Showing papers on "Optical polarization published in 1994"

Polarization properties of stimulated Brillouin scattering in single-mode fibers

Abstract: The polarization properties of stimulated brillouin scattering (SBS) in low birefringent optical fibers were evaluated using Stokes calculus. It was found that the Brillouin gain for orthogonally polarized pump and probe wave is half (in dB) of the gain for identical polarization, and not equal as often mentioned in literature. Therefore the polarization factor is K=1/sub 1/2/ for complete polarization scrambling, and not K=2. The spontaneous SBS has the same state of polarization as the pump, and its degree of polarization is 33.3% for low pump powers and near 100% for high pump powers. The experimental results agree very well with the calculations. >

The optical polarization properties of X-ray-selected BL Lacertae objects

Abstract: We discuss the optical polarization properties of X-ray-selected BL Lacertae objects (XSBLs) as determined from 3 yr of monitoring the polarization of 37 BL Lac objects and candidates The observed objects include a complete X-ray flux limited sample drawn from the Einstein Extended Medium Sensitivity Survey (EMSS) We find that the majority of the XSBLs classified solely on the appearance of their optical spectra are true members of the class of BL Lacertae objects since they possess intrinsically polarized and variable continua The duty cycle of polarized emission (fraction of time spent with the degree of polarization greater than 4%) from XSBLs is 44% The maximum observed percent polarizations are of order 10%, considerably below the maximum values observed for radio-selected BL Lacertae objects and blazars (30%-40%) While XSBLs have variable polarized emission, the majority (approximately equal to 85%) have preferred polarization position angles on timescales at least as long as 3 yr This reflects stability in the geometry of the region emitting the linearly polarized optical emission We describe the spectral dependence of the degree of polarization and discuss some of the possible mechanisms producing the observed characteristics While dilution of the polarized emission by the host galaxy starlight is certainly present in some objects, we demonstrate that the average polarization properties of XSBLs derived from our observations are not drastically affected by this effect While the confirmed BL Lac objects are shown to be photometric variables, the objects in our monitored sample did not display the larger than 1 mag variations generally used to characterize the optical variability of radio-selected BL Lacertae objects or blazars in general

A vector finite element method with the high-order mixed-interpolation-type triangular elements for optical waveguiding problems

Abstract: A vector finite element method with the high-order mixed-interpolation-type triangular elements is described for the analysis of optical waveguiding problems. It is a combination of linear edge elements for transverse components of the electric or magnetic field and quadratic nodal elements for the axial one. The use of mixed-interpolation-type elements provides a direct solution for propagation constants and avoids spurious solutions. This approach can yield more accurate results compared with the conventional approach using the lowest order mixed-interpolation-type elements, namely, constant edge elements and linear nodal elements. The accuracy of this approach is investigated by calculating the propagation characteristics of optical rib waveguides. Results obtained for both E/sup x/ and E/sup y/ polarizations are validated using benchmark results produced by established methods. >

Mach-Zehnder interferometer polarization splitter in InGaAsP/InP

Abstract: A passive TE/TM mode polarization splitter based on a Mach-Zehnder interferometer is demonstrated. Insertion loss of 1.5 db and extinction ratios of /spl minus/19 dB for TE and /spl minus/15 dB for TM have been measured at 1510-nm wavelength. The device attains large optical bandwidth employing a pair of multimode interference (MMI) couplers and a wavelength-tolerant birefringent structure. >

Polarization mode converter with polyimide half waveplate in silica-based planar lightwave circuits

Abstract: A TE/TM polarization mode converter, with an excess loss of 0.26 dB, is constructed by inserting a polyimide half waveplate into a groove formed in a silica-based single mode waveguide. The polarization dependence of an arrayed-waveguide grating multiplexer is completely eliminated with this mode converter. >

Spin beats and dynamical magnetization in quantum structures

Abstract: A femtosecond-resolved Faraday spectroscopy has been developed to directly monitor spin dynamics in magnetically tunable semiconductor quantum wells. Tunable terahertz quantum beating of the optical polarization is observed from coherent excitation of the spin states Zeeman split by a single ultrathin magnetic tunneling barrier. Subsequent spin-flip scattering of a photoinjected spin-polarized excitons deposits a magnetic ``imprint'' in the barrier which is orientation dependent and persists for orders of magnitude longer than the carrier lifetime.

