Megahertz far-infrared frequency shifting via multiple diffraction by a rotating grating

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Abstract
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 for n diffractions. Design parameters are derived for λ=119 μm (H2O laser), λ=195 μm (DCN laser), and λ=337 μm (HCN laser). The concept was successfully realized for λ=119 μm 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.