Short erbium doped phosphate fiber amplifiers

Spectroscopic properties of high concentration Er³⁺-doped phosphate glasses and performance of short Er³⁺-doped phosphate fiber amplifiers were studied and characterized. Systematic studies of cooperative upconversion of Er³⁺ ions in ⁴I₁₃/₂ level and energy transfer from Yb³⁺ to Er³⁺ in phosphate glasses were performed by a rate equation formalism. The cooperative upconversion coefficient for an Er³⁺ concentration of 4 x 10²⁰ ions/cm³ was found to be 1.1 x 10⁻¹⁸ cm³/s. An energy transfer coefficient of 1.1 x 10⁻¹⁶ cm³/s was found for an Yb³⁺ concentration of 6 x 10²⁰ ions/cm3 and an Er³⁺ concentration of 2 x 10²⁰ ions/cm³. Energy transfer efficiencies from ²F₅/₂ level of Yb³⁺ ions to ⁴I₁₃/₂ level of Er³⁺ ions higher than 95% were determined from our measurements under weak excitation. The performance of high concentration Er³⁺-doped phosphate fiber amplifiers were characterized in terms of gain, noise figure, and signal saturation for a series of active fiber lengths, pump powers, signal input powers, and signal wavelengths. A net gain of 21 dB were achieved in a 71 mm Er³⁺-doped phosphate fiber with a noise figure of ∼5.3 dB by a 980 nm pump power of 244 mW. In addition, a 10 dB net gain can be obtained with a pump power of 110 mW. Performance of short Er³⁺-doped phosphate fiber amplifiers demonstrates the potential for device applications