Showing papers on "Optical polarization published in 2016"

Fundamentals of Coherent Optical Fiber Communications

Abstract: The recently developed digital coherent receiver enables us to employ a variety of spectrally efficient modulation formats such as $M$ -ary phase-shift keying and quadrature-amplitude modulation. Moreover, in the digital domain, we can equalize all linear transmission impairments such as group-velocity dispersion and polarization-mode dispersion of transmission fibers, because coherent detection preserves the phase information of the optical signal. This paper reviews the history of research and development related to coherent optical communications and describes the principle of coherent detection, including its quantum-noise characteristics. In addition, it discusses the role of digital signal processing in mitigating linear transmission impairments, estimating the carrier phase, and tracking the state of polarization of the signal in coherent receivers.

Optical Performance Monitoring: A Review of Current and Future Technologies

Abstract: Optical performance monitoring (OPM) is the estimation and acquisition of different physical parameters of transmitted signals and various components of an optical network. OPM functionalities are indispensable in ensuring robust network operation and plays a key role in enabling flexibility and improve overall network efficiency. We review the development of various OPM techniques for direct-detection systems and digital coherent systems and discuss future OPM challenges in flexible and elastic optical networks.

Circular dichroism metamirrors with near-perfect extinction

Abstract: The efficient analysis and engineering of the polarization state is imperative in diverse disciplines, including physics, materials science, biology and quantum optics. For instance, scientists apply circularly polarized light to manipulate the spin state of electron for quantum information processing. Chrysina gloriosa (jeweled beetles) under left-handed circularly polarized light illumination appear more brilliant than those under right-handed circularly polarized light illumination, and circular dichroism spectroscopy is of critical importance to identify the structure of chiral molecules. Metallic mirrors are basic elements and widely used in optical setup to control the path of light. However, the state of circular polarization is reversed, or even degrades to elliptical polarization when it is reflected off a surface. Therefore, the original handedness of the optical signals is lost after multiple reflections in a complex optical system. Here, we propose and demonstrate a new concept of circular dichroism metamirrors, which enables selective, near-perfect reflection of designated circularly polarized light without reversing its handedness, yet complete absorption of the other polarization state. Such a metamirror can be considered as the optical analogy of Chrysina gloriosa in nature, while exhibits nearly maximal efficiency. A general method to design the circular dichroism metasmirror is presented under the framework of Jones calculus. It is analytically shown that the building block of such a metamirror needs to simultaneously break the n-fold rotational (n > 2) symmetry and mirror symmetry. By combining two layers of anisotropic metamaterial structures, we design a circular dichroism metamirror in the mid-infrared region, which shows perfect reflectance (94.7%) for left-handed circularly polarized light without reversing its handedness, while almost completely absorbs (99.3%) right-handed circularly polarized light. These findings offer new methodology to implement novel photonic devices for a variety of applications, including polarimetric imaging, molecular spectroscopy and quantum information processing.

The Imaging X-ray Polarimetry Explorer (IXPE)

Abstract: The Imaging X-ray Polarimetry Explorer (IXPE) is an exciting international collaboration for a scientific mission that dramatically brings together the unique talents of the partners to expand observation space by simultaneously adding polarization measurements to the array of source properties currently measured (energy, time, and location). IXPE uniquely brings to the table polarimetric imaging. IXPE will thus open new dimensions for understanding how X-ray emission is produced in astrophysical objects, especially systems under extreme physical conditions-such as neutron stars and black holes. Polarization singularly probes physical anisotropies-ordered magnetic fields, aspheric matter distributions, or general relativistic coupling to black-hole spin-that are not otherwise measurable. Hence, IXPE complements all other investigations in high-energy astrophysics by adding important and relatively unexplored information to the parameter space for studying cosmic X-ray sources and processes, as well as for using extreme astrophysical environments as laboratories for fundamental physics.

