Intensity distributions in nonoptical wave fields can be visualized and stored on photosensitive material. In the case of microwaves, temperature effects can be utilized with the aid of liquid crystals to visualize intensity distributions. Particular advantages for the study of intensity distributions in microwave fields presents a scanning procedure in which a microcomputer is employed for the control of a probe and the storage of the measured data. The present investigation is concerned with the employment of such a scanning procedure for the recording and the reproduction of microwave holograms. The scanning procedure makes use of an approach discussed by Farhat, et al. (1973). An eight-bit microprocessor with 64 kBytes of RAM is employed together with a diskette storage system

Hetsch, J.

English

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rt)'
NASA TECHNICAL MEMORANDUM
	
NASA TM-77349
RECORDING AND REPRODUCTION OF MICROWAVE HOLOGRAMS
USING A SCANNING PROCEDURE AND THEIR SUBSEQUENT
OPTICAL PROCESSING
"^!+! J. Hetsch
M
(NASA-TH-77349) RECORDING AND REPRODUCTION
	 M84-15519
OF B ICRONAVE HOLOGRAMS USING A SCANNING
PROCEDURE AND THEIR SUBSEQUENT OPTICAL
PROCESSING (National Aeronautics and Space
	 Unclas
Adainistration) 12 p HC A021mF A01 CSCL 14B G3/35 11421
Translation of "Aufnahme and Wiedergabe von Mikrowellenhologrammen
nach dem Abtastverfahren and deren opt?-sche Weiterverarbeitung,"
(International Union of Radio Science and Nachrichtentechnische
Gesellschaft, Gemeinsame Tagung, Kleinheubach, West Germany,
Oct. 4-8, 1982, Kleinheubacher Berichte, vol. 26, 1983, p. 51-57.
J
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHINGTON, DC 2046	 OCTOBER X983
ORIGINAL- PAGE 0
...o%^n M I AI ITY ITAYSAOS ?ttt r oAo-- e
L Rohn N..	 2. Gsnate.ent Accsssisn Ne. a Recipt"1607
 Gtoles No.
S	 TM-
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RECORDING AND REPRODUCTION OF MICROWAVE• L RVc}
^.. q.r^
-)ber 1983
HOLOGRAMS USING A SCANNING PROCEDURE AND Paye.l" argenisetisn CedeTHEIR SUBSEQUENT OPTICAL PROCESSING -
7. Audbot(a
	
