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Advances in fabrication technology have made it possible not only to vary an element's index of refraction spatially, but also its dispersion. However, most tools for dispersion engineering are based on rigorous electromagnetic theory, which is not the conventional language of optical designers. We present an approach to dispersion engineering that is congruent with conventional optical design.

I present a model for an achromatic surface lens based on the superposition of lenses that focus discrete wavelengths independent of one another. The model potentially provides insight into the physical behavior of experimentally demonstrated lenses. It also suggests a simple design algorithm.

The potential exists for new optical technologies to impact computational sensor design. We consider in a formal manner the potential impact of metamaterials.

This feature issue of Applied Optics (AO) on Optics Theory and Practice in Iberoamerica (OTPI) collects significantly expanded refereed papers presented at the multiconference RIAO-OPTILAS-MOPM, held in Cancún, Mexico, Sept. 23–27, 2019. All authors who participated at the conference were contacted and invited to contribute to this special issue. Furthermore, the AO dedicated issue on OTPI was open to contributions from other practitioners in all related areas, through a call for papers published in AO.

The potential exists for new optical technologies to impact computational sensor design. We consider in a formal manner the potential impact of metamaterials.

We use first-order optical principles to examine the ability of gradient index (GRIN) lenses to correct chromatic aberrations. We consider radial GRIN lenses with flat surfaces, with a flat diffractive surface, and with curved surfaces. We model the GRIN material system as a locally varying, subwavelength blend of three materials. In this model, we demonstrate that the color-correcting properties of each lens type can be expressed solely in terms of the dispersion properties of the base materials. We find, at this level of approximation, that the material condition for a two-material GRIN achromat with curved surfaces is identical to that for a homogeneous doublet achromat comprised of the same two materials. For the more general case of three-material, ternary GRIN elements, we use the theory to develop a figure-of-merit-based optimization approach. This allows us to identify promising material combinations without first fabricating a GRIN element. The optimization approach can be applied to alternate GRIN geometries and arbitrary glass catalogs. We use our model to search a large, commercial glass catalog to identify the best achromatic glass combinations for the three different GRIN lenses described above. Significant numerical effort was required to identify which glass combinations performed best. Ternary glass combinations are necessary to achieve good achromatic performance for flat geometries. Diffraction combined with a graded-index enables improved color correction for the same optical power or nearly a factor of two increase in power for the same level of color correction. Glass pairs that perform well as an achromatic doublet also perform well chromatically when blended in a GRIN singlet.

Fifty years ago this month, the high-profile U.S. photography and optics firm attempted to take action against apartheid. The project’s mixed results—and some details behind the story-remain relevant a half-century later.

Imaging using millimeter waves (mmWs) has many advantages and applications in the defense, security, and aviation markets. All terrestrial bodies emit mmW radiation, and these wavelengths are able to penetrate smoke, blowing dust or sand, fog/clouds/marine layers, and even clothing. However, there are many obstacles to imaging in this spectrum that have to be overcome before mmW imaging systems can be successfully realized for surveillance and defense applications. Recent developments in computational imaging have the potential to significantly improve capabilities of mmW imaging systems. Our article provides an overview of computational imaging and its implication to mmW imaging in various operation modes. We discuss the merits and drawbacks of available computational mmW imaging approaches and identify avenues of research in this rapidly evolving field.

Previous work developed a first-order theory for picking optimal pairs of materials for gradient index (GRIN) achromatic singlets. This work extends that concept to include the addition of a third material to a GRIN blend, to improve performance further. Several ternary-based GRIN lens designs are compared to binary versions. Implications for material development in gradient index optics are discussed.

The U.S. Army Research Laboratory (ARL) is the premier laboratory for land forces in the United States. A young organization that in 2017 celebrates 25 years of discovery, innovation, and transition of science and technology, ARL has already contributed significantly to several disciplines in optical sciences.

We review advances in Computational Imaging that have occurred since the mid-1990s. A similar review we presented more than a decade ago, ”Computational Imaging: Old Wine in New Bottles?” when the community was still trying to find its footing, gave a cautious assessment. Since then, the advent of highly integrated platforms, such as the smart phone, has made the linkage between optics and processing intimate and natural. This review covers key technological developments and insights over the past two decades and examines current trends to predict future capabilities.