Manipulation of Group-Velocity-Locked Vector Solitons From Fiber Lasers

Abstract: Manipulation of group-velocity-locked vector solitons (GVLVSs) is numerically proposed and experimentally demonstrated. A pseudo-high-order GVLVS could be generated from a fundamental GVLVS with the help of a polarization-resolved system. Specifically, a pseudo-high-order GVLVS with a two-humped pulse along one polarization and a single-humped pulse along the orthogonal polarization could be obtained. The phase difference between the two humps could be 180°.

Al-Pd Nanodisk Heterodimers as Antenna-Reactor Photocatalysts.

Abstract: Photocatalysis uses light energy to drive chemical reactions. Conventional industrial catalysts are made of transition metal nanoparticles that interact only weakly with light, while metals such as Au, Ag, and Al that support surface plasmons interact strongly with light but are poor catalysts. By combining plasmonic and catalytic metal nanoparticles, the plasmonic “antenna” can couple light into the catalytic “reactor”. This interaction induces an optical polarization in the reactor nanoparticle, forcing a plasmonic response. When this “forced plasmon” decays it can generate hot carriers, converting the catalyst into a photocatalyst. Here we show that precisely oriented, strongly coupled Al–Pd nanodisk heterodimers fabricated using nanoscale lithography can function as directional antenna–reactor photocatalyst complexes. The light-induced hydrogen dissociation rate on these structures is strongly dependent upon the polarization angle of the incident light with respect to the orientation of the antenna–re...

Filamentary structure and magnetic field orientation in Musca

Abstract: Herschel has shown that filamentary structures are ubiquitous in star-forming regions, in particular in nearby molecular clouds associated with Gould's Belt. High dynamic range far-infrared imaging of the Musca cloud with SPIRE and PACS reveals at least two types of filamentary structures: (1) the main ~10-pc scale high column-density linear filament; and (2) low column-density striations in close proximity to the main filament. In addition, we find features with intermediate column densities (hair-like strands) that appear physically connected to the main filament. We present an analysis of this filamentary network traced by Herschel and explore its connection with the local magnetic field. We find that both the faint dust emission striations and the plane-of-the-sky (POS) magnetic field are locally oriented close to perpendicular to the high-density main filament (position angle ~25-35°). The low-density striations and strands are oriented parallel to the POS magnetic field lines, which are derived previously from optical polarization measurements of background stars and more recently from Planck observations of dust polarized emission. The position angles are 97 ± 25°, 105 ± 7°, and 105 ± 5°. From these observations, we propose a scenario in which local interstellar material in this cloud has condensed into a gravitationally-unstable filament (with "supercritical" mass per unit length) that is accreting background matter along field lines through the striations. We also compare the filamentary structure in Musca with what is seen in similar Herschel observations of the Taurus B211/3 filament system and find that there is significantly less substructure in the Musca main filament than in the B211/3 filament. We suggest that the Musca cloud may represent an earlier evolutionary stage in which the main filament has not yet accreted sufficient mass and energy to develop a multiple system of intertwined filamentary components. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

Primary Black Hole Spin in OJ 287 as Determined by the General Relativity Centenary Flare

Abstract: OJ 287 is a quasi-periodic quasar with roughly 12 year optical cycles. It displays prominent outbursts that are predictable in a binary black hole model. The model predicted a major optical outburst in 2015 December. We found that the outburst did occur within the expected time range, peaking on 2015 December 5 at magnitude 12.9 in the optical R-band. Based on Swift/XRT satellite measurements and optical polarization data, we find that it included a major thermal component. Its timing provides an accurate estimate for the spin of the primary black hole, $\chi =0.313\pm 0.01$. The present outburst also confirms the established general relativistic properties of the system such as the loss of orbital energy to gravitational radiation at the 2% accuracy level, and it opens up the possibility of testing the black hole no-hair theorem with 10% accuracy during the present decade.

Optical Polarization Möbius Strips and Points of Purely Transverse Spin Density.

