  The theoretical calculation for nonlinear refractive index in Cr: ZnSe -
active medium predicts the strong defocusing cascaded second-order nonlinearity
within 2000 - 3000 nm spectral range. On the basis of this result the optimal
cavity configuration for Kerr-lens mode locking is proposed that allows to
achieve a sub-100 fs pulse duration. The numerical simulations testify about
strong destabilizing processes in the laser resulting from a strong self-phase
modulation. The stabilization of the ultrashort pulse generation is possible
due to spectral filtering that increases the pulse duration up to 300 fs.
