Polarimetric Performance Research Articles

Thermal stabilization of a fiber-based channeled spectropolarimetry

Channeled spectropolarimetry (CSP) is a spectral modulation technique that furnishes snapshot polarimetric measurements. Among the different spectropolarimetric methods, fiber-based techniques are fundamental requirements in specific biomedical procedures, such as endoscopy. However, the optical fiber exhibits high sensitivity to temperature changes, which subsequently incurs errors in the polarimetric reconstructions. We present a calibration technique for fiber-based CSP that leverages phase-shifting interferometry (PSI) to acquire and demodulate the fiber’s retardation phase accurately. This technique provides high robustness against temperature variations and improved polarimetric reconstruction performance. Experimental results demonstrated that the PSI calibrations offer higher stability and accuracy, at different optical fiber temperatures, as compared to reference-beam calibrations.

Polarimetric Radar Signatures and Performance of Various Radar Rainfall Estimators during an Extreme Precipitation Event over the Thousand-Island Lake Area in Eastern China

Polarimetric radar provides more choices and advantages for quantitative precipitation estimation (QPE) than single-polarization radar. Utilizing the C-band polarimetric radar in Hangzhou, China, six radar QPE estimators based on the horizontal reflectivity (ZH), specific attenuation (AH), specific differential phase (KDP), and double parameters that further integrate the differential reflectivity (ZDR), namely, R(ZH, ZDR), R(KDP, ZDR), and R(AH, ZDR), are investigated for an extreme precipitation event that occurred in Eastern China on 1 June 2016. These radar QPE estimators are respectively evaluated and compared with a local rain gauge network and drop size distribution data observed by two disdrometers. The results show that (i) although R(AH, ZDR) underestimates in the light rain scenario, it performs the best among all radar QPE estimators according to the normalized mean error; (ii) the optimal radar rainfall relationship and consistency between radar measurements aloft and their surface counterparts are both required to obtain accurate rainfall estimates close to the ground. The contamination from melting layer on AH and KDP can make R(AH), R(AH, ZDR), R(KDP), and R(KDP, ZDR) less effective than R(ZH) and R(ZH,ZDR). Instead, adjustments of the α coefficient can partly reduce such impact and hence render a superior AH–based rainfall estimator; (iii) each radar QPE estimator may outperform others during some time intervals featured by particular rainfall characteristics, but they all tend to underestimate rainfall if radar fails to capture the rapid development of rainstorms.

A novel filter wheel for multi-channel switching and polarization rotation

This study proposes an innovative filter wheel for the multi-channel switching of polarization state. The proposed device enables switching between two orthogonal linear polarization states or two orthogonal circular polarization states to achieve either linear dichroism (LD) or circular dichroism (CD). The filter wheel enables channel switching and orientation rotation with high repeatability. This paper describes in detail the methods used in the alignment of multiple polarizers and quarter-waveplates. In addition, the extinction ratio and retardation, which can be used as indications of polarimetric performance, are measured to a high degree of precision. The apparatus was applied for 6-channel polarization state switching in which three channels were used for multi-spectral broadband polarimetric applications across a range of 400–1600 nm. The remaining three channels were used for multi-wavelength precision polarimetric applications at wavelengths of 457, 532, and 632 nm in the visible range. When installed in a microspectrophotometer, the proposed apparatus can be used in multi-spectral broadband polarimetric spectroscopy as well as multi-wavelength polarimetric imaging applications to achieve LD and CD effects at the micro scale.

Polarimetric performance of a polarization modulator based on liquid crystal variable retarders for wide acceptance angles

Liquid crystal variable retarders (LCVRs) will be used in the polarization modulation packages (PMPs) of the instruments SO/PHI (Polarimetric and Helioseismic Imager) and METIS/COR (Multielement Telescope for Imaging and Spectroscopy, Coronagraph) of the Solar Orbiter Mission of the European Space Agency (ESA). Optical retarders are dependent on the angle of incidence (AOI). Since the optical retardances during the polarization modulations are optimized for a particular AOI, other angles increase the polarimetric measurement error. Coronagraphs, such as METIS, are characterized by having wide field-of-view (FoV), which involves large incidence angles through the entire instrument. METIS PMP will work with collimated beams and an AOI up to ±7.0 deg. For this reason, a double LCVR configuration with molecular tilts in opposite directions was selected for METIS PMP, which provides lower angular dependence. The polarimetric performance of the METIS PMP flight model was measured at different AOIs and compared to a single LCVR PMP. The results shown in this paper demonstrate that the functional concept used in METIS guarantees the polarimetric performances at the wide FoV expected in METIS coronagraph. Moreover, a detailed theoretical model is showed and compared to the experimental data, finding successful agreement, which can be very helpful for the design of instruments characterized by wide FoV.

Future gamma-ray missions’ polarimetric prospects

High-energy astrophysics polarimetry may significantly benefit from e-ASTROGAM and from AMEGO mission proposals, since to date limited polarimetric measurements were performed in this domain, exclusively under 1 MeV. The polarimetric potential of both missions was analyzed in the Compton regime by Monte Carlo mass model simulations with MEGAlib toolkit. The performance of e-ASTROGAM was analyzed by simulating Si tracker and calorimeter alternative configurations and detection materials, within missions’ volume, mass and power margins. The modulation polarimetric factor, Q, and the MDP were calculated for different polarized source types and for variable incidence angle measurement conditions. Finally, the polarimetric performances of both instruments was compared and analyzed. Q modulation factors obtained ranged between ~0.2 and ~0.4 in the 0.2–2.0 MeV band and MDP ~0.65% was estimated for a 100% polarized Crab type source, 1 Ms. observation time and 3σ significance.

Misalignment error analysis in polychromatic division of focal plane Stokes polarimeters

Snapshot imaging polarimeters are used in many different areas. Recently, division of focal plane (DoFP) polarimeters have proved useful as snapshot polarimeters for dynamic applications. For an optimal performance of polarimeters, different works dealing with the error analysis of such devices are proposed in literature. In terms of noise amplification from intensity measurements to the final polarization calculations, well-established quality metrics, as in the condition number or the equally weighted variance criteria, are used. Other studies analyze systematic errors due to deviations in the construction parameters. However, something not considered so far is the effect produced by misalignment between the various pixelated masks over the pixelated structure of the camera sensor, always occurring in experimental implementations of DoFPs. In this work, we study the effect of such misalignments in DoFP polarimeters and demonstrate how they lead to polarimetric systems composed of partially depolarized analyzers. We calculate the combined degree of polarization related to different amounts of misalignment and analyze the corresponding system performance. From this study, we show how an imaging polarimeter based on partially polarized analyzers can still lead to a robust and accurate polarimetric performance, and we also provide the misalignment limits in which an acceptable performance is obtained. We evaluate both the monochromatic and the polychromatic cases with special focus on the latter.

A Three-Hierarchy Evaluation of Polarimetric Performance of GF-3, Compared with ALOS-2/PALSAR-2 and RADARSAT-2.

GaoFen-3 (GF-3) is the first Chinese civilian multi-polarization synthetic aperture radar (SAR) satellite, launched on 10 August of 2016, and put into operation at the end of January 2017. The polarimetric SAR (PolSAR) system of GF-3 is able to provide quad-polarization (quad-pol) images in a variety of geophysical research and applications. However, this ability increases the complexity of maintaining image quality and calibration. As a result, to evaluate the quality of polarimetric data, polarimetric signatures are necessary to guarantee accuracy. Compared with some other operational space-borne PolSAR systems, such as ALOS-2/PALSAR-2 (ALOS-2) and RADARSAT-2, GF-3 has less reported calibration and image quality files, forcing users to validate the quality of polarimetric imagery of GF-3 before quantitative applications. In this study, without the validation data obtained from a calibration infrastructure, an innovative, three-hierarchy strategy was proposed to assess PolSAR data quality, in which the performance of GF-3 data was evaluated with ALOS-2 and RADARSAT-2 data as references. Experimental results suggested that: (1) PolSAR data of GF-3 satisfied backscatter reciprocity, similar with that of RADARSAT-2; (2) most of the GF-3 PolSAR images had no signs of polarimetric distortion affecting decomposition, and the system of GF-3 may have been improved around May 2017; and (3) the classification accuracy of GF-3 varied from 75.0% to 91.4% because of changing image-acquiring situations. In conclusion, the proposed three-hierarchy approach has the ability to evaluate polarimetric performance. It proved that the residual polarimetric distortion of calibrated GF-3 PolSAR data remained at an insignificant level, with reference to that of ALOS-2 and RADARSAT-2, and imposed no significant impact on the polarimetric decomposition components and classification accuracy.

