A Low Noise Floor 3 × 3 Coupler Phase Demodulation Scheme

Refining the noise floor and enhancing reliability are crucial for optical fiber sensing systems employing 3 × 3 coupler phase demodulation technology. In this article, we achieve the intensity noise cancellation by dividing the interference signal directly by the laser intensity. Experimental results show an average noise floor reduction of 11.2 dB and a maximum reduction of 17.3 dB at 50 kHz. We then optimize the commonly used ellipse fitting algorithm (EFA) in the 3 × 3 coupler demodulation system to overcome its inability to work properly in demodulating small signals. The improved algorithm ensures the equality of direct current (DC) values of the two interference signals by introducing gradient descent. Additionally, it rotates the coordinate system to transform the Lissajous figure into a standard ellipse with fixed phase difference and limited center. This transformation reduces the five parameters of the EFA model to three. Experimental results demonstrate that this demodulation scheme achieves an 99.98% linearity and 1.56 μrad/√Hz phase measurement resolution. Notably, it offers several advantages, including low noise floor, high reliability, and the absence of phase modulation requirements, making it highly applicable in 3 × 3 coupler demodulation systems.