Achromatic Systems Research Articles

Diffractive Achromat with Freeform Slope for Broadband Imaging over a Long Focal Depth.

We propose a method for designing a long-focal-depth diffractive achromat (LFDA). By applying rotational symmetric parameterization, an LFDA with a diameter of 10.89 mm is designed over three wavelengths at six focal planes. The smoothly changed slope designed by the binary variable slope search (BVSS) algorithm greatly reduces the discontinuity in depth, thus it is a fabrication-friendly process for grayscale laser direct writing lithography, involving less fabrication error and cost. The deviation between the designed and fabricated profiles amounts to 9.68%. The LFDA operates at multiple wavelengths (654 nm, 545 nm, and 467 nm) with a DOF of 500 mm~7.65λ × 105 (λ = 654 nm). The simulated and measured full-width at half-maximum (FWHM) of the focused beam is close to the diffraction limit. Experimental studies suggest that the LFDA possesses a superior capability to form high-quality chromatic images in a wide range of depths of field. The LFDA opens a new avenue to achieve compact achromatic systems for imaging, sensing, and 3D display.

Thermal compensation design of achromatic andapochromatic optical systems using a 3D glass chart.

It is important to determine the ideal combination of housing materials, groups of refractive materials, and their optical powers for athermalizing achromatic and apochromatic optical systems. This study proposes a combined design approach that utilizes three or more glass types to resolve thermal aberrations and defocus achromatic and apochromatic optical systems. It selects a suitable glass type using a 3D glass chart and calculates the optical power analytically. Furthermore, a temperature-insensitive optical system with a 450-750nm band based on refractive materials (CDGM Glass Co., Ltd.) is designed, with the modular transfer function value of the center field of view decreasing by less than 0.024 in the temperature range of -40∘ C to +80∘ C and the secondary spectrum aberration decreasing by over three times and being maintained within 0.08mm.

Hybrid achromatic microlenses with high numerical apertures and focusing efficiencies across the visible

Compact visible wavelength achromats are essential for miniaturized and lightweight optics. However, fabrication of such achromats has proved to be exceptionally challenging. Here, using subsurface 3D printing inside mesoporous hosts we densely integrate aligned refractive and diffractive elements, forming thin high performance hybrid achromatic imaging micro-optics. Focusing efficiencies of 51–70% are achieved for 15μm thick, 90μm diameter, 0.3 numerical aperture microlenses. Chromatic focal length errors of less than 3% allow these microlenses to form high-quality images under broadband illumination (400–700 nm). Numerical apertures upwards of 0.47 are also achieved at the cost of some focusing efficiency, demonstrating the flexibility of this approach. Furthermore, larger area images are reconstructed from an array of hybrid achromatic microlenses, laying the groundwork for achromatic light-field imagers and displays. The presented approach precisely combines optical components within 3D space to achieve thin lens systems with high focusing efficiencies, high numerical apertures, and low chromatic focusing errors, providing a pathway towards achromatic micro-optical systems.

Information systems with aperture compression

Subject of study. Variants of optical system designs with high aperture ratios at relatively low resolution are proposed. Aim of study. The study attempted to increase the energy of the optical signal received by optical information systems. Method. The proposed technical solutions are based on the design features of light-guide optical augmented reality systems with holographic mirrors utilized in reverse beam paths. Main results. The feasibility of adopting a light guide with a holographic mirror as an aperture compressor in optical systems, e.g., receiving free-space optical communication systems or laser rangefinders, is demonstrated. Variants of designs of optical systems with a light-guide aperture compressor operating with monochromatic radiation at several wavelengths and achromatic systems are presented. The use of a diffraction grating with an asymmetric groove profile instead of a holographic mirror is proposed in the last version of the optical system. Practical significance. The proposed system designs enable a significant increase in the intensity of the optical signal received by these systems by incorporating a non-detunable aperture compressor based on a light guide with a holographic mirror.

