We have measured the birefringence enhancement due to lateral selective oxidation of AlAs in GaAs/AlAs optical waveguides. The birefringence was measured by imaging the surface-emitted second harmonic generated by the nonlinear interaction of counterpropagating TE and TM modes. In a waveguide containing a single AlAs layer the birefringence is enhanced from&Dgr;n=0.017(before oxidation) to&Dgr;n=0.038(after oxidation). In an oxidized multilayer GaAs/AlAs waveguide, a birefringence as high as&Dgr;n=0.22is measured. This birefringence is sufficient to phase-match the difference frequency generation of 3–5 &mgr;m infrared radiation from two near-infrared pumps. ©1997 American Institute of Physics.