Bloch Modes of Periodically Segmented Waveguides

Abstract

Quasi-phase-matched second-harmonic generation of visible light in optical waveguides is a promising source of blue-green coherent light that may have applications to optical disk storage systems. Bierlein, et al. have studied periodically segmented waveguides in KTP and have demonstrated that it is possible to efficiently generate blue-green light. A typical structure consists of an array of ion-exchanged regions with refractive index n₀ + Δn, that is embedded in a medium with refractive index no, as shown in Fig 1. This structure is unconventional because guiding and unguiding sections alternate. Extremely low loss has been experimentally observed in these devices, indicating that guiding does exist despite the segmentation. In this paper, we show that within the limit of the paraxial approximation and neglecting back reflections, lossless guided Bloch modes exist and we numerically determine the propagation constants and mode structures.