A polarization-insensitive infrared broadband achromatic metalens consisting of all-silicon anisotropic microstructures

Infrared lenses have a wide range of applications in thermal imaging and detection. Here, by designing a single layer metasurface consisting of anisotropic microstructures, we experimentally demonstrate a polarization-insensitive long-wavelength infrared achromatic metalens. The focal lengths of the fabricated metalens with a numerical aperture of 0.33 remain unchanged and maintain diffraction-limited performance for the incident wavelength varying from 8.5 to 11.5 μm, demonstrating the elimination of chromatic aberration with 30% operation bandwidth. We envision this metalens may pave the way to the development of ultracompact achromatic thermal imaging systems.

High-resolution achromatic X-ray optical systems for broad-band imaging and for focusing attosecond pulses.

Achromatic focusing systems for hard X-rays are examined which consist of a refractive lens paired with a diffractive lens. Compared with previous analyses, we take into account the behaviour of thick refractive lenses, such as compound refractive lenses and waveguide gradient index refractive lenses, in which both the focal length and the position of the principal planes vary with wavelength. Achromatic systems formed by the combination of such a thick refractive lens with a multilayer Laue lens are found that can operate at a focusing resolution of about 3 nm, over a relative bandwidth of about 1%. With the appropriate distance between the refractive and diffractive lenses, apochromatic systems can also be found, which operate over relative bandwidth greater than 10%. These systems can be used to focus short pulses without distorting them in time by more than several attoseconds. Such systems are suitable for high-flux scanning microscopy and for creating high intensities from attosecond X-ray pulses.

Broadband achromatic metasurfaces for sub-diffraction focusing in the visible.

Conventional achromatic optical systems are matured to achieve effective chromatic aberration correction and diffraction-limited resolution by the multiple bulky lenses. The emergence of the super-oscillation phenomenon provides an effective method for non-invasive far-field super-resolution imaging. Nevertheless, most super-oscillatory lenses are significantly restricted by the chromatic aberration due to the reliance on delicate interference; on the other hand, most achromatic lenses cannot break the diffraction limit. In this article, a single-layer broadband achromatic metasurface comprising sub-wavelength anisotropic nanostructures has been proposed to achieve sub-diffraction focusing with a focal length of f=60 µm and a diameter of 20 µm in the visible ranging from 400 nm to 700 nm, which are capable of generating sub-diffraction focal spots under the left-handed circularly polarized incident light with arbitrary wavelength in the working bandwidth at the same focal plane. This method may find promising potentials in various applications such as super-resolution color imaging, light field cameras, and machine vision.

85‐2: Invited Paper: Fourth Gen Optics ‐ Planar Optics Revolutionized by LCD Technology

We will review the state‐of‐the‐art of the Fourth Generation Optics ‐ a planar optics rooted in LCD technology, ‐ and the advance it has enabled in display, AR/VR, imaging, and other critical modern technologies beyond displays. The review will lead to the conclusion that the technology is mature for many applications offering the most versatile and efficient techniques for controlling light with truly flat, thin film, achromatic, and high quality optical systems produced at low manufacturing cost inherent to LCDs.

Measuring transmitted wavefronts for non-circular apertures in broad bandwidths using discrete points

Transmitted wavefronts are usually measured by a laser interferometer of the corresponding design wavelength. The Zernike coefficient can be used to describe the transmitted wavefront of a circular aperture. However, it has some limitations in optical systems with non-circular apertures. In this paper, a method, based on discrete points, for measuring transmitted wavefronts in a broad bandwidth is presented. Simulations of monochromatic, achromatic and apochromatic systems verify the method. The system with off-axis field of view is also analyzed. The Conrady dispersion formula and apochromatic characteristic formula (ACF) are used to represent the relationship between the optical path difference (OPD) at each discrete point of the transmitted wavefront and the wavelength. We also experimentally verify this method by measuring the wavefront of the optical system with a square aperture. The results demonstrate that the method can be used to measure the non-circular wavefront in a broad bandwidth.

