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We describe an active millimeter-wave holographic imaging system that uses compressive measurements for three-dimensional (3D) tomographic object estimation. Our system records a two-dimensional (2D) digitized Gabor hologram by translating a single pixel incoherent receiver. Two approaches for compressive measurement are undertaken: nonlinear inversion of a 2D Gabor hologram for 3D object estimation and nonlinear inversion of a randomly subsampled Gabor hologram for 3D object estimation. The object estimation algorithm minimizes a convex quadratic problem using total variation (TV) regularization for 3D object estimation. We compare object reconstructions using linear backpropagation and TV minimization, and we present simulated and experimental reconstructions from both compressive measurement strategies. In contrast with backpropagation, which estimates the 3D electromagnetic field, TV minimization estimates the 3D object that produces the field. Despite undersampling, range resolution is consistent with the extent of the 3D object band volume.

A two-step iterative algorithm is described for designing pupil functions for synthesis of bipolar point spread functions in a two-channel incoherent spatial filtering system. The first step uses the method of projections onto convex sets to determine an appropriate bias for the system pupil functions. This bias determines the complementary point spread functions and therefore the magnitude of the corresponding coherent spread functions. The phase of the coherent spread functions is obtained in the second step via iterative application of a magnitude constraint and a finite support constraint. With both magnitude and phase of the coherent spread functions established, the corresponding pupil functions are realized via Fourier transformation. Results are presented for a 2-D bandpass filter and compared with analytically obtained results. The comparison suggests that the iterative algorithm yields bias functions having less energy than bias functions determined by previously proposed analytic methods.

Symbolic substitution is a method for manipulating binary data that depends on both the value of the data and its spatial location to realize logical operations. A substitution system requires only a pattern recognizer, a nonlinear device, and a pattern substituter. Using classical optical elements and phase-only holographical elements, it is possible to construct optical systems for both recognition and substitution. Systems using one or two holographical elements are presented, and procedures for designing the elements are also discussed.

This paper describes an active millimeter-wave (MMW) holographic imaging system used for the study of compressive measurement for concealed weapons detection. We record a digitized on-axis, Gabor hologram using a single pixel incoherent receiver that is translated at the detector plane to form an image composite. Capturing measurements in the MMW regime can be costly since receiver circuits are expensive and scanning systems can be plagued by their long data acquisition times. Thus, we leverage recent advances in compressive sensing with a traditional holographic method in order to estimate a 3D (x,y,z) object distribution from a 2D recorded image composite. To do this, we minimize a convex quadratic function using total variation (TV) regularization. Gabor holograms are recorded of semi-transparent objects, in the MMW, mimicking weapons and other objects. We present preliminary results of 3D reconstructions of objects at various depths estimated from a 2D recorded hologram. We compare backpropagation results with our decompressive inference algorithm. A possible application includes remote concealed weapons detection at security checkpoints.

We analyze the impact of sparse sampling on millimeter-wave (MMW) two-dimensional (2-D) holographic measurements for three-dimensional (3-D) object reconstruction. Simulations address 3-D object estimation efficacy. We present 3-D object reconstructions from experimental data.

We apply structured passive elements and computational imaging to millimeter waves to enable a mobile imager. Experimental results are presented.

Dammann gratings are binary-phase Fourier holograms that are used to generate an array of point sources. Using symmetry and separability arguments, we extend Dammann's method for generating one-dimensional symmetric arrays to general two-dimensional responses. Computer-generated holograms are used to verify the modified design method.

An examination of recent trends in imaging reveals a movement toward systems that balance processing between optics and electronics. Imaging applications include conventional imaging to produce visually pleasing images, special purpose imaging whose output is also an image but with enhanced characteristics, and functional imaging to produce information about a scene from optical data. We identify three approaches to computational imaging that are capable of achieving these goals: wavefront encoding, multiplex imaging, and feature extraction.

