Spatial Coherence Research Articles

Reliability of simplified wind fields and structure models for the response simulation of parked offshore wind turbines

Over the past few decades, offshore wind turbines (OWTs) have been popular in the wind engineering and renewable energy fields. Numerous studies have investigated the dynamic responses of OWTs by numerical simulation. However, the different numerical models gave different results, making it difficult to compare and select models for different design and research purposes. In this study, six models including the main wind field and structure model were built based on the National Renewable Energy Laboratory 5-MW OWT, and the dynamic responses were simulated to compare and quantify the effects of the models on the response of the parked OWT. The results show that a uniform distribution of the wind field is too conservative for estimating the dynamic responses of the parked OWT. The structural response calculated using a uniformly distributed wind field is twice as large as those with the spatial distribution and coherence of the wind speed. The wind profile significantly affected the response of the parked OWT, particularly on the standard deviation of the displacement. When estimating the dynamic response of the tower of a parked OWT, the simplified model, in which the blades and wind load are simplified as a concentrated mass and load, is relatively reliable. The maximum displacement at the tower top of the simplified model is close to that of the detailed model. However, a detailed model is necessary to obtain the blade responses and accurate power spectral density of the structural response. In addition, the wind-field model significantly affected the spectral characteristics of the dynamic response of the blades.

Longitudinal spatial coherence of computational ghost imaging through atmospheric turbulence

The resolution and imaging quality of ghost imaging is determined by the longitudinal spatial coherence (LSC) of speckle beams on the signal and reference arms. Based on the cross-correlation function, long-exposure and short-exposure computational ghost imaging through turbulent atmosphere is investigated analytically and numerically in the framework of the traditional imaging theory. According to the point spread function (PSF), the modulation transfer function (MTF) is derived, both of which are utilized to evaluate imaging resolution and imaging quality of computational ghost imaging (CGI), respectively. By simulating long-exposure and short-exposure ghost imaging through atmospheric turbulence, the comprehensive effects of atmospheric turbulence and beam initial parameters on the complex degree of coherence (CDC), PSF, and MTF are studied, respectively. It is found that the degradation of LSC between the two planes on the reference and signal path implies a narrower PSF and increased MTF values, which represent the better resolution and imaging quality. Thus, reducing the atmospheric turbulence strength, the speckle particle size, the wavelength and the propagation distance, and increasing the source size contribute to improving resolution and image quality of CGI because of the degradation of LSC. Furthermore, short-exposure CGI can provide imaging performance superior to long-exposure CGI in terms of resolution and imaging quality due to the decrease of LSC.

A coherence-improved and mass-balanced inflow turbulence generation method for large eddy simulation

Realistic inflow turbulence is crucial in large eddy simulations for obtaining accurate results. In this paper, an improved inflow turbulence generation method is proposed, called the coherence-improved and mass-balanced random flow generation (CMRFG) method. First, a study of the effect of the inlet mass-balanced condition is presented. Subsequently, the proposed method is explicitly deduced to satisfy arbitrary spatial coherence functions and the mass-balanced condition, so the generated turbulent fields effectively preserve the original turbulence characteristics, eliminating the need for additional inlet mass flux corrections. Meanwhile, the novel method retains the advantages of the previous method, satisfying arbitrary turbulence intensities, temporal spectra, temporal correlations, and spatial correlation coefficients. Additionally, the proposed method also ensures compliance with the divergence-free condition and Taylor's frozen hypothesis, which are essential for accurate flow field development. Finally, large eddy simulations of both homogeneous isotropic and anisotropic turbulent fields demonstrate that the turbulent fields generated by CMRFG closely align with experimental results and exhibit no artificial pressure fluctuations within the computational domain.

Directly constraining the spatial coherence of thez∼ 1 circumgalactic medium

One of the biggest puzzles regarding the circumgalactic medium (CGM) is the structure of its cool (T ∼ 104K) gas phase. While the kinematics of quasar absorption systems suggests the CGM is composed of a population of different clouds, constraining their extent and spatial distribution has proven challenging, both from theoretical and observational points of view. In this work, we study the spatial structure of thez ∼ 1 CGM with unprecedented detail via resolved spectroscopy of giant gravitational arcs. We put together a sample of Mg IIλλ2796, 2803 detections obtained with VLT/MUSE in 91 spatially independent and contiguous sight lines toward 3 arcs, each probing an isolated star-forming galaxy believed to be detected in absorption. We constrain the coherence scale of this gas (Clength) – which represents the spatial scale over which the Mg IIequivalent width (EW) remains constant – by comparing EW variations measured across all sight lines with empirical models. We find 1.4 < Clength/kpc < 7.8 (95% confidence). This measurement, of unprecedented accuracy, represents the scale over which the cool gas tends to cluster in separate structures. We argue that, ifClengthis a universal property of the CGM, it needs to be reproduced by current and future theoretical models in order for us to understand the exact role of this medium in galaxy evolution.

