Abstract
The dependence of the negative nonlinear absorption effect on modulation degree of an incident laser was investigated in an erbium doped yttrium aluminum garnet crystal which has a five-level system: absorption can occur between a ground level and an excited level (4I15/2-4I9/2) and between two other excited levels (4I13/2-2H11/2) in Er3+. According as modulation degrees of the incident laser decreased, the transmitted laser waveform was inverted against the incident one for incident modulation degree under 90% and sample lengths greater than 2mm. By calculating rate equations based on an analytical model for the five-level system, it was confirmed that the enhanced absorption was caused by stimulated emission from 4S3/2 level to 4I13/2 level and close agreement between observed and calculated values was obtained in the relation between transmitted modulation degree and incident one.
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