Eliminating Light-Shift Jumps in Rb Atomic Clocks: Active Stabilization of RF-Discharge Lamp Brightness

It has been established that the random-walk frequency noise of GPS-IIR rubidium (Rb) atomic clocks is driven by rf-discharge lamplight instability. Briefly, jumps in Rb light from the lamp result in jumps in clock frequency via the light-shift effect. These lamp jumps follow a compound Poisson process, and yield instability in the clock frequency that has an Allan deviation (ADEV) that mimics the ADEV of a continuous Wiener process. Quite likely this same mechanism drives random-walk frequency noise in all high-quality Rb atomic clocks, motivating means for its mitigation. One approach is to remove the effects of lamplight jumps in post-processing by measuring changes in lamplight brightness and knowing how those map to clock frequency via the light-shift coefficient. Another approach, and the one discussed here, actively stabilizes lamp brightness by controlling the rf-power driving the discharge. Specifically, we show that rf-power control can significantly reduce an rf-discharge lamp's brightness fluctuations, and that it can eliminate lamplight jumps defined by a compound Poisson process.