Showing papers on "Optical polarization published in 1998"

Automatic compensation of first order polarization mode dispersion in a 10 Gb/s transmission system

Abstract: We demonstrate a novel, adaptive optical polarization mode dispersion equalizer in a 10-Gb/s transmission experiment. The equalizer comprises a fast electrooptic polarization transformer an adjustable differential polarization delay line, and a simple electric distortion analyzer for automatic feedback control.

A CO/sub 2/ laser optical lattice with cold rubidium atoms

Abstract: We report here the successful trapping of rubidium atoms in the antinodes of a standing wave near 10.6 /spl mu/m. This represents the realization of an optical lattice with usually large lattice period, being approximately seven times that of the excitation wavelength of the lowest electronic resonance. In a later three-dimensional configuration, more than one atom per microscopic trap could be captured at moderate average atomic densities, whereas in conventional optical lattices, only a few percent of the lattice sites are occupied.

A Radio Study of the Seyfert Galaxy IC 5063: Evidence for Fast Gas Outflow

Abstract: We present new radio continuum (8 and 1.4 GHz) and H I 21 cm line observations of the Seyfert 2 galaxy IC 5063 (PKS 2048-572), obtained with the Australia Telescope Compact Array. The high-resolution 8 GHz image reveals a linear triple structure ~4'' (1.3 kpc) in size. This small-scale radio emission shows a strong morphological association with the narrow-line region (NLR), the inner part of the optical emission-line region. It is aligned with the inner dust lane and is oriented perpendicularly to the position angle of the optical polarization. We identify the radio nucleus as the central blob of the radio emission. At 21 cm, very broad (~700 km s-1) H I absorption is observed against the strong continuum source. This absorption is almost entirely blueshifted, indicating a fast net outflow, but a faint and narrow redshifted component is also present. In IC 5063 we see clear evidence, both morphological and kinematic, of strong shocks resulting from the interaction between the radio plasma and the interstellar medium (ISM) in the central few kiloparsecs. However, we estimate the energy flux in the radio plasma to be an order of magnitude smaller than the energy flux emitted in emission lines. Thus, although strong shocks associated with the jet/ISM interaction occur, and could contribute locally to the ionization of the NLR, they are unlikely to account solely for the global ionization of the emission-line region, particularly at large distances. The main structure of the H I emission is a warped disk associated with the system of dust lanes of R ~ 2' (~38 kpc, corresponding to ~5 effective radii). The lack of kinematically disturbed gas (both neutral and ionized) outside the central few kiloparsecs, the warped structure of the large-scale disk, and the close morphological connection between the inner dust lanes and the large-scale ionized gas all support the idea that the gas at large radii is photoionized by the central region, while shadowing effects are important in defining its X-shaped morphology. From the kinematics of the ionized and neutral gas, we find evidence for a dark halo in IC 5063, with properties very similar to those observed in some other early-type galaxies.

Polarization rotation in semiconductor bending waveguides: a coupled-mode theory formulation

Abstract: A theoretical model for the bending waveguide polarization rotator has been developed based on the full-vectorial wave equations and the coupled-mode theory. Calculation results from this model are found to agree favorably with measurement data reported in literature. It is found that the polarization conversion efficiency of this device hinges on the degree of asymmetry of the cross-sectional field profile with respect to both the x- and y-axes. Sensitivity analysis, furthermore, revealed that the device characteristics are strongly dependent on the waveguide geometry, in particular, the sidewall tilt angle and the amount of over-etch of the ridge waveguide. Finally, it is also found that the bending waveguide polarization rotator is virtually wavelength independent, making it suitable for wavelength division multiplexing (WDM) applications.

Polarization Phase-Based Method For Material Classification In Computer Vision

Abstract: A robust and accurate polarization phase-based technique for material classification is presented. The novelty of this technique is three-fold in (i) its theoretical development, (ii) application, and, (iii) experimental implementation. The concept of phase of polarization of a light wave is introduced to computer vision for discrimination between materials according to their intrinsic electrical conductivity, such as distinguishing conducting metals, and poorly conducting dielectrics. Previous work has used intensity, color and polarization component ratios. This new method is based on the physical principle that metals retard orthogonal components of light upon reflection while dielectrics do not. This method has significant complementary advantages with respect to existing techniques, is computationally efficient, and can be easily implemented with existing imaging technology. Experiments for real circuit board inspection, nonconductive and conductive glass, and, outdoor object recognition have been performed to demonstrate its accuracy and potential capabilities.

