Design of binary-phase diffusers for a compressed sensing snapshot spectral imaging system with two cameras.

We propose designs of pupil-domain optical diffusers for a snapshot spectral imaging system using binary-phase encoding. The suggested designs enable the creation of point-spread functions with defined optical response, having profiles that are dependent on incident wavefront wavelength. This efficient combination of dispersive and diffusive optical responses enables us to perform snapshot spectral imaging using compressed sensing algorithms while keeping a high optical throughput alongside a simple fabrication process. Experimental results are reported.

Dual-camera snapshot spectral imaging with a pupil-domain optical diffuser and compressed sensing algorithms.

We propose a snapshot spectral imaging method for the visible spectral range using two digital cameras placed side-by-side: a regular red-green-blue (RGB) camera and a monochromatic camera equipped with a dispersive diffractive diffuser placed at the pupil of the imaging lens. While spectral imaging was shown to be feasible using a single monochromatic camera with a pupil diffuser [Appl. Opt.55, 432 (2016)APOPAI0003-693510.1364/AO.55.000432], adding an RGB camera provides more spatial and spectral information for stable reconstruction of the spectral cube of a scene. Results of optical experiments confirm that the combined data from the two cameras relax the complexity of the underdetermined reconstruction problem and improve the reconstructed image quality obtained using compressed sensing-based algorithms.

Multifunctional binary diffractive optical elements for structured light projectors.

A set of diffractive optical elements for multiple-stripe structured illumination was designed, fabricated and characterized. Each of these elements with a single layer of binary surface relief combines functions of a diffractive lens, Gaussian-to-tophat beam shaper, and Dammann beam splitter. The optical investigations of laser light patterns at 20° fanout angle reveal up to 88% diffraction efficiency, high contrast, and nearly diffraction limited resolution. The developed technology has the potential for reducing complexity, number of optical components, power consumption and costs of structured light projectors in mobile and stationary 3D sensors.

Resonance-domain diffractive microlens arrays.

High-efficiency resonance-domain diffractive microlens arrays with high numerical apertures and 100% fill factor were designed, fabricated, and characterized. Fabricated arrays of eight off-axis microlenses with pitch 127 µm and numerical aperture 0.2 demonstrated diffraction-limited collimation of fiber light at 632.8 nm wavelength. Optical measurements revealed diffraction efficiency exceeding 93%, in match to numerical calculations with rigorous conical diffraction. The resonance-domain diffractive microlens arrays are highly suitable for applications in fiber optics, multispot optical tweezers, optical sensors, and spectrometry.

Combining and collimation of RGB laser beams with transmissive resonance domain diffractive optics.

Resonance domain diffractive optical elements for combining RGB laser beams into a single collimated beam were designed, fabricated, and experimentally investigated. The input RGB beams were angular separated up to tens of degrees and set in a nearly Bragg arrangement for high diffraction efficiency. A single resonance domain diffractive lens delivered beam combining and collimation functions with reasonable residue divergence. The resonance domain diffraction grating delivered diffraction-limited residue divergence in combining the collimated RGB beams. Optical experiments with fiber-coupled RGB lasers and e-beam-fabricated beam combiners proved low residue beam divergence, a high polarization extinction ratio, and total measured diffraction efficiency of about 80%.

Mapping of spectral signatures with snapshot spectral imaging.

We propose a snapshot spectral imaging method that enables direct reconstruction of spatial maps for spectral signatures of given materials using a monochromatic image sensor. An image-plane array of dispersive shapers converts an aerial image of an object into a tailored mixture of spectral and spatial data that is sensed and digitally processed to reconstruct weight coefficients of the spectral signatures. The feasibility of the method is proven by computer simulations.

Tunable resonance-domain diffraction gratings based on electrostrictive polymers.

Critical combination of high diffraction efficiency and large diffraction angles can be delivered by resonance-domain diffractive optics with high aspect ratio and wavelength-scale grating periods. To advance from static to electrically tunable resonance-domain diffraction grating, we resorted to its replication onto 2-5 µm thick P(VDF-TrFE-CFE) electrostrictive ter-polymer membranes. Electromechanical and optical computer simulations provided higher than 90% diffraction efficiency, a large continuous deflection range exceeding 20°, and capabilities for adiabatic spatial modulation of the grating period and slant. A prototype of the tunable resonance-domain diffraction grating was fabricated in a soft-stamp thermal nanoimprinting process, characterized, optically tested, and provided experimental feasibility proof for the tunable sub-micron-period gratings on electrostrictive polymers.

