KR20050024303A - Terahertz imaging system and method - Google Patents

Abstract

THz imaging devices and methods are provided for quickly and effectively inspecting a region of interest to determine the presence or absence of specified configurations. The apparatus includes means for generating electromagnetic radiation of a desired terahertz frequency suitable for inspection and rendering radiation incident on the region of interest. The detector means is provided with detector means for simultaneously detecting the terahertz radiation reflected or transmitted from the area at a plurality of points in the detector plane which are remote from the area of interest. Means are provided for converting the detected terahertz radiation into an image of the region of interest capable of determining the presence or absence of the specified construct.

Description

Terahertz imaging system and method

TECHNICAL FIELD This invention relates generally to imaging apparatus and methods, and more particularly to imaging systems and methods that utilize electromagnetic radiation in the terahertz (THz) range as incident energy to objects to be inspected.

The present invention relates to the detection of weapons concealed in a person or containers, such as suitcases, briefcases, sealed packages, or cardboard boxes. Such weaponry may include metal devices such as pistols, but the present invention addresses the problem of specifically monitoring, detecting and characterizing hidden explosives and biological weapons.

Plastic explosives, fertilizer bombs and biological agents are increasingly becoming weapons of war and terror, and effective means for the rapid detection and identification of concealments hiding these agents are increasingly critical. One proposed solution is to use terahertz (THz) electromagnetic waves to detect and identify explosives and biological weapons that are spectroscopically concealed through characteristic transmission or reflection spectra in the THz range (0.1-1 OTHz). Explosives (e.g., C-4, HMX, RDX, TNT, naphthalin and ammonium nitrate) are all 0.1-2.0 THz (100-2000 GHz, 3-) which can be easily distinguished from other substances such as human skin. 0.15 mm) and characteristic reflection and absorption spectra. Essentially, explosives appear in the THz detector as different "colors" compared to non-hazardous items. The use of THz for the detection of biological weapons has shown great promise. Thus, when using THz spectroscopy, THz radiation can be applied to plastics, clothing, luggage, paper products, walls, and other non-conductive (nonmetallic) materials, even when explosives and biological weapons are hidden in clothing, sealed packages, suitcases, etc. Since it is easily transmitted through the material, it is possible in principle to detect these explosives and biological weapons. By comparing the measured reflectance (or transmission) THz spectra with known calibration spectra, one can confirm the presence of these agents and distinguish them from objects such as keys, coins, human skin, and clothing. Since the metals do not pass the transmission of relatively THz wavelengths and have approximately constant reflection spectra, metal weapons such as pistols and knives can be similarly identified by THz inspection.

Most THz imaging systems proposed in the past have been based on a single THz source and detector pair that scans the object space to obtain an image. After all, these systems take a significant amount of time (typically minutes) to acquire data to generate their THz images, even a single small object (e.g. approximately a few square centimeters), which is not suitable for real-time acquisition of THz images. not. In addition, current state-of-the-art THz imaging is based on shot pulse laser or continuous wave frequency THz generation and detection. The difficulty in extending either of these techniques to continuous wave THz imaging of coherent or incoherent THz radiation is the need for coherent continuous wave or short pulse laser sources. In addition, laser sources that generate and detect THz radiation must maintain coherent phase relationships with each other. Using these methods, imaging of incoherent THz sources is not possible. The design and techniques of the present invention do not require any particular coherent or incoherent source of THz. This allows for flexibility using electron THz sources, laser-based THz illuminating sources, or THz radiation around incoherent, which may be, for example, from the sun.

1B and 1B are schematic diagrams simplifying two possible system implementations of the present invention.

2 is a schematic diagram illustrating the phase of an input THz wavefront measured at a pair of distant detectors.

3 (a) is a photograph of the actual spatial image generated by a single detector pair at fixed intervals.

3 (b) is a graph showing the expected dependence of the correlated THz electric fields measured by the detector pair as a function of the spacing (baseline) of the detector pairs, α = 10 °, k = 16.5 cm ⁻¹ (0.5 THz), A = 1.

4 is a schematic diagram of five detector antennas in a line configuration.

5 is a schematic diagram illustrating the elements used in the present invention to enable interferometer detection by heterodyne photomixing.

6 is a schematic diagram illustrating a configuration of detector antennas for use in the rotation mode.

7 is a schematic diagram showing the configuration of an intermediate frequency electronic device used to process detector outputs in the present invention.

