A Multiplexed Optical Fiber Sensor System for Distributed Measurement of Structural Strains
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Mat. Res. Soc. Symp. Proc. Vol. 503 0 1998 Materials Research Society Downloaded from https://www.cambridge.org/core. Rice University, on 26 Oct 2019 at 15:24:48, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-503-119
structure applications, due to their multi-point measurement capabilities. A distributed sensor permits measurement of a desired parameter as a function of length along the fiber. The most widely employed distributed sensing technique is based on measurement of propagation time delays of light traveling in the fiber based on the strain-induced change in the transmission of light. An optical time domain reflectometer (OTDR) is used for this purpose. A pulsed light signal is transmitted into one end of the fiber, and light signals reflected from a number of partial reflectors along the fiber length are recovered from the same fiber end. By using this concept, it is possible to determine the location and magnitude of the strain field within different sections of the structure (4). The research presented here describes the development of a new multiplexed optical fiber sensor which provides distributed measurement capability for structures. The optical fiber sensor system is based on white light interferometry. The white light sensor is highly accurate
and very sensitive since it uses interferometry for measurement of strains. The system is more efficient than the previously developed distributed sensors in terms of signal conditioning instrumentation and cost. An optical switch provides for multiplexing of strain signals from various locations in the structure. The experimental program included testing of a specimen instrumented with the white light sensor system. For comparison a number of redundant Bragg grating type fiber optic sensors as well as strain gauges were employed for verification of strain signals as measured by the new system. Fiber Optic Sensor Based on Michelson White Light Interferometry Michelson interferometers have been extensively employed for characterization of laser and light emitting diodes (LED) in terms of coherence length (5). Coherence length of a light source pertains to the ability of the lightwave to retain a stable phase difference in time. Lasers are capable of producing single wavelength emissions possessing long coherent lengths. On the other hand LED's produce white light of low coherence. A white light source such as an LED produces broad band emissions, or emissions containing a wide range of wavelengths (wide spectrum). In a Michelson white light interferometer arrangement, light from an LED is split into two beams (Fig. 1). One of the beams travels a fixed distance (reference beam). The path length of the other beam is allowed to change (sensing arm). Then the beams are recombined, and if the path length of the variable beam of light is made equal to that of the reference beam, then an interference pattern similar to that given in Fig.2 is generated. The difference in path length
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