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A fully polarimetric compact radar range operating at a center frequency of 100 GHz has been developed for obtaining radar cross section, inverse synthetic aperture radar imagery and high range resolution profiles on targets and structures of interest. The 100 GHz radar range provides scale-model RCS measurements for a variety of convenient scale factors including W-Band (1:1 scale), C-band (1:16 scale), and S-band (1:26 scale). An overview of the radar range is provided in this paper along with measurement examples of ISAR scale-model imaging, scale-model through-wall imaging, and preliminary kHz sweep-rate Doppler that demonstrate a few of the diverse and unique applications for this system. The 100 GHz transceiver consists of a fast-switching, stepped, CW microwave synthesizer driving dual-transmit and dual-receive frequency multiplier chains. The stepped resolution of the system's frequency sweep is sufficient for unambiguous resolution of the entire chamber. The compact range reflector is a CNC machined aluminum reflector edge-treated with FIRAM(trademark)-160 absorber serrations and fed from the side to produce a clean quiet zone. This range is the latest addition to a suite of compact radar ranges developed by the Submillimeter-Wave Technology Laboratory providing scale-model radar measurements at nearly all of the common radar bands., Published in Proceedings of the Antenna Measurements Techniques Association (AMTA), Columbus, OH, October 2013, pp. 276-281.

Remote sensing over long path lengths has become of greater interest in the terahertz frequency region. Applications such as pollution monitoring and detection of energetic chemicals are of particular interest. Although there has been much attention to atmospheric effects over narrow frequency windows, accurate measurements across a wide spectrum is lacking. The water vapor continuum absorption spectrum was investigated using Fourier Transform Spectroscopy. The continuum effect gives rise to an excess absorption that is unaccounted for in just a resonant line spectrum simulation. The transmission of broadband terahertz radiation from 0.300THz - 1.5THz through air with varying relative humidity levels was recorded for multiple path lengths. From these data, the absorption coefficient as a function of frequency was determined and compared with model calculations. The intensity and location of the strong absorption lines were in good agreement with spectral databases such as the 2008 HITRAN database and the JPL database. However, a noticeable continuum effect was observed particularly in the atmospheric transmission windows. A small discrepancy still remained even after accounting for continuum absorption using the best available data from the literature. This discrepancy, when projected over a one kilometer path length, typical of distances used in remote sensing, can cause a 30dB difference between calculated and observed attenuation. From the experimental and resonant line simulation spectra the air-broadening continuum parameter was calculated and compared with values available in the literature.

A coherent transceiver using a THz quantum cascade laser as the transmitter and an optically pumped molecular laser as the local oscillator has been used, with a pair of Schottky diode mixers in the receiver and reference channels, to acquire high-resolution images of fully illuminated targets, including scale models. Phase stability of the received signal sufficient to allow coherent image processing of the rotating target (in azimuth and elevation), was obtained by frequency-locking the TQCL to the free-running, highly stable optically pumped molecular laser. While the range to the target was limited by the available TQCL power (several hundred microwatts) and reasonably strong indoor atmospheric attenuation at 2.408 THz (2.0 dB/m at 40% RH), the coherence length of the QCL transmitter will allow coherent imaging over distances up to several hundred meters. In contrast to non-coherent heterodyne detection, coherent imaging allows signal integration over time intervals considerably longer than the reciprocal of the source, or signal bandwidth, with consequent improvement in the signal-to-noise ratio. Image data obtained with the system will be presented., Published in Terahertz Technology and Applications III, Proceedings of SPIE, v7601, 2010. Conference held in Bellingham, WA, January 2010. The original document contains color images.

A coherent transceiver using a THz quantum cascade (TQCL) laser as the transmitter and an optically pumped molecular laser as the local oscillator has been used, with a pair of Schottky diode mixers in the receiver and reference channels, to acquire high-resolution images of fully illuminated targets, including scale models and concealed objects. Phase stability of the received signal, sufficient to allow coherent image processing of the rotating target (in azimuth and elevation), was obtained by frequency-locking the TQCL to the free-running, highly stable optically pumped molecular laser. While the range to the target was limited by the available TQCL power (several hundred microwatts) and reasonably strong indoor atmospheric attenuation at 2.408 THz, the coherence length of the TQCL transmitter will allow coherent imaging over distances up to several hundred meters. Image data obtained with the system is presented., Published in Optics Express, v18 p16264-16272, 2010. The original document contains color images.

A fully-polarimetric compact radar range operating at 240 GHz has been developed for obtaining Ku-band RCS measurements on 1:16th scale model targets. The transceiver consists of dual fast-switching, stepped, CW X-band synthesizers driving dual X24 transmit multiplier chains and dual X24 local oscillator multiplier chains. The system alternately transmits horizontal (H) and vertical (V) radiation while simultaneously receiving H and V. Software range-gating is used to reject unwanted spurious responses in the compact range. A flat disk and rotating circular dihedral are used for polarimetric as well as RCS calibration. Cross-pol rejection ratios of better than 45 dB are routinely achieved. The compact range reflector consists of a 60" diameter, CNC machined aluminum mirror fed from the side to produce a clean 27" FWHM quiet zone. In this paper a description of this 240 GHz compact range is provided along with an ISAR measurement example., Presented at the Antenna Measurements Techniques Association (AMTA) Conference held in Atlanta, GA in October 2010. Published in the Antenna Measurements Techniques Association Proceedings, p3-8, October 2010.. U.S. Government or Federal Rights License. The original document contains color images.

