Efficient second harmonic generation with compact design: Double-pass and cavity configurations

We report on a detailed characterization of the conversion efficiency from 1064 nm to second harmonic continuous wave laser light with periodically poled potassium titanyl phosphate crystal, with the aim of building a very efficient pump for the squeezer of the Advanced Virgo gravitational wave interferometric detector. Firstly, we study the crystal in a doublepass configuration for light at both fundamental and harmonic frequencies. We discuss how the beam alignment and laser mode hopping affect the second harmonic generation (SHG) performance and we give a detailed map of the mode hopping phenomenon in our laser source. Secondly, we employ the same double-pass crystal to form a Fabry-Perot cavity that is resonant at the fundamental frequency. We then study the efficiency of the SHG cavity as a function of the crystal temperature and input power, and demonstrate a conversion efficiency as high as (99 ± 1(stat.) ± 3(syst.))%, the efficiency being limited only by the onset of subharmonic pumped parametric downconversion.