Diffractive Doublet Corrected On-axis at Two Wavelengths

Image degradation characteristics and restoration based on regularization for diffractive imaging

Metalens, the specific type of lens designed with the surfaces mading of two dimensional array at the subwavelength scale, has shown great flexibilities to control the light field, including the arbitrary modulation abilities of amplitude, phase and polarization at the subwavelength scale. Moreover, the metalens possesses the unique advantages of low loss, integratable and conformable design and ultrathin, therefore attracts immense attentions in recent years. However, in most cases, the metalens designed for a specific wavelength may penetrate through the large chromatic aberration, which limits their usefulness in multi-wavelength or broadband applications. On the other hand, the metalens has renewed new degrees of freedom due to its two-dimensional planar structure, which has the potential in the elimination of chromatic aberration. Some different typical achromatic metalens designs and their achromatic modulation mechanism were reviewed, the existing achromatic metalens were classified from the types of modulated light bands, such as the achromatic matelens for discrete and continuous wavelength respectively, and the latter can be classified as transmissive and reflective from the working mode. Finally, the application of metalenses array in imaging and their prospect of broadband achromatic devices of large depth of field were introduced.

In order to verify the technology of the membrane diffractive imaging system for Chinese next generation geo-stationary earth orbit (GEO) satellite, a series of ground experiments have been carried out using a membrane optical camera with 80 mm aperture (Φ80) lens. The inherent chromatic aberration due to diffractive imaging appears in the obtained data. To address the issue, an effective color restoration algorithm framework by matching, tailoring, and non-linearly stretching the image histograms is proposed in this letter. Experimental results show the proposed approach has good performances in color restoration of the diffractive optical images than previous methods. The effectiveness and robustness of the algorithm are also quantitatively assessed using various color deviation indexes. The results indicate that the chromatic aberration of diffractive images can be effectively removed by about 85%. Also, the proposed method presents reasonable computational efficiency.

X-ray transmission imaging has been used in a variety of applications for high-resolution measurements based on shape and density. Similarly, X-ray diffraction (XRD) imaging has been used widely for molecular structure-based identification of materials. Combining these X-ray methods has the potential to provide high-resolution material identification, exceeding the capabilities of either modality alone. However, XRD imaging methods have been limited in application by their long measurement times and poor spatial resolution, which has generally precluded combined, rapid measurements of X-ray transmission and diffraction. In this work, we present a novel X-ray fan beam coded aperture transmission and diffraction imaging system, developed using commercially available components, for rapid and accurate non-destructive imaging of industrial and biomedical specimens. The imaging system uses a 160 kV Bremsstrahlung X-ray source while achieving a spatial resolution of ≈ 1 × 1 mm2 and a spectral accuracy of > 95% with only 15 s exposures per 150 mm fan beam slice. Applications of this technology are reported in geological imaging, pharmaceutical inspection, and medical diagnosis. The performance of the imaging system indicates improved material differentiation relative to transmission imaging alone at scan times suitable for a variety of industrial and biomedical applications.

This paper introduces a novel lensless full colour diffractive computational imaging system with a planar Multilevel Phase Mask (MPM) as a diffractive optical element (DOE). The novelty concerns: a methodology of MPM design for improved depth of focus (DoF); design of PSFs for RGB imaging and an inverse imaging algorithm with sparse colour image modelling simultaneous for all RGB channels. MPMs are step-wise invariant. The cubic wavefront coding (WFC) is incorporated in MPMs with optimization of number of levels and width of invariant steps. This design of MPM makes the system robust with respect to defocus (improves DoF) and diminish chromatic aberrations typical for DOEs. Broadband multichannel test-images are exploited for design and testing of the lensless system. We consider two alternative optical setups: Wavelength Multiplexing (WM) and Wavelength Division (WD). In WM, the light beam is broadband multichannel with light sources radiating all wavelengths simultaneously and a CMOS sensor is equipped with a Bayer colour filter array (CFA) for registration of spectral measurements. In this setup, a single MPM is designed for the broadband multichannel light beams. In WD, separate exposures of RGB channels are registered by a broadband grey-scale CCD sensor. Different MPMs are designed for each of the RGB channels. Simulation experiments demonstrate the essentially extended DoF of the designed lensless systems and the advanced accuracy and quality of imaging with respect to the corresponding WM and WD systems with refractive lenses. Due to robustness of the designed lensless system to chromatic aberrations, this advantage has a place even with respect to the lens-system.

Chromatic and spherical aberration correction with hexapole and quadrupole fields.

