Imran Mehdi

Fabrication of 200 to 2700 GHz multiplier devices using GaAs and metal membranes

Multiplier device fabrication techniques have been developed to enable robust implementation of monolithic circuits well into the THz frequency range. To minimize the dielectric loading of the waveguides, some circuits are realized entirely on a 3 /spl mu/m thick GaAs membrane with metal beamleads acting as RF probes and DC contact points. Other designs retain some thicker GaAs as a support and handling structure, allowing a membrane of bare metal or thin GaAs to be suspended across an input or output waveguide. Extensive use is made of selective etches, both reactive ion (RIE) and wet chemical, to maintain critical dimensions. Electron beam (e-beam) lithography provides the small contact areas required at the highest frequencies. Planar multiplier circuits for 200 GHz to 2700 GHz have been demonstrated using a variety of metal and GaAs membrane configurations made available by these fabrication techniques.

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.

THz frequency multiplier chains based on planar Schottky diodes

The Herschel Space Observatory (HSO), an ESA cornerstone mission with NASA contribution, will enable a comprehensive study of the galactic and the extra galactic universe. At the heart of this exploration are ultra sensitive coherent detectors for high-resolution spectroscopy. Successful operation of these receivers is predicated on providing a sufficiently powerful local oscillator (LO) source. Historically, a versatile space qualified LO source for frequencies beyond 500 GHz has been difficult if not impossible. This paper will focus on the effort under way to develop, build, characterize and qualify a LO chain to 1200 GHz (Band 5 on HSO) that is based on planar GaAs diodes mounted in waveguide circuits. State-of-the-art performance has been obtained from a three-stage (×2×2×3) multiplier chain that can provide a peak output power of 120 μW (1178 GHz) at room temperature and a peak output power of 190 μW at 1183 GHZ when cooled to 113 K. Implementation of this LO source for the Heterodyne Instrument for Far Infrared (HIFI) one of three instruments on HSO will be discussed in detail.

Performance of a 1.2 THz frequency tripler using a GaAs frameless membrane monolithic circuit

The first ever planar Schottky diode multiplier working over a THz will be presented in this paper. A tunerless 1.2 THz waveguide frequency tripler has been designed, fabricated and tested. The frequency multiplier consists of a 3 micron-thick GaAs frameless-membrane monolithic circuit, mounted in a split waveguide-block, which includes a built-in Picket-Potter horn. The 1.2 THz membrane tripler is driven by a 400 GHz solid-state chain composed of HEMT based power amplifiers followed by two tunerless planar diode frequency doublers. At room temperature, output power up to 80 microwatts was measured at 1126 GHz with a peak-efficiency of 0.9% and a 3 dB bandwidth of about 3.5%. The output power of the multiplier chain increased dramatically with a decrease of the ambient temperature-up to 195 microwatts was measured at 120 K. When further cooled to 50 K the chain delivers power levels as high as 250 microwatts. To the best of our knowledge, this is the first demonstration of a fully planar multiplier chain at these frequencies, along with performance that supercedes current state-of-the-art performance of whisker-contacted sources.

MMIC power amplifiers as local oscillator drivers for FIRST

The Heterodyne Instrument for the Far-Infrared and Sub- millimeter Telescope requires local oscillators well into the terahertz frequency range. The mechanism to realize the local oscillators will involve synthesizers, active multiplier chains (AMCs) with output frequencies from 71 - 112.5 GHz, power amplifiers to amplify the AMC signals, and chains of Schottky diode multipliers to achieve terahertz frequencies. We will present the latest state-of-the-art results on 70 - 115 GHz Monolithic Millimeter-wave Integrated Circuit power amplifier technology.

THz local oscillator sources: Performance and capabilities

Frequency multiplier circuits based on planar GaAs Schottky diodes have advanced significantly in the last decade. Useful power in the >1 THz range has now been demonstrated from a complete solid-state chain. This paper will review some of the technologies that have led to this achievement along with a brief look at future challenges.

