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Concealed Weapon Detection Using Terahertz Technology

Report on Winter Project

By

Ramit Mukherjee, Krishnendu Chakraborty, Anik Batabyal,

Jayanta Das and Subham Kr. Bhagat

Department of Electronics and Telecommunication Engineering

IIEST, Shibpur

Under the supervision of

Prof. B.K. Sarkar

Kalpana Chawla Space Technology Cell

Indian Institute of Technology, Kharagpur

December 2014

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CONTENTS

Page No.

1. Acknowledgement 3

2. Abstract 4

3. Introduction 4

4. Advantages and Disadvantages of Terahertz Radiation 5

5. Generation 6

5.1. Solid State Generation 6

5.2. Optical Generation 6

5.2.1. Photoconduction 6

5.2.2. Optical Rectification 6

6. Detection 7

6.1. Photoconductive Antenna 7

6.2. Electro-Optic Sampling 9

7. Existing Techniques of Concealed Weapon Detection 10

7.1. Microwave and Millimetre Wave Imaging 10

7.2. Infrared Imaging 11

7.3. X-Ray Imaging 11

7.4. Detection using Magnetometers 11

7.5. Detection using Induction Loop 11

8. Comparison between active and passive imaging 12

9. Different Terahertz Imaging Techniques 12

9.1. Time Domain Spectroscopy (TDS) 13

9.2. Real Time Terahertz Imaging using Electro-Optic Sampling 13

9.3. Terahertz Imaging using a Microbolometer 16

10. Azimuth-Elevation Imaging Scheme 16

10.1.Resolution Cell 17

10.2.Variation of reflectivity of different materials with frequency 18

10.3.Frequency Scanning Technique 22

10.3.1.System Topology for the Frequency Scanning Technique 22

11. Typical Imaging System for Concealed Weapon Detection 23

11.1.Limitations of the typical imaging system 25

12. References 27

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ACKNOWLEDGEMENT

We would like to express our gratitude to a number of people who

supported us while preparing this report. First we want to thank the

chairman of Kalpana Chawla Space Technology Cell, Prof. D. Roy

Chowdhury for allowing us to work on the winter project. We would

also want to thank our supervisor Prof. B.K. Sarkar for the nice

advices he has given us regarding our work and for the freedom that

he allowed us to have in developing it. Secondly, we are extremely

grateful to Anindya Ghosh for his invaluable help and for his patience

in answering all our technical questions during this project. We would also like to say thanks to all the staff members of Kalpana

Chawla Space Technology Cell who had helped us throughout this

entire project.

Subham Kr. Bhagat

Jayanta Das

Anik Batabyal

Krishnendu Chakraborty

Ramit Mukherjee

Kharagpur

December, 2014

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Abstract

The use of Terahertz (THz) technology for concealed weapon detection is becoming a very

important aspect of modern day security screening at airports, railway stations, ferries

and other public checkpoints. This report initially provides a brief understanding of the

generation, detection and imaging techniques used in terahertz technology, talks about the

prospects and challenges of existing systems and establishes the superiority of the

terahertz technology over the former. However, the report primarily focuses on a typical

system with the use of these techniques aiming to achieve better resolution and hence a

more accurate detection of concealed and potentially hazardous objects in the human

body.

1. Introduction

The Terahertz frequency region extends from 0.3 THz to 3 THz [1] and lies between the

microwave and infrared regions in the electromagnetic spectrum [2] as shown in fig.1 [3].

The increase in terrorist activities like aircraft hijackings, railway bombings, etc. over the

past few years have led to an unprecedented increase in the demand for security

screening of passengers at such ports. Terahertz radiations (which are known as

submillimetre waves) have small wavelengths, high penetrability and most significantly

is not hazardous to the human body and hence its use in concealed weapon detection

has been a field of intense interest and research. The main limitation to the use of

Terahertz technology however, is the low power of the signals as shown in fig.2 [4] and

hence the difficulty in detecting it by ordinary detection systems. As a result of this, the

Terahertz region has remained inaccessible for a long period of time and the void in

scientific knowledge thus produced is known as “Terahertz gap”.

