<title>Interferometric vibration and acoustic sensor systems for airport ground traffic monitoring</title>

Extrinsic Fabry-Perot interferometer fiber-optic microphone

“…Reliable acoustic classification has been demonstrated with respect to vehicle classes such as light and heavy trucks, passenger cars, and motorcycles [1]. Fiber-optic microinterferometer sensors appear to be particularly well suited to the monitoring of traffic and for vehicle classification due to their extremely high sensitivity, full electromagnetic interference immunity, and the possibility of installation without digging up the roads [2]. No electrical equipment is required at the sensor-head location, even though the sensor itself may be separated from the processing system by several kilometers of downlead fiber.…”

“…For our case of low mirror reflectivity: % (glass-air boundary), the normalized output intensity ( and are the interference signal intensities 0018-9456/98$10.00 © 1998 IEEE reflected back from, and the input intensity into the FP cavity, respectively) of the extrinsic Fabry-Perot interferometer may be approximated by the cosinuidal characteristic of two beam interferometers. Including a factor for modeling the fringe contrast reduction by the finite spectral width of the source, assuming a Gaussian spectral distribution [7], the FPcharacteristic is approximated by (1) [2]: 1is the phase shift experienced by the light wave as it travels through the FP cavity and back, a distance of . It is the sum of the offset phase due to the initial cavity length and the additional phase shift , caused by the variation in by .…”