Radiative trapping effect of Yb3+ ions in lead-germanate glasses

Abstract

Yb is a good candidate as a pump source because there are only two manifolds in the Yb energy level scheme, the F7/2 ground state and the F5/2 excited state, and thus concentration quenching and excited-state absorption should not affect the lasing or the excitation wavelength [1]. Extensive investigations have been of Yb-doped glasses, fibers, and crystals [2–5]. The relatively long fluorescence lifetime, as well as the broad absorption and emission bands, is more prominent for Yb-doped glasses than for crystals, making the glasses attractive for diode-pumped generation of ultra-short pulses, high-power ultra-short pulse amplification and tunable laser sources [6]. In addition, the ability to achieve high concentrations of Yb ions and low loss also triggered the rapid development of various Yb-doped glasses such as phosphate, borate, and silicate glasses. However, up to now, there are few systematic investigations of the optical properties of Yb-doped germanate glasses. In this work, the spectroscopic properties of germanate glasses with different concentrations of Yb ions and different thicknesses were investigated. Glasses with compositions (mol%) 10Na2O–60GeO2– 30PbO–xYb2O3 (x = 0.2, 0.6, 1.0, 1.4, and 2.0) were prepared using regent-grade powders Na2CO3, Pb3O4, and high purity GeO2 (>99.999%). Yb 3+ was introduced as Yb2O3 with 99.9% purity. Batches of 20 g were well mixed in appropriate proportions and melted at 1050 C in a covered alumina crucible in a SiC-resistance electric furnace. The liquids were held for 15 min and the homogenous bubble-free melts were cast on a stainless plate preheated at about 150 C. The samples were well annealed in a muffle furnace to reduce thermal stress. The annealed samples were cut using a low-speed diamond saw and polished with 5 lm diamond paste. Plates of dimensions 20 mm · 10 mm · 2 mm with two polished faces were used for optical measurements. The refractive index of the studied glass was measured using the Brewster angle method. The density of the sample was measured using the Archimedes’ liquidimmersion method on an analytical balance with a precision of 0.0001 g. Absorption spectra of the glass sample were recorded with a Perkin-Elmer-Lambda 900UV/VIS/NIR spectrophotometer in the range 850–1100 nm. Fluorescence measurements were performed with a 940 nm diode laser excitation and detected by a TRIAX550 spectrophotometer controlled by a computer. The infrared fluorescence lifetime was determined by modulating the pump laser diode drive current with a square-pulse duration of 40 ms at a repetition of 10 Hz. All the measurements were performed at room temperature. The important spectroscopic parameters required include the effective emission and absorption cross sections of the F5/2 fi F7/2 transition and the upper laser level lifetimes of Yb. The emission cross section can be determined by either the method of reciprocity or the Fuchtbauer–Ladenburg formula. On the basis of the reciprocity method described by McCumber [7], the emission cross section, remi can be calculated from the measured absorption cross section, rabs, i.e.