Figure 4

Schematic representation of the optical module. The OGS beacon laser at 1530 nm is collected by the main telescope (LCO). After separation by a dichroic mirror (DBS) it is split, with part being send to the beacon polarization analyzer (consisting of a polarizing beam splitter (PBS) and two detectors), and part being focused onto a quadrant detector (QD). The quantum channel light at 808 nm collected by the main telescope is sent towards a 4-detector polarization analyzer that includes two polarizing beam splitters (PBS), one for the {HV} basis, the other for the {DA} basis, and one half-wave plate (HWP) that rotates the polarization by 45^∘ for the {DA} basis. The {HV} versus {DA} basis choice occurs randomly in the beam splitter (BS). Not shown are the corner cubes that retro-reflect the OGS beacon laser at 1560 nm, and the small diameter telescope that directs the satellite’s beacon laser at 1530 nm towards the OGS. The telescope and the polarization analyzer/detection module are not at the same scale