Infraredfhys.
Vol.32,pp. 385407, 1991
0020-0891/91
$3.00+ 0.00
Pergamon
Pressplc
Printedin GreatBritain
HETERODYNE
SPECTROSCOPY FOR SUBMILLIMETER
AND FAR-INFRARED
WAVELENGTHS
FROM
100,um TO 500pm
HANS-PETER RISER
Max-Planck Institut fiir Radioastronomie, Auf dem Hiigel 69, D-5300 Bonn 1, Deutschland
Abstract-Heterodyne
spectrometers having a resolving power of more than lo6 for the wavelength range
from 100 pm to 500 pm are described. Essentially the receiver consists of a Schottky barrier diode in an
open structure mixer, an optically pumped molecular laser as local oscillator, low noise HEMT and FET
amplifiers and an acousto optical spectrometer. Different applications of this type of receiver for remote
observations in astronomy. atmospheric physics, molecular spectroscopy and plasma diagnostics are
described.
1. INTRODUCTION
Over the last few years there has been an increasing interest in new systems that utilize the
submillimeter wave spectral region, which includes wavelengths between approximately 30 pm and
1 mm. This interest is stimulated by the steadily improved technology base for sources, receivers,
optics and other components in this band, which are used, for example, in:
-atomic and molecular spectroscopy
as in the laboratory
-plasma diagnostic experiments,
-semiconductor
physics,
-laser diagnostics,
-environmental
research,
-metrology
and
-radar
techniques.
in astronomy
and atmospheric physics, as well
This article describes high resolution heterodyne spectrometers which have been developed
by several groups for astronomical application mainly for the wavelength range 100 pm to
500pm. The application with ground-based and airborne telescopes has proved its usefulness by leading to several new discoveries and now this technique has become widely used
in different fields of research. For a more general review on submillimeter and FIR physics see
Ref. (16).
2.
SUBMILLIMETER
AND
FIR
WAVELENGTH
RANGE
2.1. Definition
At appropriately low temperatures the wavelength range from 30 pm to 1 mm (10 THz to
300 GHz/333 cm-’ to 10 cm-‘) is in the region of Planck’s black body curve where neither Wien’s
nor the Rayleigh-Jeans approximation is valid. At one end microwave techniques with waveguides
and antennae are used whereas at the other end infrared techniques with lenses and free space
propagation are used. In between, in the wavelength range of interest, both techniques will be
applied. For example, at higher frequencies the increasing effect of surface resistance losses and
the skin effect become severe recommending the use of free space propagation instead of
waveguides, but at the same time small physical dimensions of detectors compared to the
wavelength require the introduction of specially designed antennae to increase the effective detector
area.
The description of the spectral range is not yet standardized and several notations exist without
any exact definition, but the expressions “submillimeter” and “far-infrared (FIR)” are the most
385
386
HANS-PETER RISER
Table
I. The useful suectral rawze for the heterodvne soectrometer
Wavelength i,
Frequency v
Frequency J
Photon energy hv
Quantum energy hv
Quantum condition hv = kT
Wien dlnrr
100-500 pm
3.000-600 GHz
100_20cm2 x lo-“‘-4 x lo~*‘ws
12-2.5 meV
145-29 K
29-6 K
common ones. Usually in the literature “submillimeter”
denotes the range from 1 mm down to
about 300 pm, “far infrared” the range from 300 pm down to 50 pm and “mid-infrared”
the range
below 50 pm. The heterodyne
spectrometer
described below has been developed mainly for the
range from 500 pm to 100 pm which is the overlapping
region of submillimeter
and FIR. Table 1
characterizes
the spectral range of interest in both wavelength and quantum
energy.
2.2. Atmospheric
transmission
The submillimeter
and FIR wavelength range is a part of the electromagnetic
spectrum which
was underdeveloped
in all research fields, and their technical applications
until 25 years ago. The
reasons for this situation were:
~---the lack of suitable radiation sources and sensitive detectors,
-and
in particular the fact that the Earth’s atmosphere is, with the exception of some
spectral windows,
opaque and therefore
beam propagation
is limited to short
distances of only a few meters,
This latter had a particularly
strong impact upon developments
in communication
science,
astronomy and atmospheric physics. The situation changed dramatically
with use of ground-based
telescopes placed on high mountain
tops, airborne and balloon telescopes above the troposphere,
and rockets and space-platforms
outside the Earth’s atmosphere.
In addition atmospheric physics
and chemistry and the worldwide climate situation are of increasing importance and more attention
is paid to investigating
the global and local distributions
of trace gases, such as CO, NH,, 0,) Cl0
and CFC’s, which have detectable molecular transitions,
essentially in the submillimeter
and FIR
wavelength range, as a function of altitude.
Figure 1 shows the transmission
of the Earth’s atmosphere for 1 mm precipitable
water vapour
content in combination
with important molecular and atomic transitions.“”
The transmission
curve
700
0.0
IL
600
16
18
Fig. 1. Atmospheric
transmission
content in combination
500
450
20
22
4UO
24
350
26
28
3oou
30
32
cm-’
from the ground at 4200 m altitude and 1mm perceptible water vapour
with important
molecular atomic transitions
[from Ref. (78)].
Heterodyne spectrometers
387
is mostly determined by the pressure broadened absorption bands of water vapour, oxygen and
ozone. In particular the two adjacent windows at 350 ,um and 450 pm only permit measurements
from favourable sites on high-altitude mountain tops, like Mauna Kea in Hawaii at 4200 m, when
the precipitable water vapour content of the atmosphere is lower than 1 mm-which
happens
rarely. With the exception of the South Pole, where 0.1 mm of precipitable water occurs quite
frequently, opening up some more atmospheric windows, (‘06 the wavelength range from 100 pm
to 300 pm and between the atmospheric windows is only accessible from the top of the troposphere
or even higher. Figure 2 shows the atmospheric transmission at an airborne research operation
altitude of 14 km (45,000 feet). Complete transparency in the whole spectral range is given, starting
at an altitude of about 50 km.
3.
REMOTE
OBSERVATION
SPECTROSCOPY
3.1. Remote observation
In the following section we are only considering highly sensitive spectrometers for remote
observation, where it is not necessary to be at the position of the target, which in some cases would
not be possible, or desirable regarding safety aspects, or would be difficult. So we are leaving
out interesting areas such as laser spectroscopy, opto-acoustic spectroscopy and tunable-FIR
spectroscopy.
The signals to be analyzed could be either in emission from an object of interest, where it does
not matter if the object is emitting by itself or acting as some kind of reflector for an infalling
radiation, or it could be an absorption spectrum against a continuum background radiation.
Furthermore the goal is to resolve the spectral linewidth of the signal, which could be below
1 MHz (half power linewidth) due to the Doppler broadening linewidth of a small molecule. This
is equivalent to the detection of frequency shifts due to the Doppler effect with velocities of the
object down to 0.1 km/s.
