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Seeing Photons: Progress and Limits of Visible and Infrared Sensor Arrays Summary CONTEXT AND TASKING Detector technologies for both military and civilian applications have evolved over many years to a sophisticated state of current development.1 Advanced technologies, such as nanoscale-engineered materials, will provide flexibility and functionality in the design and development of future sensor systems and their components. The increasing availability of commercial products will also impact detector-based electro-optical (EO) and infrared (IR) systems and lead to new sensor system-level capabilities. At the same time, mission needs will change, and sensor system designs and capabilities over the next 10-15 years will need to evolve to match these changed mission needs. New EO-IR sensor system challenges—processing, storing, and communicating—are arising from the enormous increase in data generated as a result of the proliferation of more and ever-higher-pixel-count sensors. Generating data is not the same as providing actionable intelligence; this requires conversion of the data into usable information. Leveraging of commodity-level developments enables unprecedented capabilities for technologically advanced nation-states and, simultaneously, lowers the barrier to entry for non-state, transnational groups to pose asymmetric threats. 1 For this report, the committee defines a detector as representing a single pixel that receives photons. A focal plane array (FPA) is composed of many detectors arranged in a two-dimensional grid and generates an image. A sensor system is composed of FPAs plus other components, such as signal processing, data transmission, coolers, optics, and pointing and tracking mechanisms.
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Seeing Photons: Progress and Limits of Visible and Infrared Sensor Arrays Funding, influenced by military needs and commercial market conditions, will drive investments. Commercial funding is expected to be at a greater level than military funding, but this will be restricted to commodity areas with the potential for large-volume manufacturing. Military needs will likely leverage commercial off-the-shelf capabilities in areas such as advanced semiconductor manufacturing tools. In this overall context, the intelligence community (IC)2 asked the National Research Council (NRC) to conduct an in-depth technical assessment of detector technologies. Specifically, the NRC was asked to do the following: Consider the fundamental, physical limits to optical and infrared detector technologies with potential military utility, with priority on passive imaging systems. Elucidate trade-offs between sensitivity, spectral bandwidth and diversity, dynamic range, polarization sensitivity, operation temperature, and so forth. Compare these limits to the near-term state of the art, identifying the scaling laws and hurdles currently restricting progress.3 Identify key technologies that may help bridge the gaps within a 10-15 year time frame, the implications for future military applications, and any significant indicators of programs to develop such applications. Speculate on technologies and applications of relevance that are high-impact wild cards or have a low probability of feasible deployment within 15 years. Discuss trends in availability and format scalability and in available cooling technologies. Consider the pros and cons of implementing each existing or emerging technology, such as noise, dynamic range, processing or bandwidth bottle-necks, hardening, power consumption, weight, et cetera. Identify which entities currently lead worldwide funding, research, and development for the key technologies. Highlight the scale, scope, and particular strengths of these R&D efforts, as well as predicted trends, time scales, and commercial drivers. THE BOTTOM LINE Fundamentals of Visible and Infrared Detectors There are fundamental limits to detection. To be seen at all, an object must emit or reflect electromagnetic radiation in some spectral band. That radiation 2 According to Intelligence.gov, the IC is composed of 17 federal agencies. Accessed March 24, 2010. 3 In several consultations with the committee over a period of months, the sponsor requested that the committee address the imaging spectrum from ultraviolet to very longwave infrared.
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Seeing Photons: Progress and Limits of Visible and Infrared Sensor Arrays must pass though the medium between the object and the sensor. For cross-link, space-based sensor systems, the transmission medium is not a limitation; however, if either the sensor system or the object being viewed is within the atmosphere, atmospheric transmission must be taken into account. Difficult new mission requirements, such as viewing objects below the surface of water or behind strongly scattering media (e.g., foliage), require creative combinations of sensor technologies. Finally, the received electromagnetic information must be transduced into another form, usually an electrical signal, with sufficient signal-to-noise ratio to allow further extraction of information. Developments in detection have a long history. Both visible and IR detector technologies have undergone significant maturation, and high-performance detectors are available across most spectral bands from 0.2 to 20 μm. The most sensitive IR detectors require cooling to reduce dark current noise and reach background-limited IR photo detection (BLIP), resulting in an increase in size, weight, and power (SWaP), as well as cost, along with a reduction in reliability. Single-photon detectors are available today in the visible and near IR; there are active research efforts to extend this capability throughout the IR. In accordance with the statement of task, this report emphasizes passive sensing; however, developments in active sensing are included as appropriate throughout the report. In spite of the fact that there are high-end sensor systems capable of close to theoretical sensitivity limits in most bands, significant improvements remain possible for sensor systems by adding functionality, such as multi- and hyperspectral response, polarimetric sensitivity, dynamic resolution, and sensitivity adaptation, as well as reductions in SWaP and cost. Certainly, processing and communications requirements and capabilities will continue to drive improvements in