DARPA researchers have created the world’s first solid state receiver to demonstrate gain at 0.85 terahertz (THz). This is the latest breakthrough in the DARPA THz Electronics program in its quest for transistor-based electronics that will enable electronic capabilities at THz frequencies. This represents progress toward the second major technical milestone on the way to 1.03 THz integrated circuits. Previous milestones included demonstrations at 0.67 THz. Operating at these high frequencies enables a host of DoD electronics capabilities such as advanced communication and sensor systems.
“Realizing circuits at 0.85 THz is a remarkable achievement for the program and is the latest success from a long-term investment in frequency-scaled RF transistors,” explained John Albrecht, DARPA program manager. “The ability to coherently process signals at 0.85 THz provides a means to generate and radiate the high frequency signals needed for applications such as DARPA’s Video Synthetic Aperture Radar (ViSAR) program. VISAR seeks to develop and demonstrate a targeting sensor which operates through clouds as effectively as today’s infrared (IR) sensors operate in clear weather. This revolutionary advance would give U.S. warfighters an advantage in an especially challenging portion of the RF spectrum.”
The objective of the Terahertz (THz) Electronics program is to develop the critical device and integration technologies necessary to realize compact, high-performance electronic circuits that operate at center frequencies exceeding 1.0 THz. The program will focus on the developments of two critical THz technical areas.
Imaging, radar, spectroscopy, and communications systems operating in the millimeter wave sub-MMW frequency bands have been elusive due to a lack of effective means to generate, detect, process, and radiate the necessary high frequency signals. In order to control and manipulate radiation in this especially challenging portion of the RF spectrum, electronics must be developed that can operated at frequencies past 1 THz.
The sub millimeter wave (sub-MMW) frequency band begins at frequencies above 300 GHz where the wavelengths become less than 1 mm. Until recently, active electronics using solid-state technologies were unable to access sub-MMW frequencies directly due to insufficient transistor performance. The compromise electronic option was to use frequency conversion to multiply circuit operating frequencies up from millimeter wave frequencies. Such an approach limited the output power level of the devices and the achievable signal-to-noise ratio. It also restricted the devices to relatively large sizes in terms of footprint and weight. These limitations and restrictions prevented widespread implementation and the subsequent exploitation of the sub-MMW frequency band. The enabling technology necessary to exploit the sub-MMW band is monolithic microwave integrated circuits (MMICs) that will operate up to THz frequencies. These THz MMICs or TMICs, require THz transistors with maximum oscillation frequencies (fmax) well above 1 THz.
Terahertz Transistor Electronics. The program will aggressively develop multi-THz InP HBT and InP HEMT transistor technologies to enable TMICs. In addition, THz low-loss inter-element interconnect and integration technologies will also be developed to build compact THz transmitter and receiver modules.
Terahertz High Power Amplifier Modules. Compact, micromachined vacuum electronics devices will be developed to produce a significant increase of output power at frequencies beyond 1.0 THz and to radiate this energy at an antenna.
The success of the THz Electronics program will lead to revolutionary applications by enabling coherent THz processing techniques such as THz imaging systems, sub-MMW, ultra-wideband, ultra-high-capacity communication links and sub-MMW, single-chip widely-tunable synthesizers for explosive detection spectroscopy.
Video Synthetic Aperture Radar
DARPA’s Video Synthetic Aperture Radar (ViSAR) program seeks to develop and demonstrate an Extremely High Frequency (EHF) targeting sensor which operates through clouds as effectively as today’s infrared (IR) sensors operate in clear weather.
“The goal is a synthetic aperture radar (SAR) that provides high-resolution, full-motion video to engage maneuvering ground targets through clouds or in the clear, without having to change tactics, techniques and procedures,” said Bruce Wallace, DARPA program manager. “Ultimately, we intend to demonstrate a cloud-penetrating EHF sensor in a moveable gimbal that could be mounted on a variety of aerial platforms.”
DARPA seeks technology proposals in flight-worthy electronics, including power amplifiers and integrated receiver and exciters that are small enough to fit easily aboard aircraft. Another key proposal area is the development of new algorithms which could exploit the features of this sensor technology.
“We’re looking for proposers with advanced expertise in scene simulation software to simulate realistic synthetic EHF radar data sets,” Wallace said. “We anticipate that the system developer will use these raw data sets to test image formation, autofocus, detection and geolocation algorithms.”
The ViSAR system expects to create SAR images of the background at frame rates greater than currently available. In addition, the system should have Ground Moving Target Indicator (GMTI) capability to detect moving targets and reposition their returns in the correct location within the scene. The GMTI processing is done in parallel with SAR processing.
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