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Awareness of the existence of terrorist threats against the facilities of particular importance has placed a priority on the task of providing these facilities with modern means of protection. A multispectral complex for detecting concealed objects in the infrared and terahertz ranges of electromagnetic waves is described, as well as its main technical and design characteristics are given.
Теги: multispectral system of inspection terahertz radiation мультиспектральная система досмотра терагерцевое излучение
Different materials and biological tissues have different absorption in the terahertz range, which provides the necessary contrast of images. The developed complex is designed to detect forbidden organic, metallic and polymeric substances with concealed carrying in clothing elements, in the terahertz range of radio waves and in a combined form in the visible, infrared, and terahertz ranges. A brief review of the development of infrared detection technology was previously presented in the journal "Photonics" (see Photonics, 2017, No. 3, "IR and video system based multispectral module of detection and analysis of threats for the protection of extended objects"). A terahertz complex for detecting forbidden items has been developed and created by JSC "OKB Astron". This article describes the purpose and scope of the complex, its main technical and design characteristics.
TERAHERTZ RADIATION
The frequency range of terahertz (THz) radiation ranks an intermediate position between the infrared (IR) and microwave regions of the electromagnetic spectrum (Fig. 1). Conditionally, the limits of the THz range are the frequencies from 0.02 THz to 10.00 THz [1]. This frequency interval occupies a part of the electromagnetic spectrum between the infrared (IR) and microwave ranges, so it is often also called the far IR or submillimeter range. The spectra of complex organic molecules (molecules of proteins and DNA, explosives, etc.) lie in the terahertz range.
Terahertz radiation is non-ionizing, in contrast to X-ray radiation. Different biological tissues have different absorption in this range, which allows for the contrast of the images. Compared with visible and IR radiation, terahertz radiation is less susceptible to scattering. As a result, many dry dielectric materials, such as textiles, wood, paper, plastics, are transparent in this range. Therefore terahertz radiation can be used for non-destructive material control, scanning at airports and other controlled services. Furthermore, the wavelength of the radiation is sufficiently small to provide submicron spatial resolution when using freely propagating radiation. Molecules of different substances have their own unique resonance lines in the terahertz spectrum, which can be used for accurate identification. This makes it possible to identify the molecules by their spectral "imprints". In combination with obtaining an image in the terahertz range, this makes it possible to determine not only the shape, but also the composition of the object under study.
Terahertz radiation can be detected in the time domain, i. e., it can be measured as the amplitude and phase of the electromagnetic field. This makes it possible to directly measure the phase shift introduced by the object under investigation, and therefore allows one to investigate fast processes. The peculiarity of terahertz analysis is that it has the ability to simultaneously measure the amplitude and phase of a wave. In conventional spectroscopes used by the services of control of concealed items, only the intensity of radiation at certain frequencies is measured. Terahertz experiments often include measurements of the temporal characteristics of the electromagnetic field of terahertz pulses that interacted with the sample (i. e., reflected or passed through). The Fourier transform applied to these time domain data provides information on the phase and amplitude of the pulse, as well as a variety of additional information about the sample with which the terahertz pulse has interacted [2–4].
HISTORY AND STATE OF THE ART DEVELOPMENTS
The first experiments related to the detection of THz radiation were carried out back in 1897 by Henrik Rubens and Fox Nicholas [5]. At that time they have been solving the problem of determining the spectral composition of the radiation of an absolute black body using a bolometer. Much later, in 1975, Auston [6] developed a generator of pulsed THz radiation, built on the basis of a photo-switch: a photoconductive antenna excited by femtosecond laser pulses of the optical range. The creation of powerful pulsed lasers, in particular, femtosecond lasers generating pulses of duration about 100 fs (1 fs = 10–15 s) in the 80’s opened the way for the creation of compact sources of terahertz radiation, based on the interaction of laser radiation with the substance. It was found that terahertz radiation can be obtained through the nonlinear conversion of high-intensity laser radiation in crystals (generation of a difference frequency), and also due to certain linear effects, such as laser-induced carrier generation in semiconductors.
At the end of the 1980s, a breakthrough in terahertz studies occurred: Terahertz Time-Domain Spectroscopy (THz TDS) method was demonstrated for the first time. This method is based on the generation and detection of coherent terahertz radiation using pulses from the same laser. Unlike other methods, where only the pulse envelope or the radiation power is measured, terahertz time-domain spectroscopy allows for the direct detection of the electric field of a terahertz pulse with a high resolution, the duration of which is only about 1 ps (10–12 s). Direct measurement of the electric field makes it possible to extract information on the phase shift of the terahertz field when it interacts with the object, and thus offers opportunities for studying ultrafast processes (occurring over a fraction of picoseconds).
The development of laser technology, the creation of installations capable of generating stable and powerful ultrashort laser optical pulses, has led to the further development of THz spectroscopy [7, 8]. The first studies in this field were conducted by Martin Nuss et al. at the Bell Labs in the early 1990s. Historically, the first leader in terahertz studies was the research team from the Rensselaer Polytechnic Institute, headed by Dr. Xi-Cheng Zhang. The Rensselaer Polytechnic Institute was one of the first to open a separate laboratory, which only deals with terahertz research. At CLEO (Conference on Lasers and Electro-Optics) in 2005, Xi-Cheng Zhang et al. made a report on the first results of using terahertz waves to detect explosives and biological weapons. Using terahertz radiation at a frequency of 0.3 THz, they were able to remotely identify samples of flour, salt, soda and bacteria spores.
TeraView, Cambridge (England), separated from the Toshiba research laboratory in early 2001. TeraView researches were the first to conduct a series of experiments on multi-layer scanning of semiconductor chips, which has already offered opportunities of practical use of the new technology in non-destructive testing.
StarTiger research group (Space Technology Advancements by Resourceful, Targeted and Innovative Groups of Experts and Researchers) was founded in 2002 by the European Space Agency based on the Rutherford and Appleton lab (England, Oxford). StarTiger Group has obtained the world’s first terahertz photo of a human hand. The sensor developed by StarTiger Group uses terahertz radiation of 2 frequency bands: 0.25 and 0.30 THz. Thus, an image is obtained in two spectra and a contrast is created between the materials with different transmittance and reflection parameters in different spectra. The main advantage of this sensor is due to the fact that it does not emit electromagnetic waves. This is a passive camera of the terahertz range emitted by almost all objects, both biological and non-living ones.
During the same years, QinetiQ Defense Company from the UK has also developed a two-spectral terahertz sensor. Since the sensor operates at two frequencies, 250 and 300 GHz, it becomes possible to distinguish materials using their optical characteristics, for example, reflectivity. In the terahertz spectrum, the reflectivity of the body decreases, and the emissivity increases with increasing radiation frequency. This feature can be used to reduce the number of false alarms. It was possible to obtain the image of various objects through different materials: knives and non-metallic objects, hidden in pockets and newspaper.
The English company ThruVision (separated from the Rutherford and Appleton lab in the early 2000s) developed and tested a new type of THz detector for the security industry in 2008. The detectors are based on the use of GaAs and GaInAs based microstructures with photonic crystals. The detector allows capturing radiation in the range from several hundred gigahertzes to several terahertzes and has sensitivity at the picowatt level. Especially valuable is the established industrial production of matrixes of terahertz range detectors by this company. The advantage of this technology is that it is passive and allows you to obtain images using the radiation of the object itself. The detectors operate at room temperature with a frequency of more than 1 Hz, which allows you to capture moving objects.
The basics of using a combined visible and terahertz image in the early 2000s were suggested by Rick Blum, a professor at LeHigh University (USA). The resulting combined image gives more information than is contained in each original image separately. The combined image has enough information to see, for example, a weapon concealed under clothing. The basic idea of combining two images is as follows. The digital pattern of the original image is subjected to wavelet transform, superimposed by a terahertz image. The inverse wavelet transform allows you to obtain a combined image. This approach was implemented by us in the development of the complex.
Nowadays, terahertz technology is developing as a completely new diagnostic direction. Terahertz waves allow us to see through many optically opaque materials. With the help of terahertz radiation it is possible to obtain a picture of the structure similar to the X-ray, but without the use of potentially dangerous X-ray radiation. Terahertz sensor is a passive sensor and absolutely harmless to human, since the source of signals here is the natural radiation of the objects. At airports, a terahertz sensor can be used to detect weapons concealed under clothing. Moreover, unlike metal detectors, which are used now, a terahertz sensor is also capable of detecting non-metallic explosives [9–14].
COMPLEX STRUCTURE
The terahertz complex for detecting forbidden objects created by JSC "OKB ASTRON" (Fig. 2) includes:
• Terahertz camera.
• Surveillance camera.
• Thermal imaging camera.
• Operator’s AWS.
• Means of transferring information from the means of inspection to the operator’s workstation
• Uninterruptible power supply system.
ELEMENT BASE OF THE DETECTION COMPLEX
Photoconductive antennas
Photoconductive antenna (PC-antenna, photoconducting switch) is one of the most commonly used terahertz radiation generators and receivers. The PC-antenna consists of two metal electrodes (Fig. 3), located at some distance from each other on a semiconductor substrate. A voltage of the order of several kilovolts is applied to the electrodes. When the gap between electrodes is illuminated by an ultrashort laser pulse, the density of charge carriers in a semiconductor increases sharply for a short time (of the order of one or tens of picoseconds). For effective absorption of laser radiation with the release of carriers, the energy of laser radiation photon should exceed the width of the semiconductor forbidden band. The resulting free carriers are accelerated by a field applied to the gap, resulting in a short-time current pulse, which is the source of terahertz radiation. Thus, the laser pulse serves as an ultrafast switch for the antenna, which transfers it from the insulating state to the conducting state. The duration of the current pulse and the spectrum of the emitted terahertz wave are determined, basically, by the lifetime of the carriers in the semiconductor.
In a real-world situation, the energy of the terahertz pulse experiences saturation with increasing energy of the laser pulse. This is due to the fact that the photoinduced carriers shield the displacement field. The increase in the displacement field also encounters the limitations associated with possible electrical breakdown of the substrate. The electric breakdown field is about 400 kV/cm for gallium arsenide (GaAs).
Radiation receiver (RR)
The selection of the radiation receiver (RR) and the wavelength at which it will operate determines all other elements of the product design. Modern techniques for detecting explosive (ES), toxic (TS) and narcotic (NS) substances mainly apply the methods of sampling air and its chromatographic analysis. The presence of characteristic absorption bands in the terahertz region of the spectrum in ES, TS and NS, as well as the absence of such absorption in the container materials, indicate the promise of using THz radiation for their detection by spectral methods, as well as the possibility of developing and creating stationary and portable security systems based on Terahertz vision.
The main issue in choosing a receiver is the method of use in the active or passive mode. However, there are significant difficulties in the production of terahertz radiation sources for backlighting in the active mode. The complex developed by us detects its own terahertz radiation of objects without additional illumination.
TERAHERTZ CAMERA STRUCTURE
The terahertz camera (THZC) consists of the following units (Fig. 4):
• Scanning unit
• Focusing system
• Microwave units
• Control controller
• Control calculator and primary image processing.
Focusing system and optical scheme
The mirror systems (MS) take significant precedence over the lens systems. They consist of reflective surfaces that do not introduce chromatic aberrations, so purely mirror systems are best used in the ultraviolet (UV) and infrared (IR) spectral regions. With relatively simple MS designs, it is possible to obtain a rather perfect correction for spherical aberrations. These systems have a high aperture ratio and resolution, with greater compactness. Since the optical glass does not work in the THz region of the emission spectrum, the components of the MS optical scheme were made of metal.
The first mirror is scanning, it provides an image of the necessary space of objects, and transfers the image to the surface of the second mirror, which forms a light spot transmitted to the radiation receiver (Fig. 5). Correction of distortions of the optical system is performed by the method of Gaussian quadrature, wherein the density of traced rays is determined by the number of rays located along the radius. The maximum number is 18. This is sufficient for pupil aberrations up to 36 orders of magnitude.
The focusing system provides the concentration of the terahertz radiation flux from the object to a ruler of 8 conical waveguides of the receiving detector. The terahertz flux focusing system consists of 4 aluminum mirrors mounted on a rigid chassis. The flat input mirror is scanning; the scanning assembly provides a mechanical sweep of the terahertz radiation flux from the object to the receiving detector. The second and fourth mirrors are cylindrical, and the third one is toric.
The diameter of the focused spot of the terahertz radiation flux is approximately equal to the diameter of the entrance aperture of the conical waveguide of the horn antenna of the receiving detector, i. e. about 7 mm. This ensures the reception of the whole terahertz flux coming from the object to the THzC input window, which ensures the maximum sensitivity of the system.
Microwave unit, radiation receiver
The receiver of the terahertz radiation flux is a microwave unit that receives a terahertz signal from the focusing system. The microwave unit consists of an eight-channel microwave detector, an auxiliary module and an air cooling assembly. The microwave detector consists of eight identical channels operating simultaneously to obtain an eightfold increase in the scan rate.
The microwave receiver is performed by heterodyne scheme. The frequency of the heterodyne is 125 GHz, and the conversion is performed at the second heterodyne harmonic of the local oscillator in a mixer of counter-parallel Schottky diodes. The operating band of the intermediate frequency amplifier is 20 GHz. The microwave detector consists of eight receiving conical waveguides of horn antennas arranged in a line, but with a staggered offset. This arrangement is made in order that there are no gaps between the effective apertures of the waveguides, since the effective apertures are smaller than the physical input diameters of the receiving conical waveguides. A mixer is mounted on the counter-parallel Schottky diodes inside each receiving waveguide. The local oscillator signal is fed to the mixers via waveguides of a special configuration to ensure the distribution of the same power and one phase for each diode mixer (Fig. 6).
The FC signal is amplified by low-noise microwave amplifiers and detected by an active detection circuit. The auxiliary module of the microwave detector ensures the installation and maintenance of the operating modes of all microwave modules in order to obtain the optimum amplification and the dynamic range of the useful signal from the object. The resulting signal is digitized and fed to the control and processing controller to further form the image frame.