Abstract: Tightly focused light beams can exhibit complex and versatile structured electric field distributions. The local field may spin around any axis including a transverse axis perpendicular to the beams' propagation direction. At certain focal positions, the corresponding local polarization ellipse can even degenerate into a perfect circle, representing a point of circular polarization or $C$ point. We consider the most fundamental case of a linearly polarized Gaussian beam, where---upon tight focusing---those $C$ points created by transversely spinning fields can form the center of 3D optical polarization topologies when choosing the plane of observation appropriately. Because of the high symmetry of the focal field, these polarization topologies exhibit nontrivial structures similar to M\"obius strips. We use a direct physical measure to find $C$ points with an arbitrarily oriented spinning axis of the electric field and experimentally investigate the fully three-dimensional polarization topologies surrounding these $C$ points by exploiting an amplitude and phase reconstruction technique.

Primary black hole spin in OJ287 as determined by the General Relativity centenary flare

Abstract: OJ287 is a quasi-periodic quasar with roughly 12 year optical cycles. It displays prominent outbursts which are predictable in a binary black hole model. The model predicted a major optical outburst in December 2015. We found that the outburst did occur within the expected time range, peaking on 2015 December 5 at magnitude 12.9 in the optical R-band. Based on Swift/XRT satellite measurements and optical polarization data, we find that it included a major thermal component. Its timing provides an accurate estimate for the spin of the primary black hole, chi = 0.313 +- 0.01. The present outburst also confirms the established general relativistic properties of the system such as the loss of orbital energy to gravitational radiation at the 2 % accuracy level and it opens up the possibility of testing the black hole no-hair theorem with a 10 % accuracy during the present decade.

Optical polarization and intervalley scattering in single layers of MoS2 and MoSe2

Abstract: Single layers of MoS2 and MoSe2 were optically pumped with circularly polarized light and an appreciable polarization was initialized as the pump energy was varied. The circular polarization of the emitted photoluminescence was monitored as a function of the difference between the excitation energy and the A-exciton emission at the K-point of the Brillouin zone. Our results show a threshold of twice the LA phonon energy, specific to the material, above which phonon-assisted intervalley scattering causes depolarization. In both materials this leads to almost complete depolarization within ~100 meV above the threshold energy. We identify the extra kinetic energy of the exciton (independent of whether it is neutral or charged) as the key parameter for presenting a unifying picture of the depolarization process.

LiNbO 3 Thin-Film Modulators Using Silicon Nitride Surface Ridge Waveguides

Abstract: Lithium niobate (LiNbO3) is an excellent electrooptic material due to its low loss, large electrooptic coefficient, linear modulation response, and large modulation bandwidth. In this letter, we present the first LiNbO3 thin-film Mach–Zehnder optical modulator that employs silicon nitride surface ridge optical waveguides. The modulator contains a 1.2-cm-long push–pull phase modulation section and a pair of multimode interferometric 3-dB couplers. It demonstrated $V_{{\pi }} \cdot {L}$ of $\sim 3\text {V}\cdot \text {cm}$ and a 3-dB bandwidth of $\sim 8$ GHz.

