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.

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The paper describes a computational model to estimate the reduction of nonlinear distortions due to a fiber-fiber joint by properly focusing the optical beam from a single transverse-mode semiconductor laser source to the input end of the first fiber. Mode mixing effects are included in the model. Material dispersion is found to make a negligible contribution in comparison with a typical source coherence time. In a typical situation, where the fiber joint coupling efficiency eta is better than 0.7, all harmonic distortion components produced in the joint could be reduced by ~10 dB by halving the input beam spot radius from the value equal to the fiber core radius.

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