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

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

Since the discovery of photosensitivity in optical fibers there has been great interest in the fabrication of Bragg gratings within the core of a fiber. The ability to inscribe intracore Bragg gratings in these photosensitive fibers has revolutionized the field of telecommunications and optical fiber based sensor technology. Over the last few years, the number of researchers investigating fundamental, as well as application aspects of these gratings has increased dramatically. This article reviews the technology of Bragg gratings in optical fibers. It introduces the phenomenon of photosensitivity in optical fibers, examines the properties of Bragg gratings, and presents some of the important developments in devices and applications. The most common fabrication techniques (interferometric, phase mask, and point by point) are examined in detail with reference to the advantages and the disadvantages in utilizing them for inscribing Bragg gratings. Reflectivity, bandwidth, temperature, and strain sensitivity of the Bragg reflectors are examined and novel and special Bragg grating structures such as chirped gratings, blazed gratings, phase-shifted gratings, and superimposed multiple gratings are discussed. A formalism for calculating the spectral response of Bragg grating structures is described. Finally, devices and applications for telecommunication and fiber-optic sensors are described, and the impact of this technology on the future of the above areas is discussed.

Fiber Bragg gratings

In-service structural health monitoring (SHM) of engineering structures has assumed a significant role in assessing their safety and integrity. Fibre Bragg grating (FBG) sensors have emerged as a reliable, in situ, non-destructive tool for monitoring, diagnostics and control in civil structures. The versatility of FBG sensors represents a key advantage over other technologies in the structural sensing field. In this article, the recent research and development activities in structural health monitoring using FBG sensors have been critically reviewed, highlighting the areas where further work is needed. A few packaging schemes for FBG strain sensors are also discussed. Finally a few limitations and market barriers associated with the use of these sensors have been addressed.

Fibre Bragg gratings in structural health monitoring—Present status and applications

The spectral behaviour of two superimposed fibre gratings having different Bragg wavelengths (850 and 1300 nm) with respect to strain and temperature has been studied. The results show that the ratio of sensitivity at two Bragg wavelengths is dependent on strain and temperature, which can be used for simultaneous measurement of these parameters using a single sensing element.

/pdf/discrimination-between-strain-and-temperature-effects-using-1xlw6ng6ys.pdf

Discrimination between strain and temperature effects using dual-wavelength fibre grating sensors

We report what is to our knowledge the first fiber distributed-feedback laser using a single Bragg grating at 1.5 microm written directly into a 2-cm-long Er(3+)-doped fiber codoped with Yb(3+). We obtained robust single-frequency operation by either using one end reflector or locally heating the center of the grating to create the necessary phase shift.

/pdf/er-3-yb-3-codoped-fiber-distributed-feedback-laser-3268ibw7mm.pdf

Er(3+):Yb(3+)-codoped fiber distributed-feedback laser.

A new approach for the interrogation of a large number of fiber-optic grating sensors is proposed and demonstrated for a small number of sensors in which signal recovery is achieved by matching a receiving grating to its corresponding sensor. This technique is demonstrated for both quasi-static and periodic measurands, and the resolution achieved for a single sensor–receiving grating pair for quasi-static strain is 4.12 μ∊.

Simple multiplexing scheme for a fiber-optic grating sensor network.

It is shown for the first time that fibre gratings with reflectivities in excess of 99.8% and fractional bandwidths of 0.005 (7.5 nm or 970GHz at 1550nm) can be written by interferometric side exposure of a fibre core to a single 40mJ UV pulse of 20ns duration at 248nm. These highly efficient reflectors are formed by a new mechanism in which the core glass fuses, resulting in a very large periodic modulation of the refractive index. They are stable at temperatures as high as 800°C.

/pdf/100-reflectivity-bragg-reflectors-produced-in-optical-fibres-1ay9lsh6nl.pdf

100% reflectivity Bragg reflectors produced in optical fibres by single excimer laser pulses

A new technique for the interrogation of in-fibre Bragg grating sensors using an acousto-optic tunable filter (AOTF) is demonstrated. The scheme involves frequency shift keying (FSK) of the RF drive to the AOTF to track the wavelength shifts of a Bragg grating. Experimental results are presented for temperature measurement. This technique provides a frequency-agile system, capable of rapid, random access and very wide tuning range.

/pdf/novel-interrogating-system-for-fibre-bragg-grating-sensors-sz4xqdlr6v.pdf

Jean-Luc Archambault

Papers

Discrimination between strain and temperature effects using dual-wavelength fibre grating sensors

Er(3+):Yb(3+)-codoped fiber distributed-feedback laser.

Simple multiplexing scheme for a fiber-optic grating sensor network.

100% reflectivity Bragg reflectors produced in optical fibres by single excimer laser pulses

Novel interrogating system for fibre Bragg grating sensors using an acousto-optic tunable filter