phased - array optics
phased - array optics phased array optics (pao) is the technology of controlling the phase of light waves transmitting or reflecting from a two - dimensional surface by means of adjustable surface elements. it is the optical analogue of phased array radar. - video is targeted to blind users attribution: article text available under cc - by - sa image source in video urltoken
phased array optics is a technology that will produce three dimensional views of objects and scenery using only two dimensional displays. display systems based on this technology - - optical phased arrays - - will behave quite literally as windows onto whatever scenery we can imagine .
the topics in this list come from the osa optics and photonics topics applied to this article .
welcome to a website about a technology that doesn' t exist, but is beginning to exist. phased array optics is the use of spatial light modulators built to nanometer specifications for general manipulation of light. the most compelling application of phased array optics (pao) is the production of three dimensional scenery that is indistinguishable from real life. the display application has been compared to a star trek holodeck .
a triangle shape projected using hajimiri’s optical phased array chip. the image shown was projected in infrared light .
the term
phased array optics
can be confusing because
optical phased arrays
usually means the specific use of spatial light modulators to steer laser beams, or aim sensors, by progressive phase modulation. this technology exists today. however in the nanotechnology community, phased array optics has come to mean all optical applications of nanoscale arrays of addressable phase and amplitude modulators. these applications are more general than beam steering, and include real - time diffractive optical elements (doe) and dynamic holography .
the compact optical array used for the excalibur phased array laser test included three modules, each comprising seven separate laser channels and measuring only 10 centimeters across. photo: darpa
, 1996) first popularized the long - term potential of phased array optical technology. it is reproduced here with permission of mit press .
projections of the letters
cit
made with the optical phased array (opa) chip. the projections were made in infrared light .
the letters “c, ” “i, ” and “t” projected using hajimiri’s optical phased array chip. the images shown were projected in infrared light .
“in addition to scalability, excalibur demonstrated near - perfect correction of atmospheric turbulence — at levels well above that possible with conventional optics. ”
omega optics, inc. was founded in 2001 to design and fabricate optical components based on the planar integrated polymer and silicon waveguide technology .
imagine you are back in your teleporter room enjoying the martian panorama. this time the floor is also covered with phased array optics. martian boulders now appear at your feet, amongst the furniture, and in one case right in front of your nose! now that' s realism .
of course, you were never really on mars. the
platform
you were standing on was merely the floor of a small, comfortably furnished room called a teleporter. the walls and ceiling of this room are covered with one of the technological wonders of the 21st century: phased array optics .
j. l. hughes and a. k. ghatak ,
phased - array optical scanning ,
appl. opt. 18, 2098 - 2098 (1979 )
under the excalibur program darpa explores the development of coherent optical phased array technologies able to reduce the size and weight of laser weapons 10 times lighter and more compact than existing high - power chemical laser systems. the optical phased array architecture provides electro - optical systems with the same mission flexibility and performance enhancements that microwave phased arrays provide for rf systems. as such, future multifunction excalibur arrays may also perform tasks including laser radar, target designation, laser communications, and airborne - platform self protection .
if a transparent planar waveguide is used to carry laser light across the back of a phased array, then the array will be semitransparent with perhaps 30% transmittance (like tinted glass). this raises all sorts of interesting possibilities. for one, the array could superpose images on real scenery behind it. (a glide path for a landing aircraft is a nice example .) more profound, though, is what the array could do to the scenery itself .
it' s been argued that pao is sufficiently distinct from holography that it shouldn' t be considered holography. however given the lack of success of other holography offshoots, such as kinoform, at distinguishing themselves from holography, it must admitted that the term
holography
is now used so expansively that pao could be considered a type of holography. phased array optics may therefore be seen as the physical limit of holography: real - time dynamic holograms displayed by spatial light modulators of sub - wavelength resolution. a multiplexed color phased optical array could act as a window into any 3d world that can be imagined .
about optics & photonics topics osa publishing developed the optics and photonics topics to help organize its diverse content more accurately by topic area. this topic browser contains over 2400 terms and is organized in a three - level hierarchy. read more. topics can be refined further in the search results. the topic facet will reveal the high - level topics associated with the articles returned in the search results .
traditional cameras—even those on the thinnest of cell phones—cannot be truly flat due to their optics: lenses that require a certain shape and size in order to function. at caltech, engineers have developed a new camera design that replaces the lenses with an ultra - thin optical phased array (opa). the opa does computationally what lenses do using large pieces of glass: it manipulates incoming light to capture an image .
darpa successfully tested an optical phased array (opa) combining 21 laser beams, as part of the excalibur program. with each of the 21 array elements driven by fiber laser amplifiers the low power array was able to precisely hit a target at a distance of 7 kilometers (4. 3 miles). in three years the agency expects to scale up the design, delivering 100 kw weapon - class energy levels on target .
