Relationship between gain and Yb3+ concentration in Er3+-Yb3+ doped waveguide amplifiers

We have used a rate equation propagation model of an Er³⁺/Yb³⁺ doped Al₂O₃ waveguide amplifier with copropagating pump at 980 nm to investigate the dependence of gain on Yb³⁺ concentration. The model includes excited state absorption and energy transfer upconversion processes within the Er³⁺ as well as the relevant energy transfer processes between Yb³⁺ and Er³⁺. The results of the calculations indicate a close relationship of the parameters gain, launched pump power, waveguide length, and Yb³⁺ concentration. Codoping with a well-chosen Yb³⁺ concentration is shown to increase the gain around 1530 nm for all combinations of these parameters. The gain is improved most by Yb³⁺ codoping at pump powers around the amplifier threshold. At high pump powers the increase in gain of an Er³⁺/Yb³⁺ doped waveguide is insignificant compared to that of its Er³⁺ doped counterpart. Furthermore for each launched pump power, a nonzero Yb3 + concentration