Optics in National Defense
We are not the only nation with competence in defense science and technology. To sustain the lead which brought us victory during Desert Storm . . . recognizing that over time other nations will develop comparable capabilities, we must . . . invest in the next generation of defense technologies.
William J. Perry (1996)
Combat is repugnant to Western civilization, but it is a reality that we must face squarely because alternatives to victory are not an option. The stark realism of Desert Storm emerged from the television coverage with a technological perspective never before seen. Did anybody miss the striking footage of a precision laser-guided bomb zeroing in and then obliterating a military headquarters building in Baghdad? Could anyone have possibly not seen an entire field of Iraqi tanks, personnel carriers, and artillery units being devastated by the allied forces in just a few minutes, with hardly a response from the enemy? Also, there were very few allied casualties, unlike previous conflicts such as Vietnam and Korea. (See Box 4.1.) From this it is clear that a higher technological level has been achieved in modern warfare, serving as a powerful deterrent to potential aggressors. Technology is the centerpiece in modern warfare, enabling us to deploy and obliterate enemy forces while sustaining minimal casualties.
Throughout history, new technology has had a profound effect on how wars are conducted. Usually, the victors were those best able to apply the new technology. Over the course of the past 50 years, nuclear weapons, microwave radar, guided missiles, and other developments have led to major realignments of defense strategy. Today, the traditional modern strategy of massing large numbers of military personnel and materiel to engage enemy forces is giving way to high-tech methods of conducting warfare that minimize casualties. The U.S. military mission now requires a versatile fighting force capable of both conventional field and urban warfare in a global venue. To improve the effectiveness
BOX 4.1 OPERATION DESERT STORM
"Operation Desert Storm was primarily a sustained 43-day air campaign by the United States and its allies against Iraq between January 17,1991, and February 28,1991. It was the first large employment of U.S. air power since the Vietnam war, and by some measures (particularly the low number of U.S. casualties and the short duration of the campaign), it was perhaps the most successful war fought by the United States in the 20th century. The main ground campaign occupied only the final hours of the war."
U.S. General Accounting Office (1966)
of the combatant while reducing casualty rates, the military has a number of efforts under way that include reliance on speed and stealth to overcome opposing forces; a better equipped land warrior; rapid detection and control of nuclear, chemical, and biological threats; and dissemination of real-time intelligence on enemy targets. Optics plays a key enabling role in these plans. For the future, optical systems are sure to be the basis for entirely new classes of defense applications that will change yet again the way wars are conducted.
Since World War II, the U.S. technological approach has featured defense sponsorship of leading-edge research and development at levels necessary to maintain strong defense leadership in the world. Defense support of research has included activities that range from basic research at universities to system-level developments in industry. When the Cold War was at its peak and the Soviet Union presented a severe threat, the Department of Defense (DOD) actively pursued R&D in virtually every technical discipline. The overarching goal was the anticipation of potential breakthroughs that could upset the balance of power. To be sure, this strategy was expensive, but it has been effective and has served society well. U.S. defense capability today is preeminent in the world. Much of the resulting technology has eventually found its way into everyday life in various forms. It is arguable that national defense has been the mainspring carrying basic research discovery into applications and driving our society into the technological era in which we now reside.
Since the end of the Cold War in the early 1990s, U.S. defense strategy has undergone a seminal change. The need for the U.S. military to stay at the forefront of technology has diminished considerably. The strategies for Defense Department R&D and weapons system acquisition are consequently being realigned as a result of comparatively weak threats to U.S. security, the national desire to direct more resources toward improving U.S. competitiveness in the global marketplace, and the increasing complexity and cost of military systems.
The current major defense trends affecting optical technology include increasing reliance on commercial components to reduce costs, use of lower-risk technology to cut system development time in half, and pursuit of only those technologies judged to have the greatest potential impact on national defense (Defense News, 1996). The cost advantages of using commercial components for niche military applications arise from their large market base and designs for minimal product cost. For example, the use of commercial computer displays in military field equipment could result in considerable savings, provided the displays are fully functional in the military environment. A grand challenge for developers of military systems is the incorporation of commercial designs, components, and test capabilities into military systems that must not only work in extreme conditions, but also interoperate reliably with other military equipment. Economic considerations also favor small suppliers with lower overhead costs and in some cases the use of technology developed with government support. These are profound changes for system acquisition.
Even in this age of reduced threat levels, there is still an overriding requirement for DOD to invest in technology that provides unique military advantages. Some argue that the return on R&D investment, in this time of declining defense budgets, is larger than for any other investment and essential for DOD to preserve its edge. This argument would tilt investment to new technology to preserve the U.S. edge, since the use of commercial components in current systems levels the playing field for our competition. Systems and system components must meet military field requirements; these are severe environments in which commercial equipment often experience high failure rates because commercial design requirements do not encompass severe stress conditions. There is little overlap between optical systems for defense and commercial markets. Commercial industry has little incentive to include DOD requirements in its designs since DOD makes up a small part of its market, particularly when the changes would increase product cost. Compounding the problem are reductions in R&D funding discretion by the services, with greater reliance on work at small commercially oriented firms funded through the Small Business Innovative Research (SBIR) program.
