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

We show how the mode of the photoinduced second-harmonic light in an optical fiber changes with preparation time. We also discuss what type of nonlinear interaction can be causing the induced second-harmonic light and give experimental evidence that the initial second-harmonic light is a core–cladding interface effect.

Modal evolution of induced second-harmonic light in an optical fiber.

Thermal stabilities of photoinduced paramagnetic defects in second-harmonic (SHG) generation Ge-P-doped silica fibers and in Ge- and Ge-P-doped silica preforms are studied by electron-spin-resonance and compared with that of the reported self-organized gratings. Our data suggests that the charge trapping sites for the electric-field-induced SHG are Ge(1), Ge(2) and Ge E' d1 centers. The defects responsible for the Bragg gratings are Ge E'-type centers. Thermal darkening in Ge-doped silica core fibers reported is likely due to the thermal induced Ge E' d1 center.

Thermal stability of self-organized gratings and defects in Ge- and Ge-P-doped silica core fibers

The 11B NMR spectra of borate glasses can be accurately computer-simulated using a simple model for the electric field gradient at a boron site in the BO3 and BO4 structural units, assuming these structural units are relatively undistorted. Deviations from the average OBO bond angle in these two sites are determined from the simulations to be less than ± 2° in borate glasses. There is no evidence for increasing distortion in the fundamental structural units with increasing metal oxide content.

On the nature of unique boron sites in borate glasses

A new technique for recoating optical fibre with a polyimide jacket is demonstrated and characterised. Single and double layer coatings of 7 and 15 /spl mu/m, respectively, have been applied to 85 and 125 /spl mu/m fibre. The coatings are uniform and pinhole free, and the recoated sections have demonstrated high strength.

Method for recoating optical fibres with polyimide

The discussion of Bragg fiber gratings (FBGs) has expanded sharply in the last six years. FBGs were given versatility by the results at the United Technology Research Center when the technique of side-writing was pioneered (1989), which made the Bragg wavelength independent of the writing laser. The promise of economical production of these devices was advanced by the generation of FBGs with a single laser pulse at the Naval Research Laboratory (1992), and this result was expanded by the University of Southhampton in achieving saturated reflectivity with a single-pulse. The basis for manufacturing these excitingly versatile devices is near the point of supporting a new area of applications and industry. The intent of this article is to briefly discuss the conditions and requirements for the practical production of FBGs in support of the technology of distributed sensing.

E. J. Friebele

Papers

Modal evolution of induced second-harmonic light in an optical fiber.

Thermal stability of self-organized gratings and defects in Ge- and Ge-P-doped silica core fibers

On the nature of unique boron sites in borate glasses

Method for recoating optical fibres with polyimide

Considerations for producing single-pulse fiber Bragg gratings