The light beam in a single-mode dielectric taper establishes a field that is evanescent outside the taper core when the tapering angle is small. The dominant reason for the radiation loss from tapers is the mismatch between the taper field and the field of the straight fiber at the ends of the taper. The physical implication behind this is that the energy radiated from the taper interior is concentrated in the close vicinity of the taper core for the whole length of the taper, no matter how long the taper is, whereas the radiation loss is found to be of second order in the tapering angle.
© 1989 Optical Society of America
Xue-heng Zheng, "Understanding radiation from dielectric tapers," J. Opt. Soc. Am. A 6, 190-201 (1989)