Multi-octave LFM signal generation based on an optoelectronic recirculating frequency shift loop

A scheme to generate multi-octave linear frequency modulation (LFM) signals based on an optoelectronic recirculating frequency shift loop (RFSL) is proposed and demonstrated by simulation. In optoelectronic RFSL, the Mach-Zehnder modulator (MZM) driven by the initial narrowband LFM signal operates in the CS-DSB pattern to generate ±1st-order optical sidebands. The two sets of frequency components beat in a photodetector and generate an LFM signal with a doubled bandwidth. Then, the generated signal is used to replace the initial LFM signal and fed to the MZM to close the loop. As the circulating turn increases, the loop can output a multi-octave LFM signal. In addition, this scheme can easily suppress the influence of laser phase noise, thereby ensuring the signal performance. In a proof-of-concept simulation, the loop generates an 8.39 GHz LFM signal by using the initial LFM signal with a bandwidth of 0.524 GHz. Its bandwidth has increased by 16 times, resulting in the time-bandwidth product increasing to 8590. The bandwidth and center frequency of the generated LFM signals can also be adjusted by changing the parameters of the initial LFM signal and the bandwidth of the electrical band-pass filter. By selecting lasers with linewidths of 1 kHz, 1 MHz, and 0.1 GHz to change phase noise, the phase noise performance of the generated signals remains unchanged, indicating that the system effectively suppresses the laser phase noise.