Application of the infrared (IR) Free-Electron Laser (FEL) was started in October 2003 at the Laboratory for Electron Beam Research and Application (LEBRA) of Nihon University. The FEL system consisted of silver-coated copper mirrors has demonstrated wavelength tunability ranged from 940 to 6100 nm as a function of the electron energy and the undulator K-value. Wavelength dependence of the FEL output power has been measured in term of different electron beam currents, electron energies and the undulator K-values. Approximate 25 mJ/macropulse has been obtained in the range 2 to 3 microns, which corresponds to peak power of 2 MW, provided that the FEL pulse length is 0.4 ps as resulted from the measurement by an interferometric method. The power decrease observed in the longer wavelength range is due to a large diffraction loss in the FEL guiding optics and the vacuum ducts.

A guiding optics system for LEBRA FEL facility was completed in 2003. The optical beam extracted from the Infrared FEL resonator is guided through a long vacuum system to user’s experimental rooms, where maximum 17 aluminum-coated mirrors are used in the guiding optics. The maximum length of the optical line is approximately 50 m. Therefore, the FEL extracted through a coupling hole in a resonator mirror has to be converted to a parallel beam. The conversion optics consists of an elliptic mirror and a parabolic mirror aligned to form a confocal configuration. The diffraction profiles of a guide laser extracted through the coupling hole have shown approximately identical patterns at output ports of the experimental facility. The guiding optics has two monitoring ports consisting of fluorite beam splitters, which is useful for simultaneous measurement of the power and the spectrum of the FEL during user experiments.