William E. Nixon

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.

Terahertz inverse synthetic aperture radar (ISAR) imaging with a quantum cascade laser transmitter

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.

Transformation of the multimode terahertz quantum cascade laser beam into a Gaussian, using a hollow dielectric waveguide

We demonstrate that a short hollow dielectric tube can act as a dielectric waveguide and transform the multimode, highly diverging terahertz quantum cascade laser beam into the lowest order dielectric waveguide hybrid mode, EH(11), which then couples efficiently to the free-space Gaussian mode, TEM(00). This simple approach should enable terahertz quantum cascade lasers to be employed in applications where a spatially coherent beam is required.

Frequency stabilization of a single mode terahertz quantum cascade laser to the kilohertz level.

A simple analog locking circuit was shown to stabilize the beat signal between a 2.408 THz quantum cascade laser and a CH(2)DOH THz CO(2) optically pumped molecular laser to 3-4 kHz (FWHM). This is approximately a tenth of the observed long-term (t approximately 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.

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.

Terahertz sideband-tuned quantum cascade laser radiation

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≥several sec) bandwidth of the source. A spectral resolution of 2 MHz in CW regime was observed.

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.

Submillimeter-wave polarimetric compact ranges for scale-model radar measurements

Fully-polarimetric, wide-band compact radar ranges based on transceivers operating In the submillimeter-wave regime have been developed for obtaining radar measurements on scale models (nominally 1:16). These transceivers use fixed-tuned Schottky-diode mixers and varactor multiplier sources to obtain reasonably wide-band performance. Optically pumped gas lasers, combined with tunable microwave sideband generation In corner-cube-mounted Schottky diodes, have been implemented to extend the operating frequencies into the THz regime. A dielectric material fabrication and characterization capability has also been developed to fabricate custom anechoic materials for the ranges as well as scaled dielectric parts for the models and clutter scenes. The general approach to designing submillimeter-wave compact ranges and the particular details of systems operating at 524 GHz and 1.56 THz will presented in this paper.

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.