Compact mode-size converters for efficient coupling between fibers and integrated optical waveguides

We present a theoretical and numerical investigation of compact fiber-waveguide couplers that are based on lensing and high index-contrast and are only a few microns long. These structures are characterized by very broadband operation which makes them suitable for WDM systems. We assume that the input mode is from a lensed single-mode fiber and can be approximated by a Gaussian. Thus a preliminary design can be obtained analytically using Gaussian beam optics and ray matrices. Finite difference time domain (FDTD) simulations are performed for evaluation of the performance and optimization of the design.