Session P1b: Environmental Sensitivity of Clocks and Timing Systems

Optical Measurements of Buffer Gas Pressure Ratios
Andrew J. Householder, The Aerospace Corporation
Location: Seaview A/B
Date/Time: Tuesday, Jan. 28, 4:23 p.m.

Vapor cell microwave atomic clocks are the dominant atomic clock technology in today’s space systems due to their low size, weight, and power. In these systems the resonance cell itself – a sealed glass cell containing the atomic sample – can limit the long-term performance of the atomic frequency standard (AFS) through the buffer-gas collision shift, which is temperature dependent. To mitigate the temperature sensitivity, the resonance cell is filled with a properly chosen mixture of buffer gasses, which have opposite sign temperature shift coefficients. Of course, this requires that the resonance cell be filled with the proper ratio of buffer gases, and verifying that a given cell is filled with that proper ratio is challenging. In particular, since there are two unknowns (i.e., the two buffer-gas’s partial pressures) two independent measurements of “buffer-gas effects” must be made. A standard approach is by measuring both an optical collision shift and a microwave collision shift. This requires performing two separate spectroscopy experiments, which can be time consuming. Here, we investigate the fidelity of another method, which involves measuring the optical collision shift and collision broadening of an optical spectrum. In this presentation we will discuss our measurement of the partial pressure ratio of N2and Ar in an 87Rb vapor cell, deconvolving the overlapped (Voigt lineshape) hyperfine resonances to extract the collision shift and collision broadening values. The presentation will conclude with an analysis of the viability of this alternate approach compared to the standard approach of optical and microwave collision shifts for verifying the pressure fill of vapor cells for space atomic clocks.
Approved for public release. OTR 2024-01126.