It is an object of the present invention to provide a spatial THz imaging technique capable of simultaneously detecting a plurality of THz sources within a wide field-of-view (FOV). In order to achieve the same functionality with a single line of sight measurement, the line of sight must be scanned over the FOV to be measured, but this leads to the aforementioned challenges. Spatial resolution by the present invention is sufficient to determine, for example, the presence of explosives or biological agents as well as to determine the physical size and location of an object. This information is difficult to determine by eye technology. The THz imaging approach of the present invention has sufficient spatial resolution to detect the explosives or biological agents hidden in people or packages, containers or vehicles at a distance. An advantage of the present invention is that it provides more information than is driven by a single gaze system. Imaging processing techniques that can eliminate noise and reduce false alarms in a complete system by obtaining multiple images over time can be applied to multiple images and multiple THz sources.

Summary of the Invention

In accordance with the present invention, THz imaging apparatus and methods are provided with terahertz imaging apparatus and methods for quickly and effectively inspecting a region of interest to determine the presence or absence of specified configurations. The apparatus includes means for generating electromagnetic radiation of a desired terahertz frequency suitable for inspection and rendering terahertz radiation incident on the region of interest. At a plurality of points in the plane of the region of interest, detector means are provided for simultaneously detecting terahertz radiation reflected from or transmitted through the region. Means are provided for converting the detected terahertz radiation into an image of a region of interest that can determine the presence or absence of a specified construct.

As a courtesy, a large area is illuminated with bright THz sources to guard away. The source may be wideband and incoherent (such as radiation from the sun), or narrowband and tunable. At this time, the transmitted or reflected THz radiation is detected by the THz imaging array. The decisive technical limitation to this approach has conventionally been no imaging THz detector array (regardless of spatial resolution or tuning possibilities). In fact, no equivalent digital camera existed in the THz regime. The limitation in the THz regime was not a "camera" lens, but rather a detector array that digitized the image.

The THz imaging array of the present invention relates to previous work in radio astronomy. Unlike radio astronomy, where the positions and spectral content of radio sources (stars) cannot be known in advance, in the detection of explosives / biological agents on a THz-based basis, the spectral content and location of the sources are already known, but the THz transmission of intervening objects is known. Properties need to be determined.

In the present invention, the detector means may comprise a tunable interferometer array in which the detectors are spaced apart. The signal outputs from the pairs of detectors combine the signal outputs from the pairs of detectors with the appropriate delay and in phase and quadrature correlation to produce components for the Fourier transform plane corresponding to the detector plane. The detector array may include a plurality of semiconductor photomixers. The photomixer drive means comprising a frequency stabilized tunable optical heterodyne source is coupled to the photomixers by a common fiber optic optical connector. The array can include a series configuration of detectors, and means for rotating the array about a fixed axis. The original brightness distribution in the region of interest is recovered by the Fourier inverse transformation of the Fourier components.

The photomixers may be photoconductive devices, wherein the driving means for each pair of these devices is a pair of lasers having a difference frequency that gates the photomixers, and input terminators to respective members of the pairs of photomixers. Hertz emission is mixed with the difference frequency to provide modified signal outputs at intermediate frequencies to facilitate signal processing.

The wide FOV THz imaging interferometer array used can have high spatial resolution and spectral resolution in the 0.2-10 THz range. This array can image multiple sources of coherent or incoherent THz light simultaneously without the need for expensive short pulse laser systems. Accordingly, the present invention may include a continuous wave terahertz imaging spectrometer that is tunable from 0.2 to 3 THz for remotely detecting, monitoring and identifying hidden explosives such as C4 and RDX. The system utilizes a high brightness THz source that is scanned by the region of interest and detected by a tunable THz interferometer imaging array. THz imaging arrays have wide FOV and high spatial resolution. The system preferably uses heterodyne photomixing detection techniques, but homodyne photomixing detection may also be used. High speed photomixing devices for heterodyne detection are preferably designed to operate at optimal intermediate frequencies. Photomixing devices can be used as sources and detectors. A database of THz-frequency spectral signatures required for the target explosive is provided for use in the system, and a neural network algorithm may be used in combination to identify the explosives selected from the THz images.

By way of example, the invention is shown schematically in the accompanying drawings.

By convention, people entering the airport terminal, base, ship, or post office, or when non-intrusive screening of packages or pallets, the person (or packages 8) are not shown in FIG. 1 (a). As shown, the THz source or sources 10 and the THz imaging array 12 may be located or moved between. The schematic diagram of FIG. 1A is based on the transmission mode. In FIG. 1 (b), the THz source 14 is incident on people and objects 13 at a distance from each other, and the detection at imaging array 12 is by reflection. In both cases (a) and (b), THz sources are investigated for humans, pallets, vehicles or other research subjects.

