Five-wavelength demodulation for three-beam interferometry based on elliptic fitting

In this paper, we develop a passive five-wavelength demodulation technique for three-beam laser interferometry based on the Elliptic Fitting Algorithm (EFA). Theoretical demodulation performance is thoroughly analyzed via simulations, demonstrating the algorithm's applicability to dual-cavity sensors with a wide range of cavity lengths (excluding specific singular points). Experimental validation is conducted using a dual-cavity extrinsic Fabry-Perot interferometer (EFPI) sensor with initial cavity lengths of 168.5321 µm and 86.5592 µm. Results show that the algorithm successfully demodulates a 1 kHz sinusoidal signal regardless of whether the modulation amplitude exceeds 2π. This confirms the technique's capability for high-speed demodulation of dual-cavity interferometers with various cavity configurations (except for identified singular points).