Erbium-ytterbium co-doped aluminium oxide waveguide amplifiers fabricated by reactive co-sputtering and wet chemical etching

We report on the fabrication and optical characterization of erbium-ytterbium co-doped aluminum oxide (Al2O3:Er3+:Yb3+) waveguides using low-cost, low-temperature deposition and etching steps. We deposited Al2O3:Er3+:Yb3+ films using reactive co-sputtering, with Er3+ and Yb3+ ion concentrations ranging from 1.4-1.6 x 10(20) and 0.9-2.1 x 10(20) ions/cm(3), respectively. We etched ridge waveguides in 85% pure phosphoric acid at 60 degrees C, allowing for structures with minimal polarization sensitivity and acceptable bend radius suitable for optical amplifiers and avoiding alternative etching chemistries which use hazardous gases. Scanning-electron-microscopy (SENT) and profilometry were used to assess the etch depth, sidewall roughness, and facet profile of the waveguides. The Al2O3:Er3+:Yb3+ films exhibit a background loss as low as 0.2 +/- 0.1 dB/cm and the waveguide loss after structuring is determined to be 0.5 +/- 0.3 dB/cm at 1640 nm. Internal net gain of 4.3 +/- 0.9 dB is demonstrated at 1533 nm for a 3.0 cm long waveguide when pumped at 970 nm. The material system is promising moving forward for compact Er-Yb co-doped waveguide amplifiers and lasers on a low-cost silicon wafer-scale platform. (C) 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement