High-efficiency second harmonic generation in four-pass quadrature frequency conversion scheme

A four-pass quadrature frequency conversion scheme was developed to generate green output with high efficiency for pumping an ultrashort pulse laser system. With this scheme, a efficiency from fundamental energy into total second harmonic energy in excess of 80% was achieved for frequency doubling of 1064-nm in KTP with a low input fundamental laser intensity of 76 MW/cm2. A total second-harmonic output of 486 mJ was obtained with 607 mJ of the input 1064- nm fundamental laser at 10 Hz.