Free space optical link phase noise measurement

Dissemination of precise time and frequencies and its corresponding infrastructure has become a key tool for many scientific and industrial applications (optical clocks comparisons, global navigation and positioning systems, security). Although most of these transfers use optical fibre networks, free-space photonic links represent an essential supplement in cases where the fibres cannot be utilised. This work presents pilot experimental results of free-space optical (FSO) link phase noise measurement, an initial requirement for further intended phase-coherent transfer of precise time and frequency. The measurement setup consists of a 30 m long unidirectional FSO (DUT) supplied by an ultra-narrow 1540 nm fibre laser (locked to the ultra-stable optical cavity), and it is supplemented with parallel phase-noise stabilised optical fibre delivering the reference beam to the beat-note with the signal obtained from the FSO receiver.