Study Of A Transportable,High-precision Absolute Gravimeter Based On ⁸⁵Rb Cold Atom Interferometry

Measuring the value of gravitational acceleration g is of wide interest in metrology,geoscience and navigation.Compared with the classical gravimeter which is based on the laser interferometry of a free-falling corner-cube,the cold atom gravimeter has the advantages of high sensitivity,self-calibration,continuous observation,infinite mechanical stiffness and low maintenance cost.However,the experimental setup and operation of the cold atom gravimeter are usually very complicated,which does not satisfy field applications where accurate and integrated instruments are required.In view of this,this thesis mainly studies the transportable,high precision cold atom absolute gravimeter named WAG-H5-1which is based on ⁸⁵Rb atom interferometry.We first trap the atoms via the magneto-optical trap,and then measure the gravity acceleration by the interferometer sequence of polarization gradient cooling,lauching,state selection,Raman interferometry and detection.The sensitivity of gravity measurement is typically 0.16?Gal/mrad?1?Gal=10 nm/s²?.The major results are summarized as follow:?1?In order to realize the portability of the system,we make great efforts to integrate the gravimeter.We build a compact titanium system with ultra-high vacuum,a miniaturized optical system and an integrated electronics system.The gravimeter has a total volume of1.8m³ and can be transported separately in four packages.After a long period of transportation test with high vibrational noise,the system can still work normally and stably.In terms of precision,we conduct the high-performance active vibration isolation,magnetic shielding,normalized detection and other key technologies to enhance the gravity measurement stability to 0.7?Gal for the integration time of 2000 s.The long-term measurement sensitivity of the gravimeter reaches 28?Gal/(Hz^1/2).?2?To improve the accuracy of absolute measurement of g,we carefully evaluate the systematic errors in the system.On the one hand,we use the method of reversing the effective wave vector of Raman beams to eliminate the systematic errors which are independent of the effective wave vector direction,such as the second-order Zeeman shift.On the other hand,for the systematic errors associated with the effective wave vector direction,we use modulation,compensation or theoretical simulation to carry out an all-round evaluation.These effects include the tip/tilt of the Raman beams,Coriolis effect,self-gravitation effect,etc.In the process,we find two new systematic errors,which are ac Stark shift unbalance and Raman laser chirp effect.We figure out their generation mechanism and evaluate the systematic biases caused by them.The total systematic error uncertainty of the gravimeter is evaluated to be 9?Gal.?3?To further assess the stability and accuracy of its gravity measurements,we perform three comparisons of absolute gravity on the gravimeter.The report given by the International Comparison of Absolute Gravimeters in 2017?ICAG-2017?determines that the degree of equivalence of WAG-H5-1 is-3.8?Gal with the extended uncertainty of 20.4?Gal at 95%confidence level.As the first transportable cold atom absolute gravimeter based on ⁸⁵Rb atom interferometry in the world,it not only increases the types of test mass but also the technical diversity of gravimeters taking part in the international comparisons.Thus,it may reveal some unknown biases of the international absolute gravity measurement reference network nowadays.By simultaneous comparison with FG5X,we make sure that WAG-H5-1 has advantages in gravity measurement accuracy,continuity,sampling rate and stability.This comparison also sheds light on the enormous potential of cold atom absolute gravimeters.