Novel Schemes For Compact And High-performance Coherent Population Trapping Atomic Clocks

The passive coherent population trapping（CPT）atomic clock can be miniaturized or chipped because it requires no microwave cavity.Currently,the chip-scale atomic clock（CSAC）is based on the CPT principle.It can be applied to deep space exploration,drone cruising,portable navigation receivers,underground rescue,and underwater resource exploration.As one of the core components of Micro-Technology for Positioning,Navigation,and Timing（μPNT）,the passive CPT atomic clock has significant value in defense and civil domains.Therefore,it is the direction of crucial investment support and continuous research in the United States,Europe,China,Russia,Japan,etc.The frequency stability of the CSAC is at the level of 10^-10@1 s.However,there is still considerable room for improvement to meet the demands of applications requiring higher precision frequency signals while being sensitive to size,weight,and power consumption（SWa P）,such as microsatellite formation networking,high-speed aircraft guidance,and submarine navigation.Therefore,while maintaining the miniaturization of CPT atomic clocks,further improving its performance is of great significance for expanding the application range of CPT atomic clocks and for the development of next-generation CSAC.The dissertation mainly focuses on improving signal contrast,reducing the common-mode noise,and exploring the limits of high-performance CPT atomic clock frequency stability through a novel configuration.The main research results and innovations obtained during the doctoral period are as follows:（1）The design and implementation of a highly homogeneous magnetic field for a physical system were achieved.The highly homogeneous magnetic field produced within a compact space effectively suppresses the linewidth of the Zeeman spectrum,improves the quality of CPT signal,and reduces the second-order Zeeman shift of clock transitions,providing a guarantee for the CPT atomic clock experiments.By using a seven-section coil instead of a straight solenoid coil and optimizing various critical parameters of the coil,the inhomogeneity of the magnetic field was effectively reduced.In practical applications,the inhomogeneity of the magnetic field due to the permalloy magnetic shielding layer has a significant impact on the physical system.In the region where the length and diameter of the vapor cell are 35mm and 20mm,respectively,the optimized seven-section coil generates a magnetic field with an inhomogeneity of less than 1%when considering the effect of a double-layer magnetic shield.Compared with the straight solenoid coil under the same conditions,the magnetic field inhomogeneity of the seven-section coil is greatly optimized,and the measured data and simulation results are in good agreement,which meets the requirements of the CPT experiment.（2）Miniaturization of a high-performance CPT atomic clock with double modulation.In current high-performance CPT atomic clock schemes,using two lasers or an external modulator to generate the required coherent dual-color light for experiments is not conducive to miniaturizing system.Therefore,we use direct modulation to replace the external modulator and simplify the system structure.By directly modulating a distributed Bragg reflector（DBR）laser with a microwave signal to generate bichromatic light and applying synchronous phase modulation and polarization modulation to the microwave signal and laser beams,respectively,we achieve constructive polarization modulation（double modulation）.This method can effectively suppress extreme Zeeman trap states,improve atomic utilization,and obtain constructive interference and amplitude-enhanced CPT signals.Moreover,by combining with the time-domain Ramsey technique,we further narrow the linewidth of the CPT signal and obtain Ramsey-CPT fringes,where the contrast（C）and linewidth（Δν）of the central fringe are 12.1%and 264 Hz,respectively.Compared with the continuous-wave regime,the q value（q=C/Δν）of the Ramsey-CPT fringe was increased by approximately 30%,which provides a potential avenue for the development of compact high-performance CPT atomic clocks.Furthermore,using direct modulation instead of external modulation in this scheme dramatically reduces the volume,power consumption,and complexity of the atomic clock system,verifying the feasibility of miniaturized high-performance CPT atomic clocks.（3）A novel scheme for common-mode noise suppression in differential detection CPT atomic clocks was designed and implemented.A circularly polarized pump light first prepares the atomic ensemble into a CPT state in the configuration.Then the transmitted light is linear polarized and reflected as a probe beam to interrogate the atomic system again.The two counter-rotating circular polarization components of the linear polarized probe beam can be constructive or destructive to the previous CPT state according to their Raman phase relative to the pumping beams.By combining this configuration with a differential detection technique,electromagnetically induced transparency（EIT）and absorption（EIA）signals are simultaneously observed.The differential CPT（diff-CPT）signal is extracted by subtracting these two signals.The experiment results show that the diff-CPT signal obtained by this scheme has a higher resonance amplitude and signal-to-noise ratio（SNR）.Moreover,the common-mode noise of the system is effectively suppressed,especially the laser amplitude modulation-amplitude modulation（AM-AM）,frequency modulation-amplitude modulation（FM-AM）noise,and detector noise.Compared with the case of EIT and EIA signals,the diff-CPT signal maintains the same linewidth while achieving an order of magnitude improvement in signal-to-noise ratio,and the expected short-term frequency stability of the CPT atomic clock has the potential to improve by an order of magnitude,reaching a level of 1.6×10^-12@1 s.This scheme provides a feasible way for miniaturized high-performance CPT atomic clock.