Several papers were reported on spectroscopic properties of rare earth doped different host glasses. A complete knowledge of fluorescence properties of rare earth ions in laser materials is necessary to achieve efficient, compact and cheap sources of laser radiation for NIR and mid-IR region. Tellurite glasses are potentially useful for generation of NIR and mid-IR laser radiation due to its special features such as lowest phonon energy (750 cm<sup>-1</sup>) among oxide glasses, reasonably wide transmission region (0.35 - 5μm), good glass stability, good rare earth ion solubility, high linear and non-linear refractive index. In the present work, authors prepared Ho<sup>3+</sup> and Tm<sup>3+</sup> singly doped and Ho<sup>3+</sup>/Tm<sup>3+</sup> co-doped tellurite glasses using conventional melt-quenching method. Spectroscopic measurements and analysis of energy transfer process in Ho<sup>3+</sup>, Tm<sup>3+</sup> and Ho<sup>3+</sup> /Tm<sup>3+</sup> co-doped glasses pumped with 785nm and 451 nm excitation wavelengths have been performed. There are some spectroscopic properties which are important in understanding and modeling of rare earth doped laser materials. Using Judd-Ofelt theory, radiative transition rates (A<sub>rad</sub>), radiative lifetimes (τ<sub>R</sub>) and branching ratios (β) were estimated for certain excited states of Ho<sup>3+</sup> and Tm<sup>3+</sup> doped tellurite glasses. The emission cross-sections and gain coefficients have been determined from the absorption spectra of Ho<sup>3+</sup> and Tm<sup>3+</sup> ions in tellurite glasses. The energy transfer process such as ion cross-relaxation, Tm<sup>3+</sup>-Ho<sup>3+</sup> energy transfer and energy transfer upconversion were studied and identified to specific candidate for laser operation.
In the present paper, optical absorption and emission spectra and luminescence decay lifetimes of different concentrations, 0.1, 0.3, 0.5, 0.7 and 1.0 mol% of Er<sup>3+</sup> and 0.1Er<sup>3+</sup>/0.5Yb<sup>3+</sup> co-doped tellurite glasses (TeO<sub>2</sub>-Bi<sub>2</sub>O<sub>3</sub>-ZnONb<sub>2</sub>O<sub>5</sub>) were reported. Judd-Ofelt intensity parameters were determined and used to calculate spontaneous radiative transition probabilities (A<sub>rad</sub>), radiative lifetimes (τ<sub>R</sub>), branching ratios (β) and stimulated emission cross-sections (σ<sub>P</sub>) for certain emission transitions. NIR emission at 1.5μm and up-conversion spectra of Er<sup>3+</sup> and Er<sup>3+</sup>/Yb<sup>3+</sup> co-doped tellurite glasses were measured under excitation wavelength of 980 nm. The absorption, emission and gain cross-sections for <sup>4</sup>I<sub>13/2</sub>→<sup>4</sup>I<sub>15/2</sub> transition of Er<sup>3+</sup> are determined. The peak emission cross-section of this transition is found to be higher (9.95×10<sup>-21</sup> cm<sup>2</sup>) for 0.1 mol% of Er<sup>3+</sup> and lower (6.81×10<sup>-21</sup> cm<sup>2</sup>) for 1.0 mol% of Er<sup>3+</sup> doped tellurite glasses, which is comparable to other oxide glasses. The larger peak emission cross-section for lower concentration of Er<sup>3+</sup> is due to the high refractive index of glass matrix (2.1547), relation established from Judd-Ofelt theory. The observed full-widths at half maxima (FWHM) for lower and higher concentrations of Er<sup>3+</sup> are 64nm and 96 nm respectively. The larger values of FWHM and peak emission cross-sections are potentially useful for optical amplification processes in the design of Erbium doped fiber amplifiers (EDFs). Under 980 nm excitation three strong up-conversion bands were observed at 530nm, 546nm and 665nm. The pump power dependent intensities and mechanisms involved in the up-conversion process have been studied. The luminescence decay profiles for <sup>4</sup>I<sub>13/2</sub> level were reported for all glass matrices.