- L Petlssotllg 0►"tlseuse Rsgsvt me.
J. Hetsch.	 .-
IL V%A Utdt Ne.•
9. Psrfane 9 Oraewiset.on Noise end Addass 11. Centred er G	 1
NASw
Leo Kanner Assoc. Redwood City, CA
13. Type of now# end Period Covered•
Translation
12. ^po ►sering Agency None and Address'
NATIONAL AERONAUTICS AND SPACE ADMINISTRA f
 ION 11. Sreessrin9 Agency Cods
t;ASHINGTON_ D.C.
	 20546
15.	 tom at	 Notes
a^rls^111"a of Aefnabme tmd Wiedergabe von Mikrowellenhologrammen
nach dem Abtastverfa•hren uizd deren optische Weiterverarlieitung(International Union crf Radio Science and Nachrichtentechnische
GesellsChaft, Gemeinsame Tagung, Kleinheubach, West Germany,
Oct. 4-8,	 1982). Kleinhet^ bncher Berichte, Vol. 26, 1983, pp. 51 - 57.
(A83-23379)
16. Abstreit
	 .
•	 A microcomputer ^)ntrolled measurement set-up is
introduced that allows the recording of intensity distributions
in microwave fields-and their.visual reproduction by quasi-
rotational.ly symmetrical scanning.	 The methods chosen for
information processing are deduc-ed from-diffraction- and
system theory. *The applications descri-bed comprise diagnostics
of aperture distributions and the determination of resulting
far-field patterns of array$.of antennas.
17. Keywords (Stlectea Dr Auth*f(s)) ll. Distribution Slstment
• Unclassified - Unlimited
19. Security Clesell. (el this topers) 20. Sseurlty Clesell. (e1 this peg*) 21. No. of Pages 2L
Unclassified Unclassified
r
RECORDING AND REPRODUCTION OF MICROWAVE HOLOGRAMS
USING A SCANNING PROCEDURE AND THEIR SUBSEQUENT
OPTICAL PROCESSING
J. Hetsch
Lehrstuhl fur Theoretische Elektrotechnik, Ruhr-Universitat Bochum
1. Introduction
Intensity distributions in non-optical wavef^elds have been
made visible for some time and have been stored on photo-sensitive
material [1]. Holographic techniques also provide the opportunity
to freeze-in the complex amplitude of a stationary field at any
point and to reproduce it in the optical range [2, 3].
Of course there is no medium for microwaves comparable to
photographic material, but liquid crystals and polaroid films can
be used for such purposes via temperature effects [4, 5]. Compared
to this method, the scanning methods where the field is scanned
point-by-point or line-by-line with one or more probes, offers
advantages of relatively small transmission power with free selec-
tion of the wavelength and the potential for measured-value pro-
cessing before its transferral [6, 7]. If computers are used to
control the probe and for storage of measured data, reproductions
can be performed as often as desired to enhance picture quality,
or a wholly numerical process can be used. Therefore, a computer-
based scanning method was selected for the recording and reproduc-
tion of microwave holograms.
2. The Computer-Based Measurement Center
The recording framework (fig. 1) operating by the scanning
principle is made of plastic--as much as possible--and permits a
3-dimensional positioning of the probe on the lines of a cylin-
drical coordinate system. This permits the probe to be moved
efficiently over the sphere surface [8]. Five-phase stepping motors
provide the drive power to shift the probe in the R, A and z-
directions; these motors have the advantage of eliminating in
Numbers in the margin refer to pagination in the original text.
/51
w
s
Fig. l: Recording
Frame and Scanning
Electronics
Fig. 2: Hollow Wave-
guide Probe
ORIGINAL r:
,F POOR QUAUTY
principle, a rcoort on the position of the probe. Since the probe
is moved quasi-continually in a circular direction, an angle-
encoder is used to provide a positioning control. By means of a
mechanical system, the probe orientation in space is held constant
during its circular	 motion.
The maximum diameter of the scanned	 /52
surface amounts to 800 mm, the path reso-
lution in the r-direction is 3 micrometers,
in the 8-direction, 10-4
The electronics needed for the stepping
motors and angle encoder are placed in a
19" housing.
An open hollow waveguide with outlet-
connected crystal detector for a range of
26.5 to 40 CHz (fig. 2) isused as measuring
probe; it sends its values via a DC-amplifier
to the controlling computer. At an S/N-
ratio of 60 dB, the minimum detectable
power is about 15 nW.
The block diagram (fig. 3) shows the
interaction of recording frame, scanning
electronics and computer. The used micro-
computer has an 8-bit architecture; it also
has an expandable memory of 64 kbytes RAM
and 4 disc drives with a total of 2 mega-
byte capacity. In 30 minutes, about 300,000
position and intensity values can be recor-
ded and stored.
P1
Figure 4 shows the reproduction frame which represents the
smaller pendant to the recording frame and is also controlled by
the computer. Instead of the probe, a semiconductor light source
- -,
	 2
b oye	 ;
^cd '^)
	
:	
I
MCIor
Ste
7_7 Inler.„'CE
^— J I	 I
:9^CP^ ms 's' • ^^'%1	 ^	 ='
M,w ^f Cep C.'Inr'L Md5 8C 
ORIGINAL PHUs tai
OF POOR QUALITY
Fig. 3: Block Diagram of the
Scanning System for Record-
ing Operation
Key: 1-scanning frame
2-scanning electronics
3-control 4-radial motor
5-circular motor 6-angle
encoder 7-probe 8-probe
amplifier 9-microcomputer
10-interface, angle encoder
11-mains section
Fig. 4: Reproduction Frame
Ir, 7
Fig. 5: Probe LED Path
is accurately positioned here, and its brightness is pulse-modulated.
The motion path chosen for the probe and L,ED is shown in fig. 5;
the alternating clockwise and counter-clockwise opera t ion with
interim variation of the radial posit'-on from outside to inside is
indicated in the figure.
The image produced on picture film as a darkened distribution
of the intensity present during tha photograph, is reduced in a
final step (photographically) to a diameter of about 5mm. Figure
6 shows a typical set-up of the subsequent, optical information
3
li)
OR1GIMAL PAU ti^
^r L31 n pi t, nl 1'N
Fig. 6: Optical Computer
r+)l
	
Y	 Y
X	 x
h (r, Z)
z
	
7 =o	 7= onsf
Fig. 7: System of Parallel Planes
processing. This part of the entire process of production and
processing of microwave holograms is explained in more detail
	
.	 below.
3.	 Optical Information Processing
It is appropriate to describe the recording of microwave
holograms and their reconstruction in the optical range with the
propagation of waves between planes. Such a procedure will be
sufficient for linear polarization of the illuminating waves, and
	
