A High-Sensitivity Phase Demodulation Scheme for 3×3 Fiber Couplers Based on the Clarke Transform

A high-sensitivity phase demodulation scheme for 3× 3 fiber couplers based on the Clarke transform is proposed.Current passive demodulation algorithms exhibit limitations in weak phase signal demodulation. The Clarke transform is used to reduce the dimensionality of three-channel signals, and a THD extremum optimization strategy is implemented, enabling all-passive and high-precision demodulation of weak signals. Under a disturbance amplitude of 0.2 rad, the demodulated signal achieves a THD of 0.21% and a SINAD of 23.36dB. The scheme exhibits high linearity (99.98%) and excellent frequency response (99.75%). Long-term stability evaluation indicates a relative standard deviation (RSD) of 0.08% for signal amplitude and a RSD of 5.97% for THD, affirming the system's excellent temporal stability. The approach combines high measurement precision with a fully passive architecture, offering robust performance under extended operational conditions. The proposed demodulation scheme provides an innovative solution for distributed fiber optic sensing technologies in weak-signal detection scenarios.