Phase-mismatched localized fields in A-PPLN waveguide devices.

Highly phase-mismatched nonlinear interactions can generate spatially localized optical fields that can affect the performance of nonlinear optical devices. We present a theoretical description of the generation of such spatially localized optical fields by ultrafast pulses. The effects of temporal walk-off and pump depletion are discussed, along with methods for suppression of the localized field while maintaining the performance of the nonlinear device. The model is validated by the measurement of the spatial profile of the localized field in a quasi-phase-matched (QPM) aperiodically poled lithium niobate (A-PPLN) waveguide. Finally, we fabricate and characterize A-PPLN devices with a 33% duty cycle to reduce the locally generated field by 90%.