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

Ferroelectric, dielectric and piezoelectric properties of ferroelectric thin films and ceramics are reviewed with the aim of providing an insight into different processes which may affect the behaviour of ferroelectric devices, such as ferroelectric memories and micro-electro-mechanical systems. Taking into consideration recent advances in this field, topics such as polarization switching, polarization fatigue, effects of defects, depletion layers, and depolarization fields on hysteresis loop behaviour, and contributions of domain-wall displacement to dielectric and piezoelectric properties are discussed. An introduction into dielectric, pyroelectric, piezoelectric and elastic properties of ferroelectric materials, symmetry considerations, coupling of electro-mechanical and thermal properties, and definitions of relevant ferroelectric phenomena are provided.

https://iopscience.iop.org/article/10.1088/0034-4885/61/9/002/pdf

Ferroelectric, dielectric and piezoelectric properties of ferroelectric thin films and ceramics

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

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

A new class of active fiber devices based on piezoelectric coated optical fibers is currently being developed. The piezoelectric coating is used to produce acoustic waves within the optical fiber and these acoustic waves interfere with optical signals passing through the fiber waveguide. Optical phase modulation devices based on piezoelectric coated fibers have been demonstrated and the behavior of these devices will be reviewed. A second type of active optical fiber device has been fabricated by integrating a piezoelectric fiber coating with an intra-core Bragg grating, which acts as a reflection filter. The primary interest of this work is to produce a piezoelectric fiber optic tunable filter. These types of active all-fiber devices show promise for a variety of applications including telecommunications and sensing networks. The performance of active optical fiber devices that use piezoelectric coatings is dependent on the device structure and the piezoelectric properties of the fiber coating. Several important geometric parameters (e.g. coating thickness and device length) that influence device performance have been experimentally identified. Additionally, several relationships between the fiber coating sputter deposition process, microstructure, and properties have been observed for piezoelectric ZnO fiber coatings. Recent advances in the development of active all-fiber devices are presented.

Piezoelectric coatings for active optical fiber devices

The formation of short Bragg gratings in SMF-28e non-hydrogen loaded pristine fibre using a cw Ar+ laser operating at 244 nm is reported. Mean refractive index changes >10-3 were obtained for a laser intensity of about 513 W/cm2 and a total dose of 5.9 MJ/cm2. Strong stress changes indicate a contribution of compaction to the refractive index change.

Formation of Bragg gratings in pristine SMF- 28e fibre using CW 244-nm Ar + laser

A constant high-speed optical low-coherence reflectometer is presented. The use of both the reference and sample arms as delay lines results in highly linear optical path length changes. Scan speeds of up to 96 m/s are maintained over a range of typically 100 mm with a speed variation of only 0.1%.

Constant high-speed optical low-coherence reflectometry over 0.12 m scan range

A fiber Bragg grating tunable filter based on traction and compression has been realized using two piezoelectric actuators. A linear timing range of 28 nm was obtained.

Tension and compression tuned Bragg grating filter

Water absorption and thermal response of adhesive composite joints were investigated by measurements and numerical simulations. Water diffusivity, saturation, swelling, and thermal expansion of the constituent materials and the joint were obtained from gravimetric experiments and strain measurements using embedded fiber Bragg grating (FBG) sensors. The mechanical response of these materials at different temperatures and water content was characterized by dynamic mechanical analysis. Thermal loading and water absorption in joint specimens were detected by monitoring the FBG wavelength shift caused by thermal expansion or water swelling. The measured parameters were used in finite element models to simulate the response of the embedded sensor. The good correlation of experimental data and simulations confirmed that the change in FBG wavelength could be accurately related to the thermal load or water absorption process. The suitability of the embedded FBG sensors for monitoring of water uptake in adhesive composite joints was demonstrated. (C) 2015 Elsevier Ltd. All rights reserved.

Hans G. Limberger

Papers

Piezoelectric coatings for active optical fiber devices

Formation of Bragg gratings in pristine SMF- 28e fibre using CW 244-nm Ar + laser

Constant high-speed optical low-coherence reflectometry over 0.12 m scan range

Tension and compression tuned Bragg grating filter

An experimental–numerical investigation of hydrothermal response in adhesively bonded composite structures