I made many tests about polarization and we can see some points thanks to the following figures :

1- The ellipsicity of the NKT is near to 0 (between -0,2 and 0,2) which means the polarization is rectilinear horizontale (attachement 1).

2- The power of the NKT has no influence on the polarization, verticale or horizontale (attachement 2 and 3).

3- The polarization is not changed by the type of mirror (Ag or Diélec) (attachement 4)

The next step is to measure the polarization after the cavity to know the ellipsicity.