The eikonal or local-normal-mode (LNM) approximation method is presented and applied to the investigation of the radiation loss of a TE guided mode that is due to a narrowing linear taper in slab geometry using a weak-coupling approximation. An exact expression for the total radiation loss is given within the limits of the theory, these limits being small refractive-index changes per wavelength. An approximation to the loss expression is evaluated numerically, indicating that the total radiative loss is roughly exponentially dependent on mode number and on taper step size, at least for weak tapers. The method developed here is not restricted to tapered structures but can be readily adapted to any type of longitudinal disturbance.
© 1981 Optical Society of America
N. Tzoar and R. Pascone, "Radiation loss in tapered waveguides," J. Opt. Soc. Am. 71, 1107-1114 (1981)