ami yaacobi, jie sun, michele moresco, gerald leake, douglas coolbaugh, and michael r. watts ,
integrated phased array for wide - angle beam steering ,
opt. lett. 39, 4575 - 4578 (2014 )
phased array optics refers to hardware capable of altering the phase and amplitude of light on scales smaller than a wavelength over large areas of addressable control. the technology to do this doesn' t exist yet, but spatial light modulator (slm) and computer technology are moving in this direction. when the technology is achieved, artifacts traditionally associated with holographic image production will disappear. there will be no visible reconstruction beam, false images, or unwanted orders of diffraction .
the sources in a phased array must radiate coherently. that is, they must be able to interfere with each other. the easiest way to achieve this is to illuminate the back of the array with light from a single laser. a diverging laser beam could be aimed at the array from behind, or the beam could be transported through a thin planar waveguide on the rear surface of the array. since lasers can be made with coherence lengths of kilometers, it doesn' t really matter how the laser light gets to the sources. it will always be coherent across the array, and each source can have its own phase and amplitude calibration factors .
phased arrays and holography are both methods of wavefront reconstruction, and both can produce three dimensional images. however they differ in some important ways .
a similar principle is used in reverse in an optical phased array receiver, which is the basis for the new camera. light waves that are received by each element across the array cancel each other from all directions, except for one. in that direction, the waves amplify each other to create a focused
gaze
that can be electronically controlled .
by choosing the correct phase and amplitude distribution across the array we can, in fact, create images of any number of points at any number of locations behind the array. since any three dimensional scene can be represented as a collection of discrete points in space, it follows that our array can reproduce any three dimensional scene .
this interference pattern is analogous to the phase and amplitude modulation of sources in a phased array. when a laser beam is shone on the developed film, the transparent areas of the interference pattern select out those parts of the beam that have the correct phase to reconstruct the recorded scene .
virtual teleporters covered on all sides by phased arrays bear a striking similarity to the
holodeck
concept of star trek. anything could happen in such a room. anything .
matching the phase of sources in a phased array has interesting consequences. suppose an observer looking at the array sees one square meter worth of sources radiating at exactly the same phase relative to him. the sources will interfere constructively, depositing their entire output energy to a very small point at his location. in other words, the observer will be at the focus of a one kilowatt laser (not a very healthy place to be) .
phased arrays, which are used in wireless communication and radar, are collections of individual transmitters, all sending out the same signal as waves. these waves interfere with each other constructively and destructively, amplifying the signal in one direction while canceling it out elsewhere. thus, an array can create a tightly focused beam of signal, which can be steered in different directions by staggering the timing of transmissions made at various points across the array .
the number of scene elements recorded by an array is roughly equal to the number of sources in the array. to compute the phase and amplitude for each source, we must add the contributions of spherical waves from each scene element to each source point. for a square meter array, this is 1000 trillion trillion floating point operations. a single computer would have trouble finishing this calculation within the lifetime of the universe .
the data storage requirements of arrays cannot yet be met economically. (a phased array movie would consume several hundred terabytes !) nevertheless, storage technologies able to handle arrays are on the horizon. certainly molecular technology, with its promise of 1 gigabyte per cubic micron (drexler 1986, 247), will be more than sufficient .
phased arrays can produce images of objects anywhere in space behind them. but that is not all they can do. with certain restrictions, they can also make objects appear in front of them .
phased arrays are based on the theory of diffraction from physical optics. this theory says that patterns of light waves travelling beyond an aperture (such as a window) are entirely determined by the amplitude and phase distribution of light at the surface of the aperture. this means that if we produce light with the right phase and brightness distribution across a two dimensional surface, we can reproduce the same light waves that would emanate from a three dimensional scene behind the surface. in other words, we can make that surface appear as a window onto the scene .
consider putting a tiny laser at the base of each phase shifter. (quantum well semiconductor lasers would be ideal .) these lasers provide a coherent boost to light passing through the array, essentially transforming the whole array into a resonant cavity. in other words, the array becomes one big laser with adjustable emission phase across its surface. color could be dealt with by making the lasers tunable, or by laying them out in a red - green - blue hexagonal lattice .
it is now obvious why the array contains so much information. a meter wide array (or window for that matter) has a diffraction limited resolution of about one millionth of a radian, or 0. 2 seconds of arc. the array image therefore contains every scene element subtending a solid angle greater than a trillionth of a steradian: the fly on the man' s hat, leaves on trees five miles away, most every blade of grass in the meadow, and so on .
real time arrays still pose formidable data storage and transmission problems. a square meter array operated at human eye resolution will require about a gigabyte per frame, or 50 gigabytes per second. interestingly light itself is ideally suited to transmission at this rate. modulated light could be used for both long distance transmission and local control of array sources. instead of trillions of tiny wires, light traveling through the same waveguide as the primary laser could carry image information to decoders at periodic intervals in the array .
