C

Additional Technology Examples

ENABLING TECHNOLOGIES

This appendix gives examples of additional technologies in the area of optics and photonics as they relate to many of the fields described in the chapters of this report. The Committee on Harnessing Light: Capitalizing on Optical Science Trends and Challenges for Future Research believes that this compilation puts additional emphasis on how optics and photonics truly are enabling technologies, and at the same time it provides the reader with further examples that highlight the many complex ways that optics and photonics support the foundation of many common areas not always directly associated with the fields of optics and photonics.

DEFENSE AND NATIONAL SECURITY

This section discusses the changes in many of the areas that were addressed in “Optics in National Defense,” Chapter 4 of the National Research Council’s (NRC’s) 1998 Harnessing Light: Optical Science and Engineering in the 21st Century.1 The subsections below provide an update for the areas of surveillance, night vision, laser systems operating in the atmosphere and in space, fiber-optic systems, and special techniques (e.g., chemical and biological species detection, laser gyros, and optical signal processing).

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1 National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, D.C.: National Academy Press.



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C Additional Technology Examples ENABLING TECHNOLOGIES This appendix gives examples of additional technologies in the area of optics and photonics as they relate to many of the fields described in the chapters of this report. The Committee on Harnessing Light: Capitalizing on Optical Science Trends and Challenges for Future Research believes that this compilation puts ad- ditional emphasis on how optics and photonics truly are enabling technologies, and at the same time it provides the reader with further examples that highlight the many complex ways that optics and photonics support the foundation of many common areas not always directly associated with the fields of optics and photonics. DEFENSE AND NATIONAL SECURITY This section discusses the changes in many of the areas that were addressed in “Optics in National Defense,” Chapter 4 of the National Research Council’s (NRC’s) 1998 Harnessing Light: Optical Science and Engineering in the 21st Century.1 The subsections below provide an update for the areas of surveillance, night vision, laser systems operating in the atmosphere and in space, fiber-optic systems, and special techniques (e.g., chemical and biological species detection, laser gyros, and optical signal processing). 1  National Research Council. 1998. Harnessing Light: Optical Science and Engineering for the 21st Century. Washington, D.C.: National Academy Press. 288

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A pp e n d i x C 289 Surveillance Surveillance still plays a critical role in the detection and assessment of hostile threats to the United States. High-resolution imaging satellites have been deployed for more than 50 years to provide critical data for U.S. defense experts over de- nied airspace. The progress in optical sensors over the past decade has created an exponential growth in intelligence, surveillance, and reconnaissance (ISR) data from both passive and active sensors. This progress includes not just an increase in area coverage rate, but also an increase in sensor capabilities and performance. Material advances have made collection at new wavelengths feasible, and improved components provide new data signatures including vibrometry, polarimetry, hy- per-spectral signatures, and three-dimensional data that mitigate camouflage for targets of interest. A key advance since the NRC’s 1998 study is the dramatic increase in the ap- plication of active optical sensors for surveillance. The primary impetus for this increase has been the advances made in laser technology, including advances in robustness, efficiency, and optical power (see Figure C.1) for many wavelengths. In order for optical sensors to be widely fielded, they must also meet eye-safety requirements, which have driven advances in sources and amplifiers for 1.5 and 2 µm wavelength lasers. In order to maximize atmospheric transmission, there has been a push for longer wavelength amplifiers in regimes with efficient detectors. Recent advances in thulium (Tm)-doped fiber amplifiers enable laser sensor operation in the 1.9 to 2.1 µm wavelength range. Average power levels are approaching the kilowatt level, and pulsed amplifiers with peak powers approaching 100 kW have been demonstrated.2,3,4,5 Several vendors are offering lasers with output powers up to 150 W. There are also several vendors offering narrow line-width, rapidly tunable 2.1 µm laser sources. The atmospheric transmission at this wavelength combined with the availability of commercial amplifiers, sources, and detectors makes 2.1 µm an attractive wavelength for long-range laser sensing. The efficiency of the current 2  Cristensen, S., G. Frith, and B. Samson. 2008. “Developments in Thulium-Doped Fiber Lasers Offer Higher Powers.” SPIE Newsroom. DOI: 10.1117/2.1200807.1152. Available at http://spie.org/ x26003.xml. Accessed July 31, 2012. 3  Moulton, P.F., G.A. Rines, E.V. Slobodtchikov, K.F. Wall, G. Firth, B. Samson, and A.L.G. Carter. 2009. Tm-doped fiber lasers: Fundamentals and power scaling. IEEE Journal of Selected Topics in Quantum Electronics 15(1):85-92. 4  Sudesh, V., T.S. McComb, R.A. Sims, L. Shah, M. Richardson, and J. Stryjewsky. 2009. Latest de- velopments in high power, tunable, CW, narrow line thulium fiber laser for deployment to the ISTEF. Proceedings of SPIE 7325:73250B. 5  McComb, T.S., R.A. Sims, C.C.C. Willis, P. Kadwani, V. Sudesh, L. Shah, and M. Richardson. 2010. High-power widely tunable thulium fiber lasers. Applied Optics 49(32):6236.

