Alignment Requirements Research Articles

Angular MIMO for Underwater Wireless Optical Communications: Link Modeling and Tracking

Angular imaging multiple-input--multiple-output (A-MIMO) is investigated for short-range, high-speed underwater wireless optical communications (UWOCs) where, unlike conventional imaging MIMO (C-MIMO), data are transmitted in an angle rather than in space. In this approach, the strict requirements of on-axis alignment and fixed channel length are relaxed. This technique also allows for simpler estimation of the relative misalignment between the transmitter and the receiver from the received image. For the first time, we derive a comprehensive model for the underwater A-MIMO link by taking into account link misalignment, background noise, as well as seawater absorption and scattering. Power distributions at the receiver are modeled by the angle of arrival of the received signal on the lens and its position of arrival on the focal plane of the detector. We further propose and model a tracked A-MIMO (TA-MIMO) system that maintains the alignment between the two ends of the link, for which the distribution of the residual tracking error is calculated. The UWOC channel capacity is then estimated for buoyed-to-fixed (B2F) (which has dominant angular misalignments) and mobile-to-fixed (M2F) (which has dominant off-axis misalignment) communication scenarios. Numerical results indicate that in the B2F scenario, A-MIMO is sensitive to angular misalignments; however, TA-MIMO outperforms C-MIMO. In the case of M2F links, A-MIMO greatly outperforms C-MIMO when off-axis misalignments are present. This work serves as a design guide to determine the selection of A-MIMO, TA-MIMO, or C-MIMO receivers depending on the misalignment conditions for a particular underwater application.

Generalized reverse projection method for grating-based phase tomography.

The reverse projection protocol results in fast phase-contrast imaging thanks to its compatibility with conventional computed-tomography scanning. Many researchers have proposed variants. However, all these reverse projection methods in grating-based phase-contrast imaging are built on the hypothesis of the synchronous phase of reference shifting curves in the whole field of view. The hypothesis imposes uniformity and alignment requirements on the gratings, thus the field of view is generally limited. In this paper, a generalized reverse projection method is presented analytically for the case of non-uniform reference in grating-based phase tomography. The method is demonstrated by theoretical derivation, numerical simulations and synchrotron radiation experiments. The influence of imaging position to sensitivity, and the phase-wrapping phenomenon are also discussed. The proposed method combines the advantages of the high efficiency of the reverse projection method and the universal applicability of the phase-stepping method. The authors believe that the method would be used widely in fast and dose-constrained imaging.

Towards self-similarity consistency and feature discrimination for unsupervised domain adaptation

Recent advances in unsupervised domain adaptation mainly focus on learning shared representations by global statistics alignment, such as the Maximum Mean Discrepancy (MMD) which matches the Mean statistics across domains. The lack of class information, however, may lead to partial alignment (or even misalignment) and poor generalization performance. For robust domain alignment, we argue that the similarities across different features in the source domain should be consistent with that in the target domain. Based on this assumption, we propose a new domain discrepancy metric, i.e., Self-similarity Consistency (SSC), to enforce the pairwise relationship between different features being consistent across domains. The Gram matrix matching and Correlation Alignment is proven to be a special case, and a sub-optimal measure of our proposed SSC. Furthermore, we also propose to mitigate the side effect of the partial alignment and misalignment by incorporating the discriminative information of the deep representations. Specifically, a simple yet effective feature norm constraint is exploited to enlarge the discrepancy of inter-class samples. It relieves the requirements of strict alignment when performing adaptation, therefore improving the adaptation performance significantly. Extensive experiments on visual domain adaptation tasks demonstrate the effectiveness of our proposed SSC metric and feature discrimination approach.

Improvement of a computer-aided alignment algorithm for the nonsymmetric off-axis reflective telescope