in addition to scalability, excalibur demonstrated near - perfect correction of atmospheric turbulence — at levels well above that possible with conventional optics. while not typically noticeable over short distances, the atmosphere contains turbulent density fluctuations that can increase the divergence and reduce the uniformity of laser beams, leading to diffuse, shifted and splotchy laser endpoints, resulting in less power on the target .
now consider a building covered on all sides by a high power phased array. by day it could produce an image of a landscape contiguous with its surroundings, thereby rendering itself effectively invisible. by night it could do more of the same, or, for variety, create the image of a floodlit building. perhaps it would supplement this scene with the image of a powerful, sweeping searchlight to detect and / or intimidate intruders. for those occasions when the searchlight was not intimidating enough, it could match the phase of a few array sources relative to an intruder .
the phase shifter is made of an electrooptic birefringent material. this means that the shifter changes its refractive index along one polarization direction in response to an electric field. a polarizing filter down the back of the array ensures light of the proper linear polarization enters the shifter. a quarter wave plate on the front of the array to restore elliptical polarization is optional .
optical phased arrays pose substantial challenges of information processing and nanofabrication. nevertheless it seems certain that these challenges will be met within the next twenty to thirty years. some precursor technologies, such as lcd holography, are at the edge of even current capability .
for many applications the scenery produced by an array will originate entirely in a computer. when it doesn' t, a way must exist to record real scenery that allows reproduction by an array. in the general case of an incoherent (naturally illuminated) scene, the only way to do this is brute force: record conventional flat images from every angle you can get .
while the synthesis of array images may be mathematically simple, it will be computationally severe (by 1991 standards). a source separation of 0. 2 microns gives 25 million sources per square millimeter, 2. 5 billion per square centimeter, and 25 trillion array sources per square meter. each one of these sources will have to radiate with its own calculated phase and amplitude .
in the theory of diffraction, most every element of an optical system can be modelled as a two dimensional surface with a complex transmission function. (a lens, for example, is equivalent to a flat surface with particular phase changing properties .) phased arrays will turn this mathematical abstraction into technological reality. a single transparent array could behave as a programmable phase plate, fresnel zone plate, hologram, prism, diffraction grating, or variable focal length low f number diffraction limited lens. all this from an array microns thick, and able to change roles in a microsecond. you could hold such a thing in your hand, and swear it was from another world .
traditional projectors—like those used to project a film or classroom lecture notes—pass a beam of light through a tiny image, using lenses to map each point of the small picture to corresponding, yet expanded, points on a large screen. the caltech chip eliminates the need for bulky and expensive lenses and bulbs and instead uses a so - called integrated optical phased array (opa) to project the image electronically with only a single laser diode as light source and no mechanically moving parts .
while this example is a bit extreme, it illustrates that very small coherent sources can give very big laser power. it follows that generating images of lasers and laser light will be an important application of phased arrays (particularly in view of the high efficiency of semiconductor lasers) .
figure 3 illustrates the principle for a point source. by producing converging spherical waves, the array has created the image of a point source in front it. once again, creating a whole object is just a matter of assembling the points which comprise it. by controlling which areas of the array reproduce various points, hidden lines and surfaces are removed, and even front - projected objects will appear solid and opaque .
this is a staggering amount of information. to understand the origin of this information, recall that the array behaves as a window onto the scene it is reproducing. suppose a square meter array is reproducing a wilderness scene. looking through the array, one sees a meadow, a small lake and a mountain beyond. one mile away, on the far shore of the lake, a man is fishing. if you aimed a telescope in his direction you would see him clearly. and if you had a really big telescope (one with a two foot wide lens) you would be able to see a housefly on his hat .
my own thinking about this subject was the result of taking an optics course while thinking about nanotechnology in 1990. it seemed clear that addressable nanoscale optical elements could be a real - life implementation of huygens' principle, allowing any possible pattern of light waves to be generated from a planar array of sources. the idea was proposed and accepted as a book chapter in 1991, although the book, nanotechnology: molecular speculations on global abundance (mit press) didn' t finally make it into print until 1996. i later learned that the idea of holographic television, or holovideo, is traceable at least as far back as the nobel acceptance speech of dennis gabor in 1971. the first forms of electro - holography appeared in the 1980s, and the field continues to advance .
these results were described in a presentation titled
electronic two - dimensional beam steering for integrated optical phased arrays .
along with hajimiri, abiri, and aflatouni, caltech senior angad rekhi also contributed to the work. the study was funded by grants from the caltech innovation initiative, and the information science and technology initiative at caltech .