Diffractive membrane imaging systems have been an important development trend for high-orbit satellite cameras owing to their advantages of large aperture, light weight, rapid manufacture, and low cost. However, caused by the cross-coupling effects of diffraction imaging, membrane properties, subaperture stitching, on-orbit disturbances, and other physical factors, lager-aperture space diffractive membrane imaging systems have specific and complex degradation characteristics: the modulation transfer function (MTF) and signal-to-noise ratio (SNR) have more prominent degradation and serious space-variant characteristics over fields of view, with obvious background radiation properties that seriously affect the application of imaging products. To address this problem, this study established a global information transmission model by characterizing the PSF and background radiation in a full field of view to represent the imaging law of an on-orbit system. Aiming at the inverse problem of the information transmission model, we also propose a novel image inversion restoration method for the special degradation characteristics. In particular, the effect of diffraction efficiency is introduced into the inversion restoration method to solve the background radiation problem. Moreover, we innovatively designed matrix regularization parameters to further improve the correction ability of spatial variation. When the diffraction efficiency was experimentally higher than 60% and the mean measured spatial variability was less than 0.2, the proposed method exhibited a satisfactory processing performance, and could improve multiobjective comprehensive processing, such as transfer function compensation, spatial variation correction, and background radiation removal.

Programmable diffractive optics utilizing a high-resolution liquid-crystal phase modulator is demonstrated as a versatile technique for wide-dynamic-range, two-dimensional wavefront control. A high-resolution phase modulator system introduces modulo-2π phase modulation and operates with 307,200 independently addressable elements, 100% fill factor and total optical efficiencies of up to 93%. Demonstrations include beam steering, large aberration compensation, and near-diffraction-limited imaging and beam directing in a telescope system with large defocus and off-axis aberrations.

A new carotenoid has been isolated from the chicken retina for which the name galloxanthin is proposed. This substance has the properties of a hydroxy carotenoid or xanthophyll. It has not yet been crystallized. On a chromatogram of calcium carbonate it is adsorbed just below astaxanthin and above lutein. The absorption spectrum of galloxanthin lies in a region where natural carotenoids have not ordinarily been found. Its main, central absorption band falls at about 400 mµ. The position of its spectrum suggests a conjugated system of eight double bonds. This relatively short polyene structure must be reconciled with very strong adsorption affinities. With antimony trichloride, galloxanthin yields a deep blue product, possessing a main absorption band at 785 to 795 mµ, and a secondary maximum at about 710 mµ which may not be due to galloxanthin itself. Galloxanthin appears to be one of the carotenoid filter pigments associated with cone vision in the chicken. It may act as an auxiliary to the other filter pigments in differentiating colors; or its primary function may be to exclude violet and near ultraviolet radiations for which the eye has a large chromatic aberration.

Astronomical imaging with micro-arcsecond (µas) angular resolution could enable breakthrough scientific discoveries. Previously proposed µas x-ray imager designs have been interferometers with limited effective collecting area. Here we describe x-ray telescopes achieving diffraction-limited performance over a wide energy band with large effective area, employing a nested-shell architecture with grazing-incidence mirrors, while matching the optical path lengths between all shells. We present two compact nested-shell Wolter Type 2 grazing-incidence telescope designs for diffraction-limited x-ray imaging: a micro-arcsecond telescope design with 14 µas angular resolution and 2.9m2 of effective area at 5 keV photon energy (λ=0.25nm), and a smaller milli-arcsecond telescope design with 525 µas resolution and 645cm2 effective area at 1 keV (λ=1.24nm). We describe how to match the optical path lengths between all shells in a compact mirror assembly and investigate chromatic and off-axis aberrations. Chromatic aberration results from total external reflection off of mirror surfaces, and we greatly mitigate its effects by slightly adjusting the path lengths in each mirror shell. The mirror surface height error and alignment requirements for diffraction-limited performance are challenging but arguably achievable in the coming decades. Because the focal ratio for a diffraction-limited x-ray telescope is extremely large (f/D∼105), the only important off-axis aberration is curvature of field, so a 1 arc sec field of view is feasible with a flat detector. The detector must fly in formation with the mirror assembly, but relative positioning tolerances are on the order of 1 m over a distance of some tens to hundreds of kilometers. Although there are many challenges to achieving diffraction-limited x-ray imaging, we did not find any fundamental barriers.