Full-vectorial wave propagation in semiconductor optical bending waveguides and equivalent straight waveguide approximations

Abstract: Device characteristics of optical polarization rotators are founded upon the vector properties of the Maxwell equations. Recently, a bending waveguide based polarization rotator has been proposed and demonstrated. To provide a rigorous basis for the analysis and design of this polarization rotator, the full-vectorial wave equations for both E/spl I.oarr/ and H/spl I.oarr/-field in bending waveguides are derived. It is found from these wave equations that under a broad range of circumstances, a bending waveguide can be analyzed using the equivalent straight waveguide approximation. Details of the model for optical polarization rotators, which is based on the coupled-mode theory, will be described in a companion paper.

Evidence for very large-scale coherent orientations of quasar polarization vectors

Abstract: On the basis of a new sample of quasar optical po- larization measurements, we have found that, in a region of the sky, the quasar polarization vectors are not randomly oriented as naturally expected, but appear concentrated around one pref- erential direction. In order to verify this surprising although preliminary re- sult, we have compiled a large sample of quasar polarization measurements from the literature. With quite severe criteria to eliminate at best the contamination by our Galaxy, a sample of 170 quasars with good quality polarization measurements has been dened. Maps in redshift slices reveal a few regions where the polarization vectors are apparently aligned. To handle the problemmorequantitatively,non-parametric3Dstatisticaltests were designed, as well as a method for visualizing spatially the results. The signicance is evaluated through Monte-Carlo sim- ulations. Applied to our sample of 170 polarized quasars, two differ- ent statistical tests provide evidence, with signicance levels of 0.005 and 0.015 respectively, that the optical polarization vec- tors of quasars are not randomly distributed over the sky but are coherentlyorientedonverylargespatialscales.Thisorientation effectappearsspatiallydelimitedinthe3DUniverse,mainlyoc- curinginafewgroupsof10-20objects.Thepolarizationvectors of objects located along the same line of sight but at different redshifts do not appear accordingly aligned. Essentially for this reason, instrumental bias and contamination by interstellar po- larization in our Galaxy are unlikely to be responsible for the observed effect. The very large scale at which this local orientation effect is observed indicates the presence of correlations in objects or elds on spatial scales 1000h 1 Mpc at redshifts z ' 1-2,

Submilliarcsecond Polarimetric Imaging of Blazar Jets at 43 GHz

Abstract: We present 43 GHz polarization images obtained with the Very Long Baseline Array of four blazars (0829+046, 1055+018, 1334-127, and AP Librae), as well as four active galactic nuclei, which served as polarization calibrators (DA 193, OJ 287, 3C 279, and 1611+343). Six of these objects are members of the Nartallo et al. millimeter-wave polarization blazar monitoring sample. We find no differences in the overall millimeter-wave polarization properties of the BL Lacertae objects and quasars in our sample. This is in contrast to previous findings at centimeter wavelengths, in which the inferred magnetic fields of quasars are found to be predominantly aligned with the jet, while perpendicular configurations are found in BL Lac objects. With the exception of 1611+343, a low optical polarization quasar, all of the unresolved blazar cores in our sample have inferred magnetic field orientations perpendicular to the inner jet direction. Past nonimaging millimeter-wave polarization monitoring data have shown that these core orientations are stable in three of our sample objects; this may be due to strong, unresolved standing shocks located very close to the base of the jet. We also detect in the jets of blazars a moderate-sized population of polarized components having electric vectors that lie at an oblique angle to the local jet direction. We find that the observed distribution of electric vector misalignment angles cannot be fitted by a single population of oblique shocks having arbitrary inclinations with respect to the jet axis. Such a population predicts an overabundance of shocks with electric polarization vectors aligned with the jet axis, produced by relativistic effects associated with the jet flow. We find the data to be more consistent with a scenario in which the polarized jet components are merely enhanced regions whose magnetic field orientations are controlled by some mechanism other than shocks.