Heterodyne radiometer development for the Earth Observing System Microwave Limb Sounder

NASAs Mission to Planet Earth attempts to address issues related to environmental change through extensive scientific investigation and global monitoring. As part of this effort, the Earth Observing System Microwave Limb Sounder (EOS-MLS), Joe W. Waters principal investigator, was proposed and is currently in development. The Submillimeter-Wave Radiometer Development group at JPL along with collaborators at the Rutherford Appleton Laboratory in the United Kingdom and a small number of US laboratories are developing space-borne radiometer components to satisfy the detection requirements for EOS-MLS from 200 to 650 GHz with possible extension up to 2.5 THz (119 micrometers ). This conference paper summarizes the development that has been ongoing, with emphasis on the millimeter- and submillimeter-wave mixers. Detailed design and performance data for a subharmonically pumped antiparallel-pair planar-diode mixer are presented including computational simulations and measured mixer noise and conversion loss at 215 and 640 GHz. Results from a modest test program comparing the performance at 215 GHz of planar GaAs antiparallel-pair mixer diodes, planar In53Ga47As devices, GaAs planar-doped-barrier diodes and a GaAs millimeter-wave integrated circuit (MMIC) mixer are also presented. Finally, current and future development efforts in the areas of submillimeter-wave local oscillators, integrated planar-diode mixers, IF amplifiers, and THz radiometers are outlined.

Development of 200 GHz to 2.7 THz multiplier chains for submillimeter-wave heterodyne receivers

Several astrophysics and Earth observation space missions planned for the near future will require submillimeter-wave heterodyne radiometers for spectral line observations. One of these, the Far InfraRed and Submillimeter Telescope will perform high-sensitivity, high-resolution spectroscopy in the 400 to 2700 GHz range with a seven channel super- conducting heterodyne receiver complement. The local oscillators for all these channels will be constructed around state-of-the-art GaAs power amplifiers in the 71 to 115 GHz range, followed by planar Schottky diode multiplier chains. The Jet Propulsion Laboratory is responsible for developing the multiplier chains for the 1.2, 1.7, and 2.7 THz bands. This paper will focus on the designs and technologies being developed to enhance the current state- of-the-art, which is based on discrete planar or whisker contacted GaAs Schottky diode chips mounted in waveguide blocks. We are proposing a number of new planar integrated circuit and device topologies to implement multipliers at these high frequencies. Approaches include substrateless, framed and frameless GaAs membrane circuitry with single, and multiple planar integrated Schottky diodes. Circuits discussed include 200 and 400 GHz doublers, a 1.2 THz tripler and a 2.4 THz doubler. Progress to date, with the implications of this technology development for future Earth and space science instruments, is presented.

Radiometer-on-a-chip: a path toward super-compact submillimeter-wave imaging arrays

A novel approach for submillimeter-wave heterodyne imaging arrays is presented in this paper. By utilizing diverse technologies such as GaAs membrane based terahertz diodes, wafer bonding, bulk Si micromachining, micro-lens optics, and CMOS 3-D chip architectures, a super-compact low-mass submillimeter-wave imaging array is envisioned. A fourwafer based silicon block for a working W-band power amplifier MMIC is demonstrated. This module drastically reduces mass and volume associated with metal block implementations without sacrificing performance. A path towards super compact array receivers in the 500-600 GHz range is described in detail.

Development of millimeter and submillimeter-wave local oscillator circuits for a space telescope

FIRST (Far InfraRed and Submillimeter Telescope) is a European science mission that will perform photometry and spectroscopy in the 80 - 670 micrometers range. The proposed heterodyne instrument for FIRST is a seven-channel receiver, which combines the high spectral resolving capability (0.3 - 300 km/s) of the radio heterodyne technique with the low noise detection offered by superconductor-insulator- superconductor and hot electron bolometer mixers. It is designed to provide almost continuous frequency coverage from 480 - 2700 GHz. The Jet Propulsion Laboratory is responsible for developing and implementing the local oscillator sources for the 1200 - 2700 GHz mixers. The present state-of-the-art approach for millimeter-wave multipliers, based on waveguide blocks and discretely mounted devices, becomes harder and harder to implement as the frequency range is extended beyond 300 GHz. This talk will focus on the technology that is being developed to enhance and extend planar integrated Schottky devices and circuits to meet mission local oscillator requirements. The baseline approach is to use GaAs power amplifiers from 71 to 115 GHz followed by a series of planar Schottky diode varactor multiplier stages to generate the required LO signal. The circuits have to be robust, relatively easy to assemble, and must provide broad fix-tuned bandwidth. A number of new technology initiatives being implemented to achieve these goals will be discussed. Approaches include quartz-based and substrate-less diode circuitry and integrated GaAs membrane technology. Recent results and progress-to-date will be presented.