| Summary: | A flat 2.0 μm ultra broadband emission with a full width at half maximum (FWHM) of 329 nm is achieved in 1 mol.% Tm<sub>2</sub>O<sub>3</sub> and 0.05 mol.% Ho<sub>2</sub>O<sub>3</sub> co-doped gallium tellurite glasses upon the excitation of an 808 nm laser diode. The influence of Tm<sup>3+</sup> and Ho<sup>3+</sup> contents on 2.0 μm spectroscopic properties of gallium tellurite glasses is minutely investigated by absorption spectra, emission spectra, and lifetime measurement. In addition, emission cross section and gain coefficient of Ho<sup>3+</sup> ions at 2.0 μm are calculated, and the maximum values reach 8.2 × 10<sup>−21</sup> cm<sup>2</sup> and 1.54 cm<sup>−1</sup>, respectively. Moreover, forward and backward energy transfer probability between Tm<sup>3+</sup> and Ho<sup>3+</sup> ions are qualitatively evaluated by the extended spectral overlap method. Large ratio of the forward energy transfer from Tm<sup>3+</sup> to Ho<sup>3+</sup> to the backward one (19.7) and high forward energy transfer coefficient (6.22 × 10<sup>39</sup> cm<sup>6</sup>/s) are responsible for effective 2.0 μm emission from Ho<sup>3+</sup> ions. These results manifest that Tm<sup>3+</sup>/Ho<sup>3+</sup> co-doped gallium tellurite glass is suitable for potential applications of broadband light sources and tunable fiber lasers operating in eye-safe 2.0 µm spectral region.
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