Simple model for polarization sensitivity of silica waveguide Mach-Zehnder interferometer

We propose a model for polarization response of Mach-Zehnder interferometer (MZI). Our model explains in particular the phase shift between two polarization states with low power applied to thermooptical heaters. We reproduce test results and, based on the model, propose ways to reduce the polarization sensitivity of MZI-based devices such as variable optical attenuators.