Optical Polarization Encoding Using Graphene‐Loaded Plasmonic Metasurfaces

Abstract: Moreover, the development of a feasible metadevice design that has the ability to dynamically tune the light polarization state has attracted enormous interest. Such metadevices can be employed for realizing not only the polarization encoding but also polarization-division multiplexing (PDM), which is a crucial technique that can signifi cantly increase the transmission capacity of a single physical channel. [ 34 ] The traditional technique for realizing PDM requires a complex optical system and cumbersome volume. Therefore, a metadevice-based polarization modulator offers a new approach for simplifying the optical process and miniaturizing the required volume. Although metasurfaces can transform light polarization, the change of polarization-control characteristic requires variation of the size, shape, and material properties of the structure. Hence, the dynamical manipulation of the polarization state using a metasurface is still challenging. In this work, we propose a metadevice by integrating a single layer of graphene with an anisotropic metasurface, which can dynamically modulate the polarization state of light with wide tunable range in MIR wavelengths. By switching gate voltage applied on the graphene among three different values, the incident linearly polarized (LP) light can be dynamically converted into left circularly polarized (LCP) light, right circularly polarized (RCP) light, or linearly cross-polarized light in the refl ection direction by the proposed metadevice. A continuous polarization evolution from LCP to RCP light can be achieved as the gate voltage gradually increases. In addition, two mutually perpendicular LP light beams and an elliptically polarized (EP) light beam can also be generated in the refl ection direction by the proposed metadevice under EP light illumination. Based on these polarization-control characteristics, the proposed metadevice can realize polarization encoding and the PDM technique. The process of multiplexing and demultiplexing in the PDM technique can be accomplished by metadevices with a more simple approach and in a more compact space than traditional PDM techniques. The new degrees of freedom enabled by the metadevice facilitate the arbitrary manipulation of light polarization states and will profoundly affect a wide range of modern optical communication devices. Control of the light polarization state is an important area of research for metasurfaces. The anisotropy of plasmonic metasurfaces plays a dominant role in manipulating the light polarization state. The different optical responses along two orthogonal principal axes of the nanostructures result in different amplitude attenuation and phase retardation of the Optical Polarization Encoding Using Graphene-Loaded Plasmonic Metasurfaces

RoboPol: the optical polarization of gamma-ray-loud and gamma-ray-quiet blazars

Abstract: We present average R-band optopolarimetric data, as well as variability parameters, from the first and second RoboPol observing season. We investigate whether gamma- ray--loud and gamma-ray--quiet blazars exhibit systematic differences in their optical polarization properties. We find that gamma-ray--loud blazars have a systematically higher polarization fraction (0.092) than gamma-ray--quiet blazars (0.031), with the hypothesis of the two samples being drawn from the same distribution of polarization fractions being rejected at the 3{\sigma} level. We have not found any evidence that this discrepancy is related to differences in the redshift distribution, rest-frame R-band lu- minosity density, or the source classification. The median polarization fraction versus synchrotron-peak-frequency plot shows an envelope implying that high synchrotron- peaked sources have a smaller range of median polarization fractions concentrated around lower values. Our gamma-ray--quiet sources show similar median polarization fractions although they are all low synchrotron-peaked. We also find that the random- ness of the polarization angle depends on the synchrotron peak frequency. For high synchrotron-peaked sources it tends to concentrate around preferred directions while for low synchrotron-peaked sources it is more variable and less likely to have a pre- ferred direction. We propose a scenario which mediates efficient particle acceleration in shocks and increases the helical B-field component immediately downstream of the shock.

Long-Distance Wireless mm-Wave Signal Delivery at W-Band

Abstract: W-band (75–110 GHz) is a potential radio frequency band to provide long-distance wireless links for mobile data transmission. We proposed a high-speed long-distance wireless transmission link at W-band based on some enabling technologies and advanced devices, such as antenna polarization multiplexing combined with multiple-input multiple-output, large-gain/high-power W-band electrical amplifiers, high-gain small-beamwidth Cassegrain antennas, and wideband optical/electrical components. We experimentally demonstrated that our proposed wireless transmission link can realize up to 1.7-km wireless delivery of 20-Gb/s@85.5-GHz millimeter-wave signal with a bit-error rate less than $3.8 \times 10^{-3}$ .

