A novel multipass scheme for enhancement of second harmonic generation.

We present experimental evidence of enhancement of the second-harmonic generation by using a multipass non-collinear phase-matching configuration for a Q-switched nanosecond-kilohertz Nd:YVO4 laser. In comparison with the single-pass configuration, the enhancement factor of the second-harmonic laser with the two-pass configuration is up to 2.5 in a type I beta-barium borate crystal, which can be further increased by adding the third-pass through the crystal. In addition, we provide a general relationship between the phase-matching angle and tilting azimuth angles of the unconverted fundamental and second-harmonic lasers. The multipass non-collinear phase-matching configuration is capable of reducing a thermal effect due to the absorption in the crystal and effectively avoiding damages on the crystal surfaces.