A new broadband hybrid circulator has been developed for use at millimeter-wave (MMW) frequencies. The hybrid circulator comprises a Faraday rotator and an orthomode transducer (OMT) which work together to produce the circulator function. The initial hybrid circulator prototypes employ an asymmetric OMT and are designed to have 20 dB isolation over 24% fractional bandwidths. However, the architecture can be modified to cover full rectangular waveguide bands by using a symmetric OMT. The hybrid circulator bandwidth does not degrade at the higher MMW frequencies like the Y-junction. For example, a well-tuned Y-junction circulator designed to operate at 60 GHz has a 20 dB fractional bandwidth near 10%. But the bandwidth drops to 2% for a Y-junction circulator operating at 160 GHz. Test data are presented for two hybrid circulator prototypes covering the bands 53–68 GHz in WR-15 and 150–190 GHz in WR-5. The measured insertion loss of the hybrid circulator is typically less than 0.8 dB over the 53–68 GHz band and near 2 dB over the 150–190 GHz band. The measured VSWR is less than 1.4:1 in both bands. The hybrid circulator can be scaled to at least 400 GHz.

Radiological protection is a matter of concern for members of the public and thus national authorities are more likely to trust the quality of radioactivity data provided by accredited laboratories using common standards. Normative approach based on international standards aims to ensure the accuracy or validity of the test result through calibrations and measurements traceable to the International System of Units. This approach guarantees that radioactivity test results on the same types of samples are comparable over time and space as well as between different testing laboratories. Today, testing laboratories involved in radioactivity measurement have a set of more than 150 international standards to help them perform their work. Most of them are published by the International Standardization Organization (ISO) and the International Electrotechnical Commission (IEC). This paper reviews the most essential ISO standards that give guidance to testing laboratories at different stages from sampling planning to the transmission of the test report to their customers, summarizes recent activities and achievements and present the perspectives on new standards under development by the ISO Working Groups dealing with radioactivity measurement in connection with radiological protection.

Fourth International Symposium on Space T'erahertz Technology Page 377 GaAs SCHOTTKY DIODES FOR Ilk MIXING APPLICATIONS P.A.D. Wood, W.C.B. Peatman, D.W. Porterfield, and T.W. Crowe GaAs Schottky diodes are currently the most sensitive heterodyne receiver elements for applications above 1 THz which require high spectral resolution and broad bandwidth. Diode performance can be further improved by optimizing parameters such as anode diameter and doping concentration. Based on previous experimental and theoretical research, improved diodes have been fabricated in the Semiconductor Device Laboratory of the University of Virginia. These diodes have an epitaxial layer doping density of 1x10 18 cm 3 and zero-bias junction capacitance as low as 0.25 IF. Diode performance was evaluated using video responsivity, mixer noise temperature and mixer conversion loss. These measurements have confirmed earlier predictions that for higher frequencies higher doping densities and smaller anode diameters must be used. Several current and planned NASA programs require high sensitivity heterodyne receivers for the frequency range from 600 GHz through 2.5 THz. These include both atmospheric and radio astronomy missions. The Microwave Limb Sounder (MLS), which will be flown on the Earth Observing System, will have receivers at 215 GHz, 440 GHz, 640 GHz and 2.5 THz for atmospheric measurements related to ozone depletion [1]. A planned astronomy mission (SMIM) will require receivers covering the spectrum from 400 GHz through 1.2 THz [2]. Although superconducting technology is being pursued in these frequency ranges, and is in fact better than Schottky technology by at least a factor of two at 500 Gliz [3,4,5], it is not clear when, or if, the SIS junctions will be extended to THz frequencies. Also, Schottky diodes are much more convenient for many applications due to their ability to operate at any temperature in the range Page 378 Fourth International Symposium on Space Terahertz Technology from below 20K to above 300K. In the frequency range above about 600 GHz the only receivers presently available for these missions are based on GaAs Schottky mixer diodes. Thus, any improvement in the performance of these diodes can have a great impact on these NASA programs. GaAs Schottky diodes were first used in heterodyne receivers at microwave and millimeter wavelengths. In the 1970s it was shown that diodes with low epitaxial layer doping density can be extremely sensitive mixer elements when operated at low temperature [6]. This led to the development of greatly improved Schottky receivers at millimeter wavelengths, including a system that yielded a mixer noise temperature of only 35K DSB at about 100 GHz (<8 hv/k) [7]. It is remarkable that nearly ten years later this result is quite comparable to the best results obtained with SIS receivers at this frequency, particularly when one considers that the Schottky receiver need only be cooled to 20K. Based on this success, there has been a large bias toward using low doped diodes at all frequencies. However, recent investigations of submillimeter wavelength receivers have shown that this may not be the optimum diode design. In 1989 Harris et. al published a detailed study of noise in an 800 GHz Schottky receiver [8]. This study was later extended to consider the diode noise [9]. It was shown that at 800 GHz the receiver noise was not dominated by the diode shot noise, as was the case at millimeter wavelengths, but rather by hot-electron noise. Thus, it was proposed that diodes with higher doping density and smaller anode diameter would reduce receiver noise by lessening the hot-electron noise. Other studies have led to similar conclusions. For example, Bhapkar investigated the series impedance of the Schottky diodes by a finite difference technique that included all of the most important phenomena at THz frequencies [10]. This work showed clearly that higher doping density and smaller anodes were required to increase the cut-off frequency (related to the RsCjo product) to the point where THz performance was optimized. At the same time several empirical studies were indicating that such diodes always performed better at high frequency than the older, lower doped diodes. Thus, in 1991 we decided to fabricate diodes with epitaxial layer doping densities as high as 1x10' 8 cm and anode diameters substantially below one micron. The fabrication and RF performance of these diodes is described in the remainder of this paper.

