Showing papers on "Spatial filter published in 2021"

Diffusion-Based Smoothers for Spatial Filtering of Gridded Geophysical Data

Abstract: We describe a new way to apply a spatial filter to gridded data from models or observations, focusing on low-pass filters. The new method is analogous to smoothing via diffusion, and its implementa...

Observation of transition between multimode Q-switching and spatiotemporal mode locking

Abstract: We report experimental observation of multimode Q-switching and spatiotemporal mode locking in a multimode fiber laser A typical steady Q-switching state is achieved with a 188 μs pulse duration, a 7014 kHz repetition rate, and a 2158 mW output power, corresponding to the single pulse energy of 308 μJ We find weak spatial filtering is essential to obtain stable Q-switched pulses, in contrast to the relatively stronger spatial filtering for spatiotemporal mode locking Furthermore, a reversible transition process, as well as a critical bistable state, between multimode Q-switching and spatiotemporal mode locking, is achieved with specific spatial coupling and waveplates sets We believe the results will not only contribute to understanding the complicated nonlinear dynamics in multimode, fiber-based platforms, but also benefit the development of promising high-pulse energy lasers

Fano-like resonances in nanostructured thin films for spatial filtering

Abstract: Fano-like resonant coupling of electromagnetic radiation with planar waveguiding modes of nanostructured thin films is proposed and realized experimentally Different from conventional Fano coupling to compact resonators with the discrete spectrum, we report Fano-like coupling to infinitely extended planar waveguiding modes of the spatially unbound system We fabricated the films by the ion beam sputtering method on nano-modulated substrates The observed Fano-like process shows extremely strong sensitivity with respect to the wavelength and especially to the incidence angle of the radiation and can potentially be used for frequency and spatial filtering of light in transmission/reflection through/from such nanostructured thin films

High-resolution digital spatial control of a highly multimode laser

Abstract: We developed a rapid and efficient method for generating laser outputs with arbitrary shaped distributions and properties that are needed for a variety of applications. It is based on simultaneously controlling the intensity, phase, and coherence distributions of the laser. The method involves a digital degenerate cavity laser in which a phase-only spatial light modulator and spatial filters are incorporated. As a result, a variety of unique and high-resolution arbitrary shaped laser beams were generated with either a low or a high spatial coherence and with a minimal change in the laser output power. By controlling the phase, intensity, and coherence distributions, a shaped laser beam was efficiently reshaped into a completely different shape after free space propagation. The generation of such laser beams could lead to new and interesting applications.

Analysis and design of a passive spatial filter for sub-6 GHz 5G communication systems

Abstract: A frequency-selective surface (FSS) is able to transmit or reflect incoming electromagnetic waves, and these properties of FSS can be utilized in printed antennas to improve the performance of these antennas. Sub-6 GHz frequency bands are used in fifth-generation (5G) systems for various applications. This paper presents the design and analysis of a wideband band-pass spatial filter using a double square loop frequency-selective surface (DSLFSS) for the sub-6 GHz 5G frequency range 1 (FR1). The proposed spatial filter consists of DSLFSS elements and can be placed on the patch radiator to increase the radiation characteristics in n77, n78, and n79 bands of the sub-6 GHz 5G spectrum. The effect of varying the width of the loops, angle of incidence. and polarization on the transmission coefficient in the frequency band of operation is analyzed. The design is synthesized using the closed form mathematical expressions for finding the physical dimensions of the spatial filter. Design trade-offs are reported based on the proposed mathematical formulation and simulations. The designed DSLFSS structure is fabricated and measured. The design results are authenticated by comparing results from the Ansys HFSS v20 Electronic Desktop Circuit Editor and measurement setup. In addition, the extension of the results from a unit cell is taken to the $$2 \times 2$$ array and $$10 \times 10$$ array, which shows nearly the same performance, hence confirming the stability of the DSLFSS structure. The proposed DSLFSS-based spatial filter has the potential for use in the design and development of patch radiators with improved radiation characteristics.