SURVEILLANCE CAMERA
AXIS M1011 video surveillance camera with the Axis Communications built-in lens was used by us as a finished product. The surveillance camera is installed next to the input window of the terahertz camera.
THERMAL CAMERA
We used a thermal imaging camera ASTRON384. The Astron thermal imaging lens is a finished product. The lens is made of a germanium single crystal with a bandwidth of 7–14 µm and a focal length of 28 mm, IFOV spatial resolution of 0.89 mrad (IFOV – instantaneous field of view). The only additional feature of the product is the increased requirements for the quality of germanium optics, which is achieved using high-quality single crystals with low dislocation density and the absence of dislocation clusters for manufacturing blanks. The thermal camera is installed next to the input window of the terahertz camera.
CONTROL AND PROCESSES CONTROLLER OF THE MICROWAVE UNIT AND THE SCANNING UNIT
The signals in digital form from each of the eight channels are synchronized with the position of the input scanning mirror of simultaneously by two coordinates. The image frame is consistently constructed from 6 vertical bands, which in turn consist of eight columns (by the number of input microwave conical waveguides and detectors). The useful signal from the object is very small by its nature of origin and is masked by the inevitable thermal noise of the entire receiving path; therefore, digital accumulation of information is made for each frame element (pixel). Furthermore, for several frames, the useful component of the signal grows faster than the noise component, which makes it possible to better see the various sections of the object. The generated frames are sent to the computer for further processing with special software to identify hidden objects and eliminate jamming signals.
SCANNING UNIT
The scanning unit is designed to rotate the first focusing mirror in two planes in order to sequentially bypass the area of the object and form an image frame in the terahertz range. The rotating mirror is attached to the mechanism of the scanning unit. Two stepper motors provide a vertical and horizontal mechanical sweep of the terahertz radiation flux from the object. The main parts are made of aluminum to reduce weight and therefore dynamic loads. Each stepper motor is controlled from its own control controller. The control controllers receive control signals from the microwave unit control controller. The scanning unit outputs two synchronization signals to the microwave unit control controller, which correspond to the average positions of the rotating mirror. These signals synchronize the center of the image frame in the terahertz range with the main optical axis of the focusing system. The scanning unit is mounted on the chassis of the focusing system to ensure the accuracy and stability of the optical system of the terahertz camera.
TERAHERTZ COMPLEX OPERATION RESULTS
The test objects were placed on the human body and concealed under various materials. Fig. 7а shows the images (screenshot of the AWS screen) of the experimenter with the model belt concealed under the clothes. There is an image obtained from the terahertz channel in the central part (Fig. 7b) (the tested object is already visible). Fig. 7c gives a combined image processed by the AWS analyzer. It can be seen that the complex positively detects the tested object.
CONCLUSION
JSC "OKB Astron" has formulated the concept of an active detection system based on THz sources and detectors, which makes it possible to detect concealed substances, a scheme for generating THz radiation and receiving reflected and scattered radiation was selected. Also, the fundamentals of the technology of compact sources and receivers of THz radiation based on GaAs and InGaAs nanostructures have been developed. A design was developed and a model of a compact THz radiation source (up to 1.2 THz) was manufactured based on semiconductor nanostructures with radiation delivery, including nanostructured fiber. A design was developed and a model of a THz radiation receiver (up to 1.2 THz) was manufactured with a sensitivity that allows recording signals from detection objects in real time.
JSC "OKB Astron" has manufactured an active system for detecting covertly carried substances based on THz sources and detectors. The system consists of a passive terahertz camera, a video surveillance camera and a thermal imaging camera, the product is configured as a single unit. The design of the complex together with the THz band (implemented by a passive terahertz camera) provides for a video image channel in the visible optical range and an image channel in the infrared spectral region (8–14 µm) with the possibility of subsequent imposition of the images and their joint processing.
System parameters:
• Video signal frame rate is 6 fps,
• Resolution of the video signal frame: 320 Ч 240 pixels for the optical and IR range,
• Image format is jpg, video is avi
• Video transmission range: from the complex to the operator’s workstation is up to 100 m,
• Storage time of the recorded video information is not less than 30 days.
The display of the result of the security screening is carried out in two modes:
• by a terahertz camera
• by combined video surveillance with a graphic isolation of the concealed object location area (embedding location) on the image of the screened subject.
Special software has been developed for the operator’s workstation. It allows creating a large-scale projection model of the object of security screening with the subsequent automatic processing of video signals received from a passive terahertz camera, a video surveillance camera, a thermal camera, in order to detect forbidden objects.
The special software provides automatic signaling to the operator about attempts to resist recognition; it yields photo and video images to the external system in the visible range for photo identification of the face; it determines the exact spatial position and the corresponding scale factors for comparing the recorded image anomalies with a catalogue of forbidden objects; with the purpose of detecting forbidden objects, it synthesizes the image of the subject by analyzing the video signals received from a passive terahertz camera, a video surveillance camera and a thermal camera.
The recently increased terrorist threat with respect to the facilities of particular importance places a priority on the task of providing these facilities with modern means of protection. The fight against terrorism is one of the highest priorities for all Russian security agencies. The new scanning system offered by JSC "OKB Astron" will significantly increase the level of safety of the most important facilities of the Russian infrastructure.
TERAHERTZ RADIATION
The frequency range of terahertz (THz) radiation ranks an intermediate position between the infrared (IR) and microwave regions of the electromagnetic spectrum (Fig. 1). Conditionally, the limits of the THz range are the frequencies from 0.02 THz to 10.00 THz [1]. This frequency interval occupies a part of the electromagnetic spectrum between the infrared (IR) and microwave ranges, so it is often also called the far IR or submillimeter range. The spectra of complex organic molecules (molecules of proteins and DNA, explosives, etc.) lie in the terahertz range.
Terahertz radiation is non-ionizing, in contrast to X-ray radiation. Different biological tissues have different absorption in this range, which allows for the contrast of the images. Compared with visible and IR radiation, terahertz radiation is less susceptible to scattering. As a result, many dry dielectric materials, such as textiles, wood, paper, plastics, are transparent in this range. Therefore terahertz radiation can be used for non-destructive material control, scanning at airports and other controlled services. Furthermore, the wavelength of the radiation is sufficiently small to provide submicron spatial resolution when using freely propagating radiation. Molecules of different substances have their own unique resonance lines in the terahertz spectrum, which can be used for accurate identification. This makes it possible to identify the molecules by their spectral "imprints". In combination with obtaining an image in the terahertz range, this makes it possible to determine not only the shape, but also the composition of the object under study.
Terahertz radiation can be detected in the time domain, i. e., it can be measured as the amplitude and phase of the electromagnetic field. This makes it possible to directly measure the phase shift introduced by the object under investigation, and therefore allows one to investigate fast processes. The peculiarity of terahertz analysis is that it has the ability to simultaneously measure the amplitude and phase of a wave. In conventional spectroscopes used by the services of control of concealed items, only the intensity of radiation at certain frequencies is measured. Terahertz experiments often include measurements of the temporal characteristics of the electromagnetic field of terahertz pulses that interacted with the sample (i. e., reflected or passed through). The Fourier transform applied to these time domain data provides information on the phase and amplitude of the pulse, as well as a variety of additional information about the sample with which the terahertz pulse has interacted [2–4].
HISTORY AND STATE OF THE ART DEVELOPMENTS
The first experiments related to the detection of THz radiation were carried out back in 1897 by Henrik Rubens and Fox Nicholas [5]. At that time they have been solving the problem of determining the spectral composition of the radiation of an absolute black body using a bolometer. Much later, in 1975, Auston [6] developed a generator of pulsed THz radiation, built on the basis of a photo-switch: a photoconductive antenna excited by femtosecond laser pulses of the optical range. The creation of powerful pulsed lasers, in particular, femtosecond lasers generating pulses of duration about 100 fs (1 fs = 10–15 s) in the 80’s opened the way for the creation of compact sources of terahertz radiation, based on the interaction of laser radiation with the substance. It was found that terahertz radiation can be obtained through the nonlinear conversion of high-intensity laser radiation in crystals (generation of a difference frequency), and also due to certain linear effects, such as laser-induced carrier generation in semiconductors.
At the end of the 1980s, a breakthrough in terahertz studies occurred: Terahertz Time-Domain Spectroscopy (THz TDS) method was demonstrated for the first time. This method is based on the generation and detection of coherent terahertz radiation using pulses from the same laser. Unlike other methods, where only the pulse envelope or the radiation power is measured, terahertz time-domain spectroscopy allows for the direct detection of the electric field of a terahertz pulse with a high resolution, the duration of which is only about 1 ps (10–12 s). Direct measurement of the electric field makes it possible to extract information on the phase shift of the terahertz field when it interacts with the object, and thus offers opportunities for studying ultrafast processes (occurring over a fraction of picoseconds).
The development of laser technology, the creation of installations capable of generating stable and powerful ultrashort laser optical pulses, has led to the further development of THz spectroscopy [7, 8]. The first studies in this field were conducted by Martin Nuss et al. at the Bell Labs in the early 1990s. Historically, the first leader in terahertz studies was the research team from the Rensselaer Polytechnic Institute, headed by Dr. Xi-Cheng Zhang. The Rensselaer Polytechnic Institute was one of the first to open a separate laboratory, which only deals with terahertz research. At CLEO (Conference on Lasers and Electro-Optics) in 2005, Xi-Cheng Zhang et al. made a report on the first results of using terahertz waves to detect explosives and biological weapons. Using terahertz radiation at a frequency of 0.3 THz, they were able to remotely identify samples of flour, salt, soda and bacteria spores.