Highly spectrum efficient OFDM/PDM wireless networks by using optical SSB modulation

Abstract: Optical frequency division multiplexing (OFDM) wireless access network using optical single-sideband modulation is proposed, which allows to deliver different wireless signals from a central station to each base station over an optical fiber network. The optical single-sideband (SSB) OFDM can achieve the highest spectrum efficiency of 0.25 b/s/Hz. A 0.25 b/s/Hz has been experimentally verified by the error-free transport of two-channel 4.98 Gb/s BPSK signal on 10 GHz wireless carrier with the frequency channel interval of 20 GHz at 1550 nm over 50 km-long standard single-mode fiber link. Furthermore, it has been also experimentally confirmed that by introducing the polarization division multiplexing (PDM), the ultimate spectrum efficiency of 0.5 b/s/Hz can be achieved by the hybrid OFDM/PDM.

A Mueller matrix formalism for modeling polarization effects in erbium-doped fiber

Abstract: The effects of erbium anisotropy in erbium-doped fiber lasers, sources, and amplifiers are examined. Starting from basic ion properties, inversion and gain equations are derived analytically to describe polarization dependencies. A novel matrix form of the Er/sup 3+/ rate equations is presented to propagate powers and polarization states. These equations are then numerically integrated and compared to experimentally observed polarization hole burning and polarization dependent gain. The theoretical predictions agree strongly with experiment in all cases.

Statistical prediction and experimental verification of concatenations of fiber optic components with polarization dependent loss

Abstract: Due to the statistical nature of the evolution of the polarization state in optical fibers and components, it is difficult to measure the global polarization dependent loss (PDL) of a series of concatenated components, each having a different PDL value. The global PDL of concatenated components with PDL cannot be obtained by a simple addition of the single PDL values, moreover, it requires a statistical description. In the present work the statistics of the PDL resulting from the concatenation of several components with PDL, connected by standard optical fibers or by elements having some polarization mode dispersion (PMD) is discussed. Simulations and experimental results supporting the existing theory are presented.

Digital control polarization based optical scanner

Abstract: High speed low cost billion point optical scanner using a binary optical polarization sensitive cascaded architecture network having binary switchable optical plates for scanning in one dimension(x-direction), two dimensions(x and y directions) and three dimensions(x, y and z directions). The scanner has a fast microsecond beam switching speed and low control power and realizes high space bandwidth products of up to one billion points using minimum control signals and hardware. The binary switching optical plates are modules for the x, y, and z directions. For each of the directions, each module contains an active polarization switches and passive beam steering devices. The switches can be high speed flat panel thin film polarization switches such as ferroelectric liquid crystal devices or multiple quantum well binary polarization rotation devices or nematic liquid crystal(NLC) polarization rotator devices. The beam steering devices can be birefringent plates such as birefringent mode nematic liquid crystals(BM-NLC), diffractive optical elements(DOEs), polymer dispensed liquid crystals(PDLCs), and computer generated birefringent nanosecond fabricated structures.

Strain-induced birefringence in vertical-cavity semiconductor lasers

Abstract: We describe a new technique to study and control the polarization properties of planar vertical-cavity semiconductor lasers. The technique consists of the application of a controllable amount of strain by means of the thermal expansion that results from local heating in the vicinity of the device. Analytical expressions are derived for the strain and birefringence induced with this hot-spot technique. Experimentally, the relation between strain and birefringence is found to be highly anisotropic; this allows a natural interpretation of the distribution of the native polarization angles.

Single crystallites in “planar polycrystalline” oligothiophene films: Determination of orientation and thickness by polarization microscopy

Abstract: Thin films of evaporated oligothiophenes (α-nT, n=4–8) show a “planar polycrystalline” structure: each of the individual crystallites has a random azimuthal orientation, the (a,b) face of its unit cell is aligned with the surface plane. We introduce a technique to determine the orientation and thickness of such aligned thiophene crystals by optical polarization microscopy. Due to the optical birefringence of the crystal, it appears with different colors in the microscope dependent on its orientation and thickness. To support the method proposed, we solve Maxwell’s equations and obtain quantitative agreement with the observed colors. The organic crystal shows biaxial anisotropy. For unsubstituted quaterthiophene, α-4T, we find effective refractive indices nb=1.84±0.1 and na=1.61±0.1 for waves under normal incidence. Our conclusions are fully confirmed by atomic force microscopy with molecular resolution. Our analyses result in a simple recipe to obtain the directions of the a and b crystal axes from the optical experiment.

