Mode Locking At and Below the CW Threshold

We explore experimentally a new regime of operation for mode locking in a Ti:Sapphire laser with enhanced Kerr nonlinearity, where the threshold for pulsed operation is lowered below the threshold for continuous-wave (CW) operation. Even though a CW solution cannot exist in this regime, pulsed oscillation can be realized directly from zero CW oscillation. In this regime, the point of maximum strength of the Kerr nonlinear process provides a "sweet spot" for mode locking, which can be optimized to considerably lower the pump power threshold. The properties of the "sweet spot" are explained with a qualitative model.