Showing papers on "Spatial filter published in 2007"

Generation of arbitrary vector beams with a spatial light modulator and a common path interferometric arrangement

Abstract: We describe a convenient approach for generating arbitrary vector beams in a 4-f system with a spatial light modulator (SLM) and a common path interferometric arrangement. A computer-generated hologram is introduced onto SLM for performing the beam conversion. Optical realization of a variety of polarization configurations confirms the reliability and flexibility of our method.

Semiparametric Filtering of Spatial Autocorrelation: The Eigenvector Approach

Abstract: In the context of spatial regression analysis, several methods can be used to control for the statistical effects of spatial dependencies among observations. Maximum likelihood or Bayesian approaches account for spatial dependencies in a parametric framework, whereas recent spatial filtering approaches focus on nonparametrically removing spatial autocorrelation. In this paper we propose a semiparametric spatial filtering approach that allows researchers to deal explicitly with (a) spatially lagged autoregressive models and (b) simultaneous autoregressive spatial models. As in one non-parametric spatial filtering approach, a specific subset of eigenvectors from a transformed spatial link matrix is used to capture dependencies among the disturbances of a spatial regression model. However, the optimal subset in the proposed filtering model is identified more intuitively by an objective function that minimizes spatial autocorrelation rather than maximizes a model fit. The proposed objective function has the a...

PROPER: an optical propagation library for IDL

Abstract: PROPER is a library of IDL (Interactive Data Language) routines for simulating optical propagation in the near and far fields using Fourier-based Fresnel and angular spectrum methods. The goal of PROPER is to provide a free, easy-to-use, and versatile means for simulating systems that require diffraction-based rather than geometrical analyses, such as spatial filtering systems with intermediate optics (e.g. a stellar coronagraph for extrasolar planet imaging). It has routines for creating complex apertures and obscurations, wavefront errors (defined by Zernikes, power spectra, or user-supplied maps), amplitude modulators (e.g. coronagraphic occulters), simple lenses, and deformable mirrors. The routines automatically select which propagator (near or far-field) is best at each surface based on analytically propagating a Gaussian pilot beam. The library includes a comprehensive manual and is distributed as IDL source code.

Dual resonator 1-D EBG antenna with slot array feed for improved radiation bandwidth

Abstract: A one-dimensional (1-D) electromagnetic bandgap (EBG) resonator antenna with both high gain and wide radiation bandwidth is described. The use of dual resonators in combination with an array of sources leads to a dramatic increase in the radiation bandwidth of the antenna. The effect of the dual resonators is to increase the bandwidth over which the EBG material acts as a spatial filter, while the array feed increases both the directivity and radiation bandwidth. Theoretical results for a prototype designed to operate at Ku-band are presented and shown to be in good agreement with measured results. The effect of increasing the number of array elements on directivity and radiation bandwidth is examined, and the improvements of the proposed configuration compared with a classical 1-D EBG resonator antenna with single source is presented.

Generation of hollow Gaussian beams by spatial filtering

Abstract: We demonstrate that hollow Gaussian beams can be obtained from Fourier transform of the differentials of a Gaussian beam, and thus they can be generated by spatial filtering in the Fourier domain with spatial filters that consist of binomial combinations of even-order Hermite polynomials. A typical 4f optical system and a Michelson interferometer type system are proposed to implement the proposed scheme. Numerical results have proved the validity and effectiveness of this method. Furthermore, other polynomial Gaussian beams can also be generated by using this scheme. This approach is simple and may find significant applications in generating the dark hollow beams for nanophotonic technology.

Laser beam quality and pointing measurement with an optical resonator.