A 0.7–5.7 GHz Reconfigurable MIMO Receiver Architecture for Analog Spatial Notch Filtering Using Orthogonal Beamforming

Abstract: A highly reconfigurable direct-conversion software-defined multiple-input multiple-output (MIMO) receiver with four RF inputs and four I/Q baseband outputs is proposed. It allows for digital MIMO but also analog interference rejection by spatial notch filtering through four flexible and simultaneous orthogonal beams. A segmented constant-Gm vector modulator (VM) with improved interference tolerance and wide RF frequency range targeting the sub-6-GHz bands is proposed. It exploits current-domain beamforming before $I$ – $V$ conversion by transimpedance amplifiers. A 0.7–5.7-GHz 22-nm fully depleted silicon-on-insulator (FD-SOI) prototype chip achieves >29 dB spatial filtering for a single notch and an ultrawideband 20-dB notch suppression bandwidth of 2.3 GHz at broadside excitation at an local oscillator (LO) frequency of 2.5 GHz. In the notches, an IIP3 of +16 dBm and B1dB of −11.5 dBm at a 41-dB gain is achieved, improving IIP3 and B1dB by 35 and 27 dB, respectively, by spatial filtering. A single-element noise figure (NF) of 5.5–7 dB is achieved on the VM constellation corners, degrading about 2 dB on the points nearby the biggest circle fitting into a square constellation. However, sub-3-dB system NF is potentially achievable, taking into account up to 6-dB improvement by the four-element beamforming. Given both gain and phase control provided by the VM, spatial patterns with up to three independent nulls can be synthesized with the four-element antenna array. The chip of 0.52 mm2 active area consumes 77–139 mW at an LO-frequency of 0.7–5.7 GHz from a 0.8-V supply.

Transmission Mueller matrix imaging with spatial filtering.

Abstract: In this Letter, we report a study on the effects of spatial filtering for a transmission Mueller matrix imaging system. A spatial filter (SF) is placed on the back Fourier plane of the imaging lens in a dual-rotating-retarders Mueller matrix imaging system to select photons within a certain scattering angle. The system is then applied to three types of human cancerous tissues. When imaging with a small-aperture SF, some polarimetry basis parameters show sharp changes in contrast in the cancerous regions. Monte Carlo simulations using a simple sphere–cylinder scattering model also show that spatial filtering of the scattered photons provides extra information on the size and shape of the scattering particles. The results indicate that spatial filtering enhances the capability of polarization imaging as a powerful tool for biomedical diagnosis.

Nanostructured Multilayer Coatings for Spatial Filtering

Abstract: Spatial filtering is an important mechanism to improve the spatial quality of laser beams. Typically, a confocal arrangement of lenses with a diaphragm in the focal plane is used for intracavity spatial filtering. Such conventional filtering requires access to the far-field domain. In microlasers, however, conventional filtering is impossible due to the lack of space in micro-resonators to access the far-field. Therefore, a novel concept for more compact and efficient spatial filtering is necessary. In this study, we propose and demonstrate a conceptually novel mechanism of spatial filtering in the near-field domain, by a nanostructured multilayer coating - a 2D photonic crystal structure with a periodic index modulation along the longitudinal and transverse direction to the beam propagation. The structure is built on a nano-modulated substrate, to provide the transverse periodicity. The physical vapor deposition is used to provide self-repeating modulation in the longitudinal direction. We experimentally demonstrate a 5 micron thick photonic multilayer structure composed of nanostructured multiple layers of alternating high- and low-index materials providing spatial filtering in the near-infrared frequencies with 2° low angle passband. The proposed photonic structure can be considered as an ideal component for intracavity spatial filtering in microlasers.

Non-line-of-sight object location estimation from scattered light using plenoptic data

Abstract: We investigate the use of plenoptic data for locating non-line-of-sight (NLOS) objects from a scattered light signature. Using Fourier analysis, the resolution limits of the depth and transversal location estimates are derived from fundamental considerations on scattering physics and measurement noise. Based on the refocusing algorithm developed in the computer vision field, we derive an alternative formulation of the projection slice theorem in a form directly connecting the light field and a full spatial frequency spectrum including both depth and transversal dimensions. Using this alternative formulation, we propose an efficient spatial frequency filtering method for location estimation that is defined on a newly introduced mixed space frequency plane and achieves the theoretically limited depth resolution. A comparison with experimental results is reported.

