Excited state absorption and energy transfer in Ho3+-doped indium fluoride glass

Abstract

This investigation examines in detail the rates of energy transfer relevant to the I-5(6) -> I-5(6) transition (at 3930 nm) in Ho3+-doped InF3 glass as a function of the Ho3+ concentration. The decay times, branching ratios and rate parameters for energy transfer were measured in this investigation for Ho3+ (x)-doped InF3 glass with x = 2, 4 and 10 mol.% and they were used as the input parameters for a rate equation analysis. Excited state absorption (ESA) initiating from the lower laser level is included in the study. Numerical simulation of CW laser emission at 3.9 pm was performed using two pump wavelengths, one for upper laser level excitation (i.e., I-5(8) -> (5)1(5) = lambda(P1)) and the other for lower laser level de-excitation (i.e., I-5(6) -> S-5(2) = lambda(P2)). The pump wavelength lambda(P2) = 962 nm was chosen based on the measurements of ESA and the application of the McCumber method. Critically, the estimated ESA cross section at lambda(P2) = 962 nm (sigma(ESA) = 7.1 x 10(-21) cm(2)) is approximately sixteen times larger than ground state (I-5(8)) absorption cross section (sigma(GSA) = 4.3 x 10(-22) cm(2)) and ESA does not overlap with any ground state absorption process. Our calculations suggest that even for high Ho3+ concentration in which cross relaxation has been shown in a previous study to quench the I-5(5) level, ESA is nevertheless strong enough to allow a sufficient population inversion required for practical CW emission.