In 1985 I joined Texas Instruments’ (TI’s) Deformable Mirror Device (DMD) group to develop applications of the cantilever device in coherent optical signal processing. At that time I witnessed the “aha discovery” that led to the invention of the DLP. It is interesting to consider the many years of effort that led Larry Hornbeck to this commercially successful implementation, not just the technology, but the efforts to sustain the project through sponsored R&D. While TI viewed the only sustainable market as (incoherent) display applications, the DMD group sustained the effort with DoD funding for coherent and incoherent optical signal processing systems, including matched filter correlators, digital optical switches, optical crossbar switches and related neural network processors. For coherent signal processing the need for a 2π phase-only (piston-motion pixel) spatial light modulator (SLM) was readily apparent to the sponsors. While TI saw little commercial justification for the phase-only device, this need inspired me around 1991 to develop a new class of real-time computer-generated holography algorithms referred to a pseudorandom encoding, in which each phase-only pixel is encoded with a desired magnitude and phase. The optical Fourier transforms of the modulation enabled my developments of multi-spot object targeting and laser tweezer systems. Around 2005 I began using Digital Light Processing (DLP) developer kits in place of scanners to time-share images with a small number of detectors. One system using a single, high sensitivity detector together with well-chosen DLP frames quickly forms a “partial image” of a point-like scene objects – arguably, an early version of compressive sensing. This paper concludes with recommendations on optimizing the performance and applications of, and potential markets for TI’s recently demonstrated phase-only DLP.
This report reviews progress in capillary-force-directed self-assembly fabrication methods together with applications of the suspended structures as fiber-based optical components, cells scaffolds for tissue regeneration, and as templates for suspended microfluidic networks and nanomaterials. Capillary forces can direct polymeric solutions, melts and nanocomposites to form near constant diameter fibers, not just over long distances, as in electrospinning, but over micron to centimeter distances typical of integrated circuits and MEMS. Crude hand-brushing of polymeric solutions over micropillar arrays has produced, in a matter of seconds, uniform arrays of near identical nanofibers and trampolinelike membranes suspended between the micropillars. Direct point-to-point writing has also been accomplished with AFM tips, capillary syringes and electrospinning jets. The brush-on method, while fast, does not produce arbitrary patterns. The direct-write method, while producing arbitrary patterns, is slow. Recently, fast arbitrary patterning has been demonstrated by photopatterning holes through thin suspended polymer films, followed by thermal annealing which causes holes, driven by capillary force, to expand, form threads and thin into fibers. For an ablation threshold of polystyrene of 10 mJ/cm2 at 193 nm there are many adequately intense excimer lasers that could be incorporated into roll-to-roll systems. At a hole exposure threshold of ~1 mJ/cm2 , even current 193 nm wafer stepping projection printers could pattern wafers with arbitrary suspended structures at economically sustainable production rates of greater than 50 wafers per hour. A new method of extending this fabrication method to three-dimensions is also described, that additionally overcomes the need for prefabricated micropillar arrays.
Several high aspect ratio nanostructures have been made by capillary force directed self-assembly including polymeric nanofiber air-bridges, trampoline-like membranes, microsphere-beaded nanofibers, and intermetallic nanoneedles. Arrays of polymer air-bridges form in seconds by simply hand brushing a bead of polymeric liquid over an array of micropillars. The domination of capillary force that is thinning unstable capillary bridges leads to uniform arrays of nanofiber air-bridges. Similarly, arrays of vertically oriented Ag2Ga nanoneedles have been formed by dipping silvercoated arrays of pyramidal silicon into melted gallium. Force-displacement measurements of these structures are presented. These nanostructures, especially when compressively or torsionally buckled, have extremely low stiffnesses, motion due to thermal fluctuations that is relatively easily detected, and the ability to move great distances for very small changes in applied force. Nanofibers with bead-on-a-string structure, where the beads are micron diameter and loaded with magnetic iron oxide (maghemite), are shown to be simply viewable under optical microscopes, have micronewton/ m stiffness, and have ultralow torsional stiffnesses enabling the bead to be rotated numerous revolutions without breaking. Combination of these high aspect ratio structures with stretched elastomers offer interesting possibilities for robotic actuation and locomotion. Polydimethylsiloxane loaded with nanomaterials, e.g. nanotubes, graphene or MoS2, can be efficiently heated with directed light. Heating produces considerable force through the thermoelastic effect, and this force can be used for continuous translation or to trigger reversible elastic buckling of the nanostructures. The remote stimulation of motion with light provides a possible mechanism for producing cooperative behavior between swarms of semiautonomous nanorobots.
