Study On The Growth And Spectral Properties Of Er³⁺ Activated 1.5-1.6μm Er:YPO₄ And Er/Yb:YPO₄ Crystal

At present,the commercial laser material in the 1.5-1.6μm band is phosphate glass co-doped with Er³⁺and Yb³⁺ions.Compared with this,phosphate single crystal material not only has similar phonon energy,but also has more excellent thermal properties,basic physical and chemical properties such as mechanical strength show more powerful application value and potential in the 1.5-1.6μm band.Centimeter-scale Er³⁺（0.75 at.%）ion-activated YPO₄ single crystals were successfully prepared by high temperature solution method with Pb₂P₂O₇ as flux.The crystal growth process,structural characteristics,polarization absorption and fluorescence spectra and fluorescence lifetime were studied in detail.Base on Judd-Ofelt（J-O）theory,the important spectral parameters of Er³⁺ions in YPO₄ crystal,such as oscillator strength,J-O intensity parameters,spectral line intensity,spontaneous emission transition probability,fluorescence branching ratio and radiation lifetime are calculated systematically.Its fluorescence emission spectrum shows obvious anisotropy and multi-wavelength emission characteristics.The central wavelengths of four main emission peaks in the 1.5-1.6μm band are located at 1505,1538,1562 and 1586 nm,respectively,and the wavelength frequency difference is about 2.9-10.2 THz.The emission cross section was calculated by the Füchtbauer-Ladenburg（F-L）method,the largest emission cross section was1.66×10^-20 cm²,corresponding to 1538 nm underσ-polarization,and the smallest emission cross section was 0.99×10^-20 cm²,corresponding toπ-polarization at 1586nm.Centimeter-scale YPO₄ single crystals（7.5Yb/Er:YPO₄,11.5Yb/Er:YPO₄）co-doped with different concentrations of Yb³⁺（7.5 at.%and 11.5 at.%）and Er³⁺（0.75at.%）ions were successfully prepared by high temperature solution method and Pb₂P₂O₇ as flux.The structural characteristics,polarization absorption and fluorescence spectra and fluorescence lifetime of the two crystals were tested and compared in detail.The absorption coefficient,absorption cross section,spectral line/oscillator/J-O intensity parameters,radiation transition probability,radiation lifetime,fluorescence branching ratio,emission cross section and gain cross section are calculated by J-O theory,reciprocity method（RM）and F-L method.The results show that the comprehensive spectral performance of the 7.5Yb/Er:YPO₄ crystal is better than that of the 11.5Yb/Er:YPO₄ crystal and Er:YPO₄ crystals,and its absorption full width at half maxima around 975 nm are about 17.1 nm（σ-polarizations）and 12.0 nm（π-polarizations）,and the energy transfer efficiency form Yb³⁺to Er³⁺is as high as 95.1%,which is very suitable for In Ga As laser diode（LD）to achieve high-efficiency pumping.Comparing the spectral parameters with other known Er³⁺/Yb³⁺co-doped laser crystal materials,it shows that the 0.75 at.%Er³⁺/7.5 at.%Yb³⁺:YPO₄ crystal prepared in this paper has relatively larger absorption cross section and emission cross section,better spectral quality,suitable fluorescence lifetime,and population inversion fraction to achieve laser output further indicate that it is an excellent laser material that can be pumped by LD and has the potential to be applied in the 1.5-1.6μm band.