Gerald Meltz

Multiplexing fiber Bragg grating sensors

Bragg reflection gratings and out-coupling taps for sensors can be written holographically within the core of many commercial fibers available today. The gratings appear to be permanent and have been tested to temperatures in excess of 500 degree(s)C. Quasi-distributed temperature, strain, pressure, chemical, and interferometric type sensors can be made with the wavelength selective, reflection gratings, and taps. The fiber gratings, and the different types of sensors they can make, conveniently lend themselves to WDM, TDM, and FDM types of multiplexing schemes. Instrumentation to detect the multiple sensors and measure their spectral shift for localized and quasi-distributed sensing is currently under development.

Multifunction, distributed optical fiber sensor for composite cure and response monitoring

Bragg gratings have been exposed into the core of optical fibers at specific locations to implement distributed, discrete gages for measurement of strain and temperature. Demonstrations using these devices to monitor composite specimen curing and bending are discussed. The sensor signals are also used in a control loop to drive an actuator for active damping of the test sample.

Fiber Bragg grating chemical sensor

A new fiber-optic method for quasi-distributed remote chemical sensing is described. The technique is based on using a wavelength selective Bragg-grating radiation coupler for localized excitation and detection of fluorescent species on or near the surface of the fiber cladding. A grating can be formed in germanosilicate fiber by exposing the core, through the side of the cladding, to an ultraviolet, two-beam interference pattern. With a suitable choice of index modulation, the grating can be designed to couple light efficiently from a guided mode into a narrow radiation beam. The index perturbation also enhances the capture of the emitted luminescence. Since the gratings are blazed to optimize the coupling efficiency for a desired wavelength band, each sensing region can be designed for a specific measurement. An array of identical sensors can be sampled by optical time domaifl reflectometry.

High-temperature capabilities and limitations of fiber grating sensors

Results from long term testing of fiber grating sensors at high temperatures are presented. Experimental test results are also presented on their high temperature strain response. Creep of the fiber under load appeared to be a problem at 650 degree(s)C and higher, and calculations were made to estimate the significance of Ge diffusion from the core at high temperatures.

Optical Fiber Stress Wave Sensor

The attenuation and dispersion of intense stress waves in solids can be measured by fiber optic techniques. This paper describes an in-situ optical fiber strain sensor that can be used to measure transient subsurface strains with a bandwidth of 1 GHz. The sensor consist of a length of polarization maintaining fiber which is illuminated by a coherent polarized light source. The presence of a stress wave is detected and measured by monitoring the state of polarization and phase of the transmitted light, in a manner closely analogous to a classical photoelastic diagnostic-except on a sub-miniature scale. By using two wave-lengths, both components of transverse stress in an incident disturbance can be determined. Experimental results are shown and the sensor principles and measurement techniques are discussed.

Multi-Wavelength Twin-Core Fiber Optic Sensors

Fiber optic sensors have unique advantages and capabilities when applied to experimental stress analysis, particularly at high temperatures1, and to the nondestructive evaluation of composite materials and structures2 . For many applications, practical considerations dictate the need for simultaneous temperature and strain observations or the resolution of serially distributed flexural strain along the lightguide sensor. In a dual mode fiber, wherein a perturbation is sensed by measuring the change in differential phase between modes, multiple wavelength operation can be used to separate thermal and stress effects or to resolve the distribution of flexural strain along a bent fiber.

UV-induced Bragg gratings in optical fibers and thin-film waveguides

The side-writing of Bragg gratings in fibers by exposure to a periodic UV intensity pattern is discussed. The spectral characteristics of gratings in depressed index fiber are described including the formation of higher-order reflection bands and coupling to cladding modes. Exposure requirements are compared with photosensitivity measurements of the refractive index change induced by individual KrF laser pulses. Commercial germaosilicate thin film waveguides are also found to be photosensitive. Results are presented on UV-induced lateral wave confinement in H2-loaded film and grating formation in standard buried channel guides.

Instrumentation concepts for multiplexed Bragg grating sensors

The exposure of multiple Bragg gratings into a single optical fiber has been demonstrated. As sensors of strain and temperature, these devices promise an architecturally efficient means to conduct quasi-distributed measurements with an array of discrete gages. Such a configuration can have great benefit when embedded within composite structures. This paper will present instrumentation concepts and results from initial laboratory work to develop methods for multiplexed strain and temperature measurements using Bragg grating sensors.