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290 Optics and Photonics: Essential Technologies for O u r N at i o n FIGURE C.1  Progress in output power for 1 µm, 1.5 µm, and 2 µm optical amplifiers. This plot tracks the historical progress in output power from a single fiber amplifier. Ytterbium (Yb)-fiber amplifiers (1 µm) have a significant lead over other technologies primarily due to their low quantum defect and the abundance of high-brightness pump sources at 915 to 975 nm. However, thulium (Tm)-fiber am- plifiers (1.9 to 2.1 µm) have been demonstrated with average output powers approaching 1 kW and pulsed operation approaching 100 kW without sacrificing beam quality. There are several commercial units available with an average power ~150 W. Their efficiency is set by the efficiency of the pump source, which should continue to improve over time as demand for Tm-lasers increases. SOURCE: Cristensen, S., G. Frith, and B. Samson. 2008. “Developments in Thulium-Doped Fiber Lasers Offer Higher Powers.” SPIE Newsroom. DOI: 10.1117/2.1200807.1152. Available at http://spie.org/x26003. xml. Reprinted with permission. commercial amplifiers is approximately 6.25 percent6 limited by the efficiency of the pump source. However, there have been many investments made in these areas, which should improve the efficiency over time. 6  See, for example, IPG Photonics, “2 Micron CW Fiber Lasers.” Available at http://www.­ pgphotonics. i com/products_2microns.htm. Accessed July 31, 2012.

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A pp e n d i x C 291 Night Vision Night-vision capabilities continue to be an important tactical tool for the warfighter. In fact, the proliferation of equipment over the past few decades has led to a significant amount of surplus equipment available at very low cost, which has eroded the tactical advantage for the United States that existed for some time. During the First Gulf War, the United States “owned the night,” with U.S. night- vision systems significantly outperforming Iraqi night-vision sensors.7 However, the current commercially available night-vision sensors are nearly equivalent to the best U.S. night-vision systems. Since the NRC’s 1998 Harnessing Light8 study, there have been substantial improvements in sensitivity, performance for uncooled systems, and expanded wavelength applicability, which have enabled practical ther- mal imaging systems for size, weight, and power (SWaP)-constrained platforms. Laser Rangefinders, Designators, Jammers, and Communicators The significant increase in laser diode efficiency coupled with the decrease in cost has enabled recent advances in laser designators. However, similar advances in night vision and imaging detector arrays have limited the use of laser designa- tors and led to ground force casualties in recent engagements. Therefore, there is a greater push for SWaP improvements to enable designators on small unmanned platforms (e.g., micro-unmanned aerial vehicles [UAVs]), which will also carry over to active sensors and optical communication systems. Early laser designator systems used neodymium-doped yttrium aluminum garnet (Nd=YAG) lasers at 1 µm. However, improvements in laser materials and efficiency have enabled a wider range of wavelengths to be implemented. A large investment in laser communications had been made prior to publica- tion of the NRC’s 19989 report and has continued since that time. Optical commu- nication in fibers has been steadily advancing in the past decade. The high carrier frequency of light, combined with the low attenuation in fiber, makes it attractive for telecommunications applications. For free-space applications, the short wave- length improves directivity by minimizing diffraction when compared to radio- frequency (RF) communications. This is one of the key motivators for moving to optical communications, which minimize the probability of interception, jamming, and detection while dramatically minimizing the power needed for a given com- munication bandwidth, since most of the energy can be focused on the receiver. 7  National Research Council. 2010. Seeing Photons: Progress and Limits of Visible and Infrared Sensor Arrays. Washington, D.C.: The National Academies Press. 8  National Research Council. 1998. Harnessing Light. 9  National Research Council. 1998. Harnessing Light.

