Showing papers on "Birefringence published in 1991"

Birefringent interference polarizer

Abstract: A birefringent interference polarizer which may be fabricated from readily available materials using established coextrusion techniques is provided. The polarizer has a level of light absorption near zero and can be fabricated to polarize and reflect light of specific wavelengths while transmitting light of other wavelengths. The polarizer includes multiple alternating oriented layers of at least first and second polymeric materials having respective nonzero stress optical coefficients which are sufficiently different to produce a refractive index mismatch between the first and second polymeric materials in a first plane which is different from the refractive index mismatch between the first and second polymeric materials in a second plane normal to the first plane. The refractive index mismatch in the first plane is preferably at least 0.03.

Stability of solitons in randomly varying birefringent fibers

Abstract: The effects of randomly varying birefringence on solitons are studied. It is shown analytically that the evolution equation can be reduced to the nonlinear Schrodinger equation if the variation length is much shorter than the soliton period. The soliton does not split at high values of the average birefringence, but it does undergo spreading and loss of polarization. A soliton with a temporally constant initial state of polarization is still largely polarized after 40z0 if the normalized birefringence is δ ≤ 1.3.

Polarisation-insensitive fibre optic Michelson interferometer

Abstract: A polarisation-insensitive fibre optic Michelson interferometric sensor configuration is demonstrated. The approach is based on the use of birefringence compensation in a retraced fibre path using Faraday rotator mirror elements.< >

Chiral smectic liquid crystal optical modulators having variable retardation

Abstract: The present invention provides optical modulators which comprise aligned chiral smectic liquid crystal cells within an optical resonance cavity. The cavity configurations include symmetric and asymmetric Fabry-Perot etalons. The liquid crystal cells can be planar- or homeotropically-aligned and can be discrete state or analog cells. The device configurations of the present invention provide discrete or continuous optical modulation of the phase, intensity, and wavelength of elliptically polarized light, without requiring polarization analyzers. The modulators are optically or electronically addressable in single pixels or arrays of multiple pixels. Certain homeotropically-aligned cells are provided as an aspect of this invention, as are certain variable retarders comprised of planar-aligned cells in combination with birefringent elements.

Polarization mode dispersion of short and long single-mode fibers

Abstract: Polarization mode dispersion (PMD) in short and long single-mode fibers was measured by a polarization-maintaining Michelson interferometer. A nonnegligible PMD was found in some standard fibers. The sensitivity enables PMD to measure the bend-induced PMD of a fiber rolled on a 28-cm diameter drum. A theoretical model for PMD with random mode coupling is developed, and an explicit equation for the time-of-flight distribution is presented. Comparison between measurements on short and long fibers with residual birefringence leads to an estimation of the coupling length on the order of 20-30 m. >

Rotatory Power and Other Optical Properties of Certain Liquid Crystals

Abstract: A group of liquid crystals, mainly derivates of cholesterol, shows remarkable optical properties, including strong rotatory power and selective reflexion of circularly polarized light in a narrow region of wave-lengths. In this paper it is shown how these properties can be explained by the assumption that the molecules are arranged in a special way, so that the electrical axes rotate screw-like. It is inessential whether this occurs in small steps or continuously. When the axes make one revolution over a thickness p, then light in a region around λ=pn will be reflected (n = refractive index). The second important parameter is the value of the double refraction α = (n 2 - n 1)/n. From p and α all optical properties can be calculated. No accurate data for testing the theory are available but qualitatively the agreement is complete.

Dynamical equation for polarization dispersion

Abstract: Polarization dispersion in single-mode fiber that contains arbitrary birefringence is described through a vector differential equation. Monte-Carlo simulations using this equation show good agreement with experimental measurements in a randomly birefringent fiber and with a previously reported analytic expression for the length dependence of the dispersion. We also correct an error made in earlier research and show that the probability density function for the magnitude of the dispersion at long lengths is Maxwellian rather than Gaussian as previously reported.

