Heterodyne infrared astronomy was carried out using CO2 lasers and some solid state tunable lasers. The best available detectors are mercury cadmium telluride photodiodes. Their quantum efficiencies reach values near 0.5 and in an overall system an effective quantum efficiency, taking into account optical losses and amplifier noise, of about 0.25 was demonstrated. Initial uses of 10 micron heterodyne spectroscopy were for the study of planetary molecular spectra

Townes, C. H.

English

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y	 Heterodyne detection has a theoretical sensitivity
disadvantage by comparison with direct detection
(photon counters or bolometers), b ,it an actual prac-
tical advantage when either lonx wavelength or very
high spectral resolution is involved. For unpolarized
radiation the ideal theoretical ratio of sensitivity
of heterodyne detection compared to direct detection
°	 is
U	 e-hv/kT 112
^:	 2
?n+'rared Heterodyne S ectro6copy for
Astronomical Purposes
University of California, Berkeley,
USA
0. H. Townes
Here v is the frequency, T the effective temperature
of background radiation falling on the detector, and
It is assumed b y >> kT. For polarized radiation at
low frequencies such that b y << kT, the ideal theoret-
ical sensitivity of the two types of detP.:tion is
Identical. However, in the microwave re,6 ion, where
b y << kT, heterodyne detection has an important prac-
tical advantage. This i s that heterodyne detectors
exist which come reasonably close to the theoretical
sensitivity limit, while photon counters or holometers
miss this limit by a wide margin, so that in practice
heterodyne detection is much more sensitive and gener-
ally used at these low frequencies. As the frequency
Is increased from the microwave region, not only does
the theoretically expected ratio of sensitivity
decrease monotonically, but also the quality of tech-
nical devices available varies considerably. At
10 microns wavelength and shorter, detectors of both
types are available which under some conditions come
within a factor of 2 of ideal performance.
The availability of good detectors and good l-secs
for	 'il oscillators has meant that, while the lc._ ,;er
infra. •.d wave-lengths may be r ,)re favorable in prin-
ciple, the 10 micron atmospheric window has been the
most fruitful region for infrared astronomical work
with heterodyne detection. Even thourh for any band-
width a direct detector should in principle be about
twenty times more sensitive than heterodyne at 10u
k
wavelength, direct detector.- miss ideal sensitivity
for very narrow bandwidths by a substantial factor AtO
heterodyne
	
tectors can be more sensIt ive. Nrwever,
with continuing detector development one must. in the
long run look to Infrared heterodyne detection prirnar-
tly for convenience rather , than for ultimate sensitiv-
ity, in particular convenience and efficiency in ob-
taining very high spectral resolution. Heterodyne
detection translates the problem of spectral resolu-
tion into a lower , frequency dnmain where it. is easier
to build very narrow filters; with direct detection
high resolut ion
 
must be obt • i1 ne d essentially by the
use of very long pathlengt h di fforences berovc detec-
tion, and these can be very awkward whorl it. ic l dos fired
to resolve bandwidths less than about 0.01 viii -
While primarily a narrow hand techrni,jne , Y;etero-
dyne spectroscopy for astronomical purF e ms : e	 In the
10 micron region requires binAwldths which ;ire very
large by comparlson with the normal situat loo at
microwave frequencies. We Have built a t>4-f1le.er
system of bandwidth about 1300 MFi::; neverthe1c.;o, ft_^r
a fixed frequency local oscillator at l0u wavcicnjrth,
even the earth's orbital motion can produce doppler
shifts which change the frequene l es of 1 tries to be
observed by several times the width of this :e • ± of
filters. Furthermore, althoui-h tunable la cr local
oscillators can he built which come close to the per-
formance destred for, heterodyne spectro^;c. • p , they
still do not conveniently t71ve the required r.t -1 h i 1 l t y
and power.
	
