Improved phase demodulation algorithm for a 3 × 3 fiber coupler based on ellipse fitting

The ellipse fitting algorithm (EFA) is widely used to compensate for asymmetric parameters in phase demodulation based on a 3×3 fiber coupler. However, it is prone to demodulation distortion of the measured phase signals under small signal conditions. In this paper, an improved demodulation algorithm, referred to as EFA-C-ATAN, is proposed to enhance demodulation stability and accuracy. The proposed algorithm employs a geometric transformation method, including translation, rotation, and normalization, to remove asymmetric parameters in the interference signals, thereby achieving phase demodulation. Besides, a lower ellipse eccentricity leads to improved fitting precision during the demodulation procedure. Experimental results show that the signal-to-noise and distortion ratio (SINAD) is improved, and the total harmonic distortion (THD) is reduced, especially under the small signal case. In addition, the standard deviation (STD) of amplitude fluctuations is decreased in small signal conditions, and a linearity response of 0.9998 is achieved. Finally, the computation time and real-time implementation feasibility of the proposed algorithm are discussed.