This PDF file contains the front matter associated with Proceedings of SPIE Volume 6548, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Passive imaging for security and other applications has reached an important level of development. Ka and W band imaging systems are now commercial products, thanks to cheap and stable amplifiers. While deployment numbers are still modest, improvements to these systems will come from engineering and algorithm development, and not fundamental research. What research should focus on are system volume reduction and resolution improvements. Fundamental research has several potential paths to solve these problems. Silicon Germanium CMOS can build acceptable millimeter wave amplifiers, and while SiGe noise figure is higher than GaAs, the ability to integrate RF and back end processing will push us closer to a CCD-like sensor. Antimony Arsenide features higher mobility than GaAs, with very low flicker noise and operation above 200 GHz which will reduce aperture size for equivalent resolution. More focus is needed in material characterization, particularly for clothing and common commercial materials. Finally, Sparse Array technology may build flat, conformal structures with high resolution and relatively low detector count. This paper will briefly discuss the time line of past innovations, and explore the advantages and challenges of the new technologies that will drive this field forward.

A one-week experiment was conducted to determine the millimeter-wave transmission loss due to dust. Transmission data was collected at 35, 94, and 217 GHz through a recirculating dust tunnel. Dust clouds of various densities were measured during the experiment. The millimeter-wave measurements were non-coherent, using transmitting sources on one side of the dust tunnel and antenna/detectors on the other. The hardware was designed to minimize noise and drift. Even so, it was found that the transmission loss across the 1-m dust tunnel at high dust densities was lower than could be measured accurately with the equipment. Therefore, the results given are limited to system noise and represent maximum transmission losses at the various frequencies. The results show losses less than 0.02 and 0.08 dB for 94 and 217 GHz respectively across one meter of dust with density 3000 mg/m³. The actual losses are lower and a long baseline interferometer will be required to determine the loss values precisely. Despite the limitations of the experiment, the data show that millimeter-wave imager performance will not be significantly impacted by even a very dense dust cloud.

The design and testing of a 190 GHz imaging system is presented. The system features two beam-scanning antennas; the first transmits a horizontal fan beam and the second receives a vertical fan beam. By correlating the signals from the antennas, an estimate of the millimeter-wave reflectivity at the intersection of the fan beams is obtained. Each fan beam is scanned by rotating a small subreflector within the antenna; this simple rotation motion allows rapid scanning. The system is portable, currently approximately 0.6m × 0.6m × 2m high; the key size constraint is imposed by the 450 mm aperture length of the antennas. The imager has an angular resolution of 0.25° and a field of view of 14°×14°, resulting in a raw image of approximately 50 × 50 pixels. The raw image is processed using super-resolution techniques. Images will be presented which show the capability of the system to image metallic and ceramic objects beneath clothing. These images were obtained by illuminating the scene with signals from a frequency-doubled Gunn oscillator. While this paper focuses on active imaging, the system can also operate in passive mode with reduced sensitivity.

A ground-based radiometer has been used to measure airborne targets at low elevation angles, the complicated background noise received by the radiometer has been classified, and its influence on the main lobe and side lobe of the antenna analyzed. Firstly the influence of elevation angle and the operating frequency on the background noise received by the main lobe has been analyzed, and corresponding plots have been presented. Secondly, the influence of elevation angle and terrestrial surface parameters on background noise received by the side lobe has been analyzed and corresponding equations and plots have also been presented. Mathematical models and theoretical curves are in good agreement with the experimental data collected at 8mm from sky brightness temperature measurements with different terrestrial surface brightness temperatures.

Trex Enterprises has applied the frequency scanned antenna architecture found in the ST-150 stand-off imager to closein personnel screening devices, including a full-body imager and a handheld scanning imager. These devices present the user with an image with 3mm square pixels and 10-18 mm spatial resolution using few amplifiers and a low level of mechanical complexity. The frequency scanned architecture permits the real-time imaging of a linear array of 64 pixels with a single amplifier module. The linear imager or imagers are slowly mechanically scanned to provide a twodimensional image. The imagers were used to capture images of concealed threat items at thermal resolutions from 1 K to 0.2 K, indoor and outdoors. Image quality is generally superior to that of stand-off detectors, detecting items as small as 10 mm.