Physical processes in eclipsing pulsars: Eclipse mechanisms and diagnostics

Abstract: We investigate how the radio emission of a pulsar interacts with plasma derived from a stellar companion. Various physical mechanisms that can cause radio pulse eclipse are discussed, and predictions are made for the polarization properties of the emergent radio wave. We consider eclipses by a wind from the stellar companion, by a stellar magnetosphere, or by material entrained in the pulsar wind. Eclipses due to refraction require either a relatively high plasma density or a sharp edge to the plasma distribution. The conditions that must prevail for free-free absorption to be effective in eclipsing a radio beam are also outlined. Pulse smearing may be important at higher frequencies; related eclipse mechanisms include pulse spreading due to a rapidly changing electron column, and scattering by Langmuir turbulence. The high brightness temperature radio beam can generate its own plasma turbulence via a number of nonlinear parametric instabilities, such as the instability associated with stimulated Raman scattering. When the plasma turbulence is heavily damped, the radio bean can still undergo induced Compton scattering. Stimulated scattering effects such as these are very sensitive to the presence of narrow-band substructure in the pulsar radio emission. Finally, we consider the possibility that plasma derived from a stellar companion may mix with the relativistic pulsar wind and cause cyclotron absorption at low radio frequencies. Even if the cyclotron optical depth is small, fluctuations in the emergent polarization of the radio beam on the timescale of a few seconds are a very sensitive probe of the spatial structure of the magnetic field in the pulsar wind. The current observational properties of two known eclipsing pulsar systems, PSR 1957+20 and PSR 1744-24A, are used to construct tentative eclipse models. The favored model for PSR 1957+20 is cyclotron or synchrotron absorption by plasma embedded in the pulsar wind combined with pulse smearing at high frequency, and the favored model for PSR 1744-24A is backscattering off plasma turbulence generated by the stimulated Raman scattering parametric instability. Pulsar eclipses promise to provide a good diagnostic of pulsar winds and possible of the pulse emission mechanism.

Polarimetric fiber laser sensors

Abstract: Apparatus for measuring a physical quantity such as pressure, temperature, strain etc., by using a beat frequency of a longitudinal mode, a transverse mode or two polarization modes of the optical fiber laser. In the rare-earth optical fiber laser as a resonator, there are two different polarization modes normal to each other. Since the optical path lengths of the optical fiber laser for the two polarization modes are different, longitudinal mode frequencies of each polarization mode are different depending on the birefringence of resonator which is varied as an external physical quantity such as pressure, temperature, strain etc. Due to the variation in the birefringence of the resonator, the longitudinal mode frequency difference between two polarization modes becomes varied. This variation is linearly proportional to the physical quantity externally applied. The difference in longitudinal mode frequency between polarization modes can be measured from a beat signal of the polarization modes. Accordingly, it is possible to measure the physical quantity by measuring the variation in polarization mode beat frequency.

Theory of anisotropy of two-photon absorption in zinc-blende semiconductors

Abstract: The influence of the higher ${\mathrm{\ensuremath{\Gamma}}}_{15}^{\mathit{c}}$ conduction-band set on the strength of the two-photon-absorption coefficient, the two-photon linear/circular dichroism, and the two-photon-absorption anisotropy is calculated across the spectral region below the fundamental absorption edge, for the cubic materials GaAs and InSb. The anisotropy is entirely due to the higher band and is predicted to produce up to a 70% variation in the two-photon-absorption coefficient of GaAs as the relative orientation of the optical polarization to the crystal axes is altered. This is in good agreement with recent experiments.

Analysis of the POLDER polarization measurements performed over cloud covers

Abstract: The POLDER instrument is designed to provide wide field of view bidimensional images in polarized light. During campaigns of the airborne version of the instrument, images of homogeneous cloud fields were acquired in polarized bands centered at 450 and 850 nm. The polarization of these images is analyzed. The bidirectional polarization distribution function measured in the 850 nm band is shown to make evident the liquid phase of the cloud droplets, by the large characteristic polarization of the cloudbows detected in backward scattering directions. The sensitivity of this feature to cloud parameters is discussed. On the contrary, for observation directions at about 90/spl deg/-100/spl deg/ from the Sun, the cloud polarization is negligible. In these directions, the polarized light observed in the 450 nm band is characteristic of the molecular scattering about the cloud, which allows the cloud top altitude to be derived. The feasibility of the method is analyzed and is tested on cloud pictures acquired at different altitudes above cloud fields. >

Xenon NMR Spectroscopy

Abstract: Xenon NMR spectroscopy has become an important and widely used tool for characterizing complex chemical systems and host phases. Issues central to new and existing uses of xenon NMR in materials research are emphasized, including xenon mass transport considerations, the sensitivity of xenon’s NMR parameters to local and macroscopic material structure and dynamics, and novel optical polarization techniques. After reviewing the foundations of 129Xe and 131Xe NMR spectroscopy methods, a number of applications are presented, particularly those focusing on new developments in the field.