Coherent imaging with a resonance domain diffractive lens in laser light.

We investigated coherent imaging with a binary off-axis resonance domain diffractive lens using three lasers in visible wavelengths. The relations between the dispersion of this lens, shape of its point spread function, and spectral properties of these lasers were analyzed theoretically and experimentally. In particular, we measured the point spread function, imaging contrast, and diffraction efficiency. Experimental results proved the feasibility of imaging with low distortion and more than 83% diffraction efficiency in laser light.

Shift-bonded resonance-domain diffraction gratings.

Resonance-domain-transmission diffractive optics with grating periods comparable to those of the illumination wavelength offers large angles of light deflection and nearly 100% Bragg diffraction efficiency. Optical design preferences for nearly normal incidence can be met by proper choice for the slant of the diffraction grooves relative to the substrate. However, straightforward fabrication of the slanted submicron high-aspect-ratio grooves is challenging. In this paper, optical performance comparable to that of the slanted grooves was achieved by an alternative solution of bonding two half-height symmetrical gratings with a lateral shift and an optional small longitudinal spacing. Results of design, nanofabrication, and optical testing are presented.

Compressed sensing snapshot spectral imaging by a regular digital camera with an added optical diffuser.

We propose a spectral imaging method that allows a regular digital camera to be converted into a snapshot spectral imager by equipping the camera with a dispersive diffuser and with a compressed sensing-based algorithm for digital processing. Results of optical experiments are reported.

Design of dense transmission diffraction gratings for high efficiency.

We propose a design method for dense surface-relief diffraction gratings with high efficiency in transmission mode. Closed-form analytical relations between diffraction efficiency, polarization, and grating parameters are derived and verified in the resonance domain of diffraction under general three-dimensional angles of incidence traditionally termed conical mounting. A powerful tool for rigorous design of computer-generated holograms and diffractive optical elements with spectroscopic scale periods is now enabled.

Chromatic dispersion of a high-efficiency resonance domain diffractive lens.

Inherent strong lateral and longitudinal chromatic dispersion of a transmission resonance domain off-axis diffractive lens were studied theoretically and experimentally. It is shown that a 4 mm diameter and 0.14 NA diffractive lens provides both focusing and dispersion with a spectral resolution of up to 0.09 nm, which is suitable for laser line spectral measurements. Experimental results for measured spectra of a mercury-argon source, a helium-neon laser, and RGB laser diodes pave a technological path to compact spectral sensors and microspectrometers.

Diffractive optical elements for mode-division multiplexing of temporal signals with the aid of Laguerre-Gaussian modes.

Optical aspects of space-division multiplexing with orthogonal modes of coherent light were considered in theory and experiments with the coherent optical correlator. We resorted to the mathematical tool of generating functions and technologies of diffractive optical elements to implement complex spatial filters matched to rotationally symmetrical transverse modes. Successful multiplexing and demultiplexing in free-space transmission of low-frequency temporally modulated signals through different spatial modes was demonstrated. Experimental results show low cross talk between different mode channels and feasibility of further applications in multimode fiber optical communication data links.

Resonance domain surface relief diffractive lens for the visible spectral region.

Early expectations for a role of diffractive lenses were dramatically lessened by their high order overlapping foci, low optical powers, and competing advances in refractive micro-optics. By bringing the Bragg properties of volume holograms to diffractive lenses we got rid of ghost diffractive orders and the critical trade-off between diffraction efficiency, number of phase levels, and spatial feature-size. Binary off-axis resonance domain diffractive lens with high numerical aperture of 0.16 was designed with analytical effective grating theory, fabricated by direct e-beam writing, etched in fused silica and experimentally investigated. More than 81% measured diffraction efficiency exceeds twice the limits of thin binary optics.

Design and experimental investigation of highly efficient resonance-domain diffraction gratings in the visible spectral region.

Surface-relief resonance-domain diffraction gratings with deep and dense grooves provide considerable changes in light propagation direction, wavefront curvature, and nearly 100% Bragg diffraction efficiency usually attributed only to volume optical holograms. In this paper, we present design, computer simulation, fabrication, and experimental results of binary resonance-domain diffraction gratings in the visible spectral region. Performance of imperfectly fabricated diffraction groove profiles was optimized by controlling the DC and the depth of the grooves. Indeed, more than 97% absolute Bragg diffraction efficiency was measured at the 635 nm wavelength with binary gratings having periods of 520 nm and groove depths of about 1000 nm, fabricated by direct electron-beam lithography and reactive ion etching.

