We review the recent developments in the area of optical fiber grating sensors, including quasi-distributed strain sensing using Bragg gratings, systems based on chirped gratings, intragrating sensing concepts, long period-based grating sensors, fiber grating laser-based systems, and interferometric sensor systems based on grating reflectors.

Fiber grating sensors

In this paper, we describe the spectral characteristics that can be achieved in fiber reflection (Bragg) and transmission gratings. Both principles for understanding and tools for designing fiber gratings are emphasized. Examples are given to illustrate the wide variety of optical properties that are possible in fiber gratings. The types of gratings considered include uniform, apodized, chirped, discrete phase-shifted, and superstructure gratings; short-period and long-period gratings; symmetric and tilted gratings; and cladding-mode and radiation-mode coupling gratings.

Fiber grating spectra

The historical beginnings of photosensitivity and fiber Bragg grating (FBG) technology are recounted. The basic techniques for fiber grating fabrication, their characteristics, and the fundamental properties of fiber gratings are described. The many applications of fiber grating technology are tabulated, and some selected applications are briefly described.

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Fiber Bragg grating technology fundamentals and overview

We present a new class of long-period fiber gratings that can be used as in-fiber, low-loss, band-rejection filters. Photoinduced periodic structures written in the core of standard communication-grade fibers couple light from the fundamental guided mode to forward propagating cladding modes and act as spectrally selective loss elements with insertion losses act as backreflections <-80 dB, polarization-mode-dispersions <0.01 ps and polarization-dependent-losses <0.02 dB.

Long-period fiber grating as band-rejection filters

Long-period fiber gratings as band-rejection filters

High pressure 'hydrogen loading' has been used to sensitise standard singlemode fibres, resulting in the largest reported UV induced index changes for low GeO/sub 2/ fibres. Grating bandwidths of 4 nm and peak nu ns of 5.9*10/sup -3/ have been reproducibly achieved. Substantial index changes have also been achieved by rapidly heating H/sub 2/ loaded fibres of various compositions.<
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High pressure H/sub 2/ loading as a technique for achieving ultrahigh UV photosensitivity and thermal sensitivity in GeO/sub 2/ doped optical fibres

Long-period fiber gratings are used to flatten the gain spectrum of erbium-doped fiber amplifiers. A broadband amplifier with <0.2-dB gain variation over 30 nm is presented. We also show that a chain of amplifiers can be equalized, leading to a bandwidth enhancement by a factor of 3.

Long-period fiber-grating-based gain equalizers.

Optical fiber communication systems that use optical amplifiers are increasingly seeking high-performance devices that function as spectrally-selective band-rejection filters. For example, ASE filters that improve erbium amplifier performance and band-rejection filters in Raman lasers/amplifiers1 must have low insertion losses and low back-reflections. In addition, they must be relatively inexpensive to mass-produce and should be compact after packaging. While bulk-optic filters and short-period Bragg gratings2,3 can be used for some of the applications, all the aforementioned requirements are rarely met. In this paper, we present a novel class of photoinduced, long-period fiber gratings that function as highly-efficient band-rejection filters.

Hydrogen-induced loss increases are known to occur in optical fibers, due both to the presence of molecular H2 and to the reaction of hydrogen with defects in the fibers. To predict the long-term loss increases expected for fibers under normal conditions, it is necessary to rely on accelerated aging experiments in which fiber loss increases are measured at high temperatures and high PH2'S. For the predictions to be reliable, the dependences must be characterized on temperature, PH2, and time for each of the hydrogen-induced loss mechanisms. In some cases, simplifying assumptions can be made in arriving at conservative lifetime predictions. Hermetic coatings can be used as one means of minimizing the amount of hydrogen seen by a fiber. The performance of a hermetic coating can often be evaluated by using accelerated experiments performed at high PH2's and moderate temperatures. The presence of reactive gettering sites can have a significant effect on the loss increases measured in high-temperature experiments with hermetic fibers. Experimental resuIts are presented for hermetic and nonhermetic fibers as examples of how long-term predictions can be made.

Paul Joseph Lemaire

Papers

Long-period fiber gratings as band-rejection filters

High pressure H/sub 2/ loading as a technique for achieving ultrahigh UV photosensitivity and thermal sensitivity in GeO/sub 2/ doped optical fibres

Long-period fiber-grating-based gain equalizers.

Long-period fiber gratings as band-rejection filters

Reliability of optical fibers exposed to hydrogen: prediction of long-term loss increases