Steven Peil, Thomas G. Akin, Joseph Whalen, United States Naval Observatory

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Optical clocks based on a calcium atomic beam are being investigated as robust operational devices for inclusion in USNO’s master clock ensemble. The 400Hz-wide 1S0-3P1 transition at 657nm is a high-Q resonance for laser stabilization and has a long history in optical spectroscopy. This narrow transition requires a Ramsey-Borde interaction to optimize the interaction-time limited resolution, and it also requires a cavity-narrowed laser for interrogation. The complication of a high-finesse optical cavity makes this system more complex than the lower SWaP vapor-cell optical clocks, while remaining less complex than trapped-atom clocks. Still, an atomic-beam clock can be made with as few as 1 clock laser and a frequency comb, promising an interesting tradeoff between performance and robustness. A second detection laser is often incorporated to increase the signal-to-noise ratio (SNR). We will review progress that we have made on this system in the past several years, including investigations of SNR with and without an additional detection laser, consideration of optimal Ramsey-Borde beam separation, residual amplitude modulation (RAM) mitigation, and measurement of a preliminary stability budget. We will discuss various laser architectures in terms of trade-offs among available laser power, complexity, and robustness.