Dynamical features of loss and pump modulation in an erbium-doped fibre ring laser

Resonance characteristics, chaotic dynamics and bistable response of the erbium-doped fibre ring laser (EDFRL) are studied in an experiment under cavity-loss modulation at frequencies in the range of 1-35 kHz. With an increase in pumping ratio and modulation index, the resonance frequencies of the ring cavity laser change in agreement with the expectations from the model. A comparison with the calculated results of the pump-modulated laser helps to bring out the differences in the two modulation schemes. The loss-modulated laser is found to be more sensitive in the sub-harmonic range to its dynamical state while the pump-modulated laser is more sensitive in the super-harmonic range. To achieve chaos, pump modulation requires high values of modulation index (similar to 28%) while it is seen even at very low values (<1%) in loss modulation. Even though both the schemes follow the same period-doubling route to chaos, features such as the presence of bifurcations or chaos at the resonance frequencies are quite different. Loss modulation is more suited to study the chaotic features in EDFRL due to the range of available control parameters. By comparing the strange attractors and Lyapunov exponents of both cases, it is found that the quality of chaos is better in loss modulation.