Abstract: We present a compact diagnostic breadboard that is based on an optical ring resonator for measuring beam quality and pointing of single-frequency continuous wave lasers at a wavelength of 1064nm. To determine the beam quality of the coherent test beam, this optical resonator is used to perform a mode decomposition into Hermite-Gaussian modes. For our laser system, a power fraction in the fundamental Gaussian mode of 97.2%±0.2% was measured. Residual misalignment and mis-mode-matching to the resonator as well as the astigmatism and/or ellipticity of the test beam have been determined. Numerical simulations showed that measurements of the M2 factor and transversal intensity distribution are not suitable for determining this power fraction. To measure the beam pointing, the fundamental mode of the optical resonator was used as a stable reference. The pointing of the test beam was measured with the differential wave front sensing technique up to Fourier frequencies of 1kHz with a sensitivity to relative pointing of ∣ϵ∣=1×10−6∕Hz. Pointing measurements with an alternative method were performed and showed good agreement.

Binary-phase spatial filter for real-time swept-source optical coherence microscopy.

Abstract: We report a novel scheme to optimize the focusing condition for real-time, swept-source optical coherence microscopy. The axial and lateral behaviors of four-zone binary-phase spatial filters are presented numerically. A nearly constant axial intensity distribution along an extended depth of focus of 1.5 mm and a lateral resolution of 5 μm are experimentally verified. The A-line scan rate is up to 16 kHz, yielding a frame rate of 25 Hz and 640 lines per image.

From filters to features: scale-space analysis of edge and blur coding in human vision.

Abstract: To make vision possible, the visual nervous system must represent the most informative features in the light pattern captured by the eye. Here we use Gaussian scale-space theory to derive a multiscale model for edge analysis and we test it in perceptual experiments. At all scales there are two stages of spatial filtering. An odd-symmetric, Gaussian first derivative filter provides the input to a Gaussian second derivative filter. Crucially, the output at each stage is half-wave rectified before feeding forward to the next. This creates nonlinear channels selectively responsive to one edge polarity while suppressing spurious or "phantom" edges. The two stages have properties analogous to simple and complex cells in the visual cortex. Edges are found as peaks in a scale-space response map that is the output of the second stage. The position and scale of the peak response identify the location and blur of the edge. The model predicts remarkably accurately our results on human perception of edge location and blur for a wide range of luminance profiles, including the surprising finding that blurred edges look sharper when their length is made shorter. The model enhances our understanding of early vision by integrating computational, physiological, and psychophysical approaches. © ARVO.

Effect of Contour Intervals and Grid Cell Size on the Accuracy of DEMs and Slope Derivatives

Abstract: Digital Elevation Models (DEMs) are indispensable tools in many environmental and natural resource applications. DEMs are frequently derived from contour lines. The accuracy of such DEMs depends on different factors. This research investigates the effect of sampling density used to derive contours, vertical interval between contours (spacing), grid cell size of the DEM (resolution), terrain complexity, and spatial filtering on the accuracy of the DEM and the slope derivative. The study indicated different alternatives to achieve an acceptable accuracy depending on the contour interval, the DEM resolution and the complexity of the terrain. The effect of these factors on the accuracy of the DEM and the slope derivative was quantified using models that determine the level of accuracy (RMSE). The implementation of the models will guide users to select the best combination to improve the results in areas with similar topography. For areas with variable terrain complexity, the suggestion is to generate DEMs and slope at a suitable resolution for each terrain separately and then to merge the results to produce one final layer for the whole area. This will provide accurate estimates of elevation and slope, and subsequently improve the analyses that rely on these digital derivatives.

