Optical clocks have demonstrated uncertainties in the region in fractional frequency. This makes them prime candidates for applications like chronometric geodesy, navigation and fundamental physics. Such applications require frequency transfer between a reference clock in a static place and a transportable one, e.g. enabling geopotential mapping over a region. It is thus necessary to develop a free-space optical link, in order to extend the current fiber link network in a flexible way. I will present the TOFU (“Transfert Optique de Fréquence Ultrastable”) project. It consists in developing a free space phase-stabilized optical link through an airborne relay, over expected distances up to ~100km. We already set up a 300m-folded link between a ground transceiver and a retroreflector carried by a balloon at CNES premises. I will present the design of this system, which includes a phase measurement and compensation unit and an optical transceiver. Following, I will show the performance and limitations of the system, that we evaluated during a measurement campaign lead in March 2023. Finally, I will describe the further technical developments in order to build an active airborne transceiver, replacing the passive retroreflector.