| The measurement of time is the most accurate one among the seven units of international units system.A precise time and frequency standard could help studying many fundamental physics researches,for example,measuring the variation of fine structure constant,searching dark matter,verifying Einstein's relativity.Furthermore,it has great value in geodesy and satellite navigation system.since 1967,the Cs microwave frequency standard redefined“second”.However,the atomic optical frequency standard also known as optical clocks are more stable and have better uncertainty compared with microwave frequency standard.And it maintains rapid development.The ytterbium atomic optical clock has great potential in many kinds of optical clocks.The state key laboratory of precision spectroscopy in ECNU had built two sets of functional ytterbium optical clock.An optical clock is a complex measuring system with high precision.Errors in every stages will affect the final performance.the control system of an optical clock mainly includes lasers control and feedback loop for clock laser.And they have been optimized through my work.first of all,I developed a high-speed digital PID?Proportion-Integration-Differentiation?controller based on FPGA?Field-Programmable Gate Array?with 1 MHz bandwidth.And it is employed in the fast control of laser frequency locking.The 649&770 nm pumping lights and 759 nm lattice lights are locked at the same F-P cavity.And an optical frequency comb is used to monitor the frequency drifting of the cavity.Then an active compensation system correct the drift.As a result,the uncertainty of the lattice light will be two orders of magnitude smaller at least.In the second part,the servo system for an optical clock feedback loop is analyzed.The error signal which reflect the frequency offset between clock laser and atomic reference is calculated from the frequency hopping detections results.the correction frequency which is given out by a prediction algorithm is sent to the RF driver of the AOM?Acousto-optical Modulators?which can compensate the drifting and fluctuation of the clock laser.After simulating the feedback loop on MATLAB,I found that the typical PID control rule cannot restrain the residual error because of the constant PID parameters and the changing noise characteristics of the clock laser.It will limit the long time instability and servo error uncertainty.In order to solve these problems,a neural network adaptive PID combined with feedforward system is employed.By Tracking the clock laser's drifting and self-tuning the PID parameters,the short-time and long-time instability are both optimized.This method have been used in two-peak locking and compare experiments.Finally,the instability of the synchronous comparison reaches5.0?10-17within5000 second averaging time.The last part is about the evaluation of the servo error.The correction and statistical uncertainty of the servo error are calculated through experimental data.The correction is3.01?10-19and the statistical uncertainty is 1.02?10-17.The residual error at different drifting rate can be simulated.Then the drifting rate of the clock laser and its uncertainty can be measured by several locking experiments.finally the systematic uncertainty of the servo error can be work out which is7.68?10-20. |
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