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Polarization control optimization of the Gemini South beam transfer optics

The Beam Transfer Optics (BTO) is a sub-system of the Gemini Multi-Conjugate Adaptive Optics System (GeMS). The main purpose of the BTO is to relay the laser light from the laser service enclosure up to the Laser Launch Telescope (LLT), located behind the telescope secondary mirror, where the five laser beams are propagated to the sky. Other functionalities besides relaying the laser light from the laser to the LLT, is the laser polarization control, which is crucial to any AO related system. The polarization state of the laser output beam influences the photon return flux. It is proven that the backscattering efficiency is higher when exciting the sodium layer with a circular polarized beam than one with linear polarization. For this reason circular polarization of our five laser beams that exit the LLT is desired for any telescope position. The paper reviews the current status of the Gemini South Beam Transfer Optics polarization and its control scheme. It reports on the improvements already done on the polarization control and measurement data of the polarization state at different BTO sections. In addition we discuss further optimization and upgrade ideas of the system.