Signature stability in laser doppler vibrometry

Speckle can complicate signal acquisition in coherent laser systems such as Laser Doppler Vibrometry (LDV). Variations in the speckle pattern at the receiver 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 thesis, 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. An equation is established for the Doppler shift that is used to calculate the frequency shift contained in a heterodyned signature. The heterodyne signature is derived and the intensity imaged on the photodetector inside and LDV device is discussed. A signature stability simulation is conducted and the effects of pointing jitter on a generated electric field are investigated by a numerically changing beam spot diameter. Two methods in which speckle spot size is calculated are composed. We show experimental results on the effects of speckle size and decreasing signal strength have on the stability of an LDV signature. A kurtosis metric previously reported in the literature is used to assess the stability and quality of the return signature