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292 Optics and Photonics: Essential Technologies for O u r N at i o n Laser Weapons The Missile Defense Agency demonstrated the potential use of directed energy to defend against ballistic missiles when the Airborne Laser Test Bed (ALTB) suc- cessfully destroyed a boosting ballistic missile on February 11, 2010.10 The experi- ment, conducted at Point Mugu Naval Air Warfare Center-Weapons Division Sea Range off the coast of central California, served as a proof-of-concept demonstra- tion for the directed-energy technology. The ALTB is a pathfinder for the nation’s directed-energy program and its potential application for missile defense technol- ogy. For the demonstrations, a short-range threat-representative ballistic missile was launched from an at-sea mobile launch platform. Within seconds, the ALTB used onboard sensors to detect the boosting missile and used a low-energy laser to track the target. A second low-energy laser was fired to measure and compen- sate for atmospheric disturbances. Finally, the ALTB fired its megawatt-class high energy laser, heating the boosting ballistic missile to critical structural failure. The entire engagement occurred within 2 minutes of the target missile launch, while its rocket motors were still thrusting. This was the first directed-energy lethal intercep- tion demonstration against a liquid-fuel boosting ballistic missile target from an airborne platform. This revolutionary use of directed energy is very attractive for missile defense, with the potential to attack multiple targets at the speed of light, at a range of hundreds of kilometers, and at a low cost per interception attempt compared to the cost with current technologies. Fiber-Optic Systems Fiber-optic systems have continued to evolve to achieve higher performance with lower power in a smaller volume. Fiber-optic systems (e.g., gyros, communi- cation links) have several attractive attributes including low loss, high transmis- sion rates, and freedom from electromagnetic interference. Therefore, they have continued to be adopted into military platforms as they are upgraded. Special Techniques The special techniques (i.e., chemical and biological species detection, laser gyros, and optical signal processing) evaluated in the NRC’s 1998 report11 have evolved in different ways. Optical signal processing has also advanced, but not at the pace forecasted at that time. Importantly, recent advances in optical integrated 10  Missile Defense Agency, U.S. Department of Defense. 2010. “Airborne Laser Test Bed Suc- cessful in Lethal Intercept Experiment.” MDANews Release. Available at http://www.mda.mil/ news/10news0002.html. Accessed August 2, 2012. 11  National Research Council. 1998. Harnessing Light.

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A pp e n d i x C 293 circuits should enable significant advances in optical signal processing over the next decade. Chemical and Biological Species Detection Weapons of mass destruction, including nuclear, biological, and chemical weapons, continue to be a high-priority threat. Long-range chemical and biological detection has advanced considerably since the 1998 report.12 One example is the Joint Biological Stand-off Detection System (JBSDS), a light detection and ranging (LIDAR)-based system that is designed to detect aerosol clouds out to 5 km in a 180 arc and to discriminate clouds with biological content from clouds without biological material at distances of 1 to 3 km or more. This system will provide advance warning of the presence of potential biological weapon aerosol cloud hazards so that a commander can implement individual and collective protective measures for assigned forces. Laser Gyros for Navigation Laser gyros were already very mature at the time of the 1998 report.13 They are critical in maintaining precision navigation when the Global Positioning System (GPS) is unavailable due to platform constraints or jamming. A new advance since the 1998 report is in the area of star-trackers, which can augment inertial naviga- tion systems to improve long-term stability. Optical Signal Processing Optical processing has not changed very much since the NRC’s 1998 study.14 It continues to be very promising, since some mathematical functions can be performed very rapidly using optical analog techniques. One example is optical correlations that rely on Fourier transforms. Optical correlators compare two- dimensional image data at very high speeds. They were invented in the mid-1960s and have traditionally been used in high-cost military applications such as the analysis of satellite photographs. With recent advances in liquid-crystal technology, optical correlators have become more commercially viable—at a fraction of the high costs previously associated with such high-performance systems. Image data that are entered into the optical system are compared during the correlation process in terms of two criteria, similarity and relative position. Typically, the comparison 12  National Research Council. 1998. Harnessing Light. 13  National Research Council. 1998. Harnessing Light. 14  National Research Council. 1998. Harnessing Light.