Due to complications of the off-axis three-mirror anastigmat (TMA) telescope, each optical element in the off-axis TMA telescope is introduced with theoretical eccentricity and tilt. Moreover, the introduction of freeform surfaces and other optical elements with complex surface features generally causes the initial alignment accuracy of the optical path to be low. A large initial alignment error amplifies the sensitivity of the misalignment calculation accuracy to the measurement error of the Zernike coefficient, resulting in difficulty obtaining convergence results for a computer-aided alignment algorithm. Considering the above issues, the alignment sensitivity of each component in the optical path is analyzed in this. The large conditional number of the sensitivity matrix results in poor algorithm robustness. Thus, an adaptive damping factor least-squares algorithm model is proposed and derived to improve the efficiency of the classical least-squares algorithm. A method for piecewise optimization of the damping factor is also deduced. Experiments based on a 0.6 m off-axis TMA telescope verify the effectiveness of the algorithm. Simulation and integration experiments show that the proposed method can reduce the accuracy requirements of the initial alignment and improve the adaptability of Zernike coefficient measurement noise. The alignment procedure is carried out for three iterations, and the average of the five field-of-view wave aberration values is enhanced from 2.1λ (RMS; λ=632.8nm) to 0.09λ (average). The improved algorithm can solve the large initial alignment error of a nonsymmetrical off-axis reflective optical system with a freeform surface as well as the problem of the low success rate of the misalignment value due to low Zernike coefficient measurement accuracy.

Beam Aggregation-Based mmWave MIMO-NOMA: An AI-Enhanced Approach

Millimeter wave (mmWave) multiple-input-multiple-output (MIMO) with hybrid beamforming enables high data rate broadband services for 5G. To further improve the spectral efficiency and connectivity, non-orthogonal multiple access (NOMA) has been considered to be integrated with mmWave MIMO. Nonetheless, the existing power-domain mmWave MIMO-NOMA with narrow analog beams suffers from beam misalignments which deteriorate the data rates of the misaligned users as well as the system fairness. Thereby, we propose a beam aggregation-based mmWave MIMO-NOMA scheme to loosen the requirement of beam alignment and improve the system fairness. The proposed scheme generates virtual beams with wider beamwidth by aggregating adjacent analog beams. NOMA transmissions are utilized within each aggregated virtual beam. Then, we propose a non-orthogonal multiuser precoding scheme to ensure the fairness within the aggregated beam by maximizing the minimum achievable rates of the grouped users. To address this issue, a max-min problem is proposed and artificial intelligence (AI) technology is exploited to solve this non-trivial problem. The problem is firstly converted to an equivalent penalized minimization problem and then an unsupervised deep neural network (DNN) is trained to map the instantaneous channel coefficients to the precoders in a data-driven fashion. Specifically, we explicitly introduce the transmit power as the DNN input to achieve better generalization in different signal-to-noise ratio regions. Performance evaluations reveal that the proposed scheme can achieve significant gain on the max-min data rate compared with the conventional scheme.

A review of silicon-based wafer bonding processes, an approach to realize the monolithic integration of Si-CMOS and III–V-on-Si wafers

The heterogeneous integration of III–V devices with Si-CMOS on a common Si platform has shown great promise in the new generations of electrical and optical systems for novel applications, such as HEMT or LED with integrated control circuitry. For heterogeneous integration, direct wafer bonding (DWB) techniques can overcome the materials and thermal mismatch issues by directly bonding dissimilar materials systems and device structures together. In addition, DWB can perform at wafer-level, which eases the requirements for integration alignment and increases the scalability for volume production. In this paper, a brief review of the different bonding technologies is discussed. After that, three main DWB techniques of single-, double- and multi-bonding are presented with the demonstrations of various heterogeneous integration applications. Meanwhile, the integration challenges, such as micro-defects, surface roughness and bonding yield are discussed in detail.

Design and dynamic analysis of low-frequency mounting system for marine thrust bearing

Longitudinal vibration reduction of marine propulsion shafting has drawn great attention of research. In this paper, a low-frequency mounting system (LFMS) is designed. LFMS consists of three sub-systems: floatable self-alignment thrust bearing, floating raft, and air spring mounting system. Floatable self-alignment thrust bearing is a new type of thrust bearing, which can provide small displacement compensation for shafting. In order to maximize the overall vibration behavior of the control system, thrust bearing and marine propulsion unit are integrated on the floating raft air spring mounting system can adjust the floating raft attitude to meet the shafting alignment requirements. A theoretical model is presented to estimate the isolation effect of LFMS. The first-order longitudinal vibration frequency of shafting is 9.8 Hz. Experiments are carried out to verify the isolation effect of LFMS. Experiments show that LFMS has an obvious vibration isolation effect above 10 Hz.