Optics has matured during the past 30 years to the extent that it is now on a par with electronics and microwaves in defense systems. Optics is the nucleus of entirely new systems and system concepts that are essential to U.S. national defense. Measuring the stature and effectiveness of a technology is always problematical. Since fieldable (operational) military capability is the "bottom line" for the military, implementation of a technology is one measure of its effectiveness. Figure 4.1 is a broad depiction of the impact of optical technology
on DOD systems. The optical items cited in Figure 4.1 are those that provide the warfighter with significant leverage. The trend indicates dramatic improvement of field effectiveness in both tactical and strategic areas. Examples are laser designators that have enabled a generation of "smart" weapons to surgically strike a specific target and highresolution surveillance of military capability unobtrusively from space. Desert Storm pilots stated that they "owned the night" with their night vision capability; the strategy of shunning daylight flights in Iraq and Kuwait during the war contributed greatly to their low casualty rate.
The level of activity in a technology can be gauged by the amount of DOD resources devoted to potential future system applications, as well as to exploratory and advanced development. Annual U.S. defense expenditures for optics are large, exceeding $5 billion per year, consistent with the capability depicted in Figure 4.1. Microwaves, optics, and electronics R&D for fiscal years (FY) 1996-1998 had a budget of $600 million per year;1 approximately 27% of this is devoted to optics. Within this "electronics technology area," which includes funding for the Defense Advanced Research Projects Agency (DARPA), the amount of resources devoted to optics technology is on a par with its sister technologies, but erosion of these resources is anticipated. For instance, DOD reprogrammed resources from this budget category in FY 1996 and 1997 to cover shortfalls in the Bosnia peacekeeping operations, and DARPA, the principal organization conducting R&D, is downsizing by 20% (Military and Aerospace Electronics, 1996).
The role of optics in defense is pervasive and ubiquitous. Its roles in military systems are so broad that simply to categorize and describe them is difficult; they span the range from low-cost to expensive and from enabling to dominance of system designs. Laser propagation in free space can be used like radio waves and microwaves for radar and communications. Data can be transmitted over communication cables and imaged on displays. Unlike radio waves and microwaves, light is ideal for beam weapons, targeting (designating), and passive surveillance applications. Also, totally new applications have emerged that have no direct electronic analogue, such as laser gyros for navigation and gigabit cable communications.
The remainder of this chapter discusses optical advances that are important for national defense in surveillance, night vision, laser systems, fiber optics, displays, and special technologies. Examples are presented in terms of the way technologies are applied to various defense missions. However, the demarcation between defense-unique and civil
applications is becoming blurred. For example, the National Ignition Facility will advance our understanding of fusion energy, as discussed in Chapter 3, and will make use of the large laser facilities developed for the Department of Energy's (DOE's) nuclear weapons mission. The optics field is still relatively young, and these examples are only a small sample of the possibilities that the next 30 years will bring.
Surveillance has played a central, critical role in detecting and assessing hostile threats to the U.S. homeland and to its forces stationed around the world. For instance, as discussed in Box 4.2 and Figure 4.2, high-resolution imaging satellites have been deployed for three decades to provide data to U.S. defense experts in a broad spectral band recorded over land and sea that could be obtained by no other means.
For the purposes of this chapter, surveillance is defined as the process of collecting optical data, usually in the form of images. This can be done with an inexpensive throw-away camera or a multimillion-dollar sophisticated reconnaissance satellite system. Warfighters and government decision makers rely on these sources of on-line information to provide input to their assessments and to assist in identifying strategy options. They not only demand the best-quality data, but also often need it immediately. Thus, older film-based surveillance systems have generally been replaced by real-time electronic image formation systems installed aboard airplanes, ground vehicles, and satellites.
The heart of current surveillance systems is a very sensitive focal plane array (FPA) of detectors that capture very weak optical and
BOX 4.2 MILITARY USE OF OPTICS THROUGHOUT HISTORY
Historical uses place in perspective the accelerating rate of change that is occurring in the military uses of optics:
Archimedes, advisor to Hiero the tyrant of Syracuse, reportedly used polished metal sunlight reflectors to set fire to Roman ships. This concept was given new birth with the development of high-power lasers.
Telescopes have been used by the military for centuries. Today high-resolution imagery can be obtained on call from airborne and spaceborne surveillance systems.
Simple mirror sunlight reflectors and semaphores, used in the nineteenth century for communication, presaged the advent of laser communications in development today.
infrared (IR) emissions arising from a multitude of sources. Also, common to most systems is an optical collection telescope structure operating in the visible or infrared spectrum. Advanced signal processing methods use the timing, location, and spectral signature of events to identify those of potential military interest. These satellite systems are generally expensive (more than $100 million) but provide worldwide surveillance.
Major advances have recently been made in manufacturing FPAs to achieve pixel densities of approximately 106 per chip that parallel the advancements made in achieving ever higher densities on silicon integrated circuits. This has enabled greater sensor sensitivity over a wide field of view. Optical collection and detection technology operates close to fundamental theoretical limits to the extent that image data processing and transmission are now the bottleneck for achieving the next major improvement in system capabilities. System designers face the awesome challenge of selecting affordable technologies and platform configurations to best take advantage of these advanced capabilities.
Consistent with current acquisition policy for all new systems, the cost-performance-requirements envelope must be optimized for new surveillance programs. As a result, DOD is now designing new surveillance systems with today's available optics technology that emphasize system demonstrations to mitigate development risk. Opportunities to insert new technology exist at each of the primary data collection steps: wide-area survey, target acquisition, target characterization, target tracking, and system parameter updating.
To implement and optimize these observational sequences, the following system-level challenges for airborne and spaceborne sensor optics must be addressed:
Innovation of solutions to technology insertion to obtain the most ''bang-for-the-buck";
Fast dissemination of data to the digital battlefield;
Use of many spectral bands, multispectral or hyperspectral segments extending across the visible and IR, to penetrate enemy camouflage and locate targets (Marmo, 1996); and
On-board fusion of other sensor data for more accurate target identification and tracking.
