Aperture Optical System Research Articles

Orthonormal aberration decomposition for circular aperture and rectangular field with extension to multiple-aperture systems

A new set of orthogonal polynomials and the traditional Zernike polynomials are combined to construct the wavefront of the sparse aperture (SA) optical system. The new set of orthogonal polynomials in the rectangular domain is derived based on the Zernike polynomials. The modulation transfer functions (MTFs) of the SA system under different fields of view are calculated. The effects of some wavefront terms on the field-related MTF are analyzed. Imaging simulation and restoration are conducted for the SA system. The results indicate that the wavefront of the SA system can be represented by the combination of the new set of orthogonal polynomials and the Zernike polynomials. The former represents the field term, and the latter represents the pupil term of the wavefront. The field-related Wiener filter is constructed to restore the imaging results of the SA system, and the results show that the image quality can be improved greatly through restoration.

Simulation method for multi-source information fusion space target

Current space target simulation systems suffer from limitations, such as a single simulated spectral band, inability to allocate spectral ratios between bands, and poor imaging quality of multi-spectral simulated images. We propose a multi-source information fusion spatial target simulation method system that features a “series-parallel” hybrid connection. We optimize the design of a multi-spectral common aperture optical system and experimentally verify the proposed method. The experimental results show that the proposed system can simulate a wide range of spectral bands ranging from ultraviolet to long-wave infrared. Furthermore, it achieves precise control over the ratio of simulated radiant energy among spectral bands, with a spectral simulation error of less than 4.8%. Notably, it effectively images stars against the cosmic background and spacecraft across different spectral bands. This system serves as a theoretical cornerstone and provides crucial technological support for performance testing and rapid iterative development of multi-source information fusion space exploration systems.

Synthetic aperture optical image restoration based on multi-scale feature enhancement

With the wide applications of high-resolution imaging technology in topographic mapping, astronomical observation, and military reconnaissance and other fields, the requirements for imaging resolution of optical system are becoming higher and higher . According to the diffraction limit and Rayleigh criterion, the imaging resolution of the optical system is proportional to the size of the aperture of the system, but affected by the material and the processing of the optical component: the single aperture of the optical system cannot be infinitely enlarged. Therefore the synthetic aperture technology is proposed to replace the single large aperture optical system. Owing to the effect of sub-aperture arrangement and light scattering, the imaging of synthetic aperture optical system will be degraded because of insufficient light area and phase distortion. The traditional imaging restoration algorithm of synthetic aperture optical system is sensitive to noise, overly relies on degraded model, requires a lot of manually designed models, and has poor adaptability. To solve this problem, a multi-scale feature enhancement method of restoring the synthetic aperture optical image is proposed in this work. U-Net is used to obtain multi-scale feature, and self-attention in mixed domain is used to improve the ability of of the network to extract the features in space and channel. Multi-scale feature fusion module and feature enhancement module are constructed to fuse the information between features on different scales. The information interaction mode of the codec layer is optimized, the attention of the whole network to the real structure of the original image is enhanced, and the artifact interference caused by ringing is avoided in the process of restoration. The final experimental results are 1.51%, 4.42% and 5.22% higher than those from the advanced deep learning algorithms in the evaluation indexes of peak signal-to-noise ratio, structural similarity and perceived similarity, respectively. In addition, the method presented in this work has a good restoration effect on the degraded images to different degrees of synthetic aperture, and can effectively restore the degraded images and the images with abnormal light, so as to solve the problem of imaging degradation of synthetic aperture optical system. The feasibility of deep learning method in synthetic aperture optical image restoration is proved.