Quantum cascade lasers (QCL's) employ the mid- and far-infrared intersubband radiative transitions available in semiconducting heterostructures. Through the precise design and construction of these heterostructures the laser characteristics and output frequencies can be controlled. When fabricated, QCL's offer a lightweight and portable alternative to traditional laser systems which emit in this frequency range. The successful operation of these devices strongly depends on the effects of electron transport. Studies have been conducted on the mechanisms involved in electron transport and a prediction code for QCL simulation and design has been completed. The implemented approach utilized a three period simulation of the laser active region. All of the wavefunctions within the simulation were included in a self-consistent rate equation model. This model employed all relevant types of scattering mechanisms within three periods. Additionally, an energy balance equation was studied to determine the temperature of electron distributions separately from the lattice temperature. This equation included the influence of both electron-LO phonon and electron-electron scattering. The effect of different modelling parameters within QCL electron temperature predictions will be presented along with a description of the complete QCL prediction code., Presented at the Modeling and Simulation for Defense Systems and Applications V, SPIE Vol 7705, held in Bellingham, WA, Apr 2010. Prepared in cooperation with University of Massachusetts, Lowell, MA.

The frequency of four previously reported far-infrared laser lines have been measured to an accuracy of 100 kHz. These laser lines were measured using a heterodyne system which allowed for more accurate measurement. The four far-infrared laser lines which originated from Formic Acid (HCOOH), O-Deutero-Formic Acid (HCOOD), C-Deutero- Formic Acid (DCOOH), and Methyl Chloride (CH3Cl) were optically pumped by an ultra-stable, grating-tunable, CO2 laser. Previous measurements had been conducted using Fabry-Perot cavities not typically known for their high accuracy. The difference between the frequencies measured here and the listed frequencies for these laser lines ranged from 59 MHz to 3.9 GHz., Published in the IEEE Journal of Quantum Electronics, v46 n4 p474-477, Apr 2010. Prepared in cooperation with National Ground Intelligence Center, Charlottesville, VA.

A simple analog locking circuit was shown to stabilize the beat signal between a 2.408 THz quantum cascade laser and a CH2DOH THz CO2 optically pumped molecular laser to 3-4 kHz (FWHM). This is approximately a tenth of the observed long-term (t ~ sec) linewidth of the optically pumped laser showing that the feedback loop corrects for much of the mechanical and acoustic-induced frequency jitter of the gas laser. The achieved stability should be sufficient to enable the use of THz quantum cascade lasers as transmitters in short-range coherent transceivers., Published in Optics Express, v17 n9, 27 April 2009.

Continuous wave terahertz imaging has the potential to offer a safe, non-invasive medical imaging modality for detecting different types of human cancers. The aim of this study was to identify intrinsic biomarkers for non-melanoma skin cancer and their absorption frequencies. Knowledge of these frequencies is a prerequisite for the optimal development of a continuous wave terahertz imaging system for detecting different types of skin cancers. The absorption characteristics of skin constituents were studied between 20 and 100 cm-1 (0.6 THz - 3 THz). Terahertz radiation is highly absorbed by water. Thus, the high water content of human tissue necessitates a reflection based imaging modality. To demonstrate a reflection based, high resolution, terahertz imaging system, a prototype imaging system was constructed at 1.56 THz. The system resolution was determined to be 0.5 mm and the system signal to noise ratio was found to be 70 dB. Data from the terahertz spectroscopy experiments and reflection based terahertz images at 1.56 THz are presented., Published in the Proceedings of SPIE 2009 v7215. The original document contains color images.

A compact, tunable, narrowband terahertz source was demonstrated by mixing a single longitudinal mode 2.408 THz, free running quantum cascade laser with a 2-20 GHz microwave sweeper in a conventional corner-cube-mounted Schottky diode. The sideband spectra were characterized with a Fourier transform spectrometer, and the radiation was tuned through several D2O rotational transitions to estimate the longer term (t greater than or equal to several sec) bandwidth of the source. A spectral resolution of 2 MHz in CW regime was observed., Published in Optics Express, v16 n8 p5171-5180, 31 Mar 2008. Sponsored in part by the U.S. Army National Ground Intelligence Center and the Department of Homeland Security.

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., Sponsored in part by the Department of Homeland Security. Prepared in collaboration with the Spire Corporation, Bedford, Ma and the National Ground Intelligence Center, Charlottesville, VA. Presented at Integrated Optoelectronic Devices 2008 in San Jose, CA on 19-24 Jan 2008. Published in Terahertz Technology and Applications, Proceedings of SPIE v6893, paper 68930j, 14 Feb 2008.

