Multipass holographic interferometer forms a hologram of high diffraction efficiency, and hence provides a bright and high contrast interferogram. It is used to study any effect which changes the index of refraction and to study surface deformations of a flat reflecting surface

Brooks, R. E.

Heflinger, L. O.

English

NASA Technical Reports Server

November 1970
	
Brief 70-10426 
NASA TECH BRIEF H 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage commercial 
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nology Utilization Division, NASA, Code UT, Washington, D.C. 20546. 
Multipass Holographic Interferometer Improves Image Resolution 
Beam Splitter	 Hologram Roll-Film Camera Back 
Aperture Plate
Collimating Lens 
Subject Cavity 
Laser 
Beam 
Input
Partial 
Mirror 
eference Arm - Adjustable Length 	 End Mirror 
An interferometer which measures small phase 
differences of light is a valuable instrument used 
in the study of physical phenomena occurring in gases, 
such as shock waves and combustion fronts of ignited 
mixtures. The ability of the instrument to measure ex-
tremely small phase differences, such as those as-
sociated with low pressure phenomena, is greatly in-
creased by passing the light thro.ugh the gas a number 
of times. The cumulative phase changes caused by the 
phenomena build up with the number of passes. 
The effectiveness of conventional multipass tech-
niques is highly dependent on the quality of the optical 
elements used. Holography is well suited to multipass 
interferometry since it is essentially independent of the 
quality of the optics. A novel multipass holographic 
interferometer, shown in the figure, has been used with 
10 passes and, represents a significant improvement 
over conventional techniques. 
To achieve multipass operation, the subject is placed 
between two plane mirrors (forming a reflecting 
cavity) and the light is introduced at right angles to the 
mirror so that it is repeatedly reflected through the 
subject. If one of the mirrors is partially transparent,
a portion of the light will be transmitted through 
it with each round trip in the cavity. In order to sepa-
rate the beams corresponding to different numbers of 
passes, a slight angle exists between the two mirrors so 
that each beam will emerge at a slightly different angle. 
By focusing the emerging beam and placing a narrow 
slit in the focal plane, the desired beam can be selected 
and those not desired rejected. 
In order to form a hologram of high diffraction 
efficiency, and hence to provide a bright and high con-
trast interferogram, the reference beam is attenuated 
so that it matches the intensity of the desired subject 
beam. Image resolution is degraded by three effects 
which tend to cause a smearing of each point of the 
subject. One is "beam walking," which is due to the 
slight angle between the mirrors of the subject cavity. 
A second is multiple imaging due to the successive 
reflection of the subject in the cavity. A third effect 
is the (one-dimensional) low-pass spatial filtering 
associated with the angle-selecting slit. Through means 
of analysis and experimentation, criteria have been 
developed for selecting system parameters in the in-
strument that will minimize the total image smear. 
(continued overleaf) 
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Alternate means for selecting the subject beam in-
volve a time or coherence gating of the light. These 
can be used independently or in combination with the 
angle-selecting aperture and promise greatly increased 
numbers of subject passes without serious loss in sub-
ject resolution. Time gating is accomplished with the 
use of electro-optic shutters or an electronically gated 
image tube instead of the standard hologram recording 
film. Coherence gating uses a laser source with a coher-
ence length less than twice the subject cavity separation 
so that only one subject beam is coherent with the refer-
ence beam and, therefore, useful in forming the holo-
gram. 
Note: 
The multipass holographic interferometer can be 
used to study any effect which changes the index of
refraction (including water studies). It can also be used 
to study the surface deformations of a flat reflecting 
surface which constitutes the end mirror of the subject 
cavity. 
Patent status: 
Title to this invention has been waived under the

provisions of the National Aeronautics and Space

Act [43 U.S.C. 2457 ( f)], to the TRW Systems Group,

One Space Park, Redondo Beach, California 90278.

Source: L. 0. Heflinger and R. E. Brooks of

TRW Systems Group

under contract to

NASA Headquarters 
(HQN- 10499) 
Brief 70-10426	 Category 03


Multipass holographic interferometer improves image resolution

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