Mid-infrared frequency synthesizers: novel precise rulers for molecular spectroscopy

In this work, we demonstrate a mid-IR frequency comb based on a difference-frequency-generation (DFG) process, which takes place in a periodically-poled lithium niobate (PPLN) crystal between a continuous-wave (CW) laser source and a near-IR, fiber-based optical frequency synthesizers (OFS). Then, in order to demonstrate that the generated comb can be properly used as a direct frequency ruler, an external CW DFG source is phase-locked to the closest mid-IR comb tooth and used for high-precision molecular spectroscopy.