LAUSR

A flexible ultra-wide frequency photonic-assisted method is proposed for microwave harmonic down-conversion based on re-circulating four-wave mixing (RFWM) in a semiconductor optical amplifier (SOA). The proposed down-converter consists of a RF-driven electro-optic modulator (EOM) and a RFWM-intensified optical local oscillator (LO) located in a ring-assisted Mach-Zehnder interferometer (R-MZI). In the optical LO, the optical carrier is first modulated by a low-frequency electrical LO through an EOM for triggering high-order harmonics sideband generation in the optical domain through the FWM effect in the SOA, and the generated harmonics sidebands are further intensified by re-circulating the FWM products back to the EOM and the SOA successively with an amplified ring loop. The RFWM-intensified optical LO enables accurate tunable and ultra-wide frequency operation of down-conversion by simply adjusting the low-frequency electrical LO. In the experiment, the RFWM-based optical LO is operated with wide spectrum of more than 0.8 nm (15-dB bandwidth) and 1.2 nm (20-dB bandwidth) and tunable frequency spacing from 4 GHz to 12 GHz. The microwave frequency conversion is successfully demonstrated in the RF range of 5-40 GHz down-converted to IF band below 2 GHz with a low-frequency electrical LO at about 4.8 GHz.