Mathematical simulation of random coupling of polarization modes in single-mode optical fibers: IV. Zero drift in various schemes of fiber ring interferometers with depolarizers of polychromatic radiation

Four variants of a scheme of a fiber ring interferometer (FRI) with a polar izer, a loop of a weakly anisotropic single-mode optical fiber (SOF), are c onsidered, which differ by the location in the FRI scheme of a depolarizer of polychromatic radiation made of a strongly anisotropic SOF Based on the model of random coupling of orthogonal polarization modes in the SOF sugges ted in the first part of this work, the zero drift and fading of the interf erence signal at the FRI output caused by the polarization nonreciprocity a re calculated. In particular, it is shown that if the length of the FRI loo p considerably exceeds the h(-1) value, the depolarizer of polychromatic ra diation placed before the first beamsplitter or inside the FRI loop does no t efficiently decrease the zero drift of the FRI, whereas when the depolari zer is placed between the polarizer and the second beamsplitter, it provide s a substantial decrease in the zero drift. In the latter case, it is best to use a composite depolarizer with the ratio of lengths of sections equal to 1 : 3 and an angle of 45 degrees between its birefringence axes, as in a Lyot depolarizer. Numerical estimates of the limiting sensitivity of FRIs with large-area loops are made.