Multilayer integrated optical device and a method of fabrication thereof

A method of fabricating a three-dimensional integrated optical device comprising at least two vertically aligned waveguide structures, each including at least one waveguiding element, the method comprising the steps of:

• (a) forming a basic structure containing a semiconductor substrate carrying a buffer layer coated with a core material layer of a higher refractive index as compared to that of the buffer layer;
• (b) defining said at least one waveguiding element of the lower waveguide structure in a guiding layer on top of said basic structure, wherein said guiding layer is made of a material with a refractive index higher than the refractive index of said buffer layer and the core layer, is chosen so as to minimize a height of said at least one waveguide element and to provide effective guiding of light in the core layer;
• and
• (c) forming an upper cladding layer on top of a structure obtained in step (b), wherein a height difference between a height of the cladding layer region above said at least one waveguiding element and a height of the cladding layer region outside the waveguiding element is substantially small, thereby providing a sufficient flatness of said cladding layer to allow direct formation of the upper waveguide structure thereon;
• (d) forming said upper waveguide structure on top of said upper cladding layer by depositing a further buffer layer and repeating steps (b) and (c) with respect to a further guiding layer, significant perturbation in light propagation within said upper waveguide structure being thereby prevented.