Comparative Analysis of Polarimetric SAR Calibration Methods

In the diverse applications of polarimetric Synthetic Aperture Radar (SAR) systems, it is a crucial to conduct polarimetric calibration, which aims to remove the radar system distortion effects prior to utilizing polarimetric SAR observations. The objective of this study is to evaluate the performance of different polarimetric calibration methods. Two widely used methods, the Van Zyl and Quegan methods, and one recently proposed method, such as the Villa method, have been selected among various calibration methods in literature. The selected methods have basic differences in their assumptions that are applied to the polarimetric system model. In order to evaluate the calibration performances under different system parameters and ground characteristics, comparative analysis of the calibration results were conducted on synthetic polarimetric SAR data and ALOS PALSAR quad-pol mode data. Based on the experimental results, the advantages and limitations of different methods were clarified, and a simple hybrid calibration method is presented to further improve the polarimetric calibration performance.

Compton polarimetry with a multi-layer CdTe focal plane prototype

In high-energy astrophysics, polarimetry is a promising subject with a wide scientific potential that is relatively unexplored due to the complexity of the design and technical requirements of the sensors. Often gamma-ray telescope proposals are based on multi-layer spectro-imager instruments with polarimetric capabilities. Herein we study a new Compton polarimeter prototype based on a two-layer CdTe pixelized spectro-imager operated in coincidence. The two CdTe detectors are 2 mm thick anode 8×8 pixels’ segmented matrices with 2 mm pitch. This detection system configuration allows an assessment of the polarimetric potential of multi-layer solution focal planes as well as the polarimetric potential of a possible 3D spectro-imager by analysing the polarimetric performance when varying the distance between the two CdTe detection layers. The polarimetric modulation factors for single-layer (Q∼0.4) and two-layer (up to 0.13) double-events were measured for 6 mm and for 10 mm distance between planes. The measured polarization angle resolution was lower than 10°. The potential of CdTe spectro-imager focal plane solutions with polarimetric capabilities for the next generation space missions based on both Laue lenses and 3D segmented focal planes is also discussed.

