Simple Linear Space Formalism for Polarization-Dependent Interferometers: Theory and Application to Phase-Modulated Photonic Links

We outline a formalism for modeling interferometers, such as asymmetric Mach-Zehender interferometers used in both microwave photonic links and modern transmission systems. The formalism permits modeling elements with birefringence and polarization-dependent loss. By introducing a coordinate transformation between the standard “waveguide” view (coupled polarizations and independent waveguides) and the “coupler” view (coupled waveguides with independent polarizations), we reduce modeling to a concatenation of block diagonal operators and coordinate transformations. This connects to, and generalizes, an earlier approach. We illustrate the formalism by calculating the phase shift in a birefringent interferometer suffering differential normal mode losses in the couplers. Such phase shifts can be a significant source of even-order distortion in phase-modulated links employing an interferometer-based receiver.