On the Design of Sparse Arrays With Frequency-Invariant Beam Pattern

Abstract: Beamformer performs spatial filtering to preserve the desired signal while suppressing interfering signals and noise arriving from directions other than the direction of interest. However, the beam pattern of the conventional beamformer is dependent on the frequency of the signal. It is common to use dense and uniform arrays for a broadband signal to achieve some essential performances together, such as frequency-invariant, white noise gain, directivity factor, front-to-back ratio, etc. Recently, the interest in sparse arrays is growing, mainly due to the capacity to reduce the number of sensors. Nevertheless, in general, finding a suitable sparse array layout is still a challenging task. Many studies have focused on optimization procedures to seek the sparse array deployment. This paper presents an alternative approach to determine the location of sensors. Starting with a weight spectrum of a virtual uniform array, some techniques are used, such as analyzing the weight spectrum to determine the critical sensors, applying the clustering technique to group the sensors into the different groups, and selecting the representative sensors for each group. After the sparse array deployment is specified, the optimization technique is applied to find the beamformer coefficients. The proposed method helps to save the computation time in the design phase, and its beamformer performance outperforms other state-of-the-art methods in several aspects.

Ring-shaped optical trap based on an acousto-optic tunable spatial filter.

Abstract: We report on a novel, to the best of our knowledge, optical scheme of an annular optical trap based on an acousto-optic tunable spatial filter. Design of the optical trap is proposed and validated. Experimental demonstration with polystyrene microspheres includes controllable arrangement of freely floating particles into a circular pattern, aggregation, and disaggregation of the particles. Dynamical adjustment of the trapping field potential diameter is achieved by programmable frequency-swept controlling of the acousto-optic filter.

Spatial and temporal domain filtering for underwater lidar.

Abstract: Combined spatial and temporal processing techniques are presented to enhance optical ranging in underwater environments. The performance of underwater light detection and ranging (lidar) is often limited by scattering. Previous work has demonstrated that both hybrid lidar–radar, which temporally modulates the amplitude of light, and optical spatial coherence filtering, which spatially modulates the phase of light, have independently reduced the effects of scattering, improving performance. The combined performance of the processing methods is investigated, and experimental results demonstrate that the combined filtering improves the performance of underwater lidar systems beyond what either method provides independently.

Local spatial analysis: an easy-to-use adaptive spatial EEG filter.

Abstract: Spatial EEG filters are widely used to isolate event-related potential (ERP) components. The most commonly used spatial filters (e.g., the average reference and the surface Laplacian) are "stationary." Stationary filters are conceptually simple, easy to use, and fast to compute, but all assume that the EEG signal does not change across sensors and time. Given that ERPs are intrinsically nonstationary, applying stationary filters can lead to misinterpretations of the measured neural activity. In contrast, "adaptive" spatial filters (e.g., independent component analysis, ICA; and principal component analysis, PCA) infer their weights directly from the spatial properties of the data. They are, thus, not affected by the shortcomings of stationary filters. The issue with adaptive filters is that understanding how they work and how to interpret their output require advanced statistical and physiological knowledge. Here, we describe a novel, easy-to-use, and conceptually simple adaptive filter (local spatial analysis, LSA) for highlighting local components masked by large widespread activity. This approach exploits the statistical information stored in the trial-by-trial variability of stimulus-evoked neural activity to estimate the spatial filter parameters adaptively at each time point. Using both simulated data and real ERPs elicited by stimuli of four different sensory modalities (audition, vision, touch, and pain), we show that this method outperforms widely used stationary filters and allows to identify novel ERP components masked by large widespread activity. Implementation of the LSA filter in MATLAB is freely available to download.NEW & NOTEWORTHY EEG spatial filtering is important for exploring brain function. Two classes of filters are commonly used: stationary and adaptive. Stationary filters are simple to use but wrongly assume that stimulus-evoked EEG responses (ERPs) are stationary. Adaptive filters do not make this assumption but require solid statistical and physiological knowledge. Bridging this gap, we present local spatial analysis (LSA), an adaptive, yet computationally simple, spatial filter based on linear regression that separates local and widespread brain activity (https://www.iannettilab.net/lsa.html or https://github.com/rorybufacchi/LSA-filter).

