Infraredfhys. Vol. 32, pp. 385407, 1991 Printed in Great Britain

HETERODYNE SPECTROSCOPY

0020-0891/91 $3.00 + 0.00 Pergamon Press plc

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 in astronomy and atmospheric physics, as well as in the laboratory

-plasma diagnostic experiments, -semiconductor physics, -laser diagnostics, -environmental research, -metrology and -radar techniques.

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 useful- ness 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, 100-500 pm Frequency v 3.000-600 GHz Frequency J 100_20cm- Photon energy hv 2 x lo-“‘-4 x lo~*‘ws Quantum energy hv 12-2.5 meV Quantum condition hv = kT 145-29 K Wien d lnrr 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

700 600 500 450 4UO 350 3oou

0.0 IL 16 18 20 22 24 26 28 30 32 cm-’ Fig. 1. Atmospheric transmission from the ground at 4200 m altitude and 1 mm perceptible water vapour

content in combination 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.

388 HANS-PETER R&R

RADIATION FILTERS d/AR TUNABLE

SPECTROMETS SIGNAL DETECTOR

Fig. 3. Direct detection 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. More recently Poglitsch et al. (70) have developed an imaging spectrometer with two or three 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 r-l co k12-41

h vs,,:1,382GHz

GaAs SCHOTTKY

DIODE

MIXER

HEMT-

AMPLIFIER

FILTER

SIGNAL

1,397GHz LASER

LOCAL

‘KILLATOR

vIF ~lv~,~-vt~l= 15GHz

[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 specifica- tions, 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 superconduc- tor-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

390 HANS-PETER RISER

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 L

90” CORNER REFLECTOR

GaAs SCHOTTKY BARRIER DIODE

Fig. 6. Scaled cross-section of a Schottky contact with Fig. 7. Quasi optical mixer structure with long wire antenna equivalent circuit of the junction. 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 submil- limeter 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-wave- length.““’ 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-PETER RISER

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 Martin- Puplett 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-multi- pliers.‘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.

The first pioneering work in this area (3’,73) has been done with bulky prototypes of nearly 1000 kg 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

X=287um

393

-LO -20 0 t20 tL0

[degl

Fig. 8. Antenna patterns of an open structure 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.

394

Laser molecule

HANS-PETER R&ER

Table 2. Powerful laser local oscillator lines for the detection of CO with associated IF ranges

Wavelength Frequency Pumping-line Frequency IF f urn) (GHz) CO,-laser Molecule Transition (GHz) (GHz)

HCOOH 432.6 “NH, 373.4 CH,F, 289.4 CH,F, 281.7 CH,F, 235.1 CH,F, 214.5 CH,F? 184.3 CH,OH 163.0 CH,OH 118.8

692.95 I ,4 9 R(20) 802.987,O IO R(42)

1035.552,7 9 P(4) 1042.150,4 9 R(34) 1272.171,4 9 R(32) 1397.118,6 9 R(34) 1626.602,6 9 R(32) 1838.839,3 IO R(38) 2522.781.6 9 P(36)

co J= 65 co J= 7-6 co J = 9-8 co J = 9-8 co J= II-IO co J=l?-II co J = 14-13 co J = 16-15 co J = 22-21

691.5 I.4 806.7 3.7

1036.9 I.3 1036.9 5.2 1267.0 5.2 1381.9 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 1 j K

Heterodyne spectrometers 395

require a long integration time, non-sweeping spectrometers have to be applied, such as filter- banks, 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 band- widths 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,)min N 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

Fig. 12. Diagram

OIPLEXER

I L I

of the MPIfR heterodyne spectrometer for the range 100 pm to 500 the KAO telescope.

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 Tsys and 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 Tsys with 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 .O ff 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 397

WAVELENGTH Ivml 600 300 200 150 120 100

O500 I I I I

1000 1500 2000 2500 3000 FREQUENCY IGHzl

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.

Fig. 14.

0 / ' 1111"' I tllll" d 'g'L1'J I '"'ll'J 10-j 10-I IO“ 1 PLo [mWl 10

System noise temperature of the GaAs Schottky barrier diode 1112 as a function at 432, 214 and 118 am.

of LO power

398 HANS-PETER R0.m

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 from about 70pm to 300pm. (1.57~‘07) The pulsed output power is in the range l-10 W with pulse 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-1 500 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 10m6 W, 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 : 8.4 MHz

1230 1266 1330

Frequency IMHzl

Fig. 15. Spectral linewidth of a p-type Ge-laser emission at 120pm measured with a heterodyne spectrometer.