active invisibility means an array would acquire and synthesize contiguous scenery in real time. this could be accomplished with a system a few millimeters thick. the interior layer of our hypothetical
invisibility suit
consists of photosensors comparable to a human retina. the exterior is a phased array with phase shifters of extra close spacing and high refractive index. half of the shifters are dedicated to image production, while the other half transmit light through to the photosensitive layer. the transmitting shifters are adjusted to form a multitude of focussed
fly' s eye
images on the photosensitive layer. the system is thus able to both produce and view scenery from all angles at the same time. a powerful computer network would manage the scenery synthesis and other details, such as how suit movement affected phase relationships. other designs are possible .
using a series of pipes for the light—called phase shifters—the opa chip similarly slows down or speeds up the timing of the waves, thus controlling the direction of the light beam. to form an image, electronic data from a computer are converted into multiple electrical currents; by applying stronger or weaker currents to the light within the phase shifter, the number of electrons within each light path changes—which, in turn, changes the timing of the light wave in that path. the timed light waves are then delivered to tiny array elements within a grid on the chip. the light is then projected from each array in the grid, the individual array beams combining coherently in the air to form a single light beam and a spot on the screen .
hajimiri and his colleagues were able to bypass traditional optics by manipulating the coherence of light—a property that allows the researchers to
bend
the light waves on the surface of the chip without lenses or the use of any mechanical movement. if two waves are coherent in the direction of propagation—meaning that the peaks and troughs of one wave are exactly aligned with those of the second wave—the waves combine, resulting in one wave, a beam with twice the amplitude and four times the energy as the initial wave, moving in the direction of the coherent waves .
“the success of this real - world test provides evidence of how far opa lasers could surpass legacy lasers with conventional optics, ” said mangano, “with power efficiencies of more than 35 percent and the near - perfect beam quality of fiber laser arrays, these systems can achieve the ultra - low size, weight and power requirements (swap) required for deployment on a broad spectrum of platforms, ” said mangano. “beyond laser weapons, this technology may also benefit low - power applications such as laser communications and the search for, and identification of, targets. ”
phased arrays reproduce scenery using coherent light. this raises certain problems that have been glossed over so far. it was assumed earlier that different parts of a scene could be reproduced independently. this is not quite true for coherent light. light waves from different points in a coherent scene would interfere with each other, generating unnatural effects such as laser speckle .
the procedure for calculating the phase and amplitude distribution required to produce a scene is straightforward. all visible objects in the scene are represented in a computer in terms of discrete surface points. the spacing of the points is determined by the resolution of the array. each point is assumed to produce a spherical wave. the complex amplitudes of these waves are summed at each source position on the array. the resultant complex amplitude at each source point determines the phase and intensity that source must radiate to reproduce the scene .
fortunately several considerations come to our rescue. first, this type of calculation is amenable to fast fourier methods, which tend to reduce n squared operations to nlogn operations (a trillion fold decrease in our case). second, this problem is well suited to massive parallelism. with enough processors, there is no reason why array image synthesis couldn' t proceed at movie frame rates. finally, not all array images will require resolution beyond that of the unaided eye. this concession alone reduces the computation and bandwidth requirements by several orders of magnitude .
we demonstrate an on - chip optical phased array fabricated in a cmos compatible process with continuous, fast (100 khz), wide - angle (51°) beam - steering suitable for applications such as low - cost lidar systems. the device demonstrates the largest (51°) beam - steering and beam - spacing to date while providing the ability to steer continuously over the entire range. continuous steering is enabled by a cascaded phase shifting architecture utilizing, low power and small footprint, thermo - optic phase shifters. we demonstrate these results in the telecom c - band, but the same design can easily be adjusted for any wavelength between 1. 2 and 3. 5 μm .
although they would appear perfectly authentic to human eyes, scenes produced in this way would be easily detectable by a prism. they might not fool animals either. the ultimate solution is to cycle very quickly through the entire visual spectrum at random increments. such an array would be difficult to detect .
transparent arrays are well suited to this application. a transparent array could behave as a fly' s eye lens to record nearby objects, and later become a single huge lens to record details miles away. this type of system could quickly gather all the information needed to reproduce scenes with telescopic resolution .
last year, hajimiri' s team rolled out a one - dimensional version of the camera that was capable of detecting images in a line, such that it acted like a lensless barcode reader but with no mechanically moving parts. this year' s advance was to build the first two - dimensional array capable of creating a full image. this first 2d lensless camera has an array composed of just 64 light receivers in an 8 by 8 grid. the resulting image has low resolution. but this system represents a proof of concept for a fundamental rethinking of camera technology, hajimiri and his colleagues say .
diffraction theory is mathematically continuous; it is assumed that surfaces can be reduced to an infinite number of small elements radiating with different amplitude and phase. this introduces complications from the standpoint of developing a practical technology. fortunately we can achieve the same results with a discrete array of sources, provided they are coherent and less than 1 / 2 wavelength apart. the wavelength of visible light ranges from 0. 4 microns (violet) to 0. 7 microns (red). a two dimensional array of programmable sources 0. 2 microns apart will therefore be sufficient to reconstruct any light wave pattern we desire .
