The power coupling efficiency of an elliptical-spot-size Gaussian beam into a multimode step-index fiber is derived using a full-wave analysis. Numerical results indicate that the power coupling efficiency increases not only with decreasing index mismatch but also with increasing spot size of the Gaussian beam at its waist and the relative core–cladding refractive-index difference of the fiber. The coupling loss due to beam spread is found to be large when the spot size at the beam waist is small. The coupling loss is also found to increase with increasing wavelength of the Gaussian beam. A slight difference of power coupling efficiency is found with the different direction of electric polarization for a Gaussian beam of large spot size at its waist.
© 1983 Optical Society of America
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