Showing papers on "Spatial filter published in 1981"

Measurement of submicron laser beam radii

Abstract: Three methods for direct measurement of the intensity distribution in laser beams focused by microscope optics to waists of submicron width are described and compared. They use scans of the beam waist with (1) a knife-edge, (2) a submicroscopic point fluorescent source, and (3) convolution scans generated by the photobleached pattern of the focused beam. An indirect photographic technique is also evaluated. The laser beam is found to propagate ideally down to a minimum size usually limited by the aberrations of the optics.

Light propagation through large laser systems

Abstract: The evolution of solid-state laser systems over the past decade, both through technological advances and through increased understanding of the interplay between nonlinear effects and linear diffraction, is reviewed. The role of numerical methods to simulate the several physical processes (diffraction, self-focusing, gain saturation) involved in coherent beam propagation through large laser systems is discussed. A comprehensive simulation code for modeling all of the pertinent physical phenomena observed in laser operations (growth of small-scale modulation, spatial filtering, imaging, gain saturation, and beam-induced damage) is described in detail. The realism and accuracy of results obtained with this numerical code stem from an unambiguous identification of the sources of spatial noise, and from the use of spatial filters in modern lasers to limit the transverse beam modulation scale within the practical computational range of a two-dimensional numerical analysis. Several comparisons between code results and solid-state laser output performance data are presented. Finally, the design and performance estimation of the large Nova laser system presently under construction at the Lawrence Livermore National Laboratory (LLNL) are given.

Ultrasonic multiple-beam technique for detecting cracks in bimetallic or coarse-grained materials

Abstract: A reliable ultrasonic method for testing inner diameter cladded pipe adjacent to welds comprising use of multiple beams, bands and pulses in addition to pulseshaping and beamforming, spectral and directional averaging, as well as spatial filtering and pattern recognition. An automated inspection mode aims short shear-wave pulses from different source locations and different beam orientations to detect signals characteristic of defects. A manual confirmation mode aims both longitudinal waves and shear waves at the suspected defect and analyzes the detected returned pulses by an associated pulse pattern recognition method.

Radial intensity filters using radial birefringent elements

Abstract: A birefringent crystal with a surface curvature ρ provides a radial variation of polarization retardation. One or more such radial birefringent elements in combination with a polarizer form a radial intensity filter (RIF). RIF’s permit the generation of a variety of radial transmittance and reflectance profiles that are useful in resonator and spatial filter applications.

Optical fiber mode separation systems

Abstract: When a multimode optical fiber is deformed in accordance with an input signal to be sensed, light from lower order core modes is coupled to higher order core modes. A lens assembly is provided which separates the light in the higher order core modes from the light remaining in the lower order core modes. A spatial filter in the lens assembly enables the lens or lenses therein to couple the separated modes to two different output fibers with high efficiency and low coupling loss. The two signals in the output fibers are processed so that the common components which constitute noise are cancelled, while the signal components are additively combined and appear at the system's output so as to provide a highly sensitive, low noise fiber optic sensor system.

Diffraction effects in free‐electron lasers

Abstract: The spatial photon intensity distribution is calculated for geometries pertinent to tapered‐wiggler free‐electron lasers. A large‐radius incident photon field interacts with a small‐radius e‐beam gain medium, and the diffraction of extracted energy from the gain medium to larger radii is analyzed in the limit of small gain. The results are compared with a simple fill‐factor approximation.

