Continuous optical generation of microwave signals for fountain clocks

For the optical generation of ultrastable microwave signals for fountain clocks we developed a setup, which is based on a cavity stabilized laser and a commercial frequency comb. The robust system, in operation since 2020, is locked to a 100 MHz output frequency of a hydrogen maser and provides an ultrastable 9.6 GHz signal for the interrogation of atoms in two caesium fountain clocks, acting as primary frequency standards. Measurements reveal that the system provides a phase noise level which enables quantum projection noise limited fountain frequency instabilities at the low $10^{-14} (\tau /\mathrm{s})^{-1/2}$ level. At the same time it offers largely maintenance-free operation.