Polarization-Insensitive Coherent Detection for Continuous, Low-Noise Tracking of Optical Beatnotes

We describe a novel approach to coherent detection which makes use of a dual polarization coherent receiver. The proposed technique, adapted from telecommunication technology, allows the analysis of the polarization state of an optical signal in addition to its phase. We demonstrate that, with proper digital signal processing, the optical phase between two lasers can be always extracted from a beatnote, regardless the polarization state of the two. In addition, birefringence-induced phase noise can be rejected from the measurement. Such capabilities cannot be achieved with traditional detection based on a single photodiode. This technique is particularly useful in frequency dissemination over fiber, where the polarization of received light changes randomly and on timescales that can be as fast as milliseconds. The proposed approach allows continuous tracking of the fiber-delivered signal as required in many scientific applications, and reduces the phase noise at detection.