Keyway Alignment Using GRNN in Robotic Pipe Handling

Perforation pipes are widely used in oil&gas exploration. Aligning a perforated pipe in different workstations is very important for quality control. Typically, a keyway in a perforated pipe is machined as the alignment feature. Once the angle of a keyway is measured, an industrial robot is used to rotate the perforated pipe to the desired location. However, due to various errors, the industrial robot cannot rotate the perforated pipe to the desired location using the measured angle once. In this paper, a method based on generalized regression neural network (GRNN) is proposed to predict the rotation angle such that an industrial robot can rotate a perforated pipe to the desired location. Training datasets are collected first to train a GRNN model. The model is then deployed to predict the rotation angle. Experiments were performed to validate the proposed method and the results show that the final keyway angle satisfies the requirement of keyway alignment.

High-resolution X-ray flash radiography of Ti characteristic lines with multilayer Kirkpatrick–Baez microscope at the Shenguang-II Update laser facility

Abstract High-resolution X-ray flash radiography of Ti characteristic lines with a multilayer Kirkpatrick–Baez microscope was developed on the Shenguang-II (SG-II) Update laser facility. The microscope uses an optimized multilayer design of Co/C and W/C stacks to obtain a high reflection efficiency of the Ti characteristic lines while meeting the precise alignment requirement at the Cu Kα line. The alignment method based on dual simulated balls was proposed herein, which simultaneously realizes an accurate indication of the center field of view and the backlighter position. The optical design, multilayer coatings, and alignment method of the microscope and the experimental result of Ti flash radiography of the Au-coned CH shell target on the SG-II Update are described.

Solving Ontology Meta-Matching Problem Through an Evolutionary Algorithm With Approximate Evaluation Indicators and Adaptive Selection Pressure

Ontology applies commonly to solve the problem of heterogeneity of data in the Semantic Web, but the heterogeneity problem between two ontologies seriously affects their communication. As an effective method, ontology matching can address the problem above, whose core technique is the similarity measure. A single similarity measure calculates the similarity value about a feature between two concepts, but none of the similarity measures can ensure their effectiveness in all context due to the diverse heterogeneous features between two ontologies. Therefore, multiple similarity measures are usually aggregated to improve the result's confidence. The problem that how to determine the optimal aggregating weights for the different similarity measures to obtain a high-quality alignment is called the meta-matching problem of ontology, which is modeled as a nonlinear problem with many local optimal solutions. Evolutionary Algorithm (EA) can represent an efficient methodology to address the ontology meta-matching problem, but EA-based ontology matching techniques suffer from the premature convergence and the requirement of a reference alignment to evaluate the solutions. To overcome the defects mentioned above, in this work, an improved EA-based matching approach is proposed, where two approximate evaluation indicators, i.e. pseudo-recall and pseudo-precision, are presented to evaluate the solution's quality, and an adaptive selection pressure is utilized to overcome the premature convergence. The experiment utilizes the Ontology Alignment Evaluation Initiative (OAEI)'s benchmark, and the experimental results will prove the effectiveness of our proposed method.

Fabrication-Tolerant and Low-Loss Hybrid Plasmonic Slot Waveguide Mode Converter

Integrated plasmonic structures can concentrate light to deep-subwavelength volumes below the diffraction limit and have been explored in many applications including label-free nanophotonic sensing with single molecule sensitivity, nonlinear conversion, and lasing. However, conversion between the dielectric waveguide mode and the plasmonic mode typically suffers from large mode conversion loss and requires high nanofabrication precision because of the mode mismatch and the precise alignment requirements with the plasmonic slot structure, respectively. Here, we report low-loss conversion between a dielectric waveguide mode and a hybrid plasmonic slot waveguide mode in a structure with a large fabrication tolerance. The measured conversion loss between the dielectric waveguide mode to the hybrid plasmonic slot waveguide mode can be <; 2 dB in the wavelength range of 1.54-1.60 μm under the optimal phase match condition. The loss remains low with a lateral misalignment tolerance of >500 nm. We confirmed enhanced light-matter interaction in the plasmonic slot waveguide region by measuring the enhancement in optical absorption of graphene with and without the plasmonic slot structure. Our results demonstrate a new approach to achieve deep-subwavelength light concentration by using an easy-to-fabricate interface with dielectric waveguides.

Alignment-free filter array: Snapshot multispectral polarization imaging based on a Voronoi-like random photonic crystal filter.

