Print A rubidium atomic fountain clock with excellent long-term frequency stability can improve the performance of time scale.
The rubidium atomic fountain clock used for timekeeping should output the 10MHz and PPS signal continuously. The laser system may be interrupted because of highly susceptible to external environmental factors and requires a frequent maintenance with the traditional free-space optical assembly.
Recently, a research team led by Prof. ZHANG Shougang from the National Time Service Center (NTSC) of the Chinese Academy of Sciences (CAS) has developed a fiber laser system for rubidium atomic fountain clocks.
The fiber laser system utilizes fiber frequency doubling technology, comprising high quality fiber components that can enhance compactness and robustness while reducing maintenance requirements. This new system reduces its sensitivity to thermal and mechanical environments and maintains continuous running time exceeding one year.
The laser sources are based on the frequency doubling of two seed lasers at a wavelength of 1560 nm, which are locked using digital frequency locking and modulation transfer spectroscopy (MTS).
During the Sisyphus cooling period, the PZT control voltage of the fiber laser was ramped to detune the laser frequency to 170 MHz, and the atomic temperature is 1.9 K. A series of customized optical fiber splitters, acousto-optic modulators (AOMs), and shutters were integrated into two 2U enclosures as cooling and repumping light modules. Several fiber splitters are used to divide the laser for cooling and detecting atoms in fixed proportions.
Fiber shutters are mechanical shutters packaged in small aluminium boxes, and are coupled with polarization-maintain fiber at the front and back. A specific fiber acoustic optic modulator is used to shift the laser frequency and control the laser power with a high extinction ratio of >23 dB.
(a) Diagram of the laser system; (b) MTS signal of cooling laser and repumping laser; (c) Diagram of the laser frequency detuning.(Imaged by NTSC)
The entire laser system is integrated into a 22U cabinet and is characterized via polarization, power, and frequency stability measurements over 100 days. The laser power stability is 4×10-3 over a 15-day interval, and the laser frequency stability is 1.21×10-11 over the same 15-day interval.
(a) Relative laser power stability of six cooling beams; (b) Long-term frequency stability of laser system.(Imaged by NTSC)
Apply the fiber laser system to the rubidium atomic fountain clock NTSC-RbF2, a Ramsey fringe characterized by a central fringe half-width of 1.0 Hz and a contrast of 97% is generated. Employing a hydrogen maser as the local oscillator, an Allan deviation with a white-frequency noise reference line of σy (τ) =1.4× 10−13 τ−1/2 is achieved. The average (in)stability for the Rb fountain clock is less than 4.5×10-16 at the interval of 24 h.
Physical diagram of the rubidium atomic fountain clock NTSC-RbF2.(Imaged by NTSC)
(a) Ramsey fringes of Rb atomic fountain clock; (b) Allan deviation for Rb atomic fountain clock.(Imaged by NTSC)
The relevant results were published in the academic journal Optical Fiber Technology under the title "Fiber laser system for Rb atomic fountain clock".