Cerenkov-type second-harmonic generation in KNbO3 channel waveguides

Cerenkov-type second-harmonic generation using KNbO3 channel waveguides produced by MeV He+-ion implantation is presented from the viewpoint of device design. We derive the Cerenkov phase-matching condition for multimode waveguides and utilize Cerenkov-angle analysis as a tool for contact-free measurement of the effective indexes of guided modes of ion-implanted KNbO3 channel waveguides at a wavelength of 860 nm. The measured mode indexes are in full agreement with calculations based on the effective-index method and the refractive index-depth profiles of ion-implanted KNbO3 waveguides. The efficiency of Cerenkov-type second-harmonic generation is modeled using analytical approximations of the field distributions of the fundamental and the Cerenkov-radiation modes in embedded-channel waveguides. The acceptance width for Cerenkov-type frequency doubling in these ion-implanted waveguides is about one order of magnitude wider than for noncritical phase-matched second-harmonic generation in bulk KNbO3 crystals. Based on the theoretical simulations, guidelines for optimum device design are given, and the possibility to increase the ultimate conversion efficiency to about 30% W-1 cm-1 through lateral-resonance enhancement of the second-harmonic field in KNbO3 channel waveguides is demonstrated.