Low-pass Spatial Filter Research Articles

1D Confocal Broad Area Semiconductor Lasers (Confocal BALs) for Fundamental Transverse Mode Selection (TMS#0)

Previously in this journal we have reported on fundamental transverse mode selection (TMS#0) of broad area semiconductor lasers (BALs) with integrated twice-retracted 4f set-up and film-waveguide lens as the Fourier-transform element. Now we choose and report on a simpler approach for BAL-TMS#0, i.e., the use of a stable confocal longitudinal BAL resonator of length L with a transverse constriction. The absolute value of the radius R of curvature of both mirror-facets convex in one dimension (1D) is R = L = 2f with focal length f. The round trip length 2L = 4f again makes up for a Fourier-optical 4f set-up and the constriction resulting in a resonator-internal beam waist stands for a Fourier-optical low-pass spatial frequency filter. Good TMS#0 is achieved, as long as the constriction is tight enough, but filamentation is not completely suppressed.

$4\times8$ Patch Array-Fed FR4-Based Transmit Array Antennas for Affordable and Reliable 5G Beam Steering

This paper presents a novel method of designing affordable 28-GHz transmit array antennas utilizing FR4 substrates, which are low-cost but lossy. It is demonstrated that low insertion loss can be achieved by employing appropriate combinations of spatial filter unit cells, where each unit cell is selected to minimize the loss factors defined by lossy spatial filter modeling. The loss factor with inter-layer couplings was found to be more variable than that without inter-layer couplings, although inter-layer couplings have previously been utilized to increase the tunable range of the phase shift. Therefore, the number of metal layers in the low-pass spatial filter more affected by the inter-layer coupling is selected to be less than the number of metal layers used in the bandpass spatial filter for a given thickness in the proposed method. In addition, a novel transmits array in which some of the unit cells are sinusoidally arranged is described. This can achieve up to 1.6-dB gain enhancement at some steered angles compared with the conventional design. To simulate transmit arrays rapidly, an effective simple medium structure representing a transmit array is presented. Finally, the measured results confirm the effectiveness of the proposed design approaches for affordable transmit array antennas. Only a small difference of 0.8 dB between the simulated and measured results confirms the successful manipulation of the lossy characteristic of FR4.

Design and fabrication of multilayer dielectric film for combination device spatial filter

Spatial low-pass filters are of great importance to high-power laser systems. Different from the conventional multilayer dielectric film spatial filter device that consists of single component, a combination device of spatial filter that includes multiple film components is more appealing due to its higher laser-induced damage threshold. A novel multilayer dielectric film used for combination device spatial filter is presented in this paper. The sensitivity of the multilayer dielectric film was analyzed, and the main studies are given to the design, fabrication and performance analysis of cutoff filter-combination device. The laser-induced damage threshold of the multilayer dielectric films was tested. It is anticipated that the combination of two cutoff filters can be applied in practical applications of devices of high-power laser systems.

Extraction of Blood Vessels in Retina

This article presents a novel method to extract retinal vascular tree automatically. The proposed method consists of four steps; smoothing image using low pass spatial filter to reduce spurious noise in the image; extracting candidate borders of the vessels based on a local window property; tracking process, starting with a candidate pixel and following in the optimum direction with monitoring the connectivity of the vessel twin border; constructing the whole tree of retinal blood vessels by connecting the vessel segments based on their spatial locations, widths and directions. The algorithm was trained with 20 images from the DRIVE dataset, and tested using the remaining 20 images.

