An atomic clock with excellent long-term stability is important for maintaining the Local Atomic Time (TA(k)). The ⁸⁷ Rb atomic fountain clock has the potential to fulfill these characteristics. It is required to operate continuously, similar to commercial cesium beam clocks and hydrogen masers. This paper describes the design and preliminary operation results of an ⁸⁷ Rb atomic fountain continuous clock ( ⁸⁷ RFCC) developed at the National Time Service Center (NTSC) of the Chinese Academy of Sciences. We adopted some improved design aspects and suppressed Majorana effect and microwave leakage to enhance the clock’s stability. Its continuous operation has been improved owing to the advantages of the high-reliability optical system and servo high-resolution offset generator (HROG) algorithm. We presented an algorithm for the 5 MHz/1 PPS signal output of the ⁸⁷ RFCC through the HROG. The ⁸⁷ RFCC exhibited a frequency stability of 1.91×10 ^-13 τ ^-1/2 , reaching 4.7×10 ^-16 at 200000 s. We referenced the 1 PPS output signal to the UTC(NTSC) and obtained the relative phase fluctuation of the ⁸⁷ RFCC, which was within ±1.75 ns for approximately 90 days. We estimated the preliminary drift of the ⁸⁷ RFCC relative to the UTC(NTSC) for three different periods at 0.000019±0.00148 ns/d/d, 0.00119±0.00182 ns/d/d, and -0.00155±0.00060 ns/d/d and discussed the results. During the 90-day evaluation period, although the frequency of the ⁸⁷ RFCC relative to the UTC(NTSC) had changed, it still exhibited a low drift compared to cesium beam clocks and hydrogen masers, which highlights its potential for excellent long-term stability. The ⁸⁷ RFCC investigated here has the potential to improve the local time scale.

The modulation transfer spectroscopy (MTS) technique used to stabilize two different transition lines of rubidium, required for laser cooling of atoms, is investigated. The laser source is based on a 1560 nm semiconductor laser diode that is amplified by an erbium doped fiber amplifier (EDFA). The laser output is frequency doubled with a fiber coupled periodically poled Lithium Niobate (PPLN) crystal to 780 nm. To address the weak spectral features of repumping laser, a theoretical study is conducted followed by experimental verification. The 87Rb D2 line F = 1 → F' = 0 transition temperature-enhanced MTS signal is observed, which provides a choice to stabilize the repumping laser frquency on the F = 1 → F ' = 2 transition using frequency shifting with acousto-optic modulators (AOMs). Compared with the orignal MTS signal, this method is shown to be more stable and accurate for laser frequency locking. Laer frequency locking is realized by controlling the driving current of the seed laser using digital PID feedback. After locking, the laser linewidth is 35.36 kHz.

The cesium atomic fountain clock is the world’s most accurate microwave atomic clock. The uncertainty of blackbody radiation (BBR) shift accounts for an increasingly large percentage of the uncertainty associated with fountain clocks and has become a key factor in the performance of fountain clocks. The uncertainty of BBR shift can be reduced by improving the system environment temperature. This study examined the mechanism by which the BBR shift of the transition frequency between the two hyperfine energy levels of the ¹³³Cs ground state is generated and the calculation method for the BBR shift in the atomic fountain. Methods used to reduce the uncertainty of BBR shift were also examined. A fountain system structure with uniform temperature and good heat preservation was designed, and related technologies, such as that for measuring the temperature of the cesium fountain system, were studied. The results of 20 days of measurements, in combination with computer simulation results, showed that the temperature uncertainty of the atomic action zone is 0.12 °C and that the resulting uncertainty of BBR shift is 2.4 × 10⁻¹⁷.

We demonstrated a high-performance frequency stabilization system based on a 1560 nm fiber laser frequency-doubled to 780 nm and locked to the ⁸⁷ Rb D ₂ line via modulation transfer spectroscopy. By optimizing spectroscopic parameters and suppressing noise, aided by the introduction of an intensity-noise-equivalent laser power (INELP) concept for accurate in-loop noise assessment, the short-term stability reached 3.32 × 10 ⁻¹³ / τ . With additional active power stabilization using a fiber-coupled AOM, the system achieved 2.8 × 10 ⁻¹³ at an integration time of 20 s and did not exceed 4 × 10 ⁻¹² until 10 ⁴ s. The current stability is primarily limited by the servo bandwidth constrained by the piezoelectric actuator resonance (∼20 kHz); a fast feedback loop via an AOM is envisioned to further improve the short-term stability.

Rubidium fountain clocks are operated by a control system. In this study, first, the control requirements of a rubidium fountain clock are analyzed; then, a control system is designed and divided into a timing sequence control system, a data acquisition system, and a servo control system. Multiple boards based on PCI extensions for the instrumentation (PXI) bus and Labwindows/CVI software 2019 are adopted. The timing sequence control system outputs 16 transistor–transistor logic (TTL) signals and three arbitrary waveforms. The results show that these signals are synchronized within 380 ns. Moreover, the digital locking frequency of the master laser is triggered by one of the timing signal, sweeping the laser frequency in a wide range during polarization gradient cooling. Two time-of-flight signals are acquired, and Ramsey fringes are scanned using the data acquisition system. An error signal is obtained every two cycles, and it is feed back to the frequency synthesizer and a high resolution offset generator using the servo control system. Then the frequency synthesizer is locked to the Ramsey center resonance. The rubidium atomic fountain clock exhibited a frequency stability of 7.4 × 10⁻¹⁶ at 86,400 s. Moreover, the phase of the Rb fountain clock is compared with that of UTC (NTSC) in real time, demonstrating a frequency drift of 0.00044 ns/day/day. The whole control system is full-featured, robust and flexible, thus satisfying the requirements of rubidium fountain clocks.