Imaging and sensing systems for operation in both the millimetre wave and terahertz regions of the spectrum are reviewed. Advances in components are outlined, for both types of system, and these include: electronic, optical and vacuum-tube approaches. System and device challenges are summarised, and promising growth directions are discussed.

This paper reviews the formation of an image with coherent and incoherent radiation. It discusses the various mm-wave methods for electronic beam-forming and beam-steering such as phased array, leaky-wave antennas, up-conversion, tapped delay lines and digital beam-forming techniques. These methods are related in the paper to their optical analogues of beam-forming and steering by a lens and the measurement of the aperture function in the case of holography. It concludes that digital techniques will be used in the future when the cost of receivers is reduced but that at present opto-mechanical techniques are more cost effective. A high efficiency, compact opto-mechanical system is described. This is able to operate at any wavelength and be active or passive. Typical 94GHz images are presented.

This paper describes two systems developed by Roke Manor Research in partnership with HMG's Home Office Immigration and Nationality Directorate and which are based upon passive millimetric microwave radiometry techniques. Their purpose is to aid the detection of people concealed in curtain-sided and plastic-sided freight vehicles. The paper covers the basic physics of radiometry, the history of these developments and concludes with an account of the future directions of this work.

It is well known that millimetre wave systems can penetrate poor weather and battlefield obscurants far better than infrared or visible systems. Imaging in this band offers the opportunity for passive surveillance and navigation allowing military operations in poor weather. We have previously reported a novel prototype real time mechanically scanned passive millimetre wave imager operating at 94GHz. This 94GHz imager has diffraction limited performance over the central two thirds of the 30 x 60 degrees field of view with and a 25Hz frame update rate. This paper reports the redesign of the prototype to operate in a helicopter environment. The vibration resulting from the helicopter rotors can typically range up to 3g peak at specific frequencies and is the major challenge for imaging systems. The mechanical design of the new imager is based on a rigid space frame utilising expanded polystyrene as a structural element to support the receiver array. The optical design of the imager has also been toleranced so that the impact of vibration on image quality is minimised. The use of novel design techniques and materials support a development path to a low cost low mass production unit.

We present a summary view of the DARPA MIATA program. This program is an effort to push the technology of mmW focal plane arrays towards smaller, lighter, and less expensive implementations, thereby increasing the military utility of this technology.

Passive mm-wave imaging has great potential for all-weather flying aids and security applications. To achieve useful real time images of an ambient temperature scene, >100 detectors are required, and uptake of the technology has been limited by the high cost of detectors. We propose a novel, cost effective solution using integrated arrays of antenna-coupled HTS Josephson junction video detectors. Arrays of 16 antenna coupled YBa₂Cu₃O₇ interface engineered junctions have been fabricated with a design centre frequency of ~100GHz, optically immersed behind a single lens. Detector characteristics have been compared to stochastic RCSJ model simulations, with the antenna represented by an equivalent circuit. A real time demonstration imager has been built, and the first images obtained.

We present the development and field testing results of a dual-mode radar/radiometric imager operating at 94GHz. This instrument combines an FMCW radar with a total power radiometer in a compact, portable unit which is designed for ground based remote sensing. The radar produces range maps with a range bin resolution of 1m out to a maximum range of approximately 5km and target reflectivity can also be retrieved. The radiometer produces co-aligned thermal images of the scene with a thermal resolution of the order of one kelvin. The instrument, which uses a single 0.45m Cassegrain antenna, is rastered over the scene using a commercial pan and tilt gimbal. Image acquisition times are of the order of tens of minutes. The principal application for which the instrument was designed is to survey volcanic lava domes, irrespective of weather conditions - it has been named AVTIS for All-weather Volcano Topography Imaging Sensor. We will present results obtained with AVTIS from the Soufrière Hills Volcano, Montserrat, as well as more general terrain mapping imagery gathered locally in Scotland. Besides volcano surveying, AVTIS could be deployed for other remote sensing applications.