High-Frame-Rate Volumetric Porcine Renal Vasculature Imaging.

The aim of this study was to assess the feasibility and imaging options of contrast-enhanced volumetric ultrasound kidney vasculature imaging in a porcine model using a prototype sparse spiral array. Transcutaneous freehand in vivo imaging of two healthy porcine kidneys was performed according to three protocols with different microbubble concentrations and transmission sequences. Combining high-frame-rate transmission sequences with our previously described spatial coherence beamformer, we determined the ability to produce detailed volumetric images of the vasculature. We also determined power, color and spectral Doppler, as well as super-resolved microvasculature in a volume. The results were compared against a clinical 2-D ultrasound machine. Three-dimensional visualization of the kidney vasculature structure and blood flow was possible with our method. Good structural agreement was found between the visualized vasculature structure and the 2-D reference. Microvasculature patterns in the kidney cortex were visible with super-resolution processing. Blood flow velocity estimations were within a physiological range and pattern, also in agreement with the 2-D reference results. Volumetric imaging of the kidney vasculature was possible using a prototype sparse spiral array. Reliable structural and temporal information could be extracted from these imaging results.

Unveiling detection probability for multi-Gaussian correlated anomalous vortex modes in maritime atmospheric turbulence

In this study, we employ the Rytov approximation to investigate the detection probability of orbital angular momentum (OAM) in multi-Gaussian correlated anomalous vortex (MGCAV) beams under non-Kolmogorov maritime atmospheric turbulence. Our results demonstrate that the OAM detection probability of a MGCAV beam is influenced by various factors, including beam parameters and the characteristics of maritime atmospheric turbulence. Specifically, an increase in propagation distance, beam order, and beam index, or a decrease in inner scale, spatial coherence width, and non-Kolmogorov parameter, leads to a decrease in the OAM detection probability. The phase characteristics of partially coherent vortex modes are affected by both atmospheric turbulence phase and initial random phase, resulting in reduced robustness compared to fully coherent vortex modes. Furthermore, a comparative analysis between Gaussian–Schell correlated anomalous vortex (GSCAV) beams and MGCAV beams reveals the superior resilience of GSCAV beams in mitigating the impact of maritime atmospheric turbulence. Moreover, specific combinations of beam order, topological charge, and beam waist, or the optimal beam width, yield maximum OAM detection probability or minimum scintillation. These findings provide valuable insights applicable to optical communication, particularly in scenarios above sea and ocean levels.

Scalable Printing of Metal Nanostructures through Superluminescent Light Projection.

Direct printing of metallic nanostructures is highly desirable but current techniques cannot achieve nanoscale resolutions or are too expensive and slow. Photoreduction of solvated metal ions into metallic nanoparticles is an attractive strategy because it is faster than deposition-based techniques. However, it is still limited by the resolution versus cost tradeoff because sub-diffraction printing of nanostructures requires high-intensity light from expensive femtosecond lasers. Here, this tradeoff is overcome by leveraging the spatial and temporal coherence properties of low-intensity diode-based superluminescent light. The superluminescent light projection (SLP) technique is presented to rapidly print sub-diffraction nanostructures, as small as 210nm and at periods as small as 300nm, with light that is a billion times less intense than femtosecond lasers. Printing of arbitrarily complex 2D nanostructured silver patterns over 30µm×80µm areas in 500ms time scales is demonstrated. The post-annealed nanostructures exhibit an electrical conductivity up to 1/12th that of bulk silver. SLP is up to 480 times faster and 35 times less expensive than printing with femtosecond lasers. Therefore, it transforms nanoscale metal printing into a scalable format, thereby significantly enhancing the transition of nano-enabled devices from research laboratories into real-world applications.

