Showing papers on "Spatial filter published in 2004"

A surface EMG generation model with multilayer cylindrical description of the volume conductor

Abstract: We propose a model for surface electromyography (EMG) signal generation with cylindrical description of the volume conductor. The model is more general and complete with respect to previous approaches. The volume conductor is described as a multilayered cylinder in which the source can be located either along the longitudinal or the angular direction, in any of the layers. The source is represented as a spatio-temporal function which describes the generation, propagation, and extinction of the intracellular action potential at the end-plate, along the fiber, and at the tendons, respectively. The layers are anisotropic. The volume conductor effect is described as a two-dimensional spatial filtering. Electrodes of any shape or dimension are simulated, forming structures which are described as spatial filters. The analytical derivation which leads to the signal in the temporal domain is performed in the spatial and temporal frequency domains. Numerical issues related to the frequency-based approach are discussed. The descriptions of the volume conductor and of the source are applied to the cases of signal generation from a limb and a sphincter muscle. Representative simulations of both cases are provided. The resultant model is based on analytical derivations and constitutes a step forward in surface EMG signal modeling, including features not described in any other analytical approach.

Independent phase and amplitude control of a laser beam by use of a single-phase-only spatial light modulator.

Abstract: A phase-only spatial light modulator is used in conjunction with a spatial filter to provide independent control of the phase and amplitude of a laser beam. Continuous amplitude modulation of the beam is achieved with a resolution relevant to beam shaping of high-energy laser beams. Amplitude beam correction in a closed loop is demonstrated.

Spatially filtered wave-front sensor for high-order adaptive optics.

Abstract: Adaptive optics (AO) systems take sampled measurements of the wave-front phase. Because in the general case the spatial-frequency content of the phase aberration is not band limited, aliasing will occur. This aliasing will cause increased residual error and increased scattered light in the point-spread function (PSF). The spatially filtered wave-front sensor (SFWFS) mitigates this phenomenon by using a field stop at a focal plane before the wave-front sensor. This stop acts as a low-pass filter on the phase, significantly reducing the high-spatial-frequency content phase seen by the wave-front sensor at moderate to high Strehl ratios. We study the properties and performance of the SFWFS for open- and closed-loop correction of atmospheric turbulence, segmented-primary-mirror errors, and sensing with broadband light. In closed loop the filter reduces high-spatial-frequency phase power by a factor of 103 to 108. In a full AO-system simulation, this translates to a reduction by up to 625 times in the residual error power due to aliasing over a specific spatial frequency range. The final PSF (generated with apodization of the pupil) has up to a 100 times reduction in intensity out to λ/2d.

Spatially incoherent illumination as a mechanism for cross-talk suppression in wide-field optical coherence tomography

Abstract: Comparison of two illumination modes for wide-field optical coherence tomography has revealed that spatially coherent illumination generates coherent cross talk, causing significant image degradation, and that spatially incoherent illumination, with an adequate interferometer design, provides an efficient mechanism for suppression of coherent cross talk. This is shown by comparison of a pulsed laser with a thermal light source for a U.S. Air Force resolution target covered with a scattering solution made from microbeads as well as for an ex vivo tooth.

High‐resolution wide‐field surface plasmon microscopy

Abstract: This paper describes the application of a Kohler illuminated high-resolution wide-field microscope using surface plasmons to provide the image contrast. The response of the microscope to a grating structure in both the Fourier and the image planes is presented to demonstrate image formation by surface waves. The effect of spatial filtering in the back focal (Fourier) plane to enhance image contrast is described. We also discuss how the surface wave contrast mechanism affects the imaging performance of the microscope and discuss factors that can be expected to lead to even greater improvements in lateral resolution and sensitivity.

First- and second-order information in natural images: a filter-based approach to image statistics.

Abstract: Previous analyses of natural image statistics have dealt mainly with their Fourier power spectra. Here we explore image statistics by examining responses to biologically motivated filters that are spatially localized and respond to first-order (luminance-defined) and second-order (contrast- or texture-defined) characteristics. We compare the distribution of natural image responses across filter parameters for first- and second-order information. We find that second-order information in natural scenes shows the same self-similarity previously described for first-order information but has substantially less orientational anisotropy. The magnitudes of the two kinds of information, as well as their mutual unsigned correlation, are much stronger for particular combinations of filter parameters in natural images but not in unstructured fractal images having the same power spectra.

