Attenuation of Unwanted Cladding Modes

Losses in dielectric optical waveguides have been discussed by several authors. 1 ' 2 In two earlier papers 3,4 we explored the problem of crosstalk between two parallel optical waveguides. T w o types of crosstalk were considered. The directional coupler mechanism couples two dielectric waveguides even if they have no imperfections of any kind. The coupling, in this case, is caused by the exponentially decaying field tail of one guid'e reaching to the region of the second guide. T h e second mechanism causing crosstalk involves light scattering from waveguide imperfections. T h e light that is scattered out of the guided mode of one guide can be scattered j^ack into the guided mode of a neighboring waveguide. Both types of crosstalk can, in principle, be reduced by placing a lossy material between the two guides. However, it was concluded that isolation of the waveguides by means of a lossy surrounding medium is really not necessary to avoid crosstalk. T h e waveguides must be designed in such a way that their mode losses are low. T h e same mechanism that influences crosstalk also influences the mode loss. For example, if two guides couple appreciably by means of the directional coupler mechanism their mode fields penetrate each other to a con2565