The goal of nuclear forensics is to establish an unambiguous link between illicitly trafficked nuclear material and its origin. The Los Alamos National Laboratory (LANL) Nuclear Materials Signatures Program has implemented a graded "conduct of operations" type analysis flow path approach for determining the key nuclear, chemical, and physical signatures needed to identify the manufacturing process, intended use, and origin of interdicted nuclear material. This analysis flow path includes both destructive and non-destructive characterization techniques and has been exercized against different nuclear materials from LANL's special nuclear materials archive. Results obtained from the case study will be presented to highlight analytical techniques that offer the critical attribution information.

The performance of terahertz sources based on frequency multipliers continues to improve in terms of power, maximum frequency and frequency agility. This talk will describe the range of performance that is possible with this technology; Including narrow-band sources that generate greater than 400mW above 100 GHz and broadly tunable sources that generate 10's of microwatts above 1THz. This talk will also consider the unique source characteristics required by the wide range of applications being explored, and their Impact on source performance and design. This includes the need for power modulation and pulsing, rapid frequency sweeping, achieving highly stable and phase locked signals, as well as phase locking the source to a heterodyne receiver

Higher power and more frequency agile sources are required for plasma diagnostics, both for fusion research and for industrial process control. For example, reflectometers are planned for the ITER program that must generate of order 150 milliwatts of transmit power that can be rapidly scanned from 75 GHz through 230 GHz. In this frequency band the most successful solid-state source technology uses diode based frequency multipliers to extend the range of microwave amplifiers. For example, VDI has demonstrated a 300 mW source at 100 GHz that uses a high-power amplifier at 25 GHz and a cascaded pair of varactor doublers. The varactor doublers rely on a voltage-variable capacitance and large diode breakdown voltage to achieve large conversion efficiency and power handling. Unfortunately, these same device characteristics result in the very restricted tuning band of the doublers, and the typical 3dB bandwidth is of order 10% of the center frequency. Using this style of doubler would require an array of nearly a dozen sources to achieve the frequency bandwidth desired for ITER (75-230 GHz). This is technologically challenging and cost prohibitive. Additionally, the power available is reduced at higher frequencies and the requirement for rapid frequency tuning, even in these smaller tuning bands, often creates reliability problems. VDI is investigating the causes of these problems and possible solutions. Specifically, the performance of varactor and varistor frequency multipliers is under study, with the goal of improving the performance of cascaded multiplier chains and thereby reducing the number of channels required to cover the desired frequency range. Further, the reliability of these systems is being investigated, with the goal of understanding failures that occur during rapid frequency tuning and developing methods to alleviate these problems. Finally, VDI is developing prototype sources based on the results of these studies for use throughout the 75-300 GHz frequency band. This talk will focus on the performance and operation of these all-solid-state sources of millimeter-wave power, with emphasis on the improved power handling of the multipliers, efforts to increase tuning bandwidth and techniques that have been developed to achieve rapid frequency tuning with high system reliability. The performance of prototype systems throughout the frequency band will be presented at the conference.

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.