Electrooptic polarization scramblers for optically amplified long-haul transmission systems

Abstract: To combat polarization hole burning in erbium-doped fiber amplifiers, we modulate the input polarization state to a 8800-km long 5.33-Gb/s NRZ transmission system at rates between 10 kHz and 10 GHz using integrated-optic polarization scramblers in lithium niobate. At modulation frequencies of 40 kHz and 10.66 GHz, we observe improvements in the signal-to-noise ratio of the received electrical signal of 4 and 5 dB, respectively. >

Penalties in long-haul optical amplifier systems due to polarization dependent loss and gain

Abstract: The impact of polarization dependent gain (PDG) and polarization dependent loss (PDL) in long-haul optical amplifier systems is studied by means of a power analysis accounting for PDG, PDL and polarization mode dispersion (PMD) in the saturation regime of the erbium-doped fiber amplifiers (EDFAs). System penalties are characterized in terms of mean values and standard deviations. Mean penalties on the bit-error-ratio (BER) are directly related to PDG. BER fluctuations are found to be greatly enhanced when PDG and PDL are combined. The impact of PMD is dependent on the values for PDG and PDL. >

Positive orientations of liquid crystal molecules in a multi-domain liquid crystal display cell

Abstract: A liquid crystal display cell comprising a pair of substrates, a liquid crystal material sandwiched between the pair of substrates, and an optical polarization memory film provided on one or both of the substrates. The optical polarization memory film is exposed by polarized lights to produce a positive orientation means including a multiple micro domains. In the micro domains, orientation direction is uniform in each domain and substantially isotropic as a whole. The visual angular dependency is eliminated. No rubbing treatment is done, and problems caused by rubbing can be solved.

A study of the Chamaeleon I dark cloud and T-association – VI. Interstellar polarization, grain alignment and magnetic field

Abstract: We present new measurements of optical and near-infrared linear polarization towards 39 field stars reddened by dust in the Chamaeleon I dark cloud. New and previously published data are combined in a detailed investigation of the wavelength dependence of interstellar polarization in the cloud. The observations are well represented by the Serkowski formula, with values of the wavelength of maximum polarization (lambda(max)) in the range 0.47-0.75 mum. The highest values of lambda(max) are found in lines of sight that intercept the dense central region of the cloud. The ratio of total to selective extinction (R(V) = A(V)/E(B-V)) is only weakly correlated with lambda(max), suggesting a degree of independence among the populations of grains responsible for optical extinction and polarization. We show that the ratio of polarization to reddening is unusually high in Cha I, with p(max)/A(V) congruent-to 4.5 per cent mag-1 in some lines of sight, indicating a remarkable degree of alignment efficiency in comparison to other dark clouds. The mean direction of the magnetic field in the plane of the sky is perpendicular to the long axis of the cloud and parallel to the external magnetic field in the Galactic neighbourhood of Chamaeleon, suggesting that the cloud formed by uniform collapse along field lines. Our observations provide important constraints on theoretical models for both grain alignment and the relation between magnetic field, cloud collapse and star formation.

Relation between return voltage and other methods for measurements of dielectric response

Abstract: Computer simulations, based upon a phenomenological description of the fundamental dielectric processes of conduction and polarisation, have been performed in order to explain the relations between the different methods and to facilitate a better interpretation of experimental results. Experiments have been performed on a 2 m long sample of a 450 kV mass impregnated HVDC cable. In general, good agreement was obtained between measurements and computer simulations. >

Definitions and measurements of polarization mode dispersion: interferometric versus fixed analyzer methods

Abstract: We compare, from a theoretical and an experimental point of view, the interferometric and the fixed analyzer (also known as the wavelength scanning) techniques for measuring polarization mode dispersion. The information provided by both techniques is shown to be identical, up to a Fourier transform. This information is related to a natural definition of polarization mode delay, /spl Delta//spl tau/. For standard communication fibers, /spl Delta//spl tau/ is itself related to the mean delay between the principal polarization modes, /sub /spl omega//, by a simple numerical factor. >

