Showing papers on "Spatial filter published in 2015"

Active illumination using a digital micromirror device for quantitative phase imaging.

Abstract: We present a powerful and cost-effective method for active illumination using a digital micromirror device (DMD) for quantitative phase-imaging techniques. Displaying binary illumination patterns on a DMD with appropriate spatial filtering, plane waves with various illumination angles are generated and impinged onto a sample. Complex optical fields of the sample obtained with various incident angles are then measured via Mach-Zehnder interferometry, from which a high-resolution 2D synthetic aperture phase image and a 3D refractive index tomogram of the sample are reconstructed. We demonstrate the fast and stable illumination-control capability of the proposed method by imaging colloidal spheres and biological cells. The capability of high-speed optical diffraction tomography is also demonstrated by measuring 3D Brownian motion of colloidal particles with the tomogram acquisition rate of 100 Hz.

Security loophole in free-space quantum key distribution due to spatial-mode detector-efficiency mismatch

Abstract: In free-space quantum key distribution (QKD), the sensitivity of the receiver's detector channels may depend differently on the spatial mode of incoming photons. Consequently, an attacker can control the spatial mode to break security. We experimentally investigate a standard polarization QKD receiver and identify sources of efficiency mismatch in its optical scheme. We model a practical intercept-and-resend attack and show that it would break security in most situations. We show experimentally that adding an appropriately chosen spatial filter at the receiver's entrance may be an effective countermeasure.

Electromagnetic Field Transformations for Measurements and Simulations (Invited Paper)

Abstract: Electromagnetic field transformations are important for electromagnetic simulations and for measurements. Especially for field measurements, the influence of the measurement probe must be considered, and this can be achieved by working with weighted field transformations. This paper is a review paper on weighted field transformations, where new information on algorithmic properties and new results are also included. Starting from the spatial domain weighted radiation integral involving free space Green's functions, properties such as uniqueness and the meaning of the weighting function are discussed. Several spectral domain formulations of the weighted field transformation integrals are reviewed. The focus of the paper is on hierarchical multilevel representations of irregular field transformations with propagating plane waves on the Ewald sphere. The resulting Fast Irregular Antenna Field Transformation Algorithm (FIAFTA) is a versatile and efficient transformation technique for arbitrary antenna and scattering fields. The fields can be sampled at arbitrary irregular locations and with arbitrary measurement probes without compromising the accuracy and the efficiency of the algorithm. FIAFTA supports different equivalent sources representations of the radiation or scattering object: 1) equivalent surface current densities discretized on triangular meshes, 2) plane wave representations, 3) spherical harmonics representations. The current densities provide for excellent spatial localization and deliver most diagnostics information about the test object. A priori information about the test object can easily be incorporated, too. Using plane wave and spherical harmonics representations, the spatial localization is not as good as with spatial current densities, but still much better than in the case of conventional modal expansions. Both far-field based expansions lead to faster transformations than the equivalent currents and in particular the orthogonal spherical harmonics expansion is a very attractive and robust choice. All three expansions are well-suited for efficient echo suppression by spatial filtering. Various new field transformation and new computational performance results are shown in order to illustrate some capabilities of the algorithm.

Spatial filtering with photonic crystals

Abstract: Photonic crystals are well known for their celebrated photonic band-gaps—the forbidden frequency ranges, for which the light waves cannot propagate through the structure. The frequency (or chromatic) band-gaps of photonic crystals can be utilized for frequency filtering. In analogy to the chromatic band-gaps and the frequency filtering, the angular band-gaps and the angular (spatial) filtering are also possible in photonic crystals. In this article, we review the recent advances of the spatial filtering using the photonic crystals in different propagation regimes and for different geometries. We review the most evident configuration of filtering in Bragg regime (with the back-reflection—i.e., in the configuration with band-gaps) as well as in Laue regime (with forward deflection—i.e., in the configuration without band-gaps). We explore the spatial filtering in crystals with different symmetries, including axisymmetric crystals; we discuss the role of chirping, i.e., the dependence of the longitudinal period along the structure. We also review the experimental techniques to fabricate the photonic crystals and numerical techniques to explore the spatial filtering. Finally, we discuss several implementations of such filters for intracavity spatial filtering.

Atom interferometry in an optical cavity.

Abstract: We propose and demonstrate a new scheme for atom interferometry, using light pulses inside an optical cavity as matter wave beam splitters. The cavity provides power enhancement, spatial filtering, and a precise beam geometry, enabling new techniques such as low power beam splitters ( 75% contrast and measure the acceleration due to gravity, g, to 60 μg/sqrt[Hz] resolution in a Mach-Zehnder geometry. We use >10(7) cesium atoms in the compact mode volume (600 μm 1/e(2) waist) of the cavity and show trapping of atoms in higher transverse modes. This work paves the way toward compact, high sensitivity, multiaxis interferometry.

Directional support value of Gaussian transformation for infrared small target detection.

Abstract: Robust small target detection is one of the key techniques in IR search and tracking systems for self-defense or attacks. In this paper we present a robust solution for small target detection in a single IR image. The key ideas of the proposed method are to use the directional support value of Gaussian transform (DSVoGT) to enhance the targets, and use the multiscale representation provided by DSVoGT to reduce the false alarm rate. The original image is decomposed into sub-bands in different orientations by convolving the image with the directional support value filters, which are deduced from the weighted mapped least-squares–support vector machines (LS–SVMs). Based on the sub-band images, a support value of Gaussian matrix is constructed, and the trace of this matrix is then defined as the target measure. The corresponding multiscale correlations of the target measures are computed for enhancing target signal while suppressing the background clutter. We demonstrate the advantages of the proposed method on real IR images and compare the results against those obtained from standard detection approaches, including the top-hat filter, max–mean filter, max–median filter, min–local–Laplacian of Gaussian (LoG) filter, as well as LS–SVM. The experimental results on various cluttered background images show that the proposed method outperforms other detectors.

Analysis of focused laser differential interferometry

Abstract: A computational method for predicting the output of a focused laser differential interferometer (FLDI) given an arbitrary density field is presented. The method is verified against analytical predictions and experimental data. The FLDI simulation software is applied to the problem of measuring Mack-mode wave packets in a hypervelocity boundary layer on a 5° half-angle cone. The software is shown to complement experiments by providing the necessary information to allow quantitative density fluctuation magnitudes to be extracted from experimental measurements.

An Adaptive Spatial Filter for User-Independent Single Trial Detection of Event-Related Potentials

Abstract: Goal: Current brain–computer interfaces (BCIs) are usually based on various, often supervised, signal processing methods. The disadvantage of supervised methods is the requirement to calibrate them with recently acquired subject-specific training data. Here, we present a novel algorithm for dimensionality reduction (spatial filter), that is ideally suited for single-trial detection of event-related potentials (ERPs) and can be adapted online to a new subject to minimize or avoid calibration time. Methods: The algorithm is based on the well-known xDAWN filter, but uses generalized eigendecomposition to allow an incremental training by recursive least squares (RLS) updates of the filter coefficients. We analyze the effectiveness of the spatial filter in different transfer scenarios and combinations with adaptive classifiers. Results: The results show that it can compensate changes due to switching between different users, and therefore allows to reuse training data that has been previously recorded from other subjects. Conclusions: The presented approach allows to reduce or completely avoid a calibration phase and to instantly use the BCI system with only a minor decrease of performance. Significance: The novel filter can adapt a precomputed spatial filter to a new subject and make a BCI system user independent.

Electromagnetic Field Transformations for Measurements and Simulations

Abstract: Electromagnetic field transformations are important for electromagnetic simulations and for measurements. Especially for field measurements, the influence of the measurement probe must be considered, and this can be achieved by working with weighted field transformations. This paper is a review paper on weighted field transformations, where new information on algorithmic properties and new results are also included. Starting from the spatial domain weighted radiation integral involving free space Green's functions, properties such as uniqueness and the meaning of the weighting function are discussed. Several spectral domain formulations of the weighted field transformation integrals are reviewed. The focus of the paper is on hierarchical multilevel representations of irregular field transformations with propagating plane waves on the Ewald sphere. The resulting Fast Irregular Antenna Field Transformation Algorithm (FIAFTA) is a versatile and efficient transformation technique for arbitrary antenna and scattering fields. The fields can be sampled at arbitrary irregular locations and with arbitrary measurement probes without compromising the accuracy and the efficiency of the algorithm. FIAFTA supports different equivalent sources representations of the radiation or scattering object: 1) equivalent surface current densities discretized on triangular meshes, 2) plane wave representations, 3) spherical harmonics representations. The current densities provide for excellent spatial localization and deliver most diagnostics information about the test object. A priori information about the test object can easily be incorporated, too. Using plane wave and spherical harmonics representations, the spatial localization is not as good as with spatial current densities, but still much better than in the case of conventional modal expansions. Both far-field based expansions lead to faster transformations than the equivalent currents and in particular the orthogonal spherical harmonics expansion is a very attractive and robust choice. All three expansions are well-suited for efficient echo suppression by spatial filtering. Various new field transformation and new computational performance results are shown in order to illustrate some capabilities of the algorithm.

