Speckles and their dynamics for structured target illumination: optical spatial filtering velocimetry

Abstract

This contribution analyses the dynamics of laser speckle patterns, designed for sensing with a receiver, based on spatial filtering. Speckle translation is probed after free-space propagation of light scattered from non-specular moving surfaces. By structuring the target illumination the speckle patterns can be modulated with a fine structure, enhancing the performance of the spatial filter. The space-time normalized cross-covariance function of speckle patterns incident on the spatial filter is presented for a simple structure of the field distribution on the target. This function provides the corresponding statistical parameters, i.e. structural size, velocity and decorrelation length, for both the speckles and the fine structure. The analysis also reveals that the speckles and the fine structure do not necessarily translate as a rigid structure. The spatial power spectrum for the speckle patterns and the spatial filter defines the optimum geometry for the structured illumination. The temporal power spectrum of the photocurrent from the optical spatial filter illustrates the direct influence of the speckle statistics on the sensor performance. Experimentally, a significant increase in signal level is obtained when illuminating the target with a structured field distribution, rather than using a single Gaussian beam. Moreover, the option of improving the signal quality by manipulating the target illumination will be discussed theoretically and experimentally.