Mode-locking with ultra-low intra-cavity pulse intensity using enhanced Kerr nonlinearity (Conference Presentation)

High repetition rate mode-locked lasers (in the GHz range), are desired for frequency comb spectroscopy. A major obstacle that currently impedes high repetition mode-locking is the inevitable subsequent reduction of the single pulse energy, which reduces the efficiency of the nonlinear Kerr effect in the cavity, eventually precluding mode-locking. The standard methods to overcome this restriction is to try to maintain the intra-cavity peak intensity in spite of the reduction of the single pulse energy, by tightening the focus of the intra-cavity beam on the crystal, or by enhancing the output coupler reflectivity and increasing the pump power. This standard approach is however limited since these actions also provide better conditions for CW operation, which reduces the ML robustness, and eventually spurs the laser to operate in CW. We demonstrate a fresh attack on this problem: instead of aiming to preserve the peak intensity by tightening the focus and lowering the OC loss, we enhance the nonlinearity inside the cavity to compensate for the reduction in intra-cavity peak intensity. We harness the enhanced nonlinearity to specifically target the intra-cavity peak intensity and explore its limits and effects. With an additional nonlinear window in the cavity we enhanced the Kerr nonlinearity by an order on magnitude compared to the standard Ti:S, allowing to maintain mode-locking with an output coupler reflectivity as low as R=55% and record-low intra-cavity peak intensity (10GW/cm 2 , 50 times less than without enhanced non-linearity). Our results provide an important new knob for high-repetition rate mode-locking.