Spatial optical crosstalk in CMOS image sensors integrated with plasmonic color filters.

Abstract: Imaging resolution of complementary metal oxide semiconductor (CMOS) image sensor (CIS) keeps increasing to approximately 7k × 4k. As a result, the pixel size shrinks down to sub-2μm, which greatly increases the spatial optical crosstalk. Recently, plasmonic color filter was proposed as an alternative to conventional colorant pigmented ones. However, there is little work on its size effect and the spatial optical crosstalk in a model of CIS. By numerical simulation, we investigate the size effect of nanocross array plasmonic color filters and analyze the spatial optical crosstalk of each pixel in a Bayer array of a CIS with a pixel size of 1μm. It is found that the small pixel size deteriorates the filtering performance of nanocross color filters and induces substantial spatial color crosstalk. By integrating the plasmonic filters in the low Metal layer in standard CMOS process, the crosstalk reduces significantly, which is compatible to pigmented filters in a state-of-the-art backside illumination CIS.

High-speed incoming infrared target detection by fusion of spatial and temporal detectors.

Abstract: This paper presents a method for detecting high-speed incoming targets by the fusion of spatial and temporal detectors to achieve a high detection rate for an active protection system (APS). The incoming targets have different image velocities according to the target-camera geometry. Therefore, single-target detector-based approaches, such as a 1D temporal filter, 2D spatial filter and 3D matched filter, cannot provide a high detection rate with moderate false alarms. The target speed variation was analyzed according to the incoming angle and target velocity. The speed of the distant target at the firing time is almost stationary and increases slowly. The speed varying targets are detected stably by fusing the spatial and temporal filters. The stationary target detector is activated by an almost zero temporal contrast filter (TCF) and identifies targets using a spatial filter called the modified mean subtraction filter (M-MSF). A small motion (sub-pixel velocity) target detector is activated by a small TCF value and finds targets using the same spatial filter. A large motion (pixel-velocity) target detector works when the TCF value is high. The final target detection is terminated by fusing the three detectors based on the threat priority. The experimental results of the various target sequences show that the proposed fusion-based target detector produces the highest detection rate with an acceptable false alarm rate.

A Neural Network-Based Optimal Spatial Filter Design Method for Motor Imagery Classification

Abstract: In this study, a novel spatial filter design method is introduced. Spatial filtering is an important processing step for feature extraction in motor imagery-based brain-computer interfaces. This paper introduces a new motor imagery signal classification method combined with spatial filter optimization. We simultaneously train the spatial filter and the classifier using a neural network approach. The proposed spatial filter network (SFN) is composed of two layers: a spatial filtering layer and a classifier layer. These two layers are linked to each other with non-linear mapping functions. The proposed method addresses two shortcomings of the common spatial patterns (CSP) algorithm. First, CSP aims to maximize the between-classes variance while ignoring the minimization of within-classes variances. Consequently, the features obtained using the CSP method may have large within-classes variances. Second, the maximizing optimization function of CSP increases the classification accuracy indirectly because an independent classifier is used after the CSP method. With SFN, we aimed to maximize the between-classes variance while minimizing within-classes variances and simultaneously optimizing the spatial filter and the classifier. To classify motor imagery EEG signals, we modified the well-known feed-forward structure and derived forward and backward equations that correspond to the proposed structure. We tested our algorithm on simple toy data. Then, we compared the SFN with conventional CSP and its multi-class version, called one-versus-rest CSP, on two data sets from BCI competition III. The evaluation results demonstrate that SFN is a good alternative for classifying motor imagery EEG signals with increased classification accuracy.

