Experimental study of the dynamics of semiconductor ring lasers

Semiconductor ring lasers (SRLs) are microlasers whose cavity is formed by a ring-shaped waveguide. Therefore, SRLs can generate laser light in two counter-propagating directions, usually referred to as the clockwise and the counter-clockwise modes. It has been shown that SRLs are the promising sources in photonic integrated circuits. In particular, the bistable directional operation possible in SRLs paves the way to encoding digital information in the emission directions. SRLs have been implemented in all-optical regenerators and memory elements based in the switching between the SRL's directional modes via injection of optical pulses. Semiconductor lasers are known to be very susceptible to external perturbations. For example, if a small amount of emitted laser light is reflected back in the cavity, this can easily destabilize the laser emission leading to chaotic intensity fluctuations. Although such effects are unwanted in many applications, they can sometimes be useful. In semiconductor ring lasers, the dynamics resulting from external perturbations such as delayed-optical feedback and noise injection have not been studied in detail. In this thesis, we will undertake an investigations into these effects. The goal of this work is to understand the dynamical behaviour resulting from feedback and noise in order to be able to either avoid these effects when they are unwanted, or to use them in novel applications. We first investigate the effect of external noise injection on the SRL behaviour by injecting amplified spontaneous emission from an external source. Because of noise, mode-hopping between the counter-propagating modes is possible in a solitary SRL. We study - both experimentally and theoretically - the effect of the injected noise's strength on the mode-hopping statistics and time-scale. We show that the injecting amplified spontaneous emission results in a sharp decrease of the average residence time and plays a role similar to that of the noise of the laser field itself. Another dynamical regime that we have studied experimentally is the excitability that can be observed when tuning the coupling between the counter-propagating modes. In this regime, the SRL generates large pulses in response to small perturbations that cross a specific threshold level. In order to fully probe the dynamics in this regime, we again use the external amplified spontaneous emission source in order to inject noise with a variable amplitude. This way, our experiments reveal a statistical distribution of the noise-triggered optical pulses that is not observed in other excitable systems. When SRLs are implemented in a photonic integrated circuit along with other optical components, small reflections from these other components are hard to avoid and can lead to instabilities. Therefore, in this thesis we also study the influence of optical feedback on SRLs. We experimentally and numerically study the dynamics of SRLs subjected to delayed self-feedback, in which case each directional mode receives feedback from itself. In particular, we study the appearance of low frequency fluctuations and their dependence on the system parameters such as pump current and the feedback strength. We observe different routes to the recovery to the initial power level. We also study SRLs subject to cross-feedback (when each directional mode received delayed feedback from the counter-propagating mode) and show that under appropriate conditions, injecting of only one directional mode in the counter-propagating direction leads to square-wave oscillations.