# QCL SPECTROSCOPY AT 9 $\mu$M CALIBRATED WITH A HIGH-POWER THULIUM-BASED FREQUENCY COMB

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 Title: QCL SPECTROSCOPY AT 9 $\mu$M CALIBRATED WITH A HIGH-POWER THULIUM-BASED FREQUENCY COMB Creators: Mills, Andrew A.; Jiang, Jie; Hartl, Ingmar; Fermann, Martin; Gatti, Davide; Marangoni, Marco Issue Date: 2012 Publisher: Ohio State University Abstract: 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~nderline{\textbf{PDB\_1}}, 2001} with emission wavelengths near 2 $\mu$m is shown to be an excellent candidate to obtain absolute frequency calibration of quantum cascade lasers (QCL) operating at wavelengths as long as 9 $\mu$m. Specifically, by combining the frequencies of a 9 $\mu$m QCL with the high power 2 $\mu$m comb in a AgGaSe$_2$ crystal, SFG light is created near 1.6 $\mu$m. A portion of the 2 $\mu$m comb is non-linearly shifted to 1.6 $\mu$m. As the carrier envelope offset frequency ($f_{ceo}$) is the same for the SFG radiation and the shifted comb at 1.6 $\mu$m, 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 NH$_3$ at 9 $\mu$m, enabled by rapid scanning of the repetition rate of the comb enabled to increase the signal-to-noise ratio.~nderline{\textbf{19}}, 17520 2011} %%The first OFCS referencing of a QCL was performed at 4.4 $\mu$m via sum-frequency generation process with a Nd:YAG laser.~nderline{\textbf{32}}, 988 2007} The SFG radiation was heterodyned with a external-cavity diode laser (ECDL), where both the YAG and ECDL were locked to the OFCS. The active stabilization of $f_{ceo}$ requires an octave spanning comb and a f-2f interferometer. Description: Author Institution: IMRA America, Ann Arbor, MI; Campus Point, Dipartimento di Fisica del Politecnico di Milano, Milano, Italy URI: http://hdl.handle.net/1811/52574 Other Identifiers: 2012-MF-07