Staring X-ray backscatter imaging based on ultra-high aspect ratio lobster eye lens: erratum.

An erratum to correct a mistake for the article titled "Staring X-ray backscatter imaging based on ultra-high aspect ratio lobster eye lens" published in Opt. Express32(7), 11600 (2024)10.1364/OE.514941. The corrections have no influence on the results and conclusions of the original paper.

Staring X-ray backscatter imaging based on ultra-high aspect ratio lobster eye lens.

In contrast to conventional X-ray imaging systems, the lobster eye lens, serving as a pivotal component for X-ray focusing, presents the potential for downsizing X-ray backscatter imaging systems. This study reports the successful implementation of a pioneering non-contact staring X-ray backscatter imaging experiment, with the target positioned 1.5 meters away from the system and employing a tube voltage of 60 kV for the X-ray light source. The system is built upon a novel high aspect ratio (500) meridian lobster eye lens, employing a laboratory low illuminance desktop light source and a commercial X-ray detector to achieve high-resolution focused imaging of hard X-rays. Point spread function testing and a series of imaging experiments were carried out to assess the resolution and optimal imaging photon energy of the proposed system. Furthermore, according to the characteristics of the point spread function of the cross image of the lobster eye lens, we proposed an image processing algorithm. The experimental results demonstrate that, after processing, the Structural Similarity (SSIM) Index of the backscatter image and the ground truth image can be improved from an average of 0.0526 to 0.5758. Our research significantly contributes to the advancement of a new generation of X-ray backscatter imaging systems.

Large dynamic range stellar radiation simulation optical system.

For the current stellar spectral simulation can not realize the stellar color temperature information with large dynamic range simulation, this paper proposes a broad spectrum high-resolution subdivision and spatial beam zoning modulation combined with a large dynamic range of stellar radiation information simulation method, designed a kind of imaging and non-imaging stellar radiation information simulation optical system, using an optical system to achieve multi-color temperature spectrum and large dynamic range stellar simulation. The experimental results show that the designed system can simultaneously achieve the spectral simulation accuracy (single point evaluation) better than ± 7% in the range of spectral 450-1000 nm and color temperature 3000-11,000 K; on the premise of ensuring the spectral simulation accuracy, the magnitude simulation range reaches 0 to + 12 Mv, and the magnitude simulation accuracy is better than ± 0.05 Mv; Accurate simulation of stellar spectral information and energy large dynamic range tuning is realized, and the system is extended. The system function has been extended to realize the switching of broadband and narrowband modes, The half-peak width of the narrowband output beam is better than 4.1 nm, which extends the application of the spectral simulation technology and provides the theoretical and technical basis for the ground calibration of the development of the high-precision stellar radiation information ground simulation system.

Super-resolution lensless imaging system based on a fast anti-diffraction algorithm.

Conventional lens imaging systems modulate incident rays with a set of lenses and focus these rays on their imaging planes. A lensless imaging system uses a single mask instead of lenses to project incident rays onto the imaging plane. These rays pass through or are blocked off according to the binary mask pattern. These systems are thin, lightweight, and inexpensive. However, they do not converge the rays, causing the local images corresponding to individual light transmission units to heavily overlap in a global scene, requiring a specific algorithm for decoding. Additionally, diffraction is unavoidable when the holes on the mask are extremely small, which can degrade the imaging quality. To address these difficulties, we propose a decoding algorithm called Fourier-ADMM algorithm to unwrap the overlapped images rapidly. In addition to providing high decoding speed, the proposed technique can suppress the diffraction from the tiny holes, owing to its conjugated structure. Based on this novel decoding algorithm, a lensless imaging system is proposed, which can handle overlapped and diffracted images with a single random mask. The camera can work beyond the theoretical diffraction limit and tremendously enhance the resolution. In summary, the super-resolution lensless camera provides users with additional options to suit different situations. It can facilitate robust, high-resolution, fast decoding without sophisticated calibration.

Analysis and correction of misalignment-induced aberrations in two-mirror telescopes with stop on the secondary mirror.

We study the problem of misalignment aberration analysis and correction of the two-mirror telescopes with stop on the secondary mirror. The variation law of the system's aberration field is analyzed with nodal aberration theory when the primary mirror with an astigmatic figure error is misaligned. The analytic expression among the system wave aberration, misalignments, and astigmatism figure error is given, and the correction model of system misalignment aberration is established. The simulation experiment shows that the relative error of the prediction of system misalignment coma and astigmatism based on this model is less than 4.1%.

