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Preface
Despite the lapse of a decade since the previous edition of this book was pub-
lished, fiber Bragg gratings continue to flourish and their applications expand.
As has been the experience with optical fibers in the past, new discoveries have
continued to remain a driver for technological developments. In this respect, the
past decade has seen further activity in the poling of glass, fiber Bragg grating
sensors, high-power fiber lasers, and the opening of a new research on femtosec-
ond (fs) laser processing, which was just beginning to grow when the first edi-
tion came out in print. To reflect these developments, this edition has three
new chapters that touch on the topics of sensing, fs laser writing of fiber Bragg
gratings (FBGs), and poling of glass and optical fibers. It is hoped that these
chapters will bring the book into the mainstream of topical research interest.
The basis of the FBG, the refractive index change induced by ultraviolet or fs
laser pulses, now stands around a record 0.1, having met the prediction made
in 1999. Truly broadband mirrors spanning 300 nm are now possible in fiber
with high reflectivity (99%), challenging thin-film technology. In fact, some of
the periodic nanostructured gratings formed by fs laser pulses have a glass-air
boundary, which leads to the possibility of miniaturizing devices still further
with the large refractive index contrast of 0.45. The fs laser has allowed the
writing of strong gratings in materials that have traditionally been nonphoto-
sensitive, such as pure silica and ZBLAN glass. The use of high-intensity pulses
enables multiphoton absorption to occur, and these pulses also literally rip the
electrons out of their orbits to the conduction band, inducing plasmas and car-
rier heating. The optical damage that results has interesting applications in
strong gratings for high-temperature sensing. Indeed, sapphire fiber gratings
for high-temperature turbine measurements would not have been possible with-
out fs lasers. High-power lasers have suddenly become commonplace at unusual
wavelength, fueled by the downturn in the telecommunications and the rise of
the multibillion-dollar biophotonics and sensing industries. FBGs have found
their place in peculiar applications such as in the investigation of strain in the
human lumbar column. Glass poling, too, has evolved, even though the goal
of the 10 pm-V-1 electrically induced nonlinearity remains elusive. New polari-
zation controllers, fiber-based Q-switches, and other tunable FBG devices have
come of age with optical fiber poling. The low-loss optical fibers for telecommu-
nications made of a fused silica cladding and a germania-doped core still
xvmaintain their pride of place in optical fiber technology. Rare earth dopants in
silica and other glasses have made many more applications possible. The advent
of photonic crystal fibers are now demonstrating a way to increase the power-
handling capacity of optical fibers, although high-quality gratings remain diffi-
cult to implement in these fibers. Gratings are being applied to reduce the
impact of nonlinearities in fibers, pulse shaping and compression, and signal
processing. The mechanisms contributing to photosensitivity continue to be
debated, although major advances have been made in this area. There are a
number of methods of the holographic inscription of Bragg gratings using ultra-
violet radiation or infrared fs pulses, with the phase-mask technique holding a
prominent position. These methods have multiplied, with several techniques
demonstrated for the fabrication of ultralong gratings. New areas just on the
brink of breakthroughs, such as random lasers, are highly compatible with the
FBG. It is impossible to cover the massive advances made in this field in a book
of this size (even though the second edition is now vastly expanded), a field in
which the number of applications has exploded. The book therefore continues
to be an introduction to the extremely rich area of the technology of fiber grat-
ings, with a view to providing an insight to some of the exciting prospects,
including the principles of fiber Bragg gratings, the photosensitization of optical
fibers, Bragg grating fabrication, theory, properties of gratings, specific applica-
tions, sensing technology, glass poling, femtosecond processing of glass, and
FBG measurement techniques.
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发表于 2010-7-13 20:47:26
