atmospheric absorption of em waves

8/7/2019 Atmospheric Absorption of EM Waves

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Millimetre Wave and Terahertz Technology for the Detection of

Concealed Threats A Review

Michael C Kemp*

Iconal Technology Ltd, St Johns Innovation Centre, Cambridge, United Kingdom

ABSTRACT

There has been intense interest in the use of millimetre wave and terahertz technology for the detection of concealed

weapons, explosives and other threats. Electromagnetic waves at these frequencies are safe, penetrate barriers and have

short enough wavelengths to allow discrimination between objects. In addition, many solids including explosives havecharacteristic spectroscopic signatures at terahertz wavelengths which can be used to identify them.

This paper reviews the progress which has been made in recent years and identifies the achievements, challenges andprospects for these technologies in checkpoint people screening, stand off detection of improvised explosive devices

(IEDs) and suicide bombers as well as more specialized screening tasks.

Keywords: Millimetre wave, terahertz, terahertz imaging, terahertz spectroscopy, security screening, explosives

detection, stand-off detection

1. INTRODUCTION

The increased threats of criminal or terrorist action in recent years have led to the development of many techniques for

the detection of concealed weapons, contraband, explosives or other threats. These include metal detectors, X-ray

scanners, trace detection systems for the detection of vapours or particles of explosive left behind during handling,nuclear quadrupole resonance, neutron activation and other systems based on energetic radiation 1. Systems based on

electromagnetic radiation between, 1cm and 100 micron (30GHz 3THz) have particular advantages that:

Radiation penetrates many common barrier materials enabling concealed objects to be seen

Wavelengths are short enough to give adequate spatial resolution for imaging or localisation of threat objects Radiation at these frequencies is non-ionising and, at modest intensities, safe to use on people

This has led to the development of imaging systems based on millimetre waves, usually defined as wavelengths 1cmdown to 1mm (30 300GHz) and the investigation of even shorter wavelengths 1mm down to 100micron (300GHz

3THz) in the terahertz region. The motivation for working in each of these regions is different.

In the millimetre wave region, it is possible to produce purely passive systems which detect objects through acombination of their own thermal black body radiation as well as reflection from the sky or other ambient illumination.

Source and detector technology is relatively well developed in this region.

It is harder to work at terahertz frequencies, indeed the lack of practical sources and detectors for many years led to the

region becoming known as the terahertz gap. The region, however, has two potential advantages. Firstly, the higher

frequency means that systems can be physically smaller for the same resolution. Secondly, many materials, includingcommon explosives, exhibit characteristic terahertz spectral features which can be used to identify them. This leads tothe promise of direct detection of threat materials rather than simply inferring their possible presence by detecting an

anomaly which has to be resolved through further, physical inspection.

Interestingly, the generation and detection of millimetre waves dates back to the very earliest days of radio. J C Bose in1896 only a few years after Hertzs pioneering work used a spark gap transmitter and point-contact metal-oxide detectors

*[email protected]; phone: +44 7768 258965; www.iconal.com

Invited Paper

Optics and Photonics for Counter-Terrorism and Crime Fighting II, edited by Colin Lewis, Gari P. Owen,Proc. of SPIE Vol. 6402, 64020D, (2006) 0277-786X/06/$15 doi: 10.1117/12.692612

Proc. of SPIE Vol. 6402 64020D-1


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to generate and detect radio waves of 5mm wavelength (60GHz) and carry out a wide range of experiments into

reflection, refraction and polarization of this newly discovered radiation2,3,4. More sensitive systems however, requireamplifiers and oscillators and the development of active electronic devices capable of operating at higher and higher

frequencies began in, and continued throughout, the 20th century. One driver was the development of higher frequency

and more compact radar systems from WW2 onwards. Another was radio astronomy, where again shorter and shorter

wavelengths were used, both to increase the resolution of telescopes, to detect cool dust clouds whose peak of thermal

emission is in the millimetre wave region and to detect molecular species in gas clouds through their rotational spectra.Much of the early development of passive millimetre wave imaging has been for military applications such as aircraft

landing aids for seeing through rain and fog and a number of the millimetre wave security imagers being developed

today have their origins in these developments.

Generation and detection of terahertz radiation presents further challenges. The maximum frequency of an electronic

device is inversely proportional to the transit time, the time taken for a charge carrier to travel across the device.Fabrication limits and electrical breakdown mechanisms mean that it is hard to make devices operate above a few

hundred GHz and the power of electronic sources falls as 1/f2 or 1/f3. The alternative method is to use approaches from

optics, but the low photon energy (h) at terahertz frequencies compared with thermal effects (kT) limits the

performance of optical techniques hence the terahertz gap. Development of a variety of source and detectorapproaches to fill the gap, both electronic and optical, from the 1970s has been chronicled by Chamberlain5 and has led

to the realisation of practical laboratory systems and stimulated significant scientific research.

These advances have been accompanied by much interest in possible applications of the technology, just as has

happened in the past as other parts of the spectrum have become accessible. Interest in terahertz technology has focussedon the security6,7, pharmaceutical8,9, non-destructive testing (NDT)10, and medical industries.11,12 as well as continuing

applications in astronomy and space science.

A number of different concealed-threat detection applications have been explored including the screening of

people13,14,15, screening for people such as stowaways and illegal migrants at border crossings16, postal screening of

letters and parcels17, and, baggage screening18. People screening applications focus on the detection of non-metallic

weapons and explosives in aviation security and in the protection of sensitive facilities for example, as well as on stand-off suicide bomber detection and the detection of weapons carried by potential intruders or assailants. Mail screening

applications under development include the detection of drugs-of-abuse through their characteristic terahertz spectra as

well as explosives. In addition, the anthrax letter attacks in the USA in 2001 have prompted investigations into the

detection of white powders using both spectroscopy and imaging. Baggage screening has also been investigated

although as we shall see, absorption and scattering by the many layers of clothing in a typical, heavily-packed suitcaserepresents a significant barrier.

There have been several valuable reviews of related areas: Siegel19 and Chamberlain5 review terahertz sources and

detector technology; Woolard et al.20 describe a range of terahertz applications including detection of biological material;

Lettington21 describes millimetre wave systems and approaches; Federici et al.22 focus on security applications at

terahertz frequencies whilst Appleby & Wallace23 cover stand-off detection from 100GHz through 1THz. This paper

provides a broad overview of homeland security hidden threat detection applications across the frequency range 30GHzto 3THz, at close range and stand-off, using both imaging and spectroscopy.

We begin in section 2 by considering the properties of threat, confusion and barrier materials at millimetre and terahertz

frequencies. Sections 3 and 4 introduce the physics of imaging and spectroscopy systems, source and detectortechnology. In section 5 we describe some of the practical systems which have been developed for millimetre wave

imaging, whilst section 6 covers terahertz system developments. Section 7 discusses some issues and trends for futuredevelopment.

2. PROPERTIES OF MATERIALS

Active imaging systems rely on the reflection of radiation, whilst in passive systems, image contrast results from a

combination of emissivity and reflectivity. Detection of hidden objects depends on the transmission of radiation through



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barrier materials as well as through the atmosphere. Inhomogeneous materials also scatter incident radiation to a greater

or lesser extent. All these properties of materials at millimetre and terahertz frequencies are relevant to the design ofdetection systems.

In gases, millimetre wave and terahertz radiation excites mainly rotational modes in molecular spectra 24. The line

structure is complicated by interactions between vibrational and rotational modes. At low pressures, the existence of

many lines provides distinct fingerprints but at atmospheric pressures, lines are pressure broadened with typical line-widths of a few GHz.

At millimetre wave frequencies, non-conducting solids and liquids behave as dielectrics with a typical refractive index

range between 1 and 3. Since reflectivity is related approximately to the refractive index by:2

1)(

1)()(

+

n

nr

This means that most materials reflect between 1% and 25% of the incident radiation. Absorption coefficients are

typically a few dB/mm at 100GHz and rise with frequency due to a number of mechanisms including scattering.

Conducting liquids such as water have a reflectivity of 40% at 100GHz falling quickly to 20% at around 500GHz and

then levelling off25

, and are very strong absorbers such that penetration into water or the human body is only a millimetre

or so26

. Skin behaves similarly to water with a somewhat lower reflectivity11,13

. Metals are more or less perfectreflectors.

At terahertz wavelengths materials (solid, liquid and gas) absorb more strongly and refractive indices tend to be lower

leading to smaller reflectivity. The increased absorption is due both to resonances in the materials and scattering by the

microstructure of many substances. Absorption coefficients vary very widely. Some materials, such as plastics, remainvirtually transparent (


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A t m o s p h e r i c A t t e n u a t i o n

R A I N F O G

1 0 0 1 0 0 0 1 0 0 0 0F r e q u e n c y - G H z

A V U " k , J i t f t t H) - j 1 h I l 1 I I L0 .

0 . 0 1

S T D

/ d1 0 1 0 0 0 0 0 1 0 0 0 0 0 0

Figure 1. Atmospheric attenuation at sea level from ref. 13, calculated using currently accepted models at sea level. Rain=4mm/hr,fog=100 metre visibility, standard atmosphere (STD)= 7.5g/m3 water vapour, 2xSTD=15g/m3 water vapour.

Continuing into the terahertz region, we see that atmospheric absorption gets progressively stronger and the number ofwater vapour lines increases. Whilst from a communications engineering perspective, the atmosphere has often been

regarded as opaque at terahertz frequencies, all is not lost. Again at distances less than a few tens of metres, there are

significant windows where, at least for active systems where the signal-noise ratio is 50dB or above, the atmosphere is

transparent enough to allow hidden object detection at modest stand-off distances. Also, the water vapour lines arenarrow enough, and have known positions, to allow their effect to be removed in spectroscopic applications.

Figure 2. Atmospheric attenuation measured at 293K, 27% RH. using a terahertz time domain spectrometer

Characteristics of Threat and Confusion Materials

We first reported the spectra of common energetic compounds (RDX, PETN, HMX, TNT) and commercial explosives

based on these compounds (PE4, Semtex-H) in 20036. These results are shown in figure 3 and have subsequently beenvalidated and extended by a number of groups17, 30-33 using time-domain terahertz spectroscopy and FTIR. The strong

absorption features, particularly of RDX-based explosives around 800GHz, open up the possibility of material specific

detection of these materials. It should, however be noted that practical detection systems will need to operate inreflection rather than transmission mode due to the high absorption coefficients of the explosives themselves and, for

people screening applications, due to absorption by the body. Figure 4 shows the refractive index of RDX and PETN; the

spectral features due to the resonances are still visible, but are much less strong in reflection geometry measurements.

0 1 2 3 4100

80

60

40

20

0

Attenuation(db/m)

Frequency (THz)



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The absorption features are also present in scattered radiation, but here the overall signal strength is much lower, again

making detection more challenging.