Detection and tracking of self-luminous point sources and small targets is implemented using a single, low noise
photodetector together with a programmable micromirror array. The array is programmed either to sample the scene
image with a series of harmonically related stripe patterns or the array is partitioned into multiple regions modulated at
several different temporal frequencies. The spatial subdivision method proves more effective at tracking slow moving,
point-like targets, while the frequency based method is more effective at tracking and maintaining a spatially extended
target within a track gate. The two methods have complementary features that have been combined in a hybrid algorithm,
that is more effective at acquiring and maintaining track on an erratically moving object than either method individually.
The tracking system concepts are introduced and an overall summary of the breadboard and simulation results are
presented.
LIDAR-based systems measure the time-of-flight of a laser source onto the scene and back to the sensor, building a wide
field of view 3D raster image, but as a scanning process, there are problems associated with motion inside the scene over
the duration of the scan. By illuminating the entire scene simultaneously using a broad laser pulse, a 2D camera
equipped with a high speed shutter can measure the time-of-flight over the entire field of view (FOV), thereby, recording
an instantaneous snap-shot of the entire scene. However, spreading the laser reduces the range. So what is required is a
programmable system that can track multiple regions of interest by varying the field of regard to (1) a single direction, (2)
the entire FOV, or (3) intermediate views of interest as required by the evolving scene environment. In this project, the
investigators intend to add this variable illumination capability to existing instantaneous ranging hardware by using a
liquid crystal spatial light modulator (SLM) beam steering system that adaptively varies the (single or multi) beam
intensity profiles and pointing directions. For autonomous satellite rendezvous, docking, and inspection, the system can
perform long-range sensing with a narrow FOV while being able to expand the FOV as the target object approaches the
sensor. To this end in a previous paper, we analyzed the performance of a commercially available TOF sensor
(3DVSystems' Zmini) in terms of the depth sensitivity versus target range and albedo. In this paper, we will analyze the
laser system specifications versus range of field-of-view when beam steering is performed by means of a Boulder
Nonlinear Systems' phase-only liquid crystal SLM. Experimental results show that the adjustable laser beam FOV
extensively compensate the reflected image grayscale from objects at long range, and prove the feasibility of expanding
range with the projection from the SLM.
A multi-beam, variable footprint, laser beam steering and shaping system is described and used with a microscope to demonstrate multi-particle laser trapping. It is built around a computer-interfaced 512x512 pixel analog phase-only spatial light modulator (SLM) and a 1 W, 1064 nm wavelength laser. Hand sketches on paper made with a digital pen are used to prescribe the footprints, velocities and trajectories of multiple, independently-controlled diffracted spots. Continuous scanning is approximated by automatically designing a sequence of phase-patterns that are run through and diffracted by the SLM. Very complex scanning sequences of dozens of independently controlled spots can be quickly designed and run. The number of beams that we can trap with is necessarily limited due to the low throughput (~23 mW) of the IR light through the microscope optics. Among the trapping experiments done with the system a triangular shaped vortex ring tends to stop single particles at the apexes of the triangle. However, collision with a second particle pushes the first particle past the apex and sets it into motion, leaving the second particle stopped until collision with a third particle. The discrete motion conditioned on collisions is suggestive of a queuing process or a Markov chain.
The advantages of laser communications including high bandwidth, resistance to jamming and secure links have made it a key technology for current and future C4ISR capabilities. Laser Communications between space/air/ground/sea-based assets may require multiple links. One advantage of this redundancy is that the signal is more likely to reach the intended receiver even if the environmental conditions are poor for laser transmission. In addition, multiple links provide simultaneous receipt of information to various assets engaged in activities that may need to be coordinated. That is, multibeam laser communication mimics the "broadcast" advantage of RF communications but with less likelihood of jamming or intercept. Liquid Crystal spatial light modulators are a versatile optical head that can be used for multispot beam steering applications. One advantage of the liquid crystal approach to multibeam laser communication is that the device is a modulator in addition to a mirror, so that one could conceivably send different signal amplitudes to different locations simultaneously. This paper discusses recent improvements to a 5 12x5 1 2 spatial light modulator that is specifically implemented as a multispot beamsteerer. This will include characterization of the device, analysis of its performance, and what improvements should be incorporated into the next generation device.
