Research On The Key Technology Of A Pulsed Optically Pumped Rubidium Frequency Standard

On-board atomic clock is one of the core components of satellite navigation systems,with the rapid development of of national defense,science and technology,new type of atomic clock with better performance is required.Pulsed Optically Pumped（POP）rubidium clock has smaller optical frequency shift and cavity pulling shift compared with traditional rubidium clock,which is the main on-board atomic clock in China.In this paper,we have developed an automated control system for POP rubidium clocks,and achieved Ramsey fringes acquisition and servo-loop of POP rubidium clocks.Measured the relaxation time associated with rubidium clock transition levels,reducing the fitting error of ODSE method,and measured the relaxation time using microwave detection.New detection methods were explored,and the atomic clock signal-to-noise ratios of absorption,differential detection,and cross-polarization detection methods were experimentally compared.The details of the study are as follows.（1）Auto-test and servo-loop procedures for POP Rb atomic clocks are designed based on labwindows.（a）Using the data acquisition card to output the timing sequence for POP rubidium clock.the interval time between adjacent pulses was optimized within 5ns,such as the interval between the pumped light pulse and the first microwave pulse signal and the interval between the second microwave pulse and the detection pulse,reducing the overlap between the optical and microwave signals.（b）The auto-test program of the POP rubidium atomic clock synchronously controls the frequency and power of the microwave source,acquires the atomic clock data from the photodetector or microwave detector,realizes an efficient Ramsey fringes scan,calculates and outputs the feedback voltage of the servo-loop to lock the local crystal to atomic clock transition.The automatic signal acquisition program allows quick and easy adjustment of physical parameters（microwave power,and frequency,timing sequence etc.）,and measurement their influence on the atomic clock,thus improving the efficiency of the optimization.（2）Measurement of relaxation time.Several kinds of data fitting formulas for relaxation time measurements are established,including theoretical analysis.In particular,for the optical detection spin-echo（ODSE）method,more accurate data fitting formulas are obtained,reducing the experimental data fitting error,which is only3%much better than 10%for other optical methods.Experimental results show that this method can suppress the effect of magnetic field gradient,since the relaxation time measured by the photo detection methods includes all Zeeamn sub-levels,we measured the transverse and longitudinal relaxation times of atomic clock levles by the microwave detection method.The measurement results show that the error of the longitudinal relaxation time is only 4%and the error of the transverse relaxation time is 7%.（3）Three detection methods of POP rubidium atomic clocks are compared:absorption method,differential detection method,and cross-polarization detection method.In the absorption detection method,various physical parameters of the whole system are optimized.In the differential detection method,the relationship between the rotation angle of one-half waveplate before the physical package and the signal-to-noise ratio of Ramsey’s central fringe is measured,and the results show that the signal-to-noise ratio is maximum at 27.5 degree The signal-to-noise ratio of the differential detection method and the cross-polarization detection method were experimentally compared,which was 2.5 times higher.（4）A new TE_01U microwave cavity for rubidium atomic clocks was designed and simulated by HFSS software.The axial and radial magnetic field distributions of the new TE_01U microwave cavity and the conventional TE₀₁₁ microwave cavity were compared,showing that it has the advantages of high uniformity.（5）From the point of Ramsey fringe amplitude and the temperature frequency shift,the optimized operating temperature of the physical package for short-and long-term stability was measured.The results show that the amplitude of the Ramsey central fringe was the largest and the temperature frequency shift was the smallest at temperature of65°C,with a pressure ratio of 1:1.5 argon to nitrogen.