Showing papers on "Spatial filter published in 2018"

Spiral phase contrast imaging in nonlinear optics: seeing phase objects using invisible illumination

Abstract: Spiral phase contrast (SPC) imaging offers a vital, convenient tool for edge detection in image processing. Despite significant experimental and theoretical progress in this area, SPC imaging with invisible light is still lacking. In contrast to the general SPC scheme, here we construct a nonlinear spatial filter by equivalently imprinting the vortex phase plate onto the potassium titanyl phosphate crystal using second harmonic generation (SHG). The phase or intensity objects are displayed by a spatial light modulator (SLM) and illuminated with 1064 nm infrared light. Then the combination of our nonlinear filter with SHG in the Fourier domain enables concise, yet highly efficient SPC imaging, leading to a visible edge enhancement with invisible illumination. By programming a running dog cartoon with SLM, we also demonstrate the capacity of our scheme to detect edges and contours in real time. Our present scheme could find direct applications in infrared monitoring.

Spin-Wave Drop Filter Based on Asymmetric Side-Coupled Magnonic Crystals

Abstract: Brillouin spectroscopy and micromagnetic simulations are used to study the propagation of magnetostatic spin waves across adjacent nonidentical magnonic crystals. The characteristics of the spin-wave modes in such a structure depend on the geometry of the slabs, and efficient spin-wave coupling at the magnonic-band-gap frequency can be achieved. This combination of spatial filtering features and spin-wave coupling leads to the realization of a frequency-selective magnonic drop filter, which is expected to extend functionality in spin-wave demultiplexing applications.

A Spatial Coherence Approach to Minimum Variance Beamforming for Plane-Wave Compounding

Abstract: A new approach to implement minimum variance distortionless response (MVDR) beamforming is introduced for coherent plane-wave compounding (CPWC). MVDR requires the covariance matrix of the incoming signal to be estimated and a spatial smoothing approximation is usually adopted to prevent this calculation from being underconstrained. In the new approach, we analyze MVDR as a spatial filter that decorrelates signals received at individual channels before summation. Based on the analysis, we develop two MVDR beamformers without using any spatial smoothing. First, MVDR weights are applied to the received signals after accumulating the data over transmits at different angles, while the second involves weighting the data collected in individual transmits and compounding over the transducer elements. In both cases, the covariance matrix is estimated using a set of slightly different combinations of the echo data. We show the sufficient statistic for this estimation that can be described by approximating the correlation among the backscattered ultrasound signals to their spatial coherence. Using the van Cittert–Zernike theorem, their statistical similarity is assessed by relating the spatial coherence to the profile of the source intensity. Both spatial-coherence-based MVDR beamformers are evaluated on data sets acquired from simulation, phantom, and in vivo studies. Imaging results show that they offer improvements over simple coherent compounding in terms of spatial and contrast resolutions. They also outperform other existing MVDR-based methods in the literature that are applied to CPWC.

Improving the accuracy of magnetic field tracing by velocity gradients: principal component analysis

Abstract: Tracing of the magnetic field with Velocity Gradient Technique (VGT) allows observers to probe magnetic field directions with spectroscopic data. In this paper, we employ the method of Principal Component Analysis (PCA) to extract the spectroscopic information most valuable for VGT. By using synthetic observation data from numerical simulations, we show that PCA acts in a way similar to spatial filtering along the velocity axis. We study both subsonic and supersonic simulations and show that with the PCA filtering the tracing of magnetic fields by the VGT is significantly improved. Using 21 cm GALFA data, we demonstrate that the PCA filtering improves the alignment of the velocity gradients and the Planck dust polarization.