Production of dynamic frozen waves: controlling shape, location (and speed) of diffraction-resistant beams.

Abstract: In recent times, we experimentally realized quite an efficient modeling of the shape of diffraction-resistant optical beams, thus generating for the first time the so-called frozen waves (FW), whose longitudinal intensity pattern can be arbitrarily chosen within a prefixed space interval of the propagation axis. In this Letter, we extend our theory of FWs, which led to beams endowed with a static envelope, through a dynamic modeling of the FWs whose shape is now allowed to evolve in time in a predetermined way. Further, we experimentally create such dynamic FWs (DFWs) in optics via a computational holographic technique and a spatial light modulator. Experimental results are presented here for two cases of DFWs, one of zeroth order and the other of higher order, the latter being the most interesting exhibiting a cylindrical surface of light whose geometry changes in space and time.

Retrieval of contaminated information using random lasers

Abstract: Data retrieval are an important information processing task. In optical information processing, the usual method is spatial filtering based on Fourier optics. However, these methods are very difficult to implement in practical applications. In recent years, random lasers due to its cavity free property have attracted widespread attention, but few applications have been reported. Here, we develop an information retrieval method based on random lasers, where the spatial frequency spectrum of a contaminated Fourier transform hologram can be obtained by detecting the temporal frequency spectrum information from random lasing. The hologram information can be reconstructed from an inverse Fourier transform of the spatial frequency spectrum obtained after data processing. This method may potentially find applications in information optics and optical data storage.

One-dimensional rainbow technique using Fourier domain filtering

Abstract: Rainbow refractometry can measure the refractive index and the size of a droplet simultaneously. The refractive index measurement is extracted from the absolute rainbow scattering angle. Accordingly, the angular calibration is vital for accurate measurements. A new optical design of the one-dimensional rainbow technique is proposed by using a one-dimensional spatial filter in the Fourier domain. The relationship between the scattering angle and the CCD pixel of a recorded rainbow image can be accurately determined by a simple calibration. Moreover, only the light perpendicularly incident on the lens in the angle (φ) direction is selected, which exactly matches the classical inversion algorithm used in rainbow refractometry. Both standard and global one-dimensional rainbow techniques are implemented with the proposed optical design, and are successfully applied to measure the refractive index and the size of a line of n-heptane droplets.

In Vivo Ultrasound Thermography in Presence of Temperature Heterogeneity and Natural Motions

Abstract: Real-time ultrasound thermography has been recently demonstrated on commercially available diagnostic imaging probes. In vitro experimental results demonstrate high sensitivity to small, localized temperature changes induced by subtherapeutic focused ultrasound. Most of the published results, however, are based on a thermally induced echo strain model that assumes infinitesimal change in temperature between imaging frames. Under this assumption, the echo strain is computed using a low-pass axial differentiator, which is implemented by a finite-impulse response digital filter. In this paper, we introduce a new model for temperature estimation, which employs a recursive axial filter that acts as a spatial differentiator–integrator of echo shifts. The filter is derived from first principles and it accounts for a nonuniform temperature baseline, when computing the spatial temperature change between two frames. This is a major difference from the previously proposed infinitesimal echo strain filter ( $\delta$ -ESF) approach. We show that the new approach can be implemented by a first-order infinite-impulse response digital filter with depth-dependent spatial frequency response. Experimental results in vitro demonstrate the advantages over the $\delta$ -ESF approach in terms of suppressing the spatial variations in the estimated temperature without resorting to ad hoc low-pass filtering of echo strains. The performance of the new recursive echo strain filter (RESF) is also illustrated using echo data obtained during subtherapeutic localized heating in the hind limb of Copenhagen rat in vivo . In addition to the RESF, we have used an adaptive spatial filter to remove motion and deformation artifacts during real-time data collection. The adaptive filtering algorithm is described and comparisons with uncompensated estimated spatio-temporal temperature profiles are given. The results demonstrate the feasibility of in vivo ultrasound thermography with high sensitivity and specificity.

