Raymond M. Measures

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

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.<<ETX>>

Fiber-optic Bragg grating sensors for bridge monitoring

Abstract 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.

A Bragg grating-tuned fiber laser strain sensor system

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.<<ETX>>

Bragg intragrating structural sensing

When a fiber-optic intracore Bragg grating is subject to an appreciable strain gradient, its reflective spectrum will not only be shifted but also be distorted because of the chirp of the grating. We employed the J-matrix formalism to calculate the influence of different strain gradients on the reflective spectra of Bragg gratings and have undertaken experiments to test these calculations. The results of these experiments have confirmed that intracore Bragg gratings can be used to evaluate strain gradients and can be thought of as quasi-distributed strain sensors. This adds a new dimension to structural sensing, permitting measurements in any situation where strain gradients exist. It also provides a warning of any sensor/host debonding.

Practical fiber-optic Bragg grating strain gauge 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.

Distributed strain measurement based on a fiber Bragg grating and its reflection spectrum analysis

A method of extracting the strain profile along a fiber Bragg grating from the intensity reflection spectrum is described. The procedure is based on a filter synthesis theory that relates the aperiodicity of a grating with its reflection spectrum. To illustrate the approach, we measured the strain profile near a hole in a plate and obtained a strain resolution of 80 micro. The spatial resolution depends on the strain gradient; i.e., the higher the gradient, the better the resolution. A resolution of 0.8 mm was achieved for a 5-mm grating with a gradient of 250 micro/mm.

A multiplexed Bragg grating fiber laser sensor system

A technique for multiplexing Bragg gratings in a fiber laser arrangement is described. This technique has successfully been used to multiplex two and three Bragg gratings with very little crosstalk. The Bragg grating laser sensors were used to measure both strain and temperature. Independent strain and temperature tuning of the gratings shows no crosstalk.<<ETX>>

Smart composite structures with embedded sensors

Abstract Fiber-optic sensors embedded within advanced composite materials represent a new branch of engineering with the potential to greatly enhance the confidence and use of these materials. An overview of our developments toward fiber-optic-based Smart Composite Structures is presented. This includes a review of our development of a full-scale fiber-optic damage assessment system for an aircraft composite leading edge and the confirmation of the feasibility of such a resident fiber-optic structural integrity monitoring system. We report on the development and characterization of fiber-optic strain gauges that have been embedded within composite materials and used to measure the internal strain field or detect load-induced acoustic emission from within composite specimens, and explore the feasibility of undertaking optoacoustic cure monitoring. A discussion is also provided of two issues: fiber-optic strain sensitivity in light of the recent theoretical work of Sirkis and Haslach and thermally induced apparent strain. Lastly, mention is made of our passive, fast response, wavelength demodulation system for the Bragg grating sensor as this holds promise for the eventual development of a multiplexed, multisensing optoelectronic chip that could overcome the interconnect barrier to the practical implementation of Smart Structure technology.

Structurally integrated fiber optic damage assessment system for composite materials

Progress toward the development of a fiber optic damage assessment system for composite materials is reported. This system, which is based on the fracture of embedded optical fibers, has been characterized with respect to the orientation and location of the optical Fibers in the composite. Together with a special treatment, these parameters have been tailored to yield a system capable of detecting the threshold of damage for various impacted Kevlar/epoxy panels. The technique has been extended to measure the growth of a damage region which could arise either from impact, manufacturing flaws, or static overloading. The mechanism of optical fiber fracture has also been investigated. In addition, the influence of imbedded optical fibers on the tensile and compressive strength of the composite material has been studied. Image enhanced backlighting has been shown to be a powerful and convenient method of assessing internal damage to translucent composite materials.

Optical fibre damage detection for an aircraft composite leading edge

Abstract Recent developments towards the development of a fibre optic damage detection system for composite materials are described. Results of experiments designed to measure the influence of optical fibre orientation and depth on the sensitivity of the system are reported and optimal configurations have been determined for both. A surface treatment for controlling the damage sensitivity of the optical fibres to the point where they can detect barely visible damage is described. A study into the failure mechanisms of embedded optical fibres is also reported. The technique has been demonstrated to be capable of detecting both impact and quasi-statically induced damage and can be used to map the growth of a region of damage with increasing load.