An ultrahigh-speed AlGaAs-GaAs polarization converter using slow-wave coplanar electrodes

Abstract: A wide-band electrooptic polarization converter has been fabricated using slow-wave coplanar-strip electrodes on an AlGaAs-GaAs substrate. Microwave characteristics of the electrode have been measured up to 40 GHz. The microwave loss and the microwave index at 40 GHz are 0.4 Np/cm and 3.4, respectively. The optical measurements were done up to 20 GHz; a flat frequency response was observed.

Polarization selection and sensitivity of external cavity vertical-cavity surface-emitting laser diodes

Abstract: A theoretical analysis has been undertaken of the polarization properties of birefringent vertical-cavity surface-emitting lasers (VCSELs) subject to weak optical feedback in an external cavity configuration. Attention is focussed on the competition between two orthogonal polarizations of the fundamental (LP/sub 01/) transverse mode of the device. It is shown that control of the emission polarization can be exercised even for very small external reflectivities (10/sup -4/%) by appropriate choice of optical feedback delays. The polarization selectivity is shown to be dependent upon the strength of optical feedback. Polarization is also shown to be highly sensitive to small changes in optical feedback delay. Thermally induced wavelength shift is shown to affect polarization behaviour.

Polarization phase shifting shearography for optical metrological applications

Abstract: We present here a different and modified configuration in order to obtain the curvature fringes by incorporating phase shifting and image processing techniques. Optical fibers are used for obtaining shear fringes in which we have used Wollaston prism as the shear element.

One- and two-dimensionally conjugated tetrathynylethenes: Structure versus second-order optical polarizabilities

Abstract: Donor−acceptor substituted molecules, based on the two-dimensionally conjugated tetraethynylethene (TEE) framework, have been investigated for the first time for their first-order hyperpolarizabilities β by electric-field-induced second-harmonic generation (EFISH). We measured high values of up to βEFISH = (510 ± 100) × 10-40 m4/V at λ =1.907 μm. We derived structure−property relationships by performing semiempirical calculations for all compounds. We show that such calculations give the correct trend in the hyperpolarizability β, and more importantly, give information as to the tensorial structure of β for molecules which are two-dimensionally donor−acceptor conjugated.

Intensity dependence of superradiant emission from radiatively coupled excitons in multiple-quantum-well bragg structures

Abstract: Linear time-resolved reflection on the heavy-hole exciton transition of high-quality multiple GaAs quantum-well Bragg samples reveals enhanced radiative emission and accelerated decay of the coherent optical polarization due to radiative interwell coupling. It is shown that this superradiant mode gradually vanishes with increasing excitation intensity. Microscopic calculations attribute this decoupling to the carrier-carrier Coulomb interaction in the individual quantum wells leading to excitation-induced dephasing. The intricate density dependence is discussed comparing computed results for the excitation-dependent decay in single quantum wells and multiple-quantum-well Bragg structures.

Optical Polarization of 52 Radio-loud QSOs and BL Lacertae Objects

Abstract: Polarization measurements are presented for 52 radio-loud QSOs and BL Lac objects. For nine highly polarized (p > 3%) active galactic nuclei (AGNs), these are the first published polarization measurements. Of these nine, seven are highly polarized QSOs (HPQs), one is a BL Lac object, and another is a likely BL Lac object. Polarization variability is confirmed for some of these new and previously known highly polarized AGNs. While six of the HPQs have flat radio spectra and are almost certainly blazars, PKS 1452-217 is probably a new member of the rare class of radio-loud QSOs that show high polarization by scattering, and it is therefore important for testing orientation unified schemes. In competition for the highest redshift HPQ are the well-observed QSO PKS 0438-43 at z = 2.85, with maximum p ~ 4.7%, and PKS 0046-315 at z = 2.72, for which we find p ~ 13%.