We show that a gradient-index element designed from a blend of three materials allows a designer to specify independently the element's refractive index and its change in refractive index with respect to wavelength. We show further the effectiveness of this approach by comparing modeled chromatic performance of deflectors consisting of a single material, a binary blend of materials, and a ternary blend.

A new figure of merit is developed for ranking pairs of materials as candidates for gradient index (GRIN) optics capable of good color correction. The approach leverages recent work which derives a connection in GRIN lenses between the optical properties of constituent materials and the wavelength dependence of the lens power. We extend the analysis here, the effectiveness of which is evidenced by a simulated f/3 GRIN lens with diffraction-limited performance over the visible spectrum, using the top material pair selected out of a database of >60,000 possible candidates.

The addition of a third material to a binary blend allows designers to specify refractive index and dispersion independently in gradient index optics. The effectiveness is demonstrated via a diffraction-limited, f/1 achromatic singlet design.

A new method for synthesizing broadband antireflective (AR) surfaces at microwave and millimeter wave frequencies is demonstrated. The AR surface, we call an inverse motheye, was formed by machining a multi-layer grating of subwavelength circular holes into a non-absorptive dielectric. This created low reflected energies (

A method for synthesizing broadband antireflective (AR) surfaces at millimeter wave frequencies is demonstrated. AR surfaces were formed by machining a multilayer subwavelength structures into nonabsorptive dielectrics. This created low-reflected energies (

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 17rdquo to greater than 68rdquo.

From the expression for optical power of a radial first-order graded-index (GRIN) lens with curved surfaces, we derive an expression for chromatic aberration. Our expressions for optical power and chromatic aberration are valid under the paraxial approximation. By applying a series of further simplifying assumptions, namely a thin lens and thin GRIN, we derive a set of equations with which one can design an achromatic GRIN lens. We also derive expressions for the dispersive property of a GRIN element. Our analysis enables us to derive the relationship between material pairs that indicate their suitability as a material pair for a GRIN achromat. We use this relationship to search a standard glass catalog for attractive GRIN material pairs for a particular achromat design. We compare the optical performance of our GRIN design to that of a conventional homogeneous doublet and demonstrate that our approach is capable of identifying material pairs that perform well for achromatic GRIN lenses which would not generally be considered for conventional achromatic design. We also demonstrate our approach is capable of designing GRIN achromats with superior performance.

We present a procedure for designing two point spread functions in a millimeter wave imaging system whose difference produces a desired response with minimum bias and maximum signal-to-noise ratio.

Millimeter wave technology has become increasingly important in applications such as imaging and communications. We discuss an architecture that uses a phase array with optical upconversion of the detected millimeter wave signals to achieve a real-time, video rate passive millimeter wave imager.

Beginning with the expression for the focal length of a GRIN element that is valid under paraxial approximations, we derive an expression for chromatic aberration. Together with the expression for focal length, we derive the design equations for a GRIN achromat.

We consider the design of a first-order radial GRIN achromat assuming it is fabricated using nanolayers of a two-polymer blend.

The first experimental characterization of an eight-level, phase-only diffractive optical implementation of an error diffusion filter for use in digital image halftoning is reported. Ten identical elements were fabricated through a DARPA-sponsored workshop and subsequently experimentally characterized. A detailed statistical error analysis including both fabrication and instrumentation errors was performed to quantify the performance of the fabrication process as well as the expected system performance of the filters. Over the 10-sample ensemble, an average diffraction efficiency of 83.7% was measured with an RMS error between the designed and experimental coefficients of 0.2411%, producing nearly diffraction limited performance.