Array Configuration-Agnostic Personal Voice Activity Detection Based on Spatial Coherence

Personal voice activity detection (PVAD) has received increased attention due to the growing popularity of personal mobile devices and smart speakers. PVAD is often an integral element to speech enhancement and recognition for these applications in which lightweight signal processing is only enabled for the target user. However, in real-world scenarios, the detection performance may degrade because of competing speakers, background noise, and reverberation. To address this problem, we proposed to use equivalent rectangular bandwidth (ERB)-scaled spatial coherence as the input feature to train an array configuration-agnostic PVAD (ARCA-PVAD) network. Whereas the network model requires only 112k parameters, it exhibits excellent detection performance and robustness in adverse acoustic conditions. Notably, the proposed ARCA-PVAD system is scalable to array configurations (geometry, number of microphones, and spacing). Experimental results have demonstrated the superior performance achieved by the proposed ARCA-PVAD system over a baseline in terms of the area under receiver operating characteristic curve (AUC) and equal error rate (EER).

Spatial resolution and reconstructed size accuracy using advanced beamformers in linear array-based PAT systems

Limitations associated with linear-array probes in photoacoustic tomography are partially compensated by using advanced beamformers that exploit the temporal and spatial coherence of the recorded signals, such as Delay Multiply and Sum (DMAS), Minimum Variance (MV) or coherence factor (CF), among others. However, their associated signal processing leads to an overestimation of the spatial resolution, as well as alterations in the reconstructed object size. Numerical and experimental results reported here support this hypothesis. First, we show that the Rayleigh criterion (RC) is the most suitable choice to characterize the spatial resolution instead of the Point Spread Function (PSF) when considering advanced beamformers. Then, we observe that several advanced beamformers fail to properly reconstruct target sizes slightly above the spatial resolution, underestimating their size. This work sheds light on the suitability of this type of beamformers combined with linear probes for determining sizes and morphology in photoacoustic images.

A micro random laser of dye solution-filled tube system based on electrospun fibers

Random lasers (RLs) have good application prospects in speckle-free imaging, micro sensing and micro-nano photonic devices due to their properties such as cavityless, miniaturization and low-spatial coherence. The minimization, emission mechanism and lasing property control of RLs still need to be explored. In this paper, we present a novel micro RL system in a dye solution-filled tube, where the coherent random lasing action is facilitated by the integration of electrospun fibers and gold nanoparticles (Au NPs). The electrospun fibers serve as the scattering medium, while the Au NPs produce the local plasmon resonance effect. The interaction effect improves the photon scattering rate, makes the photons form a ring resonator in the system, and realizes the coherent RL in the tube system. The microtube RL has a very small size, a convenient preparation method and stable emission properties. It can withstand more than 2000 times pumping while maintaining a stable emission intensity. The electrospun fibers are stable and uniform in morphology, which makes it easy to achieve mass production of microtube RL. The speckle-free imaging experiment shows the RL has low spatial coherence, which indicates that it’s a good choice for micro speckle-free imaging. This work provides a convenient way to realize micro coherent RLs by electrospun fibers and could pave a way for applications in photonic devices.

Improving optical coherence of light-emitting diodes by surface plasmons via shallow-etched conic pit array.

We propose the coupling of multiple quantum wells and surface plasmons can improve coherence of light emitted from LED wafers, as evidenced herein by a shallow-etched conic pit array with evaporated Ag (V-Ag) on a GaN-based LED wafer. The improvement in spatial coherence is critically verified by angle-resolved spectra. The temporal coherence length of the V-Ag wafer is 1.4 times larger than that of the plain wafer. The coherence-enhanced wafer achieves anisotropic and deflective emission in micro area and at far field by diffraction. This research provides a novel perspective on research of plasmonic LEDs and a new straightforward architecture to acquire partially coherent light from LEDs.

Investigation of longitudinal spatial coherence induced by a partially spatially coherent source

In this study, we report longitudinal spatial coherence (LSC) generated from an extended partially spatially coherent source. This treatment differs from traditional LSC studies, which consider the source to be strictly spatially incoherent. We found that the higher transverse spatial coherence (TSC) of the source resulted in a smaller longitudinal coherence length with strong modulations. In addition, the influence of propagation distance is studied through the position of the reference plane, which was not apparent in conventional LSC formulations. The result of the assumption of TSC and propagation distance is reduced LSC contrast, which may affect the quality of the resulting images. The spatial correlation between two transverse points on different planes is demonstrated to compensate for the degradation in longitudinal coherence caused due to some misalignment. Understanding the effects of parameters such as propagation distance and partial coherence of the source can help in designing optical systems to achieve optimal LSC contrast in optical imaging modalities.