In February 2008 the US Army Research Laboratory established the Micro Autonomous Systems and Technology Collaborative Technology Alliance, in which research is performed by an alliance of government, academic and industrial researchers. The research, grounded in microsystem mechanics, microelectronics, algorithms for autonomy, and integration, is focused on addressing four cross-cutting thrusts: mobility and navigation of palm-sized platforms, sensing and perception on palm-sized platforms, sensing, perception, and navigation of a collection of platforms, and the modeling, simulation, experimentation, and validation of such platforms individually and as a collective ensemble. This article reviews the operational and technical objectives of the program, and overviews some of the research conducted to achieve them.

A research effort was conducted to explore the ways in which sub-wavelength gratings can be used to reduce the reflections from the optics of millimeter-wave imaging systems. A moth-eye lens is sometimes used at optical and infrared frequencies for this purpose, but it is too fragile to be applied directly to the plastic lenses used in the millimeter-wave. A modification of this structure, called the inverse moth-eye lens, was designed, fabricated, and tested. Within the report we present results at 35 GHz showing a 15-dB reduction of surface reflections in Rexolite. The technique was also applied to a material with a dielectric constant of 9. Transmission increased an average of about 30 dB between 30 and 40 GHz with the inverse motheye, anti-reflection (AR) surface. The implication of these results is that the weight and bulk of millimeter-wave imaging systems could be significantly reduced by using optical systems with high dielectric materials and etched, sub-wavelength AR surfaces.

We apply compression in the spatial frequency domain to generate millimeter wave images. Simulations indicate the efficacy of the approach. We are in the process of testing the system experimentally.

The Micro Autonomous Systems and Technology program is conducting research to develop small air and ground robotic systems for acquisition and delivery of improved situational awareness to warfighters.

We describe a computational imaging technique to extend the depth-of field of a 94-GHz imaging system. The technique uses a cubic phase element in the pupil plane of the system to render system operation relatively insensitive to object distance. However, the cubic phase element also introduces aberrations but, since these are fixed and known, we remove them using post-detection signal processing. We present experimental results that validate system performance and indicate a greater than four-fold increase in depth-of-field from 17" to greater than 68".

Computational imaging systems are characterized by a joint design and optimization of front end optics, focal plane arrays and post-detection processing. Each constituent technology is characterized by its unique scaling laws. In this paper we will attempt a synthesis of the behavior of individual components and develop scaling analysis of the jointly designed and optimized imaging systems.

Most improvements in radar have concentrated solely on the temporal characteristics of the electromagnetic pulse. The spatial dimension is typically exploited only through arrays of antennas. The limits to using spatial characteristics of a transmitted beam to perform ranging were investigated, specifically, Laguerre-Gaussian beams as potential carriers for encoding range in spatial characteristics. The results indicate that whereas the beams are indeed capable of encoding range, aberrations and imaging fidelity limit the utility of the approach to small ranges. The system is better suited for microscopy than it is for battlefield ranging.

The design, fabrication, experimental characterization, and system-performance analysis of a diffractive optical implementation of an error-diffusion filter for use in digital image halftoning is reported. A diffractive optical filter was fabricated as an eight-level phase element that diffuses the quantization error nonuniformly in both the weighting and the spatial dimensions, according to a prescribed algorithm. Ten identical diffractive elements were fabricated on ten different wafers and subsequently characterized experimentally. A detailed error analysis including both fabrication and instrumentation errors was carried out to quantify the performance of the fabrication process as well as the expected system performance of the filters. Halftone system performance was evaluated by use of the experimental filter’s performance and both quantitative and qualitative performance metrics. The results of this analysis demonstrate that multiple identical copies of a diffractive optical filter can be produced with sufficient accuracy that no loss in the halftoning system performance results.

This issue of Applied Optics features 16 papers related to the fabrication, design, and application of diffractive and micro-optical elements. A companion feature in the Journal of the Optical Society of America A [J. Opt. Soc. Am. A 16(5), 1091 (1999)] includes papers on the modeling of diffractive elements.

