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

Summary form only given. The mid-infrared (MIR) spectral range (λ > 4.5 μm) is of interest in both applied and fundamental spectroscopy, for diverse applications such as trace gas detection and molecular frequency metrology. Approaches currently pursued for enabling MIR spectroscopy with accurate frequency control are based on generating the desired radiation by down-conversion, either of frequency combs or of cw near-infrared (NIR) sources. Such downconverted sources, however, typically have low power per frequency interval.Upconversion of the MIR radiation to the NIR range in principle provides a way to take advantage of the frequency measurement capabilities of the standard Erbium-fiber frequency comb. In previous work, the spectral range λ <; 4.5 μm (Muecke et al. (2004), Borri et al (2010), Gatti et al (2011)) and λ U 9.1 μm (Amy-Klein et al. (2004, 2005), Mills et al. (2012)) were successfully covered, by using the standard nonlinear-optical material periodically poled lithium niobate and AgGaS2, respectively. In this work we present a simple and robust solution applicable to the whole spectral range 4.5 μm <; λ <; 12 μm, based on the use of quantum cascade lasers (QCL) as the source of spectroscopic radiation. Orientationpatterned gallium arsenide is used as the nonlinear material to generate the sum-frequency wave of a QCL with a standard high-power cw Erbium fiber laser (1.5 μm). The sum-frequency wave is further amplified by a semiconductor amplifier. Continuous measurements of this wave's and the fiber laser's frequency by a standard Erbium 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 μm, which is upconverted to 1.2 μm. Both the QCL and the cw fiber laser are stabilized to the frequency comb using feedback control. At the same time, the absolute QCL frequency is determined, with 100 kHz-level inaccuracy, relative to an atomic frequency reference. We achieved a frequency instability to sub-10 kHz level and also long-term stability (Fig. 1 left) and controlled frequency tuning (Fig. 1 right). The implementation of the method is robust and relatively simple. All components except the OP-GaAs crystal are standard, i.e. frequency comb, atomic reference, 1.5 μm cw high-power single-frequency fiber laser, semiconductor amplifier, detectors, etc. The whole system is nearly turn-key, requiring only short warm-up time, and operates frequency-stably over multiple hours. This is an important advantage for use of the apparatus as part of more complex experimental set-ups. With its current performance and its ease of use, this type of spectrometer could be used e.g. for photoacoustic spectroscopy, multipass-cell spectroscopy, integrated cavity output spectroscopy, or Lamb-dip spectroscopy.