Upgrade of the ISIR-FEL at Osaka University and oscillation experiments in the sub-millimeter wavelength region

The far-infrared free electron laser (FEL) at the Institute of Scientific and Industrial Research (ISIR), Osaka University has been upgraded to extend the wavelength region further to the long wavelength side. The present modification includes replacement of the optical resonator and installation of an evacuated optical transport line from the FEL to the measurement room as well as installation of a far-infrared monochromator for measuring the wavelength of FEL light. We recently started experiments using the new FEL system. The loss of light in the optical resonator was measured and it is experimentally confirmed that the diffraction loss is reduced considerably after the upgrade of the optical resonator