The effect of 1.5 mm cascading on 3 mm er3+ laser emission

Laser emission in various regimes was obtained at room temperature in the three-micron range on Er3+ transition 4I11/2→4i13/2, despite a much larger lifetime of the terminal level. Various mechanisms have been considered in order to obtain efficient depopulation of 4I13/2 necessary to avoid the self-saturation of the laser emission: excited state absorption of the pump radiation from the terminal level,1 energy transfer to impurities,2 up-conver-sion,3 and, most recently, cascading with 1.5µm emission from 4I13/2 to the ground level 4I15/2,4 While the up-conversion is an internal mechanism of the system of Er3+ ions and a high Er concentration is necessary in order to be effective, all the other mechanisms require additional external conditions and could act at any Er concentration.