Multi-aperture Systems Research Articles

Sequential phase diversity for wavefront correction using a deformable mirror with modeling errors.

The phase diversity (PD) method is effective for scene-based wavefront sensing and control (WFSC) in spaceborne high-resolution imagers for Earth observation. The simplest way of performing the PD WFSC is offering a diversity wavefront by directly actuating a corrective device, such as a deformable mirror. However, this strategy faces a challenge in constructing a numerical model of the provided diversity wavefront because some corrective actuators' properties prevent us from precisely determining their deflection behaviors. To avoid this modeling issue, we propose the sequential PD (SPD) method to compensate for static aberration using a corrective device with modeling errors. The SPD WFSC repeats the PD WFSC to gradually correct the aberration, where the estimated corrective wavefront is regarded as the known diversity in the subsequent PD WFSC. The numerical simulation validated that the proposed idea improved the correction performance when a corrective device had a linear modeling error. Additionally, a demonstration experiment succeeded in aberration removal using a face-sheet deformable mirror with inter-actuator coupling and non-linear responses. An additional simulation demonstrated that the proposed method effectively corrected the discontinuous wavefront aberration in multi-aperture imaging systems. The SPD WFSC can potentially bring us optical remote sensing systems with unprecedentedly high resolution.

Diversity Analysis of Multi-Aperture UWOC System Over EGG Channel With Pointing Errors

The existing single-aperture reception for underwater wireless optical communication (UWOC) is insufficient to deal with oceanic turbulence caused by the combined effect of temperature gradient and air bubbles. This paper analyzes the performance of multi-aperture reception for UWOC under channel irradiance fluctuations characterized by the mixture exponential–generalized gamma (EGG) distribution. We analyze the system performance by employing selection combining (SC) and maximum ratio combining (MRC) receivers. In particular, we derive the exact outage probability expression for the SC-based multi-aperture UWOC receiver and obtain an upper bound on the outage probability for the MRC-based multi-aperture UWOC receiver. With the help of the derived results, we analytically obtain the diversity order of the considered multi-aperture UWOC systems outperforming the single-aperture-based system by a factor of the number of multi-aperture elements. The simulation results validate the accuracy of derived analytical expressions and demonstrate that the low-complexity SC technique effectively improves the performance of the UWOC system under oceanic turbulence with pointing errors at a moderately high transmit power.

Robust dispersed fringe sensing for all-stage cophasing

As a technique for quantitatively measuring wavefront piston of optical multi-aperture systems, dispersed fringe sensor (DFS) is considered to be a coarse phasing sensor which can be complemented by fine cophasing sensors. In this work, we presented a strategy for the DFS to cover coarse and fine co-phasing stages. The strategy that combines two estimating algorithms and the optical features makes the DFS feasible for both coarse and fine cophasing stages. Subsequent analysis proves the ability of DFS in coarse and fine stages and in particular the robustness of this strategy in fine co-phasing stage. Further, we use demonstration experiment to show the ability of this strategy in different situations compared with other methods. In addition, the robust anti-noise ability is proved by both demonstration and analysis. Thus, the strategy gives phasing system the ability to robustly cover coarse and fine co-phasing stages without any further hardware cost for DFS.

Multi-aperture optical imaging systems and their mathematical light field acquisition models

Multi-aperture optical imaging systems and their mathematical light field acquisition models

High-throughput and controllable manufacturing of liquid crystal polymer planar microlens array for compact fingerprint imaging.

The microlens array (MLA) with a small geometric footprint and unique performances, is the key enabler to push the development of photonic devices toward miniaturization, multi-function and large-scale integration. However, the realization of 100% fill-factor (FF) MLAs with high controllability and its mass manufacturing without complex steps has always been a difficult issue. Here, we propose an efficient, highly flexible and low-cost manufacturing approach for MLAs with a high FF via snapshot polarization patterning. The digitalized linear polarization pattern was distributed across the photo-alignment layer with both high efficiency and accuracy, enabling large-area liquid crystal MLA with parameter controllability from element to element. The MLA manufacturing process does not involve developing, etching and deposition steps and is suitable for industry up-scaling. We further proposed a novel compact compound-eye imaging system for biometrics with the obtained MLAs. The 100% FF MLA enables high light utilization efficiency and low background crosstalk, yielding compact biometrics indentation with high recognition accuracy. The realization of such planar optics would lead to a plethora of different miniaturized multiaperture imaging systems in the future.