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2. Advantages and Disadvantages of Terahertz Radiations

The Terahertz radiations have particular advantages over the existing techniques of

security screening:

a) They are non-ionising radiations and hence possess no threat at all to the human

body and biological tissues.

b) The radiations have a high penetrability making them suitable for detection of

weapons concealed by plastic or even cardboard covers.

c) Sub-millimetre waves, having high frequencies provide better resolution for physical

systems of optimal size.

d) Explosive substances which cannot be detected by metal detectors, show

characteristic spectroscopic signatures on absorption of Terahertz radiations and can

hence be detected by using such radiations.

e) Radiations in the Terahertz frequency range have high Signal-to-Noise ratio (SNR),

upto the order of 100,000 [5].

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The Terahertz imaging and detection systems are mostly limited by high absorption in

water, biological tissues and other polar liquids. The present systems of Terahertz

imaging is constrained by low acquisition speed while the generation systems are

controlled by low frequency bandwidth. While frequencies upto 3 or 4 THz can be

generated by photoconduction techniques, higher frequencies about 30 THz can be

generated by processes like optical rectification which seriously compromise the Signal-

to-Noise Ratio (SNR). The Terahertz sensing and imaging techniques although affordable

for research purposes are quite expensive for commercial usage.

3. Generation

One of the reasons for the limited usage of Terahertz frequencies in concealed weapon

detection is the difficulty in its generation. Two relevant methods of Terahertz wave

generation is the process of Solid-State Generation and the process of Optical

Generation. Recently, processes related to and properties of Semiconductor sources are

being used to generate sub-millimetre waves. The following section discusses about the

various methods of generation of Terahertz waves.

I. Solid-State Generation

Although a very effective way for generation of Infrared waves, the main limitation this

method faces in the generation of THz waves is the unsuitability of the semiconductor

devices for this type of excitation. The method presently being researched is the

quantum cascade concept. Unlike semiconductor lasers that emit electromagnetic

radiations through electron-hole pair recombination, the quantum cascade lasers

achieves laser emissions through intersubband transitions [6]. This method is however

incapable of generating frequencies less than 10 THz at room temperature. At cryogenic

temperatures, this process can generate waves of frequencies as low as 4.4 THz.

II. Optical Generation

This method does not make use of any semiconductor bands for emission of THz waves.

Instead it uses laser of pulse width 10fs to 200fs which is either reflected or transmitted

through a THz generator. Photoconduction and Optical Rectification are the two sub

parts of the method of optical generation.

A. Photoconduction

In this process an optical laser (pulse width = 100 fs or less) is used to overcome the

forbidden energy gap GaSe semiconductor. This process results in a small amount of

current which is used to drive the radiating antennae. This process generates THz waves

of more power as compared to the optical rectification process.

B. Optical Rectification

In this technique χ(2) crystals and optical pumps are used to convert optical frequency to

Terahertz frequency. This frequency conversion technique is a non-linear one and is

called optical rectification. The generation of Terahertz frequency from the setup as

shown in fig.3 [7] may be described as follows: the 115 fs pulse generated by the laser

pump contains numerous Terahertz frequency components. So the ‘red’ and ‘blue’

components of a single optical pulse is made to ‘mix’ in a χ(2) crystal producing a

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particular Terahertz radiation of desired frequency. The “red” and “blue” components are

dispersed into different angles by a diffraction grating and focusing lens. Hence, the

generated Terahertz radiation exits at a large angular deviation from the optical pump’s

direction. In this technique when an amplified Ti-sapphire pump and lithium niobate

crystal is used, the Terahertz source will emit 1 mW average and 1 MW peak, from 0.1 to

1.6 THz at a 1 kHz repetition rate. The χ(2) crystal is 0.6% MgO-doped stoichiometric

LiNbO3 prism which is cut at 63 and 73 degree angles. The grating to lens distance is 24

cm and the lens to prism distance is 12 cm which can double the size of the Ti-sapphire

beam [7].