If it comes to radiation pulses the spectrometer should be able to resolve a single pulse in time
and in frequency where the pulse length could be as short as lo-” s.
For some applications the signals are very weak which requires a spectrometer sensitivity as close
as possible to the quantum noise level with integration capabilities of several hours.
3.2. Direct detection spectrometers
Most of the spectrometers use the concept of direct detection with a broadband video detector
(Fig. 3). The idea is to block the spectral range which is of no interest by absorption and reflection,
whereas the signal radiation should reach the detector with virtually no loss. A preselection of the
1000
500 400 300 250
200
150
lpml
Fig. 2. Atmospheric transmission at 14 km altitude. Those transitions marked with an asterisk are also
observable from the ground.
HANS-PETER
R&R
388
RADIATION
FILTERS
d/AR
Fig. 3. Direct
TUNABLE
SPECTROMETS
detection
SIGNAL
DETECTOR
spectrometer.
spectral bandwidth
is done by passive filters, which are a combination
of low and high pass
filters. Further
“blocking”
and spectral resolution
are achieved with tunable
dispersive
or
interferometric
instruments
e.g. grating
spectrometer,
Fourier
transform
spectrometer
and
Fabry-Perot
spectrometer.
For very high sensitivity one has the choice between bolometer and
photoconductive
detectors
which require,
for the background
limited
situation,
detector
physical temperatures
between 0.3 and 4.2 K. For the same reason the spectrometer
and part
of the filters in front of the detector have to be cooled as well. There are many pros and cons
for bolometers’55’ and photoconductors
but the fact is that, as yet, more or less all sensitive
spectrometers
use photoconductors.
A large selection of photoconductor
elements exists for the
wavelength
range
beyond
200 pmm(7’) with a noise equivalent
power
(NEP)
as low as
4x 10-m’7W/Hz:.’ ’ (38)The advantage of a direct detection spectrometer is the scanning facility over a
broad bandwidth
and detection of both polarizations
and many spatial modes simultaneously.
The disadvantage,
if you need to detect and to resolve very narrow spectral lines, is the somewhat
limited spectral resolving power usually in the range 10’ to 104, which depends on the performance
of the grating or the Fabry-Perot.
References
(29,38,47,
70,72,97, 108, 113) provide more
detailed descriptions
of the performance
of the different instruments
and their applications.
an imaging spectrometer
with two or three
More recently Poglitsch et al. (70) have developed
Fabry-Perot
interferometers
in series for the range 40-200 pm with a resolving power of 7 x 10’
and 6 x lo4 respectively. A 5 x 5 array of photo-conductor
detectors demonstrate
the potential
advantage in integration
time and signal-to-noise
ratio over a single detector element for extended
sources.
As we will see in the next section if one needs very high resolution (> 105) the signal radiation
itself, instead of the unwanted wavelengths,
have to be worked upon.
3.3. Coherent spectrometers
By definition a detector is called “coherent detector” if it determines both the intensity and the
phase of a signal. In the submillimeter
and FIR wavelength range this can only be carried out with
heterodyne
techniques at present. This technique is well known in radio communication
as the
“superheterodyne”
receiver. The primary purpose of a heterodyne
receiver with a diode mixer is
to translate a signal at one frequency to another where it can be amplified or processed more
effectively. The ability to shift a signal in frequency with minimum
added noise or distortion
is
important
because the properties of amplifiers, filters, and detectors are all different at different
frequencies. One of the major advantages of the diode mixer is that it can often be used successfully
at frequencies where nothing else will work, and that is particularly
the case at submillimeter
and
FIR wavelengths where no amplifiers exist and up to now direct detection spectrometers
with high
sensitivity do not cover the range from 200 pm to 500 pm.
As an example, Fig. 4 illustrates
the high resolution
heterodyne
spectrometer
used for the
detection of the carbon monoxide
rotational
line CO (J = 12-11) at 1382 GHz/217 pm in the
interstellar
medium. A very weak signal of the order of 1O-‘4 W (integrated
line profile) can be
detected and the linewidth resolved.
The incoming signal is mixed in a nonlinear resistance (diode mixer) with a local oscillator (LO)
beam with a frequency close to the signal frequency.
In this way all the signal information
is
downconverted
to the microwave range where low noise amplifiers and suitable filters exist. At the
Heterodyne spectrometers
389
Par!o+Y co k12-41
vs,,:1,382GHz
h
r-l
SIGNAL
1,397GHz
LASER
GaAs SCHOTTKY
DIODE
MIXER
LOCAL
‘KILLATOR
vIF
~lv~,~-vt~l=
15GHz
HEMTAMPLIFIER
FILTER
[q---(hlj
AOS
MONITOR
1024 DETECTORS
RES.={ ;,; En’:!?
Fig. 4. Schematic block diagram for a high resolution heterodyne spectrometer.
end the signal line profile is obtained with a microwave spectrometer, displayed on a monitor and
stored with a personal computer.
This technique allows more or less arbitrarily high spectral resolution.
4.
SUPER
HETERODYNE
RECEIVERS
The goal of very high sensitivity and spectral resolution with at the same time a suitable
bandwidth, together with the need for a compact receiver which should meet airborne specifications, requires the fulfillment of many aspects from the physical and technical point of view for
all receiver components. The following sections describe the different heterodyne receiver parts in
their state-of-the-art status. Although some of the solutions are still not ideal and need further
improvement to become a real facility instrument, the heterodyne receiver or parts of the system
have already been quite successfully used in many research areas. For heterodyne receivers for use
at wavelengths longer than 500pm see the reviews by Schneider, Archer and Payne.(89~2.67)
4.1. Detector: GaAs Schottky barrier diode
The basic coherent detector technology and physics at submillimeter and FIR come primarily
from extending the concept of microwave mixer to much shorter wavelengths. Beside superconductor-insulator-superconductor
(SIS) mixer elements and InSb hot-electron bolometers for the range
> 500 pm, GaAs Schottky barrier diodes are the mixer element of choice and the only ones which
cover the submillimeter and FIR region of interest.
Although there are only a few research laboratories worldwide fabricating GaAs Schottky barrier
diodes optimized for the submillimeter range,(3~‘9.4s.52.63’
th e performance of the devices are being
steadily improved, by evaluating the best possible design for a specific frequency range and
operating temperature.(20*2’)
This translates to concerns centering on the optimum impurity profile and device geometry, as
well as attempting to get a better understanding of the detection and mixing process at about
3000 GHz/lOO pm. Very recently new diodes with submicron structures have been made by the
University of Virginia which have been specially designed for the range 100-300 ,um.@) Figure 5
shows the cross-section of the new Schottky barrier diode with the honeycomb structure contacted
by a whisker and in Fig. 6 an enlarged view of the Schottky contact is illustrated including the
equivalent circuit of the junction. The actual design has been determined by the smallest physical
HANS-PETER
RISER
390
L- 2Spm
-
AU-I-WHISKER
Pt-Au’ SCHOTTKY
CONTACT
SiO2 tAYERh
EPITAXIAL LAYER
N-GaAs
600A
EPITAXIAL LAYER
N’-GaAs
SUBSTRATE
N’-GaAs
Fig. 5. Scaled cross-section
of a GaAs Schottky
barrier
diode contacted
by a whisker.
size of the Schottky contact which can be realized in the laboratory (- 0.5 pm diameter), and
therefrom the optimum epilayer doping and epilayer thickness.