A multichannel diffuse power estimator for dereverberation in the presence of multiple sources

Abstract: Using a recently proposed informed spatial filter, it is possible to effectively and robustly reduce reverberation from speech signals captured in noisy environments using multiple microphones. Late reverberation can be modeled by a diffuse sound field with a time-varying power spectral density (PSD). To attain reverberation reduction using this spatial filter, an accurate estimate of the diffuse sound PSD is required. In this work, a method is proposed to estimate the diffuse sound PSD from a set of reference signals by blocking the direct signal components. By considering multiple plane waves in the signal model to describe the direct sound, the method is suitable in the presence of multiple simultaneously active speakers. The proposed diffuse sound PSD estimator is analyzed and compared to existing estimators. In addition, the performance of the spatial filter computed with the diffuse sound PSD estimate is analyzed using simulated and measured room impulse responses in noisy environments with stationary noise and non-stationary babble noise.

Recursive approximation of the bilateral filter.

Abstract: This paper presents a complete proof that the bilateral filter can be implemented recursively, as long as: 1) the spatial filter can be implemented recursively and 2) the range filter can be decomposed into a recursive product. As a result, an O(ND) solution can be obtained for bilateral filtering, where N is the image size and D is the dimensionality.

Integrating Mach–Zehnder interferometry with TPIV to measure the time-resolved deformation of a compliant wall along with the 3D velocity field in a turbulent channel flow

Abstract: A system combining tomographic PIV (TPIV) and Mach–Zehnder interferometry (MZI) simultaneously measures the time-resolved 3D flow field and 2D distribution of wall-normal deformation in a turbulent channel flow over a transparent compliant surface. This paper focuses on the experimental techniques and data analysis procedures, but includes sample results. Standard TPIV analysis resolves the log layer of the mean velocity and the linear decrease in total shear stress with distance from the wall. Single-pixel ensemble correlations reveal the buffer layer and top of the viscous sublayer. Analysis of the MZI data consists of two steps, namely critical spatial filtering of interferograms to remove noise and phase demodulation to calculate the surface shape. A new technique to improve the filtration of noise from interferograms based on spatial correlations of small windows is introduced and optimized. Taking advantage of this enhancement, the phase/deformation distribution is calculated directly from arccosines of the intensity, which avoids edge artifacts affecting spectral calculations. Validations using synthetic noisy interferograms indicate that errors associated with correlation-based enhancement are consistently lower and much less sensitive to fringe shape than spectral band-pass filtering. The experimental wavenumber–frequency spectra show that the deformation consists of patterns that are larger than the field of view, surface waves and small-scale patterns. Some of the latter are advected at the freestream velocity, but mostly at 70 % of the freestream, the mean speed at 10 % of the channel half height. Indeed, spatial correlations of the deformation with velocity components peak at this elevation.

Concurrent spatial and spectral filtering by resonant nanogratings.

Abstract: Optical devices incorporating resonant periodic layers constitute an emerging technological area. Recent advances include spectral filters, broadband mirrors, and polarizers. Here, we demonstrate concurrent spatial and spectral filtering as a new outstanding attribute of this device class. This functionality is enabled by a unique, near-complete, reflection state that is discrete in both angular and spectral domains and realized with carefully-crafted nanogratings operating in the non-subwavelength regime. We study the pathway and inter-modal interference effects inducing this intriguing reflection state. In a proof-of-concept experiment, we obtain angular and spectral bandwidths of ~4 mrad and ~1 nm, respectively. This filter concept can be used for focus-free spectral and spatial filtering in compact holographic and interferometric optical instruments.

Non-local means image denoising with an adaptive directional spatial filter

Abstract: System, apparatus, method, and computer readable media for edge-enhanced non-local means (NLM) image denoising. In embodiments, edge detail is preserved in filtered image data by weighting of the noisy input target pixel value with other pixel values based on self-similarity and further informed by a data-driven directional spatial filter. Embodiments herein may denoise regions of an image lacking edge characteristics with a more uniform spatial filter than those having edge characteristics. In embodiments, directionality of a spatial filter function is modulated based on an edge metric to increase the weighting of pixel values along an edge when there is a greater probability the edge passes through the target pixel. In further embodiments, the adaptive spatial filter is elliptical and oriented relative to a spatial gradient direction with non-uniform filter widths that are based on the edge metric.

Preparing arbitrary pure states of spatial qudits with a single phase-only spatial light modulator

Abstract: We present a new method for preparing multidimensional spatial qudits by means of a single phase-only spatial light modulator (SLM). This method improves previous ones that use two SLMs, one working in amplitude regime and the other in phase regime. To that end, we addressed diffraction gratings on the slits that define the state and then we performed a spatial filtering in the Fourier plane. The amplitude of the coefficients of the quantum state are determined by the modulation deep of the diffraction gratings, and the relative phase is the mean phase value of the diffraction gratings. This encoding result to be more compact, less expensive and use the photons more efficiently.