We present a procedure for the design of binary diffractive lenses with pulse-width-modulated subwavelength features. The procedure is based on the combination of two approximate theories, effective medium theory and scalar diffraction theory, and accounts for limitations on feature size and etch depth imposed by fabrication. A design example is presented.

In this paper we demonstrate the miniaturization of a loop antenna through simulation and experiment using synthetic reactive elements, namely split ring resonators (SRRs), embedded in the substrate of the antenna. Two designs were considered with SRRs both in and out of the plane of the substrate. Simulation results show that antenna size can be reduced up to thirty eight percent for both designs. However, due to ease of fabrication, the in-plane configuration is the preferred design. Simulation and experimental results are presented.

We apply extended depth-of-field imaging using a cubic phase element to a 94 GHz imager. Simulations indicate the efficacy of the approach. We are in the process of testing the system experimentally using a cubic phase element fabricated in Rexolite™.

Linear superpositions of Laguerre-Gaussian beams are used to measure range by encoding the propagation distance as a unique spatial intensity pattern. Strategies for beam synthesis and the recovery of range information are discussed.

Superpositions of Laguerre-Gaussian modes are used to measure range. Beam patterns are experimentally realized using a spatial light modulator. Fundamental and practical limits of the 3-D beam synthesis problem are investigated.

On 10–11 April 2014, the Optical Society of America (OSA) hosted an incubator on compressive sensing at its Washington, D.C., headquarters.

While all the lessons of Operation Iraqi Freedom will not be known for some time, one already seems clear: large, well-armed ground forces are still needed for many expeditionary wars. Heavy U.S. Army forces, however, still lack the capacity to deploy overseas swiftly enough to wage these wars. The result is a deficiency in the capacity of U.S. military forces to prosecute modern wars in distant areas. This will be especially true for wars in which air power is not a cure-all and major ground combat operations must be launched. Addressing the problem of slow Army deployment rates, the Army Transformation Roadmap 2003 states the goal of transformation is to "identify and build required capabilities now and while developing future force capabilities essential to provide relevant, ready, responsive, and dominant land power to the Future Joint Force." Indeed, one aspect of the Army's transformation efforts is force redesign to develop an active component capable of deploying a responsive, agile expeditionary force in the first fifteen days of an operation.

Laguerre-Gaussian beams are considered as basis functions for inverse scattering applications. First order perturbation theory is applied to paraxial higher order Gaussian beams. Information about the scattering potential is deduced from the coupling coefficients between otherwise orthogonal modes. This leads to a theoretical description analogous to plane wave diffraction tomography. Differences between the plane wave model and the Laguerre-Gauss formalism highlight both limitations as well as opportunities for applying singular Gaussian beams to the inverse scattering problems. The perturbation analysis is applied to a number of examples illustrating how information about the scattering object can be deduced from measurements of the scattered paraxial field.

For most radar or ladar systems range information is obtained from the time necessary for an electromagnetic pulse to propagate to a target and return to a receiving antenna. In contrast, we investigate a method that replaces temporal encoding of distance with spatial encoding. In particular, we use a self-referencing superposition of Laguerre-Gaussian beams to translate propagation distance into transverse rotation of cross-section of the beam intensity. We review the mathematical foundations of the technique and discuss models for simulating its performance in turbulent atmosphere. In addition, we present a simple technique to extract the rotation angle from a two-dimensional cross-section of the beam. Preliminary results indicate that the technique is robust with respect to propagation in a turbulent atmosphere.

The history of imaging is a continuous cycle of advances in science and technology beginning with observations made with crude technology, which lead to new science and, finally, to advances in technology. I recount the history of optical imaging to place in perspective three distinct trends present in imaging research today: computational imaging, engineered materials, and quantum imaging.

The large numerical apertures employed in millimeter wave imaging are necessary for light gathering and resolution, but result in limited depth-of-field. We explore wavefront coding to extend the depth-of-field and examine effects on system performance.

Coherent superpositions of Laguerre-Gaussian beams are used for range measurements. The impact of atmospheric turbulence is investigated. A scheme is introduced to synthesize beams which are robust against a specified class of perturbations.