SPHERE/ZIMPOL high resolution polarimetric imager

Context. The SPHERE “planet finder” is an extreme adaptive optics (AO) instrument for high resolution and high contrast observations at the Very Large Telescope (VLT). We describe the Zurich Imaging Polarimeter (ZIMPOL), the visual focal plane subsystem of SPHERE, which pushes the limits of current AO systems to shorter wavelengths, higher spatial resolution, and much improved polarimetric performance. Aims. We present a detailed characterization of SPHERE/ZIMPOL which should be useful for an optimal planning of observations and for improving the data reduction and calibration. We aim to provide new benchmarks for the performance of high contrast instruments, in particular for polarimetric differential imaging. Methods. We have analyzed SPHERE/ZIMPOL point spread functions (PSFs) and measure the normalized peak surface brightness, the encircled energy, and the full width half maximum (FWHM) for different wavelengths, atmospheric conditions, star brightness, and instrument modes. Coronagraphic images are described and the peak flux attenuation and the off-axis flux transmission are determined. Simultaneous images of the coronagraphic focal plane and the pupil plane are analyzed and the suppression of the diffraction rings by the pupil stop is investigated. We compared the performance at small separation for different coronagraphs with tests for the binary α Hyi with a separation of 92 mas and a contrast of Δm ≈ 6m. For the polarimetric mode we made the instrument calibrations using zero polarization and high polarization standard stars and here we give a recipe for the absolute calibration of polarimetric data. The data show small (< 1 mas) but disturbing differential polarimetric beam shifts, which can be explained as Goos-Hähnchen shifts from the inclined mirrors, and we discuss how to correct this effect. The polarimetric sensitivity is investigated with non-coronagraphic and deep, coronagraphic observations of the dust scattering around the symbiotic Mira variable R Aqr. Results. SPHERE/ZIMPOL reaches routinely an angular resolution (FWHM) of 22−28 mas, and a normalized peak surface brightness of SB0 − mstar ≈ −6.5m arcsec−2 for the V-, R- and I-band. The AO performance is worse for mediocre ≳1.0″ seeing conditions, faint stars mR ≳ 9m, or in the presence of the “low wind” effect (telescope seeing). The coronagraphs are effective in attenuating the PSF peak by factors of > 100, and the suppression of the diffracted light improves the contrast performance by a factor of approximately two in the separation range 0.06″−0.20″. The polarimetric sensitivity is Δp < 0.01% and the polarization zero point can be calibrated to better than Δp ≈ 0.1%. The contrast limits for differential polarimetric imaging for the 400 s I-band data of R Aqr at a separation of ρ = 0.86″ are for the surface brightness contrast SBpol( ρ)−mstar ≈ 8m arcsec−2 and for the point source contrast mpol( ρ)−mstar ≈ 15m and much lower limits are achievable with deeper observations. Conclusions. SPHERE/ZIMPOL achieves imaging performances in the visual range with unprecedented characteristics, in particular very high spatial resolution and very high polarimetric contrast. This instrument opens up many new research opportunities for the detailed investigation of circumstellar dust, in scattered and therefore polarized light, for the investigation of faint companions, and for the mapping of circumstellar Hα emission.

Polarimetric precision of micropolarizer grid-based camera in the presence of additive and Poisson shot noise.

Polarimetric cameras based on micropolarizer grids make it possible to design division of focal plane (DoFP) polarimeters. However, the polarimetric estimation precision reached by these devices depends on their realization quality, which is estimated by calibration. We derive the theoretical expressions of the estimation variance of such polarimetric parameters as an angle of linear polarization and degree of linear polarization as a function of the calibrated micropolarizer characteristics. These values can be compared with the variances that would be obtained with ideal micropolarizers in order to quantitatively assess the effect of manufacturing defects on polarimetric performance. These results are validated by experimental measurements on a real-world camera.

Hard X-ray and Soft Gamma Ray Polarimetry with CdTe/CZT Spectro-Imager

CdTe/CZT based spectroscopic two-dimensional (2D)/three-dimensional (3D) imagers when operated in the Compton regime can work as high performance scattering polarimeters, for high-energy astrophysics. Polarimetry in high-energy astrophysics has been little explored. To date, X- and γ-ray source emissions have been studied almost exclusively through spectral, imaging, and timing analysis. Polarization measurements provide two additional observational parameters: the polarization angle and the level of linear polarization. These additional parameters should allow for a better discrimination between the physical mechanisms of different emission models characterizing a celestial object. Therefore, polarimetry will play a strategic role in new instrumentations for future high-energy astronomy missions. 2D and 3D CZT/CdTe spectroscopic imagers provided with coincidence readout logic can efficiently handle scattering events to perform simultaneously polarization, spectroscopy, imaging, and timing measurements. Herein, we describe the results obtained, both experimentally and by MC simulations, with CdTe/CZT pixel detector prototypes in high-energy polarimetry. We give an overview on the achievable polarimetric performance with spectroscopic imagers and on how these performances are affected by detector configuration parameters. Finally, we address the perspective of scattering polarimetry opened by the recent implementation of new high energy focusing optics, as broadband Laue lens, in next generation of hard X- and soft γ-ray astronomy instrumentation. The unprecedented sensitivity achievable by these telescopes will definitely open the window of polarimetry in this high-energy range.

