Robert J. Dengler

THz Imaging Radar for Standoff Personnel Screening

A summary of the NASA Jet Propulsion Laboratorys 675 GHz imaging radar is presented, with an emphasis on several key design aspects that enable fast, reliable through-clothes imaging of person-borne concealed objects. Using the frequency-modulated continuous-wave (FMCW) radar technique with a nearly 30 GHz bandwidth, sub-centimeter range resolution is achieved. To optimize the radars range resolution, a reliable software calibration procedure compensates for signal distortion from radar waveform nonlinearities. Low-noise, high dynamic range detection comes from the radars heterodyne RF architecture, low-noise chirp source, and high-performance 675 GHz transceiver. The radars optical design permits low-distortion fast beam scanning for single-pixel imaging, and a real-time radar image frame rate of 1 Hz is now possible. Still faster speeds are on the horizon as multi-beam THz transceivers are developed.

Penetrating 3-D Imaging at 4- and 25-m Range Using a Submillimeter-Wave Radar

We show experimentally that a high-resolution imaging radar operating at 576-605 GHz is capable of detecting weapons concealed by clothing at standoff ranges of 4-25 m. We also demonstrate the critical advantage of 3-D image reconstruction for visualizing hidden objects using active-illumination coherent terahertz imaging. The present system can image a torso with <1 cm resolution at 4 m standoff in about five minutes. Greater standoff distances and much higher frame rates should be achievable by capitalizing on the bandwidth, output power, and compactness of solid state Schottky-diode based terahertz mixers and multiplied sources.

A High-Resolution Imaging Radar at 580 GHz

We have developed a high-resolution imaging radar at 580 GHz. Coherent illumination in the 576-589 GHz range and phase-sensitive detection are implemented in an all-solid-state design based on Schottky diode sensors and sources. By employing the frequency-modulated continuous wave (FMCW) radar technique, we achieve centimeter-scale range resolution while utilizing fractional bandwidths of less than 3%. Our high operating frequencies also permit centimeter-scale cross-range resolution at several-meter standoff distances without large apertures. Scanning of a single-pixel transceiver enables targets to be rapidly mapped in three dimensions, and here we apply this technology to the detection of concealed objects on persons.

A 140-GHz monolithic low noise amplifier

The design, fabrication, and performance of a single-stage 44 GHz monolithic HEMT low noise amplifier are described. The chip includes a single heterojunction HEMT with matching and biasing circuits. Greater than 5 dB gain was measured from 43.5 to 45.5 GHz and a noise figure of 5 dB with the associated gain of 5.5 dB was achieved at 44.5 GHz. The chip size is 1.25mm x 1.0mm.This paper presents the development of a 140-GHz monolithic low noise amplifier (LNA) using 0.1-μm pseudomorphic InAlAs-InGaAs-InP low noise HEMT technology. A two-stage single-ended 140-GHz monolithic LNA has been designed, fabricated and tested. It exhibits a measured small signal gain of 9 dB at 142 GHz, and more than 5-dB gain from 138-145 GHz. This is the highest frequency monolithic amplifier ever reported using three terminal devices.

Confocal Ellipsoidal Reflector System for a Mechanically Scanned Active Terahertz Imager

We present the design of a reflector system that can rapidly scan and refocus a terahertz beam for high-resolution standoff imaging applications. The proposed optical system utilizes a confocal Gregorian geometry with a small mechanical rotating mirror and an axial displacement of the feed. For operation at submillimeter wavelengths and standoff ranges of many meters, the imaging targets are electrically very close to the antenna aperture. Therefore the main reflector surface must be an ellipse, instead of a parabola, in order to achieve the best imaging performance. Here we demonstrate how a simple design equivalence can be used to generalize the design of a Gregorian reflector system based on a paraboloidal main reflector to one with an ellipsoidal main reflector. The system parameters are determined by minimizing the optical path length error, and the results are validated with numerical simulations from the commercial antenna software package GRASP. The system is able to scan the beam over 0.5 m in cross-range at a 25 m standoff range with less than 1% increase of the half-power beam-width.

