Polarization Design in Simulation, Laboratory, and Field Studies

Abstract

Simulation: Vector Vortex Coronagraph Vector vortex retarders (VVR) are halfwave retarders with a spatially varying fast-axis. They are often used in coronagraph systems to create a vortex beam which blocks the dominant on-axis starlight. The VVR’s inherent polarization sensitivity creates the potential for deleterious polarization aberrations. Two polarization ray traces (PRT) for the first half of the Coronagraphic Debris Exoplanet Exploring Payload (CDEEP) vector vortex coronagraph (VVC) were performed to observe the Jones pupils for the field incident on the VVR. The first ray trace observed the system sans polarization elements to observe overall transmission. Amplitude plots showed 0.68 maximum in the on-diagonals compared to 0.01 maximum in the off-diagonals. The next PRT included a linear polarizer followed by a λ/4 wave-plate upstream to induce circular polarization incident on the VVR. Results showed a 0.30∼0.35% maximum for all amplitude plots and near π/2 differences between xx and yx, and xy and yy phases, indicating the expected circular polarization. This work was presented at Space Telescopes and Instrumentation 2020: Optical, Infrared, and Millimeter Wave conference for which I was a co-author. Lab Experiment: Mueller Matrix PolarimetryDichroic filters are used by instrument designers to split a field of view into different optical paths for simultaneous measurement of different spectral bands. Quantifying the polarization aberrations of a dichroic is relevant for predicting the polarization states downstream. Polarization induced by the optical system could limit the performance of diffraction-limited systems, such as exoplanet imaging coronagraphs. A rotating retarder Mueller matrix imaging polarimeter was used to quantify the polarization properties of a 650 nm roll-off dichroic. The polarization properties of this commercial dichroic are compared at normal and 45◦ angle of incidence. These effects are not due to instrument calibration since no polarization-dependence was measured at normal incidence. Transmission measurements at 680 nm and 45◦ yield a 2.9 rad magnitude of retardance and 0.95 diattenuation. The dichroic is effectively a λ/4 waveplate at 630 nm and 45◦ angle of incidence. Therefore, large bandwidth illumination would produce polarization effects which depend upon the spectrum of incident light and if from an unknown source, could be non-correctable. This work was presented at the SPIE Space Telescopes and Instrumentation 2020: Optical, Infrared, and Millimeter Wave conference for which I was first author. Field Experiment: UV Sky PolarimetryCirrus clouds are important to the radiation energy budget due to their temporal duration and >50% global coverage. The variety of ice crystal shapes and sizes in a cirrus cloud create challenges differentiating radiation insulated by the Earth’s atmosphere from that reflected back to space. The optical thickness of these clouds is often too thin to be sensed using any current passive satellite radiometers. Sensitivity studies in the UV have shown that the angle of linear polarization (AoLP) of solar radiation backscattered from thin cirrus clouds and thin liquid water clouds is rotated. In the context of insect vision, seminal studies have shown that shorter wavelengths are more useful for navigation under cloudy-sky conditions. This cloudy sky invariance in the UV will afford confidence that backscattered AoLP deviations can be attributed to the presence of thin cirrus clouds as opposed to nearby clouds. Pust and Shaw also demonstrated subvisual cloud detection in degree of linear polarization (DoLP) and AoLP in wavelengths as low as 452 nm. An Ultraviolet Stokes Imaging Polarimeter (ULTRASIP) was designed and developed for optically thin clouds and sky observations in the 360 nm - 450 nm range. Validation measurements were taken at 394 nm and show DoLP and AoLP sky-cloud difference in visually thin clouds. ULTRASIP is a time-modulated polarimeter rotating a wire-grid polarizer in front of a 16-bit, back-illuminated CCD sensor. Sky scanning on cloudy days will be compared to polarized light scattering models. This work will be presented at SPIE Polarization Science and Remote Sensing X conference in August 2021 for which I am a co-author.