Coupling from optical fibres to fast silicon modulators

In silicon based photonic circuits, optical modulation is usually performed via the plasma dispersion effect, which is a relatively slow process. Until recently, most researchers utilised Silicon on Insulator (SOI) waveguides with cross sectional dimensions of the order of 5 microns. This limits the speed of devices based on the plasma dispersion effect due to the finite transit time of charge carriers. Consequently moving to smaller dimensions will increase device speed, as well as providing other advantages of closer packing density, smaller bend radius, and cost effective fabrication. As a result, the trend in recent years has been a move to smaller waveguides, of the order of I micron in cross sectional dimensions. However, coupling light to such small waveguides is relatively inefficient. In the literature, the problem of coupling optical fibres to thin semiconductor waveguides has not been solved sufficiently well to obtain both high coupling efficiency and good fabrication tolerances, due to large difference between the fibre and the waveguide in both dimensions and refractive indices. In this paper we discuss both the design of small waveguide modulators (of the order of similar to1micron) together with a novel theoretical solution to the coupling problem. An example of coupling light to a thin silicon waveguide is given, as well as a discussion of a number of modulator design issues.