0 1 2 3 4

HMX

PETN

RDX

PE4

TNT

Semtex

Absorption(offsetforclarity)

Frequency (THz)

Figure 3. Terahertz transmission spectra of the raw explosive materials TNT, HMX, PETN and RDX together with the spectra of thecompound explosives PE4 and Semtex H.

10 20 30 40 50 60 70 80 90 100 110 1200

50

100

150

2000.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0 3.3 3.6

n

Frequency/THz

Wavenumber/cm-1

Absorption/cm-1

Absorption

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

Refr

active

index

10 20 30 40 50 60 70 80 90 100 110 1200

50

100

150

200

0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0 3.3 3.6

n

Frequency/THz

Wavenumber/cm-1

Absorption/cm-1

Absorption

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0

Refr

active

index

Figure 4. The THz absorption spectra (solid line) and refractive index (dashed line) of (a) RDX and (b) PETN.

It is not sufficient that threat materials have characteristic spectral features, these features must be distinct from harmless,

potential confusion materials. Also, barrier materials such as clothing or packaging used to conceal the threats must also

be free of confusing spectral features and, of course, must also be reasonably transparent at terahertz frequencies. We

and others have carried out several studies and measurement programmes to characterise common barrier materials andpotential confusion materials 34-36. Indicative results are shown in figure 5. Most barrier materials such as different typesof cloth, paper, cardboard, plastics are semi-transparent to terahertz with an absorption which rises smoothly with

frequency. Confusion materials, such as foodstuffs, confectionery, cosmetics, pharmaceuticals either have featureless

spectra or may have spectroscopic features in the THz range. However, having examined a large number of substances,we find spectra are distinct and we have not observed significant confusion between explosives and other materials.



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body radiation. In active systems, the reflection coefficient r becomes the dominant factor. Active systems usually use

coherent illumination and whilst the ability to measure amplitude and phase can provide additional resolution it can also be the source of significant interference effects or speckle. This effect can be minimised by including significant

frequency or angle diversity in the illumination. Since speckle will also be very sensitive to the exact position of objects

in the scene, being able to see a moving image significantly enhances the eyes ability to detect objects.

Resolution

The diffraction-limited angular resolution of an imaging system, is approximately /D where is the wavelength and

D is the aperture size. For an object at distance d, the spatial resolution is approximately d/D. For a resolution of 1cm

at a distance of 3m, this implies that a 35 GHz system requires an aperture of some 2.5m and 1m at 94GHz. At 300GHzthe aperture size is 30cm and at 1THz, just 10cm.

As pointed out by Mann40, the volume of a system for a given resolution scales as 3 and several other factors such as

weight and power consumption can also be expected to improve as the frequency of operation is increased. Since

detection systems will often be deployed in situations where space is limited, or where covert operation is required, this

provides a strong driver towards higher frequency operation. Set against this, is the increased cost of source and detectortechnology as frequencies increase and the increased absorption of most barrier materials.

Depth of field is also an important design factor. Systems operating at a low f-number (aperture/focal length) have asmall depth of field and may require a focussing element in order to capture elements in a scene at different distances.

Operating at higher frequencies allows larger f-numbers and more depth of field.

Noise and sensitivity

The sensitivity of an imaging system is the lowest temperature difference which it can detect. This is the temperature

difference equivalent to that of the thermal and other noise in the antenna and detector circuits and is known as Noise

Equivalent Temperature Difference (NETD) or alternatively NET.

NETD = NA+NT/(B)

where NT is the Noise Temperature of the detector, NA is the effective temperature of the antenna, B is the RF bandwidth

and is the post-detection integration time. Receiver Noise Temperatures can range from a few hundred to a few

thousand K. An NT of 1000K, an RF bandwidth of 20GHz and an integration time of 50us, leads to an NEDT of 1K.Cooled receivers will have improved noise performance, although an ambient temperature scene will have its own

thermal fluctuations due to its radiometric temperature T and there is no point in the receiver having significantly lowernoise than this. It should be noted that the system NETD will be greater than the raw NETD of the receivers due to

losses in the imager optical system, which, in a multi element scanned system can be a factor of 5 or more. An extensive

treatment of noise issues and calculations can be found in Brown41.

With up to 200K contrast available from the cold sky, outdoor millimetre wave imaging requires system sensitivity in the

order of 5K whereas for passive imaging indoors where the maximum contrast is significantly smaller, a sensitivity of

1K or better is needed.

Pulsed systems such as radar imagers or pulsed terahertz systems can also measure depth information 6,42. This canimprove detection and system sensitivity by techniques such as range-gating to improve contrast. If, for example, a

bright reflection from the surface of an object such as clothing can be gated out, this can increase the contrast from anobject hidden underneath.

Multispectral and spectroscopic detection and imaging

Since the absorption and reflectivity of many objects is frequency dependent, measurements at two or more frequenciescan be used to increase discrimination. Doyle42 has investigated the use of multifrequency measurements at 74, 94 and

140 GHz, however at these frequencies the differences in refractive index and absorption are relatively modest. At



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terahertz frequencies, there is more potential since pulsed terahertz systems, for example, can produce images with a

frequency range of 30:1. Kemp et al43 have shown how multi-spectral imaging can greatly improve the interpretation ofB-scan images see figure 6. In addition, the existence of true spectroscopic features in certain materials such as drugs

and explosives at terahertz frequencies can be used for detection purposes, and may lead to automatic detection of threat

materials based on their spectra rather than having to rely on shape clues from an image32,44,45.

