Er-doped ortho- and metha-phosphate glassy mixtures for 1.54 μm laser construction

The goal of our work was preparation and investigation of Er-doped potassium ytterbium lanthanum orthoand metha-phosphate glassy mixtures developed as a solid-state laser active medium. The tested samples were prepared by rapid quenching of molten mixture of starting K2CO3, YbPO4, LaPO4, YbPO4 and P2O5. Their cations molar concentration was as follows n(K) = 0.7, n(Yb) = 0.135, n(La) = 0.16 and n(Er) = 0.005 and it was the same in all tested samples. The additions of the P2O5 to the individual starting charges were batched so the following compositions of resulting glasses should be obtained: (i) pure meta-phosphate, (ii) mixture of 80 mol% of meta-phosphate and 20 mol% of ortho-phosphate, (iii) mixture of 50 mol% of meta-phosphate and 50 mol% of ortho-phosphate, (iv) mixture of 25 mol% of meta-phosphate and 75 mol% of ortho-phosphate, and (v) pure ortho-phosphate. The glassy samples were prepared in the form of discs about 8mm in diameter and 2mm in height. The absorption spectra were measured in broad range from 200 up to 2500 nm to identify possible impurities, mainly the residual OH-absorption and to calculate absorption cross-section for pumping and laser transition 4I15/2 → 4I11/2 and 4I13/2 → 4I15/2, respectively. For particular transitions fluorescence spectra and fluorescence decay time were recorded simultaneously. It was found that the fluorescence decay time, corresponding to upper laser level 4I13/2 depopulation, progressively increases with the content of orthophosphate in glass composition starting from 2ms for sample (i) up to 8ms for sample (iv). The laser action at 1.54 μm under 975nm pulsed laser diode pumping was successfully demonstrated using the sample with the longest upper laser level lifetime.