Erbium-doped spiral amplifiers with 20 dB gain on a silicon chip

We report the fabrication and optical characterization of long, spiral-shaped erbium-doped aluminum oxide (Al2O3:Er3+) channel waveguides for achieving high overall signal amplification on a small footprint. Al2O3:Er3+ films with Er3+ concentrations in the range between 0.443.1×10^20 cm^-3 were deposited by reactive co-sputtering onto standard, thermally oxidized silicon substrates. Spiral-shaped waveguides were designed and structured into the films by chlorine-based reactive ion etching. In the current design, each spiral waveguide occupies an area of 1 cm^2. Typical background propagation losses near 1500 nm are (0.2±0.1) dB/cm. A commercially available, pigtailed diode laser at 976 nm was employed as the pump source. The erbium-doped waveguide amplifiers were characterized in the small-signal-gain regime at the peak-gain wavelength (λ = 1532 nm) of Al2O3:Er3+. A maximum of 20 dB of internal net gain was measured for a 24.5-cm-long spiral waveguide with an Er3+ concentration of 0.95×10^20 cm^-3. Similar results were obtained for a shorter spiral with an Er3+ concentration about twice as high. Samples with lower concentration exhibited lower gain because of insufficient pump absorption, while samples with higher concentration showed less gain because of migration-accelerated energy transfer up-conversion and, more importantly, a fast quenching process.