Pitfalls of statistics-limited X-ray polarization analysis

Context. One of the difficulties with performing polarization analysis is that the mean polarization fraction of sub-divided data sets is larger than the polarization fraction for the integrated measurement. The resulting bias is one of the properties of the generating distribution discussed in this work. The limitations of Gaussian approximations in standard analysis based on Stokes parameters for estimating polarization parameters and their uncertainties are explored by comparing with a Bayesian analysis. The effect of uncertainty on the modulation factor is also shown, since it can have a large impact on the performance of gamma-ray burst polarimeters. Results are related to the minimum detectable polarization (MDP), a common figure of merit, making them easily applicable to any X-ray polarimeter. Aims. The aim of this work is to quantify the systematic errors induced on polarization parameters and their uncertainties when using Gaussian approximations and to show when such effects are non-negligible. Methods. The probability density function is used to deduce the properties of reconstructed polarization parameters. The reconstructed polarization parameters are used as sufficient statistics for finding a simple form of the likelihood. Bayes theorem is used to derive the posterior and to include nuisance parameters. Results. The systematic errors originating from Gaussian approximations as a function of instrument sensitivity are quantified here. Different signal-to-background scenarios are considered making the analysis relevant for a large variety of observations. Additionally, the change of posterior shape and instrument performance MDP due to uncertainties on the polarimeteric response of the instrument is shown.

Stokes polarimeter performance: general noise model and analysis.

We calculate the photometric Stokes parameter covariance matrices and SNRs estimated by polarimeters exposed to general noise distributions, such as mixed Poisson-Gaussian (PG) noise. The measurement model includes the effects of optical losses and detector quantum efficiency, enabling quantitative comparison of instruments that have different photometric efficiencies. We demonstrate this capability by comparing the performance of many common polarimeter configurations, including diattenuator-based systems, such as Azzam's four-detector polarimeter [Opt. Lett.10, 309 (1985)OPLEDP0146-959210.1364/OL.10.000309] and Kudenov's stacked photovoltaic polarimeter [Opt. Express24, 14737 (2016)OPEXFF1094-408710.1364/OE.24.014737]. Working with the full covariance matrix under mixed PG noise, we also show that instruments optimized under assumptions of Gaussian noise simultaneously exhibit optimal behavior under Poisson noise.

Instrumentation and Future Missions in the Upcoming Era of X-ray Polarimetry

The maturity of current detectors based on technologies that range from solid state to gases renewed the interest for X-ray polarimetry, raising the enthusiasm of a wide scientific community to improve the performance of polarimeters as well as to produce more detailed theoretical predictions. We will introduce the basic concepts about measuring the polarization of photons, especially in the X-rays, and we will review the current state of the art of polarimeters in a wide energy range from soft to hard X-rays, from solar flares to distant astrophysical sources. We will introduce relevant examples of polarimeters developed from the recent past up to the panorama of upcoming space missions to show how the recent development of the technology is allowing reopening the observational window of X-ray polarimetry.

Optimization of the design of X-Calibur for a long-duration balloon flight and results from a one-day test flight

X-ray polarimetry promises exciting insights into the physics of compact astrophysical objects by providing two observables: the polarization fraction and angle as function of energy. X-Calibur is a balloon-borne hard x-ray scattering polarimeter for the 15- to 60-keV energy range. After the successful test flight in September 2016, the instrument is now being prepared for a long-duration balloon (LDB) flight in December 2018 through January 2019. During the LDB flight, X-Calibur will make detailed measurements of the polarization of Vela X-1 and constrain the polarization of a sample of between 4 and 9 additional sources. We describe the upgraded polarimeter design, including the use of a beryllium scattering element, lower-noise front-end electronics, and an improved fully active CsI(Na) anticoincidence shield, which will significantly increase the instrument sensitivity. We present estimates of the improved polarimeter performance based on simulations and laboratory measurements. We present some of the results from the 2016 flight and show that we solved several problems, which led to a reduced sensitivity during the 2016 flight. We end with a description of the planned Vela X-1 observations, including a Swift/BAT-guided observation strategy.

