Partial Aperture Research Articles

Demodulation method of orbital angular momentum vortex wave by few fields sampled on an extremely small partial aperture in radiation region

In this paper, a demodulation method of electromagnetic wave carrying orbital angular momentum (OAM) has been developed by using few electric fields sampled on an extremely small portion of the whole circumference in the radiation region. Considering that the field of the OAM wave is naturally of the form of exp (ilφ) in the far-field region, the matrix pencil method is used to extract the magnitude terms and the exponential terms according to the real and imaginary parts of the fields sampled on a small portion of the whole circumference, and further, the mode purities corresponding to the desirable OAM modes can be easily solved. The proposed demodulation method only uses four sampling electric fields with the sampling angle ranges of 3.3° and 9° to well demodulate the mode purities of the OAM waves with the single mode and the mixed dual-mode, respectively. The prototype of a dual uniform circular array with the equal divergent angle for the OAM waves of the modes l = 1 and l = 2 is fabricated and measured. Good agreement between the simulation and the measurement is observed to verify the proposed demodulation method.

Suppressing White-Noise Interference for Orbital Angular Momentum Waves via the Forward–Backward Dynamic Mode Decomposition

When the orbital angular momentum (OAM)-carrying beam propagates in a highly boisterous environment, it causes the degradation of the OAM modes’ purity, which brings the crosstalk in the demultiplexing process. To address this issue, we extend the dynamic mode decomposition (DMD) method to suppress white-noise interferences of OAM by using the forward–backward DMD (FBDMD) approach. The FBDMD-based scheme retrieves the noise-free DMD mapping matrix corresponding to the actual OAM’s topological charges by combining the forward and backward DMD mapping matrix in the noisy environment and consequently reduces the crosstalk, particularly for sorting the superposed OAM modes. Numerical examples are provided to demonstrate the effectiveness and advantages of the proposed approach. It is found that the white-noise interference can be significantly suppressed in sorting the OAM modes process compared with previous works. Also, FBDMD is feasible for the whole aperture receiving and partial aperture receiving (PAR). This work offers a practical tool for OAM coaxial demultiplexing in a noisy environment.

Compressive Interferenceless Coded Aperture Correlation Holography With High Imaging Quality

Interferenceless coded aperture correlation holography (I-COACH) provides an alternative way for the 3D imaging of spatial incoherent illuminated or fluorescent sample. However, the low imaging signal-to-noise ratio (SNR) is one of the bottlenecks that restrict the application of I-COACH. The limitation is mainly originated from the strong bias level that presents in the recorded holograms. Phase shifting methods were implemented in I-COACH to eliminate the background noise while the multiple-exposures recording mechanism significantly reduces the temporal resolution of the system. In this paper, we proposed a compressive I-COACH imaging method with high reconstruction quality and without the sacrifice of the imaging speed. The 3D holographic image reconstruction was implemented under compressive sensing framework while only one single-exposure object hologram and one point spread hologram are necessary. High quality reconstructions were obtained using the proposed method, even for the down-sampled holograms. The imaging SNR of the I-COACH system was improved by a factor of more than 16.5% when comparing with the imaging SNR obtained by the conventional cross-correlation reconstruction method. The proposed method provides a fast and high-fidelity imaging method that can potentially benefit the imaging through scattering medium, partial aperture imaging, and other fields.

Effect of Quartz Aperture Covers on the Fluid Dynamics and Thermal Efficiency of Falling Particle Receivers

Abstract Falling particle receivers are an emerging technology for use in concentrating solar power systems. In this study, quartz half-shells are investigated for use as full or partial aperture covers to reduce receiver thermal losses. Quartz half-shell aperture covers offer the ability to minimally interfere with incoming solar radiation from the heliostat field while obstructing thermal radiation and advection from leaving the receiver cavity. The fluid dynamics and heat transfer of a receiver subdomain and surrounding air are modeled using ansys®fluent. We compare the percentage of total incident solar power lost due to conduction through the receiver walls, advective losses through the aperture, and radiation exiting the aperture. Contrary to expected outcomes, results show that quartz aperture covers can increase radiative losses and result in modest to nonexistent reductions in advective losses. The increased radiative losses are driven by elevated quartz half-shell temperatures and have the potential to be mitigated by active cooling and/or material selection. Quartz half-shell total transmissivity was measured experimentally using a radiometer and the National Solar Thermal Test Facility heliostat field with values up to 0.97 ± 0.01. Quartz half-shell aperture covers did not yield expected efficiency gains in numerical results due to increased radiative losses, but efficiency improvement in some numerical results and the performance of quartz half-shells subject to concentrated solar radiation suggest that quartz half-shell aperture covers should be investigated further.

Research on a Partial Aperture Factor Measurement Method for the AGRI Onboard Calibration Assembly.

A partial aperture onboard calibration method can solve the onboard calibration problems of some large aperture remote sensors, which is of great significance for the development trend of increasingly large apertures in optical remote sensors. In this paper, the solar diffuser reflectance degradation monitor (SDRDM) in the onboard calibration assembly (CA) of the FengYun-4 (FY-4) advanced geostationary radiance imager (AGRI) was used as the reference radiometer. It was designed for measuring the partial aperture factor (PAF) for the AGRI onboard calibration. First, the linear response count variation relationship between the two was established under the same radiance source input. Then, according to the known bidirectional reflection distribution function (BRDF) of the solar diffuser (SD) in the CA, the relative reflectance ratio coefficient between the AGRI observation direction and the SDRDM observation direction was calculated. On this basis, the response count value of the AGRI and the SDRDM was used to realize the high-precision measurement of the PAF of the AGRI B1~B3 bands by simulating the AGRI onboard calibration measurement under the illumination of a solar simulator in the laboratory. According to the determination process of the relevant parameters of the PAF, the measurement uncertainty of the PAF was analyzed; this uncertainty was greater than 2.04% and provided an important reference for the evaluation of the onboard absolute radiometric calibration uncertainty after launch.

IRS-Assisted OAM-MIMO Communication with Partial Arc Sampling Receiving(PASR) Scheme

Orbital Angular Momentum-based Multiple Inputs Multiple Outputs (OAM-MIMO) Communication has recently been explored in wireless communication to provide an alternative way of multiplexing different orthogonal OAM modes over a common medium and significantly improve reliability, capacity, and energy efficiency. Although the near-field applications of radio vortex have been successfully verified in a near-field line-of-sight scenario, perfect antenna misalignment should be required to maintain orthogonality, which is difficult to be satisfied in a multipath environment consisting of many non-line-of-sight(NLOS) paths and LOS path. In addition, beam spread and divergence reduce the transmission distance accompanied by large performance degradation, thus requiring a large aperture size receiving for efficient detection of OAM modes. This paper studies the communication performance of OAM-MIMO with the PASR (Partial Aperture Sampling Receiving) scheme by deploying an intelligent reflective surface (IRS) of a large number of reconfigurable passive elements to achieve an intelligent wireless propagation environment through software-controlled reflection and address the above issues and limitations under the Perfect Channel State Information (CSI). We analyze the concepts of PASR and OAM mode selection by preserving orthogonality according to the required aperture scale, and the influence of the Rayleigh and Rician channels on the system performance of IRS-Assisted OAM-MIMO Communication with the PASR scheme in a multipath environment. The simulation results show that the channel capacity for IRS-assisted OAM-MIMO Communication with the PASR scheme is greatly improved compared with IRS-assisted OAM-MIMO Communication, which can meet the performance requirements, and provide insights into the angular aperture needed for efficient (de)-multiplex OAM modes, and performance limit over propagation distance.

