Abstract

We report an observation of nonsaturable absorption of light. When a laser medium in a small aspect ratio geometry is uniformly and transversely pumped, amplified spontaneous emission (ASE) builds up along the longitudinal axis. The higher ASE intensity at the ends of the cell repopulates the ground state through the stimulated emission process more rapidly than at the center of the cell. Therefore, the absorption of the pump light is enhanced at the ends and the transmitted pump beam shows a nonuniform profile with a peak at the center. When the occupation of the states other than the upper lasing state and the ground state can be ignored, the normalized profile of the transmitted pump beam becomes identical to that of the fluorescence depicting the upper state population distribution. This fact has been confirmed experimentally with a R6G dye cell pumped with various intensities of frequency doubled Nd:YAG laser (λ = 532 nm, 10-ns pulse duration). The percentage of the absorbed pump energy at the ends of the cell remains the same, virtually nonsaturable, whereas at the center it decreases as the pump intensity increases.

© 1989 Optical Society of America