David S. Kurtz

Submillimeter-wave sideband generation using varactor Schottky diodes

A submillimeter-wave sideband generator based on phase modulation is described. The sideband generator consists of a whisker-contacted Schottky varactor mounted in a reduced-height waveguide. A microwave pump signal modulates the reflection coefficient the varactor presents to an incident submillimeter-wave carrier. The circuit discussed in this paper has exhibited a carrier-to-single sideband conversion loss of 14 dB and an output power of 55 /spl mu/W at 1.6 THz.

Terahertz sources and detectors

Through the support of the US Army Research Office we are developing terahertz sources and detectors suitable for use in the spectroscopy of chemical and biological materials as well as for use in imaging systems to detect concealed weapons. Our technology relies on nonlinear diodes to translate the functionality achieved at microwave frequencies to the terahertz band. Basic building blocks that have been developed for this application include low-noise mixers, frequency multipliers, sideband generators and direct detectors. These components rely on planar Schottky diodes and integrated diode circuits and are therefore easy to assemble and robust. They require no mechanical tuners to achieve high efficiency and broad bandwidth. This paper will review the range of performance that has been achieved with these terahertz components and briefly discuss preliminary results achieved with a spectroscopy system and the development of sources for imaging systems.

A micromachined 585 GHz Schottky mixer

Standard semiconductor fabrication processes have been used to form waveguide components for the submillimeter wavelength range. A 585 GHz fundamentally pumped Schottky mixer with record performance demonstrates this technology. It consists of an etched silicon horn, a diced waveguide, and a lithographically formed microstrip channel for the diode circuit. The block dimensions are precisely controlled and extremely sharp. The measured mixer noise temperature is 1200 K (DSB), which is equivalent to the best result obtained with standard metal machining.

Development of multiplier based sources for up to 2 THz

Nonlinear diodes are used to extend the functionality of microwave electronics into the terahertz frequency band. Systems using this technology achieve useful transmitter power and receiver sensitivity throughout the frequency range from about 100 GHz through several terahertz. This talk reviews this nonlinear diode technology, with emphasis on the ongoing research and development that will enable this terahertz technology to transition from a tool for basic science into commercial systems suitable for broader applications. Emphasis is placed on terahertz sources. Three recent VDI sources are described.

Submillimeter-wave sideband generation using a planar diode array

A 36 element array of planar Schottky diodes is used to mix the output of a CO/sub 2/ pumped far infrared laser with a 1-20 GHz microwave source to generate tunable sidebands at 1.6 THz. The double sideband power was measured by heterodyne detection with a 1T23 corner cube Schottky diode for a calculated output power of 5.9 /spl mu/W with a 28 dB conversion loss.

A high pulsed power frequency doubler to 190 GHz

A high-pulsed-power varactor doubler has been developed to efficiently transfer the power from a pulsed 95 GHz IMPATT oscillator to the 190 GHz band. The frequency doubler uses waveguide based embedding structures employing high-thermal conductivity circuits and Virginia Diodes, Inc. (VDI) proprietary GaAs Schottky varactor diode technology. The embedding circuitry is based on a balanced doubler topology that delivers state-of-the-art power and fixed-tuned bandwidth at millimeter-wave frequencies. The waveguide structure is modified to provide ample room for the large diode arrays while simultaneously blocking propagation of the unwanted TM modes. Special attention was given to maximizing the heat conduction pathways in the embedding structure to minimize heating of the varactor devices.

Millimeter-wave sideband generation using varactor phase modulators

A sideband generator based on phase modulation is presented. The sideband generator consists of a Schottky varactor diode mounted in a WR-10 waveguide tuned resonant circuit. A microwave pump signal modulates the phase of the reflection coefficient the circuit presents to an incident millimeter-wave signal. This proof-of-principle circuit has shown a sideband conversion loss of 9 dB and bandwidth of nearly 10% at 80 GHz. These results represent significant improvements over the performance of sideband generators based on resistive mixing in corner-cube mounts.

Frequency domain terahertz spectroscopy

A frequency domain terahertz spectroscopy system was developed to operate from 210-270 GHz. A multiplier chain ending with a broadband sextupler provides several milliwatts of power, and a heterodyne receiver measures the transmission through materials under test with better than 0.5% accuracy.

Integrated terahertz transmitters and receivers

Through a US Army sponsored SBIR project we are developing terahertz components based on integrated GaAs Schottky diodes for the frequency range from 200 - 700 GHz. These new components are inherently broadband and therefore require no mechanical tuners. Rather, they can be electronically swept across significant frequency bands and are therefore useful for chemical and biological spectroscopy. This talk will focus on our demonstration of a terahertz frequency Transmit / Receive capability which may be of use for CB detection and secure communications.

Solid-state sources, receivers and systems for plasma diagnostics and THz frequency extenders for VNAs

Summary form only given.Higher power and more frequency agile sources and receivers are required for plasma diagnostics, both for fusion research and for industrial process control. VDI generates power in the 100 GHz - 2 THz frequency range through the use of microwave power amplifiers and frequency multipliers based on GaAs Schottky barrier diodes. For example, VDI has demonstrated sources with -450 mW near 100 GHz and several microwatts at 1.9THz. For receivers, VDI has developed both direct diode detectors and heterodyne receivers. The direct detectors operate well across complete waveguide bands and achieve NEP of about lxl 0~Iumleuro WIcirc/Hz at 300GHz and lxl0-10 WIcirc/Hz at 1.5 THz. Heterodyne receivers are achieved with Schottky barrier mixer diodes and their room temperature sensitivity matches the best that has been achieved. These receivers operate across broad frequency bands and VDI is developing full waveguide band systems throughout this frequency range. VDI is also using these components to develop advanced systems. These include a modular extension system for Vector Network Analyzers that will cover the full frequency range for 140 GHz to over 1 THz. This system will use a set of VDI multipliers and mixers in a re configurable fashion to cover each of the waveguide bands in this frequency range. For example, the WR2.2 (330-500GHz) system uses a source consisting of a W-Band amplifier and two doublers. Removing the last doubler yields the WR5.1 (140-220GHz) source. Replacing this doubler with a tripler yields the WR1.5 (500-750GHz) band. A similar modular system is used for the receiver local oscillators. The prototype WR2.2 system has yielded a dynamic range of ~90dB across the entire waveguide band. Similar performance is expected to 1 THz. This talk will focus on the performance and operation of these high frequency components and systems and their potential applications for plasma diagnostics. Particular emphasis will be placed on the VNA extenders, which will be invaluable tools for all manner of laboratory measurements in this frequency range.