Adaptive mean filtering for noise reduction in CT polymer gel dosimetry

Abstract: X-ray computed tomography (CT) as a method of extracting 3D dose information from irradiated polymer gel dosimeters is showing potential as a practical means to implement gel dosimetry in a radiation therapy clinic. However, the response of CT contrast to dose is weak and noise reduction is critical in order to achieve adequate dose resolutions with this method. Phantom design and CT imaging technique have both been shown to decrease image noise. In addition, image postprocessing using noise reduction filtering techniques have been proposed. This work evaluates in detail the use of the adaptive mean filter for reducing noise in CT gel dosimetry. Filter performance is systematically tested using both synthetic patterns mimicking a range of clinical dose distribution features as well as actual clinical dose distributions. Both low and high signal-to-noise ratio (SNR) situations are examined. For all cases, the effects of filter kernel size and the number of iterations are investigated. Results indicate that adaptive mean filtering is a highly effective tool for noise reduction CT gel dosimetry. The optimum filtering strategy depends on characteristics of the dose distributions and image noise level. For low noise images (SNR approximately 20), the filtered results are excellent and use of adaptive mean filtering is recommended as a standard processing tool. For high noise images (SNR approximately 5) adaptive mean filtering can also produce excellent results, but filtering must be approached with more caution as spatial and dose distortions of the original dose distribution can occur.

Generalized phase contrast matched to Gaussian illumination

Abstract: We show that the generalized phase contrast method (GPC) can be used as a versatile tool for shaping an incident Gaussian illumination into arbitrary lateral beam profiles. For illustration, we use GPC in an energy-efficient phase-only implementation of various apertures that do not block light but instead effectively redirect the available photons from a bell-shaped light distribution. GPC-based generation of lateral beam profiles can thus be achieved using a simplified optical implementation as it eliminates the need for a potentially lossy initial beam shaping. The required binary phase input is simple to fabricate for static applications and can be easily reconfigured up to device frame refresh rates for dynamic applications.

Chromatism compensation of the PETAL multipetawatt high-energy laser.

Abstract: High-energy petawatt lasers use series of spatial filters in their amplification section. The refractive lenses employed introduce longitudinal chromatism that can spatially and temporally distort the ultrafast laser beam after focusing. To ensure optimum performances of petawatt laser facilities, these distortions need to be corrected. Several solutions using reflective, refractive, or diffractive optical components can be addressed. We give herein a review of these various possibilities with their application to the PETAL (Petawatt Aquitaine Laser at the Laser Integration Line facility) laser beamline and show that diffractive-based corrections appear to be the most promising.

Mode-selective spatial filtering for increased robustness in a mode group diversity multiplexing link

Abstract: We propose a very simple optical method to reduce the cross talk among the channels of a mode group diversity multiplexing (MGDM) link. MGDM is an intensity modulation, direct detection, multiple-input, multiple-output technique that creates independent communication channels over a multimode fiber (MMF). The cross talk among the channels is mitigated electronically. However, by properly employing a lens between the output of a graded-index MMF and the detectors, we achieve mode-selective spatial filtering (MSSF) and optically reduce the cross talk. The robustness of the link is then increased when compared with an implementation without MSSF. This allows for a larger number of channels.

Methods and apparatus for edge-based spatio-temporal filtering

Abstract: There are provided methods and apparatus for edge-based spatio-temporal filtering. An apparatus for filtering a sequence of pictures includes a spatial filter (110, 190), a motion compensator (130), a deblocking filter (140), and a temporal filter (150). The spatial filter (110, 190) is for spatially filtering a picture in the sequence and at least one reference picture selected from among previous pictures and subsequent pictures in the sequence with respect to the picture. The motion compensator (130), in signal communication with the spatial filter, is for forming, subsequent to spatial filtering, multiple temporal predictions for the picture from the at least one reference picture. The deblocking filter (140), in signal communication with the motion compensator, is for deblock filtering the multiple temporal predictions. The temporal filter (150), in signal communication with the deblocking filter, is for temporally filtering the multiple temporal predictions and combining the multiple temporal predictions to generate a noise reduced version of the picture.

Fiber coupled artificial compound eye

Abstract: A multiple aperture array, wide angle imaging system incorporates compound refractive optics modeled after the eyes of insects. The system channels light through the apertures of a convex spatial filter and a pair of lenslet arrays hot press molded on a positive meniscus form. The lenslets act as afocal Keplerian telescopes to superpose light from hundreds of adjacent channels to a common point on the convex surface of a fiber optic imaging taper. The superposed light from all the channels form a curved, high intensity image that is transformed by the taper into a flat format for readout by a mosaic detector array. The image is upright and distortion free with an infinite depth of field. Ghost images are blocked by a honeycomb louver baffle positioned between the lenslets and the imaging taper. The system is conformable to the geometry of any convex mounting surface, whether spherical, aspherical, or cylindrical.

