Preliminary Construction Of Electronic Control System For A Cesium Fountain Clock

The Cesium fountain clock is currently the device with the highest definition accuracy of seconds.It plays an important role in basic scientific fields such as testing the theory of relativity,absolute frequency measurement,and application fields such as international atomic timekeeping,navigation and positioning.A cesium fountain clock is mainly composed of four parts:a vacuum physical system,an optical system,a microwave source,and an electronic control system.The electronic control system provides functions such as timing control and parameter acquisition and monitoring for the operation of the entire system.This thesis focuses on the construction of the electronic control system of the cesium fountain clock,and describes the author’s main work during the postgraduate period,including timing control,RF driver of acousto-optic modulator,magnetic field control,microwave power control and the development of cavity temperature control,etc.The timing control module provides time-sequence control for the entire cesium fountain clock system.The developed time-sequence control module is based on two analog output boards and the software is based on the Lab VIEW program.It can output multiple channels of specific analog voltage timing in parallel,which makes it possible to provides triggers for a variety of subsquent modules,such as acousto-optic modulator（AOM）driver,magnetic field control,fluorescence acquisition,and etc.Among them,the time synchronization accuracy between channels is better than 1μs,which satisfies the requirements of the cesium fountain clock on time-sequence control accuracy.The RF driver drives the AOM to provide acousto-optic frequency shift operation for the optical system of the cesium fountain clock.Based on the design scheme of"Lab VIEW+FPGA+DDS+VCA",the RF driver of the AOM was developed,which has functions of frequency hopping,frequency sweep and power sweep,and enables precise control of frequency and power of the laser field.The measured frequency control accuracy of the RF driver is better than 0.1 Hz,the frequency hopping and sweep delays are both less than 400 ns,and the power control dynamic range is greater than 40 d B,which satisfies the requirements of the cesium atom cooling and trapping,launching,post-cooling and fluorescence detection operations on manipulation of optical fields.A coil-based magnetic field control scheme is used to provide a linear gradient magnetic field and a geomagnetic compensation magnetic field in the magneto-optical trap,and a quantization axis for the interaction of atoms and microwaves in the upper vacuum region.A pair of anti-Helmholtz coils were constructed to generate a linear gradient magnetic field.At a driving current of 1.8 A,the measured axial magnetic field gradient was 5.80（6）Gauss/cm;three pairs of orthogonal rectangular coils were constructed for geomagnetic compensation in the magneto-optical trap,the result of coarse adjustment is that the background magnetic field in the center of the MOT is suppressed by an order of magnitude.In the magnetic shielding cylinder in the upper vacuum area,based on the wounded C field coil and C field compensation coil,a uniform magnetic field with a length of 50 cm and a strength of 128.5 n T was generated,and the maximum static variation of the magnetic field was 1.4 n T.In the process of microwave state selection and Ramsey interaction,high-resolution control of the microwave power fed by the microwave source is required to ensure the control accuracy of the Rabi frequency.A scheme of"Lab VIEW+FPGA+digital attenuator"is adopted to control the microwave power,in which a 10-bit digital attenuator is utilized,and its minimum resolution is 1/16 d B.Synchronous switching of multiple sets of power values has been achieved,with a time delay no more than 2μs.The ambient temperature of the microwave cavity needs to be accurately controlled to ensure the stability of the microwave cavity coupling coefficient.Based on the high-precision PT100 and carbon fiber wire attached to the surface of the upper vacuum chamber as temperature probes and heating wires respectively,we have developed a temperature control system.In more than 100 days,the maximum jitter of single-point monitoring temperature is less than 15 m K.The S₁₁ parameter of the state selection cavity measured at the caesium atom transition frequency is better than-30 d B.The various modules of the developed electronic control system laid an important foundation for the construction and operation of the cesium fountain clock system.