Quasi phasematched domain periods for bulk and waveguide GaAs nonlinear frequency conversion were calculated. Waveguide beam confinement resulted in a decrease in effective refractive index and the magnitude of this index change was found to increase supralinearly with wavelength. Domain periodicity required for a 2.056-μm pumped, 10x10 μm2 GaAs core, Al0.5Ga0.5As cladding waveguide OPO at degeneracy decreased from 60.4 μm for bulk GaAs to 57.9 μm for a rib waveguide and 49.8 μm for a buried waveguide in a GaAs OPO. Careful accounting for waveguide effects is necessary to fabricate a periodic waveguide structure with the desired function.© (2009) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.