The cesium atomic fountain clock is a device that can provide high-precision time and frequency signals by reproducing the SI unit of the second. It is employed in various fields such as quantum precision measurement, satellite positioning and navigation, and geological exploration. With advancements in laser technology and changing application scenarios, the development of cesium atomic fountain clocks has increasingly emphasized compactness, automation, and high reliability. The laser system, being the core component of the cesium atomic fountain clock, traditionally occupies significant space due to spatial light paths and optic components. Larger systems are more susceptible to environmental influences. Therefore, developing compact and highly reliable laser systems is crucial for small cesium atomic fountain clocks.

The proposed laser system is composed of three main modules: laser frequency stabilization, laser amplification, and laser beam splitting. In the frequency stabilization and amplification modules, a folded double-sided optical path design is employed to control the volume within 210 mm×210 mm×165 mm. The traditional double-pass acousto-optic modulator (AOM) solution is replaced by an open-loop frequency shifting method, eliminating the need for the AOM and associated components. Moreover, in the laser beam splitting module, an electro-optical modulator with a fiber interface is used for 9 GHz modulation, utilizing the edge frequency light generated by phase modulation as repumping light. This eliminates the need for separate repumping lasers and their complex frequency stabilization paths. These improvements significantly simplify the optical path design. Furthermore, the laser beam splitting adopts an all-fiber beam splitting scheme, further compressing the system’s volume.

The frequency of the distributed feedback (DFB) laser remains locked within 50 ms, with the voltage error signal fluctuating within the ±1 V range without any jumps. The central slope of the frequency identification curve is 0.6 V/MHz, and the laser frequency locking range is 3.3 MHz. Upon opening the closed loop at 50 ms, the control voltage of the laser gradually increases, achieving frequency detuning by altering the laser injection current. This results in a 150 MHz frequency shift lasting 2 ms. The laser system begins to return to the initial control voltage at 52 ms, and by 55 ms, the frequency locking state is restored. These results indicate that the open-loop frequency shifting method can achieve rapid and large-scale detuning, meeting the requirements for atomic polarization gradient cooling in small cesium atomic fountain clocks. The laser system demonstrates reliable re-locking capability (Fig. 9). A small cesium atomic fountain clock is operated continuously for 24 h, with cold polarization gradient cooling (open-loop frequency shifting) occurring 43200 times, maintaining atomic number signal fluctuation within 10%. These findings confirm that the open-loop frequency shifting method is repeatable and effective (Fig. 10), replacing the double-pass AOM method.

The compact laser system proposed in this paper utilizes DAVLL technology for frequency stabilization, the open-loop frequency shifting method for rapid and wide-range frequency detuning, and a 9 GHz frequency-stabilized laser for generating repumping light. Compared to existing fountain clock laser systems, this design offers higher miniaturization and reliability. Verified through laser frequency stabilization and frequency shifting experiments, this compact laser system meets the operational requirements of a small cesium atomic fountain clock. In addition, the system can be readily applied to other small cold atom quantum precision measurement instruments, such as portable atomic gravimeters and atomic gyroscopes.