A direct-write ablation technique has been implemented in a photon scanning tunneling microscope setup. This combination allows us to study surface-plasmon (SP) scattering by in situ created individual surface defects, while the sizes and shapes of the defects are varied continuously. It is found that within a certain range of its size, a ``hill'' on an otherwise flat surface can be the source of a very narrow plasmon beam. This effect is explained using the Huygens-Fresnel principle. Shadowing and refraction of the SP field by smaller defects has also been observed. In order to explain these results we introduce an effective SP refractive index for two classes of surface defects: shallow topographical defects and areas covered with absorbed molecular layers. This concept allows us to achieve a qualitative understanding of plasmon scattering in many practical cases. Some simple optical elements for the control of SP propagation are suggested and demonstrated. Our observations suggest numerous practical applications in multichannel chemical sensing, biosensing, and integrated optics.

Recently, new techniques for passive imaging based on distributed aperture imagers have been developed. One promising approach utilizes photonic sampling, routing, and processing of the millimeter-wave energy for image generation. Many signal processing aspects of this approach that have been explored to date, from engineering phase profiles across the array to achieve dynamically adjustable point spread functions, to noise reduction techniques in the optical image regeneration. Herein, we focus on a method to reduce image bias in a distributed aperture millimeter wave imager by switching the phase in individual channels by π-phase. Our simulations indicate this approach has better bias performance than random phase switching.

Following an editorial emphasizing the importance of reviewers, Applied Optics’ Editor in Chief Joseph N. Mait explains the critical role of the editor.

We apply a two-dimensional hybrid finite element-boundary element method to the analysis of diffractive optical elements (DOEs). We use the finite element method to solve Helmholtz's equation in the interior of a DOE and use the boundary element method, a Green's function approach, to determine the fields exterior to the DOE. We couple the two methods at the DOE surface by enforcing field continuity conditions. We validate our implementation by comparing its results to the analytic solution for diffraction from a dielectric cylinder and to the scalar solution for far-field diffraction from a dielectric slab. We use the hybrid method to analyse the diffraction from an eight-level phase lens and a binary-phase lens that has subwavelength features.

This special Applied Optics issue pays tribute to Adolf Lohmann and his accomplishments as a scientist and teacher. A pioneer of information optics, many of his scientific contributions remain relevant even today. We thank all the authors who have contributed papers to this special feature, each of which proves the importance of Adolf Lohmann’s work for contemporary research.

The U.S. Army Research Laboratory (ARL) is the Army's premier laboratory for land forces. The Army relies on ARL for scientific discoveries, technological advances, and analyses that enable capabilities a future Army will need to persevere over adversaries. Although a relatively young organization that will celebrate 25 years of the discovery, innovation, and transition of science and technology in October 2017, ARL has already had significant impact in a wide range of scientific and technological disciplines. In this paper, we highlight some of its past and recent achievements in optics and photonics.

We present a model for noise generated in a two-channel millimeter-wave imaging system and use it to develop a phase switching scheme that generates a minimum amount of noise in the difference image between the two channels. The method switches between zero and pi over half the apertures in a distributed-aperture millimeter-wave imager. Simulations and quantitative experiments validate that this approach improves performance over switching between zero and a random phase value. The improved performance is visually evident in images of an extended object.

I describe a framework useful for designing computational imaging systems and for assessing their efficacy and performance. I cast the problem as the design of a measurement matrix H followed by post-detection processing with an estimation matrix T. I consider only simple imaging as a vehicle to identify the framework and to motivate discussion.

We demonstrate experimentally the ability to shape the point spread function of a distributed aperture millimeter wave imaging system by modifying its aperture phase. We also show how to exploit this capability to perform low-resolution analog image processing. A preliminary investigation of system performance reveals nonuniformity in amplitude response across the array is a major contributor to deviations from predicted PSFs.

In a Defense and Technology Paper (DTP) entitled "A Methodology for Assessing the Military Benefits of Science and Technology Investments," the National Defense University (NDU) Center for Technology and National Security Policy (CTNSP) presented a variety of approaches for deriving the return on investment--in terms of warfighting capabilities---for Army science and technology (S&T) efforts. As a follow-up to the methodology study that generated the DTP, the CTNSP wished to demonstrate parts of the methodology in the evaluation of an actual Army S&T effort. The Army Research Laboratory's (ARL's) Micro-Autonomous Systems and Technology (MAST) Collaborative Technology Alliance (CTA)2 program was chosen to demonstrate the utility of the methodology because it offers significant future capabilities for our Army, provides a set of very robust present-day technical challenges, and offers a significant assessment challenge since it is focused on basic research.

