This paper proposes, for the first time, an integrated optical isolator independent of light polarization. A Mach-Zehnder interferometer (MZI) with two nonreciprocal phase shifters, one for transverse electric (TE) modes and another one for transverse magnetic (TM) modes can be adjusted so that it blocks the fundamental modes of the waveguides constituting the interferometer propagating in one direction and is transparent for the modes propagating in the opposite direction. If the interferometer branch waveguides are in single mode regime, the performance of the device will not depend on the polarization of incoming light. The nonreciprocal phase shifters can be realized on structures with magnetization tangential to the propagation direction. Three geometries of nonreciprocal phase shifters are discussed and tolerances are estimated.
Oleksandr Zhuromskyy, Manfred Lohmeyer, Norbert Bahlmann, Horst tsch, Peter Hertel, and A. F. Popkov, "Analysis of Polarization Independent Mach-Zehnder-Type Integrated Optical Isolator," J. Lightwave Technol. 17, 1200- (1999)