4. Detection

The very low signal power and the high atmospheric attenuation of THz frequency waves,

makes the detection of such waves substantially difficult. Similar to generation, THz

detection can be done by methods of photoconductive antennas and electro-optic

sampling. Firstly, the waves are focused onto a detection medium. The femtosecond

laser that had been used to generate the waves is split into optical and infrared rays and

are simultaneously used to detect the different refractive indices of the material. Then

the probe beam is measured along with the measurement of time delay between the

probe pulse and the signal. As a result, we get electric field of the signal in time domain,

which is used for detection purposes.

I. Photoconductive Antenna

A photoconductive antenna (PCA) for terahertz (THz) waves consists of a highly resistive

direct semiconductor thin film with two electric contact pads. The film is made in most

cases using a III-V compound semiconductor like GaAs and is as shown in fig.4 [8]. The

PCA can act as a generator as well as a detector of THz frequency. When it acts as a

detector, a current amplifier is connected to the electrical contacts as shown in fig.5 [9].

When the THz radiations fall on the PCA, electron transition occurs from the valence to

the conduction band of the semiconductor layers, thereby generating a small transient

current of current density J(t). Now the time domain electric field is given by

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E(t)THz = �

���(�) (4.1)

and the maximum value of E can be given by

Emax = e.µ.(��)

�.�.�

� (4.2)

where µ is the mobility of the carriers, hν is the frequency of the laser pulse, P is the

average laser power, V is the biasing voltage, R is the reflectance of the substrate and L

is the size of the antenna.

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The bandwidth of the antenna is affected by factors like the physical size of the antenna,

the carrier scattering time and the THz beam scattering. The key factor affecting however

is the duration of the laser pulse. The table in Table 1 [8] shows the variation of antenna

length with frequency.

A major limitation of the PCA however is its inability to discriminate with certainty

between the reflections from the target and the interference caused. Thus weak targets

and highly strong interference can result in incorrect decisions. Fig.6 [10] shows the

incorrectness in detection due to interference which makes the signal amplitude cross

the threshold value.

II. Electro-Optic Sampling

This is a technique of optical sampling where we use certain materials exhibiting

different refractive indices under the influence of an electric field. This is a particularly

important technique of THz wave detection, especially because of its superiority over the

PCA method in terms of wider bandwidth and parallel imaging capabilities. This is

essentially a coherent pulse detection technique. Fig.7 [11] shows a schematic for the

process of electro-optic sampling. The electric field applied causes a birefringence which

results in the polarisation of optical beam passed through the crystal. This polarisation

can be analysed and it gives not only the amplitude of the THz beam, but also the phase

correct to the order of 10-2 radians [11].

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The process of electro-optic sampling is however complex and is limited by the following

factors.

(a) Dispersion and absorption inside the crystal and at the air-crystal interface;

(b) Finite pulse duration of the probe beam;

(c) Phase-mismatch between probe and THz pulses; and

(d) The geometrical overlapping between the probe and THz beams

5. Existing Techniques for Concealed Weapon Detection

Concealed Weapon Detection (CWD) is one of the most important aspects of modern time

security. Existing image sensor technologies are included in the following section.

I. Microwave and millimetre wave Imaging

Microwave and Millimetre wave have properties which are very similar to each other.

While the millimetre waves have high resolution, the microwaves have a relatively low

resolution which is still sufficient for Concealed Weapon Detection. This eliminates to a

large extent the privacy concerns inherent in the millimetre wave technique. Fig.8 and

fig.9 [12] shows the difference in images captured by the millimetre wave and microwave

imaging techniques. While the millimetre wave technique is capable of penetrating dust

and smoke, its drawback lies in the lack of spectral features for identification. This

technique is further divided into two types – active and passive imaging which are

described in details later in the report in article 6.

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II. Infrared Imaging

The infrared imaging technique is based on the principle of thermal radiation difference

due to difference in temperatures of the targets. Hence it is a type of passive imaging

(details about which are later discussed). IR sensors however have long wavelengths and

hence cannot penetrate clothing or other packaging materials. In addition, if the weapon

is concealed in the human body for a long time, the temperature difference between the

body and the target reduces and hence detection becomes difficult. Hence it is a very

elementary CWD technique. However it can be combined with millimetre wave sensors to

improve results by the method of image fusion.

