Showing papers on "Optical filter published in 1985"

Spectral windowing of frequency‐modulated optical pulses in a grating compressor

Abstract: Spectral windowing or apodization of the spatially dispersed frequency components within a grating compressor has been used to control the temporal shape of compressed optical pulses. Windowing of undesirable high and low‐frequency components, which are not linearly chirped, results in nearly complete elimination of the energy in the wings of the compressed pulse. We have applied this technique to improve the quality of pulses from a mode‐locked neodymium: yttrium aluminum garnet laser compressed to less than 2 ps using a fiber and double‐pass grating pair with an internal aperture.

Nonlinear guided wave applications

Abstract: Potential device applications of third-order nonlinear phenomena in guided wave structures are discussed. One of the waveguiding media is assumed to exhibit an intensity-dependent refractive index that results in both an intensity-dependent propagation wavevector and field distribution for the guided waves. These two characteristics lead to many interesting all-optical devices whose operating principles and power levels are outlined here. For example, the intensity-dependent field patterns can be used for thresholding and switching operations in a waveguide context. The intensity-dependent wavevector, used in conjuction with a distributed input or output coupler, can lead to devices such as optical limiters and light-controlled spatial scanners. When gratings are used as distributed feedback elements within a nonlinear waveguide, a whole class of novel devices such as bistable switches and optically tunable optical filters should be feasible.

Multimirror Fabry–Perot interferometers

Abstract: The addition of one or more partially reflecting mirrors to a traditional two-mirror Fabry–Perot interferometer results in a multimirror Fabry–Perot interferometer. A superposition of all possible multiple reflected beams is described with a general theory for multimirror interferometers, featuring matrices analogous to the theory of multilayer thin films. However, the parameters in the matrix elements are mirror reflection coefficients and spacings instead of the usual refractive indices and layer thicknesses of thin films. The transmission characteristics of two-, three-, and four-mirror Fabry–Perot optical filters are described. It is shown that a suitable choice of reflection coefficients results in transmission properties that can be described approximately with Butterworth profiles, which are known from network analysis of electrical circuits.

Molecular gas analysis by Raman scattering in intracavity laser configuration

Abstract: The concentration of multiple polyatomic gases are determined almost simultaneously by Raman scattering The gas sample is placed in a sampling cell located in the resonance cavity of a laser and a polarized laser beam having sufficient intensity to produce detectable signals of Raman scattered light is passed through the cell The scattered light is captured and redirected by means of a reflection mirror located parallel to the axis of the laser beam adjacent to and outside of the cell Signals of both inelastic Raman scattered light and elastic laser scattered light are collected by a collection lens means opposite the reflection mirror and outside the gas cell The collection lens is also parallel to the axis of the laser beam The collected scattered signals are directed onto a laser line rejection filter where the scattered elastic laser signals are filtered out and the inelastic Raman scattered signals are transmitted to come in contact with a rotating filter wheel containing a series of interference filters with each filter being specific to the transmission of one Raman line The Raman lines passing through the rotating filters are sensed sequentially by a single detector means and amplified and converted into digital electrical pulses which are processed and converted into visual readouts indicative of the concentration of each of the polyatomic molecules in the gas being determined

Nonlinear optical circuit elements as logic gates for optical computers: the first digital optical circuits.

Abstract: The first digital all-optical circuits based on intrinsic bistable devices have now been realized. Experimental results with two systems, InSb etalons and nonlinear interference filters containing ZnSe, are presented and discussed. These results indicate that a digital optical processor incorporating devices of this type would appear to be feasible.

Applying the phase-only filter concept to the synthetic discriminant function correlation filter.

Abstract: A modified form of a composite or multiplexed matched filter has been computer simulated and tested. The modification consists of using only the phase function and setting the amplitude function equal to unity—a so-called phase-only filter (POF). The original filter was D. Casasent's synthetic discriminant function (SDF) filter. The filter and test images were made from actual IR imagery. The results are compared in terms of efficiency, correlation peak height and width, and signal/noise ratio. A binary phase version of the SDF/POF was also tested. Its performance is between the SDF and the SDF/POF.

Thermally induced optical bistability in thin film devices

Abstract: A simple theoretical model describing thermally induced optical bistability is discussed. Results of experimental studies of optical bistability in ZnSe interference filters near the band edge are in good agreement with the predictions of this model.

Broad-passband-width optical filter for multi/demultiplexer using a diffraction grating and a retroreflector prism

Abstract: The letter describes the experimental results of a novel optical filter for a multi/demultiplexer using a diffraction grating and a retroreflector prism. A broader passband width, which is proportional to the difference between the prism base length and the input fibre core diameter, is obtained. The minimum insertion loss is 2.2 dB.

