Optical Bistability and Negative Nonlinear Absorption Effect at 1.5μm in Highly Er3+-Doped Glasses

The dependence of the negative nonlinear absorption effect at 1.5μm on the sample temperature, the modulation frequency and the sample length were investigated in glasses containing amounts higher than 20mol% Er3+. Reversed-phase waveforms were observed in the temperature range -260-600°C and at modulation frequencies from 1kHz to 10GHz in an erbium-doped phosphate glass. With increasing temperature, the amplitude of the reversed-phase waveforms decreases exponentially according to two separate steps. Optical bistability in the transmitted laser was observed in the range of incident laser intensity of 30μW/cm2-2.4mW/cm2. The transmitted laser intensity propagating through the glass decreased exponentially according to the well-known exponential law. However, the amplitude of the optical bistability increased logarithmically as the sample length was increased. The negative nonlinear absorption effect can be explained by considering an enhanced absorption model for a four-level system of the Er3+ ion.