Microwave local oscillator signal generation with low near-carrier phase noise based on cascaded optoelectronic oscillators

A new scheme to generate microwave local oscillator (LO) signals with low near-carrier phase noise is proposed based on cascaded optoelectronic oscillators (OEOs). In the proposed scheme, an active mode-locking OEO is utilized to generate a microwave comb with low near-carrier phase noise. Through injecting the generated microwave comb into another OEO with a well-designed loop delay, microwave LO signals with low near-carrier phase noise can be generated. The feasibility of the scheme is numerically and experimentally demonstrated. In the experiment, a microwave LO signal at 10.0005 GHz is generated with a spurious suppression ratio of 25 dB and a near-carrier phase noise of -91 dBc/Hz@100 Hz. Compared with the microwave LO signal directly generated by using the OEO without injection, the near-carrier phase noise is reduced by 23 dB. The proposed scheme can thus be used to enhance the sensitivity of the pulse Doppler radar system for detecting small targets with low altitude and slow speed.