The Mamyshev oscillator (MO) is a variety of fiber laser capable of manufacturing high-energy laser pulses at a tunable repetition rate. It’s a mode-locked laser which uses light travelling inside a closed-loop cavity to supply laser emission. Harmonic mode-locking (HML) is a sophisticated type of mode-locking process where multiple laser pulses are produced inside one round trip of sunshine. MOs employing HML are used for several advanced applications similar to optical communication, frequency metrology, and micromachining.
Despite increasing applications of HML MOs, understanding the sunshine buildup dynamics of HML inside these lasers is experimentally difficult. In a recent study published in Journal of Lightwave Technology, researchers from Hunan University, China have uncovered the buildup dynamics of HML in an all-fiberized erbium-doped MO. They successfully obtained HML pulse outputs of various orders. In these results, the signal-to-noise ratio of all harmonic pulse trains from the all-fiber MO exceeded 80 dB, demonstrating the high stability of the output. Furthermore, they investigated the transient dynamics in the course of the startup technique of HML within the MO.
“The starting dynamics of HML within the MO, characterised by the time-stretch dispersive Fourier transform technique (TS-DFT) revealed that the generation of HML will not be dominated by the splitting effect of the one pulse however the amplification of the multiple seeding pulses within the oscillator,” explains writer Dr. Ning Li.
Using rigorously designed experiments, the researchers identified five distinct ultrafast phases that occur between the injection of seed pulses into the laser cavity and the stable emission of HML pulses from the MO. These phases include rest oscillation, multi-pulses operation, pulse collapse reconstruction, unstable HML, and a stable HML state. Notably, the identified technique of stable HML generation was different from the traditional pulse splitting effect thought to end in laser emission dynamics in MOs. The experimental findings were further supported using numerical simulations.
Using the TS-DFT technique, they monitored the spectra evolution throughout the MO cavity in real-time and performed an in depth evaluation of the dynamic process during HML initiation. Observations revealed that the generation of HML within the MO was not dominated by the traditional single pulse splitting effect but fairly by the amplification of multiple seeding pulses throughout the oscillator.
“Our experimental and simulation results showed that under these conditions, the initial seed pulses throughout the cavity evolve into stable independent pulses through processes similar to gain amplification and energy redistribution, eventually resulting in a stable HML state throughout the resonator,” observes Dr. Li. “Results from our study can deepen the understanding of HML operation in MOs, and should provide an energetic solution to control the transient pulse dynamics within the high-performance ultrafast laser systems,” he adds.
Overall, this study has prolonged our understanding of sunshine buildup dynamics in MOs, specifically for advanced lasers using HML. Moreover, the study challenges the traditional understanding of the sunshine buildup and emission process in MOs.
Besides clarifying the underlying physics, the insights offered by the study may result in improved designs of MOs – advancing their use across several fields.