Continuous optical generation of microwave signals for fountain clocks

For the optical generation of ultrastable microwave signals for fountain clocks, we developed a setup 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 cesium fountain clocks, acting as primary frequency standards. Measurements reveal that the system provides a phase noise level that enables quantum projection noise limited fountain frequency instabilities at the low 10^-14(τ/s)^-1/2 level. At the same time, it offers largely maintenance-free operation.