A 3×3 Coupler Phase Demodulation Scheme Based on Three-Dimensional Ellipsoid Fitting

A 3×3 coupler phase demodulation scheme based on three-dimensional ellipsoid fitting algorithm (3D-EFA) is introduced in this paper. Compared to scheme based on two-dimensional ellipse fitting algorithm (2D-EFA), it performs better in low signal-to-noise ratio (SNR) conditions. Specifically, in the case of 1/4 arc, it reduces total harmonic distortion (THD) by 0.94% at 40 dB SNR and by 29.3% at 20 dB SNR. The scheme offers high linearity (99.99%), flat frequency response, and low relative standard deviation (RSD) of 0.67%. It effectively demodulates shock signals, achieving ±50 m localization error. The demodulation scheme is stable and has potential applications in optical fiber sensors.