The terahertz spectrum of the explosive RDX has been measured using a conventional Fourier transform infrared spectroscopy and by terahertz pulse spectroscopy in transmission and reflection modes. Seven absorption features in the spectral range 5-120 cm^-1 have been observed and identified as the fingerprints of RDX explosive. Furthermore, a sample consisting of RDX-based explosive, mounted side by side with lactose and sucrose pellets, has been imaged using a terahertz pulse imaging system. The recorded terahertz images and their spectral data have a spectral resolution of 1 cm^-1 and cover a spectral range of 5-80 cm^-1. This broad spectral coverage enables the spatial distribution of individual chemical substances of the sample to be mapped out. We also discuss the application of Principal Component Analysis and Component Spatial Pattern Analysis to the automatic identification of materials, such as explosives, from terahertz imaging.

The use of millimetre-wave imaging to identify threat objects such as guns and bomb material concealed on the person is well documented. However, the technology has been hindered by the performance and cost barriers typically associated with imaging at mm-wave frequencies. A novel scanning technique that minimises the receiver count while operating at very high efficiency levels has made it possible to build a cost-effective and high-performance mm-wave imager that can make security screening a commercial reality. The imager design allows for either passive or active operation and its compact form factor is suitable for practical installation in security channel situations. The uses of this technology include portal screening of personnel for high-resolution imaging of concealed threat objects or longer distance surveillance type monitoring of checkpoints and crowds. This presentation details the use of the imager in an active configuration to observe a checkpoint or crowd scene at stand-off distances of up to 50 metres. Target objects to be detected are the hidden metal components associated with suicide bomb constructions. A typical bomb consists of several explosive filled pipes strapped to the body or clusterings of small metallic objects embedded in explosives. Trials at 94GHz have yielded positive results by showing the presence of concealed metallic objects on people at distances of 25 metres. Objects detected have included simulated bomb constructions such as groups of metal pipes and clusters of nuts and bolts. These tests have been conducted using a Gunn based CW source and direct detect receiver unit. Further enhancement of the system includes the use of an FMCW front-end configuration.

Terahertz imaging is becoming more viable for many applications due to advances in detector and emitter technologies. One of the applications for THz imaging is the detection and identification of concealed weapons (e.g., in airport security screening lines). The path described here provides an imaging performance model for the application of concealed weapon identification. The approach is the typical U.S. Army target acquisition model for sensor performance prediction coupled to the acquire methodology for weapon identification performance prediction.

It is well known that the image detected by a camera is only a subset of the information available in a scene. The finite aperture of an optical system leads to a certain amount of diffraction and image blurring, as described by Rayleigh's diffraction criterion. The image of the scene is then sampled with a finite spatial frequency by the detector(s), be they individual elements in a CCD sensor, light-sensitive cells in the eye, or grains on a photographic film. It is important, when designing a camera system, to ensure that as much of the available image information as possible is collected by the sensor. This involves an analysis of the spatial frequencies that are present in the blurred image, and the design of the image plane detectors in order to best sample the image information. This paper describes the image sampling requirements of millimetre-wave imagers, in particular mechanically scanned imagers that use a conical scanning technique. The trade-off between the requirement to make optimum use of limited image resolution with the high cost of millimetre-wave receiver arrays is considered. Different array layouts used in existing millimetre-wave imagers are presented.

Video-frame-rate millimetre-wave imaging has recently been demonstrated with a quality similar to that of a low-quality uncooled thermal imager. In this paper we will discuss initial investigations into the transfer of image processing algorithms from more mature imaging modalities to millimetre-wave imagery. The current aim is to develop body segmentation algorithms for use in object detection and analysis. However, this requires a variety of image processing algorithms from different domains, including image de-noising, segmentation and motion tracking. This paper focuses on results from the segmentation of a body from the millimetre-wave images and a qualitative comparison of different approaches is presented. Their performance is analysed and any characteristics which enhance or limit their application are discussed. While it is possible to apply image processing algorithms developed for the visible-band directly to millimetre-wave images, the physics of the image formation process is very different. This paper discusses the potential for exploiting an understanding of the physics of image formation in the image segmentation process to enhance classification of scene components and, thereby, improve segmentation performance. This paper presents some results from a millimetre-wave image formation simulator, including synthetic images with multiple objects in the scene.