Figure 6. Broadband and multispectral terahertz B-scan images of a number of objects hidden beneath two layers of cloth.

Imaging techniques

A variety of methods have been used to produce two and three dimensional imaging systems. A single detector may be

mechanically scanned across a scene using mirrors. Scanning time can be reduced by using a line array of detectors or afull two dimensional array, at the cost of providing many detectors. Lettington21 gives a review of the different possible

approaches, whilst this is aimed at millimetre waves, the same principles apply to terahertz imaging.

Array systems are of two basic types focal plane arrays (FPA) and pupil plane arrays, often called phased arrays. In the

former a lens, or more commonly at millimetre wave and terahertz frequencies, a mirror, is used to focus incoming

radiation onto the detector array to form the image. In the latter, the beam is steered electronically by varying ormeasuring the phase relationship between array elements. An alternative way of looking at these systems is to note that

the image plane is the Fourier transform of the radiation field across the entrance pupil of the system. In an FPA, this

transformation is carried out using a lens or mirror; in a phased array, it is carried out electronically. Pupil plane arrays

have the advantage that systems do not have to be physically deep in order to accommodate the focusing optic, althoughthe electronics required are significantly more complex.

Mechanically scanned millimeter wave systems typically use rotating tilted mirrors13,21,42 and, with multiple detectors,

can operate up to video rates (15 frames/second). Terahertz imaging systems, with few exceptions, are currently limited

to a single detector and these currently take several minutes to capture an image (in order to allow longer integrationtimes to reduce noise) with scanning effected by translating off-axis parabolic mirrors, or moveable fibre-coupled

detectors. Pulsed, time-domain systems also require time to scan the whole terahertz waveform, although this does

generate depth or spectroscopic information at each pixel. Electronic beam forming using interferometry or aperturesynthesis has been used in radio astronomy and synthetic aperture radar for many years and has recently been explored

by Federici et al22 for use at terahertz frequencies.

4. SOURCES AND DETECTORS

Millimeter wave sources and detectors

Millimeter wave sources may be required, either for illumination of active systems or as local oscillators in a heterodynereceiver. Up to 94GHz Gunn diode sources are available with powers of a few hundred mW. Above this frequency it is

common to use frequency multiplication. Most existing electronic solid state electronic sources are negative differential

resistance superlattice devices, such as Gunn diodes, where the negative differential electron velocity in GaAs or n-type

silicon induces oscillations in the circuit. Gunn diodes are robust and compact, and operate at room temperature.

5 mm

(free space)

Spectral plot: Channel 3

20 40 60 80 100 120

200

400

600

800

1000

1200

1400

1600

1800

2000

Plasticexplosive

(PETN)Plastic

explosive

(RDX)

Poly-

thene

Metal

plate



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Frequency upconversion by nonlinear reactive multiplication19 can be achieved in a chain of GaAs Schottky diode

multipliers46. Most implementations consist of doublers and triplers arranged in series47. In this manner, microwavesources at 20-40 GHz can be used to drive multiplication chains, providing approximately 1mW at 500GHz and 100W

at 1THz. Incoherent sources for artificial illumination of passive millimetre wave systems can be provided using heated

panels or electronic noise sources. Coward et al48 describe a millimetre wave light box designed to provide uniformdiffuse illumination source.

The challenge in millimetre wave detectors and receivers is the reduction of noise as well as complexity and cost,

particularly if an array of detectors is to be used to enable real-time imaging 41. Direct detection with several low noise

amplifier (LNA) stages using Micromachined Millimetre-wave Integrated Circuit (MMIC) techniques in front of a diode

detector is the generally favoured technique up to 100GHz especially in passive systems. Heterostructure High ElectronMobility Transistors (HEMTs) using GaAs or InP technology are used to minimise noise. These techniques are now

being developed up to about 200GHz49. Heterodyne or mixer-based receivers using Schottky diode mixers are also usedand this is the principal technique deployed above 100GHz at present.

Terahertz sources and detectors

A wide variety of techniques, both optical and electronic, have been considered to provide sources to help fill the

terahertz gap as described in the reviews by Chamberlain5 and Siegel19.

The most mature technology, and the one used in commercial terahertz systems for NDT imaging applications and

spectroscopy is the photoconductive switch where femtosecond pulses from an ultra-fast Ti:sapphire or similar pulsedlaser are used to induce rapid changes in carrier density, and hence conductance, of a metal-semiconductor-metal

junction50-52. By applying a bias voltage the accelerated electrical charges radiate a broad-band pulse of electromagnetic

radiation with a spectrum from below 100GHz to 4THz and above. The total power produced is typically in the 100nW-1W range. Coherent detection of the incident THz radiation can be performed in a similar photoconductive antenna

circuit. By gating the photoconductive gap with a femtosecond pulse synchronised to the THz emission, a DC signal that

is proportional to the THz electric field may be measured. Further, by varying the optical path length to the receiver, the

entire THz time domain can be sampled. In this way, both the amplitude and phase of the incident THz wave can beobtained. Pulsed terahertz systems can have a dynamic range of over 80dB and operate at room temperature. On the

downside, ultra-fast pulsed lasers are relatively complex and costly; optical power is limited so that large arrays of

sources and detectors cannot easily be constructed; and the need to sample the pulse in the time domain using some form

of mechanical delay line means that systems are relatively slow and the depth of field at any one time is small.

