Excitation and Scattering of Modes on a Dielectric or Optical Fiber

A technique similar to that of Kirchhoff is used to obtain an analytic expression for mode Iaunching on a semi-infinite rod. The approximation is quasi-optical and only valid for small angles of incidence θ. When θ = 0 only HE_1M modes are excited; however, for θ<<1 many modes can be launched. The effect on HE₁₁ mode propagation of small imperfections in a dielectric waveguide is analyzed. At the frequency of interest for optical communication (cutoff for the TM₀₁ mode) the radiated power is 160 times larger than that scattered into the HE₁₁ mode.