Optimization of Al₂O₃:Er³⁺ waveguide technology for active integrated optical devices

Amorphous $Al_2O_3$ is a promising host material for active integrated optical applications such as tunable rare-earth-ion-doped laser and amplifier devices. The fabrication of slab and channel waveguides has been investigated and optimized by exploiting reactive co-sputtering and ICP reactive ion etching, respectively. The Al2O3 layers are grown reliably and reproducibly on thermally oxidized Si-wafers at deposition rates of 2-4 nm/min. Optical loss of as-deposited planar waveguides as low as 0.11±0.05 dB/cm at 1.5-μm wavelength has been demonstrated. The channel waveguide fabrication is based on $BCl_{3}/HBr$ chemistry in combination with standard photoresist and lithography processes. Upon process optimization channel waveguides with up to 600-nm etch depth, smooth side walls and optical losses as low as 0.21±0.05 dB/cm have been realized. Rare-earth-ion doping has been investigated by co-sputtering from a metallic Er target during $Al_2O_3$ layer growth. At the relevant dopant levels $({~}10^{20} cm^{-3})$ lifetimes of the $^{4}$$I_{13/2}$ level as high as 7 ms have been measured. Gain measurements have been carried out over 6.4-cm propagation length in a 700-nm-thick Er-doped $Al_2O_3$ waveguide. Net optical gain has been obtained over a 35-nm-wide wavelength range (1525-1560 nm) with a maximum of 4.9 dB.