Atmospheric transfer of a radio-frequency clock signal with a diode laser.

Remote transfer of a radio-frequency clock signal over a 60 m open atmospheric link has been experimentally investigated using a diode laser as the clock carrier. Phase-noise spectra and Allan deviations are both measured to characterize the excess clock instability incurred during the transfer process. Different detection schemes are used to assess the contributions from different noise sources. With an 80 MHz clock frequency, the total root-mean-square noise amplitude is measured to be about 5×10(-3)  rad, with fractional frequency instability on the order of 1×10(-10) at 1 s. The majority of this excess noise is attributed to the transmitter noise, with the amplitude fluctuations of the diode laser identified as the main source. The excess phase noise caused by air turbulence is at the level of 10(-4)  rad under the current experimental conditions. Our finding suggests that suppressing the transmitter noise is critical for improving the clock-transfer fidelity.