Effect of system optical configuration on visibility profiles for circularly polarized backscattered laser beam

Based on the laser Doppler anemometry technique and Mie scattering theory, the visibility-particle size relationship (visibility profile) is computed for spherical particles of complex index of refraction. The analysis is restricted to the backscatter direction, for incident light that is circularly polarized. It is observed that the shape of the visibility profile is greatly influenced by the index of refraction of the particle and the optical parameters of the system. The effect that the optical parameters, such as beam angular separations (Ψ), collecting aperture size (R), and off-axis collecting angle (Φ₀ ), have on the visibility profiles is examined. It is established that the visibility technique is useful for particle sizing under limited values of the optical parameters. It is found that particle sizing is feasible for values of Ψ ranging from 9.5° to 1.0°, and for values of R ranging from 4.0° to 14.0°. It is observed, in many cases, that moving the collecting aperture slightly off-axis improves the quality of the visibility profile for particle sizing