Frequency shifts in the megahertz region, as required under certain conditions in far‐infrared diagnostics of magnetically confined fusion plasmas, can be achieved via multiple diffraction by a cylindrical diffraction grating rotating at moderate speed. We present a compact modular concept to correct the beam distortion at the grating. The concept is based on two elliptical mirrors and a sequence of (n−1) identical concave mirrors forndiffractions. Design parameters are derived for &lgr;=119 &mgr;m (H2O laser), &lgr;=195 &mgr;m (DCN laser), and &lgr;=337 &mgr;m (HCN laser). The concept was successfully realized for &lgr;=119 &mgr;m using four diffractions. A frequency shift of 3 MHz was achieved with a grating driven by an electric micromotor. The quality of the beam exiting from the grating was excellent. The total efficiency after four diffractions was measured as 64%, corresponding to an overall efficiency (i.e., mirror and other losses included) of 0.641/4=89% per diffraction.