrication 55

3.1.1 The bulk interferometer 55
3.1.2 The phase mask 57
3.1.3 The phase mask interferometer 62
3.1.4 Slanted grating 69
3.1.5 The scanned phase mask interferometer 71
3.1.6 The Lloyd mirror and prism interferometer 74
3.1.7 Higher spatial order masks 77
3.1.8 Point-by-point writing 80
3.1.9 Gratings for mode and polarization conversion 80
3.1.10 Single-shot writing of gratings 83
3.1.11 Long-period grating fabrication 84
3.1.12 Ultralong-fiber gratings 85
3.1.13 Tuning of the Bragg wavelength, moire,
Fabry-Perot, and superstructure gratings 88
3.1.14 Fabrication of continuously chirped gratings 93
3.1.15 Fabrication of step-chirped gratings 99
3.2 Type II gratings 101
3.3 Type IIA gratings 101
3.4 Sources for holographic writing of gratings 102
3.4.1 Low coherence sources 102
3.4.2 High coherence sources 104
References 108
Chapter 4 Theory of Fiber Bragg Gratings 119
4.1 Wave Propagation 121
4.1.1 Waveguides 122
4.2 Coupled-mode theory 125
4.2.1 Spatially periodic refractive index modulation 127
4.2.2 Phase matching 130
4.2.3 Mode symmetry and the overlap integral 131
4.2.4 Spatially periodic nonsinusoidal refractive index
modulation 133
4.2.5 Types of mode coupling 134
4.3 Coupling of counterpropagating guided modes 142
4.4 Codirectional coupling 145
4.5 Polarization couplers: Rocking filters 148
4.6 Properties of uniform Bragg gratings 152
4.6.1 Phase and group delay of uniform period
gratings 155

4.7 Radiation mode couplers 157
4.7.1 Counterpropagating radiation mode coupler:
The side-tap grating 157

4.7.2 Copropagating radiation mode coupling: Long-
period gratings 171

4.8 Grating simulation 178
4.8.1 Methods for simulating gratings 178
4.8.2 Transfer matrix method 179
4.9 Multilayer analysis 185
4.9.1 Rouard's method 185
4.9.2 The multiple thin-film stack 186
References 189
Chapter 5 Apodization of Fiber Gratings 195
5.1 Apodization shading functions 197
5.2 Basic principles and methodology 199
5.2.1 Self-apodization 200
5.2.2 The amplitude mask 203
5.2.3 The variable diffraction efficiency phase mask 205
5.2.4 Multiple printing of in-fiber gratings applied to
apodization 206
5.2.5 Position-weighted fabrication of top-hat
reflection gratings 208
5.2.6 The moving fiber/phase mask technique 211
5.2.7 The symmetric stretch apodization method 216
5.3 Fabrication requirements for apodization and chirp 221
References 223
Chapter 6 Fiber Grating Band-pass Filters 227
6.1 Distributed feedback, Fabry-Perot, superstructures,
and moire gratings 229
6.1.1 The distributed feedback grating 229
6.1.2 Superstructure band-pass filter 239
6.2 The Fabry-Perot and moire band-pass filters 242
6.3 The Michelson interferometer band-pass filter 246
6.3.1 The asymmetric Michelson multiple-band-pass
filter 255
6.4 The Mach-Zehnder interferometer band-pass filter 260

6.4.1 Optical add-drop multiplexers based on the
6.5 The optical circulator based OADM 265
6.5.1 Reconfigurable OADM 270
6.6 The polarizing beam splitter band-pass filter 272
6.7 In-coupler Bragg grating filters 276
6.7.1 Bragg reflecting coupler OADM 278
6.7.2 Grating-frustrated coupler 284
6.8 Side-tap and long-period grating band-pass filters 288
6.9 Polarization rocking band-pass filter 293
6.10 Mode converters 297
6.10.1 Guided-mode intermodal couplers 297
References 300
Chapter 7 Chirped Fiber Bragg Gratings 311
7.1 General characteristics of chirped gratings 312
7.2 Chirped and step-chirped gratings 317
7.2.1 Effect of apodization 324
7.2.2 Effect of nonuniform refractive index
modulation on grating period 330
7.3 Super-step-chirped gratings 332
7.4 Polarization mode dispersion in chirped gratings 336
7.5 Systems measurements with DCGs 339
7.5.1 Systems simulations and chirped grating
performance 342
7.6 Other applications of chirped gratings 346
References 347
Chapter 8 Fiber Grating Lasers and Amplifiers 355
8.1 Fiber grating semiconductor lasers: The FGSL 355
8.2 Static and dynamic properties of FGLs 362
8.2.1 Modeling of external cavity lasers 366
8.2.2 General comments on FGLs 369
8.3 The fiber Bragg grating rare-earth-doped fiber laser 370
8.4 Erbium-doped fiber lasers 372
8.4.1 Single-frequency erbium-doped fiber lasers 374
8.5 The distributed feedback fiber laser 377
8.5.1 Multifrequency sources 379
8.5.2 Tunable single-frequency sources 380

8.6 Bragg grating based pulsed sources 380
8.7 Fiber grating resonant Raman amplifiers 383
8.8 Gain-flattening and clamping in fiber amplifiers 385
8.8.1 Amplifier gain equalization with fiber gratings 387
8.8.2 Optical gain control by gain clamping 391
8.8.3 Analysis of gain-controlled amplifiers 395
8.8.4 Cavity stability 396
8.8.5 Noise figure 397
References 398
Chapter 9 Measurement and Characterization of Gratings 409
9.1 Measurement of reflection and transmission spectra of
Bragg gratings 410
9.2 Perfect Bragg gratings 417
9.3 Phase and temporal response of Bragg gratings 418
9.3.1 Measurement of the grating profile 426
9.3.2 Measurement of internal stress 432
9.4 Strength, annealing, and lifetime of gratings 435
9.4.1 Mechanical strength 435
9.4.2 Bragg grating lifetime and thermal annealing 436
9.4.3 Accelerated aging of gratings 440
References 441
Index 447
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