An Improved Motion-Compensated 3-D LLMMSE Filter With Spatio–Temporal Adaptive Filtering Support

Abstract: In this paper, we propose a motion-compensated spatio-temporal Locally adaptive linear minimum mean squared-error (LLMMSE) filter for noisy image sequences with both temporal and spatially adaptive filtering support. Motion compensation and an adaptive temporal filtering support (TFS) structure guarantee the uniformity of the TFS. An intelligent pixel aggregation algorithm is proposed to include homogeneous neighboring pixels and exclude the outlier pixels, resulting in uniform spatial filtering support. By using the proposed spatio-temporal LLMMSE filter with uniform spatio-temporal support, we can reduce noise efficiently without introducing visually disturbing blurring artifacts. Furthermore, we employ an adaptive weighted local mean and variance estimation algorithm to improve the accuracy of estimation. The weights provide an implicit mechanism for deemphasizing the contribution of the outlier pixels which are wrongly kept within the support to avoid blurring. The performance of the proposed filter is quantitatively evaluated and compared with state-of-the-art methods. The results demonstrate that the proposed filter can achieve superior filtering performance, both subjectively and objectively.

Confocal microscope apparatus

Abstract: A confocal microscope apparatus comprises a first optical scanning system which obtains a scan image of a sample using a laser beam from a first laser light source, a second optical scanning system which scans specific regions of a sample with a laser beam from a second laser light source that is different from the first laser light source, thereby causing a particular phenomenon, and a beam diameter varying mechanism which can change the beam diameter of the laser beam of at least one of the first optical scanning system and the second optical scanning system With this configuration, the apparatus further comprises an excitation light intensity distribution calculator which calculates and stores the excitation light intensity distribution along a depth direction on the sample surface from the beam diameter of the laser beam output from the beam diameter varying mechanism

Multi-object wavefront sensor with spatial filtering

Abstract: The present invention relates to an adaptive optics sensor intended for simultaneous detection of several wave fronts on a common camera target. The sensor is intended for use in connection with multi-conjugate adaptive optics (MCAO), where several wave front measurements are needed at the same time. The sensor includes a spatial filter taking out signals resulting from parasitic reflections of the reference 10 sources and from unwanted parts of the object.

Optical coherent tomography

Abstract: A surface emission laser light source is used as a tunable laser light source. Since the surface emission laser light source can realize a broad frequency scanning range at a high speed and in the single mode, a coherent length is longer than that of a multi mode light source. For this reason, when a tomography image is calculated by executing the Fourier transform for an output obtained from an interference optical device, measuring depth can be deepened.

Directionality of generalized acoustic sensors of arbitrary order

Abstract: For several decades there has been a great deal of interest in acoustic sensors that can make multiple measurements at a single point in the ocean The order of such sensors has been defined by linking it to the order of the Taylor series approximation of the pressure field at that point Following this definition, the pressure, vector, and dyadic sensor is of order zero, one, and two, respectively For this theoretical study, a multichannel three-dimensional spatial filter is derived for a directional acoustic sensor of arbitrary order Explicit formulas are found for the filter coefficients that maximize the array gain (directivity index) of the filter as well as an explicit expression for the maximum array gain (directivity index) This process is repeated for the case of a first-order null placed in the direction opposite to the look direction of the multichannel filter Finally, an example is presented which tracks the array gain and beamwidth of a third-order acoustic sensor as the order of the null is assigned values 0, 1, 2, and 3

Hybrid multinary modulation using a phase modulating spatial light modulator and a low-pass spatial filter

Abstract: We propose a method for performing binary intensity and continuous phase modulation of beams with a spatial light modulator (SLM) and a low-pass spatial filtering 4-f system. With our method it is possible to avoid the use of phase masks in holographic data storage systems or to enhance the phase encoding of the SLM by making it capable of binary amplitude modulation. The data storage capabilities and the limitations of the method are studied.