A Kernel-Based Multi-Featured Rock Modeling and Detection Framework for a Mars Rover.

This article presents two kernel-based rock detection methods for a Mars rover. Rock detection on planetary surfaces is particularly pivotal for planetary vehicles regarding navigation and obstacle avoidance. However, the diverse morphologies of Martian rocks, the sparsity of pixel-wise features, and engineering constraints are great challenges to current pixel-wise object detection methods, resulting in inaccurate and delayed object location and recognition. We therefore propose a region-wise rock detection framework and design two detection algorithms, kernel principle component analysis (KPCA)-based rock detection (KPRD) and kernel low-rank representation (KLRR)-based rock detection (KLRD), using hypotheses of feature and sub-spatial separability. KPRD is based on KPCA and is expert in real-time detection yet with less accurate performance. KLRD is based on KPRD with KLRR which can generate more precise rock detection results with less delay. To validate the efficiency of the proposed methods, we build a small-scale Martian rock dataset, MarsData, containing various rocks. Preliminary experimental results show that our methods are efficient in dealing with complex images containing rocks, shadows, and gravel. The code and data are available at: https://github.com/CVIR-Lab/MarsData.

Design and collaborative optimization method of multilayer coatings to correct polarization aberration of optical systems.

Multilayer coatings induce a significant polarization aberration in optical systems with high numerical aperture (NA) and wide field of view, which will cause wavefront distortion and imaging degradation. Studies have used low-polarization coatings (LPC) design to reduce the coating-induced polarization aberration. However, the polarization aberration caused by LPC remain evident in systems with large incident angles and many coated surfaces. In this paper, a hybrid optimization algorithm (HOA) is proposed to enhance the design accuracy of LPC. Based on the HOA, a collaborative optimization method is developed to simultaneously design coatings with different polarization properties for multiple surfaces, which can correct polarization aberration by mutual compensation between the coated surfaces and other optical elements in a single system. Finally, a high NA lithographic lens is simulated as an example to verify the collaborative optimization method. The simulation demonstrates that this method is superior to conventional methods. This research provides a new way to correct the polarization aberration and is applicable to any systems coated with multilayer coatings.

Optical design method of a three-mirror anastigmatic telescope with low misalignment sensitivity based on the nodal aberration theory.

We propose a design method for a three-mirror anastigmatic telescope with low misalignment sensitivity and deduce the analytic expression between the misalignment aberration and its optical parameters based on the nodal aberration theory. We establish an optical system as-built performance evaluation model. Using this model as the system's as-built performance evaluation indicator, we can get an optical system that could have both low misalignment sensitivity and good image quality after optimization. The design results of a field bias three-mirror anastigmatic telescope show that the misalignment aberration of the system can be reduced by changing the spacing of the mirrors. When the spacing between the primary mirror and the secondary mirror increases and the spacing from the secondary mirror to the third mirror and the third mirror to the image plane decreases, the misalignment sensitivity will drop significantly. If the mirror spacing is changed by 10%, the misalignment sensitivity of the telescope optimized by our method is only about 85% of that of the traditional method.

Economic tolerance analysis of optical systems based on distortion.

In optical design, the purpose of tolerance allocation is not only to satisfy all image quality requirements but also to minimize the machining cost. Therefore, to achieve these two goals easily and quickly, in this paper, we propose a calculation method that can effectively reduce the data storage amount and relatively accurately fit the function relationship between tolerance and image quality. At the same time, distortion is used as the performance criterion. The differential derivation formula of the distortion with respect to the two structural parameters of decenter and tilt is derived to improve and supplement the types of structural parameters. An example is used to demonstrate the feasibility and flexibility of the approach, which can achieve a balance between image quality and machining cost.

Optical design of an Offner coded aperture snapshot spectral imaging system based on dual-DMDs in the mid-wave infrared band.

