Ken B. Cooper

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.

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.

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.

Integrated 200–240-GHz FMCW Radar Transceiver Module

We present a 220-GHz homodyne transceiver module intended for frequency modulated continuous wave radar applications. The RF transceiver circuits are fabricated on 3-μm-thick GaAs membranes, and consist of a Schottky diode based transmitter frequency doubler that simultaneously operates as a sub-harmonic down-converting mixer. Two circuits are used in a balanced configuration to improve the noise performance. The output power is > 3 dBm over a 40-GHz bandwidth (BW) centered at 220 GHz, and the receiver function is characterized by a typical mixer conversion loss of 16 dB. We present radar images at 4-m target distance with up to 60-dB dynamic range using a 30-μs chirp time, and near-BW-limited range resolution. The module is intended for applications in high-resolution real-time 3-D radar imaging, and the unit is therefore designed so that it can be assembled into 1-D or 2-D arrays.

Submillimeter-Wave Radar: Solid-State System Design and Applications

For decades, the principal role of microwave engineering techniques in the submillimeter (submm)-wave, or terahertz (THz), regime, spanning about 300 GHz-3 THz, has been to optimize the performance of components and systems used in molecular spectroscopy measurements for astronomy, earth science, and plasma diagnostics [1]. THz applications beyond spectroscopy have been much slower to develop. Ultrahigh bandwidth communication at THz frequencies may have the most powerful market forces to support it, but no systems have been deployed beyond the prototype stage, likely because of the unavailability of commercial submm-wave components, challenges with integrating them with existing communications hardware, and the often severe atmospheric attenuation.

A High-Power 105–120 GHz Broadband On-Chip Power-Combined Frequency Tripler

We report on the design, fabrication and characterization of a high-power and broadband 105-120 GHz Schottky diode frequency tripler based on a novel on-chip power combining concept that allows superior power handling than traditional approaches. The chip features twelve anodes on a 50 μm thick GaAs substrate. At room temperature, the tripler exhibits a 17% 3 dB bandwidth and a ~ 30% peak conversion efficiency for a nominal input power of around 350-400 mW, and ~ 20% efficiency for its maximum operational input power of 800-900 mW. This tripler can deliver maximum power levels very close to 200 mW. The on-chip power-combined frequency tripler is compared with a traditional tripler designed for the same band using the same design parameters.

A Silicon Micromachined Eight-Pixel Transceiver Array for Submillimeter-Wave Radar

An eight-pixel transceiver array for operation in a 340 GHz imaging radar is presented. Silicon micromachining is applied to fabricate the submillimeter-wave front-end components to increase the density and uniformity of the array while lowering the cost compared to metal machining. Performance comparable with discrete metal machined housings was achieved with the 340 GHz transmitter nominally producing 0.5 mW and the mixers having a DSB noise temperature of 2000 K with a conversion loss of 8 dB. Radar performance is primarily limited by the isolation of the hybrid coupler, which is typically 28 dB, but excellent imaging performance is still achieved and improvements in penetration compared to higher frequency imaging radars is demonstrated.