Keep in mind that optics-based surveillance is usually passive (unlike microwave radar systems, which require a pulse and return echo). Consequently, the technology insertions are mainly to improve
very sensitive surveillance systems. These technologies include materials and systems for IR FPAs, charge-coupled devices (CCDs), lightweight optics, compact coolers, staring arrays, and efficient electronic readouts and processors. These technologies are operated on-board satellites, uninhabited airborne vehicles (UAVs), and aircraft. UAVs can relay real-time images of battlefield troop deployments to field commanders as will future satellite systems.
During World War II, the use of radar by Allied forces to see through clouds and inclement weather was invaluable. This capability was literally the difference between losing and winning battles. Now, some 50 years later, optical devices are available that can see at night with such important advantages over radar as high spatial resolution (as good as ordinary eyesight) and lack of detectable radiation emanation. U.S. soldiers and fighting machines equipped with this night vision capability have had a unique tactical advantage in the post-World War II era. U.S. military forces have essentially "owned the night" and hence have been able to fight under most favorable conditions.
Early night vision units amplified reflected starlight and demonstrated considerable tactical advantages. Battlefield night vision devices now use passive detection of IR, which senses the heat radiated from objects in the scene. The challenge is to discriminate objects such as tanks, which may be only a few degrees hotter than the background. Older devices produced images that resembled bad, noisy television signals; today, the devices have been improved to the level of quality television pictures. These devices, produced in large volume, have little in common with one-of-a-kind complex surveillance systems, although the underlying physical principles are the same. It should be noted that this is not all-weather capability since heat radiation is absorbed by rain and fog, imposing well-understood operational military limitations.
Night vision units, often termed FLIRs (a historical acronym meaning forward-looking infrareds) use FPAs in a wide variety of formats for tactical battlefield applications (Lerner, 1996). Night vision designs are mass produced (in quantities of more than 10,000) at low cost; for example, more than 100,000 first-generation units, known as Common Modules, have been built. This is in contrast to the surveillance units discussed in the previous section, which have ultrahigh performance but are produced in limited numbers (1 to 100) at high cost.
The first night vision devices used cooled detectors to gain the sensitivity required to detect weak thermal radiation. The availability of detector materials has largely driven system design. Early materials such
as lead sulfide (PbS) were hand-made and suffered from a number of technical problems. Most modern FLIRs use mercury cadmium telluride (HgCdTe, or MCT) because the composition can be varied to afford detection over different regions of the IR spectrum and the elements can be mass produced with high purity. Cooling to the vicinity of 100 K requires a mechanical device and dewar (thermos bottle) with a window and optical elements to admit thermal radiation. Either the scene is scanned over a linear array of detectors with about 105 elements or a large array of detectors with about 105 elements stares at the scene to be imaged. Cooled detectors feature excitation of electrons as photons are absorbed (photodetectors), with signal processing in chips followed by conventional display of the image.
Figure 4.3 shows a cooled thermal imaging roadmap that details the progress of this class of night vision devices for Army applications. Basic materials research drives the progression as poor-quality bulk material gave way to liquid-phase epitaxy (LPE) material amenable to mass production at low cost. Future generations of detectors may employ molecular beam epitaxy (MBE) or smart FPAs with specially designed readout integrated circuits (ROICs) to provide multifunction elements. The older rotary coolers and bulk elements that made up the Common Module class of devices for the first-generation FLIRs used in Army vehicles were characterized by "noisy TV picture" quality and poor reliability. LPE elements in Standard Advanced Dewar Assembly (SADA)-class FLIRs use a linear drive cooler to produce a quality picture with 10 times greater reliability. These second-generation FLIRs are being introduced into the vehicles indicated through a program called Horizontal Technology Insertion (HTI). Third-generation devices are now in the R&D phase.
This discussion cannot cover every type of night vision system insertion; a brief synopsis of the types of fielded units is provided here.
Early uses focused on tank target detection and missile guidance. As the technology progressed and the sensitivity increased, aircraft and helicopters began to rely on FLIRs for targeting and navigation. Today, even higher-sensitivity FLIRs are in development for long-range threat detection, termed Infrared Search Track (IRST). The progression to lower-sensitivity, lower-cost units has permitted vehicle driving at night and use by individual soldiers. The most compact imaging IR devices are found in missile guidance units for Javelin and Stinger, for example. Obviously, for expendable munition guidance applications, very low cost is paramount, but the performance is usually limited to short-range targeting. An affordable cost for FLIRs ranges from $10,000 to $200,000, depending on sensitivity and other parameters.
The most recent FLIR product breakthrough is in the area of uncooled detectors. Unlike the cooled photodetector class, uncooled detectors rely on the use of a silicon microstructure upon which a thin film of material is deposited. The temperature increase brought about by the absorption of IR changes some property of this film. The three detection methods used are (1) resistive bolometric, (2) pyroelectric, and (3) thermoelectric. The low cost of silicon microstructure devices is key to the widespread use anticipated for this detector class.
In Figure 4.4 a roadmap for this uncooled class of detectors is presented showing the progression from ferroelectric element research to bolometer element research into FPAs for short-range thermal sights. Consistent with this class, a sensitivity sufficient to image up to about 1 km is possible; material advances will improve this range. As Figure 4.4 shows, the increase in size and producibility correlates with wider use in soldiers' thermal weapon sights and in driving Army vehicles, with the ultimate use in missile seekers as detectors improve. Since the units do not require a cooler, they are much lower in cost with projections of less than $10,000 for high-volume manufacturing.
