A dive into the physics of mode-locking in lasers: from soliton dynamics to new sources of short, high-intensity pulses

I will describe two recent results in research of mode-locked lasers:

1. Dissipative solitons are fundamental wave‑pulses that preserve their form in the presence of periodic loss and gain. The canonical realization of dissipative solitons is Kerr‑lens mode locking in lasers, which delicately balance nonlinear and linear propagation in both time and space to generate ultrashort optical pulses. This linear‑nonlinear balance dictates a unique pulse energy, which cannot be increased (say by elevated pumping), indicating that excess energy is expected to be radiated in the form of dispersive or diffractive waves. I will present an experiment and simulation to demonstrate that Kerr‑lens mode‑locked lasers can overcome this expectation. Specifically, by breaking the spatial symmetry between the forward and backward halves of the round‑trip in a linear cavity, the laser can modify the soliton in space to incorporate the excess energy.
   
    Scientific Reports 12, 14874 (2022), Idan Parshani, Leon Bello, Mallachi-Elia Meller and Avi Pe’er, “Passive symmetry breaking of the space–time propagation in cavity dissipative solitons”,

2. Mode locking in semiconductor lasers is a long-saught goal in laser research. I will present our recent achievement of a mode-locked semiconductor laser oscillator with record-high performance of 5-8ps pulses and record peak power of 112W. To achieve this high power performance we employ a high-current broad-area, spatially multi-mode diode amplifier, placed in an external cavity that enforces oscillation in a single spatial mode. Mode locking is achieved by dividing the large diode chip (edge emitter) into two sections with independent electrical control: one large section for gain and another small section for a saturable absorber. Precise tuning of the reverse voltage on the absorber section allows to tune the saturation level and recovery time of the absorber, providing a convenient knob to optimize the mode-locking performance for various cavity conditions.
   
   Optics Express 31, 41979 (2023), Mallachi-Elia Meller, Leon Bello, Idan Parshani, David Goldovsky, Yosef London and Avi Pe’er, “High-energy picosecond pulses with a single spatial mode from a passively mode-locked, broad-area semiconductor laser”,