We have designed a thin, multiplexed infrared imaging system that uses a diffractive element for generating shift-encoded images in each aperture. We present preliminary experimental results that demonstrate imager performance.

We designed a diffractive element for generating shift-encoded images in a thin, multiplexed imaging system. We present simulations to validate the element's use for high resolution imaging and preliminary experimental results of diffractive image replication.

Army transformation is an attempt to provide future forces with enhanced capabilities in lethality, survivability, and mobility, both strategic and tactical. Alternatives to achieving these goals differ in emphasis on weight and reliance on technology. That is, transformation plans differ if the objective is weight reduction as opposed to weight redistribution. in one approach, platform weight is reduced to meet mobility goals. However, shedding weight has implications for platform survivability and lethality; previous attempt to design a single platform that is simultaneously lethal, mobile, and survivable have not done so satisfactorily. Thus, advances in materials are required to insure the survivability of a lightweight platform. Advances in network technology are also required to make the platform more aware of its environment.

980-nm substrate-emitting oxidized-aperture VCSELs with threshold currents in the 150 /spl mu/A to 500 /spl mu/A range have been developed for optoelectronic interconnects. 8/spl times/8 VCSEL arrays with 125 /spl mu/m pitch have been flip-chip bonded to CMOS driver circuitry having 0.5 /spl mu/m gate size. InGaAs/InP 8/spl times/8 detector arrays have been flip-chip bonded to provide good responsivity and high speed detection in dense arrays with 125 /spl mu/m pitch. Data rates as high as 1.1 Gb/s have been demonstrated with the incorporation of refractive coupling optics between transmitter and receiver arrays. Studies have been conducted of crosstalk and alignment sensitivity of these optoelectronic interconnect arrays. Design and modeling techniques have been demonstrated for the development of diffractive optics for optoelectronic interconnects. Special techniques have been developed to deal with very short working distances and subwavelength feature in the diffractive optics. Fan-out performance up to 7/spl times/7 with accurately controlled intensity weights has been demonstrated with diffractive optics. The use of diffractive optics in optoelectronic interconnects with fan-out and/or fan-in can make the interconnect an integral part of the optoelectronic processing algorithm.

We discuss the design of two diffractive optical elements that provide fan-out from one processing plane to the next. One element is used in an optical analog-to-digital converter and the other is used to shape the beam of a vertical cavity surface emitting laser. We use scalar-based diffraction theory to design the diffractive element for the analog-to-digital converter and vector-based theory to design the laser beam shaper.

We introduce a new design methodology for synthesizing two-dimensional polarization sensitive computer generated holograms. Our method uses the form-birefringence of a single binary two-dimensional grating to produce different phase profiles for the two orthogonal linear polarizations. We validated the method numerically using the finite-difference-time-domain method. We fabricated a single element but have not yet characterized its performance.

Applied Optics' Editor-in-Chief, Joseph N. Mait reflects on his experience as a reader, author, reviewer and eventual editor of the journal. Dr. Mait also introduces the incoming Editor-in-Chief, Ronald G. Driggers and acknowledges outgoing Division Editor, T.-C. Poon.

The history of diffractive optic technology is traced from the first hand-drawn computer-generated holograms created by Adolf Lohmann and Byron Brown to elements fabricated using electron beams that are smaller than a human hair. The influence of Adolf Lohmann on the field is recounted, as are the major developments in fabrication and computation. In particular we highlight the influence of technology on design techniques showing how the availability of computer plotters in the 1960s lead to early encoding techniques. The transition in the 1970s to photolithographic fabrication changed the nature of diffractive design from cell-oriented to point-oriented encoding. At the same time optimization routines were developed that incorporated these new fabrication constraints. The introduction of electron beam writing in the fabrication of diffractive optics in the 1980s brought diffractive design in the 1990s full circle to techniques that are again cell-oriented. Shrinking-features have also changed the applications for diffractive elements. First used primarily as filters for optical correlators, diffractive elements will play a critical role in telecommunications systems that are nearing deployment. However, the most visible impact of Adolf Lohmann's contributions are pattern generators sold with laser pointers. This history is dedicated to Adolf Lohmann on the occassion of his seventy-fifth birthday.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Summary form only given. We have demonstrated an integrated optoelectronic MCM that uses a glass substrate as a mechanical carrier for the VCSEL, VCSEL driver, and diffractive element. Over a distance a little longer than 1 mm, our system fanned-out a single VCSEL into 60 individual fan-outs.

