A functional optical device has cores which are trenches, different portions of the cores being formed from different core materials. The optical device can be formed by forming trenches 5,7,9 within a substrate (normally a substrate 1 covered by a cladding layer 3), covering at least part of at least one trench 7 with a cover 11, depositing a first cladding material to fill the trenches 5,9 which are not covered, removing the cover 11, depositing a second cladding layer 15 of a second cladding material to fill the trenches 7 which were previously covered, removing core material outside the trenches 5,7,9 and applying a cladding layer to cover the trenches.