Femtosecond Laser-Based Microwave Signal Generation and Distribution

We review our most recent progress in microwave photonic applications of ultralow timing jitter femtosecond mode-locked fiber lasers. Sub-femtosecond timing jitter (integrated from 10 kHz to >10 MHz offset frequency) optical pulse trains can be generated from various types of mode-locked fiber lasers using dispersion engineering and intracavity filtering. To fully utilize such ultralow-jitter lasers for microwave photonic applications, we demonstrate a sub-femtosecond-resolution (-159 dBc/Hz phase noise floor) phase detection method between optical pulse trains and microwave signals using a Sagnac-fiber-loop-based device named the fiber-loop optical-microwave phase detector (FLOM-PD). Using ultralow-jitter mode-locked Er-fiber lasers and FLOM-PDs, we generate 10-GHz microwave signals with -142 dBc/Hz absolute single-sideband phase noise at 10-kHz offset frequency. When an all-fiber Michelson interferometer-based repetition-rate stabilization method is further employed, the phase noise is suppressed to -90 dBc/Hz at 10-Hz offset frequency, which results in 3-fs absolute rms timing jitter integrated from 10 Hz to 10 MHz offset frequency. Long-distance microwave phase transfer via optical fiber links is also demonstrated using the FLOM-PD as a means for stabilizing pulse time-of-flight in fiber transfer. Relative frequency instability of 6.5 × 10
^-19
is demonstrated for 2.856-GHz microwave signals transferred over a 2.3-km-long fiber link. We anticipate that the capability of generation, characterization, stabilization, and transfer of ultralow-noise microwave signals using ultralow-jitter femtosecond mode-locked lasers will find more applications in microwave photonics in the near future.