Optical heterodyne conversion, or photomixing, can be achieved using two continuous-wave (cw) lasers53,54. The mixing

of two above-bandgap (visible or near-infrared) wavelengths produces beating, which can modulate the conductance of aphotoconductive switch (semiconductor) at the THz difference frequency. Upon application of a bias, monochromatic

continuous-wave THz (cw-THz) radiation is produced, with 1 W powers reported at 1 THz. Coherent homodyne

detection is possible55. These all-photoconductive systems can be driven by inexpensive, compact and tunable diode

lasers56,57.

Both pulsed and CW photomixer sources have the advantage that they can easily coupled with a very sensitive, phase-

sensitive detector. Other, more powerful sources are available as described below, but the detectors with which they can

be coupled are much less sensitive.

Semiconductor diode lasers based on intersubband transitions, whilst prevalent in the visible and near-infrared, are

difficult to apply as THz sources since few semiconductors with suitable band gaps exist58

. Quantum cascade lasers(QCLs) overcome this problem by using a suitably engineered semiconductor heterostructure in which the electronmotion is confined along the growth direction, splitting the conduction band into discrete states59. The resulting

minibands allow THz transitions, and repetition of the structure allows each electron to cascade from one miniband to

the next, emitting a THz photon at each transition, greatly improving the efficiency. Both pulsed and continuous-wave

operation is possible, and the frequency is dictated by the layer growth thickness. Cryogenic temperatures are needed foroperation below 10THz and the lowest frequency achieved is currently around 1.9THz60 at 5K without the need for

strong magnetic fields. Lower frequencies will doubtless be achieved, although the 1-2THz region, which would be



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valuable for security applications, remains elusive. The maximum operating temperature achieved has increased from

liquid helium temperatures up to as high as 117K for CW operation and 164K for pulsed operation at 3THz 61. Thesedevelopments are close to the ranges where Peltier electric coolers can be used, although as with superconductors, it is

not clear whether room temperature operation can be achieved. In comparison to pulsed systems, QCLs exhibit in

excess of 3 orders of magnitude increase in power, typically up to tens of milliwatts.The tunability is limited to smallsweeps around a single, design frequency.

Vacuum tube sources have been made to work at THz frequencies, with some success. Tube configurations such as

backward wave oscillators, reflex klystrons, clinotrons, magnetrons and gyrotrons have produced power levels of several

milliwatts well into the THz regime19. Backward wave oscillators are probably the most advanced tube systems62.

Electrons from a heated cathode spiral towards the anode through a comb-like, corrugated, decelerating structure in amagnetic field, emitting radiation in the reverse direction. Output powers up to 100 mW are possible in the microwave

region, but this falls rapidly to typical values of 1 mW in the THz. Consequently, BWOs are primarily sub-1 THzsources.

A THz-wave optical parametric oscillator (OPO) is formed by placing a crystal with a high electro-optic coefficient

inside a Fabry-Perot cavity and pumping with an ultrafast laser (typically a Q-switched Nd:YAG). The principle ofoperation63 is the absorption of a photon, and the subsequent re-emission of two photons of lower energy by the

interaction of the pump laser with the difference frequency generation in the crystal, often LiNbO3. The THz seed is

then amplified in the cavity taking power from the pump beam. OPOs are tuneable by altering the cavity angle over arange of 0.7-3 THz demonstrated64,65 with an output power of up to a few milliwatts.

Other sourcesinclude CO2 laser to pump low pressure flowing molecular gases in THz laser cells. A strong transition of

methanol occurs at 2.52 THz 66 and can provide output power of 20 mW.

Detectors include heterodyne detectors based on Schottky diode mixers and broadband devices such as pyro-electric

detectors and bolometers, typically cooled to liquid helium temperatures to increase sensitivity. The radio astronomy

community uses these cooled detectors including Superconductor Insulator Superconductor (SIS) and Hot Electron

Bolometer (HEB) detectors19. Other detector concepts under development include Quantum Dot - Single ElectronTransistor (QD-SET) detectors67 capable of detecting single photons, albeit requiring very low temperature (100mK or

so) operation. Although still less convenient and more costly and bulky than room temperature devices, recent advances

in electrical Peltier, closed-cycle Stirling and pulse tube coolers are starting to remove the need for liquid cryogens in

cooled detector systems.

5. COMMERCIAL MILLIMETRE WAVE IMAGING SYSTEMS

A number of companies have developed millimetre wave systems for people screening for weapons and explosives. A

representative sample is shown in table 1. These systems are all designed for people screening, either in a walkthrough

portal configuration or for stand-off operation. The portal systems are generally designed with spatial resolution of

approximately 1cm to detect small threat objects. The stand-off systems are variously designed to operate at distances

between 3m and 30m and typically to detect larger threat objects such as a suicide bomb or larger weapon. Threedifferent operating frequency ranges are in use:

25 35 GHz 94 GHz 200 300 GHz

Each corresponds to one of the relatively broad windows that exist in atmospheric propagation. Atmosphericabsorption is not a significant issue for the relatively modest distances used in people-screening applications, but by

working in the same frequency ranges as some of the longer distance applications it is possible to benefit from more

readily available and lower cost components. At the lower frequencies, equipment needs to be relatively large, or

resolution is sacrificed. Higher frequencies enable more compact systems such as those developed by ThruVision40,68.

