WKB analysis of planar surface waveguides with truncated index profiles

We present a WKB analysis of planar surface waveguides with truncated arbitrary refractive index profiles. It is shown that the phase change at the turning point on the substrate side approaches zero near the cutoff. The validity of the conventional assumption of constant phase change of
\pi/2
at the turning point is investigated, and the consequent errors near the mode cutoffs are analyzed for index profiles of practical interest.