III. X-Ray Imaging

X-Ray imaging is one of the best techniques of Concealed Weapon detection given its

short wavelength and high penetration power. It provides high spatial resolution and the

capability of detecting metal as well as non-metal targets. X-rays however are incapable

of detecting concealed weapon stored in natural or artificially made cavities in the

human body. The X-ray images are of extremely high resolution and hence there is a

privacy concern. The biggest drawback of X-rays is its ionising nature which causes

damage to the human tissue. Presently thus, X-rays are used only for screening of

luggage.

IV. Detection using Magnetometers

The principle behind the use of this technique is the change of earth’s magnetic field in

presence of another magnetic material. A magnetometer is used to detect such a change.

This type of technique can be used to detect metallic weapons or weapons having

considerable iron content. This technique is presently in use at airports and other public

screening posts. However, it cannot be used to detect non-metallic explosives or

hazardous items.

V. Detection using induction loop

An induction or inductive loop is an electromagnetic detection system which uses a

moving magnet to induce an electrical current in a nearby wire. Induction loops are used

in metal detectors for detection of metal objects. In this technique a large induction loop

forms a part of a resonant circuit which is "detuned" by the coil's proximity to a

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conductive object. Apart from metallic, conductive or capacitive objects can also be

detected.

6. Comparison between Active and Passive Imaging

Imaging sensors can be classified into active and passive types.

An active imaging sensor system as shown in fig. 10(a) [13] radiates sub-millimetre

waves from the Terahertz source and illuminates the object. The receiver array observes

the amplitude or phase of the reflected waves. Using these signals, the image is

reconstructed using a computer. Since this active imaging system uses a THz source,

the signal to noise ratio (SNR) received at the receiver antenna is relatively high. On the

other hand, a passive imaging sensor system as shown in fig. 10(b) [13] receives

incoherent sub-millimetre waves emitted from the object. The amplitude of the radiation

depends on the object’s emissivity and temperature. Passive imaging sensors avoid the

use of millimetre-wave sources thus making the system block simple when compared

with active imaging [13].

Passive imaging generally records the contrast in radiometric temperature within a scene

while active imaging systems record the contrast in scattered radiance within a scene

when it is illuminated with some type of THz source.

Passive imaging enjoys the advantage of free natural illumination but is not suitable for

indoor environment operation. Active imaging however, utilizes artificial illumination and

has a variety of waveforms to choose from. In view of image formation, passive images

are formed pixel by pixel and has poor dynamic ranges whereas active images have wide

dynamic ranges.

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7. Different Terahertz Imaging Techniques

The objective of T-ray imaging is to produce images with ‘component contrast’. T-ray

imaging techniques, having longer wavelengths compared to microwaves can provide

enhanced contrast because of lower scattering ability. Various T-ray imaging techniques

include Time Domain Spectroscopy (TDS), electro-optic sampling, tomographic and

single shot imaging. Imaging with electromagnetic pulses in the Terahertz region is also

popular for real world applications as it can provide non-invasive monitoring methods.

I. Time Domain Spectroscopy

Time Domain spectroscopy usually employs a femtosecond laser source to trigger pulsed

Terahertz radiation. The ‘fs’ laser system is based on Ti-Sapphire laser with 50-100 ‘fs’

pulse width. The pulse repetition rate is typically of the order of 100 MHz. The pulsed

signal is directed by means of lens and parabolic mirrors and focused on the sample

under test. The mirrors should possess high numerical aperture for optimum spatial

resolution. A set of parabolic mirrors will image each speckle at the focus of the first

mirror onto one speckle at focus of the second mirror as shown in fig. 11 [14]. By varying

the timing of the laser pulse using the delay line, it becomes possible to scan the

Terahertz pulse and construct the electric field as a function of time. The time dependent

waveforms are converted to frequency domain by Fourier Transform, resulting in

Terahertz image [14].

Here photoconductive antennas (PCA) are used both as emitter and receiver.

