Research On Long-optical-path Detection Technology Of Spin Exchange Relaxation Free Atomic Magnetometer Based On Herriott Multipass Cell

In recent years,the weak magnetic detection technology has developed very rapidly.The theoretical value of the sensitivity of spin exchange relaxation free（SERF）atomic magnetometers can be up to the order of atlas.The sensitivity of the all-optical alkali metal atomic magnetometer has surpassed the superconducting interference quantum magnetometer and becomes the most sensitive magnetic detection method in the world.As for SERF atomic magnetometer,the main factors which influence the noise sensitivity are the pumping rate,the number density of alkali metal atomic and so on.High pumping rate will make atomic polarization be in a saturated state and influence the sensitivity of atomic magnetometer.Alkali metal atomic density which is too high will limit the life of atomic polarization.It will also affect the performance of the SERF atomic magnetometer.Based on the analysis of the working principle of SERF atomic magnetometer and theoretical expression equations,we can find out that the optical signal which uses to measure the atomic spin polarization state is directly proportional to the factor which equals to the atomic number density multiplying by the optical path length of the detecting light in the gas chamber with alkali metal atoms.Therefore,this paper proposes that the quantum noise can be eliminated by increasing the path length of the detection light in the gas chamber.The technology of multipass cell is introduced into the SERF atomic magnetometer to increase the optical path length for detecting the interaction between detecting light and pumping light.The main contents of this paper are as follows:1.Based on the summary of the latest domestic and international development of SERF atomic magnetometer,the research on long-optical-path detection technology of SERF atomic magnetometer based on Herriott type multi-channel cell is proposed in this paper.This paper analyzes the detection principle of SERF atomic magnetometer in details,and analyzes the influence of optical path length on the detection sensitivity of SERF atomic magnetometer.2.We research three kinds of multipass cell techniques that can be used in atomic magnetometer and summarize the advantages and disadvantages of each multipass cell.The Herriott cell is selected as the part of multipass cell in this paper.We designed Herriott Cell to satisfy our reseach.We obtained the effect of the incident angle of incident light,the distance between two mirrors and the focal length of the two mirrors on the shape of the light spots in the Herriott Cell and the number of light beam reflection.We use Matlab software to simulate the model of Herriott multipass cell and obtain the simulation diagram of light spot distribution on two mirrors.3.We determine to put the Herriott cell outside the atomic gas structures that Herriott multipass cell technology can realize the complex optical path and long optical path.We design and develop more reflective cavity with none-magnetic detection system structure by using 3D design software.We complete the construction of SERF atomic magnetometer detection system based on Herriott multipass cell.4.We do experimental tests on long-optical-path SERF atomic magnetometer.A comparative experiment is conducted at multiple frequency points with the detection light single-pass SERF atomic magnetometer.We analyze their performance indexes.We get the sensitivity of the proposed SERF atomic magnetometer in this paper is not less than 34fT/Hz^1/2 at multiple frequency points.Compared with the SERF magnetometer which detection light is single-pass,its sensitivity is significantly improved,which proves that the proposed scheme is feasible.