Long distance phase-coherent link between near- and mid-infrared frequencies

We present a new 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. We demonstrate the frequency stabilization of a mid-infrared laser with fractional stability better than 4×10^-14 at 1 s averaging. 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.