The mixing occurs in the non-linear junction resistance R, .('05)
The series resistance R, and the
voltage dependent junction capacitance C, are parasitic elements which degrade the diode
performance. The design optimization is a fairly complicated combination of interdependent
parameters, but essentially for the submillimeter and FIR range there are three major limitation
factors
-the
so-called cut-off frequency given by
v,, = {27cR,C,}--’
which should be significantly higher than the operating frequency,“” and a reaction
therefrom that the capacitance Cj short-circuits more and more LO and signal power
the higher the operating frequency
50R COAXIAL
90” CORNER
L
REFLECTOR
GaAs SCHOTTKY
BARRIER
DIODE
Fig.
6. Scaled cross-section
of a Schottky
contact
equivalent circuit of the junction.
with
Fig. 7. Quasi optical mixer structure with long wire antenna
and corner reflector.
Heterodyne spectrometers
391
-the skin effect and
-the plasma resonance frequency of the semiconductor material, e.g. a bulk GaAs with
a doping of 1 x 10” cmw3 has a resonance frequency close to 3 THz/lOO pm.@)
The result is that the new design has a substrate doping of 5 x 10” cmm3, with a plasma resonance
frequency well above 3 THz, and a series resistance R, of -30 R. The zero-biased junction
capacitance is as low as 0.5 ff, caused primarily by the small anode diameter of 0.5 pm and the
doping of the epilayer of 5 x 10” cmm3, resulting in a cut-off frequency above 10 THz/30 pm. As
we will see in the section on the receiver performance, this leads to significant improvements of
the heterodyne sensitivity as well as large reductions in the required LO power to drive the diode
mixer.
As a consequence of the high cut-off frequency these devices can also be used as extremely fast
video detectors (t = - IO-l3 s) operating from room to liquid helium temperatures, with an
estimated sensitivity of about lo-” W/HZ-“~.
4.2. Open structure techniques
In contrast to optically coupled devices (e.g. IR detectors with the size of a mm), the
sensitive detector area of a Schottky diode is orders of magnitude smaller than the submillimeter
and FIR wavelengths. Therefore antenna structures have to be used to increase the effective
detector area. At the same time a heterodyne receiver requires that signal and LO must be spatially
overlapping.
In the microwave range waveguide techniques provide efficient coupling. However, at submillimeter wavelengths the problems of small physical dimensions in waveguide technology and the
increasing effect of surface resistance losses and skin effect become severe, so that for wavelengths
shorter than - 500 pm a different antenna approach has to be used.
In 1977 the quasi-optical mixer structure was introduced by our group, which turned out to be
useful in the range from 500 pm to 100 pm. ‘54,73,85)
The diode contact whisker acts as a long-wire
antenna with an antenna pattern which is symmetrical around the whisker axis.(62’The coupling
efficiency is further increased by more than a factor of ten by placing a corner reflector behind the
wire resulting in a more or less symmetrical antenna beam in E- and H-plane and suppressed
sidelobes. This open structure arrangement (Fig. 7) can be tuned into resonance for the incoming
signal and LO radiation frequencies and the produced IF frequency by choosing the appropriate
whisker length and distance between the apex of the corner reflector and the whisker. Figure 8
shows typical antenna patterns at 287 pm for a 4-wavelength whisker and a spacing of 1.2-wavelength.““’ Several types of corner reflectors and corner cubes differing in geometrical shape and
whisker length-spacing ratio have been extensively investigated(‘9.3“54@~‘03~“5’
and have turned this
fairly simple detector configuration into a widely used antenna-coupling structure. The limitation
of the open structure mixer to 100 pm comes mainly from the fact that it is rather difficult to adjust
the 3-dimensional arrangement of whisker, corner reflector and diode with a precision close to a
tenth of a wavelength and that whisker size, edges and slits due to fabrication techniques are already
in the range of quarter-wavelength.
Despite considerable recent progress, coupling efficiencies achieved so far are generally poor by
microwave standards and need to be improved, perhaps by fabrication of small antennae in planar
techniques.“9,34,“z’
4.3. Gaussian optics
A heterodyne receiver requires the coupling of two beams, signal and LO, into the diode as
efficiently as possible. The reason for the low losses for the signal is self-explanatory and for the
LO it is given by the fact that Schottky diodes require a LO-power of about 1 mW which is difficult
to produce at arbitrary submillimeter and FIR wavelengths. Beside an optimized mixer structure
the two beams, propagating in free space, have to be combined somehow and focused on to the
detector. Signal and LO radiation have power levels which in some cases could differ by 10 orders
of magnitude and more; they reach the detector from different directions, have slightly different
frequencies and, because of the antenna structure and polarising optics, the system is sensitive to
one polarization only.
392
HANS-PETERRISER
These facts dominate the methods of beam combination resulting in a dual-beam interferometer,
which could be a Mach-Zehndero0g66’ or Martin-Puplett type system.@“)
Figure 12 shows our heterodyne spectrometer with a polarization-rotating
dual-beam MartinPuplett interferometer using wire grids with free-standing linear equispaced array of 10 ,um
diameter tungsten wires with 330 wires/cm (20 pm free spacing). The first wire grid at the input
combines two linear polarizations in the incident beams. The second one in combination with the
90” corner mirrors rotates signal and LO polarization in such a manner that both beams have the
same polarization at the output. Turning of the diplexer needs to be done depending on the
appropriate IF which is a result of the difference in frequency between signal and LO.
In the wavelength range of interest it is appropriate to use Gaussian beams, which are defined
as the propagation of directed radiation with Gaussian intensity distribution perpendicular to the
axis of propagation.(36’ For our application the cross-sections range from a few mm to several cm.
The best approach for the transformation of Gaussian beams by focusing elements is given by
curved mirrors with appropriate size which conserve the phase, e.g. paraboloids and ellipsoids. In
cases where for instance the focusing element could be used as a dewar window as well, lenses made
of quartz, TPX, PE and Teflon are more favourable. The reflection losses at the air-dielectric
interfaces can be reduced by concentrically grooved surfaces or appropriate coatings e.g. PE foils
on both sides of a quartz lens.
