The phenomenon of speckle, ubiquitous throughout physics, is perhaps most familiar in the context of the light field created when a laser beam is scattered by a random medium. While speckle patterns all have their origins in interference, they nevertheless show a wide variety of statistical behaviour, depending on the characteristics of the scattering medium. On close examination, these patterns reveal regions where the phase spirals around a point of discontinuity. These 'optical vortices' can also be created within a laser cavity, allowing their study under highly stable conditions. Movement of the scatterers leads to evolution of the speckle pattern involving the creation and annihilation of innumerable optical vortices. The resulting chaotic fluctuations may display occasional very bright 'glints', and they may show fractal behaviour over a wide range of timescales. The statistics of these fluctuations can be used to obtain a wealth of information about the scattering medium. Even light from the purest of sources, a single-mode laser, will show wild temporal intensity fluctuations when it is filtered within its Schawlow-Townes linewidth. Under such conditions, the output becomes indistinguishable from chaotic scattered light, allowing some insight into the fundamental processes underlying laser behaviour.
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