Narrow band optical filter is electronically tunable over a large wavelength region. The filter utilizes collinear acousto-optic diffraction in an optically anisotropic media

Harris, S. E.

English

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Acoustic Input 
Optical Output 
Output 
Polarizer 
September 1969	 Brief 69-10466 
NASA TECH BRIEF 
NASA Tech Briefs are issued to summarize specific innovations derived from the U.S. space program, 
to encourage their commercial application. Copies are available to the public at 15 cents each 
from the Clearinghouse for Federal Scientific and Technical Information, Springfield, Virginia 22151. 
Proposed Acousto-Optic Filter 
Longitudinal To Shear 
Wave Conversion
Optical input
Input 
Polarizer 
V	 Longitudinal Wave (L) 
Shear Wave (S) 
FIGURE 1. LiNb0 3
 ACOUSTO-OPTIC FILTER. 
Recent investigations suggest the possibility ,
 of a 
narrow band optical filter that is electronically tun-
able over a large wavelength region. This conceptual 
item should be of interest to the optical industry. 
Basically, the filter would utilize collinear acousto-
optic diffraction in an optically anisotropic media. 
The band of optical frequencies through which the 
filter will pass may be changed by changing the 
driving acoustic frequency. It is shown that a LiNb03 
acousto-optic filter with a pass band approximately 
1.3 cm wide which is tunable from 4000 A to 7000A 
should be possible by changing the acoustic frequency 
from 428 MHz to 990 MHz. For this case, the angular 
aperture will be about 1.50, and it should be possible 
to attain 100% (theoretical) transmission at the filter
center frequency at an expenditure of about 14 mw of 
acoustic power per mm 2
 of filter aperture. 
The proposed acousto-optic filter consists of an 
input polarizer, a crystal with an appropriate trans-
ducer, and an output polarizer. There exist a number 
of different crystal orientations, involving either 
longitudinal or shear waves which allow collinear 
diffraction of light into the orthogonal polarization. 
One possible configuration for the filter using LiNb03 
is shown in the figure. In this case the acoustic wave 
is brought in as a longitudinal wave which is then 
converted to a shear wave upon reflection at the input 
face of the crystal. The acoustic shear wave and the 
input optical beam then propagate collinearly down 
the y-axis of the crystal, along which the acousto-
(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 Stes
	 will be free from privately owned rights.
https://ntrs.nasa.gov/search.jsp?R=19690000454 2020-03-16T18:04:07+00:00Z
optic interaction takes place. Interaction between the 
acoustic and optical waves takes place as a result 
of the photoelastic effect. 
Notes: 
1. Existing filters of the Lyot type are available with 
such narrow bandwidths and wave length tuning 
capabilities. 
2. Probably the most severe limitation of the pro-
posed filter is the difficulty of obtaining large 
apertures. Since 14 mw pf propagating acoustic 
power is required per mm2 of crystal aperture, a 
1 cm square aperture would require an acoustic 
power of 1.4 watts. Broadband r-f to acoustic 
transducers can now be constructed with about 
10 dB conversion loss, thus requiring an r-f 
power of 14 watts. Also, at frequencies in the 400 
to 1000 MHz range, the construction of transducers 
having one cm2 of area is somewhat ahead of the 
present state of the art. 
3. Two other materials which may be used for this 
type of filter are sapphire and quartz. The bire-
fringence of both of these materials is about 1/10 
that of LiNb03. As a result the necessary acoustic 
frequencies would be centered about 70 MHz in-
stead of 700 MHz as in the LiNb0 3
 filter. Both the 
tuning rate and also the bandwidth of these filters 
(for the same crystal length) would be about ten
times as large as that of the LiNb0 3 filter. The 
angular aperture would be about three times as 
large as that of a LiNb0 3
 filter of the same length. 
As a result of the low refractive indices of these 
crystals, about ten to twenty times as much acoustic 
power would probably be required to obtain the 
theoretical 100% peak transmission. However, this 
might be off-set by using longer crystals. 
4. This development is in a conceptual stage only; 
at the time of this publication no model or proto-
type exists. Technical inquiries may be directed to: 
Technology Utilization Officer 
Headquarters 
National Aeronautics 
and Space Administration 
Washington, D.C. 20546 
Reference: B69-10466 
Patent status: 
Title to this invention has been waived under the 
provisions of the National Aeronautics and Space Act 
[42 U.S.C. 2457 (f)], to the Stanford University, Stan-
ford, California 94305.
Source: S. E. Harris of
Stanford University
under contract to
Headquarters
(HQN-10440) 
Brief 69-10466	 Category 02


Proposed acousto-optic filter

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