Large enhancement of second harmonic generation in Li/sub 2/B/sub 4/O/sub 7/ crystals

A simple multipass configuration (MPC) is designed using twin crystals that substantially enhances the conversion efficiency of extra cavity SHG by simultaneous employment of focusing and compensation of double refraction. A newly developed crystal lithium tetraborate (Li/sub 2/B/sub 4/O/sub 7/ or LB4) to demonstrate the feasibility of the scheme since the crystal has effective nonlinearity which is only 1/6th times that of KDP. The configuration improves the conversion efficiency of SHG in this crystal from 4.7 to 21% at the same level of fundamental intensity. Thus the designed MPC can be used for any other nonlinear crystals with higher nonlinearity as well.