An infrared imaging technique based on the nonlinear interaction known as upconversion was used to obtain images of several astronomical objects in the 10 micrometer spectral region, and to demonstrate quantitatively the sharper images allowed for wavelengths beyond the visible region. The deleterious effects of atmospheric inhomogeneities on telescope resolution were studied in the infrared region using the technique developed. The low quantum efficiency of the device employed severely limited its usefulness as an astronomical detector

Boyd, R. W.

Townes, C. H.

English

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https://ntrs.nasa.gov/search.jsp?R=19770020104 2020-03-22T09:31:02+00:00Z
JUL 1977
A Sensitive Infrared Imaging Upconverter and Spate ial RECEIVED
Coherence of Atmospheric Propagation* is^tNPUT BRAN"
R.W. Boyd and C.H. Townes, University of Califorr%A
Berkeley, California 94720
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Theoretical reasoning has indicated that atmospheric
o00"seeing" disturbances should allow sharper astronomical
	
M	 images at infrared than at visible wavelengths, l yet imaging
Mdevices for astronomy have not generally been available at
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wavelengths beyond the near infrared. Planets and their
atmospheric structure provide interesting subjects for infrared
imaging systems, 2
 in either broad or narrow spectral regions.
So do some more distant objects, such as stars surrounded
by thick dust shells, which emit strongly in the infrared,
and the nuclei of Seyfert galaxies. All of these objects can
be spatially resolved in the larger astronomical telescopes,
and hence are valuable subjects for infrared imaging. We
describe here the exploration of an infrared imaging tech-
nique based on the nonlinear interaction known as upconver-
sion. 3
 This technique was used to obtain images of several
astronomical objects in the 10 um spectral region, and to demon-.
strate quantitatively the sharper images allowed for wave-
lengths beyond the visible region.
The upconversion process is shown schematically in.
Figure 1. Infrared radiation i:3 mixed with an intense laser
*This work is suppd?1tv4 in .pa.A by NASA Grants NGL 05-003-
272 and NGR OS-003-452
2.	 r
beam in a nonlinear crystal. The nonlinearity in the optical
susceptibility of the crystal generates a signal at the sum
frequency, which is in the visible region. The upconversion
process in in principle noise free, 4 and thus provides a
sensitive method of detecting infrared radiation by first
converting it to the visible region where sensitive, low
noise detectors are readily available. Figure 1 also illus-
trates image conversion by upconversion. Photon momentum
conservation requires that for each infrared direction of
propagation the sum frequency radiation be emitted in a
unique direction, and thus a one-to-one correspondence exists
between infrared angles of propagation and sum frequency
angles of propagation. In fact, the upconversion process
simply demagnifies the sum frequency image by approximately
the ratio of the sum frequency to the infrared frequency.
In our device, thermal infrared radiation is mixed in
a 1 em long proustite crystal with an 0.25 watt, 0.7525 um
krypton ion laser beam to produce a sum frequency signal at
about 0.7 um (see Figure 2). The 2 cm -1 phase matched band-
pass of the upconverter can be angle-tuned by tilting the
crystal about its y-axis to allow operation at any wavelength
from 9.0 u to 11.0 u. Type II phase matching  is used. In
order to provide a large usable field of view several hundred
spatial modes of the infrared field are simultaneously upcon-
verted. The quantum conversion efficiency obtained with our
device was 2 x 10 -7 , which was sufficient to allow useful
astronomical measurements.
.3.
The sum frequency radiation is detected by a Varo 3-stage
image intensifier tube, cooled to a temperature of -30 C.
Cooling the tube decreases the dark count by a factor of 300
to a level of about 0.1 photo electron per second within the
3 mm field utilized by the upconverter. The output of the
image tube is lens-coupled at unit magnification to Kodak
TRI-X film for short exposures and to Kodak IIa-D film for
long exposures. Some of the laboratory results are shown in
Figure 3.
This instrument has been used on the 150 cm McMath solar
telescope of Kitt Peak National Observatory. The field of
view on the sky is then 40 seconds of arc, and the resolution
is limited by diffraction at the telescope aperture and by
inhomogeneities in the earth's atmosphere. Resulting images
contain several hundred picture elements. Infrared pictures
of the Sun, Moon, Mercury, and the dust shell surrounding the
star VY CMa have been obtained, and are shown in Figure 4.
Integration times range from 4 seconds for the Sun to 15
minutes for VY CMa.
The deleterious effects of atmospheric inhomogeneities
on telescope resolution (or "seeing" effects) were studied
in the infrared region using this apparatus. The theory of 	 y
wave propagation in a birbulent medium  predicts a X-1/5
dependence to the angular resolution of a telescope limited
by the effects of seeing when averaged over a sufficiently
long time. Previous attempts at verification of this wave-
length dependence have met with mixed results, due primarily
4.
to the lack of true imaging devices at long infrared wave-
lengths where seeing would be expected to be significantly
better than in the visible. We have investigated the wave-
length dependence of seeing by taking pictures of the sun's
limb with the 1.5 m telescope simultaneously at 10-pm and at
0.55 um, and comparing the sharpness of the solar limb in
each case. A typical 10 um limb profile is shown in Figure
5, and the corresponding visible limb profile is shown in
Figure 6. This comparison was repeated a number of times
under a variety of atmospheric conditions. After correcting
the data for the differences in solar limb darkening  and dif-
fraction at the two wavelengths, we found a systematic
improvement in seeing at the longer wavelength, as shown in
Figure 7. The RMS width of the point spread function for
seeing is found to be 1.9 ± .2 times greater at .55 u than
at 10 u in good agreement with the theoretical value 1.80
predicted on the basis of random turbulence.
These experiments represents the first successful appli-
cation of imaging upeonversion to astronomy. The low (-2 x
10 -7 ) quantum efficiency of the device used severely limited
its usefulness as an astronomical detector; nevertheless,
these measurements demonstrate the promise of infrared imaging
and of upeonversion techniques when materials which provide
larger nonlinear effects become available.7
5.
Figure Captions
Figure 1 Schematic description of the upconversion process
(upper). Imaging property of the upconversion
process (lower).
Figure 2 Optical layout of the 10 um imaging upconverter.
The monochrometer is used to eliminate background
light from the laser discharge tube. Collimated
infrared radiation is mixed with the laser beam
in the proustite crystal. The interference filters
pass the sum frequency while rejecting the laser
frequency, providing a factor of 10 18 discrimi-
nation between the two frequencies. The sum fre-
quency image is amplified by the image intensifier
tube and recorded photographically.
Figure 3 Upconverted images_of several laboratory test
objects. Figures a, b, and c show resolution
test patterns. The smallest spacing in Figure c
is 1.3 times the diffraction limit. Figure d
shows a one minute integration on a solc..-ring iron
operating at 100 C.
Figure 4 Infrared images of a number of astronomical objects.
Computer generated plots of digitized photographic
negatives are shown. In each case the field of view
is round, and a spurious spot is introduced at the
center of the field from the hole in the colli-
mating mirror shown in Figure 2. Note the enhance-
ment in the signal from the subsolar point on
- - —	
--
	