Optical filter for protective welding lens assemblies

Abstract: An optical filter for welding protective lens assemblies has two parallel electro-optical cells (303-305; 305, 307), at least one of the cells (305, 307) being of the nematic type with admixture of dye molecules with anisotropic light absorption. The filter comprises a filter sheet (309) disposed in the ray path of the cells and allowing transmission within a wave range which is offset with respect to the wave range within which said one cell (305, 307) is transmitting residual light in its absorbing state.

Infrared mesh filters fabricated by electron‐beam lithography

Abstract: Capacitive and inductive infrared mesh filters have been fabricated using electron‐beam lithography combined with a lift‐off procedure. A single level of PMMA was used to create positive tone patterns while a two‐level metal‐on‐polymer process was used to create negative tone patterns. Resonant structures consisting of arrays of crossed metal dipoles and arrays of crossed slots in otherwise continuous metal films have been fabricated that have linewidths less than 0.25 μm, and which exhibit bandstop and bandpass transmissive properties, respectively.

All-fibre wavelength filters using concatenated fused-taper couplers

Abstract: Successive fused-taper couplers are proposed as all-fibre wavelength filters. A low-loss narrowband filter constructed from two overcoupled couplers is demonstrated.

Optical filter made of inorganic material for red light

Abstract: An optical filter made of inorganic material for red light which transfers red light and absorbs all of the other colored lights can be realized by sticking an inorganic mixture powder pasted by mixing with screen oil on a glass substrate and firing it. The mixture includes sulfates of copper, alkali metal and alkali earth metal. The contrast of the picture displayed on a display device can be increased significantly by reducing the reflection of incident ambient light without the reduction of brightness on account of the application of the above optical filter onto the displaying screen thereof.

Optical sampling using an all‐fiber optical Kerr shutter

Abstract: An all‐fiber optical Kerr shutter consisting of a short single‐mode fiber and a highly birefringent fiber which serves not only as a polarizer but also as a wavelength filter to reject pump light is successfully realized. By utilizing the Kerr shutter, optical sampling is demonstrated. Time‐varying signal light from a laser diode at λ=0.84 μm is sampled into a train of pulses through the Kerr shutter which is driven by an intense light pulse at λ=1.064 μm. The applications to high‐speed signal processing and the generation of picosecond optical pulse are promising because of the fiber’s fast response of optical Kerr effect.

Blackbody radiation sensing optical fiber thermometer system

Abstract: An optical fiber thermometer system utilizing the laws of blackbody radiation includes a sensor responsive to the temperature within a region to be tested for transmitting light energy generated as a function of the temperature according to Planck's equation along an optical transmission line, and a detector responsive to the light energy for producing an electrical current. The current is amplified by a linear amplifier having provision for automatically controlling the gain of the amplifier, depending upon the magnitude of the input current, to reduce the dynamic range of the input current. The output of the linear amplifier is a voltage which is used by a signal processing network to yield an accurate indication of the temperature within the region to be tested by solving the Planck equation between two predetermined wavelengths representing upper and lower limits of an optical filter which filters light energy entering the detector.

Rotationally invariant correlation filtering

Abstract: A method is presented for analyzing and designing optical correlation filters that have tailored rotational invariance properties. The concept of a correlation of an image with a rotation of itself is introduced. A unified theory of rotation-invariant filtering is then formulated. The unified approach describes matched filters (with no rotation invariance) and circular-harmonic filters (with full rotation invariance) as special cases. The continuum of intermediate cases is described in terms of a cyclic convolution operation over angle. The angular filtering approach allows an exact choice for the continuous trade-off between loss of the correlation energy (or specificity regarding the image) and the amount of rotational invariance desired.

Optical signal processing with magnetostatic waves

Abstract: Magneto-optical devices based on Bragg diffraction of light by magnetostatic waves (MSWs) offer the potential of large time-bandwidth optical signal processing at microwave frequencies of 1 to 20 GHz and higher. A thin-film integrated-optical configuration, with the interacting MSW and guided-optical wave both propagating in a common ferrite layer, is necessary to avoid shape-factor demagnetization effects. The underlying theory of the MSW-optical interaction is outlined, including the development of expressions for optical diffraction efficiency as a function of MSW power and frequency, device geometry, materials properties, and other relevant parameters. Recent experimental observations of anisotropic Bragg diffraction and collinear TE↔TM mode conversion induced by MSWs in yttrium iron garnet (YIG) thin films suggest that high-performance MSW integrated-optical devices are feasible. Diffraction levels as large as 4% (7-mm interaction length) and a modulation dynamic range of ∼30 db have been demonstrated. Potential signal processing applications are mentioned, including: spectrum analyzers, convolvers/correlators, deflectors, non-reciprocal optical isolators, and tunable narrowband optical filters. Advantages of these MSW-based devices over the analogous acousto-optical devices include: much greater operating frequencies, tuning through the MSW dispersion relation by varying either the rf frequency or the applied bias magnetic field, simple MSW transducer structures (e.g., a single stripline), and the potential for very high diffraction efficiencies.

