Polarimetric Sensitivity Research Articles

Observations of solar scattering polarization at high spatial resolution

The weak, turbulent magnetic fields that supposedly permeate most of the solar photosphere are difficult to observe, because the Zeeman effect is virtually blind to them. The Hanle effect, acting on the scattering polarization in suitable lines, can in principle be used as a diagnostic for these fields. However, the prediction that the majority of the weak, turbulent field resides in intergranular lanes also poses significant challenges to scattering polarization observations because high spatial resolution is usually difficult to attain. We aim to measure the difference in scattering polarization between granules and intergranules. We present the respective center-to-limb variations, which may serve as input for future models. We perform full Stokes filter polarimetry at different solar limb positions with the CN band filter of the Hinode-SOT Broadband Filter Imager, which represents the first scattering polarization observations with sufficient spatial resolution to discern the granulation. Hinode-SOT offers unprecedented spatial resolution in combination with high polarimetric sensitivity. The CN band is known to have a significant scattering polarization signal, and is sensitive to the Hanle effect. We extend the instrumental polarization calibration routine to the observing wavelength, and correct for various systematic effects. The scattering polarization for granules (i.e., regions brighter than the median intensity of non-magnetic pixels) is significantly larger than for intergranules. We derive that the intergranules (i.e., the remaining non-magnetic pixels) exhibit (9.8 \pm 3.0)% less scattering polarization for 0.2<u<0.3, although systematic effects cannot be completely excluded. These observations constrain MHD models in combination with (polarized) radiative transfer in terms of CN band line formation, radiation anisotropy, and magnetic fields.

Highly birefringent microstructured fibers with enhanced sensitivity to hydrostatic pressure

We designed, manufactured and characterized two birefringent microstructured fibers that feature a 5-fold increase in polarimetric sensitivity to hydrostatic pressure compared to the earlier reported values for microstructured fibers. We demonstrate a good agreement between the finite element simulations and the experimental values for the polarimetric sensitivity to pressure and to temperature. The sensitivity to hydrostatic pressure has a negative sign and exceeds -43 rad/MPa x m at 1.55 microm for both fibers. In combination with the very low sensitivity to temperature, this makes our fibers the candidates of choice for the development of microstructured fiber based hydrostatic pressure measurement systems.

Measurements of polarimetric sensitivity to hydrostatic pressure, strain and temperature in birefringent dual-core microstructured polymer fiber

We experimentally characterized a birefringent microstructured polymer fiber of specific construction, which allows for single mode propagation in two cores separated by a pair of large holes. The fiber exhibits high birefringence in each of the cores as well as relatively weak coupling between the cores. Spectral dependence of the group and the phase modal birefringence was measured using an interferometric method. We have also measured the sensing characteristics of the fiber such as polarimetric sensitivity to hydrostatic pressure, strain and temperature. Moreover, we have studied the effect of hydrostatic pressure and strain on coupling between the cores.

Spectral and polarimetric characterization of the Gas Pixel Detector filled with dimethyl ether

The Gas Pixel Detector belongs to the very limited class of gas detectors optimized for the measurement of X-ray polarization in the emission of astrophysical sources. The choice of the mixture in which X-ray photons are absorbed and photoelectrons propagate, deeply affects both the energy range of the instrument and its performance in terms of gain, track dimension and ultimately, polarimetric sensitivity. Here we present the characterization of the Gas Pixel Detector with a 1 cm thick cell filled with dimethyl ether (DME) at 0.79 atm, selected among other mixtures for the very low diffusion coefficient. Almost completely polarized and monochromatic photons were produced at the calibration facility built at INAF/IASF-Rome exploiting Bragg diffraction at nearly 45°. For the first time ever, we measured the modulation factor and the spectral capabilities of the instrument at energies as low as 2.0 keV, but also at 2.6, 3.7, 4.0, 5.2 and 7.8 keV. These measurements cover almost completely the energy range of the instrument and allows to compare the sensitivity achieved with that of the standard mixture, composed of helium and DME.