Design and experimental analysis of high-power and high-efficiency frequency triplers to the 220 GHz and 440 GHz bands are presented. Test data for the 220 GHz tripler show 23 mW output power with 16% efficiency. Test data for the 440 GHz tripler show 13 mW output power with 12% efficiency. The 3 dB bandwidth for both triplers is about 7%. This performance is comparable to the best reported in the literature at these frequencies. There are no mechanical tuners and thus the triplers may be electronically swept to any frequency in the band. The triplers comprise a waveguide housing, a pair of quartz microstrip circuits and a Virginia Diodes (VDI) GaAs Schottky varactor chip. The simple circuit topology makes it easy to assemble the multipliers and bias the varactors. A version to 800 GHz has been designed and should be available for testing in 2007. The design is scalable to frequencies above 1 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 will review 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 broader applications will also be discussed.

The terahertz region of the electromagnetic spectrum has unique properties that make it especially useful for imaging and spectroscopic detection of concealed weapons, explosives and chemical and biological materials. However, terahertz energy is difficult to generate and detect, and this has led to a technology gap in this frequency band. Nonlinear diodes can be used to bridge this gap by translating the functionality achieved at microwave frequencies to the terahertz band. Basic building blocks include low-noise mixers, frequency multipliers, sideband generators and direct detectors. These terahertz components rely on planar Schottky diodes and recently developed integrated diode circuits make them easier to assemble and more robust. The new generation of terahertz sources and receivers requires no mechanical tuning, yet achieves high efficiency and broad bandwidth. This paper reviews the basic design of terahertz transmitters and receivers, with special emphasis on the recent development of systems that are compact, easy to use and have excellent performance.

The terahertz region of the electromagnetic spectrum, spanning from 100 GHz through 10 THz, is of increasing importance for a wide range of scientific, military and commercial applications. This interest is spurred by the unique properties of this spectral band and the very recent development of convenient terahertz sources and detectors. However, the terahertz band is also extremely challenging, in large part because it spans the transition from traditional electronics to photonics. This paper reviews the importance of this frequency band and summarizes the efforts of scientists and engineers to span the "terahertz technology gap." The emphasis is on solid-state circuits that use nonlinear diodes to translate the functionality of microwave technology to much higher frequencies.

A high-pulsed-power varactor doubler hasbeendeveloped to efficiently transfer thepowerfromapulsed 95GHZIMPATT oscillator tothe190GHzband. Thefrequency doubler uses waveguide basedembedding structures employing highthermal conductivity circuits andVirginia Diodes, Inc. (VDI) proprietary GaAsSchottky varactor diode technology. The embedding circuitry isbased onabalanced doubler topology that delivers state-of-the-art powerandfixed-tuned bandwidth atmillimeter-wave frequencies. Thewaveguide structure is modified toprovide ampleroomforthelarge diode arrays while simultaneously blocking propagation oftheunwanted TM modes. Special attention wasgiven tomaximizing theheat conduction pathways intheembedding structure tominimize heating ofthevaractor devices.

A primary challenge in creating the next generation of submillimeter-wave receiver systems for space science is generating the required local oscillator power. Full waveguide band performance and power levels suitable for array receivers are desired. Also, the sources must be suitable for use at remote installations. This means they should be compact, reliable and electronically tunable. This papers presents the recent development and testing of a terahertz LO source that meets these requirements. This source uses GaAs Schottky barrier diodes to frequency multiply the power from a millimeter wave amplifier. The final element in the multiplier chain is a frequency tripler to the WR-0.65 waveguide band, spanning from 1.1 - 1.7 THz. This tripler generates of order ten microwatts of power when pumped with 3mW. The complete x72 active multiplier chain is about six-inches in length and requires only milliwatt power level input. It has been tested to demonstrate excellent spectral purity, frequency and power stability and low noise. The construction of the multiplier chain and the test results are reviewed.

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 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.

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.

The development of compact broadband fixed-tuned receivers at millimeter and submillimeter wavelengths is discussed. These components rely on a fusion between novel integrated planar diode technology and innovative broadband circuit topologies. A 500 GHz InGaAs subharmonic mixer has been demonstrated that has the same sensitivity as a GaAs mixer (T/sub MIX//spl sim/1500 K (DSB)) while using less than half the LO power. In addition, an integrated tripler-mixer has been developed at 180 GHz that exhibits state-of-the-art sensitivity (T/sub MIX//spl sim/550 K (DSB)) using 125 mW of LO power at 30 GHz. The ultimate goal of this research is to develop a compact, highly sensitive radiometer at 874 GHz to meet upcoming critical NASA Earth Science requirements.

Terahertz sources and receivers with exceptional bandwidth, power and efficiency have been demonstrated. These components use integrated diode circuits (no whiskers or mechanical tuners) and are therefore very compact and reliable, thus facilitating a host of new applications in science, medicine and defense.