We have integrated an 850-nm VCSEL, its driver, and diffractive lenslet array onto a single substrate to produce an integrated opto-electronic multi-chip module. The diffractive element fans the output from the VCSEL into an array of approximately 60 focal spots at a plane 1416 µm from the surface of the VCSEL. Experimental results are presented.

The presentation gives an overview of the ongoing Army Research Laboratory (ARL)/University of Maryland research effort on vertical-cavity-surface-emitting-laser (VCSEL) interconnects and OE processing and why this technology is of interest. ARL is conducting a research and development effort to develop VCSELs, VCSEL arrays, and their hybridization with complimentary metal-oxide-semiconductor (CMOS) electronics and microwave monolithic integrated circuits (MMICs). ARL is also very active in the design, modeling, and development of diffractive optical elements (DOEs). VCSEL-CMOS flip-chip optoelectronic circuits and DOEs are of interest together with detector-CMOS flip-chip circuits to provide digital and analog optoelectronic interconnects in optoelectronic processing architectures. Such optoelectronic architectures show promise of relieving some of the information flow bottlenecks that are emerging in conventional digital electronic processing as the electronic state of the art advances at a rapid pace and the electronic interconnects become a significant limitation. Such optoelectronic interconnects are also of interest in the development of analog optoelectronic processing architectures that are very difficult to implement in conventional electronic circuitry due to the incorporation of dense arrays of interconnects between electronic elements. VCSEL-MMIC- detector flip-chip circuits are of interest for the incorporation of optoelectronic interconnects into analog RF systems where the optoelectronic interconnect offers advantages of size, weight, bandwidth, and power consumption. VCSEL-MMIC interconnects may also play a role in future high- speed digital optoelectronic processing.

The Army Research Laboratory (ARL) conducts a broad-based optoelectronics R and D program that addresses a number of Army applications. This program covers the full range of activities from basic materials development to component development and integration into higher levels of optoelectronic functionality. This paper addresses technology areas of interest to ARL including IR detection and imaging, IR sources, ladar, multifunction optoelectronic integration, diffractive optics, optoelectronic interconnects/processing, waveguide integrated optics, wide bandgap optoelectronics, and nonlinear optics. These areas represent a cross-section of the work conducted in the Sensors and Electron Deices Directorate of ARL. Space does not allow comprehensive discussion of the R and D program each of these technology thrust ares, but references are provided in each case so that the interested reader can pursue each of these topics further.

We contrast both direct and indirect methods for subwavelength diffractive design. Direct methods seek to optimize the diffractive structure directly by modifying its parametric representation. However, each modification requires a complete rigorous analysis. In contrast, indirect methods optimize the complex-wave amplitude, which must then be encoded into a diffractive structure.

Recent research1–9 has shown that if a binary-phase diffractive optical element (DOE) has features that are on the order of the illuminating wavelength, the performance limits set by scalar-based diffraction theory can be overcome. In fact, diffraction efficiencies in excess of 90% have been predicted for binary gratings that have subwavelength features.1,4,5 Due primarily to the availability of tools for modeling, the analysis and design of subwavelength DOEs (SWDOEs) has concentrated primarily on gratings.1-7,10 To overcome this limitation, we have developed numerical routines that use a boundary element method (BEM) to analyze diffraction from finite extent, aperiodic DOEs.11 In this paper we consider diffractive design, in particular, the design of diffractive lenses, subject to the constraints of fabrication.

