Birefringence effect in devices based on phase modulators in a Sagnac interferometer

Abstract

Optical phase modulators (PMs) in a Sagnac interferometer (SI) have many applications in electro-optic and microwave photonic (MWP) systems. Propagating lights in an SI are combined coherently at its output, so their polarizations must be maintained. Therefore, polarization-maintaining optical fiber (PMF) is used in SI-based devices. Because PMFs have birefringence, propagating lights with different polarizations experience different phase delays in the SI. In this paper, we investigate the effects of birefringence on the performance of the devices based on PMs in an SI. Specifically, we study this effect in (i) an optical intensity modulator based on a PM in an SI and (ii) an MWP mixer based on two PMs in an SI. Closed-form expressions for transmission and reflection coefficients are derived using the Jones matrix method. Besides, conversion efficiency of the MWP mixer is calculated in the presence of birefringence. It is shown that these coefficients depend on the birefringence parameter of the PMF, and it cannot be ignored in a device based on an SI containing PMs. Our analytical predictions are verified by previously reported data for some special cases.