Fast, High-Resolution Terahertz Radar Imaging at 25 Meters

We report improvements in the scanning speed and standoff range of an ultra-wide bandwidth terahertz (THz) imaging radar for person-borne concealed object detection. Fast beam scanning of the single-transceiver radar is accomplished by rapidly deflecting a flat, light-weight subreflector in a confocal Gregorian optical geometry. With RF back-end improvements also implemented, the radar imaging rate has increased by a factor of about 30 compared to that achieved previously in a 4 m standoff prototype instrument. In addition, a new 100 cm diameter ellipsoidal aluminum reflector yields beam spot diameters of approximately 1 cm over a 50×50 cm field of view at a range of 25 m, although some aberrations are observed that probably arise from misaligned optics. Through-clothes images of concealed pipes at 25 m range, acquired in 5 seconds, are presented, and the impact of reduced signal-to-noise from an even faster frame rate is analyzed. These results inform the requirements for eventually achieving sub-second or video-rate THz radar imaging.

Measurements on a 215-GHz subharmonically pumped waveguide mixer using planar back-to-back air-bridge Schottky diodes

This paper presents design and performance data for a 215-GHz subharmonically pumped waveguide mixer using an antiparallel-pair of planar air-bridge-type GaAs Schottky-barrier diodes. The waveguide design is a prototype for a 640-GHz system and uses split-block rectangular waveguide with a 2:1 width-to-height ratio throughout. The measured mixer noise and conversion loss are below that of the best reported whisker contacted or planar-diode mixers using the subharmonic-pump configuration at this frequency. In addition, the required local oscillator power is as low as 3 mW for the unbiased diode pair, and greater than 34 dB of LO noise suppression is observed. Separate sideband calibration, using a Fabry-Perot filter, indicates that the mixer can be tuned for true double sideband response at an intermediate frequency of 1.5 GHz. Microwave scale model measurements of the waveguide mount impedances are combined with a mixer nonlinear analysis computer program to predict the mixer performance as a function of anode diameter, anode finger inductance, and pad-to-pad fringing capacitance. The computed results are in qualitative agreement with measurements, and indicate that careful optimization of all three diode parameters is necessary to significantly improve the mixer performance. >

A novel split-waveguide mount design for millimeter- and submillimeter-wave frequency multipliers and harmonic mixers

A split-waveguide mount for millimeter- and submillimeter-wave frequency multipliers and harmonic mixers is presented. It consists of only two pieces, block halves, which are mirror images of each other. The mount provides parallel and series impedance tuning with two sliding backshorts at both the input and output frequencies while utilizing E-plane arms to provide an in-line waveguide input and output. Its fabrication is much easier than that of a traditional multifrequency waveguide mount. Waveguide losses are minimized by a very compact design with very short input and output waveguides. This mount is especially well suited for planar diodes used with microstrip or suspended stripline RF filters.<<ETX>>

600 GHz Imaging Radar with 2 cm Range Resolution

We report the first submillimeter-wave imaging system that has radar ranging capabilities. By frequency-modulating the K-band synthesizers of a single-pixel 630 GHz scanning vector imager and applying a distortion compensation technique in software, we have achieved a range resolution of approximately 2 cm for targets at a range of several meters. Relief images of test objects obtained with our system demonstrate that three-dimensional THz imaging of scanned targets is feasible using a room temperature, all solid-state approach.

Improved 240-GHz subharmonically pumped planar Schottky diode mixers for space-borne applications

Low-noise broad intermediate frequency (IF) band 240-GHz subharmonically pumped planar Schottky diode mixers for space-borne radiometers have been developed and characterized. The planar GaAs Schottky diodes are fully integrated with the RF/IF filter circuitry via the quartz-substrate upside-down integrated device (QUID) process resulting in a robust and easily handled package. A best double-sideband-mixer noise temperature of 490 K was achieved with 3 mW of local-oscillator power at 2-GHz IF. Over an IF band of 1.5-10 GHz, the noise temperature is below 1000 K. This state-of-the-art performance is attributed to lower parasitic capacitance devices and a low-loss waveguide circuit. Device fabrication technology and the resulting RF mixer performance obtained in the 200-250-GHz frequency range will be described.