With the advantages of flexible encoding and high frame rate, the digital micromirror devices (DMD) have been used as a binary encoding mask in the coded aperture snapshot spectral imaging (CASSI) systems. But the use of DMD will cause the image plane to tilt at a specific angle, so it is almost impossible to realize the strict matching between the optical system of CASSI and the cold stop of the infrared focal plane array in the mid-wave infrared band. In this paper, a CASSI system with two DMDs based on the Offner spectrometer is proposed to solve the above problem. The concept and working principle are described in detail. Under the premise of the matching optical parameters, the telescopic system, Offner spectroscopic system and microscopic system are designed independently. Then the integrated optimization design method is adopted, and the aberration of the microscopic system is used to offset the astigmatic aberration of the Offner spectroscopic system, and the imaging quality of the system is improved. The results of performance measurements confirm that the system has desirable spatial resolution and spectral response functions. Thus, the concept and optical design of the proposed system are verified to be effective and valuable.

Design and optimization method of a convex blazed grating in the Offner imaging spectrometer.

The convex reflective diffraction grating is an essential optical component in Offner systems, which has been widely used in imaging spectrometers. We propose a new design and optimization method for the convex blazed grating in the Offner imaging spectrometer. The method integrates the macro- and microdesign of the optical system, and it can be used to design and optimize the convex blazed grating with high diffraction efficiency. Traditional geometric optics theory and image quality evaluation methods are used to design the macro-optical structure parameters of the Offner system. And then the incident ray information, such as the incident angle and the polarization states are calculated by using the three-dimensional polarization ray-tracing method. To improve the diffraction efficiency, we combine rigorous coupled wave analysis and a particle swarm optimization algorithm to optimize the microstructure parameters of the convex-blazed grating. Further, a convex-blazed grating in a mid-wave infrared Offner imaging spectrometer is designed as an example to illustrate our design method in detail. The design results indicate that the Offner imaging spectrometer has good imaging quality, and the average diffraction efficiency of the -1st diffraction order of the convex-blazed grating in the spectral coverage 3-5 µm is 82.24%. Compared to the traditional design method, the lowest spectral diffraction efficiency is improved from 59.88% to 69.24%, the highest spectral diffraction efficiency is improved from 90.45% to 91.84%, and the standard deviation is reduced from 7.82 to 6.62.

Design and analysis of inner focus for a large spectral bandwidth optical system.

In this paper, we present a method of solving the chromatic aberration problem of large spectral bandwidth optical systems encountered during the internal focusing process. Rational selection of the focal length of each lens group and the distance between them retained the achromatic characteristic of the optical system when the inner focus lens group was mobilized. The proposed design was experimentally validated. This paper can be useful to research on internal focusing in wide-band systems.

Modeling and analysis of a monocentric multi-scale optical system.

A monocentric lens combined with a multi-scale form can achieve a large field of view while maintaining the resolution. This report describes an analytical model that is suitable for both Galilean- and Keplerian-type monocentric multi-scale (MMS) systems; this model also analyzes the correlation between the two types of systems. Moreover, the off-axis aberration associated with the analytical model was derived, on this basis, the Galilean- and Keplerian-type MMS systems were compared. It was concluded that the Galilean-type MMS system performs better with respect to aberration performance. This report provides a useful reference for further applications and developments of MC systems.

Comprehensive understanding of the focal property of lobster-eye optics.

Lobster-eye optics is a promising option to establish an all-sky monitor in the X-ray spectrum. With the development of micromachining technology, the performance of lobster-eye optics is gradually improving and has become more practical. In this paper, from an optical design point of view, the mathematical models of the square-channel lobster-eye lens and the meridional lobster-eye lens have been established based on prism analysis, and the focusing property differences of the two lenses are analyzed. There are several key conclusions: the square-channel lobster lens has no paraxial ideal focal point; the meridional lobster eye lens has better energy concentration for focusing and a weaker capacity for energy collection than the square-channel lobster eye lens in the high-energy X-ray spectrum; and the stray light arms of the square lobster-eye lens appear earlier than those of the meridional lobster-eye lens when the photon energy decreases. These conclusions can help improve the design of a lobster-eye lens for space detection and exploration.

Structural design method of the meridional lobster-eye lens with optimal efficiency.