Upslope propagation of low frequency deep ocean signals

Over the period 18–21 March, 2017, 1-hour waveforms in the 60–120 Hz band were transmitted from a HLF-6A source deployed at 300 m depth in the deep northeast Pacific Ocean. These transmissions were recorded by two vertical hydrophone line arrays at various ranges in the deep ocean, GPS-equipped sonobuoys deployed on the continental shelf, and a bottom-mounted hydrophone in 900-m water at the western edge of Monterey Bay operated by the Monterey Bay Aquarium and Research Institute (MBARI). Although the source-receiver ranges to the sonobuoys and the MBARI hydrophone were approximately the same, the bathymetry profile up the continental slope to the sonobuoy location was significantly steeper than to the MBARI hydrophone. Only the upper part of the frequency band, above 90–95 Hz, is received with good signal-to-noise ratio, illustrating the high-pass temporal filtering of upslope propagation. Upslope propagation also acts as a low-pass spatial filter, allowing only lower-order modes to propagate onto the shelf. Numerical modeling is used to examine the predictability of the measured travel times and multipath arrival structure.

3D spatially-adaptive canonical correlation analysis: Local and global methods

Local spatially-adaptive canonical correlation analysis (local CCA) with spatial constraints has been introduced to fMRI multivariate analysis for improved modeling of activation patterns. However, current algorithms require complicated spatial constraints that have only been applied to 2D local neighborhoods because the computational time would be exponentially increased if the same method is applied to 3D spatial neighborhoods.In this study, an efficient and accurate line search sequential quadratic programming (SQP) algorithm has been developed to efficiently solve the 3D local CCA problem with spatial constraints. In addition, a spatially-adaptive kernel CCA (KCCA) method is proposed to increase accuracy of fMRI activation maps. With oriented 3D spatial filters anisotropic shapes can be estimated during the KCCA analysis of fMRI time courses. These filters are orientation-adaptive leading to rotational invariance to better match arbitrary oriented fMRI activation patterns, resulting in improved sensitivity of activation detection while significantly reducing spatial blurring artifacts. The kernel method in its basic form does not require any spatial constraints and analyzes the whole-brain fMRI time series to construct an activation map. Finally, we have developed a penalized kernel CCA model that involves spatial low-pass filter constraints to increase the specificity of the method.The kernel CCA methods are compared with the standard univariate method and with two different local CCA methods that were solved by the SQP algorithm. Results show that SQP is the most efficient algorithm to solve the local constrained CCA problem, and the proposed kernel CCA methods outperformed univariate and local CCA methods in detecting activations for both simulated and real fMRI episodic memory data.

Effect of Cycloplegia on Blur Perception Thresholds as Measured by Source Method

Abstract One of the advantages of a source method over observer method in blur perception measurements is better control of a stimulus blur level, which is achieved with computerised image processing unlike the observer method that requires optical defocusing of the observer. The aim of this study was to determine if paralysation of accommodation has effect on blur perception thresholds, thereby evaluating its necessity in such experiments. Blur perception thresholds (just noticeable blur, clear image, recognition, and non-resolvable blur thresholds) were evaluated with (using cycloplegia) and without paralysed accommodation to determine effect on blur perception. A computerised low-pass spatial frequency filter was used to process images of Landolt rings and gradually increase the blur level from the stage of clear image to the stage of unrecognisable orientation of Landolt ring opening. Ascending and descending psychophysical methods were used to evaluate the previously mentioned blur perception thresholds. Results for 10 participants revealed no significant effect of cycloplegia on any of the determined blur perception thresholds. Therefore, it may be considered that blur perception measurements with source method can be done without cycloplegia.

ESTIMATION OF PHASE DELAY DUE TO PRECIPITABLE WATER FOR DINSARBASED LAND DEFORMATION MONITORING

Abstract. In this paper, we present a method for using the estimated precipitable water (PW) to mitigate atmospheric phase delay in order to improve the accuracy of land-deformation assessment with differential interferometric synthetic aperture radar (DInSAR). The phase difference obtained from multi-temporal synthetic aperture radar images contains errors of several types, and the atmospheric phase delay can be an obstacle to estimating surface subsidence. In this study, we calculate PW from external meteorological data. Firstly, we interpolate the data with regard to their spatial and temporal resolutions. Then, assuming a range direction between a target pixel and the sensor, we derive the cumulative amount of differential PW at the height of the slant range vector at pixels along that direction. The atmospheric phase delay of each interferogram is acquired by taking a residual after a preliminary determination of the linear deformation velocity and digital elevation model (DEM) error, and by applying high-pass temporal and low-pass spatial filters. Next, we estimate a regression model that connects the cumulative amount of PW and the atmospheric phase delay. Finally, we subtract the contribution of the atmospheric phase delay from the phase difference of the interferogram, and determine the linear deformation velocity and DEM error. The experimental results show a consistent relationship between the cumulative amount of differential PW and the atmospheric phase delay. An improvement in land-deformation accuracy is observed at a point at which the deformation is relatively large. Although further investigation is necessary, we conclude at this stage that the proposed approach has the potential to improve the accuracy of the DInSAR technique.

