Uncooled Photonic Devices Shine
Advances spark new generation of small, sensitive detectors and scanners.
The U.S. Defense Department is developing miniaturized infrared detectors and sensors that do not require bulky cooling systems. These devices will be compact enough to fit in small robotic vehicles and microaircraft or will be manportable. The technology also may improve night vision and missile seeking equipment. Recent advances in physics and materials science are moving these devices from the laboratory to the battlefield.
Ongoing research by the Defense Advanced Research Projects Agency (DARPA), Arlington, Virginia, is producing a number of these new sensing and imaging technologies. DARPA officials speculate that high performance uncooled infrared sensors ultimately could replace most existing cryogenically cooled systems.
Infrared sensor systems that have high operating temperatures exist, but they are not as sensitive as cooled versions because of the sensors’ fundamental thermal noise limitations. These are suitable only for use in small robotic platforms at close range. The goal of the DARPA efforts is to develop devices that will operate near their theoretical limits when at room temperature. Besides increased performance without the need for cooling, these systems would offer increased sensitivity in a smaller aperture size, providing high performance imaging in an extremely small package, DARPA officials say.
Two DARPA programs developing applications for this technology are the steered agile beam (STAB) program and the photonic wavelength and spatial signaling program (PWASSP). Both efforts use quantum physics and new materials applications to create lightweight devices that either perform well despite high thermal noise or are engineered to operate so efficiently that they generate little heat.
The STAB program is an effort to develop miniaturized components that can move a laser beam without heavy mirrors or gimbals, explains program manager Lt. Col. John C. Carrano, USA. The goal is to use all-electronic, solid-state components to steer a laser beam over a 90-degree arc in less than a millisecond from an aperture size of less than 2 centimeters. The system also must support data rates greater than one gigabit per second for communications applications.
Two major applications for beam steering are laser communications and infrared countermeasures. The ability to conduct on-the-move pointing and tracking is a requirement for future communications technologies. Size is important, and researchers are working on miniaturizing the system so that it can conform to an aircraft’s skin or be carried easily by one soldier. Much of this effort is in developing semiconductor technologies for room-temperature operation.
The infrared countermeasures work is being carried out in conjunction with the U.S. Air Force. This research seeks to develop defensive systems for aircraft to defeat incoming heat-seeking missiles by overwhelming their sensors with an infrared laser. Col. Carrano notes that the program has put together a plan for a realistic scenario to demonstrate the technology.
STAB technologies are an improvement over existing mirror and gimbal-based beam steering systems. “Those can weigh as much as 200 pounds—not something aircraft designers really want to consider. We want something that will fit into a package well under 6 pounds and be less than a couple of inches on a side,” Col. Carrano explains.
To achieve this, researchers are concentrating on microelectromechanical systems (MEMS) technologies that operate at low to medium voltages in a compact package. The main physical aspect exploited by the program is called the “super prism” effect. This permits MEMS mirrors to steer a laser beam over a very wide angle—plus or minus 45 degrees in both azimuth and elevation.
Researchers also have developed liquid crystal optical phased arrays. One challenge with creating these systems is making them fast enough to work within the specified millisecond time frame. The systems must be ultraviolet compatible so they will not degrade in sunlight and must be nondispersive because broadband infrared is needed for applications such as infrared countermeasures and laser communications.
The PWASSP seeks to develop signal processing techniques that can simultaneously capture and analyze the spectral and spatial aspects of light. By exploiting all of the attributes of light in a given image, such as polarization and frequency, warfighters can gain additional tactically relevant information, Col. Carrano says. For example, imaging quality or spectral analysis of a hidden vehicle’s engine exhaust plume may be enhanced. This capability could lead to advances in hyperspectral imaging and signal processing systems, although some enabling component technologies must be developed first.
For hyperspectral imaging, sensors would use a broader range of electromagnetic spectrum than is currently possible. The program is examining sensing in the near ultraviolet, visible and short-wave through very long-wave infrared spectrums. Technologies developed by the PWASSP may be used in imaging and sensing systems, in biological and chemical agent detectors, and in laser communications, the colonel says.
Existing high-definition imaging equipment is unsuitable for small unmanned or manportable applications because it must be cooled with liquid nitrogen, which increases its size and weight. In the new approach, infrared devices will not require the use of cooling systems because they rely on nanoscale miniaturization and materials engineering to operate more efficiently, generating less heat. But engineering challenges remain. Col. Carrano notes that it was very difficult to make infrared laser diodes for light sources and detectors that could work at near room temperature.
To find a way for these systems to operate within required parameters, DARPA researchers delved into quantum mechanics. They developed new semiconductor materials for the diodes that exploited quantum principles. Two promising technologies that emerged from this work are quantum dots and quantum cascade lasers.
The colonel describes quantum dots as artificial atoms measuring roughly 50 angstroms across. By using techniques such as molecular beam epitaxy, scientists can engineer specific properties into the atoms to manipulate specific wavelengths of light as they pass through the material. In the PWASSP, quantum dots are manufactured to diminish the dark current normally associated with long-wave infrared detectors while maintaining sensitivity to long-wave infrared light. Although the primary benefit of quantum dots is to produce infrared sensing systems that will perform well while tolerating near room temperatures, they have potential uses in future imaging systems because they can access portions of the spectrum not easily reached with conventional semiconductors, he observes.
Quantum cascade lasers emit beams of infrared light while their electronics operate at near room temperature. To achieve this, the laser diodes are constructed from dozens of nanometer-thick layers of alternating materials. These layers create a structure that permits a single photon to undergo multiple transitions as it passes through the device. In each of those shifts, it emits an infrared photon, creating a cascading effect that greatly increases the beam’s frequency.
By using quantum mechanics to speed the process, this structure overcomes some of the nonradiative processes that slow down photons. “It helps to overcome that friction so that you can get light output in the infrared and do that at higher operating temperatures. Normally, the only way to get rid of the nonradiative process would be to cool the sample down. But you don’t want to do that,” Col. Carrano says.
The program has made a number of breakthroughs. Researchers discovered that quantum dots can perform well at higher temperatures. In addition, the scientists learned to make high-power quantum cascade lasers that are tunable in the infrared spectrum while operating with sufficient power at room temperature. Col. Carrano notes that DARPA scientists stabilized quantum cascade lasers to less than 100 hertz, which permits their use in applications such as highly sensitive chemical detectors where beam stability is essential.
The program also is employing MEMS technology to make miniaturized spectrometers. These devices feature tiny cavities on a detector array. By positioning the cavities up and down the array, the spectrometer can be tuned. Some of the detectors and emitters developed for the program are in the sub-nanometer scale. “By making these artificial structures and atoms, we exploit quantum mechanics to give us what we want,” he says.
Now in its final year, PWASSP has yielded several proof-of-concept devices that could become part of other research. One example is a quantum cascade laser transmitter for a chemical sensor. Other program components could make their way into other follow-on programs such as advanced chemical and explosives sensing systems, Col. Carrano says. However, scientists continue to refine and reduce component size. “A big challenge is coming up with ways to make miniaturized infrared optics. A lot of things don’t like to transmit infrared radiation, so it’s rather hard to do,” he explains.
Additional information on the Defense Advanced Research Project Agency’s photonic wavelength and spatial signaling program and the steered agile beam program is available on the World Wide Web at www.darpa.mil.