Investigation On Mid-infrared Spectroscopic Properties Of Rare Earth Ions Doped Germanate Glasses

Mid-infrared fiber laser operating at ~3 μm contains many important characteristic spectral lines. It has extensive applications in medical diagnosis, environmental detection, and optical communication, et al. This article mainly investigated the germanate glasses for ~3 μm applications. According to the compositions optimization of germanate glasses and the concentration adjustment of rare earth ions, germanate glasses were prepared for ~3 μm emissions.Firstly, germanate glasses with different Ga2O3 contents were prepared and the XRD analyses were performed. It is found that the prepared sample begins to crystallize when Ga2O3 content is up to 25 mol%. The DTA analyses indicate that the ΔT and kgl of prepared samples can reach to 140 °C and 0.176, respectively. The glassy structure, physical and optical properties were further investigated. Results reveal that the ultraviolet cut-off edge is red-shift with increasing Ga2O3 content. Raman spectra indicate that the maximum phonon energy of prepared glass moves to low wavenumber with the increment of Ga2O3 content. Judd-Ofelt intensity parameters and radiative properties were investigated based on absorption spectra. The infrared transmittance is as high as 84% according to the measured infrared spectra. The 2.7μm emission intensity becomes weaker firstly and then stronger with the increase of Ga2O3 content. The maximum emission cross section at 2.7μm is 4.68×10-21 cm2. Moreover, the energy transfer microparameters and energy transfer upconversion coefficients were calculated by Inokuti-Hirayama model and rate equations to elucidate the observed 2.7 μm fluorescent behavior.R2O3(R=Al/Y/Gd/La) and Nb2O5 modified germanate glasses were further prepared. We investigated the effect of R2O3 and Nb2O5 on physical and chemical properties of germanate glasses such as density and refractive index as well as the comparative investigations of thermal stability, crystallization activation energy and so on. It is found that the ΔT and kgl are as high as 175°C and 0.224, respectively, much higher than those of GGB2 sample mentioned above. Raman spectra were measured to analyze the glassy structure and the maximum phonon energy. Furthermore, the emission intensity and cross section at 2.7 μm were higher for the Y2O3 modified germanate glass. The energy transfer processes among Er3+ were discussed to explain the improved 2.7 μm emissions based on upconversion spectra and near-infrared lifetimes.Based on Y2O3 modified germanate glass, the effect of Er3+ concentration on 2.7 μm emission properties were investigated. Results indicate that no concentration quenching was observed although Er3+ content was as high as 6 mol%. Research indicates that excited state absorption(ESA2), cross relaxation(CR) and energy transfer upconversion(ETU2) processes became stronger with increasing Er3+ content, which is beneficial for population inversion of 2.7 μm emissions. To further overcome population bottleneck of 2.7 μm emissions, Er3+/Tm3+ codoped germanate glasses were prepared. It is found that Tm3+ can efficiently sensitize Er3+ and enhance 2.7 μm emissions. Energy transfer mechanism between Tm3+ and Er3+ was studied and energy transfer efficiency and microparameters from Er3+:4I13/2 to Tm3+:3F4 levels were calculated to be 41.6% and 2.94×10-39 cm6/s, respectively, larger than that of Er3+:4I11/2â†'Tm3+:3H5 process. In addition, Er3+-Yb3+ codoped germanate glasses were prepared. Mid-infrared, near-infrared and upconversion emission spectra were investigated. It is found that the codoped samples have much higher emission intensity at 2.7μm, 1.53μm and upconversion region compared with Er3+ doped sample. Their emissions became stronger with the increment of Yb3+ concentration and no observation of concentration quenching. The energy transfer mechanism and microscopic parameters between Er3+ and Yb3+ ions were analyzed. Enrgy transfer coefficient of Yb3+:2F5/2â†'Er3+: 4I11/2 transition is as high as 1.42×10-39 cm6/s.Finally, 2.9μm spectroscopic performances were investigated in Ho3+/Yb3+ codoped germanate glasses. Judd-Ofelt intensity parameters and radiative properties were calculated and discussed based on absorption spectra and Judd-Ofelt theory. It was found that the spontaneous radiative transition probability of Ho3+:5I6â†'5I7 transition was as high as 36.66s-1. The strongest 2.9 μm emission was obtained when Ho3+ concentration is 0.1 mol% and its emission cross section can reach 8.58×10-21 cm2. Moreover, the net gain in the wavelength of 2866-3000 nm is positive when P was 0.5. The energy transfer processes between Ho3+ and Yb3+ were also discussed. The energy transfer efficiency and microscopic coefficient from Yb3+: 2F5/2 to Ho3+: 5I6 levels were as high as 35.8% and 4.06×10-40 cm6/s, respectively. The energy transfer coefficients from Yb3+: 2F5/2 to Ho3+: 5I6 levels in all samples were obtained by Yokota-Tanimoto model and they became smaller with increasing Ho3+ concentration.