Airborne polarimetric Doppler weather radar: trade-offs between various engineering specifications

Abstract. NCAR EOL is investigating potential configurations for the next-generation airborne phased array radar (APAR) that is capable of retrieving dynamic and microphysical characteristics of clouds and precipitation. The APAR will operate at C band. The APAR will use the electronic scanning (e-scan) feature to acquire the optimal number of independent samples for recording research-quality measurements. Since the airborne radar has only a limited time for collecting measurements over a specified region (moving aircraft platform ∼ 100 m s−1), beam multiplexing will significantly enhance its ability to collect high-resolution, research-quality measurements. Beam multiplexing reduces errors in radar measurements while providing rapid updates of scan volumes. Beamwidth depends on the size of the antenna aperture. Beamwidth and directivity of elliptical, circular, and rectangular antenna apertures are compared and radar sensitivity is evaluated for various polarimetric configurations and transmit–receive (T/R) elements. In the case of polarimetric measurements, alternate transmit with alternate receive (single-channel receiver) and simultaneous reception (dual-channel receiver) is compared. From an overall architecture perspective, element-level digitization of T/R module versus digital sub-array is considered with regard to flexibility in adaptive beamforming, polarimetric performance, calibration, and data quality. Methodologies for calibration of the radar and removing bias in polarimetric measurements are outlined. The above-mentioned engineering options are evaluated for realizing an optimal APAR system suitable for measuring the high temporal and spatial resolutions of Doppler and polarimetric measurements of precipitation and clouds.

Quantitative performance of a polarization diffraction grating polarimeter encoded onto two liquid-crystal-on-silicon displays

We present a quantitative analysis of the performance of a complete snapshot polarimeter based on a polarization diffraction grating (PDGr). The PDGr is generated in a common path polarization interferometer with a Z optical architecture that uses two liquid-crystal on silicon (LCoS) displays to imprint two different phase-only diffraction gratings onto two orthogonal linear states of polarization. As a result, we obtain a programmable PDGr capable to act as a simultaneous polarization state generator (PSG), yielding diffraction orders with different states of polarization. The same system is also shown to operate as a polarization state analyzer (PSA), therefore useful for the realization of a snapshot polarimeter. We analyze its performance using quantitative metrics such as the conditional number, and verify its reliability for the detection of states of polarization.

Blazed wire-grid polarizer for plasmon-enhanced polarization extinction: design and analysis

We explore plasmon-enhanced wire-gird polarizers (WGPs) to achieve improved polarimetric performance with more relaxed fabrication parameters compared to conventional WGP. A WGP designed with a blazed wire-grid profile was considered for plasmonic enhancement. The results show that a blazed WGP can achieve extremely high polarimetric extinction at a longer wire-grid period (Λ) compared to conventional WGP structure. Under the optimum geometrical parameters, a blazed WGP may attain an extinction ratio of over 40 dB at Λ = 800 nm, which may allow photolithography for fabrication. In contrast, conventional WGPs obtained comparable performance at Λ = 200 nm, requiring more difficult lithographic techniques. The study can therefore be of significant importance for WGPs to be more widely available for diverse applications.

Performance of photoelectron spin polarimeters with continuous and pulsed sources: from storage rings to free electron lasers.

In this work the experimental uncertainties concerning electron spin polarization (SP) under various realistic measurement conditions are theoretically derived. The accuracy of the evaluation of the SP of the photoelectron current is analysed as a function of the detector parameters and specifications, as well as of the characteristics of the photoexcitation sources. In particular, the different behaviour of single counter or twin counter detectors when the intensity fluctuations of the source are considered have been addressed, leading to a new definition of the SP detector performance. The widely used parameter called the figure of merit is shown to be inadequate for describing the efficiency of SP polarimeters, especially when they are operated with time-structured excitation sources such as free-electron lasers. Numerical simulations have been performed and yield strong implications in the choice of the detecting instruments in spin-polarization experiments, that are constrained in a limited measurement time. Our results are therefore applied to the characteristics of a wide set of state-of-the-art spectroscopy facilities all over the world, and an efficiency diagram for SP experiments is derived. These results also define new mathematical instruments for handling the correct statistics of SP measurements in the presence of source intensity fluctuations.