Limitation of the SHG and THG efficiency and beam break-up for femtosecond pulses by Kerr lensing

For many applications the second and third harmonic of a laser system is required and quite often high pulse energies are desired — therefore efficiency of the SHG and THG processes should be high. In the literature high efficiencies can be found frequently. Values up to 60 % and higher for SHG are reported for the femtosecond pulse regime and BBO as nonlinear medium. We now find that pushing to such high efficiency leads to distorted beam profiles and broadened, structured spectra of the harmonics due to the Kerr effect in the nonlinear medium. Our calculations show that for a common collimation / focusing geometry of SHG, very high nonlinear phase shifts are acquired when the crystal gets close to the focus — see Fig. I. The nonlinear phase shift can lead to strong self-focusing even in extremely thin crystals. B-Integrals higher than I0 are reached easily. Consequently the divergence in the crystal is strongly changed and the phase matching angle in the crystal can readily be lost. This limits the doubling efficiency and leads to strongly distorted beam profiles as well as spectra.