I reinstalled the QWP and the HWP in order to get a circular polarization after the last mirror. And I add a polarizer on a motorized mount, just before the translation stage. Like this, we can get any linear polarization with enough power, without going into the clean room.
I put as sample a QWP, everything seems ready to start a measurement.
IMPORTANT : LIMIT VALUE ON Z AXIS == 23mm. (avoid to hit the polarizer)