Unified MIMO transmission and reception

Abstract: A “unified” MIMO system that supports multiple operating modes for efficient data transmission is described. Each operating mode is associated with different spatial processing at a transmitting entity. For example, four operating modes may be defined for (1) full-CSI or partial-CSI transmission and (2) with or without steering transmit diversity (STD). An appropriate operating mode may be selected for use based on various factors (e.g., availability of a good channel estimate). With steering transmit diversity, data is spatially spread and transmitted on multiple spatial channels, and a single rate may then be used for all spatial channels used for data transmission. A receiving entity may utilize a minimum mean square error (MMSE) technique for all operating modes. The receiving entity may derive a spatial filter matrix and perform receiver spatial processing in the same manner for all operating modes, albeit with different effective channel response matrices.

Method of removing noise from digital moving picture data

Abstract: Provided is a method of removing noise from digital moving picture data reducing the number of frames used in a temporal filtering operation and able to detect motion between frames easily. The method comprises a method of spatial filtering, a method of temporal filtering, and a method of performing the spatial filtering and the temporal filtering sequentially. The spatial filtering method applies a spatial filtering in a YCbCr color space, preserving a contour/edge in the image in the spatial domain, and generating a weight that is adaptive to the noise for discriminating the contour/edge in the temporal filtering operation. The temporal filtering method applies temporal filtering based on motion detection and scene change detection, compensating for global motion, the motion detection considering the brightness difference and color difference of the pixels compared between frames in the temporal filtering operation, and a weight that is adaptive to the noise for detecting the motion in the temporal filtering operation. The spatial filtering method is preferably performed first, and the temporal filtering method is performed with the result of the spatial filtering.

Noise reduction in video sequences using wavelet-domain and temporal filtering

Abstract: We develop a sequential wavelet domain and temporal filtering scheme, with jointly optimized parameters, which results in high-quality video denoising over a large range of noise levels. In this scheme, spatial filtering is performed by a spatially adaptive Bayesian wavelet shrinkage in a redundant wavelet representation. In the next filtering stage, a motion detector controls selective, recursive averaging of pixel intensities over time. The results demonstrate that the proposed filter outperforms recent single-resolution representatives as well as some recent motion-compensated wavelet based video filters. We also analyze important practical issues for possible industrial applications. In particular, we investigate the performance degradations that result from making the wavelet domain filtering part less complex, by removing the redundancy of the representation and/or by replacing a sophisticated spatially adaptive shrinkage method by soft-thresholding.

Highly coherent light at 13 nm generated by use of quasi-phase-matched high-harmonic generation.

Abstract: By measuring the fringe visibility in a Young's double pinhole experiment, we demonstrate that quasi-phase-matched high-harmonic generation produces beams with very high spatial coherence at wavelengths around 13 nm. To our knowledge these are the highest spatial coherence values ever measured at such short wavelengths from any source without spatial filtering. This results in a practical, small-scale, coherent, extreme-ultraviolet source that is useful for applications in metrology, imaging, and microscopy.

Emissive screen display with laser-based external addressing

Abstract: A display apparatus includes an emissive screen having luminescent pixels that are addressed solely by a laser addressing system. Each pixel includes a luminescent region located next to a photocathode. When struck by the laser beam, free electrons are created that are accelerated by an applied high voltage field from the photocathode to the luminescent region, thereby causing the luminescent region to emit visible light with a brightness (energy) that is substantially higher than the energy of the addressing beam. Apertures are optionally provided in hexagonal luminescent regions to relax beam-scanning requirements. Optional millichannel plates (crude versions of 2 nd generation night vision system Microchannel plates) are provided to enhance photon multiplication. A position sensitive device is implemented using the photocathode or photoanode (luminescent) material to facilitate the scanning and modulating process. Ambient light is prevented from generating unwanted pixel activation by filter coatings, spatial filtering or electronic filtering.

Common-path interferometry with one-dimensional periodic filters.

