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 new technique for recoating optical fiber with a polyimide jacket is demonstrated. Single layer coatings of 7 micrometers , and double layers of 15 micrometers have been applied to 125 micrometers fiber. The coatings are uniform and concentric, and can be applied with no loss of original fiber strength. The technique accommodates multiple fiber diameters without the need for interchanging parts.

Method for recoating fiber Bragg gratings with polyimide

The low optical attenuation of 1.3-μm single-mode fibers in telecommunication systems can be compromised by exposure to ionizing radiation sources. In spite of a number of prior studies characterizing the radiation response of these waveguides,1 there is a large diversity of sensitivities and recovery behaviors. An example is shown in Fig. 1: the initial loss immediately following exposure can vary from 2.5 to 30 dB/km, and the fibers can either recover completely in 24 h or contain permanent losses of as much as 3 dB/ km.

Systematic investigation of matched clad single-mode fiber radiation response

A fiber Bragg grating sensor array is interrogated using a broad bandwidth passively mode locked fiber laser source. A novel demodulation scheme is demonstrated using highly dispersive fiber to convert the grating wavelength shift to a temporal shift in the arrival time of the reflected pulses. The mode locked fiber laser was then modified and operated in the square pulse regime, where 4 W, 10 ns pulses with bandwidths greater than 60 nm were used successfully to illuminate 2% fiber Bragg gratings.

Sensor grating demodulation using a passively mode locked fiber laser

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.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Development of a structurally embedded in-line fiber etalon for internal strain measurement

An ultra-high sensitivity static strain sensor with 300 nɛ/°C thermal apparent strain has been fabricated by constructing a fibre etalon cavity of concatenated fluoride and silica fibre sections bonded to a low thermal expansion coefficient graphite composite strut tube. The athermal cavity is 30 times less temperature-sensitive than a simple fluoride or silica cavity, while retaining the same response to physical strain.

E. J. Friebele

Papers

Method for recoating fiber Bragg gratings with polyimide

Systematic investigation of matched clad single-mode fiber radiation response

Sensor grating demodulation using a passively mode locked fiber laser

Development of a structurally embedded in-line fiber etalon for internal strain measurement

Athermal fibre cavity etalon for ultra-high-sensitivity strain sensing