Random lasing in dye-doped electrospun PMMA fibers with different emission modes

Random lasers (RLs) have significant applications in micro-nano light emitters, micro-detection, and speckle-free imaging due to their novel properties in low spatial coherence, small size and multi-emission directions. A flexible, affordable approach for creating new scattering materials has recently been suggested for electrospun fibers, which are practical materials for RLs. In this article, we employ the electrospun PMMA fibers as scatters and the DCJTB as gain materials to generate an RL system. The sample shows obvious incoherent random lasing properties. A decrease of the lasing threshold by 32 % is realized when we increase the electrospun voltage from 12 kV to 15 kV due to the generation of a small mean diameter size and concentrated diameter distribution. The lasing threshold of the DCJTB/Au NPs-doped PMMA sample is further reduced to 19.8 μJ/mm2 by incorporating Au NPs into the electrospun dye-doped fibers. This leads to a lasing threshold reduction of 40 % from the electrospun sample without doping Au NPs and 52 % from the spin-coated film sample, respectively. It indicates that the electrospun method is an efficient and practical approach to make RLs. Furthermore, using single pulse laser stimulation, we investigated several different lasing modes, including spontaneous emission, short wavelength domain, mode competition, and single mode. The actions of photon scattering are analyzed to explain the dramatic lasing process. Due to the photon scattering optical path, there are mode competition and single mode emission regions under different pump energy. The light spot quality of RL shows low spatial coherence with the useful application of speckle-free imaging. This research provides a new method of electrospun to produce low-threshold and high-quality RLs and potentially opens the door for expanding their use in optics and optoelectronic equipment.

Robust speckle-free imaging using random lasers enhanced by TiN nanoparticles in complex scattering environments

Abstract A light source with narrowband, sufficient brightness, and low spatial coherence is required for certain applications such as optical imaging and free-space optical communication. In this study, our focus was to investigate a novel imaging laser source, specifically a low-threshold random laser enhanced by TiN nanoparticles. The results demonstrate that the random laser spectrum exhibits an impressive bandwidth of 0.23 nm, accompanied by an incredibly low spatial coherence factor of merely 0.15. Due to the low spatial coherence of random laser, the speck contrast is less than 0.02 when the light passes through a scattering system. Notably, when compared to traditional lasers, the use of a random laser yields significantly superior imaging quality in both scatterless and complex scattering environments. This finding highlights the immense potential of the random laser as a narrowband and low spatial coherence laser source for robust speckle-free imaging applications, particularly in environments with intricate scattering phenomena. Furthermore, this breakthrough can be extended to various other domains, including free-space optical communication.

Cross-spectral purity of the Stokes parameters in random nonstationary electromagnetic beams.

We consider cross-spectral purity in random nonstationary electromagnetic beams in terms of the Stokes parameters representing the spectral density and the spectral polarization state. We show that a Stokes parameter being cross-spectrally pure is consistent with the property that the corresponding normalized time-integrated coherence (two-point) Stokes parameter satisfies a certain reduction formula. The current analysis differs from the previous works on cross-spectral purity of nonstationary light beams such that the purity condition is in line with Mandel's original definition. In addition, in contrast to earlier works concerning the cross-spectral purity of the polarization-state Stokes parameters, intensity-normalized coherence Stokes parameters are applied. It is consequently found that in addition to separate spatial and temporal coherence factors the reduction formula contains a third factor that depends exclusively on polarization properties. We further show that cross-spectral purity implies a specific structure for electromagnetic spectral spatial correlations. The results of this work constitute foundational advances in the interference of random nonstationary vectorial light.

Typological diversity and morphological continuity in the modern residential fabric: The case of Ankara,Turkey

Each city has a unique fabric corresponding to the dominant modes of production, legal control, and guidance systems in which it operates. Any periodic alteration in the settled mode of housing supply, in this sense, potentially results in an intrinsic change in ‘typological diversity’ and ‘spatial continuity’ within the collective fabric. If not coordinated by design and development control, the emergent variation in the housing typology may result in spatial fragmentation through the collective urban fabric. To test that point, the paper aims to reveal the subtle relationship between the two phenomena on a morphological basis. Through rapidly changing socio-economic dynamics in the last century, the cities of Turkey are subject to different housing production forms, therefore, offering a relevant context to examine the issue.Along with a planning system without effective development control tools to ensure spatial coherence responding to the dynamic nature of the housing sector, the residential fabric of Ankara comprises all the dominant housing typologies that emerged within different periods in Turkey. Accordingly, following a historical review of the housing supply forms in Turkey, the paper maps the emerging patterns of modern housing typologies through successive development zones of the city. It examines their internal typomorphological characteristics via a series of transects. Utilizing the GIS-based coherency analysis, the level of morphological continuity on each transect is calculated. Consequently, in light of the findings of the analysis, a critical perspective on housing production and development control creating different forms of spatial fragmentation through typological variation is suggested.