Polarization Characteristics of Sparse Aperture Optical Imaging Systems

Polarization Characteristics of Sparse Aperture Optical Imaging Systems

Characterization and experimental verification of the rotating synthetic aperture optical imaging system

The rotating synthetic aperture (RSA) optical imaging system employs a rectangular primary mirror for detection. During the imaging process, the primary mirror rotates around the center to achieve the aperture equivalent to the long side of the rectangle at different rotation angles. As a result, the system’s point spread function changes over time, causing periodic time-varying characteristics in the acquired images’ resolution. Moreover, due to the rectangular primary mirror, the images obtained by the RSA system are spatially asymmetric, with a lower resolution in the short side’s direction than in the long side’s direction. Hence, image processing techniques are necessary to enhance the image quality. To provide reference for the study of image quality improvement methods, we first characterize the imaging quality degradation mechanism of the RSA system and the time–space evolution law of the imaging process. We then establish an imaging experiment platform to simulate the dynamic imaging process of the RSA system. We quantify the RSA system’s impact on image degradation using objective indexes. Subsequently, by comparing the imaging experiment results with theoretical analysis, we verify the spatially asymmetric and temporally periodic imaging characteristics of the RSA system. Lastly, we introduce image super-resolution experiments to assess the limitations of directly applying generic deep learning-based single image super-resolution methods to the images captured by the RSA system, thereby revealing the challenges involved in improving image quality for the RSA system.

Inhibiting zero-order light of a spatial light modulator with voltage optimization.

The crucial zero-order light due to the pixelation effect of spatial light modulator (SLM) has been a serious issue in the field of light modulation, especially in applications with a high numerical aperture optical system. In this investigation, we report that by properly adjusting the high-level and low-level pixel voltages of an SLM, the zero-order light caused by the pixelation effect of an SLM can be significantly eliminated. The method is further validated under an inverted fluorescence microscope. The experimental results show that the zero-order light can be inhibited up to 91.3%, accompanied by an improvement of the modulation efficiency from 77.5% to 92.6%.

A wavefront detection method based on the sparse aperture sampling

A wavefront detection method for large aperture optical system based on the sparse aperture sampling is proposed. The conversion matrix of Zernike coefficients between the sub-aperture and full aperture is derived. The full aperture wavefront is calculated by the wavefront of the sub-aperture and the conversion matrix. The condition number of the conversion matrix is applied to evaluate the accuracy of the full aperture wavefront. Numerical simulation is employed to verify the accuracy of wavefront detection. The simulation results indicate that the wavefront RMS of the full aperture decreases with increasing the fill factor, sub-aperture number and sub-aperture baseline length of the sparse aperture. The accuracy of the full aperture wavefront can also be improved by our detection method.

Optical lattices engineered by vector polarization and multisector amplitude modulation

Optical lattices have several applications including super-resolution imaging, lithography, and light-tweezers. Compared with the conventional multibeam interference method, optical lattices in a tightly focused light fields are presented by cylindrical vector polarization illumination and multisector amplitude modulation. The polarization and phase difference of the illumination beam, including the size and relative position of the sectors, were analyzed using the vector diffraction theory. The obtained results indicate that the primitive cell shape of optical lattices can be controlled by the polarization direction of the illumination beam when the relative positions of sectors in the amplitude modulation mask are set. In addition, the period and primitive cell shape of optical lattices with hyper or complex constructions can be controlled by the relative positions of sectors, while the optical lattice zone primarily depends on the sector size. By combining polarization and amplitude modulation in a high numerical aperture optical system, these engineered optical lattices are potentially beneficial in expanding their applications.

Image Restoration Method of Synthetic Aperture Optical System Based on Sparse Prior

Image Restoration Method of Synthetic Aperture Optical System Based on Sparse Prior

Comparison between the scanning pentaprism and the Hartman method for wavefront analysis

Techniques for wavefront measurement have many applications as optics systems for astronomy and as verification tests for optical surfaces. The development of large aperture optical systems drives the search for low cost and high-resolution wavefront detection sensors. The Hartman technique and its variations (i.e the Shack-Hartman sensor) are the current standards for the characterization of low-order optical aberrations, such as defocus and spherical. The scanning pentaprism method is presented as a simple and low-cost method for the verification of such aberrations. In this method, a transverse section of a wavefront is scanned and sampled into a series of sub-wavefronts with smaller apertures. The relative positions of the produced centroids are measured relative to a calibrated position. This allows for the determination of the optical path difference along the section and consequently the wavefront error. Both techniques were used as tools for collimating a telescope that is part of the on-ground support equipment of ESA PLATO mission. In this work, the results from both methodologies are compared.

