Resonant excitation of Yb 3+ at 974 nm (10267 cm −1 in codoped Yb 3+ −Tb 3+ silica based fibres has lead to luminescence clearly being visible from the Tb 3 + 5D 4 level (20661 cm −1). Through time resolved fluorescence measurements we have been able to directly assign the process responsible for the fluorescence to cooperative sensitisation of the Tb 3 + 5D 4 level by two excited Yb 3+ ions. Energy transfer times for the cooperative process have been directly measured, enabling the effects of glass structure on the cooperative process to be explored. Numerical simulations based on the experimental findings have been conducted, enabling both the mechanism for the luminescence to be confirmed and the properties associated with the luminescence to be interpreted.