We have integrated an 850-nm vertical-cavity surface-emitting laser (VCSEL), its driver, and a diffractive lenslet array onto a single substrate to produce an integrated optoelectronic multichip module for signal fan-out and distribution. The diffractive element performs optical fan-out of the output beam from the VCSEL into an array of focused spots at a plane 1, 416 /spl mu/m from the surface of the VCSEL. This corresponds to 160 /spl mu/m from the surface of the diffractive lens. System design, fabrication, integration, and experimental characterization is presented.

A kinoform-based Nipkow-disk system, as applied to a real-time confocal microscope, is presented. The major advantage of this technique must be its high light efficiency (e.g., >80%), which significantly improves the performance of a confocal microscope. Our preliminary experiment indicates that there are potential applications to three-dimensional microscopic imaging as well as to object surface detection.

We describe an incoherent image processor that uses orthogonally oriented one-dimensional acousto-optic cells to implement dynamic, arbitrary bipolar point-spread functions (PSF’s). Arbitrary PSF’s are implemented as a linear superposition in time of separable PSF’s. The use of incoherent illumination increases the input field of view over that provided by coherent illumination, and implementation of the PSF by a pupil-plane filter yields a simple, compact single-lens imaging system. The acousto-optic cells offer a faster PSF update rate than that of conventional spatial light modulators, which is a critical issue for the implementation of a bipolar PSF as a subtraction between its positive and rectified negative parts. Initial experimental results are presented that demonstrate the realization of an arbitrary nonseparable PSF, image convolution with a bipolar PSF, two-dimensional image correlation, and an increased processor field of view.

We analyze the performance of two optical shadow-casting image correlators that use two-dimensional source arrays to encode the system point-spread function (PSF). The analysis of a standard shadow-casting correlator suggests that the angular divergence of the source array is a critical parameter in the determination of the maximum space–bandwidth product of the image and of the PSF that can be used with such a system. Further, the energy efficiency of a standard shadow-casting correlator is related inversely to the size of the PSF. We show that the constraints on energy efficiency and on the space–bandwidth product of the PSF can be overcome by beam steering the source elements. A modified shadow-casting correlator is proposed that uses phase-only blazed gratings to beam steer the sources. Experimental results generated by a mechanically beam-steered array are presented.

A two-dimensional optical image processor is described and demonstrated in which the point-spread function is generated by crossed acousto-optic cells in the Fourier plane of a coherent optical correlator. Coherence effects and generation of nonseparable point-spread functions are considered.

An optical morphological processor is described and demonstrated in which the structuring element is generated by means of an acousto-optic cell in the Fourier plane of a coherent optical correlator. A magneto-optic spatial light modulator is employed in the input plane. A single PC generates the drive signals for the acousto-optic cell, processes the output image, and controls the input magneto-optic spatial light modulator. Complex morphological operations of opening and closing are experimentally demonstrated by using electronic image feedback. The results of a morphological noise-removal algorithm implemented by using the optical processor are compared with computer simulations, possible sources of discrepancies are proposed, and potential remedies are discussed.

For one-dimensional binary-phase [(0, pi) and (0, non-pi)] fanout elements and for one-dimensional continuous or multilevel quantized phase fanout elements, an upper bound on diffraction efficiency is presented for fanouts ranging from 2 to 25. The upper bound is determined by optimizing with respect to the array phase the upper bound on diffraction efficiency for a coherent array. To determine the upper bound for binary-phase gratings, restrictions on the array phase are imposed. For fanouts that are >5, the upper bound on the diffraction efficiency for continuous phase fanouts ranges between 97 and 98%; for (0, pi)-binary-phase fanouts the upper bound ranges between 83 and 84%; and for (0, non-pi)-binary-phase, between 87 and 88%.

We introduce the linear system transfer matrix H as a means to interpret classical optical sensing, such as imaging and spectroscopy, and new computational sensing. This representation allows us to identify PSF engineering and multiplexed measurements as duals of each other. We also consider several new computational sensing systems and identify the corresponding system matrix.