The useful combination of quasi-optical mixer-lens/mirrordiplexer
and astronomical telescope
has been demonstrated quite successfully by several groups by achieving diffraction limited
telescope antenna beams throughout the whole wavelength range of interest.“.3’.3’,40~7~~‘o*~“5)
4.4. Local oscillator: optically pumped laser
In principle there are two ways of heterodyning, depending on the availability of suitable local
oscillators: either a tunable oscillator and a fixed IF-amplifier and filter stage, or an oscillator with
constant and untunable frequencies in combination with a selection of appropriate IF-amplifiers
and filters to operate in the desired receiver band. However, tunable oscillators are only available
in the longer submillimeter wavelength range such as carcinotronsu3) and Gunn oscillator-multipliers.‘s3’ For the wavelength range of interest only optically pumped gas lasers with fixed
frequencies can be used and are so far the only radiation sources which fulfil all requirements for
an effective mixing process,(“.“.‘*) which are:
-To operate the diode at the optimum working point, it is necessary to have LO power
in the mW-range.
-If it is intended to have a spectral resolving power of - lo’, then the spectral purity
of the emitted LO radiation must be less than - 100 kHz (total linewidth).
-Quite often heterodyne spectrometers are used for measurement times of one hour
or longer. Therefore the stability should be better than + 1% in amplitude and
i 100 kHz in frequency for this period. This is sometimes a difficult task for a laser
because temperature changes of a few degrees have a strong impact on the resonator
length and therefore on both stabilities.
-Covering the whole wavelength range (Table 1) a large number of suitable laser lines
will be required, because laser lines are not tunable and the following microwave
components can only enable an intermediate frequency (IF) of +40 GHz. At the
moment there are a few hundred discrete laser lines which fulfil the above mentioned
conditions.(24,48)The disadvantage of non-tunability is somewhat compensated by the
fact that no active feedback control circuits are necessary such as a phase-lock system
or exact LO frequency measurement.
--In addition the optically pumped gas laser has to meet industrial standards, e.g.
airborne specifications for astronomical application.
has been done with bulky prototypes of nearly 1000 kg
The first pioneering work in this area (3’,73)
in weight but nowadays compact laser systems of only 70 kg and less are available which work in
any direction in space and even with an acceleration of several g. ~5.1’,24.~5,39.‘6~101)Figure 9 shows the
airborne laser system of the Max-Planck Institute for Radioastronomy which has a length of 1.2 m
and a weight of about 70 kg. Optimization for each desired laser line has been done by the choosing
Heterodyne
spectrometers
393
X=287um
-LO
Fig. 8. Antenna
patterns
-20
0
[degl
of an open structure
t20
tL0
mixer at 287 pm [from Ref. (103)]
of a suitable hybrid mesh which are used as output couplers. They have a constant transmission
across the resonator diameter and match the required output power to the laser gain and resonator
losses for the given resonator length. (26*271
Since 1985 this system has been used for ground-based
and airborne observations
on Hawaii and with the Kuiper-Airborne-Observatory
(KAO). Detailed
descriptions
are given in the reference. (“) Copies of this system have already been made for other
applications
and are now manufactured
by industry.
Fig. 9. Photograph
of the optically pumped submillimeter
and FIR laser of the Max-Planck
Institute for
Radioastronomy
in Bonn. The system has a length of 1.2 m, a weight of 70 kg and meets airborne
specifications.
HANS-PETER R&ER
394
Table 2. Powerful
Laser
molecule
Wavelength
f urn)
HCOOH
“NH,
CH,F,
CH,F,
CH,F,
CH,F,
CH,F?
CH,OH
CH,OH
laser local oscillator
Frequency
(GHz)
692.95 I ,4
802.987,O
1035.552,7
1042.150,4
1272.171,4
1397.118,6
1626.602,6
1838.839,3
2522.781.6
432.6
373.4
289.4
281.7
235.1
214.5
184.3
163.0
118.8
lines for the detection
Pumping-line
CO,-laser
9 R(20)
IO R(42)
9 P(4)
9 R(34)
9 R(32)
9 R(34)
9 R(32)
IO R(38)
9 P(36)
Molecule
co
co
co
co
co
co
co
co
co
of CO with associated
Transition
J=
65
J=
7-6
J = 9-8
J = 9-8
J= II-IO
J=l?-II
J = 14-13
J = 16-15
J = 22-21
IF ranges
IF
(GHz)
Frequency
(GHz)
691.5
806.7
1036.9
1036.9
1267.0
1381.9
I.4
3.7
I.3
5.2
5.2
IS.1
1611.8
14.8
1841.3
25
2528.2
5.4
4.5. Microwave components
With the proviso that mixing must be done efficiently in order to detect very weak signals,
heterodyne detection offers significant signal-to-noise advantages over normal broad band video
detectors. The fundamental reason is that the heterodyne system “sees” background noise only in
the frequency band which is downconverted to lie inside the IF bandwidth B,,. This can be
arbitrarily small if the LO source is sufficiently narrow. The maximum bandwidth is limited by the
availability of good high-frequency IF amplifiers. The heterodyne system with open structure mixer
is always used as a double-sideband (DSB) receiver and is therefore sensitive to signals at +r_+
from the LO frequency. As an example, in Fig. 4 the receiver is sensitive to signals in the lower
sideband (LSB) at 1382 GHz + 600 MHz and the upper sideband (USB) at 1412 GHz + 600 MHz.
Intermediate frequency amplifier noise contributes significantly to the total noise in the heterodyne
receiver and increases with higher receiver frequency because of increasing mixer conversion losses
(see Section 4.7). Significant separation between the LO and signal frequency is desirable for the
strongly pumped Schottky diode, to avoid noise from the pump source. The fact that laser LO lines
are not tunable requires a set of low noise amplifiers to cover as much of the spectral range as
possible. Table 2 shows as an example a list of powerful laser lines for the detection of CO with
the associated IF ranges.
Present day cooled GaAs FET amplifiers and HEMTs (High Electron Mobility Transistor) are
very attractive, covering the range from about I GHz to 25 GHz. Very recently NRA0 has
developed a low noise HEMTc7” working in the range from 8 GHz to 18 GHz in one amplifier with
a noise of ~20 K and a gain of 25 dB (see Fig. 10).
Experimental experience shows that a total amplification of 60-70 dB, with about 15.-25 dB in
the first low noise amplifier, is a favourable range for an operating heterodyne spectrometer. One
problem of the open structure mixer is that impedance matching between the antenna and the
detector must be built-in to the design at the fabrication and whisker-diode contacting stage.““’
This has also an influence on the impedance matching between the detector and the low noise IF
amplifier. When the mixer is operating at optimum LO and d.c. bias an optimized matching
transformer is used for a given IF range, to reduce noise and losses to a minimum.
4.6. Acousto-optical spectrometer
After mixing and amplification of the signal a spectrometer analyses the incoming spectrum.
