Signal-space-stabilized ellipse fitting for outlier-robust phase demodulation in passive 3 × 3 optical fiber coupler interferometers

Passive 3 × 3 coupler demodulation based on the ellipse fitting algorithm (EFA) is prone to phase-recovery distortion at small phase amplitudes. In this case, the Lissajous trajectory provides weak geometric constraints for ellipse fitting. Transient outliers from circuit disturbances or other nonstationary events can further bias the fitted ellipse. However, the suppression remains insufficiently investigated in passive 3 × 3 EFA demodulation. To address this problem, a signal-space-stabilized EFA method is proposed. The three detector outputs are mapped into a three-dimensional signal space. Sample reliability is then evaluated before signal-plane estimation and ellipse fitting. Low-reliability samples are excluded from geometric calibration, and short low-reliability intervals are restored only after the ellipse geometry has been fixed. Experiments using a Mach–Zehnder/Sagnac (MZ/Sagnac) fiber-sensing system validate the method over a phase-amplitude range of 0.39–3.00 rad. At 0.39 rad, close to the π /8 regime, the proposed method remains stable, whereas traditional EFA degrades severely. At approximately 1 rad, 50 repeated acquisitions yield an average total harmonic distortion (THD) of 0.54%, an average signal-to-noise and distortion ratio (SINAD) of 38.01 dB, and good phase-amplitude linearity with R 2 = 0.9998. The method also improves the demodulation quality of measured pulse and tapping signals, demonstrating its applicability to nonstationary phase responses. These results demonstrate improved robustness of EFA-based phase demodulation under noisy and outlier-contaminated conditions.