Concept, manufacturing and challenges of ultra-compact snapshot multi-spectral multi-aperture imaging systems

Snapshot multispectral imaging is a rising non-invasive and contact-free analysis method and technology to discriminate or identify objects based on their spectral characteristics. We demonstrate a versatile system approach for compact and real-time capable snapshot cameras for the visible (VIS) and the near-infrared (NIR) or the short-wave infrared (SWIR) wavelength range based on a micro-optical multi-aperture system and various spectral filter approaches. In addition, the manufacturing, the calibration, and the limitations of the demonstration systems are described.

Blazed grating pupil remapping and rotational synthesis with computational phase correction for a two-aperture system

Multi-aperture imaging systems can be used to decrease the size, weight, and cost of an imaging system while retaining a resolution related to the diameter of the aperture gain. One solution for phasing apertures in an array uses anamorphic pupil remapping and a computational phase correction algorithm. It is shown that using blazed gratings to remap the aperture fields reduces field dependent contrast and leads to piston corrections for partially coherent illumination. Additionally, rotational aperture synthesis is used to combine two piston corrected images and create an image with resolution increases in more than just the axis of aperture gain.

Comparison of Postdetection Correction of Short- and Long-Exposure Images Formed by Traditional and Multiaperture Observation Systems in a Turbulent Atmosphere

The efficiency of retrieval of images distorted by atmospheric turbulence formed by single- and multiaperture systems is analyzed. It is shown in numerical simulation that the use of multiaperture observation systems for computer correction of atmospheric distortions under anisoplanatism can significantly reduce the exposure time. The main distortions are well corrected when imaging for a short exposure time, which corresponds to the case of a “frozen” turbulent medium. Correction of residual small-scale distortions requires an-order-of-magnitude shorter time than in the case of long-exposure imaging with the use of common single-aperture observation systems.

Computational phase correction algorithms for multi-aperture systems.

Aperture cophasing can increase the resolution of a multi-aperture array while reducing optical system size and accounting for phase errors from hardware misalignments and atmospheric turbulence. Remapping apertures using an anamorphic pupil relay can separate the spatial frequency spectrum of a previously continuous modulation transfer function to facilitate a posteriori piston corrections between apertures [J. Opt. Soc. Am. A34, A47 (2017)JOAOD60740-323210.1364/JOSAA.34.000A47]. This study expands this method from two apertures to six using maximum likelihood estimation to calculate piston phase errors and least squares for tip and tilt. Perturbed test targets were generated for specific turbulence and noise values, providing random realizations of pistons, tips, tilts, and atmospheric phase screens. Results show improved Strehl ratios for corrected images in the presence of noise and atmospheric turbulence.

Improved Spatial Modulation Diversity with High Noise Robust Based on Deep Denoising Convolution Neural Network

Synthetic aperture imaging systems can be applied to equivalently get high-resolution images of traditional monolithic primary mirror systems with less weight and costs. Spatial modulation diversity (SMD), a newly developed post-processing technology applicable for such synthetic aperture systems, is very sensitive to Gaussian noise, which greatly limits its further application. In this paper, we propose an improved SMD strategy by introducing the deep denoising convolutional neural networks (DnCNN) into the image preprocessing to improve the robustness of SMD. Results of the numerical simulations demonstrate that the strategy proposed exhibits superior performance compared to the traditional SMD technique in terms of both the root-mean-square error (RMSE) of phase estimates and the structural similarity (SSIM) of image reconstruction. In view of the superiority and robustness, the method we proposed may have important application prospects in multi-aperture imaging systems.