Compact lensless full-color holographic projection system with digital phase

A lensless full-color holographic projection system is proposed, satisfying the requirement of compactness and flexibility. The system enables projection by illuminating a single-chip spatial light modulator (SLM) simultaneously with red (R), green (G), and blue (B) lasers, in which the SLM loads a color-multiplexed phase-only hologram. To strengthen compactness, filtering and achromatic systems are achieved by digital phase, where the digital lens phase focuses the light field onto the filter plane, and the digital blazed gratings shift the RGB images to achieve a fine alignment. Besides, the flexibility of diffraction calculation is enhanced by the cascaded D-FFT and S-FFT algorithm (CDS algorithm, where D-FFT is acronym of double fast fourier transform and S-FFT is acronym of single fast fourier transform). Both simulation and optical experiments are carried out. We conducted 2D image and animation projection and multi-image-plane projection. The results confirm the feasibility of our method.

Optical design method for afocal achromatic systems

We describe the method for optical design of afocal achromatic systems that keep their characteristics stable in a wide thermal range without refocusing. Usually, thermal stabilization of the back focal length of an optical system is considered, but these methods are not directly applicable for afocal systems. The method proposed is based on the idea of using a proper combination of optical materials and powers of optical elements to keep the whole system as an afocal one in the desired temperature range. As an additional advantage, the method has shown that afocal systems have enough parameters to achieve achromatic correction and may take into account the F-number of optical elements, which is important for the aberration balance. The research presents both theoretical statements and the example of a laser beam expander for dual wavelengths. The method can also be used to design afocal achromatic compensators that help to achieve thermal stability.

Cross-spectral purity of nonstationary light

The concept of cross-spectral purity of light, established by Mandel for stationary optical fields, is extended to nonstationary fields. Separability conditions that ensure the cross-spectral purity of nonstationary light are derived in the space-frequency and space-time domains. It is also shown that the property of cross-spectral purity is not strictly preserved on propagation of fields with any appreciable spectral bandwidth. Further, we introduce a method for generating cross-spectrally pure (stationary and nonstationary) fields from spatially incoherent light sources by means of achromatic Fourier-transform systems.

New ion-optical operating modes of the BigRIPS and ZeroDegree spectrometer for the production and separation of high-quality rare isotopes beams and high-resolution spectrometer experiments

BigRIPS is a powerful two-stage in-flight separator for the research with exotic nuclei studied in frontier experiments since more than a decade. The ion-optical system is very versatile due to the multi-stage structure of BigRIPS combined with the ZeroDegree spectrometer or the Superconducting Ring Cyclotron (SRC). Various optical modes can be flexibly realized according to the purpose of experiments. Two categories of developments are presented here. One is a new operating mode of BigRIPS aiming at higher ion-optical resolving power. BigRIPS itself has a two-stage structure. Spatial isotope separation is made at both the first and second stages. In the standard operating mode of BigRIPS, at the second stage the two spatial separations with energy degraders are subtractive in their resolving powers. Here, we present the additive mode. With the resulting increased spatial separation power, the isotopic background can be substantially reduced. Higher ion-optical resolving powers of the first and second BigRIPS degrader stages are also investigated with the goals to reduce further the background and to yield access to new isotopes of heavier elements. The other development is a dispersion-matched system with BigRIPS for high-resolution spectrometer experiments. The BigRIPS and ZeroDegree spectrometer are presently two independent, coupled achromatic systems. A new dispersion-matched mode of BigRIPS and ZeroDegree will enable novel experiments. For high-resolution spectroscopy experiments with high-intensity light projectiles, SRC and BigRIPS can be operated as a dispersion-matched system. The described different ion-optical developments are a base for a new category of experiments exploring exotic nuclei and mesic atoms. Characteristic future experiments with these new ion-optical developments are exemplified in this report.

