Polarization state generator: a polarimeter calibration standard

Metasurface-Based Solid Poincaré Sphere Polarizer

The combination of conventional polarization optical elements, such as linear polarizers and waveplates, is widely adopted to tailor light's state of polarization (SOP). Meanwhile, less attention has been given to the manipulation of light's degree of polarization (DOP). Here, we propose metasurface-based polarizers that can filter unpolarized incident light to light with any prescribed SOP and DOP, corresponding to arbitrary points located both at the surface and within the solid Poincaré sphere. The Jones matrix elements of the metasurface are inverse-designed via the adjoint method. As prototypes, we experimentally demonstrated metasurface-based polarizers in near-infrared frequencies that can convert unpolarized light into linear, elliptical, or circular polarizations with varying DOPs of 1, 0.7, and 0.4, respectively. Our Letter unlocks a new degree of freedom for metasurface polarization optics and may break new ground for a variety of DOP-related applications, such as polarization calibration and quantum state tomography.

Evaluation and characterization of imaging polarimetry through metasurface polarization gratings

Metasurfaces are a new class of diffractive optical elements with subwavelength elements whose behavior can be lithographically tailored. By leveraging form birefringence, metasurfaces can serve as multifunctional freespace polarization optics. Metasurface gratings are novel, to the best of our knowledge, polarimetric components that integrate multiple polarization analyzers into a single optical element enabling the realization of compact imaging polarimeters. The promise of metasurfaces as a new polarization building block is contingent on the calibration of metagrating-based optical systems. A prototype metasurface full Stokes imaging polarimeter is compared to a benchtop reference instrument using an established linear Stokes test for 670, 532, and 460 nm gratings. We propose a complementary full Stokes accuracy test and demonstrate it using the 532 nm grating. This work presents methods and practical considerations involved in producing accurate polarization data from a metasurface-based Stokes imaging polarimeter and informs their use in polarimetric systems more generally.

Calibration and validation of Airborne Multiangle SpectroPolarimetric Imager (AirMSPI) polarization measurements

The Airborne Multiangle SpectroPolarimetric Imager (AirMSPI), a precursor to the future Multi-Angle Imager for Aerosols satellite instrument, is a remote-sensing instrument for the characterization of atmospheric aerosols and clouds. To help discriminate between different aerosol particle types, which is crucial to improve our understanding of their impact on climate and air quality, AirMSPI acquires imagery over multiple view angles in the ultraviolet, visible, and near-infrared, and it employs dual photoelastic modulators (PEMs) to target an uncertainty requirement of ±0.005 in the degree of linear polarization (DoLP) at selected wavelengths. Laboratory polarimetric calibrations using a second-generation Polarization State Generator-2 (PSG-2) and validation measurements at 0<DoLP<1 demonstrate a systematic uncertainty of <0.002. In-flight calibrations of the temperature sensitivity of the PEMs, which could otherwise introduce DoLP errors up to 0.02, are extracted from onboard "validator" measurements as well as from the AirMSPI imagery of the Earth. After this in-flight calibration, the stability of measurements of the validator's DoLP throughout the POlarimeter Definition EXperiment and Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys field campaigns is ±0.001. Comparisons of DoLP data from these campaigns with the Research Scanning Polarimeter show root-mean-square differences ranging between 0.003 and 0.006, while the regression slopes deviate from unity by currently unexplained values up to 0.024. The observed differences are the result of measurement errors in both instruments, as well as imperfections in the intercomparison. A complete polarimetric uncertainty model for AirMSPI is presented, including the effects of absolute calibration uncertainty, in-flight modulation uncertainty, and random noise.

Generation of a coherent light beam with precise and fast dynamic control of the state and degree of polarization

Recent developments of polarized light sources with tunable state and degree of polarization (SOP and DOP) inherently provide a temporally incoherent beam, which makes them unsuitable for applications like interferometry. We present a method for generating a coherent beam with full, precise, and independent control of the SOP and DOP. Our approach is based on an imbalanced dual-frequency dual-polarization light source. We demonstrate that it offers three different working regimes, respectively providing perfectly depolarized light, DOP modulated light, or fully polarized light with a deterministic SOP trajectory. A simple implementation of this versatile approach is described and experimentally validated.

Arbitrary state of polarization with customized degree of polarization generator

An optical setup able to generate arbitrary states of polarization (SOPs) with customized degree of polarization is presented in this Letter. Compared with the few alternatives existing in literature, it presents an easy-to-build optical setup and leads to a superior performance. In fact, experimental results are presented, providing an accurate control for the generation of SOPs (maximum error of 1.7% and 3.3% for ellipticity and azimuth, respectively) as well as for the associated degree of polarization (full experimental variation from 1 up to 0.003, with a 1.7% maximum error). The system proposed may be useful for different applications, for example, for polarimeters testing, speckle metrology, and biological applications.

Use of ferroelectric liquid crystal panels to control state and degree of polarization in light beams

We propose a new technique that is able to generate a light beam with a controlled state of polarization (SoP) and a customized degree of polarization (DoP). The technique relies on the fact that effective depolarization can be achieved by temporally averaging a time-dependent SoP. Our proposed setup is based on a ferroelectric liquid crystal panel of retardance λ/2, with a fast polarization switching capability (33 Hz). A mathematical basis describing the experiment is given. In addition, simulation data is discussed, showing the possibility of generating any SoP with full control of the DoP. Finally, to prove the potential of the invention proposed, experimental results are provided as well, reaching an experimental minimum DoP of 0.14.

Simplified calibration procedure for Mueller polarimeter in transmission configuration

We address calibration of Mueller polarimeters in transmission configuration and in the presence of noise. By comparing the maximum likelihood (ML) method and the extended eigenvalue calibration method, it is found that the ML method yields higher precision in the presence of noise. Moreover, we show that by employing the ML method together with simple constraints on the calibration matrices, it is possible to perform the calibration without using a retarder, and with only polarizers. This result is of great interest for the calibration of multispectral polarimeters.