Birefringent and electro-optic effects in poled polymer films: steady-state and transient properties

Abstract: Expansion of the orientational distribution function f(θ, t) of molecular dipoles in terms of Legendre polynomials with spherical modified Bessel functions in(μE/kT) as coefficients yields an analytic relation between the steady-state birefringence Δnz(ω) and the electro-optic coefficient χxxz(2)(-ω;ω,0) for a poled nonlinear optical system. A rotational diffusion equation, with the diffusion constant D, for the distribution function describing the onset and the decay of the induced optical and electro-optic properties is solved, with the help of the recurrence relation for spherical modified Bessel functions. It is found that the onset of birefringence involves at least two time constants, with rise times of 1/2D and 1/6D, while the onset of the electro-optic effect is dominated by the rise time of 1/2D. After removal of the dc poling field, the birefringence and the electro-optic effect are found to relax in time with different decay time constants, 1/6D and 1/2D, respectively. This is due to the difference in the tensor rank describing the birefringence and the electro-optic effect.

Annealing of linear birefringence in single-mode fiber coils: application to optical fiber current sensors

Abstract: Annealing procedures that greatly reduce linear birefringence in single-mode fiber coils are described. These procedures have been successfully applied to coils ranging from 5 mm to 10 cm in diameter and up to 200 or more turns. They involve temperature cycles that last 3-4 days and reach maximum temperatures of about 850 degrees C. The residual birefringence and induced loss, are minimized by proper selection of fiber. The primary application of these coils is optical fiber current sensors, where they yield small sensors that are more stable than those achieved by other techniques. A current sensor with a temperature stability of +8.4*10/sup -5//K over the range from -75 to +145 degrees C has been demonstrated. This is approximately 20% greater than the temperature dependence of the Verdet constant. Packaging degrades the stability, but a packaged sensor coil with a temperature stability of about +1.6+10/sup -4//K over the range from -20 to +120 degrees C has also been demonstrated. >

Shear-wave polarizations and subsurface stress directions at Lost Hills field

Abstract: 2 × 2 S-wave data matrix, accomplished by computationally rotating sources and receivers. Although polarization directions obtained by assuming a homogeneous subsurface were moderately consistent with depth, considerable improvement in consistency resulted from analytically stripping off a thin near‐surface layer whose fast S-wave polarization direction was about N 6°E. S-wave birefringence for vertical travel averaged 3 percent in two zones, 200–700 ft and 1200–2100 ft (60–210 m and 370–640 m), which had closely similar S-wave polarizations. Between those zones, the polarization direction changed and the birefringence magnitude was not well defined. S-wave polarizations from two concentric rings of offset VSPs were consistent in azimuth with one another and with polarizations of the near offset VSP. This consistency argues strongly for the robustness of the S-wave polarization technique as applied in this area. The S-wave polarization pattern in offset data fits a model of vertical cracks striking N 55°E...

Birefringent photosensitivity in monomode optical fibre : application to external writing of rocking filters

Abstract: Efficient, narrowband rocking filters have been made in polarisation-maintaining monomode optical fibre by photoinducing periodic birefringent gratings along the length of the fibre. These gratings, fabricated with a period chosen to provide synchronous coupling between the two principal states of polarisation of the LP01 mode at a preselected wavelength, gently rock to and fro along the principal polarisation axes of the fibre. To photoinduce the grating, the fibre is exposed periodically along its length to a UV light beam which is incident at right angles to the geometrical axis of the fibre and is directed π/A rad with respect to the principal polarisation axes. Efficient polarisation mode conversion at 1.3 μm with the birefringent fibre grating is demonstrated.

Two-dimensional optical architecture for time-delay beam forming in a phased-array antenna

Abstract: We propose and experimentally demonstrate a new two-dimensional optical architecture based on polarization switching by N spatial light modulators that provides 2N time delays to a phased-array antenna of p × p radiating elements. Moreover the 0 to 2π phase of the microwave signals is optically controlled by a liquid-crystal spatial light modulator that operates in the birefringent mode. The microwave signal originates from the coherent detection of a dual-frequency laser beam obtained with an acousto-optic frequency shifter. Preliminary experimental results obtained at a microwave frequency f = 1.85 GHz are presented.

