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

Two distinct oxygen-associated trapped-hole centers (OHCs) are identified in samples of room-temperature γ-irradiated, high-purity fused silica. One, which we label the “wet” OHC, predominates in the high-OH-content (wet) silicas while the other, the “dry” OHC, is more prevalent in low-OH (dry) silicas. Excellent computer simulations of the low-temperature electron-spin-resonance spectra are obtained for both wet and dry silicas using only the relative abundance of the “wet” and “dry” OHCs as an adjustable parameter. Analysis of the 17 O-hyperfine structure which occrs in samples of wet silica enriched in 17 O provides direct confirmation that the “wet” OHC is a hole trapped in a single nonbonding 2p-orbital of an oxygen (presumed nonbridging). Correlation of optical absorption and electron spin resonance via isochronal pulse anneals indicates that the “dry” OHC has an optical transition ay 7.6 eV. In addition, it is reported that the “dry” OHC can be induced in the dry silicas by the fiber drawing process. From the present results, an O 2 − molecular ion model appears most attractive for the “dry” OHC.

Oxygen-associated trapped-hole centers in high-purity fused silicas

Defect centers induced by ionizing radiation (50–100‐keV x rays, 60Co γ rays) in high purity P‐doped silica glass have been observed and elucidated by ESR spectroscopy. Four generic species are well characterized on the basis of the observed 31P hyperfine splittings and g values as defects analogous to PO2−3 (phosphoryl), PO4−4 (phosphoranyl), PO2−2 (phosphinyl), and PO2−4 radicals. The latter species, also termed the phosphorus‐oxygen‐hole center (POHC), is shown to occur in two variants comprising holes trapped on one or two nonbridging oxygens. Radiation‐induced Si E′ centers with and without P next‐nearest‐neighbors were also identified, and a singlet resonance S due to E′ type defects such as (OSi2)Si⋅ and/or (O2Si)Si⋅ was observed to grow in with annealing above ∼800 K, regardless of whether or not the sample was irradiated. The structures, formation mechanisms, and precursors of these defects have been determined or inferred for all centers. Radiation‐induced optical absorption spectra over the ran...

Fundamental defect centers in glass: Electron spin resonance and optical absorption studies of irradiated phosphorus‐doped silica glass and optical fibers

A new fundamental radiation-induced defect in high-purity synthetic silica has been identified by electron-spin-resonance studies of $^{17}\mathrm{O}$-enriched Si${\mathrm{O}}_{2}$ as a peroxy radical ${\mathrm{O}}_{2}^{\ensuremath{-}}$ bonded to one Si in the glass matrix. The precursors of these defects are envisioned to be \ensuremath{\equiv} Si-O-O-Si \ensuremath{\equiv} structures, some of which preexist in the silica, are formed in greater numbers during neutron bombardment, and which may release an electron either during irradiation or subsequent annealing.

Fundamental Defect Centers in Glass: The Peroxy Radical in Irradiated, High-Purity, Fused Silica

Observation and analysis of the primary 29Si hyperfine structure of the E′ center in non-crystalline SiO2

The radiation‐induced defect centers in a low‐loss Corning germanium‐doped optical fiber have been studied. In addition to silicon E′ centers, four germanium‐related centers, corresponding to electrons trapped at the site of oxygen vacancies in s p3 orbitals of germanium ions with zero to three next‐nearest‐neighbor germaniums, were observed. A model which assumes Gaussian distributions in the excited‐state energy‐level splittings has been successfully used to computer simulate the ESR spectrum of the irradiated fiber.

E. J. Friebele

Papers

Oxygen-associated trapped-hole centers in high-purity fused silicas

Fundamental defect centers in glass: Electron spin resonance and optical absorption studies of irradiated phosphorus‐doped silica glass and optical fibers

Fundamental Defect Centers in Glass: The Peroxy Radical in Irradiated, High-Purity, Fused Silica

Observation and analysis of the primary 29Si hyperfine structure of the E′ center in non-crystalline SiO2

Defect centers in a germanium-doped silica-core optical fiber