here, like most other things in life, timing is everything. with our new system, you can selectively look in a desired direction and at a very small part of the picture in front of you at any given time, by controlling the timing with femto - second—quadrillionth of a second—precision ,
says ali hajimiri, bren professor of electrical engineering and medical engineering in the division of engineering and applied science at caltech, and the principal investigator of a paper describing the new camera. the paper was presented at the optical society of america' s (osa) conference on lasers and electro - optics (cleo) and published online by the osa in the osa technical digest in march 2017 .
the recent test employed four of these modules grouped into a laser arrays. darpa plans to test coherent array of 10’s of these subapertures, each driven with a multi - kw coherently combinable fiber laser amplifiers, to deliver ~ 100 kw class laser systems for precision strikes against both ground and air targets. photo: darpa
the array designs presented so far operate by modulating laser light passing through them. the
sources
don' t actually produce light themselves. for some applications, particularly outdoor use, a higher power design is desirable. the visual intensity of the sun (about 500 watts per square meter) is the regime we seek .
we' ve created a single thin layer of integrated silicon photonics that emulates the lens and sensor of a digital camera, reducing the thickness and cost of digital cameras. it can mimic a regular lens, but can switch from a fish - eye to a telephoto lens instantaneously—with just a simple adjustment in the way the array receives light ,
hajimiri says .
in this scheme, each
source
in the array is a passive transmission element with adjustable optical path length (phase) and transmission (amplitude). fig. 2 shows a cross section through a few elements. the elements are 0. 2 microns (2000 angstroms) apart. (a bacterium could cover five of them .) each element consists of a phase shifter and an amplitude modulator .
lenses have a curve that bends the path of incoming light and focuses it onto a piece of film or, in the case of digital cameras, an image sensor. the opa has a large array of light receivers, each of which can individually add a tightly controlled time delay (or phase shift) to the light it receives, enabling the camera to selectively look in different directions and focus on different things .
of course the same principle can be employed over the entire surface area of the building, encompassing hundreds of kilowatts of laser power. our camouflage, decorative lighting system is now a long - range missile defense system, or directional transmitter of interstellar range. (with a beam divergence less than a millionth of a degree, a stable 100 meter wide array would appear 1% as bright as the sun at any distance. )
producing colored scenes is easy. three versions of each scene would be synthesized in red, green, and blue light (the three primary colors of human vision). the array would cycle through them rapidly, alternating between red, green, and blue lasers. because scenes would tend to split apart into separate colors if you moved your eyes fast, you would want a fast cycle time (ideally less than a millisecond) .
holography is simple and works without a computer, however it has inherent drawbacks. holographic recording produces phase modulation that is too coarse for unambiguous wave reconstruction. when you shine a laser on a hologram you will always get three beams out: the reconstructed object beam, the (unwanted) conjugate beam, and the laser beam itself. this happens because of the wide spacing of interference fringes produced by holographic recording. (the three beams are actually three orders of diffraction .) phased arrays modulate phase and amplitude at half wavelength intervals. this is close enough to generate only one interference maximum, which contains the reconstructed beam and nothing else .
unlike the chemical lasers that rely on a single high power source, these phased arrays will coherently combine lower - power electrically driven lasers, such as diode lasers and fiber laser amplifiers. coherently combinable single - mode diode lasers and fiber - based systems can provide overall laser efficiencies greater than 50 percent and 30 percent, respectively, while maintaining near - diffraction - limited beam quality. to produce a weapons - grade system, however, their output power must be increased without introducing additional optical phase noise and modal instability. beam - steering technologies will be pursued to make these arrays conformal with the airframe, to provide rapid retargeting across a large field of regard, and to compensate for the effects of atmospheric turbulence .
the new thing about our work is really that we can do this on a tiny, one - millimeter - square silicon chip, and the fact that we can do it very rapidly—rapidly enough to form images, since we phase - shift electronically in two dimensions ,
says behrooz abiri, a graduate student in hajimiri' s group and a coauthor on the paper. so far, the images hajimiri and his team can project with the current version of the chip are somewhat simple—a triangle, a smiley face, or single letters, for example. however, the researchers are currently experimenting with larger chips that include more light - delivering array elements that—like using a larger lens on a camera—can improve the resolution and increase the complexity of the projected images .
the year is 2020. you are standing on a platform at the edge of a thousand foot cliff overlooking the tharsis volcano range on mars. your body casts a long shadow in the light of the setting sun as you scan the horizon for interesting features with your binoculars. walking toward the edge of the precipice, you survey the texture of the rusty red boulders around you. butterflies rise in your stomach as you peer over the cliff edge .
sixteen hundred miles overhead, phobos, one of the martian moons shines conspicuously. there is a new transport base under construction there. you move toward your telescope to get a better view, when suddenly a doorway materializes out of empty space! a leg steps through it. it' s your wife with dinner .