Also, especially for passive systems, it is easier to detect dielectric threat materials at higher frequencies since their

higher refractive indices at these frequencies makes them appear more metal like and reflective. Another compactsystem, operating at 94GHz for modest stand-off distances has been developed by Brijot69.



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Figure 7. Passive millimetre wave images from the 94GHz Smiths Tadar portal imager (courtesy Smiths Detection)

A number of different scanning systems are in use, both mechanical, electronic and hybrid. The Smiths Detection

system42 uses a pair of spinning tilted mirrors to scan in the X axis and a third mirror to raster scan in the Y axis on to the

detectors. The Qinetiq70 system uses a different arrangement with a single rotating mirror to produce a conical scan on

to a complete line array. It also exploits polarisers and a quarter-wave plate to enable a folded optic reducing the

dimensions of the scanning system.

Table 1: A sample of commercial millimetre wave imagers

Name L3

SafeView

Provision

100

Agilent Qinetiq Qinetiq

SPO 20

Smiths

Tadar

Sago

Trex

ST150

Sago

Trex Real

Time

Imager

Brijot

BIS-WDS

ThruVision

T4000

Application Portal Portal Portal Stand-off8-30m

Portal 5m Stand-off

Stand-off 3-10mStand-off

Stand-off

Active/

Passive

Active Active Passive Passive Passive Passive Passive Passive Passive

Frequency 24-30GHz 24GHz 35GHz 94GHz 94GHz 75.5-

93.5GHz

75.5-

93.5GHz

90GHz c250GHz

Bandwidth >10GHz 20GHz

Imaging

System

source &

receiverarray

rotatesaround

subject

Active

antennaarray:

programm-able

Fresnel

zone-plate

Folded

Schmidtcamera:

conicalscan, off-

axis

rotating

mirror

Mechanic

al : Tiltedrotating

mirrors

Frequency

scannedantenna

andreflector

Phased

array offreq

scannedantennas

No. of

receivers

1 64 64 24 1 232 16

Receiver

technology

InP

MMIC

InP Direct

detection

InP

HEMT

MMIC

GaAs

Schottky

mixer

System

NETD

5K 1K 1-3K 6K 1K 1-1.5K

(receivers)

Spatial

resolution

0.5cmlateral

1.5cm

depth

0.5cm 0.75cm2cm

0.3degree 10mm 6mrad 6mrad128*192

pixels

5cm 3cm

Refresh rate 6 views in3 secs 15Hz 15Hz 10Hz (24receivers) 0.5Hzvariable 30Hz 4-10Hz 1-3 Hz

Aperture 90cm 80cm 60cm 18cm 12cm

Dimensions

L x W x H

cm

150 x 150

x 270

90 x 10 x

90

250 x 160

x 220

71 x 33 x

48

Explosivesimulantsheet

Blade

Wallet



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The Trex Sago71,72 approach is to create an azimuthal line scan through a frequency dependent coupling into an antenna.

In the ST150, the second axis is provided by panning the main reflecting mirror. In their real-time imager, a phased arrayof some 230 receivers is deployed. The L-3 SafeView73,74 system is an active system based on developments from PNNL

using a holographic reconstruction from a fan-beam source and line array of detector antennas which rotate around the

subject being scanned. This system measures both amplitude and phase of the reflected signal for use in the FFT-basedholographic image reconstruction.

A novel approach has been developed by Agilent75 and demonstrated in prototype form. This employs a confocal

arrangement of a single source and detector. The source is reflected from a configurable mirror made up of a 2

dimensional array of several thousand dipole reflectors. Each dipole is connected to a large, fast switching array which

can place either a short or open circuit at the feed point of each dipole. This forms a programmable, reflecting Fresnelzone plate which can be used to focus the millimetre wave source and detector onto a chosen point in space in front of

the mirror. The antennas can be switched so as to scan over 107 voxels per second, leading to a solid state imagingsystem with a 15Hz refresh rate or higher. Although the frequency employed is relatively low, 24 GHz in the prototype

system, the programmable mirror is only a few cm thick, leading to relatively compact system.

Development directions in millimetre wave systems focus largely on methods to reduce the cost of systems, withoutsacrificing sensitivity or resolution. Since most of these systems use a number of detectors in order to be able to capture

moving images, the cost of multiple receivers is one of the major cost elements.

6. TERAHERTZ SYSTEMS7.

Above 300GHz, the field is still in the research and development phase, no commercial or operationally deployedsystems exist yet for security applications. There are several different types of approach being pursued by different

workers which can be divided into three main groups:

Sub-millimetre wave electronic component and system development aimed at single frequencyimagers operating between 300 and 600GHz

Component and system development for single frequency systems operating above 600GHz Broad-band imaging and spectroscopy

The first group is aimed mainly at reducing the footprint of current millimetre wave imagers both for security and

military imaging applications. Two DARPA programmes, SWIFT76 and TIFT77 are developing both illumination source

and focal plane array technology at frequencies around 300-400GHz and 600GHz. Source technology includes vacuumelectronic sources targeting 100mW at 600GHz as sources for active stand-off imaging at a few 10s of metres. Receiverdevelopments concentrate on arrays of heterodyne detectors and techniques for local oscillator power distribution across

the arrays. Above 600GHz, current work is mainly at the component level with work on increasing the performance of

all the sources and detectors described in section 4 as well as work in projects such as TeraSec 78 which is building aprototype stand off imaging systems using CW laser sources and a heterodyne detector array.

