Selective excitation of parabolic-index optical fibers by Gaussian beams

Excitation coefficients of the guided modes of a parabolic-index optical fiber by narrow input Gaussian beams are calculated. The effects of beam offset, tilt, width, and wave-front curvature are examined. A wave-optical procedure for optimizing the input Gaussian beamwidth (to excite as few mode groups as possible) as a function of beam offset is presented and shown to be in agreement with a simple Fourier-optics optimization.