Direct Sampling for Recovering Sound Soft Scatterers from Point Source Measurements

In this paper, we consider the inverse problem of recovering a sound soft scatterer from the measured scattered field. The scattered field is assumed to be induced by a point source on a curve/surface that is known. Here, we propose and analyze new direct sampling methods for this problem. The first method we consider uses a far-field transformation of the near-field data, which allows us to derive explicit bounds in the resolution analysis for the direct sampling method’s imaging functional. Two direct sampling methods are studied, using the far-field transformation. For these imaging functionals, we use the Funk–Hecke identities to study the resolution analysis. We also study a direct sampling method for the case of the given Cauchy data. Numerical examples are given to show the applicability of the new imaging functionals for recovering a sound soft scatterer with full and partial aperture data.

Solving an inverse acousto-elastic scattering problems by combining full-waveform redatuming and time reversal

The aim of this paper is to propose a new method to solve the inverse scattering problem in a non homogeneous elastic medium, from data recorded in an acoustic medium. We aim to determine the Young modulus of elastic unknown “inclusions”, i.e. not observable solid objects, located in the elastic medium, from partial aperture boundary measurements. This approach combines a time-reversal approach and redatuming - a technique that consists in virtually moving the sensors from the original acquisition location to an arbitrary position - which allows to reduce the size of the inspected medium. Thanks to this strategy, we can recover the properties of the scatterer, this technique being also relatively insensitive to noise in the data.

Extraction of the characteristics of vortex beams with a partial receiving aperture at arbitrary locations

Orbital angular momentum (OAM) has aroused widespread interest in communication systems because it improves spectrum efficiency. However, for the free-space link, OAM-based communications encounter problems of OAM beam divergence and misalignment, which reduces the channel quality. To address these two challenges, we propose an algorithm to detect the OAM beams based on the dynamic mode decomposition (DMD). Only the partial receiving aperture is used to address the challenge caused by the beam divergence, and the output DMD spectrum exactly sorts the OAM indices of superposed modes. Besides, when there is a misalignment, the DMD spectrum is utilized as a feedback indicator to search for the lateral displacement of the mode axis. With the differential evolution, the axis of OAM beams can be located. Compared with existing detection schemes, our work enables the separation of mixed OAM modes with less received information, and the perfect alignment is dispensable as a priori. This decentralized partial aperture receiving scheme could serve as a practical and effective detection technique for OAM-based communication systems.

Resolution Enhancement of Spherical Wave-Based Holographic Stereogram with Large Depth Range

The resolution-priority holographic stereogram uses spherical waves focusing on the central depth plane (CDP) to reconstruct 3D images. The image resolution near the CDP can be easily enhanced by modifying three parameters: the capturing depth, the pixel size of elemental image and the focal length of lens array. However, the depth range may decrease as a result. In this paper, the resolution characteristics were analyzed in a geometrical imaging model, and three corresponding methods were proposed: a numerical method was proposed to find the proper capturing depth; a partial aperture filtering technique was proposed after reducing pixel size; the moving array lenslet technique was introduced after increasing focal length and partial aperture filtering. Each method can enhance resolution within the total depth range. Simulation and optical experiments were performed to verify the proposed methods.

Capacity of a Radio Vortex Communication System Using a Partial Angular Aperture Receiving Scheme under the Horizontal Non-Kolmogorov Model.

A partial receiving scheme based on limited angular aperture multi-beam receiving and demultiplexing can solve the difficulty caused by the divergence of the vortex beam in the conventional whole beam receiving scheme and realize the long-distance transmission of the vortex wave. The propagation of the radio vortex beam in atmospheric turbulence is of significant importance in theoretical study and practical applications. In this paper, the influence of atmospheric turbulence on the performance of a radio vortex (RV) communication system based on a partial angular aperture receiving (PAAR) scheme under the horizontal non-Kolmogorov channel model is studied. The spiral spectrum of the PAAR scheme and the channel capacity of the RV communication system using the PAAR scheme are derived. Simulation results demonstrate that the selected transmission frequency range has a great influence on the RV communication system based on the PAAR scheme, and the choice of the orbital angular momentum (OAM) mode number L has an influence on the propagation distance. The capacity of RV communication systems based on the PAAR scheme increases with the increase of the transmission frequency in the selected transmission frequency range of 10 GHz–60 GHz. When the number of orbital angular momentum (OAM) modes L is small, we can improve the signal-to-noise ratio (SNR) to obtain a larger capacity of the RV communication system based on the PAAR scheme over a longer propagation distance.

