Transverse coupling in fiber optics part III: Bending losses

A general method is proposed to evaluate the radiation loss of bent open waveguides. This method consists in evaluating the coupling between the waveguide mode and the whispering-gallery modes that can be associated with the surrounding medium. The expression obtained for a reactive surface coincides with a previous result by Miller and Talanov.
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We investigate in detail the radiation loss of the fundamental (HE
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) mode of a dielectric rod coupled to a slab. This arrangement, described in Part II of this article series,
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provides a useful mode-selection technique. The radiation loss is given by a simple closed-form expression. We find that for a wavelength of 1 µm and a rod radius of 5 µm in physical contact with the slab, the bending loss is less than 1 dB/km if the radius of curvature, in the plane of the slab, exceeds 22 mm.