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294 Optics and Photonics: Essential Technologies for O u r N at i o n is done between a reference image (e.g., from a database) and an input image (e.g., from an external camera or sensor). ENERGY Cost of Solar Technology In a 2008 study, leading experts in the solar field working on various technolo- gies were asked to estimate the probability of any technology reaching two different “dollars per watt” benchmarks.15 The two price benchmarks were $1.20/W installed cost, the point at which a technology can be considered commercially viable, and $0.30/W installed cost, the point at which a technology would likely emerge as dominant in supplying utility-scale energy. The experts were asked to gauge the probability that any solar technology would meet these price points by the years 2030 and 2050. The results of this study are shown in Figure C.2, in which a larger circle corresponds to a larger number of experts giving this probability. Hybrid Solar and Wind Power Systems Another approach to harvesting the Sun’s energy is a solar updraft tower, which uses the greenhouse effect to create a hybrid of solar and wind power. This technology uses a large base area sealed by a transparent material. The air heats to approximately 70°C due to the greenhouse effect. This air is then forced out of the high central tower, referred to as a “solar chimney,” as shown in Figure C.3. This is expected to produce wind, which will then be used to power turbines. Two 200-MW plants have been proposed for installation in western Arizona.16 Supporting Technologies for Solar Power The solar power industry has several supporting technologies that are crucial to further development but not directly involved in converting the Sun’s energy into electric power. Technologies such as mounts for solar modules and electronics are also crucial to the commercialization of solar power technologies. Work has been done to model the competitiveness of a given solar technology quantitatively given the variability of many uncertain factors. Several modeling programs are being developed, one of which has become widely available: the Solar 15  Curtright, A., M. Granger Morgan, and D. Keith. 2008. Expert assessments of future photovoltaic technologies. Environmental Science and Technology 42(24):9031-9038. 16  Southern California Public Power Authority. 2008. “La Paz Solar Tower Project.” Available at http://www.scppa.org/pages/projects/lapaz_solartower.html. Accessed July 30, 2012.

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A pp e n d i x C 295 FIGURE C.2  Estimates of the probability of any solar technology reaching two dollars-per-watt price points—$1.20/W and $0.30/W—by 2030 and 2050, according to experts in the solar field. Larger circles correspond to a larger number of experts giving this probability. SOURCE: Estimates based on Curtright, A., M. Granger Morgan, and D. Keith. 2008. Expert assessments of future photovoltaic technologies. Environmental Science and Technology 42(24):9031-9038. Reprinted with permission. FIGURE C.3  An updraft solar tower power plant scheme. SOURCE: Redrawn and slightly modified by Cryonic07. Original jpg-drawing made by fr:Utilisateur:Kilohn limahn.

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296 Optics and Photonics: Essential Technologies for O u r N at i o n FIGURE C.4  A sample output graph produced from the Solar Advisor Model. SOURCE: Mooney, D., M. Mehos, N. Blair, C. Christensen, S. Janzou, and P. Gilman. 2006. “Solar Advisor Model (SAM) Overview.” P. 23 in 16th Workshop on Crystalline Silicon Solar Cells and Modules: Materials and Processes; Extended Abstracts and Papers. Sopori, B.L., ed. Proceedings of the NREL/BK-520-40423 workshop held August 6-9, 2006, in Denver, Colorado. Golden, Colo.: National Renewable Energy Laboratory. Reprinted with permission. Advisor Model (SAM), produced by the National Renewable Energy Laboratory (NREL) and Sandia National Laboratories.17 This system considers a wide range of module performance options, financing options, and government subsidies and will calculate the levelized cost of energy (LCOE) for an expected situation. A sample output of SAM is shown in Figure C.4, although the program produces an enormous amount of information and only a small part of the output is rep- resented in the figure. Models that can predict the performances of solar power relative to alternate sources of energy have been developed. Among these is the program ALTSim, developed at Hobart and William Smith Colleges to determine the viability of 17  Mooney,D., M. Mehos, N. Blair, C. Christensen, S. Janzou, and P. Gilman. 2006. “Solar Advi- sor Model (SAM) Overview.” P. 23 in 16th Workshop on Crystalline Silicon Solar Cells and Modules: Materials and Processes; Extended Abstracts and Papers. Sopori, B.L., ed. Proceedings of the NREL/ BK-520-40423 workshop held August 6-9, 2006, in Denver, Colo. Golden, Colo.: National Renewable Energy Laboratory.

