Difference frequency, Doppler scattering

Heterodyne detection method for the coherent detection of laser signals, which superposes the optical signal with a reference beam from a coherent light source (local oscillator) in contrast to homodyne detection, the local oscillator is operated at a slightly different frequency than the optical signal heterodyne measurements allow for velocity measurements via Doppler effect and are employed, e.g., in electrophoretic light scattering. 

Phase Doppler anemometry or phase Doppler analysis was proposed two decades ago, based on the then well-established flow measurement technology-laser Doppler velocimetry [110]. PDA is widely used in sizing spherical and homogeneous particles, such as liquid sprays, aerosols, air bubbles in liquid, or other spherical particles. When single particles move through the intersection of at least two focused laser beams, they scatter light from each beam with different individual Doppler frequency shifts. Multiple detectors are placed at... 

The last equality exists if (0/2+ ) = ti/2 (i.e., K and u are parallel), which is often adopted in practical instrumentation, where the Doppler shift will be maximized. When = n 2, o)s-co = 2miusin0/X . If the mobile direction is changed, the frequency difference will have the same amplitude but an opposite sign. The frequency difference between the scattered light and frequency-preshifted reference beam (o)l= cao+tOps)now becomes ... 

The laser-Doppler anemometer measures local fluid velocity from the change in frequency of radiation, between a stationary source and a receiver, due to scattering by particles along the wave path. A laser is commonly used as the source of incident illumination. The measurements are essentially independent of local temperature and pressure. This technique can be used in many different flow systems with transparent fluids containing particles whose velocity is actually measured. For a brief review or the laser-Doppler technique see Goldstein, Appl. Mech. Rev., 27, 753-760 (1974). For additional details see Durst, MeUing, and Whitelaw, Principles and Practice of Laser-Doppler Anemometry, Academic, New York, 1976. 

Chapter 3 is devoted to pressure transformation of the unresolved isotropic Raman scattering spectrum which consists of a single Q-branch much narrower than other branches (shaded in Fig. 0.2(a)). Therefore rotational collapse of the Q-branch is accomplished much earlier than that of the IR spectrum as a whole (e.g. in the gas phase). Attention is concentrated on the isotropic Q-branch of N2, which is significantly narrowed before the broadening produced by weak vibrational dephasing becomes dominant. It is remarkable that isotropic Q-branch collapse is indifferent to orientational relaxation. It is affected solely by rotational energy relaxation. This is an exceptional case of pure frequency modulation similar to the Dicke effect in atomic spectroscopy [13]. The only difference is that the frequency in the Q-branch is quadratic in J whereas in the Doppler contour it is linear in translational velocity v. Consequently the rotational frequency modulation is not Gaussian but is still Markovian and therefore subject to the impact theory. The Keilson-... 

Frequency difference between scattered and incident light ... 

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