Comprehensive time-resolved photoluminescence study of an Er/Yb co-doped phosphosilicate glass fiber

The spectroscopic behavior of a commercial Er/Yb co-doped phosphosilicate fiber is studied in detail herein. Direct and indirect excitations are carried out using an array of excitation wavelengths and excitation pulse durations. The fluorescence dynamics after the pump pulse are found to be very sensitive to excitation conditions. At low Er³⁺ inversions, there is significant energy transfer from Yb³⁺:²F_5/2 to Er³⁺:⁴I_11/2, contributing to a fast fluorescence decay in Yb³⁺:²F_5/2 → ²F_7/2, along with a slow fluorescence decay attributed to Yb³⁺ ions weakly coupled to Er³⁺ ions. As Er³⁺ inversion increases after longer pump pulses, the amplitude of the fast decay decreases due to a shortage of Er³⁺ ions at the ground state, while the lifetime of the slow decay eventually increases to the spontaneous lifetime of Yb³⁺ ions expected in a phosphosilicate host. A corresponding rise in Er³⁺:⁴I_13/2 → ⁴I_15/2 fluorescence was clearly observed due to the energy transfer from Yb³⁺ at low Er³⁺ inversions, lasting for as long as there are excited Yb³⁺ ions, but the rise disappears when Er³⁺ is nearly fully inverted. The lifetime of the decay of the Er³⁺ fluorescence was observed to decrease at high Er³⁺ inversions due to energy transfer upconversion (ETU) among Er³⁺ ions. In addition, we have identified that the Er³⁺ ions at ⁴S_3/2 and ⁴F_9/2 that are responsible for the green and red fluorescence, respectively, are likely populated via Yb³⁺:²F_5/2 + Er³⁺:⁴I_11/2 → Yb³⁺:²F_7/2 + Er³⁺:⁴F_7/2 (C₂₉ in Fig. 1). The energy transfer from Er³⁺:⁴I_11/2 to Yb³⁺:²F_5/2 was also observed when pumping Er³⁺ ions at 665 nm. This study and its methodology should serve as a valuable reference when studying Er/Yb systems in the future as well as understanding previous studies.