Femtosecond mode locking based on adiabatic excitation of quadratic solitons

We demonstrate a new approach for pulse formation in mode-locked lasers, based on exciting intracavity solitons in a two-dimensionally patterned quasi-phase-matching (QPM) grating. Through an adiabatic following process enabled by an apodized QPM crystal, we transiently excite multicolor nonlinear states within the crystal, utilize their advantageous properties for pulse formation and stabilization, and then convert the energy back to the resonating laser pulse before the end of the crystal in order to suppress losses. This idea gives access to large nonlinearities that would otherwise be too lossy for use intracavity. In our case, the states accessed are self-defocusing Kerr-like nonlinearities based on phase-mismatched second-harmonic generation. The QPM device has an additional transverse gradient, for tuning the nonlinearity and to aid in laser self-starting. We demonstrate the technique in a semiconductor saturable absorber mirror mode-locked laser with Yb:CALGO as the gain medium, producing 100 fs pulses at 540 MHz repetition rate, with 760 mW of average output power. We present comprehensive theoretical and numerical modeling of the laser to understand the new mode-locking regime. Our approach offers a flexible and compact route to managing nonlinearities inside laser cavities while suppressing the losses that could otherwise prevent or deteriorate mode-locked operation, and is particularly interesting for highly compact bulk, fiber, and waveguide lasers with gigahertz repetition rates and operating wavelengths from the near- to mid-infrared spectral regions.