Jason C. Dickinson

Terahertz Imaging of Subjects With Concealed Weapons

In response to the growing interest in developing terahertz imaging systems for concealed weapons detection, the Submillimeter-Wave Technology Laboratory (STL) at the University of Massachusetts Lowell has produced full-body terahertz imagery using coherent active radar measurement techniques. The proof-of-principle results were readily obtained utilizing the compact radar range resources at STL. Two contrasting techniques were used to collect the imagery. Both methods made use of in-house transceivers, consisting of two ultra-stable far-infrared lasers, terahertz heterodyne detection systems, and terahertz anechoic chambers. The first technique involved full beam subject illumination with precision azimuth and elevation control to produce high resolution images via two axis Fourier transforms. Imagery collected in this manner is presented at 1.56THz and 350GHz. The second method utilized a focused spot, moved across the target subject in a high speed two dimensional raster pattern created by a large two-axis positioning mirror. The existing 1.56THz compact radar range was modified to project a focused illumination spot on the target subject several meters away, and receive the back-reflected intensity. The process was repeated across two dimensions, and the resultant image was assembled and displayed utilizing minimal on-the-fly processing. Imagery at 1.56THz of human subjects with concealed weapons are presented and discussed for this scan type.

Terahertz behavior of optical components and common materials

As short range, ground based, surveillance systems operating at terahertz frequencies continue to evolve, increasing attention is being directed towards the behavior of dielectric materials at terahertz frequencies as well as the behavior of optical components used to control terahertz radiation. This work provides an overview of several terahertz optical components such as frequency selective filters, laser output couplers, artificial dielectrics, and electromagnetic absorbers. In addition, a database was established that contains terahertz properties of common materials that have been largely unexplored in this region of the spectrum. The database consists of transmittance and reflectance spectra of a variety of materials measured using Fourier transform infrared spectroscopy techniques from 175 GHz - 2 THz. In addition, ultra-stable, CO2 optically pumped, far-infrared gas lasers were used to collect fixed-frequency transmittance data at 326 GHz, 584 GHz, and 1.04 THz. A Gunn oscillator was used for measurements at 94 GHz.

1.56-THz compact radar range for W-band imagery of scale-model tactical targets

A new very high-frequency compact radar range has been developed to measure scale models of tactical targets. This compact range has demonstrated very good signal-to-noise and is useful in measuring low observable targets. In addition to normal ISAR imaging of targets (range vs. horizontal cross- range), the system can also produce two-dimensional images in azimuth and elevation (vertical cross-range vs. horizontal cross-range). The 1.56 THz transceiver uses two high-stability optically pumped far-infrared lasers, microwave/laser side- band generation for frequency sweep, and a pair of Schottky diode receivers for coherent integration. Measurements made on 1/16th scale models of tactical targets, simulating W-band frequencies, allows the formation of images of very high cross-range resolution (3.5 cm full scale) while still integrating over a reasonably small angular extent (2.5 degrees). The results from several targets that have been recently measured will be presented.

NbN hot electron bolometric mixers-a new technology for low noise THz receivers

New advances in Hot Electron Bolometer (HEB) mixers have resulted in record low receiver noise temperatures at THz frequencies recently. We have developed quasi-optically coupled NbN HEB mixers and measured noise temperatures up to 1.56 THz, as described in this paper. We project the anticipated future performance of such receivers to have even lower noise temperature and LO power requirement as well as wider gain and noise bandwidths. We introduce a proposal for integrated focal plane arrays of HEB mixers which will further increase the detection speed of THz systems.

Fully polarimetric W-band ISAR imagery of scale-model tactical targets using a 1.56-THz compact range

With the continuing interest in ATR, there is a need for high-resolution fully polarimetric data on tactical targets at all radar bands. Here we describe a newly developed system for acquiring W-band data with 1/16 scale models. The NGIC sponsored ERADS project capability for obtaining fully polarimetric ISAR imagery now extends from X to W band.

1.56 Terahertz 2-frames per second standoff imaging

A Terahertz imaging system intended to demonstrate identification of objects concealed under clothing was designed, assembled, and tested. The system design was based on a 2.5 m standoff distance, with a capability of visualizing a 0.5 m by 0.5 m scene at an image rate of 2 frames per second. The system optical design consisted of a 1.56 THz laser beam, which was raster swept by a dual torsion mirror scanner. The beam was focused onto the scan subject by a stationary 50 cm-diameter focusing mirror. A heterodyne detection technique was used to down convert the backscattered signal. The system demonstrated a 1.5 cm spot resolution. Human subjects were scanned at a frame rate of 2 frames per second. Hidden metal objects were detected under a jacket worn by the human subject. A movie including data and video images was produced in 1.5 minutes scanning a human through 180° of azimuth angle at 0.7° increment.

