We have recorded the Doppler profile of a well-isolated rovibrational line in the ν 2 band of 14 NH 3 . Ammonia gas was placed in an absorption cell thermalized by a water-ice bath. By extrapolating to zero pressure, we have deduced the Doppler width which gives a first measurement of the Boltzmann constant, k B , by laser spectroscopy. A relative uncertainty of 2×10 -4 has been obtained. The present determination should be significantly improved in the near future and contribute to a new definition of the kelvin.

We describe the realization of a 5 km free-space coherent optical link through the turbulent atmosphere between a telescope and a ground target. We present the phase noise of the link, limited mainly by atmospheric turbulence and mechanical vibrations of the telescope and the target. We discuss the implications of our results for applications, with particular emphasis on optical Doppler ranging to satellites and long-distance frequency transfer.