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

With the goal of being able to create optical devices for the telecommunications industry, we investigated the effects of 810-nm, femtosecond laser radiation on various glasses. By focusing the laser beam through a microscope objective, we successfully wrote transparent, but visible, round-elliptical damage lines inside high-silica, borate, soda lime silicate, and fluorozirconate (ZBLAN) bulk glasses. Microellipsometer measurements of the damaged region in the pure and Ge-doped silica glasses showed a 0.01–0.035 refractive-index increase, depending on the radiation dose. The formation of several defects, including Si E′ or Ge E′ centers, nonbridging oxygen hole centers, and peroxy radicals, was also detected. These results suggest that multiphoton interactions occur in the glasses and that it may be possible to write three-dimensional optical circuits in bulk glasses with such a focused laser beam technique.

Writing waveguides in glass with a femtosecond laser

Sensitive optical biosensors for unlabeled targets: a review.

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

The observation of photosensitivity in Ge‐doped core optical fibers is reported. The photosensitivity is manifested by light‐induced refractive‐index changes in the core of the waveguide. Narrowband reflectors in a guide structure have been fabricated using this photosensitivity and the resulting DFB reflectors employed as laser mirrors in a cw gas laser in the visible.

Photosensitivity in optical fiber waveguides: Application to reflection filter fabrication

Strong continuous three‐wave mixing of 514.5‐nm argon laser light in a single‐mode fiber is reported. The effect, due to the third‐order nonlinearity of silica, has been observed for light whose frequency spectrum consists of either a few discrete monochromatic frequency components separated by ∼1 GHz or a quasicontinuous distribution of frequencies having a spectral envelope ∼4 GHz wide. We show that the effect provides a simple and effective method for measuring the nonlinearity of silica. In the first manifestation of the effect, the nonlinearity mixes the frequency components to produce new frequencies. In the second, multiple mixing occurs that broadens the quasicontinuous spectrum. This manifestation of the effect is large; broadening by a factor of 4 has been observed with lower intensity levels than are required to produce stimulated Brillouin scattering in the same fiber. A theoretical model is presented to describe spectral broadening by three‐wave mixing for the case of small broadening. The effect of three‐wave mixing on the operation of continuous stimulated Brillouin and Raman oscillators is also discussed. Finally, it is noted that the presence of this effect may constrain the design of long‐haul single‐mode fiber optical communication trunks.

cw three-wave mixing in single-mode optical fibers

The observation of efficient continuous‐wave Brillouin laser action in an optical‐fiber ring resonator is reported. Internal laser conversion efficiencies of 50% and a Brillouin‐shifted output of 20 mW have been achieved.

cw Brillouin laser

Continuous‐wave laser action in multiply Brillouin‐shifted spectral lines in a novel Fabry‐Perot optical‐fiber resonator is reported. Oscillation has been observed over a 200‐GHz bandwidth and a total of 14 Brillouin‐shifted lines have been detected. This Brillouin laser configuration is potentially a source of mode‐locked picosecond pulses.

cw generation of multiple Stokes and anti‐Stokes Brillouin‐shifted frequencies

Continuous‐wave Raman oscillation has been obtained employing single‐mode glass fibers in a Fabry‐Perot resonator configuration. The pump‐power input to the Raman laser required to reach threshold is 5 W of which approximately 50% is coupled into the fiber. The dominant oscillation of the Raman laser is at 499.1 nm with a 488.0‐nm pump source.

B. S. Kawasaki

Papers

Photosensitivity in optical fiber waveguides: Application to reflection filter fabrication

cw three-wave mixing in single-mode optical fibers

cw Brillouin laser

cw generation of multiple Stokes and anti‐Stokes Brillouin‐shifted frequencies

Low-threshold cw Raman laser