The analysis of diffractive lenses that have feature sizes comparable to the illumination wavelength require a rigorous, or vector-based, model of diffraction. However, most techniques presented in the literature for the vector-based analysis of such elements are limited to two-dimensional (2D), or cylindrical, lenses. In this paper we present a vector-based technique for the analysis of three-dimensional (3D), or spherical, diffractive lenses that are perfect electrical conductors (PEC).

Diffractive optical elements (DOEs) whose features are smaller than the wavelength of illumination are referred to as subwavelength diffractive optical elements (SWDOEs). Subwavelength features provide SWDOEs with an added degree of flexibility over their superwavelength counterparts. For example, in the case of binary structures, SWDOEs have been shown to achieve diffraction efficiencies in excess of 90%[1]. This added flexibility, however, results in a significant cost. Namely, the analysis and subsequent design of SWDOEs require the use of rigorous electromagnetic (EM) models. These models, which incorporate the full vector nature of electromagnetic fields, can be theoretically challenging to construct and often demand considerable computational resources.

Applied Optics Editor-in-Chief, Joseph N. Mait, comments on the value of peer review in improving the work published by authors.

Imaging systems have numerous applications in industrial, military, consumer, and medical settings. Assembling a complete imaging system requires the integration of optics, sensing, image processing, and display rendering. This issue features original research ranging from fundamental theories to novel imaging modalities and provides a systems perspective to imaging.

The sky is not falling and the state of the journal is good. These are essentially the conclusions of the committee that reviewed Applied Optics in 2012. The conclusions speak directly to the concerns stated in my December 2010 editorial “Identity Crisis.” The committee’s report is reassuringly positive, but it also provides recommendations to ensure the journal continues to fill an important and unique niche for the optics community. The committee surveyed readers, authors, reviewers, and editors about their impressions of the journal, including the quality of the journal and their regard for it. The committee also held follow-up interviews with some of the respondents. Indeed, the journal is held in high regard. A substantial majority of respondents (more than 75 percent) feel the quality of the journal has stayed the same or increased in the past five years. A minority of respondents felt the quality had decreased. The reasons cited for this decline were the publications were not science (I actually take this as a positive comment because Applied Optics is not a science journal but an applied one), contained poor analysis, were duplicative, or provided poor interpretation. I take these criticisms seriously. They indicate our reviews need to improve and our criteria for publishing manuscripts can be more stringent. The report conveys an important message applicable to all OSA journals: Impact factor should not be the sole measure of a journal’s value. Given the applied nature of the journal, impact factor is not an accurate reflection of Applied Optics’ value. For example, a search of U.S. patents issued in the past five years reveals that, in terms of optics journal citations,Applied Optics is second only to Physics Review. It would appear that many in our reader community are involved in technology development. They rely upon published results to drive company interests. Since patent citations do not contribute to impact factor calculations, we need to highlight this fact to potential authors. The report’s primary concern was the confusion that exists between manuscripts best suited for Applied Optics versus Optics Express. This issue is being addressed by OSA’s Board of Editors. Consequently, the board asked the editors of all OSA journals to look carefully at their journal definitions and to update reviewer feedback forms to ensure the form properly reflects the journal’s character. This will continue throughout the year. The planned upgrade of OSA’s review system, ADS, should help remove some of this confusion. I have proposed that, during manuscript submission, ADS should present authors with information that helps them select the journal best suited to their manuscript. I am also working with OSA staff to develop a transfer letter that editors of other journals should send to authors pre-review when they receive an applied paper that is better suited for Applied Optics. In light of the definition process and submission process to a specific journal, the editorial staff and I will reconsider the resubmission policy we instituted two years ago. Looking to the future, the review committee cautioned against the journal content remaining static. We need to ensure that the journal scope remains sufficiently broad to meet the needs of all applied work in optics. The journal needs to be nimble and capable of drawing in developing areas searching for a technical home. My editors and I will develop a process to identify and capture new topic areas. A primary tool we will use is special issues. The committee also recommended that Applied Optics promote the use of supplementary data, such as video explanations and optical design files from 1559-128X/13/040003-02$15.00/0 © 2013 Optical Society of America