# Laser doppler

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Laser Doppler Velocimetry
Christin Kalkert
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Jona Kayser
noa-x@gmx.de

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ckalkert@web.de

PHYS 173 Biophysics Laboratory, Prof. David Kleinfeld University of California, SanDiego

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Contents
1 2 Introduction Laser Doppler Velocimetry 3 3

2.1 Theory . . . . . . . . . . . . 2.2 The Experiment . . . . . . 2.2.1 Experimental Setup 2.2.2 Data Processing . .
Results. . . .

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3.1 Small Flow Tube . . . . . . . 3.2 Big Flow Tube . . . . . . . .
Discussion Methods

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1 Introduction
In many physical experiments it isnecessary to measure ow velocities in the experiment without disturbing the natural ow patterns. A very useful method to accomplish this task is the laser Doppler velocimetry (LDV). LDV is used in thestudy of various ows like atmospheric turbulences and ows in internal combustion engines. Its noninvasive nature makes it especially interesting for medical and biophysical applications where it isof great interest to measure the ow of blood in localized areas. The motivation of this project was to set up a laser Doppler velocimeter and study the characteristics of dierent ow patterns toevaluate the potential of this method.

Figure 1: Dual-beam conguration scattered o the particles. The two beams are then heterodyned on the detector to obtain an optical beating frequency in a rangewhich can be measured. Using the formula above and some geometrical arguments (see Figure 1) we obtain
fd = f2 − f1 =
θ 2 · sin( 2 ) · |vpt | λb

(2)

2 Laser Doppler Velocimetry
2.1 TheoryThe experiment uses the Doppler eect to calculate the velocity of particles in uids. Light scattered on moving particles experiences a shift in frequencies according to
fr = fb 1− 1−
e b · vp cepr · vp c

≈ fb +

vp · (epr − eb ) (1) λb

where fb and fr are the frequencies of the beam and the light at the receiver. vp is the velocity of the particle and eb is the unit vector in beam...