Repetition-rate stabilization of femtosecond stretched-spectrum fiber laser

Passively modelocked lasers produce trains of femtosecond pulses, with the temporal separation between the pulses being determined by the length of the laser cavity. The repetition rate of the laser is inverse of this temporal separation. For a free-running laser, the repetition rate is very stable over short time scales (less than 1 ms), but drifts due to environmental effects on a longer time scale. For applications demanding a precise repetition rate to be maintained, such as optical frequency metrology, the laser needs to be locked to an RF or microwave reference source with a feedback loop acting on an actuator within the laser cavity. In this work, repetition-rate stabilization of a “stretched-spectrum” fiber laser is reported, which corresponds to a new modelocking regime. As the name implies, the laser produces pulses undergoing periodic breathing of the spectra during a complete round trip through the cavity. To the best of our knowledge, this breathing is the strongest modification observed in a laser to date. It is noteworthy that even under such strong nonlinearity the laser is more robust than the regular stretched-pulse laser. Encouraged with its robustness, it is proposed that the stretched-spectrum fiber laser is a promising alternate to laser oscillators for frequency metrology applications and laser master oscillators in use with accelerator based next-generation light sources. After photodetection of the laser output, one of the upper harmonics of the laser is locked to a highly stable dielectric resonator oscillator (DRO) at 1.3 GHz. In order to reduce the environmental effects on the laser, a handmade encasing was developed and temperature control of the fibers in the cavity was implemented. Remarkably, the custom encasing of the laser dramatically improved the laser’s stability, outperforming the DRO up to a 5 kHz bandwidth. Since the heating-loop is not sensitive enough, latter upgrade does not decrease the phase noise of the laser, but ensures the temperature stability stays within limits in unclimatized environment. With the present setup, we observe a maximum locking range of a few kHz. The system has the potential to stay in-lock indefinitely, as long as the excessive perturbations on the system are prevented.