Research On The Effect Of Raman Sideband In The Atomic Gravimeter

Precision gravity measurement has important applications in resource exploration,metrology,and fundamental physic researches.Atom interferometer（AI）,an instrument which could precisely measure the absolute value of gravity,has received extensive attention around the world due to its high potential sensitivity.Coherent manipulation of atomic wavepacket is the key point in obtaining the interference fringe in AI,and the Raman beam is the essential tool in atomic wavepacket manipulation.This means that the characteristics of Raman beam will greatly influence the precision of gravity measurement.In the process of preparing a Raman beam,high order sidebands except for the two required laser frequency componets will be generated.Since the sidebands will bring an extra systematic error to AI,it’s necessary to study the influence of Raman sidebands in AI.The reason of Raman sideband generation and its influence to AI are theoretical studied in this work.Research indicates that when the seed laser with two different frequencies was injected into a tapered amplifier（TA）,the relation between the gain of TA and the power of seed laser is an important reason in sideband generation.Numerical analysis shows that the high order sideband ratio will decrease if reducing the injection power of TA.In addition,the zero phase shift point could be found by modulating the sideband ratio and evolution time（T）of the atom interferometer through our theoretical analysis.Then,experiments are performed to study the additional sideband effect caused by the electro-optic modulator and the nonlinearity of TA in our atom gravimeter.In phase modulated Raman beam,the fringe contrast will fluctuate with T with an amplitude of 3%due to the additional sidebands,and the induced phase shift will be as large as 250 mrad（corresponding to 63 μGal systematic error）.When two different frequency laser components were injected into one TA,the amplitude of fringe constrast variation is 2%,and this variation could be reduced to 1% if reducing injection power of TA.At the same time,the induced phase shift reaches 30 mrad,corresponding to a systematic error of 8 μGal.Research suggest that additional sidebands could be avoided if two TAs are used to amplify two components of Raman laser independently,but this setup also increase the complexity of the optical system.The phase modulated Raman beam exists a remarkable additional sidebands effect,and this effect needs to be evaluated periodically.However,due to the simplicity of the optical system and robustness of laser frequency locking,this optical system is still suitable for the atom gravimeter which has the capability of long-term gravity measurements.Therefore,we demonstrated an atom gravimeter which uses a composed Raman beams.This setup could be used to envaluate the additional sidebands effect by switching the phase modulated Raman beam and phase locked Raman beam（two TA without sideband）.After envaluating the systematic errors induced by two Raman beams,we found that the results of gravity measurement obtained by two kinds of Raman beam are consistent with each other.Based on the researches above,the atom gravimeter was then further imporved.By using a low phase noise reference,improving the sampling rate of the instrument,employing a fringe lock strategy,imporving the stability of light system,and etc.,the sensitivity of the gravimeter reaches 20 μGal / （Hz）^1/2.The uncertainty of systematic error is smaller than 2μGal,and the long term stability of instrument reaches 1 μGal,which meet the requirement of national precise gravity measurement facility.