Photoconductive dipole antennas are often chosen for their bandwidth. PCAs are usually

combined with hemispherical Si lenses for the purpose of delaying the pulse reflected at

Fig.11 A Terahertz imaging setup using Time Domain Spectroscopy method

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the second surface of the antenna chip and thus preventing oscillations in the Terahertz

spectrum.

II. Real Time Terahertz Imaging using Electro-Optic Sampling

The system consists of a ‘fs’ laser source, the sample to be imaged, imaging optics, an

Electro-optic (EO) crystal (ZnTe) , a computer controlled optical delay line and a CCD

(Charge Coupled Device) camera. The ‘fs’ source is divided into pump and probe beams.

The optical source is a Ti-Sapphire regenerative amplifier. The pump beam goes through

the optical delay line, and drives the large-aperture PCA, which emits THz radiation. The

antenna consists of GaAs wafer having wide photoconductive gap between gold

electrodes. A bias voltage of 5 kV is applied between the electrodes. The fs laser pulse is

illuminated onto the gap between the two electrodes. The current rises very rapidly after

injection of photo carriers by the fs laser pulse, and then decays with a time constant

given by the carrier lifetime of the semiconductor. The transient photocurrent radiates

into free space according to Maxwell’s equations. The THz radiation amplitude is

proportional to the time derivative of this transient photocurrent. The radiation from the

THz emitter passes through the sample and is focused onto the EO crystal by two

polyethylene lenses to form an image of the sample. The probe beam is expanded at the

same time, and the polarizer ensures that the probe beam is linearly polarized. The

probe beam is then led into the same optical axis as the THz radiation by a pellicle beam

splitter. Since CCD cameras do not respond to THz radiation, the 2-D EO sampling

technique is employed for transferring the THz image into an intensity pattern in the 800

nm laser beam. At each point on the EO crystal, the refractive index is changed

depending on the THz electric field within the EO crystal, and birefringence is induced.

When the probe beam passes through the EO crystal, birefringence changes the

polarization of the probe beam. Only the light with changed polarization passes through

a crossed polarizer positioned in front of the CCD camera. Through these processes, the

THz electric field distribution in the EO crystal is converted into an optical intensity

distribution which can be recorded by the CCD camera. One disadvantage of this scheme

is due to the use of the short Terahertz pulses. So, the scheme becomes inherently

broadband (>1THz), making it unsuitable for applications that require both real time

operation and frequency sensitive measurement. To achieve frequency sensitive imaging

along with real time operations, it is desirable to use focal plane array cameras that can

directly detect THz rays with sufficient speed. The coherent radiation sources can be

frequency multipliers at sub millimetre wavelengths and by far infrared gas lasers or

quantum cascade lasers (QCLs). Fig. 12 [14] shows single frequency real time

continuous wave Terahertz imaging using focal plane array camera and infrared gas

laser as the source [14].

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Fig.12 A schematic diagram of the real time THz imaging using electro-optic sampling

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III. Terahertz Imaging using a micro-bolometer

The Terahertz imaging system shown in fig.13 [15] is an uncooled micro bolometer focal plane array camera. The Terahertz beam is allowed to expand at 1.4◦ divergence angle of the laser. The reflected beam backlights an object with a maximum area of roughly 4cm×4cm and the transmitted light is received by a germanium camera lens. The focal plane is situated approximately 1.1 cm behind the germanium camera lens. A 6.5 mm thick sheet of high density polyethylene (HDPE) is placed directly in front of the camera to provide uniform background. The distance between the off-axis paraboloid and the germanium lens is fixed to collimate the light emerging from the lens. Concentrating the signal over a smaller area improves the signal/noise ratio (SNR). At the brightest illumination point, the centre of the image, the SNR is estimated to be 13 dB. Significant improvements in SNR and spatial resolution can be made by designing focal-plane micro bolometer cameras specifically optimized for THz frequencies.

8. Azimuth-Elevation Imaging Schemes

Concealed Weapon detection using Terahertz frequency mainly involves active imaging

techniques. Passive imaging is usually not used because of some disadvantages which

have been already discussed. So, we basically require a radar like system. The Azimuth-

Elevation imaging scheme is a popular one that can be used.

