Various probe ions have been employed to improve the performance of Yb-doped laser glasses. Lanthanide ions similar to the Yb 3+, e.g. Nd 3+, Eu 3+ and Gd 3+, are sensitive to their site surroundings and that is why they are useful for obtaining the structural information needed to tailor the laser host matrix. A new promising laser glass based on these investigations has been developed. We assume that the favorable laser properties of this glass can be explained by formation of [Yb(PO 4) 3]-clusters which are nearly isolated from the main part of the network. The electronphonon coupling in optimized fluoride phosphate glasses with tetrahedral anions may efficiently depopulate the terminal laser level.