KEYWORDS: Sensors, Detection and tracking algorithms, Signal to noise ratio, Computer programming, Spatial light modulators, Phase only filters, System integration, Complex systems, Lithium, Image enhancement
A spatial light modulator can be used to actively illuminate moving objects with spot patterns. This research studies the spot patterns designed to detect and track object motion. To enhance the detection and tracking process, a system supervisor dynamically updates each successive spot pattern. The spot patterns are implemented by pseudo-randomly encoding their frequency plane into a phase-only filter to be implemented on a 128 X 128 Boulder Nonlinear System's LCD device. The modification of the spot patterns are based on the change of reflected intensity from one frame to the next. Detection, tracking, encoding and supervisor design and methodology are presented.
The performance of Fourier transform optical processors, e.g. optical correlators, beam steering systems, associative memories, etc., depends intimately both on the physical characteristics of the particular spatial light modulator (SLM) and on the particular algorithms that map the signal into the available modulation range of the device. For the most general Fourier systems the information/signal is complex-valued. This is an essential requirement for multi- spot beam steering systems and composite pattern recognition filters. Since practical and/or affordable SLM's only represent a limited range of values in the complex plane (e.g. phase-only or quantized phase), numerous approaches have been proposed and demonstrated for representing, approximating, encoding or mapping complex values to the available SLM states. The best approach depends on the space bandwidth product of the signal, the number of SLM pixels, the computation time of the encoding algorithm, the time available for the application, and the quality of the optical processor, as measured by an application-specific performance metric. Based on the low pixel count and the high cost per pixel of most current SLM's we argue for encoding algorithms that map one signal value to one pixel value, as opposed to group-oriented encoding. This maximized the usable area of the frequency plane. We also recommend algorithms that maximize the fidelity over the entire frequency range as opposed to maximum diffraction efficiency/minimum mean squared error design. These ideas are illustrated with several simulated and experimental results for pseudorandom, minimum Euclidean distance, error diffusion and hybrid/blended encoding algorithms.
Controllable surface roughness can be used to program the complex transmittance of individual pixels for the purpose of designing Fourier transform holograms. We are developing a photolithographic process for the fabrication of these custom diffuser pixels. Continuous variation of the recording parameters requires extreme accuracy. Therefore, we initially consider the possibility of designing diffractive optics with a small number of effective complex values: e.g. 0.5 magnitude at -120, 0, and 120 degrees. A recently developed ternary pseudorandom encoding algorithm then can be used to encode any desired fully complex function. Simulated designs of spot array generators are used to show that fidelity improves by increasing the number of roughness cells per pixel. Further improvements result from using more complex values, as is shown for designs with an additional zero-valued transmittance (represented by a completely randomly rough pixel). These results indicate that it is practical to fabricate high-fidelity custom diffractive optic functions with only a few recording states. The major advantage of this technique is that any desired complex-valued modulation pattern can be directly encoded and fabricated on a pixel-by-pixel basis, thereby accelerating the speed of both the design and the fabrication process.
Most spatial light modulators (SLM) are limited in that they cannot produce arbitrary complex modulations. Because phase and amplitude are usually coupled, it is difficult to computer design appropriate modulation patterns fast enough for the real-time applications for which SLMs are suited. Dramatic computational speedups can be achieved by using encoding algorithms that directly translate desired complex values into values that the modulator can produce. For coherently illuminated SLMs in a Fourier transform arrangement pseudorandom encoding can be used. Each SLM pixel is programmed in sequence by selecting a single value of pixel modulation from a random distribution having an average that is identical to the desired fully complex modulation. While the method approximates fairly arbitrary complex modulations, there are always some complex values that are outside the encoding range for each SLM coupling characteristic and for each specific pseudorandom algorithm. Using the binomial distribution leads to methods of evaluating and geometrically interpreting the encoding range. Evaluations are presented of achieving fully complex encoding with SLMs that produce less than 2 (pi) of phase shift, identifying an infinite set of encoding algorithms that encode the same value, identification of the maximum encoding range, and geometric interpretation of encoding errors.