We investigate the spectral response of a THz imaging system based on ultrawideband cryogenic microbolometers. The bandwidth if this system, nominally 0.2 - 1.8 THz, is broad enough to span large variations (>10 dB) in clothing transmittance and diffraction-limited spatial resolution (factor of x8), factors that are presumably partly responsible for the unusually high quality of the images taken with it. The chief tools we have used for this are a simple THz monochromator based on a specially designed frequency selective surface, and a specially designed blackbody source that provides an accurately known power spectral density over the full bandwidth of the imager. Two completely independent measurements of the microbolometer's spectral response, in the first case using a filtered blackbody and in the second using an ultrabroadband, THz photomixer, referred to a Golay cell, agree within 5%. Evidence of frequency-dependent scattering from ordinary clothing material, distinct from simple linear attenuation, is presented from an idealized laboratory experiment. However, the scattering is relatively weak, and unlikely to have a significant effect in practical THz imaging scenarios, particularly with ultrawide bandwidths.

The objective of this program is to demonstrate a system capable of passive indoors detection and identification of concealed threat items hidden underneath the clothing of non-cooperative subjects from a stand-off distance of several meters. To meet this difficult task, we are constructing an imaging system utilising superconducting ultrawideband antenna-coupled microbolometers, coupled to innovative room temperature read-out electronics, and operated within a cryogen-free pulse tube refrigerator. Previously, we have demonstrated that these devices are capable of a Noise Equivalent Temperature Difference (NETD) of 125 mK over a pre-detection bandwidth from 0.2-1 THz using a post-detection integration time of 30 ms. Further improvements on our devices are reducing this number to a few tens of mK. Such an exquisite sensitivity is necessary in order to achieve the undoubtedly stringent requirements for low false positive alarm rate combined with high probability of detection dictated by the application. Our technological approach allows for excellent per frame NETD (objective 0.5 K or below at 30 Hz frame rate), and is also amenable to multispectral (colour) imagery that enhances the discrimination of innocuous objects against real threats. In the paper we present results obtained with an 8-pixel subarray from our linear array of 128 pixels constructed using a modular approach. Two-dimensional imaging will be achieved by the use of conical scanning.

Wideband millimeter-wave imaging techniques and systems have been developed at Pacific Northwest National Laboratory (PNNL) for concealed weapon detection and other applications. These techniques evolved from singlefrequency millimeter-wave holographic imaging methods to wideband three-dimensional planar and cylindrical techniques and systems. The single-frequency holographic method was derived from optical and ultrasonic holography techniques. Speckle is highly significant in this case, and is caused by constructive and destructive interference from multiple scattering locations or depths within a single resolution cell. The wideband three-dimensional techniques developed at PNNL significantly reduce the speckle effect through the use of high depth resolution obtained from the wide bandwidth of the illumination. For these techniques, speckle can still be significant in some cases and affect image quality. In this paper, we explore the situations in which speckle occurs and its relationship to lateral and depth resolution. This will be accomplished through numerical simulation and demonstrated in actual imaging results. Speckle may also play a significant role in altering reflection spectra in wideband terahertz spectra. Reflection from rough surfaces will generate speckle, which will result in significant variation in the reflection spectrum as measured over very wide bandwidths. This effect may make if difficult to interpret spectral absorption features from general reflectance data. In this paper, physical optics numerical simulation techniques will be used to model the reflection from arbitrary random surfaces and explore the effect of the surface on the reflection spectra and reconstructed image. Laboratory imaging and numerical modeling results in the millimeter-wave through the terahertz frequency ranges are presented.

For many applications, the usefulness of millimeter-wave imagers is limited by the large aperture sizes required to obtain images of sufficient resolution. Sparse aperture techniques could open up wider range of applications by mitigating the volume requirements of high resolution imagers. In previous proceedings, we have presented an approach towards the realization of millimeter-wave, sparse-aperture imagers using optical techniques. By using electro-optic modulators to upconvert received millimeter-wave fields onto an optical carrier, such fields can be readily captured, routed, and processed using optical techniques. Such techniques could provide significant advantages over traditional heterodyne techniques. Herein, we present progress towards the physical realization of such an imager. Specifically, we discuss the implementation challenges that must be addressed to create such an imager and present in further detail the numerous advantages such an approach will yield. We also present results obtained from a working prototype system and show that these results are in good agreement with theoretical performance models.

