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.

/pdf/fiber-bragg-grating-technology-fundamentals-and-overview-460zsnwohq.pdf

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

This paper describes a new method that we have developed to generate Bragg grating filters in germania-doped communication fibers and discusses their applications as sensors. The gratings are formed by exposing a short section of the core, through the side of the fiber, to an interference pattern of intersecting coherent beams of UV light. A permanent periodic index modulation is produced by the interference fringes. It forms a phase grating which acts as a band rejection filter, passing wavelengths that are not in resonance with the grating and strongly reflecting wavelengths which satisfy the Bragg condition. The grating wavelength is sensitive to changes temperature and strain. Measurements of the shift in Bragg wavelength with these quantities are reported and compared with computed estimates of the sensitivity. A large number of independent gratings can be easily written along a length of fiber to make a distributed sensor system. We have also demonstrated that Bragg gratings can wavelength tune laser diodes, which could be used for detecting the wavelength shift of the Bragg sensors.

Fiber Optic Bragg Grating Sensors

A single-mode linear-cavity fiber laser that utilizes intracore Bragg reflectors for cavity feedback has been continuously tuned, without mode hopping, when both the gratings and enclosed fiber are stretched uniformly. Continuous tuning is achieved in a 1.54-microm erbium fiber laser since the change in the reflected wavelength from a Bragg reflector tracks the change in the cavity resonance wavelength.

Continuously tunable single-mode erbium fiber laser

A variable light filtering arrangement (10) includes at least one optical fiber section (11) including a waveguiding core (13), and at least one permanent Bragg grating region (12) in the optical fiber section. The grating region includes a plurality of grating elements constituted by periodic refractive index variations of a predetermined initial periodicity and cumulatively redirecting, of the light launched into the core for guided propagation therein, that having an axial wavelength within a narrow band around a central wavelength that is determined by the periodicity and refractive index variations of the grating elements. At least one of the periodicity and refractive index variations of the grating region is controlledly modified in such a manner as to temporarily change the central wavelength within a predetermined wavelength range.

Variable optical fiber bragg filter arrangement

The refractive index of germanosiicate and germano—aluminosilicate fiber can be increased by a few parts in iO toupwards of five parts in i04 by photobleaching the oxygen vacancy defect band of germania. Typically, the fiber core is exposed through the side of the cladding to UV radiation at a wavelength from 240—250 nm. Permanent phase gratings are"written" with a specified period by using a pair ofintersecting coherent beams. We have investigated the fiber photosensi-tivity by monitoring the grating reflectivity and wavelength spectrum during the exposure. The Bragg wavelength shifts asthe grating is "written" due to a small increase in the average refractive index of the core. From these measurements, onecan determine the growth and saturation characteristics of the photoinduced changes and study the effects of composition and high temperature hydrogen annealing. We have also measured the polarization sensitivity and thermal stability ofgratings and the transient and permanent changes in fiber absorption.

Bragg grating formation and germanosilicate fiber photosensitivity

Bragg reflection gratings and out-coupling taps for sensors can be written holographically within the core of many commercial fibers available today. The gratings appear to be permanent and have been tested to temperatures in excess of 500°C. Quasi-distributed temperature, strain, pressure, chemical, and interferometric sensors can be made with the wavelength selective, reflection gratings, and taps. The fiber gratings, and the different types of sensors they can make, conveniently lend themselves to (wavelength-division multiplexing) WDM, (time-division multiplexing) TDM, and (frequency-division multiplexing) FDM types of multiplexing schemes. Instrumentation to detect the multiple sensors and measure their spectral shift for localized and quasi-distributed sensing is currently under development.

William W. Morey

Papers

Fiber Optic Bragg Grating Sensors

Continuously tunable single-mode erbium fiber laser

Variable optical fiber bragg filter arrangement

Bragg grating formation and germanosilicate fiber photosensitivity

Multiplexing fiber bragg grating sensors