Mamyshev oscillators (MOs), a type of fiber laser, are known for emitting high-energy pulses through a laser generation technique called harmonic mode-locking (HML). Although MOs using HML have shown promise in fields such as optical communication, frequency metrology, and micromachining, the underlying physics of how they operate has remained poorly understood—until now.
In a recent breakthrough, researchers have investigated the internal light buildup dynamics within a MO and discovered that the process deviates significantly from the widely accepted pulse-splitting mechanism long believed to drive laser emission in these systems.
MOs operate as mode-locked fiber lasers, generating pulses by circulating light within a closed-loop cavity. HML, a more advanced form of mode-locking, allows the generation of multiple laser pulses in a single round trip of light through the cavity. Despite the increasing application of HML-based MOs, directly studying the dynamics involved in pulse generation has remained a major experimental challenge.
In a new study published in the Journal of Lightwave Technology, a research team from Hunan University, China, explored these dynamics using an all-fiberized erbium-doped MO. They successfully achieved harmonic pulse outputs of varying orders, all demonstrating excellent stability with signal-to-noise ratios exceeding 80 dB.
Crucially, the researchers examined how these harmonic pulses build up during the laser startup phase.
“The starting dynamics of HML in the MO, characterized by the time-stretch dispersive Fourier transform technique (TS-DFT), revealed that the generation of HML is not dominated by the splitting effect of the single pulse but the amplification of the multiple seeding pulses in the oscillator,” explains Dr. Ning Li, one of the study’s authors.
By designing precise experiments, the team identified five distinct ultrafast phases that occur between the injection of seed pulses into the laser cavity and the onset of stable HML pulse emission. These phases include relaxation oscillation, multi-pulse operation, pulse collapse reconstruction, unstable HML, and ultimately, a stable HML state. Remarkably, this buildup process differs from the conventional pulse-splitting mechanism historically believed to be responsible for pulse formation in MOs.
To capture and analyze these phenomena in real-time, the researchers employed the TS-DFT technique to monitor the spectral evolution inside the MO cavity during the HML initiation process. Their observations confirmed that the key mechanism was not pulse splitting, but the amplification of several seed pulses coexisting in the cavity.
“Our experimental and simulation results showed that under these conditions, the initial seed pulses within the cavity evolve into stable independent pulses through processes such as gain amplification and energy redistribution, eventually leading to a stable HML state within the resonator,” observes Dr. Li. “Results from our study can deepen the understanding of HML operation in MOs, and may provide an active way to control the transient pulse dynamics in high-performance ultrafast laser systems,” he adds.
Overall, the study provides new insights into the internal processes governing MOs, specifically those using HML techniques. By revealing that laser emission in MOs is driven by the growth of multiple seed pulses—rather than by the splitting of a single pulse—these findings challenge long-standing assumptions in the field.
Beyond improving our fundamental understanding, the research opens the door to new approaches in designing and optimizing Mamyshev oscillators, potentially expanding their effectiveness in cutting-edge scientific and industrial applications.