Physical scale modeling the millimeter-wave backscattering behavior of ground clutter

The VV-polarized W-band backscattering behavior of homogeneous ground clutter has been investigated by measuring the radar cross section per unit area of 1/16th scale rough surface terrain in a 1.56 THz compact radar range. An array of scale model ground planes was fabricated with the appropriate roughness to model smooth to rough soil terrain. In addition to studying the backscattering behavior as a function of surface roughness, the dependence on soil moisture content was also characterized by tailoring the dielectric constant of the scale models. Radar imagery of the rough surfaces were acquired in a 1.56THz compact radar range by collecting single frequency backscatter data over a solid angle in both azimuth and elevation. The data were Fourier transformed in both the azimuth and elevation directions to produce two-dimensional imagery. The backscattering coefficient per unit illuminated area ((sigma) 0) was calculated as a function of elevation angle between 5 degree(s) and 85 degree(s). The results of this work have been used in the fabrication of scale model ground planes for collection of W-band radar imagery from scaled threat targets in realistic environments. Backscattering data, including clutter statistics, are compared to W-band clutter data found in the literature.

Three-dimensional fully polarimetric W-band ISAR imagery of scale-model tactical targets using a 1.56-THz compact range

Using the high-frequency terahertz compact range developed recently for measurement of polarimetric return of scale modesl of tactical targets, we have developed several techniques to produce 3D data sets. Fully polarimetric 3D ISAR data has been collected on several 1/16th scale model tactical targets in free space at individual look angles. The 3D scattering coordinates are calculated by viewing the target through a 2D angular aperture in both azimuth and elevation while simultaneously performing a linear frequency chirp to measure the down-range coordinate. Due to the high frequency of W-band radar, this technique produces high-resolution cross-range images from relatively small (approximately 1 degree) angular integrations. Several techniques for calculation of the 3D coordinates have been developed. In addition to the technique described above, a new method utilizing the phase change of the scattering centers due to differentially small changes in angle will be described. Data collected using this technique can be processed to produce 3D scattering information similar to that obtained by monopulse systems. Results from this analysis will be shown.

Polarimetric Backscattering Behavior of Ground Clutter at X, Ka, and W-band

The HH and VV-polarized backscattering behavior of homogeneous ground clutter has been investigated by measuring the radar cross section per unit area of rough surface terrain. The X, Ka, and W-band behavior was investigated by analyzing ISAR imagery of 1/16th scale terrain collected in compact radar ranges operating at 160 GHz, 520 GHz, and 1.56 THz. An array of scale model ground planes was fabricated with the appropriate roughness to model relatively smooth to rough soil terrain. In addition to studying terrain backscatter as a function of surface roughness, the dependence on soil moisture content was also characterized by tailoring the dielectric constant of the scale models. The radar cross section per unit illuminated area (?0) was calculated as a function of elevation angle between 15° and 75°. The results of this work have been used in the fabrication of scale model ground planes for collection of radar imagery from scaled threat targets situated in realistic environments. Backscattering data are presented and compared to clutter data found in the literature.

X-band ISAR imagery of scale-model tactical targets using a wide-bandwidth 350GHz compact range

The demand for high-resolution ISAR data on tactical targets at all radar bands has been growing steadily. Here we describe a new 350GHz compact range currently being constructed to acquire fully polarimetric X-band data using 1/35th scale models. ERADS currently operates compact ranges from X to W-band using 1/16th scale models. The addition of this new compact range using 1/35th scale models will permit the measurement of larger targets and the measurement of multiple targets arrangeed in a scene. It will also allow us to take advantage of teh large number of commercially available models at 1/35th scale. The 350GHz transceiver uses two high-stability optically pumped far-infrared lasers, microwave/laser 350GHz mixer side-band generation for frequency sweep, and a pair of waveguide mounted diode receivers for coherent integration. The 35GHz bandwidth at a center frequency of 350GHz will allow the X-band transceiver system to collect data with up to 6-inch down range resolution, with a round trip half power beam diameter corresponding to 60 feet. Tactical targets may be measured in free space or on various ground planes, which simulate different types of terrain. Compact range measurements of simple calibration objects have been performed and compared to theoretical results using computer code predictions. A correlation study of X-band data using field measurements, 1/35th scale models and 1/16th scale models is planned upon completion of compact range construction. Available results of the diagnostic testes and the correlation study will be presented.