TeraView, Cambridge (England), separated from the Toshiba research laboratory in early 2001. TeraView researches were the first to conduct a series of experiments on multi-layer scanning of semiconductor chips, which has already offered opportunities of practical use of the new technology in non-destructive testing.
StarTiger research group (Space Technology Advancements by Resourceful, Targeted and Innovative Groups of Experts and Researchers) was founded in 2002 by the European Space Agency based on the Rutherford and Appleton lab (England, Oxford). StarTiger Group has obtained the world’s first terahertz photo of a human hand. The sensor developed by StarTiger Group uses terahertz radiation of 2 frequency bands: 0.25 and 0.30 THz. Thus, an image is obtained in two spectra and a contrast is created between the materials with different transmittance and reflection parameters in different spectra. The main advantage of this sensor is due to the fact that it does not emit electromagnetic waves. This is a passive camera of the terahertz range emitted by almost all objects, both biological and non-living ones.
During the same years, QinetiQ Defense Company from the UK has also developed a two-spectral terahertz sensor. Since the sensor operates at two frequencies, 250 and 300 GHz, it becomes possible to distinguish materials using their optical characteristics, for example, reflectivity. In the terahertz spectrum, the reflectivity of the body decreases, and the emissivity increases with increasing radiation frequency. This feature can be used to reduce the number of false alarms. It was possible to obtain the image of various objects through different materials: knives and non-metallic objects, hidden in pockets and newspaper.
The English company ThruVision (separated from the Rutherford and Appleton lab in the early 2000s) developed and tested a new type of THz detector for the security industry in 2008. The detectors are based on the use of GaAs and GaInAs based microstructures with photonic crystals. The detector allows capturing radiation in the range from several hundred gigahertzes to several terahertzes and has sensitivity at the picowatt level. Especially valuable is the established industrial production of matrixes of terahertz range detectors by this company. The advantage of this technology is that it is passive and allows you to obtain images using the radiation of the object itself. The detectors operate at room temperature with a frequency of more than 1 Hz, which allows you to capture moving objects.
The basics of using a combined visible and terahertz image in the early 2000s were suggested by Rick Blum, a professor at LeHigh University (USA). The resulting combined image gives more information than is contained in each original image separately. The combined image has enough information to see, for example, a weapon concealed under clothing. The basic idea of combining two images is as follows. The digital pattern of the original image is subjected to wavelet transform, superimposed by a terahertz image. The inverse wavelet transform allows you to obtain a combined image. This approach was implemented by us in the development of the complex.
Nowadays, terahertz technology is developing as a completely new diagnostic direction. Terahertz waves allow us to see through many optically opaque materials. With the help of terahertz radiation it is possible to obtain a picture of the structure similar to the X-ray, but without the use of potentially dangerous X-ray radiation. Terahertz sensor is a passive sensor and absolutely harmless to human, since the source of signals here is the natural radiation of the objects. At airports, a terahertz sensor can be used to detect weapons concealed under clothing. Moreover, unlike metal detectors, which are used now, a terahertz sensor is also capable of detecting non-metallic explosives [9–14].
COMPLEX STRUCTURE
The terahertz complex for detecting forbidden objects created by JSC "OKB ASTRON" (Fig. 2) includes:
• Terahertz camera.
• Surveillance camera.
• Thermal imaging camera.
• Operator’s AWS.
• Means of transferring information from the means of inspection to the operator’s workstation
• Uninterruptible power supply system.
ELEMENT BASE OF THE DETECTION COMPLEX
Photoconductive antennas
Photoconductive antenna (PC-antenna, photoconducting switch) is one of the most commonly used terahertz radiation generators and receivers. The PC-antenna consists of two metal electrodes (Fig. 3), located at some distance from each other on a semiconductor substrate. A voltage of the order of several kilovolts is applied to the electrodes. When the gap between electrodes is illuminated by an ultrashort laser pulse, the density of charge carriers in a semiconductor increases sharply for a short time (of the order of one or tens of picoseconds). For effective absorption of laser radiation with the release of carriers, the energy of laser radiation photon should exceed the width of the semiconductor forbidden band. The resulting free carriers are accelerated by a field applied to the gap, resulting in a short-time current pulse, which is the source of terahertz radiation. Thus, the laser pulse serves as an ultrafast switch for the antenna, which transfers it from the insulating state to the conducting state. The duration of the current pulse and the spectrum of the emitted terahertz wave are determined, basically, by the lifetime of the carriers in the semiconductor.
