Enhanced 1.8 μm emission in Yb3+/Tm3+ codoped tungsten tellurite glasses for a diode-pump 2.0 μm laser

Enhanced 1.8 μm emission from Tm3+ by Yb3+ sensitization and bubbling in tungsten tellurite glasses has been demonstrated under the excitation of a 976 nm laser diode. Radiative properties such as spontaneous emission probabilities, radiative lifetimes and fluorescence branching ratios along with absorption and emission cross sections have been calculated from the absorption spectra. The influences of Yb3+ concentration, Tm3+ concentration, and bubbling time on spectroscopic properties have been thoroughly investigated. It is found that with increasing the bubbling time to 35 min, OH− content deceases sharply while 1.8 μm emission intensity improves greatly and 3F4 level lifetime prolongs to 1.57 ms close to radiative time. After 35 min, they change very little due to almost reaching the dynamic balance. The strength of interaction between Tm3+ and OH− is determined. The energy transfer mechanism between Yb3+ and Tm3+ is proposed by photoemission spectroscopy and lifetime measurement. The energy transfer coefficients have been analyzed by the extended spectral overlap method and hopping model. Large ratio of the forward energy transfer from Yb3+ to Tm3+ to the backward one (223) and high energy transfer efficiency (97%) from Yb3+ to Tm3+ ensure efficient 1.8 μm emission. Hence, Yb3+ sensitization and the reduction of OH− content could be expected to open up a possibility to achieve high efficient 2.0 μm lasing from Tm3+ under a 976 nm LD pump.