In the manufacture of optical devices such as, e.g., optical communications assemblies and optical gyroscopes, low-loss substrate-supported optical waveguides are desired. Such waveguides can be obtained by patterning a layer of deposited waveguide material and, in the interest of minimizing loss as may be due to processing-induced surface roughness, waveguides as patterned preferably undergo a heat treatment which results in surface smoothing and in rounding of the waveguide cross section. Furthermore, in the interest of avoiding re-entrant corners between substrate and waveguide, a wetting layer may be applied to the waveguide prior to heat treatment.