Showing papers on "Optical filter published in 1987"

Angular discrimination of light transmission through polymer-dispersed liquid-crystal films

Abstract: The refractive index of the polymer np is adjusted relative to the ordinary refractive index of dispersed droplets of nematic liquid crystal no to regulate the angle of maximum transmission through a film in the presence of an applied field. Maximum transmission occurs at normal incidence when np≤no with the breadth of the angle of view being largest at np=no. When np>no, maximum transmission is peaked at an angle away from normal incidence, the value of the angle depending upon the ratio np/no. Angular discriminating filters are therefore possible with these films. The optical response of these materials as a function of applied field is also shown to be dependent on the ratio np/no as is the contrast exhibited by information displays. Methods for measuring the scattering cross section of a nematic droplet are also described.

Wavelength-selective filters for single-mode fiber WDM systems using Fabry-Perot interferometers.

Abstract: The applications of the Fabry-Perot interferometer to high capacity wavelength division multiplexed optical systems are discussed. The performances of several practical embodiments, their anticipated role and ultimate performance limitations are described.

Optical tracking novelty filter.

Abstract: We demonstrate an optical filter that displays the time-dependent features of a scene. The heart of the device is an interferometer that is sensitive not to the difference between two optical paths lengths but to changes in the path-length difference. The interferometer arms share a phase-conjugating mirror. The phase conjugator ensures that, at steady state, the output of the interferometer is dark. The response of the interferometer to a step differential change in the optical lengths is a decaying exponential having a time constant governed by the time response of the phase conjugator. The interferometer may be used to monitor time- and space-dependent optical phase changes that are due, for example, to transparent fluid motion. With a modified liquid-crystal television used as a spatial light phase modulator in the interferometer, we detect time-dependent features of an image viewed by a video camera.

Color wheel assembly for lighting equipment

Abstract: A lighting fixture includes a source for producing a beam of light and pair of color wheels. Each of the color wheels has a plurality of dichoric filters mounted on the periphery of the wheel with the filters positioned contiguous to each other. The light beam is directed to a focal point. Each of the color wheels can be rotated to place the peripheral dichroic filters in position to intercept the beam. One of the color wheels is equipped with long wave pass dichroic filters while the other color wheel is equipped with short wave pass dichroic filters. By aligning various combinations of these filters, a large number of different colors with different saturations can be produced. The cutoff wavelengths for the dichoric filters are selected to be different at the long and short wavelengths of the filter set, such that there is produced a perceived uniform graduation of colors across the spectrum. Each of the dichroic filters mounted on the color wheels is in a shape of a trapezoid and is mounted adjacent other filters, such that there is no blanking of light or leakage of light in the process of changing from one filter to the next.

Optical branching filter

Abstract: An optical branching filter branches or mixes lights of n-kind of wavelengths. An input/output port for the light mixed n-kind of wavelengths is provided to one of two surfaces of the transparent block and second input/output parts for forming optical filters allowing only the lights having the predetermined wavelengths to pass therethrough, respectively among the lights in n-kind of wavelengths, but reflecting the lights having other wavelengths, respectively is provided to any one of two surfaces. A first input/output port and n second input/output ports, respectively is formed by an optical waveguide and a lens which optically connects the optical waveguide and first input/output part or a second input/output part. Positions of each second input/output port are selected so that the length of optical paths formed between the lenses of the first input/output port in the second input/output port are in the reverse proportion to the lengths of wavelengths corresponding to the second input/output portions.

Resonator Memories And Optical Novelty Filters

Abstract: Optical resonators having holographic elements are potential candidates for storing information that can be accessed through content-addressable or associative recall. Closely related to the resonator memory is the optical novelty filter, which can detect the differences between a test object and a set of reference objects. We discuss implementations of these devices using continuous optical media such as photorefractive ma-terials. The discussion is framed in the context of neural network models. There are both formal and qualitative similarities between the resonator memory and optical novelty filter and network models. Mode competition arises in the theory of the resonator memory, much as it does in some network models. We show that the role of the phenomena of "daydream-ing" in the real-time programmable optical resonator is very much akin to the role of "unlearning" in neural network memories. The theory of programming the real-time memory for a single mode is given in detail. This leads to a discussion of the optical novelty filter. Experimental results for the resonator memory, the real-time programmable memory, and the optical tracking novelty filter are reviewed. We also point to several issues that need to be addressed in order to implement more formal models of neural networks.

Application of the strongly coupled-mode theory to integrated optical devices

Abstract: A theory for strongly coupled waveguides is discussed and applied to two- and three-waveguide couplers and optical wavelength filters. This theory makes use of an exact analytical relation governing the coupling coefficients and the overlap integrals. It removes almost all of the constraints imposed by a simpler and approximate coupled-mode theory by Marcatili. It also satisfies the energy conservation and the reciprocity theorem self-consistently. We show very good numerical results with the overlap integral as large as 49 percent. The applications to electrooptical modulators, power dividers, power transfer devices, and optical filters are all presented with numerical results.

Electro-optically tunable, narrowband Ti:LiNbO3 wavelength filter

Abstract: We report a tunable optical transmission filter for 1.5?n with a bandwidth of 12 A and a tuning range of at least 80 A. The filter is based on wavelength-dependent polarisation conversion in a birefringent waveguide and employs a periodic electrode structure with interleaved birefringence tuning electrodes.

Rapid scanning fluorescence spectroscopy using an acousto‐optic tunable filter

Abstract: A new method for rapidly acquiring fluorescence spectra from optical probes using an acousto‐optic tunable filter (AOTF) has been developed. The AOTF consists of a piezoelectric transducer bonded to a birefringent crystal. Acoustic waves are generated in the crystal by an applied radio frequency. For a given radio frequency, only a narrow band of optical frequencies will be diffracted. Unlike normal Bragg diffraction which can only be used with collimated light, the AOTF has a large angular aperture (up to 28 deg) and can therefore be coupled to an uncollimated broadband white light source, photomultiplier tube, or camera. Using the device and a xenon–mercury arc lamp, excitation and emission spectra of the fluorescent pH probe BCECF were acquired in 17 ms (spectral resolution=4 nm). Rapid wavelength switching or modulation of selected wavelengths was achieved at ∼105 Hz. The device is also capable of spectral imaging with a spatial resolution of greater than 100 lines/mm.

Flat passband birefringent wavelength-division multiplexers

Abstract: Birefringent filter synthesis techniques are demonstrated to achieve very flat channel passbands for wavelength-division multiplexers. The crystal thickness and birefringence can be used to scale a given design to very small channel separations, limited primarily by the spectra of the lasers and their drift. Experimental results are reported, with typical optical crosstalk values of ?19 dB.

Multilayer optical filter for producing colored reflected light and neutral transmission

Abstract: A short-wave-pass optical filter, that includes a partially absorbing or transparent substrate coated by a set of layers having specified quarter-wave optical thickness, and is designed to reflect visible radiation of a selected color while transmitting visible radiation and maintaining a neutral color balance The coating includes layers having a high refractive index alternating with layers having a low refractive index The refractive indices and layer thicknesses are selected so that the filter's reflectance spectrum exhibits a ripple over a first segment of the visible spectrum but no significant ripple over a second segment of the visible The filters may be used as sunglass lenses that have a selected color (such as blue, orange, or violet) when viewed by one other than the sunglass wearer, while permitting the wearer to perceive transmitted light with a correct color balance The design of the inventive filter is preferably optimized to have the desired optical properties while being conveniently and repeatably manufacturable

Narrowband optical filter with partitioned cavity

Abstract: A method of designing a thin film optical bandpass filter, for transmitting light in a narrow band of wavelengths centered about a bandpass wavelength, begins by defining a first quarter-wave stack, including a first plurality of layers each having an optical thickness equal to one fourth the bandpass wavelength. A second quarter-wave stack includes a second plurality of layers each having an optical thickness equal to one fourth the bandpass wavelength. Next, a cavity layer is defined with an optical thickness equal to an integral multiple of one half the bandpass wavelength, to be interposed between the first quarter-wave stack and the second quarter-wave stack. The cavity layer is then replaced with a partitioned cavity, including at least one layer of a first optical material with a relatively high index of rfraction and at least one layer of a second optical material with a relatively low index of refraction. The partitioned cavity layers, which initially have a total optical thickness equivalent to the optical thickness of the cavity layer, are adjusted in physical thickness to center the transmission peak of the filter on the bandpass wavelength. Thin film optical bandpass filters produced according to the method are also disclosed.

Application of guided-wave optics in signal processing and sensing

Abstract: Research in the use of guided-wave optics for signal processing and sensing is reviewed, and advantages and limitations of the technology are discussed. The signal processors employ electrooptic, acoustooptic, and fiber-optic components to perform such functions as spectral analysis of radio-frequency signals, correlation and matched filtering, code and waveform synthesis, signal delay and storage, and analog-to-digital and digital-to-analog conversion. In most cases, the guided-wave approach is distinguished by the ability to perform a particular function at very high analog bandwidths or digital data rates. The second part of the review is concerned with sensor applications for guided-wave elements fabricated on planar substrates ("integrated optics"). Waveguides on an integrated optics chip have been used for sensing temperature, humidity, electric field, wavefront angle, and optical disc reflectivity. Also described are integrated optic phase and frequency shifters developed for use in fiber-optic sensors. In concluding remarks, an effort is made to provide some perspective on competing technologies and to indicate some areas where future research might prove fruitful.

Narrow-bandwidth optical waveguide transmission filters

Abstract: A general method for fabricating narrowband transmission filters at optical frequencies is proposed. For fibre- or integrated-optic applications the method consists of forming a waveguide loop by connecting the output ports of a 3dB coupler to each other. A Bragg reflection grating in the loop determines the filters frequency response.

Reflectively tapped optical fibre transversal filters

Abstract: A continuous length of silica fibre containing dielectric mirrors produced by a novel fusion-splicing technique is used in demonstrating delay-line signal processors.

All-optical switching and intensity discrimination by polarization instability in periodically twisted fiber filters

Abstract: Self-induced birefringence effects in a periodically modulated nonlinear medium are investigated An input beam linearly polarized along the axis of a fiber polarization-rocking filter undergoes a spatial instability as the input power crosses a threshold value that scales linearly with the coupling strength We describe the potential use of this effect for relatively low-power intensity discrimination, all-optical polarization switching, and small-signal amplification using linearly polarized beams The wavelength for maximum polarization conversion between the axes can be tuned by varying the input power, and the filter bandwidth narrows with increasing power

FDM-FSK star network with a tunable optical filter demultiplexer

Abstract: An optical frequency-division-multiplexed star network is analysed and demonstrated experimentally using two 45Mbit/s frequency-shift-keyed laser channels at 1.5 μm. A tunable fibre Fabry-Perot filter is used as demultiplexer. The analysis predicts and experiment supports a minimum channel spacing of about six times bit rate B for a single FFP and 3B for a tandem FFP.

Arrangement for optical image processing

Abstract: For electro-optical image conversion as well as for optical image processing by spatial filtering, a two-dimensional optical filter is provided which comprises a microstructured grid, the openings of which are designed as a matrix of windows, the windows being provided with electrostatically movable thin-film closures. The window openings are controlled by electrostatic charges which are provided by an electron beam. This matrix of light valves can preferably be used as a electro-optical converter as well as a Fourier filter in an optical computer.

A Novel Optical Fibre Methane Sensor

Abstract: This paper describes a novel fibre optic methane sensor which has several advantages over previously published fibre optic techniques. It is based on a scanning Fabry Perot etalon filter and is self-referencing, requiring only a single optical sensing path and photodetector. A laboratory version of the sensor, employing a white light source, has been constructed and tested, and was found to be capable of detecting 0.25% by volume methane in air, with a response time of approximately 1.5 seconds.

Optical Information Processing With The Double Phase Conjugate Mirror

Abstract: The operation of the double phase conjugate mirror (DPCM) is described. It can be regarded as a bidirectional spatial light modulator and controllable filter. Two independent image-bearing beams that may be derived from different lasers exchange their spatial information as they are coupled into each other in a photorefractive crystal. The,DPCM is also shown to be an optical thresholder and spatial filtering device, displaying edge enhancement. We propose the use of a resonator with two facing DPCMs to implement iterative image processing algorithms.

Fiber optic displacement transducer with dichroic target

Abstract: A fiber optic displacement transducer is provided by a fiber optic reflective probe (2), and an axially moveable target (4) having a dichroic optical filter (16) with first and second reflection surfaces (20 and 24) axially spaced by a fixed distance t and reflecting different wavelength light.

Optical ranked-order filtering using threshold decomposition

Abstract: A hybrid optical/electronic system performs median filtering and related ranked-order operations using threshold decomposition to encode the image. Threshold decomposition transforms the nonlinear neighborhood ranking operation into a linear space-invariant filtering step followed by a point-to-point threshold comparison step. Spatial multiplexing allows parallel processing of all the threshold components as well as recombination by a second linear, space-invariant filtering step. An incoherent optical correlation system performs the linear filtering, using a magneto-optic spatial light modulator as the input device and a computer-generated hologram in the filter plane. Thresholding is done electronically. By adjusting the value of the threshold, the same architecture is used to perform median, minimum, and maximum filtering of images. A totally optical system is also disclosed.

Light filter with automatic regulationof optical transmission

Abstract: The invention relates to improvement in a light filter with automatic regulation of the optical transmission. The filter comprises a filter element (1) with a liquid crystal layer located between two transparent plates (2) and assigned polarizers. The optical transmission of the light filter is varied by applying a variable electric voltage. A first optical sensor (4) is arranged behind the filter element (1) in the radiation emitting direction, and a second optical sensor (8) is arranged beside or in front of the filter element (1) in the radiation emitting direction. The two sensors are connected to a regulating circuit (6), which comprises a subtraction circuit (15) which subtracts the signal generated by the second optical sensor (8) from the signal of the first optical sensor (4), or the signal of the first optical sensor (4) from the signal of the second optical sensor (8), so that the output signal of the regulating circuit is at least approximately proportional to the amount of visible light. As the second optical sensor (8) a sensor sensitive only to infrared radiation, may be used or, if this is also sensitive to other radiation, an infrared band-pass filter may be placed in front of it.

Bandwidth of forward and backward coupling directional couplers

Abstract: This paper deals with the filter properties of directional couplers. Formulas are derived for the bandwidth in wavelength units of couplers consisting of two waveguides that exchange their power either in the direction of the incident wave (forward couplers) or in the opposite direction (backward couplers). Backward couplers require the presence of a diffraction grating on one or both waveguides but they can achieve very much smaller bandwidth than forward couplers and hence are useful for constructing narrow-band optical wavelength filters.

Color filters integrated with the detector in silicon

Abstract: The usual single-chip color imager consists of an alternating pattern of three types of color filters deposited on top of a regular array of photodetectors. The novel principle to be presented is comprised of a method for integrating the filtering element with the photodetector in silicon, which reveals an intrinsic color filtering capability based on the strong wavelength dependence of the absorption coefficient. The band-pass central wavelength of the optical filter is electronically tunable using the reverse voltage applied across the diode, so that only one diode is required for each color element.

Symbolic Substitution Using ZnS Interference Filters

Abstract: Pattern recognition is demonstrated using two-dimensional nonlinear optical logic gate arrays on ZnS and ZnSe interference filters defined by fly's eye lens arrays. Fan-out is demonstrated, allowing symbol scription using these arrays.

Apparatus for the measurement of the stress birefringence of an optical sensor

Abstract: Apparatus for measurement of the stress birefringence of an optical sensor that is influenced by a quantity to be measured. An optical emitting device sends light of two adjacent wavelengths λ 1 and λ 2 through an emitting light guide and via a polariser and a retarder plate to the sensor. An analyser conducts mutually parallel polarization components of the light emerging from the sensor on a common path to a receiving and evaluating device which derives from the measured-value-dependent intensities of such polarization components information concerning the magnitude of the quantity to be measured. The thickness of the retarder plate is dimensioned to provide a path length w of the light through the retarder plate such that the two light components λ 1 and λ 2 that leave the analyzer are influenced in mutually opposite senses by the measured quantity via the sensor, so that they do not need to be separated by optical filters and combined again by a coupler.

System design and long-span transmission experiments on an optical FSK heterodyne single filter detection system

Abstract: The influence of LD phase noise on a heterodyne noncoherent detection system was evaluated. Based on the evaluation, an optical FSK heterodyne single filter detection system with large frequency deviation and wide-band IF filter has been developed to allow use of stand-alone DFB LD's. In the system, a phase tunable DFB LD was used as an FSK transmitter light source to improve the FSK modulation characteristics. An IF filter with appropriate bandwidth evaded the influence of LD phase noise. With these configurations, long-span (243 km at 140 Mbit/s and 204 km at 280 Mhit/s) transmission experiments have been successfully carried out on this single filter detection system. To the contrary, influence of LD phase noise appeared in a limited IF bandwidth case, which agrees well with the theoretical evaluation.

Direct construction of phase-only filters

Abstract: Robert R. Kallman North Texas State University, Mathematics Department, Denton, Texas 76203. Received 18 July 1987. 0003-6935/87/245200-02$02.00/0. © 1987 Optical Society of America. The purpose of this Letter is to give a new direct construc­ tion of phase-only filters which might be useful for threshold optical correlation detectors. This construction is a descen­ dant of the constructions introduced in Kallman and leads to a significant improvement in signal-to-noise ratio (SNR) over previous methods. Simulations suggest that the result­ ing filters and their optimized binarizations can be designed to contain a great deal of information, to be stable under perturbations in the training set, and to have a very low false alarm rate. An intrinsic numerical measure of the performance or SNR of a discriminant function h against a training set will first be formulated. This formulation takes into account that a phase-only filter (POF) or binary phase-only filter (BPOF), unlike a synthetic discriminant function (SDF), cannot control the actual size of the recognition spike in the output correlation plane when a valid target is centered in the filter input plane. Start with 2-D images ƒ1,... ,fn,..., ƒm, the training set. Think of ƒ 1 t . . . , ƒn as objects one is seeking and ƒ n + 1 , . . . , ƒm as objects one is not seeking and definitely does not want to confuse with ƒ 1 , . . . , ƒn. n may well be equal to m. For any x = (x1,x2) in the plane and any function g, let gx(y) = g(y x) for any y = (y1,y1) in the plane. If one initially thinks of g as centered over the origin, think of gx as being g translated so as to be centered over x. One wants the measured optical intensities |+hx,ƒi,| (1 ≤ i ≤ n) in the output plane all to be large for x the origin [note that h(0.0) = h] and that |+hx,ƒi,| (1 ≤ i ≤ m) be as small as possible for all x values outside of some a priori chosen box (or any other region) Bi about the origin. Here Bi is empty for n + 1 ≤ i ≤ m, and +p,q, denotes the usual inner product between the pair of functions p and q or the pair of vectors p and q. The SNR of h, a number intrinsically associated with h, is defined to be

Wavelength sensitive optical devices

Abstract: An optical device for use as an optical filter or modulator comprises an optical waveguide including a portion having a refractive index that varies with optical intensity and a mechanism for providing and directing at least two coherent waves at that portion such that a standing wave of a modulated refractive index is established by interference between the coherent waves. The standing wave provides a temporary modulated index grating in the waveguide.