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A pp e n d i x C 297 biofuels competing with fuels such as oil and coal.18 Elaboration and integration of economic modeling programs such as these would enable more quantitative comparisons among technologies and allow more informed investment decisions. A large portion of the module cost for solar applications, particularly those that require concentration, comes from the tracker that the system must be mounted on. These systems must be able to keep accurate alignment with the Sun, moving a large panel area. Maintaining the proper alignment is made more difficult by the requirement that the system stay aligned despite the significant wind loading generated by the solar panel carried. The tracker can comprise approximately half the cost of a current concentrating system. Reducing this cost while not compro- mising the performance or lifetime of a tracker would make these systems much more commercially viable. Although the majority of the cost for a solar plant is the solar collection mod- ule, the electronics required to interface with the power grid and power storage still form a substantial portion of the cost. Solar panels produce direct current (DC) power, which must be transformed into alternating current (AC) to be sent into the power grid, requiring the use of an inverter. In some cases solar power can be matched to the load—for example, in the southwestern United States, where air conditioning drives peak load. Solar power up to some level can be used to handle this peak and is thus matched to the load. If solar power is also to be used at night, however, photovoltaic (PV) devices require a battery, or some other method of storage to store the power produced to be used when the system is not producing power. Charging a battery requires a charge controller, further adding to the system costs. Improving the performance or reducing the cost of any of these devices will make solar power more cost-effective. An approximate cost breakdown of these components is presented in Table C.1.19 The electronics cost is substantially reduced for concentrating solar power (CSP) systems, as the generator can produce AC power directly to the grid and the battery is unnecessary. These systems do have a large additional cost of thermal storage systems for on-demand generation, which costs approximately 30 percent as much as the solar module.20 Technical or manufacturing advances in this field will drive the cost of CSP plants down. 18  Drennen, T., R. Williams, and A. Baker. 2009. Alternative Liquid Fuels Simulation Model (AltSim). Sandia Report SAND2009-7602. Albuquerque, N. Mex.: Sandia National Laboratories. 19  Solarbuzz. 2011. “Solar Buzz Retail Pricing.” SB_Retail_Pricing_111013.xls. Available at http:// www.solarbuzz.com/facts-and-figures/retail-price-environment/module-prices. Accessed June 22, 2011. 20  Greenpeace International. 2009. “Global Concentrating Solar Power Outlook 09.” Available at http://www.greenpeace.org/international/en/publications/reports/concentrating-solar-power-2009/. Accessed July 31, 2012.

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298 Optics and Photonics: Essential Technologies for O u r N at i o n TABLE C.1  Approximate Cost Breakdown Between Module Cost and Various Complementary Electronics Required Module Unit September 2010 September 2011 Module US $/Wp (?125 W) $3.61 $2.65 Euro €/Wp (?125 W) €3.23 €2.43 Inverter US $/continuous Watt $0.715 $0.714 Euro €/continuous Watt €0.558 €0.500 Battery US $/output Watt hour $0.207 $0.213 Euro €/output Watt hour €0.161 €0.149 Charge Controller US $/Amp $5.87 $5.93 Euro €/Amp €4.58 €4.15 Solar Systems Residential ¢/kWh 34.28 29.53 Commercial ¢/kWh 24.32 19.97 Industrial ¢/kWh 18.95 15.56 SOURCE: Frost and Sullivan analysis. HEALTH AND MEDICINE The roles played by imaging, optics, and photonics in modern medicine are mentioned in Chapter 6. Some of the details of the technologies used are examined below. Optics and Photonics in the Emergency Room In the modern emergency room, the technologies mention take advantage of photons with energies chosen so that they can penetrate deep into the human body. The performance of present-day CT instruments has improved dramatically over the past decades owing to advances in x-ray sources and the introduction of sophisticated multi-element, high-efficiency x-ray detectors. These instruments provide images with submillimeter resolution over large volumes of the body in mere seconds. These almost instantaneous three-dimensional images provide visual evidence of life-threatening disorders, saving precious minutes in life-and- death situations in the emergency room. Optics and Photonics in Diagnostics The high-speed blood work mentioned in Chapter 6 can also help determine the status of the patient’s immune system. When the AIDS epidemic was first detected in the early 1980s, the cause of the disease was unknown. It took several years for the human immunodeficiency virus (HIV) to be identified as the infec- tious agent and several more years for the affected immune system cell types to

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320 Optics and Photonics: Essential Technologies for O u r N at i o n FIGURE C.19 Liquid-crystal changing polarization. NOTE: TFT, thin-film transistor; ITO, indium tin oxide. SOURCE: © Merck KGaA, Darmstadt, Germany. Reprinted with permission. Three-Dimensional Technology in LCDs One method used to achieve a three-dimensional effect using an LCD display that was mention in Chapter 10 alternates showing an image for one eye, then a slightly displaced image for the other eye. Wireless connection between a pair of glasses and the display keeps the polarization of glasses in sync, only passing light to the intended eye. The first-generation three-dimensional LCD televisions used such active shutter glasses, seen in Figure C.20. The shutter glasses were themselves crude LCDs, consisting of a single large pixel per eye. One of the challenges of this approach is brightness. In an LCD, the entire screen is not switched at once, but the screen is painted line by line, typi- cally from top to bottom. As the screen is being refreshed for one eye, the top of the display shows the desired image for that eye, but the bottom of the display still shows the previous image for the other eye. The display is constantly refreshing, so only for an instant during each cycle would the screen show an image for only one eye. To overcome this problem, an image for a given eye was shown twice in suc- cession and the shutter glasses opened during the second painting. Unfortunately,

