Research On Laser Frequency Control Of Cold Atom Interferometry Gravimeter

The cold atom interferometric gravimeter based on the atom cooling and Raman transition theory is a high-precision gravity measurement instrument and has been widely used in the fields of geological exploration,autonomous navigation,geophysics,etc.Atom-light interaction is the core of the atom gravimetry.During the atom cooling and interference process,the laser system is the"hand"for manipulating the atoms and the"eye"for observing the atoms.The spatial position and internal state of the atoms can be tailored by the laser beams with modulations on the frequency,which affects the gravimetric measurement performance.Therefore,the frequency control of the laser system is important and inevitable for an atom gravimeter.Based on the atom-light interaction mechanism in the cold atom interferometric gravimeter,this thesis studied the principle and method for laser frequency control,and realized the manipulation of atoms.Specific research contents include:1.Exploration of the measurement process of the cold atom interferometric gravimeter and analysis of the frequency characteristics of the laser in different stages.According to the laser frequency control technology and the application requirements for a portable,integrated gravimeter,an integrated laser system concept based on optical fiber devices was proposed.The technology and principle of each module in the conception of the laser system are researched and the theoretical and technical support for the development of the laser system are provided.2.Frequency stabilization of 1560 nm laser based on the modulation transfer spectroscopy.The 1560 nm laser was frequency-doubled to 780 nm and was stabilized to the D2 line transition of rubidium atoms(⁸⁷Rb).This frequency was set as the reference frequency of the whole laser system,which also functioned as the blow-away beam.In order to improve the efficiency of frequency stabilization,an automatic frequency stabilization algorithm based on the program-controlled servo circuit was proposed,which reduced the stabilization time to one tenth of the original.The peak voltage and signal-to-noise ratio of the frequency discrimination signal was optimized,leading to an improved closed-loop relative frequency stability of1.4×10^-11 at an integration time of 10 s.3.Exploration of the offset frequency locking method based on the optical phase-locked loop（OPLL）.The OPLL-based feed-forward mechanism effectively shortened the locking time in offset frequency locking between the target and the reference lasers and enabled the frequency offset jumping between different frequencies.The jumping time was reduced from tens of milliseconds to sub-milliseconds,where 0.62 ms jumping time is achieved at a frequency jump of 1 GHz.This frequency jumping speed was better than that of the commercial D2-135 phase-locked circuit box.Finally,the cooling beam and the probe beam were prepared using this method.4.Coherent light preparation based on the electro-optical modulation.The fiber Bragg grating was used to filter out the unnecessary sidebands of the electro-optical modulator,and the power of the second-order sideband was suppressed by 28 d B.The corresponding modulation frequency to prepare the Raman beams and Repump beam are set using time division multiplexing on the EOM modulated light.5.Development of a fiber laser system with optimized system parameters and reinforced long-term frequency stability.The fractional frequency stability of the master laser reached10^-9 level in a time duration of 8 hours while that of the beat note between the master and the slave lasers reached10^-13 level.The cooling beam from the laser system was injected into the vacuum chamber and the trapped cold atomic cloud was observed.Compared with a commercial ILS780-181 laser system from Muquans company,the frequency jumping time of the developed laser system was shorter and the full width at half maximum of the beat note signal was slightly narrower.