Mode-Locking With Ultra-Low Intra-Cavity Pulse Intensity Using Enhanced Kerr Nonlinearity

Kerr lens mode locking strongly depends upon intensity. Low pulse intensity weakens the nonlinear Kerr lens, and eventually mode locking is not supported. We demonstrate efficient mode-locking in a Ti:Sapphire laser oscillator with record-low intra-cavity pulse intensity (18 GW/cm
²
) and intra-cavity average power (340 mW). Using an additional nonlinear window in the cavity, the Kerr nonlinearity can be enhanced by an order of magnitude compared to the standard Ti:Sapphire cavity, allowing mode locking with an output coupler reflectivity as low as R = 55% and record-low intra-cavity pulse intensity. Our results provide an important new possibility to control mode-locking in cavities with very low intra-cavity pulse intensity, such as semiconductor laser oscillators and high-repetition rate mode-locked lasers.