Michael J. Coulombe

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.

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.

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.

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

Prospects for quantum cascade lasers as transmitters and local oscillators in coherent terahertz transmitter/receiver systems

Coherent terahertz radar systems, using CO2 laser-pumped molecular lasers have been used during the past decade for radar scale modeling applications, as well as proof-of-principle demonstrations of remote detection of concealed weapons. The presentation will consider the potential for replacement of molecular laser sources by quantum cascade lasers. While the temporal and spatial characteristics of current THz QCLs limit their applicability, rapid progress is being made in resolving these issues. Specifications for satisfying the requirements of coherent short-range THz radars will be reviewed and the feasibility of incorporating existing QCL devices into such systems will be described.

Coherent imaging at 2.4 THz with a CW quantum cascade laser transmitter

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.

TREND: a low-noise terahertz receiver user instrument for AST/RO at the South Pole

Based on the excellent performance of NbN HEB mixer receivers at THz frequencies which we have established in the laboratory, we are building a Terahertz REceiver with NbN HEB Device (TREND) to be installed on the 1.7 meter diameter AST/RO submillimeter wave telescope at the Amundsen/Scott South Pole Station. TREND is scheduled for deployment during the austral summer season of 2002/2003. The frequency range of 1.25 THz to 1.5 THz was chosen in order to match the good windows for atmospheric transmission and interstellar spectral lines of special interest. The South Pole Station is the best available site for THz observations due to the very cold and dry atmosphere over this site. In this paper, we report on the design of this receiver. In particular, we report on HEB mixer device performance, the quasi-optical coupling design using an elliptical silicon lens and a twin-slot antenna, the laser local oscillator (LO), as well as the mixer block design and the plans for coupling the TREND receiver to the sky beam and to the laser LO at the AST/RO telescope site.

A prototype fully polarimetric 160-GHz bistatic ISAR compact radar range

We present a prototype bistatic compact radar range operating at 160 GHz and capable of collecting fullypolarimetric radar cross-section and electromagnetic scattering measurements in a true far-field facility. The bistatic ISAR system incorporates two 90-inch focal length, 27-inch-diameter diamond-turned mirrors fed by 160 GHz transmit and receive horns to establish the compact range. The prototype radar range with its modest sized quiet zone serves as a precursor to a fully developed compact radar range incorporating a larger quiet zone capable of collecting X-band bistatic RCS data and 3D imagery using 1/16th scale objects. The millimeter-wave transmitter provides 20 GHz of swept bandwidth in the single linear (Horizontal/Vertical) polarization while the millimeter-wave receiver, that is sensitive to linear Horizontal and Vertical polarization, possesses a 7 dB noise figure. We present the design of the compact radar range and report on test results collected to validate the system’s performance.

Effect of periodic roughness and surface defects on the terahertz scattering behavior of cylindrical objects

This paper discusses the effect of periodic roughness and surface defects on the electromagnetic scattering of terahertz waves from cylindrical objects. The cylinders, possessing periodic roughness imparted during their fabrication process, had average roughness values ranging from approximately 0.1 μm - 0.50 μm. Metallic cylinders were fabricated from lathe-turned aluminum rods and dielectric cylinders were fabricated using a rapid prototype technique (stereolithography). The scattering behavior of the rough cylinders was measured in 160 GHz and 350 GHz compact radar ranges. In addition, the effect of seams and grooves on the scattering behavior of cylinders will also be presented.

X-band radar signature characteristics for main battle tanks in operational environments

An analysis of target separability has been performed under an OSD Target Management Initiative program entitled Radar Variations. The program has concentrated on analyzing radar signatures from multiple main battle tanks (MBTs) in order to quantify the differences in Ka-band signatures of vehicles due to intraclass and interclass target variations. As a significant factor in the success of the Radar Variations program, U Mass Lowells Submillimeter-Wave Technology Laboratory (STL) and U.S. Army National Ground Intelligence Center (NGIC) fabricated 1/16th scale exact replicas of the vehicles used in the Ka-band radar signature acquisition study directed by Simulation Technologies, Inc. (SimTech) and Targets Management Office (TMO). These replicas enabled NGIC to measure statistically significant amounts of high-fidelity signature data for a variety of target configurations with an indoor compact radar range.