Aero - Optics: Controlling Light with Air

It is possible to control light in a medium comprising nothing more than a fluid, whether in gas or liquid phase. Using air as an example, it is possible to bend light by a process of continuous refraction as opposed to stepwise refraction in the case of solid state optics. This is achieved by effecting a continuous change of the refractive index from a high value for lowest temperature (or high density) to a low value for the highest temperature (or lowest density). Since the density changes gradually from a low to a high value, the refractive index is graded as well. In fact, this effect is well known in nature: the mirage effect where light refracts away from a hot surface, usually a road or desert sand, is precisely due to continuous refraction of light. If the refractive index can be customised and controlled, then one has the possibility to create aero-optics: control of light with air. Such optics are a special subset of GRIN (Graded Refractive INdex) optics, with the most studied example being the simple lens, so-called gas lenses. The efficiency at which these lenses operate is dependent on the effectiveness of the refractive index gradient – the greater the gradient, the stronger the lens. There are advantages to such optics: they are not dispersive, and hence suitable for broad bandwidth laser pulses, and have no practical damage threshold as compared to most solid-state optics (e.g., glass). This latter point remains to be exploited for high power laser beam delivery in such applications as laser fusion and peta-watt high intensity laser beam delivery. In the remainder of this chapter we will consider such gas lenses in more detail. We outline a simple approach to controlling light through a medium comprising nothing more than air in a spinning pipe that is heated along its boundary. We outline the fluid dynamics of this simple system and through the use of computational fluid dynamics (CFD), explain the mechanisms that give rise to a plethora of interesting properties, from a lensing action resulting in the focussing of light, to a simulator of optical turbulence in the laboratory. The CFD models are supported in most cases by intuitive analytical expressions, teasing out fundamental physics from the fluid dynamics of this system, while the predicted impact of the fluid dynamics on the optical field is verified experimentally. Despite the simplicity of the system – merely a spinning, heated pipe – the fluid dynamics of the system is shown to be rather complex, comprising regions of slow and fast flow, symmetric and asymmetric density gradients, and laminar and turbulence regions. We conclude the study by outlining how such aero-optics, using aerodynamics and fluid flow to control light, may be exploited in applications ranging from long range telescopes to overcoming the damage thresholds of conventional solid state glass optics for delivery of high power lasers.