  Photoluminescence spectra have been investigated in erbium doped GaLaS(O)
glasses. The samples demonstrate intense green emission bands centered at
around 525 and 550 nm due to up-conversion processes in erbium ions. The
theoretical description of up-conversion intensity as a function of excitation
intensity has been offered. It is based on a solution of a system of rate
equations taking into account three up-conversion transitions.
