1,618 research outputs found
Calibrating and Stabilizing Spectropolarimeters with Charge Shuffling and Daytime Sky Measurements
Well-calibrated spectropolarimetry studies at resolutions of 10,000 with
signal-to-noise ratios (SNRs) better than 0.01\% across individual line
profiles, are becoming common with larger aperture telescopes.
Spectropolarimetric studies require high SNR observations and are often limited
by instrument systematic errors. As an example, fiber-fed spectropolarimeters
combined with advanced line-combination algorithms can reach statistical error
limits of 0.001\% in measurements of spectral line profiles referenced to the
continuum. Calibration of such observations is often required both for
cross-talk and for continuum polarization. This is not straightforward since
telescope cross-talk errors are rarely less than 1\%. In solar
instruments like the Daniel K. Inouye Solar Telescope (DKIST), much more
stringent calibration is required and the telescope optical design contains
substantial intrinsic polarization artifacts. This paper describes some
generally useful techniques we have applied to the HiVIS spectropolarimeter at
the 3.7m AEOS telescope on Haleakala. HiVIS now yields accurate polarized
spectral line profiles that are shot-noise limited to 0.01\% SNR levels at our
full spectral resolution of 10,000 at spectral sampling of 100,000. We
show line profiles with absolute spectropolarimetric calibration for cross-talk
and continuum polarization in a system with polarization cross-talk levels of
essentially 100\%. In these data the continuum polarization can be recovered to
one percent accuracy because of synchronized charge-shuffling model now working
with our CCD detector. These techniques can be applied to other
spectropolarimeters on other telescopes for both night and day-time
applications such as DKIST, TMT and ELT which have folded non-axially symmetric
foci.Comment: Accepted to A&
SPHERE: the exoplanet imager for the Very Large Telescope
Observations of circumstellar environments to look for the direct signal of
exoplanets and the scattered light from disks has significant instrumental
implications. In the past 15 years, major developments in adaptive optics,
coronagraphy, optical manufacturing, wavefront sensing and data processing,
together with a consistent global system analysis have enabled a new generation
of high-contrast imagers and spectrographs on large ground-based telescopes
with much better performance. One of the most productive is the
Spectro-Polarimetic High contrast imager for Exoplanets REsearch (SPHERE)
designed and built for the ESO Very Large Telescope (VLT) in Chile. SPHERE
includes an extreme adaptive optics system, a highly stable common path
interface, several types of coronagraphs and three science instruments. Two of
them, the Integral Field Spectrograph (IFS) and the Infra-Red Dual-band Imager
and Spectrograph (IRDIS), are designed to efficiently cover the near-infrared
(NIR) range in a single observation for efficient young planet search. The
third one, ZIMPOL, is designed for visible (VIR) polarimetric observation to
look for the reflected light of exoplanets and the light scattered by debris
disks. This suite of three science instruments enables to study circumstellar
environments at unprecedented angular resolution both in the visible and the
near-infrared. In this work, we present the complete instrument and its on-sky
performance after 4 years of operations at the VLT.Comment: Final version accepted for publication in A&
Exploration of a Polarized Surface Bidirectional Reflectance Model Using the Ground-Based Multiangle Spectropolarimetric Imager
Accurate characterization of surface reflection is essential for retrieval of aerosols using downward-looking remote sensors. In this paper, observations from the Ground-based Multiangle SpectroPolarimetric Imager (GroundMSPI) are used to evaluate a surface polarized bidirectional reflectance distribution function (PBRDF) model. GroundMSPI is an eight-band spectropolarimetric camera mounted on a rotating gimbal to acquire pushbroom imagery of outdoor landscapes. The camera uses a very accurate photoelastic-modulator-based polarimetric imaging technique to acquire Stokes vector measurements in three of the instrument's bands (470, 660, and 865 nm). A description of the instrument is presented, and observations of selected targets within a scene acquired on 6 January 2010 are analyzed. Data collected during the course of the day as the Sun moved across the sky provided a range of illumination geometries that facilitated evaluation of the surface model, which is comprised of a volumetric reflection term represented by the modified Rahman-Pinty-Verstraete function plus a specular reflection term generated by a randomly oriented array of Fresnel-reflecting microfacets. While the model is fairly successful in predicting the polarized reflection from two grass targets in the scene, it does a poorer job for two manmade targets (a parking lot and a truck roof), possibly due to their greater degree of geometric organization. Several empirical adjustments to the model are explored and lead to improved fits to the data. For all targets, the data support the notion of spectral invariance in the angular shape of the unpolarized and polarized surface reflection. As noted by others, this behavior provides valuable constraints on the aerosol retrieval problem, and highlights the importance of multiangle observations.NASAJPLCenter for Space Researc
Dual polarized modulation and reception for next generation mobile satellite communications
This paper presents the novel application of polarized modulation (PMod) for increasing the throughput in mobile satellite transmissions. One of the major drawbacks in mobile satellite communications is the fact that the power budget is often restrictive, making it unaffordable to improve the spectral efficiency without an increment of transmitted power. By using dual polarized antennas in the transmitter and receiver, the PMod technique achieves an improvement in throughput of up to 100% with respect to existing deployments, with an increase of less than 1 dB at low Eb/N0 regime. Additionally, the proposed scheme implies minimum hardware modifications with respect to the existing dual polarized systems and does not require additional channel state information at the transmitter; thus it can be used in current deployments. Demodulation (i.e., detection and decoding) alternatives, with different processing complexity and performance, are studied. The results are validated in a typical mobile interactive scenario, the newest version of TS 102 744 standard [Broadband Global Area Network (BGAN)], which aims to provide interactive mobile satellite communications.Peer ReviewedPostprint (author's final draft
Performance Assessment of Dual-Polarized 5G Waveforms and Beyond in Directly Modulated DFB-Laser using Volterra Equalizer
International audienceWe investigate the performance of 25-Gbps dual-polarized orthogonal frequency division multiplexing (OFDM)-based modulation in a directly modulated distributed feedback (DFB)-laser over 25 km of single-mode fiber. A Volterra equalizer is used to compensate for the nonlinear effects of the optical fiber. The results show that FBMC-OQAM modulation outperforms OFDM, universal filtered multicarrier (UFMC), and generalized frequency division multiplexing (GFDM) waveforms. Indeed, a target bit error rate of similar to 3.8 x 10(-3) [forward error correction (FEC) limit] for FBMC, UFMC, OFDM, and GFDM can be achieved at -30.5, -26, -16, and -14.9 dBm, respectively. The effect of the DFB laser is also investigated for UFMC, OFDM, and GFDM, and they undergo a Q penalty of 2.44, 2.77, and 4.14 dB, respectively, at their FEC limit points. For FBMC-OQAM, the signal is perfectly recovered when excluding the DFB laser at -30.5 dBm. (C) 2020 Society of Photo-Optical Instrumentation Engineers (SPIE
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