Mid-infrared laser phase-locking to a remote near-infrared frequency reference for high-precision molecular spectroscopy

We present a method for accurate mid-infrared frequency measurements and stabilization to a near-infrared ultra-stable frequency reference, transmitted with a long-distance fibre link and continuously monitored against state-of-the-art atomic fountain clocks. As a first application, we measure the frequency of an OsO _4 rovibrational molecular line around 10 μ m with an uncertainty of 8 × 10 ^−13 . We also demonstrate the frequency stabilization of a mid-infrared laser with fractional stability better than 4 × 10 ^−14 at 1 s averaging time and a linewidth below 17 Hz. This new stabilization scheme gives us the ability to transfer frequency stability in the range of 10 ^−15 or even better, currently accessible in the near infrared or in the visible, to mid-infrared lasers in a wide frequency range.