Figure 4.5 shows a small uncooled hand-held FLIR device with a 320 x 240 pixel array and a range of ~1 km. With excellent pixel-to-pixel uniformity yielding imagery at an affordable cost, this type of unit is suitable for deployment to individual soldiers and is also expected to see widespread commercial use.
Essential to the success of this technological thrust have been the cooperative efforts of the military services and DARPA to take the "black magic" out of detector producibility and to create a quality manufacturing infrastructure. Future DOD thrusts are to develop even larger staring arrays, multicolor FPAs for fusion of detector information, wide deployment of more sensitive uncooled imagers, and development of effective automatic target recognition systems.
Laser Systems Operating in the Atmosphere and in Space
Lasers have become such a key part of our life, at the grocery checkout counter and in compact disk (CD) players, that military uses are easily anticipated. Laser light projects long distances in very narrow beams because of its short wavelength, unlike radio waves that spread out. So optical power is more efficiently delivered to a target, and this simple idea is the essence of most military uses of lasers. Early laser research workers dreamed of destroying targets at the speed of light (a million times the speed of sound) and a host of other applications. DOD supported the early work on masers that led to the discovery of the first (ruby) laser at Hughes Research Laboratory, but it took many years of concerted Defense Department R&D to build lasers with the required efficiency of tens of percent (early versions were 0.0001% efficient) needed in fielded systems. Even after lasers were improved, it was quite a chore to make laboratory units work reliably in the field.
This section includes the analogues of microwave systems that operate in the atmosphere and space, namely, laser radar, jammers, target designators, communications, and laser weapons. Solar cells, environment sensing, law enforcement, transportation, and so forth, also operating in the atmosphere or in space for civilian use, are treated in Chapter 3. In military applications we are usually concerned about preserving the power in narrow near-diffraction-limited beams over long ranges (many kilometers), which are accurately pointed and controlled to eliminate both platform jitter and atmospheric beam distortions. The technologies common to this class of laser systems include acquisition, pointing and tracking, fieldable optics with domes or aerodynamic windows, bore-sighted detectors, and laser operation somewhere in the visible to 10-µm IR spectral ranges where the atmosphere is transparent to laser radiation.
Laser Range Finders, Designators, Jammers, and Communicators
This category of lasers was developed first, then laser power levels were gradually increased to the weapons class range discussed in the next section. The first laser range finder using ruby lasers was demonstrated less than a year after the laser's discovery and marked the introduction of widespread practical use. As improvements in the technology, especially new laser materials, and new classes of lasers came along, both range and performance were greatly improved.
Typically, a tank laser range finder is used to illuminate (with great haste) an enemy tank; the range is calculated from the received laser return pulse to determine the ballistic trajectory of a tank shell. The tank's gun is elevated and fired while the vehicle is moving. The objective is to be faster than the enemy and to kill the enemy tank by delivering highly accurate munitions. With better laser materials, especially Nd:YAG (neodymium-doped yttrium aluminum garnet, a crystalline solid with outstanding performance), field units have greater reliability and performance. In simplest terms the key to successful systems of this class is the capability to design, manufacture, and deploy lasers that are affordable and reliable. Early laser systems suffered from internal degradation of optics, which blocked their widespread deployment and required a major effort to resolve. Even more years of development were needed to field other laser subsystems after the technical community sorted through thousands of options to find the right combination of power output, efficiency, reliability, and so on. Today, DOD generally utilizes the following:
Light-emitting diodes (LEDs) or laser diodes (LDs) for very short-range illumination;
D array pumped solid-state pulsed lasers for laser range finders, target designators, jammers, and so on; and
Nd:YAG and other eye-safe solid-state lasers.
The many thousands of battlefield range finders in tanks and designators for ground and aircraft were deployed using older flashlamp excitation (or pumping) of YAG in first-generation laser technology. New technology utilizes LDs to convert electrical energy into light energy for pumping solid-state crystals such as Nd:YAG with greater efficiency and reliability. In addition, many more applications have emerged for this type of laser that depend on the cost-performance trade-off. The cost of LDs has been driven down by technology advances and volume manufacturing and is expected to drop even more. Simple LD illuminators for personal weapons are the basis for the important Multiple Integrated Laser Engagement Systems (MILES) combat training system with commercial spinoff to war games for entertainment. The sportsman will recognize this illuminator as the basis for the rifle spotting beam. Retrofit and upgrade of the entire class of pulsed range finder and target designator units so important to our success in delivering precision munitions in Desert Storm are proceeding. Modern battlefield doctrine is, in fact, profoundly shaped by laser-guided bombs and missiles, enabled by our ability to cost-effectively make the approximately 100,000 reliable laser designator sources that have been fielded.
The above low-power class of laser systems operates in the <1 W average power range. Countermeasure lasers for jamming and sensor blinding require 1 to 100 W, a range difficult to achieve until LD array pumped solid-state lasers became available. In combination with various wavelength shifting schemes [optical parametric oscillators (OPO), Raman effect] to avoid sensor selective rejection filters, jammers and blinders constitute a new escalation in the "optical" battlefield. It should be noted that Secretary of Defense Perry in September 1995 announced a prohibition against the "use of lasers specifically designed to cause permanent blindness of unenhanced vision and supports negotiations prohibiting the use of such weapons"—a position probably established, in part, because an eye safety device effective against all types of lasers generally found on the battlefield has not yet been developed and deployed. However, the use of lasers for target destruction is still permissible. R&D is continuing, and future thrusts are mostly in support of laser sources and subsystems with emphasis on wavelength diversity and agility. The U.S. Air Force study New World Vistas2 anticipates that this class of systems will be fielded in the next decade.