Simulation analysis of the restraining effect of a spatial filter on a hot image

Abstract: Based on the restraining effect that spatial filtering has on the frequency spectrum of a beam, from the small-scale focusing theory of Bespalov and Talanov (B-T theory) we derive an expression for the pinhole diameter of the spatial filter corresponding to the fastest growing frequency. Then, compared with the theoretical pinhole diameter of the spatial filter, the restraining effect of the spatial filter on a hot image with different pinhole diameters is numerically investigated. The numerical results show that, if the pinhole diameter is larger than the theoretical one, the hot-image intensity will remain steady; once the pinhole diameter becomes smaller than the theoretical one, the hot-image intensity will begin to decrease. Moreover, as the pinhole diameter decreases, a more prominent restraining effect can be obtained. But reducing the diameter of the spatial filter would lead to greater beam energy loss. The parameters of the spatial filter must be chosen to guarantee that the scheme fulfills the demand for low beam energy loss and a satisfactory restraining effect simultaneously.

Spatial filters for high-peak-power multistage laser amplifiers

Abstract: We describe spatial filters used in a Nd:glass laser with an output pulse energy up to 300 J and a pulse duration of 1 ns. This laser is designed for pumping of a chirped-pulse optical parametric amplifier. We present data required to choose the shape and diameter of a spatial filter lens, taking into account aberrations caused by spherical surfaces. Calculation of the optimal pinhole diameter is presented. Design features of the spatial filters and the procedure of their alignment are discussed in detail.

Designing a holographic modal wavefront sensor for the detection of static ocular aberrations

Abstract: The extent to which holographic modal wavefront sensing can be applied to the detection of ocular aberrations was investigated. First, the idea of extending the dynamic range of the sensor by increasing the mask bias and the collection area of the pinhole detectors used in the sensor is reviewed. Errors in the detection of single-mode aberrations owing to reduced coherence from retinal scattering, photon, readout, and quantization noise are evaluated. A sensitivity-to-noise metric is introduced to evaluate sensor designs and is found to be maximized by using a pinhole detector radius of 8.6(fλ/NΔ) for every wave of mask bias (where f=transform lens focal length, λ=wavelength, and N and Δ are the number and size of the hologram pixels, respectively). The problem of detecting ocular aberrations composed of multiple modes required a generalization of the sensitivity measure to include all incident aberration modes. A

Self-focusing suppression in a system of two nonlinear media and a spatial filter

Abstract: It is shown that the rate of development of spatial instability caused by small-scale self-focusing strongly depends on the mutual arrangement of nonlinear media and spatial filters in a setup. The expressions are obtained for the arrangement of elements providing the minimal growth rate of intensity fluctuations. The results of two-dimensional calculations confirm the efficiency of this method of suppressing small-scale self-focusing.

History-based spatio-temporal noise reduction

Abstract: Implementations of a history-based temporal motion noise filtering technique that considers the temporal smooth among multiple pictures as well as the block-based technique to estimate the noise/motion history to better reflect the spatial/temporal smoothness in the local neighborhood are provided. In particular, implementations of the per-pixel blending technique in the block-based noise measurement may be used to better manipulate pixels in both the spatial and temporal domains. A global noise detection technique may be used to estimate the occurrence and/or strength of the noise. A content adaptive spatial filtering content adaptive spatial filter based on a local edge measurement may be used to reduce picture noise as well as preserve edge sharpness. Implementations may be configured for various applications. In particular, programmability options allow users to specify the filter parameters for singularity detection, spatial-only, temporal-only and spatial-temporal filters to achieve user desirable viewing experience.