RoboPol: optical polarization-plane rotations and flaring activity in blazars

Abstract: We present measurements of rotations of the optical polarization of blazars during the second year of operation of RoboPol, a monitoring programme of an unbiased sample of gamma-ray bright blazars specially designed for effective detection of such events, and we analyse the large set of rotation events discovered in two years of observation. We investigate patterns of variability in the polarization parameters and total flux density during the rotation events and compare them to the behaviour in a non-rotating state. We have searched for possible correlations between average parameters of the polarization-plane rotations and average parameters of polarization, with the following results: (1) there is no statistical association of the rotations with contemporaneous optical flares; (2) the average fractional polarization during the rotations tends to be lower than that in a non-rotating state; (3) the average fractional polarization during rotations is correlated with the rotation rate of the polarization plane in the jet rest frame; (4) it is likely that distributions of amplitudes and durations of the rotations have physical upper bounds, so arbitrarily long rotations are not realized in nature.

Are there reliable methods to estimate the nuclear orientation of Seyfert galaxies

Abstract: Together with accretion and evolution, orientation is one of the three main drivers in the grand unification of active galactic nuclei (AGNs). Being unresolved, determining the true inclination of those powerful sources is always difficult and indirect, yet it remains a vital clue to apprehend the numerous, panchromatic and complex spectroscopic features we detect. There have only been 100 inclinations derived so far; in this context, can we be sure that we measure the true orientation of AGNs? To answer this question, four methods to estimate the nuclear inclination of AGNs are investigated and compared to inclination-dependent observables (hydrogen column density, Balmer linewidth, optical polarization and flux ratios within the infrared and relative to X-rays). Among these orientation indicators, the method developed by Fisher, Crenshaw, Kraemer, and others, mapping and modelling the radial velocities of the [O III] emission region in AGNs, is the most successful. The [O III]-mapping technique shows highly statistically significant correlations at >95 per cent confidence level for rejecting the null hypothesis for all the test cases. Such results confirm that the unified model is correct at a scale ranging from kiloparsec to a fraction of a parsec. However, at a radial distance less than 0.01 pc from the central black hole, warps and misalignments may change this picture.

Black Phosphorus Quantum Dots as an Efficient Saturable Absorber for Bound Soliton Operation in an Erbium Doped Fiber Laser

Abstract: We have investigated the nonlinear and ultrafast photonics of chemically processed black phosphorus quantum dots (BPQDs) synthesized by the solvothermal treatment approach, with an average lateral size of about 2.48 $\pm$ 0.4 nm. BPQDs exhibit optical saturable absorption measured by the balanced twin-detector measurement system at the telecommunication band and have been demonstrated for the generation of passively mode-locking operation in an erbium-doped fiber laser. Either two-pulse or three-pulse bound state of soliton pulse has been obtained, making the best use of the BPQDs-based saturable absorber. Our work suggests that BPQDs might be developed as an efficient optical saturable absorber for ultrafast photonics applications.

Theoretical and experimental investigation of optical absorption anisotropy in β-Ga2O3.

Abstract: The question of optical bandgap anisotropy in the monoclinic semiconductor β-Ga2O3 was revisited by combining accurate optical absorption measurements with theoretical analysis, performed using different advanced computation methods. As expected, the bandgap edge of bulk β-Ga2O3 was found to be a function of light polarization and crystal orientation, with the lowest onset occurring at polarization in the ac crystal plane around 4.5–4.6 eV; polarization along b unambiguously shifts the onset up by 0.2 eV. The theoretical analysis clearly indicates that the shift in the b onset is due to a suppression of the transition matrix elements of the three top valence bands at Γ point.

Demonstration of Ultra-Capacity Wireless Signal Delivery at W-Band

Abstract: High-speed long-haul wireless transmission links are required to meet the demand of mobile backhauling and emergency communications. We experimentally demonstrated ultra-high-speed 432-Gb/s polarization-division-multiplexing 16-ary quadrature amplitude modulation modulated W-band millimeter-wave (mm-wave) signal delivery over a 2-m horn antenna-based (HA-based) 4 × 4 multiple-input multiple-output (MIMO) wireless link, enabled by photonic mm-wave generation and optical/antenna polarization multiplexing. We further achieved the field trial demonstration of 80-Gb/s polarization-division-multiplexing quadrature phase shift keying modulated W-band mm-wave signal delivery over a 300-m Cassegrain antenna-based (CA-based) 4 × 4 MIMO wireless link, adopting photonic mm-wave generation, multi-band multiplexing, and optical/antenna polarization multiplexing. To the best of our knowledge, 80 Gb/s or 74.7 Gb/s after removing 7% forward-error-correction overhead is a record for W-band wireless signal delivery over a few hundred meters.