figure 1 illustrates the reconstruction principle in two dimensions for a few simple cases. in fig. 1 (a) all the sources radiate with the same amplitude and phase. their waves interfere to create a plane wave propagating normal to the surface. this is the type of wave that would be produced by a distant point source. in fig. 1 (b) there is a linear variation of phase among the sources down the line. the result is a plane wave propagating at an angle to the surface. by adjusting the amount of phase variation, we can
steer
this beam in any direction we please. in fig. 1 (c) a more complicated phase relationship is used to produce a spherical wave, creating the image of a point source near the surface .
the amplitude modulator is simple. a cross polarizer like those in lcd displays should suffice .
a large variety of crystals are known to exhibit the required electrooptic effect (kaminow 1974). some of these are semiconductors that are the stock in trade of existing microfabrication technology. unfortunately the electrooptic effect in solid crystals is rather weak. even very large electric fields produce very small changes in refractive index. since each phase shifter must be able to retard light passing through it by one full wavelength, a small change in refractive index means a long phase shifter. for solid crystals this length will be on the order of a millimeter, which is probably too long to be practical .
nematic liquid crystals exhibit an electrooptic effect thousands of times stronger than that found in solids (kahn 1972). if these crystals can be made with a high refractive index and fast response, then phase shifters one micron long like those in fig. 2 should be possible .
an interesting alternative has been suggested by jeffrey soreff of ibm (soreff 1991). rather than relying on electrooptic effects, a crystal with a large fixed birefringence, such as calcite, could be used. the crystal is sandwiched between parallel linear polarizers, and rotated relative to the direction of polarization. the phase - shifting rotation could be accomplished by mechanical rotation of the crystal, or by rotating the polarizer directions. this scheme would also permit micron length phase shifters .
in all these designs each phase shifter is behaving as a waveguide near cutoff. red light, with a wavelength in free space of 0. 6 microns, will be the most difficult to deal with. to get through a waveguide less than 0. 2 microns wide, its wavelength will have to be reduced to less than 0. 4 microns. this will require a refractive index of at least 1. 5 in the phase shifting medium .
holography avoids the computational requirement of arrays with a simple and elegant solution. the scene to be reproduced is illuminated with a laser. a photographic film is placed near the scene to capture reflected light (the
object
beam). at the same time, a reference beam from the same laser is shone on the film. because they come from the same laser, the object and reference beams are coherent and produce a microscopic interference pattern which is recorded on the film .
a simple solution exists. numerous separate versions of a scene could be synthesized, each assigning a different random phase factor to points in the scene. when presented in rapid succession (say 20 versions per second) all interference effects would blur out, creating an effectively incoherent scene. in terms of eliminating laser speckle, this process is equivalent to rapidly altering the microscopic structure of a scene surface .
for low resolution reproduction, a
fly' s eye
lattice of one centimeter wide lenses would be used to record multiple views of a scene. a computer would then infer the three dimensional structure of the scene from these views, and proceed with image synthesis by methods already discussed. this system would have adequate resolution for viewing with the unaided eye. higher resolution would require bigger lenses .
displaying still scenery matching your surroundings is one form of invisibility. we could call this passive invisibility. with a view toward very fast and compact computers, an even more audacious possibility exists: active invisibility .
such speculations lie at the limits of foreseeable technology. it is nonetheless amazing that something as fantastic as invisibility could be achieved using known science .
over the long term, the future of arrays is as limitless as imagination itself. the few applications mentioned here are just a glimpse of their potential. as arthur c. clarke recently said about virtual reality (rheingold 1991), this technology
won' t merely replace tv. it will eat it alive .
kaminow, i. p. 1974, an introduction to electrooptic devices, academic press .
kahn, f. j. 1972, electronically tunable birefringence, u. s. patent 3, 694, 053 assigned to bell telephone .
drexler, k. e. 1986, engines of creation, anchor press doubleday .
in 1991 brian wowk was a graduate student in the medical physics program at the university of manitoba in winnipeg, canada. his interests included medical imaging and the medical application of nanoscale technologies, and still do .
the opa used in these experiments consisted of three identical clusters of seven tightly packed fiber lasers, each cluster measures only 10 centimeters across. according to joseph mangano, darpa program manager, the agency is planning to scale up the design over the next three years, ultimately transmitting up to 100 kilowatts of power – levels otherwise difficult to achieve in such a small package .