Scintillation and bit error rate in bidirectional laser communications between an aerial vehicle and a satellite using annular optical beams in strong turbulent atmosphere.

Abstract: Scintillation index is examined for annular optical beams in a strong atmospheric medium of a slant path. On-axis scintillations have small- and large-scale components and are formulated for the uplink/downlink of aerial vehicle-satellite laser communications. For this purpose, the unified Rytov method and the amplitude spatial filtering of the atmospheric spectrum are utilized. Performances given by the average bit error rate (BER) are investigated by employing the corresponding scintillation index, which is found by using intensity having gamma-gamma distribution. Strong atmospheric turbulence effects on the scintillation index and BER of the collimated annular optical beam having various thicknesses are reported for the up/down vertical links, and these are compared with the scintillations of the collimated Gaussian optical beams against propagation length, source size, and the zenith angle with the selected thickness. Utilizing the scintillations found, BER changes against average signal-to-noise ratio (SNR)are plotted for up/down vertical links. The scintillation index and BER in the downlink are found to be different than the scintillation index and BER in the uplink for strong atmospheric turbulence, mainly because the structure constant is a function of the altitude. Considering the location where the aerial vehicle and satellite are deployed as the reference points, annular beams are more advantageous than the Gaussian beams at up/down slant link lengths. The effect of the thickness of the annular beam is apparent for the uplink, where thin annular beams are more advantageous at small link lengths and thick annular beams are more advantageous at large link lengths. In the downlink, thin annular beams are more advantageous at all link lengths.

A Graph-Theoretic Approach for Spatial Filtering and Its Impact on Mixed-Type Spatial Pattern Recognition in Wafer Bin Maps

Abstract: Statistical quality control in semiconductor manufacturing hinges on effective diagnostics of wafer bin maps, wherein a key challenge is to detect how defective chips tend to spatially cluster on a wafer—a problem known as spatial pattern recognition . Recently, there has been a growing interest in mixed-type spatial pattern recognition—when multiple defect patterns, of different shapes, co-exist on the same wafer. Mixed-type spatial pattern recognition entails two central tasks: (1) spatial filtering, to distinguish systematic patterns from random noises; and (2) spatial clustering, to group filtered patterns into distinct defect types. Observing that spatial filtering is instrumental to high-quality mixed-type pattern recognition, we propose to use a graph-theoretic method, called adjacency-clustering, which leverages spatial dependence among adjacent defective chips to effectively filter the raw wafer maps. Tested on real-world data and compared against a state-of-the-art approach, our proposed method achieves at least 46% gain in terms of internal cluster validation quality (i.e., validation without external class labels), and about 5% gain in terms of Normalized Mutual Information—an external cluster validation metric based on external class labels. Interestingly, the margin of improvement appears to be a function of the pattern complexity, with larger gains achieved for more complex-shaped patterns.

High-NA optical edge detection via optimized multilayer films

Abstract: There has been a significant effort to design nanophotonic structures that process images at the speed of light. A prototypical example is in edge detection, where photonic-crystal-, metasurface-, and plasmon-based designs have been proposed and in some cases experimentally demonstrated. In this work, we show that multilayer optical interference coatings can achieve visible-frequency edge detection in transmission with high numerical aperture, two-dimensional image formation, and straightforward fabrication techniques, unique among all nanophotonic approaches. We show that the conventional Laplacian-based transmission spectrum may not be ideal once the scattering physics of real designs is considered, and show that better performance can be attained with alternative spatial filter functions. Our designs, comprising alternating layers of Si and SiO$_2$ with total thicknesses of only $\approx 1{\rm\mu m}$, demonstrate the possibility for optimized multilayer films to achieve state-of-the-art edge detection, and, more broadly, analog optical implementations of linear operators.

Analysis of the impact of optical aberrations in en-face full-field OCT microscopy.