Pupil coding masks for imaging polychromatic scenes with high resolution and extended depth of field.

An algorithm for the design of imaging systems with circular symmetry that exhibit high resolution as well as extended depth of field for polychromatic incoherent illumination is presented. The approach provides a significant improvement over a publication [1] where the design was carried for a single wavelength. The approach is based on searching for a binary phase pupil mask that provides imaging with the highest cut-off spatial frequency, while assuring a desired contrast value over a given depth of field. Simulations followed by experimental results are provided.

Novel approach for extending the depth of field of Barcode decoders by using RGB channels of information.

A specially designed phase mask embedded in the lens assembly of an imaging system is shown to provide different response in the three major color bands, R, G and B of a detector array. Each channel provides optimal performance for different depth of field regions, such that the three channels jointly provide an imaging system with wide depth of field. The approach is useful in particular for Barcode imagers.

Diffractive optical elements with porous silicon layers.

We investigate the basic chromatic properties of dispersive surface relief diffractive optical elements with porous silicon (PSi) layers. Rigorous and scalar wavelength-dependent diffraction efficiencies are juxtaposed and compared to reflection coefficients of uniform silicon and PSi layers. The application of the device as an enhanced sensor is discussed. A spectral covariance criterion for efficient evaluation of the spectral changes induced by analyte filling the pores is presented. Experimental results for the device reveal an increased spectral selectivity of the diffractively structured PSi layers compared to uniform PSi layers.

Spectral multiplexing method for digital snapshot spectral imaging.

We propose a spectral imaging method for piecewise "macropixel" objects, which allows a regular digital camera to be converted into a digital snapshot spectral imager by equipping the camera with only a disperser and a demultiplexing algorithm. The method exploits a "multiplexed spectrum" intensity pattern, i.e., the superposition of spectra from adjacent different image points, formed on the image sensor of the digital camera. The spatial image resolution is restricted to a macropixel level in order to acquire both spectral and spatial data (i.e., an entire spectral cube) in a single snapshot. Results of laboratory experiments with a special macropixel object image, composed of small, spatially uniform squares, provide to our knowledge a first verification of the proposed spectral imaging method.

Impaired endothelial alpha-2 adrenergic receptor-mediated vascular relaxation in the fructose-fed rat.

To investigate the vascular endothelial dysfunction in the insulin resistance syndrome, muscarinic and alpha2-adrenergic mediated relaxations were studied in the fructose-fed rat. Male Sprague-Dawley rats were fed either fructose-rich chow (FFR, n=14) or normal chow (CNT, n=13) for 8 weeks. Systolic blood pressure (SBP) was measured by the tail-cuff method. A 3 mm segment of mesenteric artery was cannulated and pressurized, pretreated with prazosin (10(-6) mol/l) and propranolol (3x10(-6) mol/l), then pre-contracted with serotonin (10(-6) mol/l). Endothelium-dependent relaxation was induced by addition of acetylcholine (ACh, 10(-9)-10(-4) mol/l) or a selective alpha2-agonist, B-HT 920 (10(-9)-10(-5) mol/l), with or without the nitric oxide (NO) synthase inhibitor, L-NAME (10(-4) mol/l). SBP was significantly elevated in FFR but not in CNT. Plasma triglyceride in FFT (241+/-115 mg/dl) was significantly (p<0.01) higher than in CNT (84+/-34 mg/dl). Insulin and insulin/glucose ratio were higher but not significantly. Plasma glucose was not different between the two groups. In the dose-response curves to ACh, maximum relaxation and ED50 were similar between FFR and CNT. Moreover, L-NAME shifted the dose-response curves similarly to the right in both groups. Dose-response curves to B-HT 920, however, showed less relaxation in FFR than in CNT (p<0.05). B-HT 920-induced relaxations were mostly abolished by L-NAME. It is concluded that endothelial alpha2-adrenergic relaxation, predominantly mediated by NO, is likely more sensitive to the development of insulin resistance than muscarinic receptor relaxation in this 8-weeks FFR model. This early impairment of endothelial alpha2-adrenergic relaxation may contribute to the development of hypertension and insulin resistance in the FFR.