“beyond laser weapons, this technology may also benefit low - power applications such as laser communications and the search for, and identification of, targets. ” joseph mangano, darpa pm
future tests aim to prove the opa’s capabilities in even more intense environmental turbulence conditions and at higher powers. such advances may one day offer improved reliability and performance for applications such as aircraft self - defense and ballistic missile defense .
the recent excalibur demonstration used an ultra - fast optimization algorithm to effectively “freeze” the deeply turbulent atmosphere, and then correcting the resulting static optically aberrated atmosphere in sub - milliseconds to maximize the laser irradiance delivered to the target. these experiments validated that the opa could actively correct for even severe atmospheric distortion. the demonstration ran several tens of meters above the ground, where atmospheric effects can be most detrimental for military applications. in addition, these experiments demonstrated that opas might be important for correcting for the effects of boundary layer turbulence around aircraft platforms carrying laser systems .
the successful demonstration helps advance excalibur’s goal of a 100 - kilowatt - class laser system in a scalable, ultra - low swap opa configuration compatible with existing weapon system platforms. continued development and testing of excalibur fiber optic laser arrays may one day lead to multi - 100 kilowatt - class high - energy lasers (hel) in a package 10 times lighter and more compact than legacy high - power laser systems .
high - energy lasers have the potential to benefit a variety of military missions, particularly as weapons or as high - bandwidth communications devices. however, the massive swap of legacy laser systems limit their use on many military platforms. even if swap limitations can be overcome, turbulence manifested as density fluctuations in the atmosphere increase laser beam size at the target, further limiting laser target irradiance and effectiveness over long distances .
brazil signs us $ 5 billion contract for 36 swedish gripen ng fighter ...
finland has selected israel' s advanced naval attack missile to replace its current mto85m system, a derivative of swedish rbs15 that will reach the end of its life cycle in the 2020s. the new missile will be installed on existing hamina - class missile boats and the new squadron 2020 vessels, the first will be launched in 2019. according to the finnish mod, the missile will also be usable from vehicular platforms on land and against land targets .
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conservatively, holographic television may develop in the 21st century at the same rate that 2d television did in the 20th century. however that pace could accelerate considerably if moore' s law takes hold for spatial light modulators. as one illustration of the difference time makes, in 1991 i wrote that a single computer would have difficulty generating a large diffraction - limited pao image within the lifetime of universe with non - fft methods. in 2007, the fastest parallel computer in the world could actually do that calculation in only a few millennia. naive extrapolation would predict large pao displays operating at movie - frame rates by the mid - 21st century .
a silicon chip developed by caltech researchers acts as a lens - free projector—and could one day end up in your cell phone .
imagine that you are in a meeting with coworkers or at a gathering of friends. you pull out your cell phone to show a presentation or a video on youtube. but you don' t use the tiny screen; your phone projects a bright, clear image onto a wall or a big screen. such a technology may be on its way, thanks to a new light - bending silicon chip developed by researchers at caltech .
the chip was developed by ali hajimiri, thomas g. myers professor of electrical engineering, and researchers in his laboratory. the results were presented at the optical fiber communication (ofc) conference in san francisco on march 10 .
by changing the relative timing of the waves, you can change the direction of the light beam ,
says hajimiri. for example, if 10 people kneeling in line by a swimming pool slap the water at the exact same instant, they will make one big wave that travels directly away from them. but if the 10 separate slaps are staggered—each person hitting the water a half a second after the last—there will still be one big, combined wave, but with the wave bending to travel at an angle, he says .
as the electronic signal rapidly steers the beam left, right, up, and down, the light acts as a very fast pen, drawing an image made of light on the projection surface. because the direction of the light beam is controlled electronically—not mechanically—it can create a sort of line very quickly. since the light draws many times per second, the eye sees the process as a single image instead of a moving light beam, says hajimiri .
in their recent experiments, hajimiri and his colleagues have used the silicon chip to project images in infrared light, but additional work with different types of semiconductors will also allow the researchers to expand the tiny projector' s capabilities into the visible spectrum .
right now we are using silicon technology, which works better with infrared light. if you want to project visible light, you can take the exact same architecture and do it in what' s called compound semiconductor iii - v technology ,
says firooz aflatouni, another coauthor on the paper, who in january finished his two - year postdoctoral appointment at caltech and joined the university of pennsylvania as an assistant professor .
silicon is good because it can be easily integrated into electronics, but these other compound semiconductors could be used to do the same thing .
in the future, this can be incorporated into a phone, and since there is no need for a lens, you can have a phone that acts as a projector all by itself ,
hajimiri says. however, although the chip could easily be incorporated into a cell phone, he points out that a tiny projection device can have many applications—including light - based radar systems (called
lidar
), which are used in positioning, robotics, geographical measurements, and mapmaking. such equipment already exists, but current lidar technology requires complex, bulky, and expensive equipment—equipment that could be streamlined and simplified to a single chip at a much lower cost .
but i don' t want to limit the device to just a few purposes. the beauty of this thing is that these chips are small and can be made at a very low cost—and this opens up lots of interesting possibilities ,
he says .
hajimiri - ali - lensless _ camera - 7094 - news - web. jpg
what the camera does is similar to looking through a thin straw and scanning it across the field of view. we can form an image at an incredibly fast speed by manipulating the light instead of moving a mechanical object ,
says graduate student reza fatemi (ms' 16), lead author of the osa paper .