sensor systems. Some of these improvements are fundamental to the detectors or sensor systems, and some are in the ancillary components, such as optics, cooling, pointing and tracking, data handling, and compression. Key Current Technologies and Evolutionary Developments A relatively new technology relates to advances in solid-state detector materials. These advances tend to be used to render immaterial the sources of noise downstream from the detector. Initially, most detectors with gain tended to use linear gain; however, more recently, the significant advantages of uncontrolled avalanche gain, called Geiger mode operation, in which an arbitrarily large number of electrons are released based upon the arrival of a single photon, are creating new imaging modalities. In addition, there remains considerable opportunity to improve other parameters, such as operating temperature, power dissipation, manufacturability, and cost. Going forward, many of the advances in detector technologies will be in “pe-
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Seeing Photons: Progress and Limits of Visible and Infrared Sensor Arrays ripheral” areas. One important area is cooling, particularly for IR sensors. Historically, for mid-wavelength IR (MWIR) and long-wavelength IR (LWIR) detectors, cooling has been a major limitation. There has been a push toward higher-temperature detector operation, based on developing detector technologies that have lower dark current at a given operating temperature. Progress has been slow, however, and considerable room for improvement remains. An alternate approach is to reduce the SWaP requirements of cooling. The increase in digital processing capabilities, fueled by the semiconductor industry, is a further trend that will continue to have a major effect on sensor systems. Digital processing systems can be adaptable and allow customization for specific applications. Lowering cost can also make a detector technology much more widely available and cause its impact at the systems level to be greatly increased. Tracking novel adaptations of widely available and inexpensive imagers will continue to be of interest to the IC. One example is the Defense Advanced Research Projects Agency’s (DARPA’s) Autonomous Real-time Ground Ubiquitous Surveillance-Imaging System (ARGUS-IS) program that involves integrating a large number of cell phone camera chips to provide a revolutionary 1.8 gigapixel imager. Consumer demands for improved, higher-pixel-count, cell phone imagers, which did not even exist until recently, made this revolutionary imaging capability possible. For countries that do not invest significant funding in purpose-built imaging technology, the development of low-cost commodity imagers has significantly lowered the barriers to having a militarily significant imaging capability. The use of large-volume commercial sensors can enable new capabilities for both less advanced asymmetric adversaries and near-peers alike. KEY FINDING The evolutionary trends are semiconductor detectors characterized by increased pixel pitch and count, higher readout speed, higher operating temperature (especially MWIR), lower power consumption, and decreased sensor thickness. The need for larger fields of regard is a significant driver for larger arrays. Even beyond the diffraction limit of the optical system, oversampling can lead to slightly enhanced resolution. KEY FINDING The global proliferation of low-cost, commodity imagers, such as cell phone cameras and automobile thermal imagers, enables adversaries to develop sensing systems at relatively low cost, reducing the barrier to achieving limited operational capabilities. As an example, the rapid proliferation of low-cost “night vision technology” is eroding the overwhelming dominance of the United States in nighttime operations, even with the superior performance of advanced systems.
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Seeing Photons: Progress and Limits of Visible and Infrared Sensor Arrays KEY FINDING The availability of very low cost imagers developed for large consumer markets is providing opportunities to develop new sensor systems and architectures, even though the component-level imagers may not have the capabilities typical of high-performance sensors developed specifically for military applications. Additionally, the technology and manufacturing base used to make these low-cost imagers will extend the manufacturing base that can be used for fabricating customized military parts. RECOMMENDATION 3-1 The intelligence community should pay careful attention to the new capabilities inherent in both the proliferation of commodity detector technologies and their integration into novel sensor systems. ARGUS-IS and Gnuradio are examples of how available, low-cost, mature commodity visible focal plane array (FPA) technology (cell phone camera chips) and commercial off-the-shelf (COTS) communications circuitry, through sensor integration, have enabled new, advanced, high-performance imaging capabilities. KEY FINDING Existing, mature mercury cadmium telluride, indium antimonide, indium gallium arsenide, silicon charge-coupled devices, silicon complementary metal oxide semiconductors, and avalanche photodiode focal plane technologies provide sensors with excellent performance and set a very high barrier to entry for any emerging technology. For some performance parameters, such as detectivity, mature imager technologies already are operating very close to fundamental limits. However, there is still considerable opportunity to improve other parameters such as operating temperature, power dissipation, manufacturability, and cost. KEY FINDING Rapid progress is being made in the development of closely related single-photon and photon counting detectors and arrays. Single-photon detection and photon counting imagers are key enablers for a wide range of new secure communications, passive sensors, three-dimensional laser detection and ranging, and active optical sensors. Specifically, quantum cryptography relies on the distribution of entangled, single-photon qubits (keys) between the transmitter and receiver; this is inherently a single-photon process. In most cases, these applications involve physical processes in which only a small number of photons are available for detection. These detectors require high quantum efficiencies, low dark count rates, fast recovery times, and capabilities for photon number resolving.
