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

A brief overview is presented on the subject of the development of more radiation tolerant fiber optic waveguides. Data are presented for both polymer clad silica (PCS) and all silica fibers. The effects of new compositional variations on radiation response in both the core and cladding glasses are reported. Recent photobleaching measurements on irradiated fibers are also summarized. The spectral variation of the observed radiation--induced losses are given for a variety of state of the art fibers during and following exposure to ionizing radiation.© (1982) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Recent Progress In The Investigation Of Radiation Resistant Optical Fibers

Widespread use of smart structures technology in industrial and commercial applications requires significant reductions in systems cost and complexity. For knowledge and control of the shape and vibration of structures, one would ideally employ a number of discrete strain sensors placed at critical locations. One significant advantage of optical fiber sensors is that they can be multiplexed along a single fiber and be individually addressed, greatly simplifying the ingress-egress- connection problems. Multielement strain sensor arrays have been fabricated during the drawing of an optical fiber from a preform at a sustained rate of approximately equals 2,000 per hour. These arrays have been surface- mounted and embedded in composites, and distributed static and vibrational strain measurements have been made. This paper will report recent developments and improvements in this low-cost technique for fabricating and applying strain sensors for smart structures applications.

Fabrication and application of low-cost optical fiber sensor arrays for industrial and commercial applications

A set of both pure and doped silica core multimode fibers was irradiated in either pure gamma, pure proton, or mixed neutron-gamma irradiation fields. All parameters were maintained as nearly identical as practical so that a comparison of the effects of each type of irradiation could be made.

A Comparison Of Gamma, Neutron And Proton Irradiations Of Multimode Fibers

This paper describes an optical fiber interferometer that uses a short segment of silica hollow- core fiber spliced between two sections of single-mode fiber to form a mechanically robust in- line optical cavity. The hollow-core fiber is specifically manufactured to have an outer diameter that is equal to the outer diameter of the single mode lead fibers, thereby combining the best qualities of existing intrinsic and extrinsic Fabry-Perot sensors. Uniaxial tension and pure bending strength tests are used to show that the new configuration does not diminish the axial strength of bare fiber and reduces the bending strength by 17% at most. Similar tests confirm that the fiber sensor has 1.96% strain to failure. Axisymmetric finite element analysis is used to investigate the reliability of the in-line etalon during typical thermoset composite cure conditions, and parametric studies are performed to determine the mechanically optimal cavity length. The sensor strain response tests demonstrate a dynamic strain resolution of 21 n(epsilon) /(root)Hz at frequencies > 5 Hz with a sensor gauge length of 137 micrometers .© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In-line fiber etalon (ILFE) for internal strain measurement

In-fiber Bragg reflectors produced with coherent, transverse UV illumination promise to be versatile optical elements for spectral selection and distributed sensing. These fiber devices were generated by repetitive exposures to a moderate intensity, frequency-doubled dye laser under holographically stable conditions until sufficient periodic index modulation had accumulated to produce a strong reflection. Fluences in the range of 1000 J/sq cm were delivered over the course of many seconds to minutes to obtain an index modulation depth of about 1 x 10 exp -4. In an alternate approach, we used single about 1 J/sq cm pulses from a line-narrowed KrF excimer to produce gratings with a modulation depth of about 2 x 10 exp -5, and more recently have achieved values above 1 x 10 exp -4. An unanticipated benefit of the single-pulse preparation method appears to be the markedly enhanced thermal stability of the grating structure, as compared to that reported for gratings produced with multiple exposures.

E. J. Friebele

Papers

Recent Progress In The Investigation Of Radiation Resistant Optical Fibers

Fabrication and application of low-cost optical fiber sensor arrays for industrial and commercial applications

A Comparison Of Gamma, Neutron And Proton Irradiations Of Multimode Fibers

In-line fiber etalon (ILFE) for internal strain measurement

Fiber Bragg reflectors by single excimer pulse