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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Novel Light-shift Measurement Method with Multiple Photo Detectors for Gas-cell Based Atomic Clocks

Masahiro Fukuoka, Akira Hanatani, Shigeyoshi Goka

Published Apr 24, 2022• 2 citations
Publisher

Abstract

SummaryA simple light-shift measurement method using multiple photodetectors (PDs) suitable for chip-scale atomic clocks (CSACs) and miniature atomic clocks (MACs) was proposed. This method can be used with at least two photodetectors in real-time operations. To increase the long-term frequency stabilities of CSACs and MACs, the proposed method can be used as a zero light shift detector for active light shift stabilization[1].

DOI
10.1109/eftf/ifcs54560.2022.9850834

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SummaryA simple light-shift measurement method using multiple photodetectors (PDs) suitable for chip-scale atomic clocks (CSACs) and miniature atomic clocks (MACs) was proposed. This method can be used with at least two photodetectors in real-time operations. To increase the long-term frequency stabilities of CSACs and MACs, the proposed method can be used as a zero light shift detector for active light shift stabilization[1].
In a chip scale atomic clock (CSAC), a laser beam with a higher intensity is usually used to achieve a good contrast of the clock transition signal. Thus the light shift (AC Stark shift) is of great importance to the performance of a CSAC device. Here we apply the reduced light shift optical pumping method to a micro-machined 87Rb vapor cell in order to reduce the sensitivity of the measured clock frequency to various operational parameters. The preliminary result shows the improved short term stability.
We describe a novel technique for stabilizing frequency shifts in laser-interrogated vapor-cell atomic clocks. The method suppresses frequency shifts due to changes in the laser frequency, intensity, and modulation index as well as atomic vapor density. The clock operating parameters are monitored by using the atoms themselves, rather than by using conventional schemes for laser frequency and cell temperature control. The experiment is realized using a chip-scale atomic clock. The novel atomic-based stabilization approach results in a simpler setup and improved long-term performance.
Abstract Chip-scale atomic clocks based on coherent population trapping (CPT) face challenges due to miniaturized gas cells, which reduces the CPT resonance signal and the absorption signals of alkali atoms. To overcome this, we propose a method that simultaneously stabilizes the laser wavelength and microwave modulation frequency without relying on absorption line detection. Our approach enables stabilization at the point where the amplitude of the CPT resonance signal is maximized, making it suitable for compact atomic clocks. The system uses three lock-in amplifiers: one is used to stabilize the RF modulation frequency, and the other two are used for detecting the amplitude of the CPT resonance and stabilizing the laser wavelength. The Allan deviation measurements demonstrate a 1.4-fold improvement in short-term stability compared to conventional methods, suggesting that the proposed technique is a promising solution for enhancing the performance of small gas-cell atomic clocks.
Research on chip-scale atomic clocks (CSACs) based on coherent population trapping (CPT) is reviewed. The background and the inspiration for the research are described, including the important schemes proposed to improve the CPT signal quality, the selection of atoms and buffer gases, and the development of micro-cell fabrication. With regard to the reliability, stability, and service life of the CSACs, the research regarding the sensitivity of the CPT resonance to temperature and laser power changes is also reviewed, as well as the CPT resonance's collision and light of frequency shifts. The first generation CSACs have already been developed but its characters are still far from our expectations. Our conclusion is that miniaturization and power reduction are the most important aspects calling for further research.
Light shift and vapor-cell temperature shift are the two most significant factors dominating the long-term instability of compact atomic clocks. Due to the different physical mechanisms, there is not yet a solution that can effectively suppress the frequency shifts induced by these two effects. Here, we propose a 'resonance-offset' locking approach that compensates for the two physical frequency shifts. In this approach, the additional offset locking shift can effectively counteract the atomic resonance shifts arising from changes in vapor-cell temperature and light power, reducing the net impact on the clock's frequency to nearly zero. We have demonstrated this strategy on the 778 nm Rubidium two-photon optical frequency standard, successfully compensating for light shift and cell-temperature shift, respectively. This general method is particularly appealing for compact vapor-cell microwave and optical atomic clocks designed for the excellent stability rather than accuracy.