Robust, frequency-stable and accurate mid-IR laser spectrometer based on frequency comb metrology of quantum cascade lasers up-converted in orientation-patterned GaAs

We demonstrate a robust and simple method for measurement, stabilization and tuning of the frequency of cw mid-infrared (MIR, 5 - 12 $\mu$m) lasers, in particular of quantum cascade lasers, allowing implementation of flexible and ``turn-key'' spectrometers for a range of high-resolution spectroscopic tasks. The MIR laser wave is upconverted by sum-frequency generation in an orientation-patterned GaAs crystal with the output of a standard high-power cw 1.5 $\mu$m fiber laser, subsequent amplification of the sum-frequency wave, Continuous measurements of this wave's and the fiber laser's frequency by a standard Er:fiber frequency comb provide signals allowing frequency control of the MIR laser. The proof of principle is performed with a quantum cascade laser at 5.4 $\mu$m, which is upconverted to 1.2 $\mu$m. The absolute QCL frequency is determined with 100 kHz-level inaccuracy relative to an atomic frequency reference. Frequency stabilization to sub-10 kHz level, controlled frequency tuning and long-term stability are demonstrated. The whole system is nearly turn-key, requiring only short warm-up time. This is an important advantage for use of the apparatus as part of more complex experimental set-ups. Further extensions of the system are possible and will be discussed.