Laser vibrometry on solid surfaces : the effects of laser speckle

This letter discusses the phase and amplitude modulation of Doppler signals generated in laser Doppler velocimetry (LDV) systems with reference to the paper entitled 'Dynamic laser Doppler velocimetry on solid surfaces' by Rajadhyaksha and Stevenson (Vol. 4 No. 2). In particular, transit time and velocity gradient frequency broadenings are described in terms of the speckle pattern behavior on the photodetector and this approach is reconciled with the scattering element approach adopted by Rajadhyaksha and Stevenson. Furthermore, it is described how the frequency components of the Doppler phase noise can coincide with the vibration components of interest, thereby introducing ambiguity in interpretation of data. As a consequence, the reduction of Doppler phase noise by averaging is not generally acceptable. It is concluded that consideration of the dynamic speckle pattern is more appropriate to the investigation of Doppler phase noise since the speckle pattern itself has the ultimate influence on the LDV performance.