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Chapter 1: IntroductionOptical fibers have revolutionized telecommunication. Much of the success of optical fiber lies in its near-ideal properties: low transmission loss, high optical damage threshold, and low optical nonlinearity. The combination of these properties has enabled long-distance communication to become a reality. At the same time, the long lengths enabled the optical power to interact with the small nonlinearity to give rise to the phenomenon of optical solitons, overcoming the limit imposed by linear dispersion. The market for optical fiber continues to grow, despite the fact that major trunk routes and metropolitan areas have already seen a large deployment of fiber. The next stage in the field of communication is the mass delivery of integrated services, such as home banking, shopping, Internet services, and entertainment using video-on-demand. Although the bandwidth available on single mode fiber should meet the ever-increasing demand for information capacity, architectures for future networks need to exploit technologies which have the potential of driving down cost to make services economically viable. Optical fiber will have to compete with other transport media such as radio, copper cable, and satellite. Short-term economics and long-term evolutionary potential will determine the type of technology likely to succeed in the provision of these services. But it is clear that optical fibers will play a crucial role in communication systems of the future. The technological advances made in the field of photosensitive optical fibers are relatively recent; however, an increasing number of fiber devices based on this technology are getting nearer to the market place. It is believed that they will provide options to the network designer that should influence, for example, the deployment of wavelength-division-multiplexed (WDM) systems, channel selection, and deployment of transmitters in the upstream path in a network, and should make routing viable. The fascinating technology of photosensitive fiber is based on the principle of a simple in-line all-fiber optical filter, with a vast number of applications to its credit.
1.1 Historical perspective
Photosensitivity of optical fiber was discovered at the Canadian Communication Research Center in 1978 by Ken Hill et al.  during experiments using germania-doped silica fiber and visible argon ion laser radiation. It was noted that as a function of time, light launched into the fiber was increasingly reflected. This was recognized to be due to a refractive index grating written into the core of the optical fiber as a result of a standing wave intensity pattern formed by the 4% back reflection from the far end of the fiber and forward-propagating light. The refractive index grating grew in concert with the increase in reflection, which in turn increased the intensity of the standing wave pattern. The periodic refractive index variation in a meter or so of fiber was a Bragg grating with a bandwidth of around 200 MHz. But the importance of the discovery in future applications was recognized even at that time. This curious phenomenon remained the preserve of a few researchers for nearly a decade [2,3]. The primary reason for this is believed to be the difficulty in setting up the original experiments, and also because it was thought that the observations were confined to the one "magic" fiber at CRC. Further, the writing wavelength determined the spectral region of the reflection grating, limited to the visible part of the spectrum.
Researchers were already experimenting and studying the even more bizarre phenomenon of second-harmonic generation in optical fibers made of germania-doped silica, a material that has a zero second-order nonlinear coefficient responsible for second-harmonic generation. The observation was quite distinct from another nonlinear phenomenon of sumfrequency generation reported earlier by Ohmori and Sasaki  and Hill et al. , which were also curious. Ulf Osterberg and Walter Margulis  found that ML-QS infrared radiation could "condition" a germania dopedsilica fiber after long exposure such that second-harmonic radiation grew (as did Ken Hill's reflection grating) to nearly 5% efficiency and was soon identified to be a grating formed by a nonlinear process [7,8]. Julian Stone's  observation that virtually any germania-doped silica fiber demonstrated a sensitivity to argon laser radiation reopened activity in the field of fiber gratings [10,11] and for determining possible links between the two photosensitive effects. Bures et al.  had pointed out the twophoton absorption nature of the phenomenon from the fundamental radiation at 488 nm.
The major breakthrough came with the report on holographic writing of gratings using single-photon absorption at 244 nm by Gerry Meltz et al. . They demonstrated reflection gratings in the visible part of the spectrum (571-600 nm) using two interfering beams external to the fiber. The scheme provided the much-needed degree of freedom to shift the Bragg condition to longer and more useful wavelengths, predominantly dependent on the angle between the interfering beams. This principle was extended to fabricate reflection gratings at 1530 nm, a wavelength of interest in telecommunications, also allowing the demonstration of the first fiber laser operating from the reflection of the photosensitive fiber grating . The LTV induced index change in untreated optical fibers was -10-4. Since then, several developments have taken place that have pushed the index change in optical fibers up a hundredfold, making it possible to create efficient reflectors only a hundred wavelengths long. Lemaire and coworkers  showed that the loading of optical fiber with molecular hydrogen photosensitized even standard telecommunication fiber to the extent that gratings with very large refractive index modulation could be written.
Pure fused silica has shown yet another facet of its curious properties. It was reported by Brueck et al.  that at 350°C, a voltage of about 5 kV applied across a sheet of silica, a millimeter thick, for 30 minutes resulted in a permanently induced second-order nonlinearity of ---1 pm/V Although poling of optical fibers had been reported earlier using electric fields and blue-light and LTV radiation [17-19], Wong et al.  demonstrated that poling a fiber while writing a grating with UV light resulted in an enhanced electro-optic coefficient. The strength of the UV written grating could be subsequently modulated by the application of an electric field. More recently, Fujiwara et al. reported a similar photoassisted poling of bulk germanium-doped silica glass . The silica-germanium system will no doubt produce further surprises.
All these photosensitive processes are linked in some ways but can also differ dramatically in their microscopic detail. The physics of the effect continues to be debated, although the presence of defects plays a central role in more than one way...
Table of Contents
- Chapter 1: Introduction.....1
- Chapter 2: Photosensitivity and Photosensitization of Optical Fibers.....13
- Chapter 3: Fabrication of Bragg Gratings.....55
- Chapter 4: Theory of Fiber Bragg Gratings.....119
- Chapter 5: Apodization of Fiber Gratings.....195
- Chapter 6: Fiber Grating Band-pass Filters.....227
- Chapter 7: Chirped Fiber Bragg Gratings.....311
- Chapter 8: Fiber Grating Lasers and Amplifiers.....355
- Chapter 9: Measurement and Characterization of Gratings.....409
- Chapter 1: Introduction.....1
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 um 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. This 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 this 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 specific applications, and concludes with measurement techniques.
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