Define the working of Optical-Fiber Infrasound Sensors.

Optical-Fiber Infrasound Sensors:

An OFIS comprises a sealed hose and a fiber-optic Mach-Zehnder interferometer which is sensitive to acoustically induced fluctuations within strain into the hose. The OFIS involves two optical fibers wrapped around and beside the hose into slight tension into two spiral patterns containing equal pitch. Both fibers are doubled back onto themselves, but along with different spacing, than the two fibers undergo various amounts of strain while the hose expands or contracts into response to changing air pressure. Light through a laser is coupled in both fibers through a fiber-optic splitter and a piezoelectric modulator that is excited at an appropriate frequency. After travelling along the optical fibers, then the laser beams are coupled out via other fiber-optic splitter and fed to a photodetector, in which the beams interfere. The output of the photo-detector is demodulated through a lock-in amplifier synchronized along with the modulator excitation. The output of the lock-in amplifier comprises two interference signals into quadrate. These signals are sampled at an appropriate rate (for example 200 Hz) and the samples are processed to acquire the strain-fluctuation signal and, therefore, the desired acoustic-pressure signal.