The physical limit for the waveguide enhancement of nonlinear-optical processes

The waveguide enhancement of nonlinear-optical processes is shown to be physically limited because of the competition of diffraction and refractive-index-step radiation confinement. In the case of the limiting refractive-index step values for fused silica fibers, the maximum waveguide enhancement of nonlinear-optical processes is achieved with submicron fiber core diameters. Microstructure fibers with high air-filling fractions of the cladding are shown to enhance nonlinear-optical processes with waveguide enhancement factors close to the physical limit. Approximate asymptotic expressions derived for several important particular cases provide an adequate qualitative description of the waveguide enhancement of nonlinear-optical processes and allow the optimal fiber core diameters for the maximum enhancement of nonlinear-optical interactions to be estimated with a satisfactory accuracy.