We report a Kerr-lens mode-locked Cr:forsterite laser operated with negative nonlinear phase shift. The nonlinear phase shift is induced by the cascade χ<sup>(2`)</sup>:χ<sup>(2)</sup> process in a lithium triborate crystal. Employing the cascade process at large phase mismatch produces a nearly linear frequency chirp. Transform-limited pulses as short as 60 fs are generated with positive cavity dispersion.
[Optical Society of America ]
L.J. Qian, X. Liu, and F.W. Wise, "Femtosecond Kerr-lens mode locking with negative nonlinear phase shifts," Opt. Lett. 24, 166-168 (1999)