Abstract
Single-shot measurement of ultrashort time with a dispersive temporal interferometer (DTI) paves the way for exploring intracavity dynamics in ultrafast lasers. Here, we report observations of pulse evolution dynamics after mode-locking buildup in ultrafast lasers. We observe the roundtrip time and phase evolution of the mode-locked pulses after buildup, and we unveil that the pulse experiences three different stages before stabilization, i.e., the soliton breathing stage, the relaxation oscillation stage, and the long-term relaxation stage. We find that the gain depletion effect makes the pulse move forward but does not shift its phase, and it can be distinguished by a DTI from the refractive index change. With this, the dynamics of the three stages is analyzed. Moreover, it is unveiled that the gain relaxation dynamics can intensively affect a laser's stability. These results provide additional perspectives on the intracavity dynamics of mode-locked lasers and of complex nonlinear systems, and they can help to improve the laser stability, which may impact laser design, ultrafast diagnostics, and nonlinear optics.
Highlights
Mode-locked lasers [1], as important ultrashort pulse sources, are widely used in industrial applications, medical applications, and fundamental research [2,3]
Since the lifetime of the gain medium is usually much longer than the roundtrip time [27], the interplay between the gain medium and the pulse pushes the laser off the steady state [28], suggesting that the pulses should experience a relaxation process before stabilization
The observation of such a stabilization process is mitigated by the lack of accurate ultrafast measurements. Keeping in mind this fundamental physical phenomenon, the salient questions that naturally arise are to guess how the laser relaxes to the steady state and how the relaxation affects the pulses. Resolving such questions can help us to understand the interplay between the soliton and the cavity components, which may lead to the observation of some nonlinear phenomena such as rogue waves [6], the Fermi-Pasta-Ulam paradox [30], breathing solitons [15], and turbulence [31]
Summary
Mode-locked lasers [1], as important ultrashort pulse sources, are widely used in industrial applications, medical applications, and fundamental research [2,3]. With a single-shot time-measurement technique, namely the dispersive temporal interferometer (DTI) [34,35], we monitor the roundtrip time and phase evolutions of modelocked pulses during the relaxation process.
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