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A pp e n d i x C 321 FIGURE C.20  Shutter glasses blocking left image from entering right eye. Image courtesy of E. Svedberg. this solution meant that the shutter glasses for any one eye were passing light only during one out of four frames. Thus, the apparent brightness of the display was only one-quarter that of the same display when used in a two-dimensional mode without glasses. More recently an alternative approach to three-dimensional displays has been introduced, placing a patterned optical retarder on the face of the display. The role of this retarder is to continually twist the polarization of every other row, as shown in Figure C.21. To view the display in three dimensions, observers wear passive glasses that have polarizers 90° opposed. The left eye can then only observe the odd rows, say, while the right eye can only observe the even rows. The screen brightness is thus increased because both eyes are constantly receiving signal. Unfortunately, the trade-off of this alternate scheme is a reduction in verti- cal resolution. Rather than each eye receiving the 1,080 rows of a standard high- definition set, each receives only 540 rows. This reduction in vertical resolution has motivated some to suggest that future sets be made with the standard 1,920 horizontal pixels but with double the number of vertical pixels, to 2,160, so that each eye then can receive full high definition when viewing in three-dimensional mode. In two-dimensional mode, the even and odd row pairs could mimic one another to result in standard high definition, as input sources with doubled vertical resolution may be rare. Another possible approach for creating three-dimensional displays puts two

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322 Optics and Photonics: Essential Technologies for O u r N at i o n FIGURE C.21  Highly simplified view of patterned retarder and the observer’s passive glasses. Image courtesy of E. Svedberg. liquid-crystal displays in series. In this active retarder scheme, the rear display has the traditional role, while the role of the front display is to rotate the polarization on a frame-by-frame basis. The viewer then wears passive glasses to filter out im- ages not intended for a given eye. One might expect brightness to be an issue with an active retarder as it is with active shutter glasses. However, the brightness issue can be greatly reduced by synchronizing the two displays and segmenting the backlight. That is, during the refreshing of a given row or set of rows, the backlight can be turned off. As the set of rows is updated for the alternate eye, both in terms of image from the rear display and polarization from the front display, the backlight is turned on for those

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A pp e n d i x C 323 rows. The dark period is then only during the refreshing of a set of rows rather than affecting the entire display. Other Three-Dimensional Display Methods The discussion of three-dimensional displays so far has involved glasses of one sort or another. Unfortunately, the need for glasses, even the relatively more comfortable passive glasses, is thought to be stifling the broad adoption of these dis- plays. So the question arises as to how to create three-dimensional displays without the need for glasses. There are actually two cases to consider: that of the individual observer and that involving a group of observers. Of the two, the case of the indi- vidual observer is the easier, provided that the position from which the individual is observing is known. This may be a reasonable assumption when considering a handheld device. In such a situation, there are several means of alternating the delivery of a left image only to the left eye and a right image only to the right eye. For example, the backlight might have an illumination source on the left and right side within the display, with those sources alternating and being steered by some projection film to one eye, then the other. The far more difficult problem is having a three-dimensional display without the need for glasses when multiple viewers can be positioned in a wide variety of locations. As was mentioned in Chapter 10, the most popular approach is the use of viewing zones created by lenticular arrays. Touch Displays The signal detection in capacitive touch displays is dependent on the grid of unit cells, defined by a unique combination of row and column electrodes. When a signal is applied to a row electrode, the proximity of column electrodes results in coupling that can be measured. By sweeping through the rows, measurement can be made of the entire screen. As illustrated in Figure C.22, the signal radiates a small distance through non- conductive materials, such as the cover glass, and one might say that such coupling projects through the cover glass. This coupling is attenuated by a finger touching the cover glass, which provides a path to ground through the body. This reduction in capacitive coupling can be measured, and based on the readings from each unit cell the center of the touch position or positions can be interpolated to higher resolution than the cell spacing. This imaging of the touch positions has enabled the multi-touch capability. While the conductors inside the display aperture area must be transparent, outside the display aperture, and beneath the black border that commonly sur- rounds the display, are metal conductors that have lower resistivity than the ITO, providing for reduced signal loss en route to an integrated circuit (not shown)