Acoustic Sensing From a Multi-Rotor Drone

Abstract: We propose a time-frequency processing method that localizes and enhances a target sound by exploiting spectral and spatial characteristics of the ego-noise captured by a microphone array mounted on a multi-rotor micro aerial vehicle. We first exploit the time-frequency sparsity of the acoustic signal to estimate at each individual time-frequency bin the local direction of arrival (DOA) of the sound and formulate spatial filters pointing at a set of candidate directions. Then, we combine a kurtosis measure based on the spatial filtering outputs and a histogram measure based on the local DOA estimation to calculate a spatial likelihood function for source localization. Finally, we enhance the target sound by formulating a time-frequency spatial filter pointing at the estimated direction. As the ego-noise generally originates from specific directions, we propose a DOA-weighted spatial likelihood function that improves source localization performance by identifying noiseless sectors in the DOA circle. The DOA weighting scheme localizes the target sound even in extremely low signal-to-noise conditions when the target sound comes from a noiseless sector. We experimentally validate the performance of the proposed method with two array placements.

Spatial integration and differentiation of optical beams in a slab waveguide by a dielectric ridge supporting high-Q resonances.

Abstract: We show that a very simple structure consisting of a single subwavelength dielectric ridge on the surface of a slab waveguide enables spatial integration and differentiation of the profile of optical beams propagating in the waveguide. The integration and differentiation operations are performed in reflection and in transmission, respectively, at oblique incidence of the beam impinging on the ridge. The implementation of these operations is associated with the resonant excitation of a cross-polarized eigenmode of the ridge. We demonstrate that the quality factor of the resonances strongly varies along the dispersion curves and allows one to achieve the required tradeoff between the integration (or differentiation) quality and the amplitude of the resulting beam. The presented rigorous numerical simulation results confirm high-quality integration and differentiation. The proposed integrated structure may find application in ultrafast all-optical analog computing and signal processing systems.

Aberration-controlled Bessel beam processing of glass.

Abstract: It is known that Bessel beam generation with a non-ideal axicon induces beam pattern distortions. In this paper, we introduce a simple method for non-ideal axicon-generated Bessel beam reconstruction by tilting the axicon perpendicular to its optical axis. We found an optimum axicon tilt angle where beam distortions can be compensated by inducing additional astigmatic aberrations. At optimal tilt angle, the central spot symmetry and focal depth was increased. By this method we could control crack formation symmetry in the bulk of glass, which is essential for many transparent material processing applications.

Fast Adaptive Bilateral Filtering

Abstract: In the classical bilateral filter, a fixed Gaussian range kernel is used along with a spatial kernel for edge-preserving smoothing. We consider a generalization of this filter, the so-called adaptive bilateral filter, where the center and width of the Gaussian range kernel is allowed to change from pixel to pixel. Though this variant was originally proposed for sharpening and noise removal, it can also be used for other applications such as artifact removal and texture filtering. Similar to the bilateral filter, the brute-force implementation of its adaptive counterpart requires intense computations. While several fast algorithms have been proposed in the literature for bilateral filtering, most of them work only with a fixed range kernel. In this paper, we propose a fast algorithm for adaptive bilateral filtering, whose complexity does not scale with the spatial filter width. This is based on the observation that the concerned filtering can be performed purely in range space using an appropriately defined local histogram. We show that by replacing the histogram with a polynomial and the finite range-space sum with an integral, we can approximate the filter using analytic functions. In particular, an efficient algorithm is derived using the following innovations: the polynomial is fitted by matching its moments to those of the target histogram (this is done using fast convolutions), and the analytic functions are recursively computed using integration-by-parts. Our algorithm can accelerate the brute-force implementation by at least $20 \times$, without perceptible distortions in the visual quality. We demonstrate the effectiveness of our algorithm for sharpening, JPEG deblocking, and texture filtering.

Optical image processing with metasurface dark modes.

Abstract: Here we consider image processing using the optical modes of metasurfaces with an angle-dependent excitation. These spatially dispersive modes can be used to directly manipulate the spatial frequency content of an incident field, suggesting their use as ultra-compact alternatives for analog optical information processing. A general framework for describing the filtering process in terms of the optical transfer functions is provided. In the case where the relevant mode cannot be excited with a normally incident plane wave (a dark mode), high-pass filtering is obtained. We provide examples demonstrating filtering of both amplitude and pure phase objects.

Dynamic spatial filtering using a digital micromirror device for high-speed optical diffraction tomography.