Optical design of athermal, multispectral, radial gradient-index lenses

Many infrared systems operate in extreme environments (such as space or military), which require stable optical performance over an extended temperature range. We present a model for the first-order optical design of athermal, radial gradient-index (GRIN) systems, based on a form of the thermo-optic glass coefficient adapted to inhomogeneous material combinations. We find that radial GRIN components can significantly reduce the optical power balance of athermal, achromatic systems, thus reducing aberration contributions from individual lens elements and improving overall performance. This introduces the scope for a class of GRIN multispectral infrared imaging solutions. We apply this enhanced first-order modeling technique to generate starting points for optimization of a short wave to long wave infrared (SWIR/LWIR) multispectral optical design. An example of SWIR/LWIR optical design for a weapon sight application is generated and shown to have significantly reduced mass and improved performance compared with a conventional non-GRIN solution.

Dispersion controlling meta-lens at visible frequency.

Dispersion management is crucial in constructing spectrometers, superprisms, and achromatic lens systems. Unfortunately, the dispersion of natural materials is determined by the molecular energy levels with limited tunability, and thus conventional methods of dispersion controlling are complex and need to trade off other aberration. Metasurface offers an alternative method to overcome those limits via utilizing dedicatedly designed nanostructures that response to special wavelength, which results in well-engineered dispersions. As proof of the concept, we design a series of flat dielectric metasurface lenses, which are able to steer the dispersion arbitrarily for three wavelengths at visible frequency (473, 532, and 632.8 nm). Based on the unique dispersion engineering ability of metasurface, the achromatic meta-lens and the super-dispersion meta-lenses are realized. Furthermore, the light of different wavelengths can be focused on any desired spatial positions.

The coherent radiation fraction of low-emittance synchrotrons.

In this work the coherence properties of the synchrotron radiation beam from an X-ray undulator in a fourth-generation storage ring are analyzed. A slightly focused X-ray beam is simulated using a wavefront propagation through a non-redundant array of slits and the mutual coherence function is directly obtained and compared with the Gaussian-Schell approximation. The numerical wave propagation and the approximate analytical approaches are shown to agree qualitatively, and it is also shown that, when the coherent fraction is selected by a finite aperture before the focusing element, even achromatic focusing systems like total reflection mirrors become slightly chromatic. This effect is also well accounted for in the Gaussian-Schell model. The wavefront propagation simulation through the non-redundant array was repeated with an imperfect mirror demonstrating that, although the wavefront is distorted, its coherent length is practically unchanged.

Optimal achromatic wave retarders using two birefringent wave plates: reply

This reply attempts to cast some more light on the achromatic systems composed by wave plates, in particular to the calculus of the overall retardation and the use of the Jones matrix equivalence theorem. An equivalent expression for the overall retardation of the system in terms of the trace is also given.

Differences in Photoreceptor Processing Speed for Chromatic and Achromatic Vision in the Bumblebee,Bombus terrestris

Fast detection of visual change can be mediated by visual processes that ignore chromatic aspects of the visual signal, relying on inputs from a single photoreceptor class (or pooled input from similar classes). There is an established link between photoreceptor processing speed (in achromatic vision) and visual ecology. Highly maneuverable flies, for example, have the fastest know photoreceptors, relying on metabolically expensive membrane conductances to boost performance. Less active species forgo this investment and their photoreceptors are correspondingly slower. However, within a species, additional classes of photoreceptors are required to extract chromatic information, and the question therefore arises as to whether there might be within-species differences in processing speed between photoreceptors involved in chromatic processing compared with those feeding into fast achromatic visual systems. We used intracellular recording to compare light-adapted impulse responses in three spectral classes of photoreceptor in the bumblebee. Green-sensitive photoreceptors, which are known to provide achromatic contrast for motion detection, generated the fastest impulse responses (half-width, Deltat = 7.9 +/- 1.1 ms). Blue- and UV-sensitive photoreceptors (which are involved in color vision) were significantly slower (9.8 +/- 1.2 and 12.3 +/- 1.8 ms, respectively). The faster responses of green photoreceptors are in keeping with their role in fast achromatic vision. However, blue and UV photoreceptors are still relatively fast in comparison with many other insect species, as well as vertebrate cones, suggesting a significant investment in photoreceptor processing for color vision in bees. We discuss this finding in relation to bees' requirement for accurate learning of flower color, especially in conditions of variable luminance contrast.

