Improved SHG phase matching response for focused Gaussian beams in Gouy compensated quasi-phase-matched structures

Nonlinear interactions such as second harmonic generation and sum and difference frequency generation are routinely used for the generation of laser wavelengths. With high power lasers it is necessary to use bulk crystals and tight beam focusing to achieve the maximum conversion efficiency. The use of Quasi- phase-matched (QPM) materials such as PPLN and PPKTP are now routinely reported; however, in this paper, a simple linear QPM crystal does not fully the conversion response. The theoretical and experimental results shows that by careful control of the crystal design it is possible to fully compensate for the phase errors associated with the focusing induced Gouy shift. However, this optimisation is only valid for linearly invariant materials, which are unable to compensate fully for the Gouy phase inherent to focused interactions.