In recent years, atomic frequency standards (AFSs) have been significantly improved and widely applied in measurement and transportation. The uncertainty of fountain AFSs has reached 10-16, and that of optical AFSs is much higher at 10-19, so AFSs are expected to replace Cs as a new definition of second. The development of AFSs is inseparable from the improvement of laser frequency stabilization. Narrow and ultra-narrow line-width lasers based on the frequency stabilization technology play significant roles in capturing and cooling atoms and ions, and trapping photons. Saturated absorption is widely employed in the frequency stabilization of AFSs because of its simple operation, high resolution, and the elimination of Doppler broadening. It is a technology commonly adopted in the microwave domain of atomic fountain clocks. The saturated absorption frequency stabilization of the atomic fountain clock is realized by locking a specific saturation absorption transition peak. The saturated absorption spectrum signal is modulated and demodulated to produce an error signal, and then a specific feedback signal is generated based on the error signal to control the laser output frequency. According to the output frequency, a series of modulation and frequency shifts are carried out to realize capturing, cooling, launching, and other functions which are required by the cold atom experiment. Generally, atoms have multiple saturated absorption peaks with only one involved in frequency locking, and unselected saturated absorption peaks cannot be utilized.

We propose an optimized method of laser frequency stabilization based on AFSs. This method applies the frequency control in laser cooling to the frequency stabilization system and realizes the transfer locking between different saturated absorption transition peaks. The sidebands are generated by fiber electro-optic modulation. Since the zero-order diffraction light cannot meet the frequency shift requirement, the fountain system generally takes +1 (or -1) order diffraction light as the effective laser. We employ the -1-order diffraction light and adjust it to the maximum. Then the frequency on the saturated absorption peak of the sideband is locked to realize the laser frequency shift. A large aperture saturated absorption scheme is adopted to improve the signal-to-noise ratio (SNR) of the saturated absorption signal and its error signal after frequency shift. The feature of this scheme is to apply an adjustable aperture in the light path to improve the SNR of fringe by adjusting the size of the light spot. Finally, the signal differences in saturation absorption signals before and after modulation and optimization are compared.

The probing light of the main optical path of the 85Rb atomic fountain system employed for two-level detection is adopted as the light source. The 1.035 GHz wide range frequency shift is realized by an optical fiber electro-optic modulator, which meets the requirements of the transition from F=3→F'=3?F'=4 of 85Rb to F=2→F'=3 of 87Rb. F=3→F'=3?F'=4 transition corresponds to the frequency locking position of 85Rb. The energy level transfer process is shown in Fig. 2. We make the frequency-shifted laser pass through the saturated absorption optical path. Figure 4 shows the saturation absorption signal and the change of its SNR with the spot diameter. When the total input optical power remains unchanged and the spot diameter increases from 2 to 10 mm, the saturated absorption signal increases and its corresponding SNR increases by about 13 dB. Figures 4(c) and 4(d) indicate the relationship between error signal, spot diameter, and optical power. With the rising spot diameter and optical power (optical power density), the error signal and its slope increase with improved SNR. The changes in saturation absorption signal before and after modulation are shown in Fig. 5. The saturated absorption signals before and after optimization are locked respectively. The signal before the optimization is too small to lock, while the optimized signal can be locked, and the atomic cloud signal is observed in the magneto-optical trap (MOT) area as shown in Fig. 6. The locked saturated absorption signal can achieve stable operation for a long time and the error signal fluctuation is kept within 5×10-4V, which meets the requirements of the atom fountain clock experiment.

In this study, we put forward an optimized method for laser frequency stabilization. The frequency of the incident light is shifted by electro-optical modulation to realize the transfer locking of 85Rb to 87Rb transition peaks through the probing light from the main optical path of the 85Rb fountain clock. The SNR of the transition spectral line is optimized by increasing the saturated absorption aperture, which has been improved by about 13 dB. After the frequency shift signal is locked, the atom cloud signal is observed on the atomic clock and can operate stably for a long time. By this method, we can achieve transfer locking of any frequency in the frequency range. It is also hopeful to optimize the main optical path by adding fiber electro-optical modulator (FEOM) in front of the main optical path and performing time control to realize some acousto-optic modulator (AOM) functions. As a result, the number of modulation devices is reduced and the optical path is simplified. Finally, the optical power transmission efficiency is improved to the possible realization of all fiber links.