Partial aperture imaging system based on sparse point spread holograms and nonlinear cross-correlations

Partial aperture imaging system (PAIS) is a recently developed concept in which the traditional disc-shaped aperture is replaced by an aperture with a much smaller area and yet its imaging capabilities are comparable to the full aperture systems. Recently PAIS was demonstrated as an indirect incoherent digital three-dimensional imaging technique. Later it was successfully implemented in the study of the synthetic marginal aperture with revolving telescopes (SMART) to provide superresolution with subaperture area that was less than one percent of the area of the full synthetic disc-shaped aperture. In the study of SMART, the concept of PAIS was tested by placing eight coded phase reflectors along the boundary of the full synthetic aperture. In the current study, various improvements of PAIS are tested and its performance is compared with the other equivalent systems. Among the structural changes, we test ring-shaped eight coded phase subapertures with the same area as of the previous circular subapertures, distributed along the boundary of the full disc-shaped aperture. Another change in the current system is the use of coded phase mask with a point response of a sparse dot pattern. The third change is in the reconstruction process in which a nonlinear correlation with optimal parameters is implemented. With the improved image quality, the modified-PAIS can save weight and cost of imaging devices in general and of space telescopes in particular. Experimental results with reflective objects show that the concept of coded aperture extends the limits of classical imaging.

Single-shot wavelength-selective quantitative phase microscopy by partial aperture imaging and polarization-phase-division multiplexing.

We propose a single-shot wavelength-selective quantitative phase microscopy by annular white-light illumination, polarization-phase-division, and parallel phase-shifting. Compared to conventional multi-wavelength incoherence digital holography, the proposed microscopy presents the following merits: no switching of illumination or mechanical scanning, high spatiotemporal phase sensitivity, and single-shot reconstruction at each wavelength. Experiments validate these characteristics by quantitative phase imaging of gratings, cells, and tissues.

Imaging in Three-Dimensional Waveguides with Partial Aperture Data

We consider the problem of imaging extended reflectors in three-dimensional acoustic waveguides using a planar array that is parallel to the waveguide’s cross-section. Our data consist of the multiple-frequency array response matrix. To form an image, we back-propagate a projection of the data on the propagating modes in the waveguide. The projection operator is adequately defined for any array-aperture size and shape that cover fully or partially the waveguide cross-section. The properties of the imaging method are analyzed theoretically and its effectiveness is assessed by numerical experiments for scatterers and arrays of varying shape and size.

Time reversal for elastic scatterer location from acoustic recording

The aim of this paper is to study the feasibility of time-reversal methods in a non homogeneous elastic medium, from data recorded in an acoustic medium. We aim to determine, from partial aperture boundary measurements, the presence and some physical properties of elastic unknown “inclusions”, i.e. not observable solid objects, located in the elastic medium. We first derive a variational formulation of the acousto-elastic problem, from which one constructs a time-dependent finite element method to solve the forward, and then, the time reversed problem. Several criteria, derived from the reverse time migration framework, are then proposed to construct images of the inclusions, and to determine their locations. The dependence of the approach to several parameters (aperture, number of sources, etc.) is also investigated. In particular, it is shown that one can differentiate between a benign and malignant close inclusions. This technique is fairly insensitive to noise in the data.

High-resolution imaging system with an annular aperture of coded phase masks for endoscopic applications.