10.1007/s11449-008-2012-y

Laser-polarimetric technique with the shot-noise-limited polarimetric sensitivity is used to study magnetic ac-susceptibility in holmium doped LiYF4 crystals in the range of Zeeman energies comparable with that of the hyperfine interaction in Ho3+ ions. Specific features of optical methods of magnetic measurements, the experimental setup, and results of measurements are discussed. Polarimetric sensitivity of the setup (∼10−8 rad) allowed us to measure the ac-susceptibility of LiYF4 single crystals containing 0.1–0.3 mol % of impurity Ho3+ ions with the signal-to-noise ratio exceeding 102. The obtained field-strength and frequency dependences of the ac-susceptibility show that the resonant peaks of the susceptibility mainly result from cross-relaxation transitions between the electronic-nuclear sublevels of Ho3+ ions.

MULTI-LINE STOKES INVERSION FOR PROMINENCE MAGNETIC-FIELD DIAGNOSTICS

We present test results on the simultaneous inversion of the Stokes profiles of the He I lines at 587.6 nm (D_3) and 1083.0 nm in prominences (90-deg scattering). We created datasets of synthetic Stokes profiles for the case of quiescent prominences (B<200 G), assuming a conservative value of 10^-3 of the peak intensity for the polarimetric sensitivity of the simulated observations. In this work, we focus on the error analysis for the inference of the magnetic field vector, under the usual assumption that the prominence can be assimilated to a slab of finite optical thickness with uniform magnetic and thermodynamic properties. We find that the simultaneous inversion of the two lines significantly reduces the errors on the inference of the magnetic field vector, with respect to the case of single-line inversion. These results provide a solid justification for current and future instrumental efforts with multi-line capabilities for the observations of solar prominences and filaments.

Pressure Measurement Based on a Multimode Phase Retarder Plastic Optical Fiber

This paper demonstrates a polarimetric pressure measurement using a multimode phase retarder plastic optical fiber fabricated from a birefringence-controllable copolymer. An optimal composition that gives the most pronounced response to pressure was studied by comparing the extinction ratio of fibers fabricated with different copolymer compositions. A simple sensor design with a single-fiber structure was proposed that has detection capabilities similar to those of the conventional dual-path structure of a Mach-Zehnder interferometer. A linear relationship between the fiber birefringence and the applied pressure was observed in a range from 0 to 0.06 MPa. A polarimetric sensitivity of 1.56 x 10(-5) MPa(-1) was obtained experimentally from the relationship between the birefringence and pressure.

Birefringent photonic crystal fibers with zero polarimetric sensitivity to temperature

We designed, fabricated, and characterized birefringent holey fibers with zero polarimetric sensitivity to temperature. The sensitivity measurements were carried out in a wide spectral range of 0.68–1.55 μm in fibers with different hole and pitch values and with birefringence induced by a pair of large holes adjacent to the core. Our results show that zero sensitivity to temperature can be obtained at certain wavelengths for the bare fibers with properly adjusted geometrical parameters. Moreover, the spectral measurements of the sensitivity to temperature are in good agreement with the modeling results for all the investigated fibers.

Dynamics of small-scale magnetic fields on the Sun: observations and numerical simulations

