Long-term stable sub-femtosecond synchronization of microwave signals with mode-locked Er-fiber lasers

We present a fiber-based optical-microwave phase detector that detects the phase error between optical pulse trains and microwave signals with sub-fs resolution over 1 MHz bandwidth. The proposed phase detector is used to synchronize microwave signals from VCO with optical pulse trains from mode-locked Er-fiber lasers. The residual phase noise between the optical pulse trains and the synchronized microwave signals is -133 dBc/Hz (-154 dBc/Hz) at 1 Hz (5 kHz) offset frequency, which results in 838 as integrated rms timing jitter [1 Hz-1 MHz]. The long-term residual phase drift is 847 as (rms) measured over 2 hours. We also used the phase detector and the low-jitter Er-fiber laser to measure the phase noise of a microwave signal synthesizer.