Today diffractive elements are routinely designed and fabricated that diffract laser beams into hundreds of spots with diffraction efficiencies in excess of 90%. Thus it is easy to imagine that an electrically addressed phase-only spatial light modulator (SLM) could be used for the simultaneous and independent scanning of multiple spots in arbitrary directions. As opposed to single spot scanning, each spot would be present for the entire frame time of the modulator, thereby increasing dwell time over raster-based scanners. Key to actually achieving arbitrary scanning operations at real-time rates is the issue of specifying the electrical control values to the SLM. The control values are not obvious because arbitrary complex modulation is needed to produce arbitrary diffraction patterns. Calculation of the control values using global optimization (as is used to design diffractive optics) is too slow. Group oriented encoding procedures are fast but the use of groups to represent individual complex values reduces the resolution of the scanner correspondingly. The recently developed method referred to as pseudorandom encoding can be computed in real- time and does not sacrifice resolution. This statistically based encoding procedure is reviewed with emphasis on its performance and suitability for applications requiring multispot beam steering.
KEYWORDS: Detection and tracking algorithms, Image filtering, Digital signal processing, Signal to noise ratio, Spatial light modulators, Composites, Target detection, Charge-coupled devices, Electronic filtering, Linear filtering
A fully implemented vision based tracker must be able to identify an object in a variety of poses or distortions and estimate its position in a scene. After locking onto the object, continuous steady state tracking is required as the object gradually changes its position and orientation. The tracker must be able to recognize loss of track and take action to recover the object during break-lock and other transient conditions. While correlators can optimally recognize an object under theoretically ideal conditions, autonomous tracking of objects would require the development of a high level controller, or intelligent supervisor to deal will an uncontrolled visual environment. The supervisor would need to configure and control the analysis of the input environment, the detection procedure of the target, the trajectory estimation for maintaining lock on the target, and the camera orientation. In this paper we review the tracking problem. We then describe supervisor design based on configuring a suite of specific operations. Some of the operations include wide-area scan and prescreening operations using vector inner product composite filters; accurate detection and location with distortion-invariant composite filters to isolate a large data base of training views; filter banks distortion and which is distinguishable from characters associated with other objects. We discuss both hardware and algorithm considerations for the tracking problem. A general conclusion is that specific composite filter designs can be combined and configured to perform the tracking process.
Pseudorandom encoding is a statstica11ybased, pixel-by-pixel mapping of complex valued modulations Auto modul6tors that do not produce a!! comp1e values.' The resulting far-field 1iffraction pattern closely approximates that from the desired, but unimplementabic, complex modulation. Since the methed is point-oriented, the desired complex modulation can be synthesized and encoded without resorting to time consuming constrained global optimizations e.g. simulated anneding,4 genetic,3 and Gerchberg Saxton4 algorithms. In addition to reducing design time, the resulting diffraction patterns can have reasonably high diffraction efficiencies and low levels of background noise.
Hybrid correlators are composed of numerous nonideal electronic and optical components that, to one degree or another, limit performance through unintended transformations of signals. Many of these eftects show up as phase errors at a spatial light modulator (SLM) plane. The errors can be described as random variables, or as systematic offsets from the correct phases, as appropriate. Sources of systematic phase errors include quantizing circuits, incorrect or nonlinear amplifier gain, limited range phase modulators and residual phase modulation of amplitude-mostly SLMs. Random phase errors arise from electronic noise and fabrication variations of SLMs. Several systematic and random filter plane errors are related through a single parameter that describes the amount of phase mismatch. A model of peak-to-noise ratio (PNR) is also presented that describes the combined effects of random and systematic errors. This expression contains the products of two functions, one that depends only on systematic, the other on random, phase mismatch. PNR is also a function of the number of pixels in the filter plane modulator and a normalized moment of the amplitude of the image spectrum. The model is useful for developing phase error budgets for correlation systems.
