Uncooled photonic devices shine

Signal, Jan 2003 by Kenyon, Henry S

Special Report

Advances spark new generation of small, sensitise 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, solidstate 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.