Scalable erbium-doped waveguide amplifier with external fiber-to-fiber gain using reactively sputtered polycrystalline aluminium oxide.

We demonstrate reactively sputtered polycrystalline Al₂O₃:Er³⁺ waveguide amplifiers exhibiting external fiber-to-fiber net gain, broadband amplification, and low noise figure. With an erbium concentration of 1.5 × 10²⁰ ions/cm³, a 30 cm amplifier length, and bi-directional pumping at 1480 nm, > 14 dB of external gain at 1550 nm is shown with off-chip output powers of over 56 mW measured at the output fiber, as well as sustained gain across 60 nm of bandwidth. The fiber-to-fiber noise figure (NF) is 6.5 ± 0.1 dB at 1550 nm, with a minimum value of 5.6 ± 0.1 dB at 1566 nm. Such a device is made possible using polycrystalline Al₂O₃:Er³⁺ capable of high-temperature LPCVD SiO₂ cladding, and therefore low coupling and background waveguide losses down to 2.5 dB/facet and below 0.1 dB/cm, respectively, due to high conformality and optical quality. This demonstration highlights the importance and advantages of using polycrystalline Al₂O₃:Er³⁺ for applications that require optical amplification on a scalable and versatile photonic waveguide platform.