Reduction of Helicity-Dependent Instrumental Laser Intensity Asymmetries

We present a new optical system that greatly reduces helicity-dependent instrumental intensity asymmetries. The optical setup is similar to that described in Fabrikant \textit{et al.} [1], where two beams with orthogonal linear polarizations are sent through a chopper, allowing only one beam to pass through the optical system at a time. The two temporally-separated beams are then spatially recombined. We now use a system, with a second active polarization changing element, that is analogous to that described in Gay and Dunning [2], which compensates for false asymmetries in Mott polarimetry. In our setup, the orthogonal linear polarizations are now circularly polarized by a Pockels cell switching between a retardance of $+\lambda $/4 and --$\lambda $/4 at the same frequency as the chopper, but with a 90-degree phase shift. Using this method, we have been able to control the standard deviation of the mean of our asymmetries, as measured by a photodiode with lock-in signal processing, to 3*10$^{-8}$. \\[4pt] [1] M.I. Fabrikant, K.W. Trantham, V.M. Andrianarijaona, and T.J. Gay, Appl. Opt.~\textbf{47}, 2465-2469 (2008).\\[0pt] [2] T.J. Gay and F.B. Dunning, Rev. Sci. Instrum.~\textbf{63}, 1635 (1992).