Abstract: Optical coherence tomography (OCT) is a powerful technique for cross-sectioning imaging. However, the lateral resolution may be degraded by optical aberrations originating from the sample or the setup. We present an extensive quantitative study of the impact of aberrations in time-domain en-face full-field OCT (FFOCT). Using an adaptive optics loop integrated in an FFOCT setup, a deformable mirror is used to introduce low-order calibrated aberrations. The experimental analysis of both the line spread functions (SF) and the complex object images has allowed us to measure the loss in contrast and the impact on lateral spatial resolution. We demonstrate that the frequency content of FFOCT image spectra in terms of signal-to-noise ratio and cutoff frequency is degraded by aberrations but remains much higher than in conventional incoherent images. Line SF profiles in conventional imaging display widening, whereas in FFOCT they display oscillations, leading to the possible perception of preserved resolution. Nevertheless, for complex objects, the aberration image blurring is strong due to the convolution process by the point SF, resulting in a significant filtering of the image spatial spectrum.

Optomechanical Coupling Active Control for Improving Beam Pointing Accuracy of the Spatial Filter in PW Laser Facility

Abstract: To improve the beam pointing accuracy of PW laser facility and reduce the optical axis deviation caused by the deflection amplitude response deviation of the confocal lens of spatial filter for microvibration action, an Optomechanical coupling vibration active control theory is proposed to make the peak value of output optical angle response lower than the pointing threshold value by 0.2 μrad. To establish an Optomechanical coupling vibration active control system, the active control parameters are introduced into the beam transmission matrix of the Optomechanical coupling system. The active control parameters and the peak value of the output light angle response are linked point to point. The algorithm flow of the active control system is designed, the control rules are established, and the control effect is verified. The results show that the peak value of the output optical angle response of the spatial filter decreases by 98.13%, and the attenuation is nearly two orders of magnitude after the active control, which effectively improves the convergence accuracy of the beam pointing of the spatial filter of the PW laser facility, and realizes the beam pointing control under the broadband excitation, and the control result is consistent with the expectation.

Direct generation of mid-infrared pulsed optical vortices at ∼ 2.7 µm

Abstract: We present the first, to the best of our knowledge, direct generation of pulsed optical vortices in the 2.7-µ m spectral range by employing polycrystalline Fe:ZnSe as a saturable absorber (SA). A modified theoretical model taking into account the propagation features of the reshaped annular pump beam is elaborated to accurately determine the excitation conditions of the Laguerre–Gaussian (LG0,l) modes, yielding a lasing efficiency comparable to the fundamental TEM00 mode in continuous-wave (CW) regime. Nanosecond scalar optical vortices with well-defined handedness are successfully produced by taking advantages of designated mode-matching, high polarization extinction ratio (PER), and the "spatial filter" effect of the SA on other transverse modes. Such scalar vortex laser pulses in the mid-infrared region will enable new applications such as frequency down conversion to produce optical vortices at longer (far-infrared) wavelengths, structuring organic materials, novel molecular spectroscopy, etc.

Distributed Spatial Filtering Over Networked Systems

Abstract: This letter concerns distributed spatial filtering over networked systems, ie, transforming signal values given for nodes to those with a desired spatial frequency characteristic via a distributed computation An existing filtering algorithm can achieve only low-pass filter characteristics, which limits its range of applications To address this limitation, we extend the aforementioned filtering algorithm using an additional design parameter We then present a characterization of all the realizable filter characteristics as a necessary and sufficient condition for achieving distributed spatial filtering As a result, it is shown that the extended algorithm increases the range of the realizable filter characteristics The proposed method is verified not only by simulation but also by denoising experiments for a real sensor network The results show that the proposed method effectively reduces spatial noise and achieves higher performance than an average consensus algorithm and an average filter