Electrically tunable and polarization insensitive Fabry–Perot étalon with a liquid‐crystal film

Abstract: A polarization insensitive, electrically tunable Fabry–Perot structure is demonstrated. The polarization insensitivity is realized by using a twisted structure of a nematic liquid crystal such that the molecules at one surface are orthogonal to those at the other surface. In the high‐field regime, this configuration can be thought of as a structure composed of two orthogonal, birefringent slabs with the electrically controllable thickness. At low voltages the device is polarization sensitive but becomes polarization insensitive at relatively higher voltages. In the polarization insensitive region, the linewidth of the transmission peak is of the order of 0.5 nm and the tuning range is about 15 nm.

A semiempirical model for liquid-crystal refractive index dispersions

Abstract: A single‐band model and a three‐band model which consists of one σ→σ* and two major π→π* electronic transitions were developed for understanding the refractive index dispersions of uniaxial liquid crystals. This information is particularly important for liquid‐crystal‐display applications. By comparing the three‐band model with experimental results of 5CB and 7CB (two cyanobiphenyl homologs), the contribution of each band to both refractive indices and birefringence is evaluated quantitatively. It is found that the σ→σ* band makes the primary contribution to refractive indices, but its contribution to birefringence is not as significant. Conversely, the π→π* bands make the primary contribution to birefringence, but their contribution to refractive indices is not comparable to the σ→σ* band. These characteristics originate from the differences in transition wavelength, intensity, and absorption anisotropy of the σ→σ* and π→π* bands.

All-optical polarisation switch with long low-birefringence fibre

Abstract: Nonlinear polarisation switching of picosecond pulses from the linear polarisation eigenstates of a 200 m-long low-birefringence spun fibre is reported. The relatively low switching peak power of 20 W is promising for applications to all-optical processing and intensity discrimination of ultra-short pulses.

Birefringent film with nx >nz >ny, process for producing the same, retardation film, elliptically polarizing plate, and liquid crystal display

Abstract: A birefringent film in which molecules oriented in a direction parallel to the film plane and molecules oriented in the direction of the thickness of the film are present in a mixed state. Also, a process for producing the birefringent film, which comprises laminating a resin film on one or both sides thereof with a shrinkable film, thereby preparing a laminate, and then heat stretching the laminate, thereby performing stretching treatment of the resin film while imparting to the resin film the ability to shrink in the direction crossing the stretching direction. A retardation film is formed by using at least one birefringent film and has refractive indexes, n x and n y , in two directions parallel to the plate plane and crossing each other at right angles and a refractive index, n z , in the direction of the thickness of the plate, said refractive indexes n x , n y , and n z satisfying the following equation: 0<(n.sub.x -n.sub.z)/(n.sub.x -n.sub.y)<1 provided that n x >n y . Also, an elliptically polarizing plate comprises a laminate of the retardation film with a polarizing plate. Additionally, a liquid crystal display comprises a liquid crystal cell and a polarizing plate disposed at least on one side of said liquid crystal cell through the retardation film.

Laser‐induced birefringence in homeotropic films of photochromic comb‐shaped liquid‐crystalline copolymers with azobenzene moieties at different temperatures

Abstract: Comb-shaped liquid-crystalline copolymers (LCP) containing azobenzene moieties have attracted much attention in the last few This particular interest in ordered photochromic polymer films is predominantly caused by their unique combination of optically anisotropic and polymer properties, which allows different types of laser-addressed, reversible optical data storage. The use of linearly polarized light offers a fundamentally new approach to induce optical anisotropy by rotational diffusion in amorphous photochromic polymers 1 3 , 14) as well as in LCP’s’s2). Recent publications6-12) were aimed at showing that azo moieties undergo E-Z photoisomerization and reorientation, in such a way that optically anisotropic properties such as birefringence and dichroism of homeotropically and planary oriented films of LCP’s are modified in the glassy as well as in the visco-elastic state. It had been shown that the photoinduced rotational diffusion of azo-chromophores does not disturb the other surrounding mesogenic side groups in the glassy state of LCP’s9s10). In contrast to these conclusions, our results from conoscopic studies of LCP’s, which differ concerning the mesogenic moieties, suggest that even in the glassy state the photoinduced reorientation of a minority of azo-chromophores triggers the physical reorientation of other mesogenic groups in the same direction, i.e. perpendicular to the plane of polarization of the incident light ’7 12) . Up to now, there is no information about the influence of temperature, type of mesophase and content of azo moieties on the kinetics of this type of photorecording. The main concern of this paper is to present quantitative results of photoinduced birefringence at different temperatures, using homeotropic films of smectic as well as nematic LCP’s with different structures and different contents of azo-chromophores.

