Since coming on-line in November 2013, the Fritz-Haber-Institut (FHI) der Max-Planck-Gesellschaft (MPG) Free-Electron Laser (FEL) has provided intense, tunable infrared radiation to FHI user groups. It has enabled experiments in diverse fields ranging from bio-molecular spectroscopy to studies of clusters and nanoparticles, nonlinear solid-state spectroscopy, and surface science, resulting in 85 peer-reviewed publications so far. A significant upgrade of the FHI FEL is now nearing completion. A second short Rayleigh range undulator FEL beamline has been added that will permit lasing from < 5 microns to > 160 microns in the far IR. Additionally, a 500 MHz kicker cavity has been installed. It will permit simultaneous two-color operation of the FEL from both FEL beamlines over an optical range of 5 to 50 microns by deflecting alternate 1 GHz pulses into each of the two undulators. We will describe the upgraded FHI FEL physics and engineering design and present the current status of commissioning.

This paper provides a summary of the analyses that led to the definition of the 2-color upgrade of the IR FEL at FHI Berlin. We briefly cover several different aspects of the design, beginning with the beam dynamics of the second far-IR beamline, engineering considerations of that physics design, and the FEL physics that defined the short-Rayleigh range undulator as well as aspects of the undulator design itself. Additionally, we touch on the approach to 2-color commissioning with pulse picking, as well as considerations for the far-IR optical transport to users. The status of commissioning is described in a parallel paper at this Conference by W. Schöllkopf et al.