Homodyne coherent demodulation method for fiber laser hydrophone using a 3×3 coupler with gains adjusted real-time for symmetry

Abstract

To develop a practical fiber laser hydrophone, demodulation method to get the wavelength shift of the fiber laser hydrophone is a key technology. A novel scheme for demodulating the phase shift is proposed to eliminate the affect of the three unsymmetrical outputs from 3×3 coupler. A special polarization-insensitive unbalanced Michelson fiber interferometer based on the 3×3 coupler is employed to transpose the wavelength shift into phase shift of the interferometer. The interferometer just has two outputs. A certain length of the fiber in an arm of the interferometer is wrapped onto a PZT tube to get a high frequency phase modulation signal whose amplitude is bigger than π rad, so the direct and alternate item of the two outputs from the interferometer can be gotten. The two outputs from the interferometer are detected by photoelectrical detectors, and the digitized acquisition data from the detectors are processed by Labview program to demodulate the phase shift. The gains of the two outputs are adjusted real-time to get two symmetric signals after their direct items were subtracted, and the two symmetric signals are processed through differentiator and cross-multiplier to demodulate the acoustic pressure signal. The sensitivity experiment shows that the new demodulation scheme can get a steady output and the phase output is linear to the pressure applied on the fiber laser sensor when the outputs from the interferometer are unsymmetrical. The amplitude fluctuate of the phase output is less than 5% when the fiber laser hydrophone is under stable acoustic pressure at a frequency point.