An azimuth is an angular measurement in a spherical coordinate system. The vector

from an observer to a point of interest is projected perpendicularly onto a reference

Fig.13 Experimental setup of THz imaging using a microbolometer

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plane. The angle between the projected vector and a reference vector on the reference

plane is called the azimuth.

Elevation is defined as the angle between the object and the observer’s local horizon.

I. Resolution Cell

Resolution is defined as the ability to specifically detect multiple features on the same

target.

The problem with low resolution radars is that very often the entire target is treated as a

single point. But a high resolution radar can scan the same target with the help of a

number of resolution cells and thus additional information about the target can be

obtained. This is explained below with the help of the images given. Fig.14 [10]

For detection and imaging, each resolution cell acts as a single unit. It is like the least

count concept in measurement. Targets or their features smaller than the resolution cell

cannot be detected and hence the resolution cell should be as small as possible.

For this scheme resolution in the elevation direction (RE) is given by,

RE = ��

� (8.1)

where ‘c’ is the velocity of light in medium concerned, ‘τ’ is the width of the pulse from

the terahertz source. For better resolution in this direction the pulse width must be less

which again reduces signal power substantially. Thus, pulse compression is performed

where the transmission pulse width is more but that in the receiver side is less.

The resolution in the azimuth direction (RA) is given by

RA = �

� (8.2)

Here, ‘λ’ is the wavelength of the terahertz radiation used and ‘d’ is the aperture width

or length of the antennae.

Fig.14 Comparison of low and high resolution radar

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The basic principle of the detection and imaging by this scheme is given next.

We can begin with the radar equations [10]. The power that is radiated effectively in the

direction of the main beam is called effective radiated power (ERP) given by

ERP (in Watts) = (PTGT) (8.3)

Here, PT is the transmit power delivered to the antenna (Watts) and GT is the gain of the

radar’s transmit antenna. The transmit power is actually the power from the THz source

(we are using an active imaging system).

Next, we consider the forward (illumination) power density at the target, P/A which is

given by

�

� (in Watts per square meter) =

��

������

(8.4)

Here, RT is the range from radar transmitted to target (in meters) and 4ПRT2 is the

surface area of the sphere of radius RT. This power density (P/A) is the amount of power

falling on each unit area of a plane perpendicular to the axis of the antenna at a

distance RT from the radar.

The effective power reflected by the target in the direction of the radar (Ptgt) is directly

proportional to P/A and reflection characteristics of the target which is given by

Ptgt = �

� σ (8.5)

where, σ is the target’s radar cross-section which depends mainly on the reflection

characteristics of the target.

The energy reflected from the target propagates away from it at the propagation velocity.

The power density in the backscattered wave at the radar is the power effectively

isotropically radiated by the target divided by the surface of a sphere of radius equal to

the range from target to radar. There is no gain factor in backscatter propagation since

the target is treated as though it were isotropic. In a monostatic radar, the range from

the target to the radar’s receiving antenna equals the range from radar’s transmitting

antenna to target. This is given by the following equation

�

�� =

�����

�������

� (8.6)

where, P/AB is the backscatter power density at the radar’s receiving antenna (Watts

per square meter) and RR is the range from the target to the radar’s receiving antenna

(meters).

II. Variation of Reflectivity of different materials with frequency

Depending on varying σ, the different parts of the target pick up different colours or

signatures on the detected image as shown in the fig.15 [17].

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Fig.15 Terahertz scanning and corresponding imaging of a person carrying concealed

weapon

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For our ready reference, the variation in reflectivity of certain materials are given below

in Table 2 [18].

For particular metals, reflectivities are given for two terahertz frequencies as shown in

Table 3 [19]

Table 2 Normal reflectivity of different materials at 94 GHz frequency

Table 3 The submillimeter wave reflectivity(R) of metals

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Reflectivity characteristics of certain explosives in the terahertz frequency range are

given in fig.16(a) and 16(b) [20].

Inspired by this imaging scheme we need to scan the target or the sample in these two

directions (azimuth and elevation). This can be achieved by two methods:

i. Using a motor to move the receiver mechanically keeping the transmitter and

target fixed,

ii. By frequency scanning technique.