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.

Table 3. Fundamental molecular rotational transitions of hydrides

NH, JK = lo-O0 523.7 firn HCI J= I*+0 479.0 pm

W J= 10,-o, 421.1 pm CH+ J= 1-O 359.0pm

Hz0 J KpKs = 1 I, -003 269.3 pm OH =Q J = S/2+3/2 119.4pm HD J= l-0 112.1 urn H* 5=2-+0 28.l;m

14 v= 1.612 Gtlz A= 186 pm

13

wl t”

12 ~1.382 GHz

2 A= 217 11 pm

v=l.267 GHz a

10 b A= 237 pm

z . P z9 v=l.O37 GHz g

8, A- 289 pm

4 7' v= 807 GHz 6 b A= 372 pm

0-

VIBRATIONAL GROUND STATE V=O Fig, 16. Energy level diagram of the CO rotational

transitions.

400 HANS-PETER R&ER

2329A

0.007eV

0 z ‘p,,, 157.71pm

24/2

Nil 2 N

r ‘5

57Sb6A

'0

6548.lA 6583.d

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 accuracy. In addition pressure broadening can also be investigated. (84.g2,94) Figure 18 illustrates the

26.7045 26.7011 26.6978 lcm-‘1 374.469 374.516 374.563 Ipml

1’

I I 1.0 +b-y w f+y r-

0.9

0.6 - Kzl

0.5 - K:O

I I I I I I 800.600 800.550 800.500 800.450 800.400 800.350

FREWENCY [GHzl Fig. 18. Phosphine spectrum of the multiplet line (J = 3-2, K = 0, 1,2) at 800 GHz/374 pm with a 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 spec- troscopy. 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 correspond- ing 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 tran- sition. 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 J=C5 433.6 pm Fetterman et al., 1981”” ‘TO J=7-6 371.7 pm Schultz er al., 1985”” ‘TO J=9-8 289.1 pm Roeser et al., 1989’80’ ‘*co J= 11-10 236.6 pm Roeser et al., 1989’w’ ‘TO J= 12-11 216.9 pm Roeser ef al.. 1989”” ‘TO J= l&l3 186.0 pm Roeser e, al.. 1989’*” “CO J= 17-16 153.3 pm Boreiko et al., 1989”01 ‘TO J = 22-21 118.6pm Boreiko et al., 1989”” “CO J=&5 453.5 pm Graf er al., 1990”” “CO J=7-6 388.8 pm Roeser 1990’ “CO J=9-8 308.4 pm Boreiko et al., 1991”” Co ‘P,-‘P, 370.4 pm Jaffe er al., 1985@” C+ 2P,:,-2P, 2

J=9-d 157.7 pm Boreiko er al., 1988”’

HCN 376.0 pm Stutzki et al., 1988’w’ HCO+ 5=9-a 373.6 /urn Jaffe et a/., 1989”” SO, 7x-6,s 376.2 Nrn Stutzki er ul., 1989”N’ CH,OH 13,-12,E 373.7 pm Stutzki et al.. 1989”w’ OH Jp = 5i2--3/2- 119.2 pm Betz et al.. 1989@” Unidentified line 370.3 urn Stutzki et al.. 1989”“’

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

Fig. 19.

1”“1”“1”“1”“I ,‘~~‘,““,““l~l’“‘l”“l””

9 -8 ORION *O -60 -180

11-l

60

V [km s-'I 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 I

20 10 0 -10 V [km s-'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 I I I I

771.185 GHz 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.

404 HANS-PETER R&R

* I\

20 -<.-*...__ * I I

lo-* vo:~;E MlXlNG %o

1O-5

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 are very low (< 10ei8 W/Hz). Here the heterodyne technique offers several unique properties:

-The sensitivity is close to the quantum noise limit -The laser pulse of about 1 1~s requires a fast detector -The 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 I I I

1,381.670 GHz FREQUENCY [lOOMHz/divl Fig. 23. Ozone spectrum of the transition (14,,,-13,,,) at 214 pm/1381.670 GHz in

against Jupiter. absorption

Heterodyne spectrometers 405

Fig. 24.

3 I I I I I I I t$ =IxlO”rn~~

= T, =670eV

+ &a =4.4

zz- o - Ti ~250eV T

2 < - d

= ,’ i7i 5 l-

z 0

?2 25

50 w-l I I

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

FREQUENCY KHz1 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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