Quantum cascade laser-based mid-IR frequency metrology system with ultra-narrow linewidth and 1 × 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 1×10−13. The absolute frequency of the QCL is measured relative to a hydrogen maser, with instability <1×10−13 and inaccuracy 5×10−13, using a frequency comb phase stabilized to an independent ultra-stable 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.