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

The first part of this two-part review focused on the fundamental and diagnostics aspects of laser-induced plasmas, only touching briefly upon concepts such as sensitivity and detection limits and largely omitting any discussion of the vast panorama of the practical applications of the technique. Clearly a true LIBS community has emerged, which promises to quicken the pace of LIBS developments, applications, and implementations. With this second part, a more applied flavor is taken, and its intended goal is summarizing the current state-of-the-art of analytical LIBS, providing a contemporary snapshot of LIBS applications, and highlighting new directions in laser-induced breakdown spectroscopy, such as novel approaches, instrumental developments, and advanced use of chemometric tools. More specifically, we discuss instrumental and analytical approaches (e.g., double- and multi-pulse LIBS to improve the sensitivity), calibration-free approaches, hyphenated approaches in which techniques such as Raman and fluorescence are coupled with LIBS to increase sensitivity and information power, resonantly enhanced LIBS approaches, signal processing and optimization (e.g., signal-to-noise analysis), and finally applications. An attempt is made to provide an updated view of the role played by LIBS in the various fields, with emphasis on applications considered to be unique. We finally try to assess where LIBS is going as an analytical field, where in our opinion it should go, and what should still be done for consolidating the technique as a mature method of chemical analysis.

https://journals.sagepub.com/doi/pdf/10.1366/11-06574

Laser-induced breakdown spectroscopy (LIBS), part II: review of instrumental and methodological approaches to material analysis and applications to different fields.

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

A novel, passive, and self-referencing wavelength detection system (WDS) that measures the wavelength of the narrowband back-reflected spectrum of guided-wave Bragg gratings is described. This letter also reports on the use of such a detection system with fiber-optic Bragg gratings used as absolute strain sensors. The wavelength detection system demonstrated a 1% strain resolution of the total strain measurement range. >

A passive wavelength demodulation system for guided-wave Bragg grating sensors

Fiber-optic Bragg grating strain sensors hold a great deal of potential for structural monitoring because of their exceptional stability and demonstrated potential for long-term monitoring. This sensing technology takes advantage of a spectrally encoded signal which provides inherent immunity from signal intensity fluctuations which plague many other fiber-optic and electronic sensing techniques. This results in measurement stability and lead/interconnect insensitivity which permit longterm and intermittent monitoring with high resolution and accuracy. Fiber-optic grating sensors are intrinsic to the optical fiber, thus capitalizing on its extremely small size and inherent strength and durability. Recent results are provided from a sensor array installed in a road bridge. The strain sensors are attached to both steel and carbon-fiber-reinforced plastic prestressing tendons, which are embedded in the precast girders of the bridge. Measurements of traffic loads and the relaxation behaviour of the tendons are presented. The potential of fiber grating technology is briefly discussed including its application in long-gage strain-sensing and strain-distribution measurements.

Fiber-optic Bragg grating sensors for bridge monitoring

The development of a fiber laser sensor system which permits efficient interrogation of Bragg grating sensors is reported. A tunable erbium doped fiber laser which utilizes a broadband mirror and an intracore Bragg grating reflector in side-pump configuration is described. The wavelength of the laser oscillation is determined by the Bragg grating, which is remotely located and used as a strain sensor. This arrangement is used in conjunction with a passive wavelength demodulation system (WDS) to form a self-contained fiber laser strain sensor system, allowing efficient interrogation of the Bragg sensor. This device provides interrupt-immune sensing of static and dynamic strains with a bandwidth of 13.0 kHz. >

A Bragg grating-tuned fiber laser strain sensor system

A fiber-optic strain gauge system for use in structural monitoring and smart-structure applications is described. The strain gauge uses a fiber-optic Bragg grating sensor to measure strain and a passive, wavelength demodulation system to determine the wavelength of the narrow-band, backreflected spectrum from the grating sensor. The fiber-optic strain gauge system permits the measurement of both static and dynamic strains with a noise-limited resolution of 0.44 microstrain/√Hz, a measurement dynamic range of 27.8 dB, and a bandwidth of 250 Hz.

Practical fiber-optic Bragg grating strain gauge system

A new distributed strain sensing technique using a fiber optic Bragg grating has been developed and tested. This is the first `true' distributed strain sensor, to the authors' knowledge, with a high spatial resolution of about 1 mm. Since gratings can be made with a length of tens of centimeters, this new fiber optic measurement technique could have broad applications to smart materials and structures where monitoring of a continuous strain profile over a length of millimeters to tens of centimeters is needed. In this paper three different demodulation approaches are reviewed indicating a trade-off between a relatively simple measurement process for selected types of strain profile and a more complete measurement process that is suitable for any strain profile. Experimental results with different approaches are presented.

https://iopscience.iop.org/article/10.1088/0964-1726/7/2/012/pdf

Raymond M. Measures

Papers

A passive wavelength demodulation system for guided-wave Bragg grating sensors

Fiber-optic Bragg grating sensors for bridge monitoring

A Bragg grating-tuned fiber laser strain sensor system

Practical fiber-optic Bragg grating strain gauge system

Continuous arbitrary strain profile measurements with fiber Bragg gratings