Contrast of a photomixing signal in a multimode ring interferometer

A simple method is proposed for determining the contrast of a photomixing signal in a fiber-optic ring interferometer. The method is based on determination of the constant component of the photocurrent in the presence and absence of harmonic phase modulation. Measurements are reported of the contrast of a photomixing signal in a multimode fiber-optic ring interferometer as a function of the number of mode groups mixed in a photodetector, length of a waveguide, and radiation wavelength. It is shown that in the case of low-index mode groups the contrast is mainly due to the intergroup exchange, whereas in the case of high-index mode groups it is due to the intragroup exchange.