Demonstration of an optimal polarization scrambler for long-haul optical amplifier systems

Abstract: Polarization scrambling of the input signal in a long-haul erbium-doped fiber amplifier (EDFA) system is studied both experimentally and theoretically. Demonstration of an optimal polarization scrambler is achieved ensuring complete suppression of the signal decay caused by polarization dependent gain (PDG) in EDFAs. Best performance of the system is attained without imposing severe requirements on the polarization scrambler in terms of residual degree of polarization. Close agreement between measurements and calculations provides a valuable validation of the assumed linear dependency of the PDG differential gain with the degree of polarization of the light. >

Method of orienting liquid crystal molecules in a multi-domain liquid crystal display cell

Abstract: A liquid crystal display cell comprising a pair of substrates, a liquid crystal material sandwiched between said pair of substrates, and an optical polarization memory film (5) provided on one or both of the substrates. The optical polarization memory film (5) is exposed by polarized lights (12) to produce a positive orientation means including multiple micro domains. In the micro domains, orientation direction is uniform in each domain and substantially isotropic as a whole. The visual angular dependency is eliminated. No rubbing treatment is done, and problems caused by rubbing can be solved.

Multilevel polarization modulation using a specifically designed LiNbO/sub 3/ device

Abstract: Binary and multilevel optical polarization modulation (POLSK) is an innovative transmission technique, suitable for both coherent and direct detection. To prove its feasibility, we have built a flexible polarization modulator based on a specifically designed LiNbO/sub 3/ device. We show that it can generate any state of polarization and therefore it is suitable for transmission of any binary or multilevel POLSK constellation. Once electrically equalized, the modulator has a 4 GHz bandwidth. To test the modulator with actual digital modulation, we set up a coherent binary experiment at 800 Mbit/s, and attained a sensitivity 1.5 dB away from the noise limit of the receiver. >

Destruction free ageing diagnosis of power cable insulation using the isothermal relaxation current analysis

Abstract: This approach describes the destruction free evaluation of the ageing behaviour of polymeric cable insulations using a new dielectric method. Ageing is influenced by electrical, physical, mechanical and morphological properties. All these changes have a strong influence on the residual strength and the lifetime of a power cable. The cable performance should be evaluated onsite. The destruction-free determination of the ageing behaviour is governed by the depolarization processes inside the insulation. After previous d.c. polarization of the cable insulation the transient part of the current is based upon the thermic release of carriers from traps. The analysis of the current from discrete trap levels hereby enables an evaluation of the ageing process inside the insulation. The Isothermal Relaxation Current (IRC)-Analysis uses the relaxation current measurement and a special mathematical calculation to gain the information about the changes of the polarization processes inside the insulation. The response of the polymeric insulation can be physically described and interpreted taking into account the phenomenon of interfacial polarization and the polymeric conduction behaviour. Therefore the development of an equivalent circuit was necessary to discuss these phenomena more explicitly. The experimental results were achieved during the DIN VDE 0273 A1/A2 (Entwurf) long term test of medium-voltage cables in different steps of ageing and degradation by a major German cable manufacturer. >

Polarization independent, integrated optical, acoustically tunable wavelength filters/switches with tapered acoustical directional coupler

Abstract: Enhanced sidelobe suppression of the filter characteristics of tunable acousto-optical mode converters in LiNbO/sub 3/ has been achieved using for the first time tapered acoustical directional couplers. A sidelobe suppression of more than 15.5 dB could be demonstrated in a single stage device. By combining the mode converter with two integrated optical polarization splitters a polarization independent filter/wavelength selective switch has been fabricated with less than 3 dB intrinsic insertion loss, a polarization dependence of less than 1 dB, and a sidelobe suppression exceeding 14 dB. >

Method and apparatus for determining the concentration of optically-active materials in a fluid

Abstract: In a method and apparatus for determining the concentration of optically-active substances in a fluid, such as glucose in the body fluids of a patient, two light sources are used to produce respective polarized light beams which pass through the substance to be analyzed and are incident on a detector. The second beam has a fixed polarization direction relative to the first beam. The light sources are alternatingly switched on and off at a known-frequency. A dispersing element is disposed in the path of each beam. A comparative signal is produced from the beams exiting the substance and incident on the detector. Due to the presence of the dispersive element in the path of each beam, it is possible to spectrally resolve the components of the comparative signals, so that the optical polarization contributed to the comparative signal by each substance in the fluid can be identified, and thus the concentration of a selected optically-active substance, even in the presence of multiple optically-active substances in the fluid, can be accurately identified.