the applications are endless ,
says graduate student behrooz abiri (ms' 12), coauthor of the osa paper .
even in today' s smartphones, the camera is the component that limits how thin your phone can get. once scaled up, this technology can make lenses and thick cameras obsolete. it may even have implications for astronomy by enabling ultra - light, ultra - thin enormous flat telescopes on the ground or in space .
the ability to control all the optical properties of a camera electronically using a paper - thin layer of low - cost silicon photonics without any mechanical movement, lenses, or mirrors, opens a new world of imagers that could look like wallpaper, blinds, or even wearable fabric ,
says hajimiri. next, the team will work on scaling up the camera by designing chips that enable much larger receivers with higher resolution and sensitivity .
the study is titled
an 8x8 heterodyne lens - less opa camera .
this research was funded by the rothenberg innovation initiative (ri2), previously known as the caltech innovation initiative (ci2) .
. such a device can be formed on any surface of interest. we can provide both design and fabrication of antenna from 1 ghz to 32 ghz. for more detailed information, please read the pdf below :
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phased array optics ( pao ) is the technology of controlling the phase of light waves transmitting or reflecting from a two-dimensional surface by means of adjustable surface elements . it is the optical analogue of phased array radar . by dynamically controlling the optical properties of a surface on a microscopic scale , it is possible to steer the direction of light beams , or the view direction of sensors , without any moving parts . hardware associated with beam steering applications is commonly called an optical phased array ( opa ) . phased array beam steering is used for optical switching and multiplexing in optoelectronic devices , and for aiming laser beams on a macroscopic scale . complicated patterns of phase variation can be used to produce diffractive optical elements , such as dynamic virtual lenses , for beam focusing or splitting in addition to aiming . dynamic phase variation can also produce real-time holograms . devices permitting detailed addressable phase control over two dimensions are a type of spatial light modulator ( slm ) . in nanotechnology , phased array optics refers to arrays of lasers or slms with addressable phase and amplitude elements smaller than a wavelength of light . while still theoretical , such high resolution arrays would permit extremely realistic three-dimensional image display by dynamic holography with no unwanted orders of diffraction . applications for weapons , space communications , and invisibility by optical camouflage have also been suggested . darpa 's excalibur program aims to provide realtime correction of atmospheric turbulence for a laser weapon . 
phased array optics (pao) is the technology of controlling the phase of light waves transmitting or reflecting from a two-dimensional surface by means of adjustable surface elements. it is the optical analogue of phased array radar. by dynamically controlling the optical properties of a surface on a microscopic scale, it is possible to steer the direction of light beams, or the view direction of sensors, without any moving parts. hardware associated with beam steering applications is commonly called an optical phased array (opa). phased array beam steering is used for optical switching and multiplexing in optoelectronic devices, and for aiming laser beams on a macroscopic scale. complicated patterns of phase variation can be used to produce diffractive optical elements, such as dynamic virtual lenses, for beam focusing or splitting in addition to aiming. dynamic phase variation can also produce real-time holograms. devices permitting detailed addressable phase control over two dimensions are a type of spatial light modulator (slm). in nanotechnology, phased array optics refers to arrays of lasers or slms with addressable phase and amplitude elements smaller than a wavelength of light. while still theoretical, such high resolution arrays would permit extremely realistic three-dimensional image display by dynamic holography with no unwanted orders of diffraction. applications for weapons, space communications, and invisibility by optical camouflage have also been suggested. darpa's excalibur program aims to provide realtime correction of atmospheric turbulence for a laser weapon.
phased array optics (pao) is the technology of controlling the phase of light waves transmitting or reflecting from a two-dimensional surface by means of adjustable surface elements. it is the optical analogue of phased array radar. by dynamically controlling the optical properties of a surface on a microscopic scale, it is possible to steer the direction of light beams, or the view direction of sensors, without any moving parts. hardware associated with beam steering applications is commonly called an optical phased array (opa). phased array beam steering is used for optical switching and multiplexing in optoelectronic devices, and for aiming laser beams on a macroscopic scale. complicated patterns of phase variation can be used to produce diffractive optical elements, such as dynamic virtual lenses, for beam focusing or splitting in addition to aiming. dynamic phase variation can also produce real-time holograms. devices permitting detailed addressable phase control over two dimensions are a type of spatial light modulator (slm). in nanotechnology, phased array optics refers to arrays of lasers or slms with addressable phase and amplitude elements smaller than a wavelength of light. while still theoretical, such high resolution arrays would permit extremely realistic three-dimensional image display by dynamic holography with no unwanted orders of diffraction. applications for weapons, space communications, and invisibility by optical camouflage have also been suggested. darpa's excalibur program aims to provide realtime correction of atmospheric turbulence for a laser weapon.