We explore novel configuration of Kerr-lens mode-locked solid-state lasers, where the intra-cavity intensity necessary for mode-locking is reduced to record-low levels. By adding to the cavity a Kerr window of highly nonlinear glass allows to reduce the intra cavity power down to 0.34W with record–low intra-cavity pulse energy of 4.5nJ.

Reduction of the intra-cavity power is specifically important for mode-locking in short cavities with a high repetition rate (in the GHz range), which is much desired for precision measurement and frequency comb spectroscopy. The major obstacles that impedes the construction of such a laser is that as the repetition rate becomes higher, the intra-cavity pulse energy is reduced, which reduces the efficiency of the nonlinear process in the cavity, eventually precluding mode-locking.

We study the connection between the nonlinearity strength and intra-cavity pulse energy that is required to obtain mode-locking. From our results we calculate the additional nonlinearity required to obtain mode-locking at a given repetition rate and output coupler reflectivity (that determines the pulse energy). We estimate that efficient mode-locking with repetition rate of 1-5 GHz are well within reach.