The cesium fountain primary frequency standard NIM5 has been developed at the National Institute of Metrology in China. The NIM5 loads atoms in an optical molasses from the background Cs vapor directly. Atoms are then cooled to a temperature of about 2 μK and launched to a height of 81 cm. The fringes of the Ramsey pattern have a width of 0.98 Hz. The NIM5 operates for more than 300 d a year, operating nearly continuously for 15 d at a time. By stabilizing the 9.19 GHz microwave frequency to the center of the central Ramsey fringe, a typical fractional frequency instability of 3 × 10 −13 (τ/s) −1/2 is obtained when running at high atom density, and a combined uncertainty, including Type A and B uncertainties, is typically 1.6 × 10 −15 . Comparisons of data between NIM5 and 5 other fountain clocks were carried out in May 2013 via two-way satellite time and frequency transfer (TWSTFT), and the results show good agreement within the uncertainties. Six groups of NIM5 data from January to June 2014 have been published in Circular T 319 and 320.
We demonstrate an 852-nm external cavity diode laser (ECDL) system whose wavelength is mainly determined by an interference filter instead of other wavelength selective elements. The Lorentzian linewidth measured by the heterodyne beating between two identical lasers is 28.3 kHz. Moreover, we test the application of the ECDL in the Faraday atomic filter. Besides saturated absorption spectrum, the transmission spectrum of the Faraday atomic filter at 852 nm is measured by using the ECDL. This interference filter ECDL method can also be extended to other wavelengths and widen the application range of diode laser.
TA(NIM) is being updated now. The new TA(NIM) uses an active hydrogen maser as a reference clock, and the frequency of TA(NIM) is steered by an algorithm to keep it the same as the NIM4 fountain clock, which is NIM's primary frequency standard. This paper introduces the generation of new TA(NIM), including the equations to predict maser frequency and the steering algorithm. The time difference and the frequency difference between the new TA(NIM) and UTC are also reported.
The NIM4 caesium fountain clock has been operating stably and subcontinually since August 2003. W e present our improvements on NIM4 in 200546 and the most recent evaluation for its frequency shifts with an uncertainty of 5 x 10-15. A 20Sday comparison between NIM4 and GPS time shows an agreement of 2 x 10-14. Finally the construction of the NIM5 transportable caesium fountain clock is briefly reported.
We have proposed a new technique to obtain highly polarized atoms for improving the sensitivity of the all-optical Cs atomic magnetometer. In a Bell–Bloom magnetometer, the resonance pumping from the ground state
F
g
=
3
to the excited state
F
e
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4
at the D1 line is used to polarize the Cs atoms and populate the sublevels of the
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m
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4
⟩
partly as the transition
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g
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4
→
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3
remains leaking. Here we utilize a modulation technique to generate a sideband frequency to pump both transitions simultaneously, which drives more atoms to the target state
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g
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4
,
m
F
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4
⟩
. This improvement significantly increases the magnetic resonance signal but with little effect on its linewidth. It is very suitable for the miniaturization of the atomic magnetometer in the future.
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