Microwave Photonic Downconversion Using Phase Modulators in a Sagnac Loop Interferometer

A new high conversion efficiency and wideband microwave photonic downconverter with infinite isolation between the RF and LO port, which enables the RF and LO to be located at different locations, is presented. It is based on two traveling wave electrooptic phase modulators inside a Sagnac loop interferometer. No dc bias voltages are required and there are no bias drift issues. High conversion efficiency downconversion is obtained by optical carrier suppression inherent from the Sagnac loop structure, which allows high-amplitude RF and LO sidebands into the photodetector. The microwave photonic downconverter topology can be extended to have multiple RF channels using a wavelength division multiplexing technique in which the phase modulators driven by the RF signals can be located in different remote locations. Experimental results are presented which demonstrate wideband operation from 5 to 20 GHz and over 26 dB conversion efficiency improvement compared to the conventional dual-series MZM-based microwave photonic downconverter.