Excitation of Parabolic-Index Fibers With Incoherent Sources

We study the excitation of a parabolic-index fiber by an incoherent source. The theory is based on approximating the fiber modes by Laguerregaussian functions. The dependence of the total light power injected into the fiber core on the separation between source and fiber, and on the transverse displacement of the source, is shown in graphic form. Also shown are far-field radiation patterns, which indicate the distribution of power versus mode number, for several launching conditions and plots of power versus azimuthal mode number for given values of the compound mode number. The study of launching efficiency versus source radius leads to prescriptions for optimizing the ratio of source to fiber core radius without a matching lens. Use of a lens for matching the image of a small source to the size of the fiber core increases the launching efficiency relative to the power consumption of the light-emitting-source diode.