A proto-type passive millimeter-wave (MMW) camera with interferometric processing has been developed and evaluated to confirm the feasibility of the interferometric MMW camera and to study the characteristics of MMW images. This proto-type camera is comprised of the minimum configuration as an interferometric imager which consists of two sets of a W-band receiver with a horn antenna, and a digital processing unit. The position of these two antennas with W-band front-end moves on the precision linear slider in horizontal and vertical axis. The coherently amplified two channel signals are digitized and processed in the hardware processor. The process is comprised of correlation of all combination of each axis data, and integration to improve the signal to noise ratio. The integrated data is processed to make an image by matched filter processing. The integration time is from 1mS to 10S depending on required integration gain. The maximum synthesized antenna aperture size is 1m for horizontal axis and 50cm for vertical axis. In this paper, the evaluation of the proto-type P-MMW camera is descried. After the evaluation, some improvement was scheduled and conducted. Also, future plan for a real-time camera using this technique is presented .

A prototype cross-correlating 190 GHz passive mm-wave imaging system has been developed. This system is based on the Mills Cross system used for radio astronomical imaging. It uses two pillbox antennas arranged in a T configuration. Each antenna generates a fan beam and the two fan beams are orthogonal to each other. By cross-correlating signals received from the two antennas, an output is obtained which is proportional to the millimeter-wave intensity radiated from the target at the intersection of the two fan beams. Beam scanning is generated by rotating a small sub-reflector inside each antenna. As a result, these relatively heavy antennas are stable during scanning and a high frame rate can be achieved. Another advantage of this approach is that only two receivers are required. The baseline (the displacement between phase centers of the two antennas) of this system is not zero, because the phase centers of the two antennas are not located at the same position. The baseline generates a fringe in the imaging system and its influence on the performance of the system is analyzed in this paper. The scanning speed of this system is also much faster than that of the Mills Cross imaging system and its influence on the resolution is also analyzed. It is found that the effect of the scanning speed is minimized when the beam scans along the equal-phase line of the fringe. This system can also be used as an active imaging system and this is discussed in another paper.

We are developing a passive W-band millimeter wave imaging array that operates without the use of RF low noise amplifiers. The work is supported by the DARPA MIATA program. Previously reported Phase I results were a noise equivalent temperature difference (NETD) of 4.8°K. The goal of Phase II, currently underway, is to decrease this to 2°K or less. There are two improvements that must be made to achieve the goal. The square law diode detector sensitivity and the RF bandwidth reaching the detector must be increased significantly. This paper mainly deals with the first issue, the effort to increase the sensitivity by decreasing the diode area and capacitance, using electron beam lithography. Brief mention will be made of the redesign of the antenna-to-diode transition that simulations indicate will provide a doubling of bandwidth from 30 to 60 GHz.

Millimetre-wave (mm-wave) imaging systems for a number of applications rely on a multitude of receiver modules mounted at the focal plane of focusing optics. Analysis shows that the receiver front-end forms a significant proportion of the overall cost of an imaging system restricting market take-up of commercial systems for security screening and all weather vision. The cost of imager front-ends can be significantly reduced by the use of low-cost multi-layer softboard technology used for RF printed circuit boards (PCBs) and new monolithic microwave integrated circuit (MMIC) chipsets with fewer MMIC devices. Modelled performance of W-band mm-wave imaging receiver using these techniques shows effective bandwidths of 38GHz and noise equivalent temperature difference (NETD) of 0.3K when using a 0.2ms integration time. This performance is achieved with a potential cost saving of about 60%.

64 MMIC receivers built for mm-wave imaging have been systematically characterized. Each receiver comprises three InP MMIC low noise amplifiers and a biased Schottky diode detector in a compact package with a horn antenna. The characterization includes spectral response, responsivity and noise. The noise has both white and 1/f components. The average (±standard deviation) receiver bandwidth is 23.8±3GHz, and the overall noise equivalent temperature difference (NETD) in an integration time of 0.2ms, as used in the imager, is 0.66±0.1K.