Partial aperture imaging is a combination of two different techniques; coded aperture imaging and imaging through an aperture that is only a part of the complete disk, commonly used as the aperture of most imaging systems. In the present study, the partial aperture is a ring where the imaging through this aperture resolves small details of the observed scene similarly to the full disk aperture with the same diameter. However, unlike the full aperture, the annular aperture enables using the inner area of the ring for other applications. In this study, we consider the implementation of this special aperture in medical imaging instruments, such as endoscopes, for imaging internal cavities in general and of the human body in particular. By using this annular aperture, it is possible to transfer through the internal open circle of the ring other elements such as surgical tools, fibers and illumination devices. In the proposed configuration, light originated from a source point passes through an annular coded aperture and creates a sparse, randomly distributed, intensity dot pattern on the camera plane. A combination of the dot patterns, each one recorded only once, is used as the point spread hologram of the imaging system. The image is reconstructed digitally by cross correlation between the object intensity response and the point spread hologram.

Upper limit for angular compounding speckle reduction.

Angular compounding is a technique for reducing speckle noise in optical coherence tomography that is claimed to significantly improve the signal-to-noise ratio (SNR) of images without impairing their spatial resolution. Here, we examine how focal point movements caused by optical aberrations in an angular compounding system may produce unintended spatial averaging and concomitant loss of spatial resolution. Experimentally, we accounted for such aberrations by aligning our system and measuring distortions in images and found that when the distortions were corrected, the speckle reduction by angular compounding was limited. Our theoretical analysis using Monte Carlo simulations indicates that "pure" angular compounding (i.e., with no spatial averaging) over our full numerical aperture (13° in air) can improve the SNR by not more than a factor of 1.3. Illuminating only a partial aperture cannot improve this factor compared to a spatial averaging system with equivalent loss of resolution. We conclude that speckle reduction using angular compounding is equivalent to spatial averaging. Nonetheless, angular compounding may be useful for improving images in applications where the depth of field is important. The distortions tend to be the greatest off the focal plane, and so angular compounding combined with our correction technique can reduce speckle with a minimal loss of resolution across a large depth of field.

Orbital angular momentum demultiplexing with synthetic partial aperture receivers

Long-distance transmitted orbital angular momentum (OAM) cannot be received completely by a single aperture receiver due to the divergence of OAM beams in free space, which results in a challenge for measuring and demultiplexing the multi-OAM modes. We propose a novel approach in measuring multi-OAM modes by using synthetic partial aperture receivers, which could significantly reduce the intensity crosstalk caused by a power leakage. The available OAM modes in a long-distance free-space communication system are greatly increased, and all of the OAM modes can be used for multiplexing or encoding. Furthermore, an OAM demultiplexing array is employed to convert the incident OAM modes into Gaussian-like beams that can be detected by commercial photodetectors with high speed. The proposed approach has great potential applications in long-distance free-space OAM detection and communication.

Review of 3D Imaging by Coded Aperture Correlation Holography (COACH)

Coded aperture correlation holography (COACH) is a relatively new technique to record holograms of incoherently illuminated scenes. In this review, we survey the main milestones in the COACH topic from two main points of view. First, we review the prime architectures of optical hologram recorders in the family of COACH systems. Second, we discuss some of the key applications of these recorders in the field of imaging in general, and for 3D super-resolution imaging, partial aperture imaging, and seeing through scattering medium, in particular. We summarize this overview with a general perspective on this research topic and its prospective directions.

Recent advances in self-interference incoherent digital holography

Self-interference holography is a common technique to record holograms of incoherently illuminated scenes. In this review, we survey the main milestones in the topic of self-interference incoherent digital holography from two main points of view. First, we review the prime architectures of optical hologram recorders over more than 50 years. Second, we discuss some of the key applications of these recorders in the field of imaging in general, and for 3D super-resolution imaging, fluorescence microscopy, partial aperture imaging, seeing through a scattering medium, and spectral imaging in particular. We summarize this overview with a general perspective on this research topic and its prospective directions.