Phase extraction method for nonlinear interference signals based on a 90° hybrid coupler in frequency scanning interferometry

Frequency scanning interferometry (FSI) is an absolute distance measurement method based on external cavity tunable lasers (ECTLs). However, nonlinear tuning of ECTLs will introduce errors in the extraction of the phase of the interference signal, further affecting the measurement accuracy of the FSI system. In this paper, an FSI measurement system is built based on a 90° hybrid coupler, and the corresponding phase extraction method is studied. After interference is performed in a 90° hybrid coupler, an error in the phase extraction of the interference signal occurs due to the unequal amplitude and orthogonal phase deviation of the two signals. This approach is different from the traditional method of first compensating for non-strictly orthogonal signals into strictly orthogonal signals and then calculating them. In this paper, the compensation process is not implemented, the composition of the error is analyzed after directly processing non-strictly orthogonal signals, and a method that accurately extracts the phase of the nonlinear interference signal is proposed. The effectiveness of the algorithm was verified through nonlinear simulation and experiments, and the computational efficiency of the algorithm was analyzed.