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Final report-Concealed weapon detection using Terahertz Technology


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Final report-Concealed weapon detection using Terahertz Technology

  1. 1. 1 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
  2. 2. 2 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
  3. 3. 3 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
  4. 4. 4 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”.
  5. 5. 5 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].
  6. 6. 6 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
  7. 7. 7 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
  8. 8. 8 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.
  9. 9. 9 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].
  10. 10. 10 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.
  11. 11. 11 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
  12. 12. 12 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.
  13. 13. 13 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
  14. 14. 14 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].
  15. 15. 15 Fig.12 A schematic diagram of the real time THz imaging using electro-optic sampling
  16. 16. 16 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
  17. 17. 17 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
  18. 18. 18 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].
  19. 19. 19 Fig.15 Terahertz scanning and corresponding imaging of a person carrying concealed weapon
  20. 20. 20 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
  21. 21. 21 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
  22. 22. 22 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
  23. 23. 23 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
  24. 24. 24 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.
  25. 25. 25 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. 26. 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
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