1. We have experimentally observed a similar qualitative behavior by measuring the ratio between the CW and ML intra-cavity powers as a function of Z for a Kerr medium with varying thickness. The Kerr strength can be experimentally expressed as the ratio between the CW and ML intra-cavity powers, while increasing the distance Z between the curved mirrors effectively increases the pulse power. We find that as the material thickness increases the “sweet spot” is pushed towards higher values of Z. The Kerr strength also increases with increasing thickness, eventually enables mode locking directly from zero CW oscillation:Results: A well known feature of mode locking is that only when the pump power crosses a certain threshold, ML can be initiated. Another characteristic feature of ML is that a certain amount of CW oscillation in the cavity is necessary, and only on top of an existing CW can a small noise seed be amplified to create the pulse. Yet, the question whether the preliminary existence of a CW oscillation is a necessary condition for ML operation was not directly explored. In this work, we show that an initial CW power need not exist. Mode Locking At and Below the CW Threshold Shai Yefet and Avi Pe’er Physics Department & Institute of Nanotechnology, Bar-Ilan University, Ramat-Gan 52900, Israel Introduction: 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 lower than the threshold for continuous-wave (CW) operation. Even though a CW oscillation cannot exist in this regime, a stable pulse oscillation can be realized by first raising the pump power to the CW threshold, locking, and then lowering the pump again below the CW threshold, shedding light on the dynamical evolution of mode locking in the cavity. The "sweet spot", where the thresholds for pulsed and CW operations meet, offers high performance for mode locking with low threshold pump power and very low intra-cavity average power. Motivation: Simulation: A qualitative model is given to explain the physical origin of the “sweet spot” in which the Kerr strength is maximal. To simulate this behavior, we calculate the strength of the Kerr effect as a function of the pulse power for a Kerr medium with varying thickness: Experimental setup and results: The key feature of our optical cavity is the addition of a lens based telescope between the curved mirrors. The nonlinearity of the cavity can be enhanced in a controlled manner by introducing an additional nonlinear plate near the imaged focus while varying the window position. The ML and CW operation parameters as a function of Z are plotted below for a Brewster cut SF6 glass plate. For off-focus SF6 window position (a+b), we see the typical behavior in which the ML threshold is always larger than the CW threshold. ML is most favorable at the “sweet spot where the CW power drops to minimum. For in focus SF6 window position (b+c). The ML threshold curve eventually crosses the CW threshold where the CW power drops to zero, and ML can be achieved directly from pure fluorescence. Above the crossing point, stable ML can still be initiated, but only by first raising the pump power up to the CW threshold. TiS M1 M2 OC HR Pump source @ 532nm f f 2f Additional window Z