Synchronized Two-Color Mode-Locked Fiber Laser Source With GHz Repetition Rate

Multi-color ultrashort pulses with GHz repetition rates are promising for spectroscopic and four-wave-mixing-based applications that specifically require both fast transient response and fast refresh rate. Among existing technologies for generating multi-color ultrashort pulses, actively synchronizing multiple lasers operating at different wavelengths advances in the technical simplicity, and shows a high coherence. However, for GHz repetition rates, the short cavity design, on one hand, is sensitive to environmental perturbations that change the cavity length (thus the repetition rate); on the other hand, it challenges the active repetition-rate synchronization by adding extra free-space components. In this work, we demonstrate long-term stable synchronization between a Yb-doped GHz-repetition-rate mode-locked fiber laser and an Er-doped GHz-repetition-rate mode-locked fiber laser. The stable active synchronization is realized by using the phase-locked-loop technology that is implemented for short fiber cavities. The repetition rates of the two short fiber laser cavities can be stably locked with millihertz precision and a timing jitter of about 20 fs.