Advanced Imaging Sensors 1. (Slide) 2. Imaging Systems - DoD Requirements Although DoD target acquisition needs are tailored to particular missions, there is a generic list of needs. Longer range is almost always desirable but with the addition of the ability to identify the target. The target identification capability, which allows the system user to discriminate friend from foe before engagement, has proven to be critical in complex battlefield scenarios. The need to direct weapons to a specific aim point on the target will also be important, not only for lethality, but to reduce collateral damage. The user must also have knowledge of weapon effectiveness, via imaging sensors to assess damage. Finally, imaging sensors provide the eyes for the robotics and extremely small air vehicles being developed for future battlefield situations. In addition to the surveillance mission, these sensors may be required to perform intricate tasks in difficult environments, necessitating the use of high quality sensing in an extremely small package. 3. Need for Precision Targeting Battlefield targeting involves rapid response in complex scenarios. Targets are often hidden to match the background and target signatures are suppressed. The target system must search wide areas, make decisions rapidly, and act decisively. 4. Advanced Imaging Sensors - Objectives Advanced Imaging Sensors addresses the technology required to add an affordable imaging capability to a wide range of defense systems and to develop new imaging sensors for future platforms, especially small robotic vehicles and micro-air vehicles. Size weight and power are major considerations for all DoD systems, but especially important for the unmanned systems. These systems do not have the space or power to integrate the cryogenic cooling currently used in most infrared systems. The micro-vehicles require sensors that operate at or near room temperature, consuming minimum power and integrated into small, compact packages. This focuses the uncooled infrared technology development. The ultimate goal of the development is to achieve uncooled sensors operating close to their theoretical limit. If this were achieved, this significantly enhanced sensitivity can be traded for smaller aperture, providing high performance imaging in an extremely small package. A high performance uncooled infrared sensor could replace most of the existing cryogenically cooled sensors, at substantailly reduced cost, weight and power. In addition, the targeting capability of the uncooled infrared sensor can be significantly enhanced with use of another sensor for high resolution, precision targeting sensor. An Uncooled or thermoelectrically cooled short wave infrared sensor, designed to provide range information at each pixel, generates a three dimensional image. The three dimensional imaging sensor assists in identifying targets and selecting precise aim-points. This sensor suite adds high imaging performance in a compact, low power package to many DoD systems. 5. Why Uncooled IR? Currently, infrared sensors, with response in the eight to twelve micron spectral region, operate at a temperature of 77 degrees Kelvin (or less) and must be packaged in a vacuum envelope or dewar. A mechanical refrigerator maintains the detector at a stable operating temperature. The requirements for refrigeration and packaging add substantially to the size and cost of the detector assembly. The cooling is necessary to achieve the sensitivity required for long range detection and recognition of targets. While not at the same level of performance of the cooled sensors, the uncooled has the potential to meet performance objectives of many DoD systems. In addition, the uncooled devices operate at or near room temperature, and are packaged in a flat-pack assembly, similar to integrated circuits. Currently, a battery operated thermoelectric temperature stabilization maintains the uncooled infrared detector array at a stable operating point. Research is underway to eliminate the temperature stabilization requirement, further reducing size weight and power. 6. Uncooled IR Applications These are several DoD and commercial applications of uncooled infrared sensors. The current technology meets the needs of some DoD and most commercial applications. The requirements for night driving sights, hand-held imaging and rifle sights are met with the current technology, defined as arrays of 240x320 elements with a two mil pixel having thermal sensitivity of 50 to 100 milli-Kelvin. More advanced DoD applications require higher density pixels of approximately one square mil in area, with the ability to detect temperature differences much less than 50 milli-Kelvin. This higher sensitivity and pixel density substantially expands the application base of uncooled infrared. With this performance, many of the cryogenically cooled sensor systems can be replaced with uncooled. 7. Uncooled IR Pay-off The uncooled infrared provides advantages in several DoD systems. It has the potential to provide advanced night vision in new applications, and to dramatically expand imaging capability in existing systems. One example is the missile seeker area. Uncooled offers the potential to significantly reduce the cost of missile seeker sensors, and even to add new capability, enhancing overall system effectiveness. For systems requiring both target acquisition and tracking sensors, a large area uncooled array may permit target acquisition through the missile seeker, eliminating the need for an ancillary target acquisition sight and resulting in significant system weight reduction. Another important area where imaging is essential is in robotics and unmanned air vehicles. The uncooled infrared provides the only imaging sensor compatible with system size, weight and power requirements. The uncooled IR micro-sensors are a step toward a new class of infrared imaging sensors, meeting the needs of unmanned and unattended systems. 