It is interesting that early forecasts of the significant use of laser communications in free space have not materialized. Development of ground, ship, aircraft, and satellite terminals has been extensive (about $1 billion) over the past two to three decades, with active work still in progress, especially for satellite-to-satellite relay links that would permit elimination of a ground relay station. This status is due largely to technical problems with lasers combined with advances in the capability of competing microwave links. (The better cost-risk trade-off of microwave links is described in Chapter 1.) The maturation of LDs as a product seems likely to result eventually in fielded commercial links, especially in the 0.01- to 1-gigabit-per-second regime for satellites and other airborne vehicles.
The concept of near instantaneous destruction of airborne and space-based targets is quite appealing. To this end, high-power lasers have been under development since the 1970s. Weapons-class laser development began with the discovery of the high-power carbon dioxide (CO2) molecular laser operating at 10-µm wavelength. There have since been many technology advances in high-energy lasers, and both ground and airborne demonstrations have validated the basic weapons concept (Figure 4.6).
Laser weapons designs and matching mission roles, such as the destruction of sensors in imagers, missile guidance, and surveillance systems, are understood well enough that advanced system development could proceed if national security required it (Knowles, 1996). The New World Vistas2 study advocates extensive use of laser weapons against missiles, satellites, and other ground assets in the next decade.
Today a number of gases have been discovered with the right properties to generate high-power in the 1-MW regime and offer the flexibility of excitation by electrical or chemical means. Entirely new areas of physics and optics are encountered in this high optical power regime. Technical challenges include the following:
Maintaining stable optical resonators under high thermal loading;
Extracting diffraction-limited beams with high-efficiency from the laser cavity;
Suppressing nonlinear optical effects along the propagation path;
Delivering beam power on target via optical control to correct for beam distortions during propagation; and
Solving operational issues such as environmental factors and lethality for different target classes.
At first glance, high-power lasers would seem to serve only military needs. However, advances in this technology have provided many other uses for scientific and commercial applications. For example, adaptive
optics technology for atmospheric compensation of laser weapons is revolutionizing the design of astronomical observatories (discussed in Chapter 3). The distortion of optical beams along an atmospheric propagation path is highly complex. It is compounded by high-power nonlinear effects, which can also break up the beam.
During the 1960s and 1970s, DOD mounted a major technical campaign to understand and resolve these problems. In addition to elucidating the physics of beam propagation, adaptive optics and phase conjugation were developed to solve the problem. These techniques have been reduced to practice and are now employed as essential elements in system designs. Adaptive optics control of laser beams constitutes a major recent breakthrough in laser weapons system developments. Ground-based lasers can now potentially negate satellite threats. However, not all elements of the technology are complete. Continued developments are required in terms of power, size, weight, reliability, and beam quality. Future activities should address reducing the manufacturing cost of high-power laser systems to make them more affordable. Adaptive optics is effective in this area as well, providing compensation for lower-cost, relaxed optical tolerance, laser resonator designs.
Laser weapons can revolutionize battlefield strategies, but they raise a new class of system engineering and battle operational issues. Knowing how and when to use this new capability takes careful planning. For example, the laser weapon must work in concert with existing defenses. A shipboard laser weapon with short time response may be the last line of defense against an incoming enemy missile that has
passed through layers of conventional weaponry; hence, the tolerable range for destruction of missile guidance or munitions is important to survival of the ship. Many such issues, involving the agility of the laser weapon and targets in comparison with the allowable time for weapons use, are still unresolved.
The 1972 Anti-Ballistic Missile Treaty constrained work on space-based platforms with lasers designed for use against space targets. An airborne system is in active development that would intercept tactical (theater) ballistic missiles such as Scuds during their boost phase and blow them up via laser heating. This is an important deterrent since the munitions would be destroyed over enemy territory. A compact chemical oxygen iodine laser (COIL) has been selected for this mission. The Airborne Laser (ABL) program, employing a COIL weapon against theater-range ballistic missile threats launched from a mobile platform, was approved in 1996 to proceed to the Program Definition and Risk Reduction phase. This action ushers in a totally new weapons system concept, capping three decades of intensive R&D. This entirely new dimension in threat deterrence cannot easily be duplicated by other nations and underscores the breakthrough potential of optics to respond to national defense needs.
Laser weapon technology and military needs seem to be converging in this decade. Ongoing programs like the DARPA/Tri-Service Mid Infrared Laser development for the 2- to 5-μm range and the Space Based Laser (SBL) project sponsored by Ballistic Missile Defense Organization enjoy broad support for multiple missions. As a result of the Air Force New World Vistas study, systems and operations analysts are very active in matching laser concepts to today's problems. For example, the notion of a "frugal kill" optimizes laser fluence on target to just the right amount for destruction without overdesigning the system. Future R&D directions feature efficient laser designs such as high-power DL-pumped solid-state lasers and better beam quality for frugal kill.
Fiber-optic (FO) systems represent an area in which commercial investment has led the way for DOD applications. For basic data transmission similar to commercial telephone service, DOD has adapted and improved the hardware for battlefield environments, with commercial organizations leading the R&D effort. For DOD, desirable attributes include freedom from electromagnetic interference, low power consumption, small size and weight, enhanced physical security, and high available data transmission rates.