Abstract: We discuss a spatial filtering interferometry setup that employs a periodic spatial filter with either cosine transmittance or binary phase modulation. The setup's input plane is formed by two separate windows, one of which supports a phase object and the other, a reference beam. Using the appropriate frequency and orientation of the filter produces an interference pattern of the two input fields at the output plane of the system. The main attributes and advantages of the setup are discussed and experimentally illustrated with the example of a binary phase periodic filter implemented with a spatial light modulator.

Phase locking in a fiber laser array with varying path lengths

Abstract: Phase locking has been demonstrated in fiber lasers coupled to a common resonator containing a spatial filter. The phase-locked mode is highly stable despite the phase variations in the individual elements caused by thermal and mechanical effects. The ability to maintain phase locking is attributed to a self-adjusting process, which can be found only in systems with a combination of high gain, long length, low Q, and broad gain bandwidth, all of which can be met in fiber laser arrays.

Spatial filtering of RF interference in radio astronomy using a reference antenna

Abstract: Radio astronomical observations are increasingly contaminated by RF interference. Assuming an array of telescopes, we have previously considered spatial filtering techniques based on projecting out the interferer array signature vector. In this paper, we consider extending the astronomical array with a reference antenna (or array), and develop spatial filtering algorithms for this situation. The information from the reference antenna improves the quality of the interferer signature vector estimation, hence more of the interference can be projected out. The conditioning of the problem improves as well. The algorithms are tested both on simulated and experimental data.

Image processing apparatus

Abstract: An input image is inputted into a pre-process circuit and a spatial filter. The pre-process circuit emphasizes a linear shadow or an edge to some extent, or reduces noises overall. Following pre-processing, the image signal is supplied to a direction detection circuit, a direction and its intensity of an area of interest are detected. These two results of the detections are correlated with peripheral areas of interest to some extent by a vector filter. An output of the vector filter is supplied to a spatial filter coefficient generation circuit, and a filter coefficient of the spatial filter is decided. The spatial filter processes the input image based on the direction of the shadow and its intensity, whereby selectively suppressing noise components of the background in real time without damaging the contrasts of a linear shadow or an edge-like shadow.

Array signal processing for radio astronomy

Abstract: Radio astronomy forms an interesting application area for array signal processing techniques. Current synthesis imaging telescopes consist of a small number of identical dishes, which track a fixed patch in the sky and produce estimates of the time-varying spatial covariance matrix. The observations sometimes are distorted by interference, e.g., from radio, TV, radar or satellite transmissions. We describe some of the tools that array signal processing offers to filter out the interference, based on eigenvalue decompositions and factor analysis, which is a more general technique applicable to partially calibrated arrays. We consider detection of interference, spatial filtering techniques using projections, and discuss how a reference antenna pointed at the interferer can improve the performance. We also consider image formation and its relation to beamforming.

4f coherent imager system and its application to nonlinear optical measurements

Abstract: We present a powerful but simple technique based on a 4f coherent imager system with top-hat beams to characterize nonlinear optical properties. We describe the theoretical model and the experimental details of the measurement for materials having nonlinear refraction with or without nonlinear absorption. We show that it is possible to characterize the nonlinearities by analyzing the intensity profile of the image after nonlinear filtering through the material placed in the Fourier plane of the setup. We will show that, as in the Z-scan technique, the use of top-hat beams instead of Gaussian beams increases the sensitivity of the measurement. Intensity-dependent nonlinearities can be studied by use of this single laser-shot technique. We validate this nonlinear imaging technique by measuring the absolute value of the n2 coefficient for CS2 and some well-known chalcogenide glasses (As2S3, As2Se3, GeSe4, and Ge10As10Se80). Our values are in good agreement with those obtained by other techniques.

