<title>Microlenses For Coupling Single-Mode Fibers To Single-Mode Thin-Film Waveguides</title>

Abstract

Philip D. BearMcDonnell Douglas Astronautics Company -St. Louis, St. Louis, Missouri 63166AbstractA new method has been devised for producing microlenses on the ends of single -modeoptical fibers. A lens is formed by dipping the fiber end into negative photoresistwhile the fiber core carries Z0.1 mW of HeNe laser light. The photoresist lenses re-quire no developing or rinsing. The lenses are shown to transform the near -Gaussianbeam emitted by the fiber into another near -Gaussian beam with a reduced waist diameter.The size of the new waist can be selected by varying the number of times the fiber isdipped into the photoresist. The waist reduction is shown to increase coupling intosingle -mode optical waveguides.IntroductionWhen coupling light between single mode fibers and single mode planar waveguides, oneshould be cognizant of the variables upon which the process depends. The degree to whichthe phase and amplitude distributions are matched determines the efficiency of coupling.First, let us address the matching of the phase distributions of the light. A single modefiber emits a beam which begins diverging upon exiting the fiber so that the waist of thebeam, which exhibits a planar phase, is located at the fiber end. The phase of the lightin a single mode planar waveguide is also planar. Therefore, to achieve maximum couplingwith respect to phase matching, the fiber end should be butt -coupled to the edge of theplanar waveguide.In order to match the amplitude distributions there are two conditions to be considered.The first is that the coupling is affected by the relative displacement of the two modes.When the regions of maximum mode intensity for each mode lie on a common optic axis thecoupling is maximized with respect to this parameter. The second condition concerns therelative sizes of the two modes. The closer the mode in the fiber is to matching the modesize in the waveguide the better the coupling efficiency which is achievable.Fabrication of microlensesIt may be difficult, in general, to vary parameters such as the indices of refractionor wavelength in order to match the mode sizes. For example, the single -mode fiber used inthis laboratory has a waist diameter of 2 Wo