The influence of polarization on canopy transmission properties at 6.6 GHz and implications for large scale soil moisture monitoring in semi-arid environments

Abstract: Nimbus/SMMR 6.6 GHz data were used together with an extensive data base of surface moisture to study the influence of polarization on radiative transfer characteristics of savannah vegetation in south-eastern Botswana. Ratios of virtual and horizontal polarization optical depths and ratios of albedos were found to be almost constant throughout a three-year period. This conclusion led to a new approach to eliminate the vegetation components in the remotely sensed brightness temperatures, which is important for soil moisture monitoring from space. Using the same data set, this dual-polarization approach was compared with an earlier described synergistic approach, in which NDVI was used to eliminate the vegetation effects. The root mean square error between ground based soil moisture and satellite estimated surface soil moisture was reduced to 1.2% by volume from 5% by volume using the dual polarization approach. >

Weighted-coupling Y-branch optical switch in InGaAs/InGaAlAs quantum well electron transfer waveguides

Abstract: We demonstrate the first weighted-coupling Y-branch switch in semiconductors The active waveguide core contains an InGaAs/InGaAlAs chopped quantum well electron transfer structure which provides large voltage-controlled refractive index changes near 155 /spl mu/m with high speed capability We obtain polarization-independent switching over a wide operating range, from 152 to 158 /spl mu/m wavelength We show that shaping the Y-branch results in significant improvements in voltage-length product and crosstalk performance over conventional, non-shaped Y-branches In push-pull configuration, the drive voltage requirement is only 35 V for a 550 /spl mu/m active length switch >

COBE DIRBE near-infrared polarimetry of the zodiacal light: Initial results

Abstract: This Letter describes near-infrared polarimetry of the zodiacal light at 2.2 micrometers, measured with the Diffuse Infrared Background Experiment (DIRBE) aboard the Cosmic Background Explorer (COBE) spacecraft. The polarization is due to scattering of sunlight. The polarization vector is perpendicular to the scattering plane, and its observed amplitude on the ecliptic equator at an elongation of 90 deg and ecliptic longitude of 10 deg declines from 12.0 +/- 0.4% at 1.25 micrometers to 8.0 +/- 0.6% at 3.5 micrometers (cf. 16% in the visible); the principal source of uncertainty is photometric noise due to stars. The observed near-infrared colors at this location are redder than Solar, but at 3.5 micrometers this is due at least in part to the thermal emission contribution from the interplanetary dust. Mie theory calculations show that both polarizations and colors are important in constraining models of interplanetary dust.

Symmetry properties of anisotropic dielectric gratings

Abstract: We consider the exact equations, obtained by application of the Bloch–Floquet theorem, that describe the propagation of light in anisotropic dielectric gratings with an arbitrary variation of the dielectric tensor along the longitudinal direction. On the basis of these equations we determine the scattering matrix and the corresponding grating efficiencies. The conditions for electromagnetic energy conservation are derived and an energy representation for the field inside the grating is defined, thus permitting determination of reciprocity relations that give rise to particular symmetries of the grating efficiencies if one neglects the reflections at the boundaries. Finally, by exploiting the symmetry properties of the matrix that describes the propagation of the electromagnetic field inside the grating, we discuss the polarization properties of the diffracted beams in chiral C smectics in the phase-grating configuration and in the periodic structures induced in certain nematic liquid crystals by the application of static magnetic or electric fields.

Adaptive polarization diversity detection scheme for coherent communications and interferometric fiber sensors

Abstract: A method and apparatus for overcoming polarization induced signal fading in both heterodyne communication and interferometric sensing. An adjustable birefringent element in series with a linear-polarization beam splitter forms an elliptical-polarization beam splitter. The birefringent element controllably evolves the states of polarization of two input signals thereby controlling the power contribution of each signal onto the orthogonal axes used by the linear beam splitter. When the states of polarization are evolved such that there are equal signal to reference power ratios on the beam splitter axes, subsequent detectors generate a constant, optimum amplitude signal without the need for weighting or decision circuits.