LaLI-POP: Lamp and Laser Integrated Pulsed-Optically Pumped Atomic Clock

Abstract

Arguably, the rubidium atomic frequency standard (RAFS) is the workhorse of precise timekeeping in space, with the GPS RAFS one of the best atomic clocks now operational in any satellite system. However, the GPS RAFS is not perfect: 1) its short-term stability (i.e., averaging times less than 10⁴ seconds) is limited by the size of the atomic signal, and 2) its long-term stability (i.e., averaging times greater than 10⁵ seconds) is limited by frequency jumps caused by the light-shift effect. In both cases, the limitation of the RAFS’s frequency stability traces to the use of an rf-discharge lamp for atomic signal generation and atomic system monitoring: low lamplight intensity results in small atomic signals, while lamplight-induced frequency jumps manifest as random-walk frequency noise. Though replacement of the lamp by a diode laser eliminates these RAFS problems, new laser-related problems arise in the laser-pumped RAFS. In particular, PM-to-AM conversion noise and intrinsic diode laser relative intensity noise can limit the device’s signal-to-noise ratio. LaLI-POP solves these problems by combining the best of laser optical pumping with lamplight signal detection. A diode laser is pulsed on/off creating efficient optical pumping and hence large atomic clock signals, while microwave absorption is detected during the laser-off cycles by monitoring the vapor’s transmission of lamplight. Since lamplight has insignificant levels of PM-to-AM conversion and relative intensity noise, large atomic signals are detected with very low noise. Moreover since the lamplight only needs to monitor the absorption and not optically pump the vapor, the lamplight intensity can be low, reducing the lamp’s light-shift of the 0–0 resonance. Consequently, lamplight-induced frequency jumps should be much less problematic. In this presentation we provide a detailed theoretical estimate of LaLI-POP’s short and long-term frequency stability, along with results from our initial experiments exploring the LaLI-POP concept.