The benefits of moving up in frequency from the millimetre wave region towards a frequency of 1 THz are those of smaller systems and better diffraction limited image resolutions. Limitations will be examined, considering effects such as the absorption in the atmosphere, various materials, the human body and fundamental radiometric noise limitations. The physics behind these considerations will be examined and results and artefacts presented using scene simulation. Conclusions are that above 500 GHz high atmospheric absorption severely limits imager to subject distances to a few hundred metres. The effect of absorption is poor subject illumination and high signal attenuation between the subject and the imager. These limitations may be over come partially, for short imager to subject distances (less than a few hundred metres), by using active illumination with narrow bandwidth radiation sources. However, transmit powers rise steeply with imager to subject distance and radiation frequency, lying typically between 1 mW and 1 W over the frequency band 500 GHz to 1 THz, for a radiation bandwidth of 1 GHz and an imager to subject distance of 20 m. Similar systems analysis for medical applications indicates that the high attenuation in human tissue limits probing or penetration distances of the radiation. Radiometric photon noise, electrical properties of human tissue and irradiation power restrictions (1 mW/cm²) limit maximum diagnostic depths to between two and one millimetres between the frequencies of 100 GHz and 1 THz.

For sensitive wideband spectroscopy at TeraHertz frequencies one needs a wide-range electrically tunable THz source and a sensitive detector. In this paper a superconducting normal metal cold electron bolometer (CEB) was used as a broadband sensor. Bolometers were integrated with broadband log-periodic antenna designed for 0.2-2 THz frequency range and double-dipole antennas designed for 300 and 600 GHz central frequency. A Josephson junction was used as a wide band electrically tuned terahertz cryogenic oscillator. Bicrystal YBaCuO Josephson junctions demonstrated a characteristic voltage I_cR_n of over 4 mV that corresponds to characteristic frequency about 2 THz. The bolometer chip is attached to a Si substrate lens at 260 mK and the oscillator chip is attached to the sapphire substrate lens at 1.8 K, with lenses facing each other at the distance of few centimeters. High signal was measured in the whole frequency range up to 1.7 THz by simple changing the bias voltage of Josephson junction from zero to 3.5 mV. A voltage response of the bolometer up to 4*10⁸ V/W corresponds to an amplifier-limited technical noise equivalent power of the bolometer NEP=1.25*10^-17 W/Hz^1/2. Combining a Terahertz band Josephson junction, a high-sensitive hot electron bolometer, and a sample under test in between, makes it possible to develop a cryogenic compact Terahertz-band transmission spectrometer with a resolution below 1 GHz corresponding to the linewidth of Josephson oscillations. For frequencies below 600 GHz a conventional Nb shunted SIS junction can be used as Josephson oscillator.

Beam-steering techniques are required to fully exploit terahertz imaging systems. We propose and model a device employing artificial dielectric techniques to provide a variable phase-control medium. The device consists of two interlocking artificial dielectric surfaces that are initially aligned parallel to each other. By mechanically introducing a relative tilt between the plates, a transmitted wave is subjected to a graded phase delay and thus the beam is steered away from the normal. Continuous and large steering angles are possible. We predict a practical device constructed from a silicon substrate could steer TE beams by up to 4.6 degrees.

In order to improve the design and analyse the performance of efficient terahertz optical systems, novel quasi-optical components along with dedicated software tools are required. At sub-millimetre wavelengths, diffraction dominates the propagation of radiation within quasi-optical systems and conventional geometrical optics techniques are not adequate to accurately guide the beams or assess optical efficiency. In fact, in general Optical design in the terahertz waveband suffers from a lack of dedicated commercial software packages for modelling the range of electromagnetic propagation regimes that are important in such systems. In this paper we describe the physical basis for efficient CAD software tools we are developing to specifically model long wavelength systems. The goal is the creation of a user-friendly package for optical engineers allowing potential systems to be quickly simulated as well as also providing an analytical tool for verification of existing optical systems. The basic approach to modelling such optical trains is the application of modal analysis e.g. [1][2], which we have extended to include scattering at common off-axis conic reflectors. Other analytical techniques are also ncluded within the CAD software framework such as plane wave decomposition and full physical optics. We also present preliminary analytical methods for characterising standing waves that can occur in terahertz systems and report on novel binary optical components for this wavelength range. Much of this development work has been applied to space instrumentation but is relevant for all Terahertz Imaging systems.

