Optics research at the U.S. Army Research Laboratory.

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.

U.S. Army Research Laboratory focus issue: introduction.

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.

Diffractive lens fabricated with binary features less than 60 nm.

We designed, fabricated, and characterized a binary diffractive lens with features less than 60 nm. The lens was designed for operation in the red portion of the spectrum. Experimental measurements of lens performance agree with predictions generated by rigorous models of diffraction.

Three-dimensional analysis of subwavelength diffractive optical elements with the finite-difference time-domain method.

We present a three-dimensional (3D) analysis of subwavelength diffractive optical elements (DOE's), using the finite-difference time-domain (FDTD) method. To this end we develop and apply efficient 3D FDTD methods that exploit DOE properties, such as symmetry. An axisymmetric method is validated experimentally and is used to validate the more general 3D method. Analyses of subwavelength gratings and lenses, both with and without rotational symmetry, are presented in addition to a 2 x 2 subwavelength focusing array generator.

Diffractive optics and micro-optics: introduction to the feature issue.

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.

Design and analysis of a diffractive optical filter for use in an optoelectronic error-diffusion neural network.

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.

Binary subwavelength diffractive-lens design.

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.

Incoherent optical image processing with acousto-optic pupil-plane filtering.

We describe an incoherent image processor that uses orthogonally oriented one-dimensional acoustooptic 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.

Kinoform-based Nipkow disk for a confocal microscope.

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.

Design and rigorous analysis of high-efficiency array generators.

One-dimensional array generators with seven to fifteen spots are designed by the method of virtual sources in combination with simulated-annealing optimization. Continuous surface-relief grating array generators and their discretized versions are analyzed with rigorous diffraction theory, and the results are compared with those obtained from Fourier-optics theory. The array-generator designs have high diffraction efficiency and good uniformity.

Performance analysis of optical shadow-casting correlators.

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 shadowcasting 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.

Incoherent pattern recognition with phase-only filters.

Phase-only and binary-phase spatial light modulators can be employed in the filter plane of incoherent pattern recognitionsystems. We employ computer simulations to show that advanced filter design techniques can be used to achieve a close approximation to classical matched filtering.

Upper bound on the diffraction efficiency of phase-only fanout elements.

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%.

Acousto-optic processing with electronic image feedback for morphological filtering.

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.

Crossed Bragg cell implementation of a Fourier-plane filter for optical image correlators.

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

Acousto-optic generation of two-dimensional spot arrays.

We demonstrate, through the use of an acousto-optic system, the generation of two-dimensional uniform spot arrays with high diffraction efficiency. The system consists of two acousto-optic cells oriented orthogonally to each other and driven by an arbitrary waveform generator. Determination of the acousto-optic drive signal is discussed, as well as techniques for generating arbitrary spot arrays.

Design of Dammann gratings for two-dimensional, nonseparable, noncentrosymmetric responses.

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.

Pupil function design algorithm for bipolar incoherent spatial filtering.

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.

Computer-generated holograms by means of a magnetooptic spatial light modulator.

A magnetooptic spatial light modulator is used to reconstruct computer-generated Fourier holograms. Different methods for designing the holgrams are considered including binary and complex quantization in conjunction with an iterative algorithm, carrier techniques, phase manipulations, and cell oriented binary coding. The limitations of binary quantization are discussed, and the trade-offs between space-bandwidth and quantization error are considered. Using a device having an array of 48 x 48 elements the best compromise is achieved using carrier techniques in conjunction with phase manipulations and binary quantization.

Modulation transfer function technique for real time radioscopic system characterization.

At the University of Virginia neutron radiography facility, a modulation transfer function technique has been developed that can easily predict and compare the resolving characteristics of the real time system and the individual system components. We desired a simple method by which new system components could be analyzed to determine their image transfer characteristics and to estimate how they would affect the composite system during data acquisition. The method employed measures a small set of constant system parameters related to data collected across a cadmium cut-edge aperture. The effects of system noise and spatial variance on the measured data are reduced so that a representation of the true signal can be obtained for analysis. Resolution parameters for the total neutron radiography system and for the individual system components are reported.