Negative Nonlinear Absorption Effect for 1.5 μm Laser in an Erbium Chloride Solution

The dependence of the negative nonlinear absorption effect on both modulation degree and frequency of the incident laser was investigated in an erbium chloride solution using a 1.5μm laser diode. The reversed-phase waveform was observed at modulation degrees smaller than 71%. As modulation frequency increased, the transmitted waveforms became asymmetrical and the effect was suppressed. The effect was obtained at wavelengths of 1510.3 to 1510.8 nm. The negative nonlinear absorption effect for 1.5 μm. can be explained by considering an enhanced absorption model for a four-level system of the Er³⁺ ion.