Full-field Spatial Mapping of Spectral Polarization Information: Experimental Schemes, Instrument Development and Calibration

S, Athira B. (2016) Full-field Spatial Mapping of Spectral Polarization Information: Experimental Schemes, Instrument Development and Calibration. Masters thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

Magnetic instabilities in sunspot structures and reconnection mediated restructuring of fields produce violent solar eruptions such as flares and coronal mass ejections (CMEs) – which have severe space weather consequences. Measurement and analysis of the components of the vector magnetic field in solar corona can potentially yield quantitative information on coronal magnetic structure, dynamics and heating, which is therefore of paramount importance. However, there is no instrument yet operational, anywhere in the world, which can routinely perform simultaneous imaging, spectroscopic diagnostics and polarimetric imaging of the corona. Among the various experimental techniques explored for coronal magnetometry, optical polarimetry (on selected spectral lines of radiation emitted by ions) has been found to be particularly promising. Specifically, measurement of full Stokes vector elements. With this motivation and with the intent of providing the proof-of-concept for Hyper-Spectral Imaging Polarimetry, in this thesis I have done the calibration of non rotating retarder and developed a system of spectral imaging polarimetry with rotating retarder approach. In the later part of the thesis I have dealt with Spin –Orbit interaction of light which creates a geometric phase. And also done a experimental set-up to measure this geometrical phase when is created by the spatial variation of polarization. We have developed a spectral imaging polarimeter using rotating retarder approach. Under the context of usage in satellites, the main drawback of using this type of polarimeter is the utilization of mechanical rotation. This can lead to additional spatial in homogeneity. So the idea was to develop LCVR based stokes polarimeter which will be completely electronically controlled. Thus no mechanical vibrations or rotations are involved in it. For that purpose we have fully calibrated the LCVRs using the Mueller matrix spectral imaging polarimeter. Initial attempts were made to develop a spectral imaging stokes polarimeter using LCVRs. A major problem encountered in using the existing LCVR is their non-ideal behavior or nature, such as (a) spatial in homogeneity of birefringence (b) temperature stability of birefringence was not good enough for accurate measurements. However, we envisage that with LCVRs with better qualities such as having better temperature stability and spatial homogeneity. One may perceive on the goal of developing a non- rotating or LCVR based stokes polarimetry which can achieve the polarization signal with high spectral and spatial accuracy. This is one of our future goals. We have also initiated some studies on using Orbital Angular Momentum of light and geometrical phase of light in the context of magnetic field mapping. So the goal was to measure the geometrical phase which arises due to the spatially varying polarization states. In the context of solar magnetic field, the magnetic field in the transverse direction changes spatially which leads to geometrical phase. The initial idea of the experiment was to quantify this geometrical phase information to indirectly extract the information about the spatial variation of magnetic field. The experiments done here are at the preliminary level does not involve inhomogeneous magnetic fields. By creating spatial in homogeneity or spatially varying geometrical phase using optical elements, the initial results suggest that the resulting gradient of geometrical phase can, in principle be used to quantify the spatial variation of polarized light.

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