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Introducing Speckle Noise Maps For Laser Vibrometry

机译：介绍用于激光振动测量的斑点噪声图

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摘要

When coherent light scatters from a surface, which is rough on the scale of the wavelength of the light, a speckle pattern is produced. The Laser Vibrometer measures target vibration velocity in the direction of the incident laser beam and typically samples a region of a speckle pattern on its photodetector. Target motions can cause the speckle pattern to change on the photodetector surface, particularly when target motions are non-normal to the direction of the laser beam. This speckle motion modulates the Doppler signal and adds noise to the demodulated output signal. Periodic target motions can cause the speckle noise to become pseudo-random and produce harmonic peaks, with the same fundamental frequency as the genuine target vibrations, which can be indistinguishable from the genuine target vibrations. Typical speckle noise levels are generally considered to be low-level, but they have not so far been adequately quantified. This paper reports preliminary results quantifying speckle noise levels using controlled experimental configurations incorporating periodic in-plane and tilt target motions. Working with commercial Laser Vibrometers, various target surface finishes and treatments are considered and speckle noise maps are produced for each configuration. For a tilting surface, speckle noise has been quantified at approximately 1μm s~(-1)/deg s~(-1) while, for surfaces with in-plane motion, the sensitivity to speckle noise has been estimated pessimistically at 0.1% of the in-plane velocity. Ultimately, these speckle noise maps will form a valuable practical resource for the Laser Vibrometer user.
机译：当相干光从表面散射时，该表面在光的波长范围内很粗糙，因此会产生斑点图案。激光测振仪在入射激光束的方向上测量目标振动速度，通常会在其光电探测器上对斑点图案区域进行采样。目标运动会导致斑点图案在光电探测器表面发生变化，特别是当目标运动与激光束方向不垂直时。这种斑点运动调制了多普勒信号，并向解调后的输出信号增加了噪声。周期性的目标运动可能会导致斑点噪声变成伪随机噪声，并产生谐波峰值，该谐波峰值具有与真实目标振动相同的基本频率，这与真实目标振动是无法区分的。典型的散斑噪声水平通常被认为是低水平的，但到目前为止尚未对其进行充分量化。本文报告了使用结合周期性的平面内运动和倾斜目标运动的受控实验配置量化斑点噪声水平的初步结果。与商用激光振动计一起使用时，应考虑各种目标表面的光洁度和处理，并针对每种配置生成斑点噪声图。对于倾斜的表面，斑点噪声已被量化为大约1μms〜（-1）/ deg s〜（-1），而对于具有平面内运动的表面，对斑点噪声的敏感度被悲观地估计为0.1％平面内速度。最终，这些斑点噪声图将为激光振动计用户提供宝贵的实用资源。

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