Comparison Between Single-Aperture and Multi-Aperture Approaches to Computer Correction of Images

The effectiveness of the reconstruction of images formed by single- and multi-aperture systems and distorted by an atmospheric turbulence is analyzed in this work. Based on numerical simulation, we show that the use of multi-aperture observation systems for the computer correction of atmospheric distortions under anisoplanar conditions can significantly reduce the exposure time. Main distortions are well corrected during image synthesis for a short exposure time, which corresponds to a “frozen” turbulence. The time required for the correction of residual small-scale distortions is an order of magnitude shorter than in the case of synthesis of long-exposure images with the use of traditional single-aperture observation systems.

Comparison of post-detection correction of shortand long-exposure images formed by commonly used and multi-aperture observation systems in a turbulent atmosphere

Comparison of post-detection correction of shortand long-exposure images formed by commonly used and multi-aperture observation systems in a turbulent atmosphere

Deep learning-based image reconstruction for multi-aperture diffractive lens

Multi-aperture imaging systems are one of the modern trends for imaging devices. The paper presents the results of developing a multi-aperture imaging system based on single diffractive lenses and postprocessing pipeline based on deep learning. The pipeline includes two processing steps: color correction using a generative adversarial network and chromatic deblurring by a convolutional neural network.

Smart Compound Eyes Enable Tunable Imaging

AbstractCompound eyes are natural multiaperture optical imaging systems and have substantial potential in the field of modern optics. However, both natural and artificial compound eyes are composed of ommatidia with fixed focal lengths, and thus incapable of variable‐focus imaging. In this study, inspired by the tunable crystalline lens of human eyes, smart stimuli‐responsive compound eyes based on the bovine serum album (BSA) protein are fabricated via femtosecond laser direct writing. Due to the swelling and shrinking effect of BSA under different pH conditions, a tunable field of view (FOV, 35°–80°) and variable focal length of ommatidia are achieved. In addition to the direct prototyping of an entire protein‐based compound eye, the ability to flexibly integrate the smart protein ommatidia with a conventional optical lens (an SU‐8 lens in this study) to form a composite compound eye is shown. The composite compound eye achieves nearly 400% of focal length tuning at a fixed FOV. It is anticipated that femtosecond laser fabrication and the integration of smart protein‐based compound eyes may emerge as an enabler for fabricating miniature tunable imaging systems.

Impact of atmospheric turbulence and weather conditions on the performance of satellite-to-ground optical communication system based on multiaperture coherent receivers

For satellite-to-ground downlink optical communication systems, it is desirable to reduce the size, weight, and power profiles of the space terminals by employing high sensitivity and large collection area ground terminals. Multiaperture coherent receivers utilizing spatial diversity techniques can satisfy this requirement and have many advantages over the single large aperture coherent receivers. The influence of both atmospheric turbulence and weather conditions on the bit error rate performance of multiaperture coherent receivers utilizing maximum ratio combining, equal gain combining, and selection combining techniques is systematically investigated and compared for the satellite-to-ground downlink communication system. The results are useful for the design and adaptive optimization of the multiaperture coherent receiver-based downlink communication systems.

Restoration of Sparse Aperture Images Using Spatial Modulation Diversity Technology Based on a Binocular Telescope Testbed

In this paper, our recently developed spatial modulation diversity technology (SMDT) is first used to correct the blurring and restore the lost midband contrast of synthetic images based upon an actual sparsely filled aperture system, specifically a binocular telescope testbed consisting of two horizontally arranged collector telescopes with 127 mm diameter. Synthetic images are obtained with the testbed cophased by an optimization algorithm. Diversity images are generated by using electronic shutter to modulate the transmittance of each subaperture, alternately. We capture two independent datasets corresponding to different objects regarded as the reference of each other. Then an improved algorithm of SMDT for real sparse aperture systems is proposed to restore the synthetic imagery by processing the datasets. The experimental results that the reconstructed images present high quality and contrast and that the repeatable wavefront measurements show a good agreement demonstrate the availability of SMDT for image restoration in actual multiaperture systems.