Fig.16(a) Variation of reflectance of RDX with THz frequency

Fig.16(b) Variation of reflectance of Tartaric acid with THz frequency

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III. Frequency Scanning Technique

Frequency scanning antennas, which scan beams by changing frequency are widely

used in imaging applications. Such antennas often utilize the characteristics of a leaky

wave antenna that comprises of a slotted waveguide, where the slot works as the

antenna aperture. The waveguide acts as the transmission line for propagating the

waves radiated from the slot [21].

The position, shape and orientation of the slots determine their nature of radiation. The

above fig.17 [22] shows the current distribution in the waveguide walls.

The slope of the wavefront and hence the angular position of the lobes, changes with

frequency, when the phase distribution along the waveguide is a function of frequency.

Thus frequency scanning can be accomplished.

System Topology for the frequency scanning technique

A submillimeter, continuous wave system is shown in the fig.18 [21]. Here the emitter is

a frequency scanning antenna array. The receiver is a diagonal horn, and two biconvex

lens are used to focus the radiation.

Fig.17 Orientation of slots in the slotted waveguide used for frequency scanning

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The frequency scanning antenna is placed on the focal point of the first lens, thus the

outgoing rays will be parallel to the focal axis. Then the rays will reflect in the target and

are collimated using the second lens. The receiving horn is placed on the focal point of

the second lens [21].

9. Typical Imaging System for Concealed Weapon Detection

Generation, detection and imaging techniques for Concealed Weapon Detection using

terahertz frequency have already been discussed. Now, we propose a complete setup

which can be used for security screening at public checkpoints.

The setup shown in the figure shows the basic block diagrams of the transmitter and

receiver and also provides insight to their internal architectures. Here, photoconductive

antennas are used as emitter and detector, femtosecond laser source is the Terahertz

source and computers are utilised for data acquisition purpose. The person to be

scanned acts as the sample who must enter the enclosure so that his/her scanned

image is obtained in the acquisition system. The transmission of the terahertz radiation

through the enclosure is a critical issue in this setup. So, the enclosure should be such

that the terahertz radiation can easily penetrate through it.

Fig.18 THz imaging system based on frequency scanning technique

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Some important points of the various components in the setup are described below

Laser: The fs laser system generates pulses of width 50 to 100 fs. The pulse repetition

rate is of the order of 100 MHz.

Delay Stage: Stepping motors are usually employed to introduce a delay for proper

synchronization. The length of time delay line determines the frequency resolution of the

setup.

Antennas: Photoconductive antennas are usually chosen as described in article 4. Spiral

antennas can also supply higher power but also emit circularly polarized THz radiation.

In the future, detector arrays of PC antennas can provide an interesting alternative.

Mirrors: Parabolic mirrors are widely used for focussing THz radiation. Compared to

lens, mirrors are advantageous as they do not produce undesired reflections and hence,

radiation loss is minimised. The mirrors should possess high numerical aperture for

higher spatial resolution.

Chopper: Since we can never achieve symmetrical configuration at the PC antenna gap,

there always exist an average current even in the absence of THz radiation, hence it is

necessary to chop the laser pump beam and use a lock in amplifier. The chopper is

typically operated at a frequency near 1 KHz. Fig.19 shows a design of a typical THz

imaging system.

Fig.19 A typical THz imaging system using the time domain spectroscopy method of

THz imaging.

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I. Limitations of the Typical Imaging System

i. As discussed earlier, a crucial issue in the design of such a system is the

material with which the enclosure is made. The enclosure should be made of a

material that can be penetrated by THz radiations. ii. Another limitation of the system is the problems that arise due to the near and

far field effects of the electromagnetic fields surrounding the antenna. Near field and Far field effects of electromagnetic fields.

Near field effect: For a certain distance from the antenna, the electric and magnetic fields

are so oriented, that instead of wave propagation energy storage takes place and hence

the desired beam from the antenna is not formed. The near field region can be

considered as a collection of dipoles with a fixed phase relationship as shown in fig.20

[23]. In this region the amplitude falls of proportional to 1/r2 and hence after a very

small distance from the transmitter EM wave propagation ceases to exist.

