Research On Key Technologies Of Optical System Of Atomic Interference Gravimeter

High precision gravimetry has important scientific significance and application value in the determination of basic physical constants,inertial navigation,resource exploration and geodesy.With the development and maturity of laser cooling and atomic manipulation technology,a new kind of gravimeter,atomic interference gravimeter,has been born.Because of its potential high precision and high sensitivity in gravimetry,the atomic interference gravimeter has been widely concerned and studied at home and abroad.The realization of the atomic interferometer includes atomic cooling and trapping,initial state preparation,atomic interference and other stages,each stage requires the participation of laser.Therefore,the laser optical system is the key subsystem of the atomic interferometer.The interference of atoms needs a pair of Raman lasers with fixed frequency difference and constant phase.The quality of Raman laser will directly affect the accuracy of measurement.According to the current research progress of the research group,the key technologies of the optical system of the atomic interference gravimeter were studied in this paper.The primary premise of atomic interference gravimeter was to prepare large flux cold atomic source at low temperature.Atomic cooling required multiple beams of cooling laser with stable frequency（four beams in a two-dimensional magneto-optical trap and six beams in a three-dimensional magneto-optical trap）and repumping laser,and at the same time the detuning amount of the cooling light（frequency shift of an acousto-optic modulator for experiments）needed to be controlled.A beam expanding collimator for magneto-optical trap was designed and developed,which could realize the beam expanding collimation of the cooling laser of polarization maintaining fiber and accurately adjusted and maintained its polarization state.A good cold atom cluster has been obtained in the magneto-optical trap.Raman laser system was the key optical system to realize cold atom interference,which required a pair of Raman laser with fixed frequency difference and phase coherence.Based on the analysis of various Raman laser preparation schemes,it was decided to adopt the optical phase-locked loop method.The feasibility of frequency modulation and phase lock of the external cavity semiconductor laser in our experiment was analyzed.The frequency stabilization techniques of saturated absorption spectrum,modulation transfer spectrum and frequency modulation spectrum used in the experiment were studied.After analysis and comparison,the modulation transfer frequency stabilization was used to lock the reference Raman laser frequency on the hyperfine energy level of D₂ line of ⁸⁷Rb atom.On the premise of frequency stabilization,the experiment completed the experimental construction of Raman optical system by using two master-slave lasers and a tapered laser amplifier.A pair of Raman laser with a frequency difference of 6.8GHz was prepared,and the power of the two coupled Raman laser was amplified to 200m W by the tapered amplifier.The Raman laser measured by real-time spectrum analyzer was about27.5khz in-3d B bandwidth,which met the experimental requirements of atomic interference.