In a real-world situation, the energy of the terahertz pulse experiences saturation with increasing energy of the laser pulse. This is due to the fact that the photoinduced carriers shield the displacement field. The increase in the displacement field also encounters the limitations associated with possible electrical breakdown of the substrate. The electric breakdown field is about 400 kV/cm for gallium arsenide (GaAs).
Radiation receiver (RR)
The selection of the radiation receiver (RR) and the wavelength at which it will operate determines all other elements of the product design. Modern techniques for detecting explosive (ES), toxic (TS) and narcotic (NS) substances mainly apply the methods of sampling air and its chromatographic analysis. The presence of characteristic absorption bands in the terahertz region of the spectrum in ES, TS and NS, as well as the absence of such absorption in the container materials, indicate the promise of using THz radiation for their detection by spectral methods, as well as the possibility of developing and creating stationary and portable security systems based on Terahertz vision.
The main issue in choosing a receiver is the method of use in the active or passive mode. However, there are significant difficulties in the production of terahertz radiation sources for backlighting in the active mode. The complex developed by us detects its own terahertz radiation of objects without additional illumination.
TERAHERTZ CAMERA STRUCTURE
The terahertz camera (THZC) consists of the following units (Fig. 4):
• Scanning unit
• Focusing system
• Microwave units
• Control controller
• Control calculator and primary image processing.
Focusing system and optical scheme
The mirror systems (MS) take significant precedence over the lens systems. They consist of reflective surfaces that do not introduce chromatic aberrations, so purely mirror systems are best used in the ultraviolet (UV) and infrared (IR) spectral regions. With relatively simple MS designs, it is possible to obtain a rather perfect correction for spherical aberrations. These systems have a high aperture ratio and resolution, with greater compactness. Since the optical glass does not work in the THz region of the emission spectrum, the components of the MS optical scheme were made of metal.
The first mirror is scanning, it provides an image of the necessary space of objects, and transfers the image to the surface of the second mirror, which forms a light spot transmitted to the radiation receiver (Fig. 5). Correction of distortions of the optical system is performed by the method of Gaussian quadrature, wherein the density of traced rays is determined by the number of rays located along the radius. The maximum number is 18. This is sufficient for pupil aberrations up to 36 orders of magnitude.
The focusing system provides the concentration of the terahertz radiation flux from the object to a ruler of 8 conical waveguides of the receiving detector. The terahertz flux focusing system consists of 4 aluminum mirrors mounted on a rigid chassis. The flat input mirror is scanning; the scanning assembly provides a mechanical sweep of the terahertz radiation flux from the object to the receiving detector. The second and fourth mirrors are cylindrical, and the third one is toric.
The diameter of the focused spot of the terahertz radiation flux is approximately equal to the diameter of the entrance aperture of the conical waveguide of the horn antenna of the receiving detector, i. e. about 7 mm. This ensures the reception of the whole terahertz flux coming from the object to the THzC input window, which ensures the maximum sensitivity of the system.
Microwave unit, radiation receiver
The receiver of the terahertz radiation flux is a microwave unit that receives a terahertz signal from the focusing system. The microwave unit consists of an eight-channel microwave detector, an auxiliary module and an air cooling assembly. The microwave detector consists of eight identical channels operating simultaneously to obtain an eightfold increase in the scan rate.
The microwave receiver is performed by heterodyne scheme. The frequency of the heterodyne is 125 GHz, and the conversion is performed at the second heterodyne harmonic of the local oscillator in a mixer of counter-parallel Schottky diodes. The operating band of the intermediate frequency amplifier is 20 GHz. The microwave detector consists of eight receiving conical waveguides of horn antennas arranged in a line, but with a staggered offset. This arrangement is made in order that there are no gaps between the effective apertures of the waveguides, since the effective apertures are smaller than the physical input diameters of the receiving conical waveguides. A mixer is mounted on the counter-parallel Schottky diodes inside each receiving waveguide. The local oscillator signal is fed to the mixers via waveguides of a special configuration to ensure the distribution of the same power and one phase for each diode mixer (Fig. 6).
The FC signal is amplified by low-noise microwave amplifiers and detected by an active detection circuit. The auxiliary module of the microwave detector ensures the installation and maintenance of the operating modes of all microwave modules in order to obtain the optimum amplification and the dynamic range of the useful signal from the object. The resulting signal is digitized and fed to the control and processing controller to further form the image frame.
SURVEILLANCE CAMERA
AXIS M1011 video surveillance camera with the Axis Communications built-in lens was used by us as a finished product. The surveillance camera is installed next to the input window of the terahertz camera.
THERMAL CAMERA
We used a thermal imaging camera ASTRON384. The Astron thermal imaging lens is a finished product. The lens is made of a germanium single crystal with a bandwidth of 7–14 µm and a focal length of 28 mm, IFOV spatial resolution of 0.89 mrad (IFOV – instantaneous field of view). The only additional feature of the product is the increased requirements for the quality of germanium optics, which is achieved using high-quality single crystals with low dislocation density and the absence of dislocation clusters for manufacturing blanks. The thermal camera is installed next to the input window of the terahertz camera.