Adaptive Particle Swarm Optimization for Automatic Design of Common Aperture Optical System

This paper presents an optimal design method to solve the initial optical parameters, which is used for the common-aperture design including transmission and reflection system. On the basis of the particle swarm optimization algorithm, the self-adaptive weight and multi-level screening principle are introduced, which effectively improves the local and global searching ability in the non-linear space. Utilizing the third-order aberration and achromatic theories, the optimal initial structural parameters satisfying the objective function can be quickly calculated. We propose a common-aperture architecture and an illustrative optical design to demonstrate the concept. The system has the ability of field of view segmentation and optical path segmentation, and realizes the three-dimensional compact optical system layout. Simulations of ray tracing acquisition and imaging performances demonstrate the potential of adaptive particle swarm optimization algorithms for flexibility and simplicity in common-aperture imaging design.

Optical imaging for Goaly3 Multi-mirror telescope system

In order to better investigate the imaging characteristics of the sparse aperture optical system, a Goaly3 multi-mirror telescope (MMT) is designed and its prototype is manufactured. The telescope adopts the Cassegrain structure. Its primary mirror is composed of three sub-mirrors with the same diameters of 80 mm. The optical imaging of the telescope is conducted in the experiment. It is shown that the experimental results are consistent with the theoretical analysis. The practical cutoff frequency of the telescope is 70.7 lp/mm with the corresponding diffraction limit being 74 lp/mm. The quality and contrast of the imaging results are improved greatly through image restoration.

Design of a common aperture optical system for multiband spectral reception and visible imaging

Design of a common aperture optical system for multiband spectral reception and visible imaging

On the feasibility of the monostatic scheme for constructing the land-based telescope at supervision of space objects

In modern large aperture optical systems for tracking astronomical objects, the technology of bistatic schemes is widely introduced, in which the laser, the main and auxiliary telescopes are spatially separated. This requires additional measures to be taken when aligning the optical axes of telescopes, especially when tracking LEO space objects. The choice of bistatic schemes in astronomical telescopes is due to the problem of “tilt uncertainty” inherent in monostatic schemes for the formation of a laser reference star. This problem is caused by the difficulty or even the impossibility of determining the tilt of the wavefront when the laser guide star jitters in the image plane. The article discusses a monostatic scheme for constructing ground-based adaptive optoelectronic systems. The monostatic scheme combines the optical axis of the laser, which forms the laser guiding star, and the optical axis of the telescope, which serves to obtain images of space objects by eliminating phase disturbances of the atmosphere due to the radiation of the laser guiding star. The proposed method for determining the tilt of the wavefront in a monostatic scheme is based on the analysis of expressions for the dispersion of the tilt jitter of the images of a laser reference star and a space object for the case when the diameter of the receiving aperture of the telescope is much larger than the diameter of the aperture of the laser forming the laser reference star. This approach is based on the long-elicited strong correlation between the instantaneous values of the tilt of the laser beam and the received beam from a natural star, transmitted towards each other. When observing low-orbit small-sized space objects and laser reference stars, it is assumed that they are in the Fresnel zone of the receiving aperture of the optoelectronic system and within the isoplanatism angle of the atmosphere, determined within the framework of an isotropic and locally homogeneous model of atmospheric turbulence. The proposed solution made it possible to determine the value of the instantaneous tilt angle for the image of an inconspicuous space object in the focal plane of the receiving aperture of the telescope on the basis of measuring the instantaneous tilt angle for the actually observed image of the laser reference star. The results can be used in the development of ground-based adaptive optoelectronic tracking systems for low-orbit small-sized space objects.