Towards optical polarization control of laser-driven proton acceleration in foils undergoing relativistic transparency

Abstract: Control of the collective response of plasma particles to intense laser light is intrinsic to relativistic optics, the development of compact laser-driven particle and radiation sources, as well as investigations of some laboratory astrophysics phenomena. We recently demonstrated that a relativistic plasma aperture produced in an ultra-thin foil at the focus of intense laser radiation can induce diffraction, enabling polarization-based control of the collective motion of plasma electrons. Here we show that under these conditions the electron dynamics are mapped into the beam of protons accelerated via strong charge-separation-induced electrostatic fields. It is demonstrated experimentally and numerically via 3D particle-in-cell simulations that the degree of ellipticity of the laser polarization strongly influences the spatial-intensity distribution of the beam of multi-MeV protons. The influence on both sheath-accelerated and radiation pressure-accelerated protons is investigated. This approach opens up a potential new route to control laser-driven ion sources.

Mode Division Multiplexing Using Orbital Angular Momentum Modes Over 1.4-km Ring Core Fiber

Abstract: Mode division multiplexing (MDM) systems using orbital angular momentum (OAM) modes can recover the data in D different modes without recourse to full (2 D × 2 D ) multiple input multiple output (MIMO) processing. One of the biggest challenges in OAM-MDM systems is the mode instability following fiber propagation. Previously, MIMO-free OAM-MDM data transmission with two modes over 1.1 km of vortex fiber was demonstrated, where optical polarization demultiplexing was employed in the setup. We demonstrate MDM data transmission using two OAM modes over 1.4 km of a specially designed ring core fiber without using full MIMO processing or optical polarization demultiplexing. We demonstrate reception with electrical polarization demultiplexing, i.e., minimal 2 × 2 MIMO, showing the compatibility of OAM-MDM with current polarization demultiplexing receivers.

Optical Switches Based on Silicon Photonics for ROADM Application

Abstract: We demonstrate compact and low-loss $8 \times 8$ silicon photonic switch modules, which are applicable to transponder aggregators (TPAs) in colorless, directionless, and contentionless reconfigurable optical add-drop multiplexers. Newly designed silicon optical switch chips incorporating spot size converters with polarization insensitive and wavelength insensitive properties over C/L bands are packaged. The developed module shows about 6-dB average excess optical loss, including optical coupling loss, on all 64 optical paths with low-polarization-dependent loss and low crosstalk. Using these compact optical switch modules, we construct a TPA prototype featuring over 100-port optical switch subsystem densely mounted on one board and confirm its feasibility.

RoboPol: do optical polarization rotations occur in all blazars?

Abstract: We present a new set of optical polarization plane rotations in blazars, observed during the third year of operation of RoboPol. The entire set of rotation events discovered during three years of observations is analysed with the aim of determining whether these events are inherent in all blazars. It is found that the frequency of the polarization plane rotations varies widely among blazars. This variation cannot be explained either by a difference in the relativistic boosting or by selection effects caused by a difference in the average fractional polarization. We conclude that the rotations are characteristic of a subset of blazars and that they occur as a consequence of their intrinsic properties.