8. Current Uncooled Detector The uncooled detector consists of an array of suspended infrared sensitive structures, built upon arms providing support and contact to the read-out integrated circuit. A thermal signal is developed across the suspended infrared material and input to a read-out amplifier under the detector. The support arms provide a dual function; however with conflicting requirements. Low thermal conductivity is necessary to thermally isolate the detector, and develop a large thermal responsivity. High electrical conductivity provides a low noise electrical contact to the input preamplifier, which is necessary for high signal to noise. Therein lies the challenge, since materials with low thermal conductivity and high electrical conductivity are not usually available. The physical support arms thus limit the thermal sensitivity achieved with this structure. If the current support structure is replaced with an ideal thermal isolation structure, new non-contact read-out schemes must be developed to convert the thermal signal to an electronic measurable signal. 9. Thermal Detector Challenges The challenges in design and fabrication of an ideal thermal detector involve the infrared sensing materials, device structure, read-out electronics, and signal processing. New material, with high responsivity in the infrared, is necessary to achieve high absorption in a very thin layers. The absorption can be enhanced through the development of an optical cavity underneath the detector. Read-out of the thermal signal may require optical or other non-contact techniques to convert the absorbed infrared radiation to an electronic signal. A major challenge confronting the uncooled infrared technology is the development of electronic temperature stabilization and non-uniformity correction. The current approaches require battery operated temperature stabilization and mechanical reference to achieve a uniform infrared picture. Although these techniques are relatively efficient, they add size and weight to the system, precluding the use of uncooled sensors in some of the more demanding micro-sensor applications. 10. Ideal Thermal Read-out Circuit In the ideal thermal detector, the infrared sensitive material absorbs radiation from the target without loss through contacts or supports. An ideal thermal isolation structure ensures the detector does not lose signal, other than loss due to the radiation exchange with the environment. This is the theoretical detection limit. In addition, in the ideal thermal device, the absorbing material must also be a very thin layer, approximately one thousand angstroms, to maintain a thermal time constant consistent with imaging requirements. A mechanism must also be developed to eliminate the diffusion of signal charge between adjacent pixels in the material. In addition, the read-out mechanism must convert the thermal information to an electrical signal without contact to the material. These characteristics must then be combined into a high density array of approximately one million picture elements, and assembled into a vacuum package. 11. Temperature coefficients stored in memory • FPA temperature data provided to processor • Interpretation of correction coefficients • Offset correction applied to FPA data Issues • NEDT achievable with electronic compensation • Temperature range of operation • Electronics size/power 12. Uncooled IR Signal Processing Uncooled infrared camera systems have unique signal processing requirements. For systems with high thermal sensitivity, the temperature non-uniformities must be compensated to the sensitivity limit of the system which is only a few milli-degrees. This requires memory to store temperature data characteristics of the focal plane array. Large scene temperature excursions require large dynamic range with local area contrast enhancement. 13. Precision Targeting The requirement to positively identify a target or to precisely select a target aim point requires higher resolution than is practical with far infrared optical systems. Near-infrared sensing, with its inherently higher resolution, adds precision targeting capability, but since signal levels are less than the far infrared, an active illumination is necessary to achieve adequate signal to noise ratio. Integration of high speed electronics at each pixel in the array provides timing information, which creates a three dimensional image of the target. The three dimensional image facilitates identification of the target, independent of target aspect, and provides image data useful in both aided and automatic target identification. 14. Three Dimensional Technology The three dimensional imaging array can be implemented with a two dimensional detector array sensitive to short wave-length radiation, typically in at the 1.54 micron eye-safe region. At each detector, high-speed electronics sample the return signal from the target, and determine the time of arrival at each pixel. Since the signal return is relatively small, only a few photons arrive at each pixel, and gain at the pixel is necessary to raise the signal level above amplifier noise. The gain mechanism can be achieved in several ways, but must not add excess noise to the detected signal. In one method, an avalanche gain in each detector, combined with a low noise amplifier in each unit cell, raises return signal above the noise. Other features, which may be included in the array, include output analog to digital conversion and gain/bias control at each pixel to optimize the detector operating point. 15. High Speed Imaging Devices with Gain - Concepts New concepts for three dimensional imaging devices incorporate a means to include gain, at low noise, at the detector. One example is the fusion of near-infrared material with silicon technology. A high gain at low noise can be achieved in the silicon circuit, while the narrow band material absorbs the near infrared radiation. Gain in the near infrared device can also be achieved by grading the material composition as the device is grown. Near infrared radiation is absorbed in the narrow band-gap region of the device, and wide band-gap region provides gain. Other methods for achieving gain, at low noise, at the detector include image intensification and electron multiplication. 16. Summary The potential for the uncooled infrared impacts nearly every class of Defense imaging systems. Recent technology advances indicate that much more is possible, even performance approaching the theoretical limit. This opens a wide range of applications for affordable night imaging, tailored to meet unique system requirements. The addition of three dimensional imaging adds a precision target engagement capability, not previously available. This sensor suite, operating without the traditional cryogenic cooling, offers DoD users wide area surveillance, with reliable target identification, in a compact, light weight package at an affordable cost.