Millimeter-wave radiation has the unique ability to penetrate atmospheric obscurations such as smoke, fog, and light rain while maintaining the capability for high-resolution imaging. However, suitable technologies for creating high-sensitivity, large pixel-count detectors are a limiting factor in the implementation of such systems. To this end, we present a technique for detecting millimeter-wave radiation based on optical upconversion that promises both high sensitivity and scalability to large pixel arrays. High-speed optical modulation is used to transfer millimeter-wave radiation onto the sidebands of a near-infrared optical carrier frequency. Optical filtering techniques are subsequently used to suppress light at the carrier frequency. The resultant signal is passed to a low-frequency photodetector, which converts the remaining sideband energy to a photocurrent proportional to the incident millimeter wave energy at the modulator input. Utilizing the low noise powers of such photodetectors, high sensitivities may be obtained even accounting for the relatively high signal losses associated with optical upconversion. Since optical upconversion inherently preserves both phase and amplitude information and fiber optics may readily be used for low-loss routing of the modulated signal, such an approach offers promise for high-resolution synthetic aperture imaging. Alternatively, since each of the required components may be fabricated in III-V materials using planar semiconductor processing techniques, integration of multi-pixel arrays is feasible. Herein, we present experimental results obtained using a baseline detector assembled from commercially available fiber-optic components as well as efforts to integrate the desired functionality into a single GaAs substrate. An initial noise equivalent power (NEP) of the proposed detector has been demonstrated at sub-nanowatt levels, with improvements to sub-picowatt NEP's anticipated as the setup is optimized.

This paper analyzes simple imaging configurations to scan a human body, suitable as passive or active millimetre-wave imaging systems for concealed weapon detection (CWD). The first cylindrical configuration allows a 360 degrees scan: N unphased diffraction-limited antennas each of size L are placed on a circular support surrounding the subject (allowing scanning in the horizontal plane with N non-overlapping independent beams), and this circle is mechanically displaced over the whole body height. An analytical formula gives the maximum obtainable spatial resolution for different dimensions of the circular scanning device and operating frequencies, and the number of receivers achieving this optimal resolution. Constraints to be taken into account are diffraction, the usable total length of the circle, and the full coverage by the N beams over the subject, which is modelled as a cylinder with variable radius, coaxial with the scanning circle. Numerical calculations of system resolution are shown for different operating microwave (MW) and millimetre-wave (MMW) frequencies; in order to study off-axis performances, situations where the subject is not coaxial with the scanning device are also considered. For the case of a parallelepiped to be imaged instead of a cylinder, a linear array configuration is analyzed similarly to the circular one. A theoretical study is carried out to design other curved arrays, filled with unphased diffraction-limited antennas, for the imaging of linear subjects with finer resolution. Finally, the application of such configurations is considered for the design of active imaging systems, and different system architectures are discussed.

We describe a new method of detecting electromagnetic radiation, operating across a broad frequency range and at least up to liquid nitrogen temperatures. The method is based on the excitation of a special type of plasma waves -- edge magnetoplasmons -- in a semiconductor structure with an embedded two-dimensional (2D) electron layer. Irradiation of the sample by electromagnetic waves induces a photovoltage (or a photoresistance) between pairs of contacts to the 2D electron gas, which oscillates as a function of an applied magnetic field. The amplitude and the period of the oscillations are proportional to the radiation power and the wavelength respectively, allowing one to use the device as a detector and spectrometer of radiation. Successful operation of such a detector/spectrometer has been experimentally demonstrated in GaAs-AlGaAs quantum-well structures at microwave frequencies from 20 GHz up to ~150 GHz and at temperatures up to ~80 K. We do not expect any principal difficulties in extending the operating frequency range into the terahertz region. The sample design is very simple and does not require submicron technology.