Context. Small-scale magnetic fields play an important role in the structure and the dynamics of the solar atmosphere. Aims. This study aims at revealing the evolution of magnetic fields, together with granular convection in the quiet Sun by means of observations with high spatial, spectral, and temporal resolution and of numerical MHD simulations. Methods. Time sequences from quiet Sun disc centre were obtained with the upgraded “Gottingen” Fabry-Perot spectropolarimeter at the Vacuum Tower Telescope, Observatorio del Teide/Tenerife, in the $\ion{Fe}{i}$ 6173 A line. The data were reconstructed with speckle methods. For comparison with the observations, numerical simulations of granular magnetoconvection were carried out with the MURaM code. The intensities and Stokes vectors emerging from the simulation box were degraded in wavelength, spatial co-ordinates, and noise to the quality of the observations. Results. The noise in the observed magnetograms from the centre-of-gravity method is $\sigma_B\approx2$ G, yielding a polarimetric sensitivity of $3 \times 10^{15}$ Mx, at a cadence of 23 s with 0$\farcs$33 spatial resolution in a field of view of ${\sim}33\arcsec\times 25\arcsec$. Many of the observed V profiles in network and internetwork (IN) areas exhibit strong asymmetries that indicate strong magnetoconvection. The temporal evolutions of IN structures and of a bright point (BP), as seen in broadband and line-minimum images, in Dopplergrams, and in magnetograms, are presented. The magnetic field structure in the numerical MHD simulations is even more complex than seen in the observations. Correspondingly, the emergent Stokes profiles are often very abnormal. The degradation yields a reduction of the intrinsic field strength to the “observed” one by a factor 4–5. The spectral resolution of the spectrometer is adequate, yet the limitation in spatial resolution and by noise filtering swamps the details seen in the non-degraded simulations. A BP was not found in the simulations, presumably because BP's are not so common and the simulated box is quiet small: only 6 Mm wide in both horizontal directions. Conclusions. The combination of high-resolution observations with numerical simulation is a highly valuable means for studying small-scale magnetic fields on the Sun. Two-dimensional, low-noise data with spectral resolution as good as achieved here and with spatial resolution of 0$\farcs$1 and better are needed to better understand this important part of the solar magnetism.

Stokes imaging polarimetry using image restoration at the Swedish 1-m solar telescope

Aims. We aim to achieve both high spatial resolution and high polarimetric sensitivity, using an earth-based 1m-class solar telescope, for the study of magnetic fine structure on the surface of the Sun. Methods. We use a setup with 3 high-speed, low-noise cameras to construct datasets with interleaved polarimetric states, particularly suitable for Multi-Object Multi-Frame Blind Deconvolution image restorations. We discuss the polarimetric calibration routine and various potential sources of error in the results. Results. We obtained near diffraction limited images, which have a noise level of ≈10 −3 Icont. We confirm that dark cores have a weaker magnetic field and a lower inclination angle with respect to the solar surface than the edges of the penumbral filament. We demonstrate that the magnetic field strength in faculae-striations is significantly lower than in other nearby parts of the faculae.

Laser-polarimetric measurements of magnetic ac-susceptibility in LiYF4: Ho3+ crystals

Laser-polarimetric technique with the shot-noise-limited polarimetric sensitivity is used to study magnetic ac-susceptibility in holmium doped LiYF4 crystals in the range of Zeeman energies comparable with that of the hyperfine interaction in Ho3+ ions. Specific features of optical methods of magnetic measurements, the experimental setup, and results of measurements are discussed. Polarimetric sensitivity of the setup (∼10−8 rad) allowed us to measure the ac-susceptibility of LiYF4 single crystals containing 0.1–0.3 mol % of impurity Ho3+ ions with the signal-to-noise ratio exceeding 102. The obtained field-strength and frequency dependences of the ac-susceptibility show that the resonant peaks of the susceptibility mainly result from cross-relaxation transitions between the electronic-nuclear sublevels of Ho3+ ions.

Narrow-band full Stokes polarimetry of small structures on the Sun with speckle methods

Aims. For the detection and the study of small-scale magnetic fields on the Sun, it is important to obtain observations with both high spatial resolution and high polarimetric sensitivity. Methods. A second narrow-band etalon and a full Stokes polarimeter, based on ferroelectric liquid crystals, were implemented in the two-dimensional Gottingen Fabry-Perot spectrometer/polarimeter at the Vacuum Tower Telescope, Observatorio del Teide/Tenerife. First observations with the Fe 6173 A line and their data analysis with speckle methods are described. Results. The new polarimeter yields magnetograms of a field of view of 31 x 52 with minimised seeing induced signals and without spurious signals as from the use of beam-splitting calcites. The achieved spatial and temporal resolution are 0.30-035 and 36s, respectively. With a detection limit for the field strength of 16G (=3σ), a polarimetric sensitivity of 7-8 x 10 15 Mx is obtained. Examples of intensity maps, Dopplergrams, and magnetograms from quiet and active regions on the Sun are discussed. Some of the results on solar magnetism are on 1) a pore with small-scale structure where we find a region with very low temperature gradient, 2) polar faculae with strong magnetic field signals and weaker signals in other areas surrounding them, and 3) small-scale inter-network magnetic fields with area fillings in the resolution elements of the order of 0.15.

