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

Review of the present status of optical fiber sensors

Recent research on fibre optic long-period gratings (LPGs) is reviewed with emphasis placed upon the characteristics of LPGs that make them attractive for applications in sensing strain, temperature, bend radius and external index of refraction. The prospect of the development of multi-parameter sensors, capable of simultaneously monitoring a number of these measurands will be discussed.

https://iopscience.iop.org/article/10.1088/0957-0233/14/5/201/pdf

Optical fibre long-period grating sensors: characteristics and application

In-fibre Bragg grating (FBG) sensors are one of the most exciting developments in the field of optical fibre sensors in recent years. Compared with conventional fibre-optic sensors, FBG sensors have a number of distinguishing advantages. Significant progress has been made in applications to strain and temperature measurements. FBG sensors prove to be one of the most promising candidates for fibre-optic smart structures. This article presents a comprehensive and systematic overview of FBG sensor technology regarding many aspects including sensing principles, properties, fabrication, interrogation and multiplexing of FBG sensors. It is anticipated that FBG sensor systems will be commercialized and widely applied in practice in the near future due to the maturity of economical production of FBGs and the availability of cost effective interrogation and multiplexing techniques.

https://iopscience.iop.org/article/10.1088/0957-0233/8/4/002/pdf

In-fibre Bragg grating sensors

The spectroscopic properties, structure and interconversions of optically active oxygen-deficiency-related point defects in vitreous silica are reviewed. These defects, the E′-centers (oxygen vacancies with a trapped hole or 3-fold-coordinated silicons), different variants of diamagnetic `ODCs' (oxygen-deficiency centers), and their Ge-related analogs play a key role in the fiber-optic Bragg grating writing processes. The controversy surrounding the structural models for the Si- and Ge-related ODCs is discussed and the similarity between the bulk and surface point defects in silica is emphasized. The possible interconversion mechanisms between 2-fold-coordinated Si, neutral oxygen vacancies and E′-centers are discussed. The effects of glassy disorder have a profound effect on defect properties and interconversion processes in silica.

Optically active oxygen-deficiency-related centers in amorphous silicon dioxide

In this paper, high-accuracy measurements of ultraviolet (UV)-induced refractive-index changes (plusmn3times10-7) in germanosilicate optical fiber as a function of intensity and exposure time are presented. To examine the early growth characteristics of the fiber, samples are irradiated with 244-nm light for 100 s at relatively low intensities (0.007-2.7 W/cm2). The combined growth data is then interpolated to generate a 3-D "index growth surface" of photo-induced index. An empirically derived mathematical expression relates the index growth to the exposure time and intensity. Evidence is presented that, after exposing the fiber at one intensity, additional growth at a different intensity is dictated by the final index change of the first exposure and the intensity of the second exposure. This "compound growth rule" permits the complete calculation of induced-grating structures produced by such a complex exposure history. Using the index-growth surface and the compound-growth rule, the growth and UV erasure of a fiber Bragg grating is successfully predicted using a modified F-matrix algorithm

Early Index Growth in Germanosilicate Fiber Upon Exposure to Continuous-Wave Ultraviolet Light

A network of distributed optical Bragg grating sensors is used for monitoring of a full scale laboratory bridge in its pristine and damaged state. Damages consist of a series of cuts that are introduced in an external girder to simulate fracture or fatigue crack of a main load carrying bridge component. The after fracture behavior is described in terms of load path redistribution and strain level changes in the structure.

Damage assessment of a full-scale bridge using an optical fiber monitoring system

Spectral measurements of the permanent and transient radiation-induced optical absorption in state-of-the-art low loss step and graded index optical fiber waveguides have revealed radiation-induced increases in the OH overtone and combination bands which limit the transmission at long wavelengths. The induced damage in Ge-doped silica core fibers codoped with B has been observed to increase at wavelengths longer than 1.05 μ, and P has been identified as a dopant which suppresses the intense uv Ge-related transient absorption in Ge-doped silica fibers but results in greater permanent damage in the visible and near ir.

Radiation-Induced Optical Absorption Spectra of Fiber Optic Waveguides in the 0.4–1.7 μ Region

This paper reports progress in the derivation of an empirical model for predicting the radiation induced loss in matched clad single-mode optical fibers. We have drawn fibers from a set of MCVD preforms which were produced over a range of fabrication conditions intended to span the variations encountered in commercial MCVD fiber production. Conclusions regarding dependence of fallout radiation sensitivity on fabrication parameters were drawn from the data through orthogonal matrix analysis.

Radiation Response Prediction In Single-Mode Optical Fibers II

Rare earth-doped glasses exhibit high initial photosensitivity but their response saturates at relatively modest values of (Delta) n (approximately 5 X 10 -7 ), which greatly limits their usefulness for device applications. In the context of our model, saturation results from either exhaustion of photosensitive rare earth sites, trap sites, or through competition between two photon creation and one photon bleaching processes. In this paper we report the results of new experiments designed to further elucidate the photosensitivity process with specific emphasis on the saturation mechanisms(s). Based on these new experimental results we present a refinement of our earlier model.

E. J. Friebele

Papers

Early Index Growth in Germanosilicate Fiber Upon Exposure to Continuous-Wave Ultraviolet Light

Damage assessment of a full-scale bridge using an optical fiber monitoring system

Radiation-Induced Optical Absorption Spectra of Fiber Optic Waveguides in the 0.4–1.7 μ Region

Radiation Response Prediction In Single-Mode Optical Fibers II

Photosensitivity of rare earth-doped glasses