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Seeing Photons: Progress and Limits of Visible and Infrared Sensor Arrays TABLE S-1 Trigger Points of Technical Progress and Their Implications Single Photon 2010 (SOA) 2015 (TP) 2020 (TP) 2025 (TP) Q efficiency 90% >90% >90% >90% Speed 10 GHz 100 GHz THz THz Wavelength Visible 1.55 μm 1.55 μm 1.55 μm Operating temperature 4 K 77 K 300 K 300 K Application QKD QKD/quantum computer Quantum computer NOTE: QKD = quantum key distribution; SOA = state of the art; TP = trigger point, which indicates a capability that should stimulate the intelligence community to do significant collection and/or analysis. RECOMMENDATION 3-2 The intelligence community should carefully track developments related to single-photon and photon counting detectors across the full spectrum, from the ultraviolet to very long wavelength infrared. Table S-1 lists trigger events that would cause a significant shift in capability and should be carefully monitored by the intelligence community. KEY FINDING There is significant opportunity to customize image sensor architectures for specific applications that can lead to dramatic improvements in system-level performance, including size, weight, and power. Advanced architectural design, including integration of sensing and processing (in-pixel and on-chip), can have greater system-level impact than making small gains in driving detector performance incrementally closer to fundamental detectivity limits. RECOMMENDATION 3-3 The intelligence community should evaluate and track system capabilities rather than focusing solely on component technical achievements. These include technologies that enable in-pixel and on-chip processing, lower-power operation, and higher operating temperatures, as well as technologies that improve manufacturability. KEY FINDING For both cryocooler and thermoelectric cooler technologies, there are a number of commercial market drivers, separate from sensor cooling applications, that will drive evolutionary improvements in SWaP. Over the next 10-15 years, it is reasonable to expect that these improvements will achieve overall reductions in SWaP on the order of 20-30 percent.
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Seeing Photons: Progress and Limits of Visible and Infrared Sensor Arrays Emerging Technologies with Potentially Significant Impacts The user always wants more resolution and a wider field of view. Resolution is limited by diffraction, but field of view is not limited in a fundamental manner. Developers continue to try to meet these ever-increasing demands. Users are willing to pay for development to meet these needs, especially an increase in field of regard without sacrificing available resolution, which directly leads to increased pixel count and larger-area arrays. Emerging thermoelectric, phononic crystal, and laser cooling technologies offer potential for improving sensor systems, because such technologies might be able to replace the cooling furnished by current bulk coolers, with their attendant SwaP penalties. Another peripheral area that has a major impact is the ability to handle the vast amount of data generated. There are many new sensors coming along that generate large amounts of data. Hyperspectral sensor systems, for example, generate significant data volumes as a result of the additional spectral dimension. Digital developments are not driven by the relatively small number of sensor systems. The gaming industry has a much more significant impact on digital progress, and advances in computation and communication will have a major impact on sensor technology. Countries around the world are poised to take advantage of nanotechnology to potentially build entirely new sensors and sensor systems. Therefore, international progress in the nanotechnology field constitutes a principal driver for significant advances in sensors. KEY FINDING Thin-film thermoelectric devices have the potential to substantially reduce size, weight, and power requirements of the active cooling component for room-temperature focal plane arrays. If these devices can meet cost and life-time metrics, they will displace the currently used bulk coolers. The near-term driver for these developments likely will be in fields such as microelectronics with much larger market potential than detectors. RECOMMENDATION 4-1 The intelligence community should monitor commercial developments in thin-film cooler technology. KEY FINDING Scaling the data throughput of focal plane sensor systems involves not only the sensor chip but also the detector-processor interface, signal processing and compression, and the communication link (wireless for remote air- and
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Seeing Photons: Progress and Limits of Visible and Infrared Sensor Arrays space-borne missions). Advanced compression and filtering with on-board processing provided by commodity multicore architectures are reducing communications demands. RECOMMENDATION 4-2 Analyses of national capabilities should include consideration of advances in processing technologies for other uses—for example, commercial developments—that could also enhance the use of detectors in future sensor systems. The Global Landscape of Detector Technologies To date, the United States has been the international leader in designing, developing, and implementing detector technologies. An exception is the migration of visible detectors, driven by consumer requirements, to an Asian manufacturing base. Significantly, existing U.S. export control policies have eroded and will continue to erode U.S. advantages in areas of military detector technologies. Significant detector technology developments will continue to occur in Europe, specifically in the United Kingdom, France, and Germany, as well as in Israel. China, today, is a second-tier nation in designing and fielding detector technologies; however, it is investing substantial resources and is anticipated to emerge as a significant competitor within the 10- to 15-year time frame of this study. KEY FINDING Current export restrictions will continue to have a significant effect on development and maturation of detector technologies over the next decade. Numerous foreign countries are already developing their own technology base rather than utilizing U.S. technology and often will compete with U.S. technology. U.S. export restrictions are a primary driver creating this competition. U.S. companies invest significant resources in obtaining, funding, and exploiting foreign products so that they can compete in foreign markets without export restrictions.