Potential and limits of holographic reconstruction algorithms

Abstract: For the purpose of ultrasonic nondestructive testing of materials, holography in connection with digital reconstruction algorithms has been proposed as a modern tool to extract crack sizes from ultrasonic scattering data. Defining the typical holographic reconstruction algorithm as the application of the scalar Kirchhoff diffraction theory to backward wave propagation, we demonstrate its general incapability of reconstructing equivalent sources, and hence, geometries of scattering bodies. Only the special case of a planar measurement recording surface, that is to say, a hologram plane, and a planar crack with perfectly rigid boundary conditions parallel to the hologram plane and perpendicular to the incident field yields a nearly perfect correlation between crack size and reconstructed image; the reconstruction algorithm is then referred to as the Rayleigh-Sommerfeld formula; it therefore represents the optimal case matched to that special geometrical situation and, hence, may be interpreted as a quasi-matched spatial filter. Using integral equation theory and physical optics, we compute synthetic holographic data for a linear cracklike scatterer for both plane and spherical wave incidence, the latter case simulating a synthetic aperture impulse echo situation, thus illustrating how the Rayleigh-Sommerfeld algorithm or its Fresnel approximation increasingly fail for cracks inclined to the hologram plane and excited nonperpendicularly. Furthermore, we point out how the physical data recording process may additionally influence the reconstruction accuracy, and, finally, guidelines for a careful and serious application of these holographic reconstruction algorithms are given. The theoretical results are supported by measurements.

Plasma-breakdown retropulse isolators for the infrared.

Abstract: The design and performance of a passive plasma isolator for suppressing retropulses in high-power CO2 laser-fusion systems are described. The device uses a gas-filled spatial filter designed to produce a plasma at the focal plane iris. General design criteria for these isolators are discussed, and the performance of a specific isolator is evaluated. For the unit tested, retropulse attenuations of 33 dB for focal plane intensities of 1.5 TW/cm2 have been demonstrated.

Fabry-Perot filters used for optical preprocessing.

Abstract: A Fabry-Perot etalon placed in the object plane of a coherent optical processing system can be used as a variable bandpass preprocessing filter. Use of this filter leads to improved correlation signals in a matched spatial filter system. Experimental results are shown and compared with results obtained by two other preprocessing methods.

Processing measurements of the directional content of Fourier spectra.

Abstract: Directional analysis and filtering make use of the most straightforward properties of optical Fourier transforms. A quantitative study of the detectability of angular maximums in Fourier spectra and its relationships with the shape and distribution of elements in the object is proposed. The problem is modeled by using a simple object which consists of the superimposition of two random distributions of rectangular grains, one being rotated with respect to the other. Computing the Fourier spectrum of this object allows the expression of the amount of light integrated through a wedge which scans the spectrum. An index of angular separability of the two distributions is made up from such measurements. It is shown to depend on the rotation angle as well as the grain shape. Using an additional spatial filter can improve this index. The influence of its radius is studied. Experimental results obtained either in real time with optical fibers or on photographic records with a microdensitometer are compared with the theoretical ones. They show the necessity of the implementation of optimal estimation schemes to reduce the influence of the noise. Two linear least squares filters are used: an angular Wiener filter and an autoregressive one. In the case of a high signal-to-noise ratio, the Wiener filter reduces to a Laplacian filter which does not depend on the shape of the grains.

Method and apparatus for detecting pattern

Abstract: PURPOSE: To obtain a detection signal faithful to an original pattern without superposing cutoff spatial frequency, by detecting a pattern by the light transmitting through a filter whose transmissivity changes in a pattern detecting direction. CONSTITUTION: The beam emitted from a laser beam source 15 is converged to the pupil 10 of an objective lens 1 through a lens 151 and a beam splitter 11 to irradiate the object 30 to be inspected on a wafer 3. The laser beam reflected from the object 30 to be inspected is incident to an imaging apparatus 13 by a lens 1 and a relay lens 12 to form the image of the object 30 to be inspected. Herein, the lens 12 forms the image of the pupil 10 on an analogue spatial filter 14 and laser beam transmits through the region 140 of the filter 14 perfectly but does not transmit through a region 142 at all and transmissivity changes in a region 141 according to a place. By this method, it is eliminated that the beam reflected from the object 30 to be inspected is steeply cut at specific spatial frequency and the pattern image of the object 30 to be inspected detected by the apparatus 13 becomes a smooth image prevented from the superposition of a specific cut-off spatial frequency and faithful to an original pattern. COPYRIGHT: (C)1989,JPO&Japio

Formation of a beam of high optical quality in a multistage neodymium laser

Abstract: A report is given of an investigation of the formation of the intensity profile of laser radiation by the application of the spatial filtering and apodization method to a high-power neodymium system. Optimal conditions are found for the filtering of spatial frequencies and the newly developed simplest vacuum spatial filter is described. The method for making soft apertures by the evaporation of metal films on glass substrates is reported together with a study of the apodizing properties of these apertures. A study of spatial coherence of apodized laser radiation is described. The results are given of the formation of a beam of high optical quality in a high-power system with an exit aperture 45 mm in diameter.

Linear versus logarithmic spatial filtering in the removal of multiplicative noises.

Abstract: Linear and logarithmic coherent spatial-filtering techniques are compared in the separation of multiplicative noise from an image in a coherent-optical image-processing system It is demonstrated that both techniques are effective in the removal of simple grating-type high-frequency noises, but that linear filtering cannot remove noise of spatial frequencies lower than 5 lines/cm, whereas logarithmic filtering can

Frequency spatial filter for two frequency microwave antenna - comprising double sandwich of metallic grids and dielectric sheets

Abstract: The filter can either separate or combine two electromagnetic waves of determinate mean angle of incidence in different frequency bands. A central sheet (2) of low-loss dielectric material (e.g. polyphenylene) carries on both faces (4,5) identical metallic grids of rectangles symmetrical about the plane of incidence (6). The central sheet is sandwiched between two others (1,3) of the same material or of other materials having the same characteristics. The combination reflects quasi-optically a beam of W-band (94 GHz) waves, and transmits without deflection a beam of X-band (10 GHz) waves arriving at the same angle of incidence on the opposite face. Also claimed are a Gregory offset antenna and a Cassegrain antenna, each incorporating such a filter.

Monitoring system for a predetermined field of view

Abstract: A lift control system includes a spatial filter for producing a signal having various features, a discriminator for discriminating one or more features of the signal, and for producing a control signal when at least one of the features discriminated falls in a predetermined range, and active devices connected to the discriminator. The spatial filter is defined by a semi-spherical bowl having a plurality of openings or by a poly-face prism which is made from a plano-convex lens with its curved face so polished or cut as to have a plurality of flat faces.

Phased array signal processing

Abstract: A phased array radar system, such as may be used in a low-angle tracking radar or direction finding equipment, includes a multi-channel digital filter to remove unwanted multipath signals. The R.F. signals from the array are first converted to zero I.F. and digitized. After n-channel Doppler filtering (13) the digital signals are subjected to filtering (14) either to effect band-pass spatial filtering, thereby attenuating generally all signals arriving outside preset angular limits, or notch filtering which heavily attenuates signals arriving from one particular angle. In one embodiment spatial filtering is followed by notch filtering. The filtered signals are then beam-formed (15) before being applied to further digital processing, such as beam peak tracking (16).

The Estimation of Seismic Body Wave Signals in the Presence of Oceanic Microseisms

Abstract: : This report describes studies of the processing of broadband seismograms (from a system with displacement response flat from around 0.1 to 10 Hz) for the estimation of signal shape rather than the maximisation of signal-to-noise ratio. By definition, Wiener filtering gives the best estimate of signal shape in the sense that filters are designed to minimise the mean square of the difference between the true signal and the estimated signal, consequently most of the report describes studies of the application of this type of filter. For seismometer array recordings Wiener filters apply both spatial and frequency filtering to extract the signal. However, if the required noise reduction can be obtained by spatial filtering alone, then no frequency filtering is applied and so the signal is passed undistorted. From the data studied in this report it is possible to get noise reductions due to spatial filtering of up to 6 with an array of 4 seismometers. (Author)

Optical information parallel processing: a technique.

Abstract: A coherent optical information parallel processing technique using a multidiffraction grating is presented. We have shown that this parallel processing technique is capable of performing multichannel spatial filtering in the spatial frequency plane. The number of parallel processing channels is about 2N times the number of processing channels of the conventional coherent optical processor. However, in practice, the number of processing channels is limited by the number of multiplex gratings that can be synthesized and the available power of the light source. Extension of the monochromatic parallel processing system to polychromatic parallel processing is also presented. It can be shown that the number of processing channels of the polychromatic parallel processor is higher than the monochromatic parallel processor. Extension of this parallel processing concept to multisignal parallel processing is also discussed. However, the multisignal processing technique is not a real-time method because it requires an initial encoding step. Nevertheless, this disadvantage may be alleviated with ingenious design of the encoding system. We also assert that this parallel processing technique may be applied to white light parallel processing.

Isolator for high power radiation

Abstract: An isolator device that permits the passage of low-intensity radiation while blocking the passage of high-intensity radiation comprises a spatial filter and a gas cell that utilizes the phenomenon of thermal blooming to deflect high-intensity radiation so that it is blocked by the spatial filter.

Scanning optical correlator.

Abstract: The operation of a scanning optical correlator is described and demonstrated. The device is shown to combine the advantages of a coherent correlator, which makes preprocessing operations easy to realize, with the advantages of direct-image-casting correlators, which are insensitive to setup misalignment and imperfections and do not require the preparation of any filter. For some applications, these attributes may compensate for the fact that scanning optical systems are inherently slower than parallel, frequency plane processors.

Spatial frequency coloring and feature coloring

Abstract: Presents a method that uses as components a dispersive element (prism or grating) and a gray spatial filter. The dispersive element produces a multitude of laterally shifted spatial frequency spectra in the Fourier domain. A suitable gray transparency in the Fourier plane assigns to every spatial frequency a typical wavelength. Hence, the texture of the object can be color encoded in the image. In a second experiment the filter is computed such that one out of two different patterns appear red and the other green. This experiment is an extension of Wiener filtering.

Optical Processing Of Photographic Images

Abstract: Recent advances in electro-optics have brought into use communication and information theory to analyze performance in coherent and incoherent optical information processing systems. An optical information processsing system can be analyzed with many of the same concepts of linear system theory (e.g., spatial impulse response, spatial frequency and spatial domain synthesis, etc.), and the photographic images to be processed can be regarded in the same manner as time signals (e.g., spatial frequency content, spatial amplitude and phase modulation, space-bandwidth product, etc.). Both coherent and incoherent optical processing systems can be treated as linear systems, and the processing operation can generally be carried out by communication theory concepts. Although coherent optical information processing operations have been used for performing complex amplitude operations, complex processing can also be performed with incoherent or white-light illumination. The importance of optical information processing operations, either coherent or incoherent, is due to the basic Fourier transform properties of lenses. In this paper, we will discuss mostly the incoherent systems because they are of more recent interest and possess certain advantages, we feel, over the traditional coherent optical processors. Experimental illustrations of the results are provided. In view of the broad area in optical processing of photographic images, we will confine ourselves to a few applications that we consider of general interest. We apologize for the omission of other techniques and applications, and for neglecting the inclusion of their references.

Electrooptic approach to an integrated optics spectrum analyzer

Abstract: An integrated optics spectrum analyzer based on using the linear electrooptic effect is investigated. This spectrum analyzer performs Fourier analysis of sampled electronic signals, where each signal is fed to an electrode of an electrode array. The electrode array acts as a spatial light modulator, and the diffracted light field, representing a weighted discrete Fourier transform (DFT), is focused on a detector array by an integrated transform lens. The theory of operation of the spectrum analyzer is outlined, numerical results relating to this theory are presented, and questions concerning efficiency, dynamic range, design, and implementation are discussed.

Spatial filter system

Abstract: A spatial filter system for filtering a matrix of data values by an m×n matrix, where the m×n matrix is the product of a 1×n matrix and a m×a matrix. The system includes a first spatial filter adapated to filter each row of the data matrix with the 1×m matrix to provide a first filtered value matrix. The system also includes a second spatial filter adapted to filter each column of the first filtered value matrix with the m×1 matrix to provide a second filtered value matrix.