Abstract

We consider the modal characteristics of a linearly tapered multimode step index optical fiber using a discrete step approximation to the smooth taper. Our method accounts for the interaction between modes and is capable of tracking the propagation through the taper by using an accurate expansion of the field on each side of the step; we use basis functions that are known to approximate the field very accurately. Integration is avoided in evaluating the coupling efficiency across the step. The circular step index fiber is convenient to test our method inasmuch as Snyder [1] gives a simple formula that predicts the interaction between the HE,, and the HE,₂ modes in a multimode fiber. However, our method is accurate for a wide range of waveguides of arbitrary cross section and arbitrary refractive index profile. Snyder’s work is based on coupled mode theory and ignores all loss and coupling except between the two modes of interest.

© 1994 Optical Society of America