Abstract
By introducing quantitative measures for the three effects of diffraction, focusing, and defocusing, physical-engineering insight is brought into the complex problem of Gaussian-beam propagation in graded-index media. Beam propagation in single-mode optical waveguides can be studied approximately by replacing them by equivalent focusing quadratic-index media. An “antiguiding” fiber is introduced for which the core index is smaller than the cladding index. Short sections of conventional and antiguiding fibers function similarly to positive and negative graded-index rod lenses. These fiber lenses are used to obtain low-loss connections between single-mode fibers with different spot sizes.