Undercladding and uppercladding layers that form part of an optical waveguide are deposited with a thermal CVD technique. The optical waveguide has a structure in which optical cores are formed over the undercladding layer and in which the uppercladding layer is formed over and between the optical cores. Generally, the optical cores have a refractive index greater than a refractive index of the cladding layers. A borophosphosilicate-glass layer may be used as one or both of the cladding layers. Thick cladding layers may be formed by cyclically depositing and annealing portions of the layer.