Highly efficient second-harmonic generation in novel four-pass quadrature frequency conversion

A four-pass quadrature frequency conversion scheme was developed for achieving high conversion efficiency. With this scheme, an efficiency 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 second-harmonic output of 486 mJ was obtained with 607 mJ of the input 1064 nm fundamental laser at 10 Hz.