We develop a photonic crystal filter with a new structure and propose a method to realize a snapshot multispectral polarization camera by mounting the filter on a monochrome imager with no requirement for a specific alignment. The developed filter is based on the Voronoi structure, which forms multilayered photonic crystals with random wave-like structures in each of the Voronoi cells. Because the transmission characteristics of the multilayered photonic crystal can be controlled simply by changing the microstructure, there is no need to change the manufacturing process and materials for each Voronoi cell. Furthermore, the Voronoi cell is randomly distributed so that the filter can be junctioned with the imager at arbitrary positions and angles without the need to position the filter during mounting, although it requires measurement of the camera characteristics and an image restoration process after filter mounting. In this experiment, we evaluated to reconstruct spectra as well as linearly polarized components and RGB images in the visible wavelength range from a single exposure image.

Alignment of DECam-like large survey telescope for real-time active optics and error analysis

To obtain relatively high imaging quality in the whole field of view and simultaneously create petabytes of data with a high signal-to-noise ratio, the alignment requirements for the dark energy camera (DECam)-like wide field active optics survey telescope are strict. Ensuring the real-time alignment of the system is the most important target for the future DECam-like large survey telescope due to the requirement for ellipticity of stellar image across the full field of view is much higher than the traditional telescopes. In such case, the alignment procedure needs to be designed more carefully to consider the large dynamical range and tight residual optical aberration at the same time The error of coarse alignment, influence of camera seeing, error of wavefront sensing, and interaction between the active optics and primary focus assembly and the active optics nonlinear factors, are analyzed by focusing on the DECam-like wide-field active optics survey telescope. Finally, the error of a real time active optics system is analyzed using the normalized point source sensitivity (PSSn). The PSSn of an ideal telescope is 0.9698 and the PSSn of a telescope with an alignment error is 0.8359. This work will be a good guide for the design of future large survey telescopes.

Performance Limits of an Alternating Current Electroluminescent Device.

The use of an alternating current (AC) voltage is a simple, versatile method of producing electroluminescence from generic emissive materials without the need for contact engineering. Recently, it was shown that AC-driven, capacitive electroluminescent devices with carbon nanotube network contacts can be used to generate and study electroluminescence from a variety of molecular materials emitting in the infrared-to-ultraviolet range. Here, performance trade-offs in these devices are studied through comprehensive device simulations and illustrative experiments, enhancing understanding of the mechanism and capability of electroluminescent devices based on alternating as opposed to direct current (DC) schemes. AC-driven electroluminescent devices can overcome several limitations of conventional DC-driven electroluminescent devices, including the requirement for proper alignment of material energy levels and the need to process emitting materials into uniform thin films. By simultaneously optimizing device geometry, driving parameters, and material characteristics, the performance of these devices can be tuned. Importantly, the turn-on voltage of AC-driven electroluminescent devices approaches the bandgap of the emitting material as the gate oxide thickness is scaled, and internally efficient electroluminescence can be achieved using low-mobility single-layer emitter films with varying thicknesses and energy barrier heights relative to the contact.

Improving the performance of reference-frame-independent quantum key distribution through a turbulent atmosphere

Reference-frame-independent quantum key distribution (RFI-QKD) can dispense with the requirements of active alignment on reference frames between legitimate users, which is beneficial for free-space implementation. However, the fluctuating transmittance due to atmospheric turbulence still remains a great challenge for improving the performance and has been seldom addressed. In this paper we extend the recently proposed prefixed-threshold real-time selection method to RFI-QKD while combining practical consideration of the transmittance probability distribution model based on the finite aperture theory. Through numerical simulations, we present an estimation for the variance of the log-normal model with respect to distance and receiving aperture radius, and demonstrate the effectiveness of using this method in the RFI protocol. Considering the finite-key effects, simulation results show that the gap of the key rate with different reference frame deviations can be alleviated by increasing the data size. By adopting this method one can tolerate more serious transmission loss, especially in strong turbulence cases, which is helpful for future free-space experimental designs.

Long-reach underwater wireless optical communication with relaxed link alignment enabled by optical combination and arrayed sensitive receivers.

