Phase characteristics of a spherical particle passing through the crossed region of two coherent beams

Light with periodic intensity is scattered when a spherical particle passes through the crossover region of two coherent laser beams. Photodetection of this light produces a periodic signal of varying amplitude. The phase of the signal varies with the particle diameter and reflective index, the beam-crossing angle and wavelength, and the position and size of the scattered-light-collecting lens. Here, the phase variation with respect to the particle absorptive index of refraction, collecting-lens size, and beam-crossing angle is calculated using Mie scattering theory and reflection theory. Variations in the phase of the Doppler signal relative to the pedestal signal are analyzed over the particle-diameter range 0.1 to 18 μm. The two theories show good agreement in phase predictions, especially for large absorptive indices and for small collection lenses. Both theories predict phase to be inversely proportional to the beam-crossing angle. It is concluded that the reflection-theory prediction of relationships between the incident beam-crossing angle and phase is accurate even for transparent particles when smaller collecting lenses are used