The ac stark shift and space-borne rubidium atomic clocks

Abstract

Due to its small size, low weight, and low power consumption, the Rb atomic frequency standard (RAFS) is routinely the first choice for atomic timekeeping in space. Consequently, though the device has very good frequency stability (rivaling passive hydrogen masers), there is interest in uncovering the fundamental processes limiting its long-term performance, with the goal of improving the device for future space systems and missions. The ac Stark shift (i. e., light shift) is one of the more likely processes limiting the RAFS' long-term timekeeping ability, yet its manifestation in the RAFS remains poorly understood. In part, this comes from the fact that light-shift induced frequency fluctuations must be quantified in terms of the RAFS' light-shift coefficient and the output variations in the RAFS' rf-discharge lamp, which is a nonlinear inductively-couple plasma (ICP). Here, we analyze the light-shift effect for a family of 10 on-orbit Block-IIR GPS RAFS, examining decade-long records of their on-orbit frequency and rf-discharge lamp fluctuations. We find that the ICP's light intensity variations can take several forms: deterministic aging, jumps, ramps, and non-stationary noise, each of which affects the RAFS' frequency via the light shift. Correlating these light intensity changes with RAFS frequency changes, we estimate the light-shift coefficient, K-LS, for the family of RAFS: K-LS = -(1.9 +/- 0.3) x 10(-12) /%. The 16% family-wide variation in K-LS indicates that while each RAFS may have its own individual K-LS, the variance of K-LS among similarly designed RAFS can be relatively small. Combining K-LS with our estimate of the ICP light intensity's non-stationary noise, we find evidence that random-walk frequency noise in high-quality space-borne RAFS is strongly influenced by the RAFS' rf-discharge lamp via the light shift effect. Published by AIP Publishing

Spectral emission from the alkali inductively-coupled plasma: Theory and experiment

“…In the future, we hope to use this model to better understand jumps in the light intensity of rf-discharge lamps employed in RAFS, which impact the RAFS' operation via the light-shift effect. 13,14 …”

“…7 Regarding atomic clocks, Rb rf-discharge lamps are crucial elements for atomic signal generation in the devices flying on GPS, Galileo, and BeiDou global navigation satellite system (GNSS) spacecraft, 8,9 playing a critical role in the achievable timekeeping performances of those atomic clocks. Not only does the Rb light photon flux determine the atomic clock's signal-tonoise ratio, 10,11 but fluctuations in Rb light brightness get mapped onto the clock's output frequency via the ac-Stark shift (i.e., the light shift), [12][13][14] defining the device's ultimate timekeeping ability. Consequently, from an applied physics and technology perspective understanding the alkali ICP is critical for understanding the performance limitations of GNSS, and the pathways for GNSS improvement.…”

Absolute hyperfine energy levels and isotope shift of Rb 5S–6S two-photon transition

“…Optical transitions of Rb are of interest due to their use in portable atomic clocks, for example possible use in the GPS system as a replacement for Rb lamp based clocks. Using a two-photon near-IR transition to the 5D or 6S state as the clock transition could improve the stability of these clocks [6][7][8]. The 5S-6S transition has an advantage that the hyperfine level pattern is identical for the two states, and they have the same Lande g-factors, making the first order Zeeman shifts the same for the two states, and thus the 5S-6S transition is less sensitive to magnetic field shifts.…”

Development of a Miniaturized Frequency Standard Comparator Based on FPGA

“…It is undeniable that any specific device that generates standard values is not absolutely stable, and the frequency standard is no exception. Under the influences of internal and external factors, the frequency standard output will change slowly and eventually lead to the failure of its standard reference function [5][6][7][8][9][10]. In this case, it will need to be calibrated.…”