Exploration And Study On New Schemes For Cold Cesium Microwave Frequency Standard

According to the principle of the atomic frequency standard,the frequency stability can be improved through two methods.One is extending the Ramsey evolution time TR.The other is developing continuous measurement system to eliminate the Dick effect.Focusing on the two methods,we carries out the exploration and study on new schemes for cold cesium microwave frequency standard in this thesis.The thesis first studies the scheme that extends the relaxation time of cold atoms by use of wall-coated cell.The Ramsey evolution time TR of the cold atom clock HORACE is about 35 ms,which is limited by the relaxation effect caused by the collisions between cold atoms and the bare glass cell wall.To suppress the relaxation effect,extend the Ramsey evolution time TR and further improve the frequency stability,the thesis proposes and studies the scheme that replaces the bare glass cell with the one coated by ?-alkene.The cesium atoms are cooled by means of isotropic light generated by an integrating sphere.The relaxation of the cold cesium atoms inside the wall-coated cell is studied theoretically and experimentally.The results show that the coating wall has the anti-relaxation property and the relaxation time of the thermal cesium atoms can reach 320 ms.The cold cesium atoms inside the cell,however,heated by various collisional effects,is only about100 ms.After 100 ms,the cesium atoms relaxes with a 320-ms time constant.Hence,the coating wall dose not extend the relaxation time markedly.With the temperature and number evolutions of the cold cesium atoms,the collision between cold cesium atoms and coating wall is analyzed in detail.The analysis result tells that there exists elastic reflection during the collisions.The reflectance is obtained roughly 0.58 and needs more precise measurements.Secondly,the thesis reports the development of a novel cold atom source,2D-HP MOT,based on a two-dimensional(2D)magneto-optical trap(MOT)with independent axial cooling and pushing.The axial cooling is accomplished by a pair of intense hollow laser beams,and the pushing is regulated by a weak and narrow Gaussian laser beam.Using the thin pushing beam,2D-HP MOT can effectively suppress the light shift of the clock transition caused by the leaking pushing light while maintaining an intense atomic flux.The dependences of the atomic flux on the pushing laser power and detuning are analyzed theoretically and studied experimentally.The experimental results are in good agreement with the theoretical model.With a 10-?W resonant pushing laser beam,the light shift of the clock transition caused by the leaking light is 20 times lower than that of the 2D+MOT.And the atomic flux reaches 2.64 × 1010atoms/s,higher than that of the2D+MOT.The axial velocity distribution of the cold cesium beam centers at 7 m/s with a spread of 3 m/s.And the radial divergence angle is less than 14 mrad.Based on the cold cesium beam,the construction of a continuous atomic frequency standard is carried out.By use of the cold cesium beam,the Ramsey signal can be measured continuously,so that the Dick effect is eliminated.In theory,this scheme can realize the 1/? atomic clock and help improve the frequency stability.With a 10-cm free-evolution length,the theoretical limit of the frequency stability is 3 × 10-14/??.In experiment,with a 3-cm free-evolution length,we obtained a Ramsey signal with central linewidth 160 Hz,contrast 26%,and SNR 40.This result lays the foundation for the systemic measurement of all kinds of frequency shift,and the closed-loop locking of the continuous cold cesium atomic frequency standard.