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

Na 2 O3SiO 2 glasses containing up to 17 wt. % H 2 O have been x-irradiated at 77K and studied by ESR and optical techniques as a function of anneal temperature. It is shown that the radiation-protected characteristic of hydrosilicate glasses containing > 2 wt. % water is related to the postirradiation reaction of H 2 O molecules with HC 1 and HC 2 hole centers to form free OH° radicals, observed for the first time in glasses.

Radiation-induced defects in glasses with high water content

This paper describes an ongoing effort to develop techniques capable of locating the position of space debris impacts and to quantify the strain energy absorbed by the space structure as a result of these impacts. The techniques under development use optical fiber sensors and neural networks as the primary sensor and decision making components. To date, this project has resulted in the development of (1) a mathematical model of plate impact dynamics for use in sensor and neural network paradigm development, (2) a sensor demodulation system specifically designed for moderate impact energies, (3) several neural network paradigms with the potential to locate impacts, and (4) a test configuration to experimentally confirm the proposed paradigms.

Development of an impact detection technique using optical fiber sensors and neural networks

Arrays of fiber Bragg gratings (FBGs) have been produced on-line during fiber draw using a KrF excimer laser and a computer-controlled interferometer. A table of the desired Bragg wavelengths (lambda) B of the gratings is used by the computer to appropriately vary the intersection angle of the two interfering beams, maintain focus and beam intersection at the proper position relative to the fiber, and trigger the laser. A large number of narrow-line Type I gratings have been produced, and in some instances small Type II gratings have been found to overlap the Type I FBGs. The reflectance of the gratings in one series was found to be polarization-dependent, lending further support to our previous model whereby the index change induced by the excimer laser occurs at the core-clad interface asymmetrically on one side of the core.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Contiguous fiber Bragg grating arrays produced on-line during fiber draw

An algorithm has been developed to determine deformations of a cantilever honeycomb plate under arbitrary loading conditions. The algorithm utilizes strain information from a set of sixteen fiber Bragg grating sensors, mounted on the plate so that all sensors measure strains along the clamped-free direction. The sensors are interrogated using a wavelength-division multiplexing scheme. A two-dimensional bi-polynomial function which represents the strain field is created using a least-squares algorithm. This function is integrated twice with the known boundary conditions applied to yield the deformation field for the plate. Maximum differences between finite-element solutions and least-squares estimates did not exceed 29.0 percent for any of the 16 investigated load scenarios. However, when considering areas of maximum deflection, the least-squares estimates did not exceed 13.3 percent difference. The algorithms used to interrogate the sensors, perform the strain-displacement calculations, and generate a real-time (approximately 4 Hz) mesh of displacement are encoded in a C program.

Cantilever-plate-deformation monitoring using wavelength-division multiplexed fiber Bragg grating sensors

Ultrahigh sensitivity strain sensors are required for precision applications where length changes on the order of nanometers must bedetected Using a single mode fiber as a cavity etalon, a new, ultrahigh sensitivity optical fiber sensor has been developed The sensor isfabricated by gold-coating the cleaved ends of'-4 m long single mode fiber to form a moderate finesse (F 50) fiber cavity etalon Thesensor has demonstrated static strain sensitivity of-1 n and low frequency noise performance of-4 p/Hz at 10 Hz The results of quasi-static stepped strain (<50 ne) and free vibration dynamic strain measurements of these sensors bonded to low expansion struts will bereportedKeywords: fiber optic sensOf, strain sensing, thermal apparent strain, microdynamics 1 iNTRODUCTION Ultrahigh sensitivity strain sensing is desirable for understanding the microdynamics of materials and for monitoring the behavior ofprecision structures For example, small transient strains are induced in spacecraft by the relaxation ofthermal gradients, attitude controlmaneuvers, or antenna slewing These transients, referred to as "snap, crackle, and pop," may affect the performance of precisiontelescopes or long baseline interferometers Likewise, temperature fluctuations or loading of low thermal expansion coefficient materialsmay cause nanometer deformations of such structuresDetection of nanometer-level motions, whether dynamic or static, places stringent requirements on the measurement apparatusConventional techniques include laser interferometry and capacitance gages The former requires the attachment of reflectors to the

E. J. Friebele

Papers

Radiation-induced defects in glasses with high water content

Development of an impact detection technique using optical fiber sensors and neural networks

Contiguous fiber Bragg grating arrays produced on-line during fiber draw

Cantilever-plate-deformation monitoring using wavelength-division multiplexed fiber Bragg grating sensors

Ultrahigh-sensitivity strain sensing using fiber cavity etalons