A novel method for the fabrication of optical slab waveguides is prese
nted. After growth of the base cladding by thermal oxidation and depos
ition of the core layer of the waveguide by PECVD of SiO(x), the top c
ladding layer is formed from the core layer material by an additional
thermal oxidation which transforms SiO(x) into SiO2. With this simple
technology, contaminations and rough interfaces between core and cladd
ing are avoided. Low propagation losses and good matching to optical f
ibres are achieved.