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We report on La-and Er-doped tellurite glasses that demonstrate improved properties for rare earth fibre lasers in the mid-infrared. IntroductionTellurite glasses are promising candidates for fibre lasers operating in the mid-infrared (IR). They have low phonon energies, high refractive indices and high rare earth solubility, resulting in IR transmission up to 5µm and high gain per unit fibre length [1,2]. Microstructured optical fibre (MOF) technology [3] is a promising route to high-power double-clad tellurite fibre lasers. Ideally, the active core in such fibres should have the same index as the material used for the passive holey cladding to make full use of the novel MOF guidance properties [3].The Er 3+ ion is attractive for IR fibre lasers since it allows lasing in the IR via pumping with commercially available laser diodes at 980nm. Tellurite glasses based on Na 2 O-ZnO-TeO 2 are promising hosts for Er 3+ [1,2]. Doping of Er 3+ in this glass system results in a decrease of the refractive index [2]. Substitution of La 3+ by Er 3+ in a ZnO-La 2 O 3 -TeO 2 glass did not change the index up to 0.3mol% Er 2 O 3 [4], which offers index matching of erbium doped core and erbium-free cladding in a MOF.This paper investigates the impact of Er 2 O 3 and La 2 O 3 doping on the base glass 5Na 2 O-20ZnO-75TeO 2 (in mol%). The potential of La-doped tellurite glass for fibre lasers is demonstrated here via low-loss bare and microstructured fibre fabrication.