CONTROL AND PROCESSES CONTROLLER OF THE MICROWAVE UNIT AND THE SCANNING UNIT
The signals in digital form from each of the eight channels are synchronized with the position of the input scanning mirror of simultaneously by two coordinates. The image frame is consistently constructed from 6 vertical bands, which in turn consist of eight columns (by the number of input microwave conical waveguides and detectors). The useful signal from the object is very small by its nature of origin and is masked by the inevitable thermal noise of the entire receiving path; therefore, digital accumulation of information is made for each frame element (pixel). Furthermore, for several frames, the useful component of the signal grows faster than the noise component, which makes it possible to better see the various sections of the object. The generated frames are sent to the computer for further processing with special software to identify hidden objects and eliminate jamming signals.
SCANNING UNIT
The scanning unit is designed to rotate the first focusing mirror in two planes in order to sequentially bypass the area of the object and form an image frame in the terahertz range. The rotating mirror is attached to the mechanism of the scanning unit. Two stepper motors provide a vertical and horizontal mechanical sweep of the terahertz radiation flux from the object. The main parts are made of aluminum to reduce weight and therefore dynamic loads. Each stepper motor is controlled from its own control controller. The control controllers receive control signals from the microwave unit control controller. The scanning unit outputs two synchronization signals to the microwave unit control controller, which correspond to the average positions of the rotating mirror. These signals synchronize the center of the image frame in the terahertz range with the main optical axis of the focusing system. The scanning unit is mounted on the chassis of the focusing system to ensure the accuracy and stability of the optical system of the terahertz camera.
TERAHERTZ COMPLEX OPERATION RESULTS
The test objects were placed on the human body and concealed under various materials. Fig. 7а shows the images (screenshot of the AWS screen) of the experimenter with the model belt concealed under the clothes. There is an image obtained from the terahertz channel in the central part (Fig. 7b) (the tested object is already visible). Fig. 7c gives a combined image processed by the AWS analyzer. It can be seen that the complex positively detects the tested object.
CONCLUSION
JSC "OKB Astron" has formulated the concept of an active detection system based on THz sources and detectors, which makes it possible to detect concealed substances, a scheme for generating THz radiation and receiving reflected and scattered radiation was selected. Also, the fundamentals of the technology of compact sources and receivers of THz radiation based on GaAs and InGaAs nanostructures have been developed. A design was developed and a model of a compact THz radiation source (up to 1.2 THz) was manufactured based on semiconductor nanostructures with radiation delivery, including nanostructured fiber. A design was developed and a model of a THz radiation receiver (up to 1.2 THz) was manufactured with a sensitivity that allows recording signals from detection objects in real time.
JSC "OKB Astron" has manufactured an active system for detecting covertly carried substances based on THz sources and detectors. The system consists of a passive terahertz camera, a video surveillance camera and a thermal imaging camera, the product is configured as a single unit. The design of the complex together with the THz band (implemented by a passive terahertz camera) provides for a video image channel in the visible optical range and an image channel in the infrared spectral region (8–14 µm) with the possibility of subsequent imposition of the images and their joint processing.
System parameters:
• Video signal frame rate is 6 fps,
• Resolution of the video signal frame: 320 Ч 240 pixels for the optical and IR range,
• Image format is jpg, video is avi
• Video transmission range: from the complex to the operator’s workstation is up to 100 m,
• Storage time of the recorded video information is not less than 30 days.
The display of the result of the security screening is carried out in two modes:
• by a terahertz camera
• by combined video surveillance with a graphic isolation of the concealed object location area (embedding location) on the image of the screened subject.
Special software has been developed for the operator’s workstation. It allows creating a large-scale projection model of the object of security screening with the subsequent automatic processing of video signals received from a passive terahertz camera, a video surveillance camera, a thermal camera, in order to detect forbidden objects.
The special software provides automatic signaling to the operator about attempts to resist recognition; it yields photo and video images to the external system in the visible range for photo identification of the face; it determines the exact spatial position and the corresponding scale factors for comparing the recorded image anomalies with a catalogue of forbidden objects; with the purpose of detecting forbidden objects, it synthesizes the image of the subject by analyzing the video signals received from a passive terahertz camera, a video surveillance camera and a thermal camera.
The recently increased terrorist threat with respect to the facilities of particular importance places a priority on the task of providing these facilities with modern means of protection. The fight against terrorism is one of the highest priorities for all Russian security agencies. The new scanning system offered by JSC "OKB Astron" will significantly increase the level of safety of the most important facilities of the Russian infrastructure.
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