Abstract: Optical diffraction tomography (ODT) is an emerging microscopy technique for three-dimensional (3D) refractive index (RI) mapping of transparent specimens. Recently, the digital micromirror device (DMD) based scheme for angle-controlled plane wave illumination has been proposed to improve the imaging speed and stability of ODT. However, undesired diffraction noise always exists in the reported DMD-based illumination scheme, which leads to a limited contrast ratio of the measurement fringe and hence inaccurate RI mapping. Here we present a novel spatial filtering method, based on a second DMD, to dynamically remove the diffraction noise. The reported results illustrate significantly enhanced image quality of the obtained interferograms and the subsequently derived phase maps. And moreover, with this method, we demonstrate mapping of 3D RI distribution of polystyrene beads as well as biological cells with high accuracy. Importantly, with the proper hardware configuration, our method does not compromise the 3D imaging speed advantage promised by the DMD-based illumination scheme. Specifically, we have been able to successfully obtain interferograms at over 1 kHz speed, which is critical for potential high-throughput label-free 3D image cytometry applications.

Isotropic wavevector domain image filters by a photonic crystal slab device.

Abstract: We show that several types of isotropic image filters in the wavevector domain can be implemented with a single photonic crystal slab device. Such a slab is designed so that the guided resonance near the Γ point exhibits an isotropic band structure. Depending on the light frequency and the choice of transmission or reflection mode, the device realizes isotropic high-pass, low-pass, band-reject, and band-pass filtering in wavevector space. These filter functions are important for various image processing tasks, including edge detection, smoothing, white noise suppression, and suppression or extraction of periodic noises. We numerically demonstrate these filter functionalities by simulations of a slab structure that is designed to operate in the visible wavelength range. Our work expands the application of nanophotonics-based optical analog computing for image processing.

Two-groove narrowband transmission filter integrated into a slab waveguide

Abstract: We propose a simple integrated narrowband filter consisting of two grooves on the surface of a slab waveguide. Spectral filtering is performed in transmission at oblique incidence due to excitation of an eigenmode of the structure localized at a ridge cavity between the grooves. For the considered parameters, zero reflectance and unity transmittance are achieved at resonant conditions. The width and location of the transmittance peak can be controlled by changing the widths of the grooves and of the ridge, respectively. The proposed filter may find application in waveguide-integrated spectrometers.

Quantum image median filtering in the spatial domain

Abstract: Spatial filtering is one principal tool used in image processing for a broad spectrum of applications. Median filtering has become a prominent representation of spatial filtering because its performance in noise reduction is excellent. Although filtering of quantum images in the frequency domain has been described in the literature, and there is a one-to-one correspondence between linear spatial filters and filters in the frequency domain, median filtering is a nonlinear process that cannot be achieved in the frequency domain. We therefore investigated the spatial filtering of quantum image, focusing on the design method of the quantum median filter and applications in image de-noising. To this end, first, we presented the quantum circuits for three basic modules (i.e., Cycle Shift, Comparator, and Swap), and then, we design two composite modules (i.e., Sort and Median Calculation). We next constructed a complete quantum circuit that implements the median filtering task and present the results of several simulation experiments on some grayscale images with different noise patterns. Although experimental results show that the proposed scheme has almost the same noise suppression capacity as its classical counterpart, the complexity analysis shows that the proposed scheme can reduce the computational complexity of the classical median filter from the exponential function of image size n to the second-order polynomial function of image size n, so that the classical method can be speeded up.

