Stabilization of the output signal from a double-arm fiber optic interferometer with multimode waveguides

An analysis is made of the specific features of the operation of a multimode double-arm interferometer with the aim of identifying the reasons for fading of the output signal and possible ways of suppressing this effect. This analysis is carried out for a most commonly used multimode Mach–Zehnder interferometer with an additional phase modulation in one of the arms. It is shown that fading of the output signal in a multimode variant is related to the possibility of interference of like waveguide modes traveling in the fiber arms and characterized by random phases. The result of such interference is zero mathematical expectation of the resultant output signal, but the variance of this signal is finite because its rms deviations may reach large values. Tuning and stabilization of this interferometer may be ensured by using a radiation source with a large coherence length and wide-band frequency tuning. The required tuning range is found from a theory of image transfer along multimode waveguides.