Femtosecond Kerr-lens mode locking with negative nonlinear phase shifts.
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 chi((2;)):chi((2)) 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.
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