Goutam Chattopadhyay

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.

An all-solid-state broad-band frequency multiplier chain at 1500 GHz

We report the results of a high-performance all-solid-state broad-band frequency multiplier chain at 1500 GHz, which uses four cascaded planar Schottky-barrier varactor doublers. The multipliers are driven by monolithic-microwave integrated-circuit-based high electron-mobility transistor power amplifiers around 95 GHz with 100-150 mW of pump power. The design incorporates balanced doublers utilizing novel substrateless and membrane device fabrication technologies, achieving low-loss broad-band multipliers working in the terahertz range. For a drive power of approximately 100 mW in the 88-99-GHz range, the doublers achieved room-temperature peak efficiencies of approximately 30% at the 190-GHz stage, 20% at 375 GHz, 9% at 750 GHz, and 4% at the 1500-GHz stage. When the chain was cooled to 120 K, approximately 40 /spl mu/W of peak output power was measured for 100 mW of input pump power.

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.

Technology, Capabilities, and Performance of Low Power Terahertz Sources

New and emerging terahertz technology applications make this a very exciting time for the scientists, engineers, and technologists in the field. New sensors and detectors have been the primary driving force behind the unprecedented progress in terahertz technology, but in the last decade extraordinary developments in terahertz sources have also occurred. Driven primarily by space based missions for Earth, planetary, and astrophysical science, frequency multiplied sources have dominated the field in recent years, at least in the 2-3 THz frequency range. More recently, over the past few years terahertz quantum cascade lasers (QCLs) have made tremendous strides, finding increasing applications in terahertz systems. Vacuum electronic devices and photonic sources are not far behind either. In this article, the various technologies for terahertz sources are reviewed, and future trends are discussed.

A 540-640-GHz high-efficiency four-anode frequency tripler

We report on the design and performance of a broad-band, high-power 540-640-GHz fix-tuned balanced frequency tripler chip that utilizes four planar Schottky anodes. The suspended strip-line circuit is fabricated with a 12-/spl mu/m-thick support frame and is mounted in a split waveguide block. The chip is supported by thick beam leads that are also used to provide precise RF grounding. At room temperature, the tripler delivers 0.9-1.8 mW across the band with an estimated efficiency of 4.5%-9%. When cooled to 120 K, the tripler provides 2.0-4.2 mW across the band with an estimated efficiency of 8%-12%.

A 1.7-1.9 THz local oscillator source

We report on the design and performance of a /spl times/2/spl times/3/spl times/3 frequency multiplier chain to the 1.7-1.9 THz band. GaAs-based planar Schottky diodes are utilized in each stage. A W-band power amplifier, driven by a commercially available synthesizer, was used to pump the chain with 100 mW of input power. The peak measured output power at room temperature is 3 /spl mu/W at 1740 GHz. When cooled to 120 K, the chain provides more than 1.5 /spl mu/W from 1730 to 1875 GHz and produced a peak of 15 /spl mu/W at 1746 GHz.

A Frequency-Multiplied Source With More Than 1 mW of Power Across the 840–900-GHz Band

We report on the design, fabrication, and characterization of an 840-900-GHz frequency multiplier chain that delivers more than 1 mW across the band at room temperature with a record peak power of 1.4 mW at 875 GHz. When cooled to 120 K, the chain delivers up to 2 mW at 882 GHz. The chain consists of a power amplifier module that drives two cascaded frequency triplers. This unprecedented output power from an electronic source is achieved by utilizing in-phase power-combining techniques. The first stage tripler uses four power-combined chips while the last stage tripler utilizes two power-combined chips. The source output was analyzed with a Fourrer transform spectrometer to verify signal purity.

200, 400 and 800 GHz Schottky diode "substrateless" multipliers: design and results

Several sub-millimeter doubler circuits have been designed and built using a new fabrication technology. To reduce the RF losses in the passive circuitry, the substrate under the transmission lines is etched away, leaving the metal suspended in air held by its edges on a GaAs frame. This allows the circuit to be handled and mounted easily, and makes it very robust. To demonstrate this technology, broadband balanced planar doublers have been built and tested at 400 GHz. The next generation 200, 400 and 800 GHz doublers with improved performance are also discussed. The 368-424 GHz circuits were measured and achieved 20% efficiency at 387 GHz. The 3 dB bandwidth of the fix-tuned doubler is around 9%. The maximum output power measured is around 8 mW and drops down to 1 mW at 417 GHz. This represents the highest frequency waveguide based planar doubler to date in the literature.

A FULL-HEIGHT WAVEGUIDE TO THIN-FILM MICROSTRIP TRANSITION WITH EXCEPTIONAL RF BANDWIDTH AND COUPLING EFFICIENCY

We describe a waveguide to thin-film microstrip transition for high-performance submillimetre wave and teraherz applications. The proposed constant-radius probe couples thin-film microstrip line, to full-height rectangular waveguide with better than 99% efficiency (VSWR ≤ 1.20) and 45% fractional bandwidth. Extensive HFSS simulations, backed by scale-model measurements, are presented in the paper. By selecting the substrate material and probe radius, any real impedance between ≈ 15-60 Ω can be achieved. The radial probe gives significantly improved performance over other designs discussed in the literature. Although our primary application is submillimetre wave superconducting mixers, we show that membrane techniques should allow broad-band waveguide components to be constructed for the THz frequency range.