A comparison of the BOLD fMRI response to achromatic, L/M opponent and S-cone opponent cardinal stimuli in human visual cortex: II. chromatic vs achromatic stimuli

Aims: To compare the responsiveness of cortical areas to stimulation of the two cone opponent systems and the achromatic system. The appropriate contrast metric for the comparison of color and luminance sensitivity is unknown and so we made the comparison using both a perceptual match between stimuli (in multiples of detection threshold), a direct contrast match (in cone contrast).Methods: MR images were acquired on a 4T Bruker MedSpec. Stimuli were sinewave counter-phasing rings (2Hz, 0.5cpd) and cardinal for the selective activation of the L/M cone-opponent (RG), S cone-opponent (BY) and achromatic (Ach) systems. In different scans, stimuli were matched either at 25x detection threshold (determined psychophysically), or in cone contrast (6%). Both stereotaxic and a region of interest analysis on early visual areas (V1-V4v, and hMT+) were performed, with the latter identified in separate scanning sessions.Results: In extra-striate visual cortex two different response profiles were observed, differential for chromatic and achromatic stimuli. These profiles were independent of the metric by which the stimuli were equated. Two loci in dorsal and lateral occipital cortex, corresponding to areas V3A and hMT+, had a markedly stronger response to achromatic than to chromatic stimuli. This is in contrast to a region in the ventral occipital cortex (beyond V4v) that exhibited stronger fMRI signals to chromatic compared to achromatic stimuli.Conclusions: In areas V3A and hMT+, a selective response to achromatic stimuli was observed, suggesting a predominantly magnocellular input in these motion-sensitive areas. On the other hand, a region in ventral occipital cortex was activated more by chromatic than achromatic stimuli, which may reflect a predominant parvocellular input. These two types of response profiles are independent of the metric used to equate the stimuli. Data on V1-V4v do not follow these patterns and are discussed in a related abstract (Mullen et al.).

A comparison of the BOLD fMRI response to achromatic, L/M opponent and S-cone opponent cardinal stimuli in human visual cortex: I. perceptually matched vs contrast matched stimuli

Aims: Our primary aim was to compare the responsiveness of the different visual cortical areas to stimulation of the two cone opponent systems and the achromatic system. The appropriate contrast metric for the comparison of color and luminance sensitivity is unknown, however, and so our secondary aim was to investigate whether equivalent fMRI responses of each cortical area could better be predicted from perceptual matches (in multiples of detection threshold) or a direct contrast match (in cone contrast). Methods:MR images were acquired on a 4T Bruker MedSpec. Stimuli were sinewave counter-phasing rings (2Hz, 0.5cpd) cardinal for the selective activation of the L/M cone opponent (RG), S cone opponent (BY) and achromatic (Ach) systems. In different scans, stimuli were matched either at 25x detection threshold or in cone contrast (6%). We performed both stereotaxic and region of interest analyses on early visual areas (V1-V4v, and hMT+). Results: For stimuli presented in multiples of detection threshold, BOLD responses in areas V1, V2, V3/VP and V4v are greatest for BY stimuli and weaker for both RG and achromatic stimuli, with the BY response significantly higher than the RG. For the same stimuli matched in cone contrast, the RG and BY response is similar in these areas. The averaged color response (RG & BY) was no different from the achromatic response with the exception of V1, which had a greater averaged color than achromatic response. Conclusions:Perceptual visibility is not a good indicator of cortical activity across the RG, BY and achromatic mechanisms. fMRI BOLD signals are better predicted by cone contrast. Regardless of the metric, our data suggest a similar response to color (averaged RG & BY) and achromatic stimuli in V2, V3, VP and V4v. V1 shows a greater response to color, particularly with the cone contrast metric. Areas hMT+ and V3A do not follow these patterns and are discussed in a related abstract (Dumoulin et al).