Improved Cross-Correlation Dynamic-Light-Scattering Method

John H. Glenn Research Center, Cleveland, Ohio

A dynamic-light-scattering (DLS) method that reduces undesired contributions from multiply scattered photons has been invented. The method involves, among other things, cross-correlation processing of the outputs of two photodetectors aimed along intersecting, nearly parallel lines of sight. This present method can be characterized as an improved version of the method described in "Multiple Scattering Suppression in Laser Light Scattering" (LEW-16517),NASA Tech Briefs, Vol. 23, No. 11 (November 1999), page 14a.

The various DLS methods constitute various implementations of the concept of extracting information on the sizes and motions of light-scattering particles from the spatial and temporal dependence of the loss of coherence of scattered laser light. Typically, the particles of interest are suspended in liquids and are in Brownian motion, so that scattering of laser light from the particles gives rise to a temporally varying speckle pattern.

Scattered Laser Light enters two adjacent optical fibers attached to photodetectors. Cross-correlations of the outputs of the photodetectors are obtained for several different values of q. The resulting data can be used to select a value of q for best discrimination against multiply scattered photons.

The underlying physical principle makes it necessary to measure only singly scattered photons in order to be able to determine particle sizes and motions. However, the speckle pattern is a result of both multiple and single scattering, and photodetectors are unable to distinguish between singly and multiply scattered photons. Thus, there is a need to design a light-scattering apparatus and to process the photodetector outputs in such a way as to suppress the contribution of multiply scattered photons arriving at the photodetectors. The present method satisfies this need.

A typical apparatus used in the present method, shown in simplified form in the figure, is similar to the apparatus described in the noted prior NASA Tech Briefs article. A laser beam is aimed horizontally through a vertically oriented cylindrical sample cell containing the particles of interest suspended in a liquid. Optionally, the cell can be placed in a vat of another liquid, the index of refraction of which approximates that of the liquid in the cell. The laser beam is focused to a waist at the middle of the cell or at any other desired depth within the cell.

Two optical fibers with gradient-index-of-refraction (GRIN) lenses at their input ends are positioned with their input ends close to each other and aimed toward the beam waist (the nominal scattering volume) to receive light scattered from the beam axis to a chosen angle,φ, in the horizontal plane. The value of φ can be chosen conveniently to be 90°, but a different value can just as well be chosen to suit a specific application. The tip of one fiber is placed a short distance above and the receiving tip of the other fiber a short distance below the light-scattering (nominally horizontal) plane, thereby forming a small angle, θ, between the lines of sight from the two input fiber tips to the center of the nominal scattering volume. The positions of the fiber tips can be adjusted, by use of a micrometer-driven mechanism, to adjust the angle θ. The output ends of the fibers deliver the collected light to photodetectors.

Preferably, a polarizing filter is placed between the sample and the input fiber tips. The filter is oriented for polarization that is either parallel or perpendicular to the plane of polarization of the incident laser beam. Although scattered light of both polarizations includes contributions from multiple scattering, only the parallel-polarized light contains a contribution from single scattering. Thus, the polarizing filter can be used to increase the signal-to-noise ratio by serving as an additional means for discriminating between single and multiple scattering.

The outputs of the photodetectors are cross-correlated in such a way as to make it possible to discriminate against the multiple-scattering contribution to the speckle pattern. The cross-correlation processing is formulated to exploit the fact that single-scattering speckle arises from inside of the incident laser beam and is correlated over an angular range wider than that of multiple-scattering speckle, which can originate from anywhere within the sample. Measurements are made at several values of θ; at each such angular setting, the outputs of the photodetectors are cross-correlated over a suitably long interval of time to obtain a single numerical value for that detector angle. From the resulting ensemble of cross-correlations at various angles, one can construct a profile that can be used to determine the angular ranges over which multiple and single scattering predominate and one can estimate an optimum (or nearly optimum) value of q to minimize the multiple-scattering contribution in subsequent measurements.

This work was done by Bruce J. Ackerson of Oklahoma State University forGlenn Research Center.

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