Optical transmission filter

Abstract: An optical transmission filter for effecting differential phase delay upon light in a beam polarized in a P dimension, as a function of position within the filter aperture. The filter employs two lenses of birefringent material (12, 14), the crystal optic axes of the respective lens material being oriented in mutually orthogonal positions and at a 45o angle to the P dimension. The lenses have their adjacent surfaces (13, 15) respectively concave and convex with the same radius of curvature and their non-adjacent surfaces (11, 17) flat. The phase response of the filter is a function of the radial distance of a beam element from the filter axis with spherical lenses and a function of a linear coordinate distance from the filter axis with cylindrical lenses. A two-part construction for one lens, which permits adjustment of the difference in center thicknesses between the two lenses, allows adjustment of the differential phase delay with respect to a spatial reference.

Directional-coupler filter using dissimilar optical fibres

Abstract: The letter discusses the filter bandwidths achievable with directional couplers composed of two dissimilar optical fibres. One fibre always has a step-index core while the other may be either a step-index fibre or a graded-index fibre whose core has a parabolic, triangular or cusp-shaped refractive-index distribution.

Optical filter and the method of preparing the same

Abstract: This invention provides an optical filter comprises of a pair of solid polymer films, wherein (1) a first solid polymer film has a planar alignment of a cholesteric liquid crystal structure of either clockwise or counterclockwise helical structure, which alignment allows the light of a specified wavelength to be scattered selectively, and (2) a second solid polymer film has a planar alignment of a cholesteric liquid crystal structure of a helical structure whose helical direction is the opposite to that of the first solid polymer film, which alignment allows the light at the specified wavelength to be scattered selectively, wherein the optical axes of the first and second solid polymer films are made substantially parallel to each other. This invention also provides preparation methods of an optical filter mentioned above.

Optical sensor system

Abstract: Present electrical expendable oceanographic instruments are vulnerable to insulation leaks and electromagnetic interference; they are also unable to measure pressure and the index of refraction. In response to these difficulties a unique combination of optical temperature, pressure, and index of refraction sensors have been developed. These sensors are coupled to an optical fiber transmission link which is contained initially within a probe vehicle and is designed to be unreeled. The remote sensing feature of this combination and technique makes the instrument also suitable for industrial and data - and tele-communications use. The principle of the three sensors is that of optical filters, whose band edges are functions of temperature, pressure, and the index of refraction; this wavelength modulation technique avoids drift and allows the signals from the sensors to be wavelength multiplexed in a single optical fiber, and to be read remotely by a single detector.

Methods of and apparatus for tuning a birefringent optical filter

Abstract: The simultaneous and equal tuning of two birefringent crystal elements of a Solc type optical filter by pairs of quarter-wave plates adjacent to each and oriented such that equal and opposite physical rotations between the plates of each pair tunes its adjacent birefringent element in the same direction without changing the relative orientations of the filter input and output. Also, the simultaneous and equal tuning of two birefringent crystal elements of a Solc type optical filter by the rotation of a single contiguous group of optical components.

Apparatus and method for remote sensing of gases, vapours or aerosols

Abstract: A remote sensor for detecting gas, vapour or aerosol comprising means to measure the change in temporal coherence of light of a selected narrow waveband when it interacts with the gas etc. The light can be provided by a laser source or spectrally filtered sunlight etc. Received radiation 41 is split in to two, beams by a Fresnel biprism 44 and then detected by a detector 46 sensitised by a modulating reticle 47 to interference fringes. A glass delay plate 45 of suitable thickness is placed in the path of one of the beams such that only received radiation having a temporal coherence greater than a minimum determined by the plate thickness produces an output signal from detector 46. The minimum temporal coherence is set higher than the temporal coherence of the illuminating radiation. In alternative arrangements a band-pass temporal coherence filter may be used and the sensor can be made to spectrally scan the field of view by using a tunable laser or a variable centre frequency band-pass optical filter.

Optical filter polarizer combination and laser apparatus incorporating this combination.

