Ultra-Wide and Tunable Microwave Frequency Down-Conversion Based on Re-Circulating Opposite Frequency Shifting

A photonic-assisted scheme is proposed and demonstrated for microwave frequency down-conversion based on re-circulating opposite frequency shifting. The proposed method consists of a microwave signal receiver and a re-circulating opposite frequency shifter. The microwave signal under down-conversion is received by electro-optical modulation (EOM). The microwave frequency is down-converted with a ring-assisted re-circulating EOM, in which the 0th and 1st optical sidebands of the microwave signal are frequency shifted in the opposite direction with the frequency step of the local oscillator (LO) signal in optical frequency domain, respectively. Two optical frequency combs (OFCs) with the same frequency spacing and different center frequency are generated. After photodetection, the frequency of the down-converted IF signal is the frequency difference between the generated two OFCs. The re-circulating opposite frequency shifter enables accurately tunable and ultra-wide frequency operation of down-conversion by simply adjusting the low-frequency electrical LO. As compared with the conventional EOM-based methods, our method eliminates the bandwidth limitation of devices for the wideband LO signals EOM. Prior to the conventional OFC-based methods, ours enables flexible frequency down-conversion by tuning the frequency of LO. In the demonstrations, 6∼40 GHz radio-frequency (RF) signals are experimentally down-converted to IF band below 3 GHz with a low-frequency electrical LO in the range of 6∼6.5 GHz.