Thomas M. Goyette

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.

Continuous Wave Terahertz Transmission Imaging of Nonmelanoma Skin Cancers

Continuous wave terahertz imaging has the potential to offer a safe, noninvasive medical imaging modality for delineating human skin cancers. Terahertz pulse imaging (TPI) has already shown that there is contrast between basal cell carcinoma and normal skin. Continuous‐wave imaging offers a simpler, lower cost alternative to TPI. The goal of this study was to investigate the feasibility of continuous wave terahertz imaging for delineating skin cancers by demonstrating contrast between cancerous and normal tissue in transmission mode.

A terahertz focal plane array using HEB superconducting mixers and MMIC IF amplifiers

We present a focal plane array (FPA) designed for operation at terahertz frequencies. The FPA is based on NbN phonon-cooled hot electron bolometer mixers directly coupled to wide-band microwave monolithic integrated circuit IF amplifiers. The array incorporates all the required dc-bias and IF circuitry in a compact split-block design. We present new experimental results describing the optical coupling efficiency to the array, as well as receiver noise temperature measurements. The measurements were performed at 1.6 THz, showing good agreement with theoretical predictions. This is the first low-noise heterodyne focal plane array to be reported for any frequency above 1 THz.

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.

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.

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.