An advantage of interferometric imaging over imaging with a corresponding digital camera is that interferometric imaging can be done with only a few individual detector elements. Consumer digital cameras typically have 780,000 individual detector elements in an imaging array or array of 1024 x 768 pixels. Such high density detectors in the THz range are currently not technically feasible. One reason is that conventional detectors in the 0.1-10 THz frequency range generally require liquid helium cooling and are not easily integrated into dense array structures. Therefore, to obtain an image in the THz range, only a few (1-10) detector elements should be used to generate the images. Interferometric imaging benefits greatly from the ability to image using only a few detector elements and simultaneously image many THz radiation sources, image incoherent and coherent sources, and provide spectral and spatial imaging information. to provide.

The THz imaging array 12 used in the present invention relates to a known technique for use in radio astronomy. Unlike radio astronomy, which does not know the positions and spectral content of radio sources (stars) in advance, THz-based explosive monitoring systems know the spectral content and location of the sources, but the THz transmission (or reflection) of the intervening objects is known. Characteristics need to be determined.

To perform imaging of THz in real time, the technique of a radio interferometer is used, in which signals at two or more points in the space (aperture plane) have a suitable delay and cause them to be in phase and quadrature ( in quadrature to produce cosine and sine components of the brightness distribution. This technique measures both the amplitude and phase of the incoming signal and, if measured from a sufficient number of points in the aperture plane, the raw brightness distribution can be synthesized through a standard Fourier inverse transform (images can be obtained). have). Constructing images using interferometer arrays is a technique that has been developed for use in astronomical imaging in both radio and X-ray wavelength ranges. The radio range from metric wavelengths to sub-mm wavelengths has been a common framework for developing Fourier image reconstruction techniques, and there are many tools for modeling and simulating the characteristics of interferometer arrays. Lean arrays with 3-10 elements require special processing to reduce ambiguity (called side-lobes) in the reconstructed images. In radio astronomy, frequency-synthesis transformation techniques can be applied to THz imaging problems when data is obtained at different THz frequencies (using different spatial information at each frequency by combining data at multiple frequencies). Several techniques for imaging with lean arrays are known that employ.

The THz system of the present invention comprises three main components: (a) a THz interferometer imaging array, (b) (preferably) heterodyne mixing of THz signals and signal processing at intermediate frequencies of 100 MHz and (c) a high brightness THz source. Include.

In FIG. 2, the imaging interferometer is similar to a phased array detector in that the difference in the arrival times of the wavefront at the detectors of the two points depends on the angle α of the wavefront 20 with respect to the two detectors. For a given wavelength λ, the angular resolution is determined by the distance d between the two detectors: θ = λ / d (radians). The FOV of the interferometer is determined by the smaller of the beam pattern (direction) of the individual detectors and the bandwidth of the detectors. In the case of Gaussian bandwidth, the angular sensitivity of the interferometer also decreases like Gaussian. The 1 / e width of the FOV is given by W _{1 / e} ≒ c / πdσ (radians), where σ is the antenna's 1 / e bandwidth. The FOV is approximately v / πσ factor larger than the angular resolution. As an example, if a THz antenna with a 1 / e bandwidth of 0.01 THz and a center detection frequency of 1 THz is 10 cm apart, the interferometer may image a 5 ° FOV with 10 'resolution. At 1 cm intervals, the FOV is 51 ° and the resolution is approximately 1.7 °.

As an example of possible spatial resolution, if a THx imaging array (center frequency is approximately 1 THz, detector baseline distance is 1 cm and see Table 1) is installed in a jeep approximately 15 m (50 ft) away from human explosives For this reason, the corresponding spatial resolution for the explosives of this target 15 meters away would be approximately 45 cm at 1 THz and approximately 4.5 cm at 10 THz. Finer resolution (factor of 10) may be achieved by a 10 cm baseline array. This is enough resolution to identify wallet-sized explosives. The thickness of the explosive will be several mm. At a distance of 50m to see from a large area, a 1cm baseline array has a spatial resolution of 1.5m and a 10cm baseline array has a spatial resolution of about 15cm.

