Attosecond-precision microwave photonics based on ultralow-jitter mode-locked lasers

We present the latest progress in attosecond-precision microwave synthesis, synchronization and remote transfer, based on ultralow-jitter passively mode-locked lasers. First, we show ultralow-noise 10-GHz microwave synthesis from an 80-MHz mode-locked Er-fiber laser with -157 dBc/Hz phase noise at 100-kHz offset frequency. We further show a simple all-fiber-optic stabilization method that enables 0.98-fs (20-fs) absolute timing jitter integrated from 100 Hz (1 Hz) to 10 MHz offset frequency. Second, attosecond-stability synchronization between mode-locked lasers and microwave oscillators is demonstrated. We developed an optical phase detector, named the fiber-loop optical-microwave phase detector (FLOM-PD), that enables direct phase detection between optical pulse train and microwave with attosecond resolution and long-term stability. By using the FLOM-PD, we show 800-attosecond-precision synchronization between an 80-MHz mode-locked Er-fiber laser and a 10-GHz microwave oscillator maintained over 2 hours. Several emerging applications of FLOM-PD, ranging from ultrafast electron diffraction apparatus to fiber-optic sensors, will be also introduced. Finally, we show remote transfer of microwave phase using optical pulse trains generated from mode-locked lasers. A 2.856-GHz microwave signal is transferred over a 2.3-km fiber link with 7×10^-19 relative fractional frequency instability. We will further show the recent work in free-space transfer of optical pulse trains for microwave phase distribution.