EXCITED-STATE ABSORPTION IN TM(3-MGO AND GD3GA5O12 AND LOOPING MECHANISM() DOPED LINBO3)

Two different mechanisms for the red excitation of Tm3+ leading to up- converted fluorescence have been observed under pulsed laser excitatio n. In LiNbO3: MgO the ground state H-3(6) --> F-3(2) absorption is fol lowed during the same laser pulse by an excited state absorption from the H-3(4) level which leads to a D-1(2) violet emission. In Gd3Ga5O12 a looping mechanism constituted by an excited state absorption and a cross-relaxation progressively feeds the long-lived F-3(4) level durin g successive pulses, leading to blue fluorescence from the G-1(4) leve l. We describe a model for the dynamics and we calculate the gain and the losses of the loop.