The use of fiber optics on aircraft, satellites, ships, and submarines has proceeded at a somewhat slower pace as the design and mainte-
nance issues associated with the military field environment have been solved. Digital FO communications are now used both to connect military communications terminals and within military facilities. The center of gravity of DOD development and use has been in the Navy, although the Army push to digitize the battlefield will bring more FO systems into play. Air Force work has featured conventional systems for large ground installations and special FO links for aircraft and satellites. Much of this work has proceeded in parallel with the maturation of commercial FO networks. DARPA has undertaken a major development program, Broadband Information Technology (BIT), to gain greater flexibility and performance for DOD use of commercial networks through test-bed field trials and component development, especially wavelength-division multiplexing (WDM). Special FO and photonic techniques have been developed by DOD. The extension of digital techniques into the terabit region, development of special sensors by the Navy, parallel and serial local area networks for avionics, and both backplane and chip-to-chip communication projects are under way.
Other nondigital uses of fiber optics include the FO gyro (discussed below) and the propagation or control of radio-frequency (RF) signals via fiber, which is important because of the widespread military use of the RF spectrum. Compared with atmospheric RF propagation, fiber offers the inherent advantages of ultrawide bandwidth and much lower propagation loss, but the laser must be modulated and the RF signal faithfully extracted at the receiver terminal. FO usually offers a lower-loss alternative to coaxial cable. At 30 to 100 GHz it will probably prove to be superior to conventional waveguide or microstrip for which the propagation loss is prohibitive at long distances (i.e., >1 m). Today the technology is practical at 20 GHz for direct diode laser modulation and at 50 GHz for external modulation (at higher cost), with further improvement to 30 and 100 GHz expected within 10 years. This technical area has spurred a thriving commercial activity (valued at $100 million) for cable television FO cables, L-band Earth terminal links, and wireless personal communications.
DOD has ground-based communication needs that range from RF transmission to and from remote antennas and within communication terminals, to shared aperture transmit/receive antennas, to very complex phased array antennas for radar. An oft-cited example of the effectiveness of this technology is the avoidance of antiradiation homing missiles launched by the enemy to disable the antenna by guiding on the outgoing radar signals. Remote placement of the antenna, enabled by low-loss FO transmission of RF signals, secures the safety of personnel and the field shelter housing the control equipment.
FO permits a ''true time delay" phased array radar by switching lengths of fiber (see Figure 4.7). This capability is not possible at
microwave frequencies; emulating time delays in RF causes a form of radar degradation termed "squint." The first full FO radar operating at 850-1400 MHz was demonstrated in September 1995. It had a wide spur-free dynamic range (no extraneous signal channel noise) nearly adequate to meet today's desired radar performance requirements. Major Navy projects are well under way to use this technology to make order-of-magnitude improvements in surface ship antenna structures, with tests of readiness planned for 1998. There are no obvious technical impediments to further improvement of FO-RF technology. Widespread future use and high payoff to DOD are likely.
In warfare, timely acquisition and distribution of information is essential. Military planners, very conscious of this basic tenet, are "digitizing the battlefield." This will allow combatants to take advantage of the veritable explosion of information gathering and distribution capability to give a tactical advantage. The Air Force New World Vistas study calls for better cockpit displays to relieve overworked pilots; hence, display technology must keep pace with the means to rapidly process and analyze data. DOD has identified flat-panel displays (FPDs) as a critical technology. Older cathode-ray tube (CRT) displays are being replaced with superior FPDs. Commercial off-the-shelf (COTS) displays are used wherever possible in DOD. However, many
military requirements cannot be satisfied by COTS displays. Since the size of the DOD market is small, accounting for about 2% of the worldwide display market, DOD has sponsored display development and manufacturing necessary to meet its specific needs.
Since 1992, DARPA has led DOD display development programs for high-definition systems (HDs) and head-mounted displays (HMDs) with triservice support. Structured as dual-use programs for commercial and military technology, these DARPA programs have spurred a U.S. presence in a technology otherwise dominated by offshore suppliers. Today, a small number of U.S. suppliers (about 10) are active in serving the special needs of the U.S. military listed in Table 4.1. All displays require tolerance to -54° to 71°C storage temperatures and worst-case shock. Pixel counts vary approximately from 500 x 500 to 2500 x 2500, and most applications require full color.
The DARPA HMD program is now over with reports of mixed success. Active-matrix liquid crystal (AMLCD) and active-matrix electroluminescent (AMEL) systems were both developed. Both technologies had a goal of providing 1280 x 1024 monochrome pixels in approximately 1 square inch. Placing color filters on these elements provides a 640 x 480 color display. Production and device yield issues remain to be solved before fieldable hardware results.
The DOD FPD technologies include plasma, electroluminescence, and AMLCD. Displays utilizing these technologies are operable under high ambient light levels and can be made rugged for field use. Newer FPD technology includes field emission, MEMS (microelectromechanical systems), and three-dimensional displays requiring no viewing aids (e.g., glasses). The U.S. manufacturers still rely on overseas sources for key materials such as polarizer-retardation films, color filter material, and phosphors.
A consortium of manufacturers works on the HDS program in partnership with the government. The following results have been reported:
6.3 x 106 pixels for a 13-inch display
1024 x 768 full-color video tactical monitor
4-inch x 5-inch panel
2-million-element digital color micromirror
This work has been done under the aegis of the National Flat Panel Display Initiative. Efforts are being made to determine these products' producibility and develop manufacturing processes for them. It should be reiterated that COTS displays cannot satisfy all military display needs (Mentley, 1996).
Future DOD R&D activities include high-definition displays with acceleration of field emission display technology and organic luminescent materials for the mobile user.
TABLE 4.1 Military Display Requirements
Command and control workstations
Vehicle and cockpit
Sensor, tactical, instrumentation
Miniature and low power
Head-mounted Weapon mounted Body worn
This section provides an overview of techniques that address special military applications and/or mission requirements. The major technology initiatives are described in detail below. Many other niche applications use optics to advantage, attesting to the many dimensions that this technology can bring to bear on solving specific problems. Examples include the following:
Use of LEDs to mark friendlies during Desert Storm;
Laser scanners that convert synthetic aperture radar (SAR) signals into images;
Moderate-power, handheld medical lasers for cauterizing battlefield wounds; and
Optical radar for tracking satellites.