General solutions for stress-induced polarization in optical fibers

Abstract: General solutions of thermal stresses and material birefringence in polarization-maintaining optical fibers are presented. The solutions give the stresses and birefringence at every point in the cross section of various fibers. The fundamental solution of the stress field is constructed by using the complex variable method in elasticity. When the cores, inner claddings, or stress-inducing regions are ellipses, circles, or bow-tie shapes, exact closed-form solutions can be found. The average core birefringence and center core birefringence of elliptical core, elliptical cladding, twin core, Panda, and bow-tie fibers are calculated and compared. Among these fibers, the bow-tie fiber shows the highest birefringence. >

Structure and dynamics of a polymer solution subject to flow-induced phase separation

Abstract: Experiments combining mechanical rheometry with polarimetry (birefringence and scattering dichroism) have been conducted on a 6% solution of polystyrene (1.86x106 molecular weight) in dioctyl phthalate. Birefringence is used to measure the extent of segmental orientation, whereas the dichroism is sensitive to orientation and deformation of concentration fluctuations associated with the process of flow-induced phase separation. The results indicate that these fluctuations grow predominately along the neutral (or vorticity axis) of a simple shear flow. At higher rates of shear, orientation in the flow direction is favored. The transition in orientation direction is accompanied by time-dependent behavior in the optical properties of the solution during shear and the onset of shear thickening of the viscosity and the first normal stress difference coefficient.

Large photoinduced birefringence in an optically nonlinear polyester polymer

Abstract: A nonlinear optical polyester with disperse red side groups exhibited a stable photoinduced birefringence, from 0.14 at 850 nm to 0.21 at 633 nm, after exposure to linearly polarized short‐wavelength visible or ultraviolet light. Thin‐film wave plates, birefringent diffraction gratings and waveguides were written in this polymer using photoexposure. Thin‐ film wave plates of <1 μm thickness showed ≳π/4 phase delay and good temporal stability. A 9:1 diffraction efficiency ratio of the two polarization eigenmodes was measured from thin birefringent gratings. This ratio is in good agreement with the theoretical prediction obtained using a simple three‐level model.

Novel photo-induced effects in chalcogenide glasses

Abstract: Novel phenomena: large photo-induced anisotropy without any photo-darkening, photo-induced gyrotropy (circular dichroism and birefringence) and photo-induced light scattering accompanied by depolarization have been discovered in As 2 S 3 chalcogenide glass on exposure to below-band-gap polarized light. Maxima for all these phenomena are observed in As 34 S 52 I 14 glass. Optical and thermal reversibility of these effects has been established and studied. The photo-induced phenomena are attributed to interaction of light with weak chemical bonds leading to creation or re-orientation of anisotropic and gyrotropic scatterers.

Electrically and Optically Controlled Light Modulation and Color Switching Using Helix Distortion of Ferroelectric Liquid Crystals

Abstract: The electro-optic effect associated with the helix distortion of chiral smectic C (SmC*) liquid crystals and its application to light modulation are investigated. Optically and electrically addressed spatial light modulators based on this effect are demonstrated. They exhibit an analog response, response times as short as 100 μs, good contrast ratio and resolution, and color switching capability. The analog response is shown to be a result of the gradual distortion of the helix at low voltages. Optically this corresponds to a linear rotation of the average optic axis and change of the average effective birefringence. From measurements of the wavelength of the maximum transmission the birefringence has been found to change as a function of dc voltage and frequency of ac voltage and magnitude. The agreement between theory and experiment suggests that color changes are due solely to birefringence changes associated with changes in the geometry of the helix. We derive expressions for the angle of rot...

