Fluid velocity is measured by focusing laser beam on moving fluid and measuring Doppler shift in frequency which results when radiation is scattered by particles either originally present or deliberately injected into moving fluid

Bourquin, K. R.

Shigemoto, F. H.

English

NASA Technical Reports Server

May 1971	 Brief 71-10120 
NASA TECH BRIEF 
Ames Research Center
	 W 
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Laser Doppler Instrument Measures Fluid Velocity Without Reference Beam 
The problem 
Measure the velocity of a fluid without inserting 
sensing devices into the fluid stream or utilizing 
a reference energy source. 
Plane Mirror
Fluid In 
Laser 
Photodetector	 I 
Beam Splitter	 Fluid Out 
Frequency 
Analyzer 
The solution: 
Focus a laser beam on the moving fluid and 
measure the Doppler shift in frequency which re-
sults when the radiation is scattered by particles 
either originally present in or deliberately injected 
into the moving fluid. 
I-low it's done: 
A laser beam is focused on a small area of the 
scattering medium. The light reflected from this 
area is viewed along two optical paths, and the 
field of view of each path is limited so that only 
the small area is imaged on a photodetector.
As shown in the figure, the laser beam is focused 
on the moving fluid. Lenses are symmetrically posi-
tioned about the laser beam in order to view the 
backscattered light from different angles. The scat-
tered beam passing through one lens is directed 
onto a beam splitter which transmits a portion of 
the light to a photodetector. The beam through 
the second lens is reflected by a plane mirror 
onto the same beam splitter, which also reflects a 
portion of that beam to the photodetector. The 
Doppler-shifted optical beams are mixed at the photo-
detector to give an output which includes a periodic 
signal at the Doppler difference frequency of the 
beams. This difference frequency, which is detected 
by the frequency analyzer, is related to the velocity 
of the fluid stream in the chamber. 
Notes: 
1. The instrument may be applicable to measure-
ments of air velocity in wind tunnels, relative 
air-speed of aircraft, cloud velocity, and liquid 
flow rate. 
2. Related instruments are described in NASA Tech 
Briefs 66-10693 and 68-10349. 
3. The following documentation may be obtained 
from: 
National Technical Information Service 
Springfield, Virginia 22151 
Single document price $3.00 
(or microfiche $0.95) 
Reference: 
NASA-TN-D-4453 (N68-20375), Investiga-
tion of Air-Flow Velocity by Laser Back-
scatter
(continued overleaf) 
Ah
This document was prepared under the sponsorship of the National 
Aeronautics and Space Administration. Neither the United States 
Government nor any person acting on behalf of the United States
Government assumes any liability resulting from the use of the 
information contained in this document, or warrants that such use 
will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19710000119 2020-03-17T02:23:01+00:00Z
Patent status: 
This invention has been patented by NASA (U.S. 
Patent No. 3,547,540, issued December 15, 1970), 
and royalty-free license rights will be granted for 
its commercial development. Inquiries about obtain-
ing a license should be addressed to: 
Patent Counsel 
Mail Code 200-11 A 
Ames Research Center 
Moffett Field, California 94035 
Source: Fred H. Shigemoto and
Kent R. Bourquin
Ames Research Center
(XAC- 10770) 
Brief 71-10120	 Category 03,02


Laser Doppler instrument measures fluid velocity without reference beam

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