High precision alignment of optical waveguide features

A method of aligning optical devices of a first optical circuit and a second overlaid optical circuit of a planar optical circuit on a substrate, the method comprising the steps of:

• creating a first circuit layer on the substrate, the first circuit layer defining a core layer;
• creating a mask on the core layer, the mask being formed of a refractory material, the mask having a first portion defining a first pattern and a second portion defining a second pattern, the first pattern corresponding to a desired pattern to be formed in the core layer and the second pattern corresponding to one or more alignment marks;
• etching the core layer through the mask to create exposed portions not overlaid by the first portion and the second portion of the mask;
• removing the first portion of the mask to expose the desired pattern in the core layer while leaving the second portion of the mask in place;
• partially oxidising the second portion of the mask;
• forming an overclad layer on the exposed portions of the core layer corresponding to the desired pattern in the core layer, while leaving the oxidized second portion of the mask in place;
• creating a second circuit layer overlaid on the first circuit layer, the second circuit layer having at least one optical feature to be disposed at a predetermined position, the predetermined position of the at least one optical feature being controlled by alignment of said at least one optical feature to the one or more alignment marks in the first circuit layer.