Calculation of a nonreciprocal geometric phase of counterpropagating wavesin a fiber ring interferometer by the method of the Poincare sphere

is shown by the method of Poincare sphere that in the case of arbitrary ell ipticity of natural orthogonal polarization modes of a single-mode optical fiber (SMOF) of a circuit of a fiber ring interferometer (FRI) and arbitrar y orientation of birefringence axes of SMOF at the ends of the FRI circuit there is an infinite set of polarization states of input radiation satisfyi ng the condition for the absence of zero shift of FRI resulting from polari zation nonreciprocity of FRI. The relation between the ellipticity and the azimuth of the major axis of the ellipse of the polarization state of radia tion that does not produce FRI zero shift is found. The corresponding expre ssions are derived for the normalized Stokes and Jones vectors of radiation at the FRI input.