Balanced optical-microwave phase detector for sub-femtosecond optical-RF synchronization.

We demonstrate that balanced optical-microwave phase detectors (BOMPD) are capable of optical-RF synchronization with sub-femtosecond residual timing jitter for large-scale timing distribution systems. RF-to-optical synchronization is achieved with a long-term stability of < 1 fs RMS and < 7 fs pk-pk drift for over 10 hours and short-term stability of < 2 fs RMS jitter integrated from 1 Hz to 200 kHz as well as optical-to-RF synchronization with 0.5 fs RMS jitter integrated from 1 Hz to 20 kHz. Moreover, we achieve a -161 dBc/Hz noise floor that integrates well into the sub-fs regime and measure a nominal 50-dB AM-PM suppression ratio with potential improvement via DC offset adjustment.