Laser vibrometry and the secret life of speckle patterns

Speckle patterns are produced when coherent light scatters from optically rough surfaces. In typical applications, a Laser Vibrometer collects a region of a speckle pattern on its photodetector. However, noise can be generated in the Laser Vibrometer output when surface motions cause changes in the speckle pattern. This paper uses high resolution images to investigate typical intensity profiles in the partially- and fully-developed speckle patterns scattered from target surfaces with roughnesses in the range 11nm to 1μm Ra (≈1/60λ to 1.6λ) and from a surface treated with retro-reflective tape. In-plane and tilt target motions cause changes in the scattered speckle patterns and sequences of images are used to investigate how the speckle patterns behave in response to these motions. In the case of retro-reflective tape, incident beam diameter is also found to have an important effect on speckle pattern changes and different incident beam diameters are considered for other target surfaces and motions too. This qualitative analysis of the speckle pattern intensity profiles is compared with established theoretical understanding of speckle patterns and their motions and related directly to the instrument noise levels ultimately encountered. Such fundamental evaluation of speckle pattern motions provides a greater appreciation of speckle noise generation in the Laser Vibrometer outputs.