The Raman lidar ability to retrieve atmospheric water vapor with high accuracy makes it a premium instrument in different research fields such as climatology, meteorology or calibration of GNSS altimetry data. In order to achieve long term stability of the measurements, the system has to be carefully calibrated. The calibration allows the reduction of uncertainties due to Raman cross sections, atmospheric transmittance and instrumental efficiency. In this work we decided to focus on reducing instrumental uncertainties, in particular the optical design of the detection system, of the IGN Raman lidar (Institut National de l’information Géographique et forestière), which has shown to have produced substantial drifts during the DEMEVAP campaign conducted at OHP (Observatoire de Haute Provence, France) in 2011. Above all, the optical fiber and the photomultipliers -which are widely used components in lidar systems- both, together, appeared to induce unwanted fluctuations. We conducted numerical simulations as well as experimental tests to identify and quantify the different instability sources and come up with practical solutions. Finally, long term calibration stability of the overall system will be assessed with regular water vapor profile recordings and calibration measurements spread over several months.