Broadband infrared emission of erbium–thulium-codoped calcium boroaluminate glasses

The near-infrared emission from calcium boroaluminate (CABAL) glasses codoped with Er³⁺ and Tm³⁺ has been investigated. It is shown that by controlling the Tm]/Er] concentration ratio a fairly flat emission with a bandwidth of 370 nm can be achieved in the wavelength range from 1.4 to 2.0 μm. The broadband emission is formed by three bands centered at 1.4, 1.5 and 1.8 μm, which are related to the emission from the Tm³⁺: ³ H ₄→³ F ₄, Er³⁺: ⁴ I _13/2→⁴ I _15/2 and Tm³⁺: ³ F ₄→³ H ₆ transitions, respectively. Compared to Er-only doping and Tm-only doping at the same concentration, codoping with both ions leads to a reduction of the intensity and lifetime of the Er-related band at 1.53 μm and to an intensity enhancement of the two Tm-related emission bands at 1.46 and 1.80 μm. This is a result of energy transfer (ET) processes between Er³⁺ and Tm³⁺ ions that are relevant when determining the emission spectrum shape. Two dominant ET processes are identified, both consisting of transferring the energy of the Er³⁺ first excited level (⁴ I _13/2) in the first case to the Tm³⁺ first excited level (³ F ₄), which is excited to the third excited level (³ H ₄), and in the second case to the Tm³⁺ ground state (³ H ₆) which is excited to the first excited level (³ F ₄).