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We demonstrate a powerful tool for high-resolution mid-IR spectroscopy andfrequency metrology with quantum cascade lasers (QCLs). We have implementedfrequency stabilization of a QCL to an ultra-low expansion (ULE) referencecavity, via upconversion to the near-IR spectral range, at a level of$1\times10^{-13}$. The absolute frequency of the QCL is measured relative to ahydrogen maser, with instability $<1\times10^{-13}$ and inaccuracy$5\times10^{-13}$, using a frequency comb phase-stabilized to an independentultrastable laser. The QCL linewidth is determined to be 60 Hz, dominated byfiber noise. Active suppression of fiber noise could result in sub-10 Hzlinewidth.

A quantum cascade laser-based mid-IR frequency metrology system with ultra-narrow linewidth and $1\times 10^{-13}$-level frequency instability