Common-mode intensity noise suppression scheme in phase demodulation of 3×3 coupler based on ellipse fitting algorithm

Addressing the issue of light source intensity noise in the phase demodulation scheme of the 3 × 3 coupler, this paper proposes a common mode intensity noise suppression scheme for differential signals based on the Ellipse Fitting Algorithm (EFA). This scheme employs the EFA to correct the direct current (DC) components of the three interference signals output by the 3 × 3 coupler. One of the interference signals is selected as a self-reference signal, and the other two interference signals are then subtracted the self-reference signal to obtain differential signals. This effectively eliminates the common-mode intensity noise in the DC components of the interference signals before demodulation, preventing noise from being aliased into the demodulation results. Then, the EFA and ATAN are applied to the two differential signals for signal demodulation. Experimental results show that, compared with the EFA demodulation scheme, the noise floor of the system is reduced by an average of 18 dB, with a maximum reduction of 26 dB to 1.58 μrad/√Hz at 50 kHz, and the total harmonic distortion (THD) is less than 1%.