MU.-	 by neglecting depolarizing effects on objects of the scalar, homo-
geneous Helmholtz equation. Thus, if we consider the propagation
of waves of wavelength - between two planes separated by distance
z in free space (fig. 7), then the response of this linear, rota-
tion-symmetric system to any input stimuli, is known to be a
convoluLion of input function and pulse response:
z=const.	 z=0	 li
We thus obtain a system-theoretical formulation of the Rayleigh-
Sommerfeld diffraction integral, from which the exact pulse response
can then be found:
z• +z
	
r• _I
The usual approximation of the integrand for large spacing z,
measured in wavelengths, and for near-axis or linear approximation
of the amplitude and square of the phase, leads to the Fresnel
approximation of the pulse response:
2r	 2- rJa z 7,-- 2z
h : Y 
= jaz e	 e	 z..:.,, r' rmax
4
a
^%r•
1
/54
V,
If we simplify and write [9, 101:
.2n r'
7-'
q(rraz^
	
a	
2z
ORIGINAL PAGE IS
OF POOR QUALITY
".0
0r
4tih
 
q c]" = 1
then the Fresnel diffraction integral can be represented as follows
(neglecting the non-convolution-relevant factor):
..
^, z=const . = u ; z=0	 `I _ " T
2
"i z=u *	 kFK kFK where we have:
1: 2-dimensional Fourier transformation, R?K: Space frequency
coordinates and RK: Space coordinates.
By using the properties of q, from the convolution there
results a Fourier transformation of the q-loaded input function,
the Fresnel transformation of u along the distance z, with a
suitable imaging of the space-frequency coordinates onto space
coordinates. if the field distribution near the axis in a plane
z is large compaed to a,, then we arrive at the [word missing]
in the initial plane through a Fresnel retransformation, i.e.
through a Fourier retransformation applied to the distribution
in the Fresnel field affected by the conjugated-complex phase
factor: 7
u ^ z=0	 1`; 1 z=const. q
4) q t
If the spacing z is large compared to the ratio of the surface
area of an aperture in the initial plane to the used wavelength,
then the phase factor in the integral dependent on the aperture
coordinates, is nearly equal to one and we obtain the Fraunhofer
diffraction integral which produces a Fourier transformation rela-
tion between the distribution in an aperture and its far-field:
^Iz=const. = q ' fI z =0} !kFK . kK for z (aperture diameter
 max) 2h
From measurements in the Fresnel field of an aperture antenna,
conclusions can be drawn about the antenna availability and about
the radiation pattern.
5
rJ1RFK+RK
i	
ORIGINAL PAGE tS
OF POOR QUALITY
The holography technique now permits the storage of such Com-
plex fields only on intensity-sensitive media by overlapping the
desired object wave by a coherent reference wave. After generating
an amplitude transmittance as linearly as possible through the
available intensity distribution, in a second step the wave field
of the object is reconstructed, generally by using a reproduction
wave which resembles the reference wave during the recording.
r.
	
	 The measuring set-up in step one delivers a hologram in the
millimeter-wave range whose structure a's stored in the computer,
and in the 2nd step, a reduced image on photo material which is
reconstructed with He-Ne-laser light of about 633 nm. The ratio
of wavelengths is about 13,500; the hologram diminution factor
attainable in practice, is 170.
Regarding the spherical lenses used in the next step one can
say that they convert a spherical wave with midpoint g into one
with midpoint b. For a lens with focal length f there follows
by Fresnel approximation, its complex amplitude transmittance:
.i L = q* (r; Y)
If observations are made in the rear focal plane of the
_	 lens, teen it acts like a two-dimensional Fourier transformer
b
for signals in the front focal plane (fig. 8), which provides
the actual reason for optical processing of microwave holograms
(besides the fact that lenses for the microwave range are complex
and of poor quality).
Fig. 8: Lens as Fourier Transformer
6
K/ystr
05 6f
Hohlleilerl
Abli;t,rrohmen
} 11
2 1Holo yrommebene)
/J Rite r3enzwelle
It Mc.9sonde
Otyektwelle
6Anlennen-Array
1m	 ~
17
ORIGINAL PAGGE k4
OF POOR QUALITY
/554.	 Applications
Figure 9 shows the recording set-up selected for the appli-
cation examples.
The source of the spherical reference wave has the same dis-
tance from the hologram plane as the object; we obtain a so-called
lensless Fourier hologram which is characterized by the inter-
ference terms appearing in the hologram plane which contain the
spectral distribution of the aperture distribution affected with
a quadratic phase factor.
Fig. 9: Principle of the Recording Set-up
Key: 1-hollow waveguide 2-scanning frame (hologram
plane) 3-reference wave 4-probe 5-object
wave 6-antenna array
As a sample application of microwave holography, the local-
ization of defective elements in antenna arrays was selected [11]
(fig. 10). The photo shows the intact array of pairs of the
same rectangular horns which are linked via magic T-pieces with
symmetry-control outputs and which are powered in counterphase
from an E-branching. Figure 11 shows the hologram taken with 236
scanning circles which has the typical interference structure.
Figure 12 shows the optically reconstructed intensity distribution
in the aperture plane and we see the 4 emitting elements and the
clear size differences of the horn pairs. TI-e far-field obtained
through Fourier transformation and unfolding (by defocussing) for
the antenna array is shown in fig. 13 together with the expected
group characteristics.
7
• ^
	