We demonstrate a high-spectral-purity continuous-wave terahertz source, using a diode pumped Yb³⁺:KGd(WO₄)₂ dual frequency laser. THz radiation is generated by photomixing the two frequencies in a low temperature grown In_:25Ga_:75As photoconductor loading a dipole antenna. The frequency difference between the two optical modes is tuneable by step from d.c. to 3.1 THz. A maximum optical output power of 120 mW CW has been obtained with a beatnote-linewidth narrower than 30 kHz. Preliminary measurements show a tunable THz emission with a maximum output power in the order of a few tens of nW.

Terahertz remote-sensing applications require high sensitivity detectors and high-power terahertz sources. The reason for this is that terahertz signals can be quickly attenuated by water molecules present in the target, as well as in the environment between the source and the target. Of the many terahertz source technologies available, the one based on the optical parametric generation technique seems to be the most promising, as it is portable and can produce a relatively high-power terahertz beam. At present we are developing a terahertz source based on an optical parametric technique. We have used a Nd:YAG Q-switched laser as the pump source, and a LiNbO₃ crystal as the optical parametric medium. With LiNbO₃ our generator could produce a terahertz beam over the frequency range of 100 GHz through 3 THz. The output power was highly dependent on the properties of the material. For terahertz detection we have used a Si-bolometer or an electro-optic (EO) detector, which was specifically developed to detect CW terahertz signals. In addition to the EO sensor, we are presently developing a new detector based on a quantum-dot structure, whose noise equivalent power (NEP) is expected to be about 10^-21 W/(Hz)^1/2. This is several orders of magnitude better than the sensitivity of our bolometer (10^-13 W/(Hz)^1/2) at 4.2 K, or our EO detector (10^-12 W/(Hz)^1/2) at room temperature.

The resolution of aerial digital image is low because of atmosphere's swaying and the Image Motion Compensation's error while imaging. In view of the limitation of ordinary interpolation an approach of image magnifying based on two-dimension discrete wavelet is proposed in this article. At beginning the original image is magnified using tri-spline sampling interpolation. Then we perform two-dimension discrete wavelet transformation to the magnified image. Subsequently the inverse transformation of wavelet to recombine digital image is carried out utilizing the acquired three high-frequency images as high-frequency part of the inverse transformation and utilizing the original image as its low-frequency part, and the obtained image is double to original image in the direction of length and width. In the end we completed the proposed approach in MATLAB. The same aerial image is magnified with above-mentioned wavelet-based algorithms, conventional bilinear interpolation and tri-spline sampling interpolation algorithm separately. After comparing the magnified image's value of mean gradient, the conclusion is deduced that the wavelet-based approach can not only magnify aerial image and enhance its resolution better, but also retain the minutia of original image, through which the aerial image is ready for the further aerial image interpretation and processing.

The new method based on the pulsed complex photomagnetoelectric (PME) effect has been proposed for generation of high frequency oscillations. The dynamics of PME photoresponse have been investigated experimentally in semiconductors InAs and Cd_xHg_1-xTe (x=0.2 and 0.26) excited by Q-switched neodymium-YAG laser. The double sign inversion of the photoresponse signal was found at laser light flow I0 > 5 × 10²⁴ photons/cm²s in InAs and at I0 > 1-4 × 10²⁴ photons/cm²s in Cd_xHg_1-xTe. Study of the frequency spectra of the doubly-sign-inversion signal of PME response applying DFT analysis reveal that, the spectra are broadened significantly in the region of high frequencies. The results exhibit the possibility to reach THz frequency range using laser pulses of the picosecond duration.

A new staring 94 GHz focal plane array (4x8 pixels) multi-chip module (MCM) using a quasi-optically pumped MMIC gate mixer is presented. The new MCM with integrated microstrip antennas, matching and bias networks are manufactured using a modified MCM-D process based on 3x15 μm BCB. The active gate mixer MMIC was manufactured by OMMIC (D01PH process) and uses a 2x25 μm gate width. The 30 cm dielectric lens antenna has a F/D number of approximately unity. To minimize the spillover loss, the beamwidth of the feed antennas (microstrip antennas) was matched to the opening angle of the lens by using 2x2 subarrays for each pixel. Preliminary measurement results show a feed antenna gain of approximately 10 dBi and a conversion loss close to zero at 94 GHz, and an optimal pumping power of -2 dBm at 92.4 GHz.