Electro-optical system for measuring very small Doppler shift displacements of spectral wavelengths emitted by atoms and ions in high velocity plasma beam is discussed. Operation of circuits is described and circuit diagram is included

Aisenberg, S.

English

NASA Technical Reports Server

August 1972	 B72-10495 
•	 NASA TECH BRIEF 
NASA Headquarters
	
-4r 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
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be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Interferometric Measurement of the Velocity of Radiating Particles 
An electro-optical system measures very small Doppler-
shift displacement of the spectral wavelengths emitted 
by atoms and ions in a high-velocity plasma beam. The 
system (see fiire) is capable of detecting shifts as 
small as 0.002A in the wavelengths of visible spectral 
lines (4000-7000A). This corresponds to detecting a 1% 
change (about 104
 cm/sec) in the velocity of a light-
emitting beam that characteristically moves with a velo-
city of about 106 cm/sec.	 - 
The system is essentially a servo-controlled Fabry- 
Perot interferometer. Three fiber-optic bundles (light 
pipes) are used to collect light emitted from the plasma 
beam in different directions. One pipe is aimed in a 
direction perpendicular to the plasma velocity, so that 
the collected light bears no Doppler shift. The other two 
pipes are inclined ahead and behind the perpendicular, 
so that the collected light bears Doppler shifts toward 
shorter and longer wavelengths, respectively. The light 
from one pipe at a time is fed through a collimator to a 
narrow-band optical filter, which is selected to pass one 
particular wavelength of the spectrum of the atomic or 
ionic species under study. 
The output of the filter then goes to the Fabry-Perot 
interferometer, which is composed of two curved partial 
reflectors operated in the confocal mode. The inter-
ferometer produces a transmission pattern of concentric 
rings which is very strongly wavelength dependent. The 
central ring of the pattern is imaged through a pinhold 
onto a very sensitive photomultiplier, which is refrig-
erated to minimize dark current and Johnson noise. The 
photomultiplier converts any change in the intensity of 
the central ring into a corresponding change in an 
electrical signal.
Two factors affect the intensity. One is the wave-
length shift which is to be measured; the other is an 
overall change in the light intensity emitted by the 
plasma. To reduce this error-causing sensitivity to input 
signal amplitude, a sophisticated modulation and phase-
detection system is used. A signal from a modulation 
oscillator is used to sweep the interferometer adjustment 
periodically across the desired wavelength band. The 
fluctuating output of the photomultiplier is then fed to 
a phase detector, together with a reference signal from 
the modulation oscillator. The detector operates in the 
quadrature mode, producing a null output when the 
center of the Fabry-Perot resonance corresponds to the 
center of the spectral line. Deviations of the line toward 
shorter or longer wavelengths produce opposite-polarity 
error signals from the detector. These signals provide the 
system output and are coupled through a feedback loop. 
When the interferometer is operated in the locked mode, 
the feedback signal is added to the modulation signal 
and applied to the electrically movable (either magnetic 
or piezoelectric drive) mirror. The feedback readjusts 
the central position of the mirror to reduce the error 
voltage. 
To align and calibrate the interferometer, it is con-
venient to use a laser beam. This is applied at the input 
by introducing the removable mirror shown. For cali-
bration purposes, the system is operated in the unlocked 
sweep mode. A low-frequency linear sawtooth from a 
sweep generator causes the interferometer to scan slowly 
across the desired wavelength band. The output signal 
voltage is then recorded as a function of wavelength. 
This technique may also be used to measure the width 
and shape of an observed spectral line. The output signal 
.
(continued overleaf) 
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	 information contained in this document, or warrants that such use 
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https://ntrs.nasa.gov/search.jsp?R=19720000494 2020-03-17T03:40:08+00:00Z
I	
Calibration And 
Alignment 
Light Source 
Removable 
Mirror 
Plasma  
Beam	
Collimator 
Light  
Pipes	 +15V	 Narrow Band
Optical Filter 
Zero 
Adjust
 
___________• f —15V 
Modulation 
Oscillator
Locked 
Mode 
Sweep 0— 
Generator Sweep 
Mode
Reference
Electrically Movable 
Interferometer Mirror
Refrigerated
	
Fabry-Perot	
)	
{ Photomultiplier 
	
Interferometer 	
Pinhole 
Y1
Photomultiplier 
Power Supply 
Amplifier 
Unlocked 
o Mode 
Locked 
Mode
Feedback  
•_j
Loop Gain 
Control 
Feedback 
Amplifier Output 
Signal 
Input Signal 
Phase Detector 
voltage directly corresponds to the derivative of the line Patent status: 
shape with respect to wavelength. Numerical or analog No patent action is contemplated by NASA. 
integration yields the actual line shape.
Source: S. Aisenberg of 
Note: Space Sciences, Inc. 
No additional documentation is available. Specific under contract to 
questions, however, may be directed to: NASA Headquarters 
Technology Utilization Officer (HQN-10371) 
NASA Headquarters 
Code KT 
Washington, D.C. 20546 
Reference: B72-10495
B72-10495	 Category 03, 02 


Interferometric measurement of the velocity of radiating particles

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