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324 Optics and Photonics: Essential Technologies for O u r N at i o n FIGURE C.22  Projected capacitive touch operation. (Top) The electric field couples between the source and drain through the dielectric touch panel. (Bottom) A grounding by means of touch effects the coupling and can be used to sense position of the touch. (Not to scale.) mounted to a flex circuit bonded to the sensor glass with anisotropic conductive film, as shown in Figure C.23. There have been efforts within the industry to eliminate the separate substrate that is dedicated to the touch sensor function. Of the existing substrates considered for integration with this function, the leading candidates are the underside of the cover glass and the face of the color filter glass. Although the underside of the cover may seem quite attractive, subtle aspects work against this choice. In particular, touch sensors based on dedicated substrates are typically fabricated on large glass sheets, which are then diced into the smaller sheets needed for the display, even though such large-scale lithography runs coun- ter to the manner in which cover glass is made. The common way for cover glass to be fabricated achieves high retained strength so that it can survive damage inflicted by everyday use. This is done by putting all surfaces under compression. After cutting the cover to shape, it is then dipped into an ion exchange acid bath where smaller sodium ions are exchanged for larger potassium ions, putting all surfaces under compression. Since glass fails

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A pp e n d i x C 325 FIGURE C.23  Dedicated touch sensor glass. SOURCE: Courtesy of Zytronic Displays Limited. under tension, not compression, the compressive stress layer increases damage resistance. Unfortunately, dipping the entire cover in the acid bath results in all exposed glass surfaces becoming ion exchanged, not just the large front and back faces. If a large sheet of uncut cover glass were to be ion exchanged, after which the transpar- ent conductors were patterned, dicing individual covers from the large sheet would be difficult because the sheet already had been ion exchanged. Even if success in dicing were achieved, this still would be problematic because none of the exposed edges would have been ion exchanged, and thus central tension would be exposed. As integrating touch into the cover is difficult, the face of the color filter glass is an alternative location to consider. However, the fundamental challenge here is that the switching noise from the transistors painting the display image can be coupled

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326 Optics and Photonics: Essential Technologies for O u r N at i o n into the touch electrodes. Designers of touch ICs have made some progress toward devising schemes to avoid and/or reject such noise.23 One of the implications of integrating touch into the face of the LCD’s color filter glass is the elimination of air gaps. Although the dedicated touch substrate is commonly bonded directly to the cover glass, the bonded assembly itself is only sometimes bonded to the front polarizer of the display. Such direct bonding results in additional coupling of display noise, which is a particularly challenging problem with larger displays, like those of tablets, since ITO is such a poor conductor. As a result, although direct bonding can improve optical transmission by eliminating Fresnel reflections on either side of the air gap, eliminating the air gap is more challenging with larger displays because of problems with achieving high yields with this process. These weaknesses and those mentioned in Chapter 10 have motivated inven- tors to look for other technologies to compete with projected capacitive. As mentioned in Chapter 10, optical touch displays have limitations. How- ever, the size and conductivity of the touching object in such a system are im- material, and there is no reduction in optical transparency as there is with the not-completely-transparent conductors and any air gap of a projected capacitive touch system; see Figure C.24. However, optical touch capability has not been achieved on a large scale and—while the multi-touch experience is now commonly expected because of the widespread use of modern handheld devices—has yet to be developed. Display Frames A trend in the use of LCDs in arrays is the reduction in the gap, or bezel, be- tween the pixels of adjacent displays for application in video walls. Although still not completely seamless, gaps have been reduced to the single-digit millimeter range, as represented in Figure C.25. This change benefits the image quality, but it has repercussions for touch. In particular, a cover glass can be bonded to a rela- tively wide bezel, but as that bezel shrinks, it becomes less feasible to use the bezel as a mount. Although very narrow brackets might be used to affix cover glass to large-format displays, in time that might change to direct bonding of cover glass. This prospect has numerous challenges, such as cutting the glass after ion exchange as mentioned above in the discussion of integrated touch. Thermal ex- pansion would be an additional challenge, as current color filter substrates match the thermal expansion of the glass with the thin-film transistors (TFTs), which are 23  See, for example, “Development of IPS LCD with Integrated Touch-Panel by Hitachi Displays.” 2010. Available at http://japantechniche.com/2010/10/08/development-of-ips-lcd-with-integrated- touch-panel-by-hitachi-displays/. Accessed July 27, 2012.