Lobster-eye optics has been proposed as a high-energy detection device with great potential for astronomical observation and safety inspection due to its large-field-of-view (FOV) focusing ability. In this paper, we study the relationship between the optimal structural parameters of the meridional lobster-eye lens and focusing efficiency. The maximum odd-reflection component, which relates only to the FOV and the channel ratio of depth to width, has been found. Furthermore, the structural constant C, which determines the optimal efficiency structure of the meridional lobster-eye lens, is revealed for hard x-rays. Meanwhile, by introducing accurate reflectivity of iridium in soft x-rays, the constraint of the effective FOV with respect to x-ray wavelength is analyzed. The research results can help to design the optimal structure of the lobster-eye lens in multiple spectra.

Fabrication of hierarchical moth-eye structures with durable superhydrophobic property for ultra-broadband visual and mid-infrared applications.

Multifunctional antireflective coatings have practical applications as important optical components in many fields, particularly for optical devices and imaging systems. However, a good antireflection application in the visible region is often unsatisfactory for mid-infrared devices, and the difficulty in obtaining multiple capabilities simultaneously is one of the main factors limiting their applications. In this work, hierarchical moth-eye structures with superhydrophobicity were fabricated via inductively coupled plasma reactive ion etching (ICP-RIE) using nanodisk-array masks, which were formed by three-beam laser interference lithography (LIL), for improving the ultra-broadband optical properties. The uniform antireflection efficiency, which was close to 1% reflectivity covering over the visible and mid-infrared wavelength range, was exhibited by the moth-eye structures with high-quality pillar arrays. Additionally, irregular nanostructures were tailored onto the top of the pillars to generate hierarchical moth-eye structures for simultaneously obtaining both the superhydrophobic and anticorrosive properties. The fabricated antireflective structures, with the features of self-cleaning and durability, have the advantage of being for long-term use in harsh environments.

Generation of arbitrary vector vortex beams based on the dual-modulation method.

A dual-modulation method that combines a liquid crystal on silicon spatial light modulator with an S-waveplate and a biplate consisting of double quarter-wave plates (DQWPs) or double half-wave plates (DHWPs) is proposed. The method is used to realize the phase and polarization dual modulation of an incident laser beam. This study focuses on the generation of an arbitrary vector vortex beam (VVB) based on the proposed dual-modulation method. The phase and polarization transformation effects of the proposed method are theoretically derived using the Stokes-Mueller matrix algorithm. Correspondingly, an experimental configuration is constructed to generate arbitrary VVBs, and correlation analyses are carried out to quantitatively evaluate the quality of the generated VVBs. The results indicate that the correlation coefficients of the generated VVBs can reach more than 0.94 whether the biplate in the experimental configuration is DQWP or DHWP.

Multiring pure-phase binary optical elements to extend depth of focus.

Multiring pure-phase binary optical elements (BOEs) are widely used to extend the depth of focus (DOF) in many optical applications. Although researchers have designed various BOEs to extend the DOF, few theories and experiments have been reported to validate the performances of different N-ring pure-phase BOEs to realize the DOF as long as possible. In this paper, aberration theory is used to obtain the simple and straightforward initial phase, and a novel modified Gerchberg-Saxton algorithm is presented for generating N-ring 0-π-phase BOEs to optimally extend the DOF. Theoretical, numerical, and experimental results demonstrate that the DOF can be extended with increased N in the same NA.

Analysis of the impact of air scattering on point source transmittance.

This paper proposes a method to simulate an average air scattering model by using random particles obeying a certain size spectrum and shape distribution, and it analyzes the influence of air scattering on the point source transmittance (PST) test using the model. The results of the analysis indicated that PST measurement errors caused by air scattering are directly proportional to the cube of the diameter of the optical system and that a one-level change in the air cleanliness may result in a one-order-of-magnitude change in the error. The cleanliness level of the measurement environment is less expensive and easier to obtain from analysis than from empirical values.

A bio-inspired model for bidirectional polarisation detection.

This study investigated a novel polarisation detection model based on the microstructure of rhabdom in mantis shrimp eyes, in which a single unit can detect two directions of orthogonal polarisation. The bionic model incorporated multi-layered orthogonal Si wire grids, and the finite-difference time-domain method was used to simulate light absorption. A single-layer Si wire grid was simulated to study the effects of thickness and duty cycle on extinction ratios. A multi-layer orthogonal wire grid was simulated to study the effects of distance between adjacent layers. The simulations revealed that the bionic model can achieve orthogonal polarisation detection. Additionally, for 600 coupled layers, the extinction ratios in both directions were greater than 60, and light absorption in the absorptive directions exceeded 96%.