The RINGO2 and DIPOL optical polarization catalogue of blazars

Abstract: We present ∼2000 polarimetric and ∼3000 photometric observations of 15 γ-ray bright blazars over a period of 936 days (2008-10-11 to 2012-10-26) using data from the Tuorla blazar monitoring program (KVA DIPOL) and Liverpool Telescope (LT) RINGO2 polarimeters (supplemented with data from SkyCamZ (LT) and Fermi-LAT γ-ray data). In 11 out of 15 sources we identify a total of 19 electric vector position angle (EVPA) rotations and 95 flaring episodes. We group the sources into subclasses based on their broad-band spectral characteristics and compare their observed optical and γ-ray properties. We find that (1) the optical magnitude and γ-ray flux are positively correlated, (2) EVPA rotations can occur in any blazar subclass, four sources show rotations that go in one direction and immediately rotate back, (3) we see no difference in the γ-ray flaring rates in the sample; flares can occur during and outside of rotations with no preference for this behaviour, (4) the average degree of polarization (DoP), optical magnitude and γ-ray flux are lower during an EVPA rotation compared with during non-rotation and the distribution of the DoP during EVPA rotations is not drawn from the same parent sample as the distribution outside rotations, (5) the number of observed flaring events and optical polarization rotations are correlated, however we find no strong evidence for a temporal association between individual flares and rotations and (6) the maximum observed DoP increases from ∼10 per cent to ∼30 per cent to ∼40 per cent for subclasses with synchrotron peaks at high, intermediate and low frequencies, respectively.

Tunable Fiber Polarization Filter by Filling Different Index Liquids and Gold Wire Into Photonic Crystal Fiber

Abstract: A tunable fiber polarization filter by filling different index liquids into the central hole of photonic crystal fiber (PCF) is proposed and demonstrated. The dispersion characteristics and loss spectra of the polarization filter are evaluated by finite element method (FEM). The gold wires are selectively filled into the cladding air holes of the PCF. When the phase matching condition is satisfied, the liquid-core mode couples to surface plasmon polaritons (SPP) mode intensely. The resonance wavelength varies with the change of the structural parameters and liquids. By adjusting the refractive index of the liquid, we realize the polarization filter at the wavelength of 1.31, 1.49, and 1.55 $\mu$ m, respectively, under the optimized structural parameters. This is the first time to propose the narrowband polarization filter at the communication wavelength of 1.31 $\mu$ m to our best knowledge based on the coupling between liquid-core mode and SPP mode, and the full width half maximum (FWHM) is only 16 nm. The loss of X-polarized mode is 44336 dB/m at $\lambda$ = 1.31 $\mu$ m, and the corresponding loss of Y-polarization mode is 224 dB/m. By comparison, we find the birefringence in our structure is further better than that in conventional structure. High birefringence is helpful to separate the resonance wavelength positions of the two orthogonal polarized modes. The result also reveals that resonance loss becomes small with increasing the distance between liquid core and gold wire.

Wideband Optical Multipath Interference Cancellation Based on a Dispersive Element

Abstract: A novel scheme is proposed for analog multipath interference cancellation based on two polarization modulators (PolMs) and a dispersive element. Multiple optical compensation branches are formed by an array of tunable lasers. By adjusting the wavelength and the optical power of each tunable laser, the delay and the magnitude of the known interference signal can be finely tuned, so that the multipath interference can be accurately reconstructed for counter-phase cancellation. The dispersion-induced RF power fading in a dispersive optical link can also be compensated because of the PolM-based intensity modulation scheme, so the best cancellation depth can be obtained at any desired frequency. Experimental result shows that the proposed system can achieve more than 44-dB cancellation depth for wideband signals.

Systematic study of gamma-ray-bright blazars with optical polarization and gamma-ray variability

Abstract: Blazars are highly variable active galactic nuclei which emit radiation at all wavelengths from radio to gamma-rays. Polarized radiation from blazars is one key piece of evidence for synchrotron radiation at low energies and it also varies dramatically. The polarization of blazars is of interest for understanding the origin, confinement, and propagation of jets. However, even though numerous measurements have been performed, the mechanisms behind jet creation, composition and variability are still debated. We performed simultaneous gamma-ray and optical photopolarimetry observations of 45 blazars between Jul. 2008 and Dec. 2014 to investigate the mechanisms of variability and search for a basic relation between the several subclasses of blazars. We identify a correlation between the maximum degree of optical linear polarization and the gamma-ray luminosity or the ratio of gamma-ray to optical fluxes. Since the maximum polarization degree depends on the condition of the magnetic field (chaotic or ordered), this result implies a systematic difference in the intrinsic alignment of magnetic fields in pc-scale relativistic jets between different types blazars (FSRQs vs. BL Lacs), and consequently between different types of radio galaxies (FR Is vs. FR IIs).