Table 1: Estimated Angle Resolution of Interferometer Array as a Function of Frequency v and Distance d

The THz imaging array in the present invention employs THz detectors made by Picometrix of Ann Arbor, MI, among other suitable detectors. Picometrix THz detectors operate as photoconductive devices. For these detectors, a gold microfabricated antenna is fabricated on top of the low temperature grown GaAs, a fast photoconductive material. These are room temperature detectors that are optically coupled to an optical fiber. For typical Picometrix detector design parameters, the FOV of the THz imaging array is determined by the orientation of the detector. The FOV of the detector can be adjusted by slightly altering the design of the THz lens to focus THz radiation on the detector (from a few degrees to approximately 50 degrees).

The imaging interferometer consists of an array of individual detectors. A pair of such detectors 16, 18 are shown in the schematic of FIG. 2. Each detector measures the amplitude and phase of incoming THz radiation. When the wavefront of the THz radiation reaches the array, each detector pair (eg 16, 18) in the array determines and determines one spatial Fourier component of the input THz radiation by the distance of the detector pair. Each spatial Fourier component appears as a point in the Fourier transform plane (uv plane). In order to determine the spatial Fourier component and thus the direction of the input THz wavefront 20, the phase delay between the pair of antennas must be measured at the arrival time of the wavefront. The relative angle between the direction to the source and the base line (an imaginary line connecting two antennas and effectively including detectors) defines the geometrical delay τ _g in the arrival of the wavefront between the two antennas. All directions forming the cone around the reference have the same phase retardation τ _g = (b sin α) / c, b is the length of the baseline, c is the speed of light, and A is the relative angle. Further measurements by different orientations of the baseline must be performed to determine the correct source direction.

At fixed intervals between detector pairs, one spatial Fourier component is measured. In real-space, this single Fourier component corresponds to the intensity fringes as shown in Fig. 3 (a). By changing the spacing between the pairs of detectors (but keeping the distance to the source constant), the spatial Fourier component is changed so that the spacing between the alternating light fringes and the female fringes in FIG. By adding together the images generated by the different detector pair intervals, a hybrid real-space image is formed. Simple experimental verification of interferometer detection can be done by varying the spacing between the detector pairs (distance b in FIG. 2). Based on this geometry, the expected correlation C between the THz signals from the point source detected at the two detectors will be:

A is the amplitude of the incident THz plane wave, k is the wave number of the input electromagnetic wave, B is the spacing between the pair of detectors, and A is the angle that makes the input wavefronts with respect to the detectors as defined in FIG. Plots of expected experimental data are shown in FIG. 3 (b) for a frequency of 0.5 THz. When Fourier components of different detector pairs are included, the resulting image reaches the point spread function of the point THz source. The point spread function can be used, for example, to cancel THz interferometer images and to remove side lobe artifacts in the interferometer image.

When the number of detectors is N, there are possible pair combinations of N (N-1) 2. It is desirable to place them so that the spacing between the antennas is nonuniform so that the Fourier plane is sampled as completely as possible. A typical in-line arrangement of five antennas is shown in FIG. 4 with the resulting baseline. The interval is typically log-periodic. By recording the correlation in the electric field in the various combinations of detector pairs, information about the emission spatial distribution from the THz source can be generated. The image is generated from spatial Fourier components of all different pair combinations. The quality of the image depends on the coverage of the u-v plane and thus on the arrangement of the detection elements of the interferometer. In designing the configuration of the antennas, it is important to obtain the coverage of the u-v plane uniformly and efficiently over the range determined by the required angular resolution. Efficient u-v planar coverage by a small number of detectors may be achieved by using a array of detectors in combination with the rotation of the array about a fixed axis. If 20 measurements are made during the rotation of the N element array, the number of detectors equivalent is 20N. This may improve image quality or reduce the number of antennas required in the array.

In FIG. 5, interferometer detection of THz is made by photoconductive antenna detectors of optical fiber-optical coupling, shown in pairs of 22 and 24. In these detectors, input THz radiation is detected by mixing input THz radiation to two infrared (˜780 nm) laser beams from lasers 26 and 28 that “gate” or “turn on” the photoconductive detectors. There are clear advantages in coupling infrared laser light to detector structures using fiber optic optical cables 29 and splitter 30. For example, combining laser light and then distributing it to a large number of detectors is simple using optical fiber couplers and star splitters. Therefore, only two infrared laser sources may be used to power all antennas in the N-element interferometer array. In addition, optical fiber coupling makes the entire array more reliable and reliable. By attaching the optical fibers to the detectors, they can be easily shifted relative to one another (ie, adjustable baseline or detector spacing) since the light transmission system is optical fiber optics. With this design, the array has a low spatial resolution and a large area This allows the possibility of very quickly surviving explosives. If a particular area represents the spectrometer signature of the agent, then the baseline of the array can be adjusted to examine the suspected area with greater spatial resolution.

