Fluorescence quenching in Er3+ doped tellurite glass due to the introduction of BO3/2

Abstract

Er3+ doped tellurite glass is considered as a promising material for broadband applications because of its large stimulated emission cross section and broad emission bandwidth at the third communication window (1.55 μm) in order to increase the transmission capacity of the wavelength division multiplexing (WDM) system. The relatively low phonon energy of tellurite glass decreases the nonradiative transition probability of I13/2 → I15/2 emission and thus increases the quantum efficiency of Er3+. On the other hand, however, the nonradiative energy transfer rate from I11/2 level to I13/2 level of Er3+ in tellurite glass is lower than those in silicate and phosphate glasses with high phonon energies, which provides a long population duration on I11/2 level, which results in several undesirable effects [1] and hence leads to the decrease of the 980 nm pumping efficiency. 980 nm pumping is preferable to the success of an Er3+ doped amplifier since 1480 nm pumping cannot contribute to sufficient population inversion and a good signal-to-noise ratio. Several efforts have been made on methods of decreasing upconversion emission of Er3+ in tellurite glass. Choi et al. [2] studied the addition of energy acceptor ions such as Ce3+, Eu3+, and Tb3+ to deplete the I11/2 level by nonradiative energy transfer, because their electronic transition energies are approximately resonant with the energy gap corresponding to the I11/2 → I13/2 transition of Er3+. Hee Cho et al. [3] reported the introduction of BO3/2, which has the largest phonon energy (1400−1), into tellurite glass to increase the maximum phonon energy of the glass host to improve the I11/2 → I13/2 transition rate. However, a strong fluorescence quenching of I13/2 level was observed in our study when BO3/2 content is more than 10 mol% in the glass. In this letter, the effect of BO3/2 on lifetime of I13/2 level of Er3+ in tellurite glass was investigated. The quenching mechanisms of lifetime results from the BO3/2 addition are discussed based on nonradiative transition probability and interactions between rare earth ions. The glasses were prepared using conventional melting and quenching method from reagent-grade powders of H3BO3, ZnO, La2O3, Er2O3, and high purity powders of TeO2 (99.999%). About 20 g batches of starting materials were fully mixed and then melted in a platinum crucible at about 800–900 ◦C. The melt was poured into a brass mold and cooled in air and subsequently annealed to room temperature gradually. The obtained glasses were cut and polished carefully in order to meet the requirements for optical measurements. The fluorescence lifetime for the I13/2 level of Er3+ was measured with a 970 nm LD and a HP546800B 100 MHz oscilloscope. In order to minimize the influence of reabsorption on emission, the excitation beam was focused immediately below the surface of the samples so that the emitted light traveled only a short distance inside the sample. The radiative lifetime of I13/2 level of Er3+ is calculated according to the well-known Judd-Ofelt theory described anywhere. The quantum efficiency for the I13/2 → I15/2 transition was evaluated using the equation: η = τmea/τrad, where τmea is the measured lifetime and τrad is the calculated radiative lifetime. Fig. 1 illustrates the compositional dependence of the lifetimes and the quantum efficiency of Er3+ on BO3/2 concentration in tellurite glasses. The radiative lifetime τrad increases monotonically owing to the decrease of the refractive index of the glass host. The measured lifetime τmea and the quantum efficiency η initially decrease slightly up to BO3/2 = 8 mol% and then decrease rapidly as BO3/2 content increases. The lifetime of I13/2 level is also an important factor to the success of Er3+-doped amplifiers, since a long lifetime permits to obtain the required high-population inversion under steady-state condition using modest pump power [4]. The measured lifetime is mainly influenced by the maximum phonon energy of the glass host. If we neglect the energy transfer process between excited ions, the measured lifetime τmea is written as: