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

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

This tutorial/survey paper: (1) provides a concise point of departure for researchers and practitioners alike wishing to assess the current state of the art in the control and monitoring of civil engineering structures; and (2) provides a link between structural control and other fields of control theory, pointing out both differences and similarities, and points out where future research and application efforts are likely to prove fruitful. The paper consists of the following sections: section 1 is an introduction; section 2 deals with passive energy dissipation; section 3 deals with active control; section 4 deals with hybrid and semiactive control systems; section 5 discusses sensors for structural control; section 6 deals with smart material systems; section 7 deals with health monitoring and damage detection; and section 8 deals with research needs. An extensive list of references is provided in the references section.

Structural control: past, present, and future

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

Bragg gratings have been produced in germanosilicate optical fibers by exposing the core, through the side of the cladding, to a coherent UV two-beam interference pattern with a wavelength selected to lie in the oxygen-vacancy defect band of germania, near 244 nm. Fractional index perturbations of approximately 3 x 10(-5) have been written in a 4.4-mm length of the core with a 5-min exposure. The Bragg filters formed by this new technique had reflectivities of 50-55% and spectral widths, at half-maximum, of 42 GHz.

Formation of Bragg gratings in optical fibers by a transverse holographic method

In many network and sensor applications, small amounts of light need to be coupled out at many points from a transmission bus fiber. In-line devices with some wavelength selectivity are particularly desirable, especially if light radiated from the fiber can be efficiently coupled into another fiber. This paper describes a new method developed for the noninvasive production of wavelength selective taps in germanosilicate optical fiber. The technique is based on a recent discovery that Bragg gratings can be formed in the core of a fiber by exposing it through the side of the cladding to a coherent UV interference pattern at a wavelength which lies in the oxygen vacancy defect band of germania.1-3 The grating period and tilt of the Bragg planes are selected to couple coherently a guided mode into a narrow radiated beam which propagates through the side of the cladding. The same arrangement is also useful for coupling light into the fiber.

In-fiber Bragg grating tap

ZA P , Z A P , ZAP...two beams from a pulsed U V laser collide on the side of an optical fiber and a wave­ length filter or out-coupling tap is produced. With this\" method, we have developed a new way for generating periodic modulations of the refractive index, or gratings, in the core of a germania-doped optical fiber. When the wavelength of a guided optical signal in the fiber is twice the grating periodicity (the Bragg condition), a fraction of the signal will be reflected into a counter propagating mode. That is, the grating produces a band rejection filter in the fiber. K .O. H i l l et a l . 1 , 2 were the first to produce such gratings in optical fibers by forming a standing wave pattern with intense visible argon laser light coupled into the end of the fiber. Lam and Garside3 later showed that this was a two photon process. By going to U V wavelengths and a single photon process, we could achieve much higher writing efficiencies.4 With U V light a side exposure is required, due to the high absorption of the germania-doped core at these wavelengths. The side exposure has important advantages: it allows the grating spacing and intensity to be varied or modified, and the gratings can be localized and distributed at any position along a length of fiber. The technique is noninvasive and requires only that the buffer coating be temporarily removed for the exposure. Many applications for the gratings in fields such as distributed sensors, data communications, and signal processing should be found.

Holographically generated gratings in optical fibers

An all-fiber Fabry-Perot interferometer was made using intracore Bragg grating reflectors fabricated with the side exposure technique.1 This method provides a new and relatively easy way to make such an interferometer that can be used in dense WDM communications. Fiber Bragg reflectors made to date with this technique have spectral bandwidths ranging from 0.1 to 1 nm at the communications band wavelengths for corresponding grating lengths of 15 to 1.5 mm. Reflectivities approaching 100% have been achieved in high germania-doped commercial fibers. Measured fiber grating losses at these wavelengths do not exceed about 0.13%.

W. H. Glenn

Papers

Formation of Bragg gratings in optical fibers by a transverse holographic method

In-fiber Bragg grating tap

Holographically generated gratings in optical fibers

Fiber Fabry-Perot interferometer using side exposed fiber Bragg gratings