Ultrafast low-noise Yb: fiber laser for precise femtosecond-level synchronization applications

We present a robust, ultra-low-noise Yb: fiber femtosecond laser that delivers 100 mW average power and 140 fs transform-limited pulses at a 40 MHz repetition rate. The free-running repetition-rate drift remains below 0.25 ppm for more than 12 hours with the laboratory temperature stabilized within ±0.5 °C, enabling straightforward and highly precise synchronization to either an identical unit or to our in-house low-noise Er-doped femtosecond laser. Synchronization is achieved using custom high-sensitivity balanced optical cross-correlator modules. With only a 3 kHz PID locking bandwidth, we demonstrate 2 fs relative timing jitter on short time scales and 1.3 fs RMS timing drift over hundreds of hours of operation in a standard laboratory environment without enclosure and vibration isolation of the experimental setup. To our knowledge, this is the first demonstration of >140 hours long-term laser-to-laser synchronization using exclusively in-house technologies under uncontrolled environmental conditions. Owing to its intrinsically low noise performance, the system requires neither fast nor complex feedback for long-term stabilization. This laser platform enables true plug-and-play operation for experiments requiring high-precision timing control, such as pump-probe measurements in large-scale free-electron laser facilities for studies of ultrafast atomic dynamics.