The free-electron laser (FEL) at the Fritz Haber Institute (FHI) generates intense pulses of infrared (IR) radiation of widely tunable wavelength (from 3 µm to 60 µm, as of September 2017).

Unlike conventional lasers, where the radiation is produced in a gas, liquid, or solid, in an FEL it is generated by an electron beam propagating freely through a vacuum tube. In a device called an undulator, strong magnetic fields of alternating polarity force the electrons to undergo a wiggling (undulating) motion, thereby causing the emission of radiation. The radiation wavelength can be tuned simply by varying the electron energy or the magnetic field strength. Before entering the undulator, however, the electrons must be accelerated to almost the speed of light, requiring a complex electron accelerator.

The FHI FEL at the Fritz Haber Institute has been operational since 2013.