Broadband Coherent Microwave Frequency Comb Generation Based on a Frequency Mixed Optoelectronic Oscillator

We propose a novel coherent microwave frequency comb (MFC) signal generation method based on a frequency mixed optoelectronic oscillator (OEO). The MFC generator is fully self-excited without using any external microwave source. The generated MFC signal has the advantages of large bandwidth and low phase noise. In the proposed scheme, two orthogonally polarized optical signals from a dual-polarization Mach-Zehnder modulator (DPolMZM) are sophisticatedly manipulated to realize an OEO in one loop and frequency mixing in the other loop. The OEO loop is constructed by the optical signal at the x-polarization state to generate a fundamental seed signal with low phase noise. In the other loop, the optical signals from both the x- and ypolarization states are detected in a photodetector (PD). The electrical signal is mixed with the seed signal from the OEO to generate high-frequency signal components. The output signal from the electrical mixer is injected into the y-MZM of the DPol-MZM to close the other oscillation loop. A stable MFC signal is thus generated and its frequency interval is decided by the frequency of the seed signal. The bandwidth of the MFC signal is only limited by the bandwidth of the modulator and the PD. The phase noise of the MFC signals is maintained at a relatively low level thanks to the excellent phase noise performance of the seed signal. Experimentally, broadband coherent MFC signals with a bandwidth of 30 GHz, and frequency intervals of 1.75 GHz and 5 GHz were generated, respectively. The supermode noise suppression ratio is 30 dB, and the phase noise is lower than -115 dBc/Hz at a 10 kHz frequency offset, which is 25 dB better than the traditional electrical MFC signal generated by an arbitrary waveform generator (AWG).