ti i
	 Mil ^	 ^,	 .t
•	 '	 (( fff
Ii	 ,i	 11
^	 v	 ^l I^,tl'I I ^	 1111) III I.
^^^il^^^^,l,l,
	 11	 ► il^lu^i
	 '
Fig. 10: Antenna Array Fig. 11: lnta-t Array 	 Fig, 14: Defective Array
rniiri er 1in1 noranc
Fig. 12: Aperture dist. int. array	 Fig. 13: Far-field, int. array
i1r.f
Fig. 15: Aperture dist. defective array Fig. 16: Far-field, def. array
Fig. 17: Two elements in H-plane
Far fields
8
Fig. 18: Two elements in E-
plane
"", 
19FORIGINAL PAGE ISOF POOR QUALITY
^i
The array was then reduced by one emitting element through
a reflection-free cover. The resulting hologram (fig. 14) shows
no great changes compared to that of the intact array, but the
reproduced aperture distribution shows the defect ( fig. 15).
The constructed far-field is also clearly deformed ( fig. 16).
The potential of performing changes in the optical range on
aperture distributions yields additional applications. For
instance, the thinning out of ( usually stochastic) antenna fields
can be simulated. As an example for such an application, one
element of the defective - array was faded-out optically. We then
obtain the far-field (fig. 17) of a double horn excited in phase
in the H-plane, whose maximum was found photometrically on the
antenna axis and the far field (fig. 18) of a double horn now
excited in phase located in the E-plane with a minimum on the axis.
The potentials of the computer had not been fully utilized,
for instance, only relatively rough measurements were performed
to linearize the final amplitude transmittances. Figure 19 pro-
vides a hint about these potentials; it shows again the aperture
distribution of the inta^ _t array, reconstructed from a non -linear-
ized hologram and thu g
 affected by diffraction patterns of higher
order. The fields evaluated photometrically already indicate good
agreement with theoretical or directly-measured value3, especially
for bundled antennae.
Additional applications are found in tracking anomalies in
opaque dielectric substances, in the investigation of scattering
fields--which could be done by turning the polarization directions
of the reference wave and the probe by 90 0 , or vectorally---and in
holographic interferometry.
9
an -
t
1
rJ
ORIGINAL PACE ISOF POOR DUALITY
11
Fig. 19: Aperture Distribution
Intact Array (not linearized)
References
[1] Kleinwachter, H.: ,faking Electromagnetic Waves Visible.
A.E.O. 9 (1955), p. 154-156.
[2] Thompson, B.J.: Applications of holography. Rep. Prog. Phys.,
V. 41 (1978), pp. 633-674.
[3] Tricoles, G. and N.H. Farhat: Microwave hologra phy: Applica-
tions and techniques. Proc. JEEE, V. 65 no. 1 (Jan 1977),
pp. 108-121.
[4] Augustine, C.1:.: Field Detector [,forks in Real tir,,e. Electronics
V. 41 (1968), 118-121.
[5] Iizuka, K.: Making it in oic t-ures. Electronics, V. 41 (1968),
p. 130-131.
[6] Dooley, R.P.: X-band Holography. Proc. IEEE, vol 	 53 (1965),
pp. 1733-1735.
[7] Farhat, N.H. and W.R. Guard: Millimeter Wave Holographic
Imaging of Concealed Weapons. Proc. IEEE, V. 59 (1971),
pp. 1383-1384.
[8] Farhat, N.H., W.R. Guard and A.H. Farhat: Spiral Scanning in
Longwave Holography. Acoustical Holography, V. 4, G. Wade,
Ed., N.Y. Plenum Press (1973), pp. 267-297.
[9] Vander Lugt, A.: Operational Notation for the Analysis and
Synthesis of Optical Data Processing Systems. Proc. IEEE 54,
no. 8 (1966), pp. 1055-1063.
[10] Gaski_ll, J.D.: Linear Systems, Fourier Transforms and Optics.
Wiley, New York 1978, pp. 360-361.
[11] Ransom, P.L. and R. Mittra: A Method of Locating Defective
155 yts in ), 66 §e,6^65ed Arrays. Proc. IEEE (Letters) V. 59PP•
10
Or r


Recording and reproduction of microwave holograms using a scanning procedure and their subsequent optical processing

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