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What is laser speckle?

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Image of laser speckle
Image of laser speckle

Laser speckle is a phenomenon that occurs when laser light is scattered by a rough or irregular surface or by a medium with inhomogeneities, such as a turbid fluid. When the scattered light waves interfere with each other, they produce a pattern of bright and dark regions, known as speckle, on a detector or observation screen.

The size, shape, and intensity of the speckle depend on the properties of the scattering medium and the incident laser beam, as well as on the observation geometry and the wavelength of the laser. For example, the speckle size decreases with increasing wavelength and increasing observation distance, and it increases with increasing roughness or inhomogeneity of the medium.

Speckle is a common occurrence in many laser applications, such as imaging, sensing, and metrology, and it can have both positive and negative effects. On one hand, speckle can be used to study the microstructure and dynamics of scattering media, such as tissues, fluids, and materials, by analyzing the statistical and spatial properties of the speckle. On the other hand, it can cause noise and degradation of the image quality and resolution in laser imaging systems, and it can reduce the accuracy and repeatability of laser measurements.

Laser speckles created by lighting the opposite white wall with a 50 mW laser pointer
Laser speckles created by lighting the opposite white wall with a 50 mW laser pointer

To mitigate the adverse effects of speckle, various techniques and approaches have been developed, such as using coherent or partially coherent laser sources, modulating the incident laser beam, or applying spatial filtering or averaging techniques. Moreover, advanced techniques have been developed to generate laser beams with reduced or minimized speckle, by using specialized optical elements or configurations, such as diffusers, holograms, or phase plates.

In conclusion, laser speckle is a complex and multifaceted phenomenon that arises from the scattering of light by rough or inhomogeneous media, and it can have both scientific and technical implications in various applications. Understanding and controlling speckle is an active research field that aims to optimize the performance and capabilities of laser systems and to explore the potential of speckle for scientific and technological purposes.

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