Electric strain gauges have been the sensor of choice in structure monitoring applications. Recently, with the invention of fiber optic Bragg gratings and the rapid advance of telecommunication industry, optical strain sensors based on fiber Bragg grating is increasingly challenging the dominating position of the electric gauges. In this paper, we first discuss the performance and cost issues of typical electric strain sensing systems. This is followed by a description of the operating principles of fiber optic Bragg grating based strain sensors and a low- cost digital wavelength domain multiplexing technology for the interrogation of Bragg grating based sensor arrays. By comparing the performances and costs of electric and fiber Bragg grating based strain sensing technology at system level, we conclude that fiber optic strain sensing system is not only superior in performance but also provide lower cost per sensor for applications that requires more than 20 sensing points. Finally, a number of structure monitoring applications currently under investigation in Smart Material and Structures Research Laboratory at University of Maryland are presented.
A fiber optic sensor using the first and the second order fiber Bragg grating spectra to simultaneously measure temperature and strain is investigated. A theoretical model for estimating the wavelength and the reflectivity of the second order Bragg resonance in a fiber grating is developed, and the results are compared with the experimental results in Corning SMF 28 fiber. Moreover, technical issues regarding the sensor design such as the spectral characteristics of the Bragg gratings, bending loss in optical fibers and the higher order propagation modes are investigated. Experiments are conducted to measure the strain and temperature response of the grating sensors and compared to conventional sensors with encouraging results.
Hybrid DBR lasers composed of a semiconductor Fabry-Perot laser diode and a fiber Bragg grating have been demonstrated. In our approach, we use a single-angled facet laser diode and a conically lensed grating to obtain stable single-longitudinal mode operation over more than 85 mA current range with high side mode suppression ratio and narrow linewidth.
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