Experimental and theoretical study of semiconductor laser dynamics due to filtered optical feedback

Abstract: We report experimental results on the nonlinear dynamical response of a semiconductor laser subjected to time-delayed (>5 ns), frequency selective, optical feedback from a Fabry-Pe/spl acute/rot interferometer type of filter. Three regimes of interest, based on the relative value of the filter bandwidth with respect to the relevant laser parameters (relaxation oscillation frequency and external cavity mode spacing), are identified, viz. a wide filter case, an intermediate filter width case, and a narrow filter case. The dynamical response of the laser is shown to be quite different in each of these regimes. The principal results are 1) the laser's linewidth enhancement factor, coupled with the nonlinear response of the filter, can be exploited to induce nonlinear dynamics in the instantaneous optical frequency of the laser light on a time scale related to the time-delay of the feedback, 2) a mode mismatch effect which arises from a detuning between the filter center frequency and the nearest external cavity mode and manifests itself in a reduction of the maximum light available for feedback, and 3) a reduction in, or even disappearance of, relaxation oscillations in the laser dynamics when a filter of appropriate width is chosen. More generally, it is observed that certain dynamics that occur due to unfiltered optical feedback may be suppressed when the feedback light is spectrally filtered.

Equalization of modal dispersion in multimode fiber using spatial light modulators

Abstract: Intersymbol interference (ISI) due to modal dispersion is the dominant limitation to the bit rate-distance product in multimode fiber-optic communication systems. If the light launched into the fiber excites only the desired principal modes, modal dispersion can be eliminated. We can achieve this by using spatial light modulators (SLMs) to perform adaptive spatial filtering on the electric fields of the light. In this paper, we develop an optimization framework for setting the SLMs to obtain an upper bound on the achievable performance and develop heuristics that nearly reach this upper bound Using this framework, we show that both a sophisticated semidefinite programming-based algorithm and a simple adaptive algorithm achieve performance close to the upper bound. Performance and system complexity tradeoff curves are constructed, showing that a 20/spl times/20 array of SLM pixels with binary phase control performs within 15% of more complex implementations. Finally, we extend the framework and present preliminary results showing the promise of further increases in the capabilities of multimode fiber by using the fiber as a multiple-input multiple-output (MIMO) transmission medium.

Source localization using subspace estimation and spatial filtering

Abstract: Subspace-tracking algorithms have traditionally been unable to deal with a large number of sources and at the same timepreserve their computationally efficiency, since, typically, efficiency goes down as the cube of the signal subspace dimension. One solution to this problem, which is presented in this paper, is to use a newly developed algorithm for the design of spatial filters in matrix form, in order to spatially filter the incoming data snapshots. The result is that the signal subspaces are confined to small angular sectors and, thus, the effective number of signals present is reduced. A method is developed for designing spatial filters in an efficient manner by formulating the design procedure as a rank-deficient linear least-squares problem. The source-bearing estimation is done using the signal-covariance matrix, which is updated using a recently developed fast algorithm, which is necessary in situations where one or more sources are nonstationary. The combination of the subspace-based bearing-estimation and spatial filter algorithms is shown to give good performance in cases of medium signal-to-noise ratio and is capable of resolving sources that are below the resolution limit of both conventional and adaptive beamforming. In addition, the use of spatial filtering makes it possible to estimate bearings for more than N narrow-band sources, using an N-element array. An example illustrating this capability is given.

Thin-film large-area coherent light source, filter and amplifier apparatus and method

Abstract: Lasing at the edge of the reflection band or at a defect state within the reflection band of a thin one-dimensional feedback structure is used to create a large-area, thin-film laser source with transverse dimensions that can be much greater than the film thickness. Angular confinement of radiation propagating perpendicular to the layers leads to a spreading of the beam inside the medium which is much greater than the diffraction divergence. This enhances the spatial extent of correlation at the output surface of the thin film. When a pump source induces gain at the lasing threshold in a wide region, a spatially coherent monochromatic light beam is emitted perpendicular to the film surface from the entire gain region. Alternate embodiments of the present invention include a passive spatial filter and an active amplifier.

Novel optical spatial filtering methods based on two-dimensional photodetector arrays

Abstract: In this paper, modified opto-electronic non-contact measuring techniques for the determination of velocity, direction of motion and particle size are presented. These techniques are based on the use of two-dimensional structured photodetector arrays such as CMOS sensors. The principle of measurement utilized is the optical spatial filtering method, which is in widespread use in industrial velocimeters.

Spatial filtering in interferometry

Abstract: Spatial filtering of beams in interferometry systems is used to reduce a displacement of the beams from an optical path corresponding to the path of the beams in an optimally-aligned system. By reducing beam displacement from the optical path, the system reduces the magnitude of beam shears and associated non-cyclic errors in linear and angular displacements measured by the interferometry systems.

