Sub-Hz relative linewidths from an interferometrically stabilized mid-infrared frequency comb

Frequency combs present a unique tool for high-precision and rapid molecular spectroscopy. Difference frequency generation (DFG) of near-infrared sources is a common approach to generate passively stabilized mid-infrared combs. However, only little attention has been paid so far to precisely measure the coherence properties of such sources. Here, we investigate these using a Raman-soliton based DFG source driven by an Yb:fiber frequency comb. A hetero-dyne beat between the second harmonic of the phase-locked DFG comb near 4 & mu;m and a 2 & mu;m Tm:fiber frequency comb locked to the same optical reference is performed. Using this method, we measure the relative phase noise power spec-tral density of both combs. This results in a sub-Hz relative linewidth between the DFG comb and the Tm:fiber comb. We also introduce a new pump/seed delay locking mecha-nism based on interferometry for long-term stable intensity noise suppression.Published by Optica Publishing Group under the terms of the Cre-ative Commons Attribution 4.0 License. Further distribution of this work must maintain attribution to the author(s) and the published arti-cle's title, journal citation, and DOI.