Simultaneous nonlinear wavelength and mode conversion for high-brightness blue sources

We investigate frequency doubling of focused Bessel-like higher order fiber modes in a one-dimensionally quasi-phase matched structured KTP crystal. A single higher-order fiber mode, LP0,7, was generated at 971 nm using a fiber optical parametric amplifier pumped by sub-nanosecond pulses at 1064 nm. Frequency doubling to 485.5 nm was achieved with a maximum conversion efficiency of 48%, which produced a highly structured blue beam without any brightness improvement with respect to the fundamental beam. By optimizing the phase mismatch, we also experimentally demonstrate clean on-axis conversion-resulting from cascaded chi((2)) : chi((2)) processes, which is in good agreement with our numerical simulations, that give rise to brightness enhancement. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement