Mode propagation losses in He^+ ion-implanted KNbO_3 waveguides

The losses of ion-implanted potassium niobate (KNbO3) waveguides are evaluated theoretically and experimentally in dependence on wavelength, irradiation dose, waveguide thickness, and waveguide width. Irradiation-induced absorption and tunneling are identified as the main sources of loss. The contributions from surface scattering and intrinsic material absorption are shown to be small. The attenuation due to irradiation-induced absorption and tunneling is calculated from the experimentally determined complex refractive-index profile. The optical loss is minimum in the red part of the spectrum and increases toward the blue because of absorption and toward the infrared because of tunneling. A minimum loss of less than 0.2 cm-1(1 dB cm-1) was measured in an ion-implanted KNbO3 waveguide at a wavelength of 0.633 μm. On the basis of a theoretical model we give guidelines for the formation of optimized waveguides for specific applications, e.g., second-harmonic generation.