Low energy polarization sensitivity of the Gas Pixel Detector

An X-ray photoelectric polarimeter based on the Gas Pixel Detector has been proposed to be included in many upcoming space missions to fill the gap of about 30 years from the first (and to date only) positive measurement of polarized X-ray emission from an astrophysical source. The estimated sensitivity of the current prototype peaks at an energy of about 3 keV, but the lack of readily available polarized sources in this energy range has prevented the measurement of detector polarimetric performances. In this paper we present the measurement of the Gas Pixel Detector polarimetric sensitivity at energies of a few keV and the new, light, compact and transportable polarized source that was devised and built to this aim. Polarized photons are produced, from unpolarized radiation generated with an X-ray tube, by means of Bragg diffraction at nearly 45 ∘ . The diffraction angle is constrained with two orthogonal capillary plates, which allow good collimation with limited size thanks to the 10 μ m diameter holes. Polarized photons at energy as low as a few keV can be produced with a proper choice of diffracting crystal, while the maximum energy is limited by the X-ray tube voltage, since all the orders defined by the crystal lattice spacing are diffracted. The best trade-off between reasonable fluxes and high degree of polarization can be achieved selecting the degree of collimation provided by capillary plates. The employment of mosaic graphite and flat aluminum crystals allow the production of nearly completely polarized photons at 2.6, 3.7, and 5.2 keV from the diffraction of unpolarized continuum or line emission. The measured modulation factor of the Gas Pixel Detector at these energies is in good agreement with the estimates derived from a Monte Carlo software, which was up to now employed for driving the development of the instrument and for estimating its low energy sensitivity. In this paper we present the excellent polarimetric performance of the Gas Pixel Detector at energies where the peak sensitivity is expected. These measurements not only support our previous claims of high sensitivity but confirm the feasibility of astrophysical X-ray photoelectric polarimetry.

Measurements of polarimetric sensitivity to temperature in birefringent holey fibres

We measured spectral dependence of the polarimetric sensitivity to temperature in three birefringent holey fibres with different geometries. Our results show that thermal properties of the birefringent photonic crystal fibres depend very much on the air hole arrangement. Using the procedure which allows us to determine the sign of temperature sensitivity, we demonstrated that one of the investigated fibres is insensitive to temperature at a certain wavelength. We also measured the temperature sensitivities for the fibres with and without polymer coating and showed that the coating changes significantly the fibre response to temperature. Furthermore, we demonstrated experimentally that the spectral dependence of the polarimetric sensitivity to temperature in birefringent holey fibres obeys a scaling law.

Authors' Reply to “Comments on 'Thermooptical Sensitivity Analysis of Highly Birefringent Polarimetric Sensing Photonic Crystal Fibers With Elliptically Elongated Veins'”

For original paper see N. J. Florous et al., ibid., vol.18, no.15, p.1663 (2006) and for comment see M. Antkowiak and R. Kotynski, ibid., vol.19, no.10, p.795 (2007). Florous et al. presented calculations of thermal sensitivity of photonic crystal fibers (PCFs) with elliptical holes. In their Comment Antkowiak and Kotynski compared these results with those obtained by them previously. They also included improved calculations of the thermooptics effect contribution to the polarimetric sensitivity of PCF and, on this basis, presented a critical analysis of the calculations given in both papers. Florous et al. reply to the comments regarding the thermooptical sensitivity calculations in birefringent photonic crystal fibers (PCFs). They address the major scientific differences in the propagation characteristics of the proposed PCFs between their original letter and the comments

Comments on “Thermooptical Sensitivity Analysis of Highly Birefringent Polarimetric Sensing Photonic Crystal Fibers With Elliptically Elongated Veins”

