Process for tapering waveguides

Claim 1

A process for making a waveguide whose cross-section changes along its length to enhance coupling, said process comprising the steps of: initially forming an elongated channel waveguide having exposed ends structured for coupling, said elongated channel waveguide being formed in a substrate and provided with a predetermined lengthwise cross-sectional geometry having a predetermined width by selectively doping the substrate with ions of a species that raises the index of refraction of the substrate, the concentration of said index raising species of ions and the width of said cross-section both being constant over the length of the waveguide; and selectively applying heat along said elongated channel waveguide to change its cross-sectional geometry and dopant per unit volume concentration of said index raising ion species as a function of length along a selected length of said waveguide by further diffusing said index raising ion species widthwise and depthwise in said substrate while retaining the initial cross-section and concentration along the remaining length of the waveguide the lengthwise distribution of said applied heat being such that the product of the concentration of said index raising ion species taken as dopant per unit volume of substrate and the width of said waveguide at any lengthwise section of the waveguide remains constant along the length of the waveguide so that the modal content of said waveguide remains unchanged throughout the length of said waveguide while its cross-sectional geometry changes from one end thereof to the other so that said waveguide ends can be coupled with external devices of matching geometry and modal content; the selectively applying step providing a taper in said channel waveguide.