UV-written directional couplers in silica on silicon

Photosensitivity in Ge-doped silica is a well-known effect used for the fabrication of fiber Bragg gratings. Because the UV-induced index change of hydrogen-loaded Ge-doped silica films can be made larger than 0.001, the direct writing of channel waveguides is possible. Channel waveguides and couplers can be patterned in this way with a mask brought in contact with the film. However, defining the channel waveguides with a tightly focused UV-beam without a mask yields a more versatile fabrication technique for photonic components. In this paper, we present UV-written directional couplers in Ge-doped silica without mask patterning. We begin with a commercially available planar waveguide that consists of a 6-μm core layer with 7 mol% GeO₂-doped silica surrounded by a 40-μm top layer and a 20-μm underlayer on a silicon substrate. The films are hydrogen-loaded to an estimated hydrogen concentration of 3 mol% to increase the photosensitivity. A frequency-doubled argon-ion laser serves as a UV source. The channel waveguides are written at a speed of 2.5 mm/min with a UV power of 100 mW (cw) at 244 nm focused down to a spot diameter of 7 μm on a x-y translation stage. Typical mode sizes are 10 μm (hor.) by 6 μm (vert.). The vertical-mode size of the channel waveguide is mainly determined by the planar waveguide while the horizontal-mode size is defined by the UV-spot diameter and the UV-induced index increase