We present several novel technologies for sensing millimeter-wave (mmW) radiation for imaging and spectroscopy based on photonic devices. Along these lines, in our high-sensitivity millimeter-wave (mmW) imaging system, which is based on optical upconversion, the power of mmW radiation is transferred to the sidebands on an optical carrier via an electro-optic (EO) modulator fed by a broadband horn antenna. The detection is realized by measuring the transferred optical power of the sidebands. The sensitivity of this detection system is primarily controlled by the conversion efficiency of the EO modulator at the desired mmW frequency (e.g. 95GHz). Thus, modulators are required that exhibit an ultra-broad bandwidth and small drive voltage. In this paper, we present the design, fabrication, and characteristics of LiNbO₃ traveling-wave modulator for the mmW detection system. In a traveling-wave modulator, the bandwidth is limited by the mismatch between electrical and optical propagation constants. We have developed several techniques to finely tune the propagation constant of the mmWs in the modulator and have thereby eliminated this mismatch. Further bandwidth limitations for the modulator arise from losses in the electrode conductor, the substrate and buffer layer dielectrics, and coupling between the traveling-wave mode and the substrate modes. Modulator structures are described to reduce those losses without increasing the device driving voltage. The bandwidth and conversion limits of these structures are also discussed. The mmW detection pixels using the fabricated modulators were assembled, characterized, and analyzed. A high-sensitivity W-band detection system with a low noise-equivalent temperature difference (NETD) has been demonstrated. In addition, we present ongoing work to improve coupling millimeter-wave energy to the modulator at the W-band using techniques viable for packaged devices.

The low vapor pressure and concentration of explosive such as TNT and RDX pose significant problems for the detection of explosive vapors in the mmW bands. For the positive identification of explosive vapors using an uncooled passive mmW imaging spectrometer with a low false alarm rate (FAR) requires an unprecedented sensitivity of <150 fW. We report on the recent development of a novel uncooled mmW antenna-coupled direct detector, which shows promise of meeting this requirement.

Superresolution reconstruction (SR-REC) algorithms combine multiple frames captured using spatially under-sampled imagers to produce a single higher-resolution image. Sub-pixel information is gained from natural motion within the image instead of active pixel scanning (dithering/micro-scanning), eliminating the reliability issues and power consumption associated with moving parts. One of the major computational challenges associated with SR-REC methods is the estimation of the optical flow of the image (i.e., determining the unknown pixel shifts between consecutive frames). A linear least squares approximation is the simplest method for estimating the pixel movements from the captured data, but the size of the problem (directly proportional to the number of pixels in the image) creates a computational bottleneck, which in turn limits the usability of this algorithm in real-time portable systems. We propose the use of a reconfigurable platform to implement these computations in a low power/size environment, suitable for integration into portable millimeter wave imagers.

Microwaves can be used to detect hidden objects behind optically opaque materials. Hence, the penetration capability through such materials is of fundamental importance. In order to characterize a material of interest in the microwave region, its permittivity should be known besides its physical structure. In many cases the permittivity is unknown, inaccurately known, or known for only specific frequencies. Also very often the range of values given in the literature can have a large variability for a specific situation. In this paper we describe a procedure to determine the permittivity from radiometric free space measurements of nearly arbitrary materials. The advantage of this method is that large material samples like brick or wooden plates, and materials like textiles, which are hard to mount in a defined way in a waveguide, can be studied. An earlier presented method has been improved to obtain more accurate results. Some representative results for those MMW measurements are shown. The first attempts showed a satisfying performance.

We describe a broadband calibration source for the millimeter-wave to terahertz (THz) frequency range, the Aqueous Blackbody Calibration (ABC) source. The blackbody in this design is a body of water, which is extremely absorptive at these frequencies, held by an expanded polystyrene (EPS) holder in a very specific shape, and kept at a uniform known temperature. The undesirable reflectance at the interfaces between the water and the walls of the EPS container is significantly reduced by the use of an "optical trap" geometry. The shape of the custom-molded container is designed so that all radiation incident within the entrance aperture and solid angle of the source undergoes two TE and two TM 45-degree reflections off the water-EPS interface. This ensures an effective emissivity > 98.5% over the operating band, and has been verified by measurements of EPS transmittance and geometric optics simulation over the entire band. In addition, the effective reflectance of the source is characterized at selected frequencies within the 0.1-2.0THz range.