Waveguides and Fibers for Nonlinear Optics

Abstract

Optical waveguides are ideal for nonlinear interactions because they provide strong beam confinement over long propagation distances. They are characterized by regions of high refractive index bounded by regions of lower refractive index. Examples of such waveguides are shown in Figure 1. Two-dimensional confinement is provided by optical fibers in cylindrical geometries and by channel waveguides in quasi-rectangular waveguides. Although planar waveguides provide guiding in one dimension, the beam can focus, defocus, and diffract in the plane of the film. The propagation distances in fibers are usually limited by material attenuation, with kilometers being typical for silica-based fibers. Although material losses can also limit propagation distance for integrated-optics waveguides, fabrication techniques invariably limit propagation distances to at most 10 cm, and more typically a few centimeters. The guided-wave fields extend into all of the waveguiding media. For example, for a planar waveguide, the fields are maximum inside the high-index region (film) and decay exponentially from the boundary into the low-index media. Hence nonlinear interactions can occur in any of the media defining the waveguide. However, the high-index region carries most of the guided-wave power and hence, with the exception of a few cases that require strong nonlinearities in the bounding media, nonlinear interactions are optimized when the nonlinearity occurs inside the high-index medium.