Study of electron beam uniformity in large-area multi-aperture diode with arc plasma cathode

The use of plasma emission cathode in the conjunction with a multiple apertured electron optical system (EOS) is promising for the multi-MW class electron beams of a large cross-sectional area. In a multi-aperture source, the beam parameters could be raised simply due to increase of the number of apertures (i.e. an effective emission area), if a uniformity of the electron emission over a large-area plasma cathode is ensured. In the presented paper, the cross-sectional distribution of the emission current density was investigated using the X-ray diagnostic technique for two versions of the diode-type EOS, with electrodes performed as flat molybdenum “grids”. The first one had 241 apertures arranged hexagonally inside a circle with a diameter of 8.3 cm and the second had 499 apertures within a circle of 11.8cm diameter. The emission plasma is produced using a single arc-discharge plasma generator placed on the axis at 20 cm from the EOS. It was demonstrated that multi-aperture systems with a single on-axis plasma generator can be effectively employed to obtain large-area beams, even in the presence of the guiding magnetic field. All apertures are emitting in the 499-apertured EOS. The beam current density is quite uniform up to the radius 2.5cm and gradually decreases to the periphery.

A Compact Neutron Source for Boron Neutron Capture Therapy

We propose a neutron generator scheme based on a high-current ion source with electron cyclotron resonance plasma heating by high-power millimeter-wave gyrotron radiation. The most promising application of this neutron generator is a medical one, namely, boron neutron capture therapy of oncological diseases. A possibility for using a multi-aperture extraction system for high-current ion beam generation to increase the total current is studied. It is shown that the parameters of the plasma flow leaving a magnetic trap permit the effective use of multi-aperture systems without a significant loss in the ion beam current density. Thus, the use of multi-aperture systems in the ion source of a neutron generator can significantly increase the total neutron yield.

Low-Light Image Enhancement for Multiaperture and Multitap Systems

Intense Poisson noise drastically degrades image quality when only a few or when a single photon hits each pixel. Multiaperture systems are able to provide multiple images of the same scene, which are acquired simultaneously. After registration and cropping, the partial scene information contained in each aperture image should be the same, while the noise will be different in each one. A similar case arises in multitap systems, which are widely used in Time-of-Flight imaging (ToF), where several integration channels per pixel exist and where several sequential acquisitions are needed to generate a depth image. In this case, raw images might be different from each other, but still, since they are images of the same scene, information redundancy can be exploited to filter out the noise. In this work, we propose two different ways of joint processing of low-light multiaperture images. One of them is an extension of bilateral filtering to the multiaperture case, while the other relies on the compressive sensing theory and aims to recover a noiseless image from fewer measurements than the total number of pixels in the original noisy images. Experimental results show that both methods exhibit very close performance, which is much higher than those of previous methods. Additionally, we show that bilateral filtering can also be applied to the raw images of multitap ToF systems, leading to a significant error reduction in the final depth image.

Synthetic aperture imaging by using spatial modulation diversity technology with stochastic parallel gradient descent algorithm.

In this paper, we propose and demonstrate the synthetic aperture imaging by using spatial modulation diversity technology with stochastic parallel gradient descent (SPGD) algorithm. Instead of creating diversity images by means of focus adjustments, the technology, proposed in this paper, creates diversity images by modulating the transmittance of individual sub-aperture of multi-aperture system, respectively. Specifically, spatial modulation is realized by switching off the transmittance of each sub-aperture with electrical shutters, alternately. Based on these multi diversity images, SPGD algorithm is used for adaptively optimizing the coefficients of Zernike polynomials to reconstruct the real phase distortions of multi-aperture system and to restore the near-diffraction-limited image of object. Numerical simulation and experimental results show that this technology can be used for joint estimation of both pupil aberrations and an high resolution image of the object, successfully. The technology proposed in this paper can have wide applications in segmented and multi-aperture imaging systems.