Bence heterodyne opectroncopy 1 , i t!ri:;
r°gion is somewhat restricted by the limit(-(] fr011uency
coverage which is readily available. 	 Nevoi-thole -s,
the many lines of different isotopes of' CO , {plus
observations at carefully chosen times of the year
allow High quality astronomical spectroscopy %-Ath ;e
reasonable coverage of frequencies.
Heterodyne infrared aGtronomy has been ..ar r 'e.i
out primarily by two Froups, one at: the • Uri1	 r :;i'	 of
California at Berkeley and the University k i t ' Tor, . . o
and the other, at Goddard Space Flight  Centsr i r,
Greenbelt, Maryland. Ire addition to CO2 lasers, the
latter group has also used some solid state tunable
lasers.' The best available detector: are mercury
cadmium telluride photodiodes built by Spears of the
Lincoln Laboratories. 2 Their quantum efficiencies
reach values near 0.5 and in an overall system ,in
effective quantum efficiency, taking into account
optical losses and amplifier noise, of about 0.25 has
been demonstarted.3
A number of the initial uses of 10 micron heter-
odyne spectroscopy were for , the study of planetary
molecular spectra."_ 10 Work on CO2 ire Mars ;:nd Venus
revealed nonthermal emission of CO 2 in the hlf-h atmos-
pheres of these planets, s w!th linewidths as-small as
about 10 -3cm-1 . At resolving powers as high as about
10 7 , the shapes of these anomalous emission lines as
well as those of absorption lines of CO2
 have been
studied In some detail. These have provided good in-
formation on the pressure-temperature curve for the
r
atmosphere of Mars,' and c,n the winds in the upper
atmosphere of Venus." ft w ► : se doppler velocities have
been measured to a precision of about. 5 m /.etc. In the
stratosphere of Versus, the observations show that
winds circulate about the e.luator' with a rot rogro le
velocity of about U m/sec. However this v:^locity
varies about 50% na a time-scale of 4 or 5 days. At
mesospheric altitudes, the kinds are accelerated from
the subsolar point towards t h .., anti solar point, af-a 1 n
with peak velocities near 100 m/sec. Thera appear to
be also vertical motions of the atmosphL'rc' of Vertu:;
as large as a few tens of meters per second. While
Instruments actually entering the planet's atmosphere
can give more precise data for a variety of parameters,
the cheaper and more extensive atmospheric Int'or'rr.at:ion
which can be obtained by a series of heterodyne ohser-
vations from the ground can give Detailed i nformnl i nn
on daily wind patterns and variation:.. On the pl:inet.
Jupiter, intense ammonia emission from i__fglone at;cve
the troposphere has been repertod. 9 However, this
must be a transient phenomenon since a number , of other
searches have resulted in no ammonia detection of this
type .11
Absorption lines of ammonia have been dotected t,y
Betz et a'	 in circumstellar material around the
star IRC +10216. Three lines of ammonia .are sho^rr-i In.$
Figure 1 with a spectral resolution o`'  abhrrt. 7 x:1.0-
cm-l . These lines differ in width and shnj , o, inn
there are additional features (marked A, 13 and C)
which seem to be above random fluctuations. While
these observed lines are all metastable, , a nurimeta-
stable line has been senrc ► .ed for and found to be very
much weaker, indicating decay between collision times
and hence giving information on the molecular density
where ammonia occurs. Somewhat similar absorption
litres have been found In the star VY Canis Ma,loris.13
Central doppler velocities in this case correspond to
,-4.5 km/sec, essentially the same as prominent fea-
tures of H 2 O and OH masers associated with this star.
While the data from VY Canis Ma,loris is still being
analyzed, its lines are remarkably similar to those
from IRC +102.16, with the (6,6) line tieing substarr-
tially broader than the (2,2) line, and the absorption
depth beln^, approxin-itely 10%. Infrared heterodyne
spatial interferome •.ry ha y recently shown that a small
central hot region in both of these stellar , complexes,
presumably the star itself, emits approximately 10% of
the total infrared enerj-y, the remainder cominE- from a
dust shell of diameter approximately 0.8 second. The
dust shells appear to have optical depths near unity.
Analysis of the lines shows that for TRC +10216 most
of the detectable ammonia lies between the central
star and the much larger dust envelope. Abundance and
densities also deduced from the spectral lines show
that the "11i 3 abundance is more likely in equi 1 I bri ur.
with the cooler temperat.ur- of the dii^t than v:ith thc-
temperature of the ceritr'i' -,tar. a 2
While the total ar... ..	 o;' astronrmicrt] ^pectroc-
copy carried out so faro infrared heterodyne tech-
niques is still small,
	
i1scovery of a nurl , er of
very narrow 1 leg s and i h, '. , -;; p ortance of detail^ of
their, shapes make thI	 f., ' i a proms ;inr* orie for tht-
future. Technical develo,, r.• rats, particularly with
respect to tunable or modul^tted laser oscillators and
less noisy ;amyltfiers can be exp ,.^cted over the next
few years which will make tho techni q ue considerably
more powerful and convenlent,, so that it may in the
future be a rather widely t;:,ed type of spectroscopy.
REFERENCES
1. Mumma, M., Kostiuk,	 .('11, S. , Duhl , 1). , and
von Thuna, P.C. Nntl',	 ;, ',14 (1'a7r'))
2. Spears, D.I.. Infrave i	 :. '. c 	 17, 5 ( 1977 }
3. Betz, A.L. Thesis, i r e,' . r:•it` of Californta,
Berkeley (1977)
4. Peterson, D . W. , Johr.. 	 1. , and 13et: , A.L.
Nature 255. 12,5
5. Johnson,M.A., bet , ,.,^	 ., Mcl.nren, R.A.,
Sutton, E.C., and Towr, , 	C.11. Ap. J. 208, I.145
(1976)
6. Betz, A.L., Johnson, M.A., t'.cl,aren, R.A., and
Sutton, E.C. Ap. J. 20':1, L141 (1976)
7. Betz, A.L., Mc Laren, A.C., 'Sutton, E.C., and
Johnson, X .A. Icarus 30, 050 (1977)
ti.
	
Betz, A.L., Sutton, E.C., Mc Laren, R.A., and
McAIary, C.W. Proc. Symposium on Planetary
Atmospheres, Ottawa, p. 29 (1977)
9. Kostiuk, T., Mumma, M.J., Hillman, J,J., Buhl, D.,
Brown, L.W., and Faris, J.L. Infrared Phys. 17,
431 (1977)
10. Mumma, M.J., Abbas, M.M., Buhl, D., and Kostiuk, T.
Bull. Am. Astron. Soc. 10, 555 (1978)
11. Betz, A.L., private communication
k
12. Betz, A.L.. , Waren, R.A. , and Spears, A.L. Ap. J.,
to be publ Pn
13. Betz, A.L., aLl McLaren, R.A,, private communica-
tion
•
This work Is sulnorted in Fart by NASA Grants
NGL 05-003-272 and NGR 00-003-452
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Infrared heterodyne spectroscopy for astronomical purposes

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