The Michelson interferometer consisting of a circulator, a 3 × 3 fiber coupler and two Faraday rotatormirrors (FRMs) is used to process and demodulatethe signal, as shown in Fig. 1. The light under test(LUT) injects into one port of the 3 × 3 optical fibercoupler through a circulator and then splits into threeparts. Two of them interfere mutually in the couplerafter being reflected by the Faraday rotator mirrorswith different delay times, while the third port of thecoupler is made reflection-free. Then the interferencefringes with 120∘-phase-difference from each other areobtained by three PDs from the left three ports of thecoupler