The dependence of the negative nonlinear absorption effect on both the modulation degree and frequency ofthe incident laser was investigated in heavily erbium-doped phosphate, borate, and fluoride glasses using a 809.2-nm laser diode. With decreases in the degree of modulation, reversed-phase waveforms were obtainedin the transmitted waveform. The modulation degrees were approximately 7, 14, and 32% for phosphate, borate, and fluoride glasses, respectively. For phosphate glass, the transmitted waveform changed to theopposite phase of the incident waveform at modulation frequencies from 0.25 to 1 MHz. For fluoride glass, however, the transmitted waveform was asymmetrical with decreasing modulation frequency. Thoseresultssuggest that the negative nonlinear absorption effect can be explained by an enhanced absorption, in which theamount of absorption increases due to the increase in incident laser intensity.