Electrically switchable spatial filter based on polymer-dispersed liquid crystal film

Abstract: This work demonstrates the feasibility of using polymer-dispersed liquid crystal (PDLC) films as electrically switchable spatial filters in the optical signal process. The fabrication relies on the fact that the size of the liquid crystal droplet formed in a PDLC film is inversely proportional to the intensity of curing. Controlling the driving voltage on the PDLC sample can filter particular spatial frequencies in the Fourier optical signal process. A simulation is also performed, and the results are highly consistent with those of experiments.

Method and system for electronic spatial filtering of spectral reflectometer optical signals

Abstract: A method for determining endpoint of plasma processing of a semiconductor wafer includes providing a light source, and providing a lens system to collimate and align light from the light source to an active surface of the semiconductor wafer. A plurality of light detector fibers are interleaved among light source fibers which transmit light from the light source to the lens system. Reflected light from the active surface of the semiconductor wafer is received by a plurality of light detector fibers and provided to an imaging spectrometer. The received reflected light is analyzed by the imaging spectrometer, and matched to a model optical signal. The matched optical signal is selected to determine endpoint or other state of the plasma processing.

Single-Mode versus multimode interferometry: a performance study

Abstract: We compare performances of ground-based single-mode and multimode (speckle) interferometers in the presence of partial Adaptive Optics correction of atmospheric turbulence. It is first shown that for compact sources (i.e. sources smaller than the Airy disk of a single telescope) not entirely resolved by the interferometer, the remarkable property of spatial filtering of single-mode waveguides coupled with AO correction significantly reduces the speckle noise which arises from residual wavefront corrugations. Focusing on those sources, and in the light of the AMBER experiment (the near infrared instrument of the VLTI), we show that single-mode interferometry produces a better Signal-to-Noise Ratio on the visibility than speckle interferometry. This is true for bright sources (K < 5), and in any case as soon as Strehl ratio of 0.2 is achieved. Finally, the fiber estimator is much more robust - by two orders of magnitude - than the speckle estimator with respect to Strehl ratio variations during the calibration procedure. The present analysis theoretically explains why interferometry with fibers can produce visibility measurements with a very high precision, 1% or less.

Ghost imaging schemes: fast and broadband.

Abstract: In ghost imaging schemes information about an object is extracted by measuring the correlation between a beam that passed the object and a reference beam. We present a spatial averaging technique that substantially improves the imaging bandwidth of such schemes, which implies that information about high-frequency Fourier components can be observed in the reconstructed diffraction pattern. In the many-photon regime the averaging can be done in parallel and we show that this leads to a much faster convergence of the correlations. We also consider the reconstruction of the object image, and discuss the differences between a pixel-like detector and a bucket detector in the object arm. Finally, it is shown how to non-locally make spatial filtering of a reconstructed image. The results are presented using entangled beams created by parametric down-conversion, but they are general and can be extended also to the important case of using classically correlated thermal-like beams.

Spectral Vorticity and Lagrangian Velocity Measurements in Turbulent Jets

Abstract: In this paper we report an experimental investigation of various statistical properties of the spatial Fourier modes of the vorticity field in turbulent jets for a large range of Reynolds numbers (530 ≤R λ≤ 6100). The continuous time evolution of a spatial Fourier mode of the vorticity distribution, characterized by a well-defined wavevector, is obtained from acoustic scattering measurements. The spatial enstrophy spectrum, as a function of the spatial wave-vector, is determined by scanning the incoming sound frequencies. Time-frequency analysis of the turbulent vorticity fluctuations is also performed for different length scales of the flows. Vorticity time-correlations show that the characteristic time of a Fourier mode behaves as the sweeping time. Finally, we report preliminary Lagrangian velocity measurements obtained using acoustic scattering by soap bubbles inflated with helium. Gathering a large number of passages of isolated bubbles in the scattering volume, one is able to compute the Lagrangian velocity PDF and velocity spectrum. Despite the spatial filtering due to the finite size of the bubble, the latter exhibits a power law, with the -2 exponent predicted by the Kolmogorov theory, over one decade of frequencies.