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

We demonstrate a powerful tool for high-resolution mid-IR spectroscopy and frequency metrology with quantum cascade lasers (QCLs). We have implemented frequency stabilization of a QCL to an ultra-low expansion (ULE) reference cavity, 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 a hydrogen maser, with instability $<1\times10^{-13}$ and inaccuracy $5\times10^{-13}$, using a frequency comb phase-stabilized to an independent ultrastable laser. The QCL linewidth is determined to be 60 Hz, dominated by fiber noise. Active suppression of fiber noise could result in sub-10 Hz linewidth.