Rare-earth-ion-doped Al₂O₃ waveguides for active integrated optical devices

Reactively co-sputtered amorphous $Al_2O_3$ waveguide layers with low propagation losses have been deposited. In order to define channel waveguides in such $Al_2O_3$ films, the etching behaviour of $Al_2O_3$ has been investigated using an inductively coupled reactive ion etch system. The etch rate of $Al_2O_3$ and possible mask materials was studied by applying various common process gases and combinations of these gases, including $CF_4/O_2, BCl_3, BCl_3/HBr$ and $Cl_2$. Based on a comparison of the etch rates and patterning feasibility of the different mask materials, a $BCl_3/HBr$ plasma and and standard resist mask were used to fabricate channel waveguide structures. The etched structures exhibit straight sidewalls with minimal roughness and etch depths of up to 530 nm, sufficient for defining waveguides with strong optical confinement and low bending losses. Low additional propagation losses were measured in single-mode $Al_2O_3$ ridge waveguides defined using the developed etch process. In initial investigations, $Al_2O_3:Er$ layers fabricated using the same deposition method applied for the undoped layers show typical emission cross-sections, low green upconversion luminescence and lifetimes up to 7 ms.