Lock-in techniques for interrogation of long- and short-gauge length optical fiber sensor arrays

Volanthen, Mark, Geiger, Harald, Xu, Ming Gang and Dakin, John P. (1996) Lock-in techniques for interrogation of long- and short-gauge length optical fiber sensor arrays. Kersey, Alan D. and Dakin, John P. (eds.) In Distributed and Multiplexed Fiber Optic Sensors VI. vol. 2838, SPIE - The International Society for Optical Engineering. pp. 87-99 . (doi:10.1117/12.259819).

Abstract

Two complimentary optical fiber strain sensors employing lock-in techniques are presented. The first system interrogates an array of long gauge length sensors, defined by broadband optical reflectors and employs multiplexing in the time domain. The second system operates over shorter gauge lengths using multiple narrowband reflectors and wavelength-division-multiplexing. The first system tracks minima in the amplitude response produced from the superposition of two sinusoidal subcarrier waves. The second uses an acousto-optic-tunable-filter (AOTF) to track the peak reflective wavelength of an array of Bragg gratings. Both systems are constructed using telecommunications components. Together, the systems may be used to examine both line-integrated strain (or temperature) over long gauge lengths and local strain at a number of selected discrete points of particular interest. Lock-in techniques using dithered signals are applicable to sensors having a transfer function containing at least one turning point. This may be a maximum or minimum when observed either in transmission or reflection. The sensor responds to the dither with an amplitude-modulated signal, which permits locking of the interrogation system to the turning point. This provides a real-time response and better noise performance than scanned measurements. High-resolution monitoring of time-varying strain is demonstrated using this method. The long gauge length system has demonstrated a resolution of 3 microstrain over discrete 5 m long sensing sections, with an interrogation time of 0.25 s. When multiplexed to interrogate an array of four sections, intersection crosstalk levels were below minus 50 dB. The short gauge length interrogation system has been demonstrated using both fiber Bragg gratings and an in-line Fabry-Perot cavity as the wavelength selective reflectors. A resolution below 1 microstrain was obtained using the gratings, whereas a resolution of 1.5 multiplied by 10^-6 in optical path-length-difference was obtained when interrogating a Fabry-Perot cavity. Simultaneous monitoring of multiple Bragg gratings has also been demonstrated by multiplexing with different dither frequencies. The versatility and the high resolution make the lock-in systems ideal for smart structures applications.

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