This manuscript details the transfer of an ultra-stable optical frequency reference by means of an optical link and its application to the phase-lock of a mid-infrared laser. An optical fiber link allows the ultrastable transfer of a frequency by using a scheme which compensates the propagation noise. We extended this system to longer links, and transferred the optical frequency reference simultaneously with Internet data. A cascaded link of 300 km and a simple link of 540 km had been demonstrated with a stability of 10-19 at 104 s. Such a link has been used to lock a CO2 laser at LPL, emitting at 10 µm, to a frequency reference developed at LNE-SYRTE, Observatoire de Paris. This reference is an ultra-stable laser, emitting at 1.54 µm, the frequency of which is measured against the primary standards of LNE-SYRTE by using a femtosecond laser. This reference is transferred by an optical link to LPL, in order to stabilize the repetition rate of a second femtosecond laser and to measure or control the frequency of a CO2 laser. When the CO2 laser is locked to a molecular reference (OsO4), the stability is 4.10-14 at 1 s. The performances are even better when the CO2 laser is locked directly to the optical reference. Then the laser could be used for the experiment of observation of the parity violation in chiral molecules, in progress at LPL. This shows the feasibility of high resolution molecular spectroscopy experiments in laboratories in which there is no primary standards.
