Saturable absorption and its consequent effects in bistable erbium-doped fiber ring laser

Abstract

The linear absorption in erbium-doped fiber (EDF) contributes to its excellent role in EDF amplifiers and lasers. A nonlinear optical contribution in the absorption of EDF is responsible for optical bistable action when it is present in a laser cavity. To quantify this effect, the variation of absorption coefficient is measured at different signal powers at multiple wavelengths in the C-band for different EDF lengths, and saturable absorption parameters such as the saturation power are extracted. Then the modification in the output characteristics of EDF ring laser with change in fiber length and in the presence of self-induced saturable absorption effect within the gain medium, which leads to optical bistability, is measured. By comparing the measured parameters obtained from saturable absorption in EDF and optical bistability in EDF ring laser, we estimated the length of the gain medium which acts as the saturable absorber inside the cavity of the laser. This is useful in constructing bistable lasers with optimized conditions. The temporal evolution of cavity loss and gain with the intra-cavity power and up- and down-thresholds helps in understanding why the down-threshold will be lesser than the up-threshold in bistable laser systems.