Frequency shift, laser Doppler

Laser Doppler Velocimeters. Laser Doppler flow meters have been developed to measure Hquid or gas velocities in both open and closed conduits. Velocity is measured by detecting the frequency shift in the light scattered by natural or added contaminant particles in the flow. Operation is conceptually analogous to the Doppler ultrasonic meters. Laser Doppler meters can be appHed to very low flows and have the advantage of sensing at a distance, without mechanical contact or interaction. The technique has greatest appHcation in open-flow studies such as the deterrnination of engine exhaust velocities and ship wake characteristics. 

An interesting modification of this technique is the fibre-optic dynamic anemometer (FODA)143. A length of fibre-optic cable carries the laser beam to the interior of the dispersion. Back-scattered light, with its Doppler frequency shift, is returned to the detector along with reflected light and, again, the resulting beat frequency pattern is analysed. Since only a very small volume around... 

The data recorded as the laser frequency is scanned consists of the fluorscence signal from the PMT, a Doppler-free I2 spectrum and frequency markers from the etalon. The etalon provides a calibration of the frequency scan. The Doppler-free I2 spectra provides an absolute frequency reference used to correct for small laser frequency drifts, separator voltage drifts and to determine the absolute acceleration voltage of the separator for the Doppler shift corrections. We are thus able to record data over long periods of time, e.g. 3 hours, and maintain a reasonable resolution of 100 MHz. Some of the first online data recorded with this system is shown in Figure 2. The overall detection efficiency has been measured to be 1/1000, i.e. one detected photon per 1000 atoms, for the largest transition in the nuclear spin 1/2 isotopes. 

When a spherical particle enters the crossing volume of two laser beams, a Doppler effect occurs not only in frequency shift but also in phase shift of the scattered light. The frequency shift yields the velocity of the sphere, whereas the phase shift gives the particle size. The phase Doppler principle has been employed to measure the size and size distributions of spheres in addition to the particle velocity. The phase Doppler principle was first reported by Durst and Zare (1975) and became a viable measurement tool one decade later [Bachalo and Houser, 1984]. 

Fig. 1. Set-up of the PTB laser system. The Nd YAG laser is frequency stabilized onto a selected iodine absorption line using the phase modulation method. The probe beam is modulated at 2.05 MHz by an electro-optic modulator (EOM), the pumb beam is frequency shifted by an acousto-optical modulator (AOM). The driving AOM rf power is chopped in order to cancel frequency offsets introduced by the Doppler background using a lock-in detection scheme. The transmitted probe beam signal is detected by a photodiode (PD) and mixed with the EOM rf in a double balanced mixer (DBM)...

The dominant uncertainties in the measurement were the dc Stark shift of the deuterium line in the discharge tube, the ac Stark shift of the Ps line, the second-order Doppler shift of the thermal Ps and the amplified laser frequency shift relative to the cw laser. Counting statistics show up in the measurement of the Ps - tellurium frequency shifts as one extrapolates to zero laser power and zero atomic velocity. 

Dynamic light scattering (DLS), also called photon correlation spectroscopy (PCS) or laser light scattering (LLS) is a technique based on the principle that moving objects cause a frequency shift due to the Doppler effect. If a solution of macromolecules with random Brownian motion is illuminated with monochromatic laser light, the scattered light should contain a distribution of frequencies about the incident frequency the spectral line is virtually broadened. The width of the distribution is related to the MMD. 

Since the velocities of the dissolved macromolecules are far less than the velocity of light, the Doppler shift will be extremely small and thus the frequency shifts are very small. The high intensity of the laser source makes it possible, however, to measure weakly scattered light and to observe very small frequency shifts. Like conventional light scattering, DLS can be used successfully to determine the molecular weight, size and shape of macromolecules. Moreover, DLS is such a powerful tool that also the diffusion coefficient can be measured, and that in a more elegant manner than many other classical tools (Sun, 1994,2004). 

Radiation which is scattered from a moving particle is shifted in frequency by the Doppler effect the magnitude of the Doppler shift allows an exact determination of the velocity of the object. Laser velocimetry in biology and medicine is reviewed by Ware 24). 

Figure 1.13 Velocity Modulation Spectra (VMS). An AC electric field applied along the laser propagation direction causes the velocity, hence the Doppler shift, of ionic but not neutral species to be modulated. Spectrum (a) shows 127 MHz = 0.0042 cm-1 Doppler shifts of a 2479.4113 cm-1 ArH+ transition. Also shown is an Ar line that does not exhibit an electric field dependent Doppler shift. This spectrum was actually recorded at a fixed (DC) electric field using counter-propagating (frequency modulated) laser beams (from Haese, et ai, 1983). Spectrum (b) shows a portion of the VMS Nj A2 Hu — X2E (7,3) band. Since the AC electric field causes the Nj line to be frequency modulated, phase sensitive detection at the modulation frequency (If) results in a first derivative lineshape. Spectra (b) and (c) were recorded simultaneously. The Nj lines are absent from spectrum (c), which was recorded in population modulation mode by phase sensitive detection at twice the modulation frequency (2f). Spectrum (c) contains a few lines belonging to a N2 B3I19 — A3eJ First Positive baud. The population of N2 A3Ej is modulated at 2f because each half cycle of the AC field results in a full cycle modulation of the population of electronically excited N2 molecules. (Spectra (b) and (c) axe from Radunsky and Saykally, 1987.)...

Laser Doppler anemometry A velocity measurement device using the principle of Doppler shifted frequency of laser Hght by particles moving with the fluid. 

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