Determination of the polarization characteristics of the ELBE free electron laser

In this work, we investigated polarization characteristics of the ELBE infrared free electron laser at the experimental position by adapting imaging ellipsometry technique. We found that the laser beam has a Gaussian-like power distribution with a diameter of 3–4 mm and exhibits a linearly polarized character throughout the whole area investigated. We also evaluated the degree of polarization amounting to 98% which demonstrates a tiny depolarization effect during the beam transfer via several optical elements from the source to the experimental position.