Millimeter-Wave Tiny Lens Antenna Employing U-Shaped Filter Arrays for 5G

Abstract: This letter presents a 28-GHz compact antenna array that employs a U-Shaped thin lens where a low-profile 1 × 4 antenna array is used as a feed source. The letter demonstrates that the proposed three-dimensional (3-D) U-Shaped lens designed by phase-shifting spatial filter arrays enables more than 3 dB peak gain enhancement even at the distance of less than half of the wavelength from the feed antenna. The first design step is to employ unit cells having a stable insertion loss and phase shift against the incident angle of electromagnetic waves without the gain degradation caused by undesired cavity effects, which is a critical bottleneck at such a short distance. In addition to this, the total gain is further enhanced by creating a novel 3-D U-Shaped architecture, enabling an increase in the effective aperture size because the U-Shaped thin lens can capture the radiated fields over a broader range of incident angles compared to the prior flat lens, which is more effective at a closer distance. This letter demonstrates that the proposed approach can achieve higher than 3.8 dB enhancement in peak gain compared to the antenna without the U-Shaped lens when the phase offset (PO) is 0°. When the levels of the PO are 45°, 90°, and 135°, gain enhancements of 3.4, 3, and 2 dB are also achieved, respectively.

Mitigation of crosstalk effects in multi-LiDAR configurations

Abstract: In this paper, we examine crosstalk effects that can arise in multi-LiDAR configurations, and we show a data-based approach to mitigate these effects. Due to the ability to acquire precise 3D data of the environment, LiDAR-based sensor systems (sensors based on “Light Detection and Ranging”, e.g., laser scanners) increasingly find their way into various applications, e.g. in the automotive sector. However, with an increasing number of LiDAR sensors operating within close vicinity, the problem of potential crosstalk between these devices arises. “Crosstalk” outlines the following effect: In a typical LiDAR-based sensor, short laser pulses are emitted into the scene and the distance between sensor and object is derived from the time measured until an “echo” is received. In case multiple laser pulses of the same wavelength are emitted at the same time, the detector may not be able to distinguish between correct and false matches of laser pulses and echoes, resulting in erroneous range measurements and 3D points. During operation of our own multi-LiDAR sensor system, we were able to observe crosstalk effects in the acquired data. Having compared different spatial filtering approaches for the elimination of erroneous points in the 3D data, we propose a data-based spatio-temporal filtering and show its results, which may be sufficient depending on the application. However, technical solutions are desired for future LiDAR sensors.

Impact of Spatial Filtering on Distortion From Low-Noise Amplifiers in Massive MIMO Base Stations

Abstract: In massive multiple-input–multiple-output base stations, power consumption and cost of the low-noise amplifiers (LNAs) can be substantial because of the many antennas. We investigate the feasibility of inexpensive, power efficient LNAs, which inherently are less linear. A polynomial model is used to characterize the nonlinear LNAs and to derive the second-order statistics and spatial correlation of the distortion. We show that, with spatial matched filtering (maximum-ratio combining) at the receiver, some distortion terms combine coherently, and that the signal-to-interference-and-noise ratio of the symbol estimates therefore is limited by the linearity of the LNAs. Furthermore, it is studied how the power from a blocker in the adjacent frequency band leaks into the main band and creates distortion. The distortion term that scales cubically with the power received from the blocker has a spatial correlation that can be filtered out by spatial processing and only the coherent term that scales quadratically with the power remains. When the blocker is in free-space line-of-sight and the LNAs are identical, this quadratic term has the same spatial direction as the desired signal, and hence cannot be removed by linear receiver processing.

Tracking a moving sound source from a multi-rotor drone

Abstract: We propose a method to track from a multi-rotor drone a moving source, such as a human speaker or an emergency whistle, whose sound is mixed with the strong ego-noise generated by rotating motors and propellers. The proposed method is independent of the specific drone and does not need pre-training nor reference signals. We first employ a time-frequency spatial filter to estimate, on short audio segments, the direction of arrival of the moving source and then we track these noisy estimations with a particle filter. We quantitatively evaluate the results using a ground-truth trajectory of the sound source obtained with an on-board camera and compare the performance of the proposed method with baseline solutions.