To extend the transmission distance and relax the strict alignment requirement of underwater wireless optical communication ((UWOC), we design and implement a UWOC system using a 3×1 fiber combiner and a high-sensitive multi-pixel photon counter (MPPC). The 50-m and 100-m transmission distances (corresponding to 24 attenuation lengths) are experimentally achieved with the data rates of 16.78 Mbps and 8.39 Mbps, respectively, in a 50-m standard swimming pool. Moreover, we also investigate and optimize the performance of misalignment tolerance of this system using two MPPCs as the detectors together with different diversity reception technologies. At the 50-m transmission distance, the maximum offset between the MPPC array and the light spot center can be extended to 9 m using the maximal ratio combining (MRC), while the maximum offset is 6 m when using single MPPC.

Resonance-filtering combo system for continuous wireless charging range coverage

AbstractDistribution of wireless power charging field uniformly on a large area pad is critical for power receivers, particularly for wearable devices, wherein small form factor coils are involved. Since the receiver coil size is quite limited in these types of applications, the device is very sensitive to the amount of field it could retain and hence, it needs special placement or snapping mechanism to fix it at an optimum location for reliable wireless charging. In order to overcome this limitation for the end-user, a dual-mode multi-coil power transceiver system is proposed; utilizing resonance filtering to increase the amount of total power delivered with the rather uniform spatial distribution. Two concentric coils; center one driven by 6.78-MHz high-frequency driver (A4WP) and the outer larger one with a 200-KHz low-frequency driver (Qi) with resonant blocker could transfer up to 50 mW standards compliant flat power to a 13-mm radius 30-turns wearable receiver coil everywhere across an 8-cm radius charging pad area without any alignment requirement or snapping. Two different feedback topologies corresponding to each of the H-Bridge power drivers were also presented as an automatic series resonance coil drive frequency lock mechanism, extracting peak powers for each system individually from a standard 5 V-1A USB wall charger.

Three-Dimensional Printed Miniature Fiber-Coupled Multipass Cells with Dense Spot Patterns for ppb-Level Methane Detection Using a Near-IR Diode Laser.

Tunable diode laser absorption spectroscopy (TDLAS) based on a multipass cell (MPC) is a powerful analytical tool and is widely applied to air quality monitoring, industrial process control, and medical diagnostics. However, the conventional MPC as a core component in TDLAS devices has a large size, low utilization efficiency of the mirror surfaces, and tight optical alignment tolerances. In this paper, we design and fabricate a mini-MPC with an optical absorption path length of 4.2 m and dimensions of 4 × 4 × 6 cm3 (open cavity), which, to our best knowledge, is the current smallest MPC in terms of the same optical path length. The mini-MPC generates a seven-nonintersecting-circle dense spot pattern on two 25.4 mm spherical mirror surfaces, providing a high fill factor of 21 cm-2. A fiber-coupled collimator and an InGaAs photodetector are integrated into the mini-MPC via a high-resolution three-dimensional printed frame, hence removing the requirement of active optical alignment. Using a 1.65 μm distributed-feedback laser, the performance of this mini-MPC for methane detection was evaluated in terms of linearity, flow response time, stability, minimum detectable limit, and measurement precision. Continuous measurements of methane near a sewer and in the atmosphere were performed to demonstrate the stability and robustness of the highly integrated mini-MPC-based gas sensor. Our analysis shows that a methane minimum detectable limit of 117 ppbv is achieved, paving the way toward a sensitive, low-cost, and miniature trace gas sensor inherently suitable for large-scale deployment of distributed sensor networks and for handheld mobile devices.

Measuring the spatial deformation of a liquid-crystal on silicon display with a self-interference effect.

We present a simple technique to characterize the spatial non-uniformity of a liquid-crystal on silicon (LCOS) spatial light modulator (SLM). It is based on illuminating the display with a wavelength out of the operation range, so there is a significant reflection at the output surface. As a consequence, a Gires-Tournois interferometer is directly created, without any alignment requirement and insensitive to vibrations. The beam reflected at the output surface is the reference beam, while the beam reflected at the silicon backplane is modulated with the addressed gray level in order to quantitatively derive its deformation. We provide an experimental demonstration using a LCOS-SLM designed to operate in the near-infrared range but illuminated with visible light.

Translationally invariant metamirrors for spatial filtering of light beams

We propose a translationally invariant metamirror for light filtering in reflection. The device consists of a thin transverse grating in the micron scale positioned in front of a flat mirror. We analyze the performance of such a photonic metamirror and find a modification of the angular spectrum of the reflected beams, leading to a significant reduction of the beam divergence for particular configurations. This study points toward a type of optical component, a flat metamirror with a filtering functionality and no alignment requirements, due to its translational invariance.