Low noise distributed acoustic sensor for seismology applications

A distributed acoustic sensor (a phase optical time-domain reflectometer) configuration with a low noise level in the hertz and sub-hertz frequency ranges is proposed. The sensor scheme uses a Mach-Zehnder interferometer to generate a dual-pulse probe signal and implements the frequency stabilization of a laser source using the same interferometer as a frequency etalon. The scheme simultaneously provides a low noise level owing to the compensation of the optical path difference of interfering backscattered fields and low drift of the output signal. It has been shown experimentally that the stabilization of the laser frequency provides up to 35 dB signal/noise gain in the sub-hertz frequencies, which are of interest for seismology. The applicability of the proposed scheme is demonstrated experimentally by teleseismic earthquakes recorded by a fiber-optic cable deployed on the seabed of the Black Sea.