Utilization of optical enhancement techniques prevents loss of light by reflections at the photocathode of a photomultiplier and increases signal detection sensitivity

Grant, G. R.

Gunter, W. D., Jr.

Shaw, S. A.

English

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Splitter 
Cube	 Lens 
September 1972	 B72-10310 
NASA TECH BRIEF 
Ames Research Center 	 -4r 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Optical Enhancement of Sensitivity in 
Laser Doppler Velocity Systems 
The problem: 
Laser Doppler velocity (LDV) systems are used to 
determine flow velocities in gas streams; the systems 
permit measurement of the Doppler shift which oc-
curs when radiation is scattered by moving particles. 
Performance of LDV systems is often marginal be-
cause signals are weak when the amount of dust or 
other particulate matter in the gas stream is very 
small. Particles can be added to the stream, but often 
this is not practical. Alternatively, the signal at the 
detector can be increased by using a more powerful 
laser, but this is undesirable in terms of cost, and 
some systems already use the largest lasers. Thus, it 
was necessary to develop methods which would in-
crease the sensitivity of detection of signals. 
The solution: 
Use optical enhancement techniques to prevent 
loss of light by reflections at the photocathode of a 
photomultiplier. 
How it's done: 
A prism is used to introduce light into the end 
window of the photomultiplier tube at a sufficiently 
oblique angle so that it is trapped in the window by 
total reflection; thus, every time light reaches either 
the air-glass or the vacuum-photocathode interface, 
it is reflected back into the window. After a few re-
flections, nearly all incident light will have been ab-
sorbed by the photocathode; thus, light which would 
have been lost is conserved by the optical enhance-
ment technique. As a result, sensitivity is increased by 
about a factor of two for blue light and about a factor
of five for red light. However, since optical hetero-
dyne systems require precise alignment in order to 
function, it might appear that the technique of optical 
enhancement of photomultipliers can not be applied 
to LDV systems; in fact, the optical enhancement 
method described above can not be used with some 
detection geometries. 
The class of LDV systems that is often called self-
aligned, self-focused, or two-beam, projects two beams 
instead of one into the gas to be measured. In these 
systems, a single parallel beam from a laser is split 
into two parallel beams separated by a small distance; 
the beams strike a lens and are brought to a common 
focus at a point in the focal plane of the lens. All of 
the interference occjirs at the common point of focus, 
and light going out in any direction from that point 
consists of rays scattered from both beams, coincident 
(continued overleaf) 
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and colinear. The light perceived at an observation 
point simply turns on and off at a heterodyne fre-
quency of the order of a few kilohertz or megahertz. 
Moreover, because the heterodyne frequency is the 
difference between the Doppler shifts of the two 
scattered beams, it is the same frequency for light 
scattered in any direction. The diagram indicates 
how optical enhancement can be used with a self-
focusing LDV system. The many arrows leaving the 
focal point imply that light scattered in any direction 
can be collected and used to generate a signal. 
In experiments with argon and helium-neon lasers, 
optical enhancement provided sensitivities two to 
four times greater than could be obtained in the same 
LDV systems without optical enhancement of the 
photomultiplier sensitivity. 
References: 
1. Gunter, W. D., Jr.; Grant, G. R.; and Shaw, S. A.: 
Optical Devices to Increase Photocathode Quan-
tum Efficiency. Applied Optics, vol. 9, p. 251, 1970. 
2. Gunter, W. D., Jr., and Grant, C. R.: Optical En-
hancement of Photomultiplier Sensitivity, NASA 
Tech Brief 71-10113. 
Note: 
No additional documentation is available. Specific 
questions, however, may be directed to: 
Technology Utilization Officer 
Ames Research Center 
Moffett Field, California 94035 
Reference: B72-10310 
Patent status: 
No patent action is contemplated by NASA. 
Source: George R. Grant, William D. Gunter, Jr.,

and Spencer A. Shaw

Ames Research Center

(ARC-10653) 
B72-10310	 Category 03


Optical enhancement of sensitivity in laser Doppler velocity systems

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