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., Produced in collaboration with the U.S. Army National Ground Intelligence Center, Charlottesville, VA. The original document contains color images.

Construction of the new 350GHz compact range has been completed and it is able to collect fully polarimetric scaled X-band radar data with 6-inch full-scale range resolution. In order to investigate the reproduction of X-band data using scale models, fully polarimetric high-resolution radar signature data has been collected on several targets which include a high-fidelity in-house built 1/16th scale T72 Main Battle Tank (MBT) and a commercially available 1/35th scale model T72 modified to match its features. A correlation study of ISAR images has been performed between the X-band data sets collected on these models, a full-scale T72, a 1/35th scale model heavy equipment transporter, and several different 1/16th scaled targets of similar size. The ISAR images formed from the data were compared using several techniques which include a two-dimensional cross-correlation of the images against one another, and the comparison of the images pixel-by-pixel to measure the percentage differences. It will be shown that the T72 data sets compare well across the three different radar platforms. It has also been found that there are persistent sharp features in the two-dimensional cross-correlation maps that are located where the real target is matched even when other parameters have changed by a significant amount. These features continue to occur when the target has been imbedded in a complex two-target scene with the heavy equipment transporter., Produced in collaboration with the U.S. Army National Ground Intelligence Center, Charlottesville, VA. Presented at the Defense & Security Symposium held in Orlando, FL on 18-20 Apr 2006; published in Algroithms for Synthetic Aperture Radar Imagery XIII, Proceedings of the SPIE, v6237, paper 6237f, May 2006. The original document contains color images.

In recent years, UHF synthetic aperture radar has become a growing area of interest among the radar community. Due to their relatively long wavelengths, UHF systems provide advantages that may not be attainable by microwave and millimeter-wave radar systems. These advantages include excellent target detection statistics in high clutter environments, wide-area surveillance, and long stand-off ranges. UHF systems also have proven synergistic properties with higher frequency radar systems in applications such as topographical mapping. However, the ability to study the characteristics of these lower frequency radar systems in a controlled and systematic environment is difficult. In this work, a physical scale modeling process is utilized to generate three-dimensional UHF imagery that may be used to study scattering phenomenology at these wavelengths. Dimensionally and dielectrically scaled targets and scenes are measured in a 6 18 GHz microwave compact range to model the backscatter of the full-size target at UHF wavelengths. The microwave compact radar range and transceiver hardware utilized to model UHF radar signature data are briefly described. A description of the image processor used to generate three-dimensional UHF imagery from wide-band/wide-angle data collections is described as well. Finally, imagery of radar signature data collected from a M1A1 Abrams main battle tank model is examined. The high resolution imagery resulting from the wide-band/wide-angle collection will show that sub-wavelength features of ground targets are resolvable at these wavelengths., Produced in collaboration with the U.S. Army National Ground Intelligence Center, Charlottesville, VA. Presented at the Defense & Security Symposium held in Orlando, FL on 18-20 Apr 2006; published in Algroithms for Synthetic Aperture Radar Imagery XIII, Proceedings of the SPIE, v6237, paper 6237A, May 2006. The original document contains color images.

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., The original document contains color images.

Radar detection and identification of ground targets in diverse environments is a subject of continuing interest. It has long been known that different radar bands have advantages for different environmental conditions. For example, it has been shown that detection of targets under foliage is more easily accomplished using longer wavelength radars since there is less attenuation at these frequencies. However, higher frequency radars offer greater resolution that is crucial in target identification. Because each radar band has its own unique strengths and weakness, one current approach is the use of dual-band radar platforms. With two radar bands working simultaneously, the strengths of each radar band can be used to compliment the other. ERADS has constructed two full polarimetric compact radar ranges to acquire X-Band and UHF ISAR imagery data using 1/35th scale models. The new compact ranges allow data to be taken that can simulate a multi-frequency radar platform with frequencies low enough to detect obscured targets and high enough to provide useful resolution to aid in target identification once they have been detected. Since both compact ranges use the same scale factor, this allows measurement of the same target at the two spectral regions simply by moving the target model from one compact range to the other. Data can thus be taken whose differences in scattering are due only to the difference in radar frequency, eliminating variations due to differences in target models as well as the surrounding ground clutter. Detailed descriptions of the new compact ranges will be presented along with results from sample data sets., Produced in collaboration with the U.S. Army National Ground Intelligence Center, Charlottesville, VA. Presented at the Defense & Security Symposium held in Orlando, FL on 28 Mar-1 Apr 2005; published in Algroithms for Synthetic Aperture Radar Imagery XII, Proceedings of the SPIE, v5808, p440-449, May 2006. The original document contains color images.

The original document contains color images. Pub. in the Proceedings of SPIE, v5427 p227-236, 2004.

Pub. in the Proceedings of SPIE, v4382 p241-251, 2001.

The original document contains color images.

The original document contains color images.