Graded Index Waveguides

Abstract

There are two ways to significantly reduce modal dispersion in a waveguide: use only single mode waveguides, or use a graded index waveguide. The first choice appears to be the simplest, but it is not always a practical solution. Single mode waveguides are much more difficult to couple light into than multimode waveguides. To help see this, consider the two planar structures shown in Fig.7.1. Both waveguides have the same indexes, but one of them has a larger guiding layer. The number of guided TE modes can be approximated from Eq.2.43

$$m \approx (hk\sqrt {n_f^2 - n_s^2} )/\pi $$

(7.1)

Given identical indices of refraction, the only way to make a waveguide operate in a single mode is to reduce the thickness, h, of the guiding film. The smaller dimension of the single-mode guiding layer makes alignment between sources and other guides much more critical than with a large (multimode) structure. Connecting and aligning between multimode waveguides is easier due to the large size.