High Conversion Efficiency Photonic Microwave Mixer With Image Rejection Capability

A new photonic microwave image rejection mixer that can achieve high conversion efficiency and has a wide bandwidth is presented. It is based on an integrated dual-parallel Mach-Zehnder modulator (MZM) with a 90° polarization rotator on one branch of the modulator. The two modulators connected in parallel are biased at the minimum transmission point to generate two sets of orthogonally polarized radio frequency (RF) and local oscillator (LO) modulation sidebands with the optical carrier being suppressed. Suppressing the optical carrier enables high RF and LO modulation sidebands to be detected by a photodiode without saturation. The LO modulation index is also designed to obtain a high conversion efficiency mixing operation. Experimental results demonstrate frequency downconversion with an image rejection function and a high conversion efficiency of > $-$ 5 dB over a wide 3 to 20 GHz frequency range. A technique for compensating the effect of coupler amplitude and phase imbalance to obtain a high image rejection ratio is also experimentally demonstrated.