Control of optical turbulence

We consider the suppression of the so-called "turbulent regimes" in optical systems with advection. It is well known that spatiotemporal systems submitted to advection can undergo a convective instability above a critical value of the drift parameter. The result is a hypersensivity to noise leading to spatiotemporal erratic regimes called noise sustained structures or "optical turbulence" (Morgner and Mitscke, 1999). This phenomenon appears with slight difference either in system with local saturation (e.g. liquid crystal with optical feedback) or with global saturation (e.g., actively mode-locked lasers). We demonstrate that it is possible to suppress those kind of instability with a rather simple method. To do this, we revisit the "coherent photon seeding" method applied empirically in the nineties to stabilized actively mode-locked laser (Beaud et al., 1990). We study numerically and experimentally the process of stabilization on a elementary advection-diffusion equation (Ginzburg-Landau equation with advection). In addition to determine the adequate parameters, this study allows to interpret the stabilization process: the feedback creates a new deterministic solution toward which the system evolves, the convective threshold has been shifted. This new solution is no more sensitive to noise. Although this process is very different from "Ott-Grebogy-Yorke" method, it requires extremely small perturbation to be achieved (just above noise level). Finally, we check experimentally this method on a free electron laser (UVSORTI FEL, Japan). We suppress the "turbulent" regimes occurring in this laser with a simple optical feedback. The ratio of the re-injected power is in the order of 10^-8. This control scheme should be applicable in other systems (optical or not).