Abstract
A negative input-output characteristic was obtained in an erbium doped yttrium aluminum garnet crystal which has multiple-level energy structure: 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 distorted and eventually inverted against the incident one in case that the modulation degree was less than 0.5. It is suggested that an enhanced absorption phenomenon takes place since the excited state absorption from 4I13/2 to 2H11/2 and that a 4S3/2 level plays an important role. Because the time-dependence of this phenomenon matches a life time of the 4S3/2 level. By considering multiple excited state absorption in detail, active functions such as an optical signal inverter could be derived from the negative nonlinear absorption effect.