Figure 8. Raw passive indoors THz imagery obtained with an antenna-coupled microbolometer from ref. 83. The centre frequency isapproximately 450GHz.



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Lee and Hu79 have explored the use of microbolometer focal plane arrays for the real-time detection of terahertz images.They used a 4.3THz QCL source and a 320 x 240 pixel room temperature vanadium oxide microbolometer array camera

originally designed for use in infra-red thermal imaging at 7.5-14um but which had sufficient sensitivity at THz

frequencies to allow the imaging of small objects hidden inside an envelope in transmission mode at 60Hz frame rate.The authors make the point that such a system could be used with a number of QCLs each operating at different

frequencies to provide a measure of frequency discrimination. QCL sources have also been used for imaging smallobjects by Barbieri and Alton using a Schottky diode-based heterodyne detectors80.

Microbolometers provide an interesting broadband solution to the problem of detector arrays. They are intrinsically

simple, low cost, devices which can be designed in arrays to operate over a wide frequency range and have beendeveloped by Grossman and Luukanen81. Whilst at room temperature, their sensitivity is modest and need to be used in

an active system82, cooled to a few degrees K to reduce thermal noise, the sensitivity is considerably higher andmicrobolometers can be used for indoor passive imaging over a wide range from 100GHz to beyond 3 THz. Luukanen83

has developed Niobium (NB) and Niobium Nitride (NBN) based microbolometers with NETD of a few tens of

milliKelvin which are being developed into linear arrays. The goal is a real time (30Hz frame rate) conical-scan system

with a line array of 128 detectors, operating across a bandwidth of 200GHz to 3.6 THz with a system NETD of 0.5K.Laboratory images have been collected with a single pixel scanned detector at 10ms/pixel with an effective bandwidth of

at least an octave centred on 450GHz and an NETD of 200mK. An example is shown in figure 8. With a 30cm primary

optic, this system achieves a spatial resolution of 8mm on the whole body target. With such a broad bandwidth it ispossible to use bandpass filters based, for example on a frequency selective surface, to collect multi-spectral images

across the whole of the terahertz region84

Kawase39 has used an OPO based system to develop prototypes for inspecting mail for concealed drugs using a two level

system. The first level looks for powdered substances hidden in the letter by the increased scattering signal obtainedfrom powdered substances. If suspicious objects are detected, spectroscopic analysis is carried out to seek to identify the

substance.

We have used pulsed terahertz systems to demonstrate proof-of-principle stand-off explosives detection starting with thematerials characterisation of explosives, barrier and confusion materials, work on reflection spectroscopy and the

development of laboratory prototype systems for stand-off detection at a distance of 1m15. The system shown in figure

9, was used to demonstrate spectroscopic detection of RDX based explosives in real time (1/15 sec integration time)

beneath several layers of clothing at a distance of 1m, figures 10 and11.

Zhang has shown that a parallel collimated terahertz beam can travel up to 30m in air and then be focussed down, with a

remote optical system close to the target, to collect reflection spectra, again of RDX-based explosive85.

Femtosecond

Pulsed Laser

Beamsplitter

Long DelayStage

Detector

Emitter

Target

15Hz FastScan Delay

Stage

Lock-in

Amplifier

PC

Signal

Reference

ParabolicMirror

Parabolic

Mirror

THz Beam

Femtosecond

Pulsed Laser

Beamsplitter

Long DelayStage

Detector

Emitter

Target

15Hz FastScan Delay

Stage

Lock-in

Amplifier

PC

Signal

Reference

ParabolicMirror

Parabolic

Mirror

THz Beam

Figure 9. Schematic of a THz photoconductive system. In this example, specifically designed for stand-off explosives detection, the

THz beam is manipulated for reflection spectroscopy of a target material.



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0.50 0.75 1.00 1.25 1.50 1.75 2.000.02

0.04

0.06

0.08

Reflectance

Frequency/THz

0.50 0.75 1.00 1.25 1.50 1.75 2.000.02

0.04

0.06

0.08

0.10

Reflecta

nce

Frequency/THz

Figure10. The measured (solid line) and calculated (dashed line) reflectance spectra of (a) Semtex-H, and (b) SX2 at a stand-off

distance of 1 m, under normal atmospheric conditions, taken real time. The calculated spectra were derived from transmissionspectroscopy data.

0.50 0.75 1.00 1.25 1.500.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

Reflectance

Frequency/THz

Increasing layers of cotton

0.50 0.75 1.00 1.25 1.50-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

0.200 layers Cotton

1 layers Cotton2 layers Cotton

3 layers Cotton

4 layers Cotton

DerivativeReflectance

Frequency/THz

Figure 11. (a) The reflectance of SX2 behind cotton clothing, and (b) the first derivative of the reflectance of SX2 behind cotton

clothing. The RDX spectral feature is clearly visible in (b) through 4 layers of cotton.

These experiments show that pulsed systems are capable of providing point spectroscopic measurements in real time at

distances of a few metres at frequencies up to about 1.5THz. For larger distances and to detect substances based onspectral features at higher frequencies more power is needed. Although small scale arrays have been demonstrated using

pulsed terahertz15, arrays of many elements suitable for spectroscopic imaging in real time, require more optical

excitation power than can easily be provided by ultrafast lasers. In addition, long and fast delay lines are needed to process signals from objects at an arbitrary distance. Accordingly, whilst pulsed terahertz applications might find

practical applications in concealed object detection at relatively small distances, or where the target object geometry is

well constrained, other approaches are needed for stand-off detection.

