【作者】 张敬芳；

【导师】 林强； 王兆英； 徐云飞；

【作者基本信息】 浙江大学 ， 光学， 2013， 博士

【摘要】 激光冷却和俘获原子的研究中,一般认为激光是空间完全相干和时间完全相干的光源。本文的工作主要是探究空间部分相干激光与中性原子之间的相互作用,以及冷却激光的空间相干性对冷原子特性的影响。我们分别在理论和实验上研究了空间部分相干激光冷却铷原子的动力学过程。实验上,利用铌酸锂晶体对激光电光相位调制,改变调制电压和频率的大小,来控制调制后激光的空间相干度,并且通过杨氏双孔干涉实验来度量激光的空间相干度。另一方面,利用调制激光和另一参考激光拍频,得到的信号带宽反映调制激光的时间相干性。实验结果和理论分析都证明：电光相位调制可以非常有效地改变激光的空间相干性,但它对激光的时间相干性的影响很小,在很多情况下可忽略。晶体的电光相位调制越大,调制后激光的空间相干性越低。在三维磁光阱和偏振梯度冷却实验中,我们使用空间相干度不同的激光去冷却原子,然后测量冷原子团的特性。实验测量结果发现：冷原子团的原子数目和高斯型的密度分布不随冷却激光的空间相干度变化,但是原子团的温度随着冷却激光空间相干度的减小而升高。理论上,本论文分析了空间部分相干激光与原子的相互作用,定性解释了冷却激光的空间相干度对冷原子团特性造成的影响。限制冷原子数目和原子空间密度分布的主要因素是平均光子散射率。因为光子散射率的平均值与冷却激光的空间相干度无关,所以冷原子数和原子密度分布不随冷却激光的空间相干度变化。但是空间部分相干激光作用在原子上的散射力在时间空间上都存在明显的涨落,这种涨落会使冷原子的速率分布增宽,从而导致原子团温度随着冷却激光的空间相干度的减小而升高。更多还原

【Abstract】 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.更多还原