Instrument system uses laser and optical heterodyne receiver to measure Doppler phase shifts produced on a laser beam by the deflection of the vibrating surface of a structure under test or in operation

Foster, J. V.

English

NASA Technical Reports Server

July 1971	 Brief 71-10249 
S NASA TECH BRIEF 
Ames Research Center 
_
.1 Eli 
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 aubscription 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.F. 20546. 
Laser Vibration Analyzer 
The problem: 
To measure the vibration of a structure under 
test or in operation without attaching vibration 
sensors, such as strain gages or inertial transducers. 
The solution: 
An instrument system which uses a laser and an 
optical heterodyne receiver to measure the Doppler 
phase shifts produced on a laser beam by the cie-
flection of the vibrating surface. 
How it's done: 
The beam from the helium-neon laser is directed 
into a Bragg cell. One part passes directl y to the 
vibrating surface; the diffracted portion passes to 
Laser	 Braug 
ciT 
25 MHz 
Generator
bined with the reference beam to produce inter-
ference with patterns which are sensed by the 
photodetector. 
If the reflecting surface remains stationary, the 
interference patterns at the photodetector vary si-
nusoidálly at the difference frequency, which is just 
Frequency-Shifted 
"Local Oscillator" Beam 
Lens Oi oUnshiftedPartial	 Signal Beam 
Mirror	 Vibrating 
- Surface 
Impedance 
	
Photodetector I- '-J Matching	 Spectrum 
	
Network	 Analyzer 
Block Diagram of Vibration Measurement System 
a beamsplitter mirror and directly into the photo-
	 equal to the shift or system intermediate frequency 
detector to serve as the reference or local oscillator
	 (IF). If the surface moves, the phase or frequency of 
beam. The frequency of the diffracted laser beam is
	 the interference changes, producing frequency modula-
shifted in the Bragg cell by plane-traveling acoustic
	 tion of the system IF. The shift provides a fre-
waves in water. The amount of the shift is large
	 quency bias so that motions toward or away from 
compared to the Doppler shifts in the light reflected
	 the instrument produce distinctive Doppler modula-
from the vibrating surface. The reflected light from
	 tions which define the direction of the motion as 
the vibrating surface returns through the instrument
	 well as its magnitude. The process signal from the 
optical system to the beamsplitter, where it is corn-
	 detector is displayed visually on a spectrum analyzer. 
(continued overleaf) 
This document was prepared under the sponsorship of the National 	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States 	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States 	 will be free from privately owned.rights.
https://ntrs.nasa.gov/search.jsp?R=19710000248 2020-03-17T02:19:58+00:00Z
Notes: 
1. Vibratidn frequencies from 1 to 0.5 MHz have 
been measured, with displacements from 1.27 cm 
peak-to-peak down to about iO 4 microns. 
2. Related material can be found in Tech Briefs 
B70-10439 and B71-10145. 
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-CR-985 (N68-14070), Study of Vibra-
tion Measurement by Laser Methods
Patent status: 
This is the invention of a NASA employee, and 
U.S. Patent No. 3,355,934 has been issued to him. 
Inquiries about obtaining license rights for its com-
mercial development should be addressed to the 
inventor, Mr. John V. Foster, at the NASA - Ames 
Research Center.
Source: J. V. Foster

Ames Research Center

(XAC-01670) 
Brief 71-10249	 Category 03


Laser vibration analyzer

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