For strong signals a conventional spectrum analyzer can be used, but for weak signals, which
‘cl
C.+t.,~.,+,,C,+,~,+
.*..+..,..+.+.,C..+..*..~..~..~~.?
'.*.,+,~,+.~..,*..+.*.~*~'+~~'
FREQUENCY[GHzl
Fig.
IO. Noise
figure
and
gain
profile
of the
8-18 GHz
HEMT
from
NRA0
cooled
down
to
1j K
Heterodyne spectrometers
395
require a long integration time, non-sweeping spectrometers have to be applied, such as filterbanks, autocorrelators or AOSs. Acousto-optical spectrometers (AOS) are now widely used,
having several advantages compared with the other types, and the development and continued
improvement of new acousto-optic materials allows the construction of spectrometers with bandwidths ranging from about 10 MHz to 2 GHz with typical resolutions from 10 kHz to several
MHz (39.59,61,78,87,92,110)
The main principle of an AOS is Bragg scattering of a coherent light beam at a phase grating.“*)
The AOS consists of four major components: a HeNe- or diode laser, beam expanding optics, a
deflector crystal and a photo detector array (see Fig. 11). The amplified IF signal generates a density
wave which functions in an acousto-optical deflector crystal in a similar manner to a phase grating.
This grating corresponds to the frequency distribution of the signal. A beam from a HeNe-laser
is diffracted in the density wave and the distribution of light over different deflection angles is then
proportional to the signal frequency power distribution. A lens system is used to focus the first
order diffracted light on to a photo diode array (PDA) or a charged coupled device (CCD) with
1024 or 2048 detector elements. The output of the photo detector is then digitized and fed to a
personal computer either directly or via a digital integrator. The two main design parameters,
bandwidth and resolution, are determined by the choice of the Bragg-cell material, e.g. TeO, is a
useful material for a bandwidth of 50 MHz with 1024 channels, resulting in a resolution of about
50 kHz/channel, whereas LiNbO, is more useful for a bandwidth of 1000 MHz with 1024 channels
and a resolution of w 1 MHz/channel.
For airborne applications we have developed a compact AOS with a self-supporting structure
(500 x 100 x 200 mm, 10 kg) having a bandwidth of 1 GHz with 1024 channels and we have used
it on board the KAO since 1988 with considerable success.
4.7. Receiver performance
With the above described components it is possible to use “ONE” heterodyne system over the
whole range from 100 pm to 500 pm having a resolution which may be chosen in the range 104-10’
and, if necessary, up to two orders of magnitude higher. Figure 12 shows the heterodyne
spectrometer together with the Kuiper airborne telescope as it has been used since 1985.
The minimum detectable signal power (P,)min of a heterodyne receiver is given by
(P,)minN const X T,,,/(t
X &)“*
where t is the integration time, B,r the receiver bandwidth and Tsys the so-called system noise
temperature. In practice, the major noise sources are mainly the noise intrinsic to the Schottky
mixer (Johnson noise, current shot noise, etc.) and in the second place IF amplifier noise followed
by amplitude modulation (instabilities) of the LO. According to microwave techniques the system
noise temperature Tsys is defined as
Fig. 11. Optical layout of the acousto-optical
spectrometer (AOS).
396
HANS-PETER
RISER
OIPLEXER
I
Fig. 12. Diagram
of the MPIfR
heterodyne
L
spectrometer
for the range
the KAO telescope.
I
100 pm to 500
together
with
referring all noise sources to the receiver input. Here, T,,,i,is the Schottky mixer noise temperature,
Ttr the IF amplifier noise and Lo the overall conversion losses including radiation coupling losses
as well as mixing losses by downconverting the signal frequency to the IF.
The heterodyne noise equivalent power NEP,, expressed in (W/Hz) is an adequate expression
for Tsysand related to it by
NEP, = k x Tsrs
usually given for B,, = 1 Hz bandwidth, where k is Boltzmann’s constant = 1.38 x 10ez3 (Ws/K).
The sensitivity of the heterodyne system is determined at different wavelengths by measuring the
system noise temperature Tsyswith a hot/cold load. Eccosorb material is used as a black body at
liquid nitrogen and room temperature. It is assumed that Eccosorb is an ideal black body and that
the Rayleigh-Jeans approximation is still valid in the whole range, even though it is a poor
approximation for low temperature T at shorter wavelengths. The sensitivity is measured as DSB
value and T,,,(SSB) = 2 x T,,,(DSB).
Figure 13 illustrates the sensitivity of the heterodyne receiver for two different diodes and
indicates the improvements achieved with the new submicron devices. (6.44.‘“‘)
The diode 117 has an
anode diameter of 0.8 pm and a capacitance of - 1.Off whereas diode 1112 has a diameter of
0.5 pm and a capacitance of 0.5 ff. Figure 14 shows another improvement achieved with submicron
diode 1112. Previously used diodes with > 1 l&m and > 1 fF characteristics require a LO power
which is about 10 times higher over the whole range. As a consequence more laser lines are now
available as LO and tunable solid state oscillator-multipliers might be useful again in the near
future, at least for the 300 pm to 500 pm range.
Cooling to liquid nitrogen temperatures gives only an improvement in sensitivity of - 35% which
is eaten up partially by losses at an additional window on the cryostat, and by the inconvenience
of cryogenic techniques; but IF amplifiers must be cooled in any event.
Heterodyne spectrometers
600
300
O500
1000
I
WAVELENGTH
Ivml
200
150
I
I
2000
1500
FREQUENCY
IGHzl
397
120
100
I
2500
3000
Fig. 13. System noise temperature of the GaAs Schottky barrier diodes 117 and 1112 as a function of
wavelength. Open circles and triangles are from our own work, solid circle from Ref. (9) and open squares
from Ref. (44) using the same diodes.
It has been shown that in principle GaAs Schottky barrier diodes can be used below 100pm
at least as a video detector,‘45*46,‘041
but the poor quasi-optical coupling structure below 100 pm
prevents them from being a highly sensitive detector at the present time.
A comparison between heterodyne detection and direct detection has been discussed by many
authors concerning sensitivity and useful applications, e.g. Refs (7,43,72,98). Summarizing, the
fundamental and important difference compared to a microwave spectrum analyzer, grating or
Fabry-Perot spectrometer is given by the fact that the described heterodyne spectrometer is not
a scanning spectrometer, but all the 1024 photodiodes, each with a bandwidth of 1 MHz, are
integrating simultaneously. This results in an enormous reduction in measurement time and allows
the detection of very weak signals. The disadvantage, compared to incoherent spectrometers, is the
limiting bandwidth of a few GHz (N +O.l% of the spectral range) and the “tuning” of +20 GHz
around the laser LO frequency.
0 / '1111"' Itllll"d 'g'L1'JI '"'ll'J
IO“
10-I
1 PLo[mWl10
10-j
Fig. 14. System noise temperature of the GaAs Schottky barrier diode 1112 as a function of LO power
at 432, 214 and 118 am.
