Investigation On Atom Cooling By Partially Spatially Coherent Lasers

The laser beams used in atom cooling and trapping are generally regarded as fully coherent both temporally and spatially. The main aim of the thesis is to explore the interaction between the partially spatially coherent laser and neutral atoms and investigate the influence of the spatial coherence of the cooling lasers on the cold atoms. The atom cooling and trapping by partially spatially coherent lasers is investigated both experimentally and theoretically.The degree of the spatial coherence of the laser beams is controlled by changing the applied voltage and frequency of the electro-optic phase modulation of LiNbO,(LN) crystal. The Young’s double pinhole experiment is used to quantify the degree of the spatial coherence of the modulated laser beam. The temporal coherence of the laser can be reflected by the measurement of the beat note bandwidth between the modulated laser and another reference laser. Both the experimental results and theoretical analysis prove that the phase modulation can effectively degrade the spatial coherence of the laser beam, but has very weak influence on the temporal coherence of the laser. When a higher phase modulation is applied on the laser beam, the corresponding degree of spatial coherence can be lower.In both three dimensional magneto-optical trap (MOT) and polarization gradient cooling (PGC) experiments, the characteristics of the atoms cooled by lasers with various degrees of spatial coherence are measured. The experimental results indicate that the atom number and atomic Gaussian density distribution keep unchanged with various degrees of spatial coherence of the cooling lasers, while the measured temperature of the atomic cloud increases as the degree of the spatial coherence of the cooling lasers decreases.A theoretical analysis of the interaction between the partially spatially coherent lasers can be used to explain the influence of the spatial coherence of the cooling lasers on the cold atoms. The unchanged atom number and atomic Gaussian density distribution is mainly due to the average photon scattering rate independent on the spatial coherence of the laser beams. For the increase of temperature with decreasing degree of spatial coherence of the cooling lasers, the reason is that the scattering force of the partially spatially coherent lasers acting on the atoms fluctuates temporally and spatially, which broadens the speed distribution of the cold atoms.