Reducing Optical Losses and Energy-Transfer Upconversion in ${\rm Er}_{x}{\rm Y}_{{2}\hbox {-}{\rm x}}{\rm SiO}_{5}$ Waveguides

Optical losses and energy-transfer upconversion (ETU) in sputter-deposited polycrystalline Er_xY_2-xSiO₅ thin film waveguides are investigated. We find that the high temperature anneal that is necessary for crystallization and optical activation of Er induces grain growth that causes large propagation losses. Through minimizing the grain growth, propagation loss was lowered to 2.2±0.2 dB/cm while maintaining a high confinement factor of 50%. Changing the target material to minimize sputtering of Er clusters reduced the ETU coefficient to an estimated value of 3.1±0.2×10^-17 cm³/s at an Er concentration of 7.6×10²⁰ cm^-3. The maximum level of population inversion was 0.47±0.02, resulting in signal enhancement of 13.3 dB/cm at 1529 nm.