Fabrication of tapered waveguide by ashed photoresist

It is necessary to interconnect various optical components with different input or output sizes in hybrid integrated optics. Integration of optical devices on an optical printed circuit board sometimes requires a tapered waveguide to match a backbone waveguide to a smaller optical device in order to minimize optical loss. In this work a backbone waveguide of 20×25μm2 cross-section is tapered to a 3×5μm2 cross-section. The tapered waveguide is fabricated by soft lithography, which imprints an optical polymer using a polydimethlysiloxane mold. A master of the tapered waveguide structure used to make the mold is prepared by plasma ashing of patterned photoresist. The ratio of the stripping rate (vertical etching) to the trimming rate (horizontal etching) can be changed as a result of the line-width-dependent stripping rate. Thus narrower portions of a line become thinner (vertically) than the wider portions of the line during a single ashing step. Fabricated tapered waveguide shows higher optical transmission than that of an abruptly matched waveguide.