A phase-tracking fiber interferometer for seismologic applications

A fiber Fabry-Pe acuterot interferometer is proposed for high precision seismic wave detection. Simple polarization optics are employed in the sensor head design to generate two inter-dependent quadrature phase-shifted interference fringes, with the added advantage of reducing/suppressing signal attenuation due to polarization scrambling in the fiber system. Interference signal demodulation exploits an algorithm which optimizes the phase calculation on the alternatively evolving quadrature pair as well as enables phase compensation for out-of-quadrature signals. Vibration measurements carried out concurrently with a seismometer under free oscillating conditions show excellent agreement. Differences obtained from both instruments in displacement amplitude and velocity amounted to ~50 nm and 5 nm/ms, respectively.