Abstract
The effects of walk-off and group velocity difference on the optical parametric generation in KTiOPO4 crystals are numerically revealed. These calculations treat the transverse-mode profile and the temporal pulse shape of the interaction pulses simultaneously. Combining calculations with experimental results, the values of a second-order nonlinear coefficient and two-photon absorption coefficient at 0.6 μm are obtained as d24=4.2 pm/V and 0.35 pm/W, respectively. Due to the back-conversion, the maximum conversion efficiency with a 5-mm-length crystal is limited to about 10%. This is because the beam diameter and the pulse duration of the signal and idler fields become small in the propagation and interaction with the pump field cannot take place at the entire portion of the pump field. The calculation results also indicate that 280 fs output pulses are obtained from a 1 ps pump pulse by the pulse narrowing effect. It is also clarified that the back-conversion causes depression and splitting of the transverse mode and walk-off plays an important role in the transformation of the transverse-mode profile. The depression and splitting can be suppressed by using a short-length crystal or decreasing pump intensity.
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