Chemical and Biological Species Detection
The 1995 release of sarin in the Tokyo subway once again brought us face-to-face with the destruction and loss of human life that a small terrorist group can cause. Some believe that the open U.S. society is particularly vulnerable to this type of attack. The Oklahoma City and World Trade Center bombings are reminders of our need for greater vigilance and better technology for early detection of this type of threat.
Weapons of mass destruction involving nuclear, biological, and chemical (NBC) species are a significant new emerging global threat and have a high priority within DOD. Because chemical and biological weapons can be produced with relatively low technology and are easily acquired, their number is increasing. More than 30 countries are now suspected of having chemical weapons capability and more than a dozen of having biological weapon competence. There are also about a dozen confirmed nuclear-capable countries.
Electro-optic technologies are central to meeting mission area requirements arising from these threats. The technologies of interest include the following:
Active detection devices, such as backscatter lidars at eye-safe wavelengths, differential absorption (DIAL) lidars over the band between 2 and 11 mm, resonance Raman lidars, and laser-stimulated biological fluorescence;
Passive detection devices, including imaging and nonimaging spectrometers and FLIR and Fourier-transform infrared (FTIR) systems; and
Lasers to decontaminate and kill biological species.
Long-range chemical detection using lidar or other stand-off techniques would give the greatest tactical advantage. An effective detection scheme must meet many practical mission requirements. The objective of achieving an ultralow species concentration detection capability is important, but what is also required is an extremely low rate of false alarms for detection for a wide range of chemical species. This places multidimensional requirements on the sensor suite, making electro-optics an essential part of strategies for addressing this threat.
In FY 1996, DOD made major investments in electro-optics for nonproliferation ($20 million), strategic and tactical intelligence, battlefield surveillance ($9 million), counterforce ($18 million), active defense ($2.5 million), and passive defense ($20 million). Furthermore, DARPA announced a new initiative funded at the $100 million level in a multifaceted approach to inject new technology into this critical problem area, and DOE efforts continue at a significant (> $20 million) level. A well-planned, well-coordinated, cohesive effort is required to greatly advance the probability of success. This would include a multiyear strategy overseen by a strong single manager to coordinate activities within DOD, DOE, and so forth.
Laser Gyros for Navigation
Laser gyros are important as inertial navigation sensors. These devices run laser light around in a closed path in both directions; if the platform rotates, the tiny differential time delay can be detected and the rotation rate deduced. Early gyros used helium-neon ring resonators, which oscillated at different frequencies; platform rotation is proportional to the differential frequency. Many years of development have yielded ring laser gyros (RLGs) with a low bias drift (< 0.005 degree per hour) and random walk (< 0.0015 degree per hour), low cost ($10,000), and long life (105 hours). With these parameters, a 4-pound, 80-cubic-inch RLG is suitable for aircraft navigation. For other uses such as missile guidance, smaller, lower-accuracy units are necessary.
Development of the fiber-optic gyro (FOG) has created another alternative, especially for lower-accuracy applications. A FOG with a size and weight of 1.3 cubic inches and 0.3 pound, having a hundredfold lower accuracy, is still adequate for missile guidance and costs only $4,000. These units are now in production as replacements for conventional mechanical gyros that have to be spun up for each mission. Inertial guidance units are still necessary, even in this age of global positioning systems, since these systems can be jammed during wartime.
Optical Signal Processing
Many signal processing functions can take advantage of the unique properties of optics. Mathematical calculations can be performed in real-time using analog optical techniques. Some computations are extremely tedious when performed on a digital computer but relatively easy on an analog optical computer. For example, Fourier transforms can be readily accomplished by placing a suitable lens in an optical telescope. Since Fourier transforms are often used to obtain the frequency spread of short-pulse radar signals, an optical subsystem could provide an important capability.
Image and electronic signal processing have long been fostered by research organizations within DOD, such as the Office of Naval Research. Acousto-optic modulator-based signal correlators have been designed and incorporated into military products, although widespread field deployment has not yet occurred. Prototype vector-matrix, matrix-matrix, and neural network optical processors have all been applied to DOD problems with varying levels of success. Much of the work describing actual field tests remains classified. (Further discussion of these techniques for commercial applications can be found in Chapter 1.)
Summary and Recommendations
Acquisition reform and the use of commercial items are important strategies for reducing DOD system cost, but many commercially available optical products require special adaptation or improvement to meet unique DOD needs. There are special DOD field requirements for military systems that are not required for commercial applications. Examples are (1) displays that must work in high ambient light levels, (2) devices that will withstand large temperature excursions, and (3) IR imaging devices with long-range detection and surveillance requirements. Commercial industry is reluctant to invest development effort in these low-volume military products. However, some products such as diode lasers, developed by the military to exacting specifications, have a large commercial market potential.
Special DOD operational requirements and low-volume production will necessitate continuing DOD support for core optical competencies from basic research to manufacturing technology. For example, DOD is financially supporting the development of a manufacturing plant that produces active-matrix LCD display products for aircraft and other military platforms.
Past producibility efforts directed at key, high-cost system components have successfully lowered costs and improved performance. The optics assembly process is currently the most expensive system manufacturing step. DOD system suppliers are trying to attack this problem with concurrent engineering and assembly-friendly designs. Examples are the triservice-DARPA FPA producibility program for night vision, extensive DOD support of diode laser arrays, and more recent work to enable cost-effective production of vertical cavity surface emission lasers (VCSELs) for photonic applications.
