Mid-IR frequency comb with sub-Hertz relative line width based on Raman soliton self-frequency shift

Optical frequency combs in the mid-infrared spectral region became an invaluable tool for molecular spectroscopy. This spectral region is filled with highly specific vibrational transitions which allow for a precise identification of the composition of gas mixtures and fundamental research of phenomena in physics and chemistry with highest precision. Fre- quency combs in the mid-infrared are often generated via nonlinear frequency conversion of near-infrared lasers. In this work, two frequency components of the same Yb:fiber comb are converted to the mid-infrared at 4 μm via difference frequency generation (DFG) inside a nonlinear crystal. The long-wavelength component is generated from the shorter wavelength driving laser via Raman soliton self-frequency shift in a highly nonlinear fiber.In the past, not much attention has been paid to the coherence properties and optical line widths of similar DFG frequency combs. To study these properties, the generated mid-infrared light is interfered with a reference frequency comb of known properties. The phase noise of the generated heterodyne beat is characterized to determine the optical line width of the mid-infrared frequency comb. The results show sub-Hz relative line widths relative between the two combs. This method also allows to characterize optical sidebands generated by a dither-and-lock technique, which was used in a previous work for long-term stabilization of the source for minimal intensity noise. In this work, this stabilization mechanism is replaced with an interferometric method which does not require modulation. Paired with the low comb line width this allows for the use of the presented mid-infrared frequency comb for molecular spectroscopy at high resolution with high long-term stability.