Showing papers on "Spatial filter published in 1999"

Integrated optics for astronomical interferometry. I. Concept and astronomical applications

Abstract: We propose a new instrumental concept for long-baseline optical single-mode interferometry using integrated optics which were developed for telecommunication. Visible and infrared multi-aperture interferometry requires many optical functions (spatial filtering, beam combination, photometric calibration, polarization control) to detect astronomical signals at very high angular resolution. Since the 80's, integrated optics on planar substrate have become available for telecommunication applications with multiple optical functions like power dividing, coupling, multiplexing, etc. We present the concept of an optical / infrared interferometric instrument based on this new technology. The main advantage is to provide an interferometric combination unit on a single optical chip. Integrated optics are compact, provide stability, low sensitivity to external constrains like temperature, pressure or mechanical stresses, no optical alignment except for coupling, simplicity and intrinsic polarization control. The integrated optics devices are inexpensive compared to devices that have the same functionalities in bulk optics. We think integrated optics will fundamentally change single-mode interferometry. Integrated optics devices are in particular well-suited for interferometric combination of numerous beams to achieve aperture synthesis imaging or for space-based interferometers where stability and a minimum of optical alignments are wished.

Infrared touch panel with improved sunlight rejection

Abstract: Ambient light can saturate the light detectors of a touch screen and cause unreliable operation. To address this concern, one or more filters may be placed proximate the detectors. A first filter may be a spatial filter, such as a microlouvre filter. A microlouvre filter is arranged so that it passes light transmitted by the light detectors and rejects off-axis light. Another filter may be a narrow band optical filter, such as a dichroic notch filter. Such an optical filter passes light having wavelengths within a narrow band selected to correspond to the spectral characteristics of the light emitters, while it rejects light of other wavelengths. The spatial filter and the narrow band optical filter may be used separately or in combination.

Time-alternating method based on single-sideband holography with half-zone-plate processing for the enlargement of viewing zones.

Abstract: The single-sideband method of holography, as is well known, cuts off beams that come from conjugate images for holograms produced in the Fraunhofer region and from objects with no phase components. The single-sideband method with half-zone-plate processing is also effective in the Fresnel region for beams from an object that has phase components. However, this method restricts the viewing zone to a narrow range. We propose a method to improve this restriction by time-alternating switching of hologram patterns and a spatial filter set on the focal plane of a reconstruction lens.

Method and system for high speed measuring of microscopic targets

Abstract: A system including confocal and triangulation-based scanners or subsystems provides data which is both acquired and processed under the control of a control algorithm to obtain information such as dimensional information about microscopic targets which may be “non-cooperative.” The “non-cooperative” targets are illuminated with a scanning beam of electromagnetic radiation such as laser light incident from a first direction. A confocal detector of the electromagnetic radiation is placed at a first location for receiving reflected radiation which is substantially optically collinear with the incident beam of electromagnetic radiation. The system includes a spatial filter for attenuating background energy. The triangulation-based subsystem also includes a detector of electromagnetic radiation which is placed at a second location which is non-collinear with respect to the incident beam. This detector has a position sensitive axis. Digital data is derived from signals produced by the detectors. In this way, data from at least one triangulation-based channel is acquired in parallel or sequentially with at least one slice of confocal image data having substantially perfect temporal and spatial registration with the triangulation-based sensor data. This allows for fusion or further processing of the data for use with a predetermined measurement algorithm to thereby obtain information about the targets.

Integrated optics for astronomical interferometry - I. Concept and astronomical applications

Abstract: We propose a new instrumental concept for long-baseline optical single-mode interferometry using integrated optics which were developed for telecommunication. Visible and infrared multi-aperture interferometry requires many optical functions (spatial filtering, beam combination, photometric calibration, polarization control) to detect astronomical signals at very high angular resolution. Since the 80's, integrated optics on planar substrate have become available for telecommunication applications with multiple optical functions like power dividing, coupling, multiplexing, etc. We present the concept of an optical/infrared interferometric instrument based on this new technology. The main advantage is to provide an interferometric combination unit on a single optical chip. Integrated optics are compact, provide stability, low sensitivity to external constrains like temperature, pressure or mechanical stresses, no optical alignment except for coupling, simplicity and intrinsic polarization control. The integrated optics devices are inexpensive compared to devices that have the same functionalities in bulk optics. We think integrated optics will fundamentally change single-mode interferometry. Integrated optics devices are in particular well-suited for interferometric combination of numerous beams to achieve aperture synthesis imaging or for space-based interferometers where stability and a minimum of optical alignments are wished.

