SOA Fiber Ring Lasers: Single- Versus Multiple-Mode Oscillation

Earlier investigations have demonstrated that a narrow bandwidth intracavity filter in a semiconductor optical amplifier (SOA) fiber ring laser results in an output spectrum comprising multiple longitudinal modes, whereas a large bandwidth filter generally leads to quasi-single-mode oscillation. In this paper, we report on a numerical model simulating the SOA fiber ring laser dynamics and compare its predictions with experimental results obtained with two different SOAs. We demonstrate that the observed experimental behavior results from the interplay between the complex transfer function of the intracavity optical filter, fast gain saturation and four-wave mixing (FWM) due to the linewidth enhancement factor. The dispersion introduced by the complex transfer function of the filter can lead to modulation instability, which plays a key role in the laser dynamics that can evolve into a single-longitudinal mode (SLM) operation. We also simulate the behavior of the laser emission when the wavelength of the optical filter is finely tuned.