Properties of rare earth doped thin film dielectric layers for upconversion laser waveguides

Thin films of erbium doped tantalum pentoxide were prepared on unheated <100> orientated silicon substrates with a thermally grown 2 μm thick SiO2 layer using reactive sputtering in an oxygen rich environment for upconversion laser experiments. Tantalum and Erbium metal targets were co-sputtered using an ion beam assisted reactive process to produce the high quality amorphous thin film layers. The Erbium dopant concentration was adjusted by varying the relative deposition rate. Slab waveguide loss before high temperature annealing was measured to be 0.8 dB/cm at 633 nm which reduced to 0.5 dB/cm after annealing at 500 °C. Micron scale waveguides were etched into the deposited thin film slab waveguides using photolithography and reactive ion etching. Finally a silicon dioxide buffer layer was deposited on top of the ridge waveguides to help constrict the laser mode and to add a protective layer. 2 cm long waveguide samples were cleaved and polished to achieve good optical quality end facets for laser experiments. Upconversion efficiency and laser gain measurements were performed on the final waveguides. Waveguide losses of 6.35 dB/cm were measured for the final ridge waveguides and a coupling efficiency of 48 % was obtained. A positive net gain of 0.25 dB was measured for upconversion to 551 nm using a pump probe optical setup.