Nonlinear Spatial Filtering in Multichannel Speech Enhancement

Abstract: The majority of multichannel speech enhancement algorithms are two-step procedures that first apply a linear spatial filter, a so-called beamformer, and combine it with a single-channel approach for postprocessing. However, the serial concatenation of a linear spatial filter and a postfilter is not generally optimal in the minimum mean square error (MMSE) sense for noise distributions other than a Gaussian distribution. Rather, the MMSE optimal filter is a joint spatial and spectral nonlinear function. While estimating the parameters of such a filter with traditional methods is challenging, modern neural networks may provide an efficient way to learn the nonlinear function directly from data. To see if further research in this direction is worthwhile, in this work we examine the potential performance benefit of replacing the common two-step procedure with a joint spatial and spectral nonlinear filter. We analyze three different forms of non-Gaussianity: First, we evaluate on super-Gaussian noise with a high kurtosis. Second, we evaluate on inhomogeneous noise fields created by five interfering sources using two microphones, and third, we evaluate on real-world recordings from the CHiME3 database. In all scenarios, considerable improvements may be obtained. Most prominently, our analyses show that a nonlinear spatial filter uses the available spatial information more effectively than a linear spatial filter as it is capable of suppressing more than $D-1$ directional interfering sources with a $D$-dimensional microphone array without spatial adaptation.

Complex-valued Spatial Autoencoders for Multichannel Speech Enhancement

Abstract: In this contribution, we present a novel online approach to multichannel speech enhancement. The proposed method estimates the enhanced signal through a filter-and-sum framework. More specifically, complex-valued masks are estimated by a deep complex-valued neural network, termed the complex-valued spatial autoencoder. The proposed network is capable of exploiting as well as manipulating both the phase and the amplitude of the microphone signals. As shown by the experimental results, the proposed approach is able to exploit both spatial and spectral characteristics of the desired source signal resulting in a physically plausible spatial selectivity and superior speech quality compared to other baseline methods.

Generation of annular femtosecond few-cycle pulses by self-compression and spatial filtering in solid thin plates

Abstract: Annular-shaped femtosecond few-cycle pulses are generated by 40fs laser pulses propagating through 6 solid thin plates in numerical simulations as well as in experiments. The generation of such pulses takes advantage of the conical emission caused by plasma effect, which introduces continuously varying off-axis plasma density along the radial direction of the propagating beam. The negative dispersion induced by the plasma causes the pulse at particular radial location to be self-compressed and to form an annular beam of short pulse, which can be extracted simply by spatial filtering. Meanwhile, by adjusting the input pulse energy and position of each thin plate relative to the laser focus, we control the plasma density in thin plates which changes the ratio between ionization and effects providing positive dispersion, and obtain a higher compression ratio indicating that the scheme of solid thin plates has the flexibility to regulate the laser intensity so as to plasma density, thus the negative dispersion the pulse experiences during propagation. Few-cycle pulses as short as 8.8 fs are generated in experiments, meanwhile the shortest pulse duration found in the simulations is 5.0 fs, which corresponds to two optical cycles at its central wavelength 761 nm. This method has great potential in high-power few-cycle pulse generation.

Time-averaged image projection through a multimode fiber.

Abstract: Many disciplines, ranging from lithography to opto-genetics, require high-fidelity image projection. However, not all optical systems can display all types of images with equal ease. Therefore, the image projection quality is dependent on the type of image. In some circumstances, this can lead to a catastrophic loss of intensity or image quality. For complex optical systems, it may not be known in advance which types of images pose a problem. Here we show a new method called Time-Averaged image Projection (TAP), allowing us to mitigate these limitations by taking the entire image projection system into account despite its complexity and building the desired intensity distribution up from multiple illumination patterns. Using a complex optical setup, consisting of a wavefront shaper and a multimode optical fiber illuminated by coherent light, we succeeded to suppress any speckle-related background. Further, we can display independent images at multiple distances simultaneously, and alter the effective sharpness depth through the algorithm. Our results demonstrate that TAP can significantly enhance the image projection quality in multiple ways. We anticipate that our results will greatly complement any application in which the response to light irradiation is relatively slow (one microsecond with current technology) and where high-fidelity spatial distribution of optical power is required.

Emphasising spatial structure in geosocial media data using spatial amplifier filtering

Abstract: In this article, a new method called spatial amplifier filtering is proposed The presented method is related to Moran eigenvector filtering and allows the accentuation of spatial structures in het