HANS-PETER
R0.m
398
4.8. Other developments
SZS mixers. In the millimeter and longer submillimeter range the most recent detector
development is the SIS (superconductor-insulator-superconductor)
mixer. These quasi-particle
junctions offer noise temperatures close to the quantum noise limit and possibly conversion
gain.@,‘4*6s)
The effective nonlinearity is very large compared with Schottky diodes. Therefore the
required LO power is reduced by a factor of - lo4 or more. Up to now SIS mixers have been used
up to 460 GHz/650 pm(“*) but photon-assisted tunnelling up to 1000 GHz/300 pm has been
reported. (**Jo)It is expected that sensitive SIS mixers might be available in the next 5-10 years
operating up to 1000 GHz.
Ge-lasers. Since 1984 p-type Ge-lasers are available, utilizing the population
inversion
between the sub-bands of light holes and heavy holes under crossed E and B fields. They are tunable
The pulsed output power is in the range l-10 W with pulse
from about 70pm to 300pm. (1.57~‘07)
lengths of lo-' s to 10e6 s and a repetition rate of 10 Hz. When using a semiconfocal resonator
the emitted spectral linewidth (HPLW) is about 10 MHz. @*)Linewidth and tunability make it a
useful spectroscopic tool for absorption spectroscopy as well as the LO in a heterodyne receiver.
Figure 15 shows the spectral linewidth of the p-type Ge-laser emission at 120 pm measured with
a heterodyne spectrometer.
Sideband heterodyne detection. At the Californian Institute of Technology a sideband generation
LO heterodyne receiver has been developed for the 100 pm range. (‘OS)
The detector is a stressed
Ge:Ga photoconductor with a useful bandwidth of 20 MHz. The LO radiation is emitted from a
Schottky diode in a corner cube by mixing a submillimeter laser at 118 pm with a tunable
(2-18 GHz) microwave source. The noise temperature is given as o-1500 K (DSB).
ZnSb hot electron mixers. Most of the first detections of submillimeter molecular and atomic lines
for wavelengths > 500 pm have been made with the InSb hot electron bolometer operating at liquid
helium temperature. (69)The available bandwidth, however, is severely limited by electron relaxation
times to < 2 MHz. With LO power of the order of 10m6W, the receiver noise temperature is about
400 K at 600 pm. Response of the InSb mixer at shorter wavelengths has been demonstrated by
subjecting it to a magnetic field of a few kG. This results in a greater quantum efficiency due to
cyclotron resonance absorption by free electrons. A receiver noise temperature of 500 K (DSB) has
been achieved at 370~rn.“~’
HgCdTe receivers. Mercury-cadmium-telluride
photodiodes have been investigated theoretically
and experimentally for the use as wide-bandwidth (1 GHz) photomixers for the range 8-30 pm.
A typical quantum efficiency of about 50% gives a NEP of about 4 x 10e2’ W/Hz at 28 pm.(53,95296)
FWHM
1330
Frequency
15. Spectral
MHz
1266
1230
Fig.
: 8.4
linewidth
of a p-type
IMHzl
Ge-laser emission
spectrometer.
at 120pm
measured
with
a heterodyne
Heterodyne spectrometers
399
With the techniques for a multi-element two-dimensional array and the expectation that
HgCdTe could work up to 50 pm or even higher, this could be an extremely powerful detection
system.
5.
APPLICATIONS
The submillimeter and FIR region of the spectrum is a particularly rich source of information
on a diverse array of physical phenomena. Part of this comes from the fact that the IR and
microwave regions overlap with their characteristics, and we might expect to observe types of
phenomena in the submillimeter and FIR usually found only in one or other of its neighbours,
for example pure rotation, rotation-vibration spectra of molecules and fine structure lines of atoms
in the ground states.
Furthermore, many phenomena are characterized by excitation energies which have equivalent
temperatures in the range -3&150 K (see Table l), and the quantum energy range -2-12 meV
which determines the material and impurity levels for photoconductive detectors as well as SIS
elements. At the same time black bodies with temperatures between -5 and 30 K have their
radiation maxima in the range 500-100 pm.
All these phenomena belong naturally in the wavelength range of interest and are a consequence
of numerical magnitudes of certain fundamental physical quantities.
The following sections give a brief and selective description of potential applications for
heterodyne spectrometers, Although not complete, they are ones which I believe are of great
interest, and moreover they have had and will still have a tremendous impact on further
instrumental development.
5.1. Atomic and molecular spectroscopy
The wavelength range of the energy of most of the motions of molecules lies in the range from
0.4 pm to the cm-range with electron transitions in the ultraviolet or visible spectral region. If the
molecular motion is simply that of vibration the wavelengths are in the IR from about 1 to 25 pm
and if the molecule just rotates the wavelengths are, in general, longer than 25 pm. Table 3 gives
examples of fundamental rotational transitions of a few hydrides and Fig. 16 shows the energy level
diagram of carbon monoxide (CO), which is one of the most prominent gases in atmospheric
physics and astronomy, but for completely different reasons.
14
v= 1.612Gtlz
A= 186 pm
13
wl 12
t”
2 11
a
z
10 .
g
8,
P
z9
4
Table
NH,
HCI
W
CH+
Hz0
OH
HD
H*
3. Fundamental
molecular
transitions of hydrides
JK = lo-O0
J= I*+0
J= 10,-o,
J= 1-O
J KpKs = 1I, -003
=Q J = S/2+3/2
J= l-0
5=2-+0
A=
b
217 pm
v=l.267GHz
A= 237 pm
v=l.O37GHz
289 pm
A-
7'
v= 807 GHz
A= 372 pm
6 b
rotational
523.7 firn
479.0 pm
421.1 pm
359.0pm
269.3 pm
119.4pm
112.1urn
28.l;m
~1.382 GHz
0VIBRATIONAL
Fig,
GROUND
STATE
V=O
16. Energy level diagram of the CO rotational
transitions.
HANS-PETERR&ER
400
z
Nil2 N
r
57Sb6A
2329A
0.007eV
6548.lA
‘p,,,
157.71pm
0
‘5
'0
6583.d
24/2
Fig. 17. Atomic fine structure lines of ionized carbon (C+) and nitrogen (N+).
The energies of the discrete rotational energy levels depend upon the quantum number of total
angular momentum, and the quantum number of angular momentum about the unique axis. If the
molecules are small (diatomic) or have a framework of the categories “linear triatomic” molecules,
“spherical tops” or “ symmetric tops” the transition frequencies between two energy levels can be
determined by theory very accurately. However, for complex polyatomic molecules the theoretical
prediction has an accuracy of > + a few 100 MHz only.
