Laser Doppler velocimeters are used for measurement of localized fluid velocities without perturbation of flow field. Technique which utilizes only two outgoing beams polarized normally to one another can be processed in such a manner that local oscillator signal is obtained and usual dual-scatter velocity is also retrieved

Orloff, K. L.

English

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-- fijiie 1973	 B73-10262 
NASA TECH BRIEF 
Ames Research Center	 4r 
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commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
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be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Laser Velocimeter with Transverse and On-Axis Sensitivity 
Laser Doppler velocimeters (LDV) are used for the 
measurement of localized fluid velocities without 
perturbation of the flow field. The basic principle of 
operation is that coherent light is scattered and 
shifted in frequency by particulate matter carried 
along in a moving field. Modem laser velocity instru-
ments employ a crossed-beam, dual-scatter system in 
which parallel incident beams are focused in the 
flowing stream through the same lens; light collected 
from the intersection volume at any angle contains a 
Doppler frequency resulting from the heterodyning 
of the scattered light from the two incident beams. 
Velocimeters of this type are only sensitive to the 
component of velocity perpendicular to the bisector 
of the angle between the two beams and in a plane 
defined by the two beams; the velocity component is 
obtained from measurement of the beam intersection 
angle and the heterodyne signal. In early instruments, 
called local oscillator velocimeters, the incident and 
scattered light were recombined at the photocathode 
of a photomultiplier tube to produce a beat signal 
linearly related to the average velocity of -the fluid 
within a small scattering volume. 
A dual-scatter system in the forward scatter, off-
axis configuration appears attractive from the view-
point of signal-to-noise and spatial resolution, but it 
is more convenient to use a confocal backscatter on-
axis velocimeter which utilizes a common lens for 
both the transmission of the outgoing beams and the 
collection of scattered light; moreover, with this con-
figuration, any velocity component in a plane normal 
to the transmitting axis can be measured. Unfortun-
ately, the "on-axis" component is difficult to obtain 
from a confocal backscatter system, but a technique
which utilizes only two outgoing beams polarized 
normally to one another can be processed in such a 
manner that a local oscillator signal is obtained (cor-
responding to the nearly on-axis velocity component), 
and the usual dual-scatter (transverse) velocity is also 
retrieved. 
When the two outgoing beams are polarized nor-
mally to each other, the usual dual-scatter fringes are 
not seen in the intersection region for observations 
made parallel to either of the incident polarizations. 
However, fringes of maximum contrast are observed 
at either +45 0 or —45° with respect to either of the 
incident-beam polarization directions; additionally, 
the fringes observed at these angles are displaced 
from each other by exactly one half of the fringe 
spacing, implying that Doppler signals generated by 
each of these sets of fringes differ in phase by 7r. This 
property has been independently applied in differ-
encing techniques to enhance the Doppler signal and 
subtract unwanted signals common to both detectors, 
such as the dc pedestal in a single-particle laser 
Doppler velocity (LDV) system and the amplitude 
fluctuations in a multiple-particle LDV arrangement. 
Although signal enhancement is potentially useful 
for LDV work, the primary utilization of the normal 
polarization technique described here is that the two 
outgoing beams are effectively distinguishable from 
each other, and observation of the scattered radiation 
at a polarization of either 0° or 900 will only allow 
light scattered from one beam or the other to be seen 
(since linear polarization is preserved in the back-
scatter geometry). Utilizing these two facts, an op-
tical system was designed which can simultaneously 
detect two independent velocity components, one 
(continued overleaf) 
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V2
very nearly along the LDV axis and the other trans-
verse to the axis. The dual-scatter signal can be 
retrieved by observing at either +45 0 or 450, con-
veniently obtained through use of a Wollastrom 
prism. (The differencing technique is optional, and 
may or may not be used as desired.) 
VI	 V 
Collected 
Scattered 
Radiation 
0 VA 
A local oscillator arrangement sensitive to the on-
axis velocity component and utilizing only one of the 
outgoing beams is quite easily incorporated. A refer-
ence beam is obtained as a spurious reflection from 
the splitter cube and aligned with the returning scat-
tered radiation. Only scattered light from the ver-
tically polarized outgoing beam heterodynes with the 
(vertically polarized) local oscillator beam. A small 
fraction of the collected backscattered light (0.20 cm2 
out of 25 cm2
 on the receiving lens) is all that is 
needed to obtain a very strong local oscillator Dop-
pler signal. Utilization of such a small fraction of the 
scattered light for the on-axis component has the 
advantages that most of the scattered radiation still 
contributes to the dual-scatter signal, and the aperture 
broadening of the local oscillator system is minimal. 
In particular, it is only that portion of scattered radi-
ation which passes through the transparent glass slide 
and is parallel with the local oscillator beam at the 
focus of the collecting lens which gives rise to a 
heterodyne signal. 
The two measured velocity components are shown 
in the diagram. They are not exactly orthogonal, but 
are separated by (r/2 + e/4). Since e is typically 
quite small, usually a few degrees, 0/4 is much
smaller than r/2 and the two components are orthog-
onal for all practical purposes. However, the measure-
ment directions are well defined, and for two-dimen-
sional flow, the exactly on-axis component can be 
computed from 
vjai = v2 sec 0/2 + v tan 0/4 
where the independent velocity components vi and V2 
and computed from the measured Doppler fre-
quencies:
(2nvi/Ao) sin 0/2 
1)2 = (2nv2/Ao) cos 0/4. 
Frequency shifting the vertically polarized beam 
with a single Bragg cell will give both components 
directional sensitivity, a vital feature if the velocity 
vector direction is not known beforehand. Frequency 
shifting one of the laser beams (by amount Vm) in 
either a dual-scatter or a reference (or local oscillator) 
system will bias the Doppler signal, VD, so that it is 
added to, or subtracted from 1)m, depending upon 
flow direction. Thus, with one Bragg cell in the 
system (Vm = 37.20 MHz), the spectrum analyzer will 
display Vm ±Vi and 1)m ±V2 from the two tubes, re-
spectively, depending on the absolute directions of 
the velocity components vi and v2. 
Note: 
Requests for further information may be directed 
to:
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94035 
Reference: TSP 73-10262 
Patent status: 
Inquiries concerning rights for the commercial use 
of this invention should be addressed to: 
NASA Patent Counsel 
Mail Code 200-1A 
Ames Research Center 
Moffett Field, California 94035 
Source: Keneth L. Orloff
Ames Research Center
(ARC-10642) 
. 
S 
. 
B73-10262	 Category 03


Laser velocimeter with transverse and on-axis sensitivity

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