Lasers 26 and 28 are two narrowband infrared lasers used with detectors to detect THz radiation via differential frequency optical heterodyne photomixing. Two external cavity diode lasers (ECDL) are used to produce two different colors (wavelengths) of infrared radiation close to 780 nm. The THz frequency can be tuned by adjusting the different frequencies of the two infrared colors. Although homodyne detection of laser mixing in photoconductive antennas has been carried out by others, heterodyne detection is not reliably performed. Heterodyne detection technology improves the sensitivity of the THz array compared to homodyne (DC) detection. Heterodyne detection separates the Thz source from the THz local oscillator (LO), which means that the THz source and LO need not be coherent or come from the same source. In photo-mixing detection technology, the THz local oscillator is provided by mixing two infrared laser wavelengths. Heterodyne detection techniques are also suitable for obtaining spatial images and spectra from an array by scanning local oscillator frequencies.

In the photo-mixing geometry, one may think about mixing two infrared laser sources as generating a local oscillator (LO) signal in a photoconductive antenna detector element. In this geometry, the intermediate frequency generated by the mixing of the local oscillator and terahertz signals may be in the range of 100-3000 MHz (the line width of the ECDL source is approximately 5 MHz). This frequency is handled because of the ease of use of electronic components at intermediate frequencies of 100 MHz. The relative phase and amplitude (ie, Fourier component in the u-v plane) of the THz field for a pair of detectors are determined by correlating the measured intermediate frequency (IF) signal frequencies at the two detectors. Once the THz signals are downconverted to the IF band of 100 MHz, the signals can be processed with the same well developed correlator technology used in radio astronomy.

In the heterodyne mixing technique used, the input THz signal (ω _signal ˜1 THz) received by the individual detectors is combined with a local oscillator signal ω _{LO that} differs slightly from the signal frequency. In this embodiment of the imaging array, the local oscillator signal [ω _LO ~ (1 + δ) THz] is generated by differential frequency mixing of the CW IR laser beams. The output of the detector is the mixing of the two THz signals with the difference frequency ω _IF = ω _LO -ω _signal . The difference frequency is in the intermediate frequency range (kHz-GHz) and can be processed electronically to obtain the original phase amplitude of the THz signal. The relative phase and amplitude of the THz field (ie the Fourier component in the uv plane) for a pair of detectors is determined by correlating the measured IF signal frequencies at the two detectors. By sweeping the local oscillator frequency of a fixed IF frequency (ie, varying the wavelength difference between two IR lasers), the interferometer can image THz sources at various frequencies. The advantage of sweeping the LO frequency is that it allows the monitoring of specific chemicals in the study subject, thereby identifying explosives by their spectral characteristics in the THz range.

A typical arrangement of antennas for an interferometer is shown schematically in FIG. 6, combining the five individual antennas and using rotation to illustrate the imaging of the interferometer THz array. The non-redundant arrangement of twelve detectors provides 66 Fourier components in every rotational orientation of the array 30. The selected array design places a total of 12 detectors along the two vertical axes of the geometric spiral as shown in the figure. Each triangle represents a detector and each detector is numbered from 1 to 12 in the counterclockwise direction.

For efficient coverage of the Fourier transform u-v plane, it is important to vary the spacing between each detector pair such that each detector pair produces an inherent spatial Fourier component rather than harmonics of any component other than the intrinsic spatial Fourier component. Multiple occurrences of the same detector spacing will not provide any additional imaging information. 5 is the interval

d = ar ₀ b ^(n-1) is modeled by (1). Where d is the distance from the origin, a is the interval constant, r ₀ is the distance of the first detector, and n is the number of detectors. The value b is a constant describing the rate at which successive detectors spiral out from the origin. If b is chosen, the interval constant a can be used as a magnification to normalize the overall size of the detector array for different applications. The total size of the array can be estimated as approximately twice the distance from the detector 12 to the origin (d ₁₂ = ar ₀ b ¹¹ ).