A similar situation is given for the atomic fine structure lines in their ground states, where the
theoretical prediction with an accuracy of + 1 GHz is even worse. The fine structure lines are
produced by magnetic dipole transitions within multiplets of given spin and orbital angular
momentum whose states of different total momentum are split by spin-orbit interaction. Figure 17
shows the atomic fine structure lines of ionized carbon C+ and nitrogen N+.
A heterodyne spectrometer can now be used in a twofold respect:
-The
transition frequency between two energy levels can be determined with an
absolute value of 100 kHz, and
-by knowing the transition frequency the atom or molecule can be detected by remote
observation.
Replacing the telescope in Fig. 12 by an absorption or emission cell equipped with discharge,
Zeeman and Stark effect capabilities, one can measure the absolute frequency of molecular and
atomic transitions in order to determine, for example, the molecular constants with higher
Figure 18 illustrates the
accuracy. In addition pressure broadening can also be investigated. (84.g2,94)
1.0
26.7045
374.469
26.7011
374.516
w
f+y
+b-y
26.6978 lcm-‘1
374.563 Ipml
I
I
r-
0.9
1’
Kzl
0.6-
K:O
I
I
I
I
800.400
800.350
800.450
800.500
FREWENCY [GHzl
Fig. 18. Phosphine spectrum of the multiplet line (J = 3-2, K = 0,1,2) at 800 GHz/374 pm with a
0.5 -
I
800.600
I
800.550
resolution of 500 kHz per AOS channel; pressure 70 mtorr, absorption cell length 170 mm, integration
time 25 s.
Heterodyne spectrometers
401
detection of the multiplet spectrum of phosphine (PH3 : J = 3-2, K = 0, 1,2) in absorption of about
800 GHz/375 pm with a resolution of 500 kHz per channel. The integration time was 25 s for an
absorption cell of 170 mm in length and a pressure of 70 mtorr.(33T9’)
For spectroscopy in the laboratory it is recommended, if the molecule has a rich spectrum, like
methanol (CH,OH), to search for transitions with a Fourier transform spectrometer (FTS) and
then to use a heterodyne spectrometer, as described above, a laser magnetic resonance spectrometer
(LMR) or a tunable FIR spectrometer (TUFIR) for detailed investigations and absolute frequency
calibration.‘94)
For remote observation, as we will see in the next two sections, the detection of a transition
proves the existence of that particular molecule or atom and the line profile gives information about
density, pressure and velocity structure due to the Doppler effect. Here the exact frequency is
necessary to distinguish between species.
It is also worth noticing that in a few cases there are also vibration-rotation
transitions in the
100-500 ,um range. For example weakly bound atoms in a linear molecular chain, such as C, with
an expected lowest transition near 150 pm.(35)
5.2. Astronomy
The wavelength band between 50 and 500pm is at present one of the last sectors of the
electromagnetic spectrum which remains to be opened up completely for astronomical spectroscopy. This band contains numerous spectral lines of prime importance for the study of
interstellar matter e.g. low energy rotational transitions of small and light molecules (e.g. hydrides),
higher transitions of somewhat heavier molecules (like CO) in excited rotational states corresponding to elevated temperatures, as well as important fine structure transitions, like in O”, C+ and N+,
which in contrast to molecules, do not have transitions in the microwave range.
Observation of these lines promises to advance the understanding of interstellar chemistry, of
the energy content of interstellar clouds, and of star formation processes in different regions of the
Galaxy. For example, the emission from lower CO transitions (J < 4) towards molecular cloud
cores, which might have a proto-star embedded in them, have in general high opacity, especially
at the line center, which prevents observations deep within the core. The cold foreground gas has
no emission in higher rotational transitions, e.g. CO J > 6, which represents hot gas with lower
opacity, allowing conditions nearer to the possible embedded source to be observed.
The higher the observed CO transition the deeper inside the cloud the physical information comes
from, With the detection of different high transitions, and observing several warm and hot
star-forming regions, one hopes to obtain temperature and density information as a function of
velocity and, in some sources, of position. This will permit correlation with the global properties
of the region.
The first heterodyne systems based on the technology described in Section 4’3’,74,75)
were used at
433 /lrn for the detection of CO (J = 65). These receivers were bulky so they could only be used
in a Coude room. Since then several groups have developed compact heterodyne spectrometers for
installation at the Cassegrain focus of ground-based telescopes’39~42~49~56’
and even more sophisticated
systems for the KAO which meet airborne specifications. (s,76,79~“4)
Up to now nearly 20 new
molecular and atomic lines have been discovered (Table 4) and more than 50 publications on
galactic and extragalactic astronomy testify the diversity of high resolution submillimeter and FIR
astronomy.
We report here on recent research with our own heterodyne spectrometer. Figure 19 shows the
first high resolution detection of CO transitions in the Orion molecular cloud 1 (OMC- 1) performed
from abroad the KAO in September 1988 and February/March 1989.(80~8’)
These are typical spectra
chosen from small maps around BN/KL (Becklin-Neugebauer/Kleinman-Low)
for each transition. The number, e.g. - 60/ - 180, indicate the off-position from BN/KL: - 60”east/ - 180”south.
Previous mapping of OMC-1 in the CO (J = 7-6) transition showed the warm gas component
to be extended over several arcminutes .(88,93)
This is remarkable because the excitation energy
required to populate the level J = 7 is equivalent to a temperature of 155 K. It then turned out
that still higher rotational lines can be seen over an extent of several arcminutes at high antenna
temperature levels (TA N 70 K), although they require considerable excitation energies: CO J = 12
has an excitation temperature of 430 K. The spectral lines in Fig. 19 have interesting velocity
402
HANS-PETER RISER
Table 4. First detections of molecular and atomic transitions in molecular clouds with
heterodyne spectrometers
‘TO
‘TO
‘TO
‘*co
‘TO
‘TO
“CO
‘TO
“CO
“CO
“CO
Co
C+
HCN
HCO+
SO,
CH,OH
OH
Unidentified line
J=C5
J=7-6
J=9-8
J= 11-10
J= 12-11
J= l&l3
J= 17-16
J = 22-21
J=&5
J=7-6
J=9-8
‘P,-‘P,
2P,:,-2P, 2
J=9-d
5=9-a
7x-6,s
13,-12,E
Jp = 5i2--3/2-
Fetterman et al., 1981””
Schultz er al., 1985””
Roeser et al., 1989’80’
Roeser et al., 1989’w’
Roeser ef al.. 1989””
Roeser e, al.. 1989’*”
Boreiko et al., 1989”01
Boreiko et al., 1989””
Graf er al., 1990””
Roeser 1990’
Boreiko et al., 1991””
Jaffe er al., 1985@”
Boreiko er al., 1988”’
Stutzki et al., 1988’w’
Jaffe et a/., 1989””
Stutzki er ul., 1989”N’
Stutzki et al.. 1989”w’
Betz et al.. 1989@”
Stutzki et al.. 1989”“’
433.6 pm
371.7 pm
289.1 pm
236.6 pm
216.9 pm
186.0 pm
153.3 pm
118.6pm
453.5 pm
388.8 pm
308.4 pm
370.4 pm
157.7 pm
376.0 pm
373.6 /urn
376.2 Nrn
373.7 pm
119.2 pm
370.3 urn
structure depending on the position in OMC-1 and the transition. A Doppler shift of 1 km/s is
equivalent to a frequency shift of - 3 MHz at - 300 pm. In several different areas we found the
high CO transition to have extremely narrow linewidths of about 2 km/s (see Fig. 20), which is
not the case for lower transitions. At these positions linewidths decrease systematically with
increasing excitation temperature, which is in contrast to what has been expected.