In this paper several systematic and random filter plane errors are related through a single parameter that describes the amount of phase mismatch. A model of peak-to-noise ratio (PNR) is also presented that describes the combined effects of random and systematic errors. This expression contains the products of two functions, one that depends only on systematic, the other on random, phase mismatch. PNR is also a function of the number of pixels in the filter plane modulator and a normalized moment the amplitude of the image spectrum. The model is useful for developing phase error budgets for correlation systems.
Phase-modulating devices, especially spatial light modulators, are often incapable of producing a full 360 degrees of phase modulation. Other limitations due to calibration errors, signal distortion, and quantization can cause the actual phase modulation to differ from the desired modulation. Such limitations on the filter plane modulator can reduce the performance of phase-only correlators. We quantify these performance losses for various phase limitations, both through simulation and through the development of an approximate model of performance. In one case we quantitatively compare the performance of phase-only filters that are optimized for limited-range phase modulation (as prescribed by Juday's `minimum Euclidean distance' principle) with the performance of nonoptimal filters. In another case we analyze the effect of not compensating a quadratic dependence of phase on signal voltage which is anticipated for some spatial modulators, e.g. deformable mirror devices.
In this paper, we introduce a unified model-based pattern recognition approach that can be formulated into a variety of techniques to be used for a variety of applications. Complex phasor addition and cancellation are incorporated into the design of filter(s) to perform implicit logical operations using linear correlation operators. These implicit logical operations are suitable to implement high-level gray-scale morphological transformations of input images. In this way we effectively project nonlinear decision boundaries into the input signal space yet maintain the mathematical simplicity of linear filter designs. We apply this approach to the automatic distortion- and intensity-invariance object recognition problem. We introduce a set of shape operators or complex filters that are logically structured into a filter bank architecture to accomplish the distortion and intensity-invariant system. This synthesized complex filter bank is optimally sensitive to fractal noise representing natural scenery. The sensitivity is optimized for a specific fractal parameter range using the Fisher discriminant. The output responses of the proposed system are shown for target, clutter, and pseudo-target inputs to represent its discrimination and generalization capability in the presence of distortion and intensity variations.
The monochromatic diffraction patterns from phase-only spatial light modulators, such as deformable mirror- and liquid crystal-SLMs, are routinely modeled using the fast Fourier transform. Random errors in phase have typically been evaluated by Monte Carlo analysis. This can require the averaging of numerous runs to find the expected value of intensity and its standard deviation. However, the pixel structure of current modulators, which allows each pixel to apply arbitrary piston or tilt modulation, greatly simplifies the form of the expectations. Specifically, we assume that the parameters of piston, and likewise tilt, describing the entire SLM transmittance are Gaussian, independent, and identically distributed random variables. Expressions for the expected value of the Fourier plane intensity spectrum and its standard deviation, and for propagation of the intensity pattern to any arbitrary observation plane using the angular spectrum of waves formulation are presented. Each expression reduces to a small number of Fourier transform operations that may be efficiently calculated by the FFT.
Piston-only spatial light modulators, such as the flexure-beam deformable mirror device, hold great promise for real-time optical processors because of their ability to accurately match the phase of arbitrary laser images. Non-ideal devices may have uncontrolled phase errors which can limit performance. A statistical analysis has been developed which models the effect of random piston and tilt errors on the diffraction pattern of phase-only SLMs. A slight modification of these equations describes the performance of these SLMs in the phase-only correlator and the phase-only correlator modulated by pseudo-random sequences. Results on correlator diffraction efficiency versus amount of phase error are presented. For the specific case of a binary phase-only matched filter a diffraction efficiency of 40.5% is found.
An optical crossbar switch is proposed that uses a semiconductor-addressed, deformable-mirror spatial light modulator to direct gigabit-per-second optical transmissions to several receivers. The switch performance is described in terms of size, number, and deflection range of micromechanical mirror elements, as well as emitter, detector, and lens dimensions. These parameters are incorporated in a link energy budget and estimates of system dimensions. Maximum-sized 128- to 1000-channel crossbars appear feasible based on current and projected performance limits of various lightwave components.
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