If the imaging array was based on a spinning platform, rotation of the array relative to the THz source can be used to improve image quality. If, for example, the measurements were made 20 times during the rotation with respect to the N element array, the corresponding detector number is 20N. This can lead to improved image quality or a reduction in the number of antennas required in the array. When using the rotation of the array platform, the THz source can be found with only three antennas in the array. In fact, three antennas placed in a triangular pattern can be used to triangulate the position of the THz source in space. It can be further improved by tuning relatively narrowband detectors to several THz wavelengths (possibly using CW infrared laser excitation). In addition to providing spectral information about the THz source, interferograms at various THz frequencies may be used to improve spatial resolution or reduce the number of antennas required.

When generating images from the array 30, the array is rotated about the origin to fill more spatial Fourier components in the uv plane to increase the overall resolution. Data is obtained from the array every 1 ° of a total of 90 ° of rotation. In processing such data it can be assumed that the source of THz radiation is in the far field. That is, the distance from the imaging array to the source is much greater than the typical spacing between detectors in the array. In this limitation, the wavefronts of the input THz radiation are planar.

Detection of hidden C4 or RDX explosives in people or packages is not easy because there are many other materials that reflect or transmit THz radiation. Metals such as coins and belt buckles have an approximately constant reflection spectrum. The system can usually distinguish centimeter-sized C4 or RDX explosives from items. RDX has a reflection spectrum that includes large peaks at certain THz frequencies that can be distinguished from other articles such as clothing, skin, metal.

In order to demonstrate the ability of the THz imaging array, imaging of the RDX was simulated and found to be distinguishable from metal articles based on its spectral signature. In the simulation, the RDX was assumed to be 30 m away from the THz imaging array. The size of the array was 5 m. Images of two 1.4 cm squared samples lying next to each other were obtained. One is RDX and the other is metal. Incorrect color images were generated by coloring in red any positions that had large reflections at the 0.08, 0.175 and 0.4 THz characteristic frequencies of the RDX. Otherwise, each individual frequency was assigned a color proportional to its THz frequency. Eventually, the metal (reflecting all THz frequencies) was colored white. RDX was easily distinguished from metal and background. The neural network algorithm can be used to analyze THz images as a function of frequency to identify spatial locations that represent the spectral characteristics of the RDX.

There are several important differences between arrays used in astronomy applications and arrays used in those used herein for the detection of hidden weapons. For astronomical radio interferometer imaging, it is assumed that the input radio waves have plane wavefronts. With suitable delays between the various detector elements, any pair of elements can sample an electric field from the same wavefront. When the same wavefront is sampled, by definition, all points on the same wavefront have the same phase (spatially coherence) so that an incoherent source of radiation can also be detected with an interferometer. In the application to the explosive detection, the base line (the spacing between the array elements) is assumed to be ˜10 cm. The distance between the THz source and the detection array will be only a few meters for portal detection up to about 50 meters for wide area detection. While the distance of 2-50 meters is several orders of magnitude larger than the spacing between array elements, the THz interferometer array will have to deal with uneven, partially curved wavefronts. Also, because the wavefronts are curved, imaging the incoherent source of THz radiation is even more problematic because the appropriate delays between the detector elements depend on the curvature of the wavefront.

This potential problem is overcome by using THz source radiation with sufficiently long coherence lengths. THz radiation produced by optical heterodyne differential frequency mixing has a very long coherence length because the ECDL lasers that produce THz have very long coherence lengths. Using a long coherence length THz source means that many sequential wavefronts will be coherent and easily generate an interferometer signal. This design constraint places a coherence length limitation on the type of THz sources that could have been imaged.

