speckle pattern - Speckle technique

A granular image formed where a laser beam strikes an unpolished surface. It is a form of interference pattern present only with laser light - a consequence of laser light's unique (coherence) properties.

A speckle pattern is a random intensity pattern produced by the mutual interference of coherent wavefronts that are subject to phase differences and/or intensity fluctuations. Prominent examples include the seemingly random pattern created when a coherent laser beam is reflected off a rough surface, and the highly magnified image of a star through imperfect optics or through the atmosphere (see speckle imaging). If the surface is rough enough to create pathlength differences exceeding a wavelength, the statistics of the speckle field will correspond to a random walk in the complex plane.

In the output of a multimode optical fiber, a speckle pattern results from a superposition of mode field patterns. If the relative modal group velocities change with time, the speckle pattern will also change with time.

Speckle technique

An object with a rough surface, when illuminated with light from laser, exhibits a speckled appearance. The formation of such a speckle pattern is due to the high coherence of the laser light. Since variations in the surface are greater than the wavelength, coherent light scattered by the individual elements of the surface interferes to form a stationary pattern. The speckle pattern appears to scintillate or sparkle when there is any relative movement of the surface and the observer. The speckle pattern was initially considered the bane of holographers as holographic reconstructions were accompanied by grainy noise. It was later realized that these speckle patterns could carry information about the object's surface deformations, and that speckled wave fronts could interfere. The speckle pattern formed in the space due to self-interference among the propagating scattered waves is called “objective speckles”. The speckle pattern formed by imaging a diffuse object illuminated by a coherent beam is called a “subjective speckle pattern”.