Excitation of the Dominant Mode of a Round Fiber by a Gaussian Beam

Communication by means of optical fibers requires that light energy can be coupled into the fiber in an efficient way. Of the different methods of exciting an optical fiber, the simplest consists of shining a beam of laser light on the end of the fiber. It is the purpose of this paper to investigate the power loss that results at the transition from a laser beam propagating in free space to the lowest order H E n mode of a round optical fiber. 1695 1700 TI-IE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1970 The geometry of the problem is sketched in Fig. 1. It is assumed that the fiber core is embedded in an infinite material, its cladding. For simplicity it is assumed that the value of the refractive index outside of the core is unity. The theory is manageable only if reflections from the end of the fiber are neglected. The transmission coefficients are calculated by matching only the transverse component of the electric or of the magnetic field at z -- 0. Finally, an average of these two values is taken. The incident beam is assumed to have a field distribution of the form Ex = A exp exp (--ikz) for z ~ 0 (1)