Joint Multi-Microphone Speech Dereverberation and Noise Reduction Using Integrated Sidelobe Cancellation and Linear Prediction

Abstract: In multi-microphone speech enhancement, reverberation and noise are commonly suppressed by deconvolution and spatial filtering, i.e. using multi-channel linear prediction (MCLP) on the one hand and beamforming, e.g., a generalized sidelobe canceler (GSC), on the other hand. In this paper, in order to perform both deconvolution and spatial filtering, we propose to integrate MCLP and the GSC into a novel framework referred to as integrated sidelobe cancellation and linear prediction (ISCLP), wherein the sidelobe-cancellation (SC) filter and the linear prediction (LP) filter operate in parallel. Further, within this framework, we propose to estimate both filters jointly by means of a single Kalman filter. While ISCLP is roughly M times less expensive than a corresponding cascade of multiple-output MCLP and the GSC, where M denotes the number of microphones, it performs equally well in terms of dereverberation and noise reduction, as shown in simulations using one localized noise source.

Highly stable wide-field common path digital holographic microscope based on a Fresnel biprism interferometer

Abstract: Quantitative phase imaging (QPI) of biological cells and tissues is an important technique useful in the determination of many biophysical parameters such as morphology, refractive index, thickness, cell dry mass, hemoglobin concentration, etc. Off-axis digital holography has been ideal for the QPI of microscopic specimens, but it has lower temporal stability compared to on-axis and common path digital holographic microscopes, which offer higher temporal stability. In this paper, we present a very simple, easy to align yet highly stable common path digital holographic microscope based on a Fresnel biprism interferometer. The system uses a biprism to divide the incoming beam into reference and object beams without the loss of optical power, unlike diffraction phase microscopy. Two methods are proposed, one by utilizing the spatial filtering mode and the other by the self-referencing mode. It offers the advantage that the reference beam can be easily created simply by translating the object in the focal plane of the microscopic objective or by spatially filtering one of the object beams in the Fourier domain. The proposed setup offers no power loss and a high phase stability of approximately 0.006 rad (6 mrad) without using any vibration isolation. The experiments on industrial and biological samples are reported demonstrating its application both for static and dynamic samples.

Self-assembled stretchable photonic crystal for a tunable color filter.

Abstract: In this Letter, we report the development of a continuously tunable color filter based on a self-assembled isotropically stretchable microbead monolayer. Spreading equidistantly upon the application of lateral strain, the isotropically stretchable monolayer serves as a dynamic diffraction grating whose diffraction angle can be mechanically modulated. Combined with a simple spatial filtering scheme, the spectra of the filtered light are solely controlled by external strain (up to 32% radial strain) to cover a broad visible spectrum. Through a finite-difference time-domain far-field diffraction simulation, we validate the working principle of the proposed color filter. The proposed continuously tunable color filter is expected to open original applications in next-generation display field.

Edge enhancement by negative Poincare–Hopf index filters

Abstract: Phase and polarization are interrelated quantities, and hence polarization elements that perform like phase elements can be designed. In this Letter, we show that a polarizing element producing a negative Poincare–Hopf (PH) index beam can be used as a spatial filter to perform edge enhancement. Either isotropic or anisotropic edge enhancement can be achieved by polarization selection of the light that illuminates the sample. A conventional microscope imaging system is modified into a polarization-selective optical Fourier processor. Experimental results are presented to show that negative PH index filters, producing a set of orthogonal polarization distribution and their superpositions, can also be used for edge enhancement in optical signal processing.

Controlling the degree of polarization of partially coherent electromagnetic beams with lenses.

Abstract: We show theoretically that the degree of polarization of a partially coherent electromagnetic beam changes dramatically as the beam is being focused. A low numerical aperture lens can considerably enhance the degree of polarization at its geometrical focus. When two identical lenses are employed in a 4f configuration, the degree of polarization of a beam can be tailored by using amplitude masks in the Fourier plane located in the middle of the two lenses. Our findings open up the possibility to control this fundamental property of random beams in a simple manner.

Toward the generation of broadband optical vortices: extending the spectral range of a q-plate by polarization-selective filtering

Abstract: Optical vortex beams in the visible and near-infrared spectrum over a wide spectral region are generated by a single S-waveplate polarization converter using polarization-selective filtering. A spectral coverage of 600 nm is demonstrated, with maximum efficiency at a wavelength of 530 nm. The broadband coverage is obtained using polarization filtering, which is applicable for any component based on geometric phase retardation. The efficiency of the filtering varies from 50% to 95% depending on the wavelength. This technique has potential application in stimulated emission microscopy and lithography.