Tunable coherent radiation in the soft X-ray and extreme ultraviolet spectral regions

Abstract: Undulator radiation, generated by relativistic electrons traversing a periodic magnet structure, can provide a continuously tunable source of very bright and partially coherent radiation in the extreme ultraviolet (EUV), soft X-ray (SXR), and X-ray regions of the electromagnetic spectrum. Typically, 1-10 W are radiated within a 1/N relative spectral bandwidth, where N is of order 100. Monochromators are frequently used to narrow the spectral bandwidth and increase the longitudinal coherence length, albeit with a more than proportionate loss of power. Pinhole spatial filtering is employed to provide spatially coherent radiation at a power level determined by the wavelength, electron beam, and undulator parameters. In this paper, experiments are described in which broadly tunable, spatially coherent power is generated at EUV and soft X-ray wavelengths extending from about 3 to 16 nm (80-430-eV photon energies). Spatially coherent power of order 10 /spl mu/W is achieved in a relative spectral bandwidth of 9/spl times/10/sup -4/, with 1.90-GeV electrons traversing an 8-cm period undulator of 55 periods. This radiation has been used in 13.4-nm interferometric tests that achieve an rms wavefront error (departure from sphericity) of /spl lambda//sub euv//330. These techniques scale in a straightforward manner to shorter soft X-ray wavelengths using 4-5-cm period undulators at 1.90 GeV and to X-ray wavelengths of order 0.1 nm using higher energy (6-8 GeV) electron beams at other facilities.

Microbend sensor structure for use in distributed and quasi-distributed sensor systems based on selective launching and filtering of the modes in graded index multimode fiber

Abstract: This paper describes a novel optical fiber microbend sensor architecture which my be utilized in distributed and quasi-distributed measurement. The actual sensor element is graded index multimode fiber coupled to the measurand field through the usual microbend inducing structures. However, the feed to the sensing section is through a single-mode fiber spliced to the multimode fiber to ensure that only the lowest order spatial mode is launched. Similarly the receiver is also coupled to the sensing element through a single mode fiber. The single mode within multimode fiber propagates with minimal mode coupling with source to receiver losses of typically 0.7 dB for short sensors ranging to approximately 0.3 dB per each additional kilometer of sensing fiber. The sensitivity of this structure to microbend induced losses has been thoroughly characterized. Typically the optical power loss for a given microbend structure and force is about three to six times higher in this architecture than for conventional fully mode filled microbend sensor. The structure is also almost totally insensitive to macrobend induced losses and allows a variety of novel designs in microbend inducing structures. Additionally, spatial mode filters allow effective control over concatenation effects that are common in microbend sensors.

Spatio-temporal video noise reduction system

Abstract: A method and system for digital video noise reduction which includes a picture analyser (103) for analysing pictures in a video sequence to determine the amount of moving regions therein, a noise level detector (102) for estimating the noise level (N) in the video sequence, a filtering level estimator (105, 103) for determining a maximum filtering level (L) for each picture based on the amount of moving regions and the estimated noise level (N) and a spatial filter (106) and a temporal filter (107), the temporal filter being coupled to the filter level estimator for controlling the level of filtering of each picture in the sequence in accordance with the maximum filtering level.

Holographic authentication element and document having holographic authentication element formed thereupon

Abstract: Device, method, and system for recording diffractive high resolution text, pictorial, and/or other graphical information is provided which is particularly suited to recording information that would be difficult to reproduce by typical counterfeiting methods. The information recorded may be used to authenticate the recorded item, or indirectly, an item to which the recording is attached. Such items may include legal, financial and commercial instruments, credit cards, and packaging for such items as software, art, and other items where forgery of the item may be a concern. In one embodiment of the invention light is selectively passed by a shutter, and a spatial filter then cleans the beam to remove undesirable frequency components. A liquid crystal display (LCD) dynamically receives a data stream from a computer where each of the displayed data values presents an optical characteristic (for example density, phase, or polarization) to the filtered beam and causes diffraction into a plurality of diffracted beams, the character of which is dependent on the data displayed. A mask selectively passes only predetermined ones of the beams so that only frequency components that will generate the desired optical interference fringes are allowed to pass. Additional optical components receive the passed beams and redirect them to overlap at an output plane to form interference fringes. These interference fringes can have a very high spatial frequency so that extremely fine lines or small objects, including text and graphics, can be recorded.

Programmable axial apodizing and hyperresolving amplitude filters with a liquid-crystal spatial light modulator.

Abstract: Amplitude-transmitting filters for apodizing and hyperresolving applications can be easily implemented by use of a two-dimensional programmable liquid-crystal spatial light modulator operating in a transmission-only mode. Experimental results are in excellent agreement with theoretical predictions. This approach can permit the analysis of various filter designs and can allow the filters to be changed rapidly to modify the response of an optical system.

Diffraction gratings: aberrations and applications

Abstract: Diffraction grating is an optical instrument, which is advantageously used for scientific research in spectroscopy and spatial filtering. It has many other applications in science and technology such as wavelength selectors for tuneable lasers, selective surfaces for solar energy, masks for photolithography, beam sampling mirrors for high power lasers, spectrometers in extreme UV and X-ray regions for space optics, metrology, phase measuring interferometry and pattern recognition, etc. Although the holographic gratings have many advantages over the ruled gratings but they have not yet eliminated completely the classical ruled gratings as the ruled gratings have their own importance in many applications. Therefore, in this article, both the ruled and holographic gratings will be discussed, in particular, their focusing properties, aberrations, and their applications.

Versatile method and system for high speed, 3d imaging of microscopic targets

Abstract: A system including confocal and triangulation-based scanners or subsystems provides data which is both acquired and processed under the control of a control algorithm to obtain information such as dimensional information about microscopic targets which may be "non-cooperative". The "non-cooperative" targets are illuminated with a scanning beam of electromagnetic radiation such as laser light incident from a first direction. A confocal detector of the electromagnetic radiation is placed at a first location for receiving reflected radiation which is substantially optically collinear with the incident beam of electromagnetic radiation. The system includes a spatial filter for attenuating background energy. The triangulation-based subsystem also includes a detector of electromagnetic radiation which is placed at a second location which is non-collinear with respect to the incident beam. This detector has a position sensitive axis. Digital data is derived from signals produced by the detectors. In this way, data from at least one triangulation-based channel is acquired in parallel or sequentially with at least one slice of confocal sensor data having substantially perfect temporal and spatial registration with the triangulation-based sensor data. This allows for fusion or further processing of the data for use with a predetermined measurement algorithm to thereby obtain information about the targets.

Glass substrate inspection apparatus

Abstract: An optical inspection station for inspecting a substrate. The substrate may include a first surface and a second surface. Light is reflected from both the first and second surfaces of the substrate. The light reflected from the first surface is detected by a light detector. A controller may determine a surface characteristic of the first surface from the detected light. The system may include a spatial filter that filters the light reflected from the second surface. The spatial filter eliminates the optical noise that may be created by the light reflected from the second surface.

Surface figure and roughness tolerances for NIF optics and the interpretation of the gradient, P-V wavefront, and RMS specifications

Abstract: In a high energy laser system such as the National Ignition Facility (NIF), the ability to focus light into as small a spot as possible at the highest possible fluence is highly dependent on the quality of the optics used in the system. Typically, surface form errors and transmitted and reflected wavefront errors are specified in terms of a peak-to-valley wavefront error (P-V), or occasionally in terms of an RMS wavefront error (RMS). It has been shown, however, that the parameter that most closely correlates with beam focusability is neither of these, but the RMS of the gradients of the wavefront error (RMS Gradient). Further, the spatial frequency of the wavefront error plays a significant role in the way that a given error effects the performance of the laser system, so careful attention must be paid to how the spatial filtering is both specified an accomplished. Since ISO 10110 has no specific provisions for a gradient specification, LLNL has developed its own notation and procedures for these critical specifications. In evaluating surface figure errors as specified by the NIF drawings, modern phase modulating interferometers (PMI) will be used. In addition to performing QA testing of the optics, LLNL intends to utilize the software capabilities of the instruments to obtain the information to model the wavefront of the 131 passes through various optical elements comprising the NIF 'front end.' A typical transmitted wavefront error call-out for an optic in the front end is: (lambda) /8 P-V, (lambda) /40 RMS, (lambda) /30/ cm RMS gradient, with these values evaluated for spatial periods greater than 2 mm only. Test will be performed and documented after coating and as installed in the specified mechanical mounts. This paper describes the evaluation of the wavefront error for NIF small optics including specifications over a given spatial period call-out, the proper low pass filtering of the data and the allowable filtering and settings that can be applied to obtain proper wavefront data. This paper also describes the origin and evolution of other NIF wavefront and roughness specifications, and gives examples. Since the wavefront requirements and hence the specifications vary for the different systems in the NIF, we will focus on one system, the injection laser system (ILS) or 'front end.' Also discussed will be the metrology and data manipulation requirements for the large aperture optics. Finally, clarification will be given to the differences between various versions of the RMS wavefront and roughness specifications allowed in ISO 10110, and how they contrast to the RMS roughness specifications used in ANSI-Y14.5.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Optical correlation with totally incoherent light

Abstract: An optical correlator that can operate with totally incoherent light is presented. Such a correlator can be designed to compensate completely for the inherent chromatic aberrations by resorting to elements with specialized, possibly impractical, dispersion characteristics. Nevertheless, a practical configuration that exploits available achromatic lenses and Fresnel zone plates was designed, built, and tested experimentally. The results reveal that detectable correlation peaks can be obtained with totally incoherent white light. The designs, experimental procedures, and results are presented.

Fourier-optical selection of higher order transverse modes in broad area lasers

Abstract: A partially and a highly antireflection coated broad area laser are operated in an external cavity Fourier-optical 4f set-up to experimentally investigate transverse mode selection. The external cavity consists of a lens and a spatial frequency filter. Running freely the lasers show non-stationary filamentation. Placing the spatial filter unit directly onto the optical axis gives cw fundamental mode operation and a transverse shift of the spatial filter in the plane of the active region allows for selective excitation of higher order modes.

Apparatus and method for calibrating a wavelength stabilized laser

Abstract: A wavelength stabilized laser system includes a laser that produces a laser light having an amplitude and a wavelength that varies as a function of a temperature of the laser. A temperature control device controls the temperature of the laser. A beam splitter deflects a portion of the laser light. A first detector receives the portion of the laser light from the beam splitter and provides a first signal representing the amplitude of the laser light. A filter has a gain that is a function of the wavelength of the laser light. The filter receives the laser light and outputs a filtered light having an amplitude that varies with the wavelength of the laser light. A second detector provides a second signal representing the amplitude of the filtered light. A sealed housing contains the laser, the first and second detectors and the filter. The housing has a window. An electromagnetic radiation source outside of the housing transmits a calibration beam through the window and through the filter, onto the second detector. One or more processors control the temperature control device based on the first and second signals, and monitor a light transmitting characteristic of the filter based on the second signal.

Detection improvement in spatially filtered x-ray fluoroscopy image sequences.

Abstract: The effect of spatial noise-reduction filtering on human observer detection of stationary cylinders mimicking arteries, catheters, and guide wires in x-ray fluoroscopy was investigated in both single image frames and image sequences. Ideal edge-preserving spatial filtering was simulated by filtering of the noise before addition of the target cylinder. This allowed us to separate the effect of edge blurring from those of noise reduction and spatial noise correlation. We used three different center-weighted averagers that reduced pixel noise variance by factors of 0.75, 0.50, and 0.25. As compared with no filtering, the effect of filtering on detection in single images was statistically insignificant. This indicated an adverse effect of spatial noise correlation on detection that countered the effect of noise reduction. By comparison, spatial filtering significantly improved detection in image sequences and yielded potential x-ray dose savings of 26–34%. Comparison of results with two observer models suggested that human observers have an improved detection efficiency in spatially filtered image sequences as compared with white-noise sequences. Pixel noise reduction, a measure commonly used to assess filter performance, overestimated the effect of filtering on detection and was not a good indicator of image quality. We conclude that edge-preserving spatial filtering is more effective in sequences than in single images and that such filtering can be used to improve image quality in noisy image sequences such as x-ray fluoroscopy.

Realization of time gating by use of spatial filtering

Abstract: A method for simulating conventional time gating in low-coherence optical imaging processes in highly scattering media is given. The method uses monochromatic instead of broadband light, and spatial filtering is substituted for time gating. The process enables the study of imaging techniques in scattering media to be carried out in an easy and highly controllable way. Experimental results are given.

Joint power control, multiuser detection and beamforming for CDMA systems

Abstract: CDMA systems are limited by the interference that users create for each other. Several methods of controlling and/or suppressing the interference through power control, multiuser detection (temporal filtering) and beamforming (spatial filtering) are studied to increase the capacity of CDMA systems. We investigate the capacity increase that is possible by combining power control with intelligent temporal and spatial receiver filter design. The SIR maximizing joint temporal-spatial receiver filters in unconstrained and constrained filter spaces are derived. Two-step iterative power control algorithms that converge to the optimum powers and the joint temporal and spatial receiver filters in the corresponding filter domains are given. A power control algorithm with less complex filter update procedure is given. It is observed that significant savings in total transmit power expended by all users in the system are possible if filtering in both domains are utilized compared with conventional power control and joint optimal power control and filtering in one domain.

Wavelength tuning of photo-induced gratings

Abstract: Optical gratings having a range of possible Bragg wavelengths can be produced using a single phase mask by exposing the mask to a non-collimated spatially filtered beam of light. A spatial filter (30) removes high spatial frequency components from the beam, and a focusing system (36) directs the filtered beam to a phase mask (40). A rate at which the beam is focused and a spacing between the phase mask and a photo-sensitive waveguide (12) are varied to produce gratings in the waveguide having a range of possible periods.

Optical displacement measuring apparatus

Abstract: PROBLEM TO BE SOLVED: To provide a displacement and vibration interference noncontact sensor and high precision which is simple and has a wide measuring range. SOLUTION: By using a directional fiber coupler 2, a light beam outputted from a laser light source 1 is made to enter two single mode fibers 3, 4 and a linear grating interference fringes is formed in a space by the output beam 6. In this case, one out of the two fibers is wound around a PZT (piezoelectric element) 5. When the interference fringes are projected on the reflecting surface of a rough surface of an object, linear grating interference fringes (the intensity changes sinusoidally in the y-direction) are projected on the reflecting surface. When a grating pattern 8 on the reflecting surface is focused by an optical system 9, a pattern 11 similar to the pattern 8 on the object surface is images on an imagery surface 10. A parallel grating spatial filter having pitches equal to or nearly equal to those of the imagery grating pattern, i.e., Ronchi grating 12 is so arranged on the imagery surface that both of the gratings become parallel. A transmission light of the Ronchi grating is condensed and received with a photodetector 13.

Adaptive computer-based spatial-filtering method for more accurate estimation of the surface velocity of debris flow

Abstract: An adaptive computer-based spatial-filtering velocimeter to measure the surface velocity of a natural debris flow with high accuracy is described that can adjust the filter parameters, specifically, the slit width of the filter, based on the surface-pattern characteristics of the flow. A computer simulation confirms the effectiveness of this technique. The surface velocity of a natural debris flow at the Mt. Yakedake Volcano, Japan, was estimated by this adaptive method, and the results were compared with those obtained by two other methods: hardware-based spatial filtering and normal computer-based spatial filtering.

Technique and apparatus for performing electronic speckle pattern interferometry

Abstract: A speckle interferometer for measuring displacement deformation, motion or strain of an optically irregular surface of a specimen is disclosed. The interferometer includes laser, a spatial filter for receiving radiation from the laser and converting it into a spherical beam and projecting it to the optically irregular surface of the specimen is located. A reference plate located in or near the second location for reflecting or scattering some or all the radiation to a fourth location, said reflection interfering with the reflection from the optically irregular surface to form a pattern of speckles. A camera and imaging system for measuring displacement and changes in intensity of the speckles is also included.

Automated set up of an energy filtering transmission electron microscope

Abstract: Electron optical aberrations of an energy filtering system (120) of an energy filtering transmission electron microscope (100) (EFTEM) are automatically corrected under computer control to set up the EFTEM for use. Optics of the electron microscope preceding an energy filter are used to scan the beam at the entrance to the filter in a pattern corresponding to a defined geometry. The beam can either be finely focused to yield a spot at each position visited during the pattern scan, or the beam can be spread out and imprinted with a well-defined intensity distribution, such as normally occurs due to passage of the beam through a specimen, so that its relative scanned displacements can be assessed using cross-correlation techniques. In the case of the finely focused beam, electron images of the scanned pattern directly yield a spot pattern image. Deviations of the recorded spot pattern image from the defined scan geometry reflect the imaging aberrations introduced by the energy filter. In the case of the spread out beam, post-filter electron images of the scanned beam are cross-correlated with an image of the beam taken without scanning yielding cross-correlation peak images that give the effective displacement of each scanned beam position due to the aberrations/distortions of the filter. Summing the cross-correlation peak images again yields a spot pattern image that is equivalent to that obtained in the focused beam case. Deviations of the recorded spot pattern image from the defined scan geometry are analyzed to assess and subsequently correct aberrations introduced by the energy filter.

Spatial vector filtering to reduce noise in interferometric phase images

Abstract: Spatial filtering to reduce noise is an important process to improve phase measurements in interferometric imaging. Currently these techniques are predominately based on filtering the interferometric phase. However the amplitude, which is ignored or only used to a minimal extent in conventional filtering, holds useful information about the nature of the phase measurement. When combining multilook images, adding the full complex vectors produces a least mean square estimate of the interferometric phase. Utilising this principle, a new spatial filtering technique is introduced. In this technique, a compromise is suggested where the square-root of the interferometric amplitude is taken prior to summing the spatially weighted vectors. To deduce the weights for the filtering function it was necessary to utilise a Monte-Carlo approach, which calculates the filter weights via an iterative minimisation procedure. The main application for this filter is to improve the quality of displacement maps obtained by a ground based interferometric radar system that monitors rock slope movement in a mining environment. For this application high SNR is expected, however, due to speckle, large errors can still be present in the image. The filter is designed to reduce these errors. Using simulated data and C-Band ERS images, the new spatial vector filter is shown to produce improved phase measurements compared with conventional filtering techniques.

Apparatus and method for coupling high intensity light into low temperature optical fiber

Abstract: A method and apparatus for coupling high intensity light (82) into a low melting temperature optical fiber (80) which uses a high temperature, low NA optical fiber as a spatial filter (84) between a source of high intensity light (33) and a low melting temperature, low NA optical fiber (80). In an alternate embodiment, the spatial filter (84B) is composed of a fused bundle of optical fibers. The source of light (33) may be a high intensity arc lamp or may be a high NA, high melting temperature optical fiber transmitting light from a remote light source.

Aperture for high density laser radiation minimizes absorption heating

Abstract: The aperture (1) absorbs between 0 and 1 per cent of the laser radiation for a defined aperture thickness. A region of the aperture surface deflects or couples out the unwanted part of the laser radiation. The aperture can have an opening (2) so that the part of the radiation absorbed for the given thickness is null. The aperture is highly transmissive and can be a dielectric, especially sapphire, glass or crystal. Independent claims are also included for use of an aperture for fiber coupling of radiation from high power diode lasers, characterizing radiation fields and stopping or spatial filtering in a laser oscillator or oscillator amplifier.