Far field effect: At a certain distance from the antenna, the near field effect ceases to

exist and electromagnetic radiations behave according to their conventional

characteristics, dominated by electric dipole types electric and magnetic fields as shown

in fig.21 [23]. The amplitude in this region falls of by a factor of 1/r in the far field

region.

Fig.20 Electric and magnetic field Fig.21 The dipole pattern for far

for near field region. field region.

26

For electromagnetically short antennas the near field region exists for r<<� while the far

field region exists for r>>2�, while the intermediate region is known as the transition

zone as shown in fig.22 [23].

For electromagnetically long antennas the near and far field regions are defined in terms

of the Fraunhoffer distance df and is given by

df = � �

� (9.1)

where D is the diameter of the antenna and � is the wavelength of the radiation.

Distances less than the Fraunhoffer distance is known as near field region, while that

greater than the Fraunhoffer distance is known as far field region.

As our system uses THz radiations which are of short wavelengths, df becomes high and

hence the near field region extends upto a greater distance. To reduce the effect of this

problem, we would have to design an antenna having an aperture diameter such that

the near field region is less extensive. We can also keep a larger distance between the

transmitter antenna and the target in order to avoid effects of the near field region.

Fig.22 The demarcation of near field, transition and far field regions for

electromagnetically short antennas

27

References

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2. Wikipedia – Terahertz radiation.

(http://en.wikipedia.org/wiki/Terahertz_radiation)

3. A disruptive terahertz technology. (http://teracascade.com/technology.php?lang=en)

4. Source: (http://opticalengineering.spiedigitallibrary.org/article.aspx?articleid=1668918).

5. X-C Zhang, ‘Terahertz wave imaging - horizons and hurdles’.

6. R.F. Kazarinov and R.A. Suris, ‘Possibility of amplification of electromagnetic waves in a semiconductor with a super lattice’.

7. Jerry C. Chen, M. Jalal Khan, Zong-Long Liau and Sumanth Kaushik ‘Terahertz generation and detection using nonlinear frequency conversion’.

8. PCA - Photoconductive Antenna for THz Applications. (http://www.batop.com/information/PCA_infos.html)

9. An Introduction to Terahertz Electromagnetic Waves Generation, Detection, Properties and Applications.

10. ‘Radar: Principles, Technology and Application’ by Byron Edde. 11. Time-resolved optoelectronics measurement techniques. 12. Source: (http://www.ptb.de/cms/en/fachabteilungen/abt2/fb-25/ag-254/time-

resolved-optoelectronic-measurement-techniques.html) 13. Leonardo Carrer, ‘Concealed Weapon Detection (Microwave Imaging Approach)’. 14. Masaru Sato and Kozi Muzino, ‘Millimeter-Wave Imaging Sensors’. 15. Michael Herrmann, Ryoichi Fukasawa and Osamu Morikawa, ‘Terahertz imaging’. 16. Alan Wei Min Lee and Qing Hu, ‘Real-Time, continuous-wave terahertz imaging by

use of a microbolometer focal-plane array’. 17. Source: (http://www.gizmag.com/prototype-handheld-scanner-police-

search/11925/). 18. Roger Appleby and Rupert N. Anderton, ‘Millimeter-Wave and Submillimeter-Wave

Imaging for Security and Surveillance’. 19. Robert H. Giles, ‘Characterization of Material Properties at Terahertz Frequencies’. 20. N.Palka, ‘THz Reflection Spectroscopy of Explosives measured by Time Domain

Spectroscopy’. 21. R. Camblor, S. Ver Hoeye, C. Vazquez, G. Hotopan , M. Fernandez , A. Hadarig

and F. Las-Heras, ‘Sub-millimeter Wave Imaging System based on Frequency Scanning Antenna’.

22. Source: (http://www.miwavetechnoventures.com/horn-antennas.html). 23. Wikipedia – Near and far field.

(http://en.wikipedia.org/wiki/Near_and_far_field)