Harmonically Mode-Locked Optoelectronic Oscillator Incorporating a Microwave Photonic Filter for Supermode Noise Suppression

An actively and harmonically mode-locked optoelectronic oscillator (OEO) is demonstrated to generate a broadband microwave frequency comb (MFC) with a low near-carrier phase noise and a suppressed supermode noise. To achieve a low near-carrier phase noise, the OEO is designed to have a long feedback loop, to increase its Q factor. However, an OEO with a long loop has a large number of densely spaced longitudinal modes, making the OEO, if harmonically mode locked, have a high supermode noise. In this work, a novel approach to suppressing supermode noise by incorporating a microwave photonic filter (MPF) is proposed and experimentally demonstrated. The MPF deployed in the OEO loop has a free spectral range (FSR) equal to the harmonic mode locking frequency. This ensures that the desired longitudinal modes are selected, while the unwanted supermodes are effectively suppressed. The proposed approach is experimentally evaluated. For an OEO with 2nd-order and 3rd-order harmonic mode locking, by using a two-tap and three-tap MPF, the supermode suppression ratio is increased from 8 dB to 59 dB for 2nd-order harmonic mode locking and from 5 dB to 55 dB for 3rd-order harmonic mode locking, indicating the suppression ratio is increased by 51 dB and 50 dB as compared with a harmonically mode-locked OEO without an MPF. The phase noise of the proposed mode-locked OEO at an offset frequency of 10 kHz are measured to be about -94 dBc/Hz and -104 dBc/Hz at a central frequency of 10 GHz and 4 GHz, respectively. The proposed technique can also be utilized for higher order harmonically mode-locked OEOs, showing its capability for high-repetition-rate MFC generation with low near-carrier phase noise and low supermode noise.