15min:

ANDREW A. MILLS, JIE JIANG, INGMAR HARTL, MARTIN FERMANN, IMRA America, Ann Arbor, MI; DAVIDE GATTI, MARCO MARANGONI, Campus Point, Dipartimento di Fisica del Politecnico di Milano, Milano, Italy.

Optical frequency comb synthesizers (OFCS) comprised of mode-locked femtosecond lasers can be stabilized with Hertz-level accuracy and used in combination with cw lasers for high resolution spectroscopy. As currently established OFCS technologies are confined to the near-IR, mid-IR spectroscopy requires either down-conversion of near-IR combs or up-conversion of the probing laser. Due to the near-IR absorption edge of the nonlinear crystals with extended mid-IR transparency, the conversion efficiency of nonlinear processes increases with the wavelength of the interacting fields. A more straightforward and efficient link between comb and probing laser is thus expected to be obtained by increasing the wavelength of the comb synthesizer. In this work, the use of a novel, powerful Thulium-based OFCS with emission wavelengths near 2 µm is shown to be an excellent candidate to obtain absolute frequency calibration of quantum cascade lasers (QCL) operating at wavelengths as long as 9 µm.

Specifically, by combining the frequencies of a 9 µm QCL with the high power 2 µm comb in a AgGaSe₂ crystal, SFG light is created near 1.6 µm. A portion of the 2 µm comb is non-linearly shifted to 1.6 µm. As the carrier envelope offset frequency (f_ceo) is the same for the SFG radiation and the shifted comb at 1.6 µm, heterodyning the two signals produces a beat signal independent of f_ceo, eliminating the need for an octave spanning comb and f-2f interferometer. We report on the development of this instrument, and the absolute line transitions of NH₃ at 9 µm, enabled by rapid scanning of the repetition rate of the comb enabled to increase the signal-to-noise ratio.