For original paper see N. J. Florous et al., ibid., vol.18, no.15, p.1663 (2006). Florous et al. presented calculations of thermal sensitivity of photonic crystal fibers (PCFs) with elliptical holes. In this Comment Antkowiak and Kotynski compare these results with those obtained by them previously. They also include improved calculations of the thermooptics effect contribution to the polarimetric sensitivity of PCF and, on this basis, present a critical analysis of the calculations given in both papers

Measurements of sensitivity to hydrostatic pressure and temperature in highly birefringent photonic crystal fibers

We report on experimental studies of polarimetric sensitivity to hydrostatic pressure and temperature in two highly birefringent index guided photonic crystal fibers, in which birefringence is induced by one row of the cladding holes with diameters smaller than the other cladding holes. The sensitivity measurements were carried out in the spectral range from 0.6 μm to 1.6 μm. Our results show that absolute value of the polarimetric sensitivity to hydrostatic pressure can reach 23 rad/MPa × m, which is almost one order of magnitude higher than in conventional fibers with elliptical core. Simultaneously, polarimetric sensitivity to temperature is at least two orders of magnitude lower than in conventional highly birefringent fibers. Moreover, we proved experimentally that one of the investigated fibers is completely insensitive to temperature at certain wavelength.

Evolving Photospheric Flux Concentrations and Filament Dynamic Changes

We analyze the role of weak photospheric flux concentrations that evolve in a filament channel, in the triggering of dynamic changes in the shape of a filament. The high polarimetric sensitivity of THEMIS allowed us to detect weak flux concentrations (few Gauss) associated with the filament development. The synoptic instruments (MDI, SOLIS) even if their sensitivity is much less than THEMIS were useful to follow any subsequent strengthening of these flux concentrations after their identification in the THEMIS magnetograms. We found that (1) the northern part of the filament develops an Hα barb at the same time that weak minority polarity elements develop near a plage; (2) a section in the southern part of the Hα filament gradually disappears and later reforms at the same time that several mixed-polarity magnetic elements appear, then subsequently cancel or spread away from each other. These changes correspond to increases in EUV emission, as observed by TRACE, EIT, and CDS. This suggests that the plasma is temporarily heated along the filament spine. An idealized sequence of force-free models of this filament channel, based on plasma-supporting magnetic dips occurring in the windings of a very weakly twisted flux tube, naturally explains the evolution of its southern part as being due to changes in the topology of the coronal magnetic field as the photospheric flux concentrations evolve.

Fibre orientation contrast for depth-resolved identification of structural interfaces in birefringent tissue

Incorporation of polarimetric sensitivity into optical coherence tomography can provide additional image contrast when structures of interest are optically anisotropic (e.g., fibrous tissue). We present a generalized technique based on polarization-sensitive optical coherence tomography to detect changes in depth-resolved fibre orientation and thus increase image contrast in multiple-layered birefringent tissues. A high contrast B-scan image of collagen fibre orientation is shown for a porcine intervertebral disc cartilage specimen that exhibited low backscattering intensity contrast. Interfaces in the annulus fibrosus identified using depth-resolved fibre orientation allowed quantification of lamellae thickness. Moreover, the technique detects changes in fibre orientation without intense processing needed to effectively quantify tissue retardation and diattenuation.

Modeling and measurement of temperature sensitivity in birefringent photonic crystal holey fibers

We analyzed theoretically the spectral dependence of polarimetric sensitivity to temperature (KT) and the susceptibility of phase modal birefringence to temperature (dB/dT) in several birefringent photonic crystal holey fibers of different construction. Contributions to dB/dT related to thermal expansion of the fiber dimensions and that related to temperature-induced changes in glass and air refractive indices were calculated separately. Our results showed that dB/dT depends strongly on the material used for manufacturing the fiber and on the fiber's geometry. We demonstrate that, by properly designing the birefringent holey fiber, it is possible to reduce its temperature sensitivity significantly and even to ensure a null response to temperature at a specific wavelength. Furthermore, we show that the temperature sensitivity in a fiber with arbitrary geometry can be significantly reduced by proper choice of the glass used in the fiber's manufacture. We also measured the polarimetric sensitivity to temperature and identified its sign in two silica-air fibers. The experimental values are in good agreement with the results of modeling.