Adaptive Ellipse Fitting Based Ameliorated Passive Interferometric Demodulation Scheme

We propose an adaptive ellipse fitting for a passive interferometric demodulation scheme using a 3 × 3 coupler to attain the optimal balance between accuracy and efficiency. The adaptive ellipse fitting outputs optimal ellipse correction parameters according to the input elliptic arc radian. The radian of the Lissajous figure is initially calculated to determine whether least squares or hyper renormalization based ellipse fitting should be executed. If the radian is small, the previous fitted ellipse correction parameters are invoked as the 3 × 3 coupler is a passive device. The desired signal is then extracted from two corrected interference signals. Simulation and experimental results show that hyper-renormalization method has higher accuracy when the radian ≥ π/2. And least squares method has higher efficiency and close accuracy when the radian ≥ 5π/4. The judgment criteria are therefore set as π/2 and 5π/4 for the adaptive ellipse fitting. The experimental results show that the proposed scheme not only exhibits high robustness and accuracy but also enhances operational efficiency. Small signals can be demodulated without failures, harmonic distortions are well suppressed and the response linearity reaches up to 99.999% in a wide stimulus range.