Optical polarization of high-energy BL Lacertae objects

Abstract: Context. We investigate the optical polarization properties of high-energy BL Lac objects using data from the RoboPol blazar monitoring program and the Nordic Optical Telescope. Aims. We wish to understand if there are differences between the BL Lac objects that have been detected with the current-generation TeV instruments and those objects that have not yet been detected. Methods. We used a maximum-likelihood method to investigate the optical polarization fraction and its variability in these sources. In order to study the polarization position angle variability, we calculated the time derivative of the electric vector position angle (EVPA) change. We also studied the spread in the Stokes Q / I − U / I plane and rotations in the polarization plane. Results. The mean polarization fraction of the TeV-detected BL Lacs is 5%, while the non-TeV sources show a higher mean polarization fraction of 7%. This difference in polarization fraction disappears when the dilution by the unpolarized light of the host galaxy is accounted for. The TeV sources show somewhat lower fractional polarization variability amplitudes than the non-TeV sources. Also the fraction of sources with a smaller spread in the Q / I − U / I plane and a clumped distribution of points away from the origin, possibly indicating a preferred polarization angle, is larger in the TeV than in the non-TeV sources. These differences between TeV and non-TeV samples seem to arise from differences between intermediate and high spectral peaking sources instead of the TeV detection. When the EVPA variations are studied, the rate of EVPA change is similar in both samples. We detect significant EVPA rotations in both TeV and non-TeV sources, showing that rotations can occur in high spectral peaking BL Lac objects when the monitoring cadence is dense enough. Our simulations show that we cannot exclude a random walk origin for these rotations. Conclusions. These results indicate that there are no intrinsic differences in the polarization properties of the TeV-detected and non-TeV-detected high-energy BL Lac objects. This suggests that the polarization properties are not directly related to the TeV-detection, but instead the TeV loudness is connected to the general flaring activity, redshift, and the synchrotron peak location.

Multifrequency Photo-polarimetric WEBT Observation Campaign on the Blazar S5 0716+714: Source Microvariability and Search for Characteristic Timescales

Abstract: Here we report on the results of the WEBT photo-polarimetric campaign targeting the blazar S5~0716+71, organized in March 2014 to monitor the source simultaneously in BVRI and near IR filters. The campaign resulted in an unprecedented dataset spanning $\sim 110$\,h of nearly continuous, multi-band observations, including two sets of densely sampled polarimetric data mainly in R filter. During the campaign, the source displayed pronounced variability with peak-to-peak variations of about $30\%$ and "bluer-when-brighter" spectral evolution, consisting of a day-timescale modulation with superimposed hourlong microflares characterized by $\sim 0.1$\,mag flux changes. We performed an in-depth search for quasi-periodicities in the source light curve; hints for the presence of oscillations on timescales of $\sim 3$\,h and $\sim 5$\,h do not represent highly significant departures from a pure red-noise power spectrum. We observed that, at a certain configuration of the optical polarization angle relative to the positional angle of the innermost radio jet in the source, changes in the polarization degree led the total flux variability by about 2\,h; meanwhile, when the relative configuration of the polarization and jet angles altered, no such lag could be noted. The microflaring events, when analyzed as separate pulse emission components, were found to be characterized by a very high polarization degree ($> 30\%$) and polarization angles which differed substantially from the polarization angle of the underlying background component, or from the radio jet positional angle. We discuss the results in the general context of blazar emission and energy dissipation models.