A 3 × 3 Phase Demodulation System Based on Active Ellipse Fitting for 3C Fiber-optic Geophone Array and Field Tests

A phase demodulation system with a 3 × 3 coupler and active ellipse fitting algorithm for 3C interferometric fiber optic geophone array is proposed. Additional phase modulation is introduced to solve the problem of EFA fitting errors in weak signals. A stable Lissajous is provided for the EFA algorithm to ensure that the ellipse parameters can be calculated at any time. It is experimentally demonstrated that the system exhibits a noise base level of -92.61 dB and a total harmonic distortion of $1.53 \times 10^{-3}$ . The system achieves common mode noise suppression by incorporating a reference sensor, thereby mitigating the impact of field noise on the demodulation process. In the field tests, the proposed system successfully demodulated all phases from a fiber-optic geophone array simultaneously while observing time-of-flight delays and seismic wave frequencies.