-
6.
_	 = Mercury.	 The detection of VY Canis Majoris is
marginal.
Figure 5 Solar Limb Profile at 10 um. 	 The solid curve
is a theoretical limb profile formed by convolving
the true limb profile with a Gaussian shaped point
spread function representing the combined effects
_ of "seeing", diffraction, and instrumental resolu-
tion.	 In this case the Gaussian has a full width
to 11e points of 4 arc seconds.
Figure 6 Solar Limb Profile at Visible Wavelengths. 	 The
solid curve is a theoretical limb profile formed
by convolving the true limb profile with a Gaussian
== shaped point spread function representing the
combined effects of "seeing", diffraction, and
instrumental resolution.	 In this case the
Gaussian has a full width to 11e points of 6 arc
seconds.
=:	 Figure 7 Visible Seeing vs. Infrared Seeing.
7.
References
1. R.E. Hufnagel and N.R.	 Stanley, J. Opt.	 Soo. Am.,	 549
52	 (1964).
2. J.A. Westphal, K. Matthews, and R.J. Terrile, Ap. J. Lett.
188 9 L111 (1974).
3. K.F. Hula*, Rep. Prog. Phys., 36 9 497 (1973) and F.
Zernike and J.E. Midwinter, A died Nonlinear 0 tics,
John Wiley and Sons, Inc., New 	 or	 .
4. C.L. Tang, Phys. Rev.,	 182,	 367	 (1970).
S. J.E. Midwinter and J. Warner, Br. J. Appl. Phys., 16,
1135	 (1965).
6. P. Lena, Astron. Astrophys., 4, 202 (1970) and J.E.
Gaustad and J.R. Rogerson, Ap. J., 134 9 323 (1961).
7. R.W. Boyd, to be published in Optical Engineering, 1977.
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A sensitive infrared imaging up converter and spatial coherence of atmospheric propagation

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