Funding of new technology developments for DOD systems is declining. There is considerable interest in using existing or commercial optical technology. This is a result of DOD's acquisition reform initiated in 1994-95, which stresses an affordability-system performance trade-off. DARPA senior management and industry sources confirm the trend.
DOD R&D funding practices and budgets have resulted in greater optical industry reliance on SBIR grants and cooperative R&D agreements (CRADAs) with federal laboratories. Numerous small firms rely totally on SBIR grants (which are increasing) and CRADAs (which are decreasing) to develop a technology into viable products. The DOD objective of keeping the military technologically sharp so that our nation will not be blindsided by a foreign power adds a longer-range dimension to R&D that firms find indispensable to innovation.
Small companies are becoming increasingly important as a source of advanced optical technology for rapid insertion into new DOD systems. Defense prime contractors have noted that they are increasingly dependent on small companies for innovative solutions as a better economic alternative to in-house work.
The downsizing of DOD programs is eroding the defense manufacturing base. DOD programs must address the producibility needs of high-leverage optical components and systems that provide strategic advantage. COTS items cannot satisfy all military requirements. There is a tendency to associate optical technologies with the use of commercial electronic computers and components. Optical systems do use many commercial parts, and 25% of DOD's optics are imported. However, most military optical systems could not be assembled solely from commercial parts. Companies have been extremely reluctant to change commercial specifications to satisfy peculiar DOD requirements. Also, it is not necessarily in the national interest to design
advanced optical systems that provide U.S. fighting forces with a strategic advantage and are also readily available to potential adversaries. For this reason and to ensure a supply of key components during periods of crisis, DOD must sustain selected manufacturing infrastructures.
Even with post-Cold War macrochanges in military doctrine, DOD will continue to favor optics for surveillance, surgical strike with precision guided munitions, and so forth. High-leverage technologies that offer crucial advantages to the nation must be maintained. It is a daunting task to bring to completion the high-leverage military systems needed in limited conflicts, antiterrorist activities, and rogue nation situations as the DOD budget shrinks. The greatest return on investment has historically come from R&D. This imperative becomes even stronger as the budget shrinks.
Maintaining a proper R&D balance is difficult as DOD downsizes. Very close coordination among all involved parties in optics R&D (e.g., DARPA, the military services, DOE) is essential. The committee's assessment of R&D balance and long-range planning leads to concern. With downsizing, DOD plans become more vulnerable to unexpected uses of R&D funds, such as peacekeeping in Bosnia. The result has been unplanned, short-notice reductions in the R&D budget, which leave key projects incomplete. Since DARPA accounts for roughly 70% of DOD's science and technology budget, very close cooperation with the military services and other agencies is needed to avoid the disruptions that have occurred in the past. With science and technology projects undertaken by many university, government, and industry laboratories, planning must be closely coordinated and continuity maintained to extract maximum benefit from these activities. The critical R&D support that DARPA provides to warfighters with ultimate technology transfer to the services dictates early, close coordination of all parties.
The confluence of a number of DOD and congressional acquisition policy changes has resulted in greater COTS use, lower-cost system design compromises, and greater dependence on small companies for new technology. This chapter has discussed the acquisition policy of cost-performance-specification optimization for new systems. Economic factors have also forced reliance on commercial and other small business enterprises for R&D and manufactured products. This trend is particularly strong in the optics field, with hundreds of small companies supplying government's needs.
DOD should ensure the existence of domestic manufacturing infrastructures capable of supplying low-cost, high-quality optical components that meet its needs via support for DARPA and the Manufacturing Technology Program.
It is fundamental that a well-founded manufacturing process saves money on the ultimate product, so this recommendation could seem highly generic. However, with increasing reliance on innovative small suppliers, affordability and quality as new program trade parameters, lower-volume system production, unique mission requirements, and new mission objectives for DOD, the manufacturing paradigm has changed. New manufacturing techniques are being developed to improve commercial productivity, and DOD should take full advantage of them.
A central, coordinated DOD-DOE time-phased plan should be developed and conducted to enable worldwide optical detection and verification of chemical species that threaten civilians and military personnel through hostile attacks.
Much has been said about this problem, and its implications are clearly very serious in both civil and military scenarios. After many years of work, partial solutions have emerged. The committee believes that concerted R&D activity with a focus on optics has the best chance for a technical solution to airborne detection in view of the rich molecular spectrum accessible by optical methods. A single federal authority should be placed in charge of these crucial programs.
A coordinated multiyear DOD plan should be conducted to develop RF photonic phased antenna-array technology for radar and communications.
An L-band version of this photonic system and many of the components for higher-frequency systems have been demonstrated. With improved modulator and switch designs, Bragg grating fibers, and higher-power diode lasers, new and better approaches are likely and it is time for a major push.
Key technologies such as high-power laser activities and new optics should continue to be pursued by DOD.
Erosion of the optical technology base through benign neglect or lack of federal agency coordination must not be allowed; the return on investment is very high. After decades of work, fieldable laser devices can now provide the power levels, spectral diversity, and adaptive optics configurations necessary as countermeasures to extreme threats, especially from missile attack. Recent program awards—the Airborne Laser program ($1.1 billion from 1996 to 2002), the Space Based Laser ($100 million per year), and two conformal optics programs ($24.6 million)—carry forward some of this essential work. New laser sources and optical technology innovations offer solutions and totally new system concepts that can provide our nation with true strategic and tactical defense advantages.
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