Imaging spectrometers are essential instruments for hyperspectral remote sensing systems that capture pupil-plane spectral radiance from a scene and ultimately form it into a hyperspectral data cube for processing, analysis, and information extraction. An imaging spectrometer is an EO/IR system, thus it contains all of the basic imaging system components outlined in the previous chapter, exhibits similar design challenges, and is characterized by some of the same performance metrics. In addition to imaging, however, an imaging spectrometer must also capture radiation spectra for each image pixel with adequate fidelity; this places additional demands on system design and adds another dimension to performance characterization. The primary design challenge is that a hyperspectral data cube is essentially a 3D construct, with two spatial dimensions and one spectral dimension, while FPAs are 2D. Therefore, some means are necessary to capture this additional dimension. Often this is done using the time dimension. Various designs that perform this function in different ways are explored in this chapter and subsequent two chapters. In this chapter, we focus on arguably the most common type, the dispersive imaging spectrometer. Dispersive spectrometers use either a prism or grating to spatially spread a spectrum of incident radiation across a detector array. In a dispersive imaging spectrometer, depicted conceptually in Fig. 7.1, the spectrum is dispersed in one direction, while the image is relayed in an orthogonal direction. In the direction of dispersion, it is necessary to limit the field of view to one spatial element; this is performed using a slit. If the field of view is not thusly limited, irradiance distribution on the detector array will resemble an image with large lateral chromatic aberration. In that way, a back-end spectrometer only captures a single line of the 2D field produced by front-end optics. A scanning mechanism, such as the pushbroom or whiskbroom method discussed in Chapter 6, is required to image from frame to frame in a direction orthogonal to the slit. As this occurs, a hyperspectral data product is formed as a sequence of spatial-spectral frames, as depicted in Fig. 7.2. The composite set forms a hyperspectral data cube.

Conventional multichannel imaging systems comprise of many optical channels having similar imaging properties, namely field-of-view (FOV) and angular resolution/magnification. We demonstrated that the different optical channels can be designed such that each optical channel captures a different FOV and angular resolution compared to its neighboring channels. We designed and experimentally demonstrated a three-channel multiresolution imaging system where the first optical channel has the narrowest FOV (7°) and highest angular resolution (0.0096°) and the third optical channel has the widest FOV (80°) and lowest angular resolution (0.078°)1. The second optical channel has intermediate properties. The performance of the demonstrated three-channel imaging system however was affected by chromatic aberrations as it was designed for a single wavelength of 587.6 nm. The first optical channel was largely influenced by longitudinal chromatic aberration while the third channel by lateral chromatic aberration. Therefore, we have replaced the aspherical refractive lenses by hybrid lenses, which contain diffractive structures on top of their refractive surfaces, in the three-channel multiresolution imaging system to reduce its chromatic aberrations. The performance of the three channels with hybrid lenses is compared with those of the corresponding channels without hybrid lenses. The longitudinal color aberration of the first optical channel has been reduced from 1.7 mm to 0.2 mm; whereas the lateral color aberration of the third optical channel has been reduced from 250 μm to 14 μm. In conclusion, the hybrid lenses have reduced the chromatic aberrations of the three channels and extended the operating spectral range of the imaging system in the visible wavelength range.

A thorough and complete assessment of microstructure of materials requires a wide variety of analytical techniques, which should be sensitive to at all length scales - from mms to atomic scale. While there have been rapid advances in imaging and spectroscopy techniques for microstructural analysis - at all length scales, techniques for crystallographic analysis of microstructure have primarily relied on bulk x-ray/neutron diffraction and TEM. X-ray and neutron diffraction techniques, though very powerful, are primarily bulk techniques and extraction of local crystallography is formidable if not impossible. On the other hand, TEM diffraction techniques provide precise crystallographic information, but at a much smaller length-scale and suffer from poor statistics and tedious specimen preparation procedures. With the advent of commercially available electron backscattered diffraction (EBSD) and orientational imaging (OIM) systems for SEM, and sophisticated pattern recognition procedures, it is now possible to bridge the length-scale gap between bulk and TEM diffraction techniques.

The next major step in transmission electron microscopy (TEM) towards improved resolution and usability is the correction of the axial chromatic aberration CC. Our quadrupole-octupole-type CC/CS-corrector consists of ten quadrupole stages. Two of them produce crossed electric/magnetic quadrupole fields for CC-correction and octupole fields for CS-correction. The design allows to compensate for all axial aberration coefficients up to fourth order. The fifth-order axial aberrations are sufficiently small to allow for a spatial resolution of 50pm at 200 and 300kV [1,2]. Moreover, all off-axial aberrations up to third order which increase linearly with the field of view are adjustable. Particularly, the azimuthal off-axial coma of the magnetic objective lens can be compensated. The corrector was built for the TEAM project [3] and is incorporated in a FEI TITAN 80–300 electron microscope below the objective.

Color correction with 3D lookup tables in diffractive optical imaging systems