Laser patterning of optical waveguide structures via selective leaching of phase-separable glass

Abstract

Patterning of refractive-index structures in glass by direct laser irradiation or masked ion exchange has been of considerable interest over the years.1–3 We describe a technique of forming step-index waveguide structures in glass based on a laser-induced selective leaching process on phase-separable glass. Code 7720, a soda lead borosilicate glass, is first phase separated by an appropriate heat treatment. It is then exposed to a scanning focused beam of a CO2 laser to produce a rapid thermal annealing effect which erases the phase separation. The irradiated area becomes impervious to subsequent acid leaching normally used to remove the borate-rich phase of phase-separated Code 7720 glass. This process produces an index-guiding clear-glass region surrounded by a low-index porous glass region. Multimode waveguide structures with a guide width in the 100–200-μm range and N.A. ~0.15 are readily produced using this CO2 laser-induced selective leaching technique. This technique has been used to produce 3-dB cross-splitters. Since the guiding region is directly controlled by the thermal profile of the writing beam, a narrower guide width is feasible when a metal mask is used to confine further the area to be irradiated.