Analysis of an Interferometric Stokes Imaging Polarimeter

Abstract

Estimation of Stokes vector components from an interferometric fringe encoded image is a novel way of measuring the State Of Polarization (SOP) distribution across a scene. Imaging polarimeters employing interferometric techniques encode SOP information in a single image in the form of fringes. The lack of moving parts and the use of a single image eliminates the problems of conventional polarimetry - vibration, spurious signal generation due to artifacts, beam wander and the need for registration routines. However, interferometric polarimeters are limited by narrow band pass operation and short exposure time operations which decrease the Signal to Noise Ratio (SNR) in the detected image.A simulation environment for designing an Interferometric Stokes Imaging polarimeter (ISIP) and a detector with noise effects is created and presented. A user is capable of imaging an object with defined SOP through an ISIP on to a detector producing a digitized image output. The simulation also includes band pass imaging capabilities, control of detector noise, and object brightness levels.The Stokes images are estimated from a fringe encoded image of a scene by means of a reconstructor algorithm. A spatial domain methodology involving the idea of a unit cell and slide approach is applied to the reconstructor model developed using Mueller calculus. The validation of this methodology and effectiveness compared to a discrete approach is demonstrated with suitable examples. The pixel size required to sample the fringes and the minimum unit cell size required for reconstruction are investigated using condition numbers. The importance of the PSF of fore-optics (telescope) used in imaging the object is investigated and analyzed using a point source imaging example and a Nyquist criteria is presented.Reconstruction of fringe modulated images in the presence of noise involves choosing an optimal sized unitcell. The choice of the unit cell based on the size of the polarization domain and illumination level is analyzed using a bias-variance tradeoff to obtain the minimum root mean square error. A similar tradeoff study is used to analyze the choice of the band pass filters under various illumination levels. Finally, a sensitivity analysis of the ISIP is presented to explore the applicability of this device to detect low degrees of polarization in areas like remote sensing.