Optical Polarization of 52 Radio-Loud QSOs and BL Lac Objects

Abstract: Polarization measurements are presented for 52 radio-loud QSOs and BL Lac objects. For 9 highly polarized (p >3%) AGN, these are the first published polarization measurements. Of these 9, 7 are highly-polarized QSOs (HPQs), one is a BL Lac object and another is a likely BL Lac object. Polarization variability is confirmed for some of these new and previously known highly-polarized AGN. While 6 of the HPQs have flat radio spectra are almost certainly blazars, PKS 1452-217 is probably a new member of the rare class of radio-loud QSOs that show high polarization by scattering, and is therefore important for testing orientation Unified Schemes. In competition for the highest redshift HPQ are the well-observed QSO PKS 0438-43 at z = 2.85, with maximum p = 4.7%, and PKS 0046-315 at z = 2.72, for which we find p = 13%.

Image understanding from thermal emission polarization

Abstract: Existing polarization-based image understanding techniques use information only from reflected light. Apart from incandescent bodies thermally emitted light radiation from elements of a scene in the visible spectrum is insignificant. However, at longer wavelengths such as in the infrared thermal emission is typically quite prevalent from a number of scene elements of interest. FLIR imagery of both indoor and outdoor scenes reveals that many objects thermally emit a significant amount of radiation. Polarization from thermally emitting objects has been observed as long as 170 years ago from incandescent objects but since then there have only been a limited number of empirical investigations into this phenomenon. This paper presents a comprehensive model for explaining polarization of thermal emission from both rough and smooth surfaces, in agreement with empirical data, that can significantly enhance the image understanding of FLIR imagery. In particular it is possible to discern metal from dielectric materials under certain conditions, and from an accurate model for thermally emitted polarization it is possible to predictively model polarization signatures from CAD models of importance to automatic target recognition.

Enhanced optical detection of minimum features using depolarization

Abstract: Optical measurement of image shortening on a lithographically formed minimum feature is enhanced using crossed-polarizer imaging. The minimum feature, which is comprised of a nested array of lines having a width and space corresponding to the critical dimension of interest, is caused to maintain a preferred optical polarization axis due to the repeating nature of the nested pattern. The preferred optical polarization axis causes the transfer of some of the linearly polarized illumination into a new polarization orientation which is sympathetic with a crossed polarizer located in the detection channel of the optical imaging system. The crossed polarizer then allows the light reflected from the nested feature to pass to a detector, while extraneous (background) light is rejected. This results in a high contrast image of the nested feature that facilitates the determination of width, and that reduces measurement variability as a function of the optical properties of the nested feature.

Theory of non-Markovian gain in strained-layer quantum-well lasers with many-body effects

Abstract: A non-Markovian model for the optical gain of strained-layer quantum-well lasers is developed taking into account the valence-band mixing, strain effects, many-body effects, and the non-Markovian relaxation using the time-convolutionless reduced-density operator formalism given in previous papers for an arbitrary driven system coupled to a stochastic reservoir. Many-body effects are taken into account within the time-dependent Hartree-Fock approximation and the valence-band structure is calculated from the 6/spl times/6 Luttinger-Kohn Hamiltonian. The optical gain with Coulomb (or excitonic) enhancement is derived by integrating the equation of motion for the interband polarization. It is shown that the vertex function for the interband polarization can be obtained exactly without relying on the Pade approximation. As a numerical example, an In/sub x/Ga/sub 1-x/As-InP quantum well (QW) is chosen for its wide application in optical communication systems. It is predicted that the Coulomb enhancement of gain is pronounced in the cases of compressive and unstrained QWs while it is negligible in the case of tensile strained QW.

Design of optical strip-loaded waveguides with zero modal birefringence

Abstract: Integrated optic devices are usually sensitive to the polarization state of light, because the two polarized modes of the waveguides that form the devices in general have different propagation constants and may also suffer from different losses. The performance of such devices becomes unstable when they are connected to single-mode fibers, as the polarization state of the output light from a practical fiber link usually fluctuates in an unpredictable manner. A simple solution could be provided by using waveguides in which the two polarized modes are degenerate, i.e., have equal propagation constants. In this paper, it is shown theoretically with the spectral index method that the quasi-transverse electric (TE) and quasi-transverse magnetic (TM) modes of a properly designed optical strip-loaded waveguide can have equal propagation constants. The conditions for achieving mode degeneracy, or zero modal birefringence, are presented and discussed. Strip-loaded waveguides with degenerate polarized modes can be used potentially for forming polarization-insensitive optoelectronic devices.