Coherent optical clock down-conversion for microwave frequencies with 10 instability.

Optical atomic clocks are poised to redefine the Système International (SI) second, thanks to stability and accuracy more than 100 times better than the current microwave atomic clock standard. However, the best optical clocks have not seen their performance transferred to the electronic domain, where radar, navigation, communications, and fundamental research rely on less stable microwave sources. By comparing two independent optical-to-electronic signal generators, we demonstrate a 10-gigahertz microwave signal with phase that exactly tracks that of the optical clock phase from which it is derived, yielding an absolute fractional frequency instability of 1 × 10 in the electronic domain.

Optical-Clock-Based Time Scale.

A time scale is a procedure for accurately and continuously marking the passage of time. It is exemplified by Coordinated Universal Time (UTC) and provides the backbone for critical navigation tools such as the Global Positioning System. Present time scales employ microwave atomic clocks, whose attributes can be combined and averaged in a manner such that the composite is more stable, accurate, and reliable than the output of any individual clock.

Accurate control of optoelectronic amplitude to phase noise conversion in photodetection of ultra-fast optical pulses.

When illuminating a photodiode with modulated laser light, optical intensity fluctuations of the incident beam are converted into phase fluctuations of the output electrical signal. This amplitude to phase noise conversion (APC) thus imposes a stringent constraint on the relative intensity noise (RIN) of the laser carrier when dealing with ultra-low phase noise microwave generation. Although the APC vanishes under certain conditions, it exhibits random fluctuations preventing efficient long-term passive stabilization schemes.

Phase noise characterization of sub-hertz linewidth lasers via digital cross correlation.

Phase noise or frequency noise is a key metric to evaluate the short-term stability of a laser. This property is of great interest for the applications but delicate to characterize, especially for narrow linewidth lasers. In this Letter, we demonstrate a digital cross-correlation scheme to characterize the absolute phase noise of sub-hertz linewidth lasers.