Design and analysis of segmented-mirror primary mirror system

with the development of space exploration, the resolution of optical system is required to be higher and higher. According to the Rayleigh criterion, the resolution of the optical system is directly proportional to the aperture. However, due to the influence of processing capacity and size of the rocket fairing, it is difficult to achieve one single large aperture. Therefore, the scheme of using small size segmented-mirror to achieve one large aperture becomes an effective form. Because location difference between the deployment and the ideal position, the imaging quality will be affected, and cannot be imaged in serious case. For the analysis of influence of the position error on the imaging quality, the traditional method is to analyze on the optical transfer function and Strehl ratio in theory, ignoring the influence of various aberrations that cannot be corrected by the optical system itself. In this paper, the segmented-mirror primary mirror system model is established to analyze the influence of the position error and the real influence of the various aberrations of the system, so as to complete the analysis of the influence more comprehensively. Through the analysis of different position errors, we can get the final wave-front aberration and Strehl ratio of the system under the influence of position error, which can be used as a reference for the analysis of position error influence of segmented-mirror primary mirror form and sparse aperture form of large aperture optical system in the future.

Tight focusing of the centrosymmetric shape of hybrid polarized beams by adjustable multi-vortex phases

We propose an approach for achieving various centrosymmetric shapes by employing hybrid polarized Bessel–Gaussian (HPBG) beams with multi-vortex phases, which are obtained by embedding a few first-order off-axis topological charges into vortices separated by equal arc lengths of a circle. According to the Debye–Wolf electromagnetic diffraction formula (which is routinely used to describe focusing by high numerical aperture optical systems), we investigate the evolution of tightly focused intensity profiles of the HPBG beams with multi-vortex phases (which are the vectorial electric field of radial and azimuthal polarization), by tuning the positional vectors of the embedded vortex phases, the number of vortex phases and the ratio of radial to azimuthal polarization of the hybrid polarization. The simulation results show that the number of vortex phases is equal to the number of vertices of hollow polygons, increasing the magnitude of polar vector leads to deformation of the hollow polygons, and that the ratio of the radial and azimuthal polarization magnitudes affects the edge sharpness of the hollow polygon in the focal plane, respectively. We can produce triangles, squares, pentagons, hexagons, and inner crosses in the central hollow region, and outer crosses, embedded stars and snowflakes by manipulating the numbers and sites of multi-vortex phase singularities. The focusing structures are robust to noise and maintain a limited thickness along the optical axis. These specific intensity profiles are significant for potential applications including the trapping of multiple micro-sized particles, nonlinear optics, optical beam shaping, and optical telecommunication applications.

Multi-frame image restoration method for novel rotating synthetic aperture imaging system

The novel rotating synthetic aperture (RSA) optical imaging system is an important development direction for future high-resolution optical remote sensing satellites in geostationary orbit. However, owing to the rotating rectangular pupil, the point spread function of the RSA system has an asymmetric spatial distribution, and the images obtained using the primary mirror from different rotation angles have nonuniform blur degradation. Moreover, platform vibration and pupil rotation have coupling effects on the RSA imaging, resulting in further radiometric and geometric quality degradation. To address these problems, the image degradation characteristics are first analyzed according to the imaging mechanism. Then, combined with the theory of mutual information, an image registration method is suggested by introducing the orientation gradient information. From this, a multi-frame image restoration model is proposed based on the directional gradient prior of the RSA system image. From the perspective of interpretation and application, when the aspect ratio is less than 3, the proposed inversion restoration method can achieve a satisfactory processing performance. This work can provide engineering application reference for the future space application of RSA imaging technology.

A Golay3 sparse aperture optical system of primary mirror with free-form surface

To image distant and dim objects, this paper designs a sparse aperture optical system with a small F number and a large filling factor. The design process is divided into three steps. To start with, the curvature radius and spacing of the primary and secondary mirrors are calculated under the guidance of the paraxial imaging theory. Then, based on the primary aberration theory, the conic coefficients of the primary and secondary mirrors are also calculated. Finally, the node aberration theory is used to increase the field of view and eliminate the residual aberrations. Through theoretical derivation and computer-aided methods, a Golay3 sparse aperture optical system with primary mirror used Zernike polynomial is designed. The full field of view is 0.3° and the F number is 5.

Design and Realization of Visible/LWIR Dual-color Common Aperture Optical System

Design and Realization of Visible/LWIR Dual-color Common Aperture Optical System

Common aperture optical system of single photon laser and medium wave infrared

Common aperture optical system of single photon laser and medium wave infrared