Speckle can complicate signal acquisition in coherent laser systems such as Laser Doppler Vibrometry (LDV). Variations in the speckle pattern at the reliever due to fluctuations in the system such as beam pointing can lead to impulsive events in the signature. The beam size at the object has a direct influence on the size of the speckle at the receiving aperture. Increasing the beam spot size reduces the average speckle size, but also decreases the strength of the signal coupled with the local oscillator in the LDV. In this paper, we derive the relationship between scattering spot size at the object and average speckle size at the receiver. Theory is presented on how increasing the beam diameter at the object can reduce the fluctuations of the heterodyned signal coupled with the Local Oscillator (LO). The Antenna theorem is presented to show the tradeoff between angular field of view and capture area. We show experimental results on the effects of speckle size and decreasing signal strength have on the stability of an LDV signature. We use a kurtosis metric previously reported in the literature to assess the stability and quality of the return signature.
Thomas Z. Iverson and Edward A. Watson, "Signature stability in laser Doppler vibrometry," Proc. SPIE 10191, Laser Radar Technology and Applications XXII, 101910G (Presented at SPIE Defense + Security: April 12, 2017; Published: 5 May 2017); https://doi.org/10.1117/12.2266269.
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