Polarization diagnosis and transmission distortion calibration of the Dalian coherent light source

Polarized light serves as a fundamental tool in various scientific research, particularly in the study of chiral molecules and magnetic materials. For Free-electron Laser (FEL) sources, the Elliptical Polarized Undulator (EPU) has been extensively used as a tool to generate tunable polarized FEL. The accurate adjustment of the polarization state critically depends on the precise diagnosis of the polarization state. This work presents a Vacuum Ultraviolet (VUV) polarimeter that employs a reflection polarizer to determine the polarization state of the Dalian Coherent Light Source (DCLS), a VUV-FEL facility in Dalian, China. This instrument has been precisely installed on the DCLS-FEL2 beamline to measure the EPU-generated FEL pulses. The linear polarization angle was measured with an accuracy of ±1°, and a polarization degree of 95% was achieved at 10.2 eV photon energy. Furthermore, beamline transmission matrix calibration has been conducted to ensure precision of polarization state at downstream end-stations.