Jesse P. Samluk

Design and performance of a distributed aperture millimeter-wave imaging system using optical upconversion

Passive imaging using millimeter waves (mmWs) has many advantages and applications in the defense and security markets. All terrestrial bodies emit mmW radiation and these wavelengths are able to penetrate smoke, blowing dust or sand, fog/clouds/marine layers, and even clothing. One primary obstacle to imaging in this spectrum is that longer wavelengths require larger apertures to achieve the resolutions typically desired in surveillance applications. As a result, lens-based focal plane systems tend to require large aperture optics, which severely limit the minimum achievable volume and weight of such systems. To overcome this limitation, a distributed aperture detection scheme is used in which the effective aperture size can be increased without the associated volumetric increase in imager size. However, such systems typically require high frequency (~ 30 - 300 GHz) signal routing and down conversion as well as large correlator banks. Herein, we describe an alternate approach to distributed aperture mmW imaging using optical upconversion of the mmW signal onto an optical carrier. This conversion serves, in essence, to scale the mmW sparse aperture array signals onto a complementary optical array. The optical side bands are subsequently stripped from the optical carrier and optically recombined to provide a real-time snapshot of the mmW signal. In this paper, the design tradeoffs of resolution, bandwidth, number of elements, and field of view inherent in this type of system will be discussed. We also will present the performance of a 30 element distributed aperture proof of concept imaging system operating at 35 GHz.

Modeling passive millimeter wave imaging sensor performance for discriminating small watercraft.

Passive millimeter wave (pmmW) imagers are quickly becoming practical sensor candidates for military and nonmilitary tasks. Our focus was to adapt the Night Vision [U.S. Army Research Development and Engineering Command, Communications and Electronics Research Development and Engineering Center, Night Vision and Electronics Sensors Directorate (NVESD)] passive thermal infrared imager performance models and apply them to pmmW imaging systems for prediction of field performance for the task of small watercraft and boat identification. The Night Vision Labs infrared sensor model has been evolving since the 1950s, with the most current model being NVThermIP [Night Vision Thermal and Image Processing (NVThermIP) Model Users Manual, Rev. 9 (U.S. Army RDECON, CERDEC, NVESD, 2006)]. It has wide recognition as an engineering tool for sensor evaluation. This effort included collecting pmmW signatures for a representative set of targets, conducting an observer perception experiment, and deriving the task difficulty criteria that can be used in NVThermIP for identification of boats. The task difficulty criteria are used by designers and managers to create systems capable of meeting specific performance criteria in the field.

Comparison of diurnal contrast changes for millimeter-wave and infrared imagery

Far-infrared outdoor imagery has a lower contrast in the morning/afternoon relative to the highest contrast, which is observed at 14:00. Millimeter-wave (mmW) imagery can also follow this pattern. However, in this paper, we show that the opposite can occur for mmW imagery, wherein a higher contrast can occur in the morning/afternoon and lower contrast at 14:00. To this end, we show that a wood and rubber sample are observed to have a difference in mmW radiometric temperature of 17 degrees C at 9:00 and a difference of only 7 degrees C at 14:00. Details of our observations are presented.

Perfect lens makes a perfect trap

In this work, we present for the first time a new and realistic application of the “perfect lens”, namely, electromagnetic traps (or tweezers). We combined two recently developed techniques, 3D negative refraction flat lenses (3DNRFLs) and optical tweezers, and experimentally demonstrated the very unique advantages of using 3DNRFLs for electromagnetic traps. Super-resolution and short focal distance of the flat lens result in a highly focused and strongly convergent beam, which is a key requirement for a stable and accurate electromagnetic trap. The translation symmetry of 3DNRFL provides translation-invariance for imaging, which allows an electromagnetic trap to be translated without moving the lens, and permits a trap array by using multiple sources with a single lens. Electromagnetic trapping was demonstrated using polystyrene particles in suspension, and subsequent to being trapped to a single point, they were then accurately manipulated over a large distance by simple movement of a 3DNRFL-imaged microwave monopole source.

Far field millimeter-wave imaging via optical upconversion

Millimeter-wave imaging has the unique potential to penetrate through poor weather and atmospheric conditions and create a high-resolution image. In pursuit of this goal, we have implemented a far-field imaging system that is based on optical upconversion techniques. Our imaging system is passive, in which all native blackbody radiation that is emitted from the object being scanned is detected by a Cassegrain antenna on a rotating gimbal mount. The signal received by the Cassegrain is passed to an optical modulator which transfers the radiation onto sidebands of a near-infrared optical carrier frequency. The signal is then passed to a low-frequency photodetector that converts remaining sideband energy to a photocurrent. Even though optical upconversion can produce loss, our system demonstrates low noise equivalent powers (NEP) due to the low-noise of the photodetection process. Herein, we present our experimental results and images obtained by using the far-field scanning system, which was assembled with commercially available components. In addition, we detail efforts to increase the resolution of the image and to compact the imaging system as a whole.

