Spectral dynamics on saturable absorber in mode-locking with time stretch spectroscopy

Area of Science:

• Ultrafast optics
• Laser physics
• Nonlinear dynamics

Background:

• Mode-locked lasers generate ultrashort pulses for diverse scientific applications.
• Semiconductor saturable absorbers are crucial for achieving stable, long-term, alignment-free mode-locking.
• Conventional spectrometers cannot capture the complex, rapid spectral dynamics during mode-locking build-up.

Purpose of the Study:

• To experimentally demonstrate the real-time spectral evolution during femtosecond pulse build-up.
• To investigate the transient dynamics leading to stable mode-locking in a fiber laser.
• To utilize time-stretch dispersive Fourier transformation (TS-DFT) for high-speed spectral measurements.

Main Methods:

• Employed a homemade passive mode-locked Yb fiber laser with a semiconductor saturable absorber mirror.
• Utilized time-stretch dispersive Fourier transformation (TS-DFT) for successive single-shot spectral measurements.
• Captured 700 consecutive spectra within a 17 µs time window to resolve transient dynamics.

Main Results:

• Observed real-time spectral evolution of femtosecond pulse build-up.
• Detected oscillating or shifting fringe patterns in spectra before stable mode-locking, indicating multiple pulses.
• Identified the presence of multiple pulses, including soliton molecules with varying relative phases.

Conclusions:

• The study provides novel insights into the birth of ultrafast mode-locked laser pulses.
• Transient dynamics, including multiple pulse formation, are linked to the fast relaxation time of saturable absorption.
• The findings enhance understanding of the transition to stable single-pulse operation in ultrafast lasers.