The Characteristics of Significant Wave Height and Sea Surface Temperature In The Sunda Strait

Sunda Strait has an important role in the water mass exchange from the Pacific Ocean and the Indian Ocean so that the oceanographic condition is strongly affected by seasonal factors. The purpose of this study is to observe the relationship and the characteristics of significant wave height (SWH) and sea surface temperature (SST) in the Sunda strait and its relationship with IOD. The method employed is spatial analysis, low-pass filter, and spectrum analysis by S-Transform, beside that the correlation between SST and SWH is analyzed by wavelet coherency. The period of SWH and SST is dominantly semiannual, at the time of winter monsoon (the Northeast Monsoon), the SWH was reaching up to 2,11 m, while at the summer monsoon, the SWH was reaching up to 3.62 m. Reversely, the SST increased during the winter monsoon. At the time of 2016 had been detecting by the negative IOD with the IOD index of -0.65 and it caused the SWH increased by 0.3 m than its average. Based on the wavelet coherence, the SWH and the SST have the coherence in the period of 8 to 16 days, especially in March to April, and June to August.

Diffraction phase microscopy realized with an automatic digital pinhole

We report a novel approach to diffraction phase microscopy (DPM) with automatic pinhole alignment. The pinhole, which serves as a spatial low-pass filter to generate a uniform reference beam, is made out of a liquid crystal display (LCD) device that allows for electrical control. We have made DPM more accessible to users, while maintaining high phase measurement sensitivity and accuracy, through exploring low cost optical components and replacing the tedious pinhole alignment process with an automatic pinhole optical alignment procedure. Due to its flexibility in modifying the size and shape, this LCD device serves as a universal filter, requiring no future replacement. Moreover, a graphic user interface for real-time phase imaging has been also developed by using a USB CMOS camera. Experimental results of height maps of beads sample and live red blood cells (RBCs) dynamics are also presented, making this system ready for broad adaption to biological imaging and material metrology.

Decomposition of the Nodal Conductance Matrix of a Planar Resistive Grid and Derivation of Its Eigenvalues and Eigenvectors Using the Kronecker Product and Sum With Application to CNN Image Filters

It is shown that an (M × N)-node planar resistive grid can be decomposed into two sub-grids; one made up of M N-node horizontal and the other of N M-node vertical linear resistive grids which corresponds to decomposing its nodal conductance matrix (NCM) into the Kronecker sum of the NCMs of horizontal and vertical linear grids. This enables the analytical expressions of the eigenvalues and eigenvectors of the NCMs of the sub-grids as well as those of the planar resistive grid to be expressed in terms of those of the two linear grids, whose analytical expressions are well known. For a Cellular Neural Network (CNN) Gabor-type filter (GTF) we define generalized nodal conductance matrices (GNCMs) that correspond to the NCMs of the resistive sub-grids, show that each Kronecker decomposition has a counterpart in CNN GTF and prove that each GNCM, its counterpart NCM and the corresponding temporal state matrices are related through unitary diagonal similarity transformations. Consequently, we prove that the eigenvalues of the temporal state matrix of a spatial band-pass CNN GTF are the same as those of its counterpart spatial low-pass CNN image filter, hence their temporal transient behaviors are similar in settling to a forced response.