Phase and frequency resolution of picosecond optical Kerr nonlinearities

Abstract: We report on a method aimed at measuring both the real and imaginary parts of the third-order Kerr-type nonlinear-optical susceptibility of isotropic materials. It is based on Kerr ellipsometry in which analysis of the polarization state of transmitted light between a nearly crossed polarizer and an analyzer allows separation of pump-induced dichroism and birefringence. The method provides a natural measurement of nonlinear phase retardations in angle units, without any calibration procedure. By using a white-light continuum as a probe, this property is used to get a simultaneous measurement of the nonlinear dispersion in the whole visible spectrum. It is illustrated in tetramethylsilane, where stimulated Stokes and anti-Stokes Raman contributions to the nonlinearity are observed.

Polymer-Dispersed Liquid Crystals: Boojums at Work

Abstract: The idea of dispersing micron-size birefringent particles in a polymer to selectively scatter light is not new. In the 1930s Land patented a light polarizing material in which small, oriented solid crystallites were suspended in a clear polymer. The polymer material was selected so that its refractive index matched one of the principal refractive indices of the crystallites while the other did not. The resuit was a light polarizer tha t would pass one component of polarized light but scatter the other component out of the beam path.This idea was substantially expanded by the introduction of liquid crystals as the birefringent material. The orientation of the particles (in this case droplets), and hence the refractive index match and the scattering, could be controlled by an electric field. Such a material could be used as a light shutter for either unpolarized or polarized light. In the mid-1970s this basic concept was applied by Hilsum, but having no way to disperse droplets of liquid crystals in a polymer, he did the opposite and put optically isotropic solid particles in the birefringent liquid crystal.Although Hilsum demonstrated the concept, no commercial device was produced, probably because the shutter contrast was limited. Since then several ways have been found to disperse droplets in a polymer: filling the pores of a microfilter; emulsifying the liquid crystal in a water soluble polymer; and using phase separation methods to create a dispersion of droplets in non-aqueous polymer materials.

Birefringent film and its manufacture, phase difference plate, elliptic polarizing plate and liquid crystal display device

Abstract: PURPOSE: To obtain a birefringent film which has large refractive index differences in respective directions and also has various refractive indexes by mixedly providing a molecule group oriented in the plane direction of the film and a molecule group oriented in a thickness direction. CONSTITUTION: When a resin film is drawn, a laminate is formed by adhering shrinkable films on one or both surfaces of the film and heated and drawn to apply the shrinking force to the resin film in a direction intersecting orthogonally with the drawing direction, thereby obtaining the birefringent film 1 which mixedly has the molecule groups oriented in the drawing direction and thickness direction. Then the phase difference plate 3 which satisfies n x >n y and 0<(n x -n z )/(n x -n y )<1 by the thickness directional molecule orientation is obtained, where n x , n y , and n z are the refractive indexes in the direction of the orthogonal axis of the platnar plane and the thickness direction. COPYRIGHT: (C)1993,JPO&Japio

Miniature Faraday current sensor based on multiple critical angle reflections in a bulk-optic ring.

Abstract: A novel optical element for a bulk glass current sensor that utilizes the Faraday effect is presented with a demonstrated sensitivity of 1 amp-turn and a flat frequency response in the range of 10–104 Hz. The new sensing element overcomes the problems associated with birefringence in optical fiber current sensors and the requirement to make bulk-optic current sensors in complex three-dimensional topologies.

Experimental verification of a form‐birefringent polarization splitter

Abstract: Experimental verification is made of a new polarization splitter which utilizes artificial anisotropic dielectrics. The splitter is composed of layers of periodically laminated SiO2/TiO2 thin films. The SiO2 and TiO2 films are alternately deposited by rf sputtering and reactive dc sputtering, respectively. The thickness of each layer is 50 nm, while the total number of the layers amounts to 2000. The measured polarization split angles are 5.7° (λ=0.63 μm) and 5.1° (λ=1.3 μm), being roughly the same as those predicted.

Polarization noise suppression in retracing optical fiber circuits.

Abstract: A novel scheme is proposed in which the presence of a Faraday rotator in a retracing fiber-optic circuit allows the complete, passive, and universal compensation of any birefringence (reciprocal) change occurring in the fiber. The action of the scheme was experimentally tested by inducing thermal and mechanical perturbations on a fiber coil. The results confirm the theoretical expectations and compensating for unwanted birefringence effects.