Acousto-optic visualization of optical wavefronts [Invited]

Abstract: The paper presents a short survey on theoretical and experimental investigations of an acousto-optic (AO) method of optical wavefront visualization proposed and developed at Moscow State University The method is based on angular selectivity of Bragg diffraction It is shown that distribution of light intensity in the visualized image is proportional to the phase gradient in the AO interaction plane Spatial resolution and contrast of the visualized image are determined primarily by the divergence angle of the acoustic beam Most attention in the paper is concentrated on the problem of AO visualization of two-dimensional phase objects The important advantage of the AO method consists of the possibility of optical field phase-structure registration in the presence of amplitude modulation of the initial optical field Examples of computer simulations as well as some experimental results are presented for illustration of the potentialities of this method

Hyperuniformity in amorphous speckle patterns.

Abstract: Hyperuniform structures possess the ability to confine and drive light, although their fabrication is extremely challenging. Here we demonstrate that speckle patterns obtained by a superposition of randomly arranged sources of Bessel beams can be used to generate hyperunifrom scalar fields. By exploiting laser light tailored with a spatial filter, we experimentally produce (without requiring any computational power) a speckle pattern possessing maxima at locations corresponding to a hyperuniform distribution. By properly filtering out intensity fluctuation from the same speckle pattern, it is possible to retrieve an intensity profile satisfying the hyperuniformity requirements. Our findings are supported by extensive numerical simulations.

Enhanced superlens imaging with loss-compensating hyperbolic near-field spatial filter

Abstract: Recently a coherent optical process called plasmon-injection (Π) scheme, which employs an auxiliary source, has been introduced as a new technique to compensate losses in metamaterials. Here, a physical implementation of the Π scheme is proposed for enhanced superlens imaging in the presence of absorption losses and noise. The auxiliary source is constructed by a high-intensity illumination (above 1 mW/μm2) of the superlens integrated with a near-field spatial filter. The integrated system enables reconstruction of an object previously unresolvable with the superlens alone. This work elevates the viability of the Π scheme as a strong candidate for loss compensation in near-field imaging systems without requiring nonlinear effects or gain media.

Enhanced superlens imaging with loss-compensating hyperbolic near-field spatial filter

Abstract: Recently a coherent optical process called plasmon injection ($\Pi$) scheme, which employs an auxiliary source, has been introduced as a new technique to compensate losses in metamaterials. In this work, a physical implementation of the $\Pi$ scheme on a thin silver film is proposed for enhanced superlens imaging. The efficacy of the scheme is illustrated by enhancing near-field imaging deeper beyond the diffraction limit in the presence of absorption losses and noise. The auxiliary source is constructed by a high-intensity illumination of the superlens integrated with a near-field spatial filter. The integrated system enables reconstruction of an object previously unresolvable with the superlens alone. This work elevates the viability of the $\Pi$ scheme as a strong candidate for loss compensation in near-field imaging systems without requiring non-linear effects or gain medium.

Single Infrared Image-Based Stripe Nonuniformity Correction via a Two-Stage Filtering Method.

Abstract: The presence of stripe nonuniformity severely degrades the image quality and affects the performance in many infrared (IR) sensing applications. Prior works correct the nonuniformity by using similar spatial representations, which inevitably damage some detailed structures of the image. In this paper, we instead take advantage of spectral prior of stripe noise to solve its correction problem in single IR image. We first analyse the significant spectral difference between stripes and image structures and utilize this knowledge to characterize stripe nonuniformity. Then a two-stage filtering strategy is adopted combining spectral and spatial filtering. The proposed method enables stripe nonuniformity to be eliminated from coarse to fine, thus preserving image details well. Extensive experiments on simulated images and raw IR images demonstrate that the proposed method achieves superior correction performance over the recent state-of-the-art methods.