Abstract: An optical filter polarizer combination providing a spacial transmission function to polarized incident light. When the light is in the form of a beam the combination may be used to reject extraneous light, or to modify the intensity profile of the beam as a radial or as a linear spacial function (orthogonal to the axis of the beam). The filter employs two lens elements of birefringent materials, with their crystal optical axes mutually orthogonal, and at 45o to the plane of polarization. Spherical lenses are employed for a radial spacial function and cylindrical lenses are employed for a linear spacial function. The combination is useful for mode selection in a stable laser oscillator, for beam quality improvement in a stable/unstable oscillator, and for fill factor enhancement in coupling energy from a laser oscillator to a laser amplifier.

Position-tunable holographic filters in dichromated gelatin for use in single-mode-fiber demultiplexers

Abstract: We describe a continuously tunable reflection filter for the 1.2-1.6-microm wavelength range. The filter is a volume-reflection hologram in dichromated gelatin and exhibits high reflection efficiencies (~95%) with narrow spectral bandwidth (~l% FWHM). These filters have been initially designed for use as position-tuned wavelength demultiplexers or channel-selection switches in multichannel optical-fiber systems. A prototype device has been constructed with an insertion loss between input and output single-mode fibers of <3 dB, with bandwidths Deltalambda((1/2))(FWHM) ~1% over the wavelength band.

Fabry-Perot with an absorbing etalon cavity

Abstract: The properties of the Fabry-Perot interferometer with an absorbing medium in the cavity have been investigated. When very large reflective finesse etalons are used, the interferometer behaves as a long-path absorption cell with a gain, defined as the ratio of the measured absorption over the sample absorption for the same path as the etalon spacer, equal to C1/2. (C is the etalon contrast.) Practical considerations make the plane Fabry-Perot unsuitable for this use because of the rather large area and surface finesse requirements. The spherical Fabry-Perot, on the other hand, is not constrained in the area requirement and thus appears to be the choice etalon for a long-path absorption cell. The results also show that interference filters, because of their high reflective finesse, have absorption effects which degrade their ultimate performance.

Recursive optical filter system

Abstract: An optical notching or filtering system in which a beam of optical radiation is passed through the optical filtering means a multiplicity of times. The optical filtering means has an optical Fourier transform means (12), a spatial filter (14), optical inverse Fourier transform means (16), and a reflector array (18). After an input optical signal beam undergoes a filtering pass by being directed through the transform means, the filter, and the inverse transform means, the reflector array re-introduces the beam for a multiplicity of subsequent filtering passes. Four embodiments of the system of the invention having various configurations of the reflector arrays for use with one- and two-dimensional spatial filters are disclosed. The various embodiments are set forth as being used in a heterodyning optical notching filter systsm.

Tunable birefringent wavelength-division multiplexer/demultiplexer

Abstract: A tunable wavelength-division multiplexer/demultiplexer, based on a polarisation-insensitive birefringent optical filter design, is described. The two channels of the device can each be tuned, with the channel separation adjustable over a wide range. Experiments with 6 to 30 nm laser channel separations have yielded optical crosstalk values of less than −20 dB.

Boron and silicon: filters for the extreme ultraviolet.

Abstract: Thin films of boron and silicon have been developed using electron beam deposition. The transmissions of these filters were measured from soft X-ray wavelengths to the far ultraviolet and at optical wavelengths. The boron filter transmission peaks near 66 A and the silicon filter peaks near 136 A as expected on theoretical grounds, but the extreme ultraviolet bandpass is narrower than expected. The peak transmission of these filters does not change with time, but the width of the silicon filter bandpass is reduced slightly as the filter ages.

Optical emission line monitor with background observation and cancellation

Abstract: A fiber optics based optical emission line monitoring system is provided in which selected spectral emission lines, such as the sodium D-line emission in coal combustion, may be detected in the presence of interferring background or blackbody radiation with emissions much greater in intensity than that of the emission line being detected. A bifurcated fiber optic light guide is adapted at the end of one branch to view the combustion light which is guided to a first bandpass filter, adapted to the common trunk end of the fiber. A portion of the light is reflected back through the common trunk portion of the fiber to a second bandpass filter adapted to the end of the other branch of the fiber. The first filter bandpass is centered at a wavelength corresponding to the emission line to be detected with a bandwidth of about three nanometers (nm). The second filter is centered at the same wavelength but having a width of about 10 nm. First and second light detectors are located to view the light passing through the first and second filters respectively. Thus, the second detector is blind to the light corresponding to the emission line of interest detected by the first detector and the difference between the two detector outputs is uniquely indicative of the intensity of only the combustion flame emission of interest. This instrument can reduce the effects of interferring blackbody radiation by greater than 20 dB.