94 GHz millimetre-wave imaging system implementing optical upconversion

Millimeter wave (mmW) imaging is continually being researched for its applicability in all weather imaging. While previous accounts of our imaging system utilized Q-band frequencies (33-50 GHz), we have implemented a system that now achieves far-field imaging at W-band frequencies (75-110 GHz). Our mmW imaging approach is unique due to the fact that optical upconversion is used as the method of detection. Optical modulators are not commercially available at W-band frequencies; therefore, we have designed our own optical modulator that functions at this frequency range. Imaging at higher frequencies increases our overall resolution two to three times over what was achieved at Q-band frequencies with our system. Herein, we present imaging results obtained using this novel detector setup, as well as key imager metrics that have been experimentally validated.

Millimeter wave sensor requirements for maritime small craft identification

Passive millimeter wave (mmW) imagers have improved in terms of resolution sensitivity and frame rate. Currently, the Office of Naval Research (ONR), along with the US Army Research, Development and Engineering Command, Communications Electronics Research Development and Engineering Center (RDECOM CERDEC) Night Vision and Electronic Sensor Directorate (NVESD), are investigating the current state-of-the-art of mmW imaging systems. The focus of this study was the performance of mmW imaging systems for the task of small watercraft / boat identification field performance. First mmW signatures were collected. This consisted of a set of eight small watercrafts; at 5 different aspects, during the daylight hours over a 48 hour period in the spring of 2008. Target characteristics were measured and characteristic dimension, signatures, and Root Sum Squared of Targets Temperature (RRSΔT) tabulated. Then an eight-alternative, forced choice (8AFC) human perception experiment was developed and conducted at NVESD. The ability of observers to discriminate between small watercraft was quantified. Next, the task difficulty criterion, V50, was quantified by applying this data to NVESDs target acquisition models using the Targeting Task Performance (TTP) metric. These parameters can be used to evaluate sensor field performance for Anti-Terrorism / Force Protection (AT/FP) and navigation tasks for the U.S. Navy, as well as for design and evaluation of imaging passive mmW sensors for both the U.S. Navy and U.S. Coast Guard.

Analytical model and optical design of distributed aperture optical system for millimeter-wave imaging

Millimeter-wave imaging is very interesting due to its unique transmission properties through a broad range of atmospheric obscurants such as cloud, dust, fog, sandstorms, and smoke, which thereby enables all-weather passive imaging. Unfortunately, the usefulness of millimeter-wave imagers is often limited by the large aperture sizes required to obtain images of sufficient resolution, as governed by the diffraction limit. To this end, we previously proposed a distributed aperture system for direct non-scan millimeter-wave imaging using an optical upconversion technique. In this proposed approach, an antenna array is employed to sample image signals in the millimeter-wave domain. The sampled millimeter-wave signals are then upconverted to the optical domain using electro-optic modulation techniques. These optical signals are mapped into a similar array on the entrance pupil of the following optical system for direct imaging. Although distributed aperture imaging is not new in both radio astronomy and conventional optical inteferometric imaging, the proposed approach is different in that it physically samples image in the millimeter-wave domain and directly forms the image in the optical domain. Therefore, specific analysis and evaluation techniques are required for the design and optimization of the proposed system. In this paper, we will address these issues, develop techniques to evaluate and enhance the system imaging performance and present methods to optimize the geometric configuration.

Polarization difference imaging for millimeter-wave in a desert environment

The low attenuation of millimeter-wave radiation propagating through sandstorms has created an interest in using millimeter-wave imagers in desert environments. The ground in desert environments can have significant differences in polarization properties depending on the angle of observation. Perturbations to the natural desert surface will change these polarization properties and by using a polarization difference technique these changes are highlighted. This technique has been applied to millimeter-wave images from a desert environment for several different objects including holes in the ground, footsteps, and changes to the surface created by digging.

Passive Millimeter-Wave Cross Polarization Imaging and Phenomenology

Passive millimeter-wave (mmW) imaging has many specific defense, security and safety applications, due to the fact that all terrestrial bodies above absolute zero are emissive, and these wavelengths are not scattered by normal obscurants such as haze, fog, smoke, dust, sandstorms, clouds, or fabrics. We have previously demonstrated results from the construction of a 94 GHz passive mmW far-field imaging system utilizing optical upconversion, which imaged in only horizontal polarization. The effective radiometric temperature of an object is a combination of the objects surface and scattered radiometric temperatures. The surface radiometric temperature is a function of the objects emissivity, which is polarization dependent. Imaging with radiometric temperature data from both polarizations will allow a greater identification of the scene being imaged, and allow the recognition of subtle features which were not previously observable. This additional functionality is accomplished through the installation of added equipment and programming on our system, thus allowing the simultaneous data collection of imagery in both polarizations. Herein, we present our experimental procedures, results and passive mmW images obtained by using our far-field imaging system, a brief discussion of the phenomenology observed through the application of these techniques, as well as the preliminary details regarding our work on a 3-D passive mmW simulator capable of true physical polarization dependent effective emissivity and reflectivity rendering, based on the open-source Blender engine.