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A pp e n d i x C 327 FIGURE C.24  Optical sensors in the upper corners of the array detecting touch. SOURCE: NextWindow. ­ 2012. “Optical Touch Overview.” Available at http://www.nextwindow.com/optical/. Reprinted with permission. FIGURE C.25  Narrow-bezel liquid-crystal display array. SOURCE: Courtesy of NEC Display Solutions of America.

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328 Optics and Photonics: Essential Technologies for O u r N at i o n switched to control the light valves that form the image. As these transistors are based on silicon, the glass substrate material is chosen to match the coefficient of thermal expansion of silicon, which is relatively low. Cover glass, by contrast, has relatively high thermal expansion, as this is currently thought necessary to achieve a glass capable of ion exchange. Nevertheless, if an internal polarizer were to be achieved, researchers would be highly motivated to create a glass that combines cover and color filter functionality. OLED Displays In order to form a light-emitting device, the light-emitting organic layer of an organic light-emitting diode (OLED) requires several other layers. These in- clude a transparent substrate, which can be either rigid or flexible, depending on the application; a transparent conducting anode; a conducting organic layer; the organic light-emitting layer; and a cathode, which may or may not be transparent, depending on the application. In operation, an electrical potential is applied across the OLED by connecting a battery or other power source between the cathode and the anode, causing a current to flow. The current flow results in electrons being removed from the molecules in the emissive organic layer, creating holes. When these holes are filled at the in- terface with the conducting organic layer, the electrons give up their excess energy as photons. The intensity of the emitted light is determined by the total current flow, and the color is determined by the energy level of the hole that is filled by the electrons. This, in turn, is determined by the properties of the organic molecules, allowing OLEDs to be used in color displays. OLEDs can be made on transparent substrates to form an all-transparent display, or on an opaque or reflective substrate. In the former case, it makes pos- sible what is known as a heads-up display, since only the displayed information interrupts the visual field. Flexible Displays “Flexible display technology” is a term used for a desirable technology for the next generation of cell phones, military devices, and reading devices. A device with flexible display technology would enable the user to overcome the fear of breaking, bending, or scratching the device. One type of flexible display technology uses organic films constructed from OLEDs, which in turn are made from layers of organic material and the conductive materials needed to inject electrons and holes. When a voltage of proper polarity is applied to the conductive layers, electrons from one layer combine with the holes

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A pp e n d i x C 329 from the other, releasing light. If these OLEDs were constructed from polymers with high flexibility, they could be the basis of lightweight, portable, rollup displays, or displays that could be used on curved surfaces. Another promising material technology is amorphous oxides. Some amor- phous oxides can form thin films that are transparent and electrically conductive, which is why they already serve as the see-through electrode layer in displays and solar cells. It was this combination of qualities that led to the surge in research that began in 1996, when Hideo Hosono and his colleagues at the Tokyo Institute of Technology first noted the merits of amorphous transparent conducting oxides. The biggest problem when amorphous silicon is deposited on flexible plastic is switching and drifts. Amorphous oxides could do more than simply serve as passive electrodes. They could also replace amorphous silicon as the active semiconducting material in TFTs. The advantages of oxide semiconductors over amorphous silicon are mo- tivating much work in the display industry. Only 2 years after the first oxide-based transistors were reported, Korea’s LG Electronics Co. revealed a prototype OLED display that used indium gallium zinc oxide (IGZO) transistors to drive its pixels. Other companies followed quickly, with oxide-based displays of their own. The U.S. $100 billion flat-panel-display industry has been built on amorphous silicon, and the new materials will have to compete with its 30-year head start. However, amorphous silicon is a mature technology, and most limitations arise from funda- mental physical and chemical properties requiring breakthroughs. Amorphous oxide semiconductors will likely challenge amorphous silicon. When this will happen depends mainly on the development time for a large-scale FIGURE C.26  Crystalline, polycrystalline, and amorphous atomic structures. SOURCE: Reprinted, with permission, from Wager, J.F., and Hoffman, R. 2011. Thin, fast, and flexible. IEEE Spectrum 48(5). Copyright 2011 by IEEE.

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330 Optics and Photonics: Essential Technologies for O u r N at i o n manufacturing capability incorporating these materials. However, the oxide TFT fabrication process is very similar to that used for amorphous (for atomic struc- tures, see Figure C.26) silicon devices; thus the display industry can leverage much of the existing infrastructure and know-how. A key advantage amorphous oxides hold over amorphous silicon is their higher charge-carrier mobility.