Defense Tech Briefs - Optical-Fiber Infrasound Sensors

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Optical-Fiber Infrasound Sensors

Defense Threat Reduction Agency, Fort Belvoir, Virginia

Jul 31 2007

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Features include directionality and relatively flat frequency response.

Optical-fiber infrasound sensors (OFISs) are being developed for detecting acoustic pressures in the frequency range from a few millihertz to a few hertz. As explained below, these sensors were conceived to overcome some of the limitations of prior infrasound sensors based on pipe filters connected to microbarographs.

Figure 1. An OFIS includes a fiber-optic Mach-Zehnder interferometer that measures acoustic strain in a sealed hose. The length and diameter of the hose are not critical and can be chosen according to considerations of sensitivity, directionality, and suppression of noise. In one prototype OFIS, the hose is 2.5 cm in diameter and 89 m long.

An OFIS includes a sealed hose and a fiber-optic Mach-Zehnder interferometer that is sensitive to acoustically induced fluctuations in strain in the hose. The OFIS (see Figure 1) includes two optical fibers wrapped around and along the hose in slight tension in two spiral patterns having equal pitch. Both fibers are doubled back on themselves, but with different spacings, so that the two fibers undergo different amounts of strain when the hose expands or contracts in response to changing air pressure. Light from a laser is coupled into both fibers via a fiber-optic splitter and a piezoelectric modulator, which is excited at a suitable frequency. After traveling along the optical fibers, the laser beams are coupled out via another fiber-optic splitter and fed to a photodetector, wherein the beams interfere. The output of the photodetector is demodulated by a lock-in amplifier synchronized with the modulator excitation. The output of the lock-in amplifier consists of two interference signals in quadrature. These signals are sampled at a suitable rate (e.g., 200 Hz) and the samples are processed to obtain the strain-fluctuation signal and, hence, the desired acoustic-pressure signal.

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