In astronomical applications, the location, physical size, and frequency content (eg, star) of the THz source is unknown. Radio interferometer arrays and data processing are designed to know the location and spectral content of radio sources. In the present invention, the location, physical size and spectra of the THz sources are already known as they are part of the imaging system. Instead, the interest is in the THz transmission or reflection spectrum of the intervening material. In other words, are there explosives or biological agents? In order to process THz images for the presence of explosives or biological agents, images at characteristic THz frequencies are obtained by changing the frequency of the THz local oscillator. Artistic Neural Network (ANN) algorithms can be used to determine the presence of explosives and biological agents from THz images. ANN is a group of mathematical functions that make up the mapping of inputs to outputs. In the present invention, the inputs are spectral image arrays from a THz interferometer imaging system. The outputs are a clear confirmation as well as the location in the detection area of the target agent. By using the values for the detected power at specific THz frequencies from inputs, the neural network can recognize the presence of different combinations of THz colors (eg, frequencies) corresponding to a particular explosive or biological agent. Can be "trained". This type of use for ANN is known. For example, ANNs have been used to establish a system for classifying nearly 150 microbial strains based on Fourier transform-infrared (FTIR) absorbance spectra (T. Udelhoven, et al., "Development of a hierarchical classification system with artificial Neural Networks and FT-IR Spectra for the Identification of Bacteria, "Applied Spectroscopy, 54, NO. 10, P. 1471 (2000)). Similarly, individual organic components in a multi-mixture of chlorinated hydrocarbons were identified with high accuracy from low signal-to-noise ratio Raman spectra using ANNs. (T. Lu and J. Lerner, "Spectroscopy and Hybrid Neural Network analysis," Proc. IEEE, 84, no. 6, p.895 (1996)). Similarly, neural network principles were used in THz spectra as an analysis tool to distinguish different sets of DNA (T. Globus, et. Al "Application of Neural Network Analysis to Submillimeter-wave vibrational spectroscopy of DNA macromoledules", in the Proceedings to the 2001 ISSSR, June 12-15, Quebec City, Canada (2001)). The artistic neural networks can be used in the present invention to analyze combined images taken at different times or at different spatial resolutions to reduce false alarm rates.

In the present invention, shown in the schematic diagram of FIG. 5, the method of detecting (and generating) THz radiation is the optical mixing of two CW infrared laser beams by photoconductive antenna structures. The frequency difference between the two external cavity diode lasers (ECDL) is tuned to approximately 1 THz (far infrared region) corresponding to 33 cm ⁻¹ . The ECDL has a center wavelength of 780 nm and a tuning range of ± 9 nm, corresponding to ± 4.2 THz. ECDL lasers have good stability (approximately 5 MHz linewidth) to be able to scan their difference frequencies from 0.1 to 2 THz. Due to the flexibility of ECDL and ease of tuning, ECDL lasers are chosen over less expensive DFB and diode lasers. The output of the lasers is combined in the fiber coupler 29. The power of the combined laser beams is split using an optical fiber-optic splitter 30, and using optical fiber optical cables, with two detection antennas 22, 24 comprising one of several pairs of spaced apart detectors. Is sent. Optical mixing of the input THz and infrared lasers produces an electrical signal of intermediate frequency ω _IF . The spatial Fourier component of the antenna pair is measured by filtering the band of IF frequencies in filters 32 and 34 and using correlation circuit 36. As the THz detectors, photoconductive detectors such as those manufactured by Picometrix can be used as described above.

The two laser sources are thus tuned to the difference frequency AM in the THz range. The combined laser beams are split using an optical fiber splitter and sent by optical mixing to a photoconductive antenna for detection of input THz radiation.

By using heterodyne detection, the intermediate frequencies of the mixed THz frequencies are in the MHz range, unlike homodyne detection in the typical DC or KHz range. [In a standard THz spectrometer setup using short-pulse laser sources, the voltage applied to the laser beam or THz emitter is KHz repetition rate to facilitate phase-sensitive detection using a lock-in amplifier. Is modulated by THz detector electronics are designed such that their antenna detector packages can transmit intermediate frequencies in the 100 MHz range.

As shown in FIG. 7, after downmixing the THz signal to the intermediate frequency (IF), the range from 0.01 to several GH is very important in wireless communication (as baseband) and optical communication (as sideband), so in this range Extremely good filters 32 and 34 and low noise amplifiers 31 and 33 are used. The intermediate frequency is processed at this frequency because electronic components are readily available at the 100 MHz frequency. The first step is to digitize the input IF band by sampling at the Nyquist frequency (1 Gsample / s for a 100 MHz bandwidth). The digital signal is then passed through a time-demultiplexer (shift register) so that each bit of the shift register is correlated (multiplied) with the corresponding bit from the second THz detector. The signal sampled at 1 GHz can be lowered to 125 MHz by using an 8-bit time-demultiplexer and then sent to a 125 MHz field programmable gate array (FPGA) for correlation. This configuration provides an N = 64 lag (8 × 8), which provides a frequency resolution of 15.6 MHz for a B = 100 MHz bandwidth of Δf = 2B / N. Larger numbers of forms can be achieved by connecting the correlator chips in series, resulting in higher frequency resolution. Considering this as a correlator unit, each correlator unit processes signals from a pair of detectors. For example, n (N-1) 12 correlator units are needed.

