The advent of optical frequency synthesizers (OFSs), based on mode-locked femtosecond (fs) lasers, has suddenly led to new advances in the field of precision spectroscopy. This has represented an immediate breakthrough for accurate frequency metrology in the visible/NIR spectrum where the first OFSs worked, enabling for measurements of atom energies with a precision approaching 1 part in 1015. However, further extension of OFSs to the IR region is crucial for absolute frequency measurements on molecular ro-vibrational spectra. 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 OFS [1]. The flexible character of our approach allows for a full and direct control of the comb features by simply acting on the parameters of the cw laser. In particular, by tuning its wavelength, the mid-IR comb covers the region from 2.9 to 3.5 μm in 180 mn wide spans. By phase-locking, the pump laser to the near-IR OFS, the mid-IR comb is directly linked to the Cs primary standard via a global positioning system (GPS) time receiver (see Fig. 1). 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. Moreover, the proposed scheme can be easily implemented in other spectral regions by use of suitable pumping sources and non-linear crystals. In this regard, our approach offers the opportunity of comb-referencing for direct laser sources operating in the mid-IR, such as quantum cascade lasers. This opens new perspectives for absolute frequency measurements on ro-vibrational molecular transitions for determination of molecular constants and frequency grids with improved accuracy. Finally, use of advanced fibre-based devices, which benefit from the continuous progress in telecom technology, will give, in the future, additional advantages for the realization of more and more effective set-ups. In this perspective, a more powerful mid-IR comb may be employed as a direct spectroscopic source for coherent coupling to high-finesse cavities, providing sensitive molecular detection across several hundreds of nanometers [2].

© 2007 IEEE

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