| Pulsed optically pumped（POP）rubidium（Rb）cell atomic frequency standard（AFS）is particularly attractive,especially as one of candidates for new-generation space AFSs in satellite navigation systems.On the one hand,it inherits advantages from traditional Rb AFS（RAFS）of simple structure and compact size;on the other hand,its periodic operation timing sequence,including pulsed laser pumping,double pulsed microwave interrogation and detection of clock transition signal,is based on the POP technique and Ramsey scheme of separated oscillating fields,which contributes to better stability performance.At present,the related research into POP RAFS focuses on two points:（1）the integration and miniaturization of laser systems,physical packages and electronics,in the process of the development of an engineering prototype;（2）further improvement of the stability performance by optimizing the key performance characteristics of the three modules and by improving technical schemes,on the condition of analyzing the main factors affecting short-term and long-term stability.This thesis focuses on the improvement of the short-term stability performance of the POP RAFS,as well as the integration and miniaturization of the light source（laser system）and microwave source（microwave synthesis chain）.The main research work and results are summarized as the following four points.（1）By theoretically analyzing the main noise sources contributing to the short-term instability,the performance characteristics of laser and microwave are considered to be the dominant limitations on the short-term stability.In the analysis of short-term stability,the contribution of transmit noise induced by Brownian motion of Rb atoms housed in the cell is taken into account,to further improve the reasonability of theoretical estimation.（2）A compact transportable laser system is designed and implemented.The laser system consists of the optical module and the laser driving and controlling electronics,which are staked up and down to reduce the volume of the POP RAFS.Compared with the reported optical modules for POP RAFS,the enclosed optical box in this thesis is partitioned into four separate sub-spaces according to the thermal,magnetic and electrical isolation requirements,which can reduce the mutual interference of optical apparatuses.Meanwhile,the two beam expanding lenses requiring large space between them are added.Therefore,the optical box without an increase in volume includes laser collimation and shaping sub-module,saturated absorption optical setup,acousto-optic modulator（AOM）optical setup and beam expanding optical setup.The experiment results show that the output laser with a maximum power of 35 m W,and with a frequency stability of 3.0×10-12@1 s and 3.5×10-12@104s,and an intensity stability of 3.5×10-4@1 s and 3.9×10-4@104s.The relative intensity noise of the detection laser is lower than 3×10-121 Hz@200～4000 Hz.The contribution of the laser noise to the short-term instability is approximately 1.96×10-13-1/2,which meets the design requirements of the compact laser system for POP RAFS.（3）A dedicated microwave frequency synthesis chain is designed and developed.The microwave synthesis chain is based on the hybrid synthesis of phase-locked loop（PLL）and direct digital frequency synthesis（DDS）.The phase-locked-loop dielectric resonance oscillator（PDRO）acts as the fixed frequency source which synthesizes 6.7GHz by 670-times multiplication,starting from the 10 MHz reference frequency.Based on the PLL driving DDS synthesis scheme,the PLL multiplies the input10MHz reference frequency by 100 times to output 1000 MHz,which is used as the clock frequency reference of the DDS;and the DDS generates～134.682 MHz,starting from 1000 MHz by adjustable～0.134628 times multiplication.By mixing～134.682 MHz with 6.7 GHz,the～6.834682 GHz microwave signal is obtained.In this way,the frequency multiplication is performed by the PDRO and the PLL,respectively,which avoid spurs and phase noise from multistage frequency multipliers and allows more stable and reliable performance.Moreover,compared with“PLL+～GHz oscillator”,PDRO has high integration,with small volume,low power consumption,and a great reduction in microwave leakage.The frequency and the power of the output microwave signal are adjustable in the range of±10 k Hz and-50 d Bm～0 d Bm,respectively.The experimental results show that the phase noise of the output～6.8 GHz microwave signal is-92 d Bc/Hz@100 Hz and-122 d Bc/Hz@1 k Hz,which contributes an instability component of 1.3×10-13-1/2to the POP RAFS.（4）The servo control loop of the POP RAFS is closed with the laser system and the microwave synthesis chain in this work,and the measured short-term stability is2.7×10-13-1/2@1～100 s,which is better than 5.0×10-13-1/2and is close to the reported best short-term stability of POP RAFS at home and abroad.Meanwhile,it is in reasonable agreement with the theoretically estimated short-term stability,which illustrates the reasonability and validity of the theoretical estimation model.The research work summarized above improves the stability performance of POP RAFS.And it helps to promote the development of an engineering prototype. |
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