Diffusive random laser modes under a spatiotemporal scope

Abstract: At present the prediction and characterization of the emission output of a diffusive random laser remains a challenge, despite the variety of investigated materials and theoretical interpretations given up to now. Here, a new mode selection method, based on spatial filtering and ultrafast detection, which allows to separate individual lasing modes and follow their temporal evolution is presented. In particular, the work explores the random laser behavior of a ground powder of an organic-inorganic hybrid compound based on Rhodamine B incorporated into a di-ureasil host. The experimental approach gives direct access to the mode structure and dynamics, shows clear modal relaxation oscillations, and illustrates the lasing modes stochastic behavior of this diffusive scattering system. The effect of the excitation energy on its modal density is also investigated. Finally, imaging measurements reveal the dominant role of diffusion over amplification processes in this kind of unconventional lasers.

Measurement of acceleration and orbital angular momentum of Airy beam and Airy-vortex beam by astigmatic transformation

Abstract: Special beams, including the Airy beam and the vortex-embedded Airy beam, draw much attention due to their unique features and promising applications. Therefore, it is necessary to devise a straightforward method for measuring these peculiar features of the beams with ease. Hence we present the astigmatic transformation of Airy and Airy-vortex beam. The “acceleration” coefficient of the Airy beam is directly determined from a single image by fitting the astigmatically transformed beam to an analytic expression. In addition, the orbital angular momentum of optical vortex in Airy-vortex beam is measured directly using a single image.

Zernike phase spatial filter for measuring the aberrations of the optical structures of the eye

Abstract: To measure directly the wavefront aberration coefficients, we propose to use the multi-order diffractive element fitted with the set of Zernike polynomials. Polynomials of lowest degree describe defocusing (ametropy) and astigmatism. Coefficients of highest degree correspond to the spherical aberration of oblique rays that occurs as a consequence of misalignment of the crystalline lens and foveola, as well as deflection at the periphery of the crystalline lens. Multi-order elements allow several tens of expansions coefficients to be measured simultaneously, which will enable to investigate insufficiently known high-order aberrations for the differentiated diagnostics of eye diseases.

Simple Broadband Quasi-Optical Spatial Multiplexer in Substrate Integrated Technology

Abstract: We present a new type of broadband spectral-spatial quasi-optical decomposer for microwave analog signal processing fully implemented in substrate integrated waveguide (SIW) technology. An input SIW is modulated to couple energy to a near-field focused surface wave inside the hosting substrate. Due to its inherent spatially dispersive response, the focal point location varies with frequency. Thus, by placing output SIW ports at the appropriate positions, the focused signal can be extracted for each frequency providing instantaneous spectral decomposition. A design with six output channels from 11 to 16 GHz with 4-dB insertion losses and 1-ns group-delay swing is illustrated. Experiments on a prototype are reported to prove the feasibility of the proposed approach.

Temporal filtering of independent color channels in image data

Abstract: An image processing pipeline may perform temporal filtering on independent color channels in image data. A filter weight may be determined for a given pixel received at a temporal filter. The filter weight may be determined for blending a value of a channel in a full color encoding of the given pixel with a value of the same channel for a corresponding pixel in a previously filtered reference image frame. In some embodiments, the filtering strength for the channel may be determined independent from the filtering strength of another channel in the full color encoding of the given pixel. Spatial filtering may be applied to a filtered version of the given pixel prior to storing the given pixel as part of a new reference image frame.

Distributed multi-channel coherent optical fiber sensing system

Abstract: A method and system are provided. The method includes converting, using a spatial mode converter, an input signal into a plurality of spatial modes and performing polarization multiplexing and mode multiplexing, using a polarization multiplexer and a mode multiplexer, respectively, on the input signal. The method further includes injecting the input signal into a fiber optic medium. The method additionally includes applying, using at least one spatial filter in each of a forward and a backward direction within the fiber optical medium, the plurality of spatial modes within the fiber optical medium to transmit the input signal and perform distributed fault sensing on the input signal simultaenously.