Since THz sources / detectors / components are very active for research, it is possible that better THz components can be developed than the components discussed herein. However, the overall design of the THz imaging array is: (a) As long as the new detection elements detect the THz electric field, not the power (some new THz mixers and THz local oscillators will be the same), this new mixer technology will be used for the THz imaging array. It may be used to improve performance, and (b) the THz sources to be imaged can be either incoherent or coherent, which is very certain. This is actually a system engineering advantage for the THz interferometer array approach. The interferometer array design can easily incorporate the advanced of THz local oscillator sources and mixers without having to completely reinvent the THz array with the bar advanced in THz technology.

Although the present invention has been disclosed by specific embodiments thereof, it will be appreciated that many modifications to the invention will be possible to those skilled in the art in light of the disclosure, and that the variations are within the teachings herein. Accordingly, the invention is to be construed broadly and limited only by the scope and spirit of the claims appended hereto.

Claims (22)

A terahertz imaging device for inspecting a region of interest to determine the presence or absence of a specified composition, (a) means for generating electromagnetic radiation of a desired terahertz frequency suitable for said inspection; (b) means for injecting said terahertz radiation into said region of interest; (c) detector means for simultaneously detecting the terahertz radiation reflected or transmitted from the area at a plurality of points in the detector plane spaced from the area of interest; And (d) means for converting the detected terahertz radiation into an image of the region of interest capable of determining the presence or absence of the specified construct. The terahertz imaging device of claim 1, wherein the detector means in (c) comprises a spaced interferometer array of spaced detectors. The terahertz imaging device of claim 2, comprising means for combining the signal outputs from the pairs of detectors with a suitable delay and in-phase and quadrature correlation to generate components for a Fourier transform plane corresponding to the detector plane. . The terahertz imaging device of claim 3, wherein the detector array comprises a plurality of semiconductor photomixers and further includes photomixer driving means. 5. The terahertz imaging device of claim 4, wherein the photomixer driving means comprises a frequency stabilized tunable optical heterodyne source coupled to the photomixers by a common optical fiber optical connector. The terahertz imaging device of claim 3, wherein the detector array comprises an array of detectors. 7. The terahertz imaging device of claim 6, further comprising means for rotating the array about a fixed axis. 4. The terahertz imaging device of claim 3, further comprising means for synthesizing the original brightness distribution of the region of interest by Fourier inverse transformation of the Fourier components. 5. The terahertz imaging device of claim 4, further comprising means for synthesizing the original brightness distribution of the region of interest by Fourier inverse transformation of the Fourier components. 5. The apparatus of claim 4, wherein the photomixers are photoconductive devices, and wherein the driving means for each pair of photomixers comprises a pair of lasers having a difference frequency that gates the photomixers. The input terahertz emission at each member of a pair of photomixers provides modified signal outputs having intermediate frequencies that are mixed with the difference frequency to facilitate signal processing. The terahertz imaging device of claim 8, further comprising image analysis means for comparing portions of the imaged region of interest to test standards to determine the presence of the specified constructs. 10. The terahertz imaging device of claim 9, further comprising image analysis means for comparing portions of the imaged region of interest to test standards to determine the presence of the specified constructs. The terahertz imaging device of claim 8, wherein the analyzing means compares the portions with a test standard corresponding to an explosive composition. 10. The terahertz imaging device of claim 9, wherein the analyzing means compares the portions with a test standard corresponding to an explosive composition. The terahertz imaging device of claim 8, wherein the analysis means compares the portions with a test standard corresponding to a biological agent. The terahertz imaging device of claim 9, wherein the analysis means compares the portions with a test standard corresponding to a biological agent. A method of terahertz imaging for inspecting a region of interest to determine the presence or absence of specified constructs, (a) generating electromagnetic radiation of a desired terahertz frequency suitable for said inspection; (b) injecting the terahertz radiation into the region of interest; (c) simultaneously detecting the terahertz radiation reflected or transmitted from the region at a plurality of points in the detector plane spaced from the region of interest; And (d) converting the detected terahertz radiation into an image of the region of interest capable of determining the presence or absence of the specified construct. 18. The method of claim 17, wherein step (c) is detected by an interferometer array of spaced detectors. 19. The method of claim 18, wherein signal outputs from the detector pairs are combined with suitable delay and in phase and quadrature correlation to generate components for a Fourier transform plane corresponding to the detector plane. 20. The method of claim 19, wherein the original brightness distribution of the region of interest is synthesized by Fourier inverse transformation of the Fourier components. The method of claim 20, wherein the region of interest comprises an explosive composition. The method of claim 20, wherein the region of interest comprises a biological agent.