Anti-aliasing filters for deriving high-accuracy DEMs from TLS data: A case study from Freeport, Texas

Terrestrial laser scanning (TLS), also known as ground-based Light Detection and Ranging (LiDAR), has been frequently applied to build bare-earth digital elevation models (DEMs) for high-accuracy geomorphology studies. The point clouds acquired from TLS often achieve a spatial resolution at fingerprint (e.g., 3cm×3cm) to handprint (e.g., 10cm×10cm) level. A downsampling process has to be applied to decimate the massive point clouds and obtain manageable DEMs. It is well known that downsampling can result in aliasing that causes different signal components to become indistinguishable when the signal is reconstructed from the datasets with a lower sampling rate. Conventional DEMs are mainly the results of upsampling of sparse elevation measurements from land surveying, satellite remote sensing, and aerial photography. As a consequence, the effects of aliasing caused by downsampling have not been fully investigated in the open literature of DEMs. This study aims to investigate the spatial aliasing problem of regridding dense TLS data. The TLS data collected from the beach and dune area near Freeport, Texas in the summer of 2015 are used for this study. The core idea of the anti-aliasing procedure is to apply a low-pass spatial filter prior to conducting downsampling. This article describes the successful use of a fourth-order Butterworth low-pass spatial filter employed in the Generic Mapping Tools (GMT) software package as an anti-aliasing filter. The filter can be applied as an isotropic filter with a single cutoff wavelength or as an anisotropic filter with two different cutoff wavelengths in the X and Y directions. The cutoff wavelength for the isotropic filter is recommended to be three times the grid size of the target DEM.

Numerical Investigation of Flow Control for a Flat-Window Cylindrical Turret

This work presents multiple high-fidelity large-eddy simulations of flow over a cylindrical turret with a flat window oriented at two angles, 90 and 100 deg. For the 100 deg case, additional computations were performed to investigate the effectiveness of multiple types of flow control including rows of short pins, tall pins, and steady blowing wall jets inserted upstream of the turret as well as a steady suction slot at the leading edge of the flat window. The large-eddy simulation computations were obtained using a well-validated high-order Navier–Stokes flow solver employing a sixth-order compact spatial discretization in conjunction with an eighth-order low-pass spatial filter. Overall, large-eddy simulation solutions compared favorably to experimental time mean and fluctuating velocity profiles as well as the general flow structure at both angles. Additionally, valuable insight was obtained on how rows of pins, blowing jets, and steady slot suction control flow separation. Slot suction at the upwind aperture lip was determined to be more effective than rows of pins and blowing jets in controlling flow separation, which is critical in reducing aero-optical aberrations. Steady slot suction was capable of eliminating massive separation at a larger look angle of 120 deg.

Analysis of low-pass filters for approximate deconvolution closure modelling in one-dimensional decaying Burgers turbulence

The idea of spatial filtering is central in approximate deconvolution large-eddy simulation (AD-LES) of turbulent flows. The need for low-pass filters naturally arises in the approximate deconvolution approach which is based solely on mathematical approximations by employing repeated filtering operators. Two families of low-pass spatial filters are studied in this paper: the Butterworth filters and the Padé filters. With a selection of various filtering parameters, variants of the AD-LES are systematically applied to the decaying Burgers turbulence problem, which is a standard prototype for more complex turbulent flows. Comparing with the direct numerical simulations, it is shown that all forms of the AD-LES approaches predict significantly better results than the under-resolved simulations at the same grid resolution. However, the results highly depend on the selection of the filtering procedure and the filter design. It is concluded that a complete attenuation for the smallest scales is crucial to prevent energy accumulation at the grid cut-off.

