Laser Vibrometry: The Effect of Speckle Size on Doppler Signal Phase and Amplitude Modulation

Abstract When coherent laser light is incident on a surface that is rough on the scale of the optical wavelength a speckle pattern is formed in the light which is backscattered. The performance of Laser Doppler Vibrometers is significantly influenced by changes which occur in the speckle pattern on the instrument’s photodetector. This paper investigates experimentally how the ratio of photodetector size to speckle size affects both the phase and amplitude of the Doppler signal component of the photodetector output. This speckle induced signal modulation contributes noise in the instrument’s output referred to as speckle noise. Minimisation of speckle noise is essential to lower the Laser Vibrometer noise floor and ensure reliable operation. Experimental data are presented in this paper to explore the combined effects of speckle size, photodetector size and speckle transition speed. Speckle noise is found to be minimised when a compromise in detector position is achieved between the competing effects of speckle size and speckle transition speed.