One such approach, being explored by several groups22,56 is to use photomixer based sources and detectors. Like pulsed

systems, these can be operated in the very sensitive coherent homodyne detection mode which compensates for the very

low powers (below 1 W) generated by this technique. Since the laser sources are low-cost CW diode lasers and since

laser amplifiers can be used, these techniques are more suitable for developing arrays of either sources or detectors.Federici22 has explored the use of this type of source and detector in work on interferometric imaging to address thestand-off detection problem. As discussed in section 4, this uses the concepts of aperture synthesis to use an array of

detector pairs to provide coverage of the u-v plane. Extensive simulations have been performed, and recent work has

shown point-spread functions and images of simple objects produced by moving a single detector pair to sample the u-vplane. Spectroscopic information can be obtained by tuning the sources and detectors. This can either be done by tuning

one of the diode lasers so as to vary the difference frequency, or by exciting the devices with several laser frequencies

(a) (b)

(a) (b)



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simultaneously to generate a number of beat frequencies86. Further work is necessary to develop these techniques,

particularly to address the very significant fall-off in source power with frequency.

An alternative approach, due to Zhang90,91, to avoid the issues of atmospheric water vapour absorption in stand-off

detection, which has recently been demonstrated in the laboratory, is to propagate the optical pulse to the target andgenerate terahertz waves locally by a so called four-wave-mixing approach between a NIR frequency fundamental pulse

and a second beam at its second harmonic focussed down so as to breakdown air into a plasma. Mixing due to a thirdorder optical process generates terahertz. Zhang has also shown that, analogously to detection in a conventional pulsed

terahertz system, probe beams (again using a fundamental and second harmonic) can be used to detect terahertz radiation

in a similar plasma. The experiment used an amplified Ti:sapphire laser, generating 800 nm, 120 fs, 800 Jpulses at a

repetition rate of 1 kHz, which propagate in the atmosphere with minimal absorption. This is an ingenious approach,although it should be noted that, unlike all the other techniques discussed here, potentially harmful laser radiation is

directed at the target, rather than just terahertz waves.

6. ISSUES AND TRENDS

So far, we have concentrated on the basic physical factors sources, detectors, imaging system geometry, target and

barrier material properties which influence the performance of a system. Two other factors should also be considered.

The first is the use of algorithms and software to process the output in some way. The second is to consider the

millimetre-wave or terahertz device as simply one sensor in an overall detection system which may combine the outputof a number of complementary sensors so as to span the detection space more effectively.

Super-resolution techniques can be used to improve resolution by deconvolving the measured image with the point

spread function of the imager and this has been applied to millimetre wave imagery87-89. These resolution improvements,

however, come at the cost of increased noise in the image.

Other contrast enhancement techniques can be used to reduce the effects of instrumental artefacts or to enhance thedetection of threats based on known characteristics of threat objects. Where the characteristics of threats cannot easily be

modelled, trained neural network techniques can be applied. Federici22 has explored the use of these techniques on

simulated Terahertz images. Object identification when data on spectroscopic features is present can be extracted by

pattern matching techniques. Principal Component Analysis has been used to make so called chemical maps fromhyperspectral terahertz images44,45. Overall, however, the use of image reconstruction and other algorithms is less

sophisticated than in related fields such as medical imaging, ground penetrating and synthetic aperture radars, etc. and

it is likely that the field would benefit from cross-fertilisation from these areas.

A particular issue in people screening systems is that of privacy since millimetre wave systems match quite well the

popular image of X-ray specs to see through clothing. Various proprietary approaches have been developed for whole-

body imagers, either to identify and blur-out the genital areas, or to extract potential threat objects and then overlay them

on a conventional video image.

Detection systems may be improved by using more than a single frequency or band of frequencies. Sensor fusion,

combining the output of disparate sensors, so as to span a larger detection space and avoid the inevitable sensitivitygaps inherent in any one technique, can be applied in a number of ways. The use of multi- and hyper-spectral terahertz

data has already been discussed. Since terahertz imaging systems are, for the time being slower than millimetre wave

systems, a millimetre wave image could be used to identify potential threat objects for further investigation by a terahertzspectrometer. Going beyond this frequency range, Millimetre wave and video (CCTV) images can be overlaid and, of

course, it is possible to combine these with completely different types of sensor such as trace detection.

8. CONCLUSIONS

Developments of millimetre wave systems over a number of years have led to a range of commercial mm wave systems

mainly operating around 30GHz or at 94 GHz designed for a range of checkpoint and stand-off people screeningapplications and these are now beginning to become more widely used in the field. Higher frequencies enable more

compact systems and these are also starting to appear with the launch of the ThruVision passive imager and other



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components and systems under development moving up into the sub-millimetre or low terahertz frequency range. Whilst

commercial terahertz systems have been developed as analytical instruments and for specific industrial inspection tasks,detection of concealed threats has, so far been limited to laboratory demonstrations. Before systems can be produced for

operational use, further development is required, both in source and detector technology and in system architectures. For

stand-off applications, there is still no obvious solution in terms of a cost-effective, powerful 100mW or above,frequency agile or broadband source operating over the band 500GHz 2.5THz, which can be coupled to a sensitive

detection scheme. Nonetheless terahertz continues to show promise as a technique for people screening due to itspotential for materials specific detection, an area where few other candidate technologies exist.

ACKNOWLEDGEMENTS

The author would like to acknowledge valuable discussions and contributions referenced below from many research

groups and companies working in the field.

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