Abstract
We report on studies on the light-shift in caesium miniature atomic clocks based on coherent population trapping (CPT) using a micro-fabricated buffer-gas cell (MEMS cell). The CPT signal is observed on the Cs D1-line by coupling the two hyperfine ground-state Zeeman sublevels involved in the clock transition to a common excited state, using two coherent electromagnetic fields. These light fields are created with a distributed feedback laser and an electro-optical modulator. We study the light-shift phenomena at different cell temperatures and laser wavelengths around 894.6 nm. By adjusting the cell temperature, conditions are identified where a miniature CPT atomic clock can be operated with simultaneously low temperature coefficient and suppressed light-shift. The impact of the light-shift on the clock frequency stability is evaluated. These results are relevant for improving the long-term frequency stability of CPT-based Cs vapour-cell clocks.
Highlights
Coherent population trapping (CPT) can be observed, for example, on the alkali D-lines by coupling two ground-state atomic levels to a common excited state, using two coherentM
The MEMS cell is placed in a clock physics package (PP) where it is heated to a cell temperature TC in the 45–90 °C range and temperaturecontrolled with an uncertainty \0.1 °C
The overall intensity light-shift from all different laser sidebands changes from negative to positive, as it is seen in our measurements. Another mechanism potentially contributing to the observed effect could be a frequency shift of the frequency-stabilized laser with changing cell temperature: due to the strong collisional broadening in the MEMS cell (Fig. 4a, points 1 and 2), the wing of the Doppler absorption line from the off-resonant excited state can introduce a temperature-dependent offset in the lock-in error signal and shift the laser frequency
Summary
Coherent population trapping (CPT) can be observed, for example, on the alkali D-lines by coupling two ground-state atomic levels to a common excited state, using two coherent. The possibility to obtain narrow CPT resonances [17, 18] led to their application in atomic frequency standards (‘‘atomic clocks’’) [19], where the CPT resonance is used as atomic reference to which the frequency of a quartz oscillator is stabilized [20,21,22] In this case the K scheme is formed by the 6S1/2 ground-state components jF = 3, mF = 0[ and jF = 4, mF = 0[ involved in the ‘‘clock transition’’ of atomic Cs [19] and one of the hyperfine-split excited states (6P1/2, F0 = 3 or 4 in our case), see Fig. 1. Unlike optically pumped double-resonance atomic clocks, CPT-based atomic clocks do not require a microwave cavity, allowing a strong miniaturization of the device [23,24,25,26]
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