More recently a first detection of the isotopic 13C0 transition (J = 74) at 388.8 pm has been
made in W 51 with the KAO in July 1990 (see Fig. 21).
5.3. Atmospheric physics
Most of the atoms and molecules which are relevant for astronomy are also of interest for
atmospheric physics, e.g. H,O, 03, ClO, NH,, CO, OH and their isotopes. Furthermore, in
astronomy the atmosphere is a hindrance, an unavoidable filter with its own emission and
,‘~~‘,““,““l~l’“‘l”“l””
1”“1”“1”“1”“I
9
-8
*O
ORION
-60
-180
11-l
60
V [km s-'I
Fig. 19. Typical spectra of CO in the Orion molecular cloud (OMC-I): J = 12-11 at 217 pm, J = 14-13
EA+
1QI:
,,m
I -0-Q
I+
3110 ,,m
I 1 I-In
It
717
,,m
Heterodyne spectrometers
403
T;lK:
60
40
20
0
I
I
I
20 10
I
0 -10
V [kms-'I
Fig. 20. CO (J = 12-l 1) emission at the ionization front in Orion with an extremely narrow line width.
absorption spectra, which could falsify the detected interstellar spectra. Therefore astronomical
heterodyne spectrometers have also been used many times from the ground and the KAO to
investigate the atmospheric transmission curve by measuring mainly absorption spectra, e.g. CO
and 0,.
The line profile of the line of sight through the atmosphere, vertical or horizontal, gives
integrated information about the molecular concentration as a function of altitude or position in
convolution with the Doppler and pressure broadening. Knowing the temperature and pressure
profile of the Earth’s atmosphere, it is possible to derive from an accurately measured line profile
the volume mixing ratio. As an example, Fig. 22 shows such a profile of CO obtained by measuring
the CO (J = 65) transition at 434 pm from the ground. (‘w The dashed curve with asterisks (*)
are results obtained with balloon flights using a gas chromatographic analysis of air samples
collected from altitudes up to 35 km. c3’)Figure 23 is another example for the detection of ozone
at 214 pm taken from an airplane with an integration time of only 300 s.
In principle the same technique can be used for almost all other molecules of interest. The great
advantage of this method is the remote observation from the ground or airplane without reaching
the stratosphere. Future space platforms will enable a view towards the Earth.
5.4. Plasma diagnostics
If one replaces the signal input in Fig. 12 by a strong and pulsed laser radiation which has been
used for collective Thomson scattering, from density fluctuations at the thermal level, one can
measure the ion temperature of a tokamak plasma. Due to the extremely small Thomson scattering
1
I
I
I
I
,I
I
771.185
GHz
I
I
I
I
FREQUENCY
[lOOMHz/divl
Fig. 21. First detection of ‘TO (J = 7-6) at 388.8 pm in W 51 with the KAO in July 1990.
HANS-PETERR&R
404
*
I\
20 -<.-*...__
* I
I
vo:~;E
MlXlNG %o
1O-5
lo-*
Fig. 22. Volume mixing ratio of CO in the Earth’s atmosphere
from Ref. (109). The dashed curve
with asterisks represents results obtained with balloon measurements
and gas chromatographic
analysis
from Ref. (32).
cross section of 6.6 x 10ez9 m’ the signal levels which have to be analyzed
(< 10ei8 W/Hz). Here the heterodyne technique offers several unique properties:
-The
-The
-The
are very low
sensitivity is close to the quantum noise limit
laser pulse of about 1 1~s requires a fast detector
operation of the spectrometer is not too sensitive to a tokamak environment.
Recent successful experiments have been carried out by research groups from the ETH in
Lausanne/Switzerland
and the University in Diisseldorf/F.R.G.
using a pulsed D,O-laser at
385 pm on a tokamak. c4)Measurements have been carried out in H, D and He plasmas with electron
densities above 5 x lOI m-3 and already a single laser shot was sufficient to determine the ion
temperature. Figure 24 shows the scattering spectrum for a He plasma recorded in a single laser
shot.
6.
SUMMARY
A single heterodyne spectrometer can be used for the wavelength range from 100 pm to 500 pm
with a sensitivity which is about a factor 200 above the quantum noise limit in the whole range.
With a resolving power of 106-10’ and a spectrometer instantaneous bandwidth of 1 GHz the
heterodyne receiver will be an important tool for spectroscopy of molecules, atoms and solid state
devices. Since its first use in astronomy this technique has become widely used in different fields
I
I
I
I
I
I
I
1,381.670 GHz FREQUENCY
Fig.
23.
Ozone
spectrum
of
the
transition
(14,,,-13,,,)
against Jupiter.
at
I
I
I
[lOOMHz/divl
214 pm/1381.670
GHz
in absorption
Heterodyne spectrometers
3
I
=
+
zz-
I
I
I
I
o T
<
2
d
405
I
I
t$ =IxlO”rn~~
T, =670eV
&a =4.4
Ti ~250eV
-
,’
=
i7i
5 lz
?2
25
50
w-l
0
0
I
0.2
I
0.4 0.6 0.8 1.0
FREQUENCY KHz1
1.2
1.4
Fig. 24. Measured spectrum for a He-plasma recorded in a single laser shot.
of research. Important in this respect was the demonstration of an airborne system which has
proved its usefulness in a hostile environment. Expected improvements in sensitivity, higher
bandwidth, reliability and extension much below 100,~m will ensure an exciting future.
Acknow/edgen?enfs-Several researchers have read the manuscript and given comments. 1 feel that their contribution has
been very important to make this work more complete. In particular I wish to thank G. Ediss, A. Harris, M. F. Kimmitt,
R. Schieder, A. Betz and R. Wattenbach for their discussions.
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K. J. Button), Chap. 1, pp. 2-87. Academic Press, New York (1986).
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A. Pechelon, Phys. Rev. Lett. 62, 2833 (1989).
5. A. L. Betz and J. Zmuidzinas, NASA Conf. Publ. 2353, pp. 320-329 (1984).
6. A. L. Betz and R. T. Boreiko, Ap. J. Letf. 346, LlOl (1989).
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