A Spectral Wave Model for Inhomogeneous Water Wave Fields Using the Quasi-Coherent Theory

A numerical stochastic wave model was developed in this study based on the quasi-coherent theoretical framework proposed by Smit and Janssen in 2013. Subsequently, the model was implemented to reproduce and cross-confirm the findings of the quasi-coherent (QC) spectral wave modeling approach. The process included simulations of experiments conducted by Vincent and Briggs regarding waves propagating over a submerged shoal. The results of the simulations agree with the expected results of the QC theory, which can account for the spatial coherence of inhomogeneous wave fields and capture wave interference more accurately than conventional spectral wave models. In addition, extra insight was gained about aspects of the overall numerical implementation of the QC theory.

A multimodal pipeline for image correction and registration of mass spectrometry imaging with microscopy

The integration of matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI MSI) and histology plays a pivotal role in advancing our understanding of complex heterogeneous tissues, which provides a comprehensive description of biological tissue with both wide molecule coverage and high lateral resolution. Herein, we proposed a novel strategy for the correction and registration of MALDI MSI data with hematoxylin & eosin (H&E) staining images. To overcome the challenges of discrepancies in spatial resolution towards the unification of the two imaging modalities, a deep learning-based interpolation algorithm for MALDI MSI data was constructed, which enables spatial coherence and the following orientation matching between images. Coupled with the affine transformation (AT) and the subsequent moving least squares algorithm, the two types of images from one rat brain tissue section were aligned automatically with high accuracy. Moreover, we demonstrated the practicality of the developed pipeline by projecting it to a rat cerebral ischemia-reperfusion injury model, which would help decipher the link between molecular metabolism and pathological interpretation towards microregion. This new approach offers the chance for other types of bioimaging to boost the field of multimodal image fusion.

Harnessing the magic of light: spatial coherence instructed swin transformer for universal holographic imaging

Harnessing the magic of light: spatial coherence instructed swin transformer for universal holographic imaging

Synchronized Structure and Teleconnection Patterns of Meteorological Drought Events over the Yangtze River Basin, China

Investigating the synchronized structure and teleconnection patterns of meteorological drought events (MDEs) contributes to elucidating drought’s evolution. In this study, the CN05.1 gridded meteorological dataset from 1961 to 2021 was utilized to calculate the 3-month standardized precipitation evapotranspiration index (SPEI-3) for each grid in the Yangtze River Basin (YRB). Based on these SPEI-3 series, the grid-based MDEs were then extracted. Subsequently, event synchronization and complex networks were employed to construct the MDE synchronized network over the YRB. This network was used to identify the MDEs’ topological structure, synchronized subregions, and representative grids. Finally, the MDE characteristics and MDE teleconnection patterns of individual subregions were investigated. The results of the MDE topological structure show that the northeastern portion of the YRB tends to experience widespread MDEs, while specific areas in the upper reaches are prone to localized MDEs. Synchronous MDEs mainly propagate along the central pathway and the eastern pathway, which display relatively low MDE spatial coherence. The YRB is partitioned into eight MDE synchronized subregions, each exhibiting distinct characteristics in terms of the frequency, duration, total severity, and peak of MDEs, as well as MDE temporal frequency distributions. Among all teleconnection factors, El Niño–Southern Oscillation (ENSO) exerts a strong influence on MDEs in all subregions, the Pacific Decadal Oscillation (PDO) shows a significant association with MDEs in all subregions except for Subregion 3 in the southeast, the North Atlantic Oscillation (NAO) displays a significant influence on MDEs in the southern subregions of the YRB, and the Arctic Oscillation (AO) has a more pronounced influence on MDEs in the northern subregions. This study provides valuable insights on drought’s evolution within the YRB and offers guidance to policymakers for advanced preventive measures.