Doppler shift monitoring

Elastic scattering is also the basis for Hdar, in which a laser pulse is propagated into a telescope s field of view, and the return signal is collected for detection and in some cases spectral analysis (14,196). The azimuth and elevation of the scatterers (from the orientation of the telescope), their column density (from the intensity), range (from the temporal delay), and velocity (from Doppler shifts) can be deterrnined. Such accurate, rapid three-dimensional spatial information about target species is useful in monitoring air mass movements and plume transport, and for tracking aerosols and pollutants (197). 

Most optical cooling experiments have been performed up to now on alkali atoms, such as Na or Rb, using a single-mode cw dye laser. The velocity decrease of the atoms is monitored with the tunable probe laser L2, which is sufficiently weak that it does not affect the velocity distribution. The probe-laser-induced fluorescence /fi(< 2) is measured as a function of the Doppler shift. Experiments have shown that the atoms could be completely stopped and their velocity could even be reversed [14.16]. An example of the compression of the thermal velocity distribution into a narrow range around V = 200 m/s is illustrated in Fig. 14.8 for Na atoms. 

There are two types of size characterization equipment based on laser Doppler shift measurement. In the first technique, the fineparticles under study move through a region illuminated by a laser beam. The Doppler shifts generated by the moving fineparticles are evaluated and related to the velocity of the fineparticles under study. The method used to study fineparticles less than 1 micron in size utilizes the Doppler shift created by Brownian motion as monitored with a laser beam [1]. This second type of technology is variously known as photoncorrelation spectroscopy (PCS) or dynamic light scattering (DLS). 

Sound waves are emitted from the probe (P) at a frequency F0. They are reflected off moving red blood cells and back towards the probe at a new frequency, FR. The phase shift can now be determined by FR-F0. The angle of incidence (9) is shown on the diagram. If a measurement or estimate of the cross-sectional area of the blood vessel is known, flow can be derived as area multiplied by velocity (m2.m.s 1 = m3.s 1). This is the principle behind oesophageal Doppler cardiac output monitoring. 

Doppler ultrasonic flowmeters depend upon the reflection of a continuous ultrasonic wave (frequency 0.5-10 MHz) from particulate matter (scatterers) contained in the fluid. Hence they may be used to monitor the rate of flow of dirty liquids. The transducer involved can act both as transmitter and receiver and is generally of the clamp-on type (Fig. 6.4). If the scatterers can be assumed to be moving at the velocity of the liquid, then the volumetric rate of flow Q is related to the Doppler frequency shift AtoD by ... 

Figure 3-9. Sub-Doppler resolution OH LIF spectra indicate a modest amount of c.m. translational energy for all of the OH levels monitored in the reaction H + N20 -> OH + N2 see Bohmer et al. (1992) for details. A Franck-Condon projection of the 1,3-hydrogen shift transition state N-N separation of 1.23 A (solid line, lower part) predicts a high degree of N2 vibrational excitation (upper part) and yields good agreement with the data (shaded circles, lower part).

The Doppler technique measures the frequency shift of scattered waves with respect to incident sound waves. The technique, therefore, requires the presence of scatterers in the flow that is being monitored. The scatterers could be turbulent eddies or vortex shedding for liquid single-phase flows, and solid particles for solid/fluid mixed-phase flows. The basic geometry of a Doppler... 

Fig. 2.36 Measurement of the isotope shift of the stable lead isotopes measured with Doppler-free two-photon spectroscopy at Aexc = 450 nm and monitored via fluorescence [250]...

Instead of a gated switch in the fluorescence detector for pulsed excitation, one may also use excitation by a cw laser that is phase-modulated with a modulation amplitude of tt (Fig. 9.81). The fluorescence generated under sub-Doppler excitation in a collimated atomic beam is observed during a short time interval A, which is shifted by the variable delay T against the time to of the phase jump. If the fluorescence intensity T) is monitored as a function of the laser frequency cu,... 

The narrow spectral line of a DL enables isotope selective analysis. For light and heavy elements (such as Li and U) the isotope shifts in spectral lines are often larger than the Doppler widths of the lines, in this case isotopically selective measurements are possible using simple Doppler-limited spectroseopy - DLAAS or laser induced fluorescence (LIF). For example, and ratios have been measured by Doppler-limited optogalvanic. spectroscopy in a hollow cathode discharge. DLAAS and LIF techniques have been combined with laser ablation for the selective detection of uranium isotopes in solid samples. This approach can be fruitful for development of a compact analytical instrument for rapid monitoring of nuclear wastes. 

Modified Laser-Doppler Anemometry is another alternative method to monitor liquid wicking in a 2D fabric plane based on the Doppler principle. When a laser beam is passed through a flowing liquid, light is scattered by the particles suspended in the liquid. The scattered light is subject to a frequency shift and contains information about the velocity of the particles, which can then be examined by electro-optical techniques. This measurement, therefore, requires the flow medium to be partly transparent and containing particles, that scatter light used this method, to obtain the local velocity of liquid flow in a nonwoven fabric. 

---