Cooperative upconversion as the gain-limiting factor in Er doped miniature Al2O3 optical waveguide amplifiers

Abstract

Erbium doped ${\mathrm{Al}}_2{\mathrm{O}}_3$ waveguide amplifiers were fabricated using two different doping methods, namely Er ion implantation into sputter deposited ${\mathrm{Al}}_2{\mathrm{O}}_3,$ and co-sputtering from an ${\mathrm{Er}}_2{\mathrm{O}}_3/{\mathrm{Al}}_2{\mathrm{O}}_3$ target. Although the Er concentration in both materials is almost identical (0.28 and 0.31 at. %), the amplifiers show a completely different behavior. Upon pumping with 1.48 μm, the co-sputtered waveguide shows a strong green luminescence from the ${}^4{S}_{\text{3/2}}$ level, indicating efficient cooperative upconversion in this material. This is confirmed by pump power dependent measurements of the optical transmission at 1.53 μm and the spontaneous emission at 1.53 and 0.98 μm. All measurements can be accurately modeled using a set of rate equations that include first order and second order cooperative upconversion. The first order cooperative upconversion coefficient $C_{24}$ is found to be ${\text{3.5×10}}^{\text{−16}}\hspace{0.17em}{\mathrm{cm}}^3 {\mathrm{s}}^{\text{−1}}$ in the co-sputtered material, two orders of magnitude higher than the value obtained in Er implanted ${\mathrm{Al}}_2{\mathrm{O}}_3$ of ${\text{4.1×10}}^{\text{−18}}\hspace{0.17em}{\mathrm{cm}}^3 {\mathrm{s}}^{\text{−1}}.$ It is concluded that the co-sputtering process results in a strongly inhomogeneous atomic scale spatial distribution of the Er ions. As a result, the co-sputtered waveguides do not show optical gain, while the implanted waveguides do.