基于截止滤光片的光束空间低通滤波技术研究

基于截止滤光片的光束空间低通滤波技术研究

非聚焦型光束空间低通滤波技术研究进展

非聚焦型光束空间低通滤波技术研究进展

Sensitivity of near-inertial internal waves to spatial interpolations of wind stress in ocean generation circulation models

The oceanic near-inertial internal waves (NIWs) have been extensively studied using ocean general circulation models (OGCMs). Currently most OGCMs use the bilinear or bicubic interpolation to interpolate wind stress onto models’ surface grids. In this study, we examine the influences of bilinear and bicubic interpolations on the wavenumber and frequency spectra of wind stress and on the simulated NIWs in the ocean. It is demonstrated that both the bilinear and bicubic interpolations are equivalent to spatial low-pass filters with the former leading to more significant loss of wind stress variance at high wavenumbers. When coarse (e.g., 2°) wind stress is used to force OGCMs, the bilinear and bicubic interpolations significantly damp the wavenumber spectrum of wind stress at mesoscales, leading to decreased near-inertial wind stress variance. Using the bilinear (bicubic) interpolation could weaken the near-inertial wind work by ∼43% (22%) in the subtropical region (10°N–30°N) and by ∼16% (4%) at the midlatitudes (30°N–50°N).We propose a new interpolation method, i.e., the bi-sinc-function interpolation, which is able to retain all the wind stress variance within the Nyquist wavenumber. Compared to the bilinear and bicubic interpolations, the bi-sinc-function interpolation improves the simulations of NIWs and should be incorporated into OGCMs especially when coarse wind stress is used.

Hydrologically Aided Interpolation of Daily Precipitation and Temperature Fields in a Mesoscale Alpine Catchment

Abstract Hydrological modeling in mountainous regions, where catchment hydrology is heavily influenced by snow (and possibly ice) processes, is a challenging task. The intrinsic complexity of local processes is added to the difficulty of estimating spatially distributed inputs such as precipitation and temperature, which often exhibit a high spatial heterogeneity that cannot be fully captured by measurement networks. Hence, an interpolation step is often required prior to the hydrological modeling step. Usually, the reconstruction of meteorological forcings and the calibration of the hydrological model are done sequentially. The outputs of the hydrological model (discharge estimates) may give some insight into the quality of the forcings used to feed it, but in this two-step independent analysis, it is not possible to easily feed the interpolation scheme back with the discrepancies between observed and simulated discharges. Yet, despite having undergone the rainfall–runoff (or snow–runoff) transformation, discharge at the outlet of a (sub)catchment is still an interesting integrator (spatial low-pass filter) of the forcing fields and is ancillary areal information complementing the direct, point-scale data collected at gauges. In this perspective, choosing the best interpolation scheme partly becomes an inverse hydrological problem. Here, a joint calibration strategy is presented where the parameters of both the interpolation model (i.e., reconstruction procedure of meteorological forcings) and the hydrological model (snow cover, soil moisture accounting, and flow-routing schemes) are jointly inferred in a multisite and multivariable approach. Interpolated fields are daily rainfall and temperature, whereas hydrological variables consist of discharge and snow water equivalent time series at several locations in the Durance River catchment.

Low-pass spatial filters with small angle-domain bandwidth based on one-dimensional metamaterial photonic crystals

By using the spatial properties of zero-index gaps in one-dimensional metamaterial photonic crystals, we have designed low-pass spatial filters with small angle-domain bandwidth less than 10 degrees. It is demonstrated by transfer matrix method that the small angle-domain bandwidth and roll-off factor of the spatial filters can be adjusted by changing the structure parameters of the metamaterial photonic crystals. In addition, the spatial filters have absolute polarization-independent properties compared with previous photonic crystal spatial filters. These spatial filters with small angle-domain bandwidth may be applied to the laser science in the future, and decrease the space occupied by the traditional spatial filters.

Filter Dependence on the Phase Error of Fourier Transform Profilometry

In phase measuring fringe reflection Fourier transform profilometry, it gets the captured image data from the projected sinusoidal fringe patterns by using the CCD cameras. Added white noise in captured fringe images and its low contrast levels affect the accuracy of three-dimensional surface profile measurements induced the phase errors. In this work, we present a phase error reduction method using the low pass spatial image filter and median filter. Experimental results have shown to validate the low pass filter and median filter. Although the effect of the filter is to be smoothed the fringe, white noise included in the pattern was able to remove effectively. In case of the median filter, it showed negative influence to the phase error, but the low-pass filter evidently reduced the phase error about 30%.