高光學密度的冷原子雲製備

Production of a high-optical-density cold atom cloud

洪舜昶(Shuen-Chang Hung)

指導教授 ： 余怡德

國立清華大學/理學院/物理學系/碩士(2015年)

https://doi.org/10.6843/NTHU.2015.00245

摘要

摘要
本論文為「高光學密度冷原子雲的備製」的架設。在許多實驗中，Optical depth ( OD )是一個重要的參數，其定義為： OD = nσL，n為原子密度、σ為吸收截面積(cross section)、L為原子團長度。在電磁波引發透明效應(Elctromagnetically Induced Transparency)中的慢光效應(Slow light)，需要夠大的OD來增加延遲時間(delay time)：是一種訊號延遲的概念，即為訊號穿過原子團所需要的時間。在量子記憶體中(Light storage)延遲時間越長，原子可儲存的資訊長度(Pulse width)越長。
與以往實驗室冷原子雲的備製不同的是，陷阱雷射(Trapping Laser)的光斑大小、功率高了許多。功率需要330 mW以上、光斑大約為30 mm，這使得在架設上我們需要提高在同樣injection power下雷射功率放大器(MOPA)的出光功率，以及提高雷射進光纖的耦合率；而相對於小的光斑，大的光斑需要架設的面積比較大，意謂著一對反向對打的陷阱雷射光會出現中心光強度不同、光強度最強處的偏移，導致冷原子雲的不對稱。
在解決原子團不對稱的問題後，我們以二能階吸收的方式來評估冷原子團的光學密度，在沒有任何優化的情況下(concentrate population, dark MOT, compress MOT and so on)OD約為100。但由於二能階吸收只能評估OD，我們必須在高光學密度的情況下優化完EIT系統後，才能進一步做增加光學密度的優化，而這些優化將會使的光學密度成倍率增加。

關鍵字

高光學密度 ； 銣原子 ； 冷原子雲

並列摘要

Abstract
This thesis is about “Production of a high-optical-density cold atom cloud”. In the experiment, Optical depth (OD) is an important parameter. The definition of OD is : OD = nσL, which n is density of atoms, σ is cross section and L is the length of media. The Slow effect in Elctromagnetically Induced Transparency needs larger optical depth to increase delay time which means how long the signal could pass through. In Quantum memory, the longer light storage have, the longer pulse width that atoms could save.
In our lab, production of cold atoms cloud is much different from the past in this time. The major difference is trapping laser we used have much larger beam size and much larger power. In the past, the power was near to 100 mW, but in this time, the power we used is near to 330 mW, and beam size’s diameter is 30 mm. Because of higher power ,we need to optimize the efficiency of MOPA which must have higher output power at lower injection power and the efficiency of coupling in bare fiber. Due to large beam size, the MOT system we erected may occur some problem. It may cause our cold atoms cloud becomes asymmetric.
After solve problems about the shape of cloud, we use two-level system to estimate optical depth, and OD is near to 100 without any optimization(concentrate population, dark MOT, compress MOT and so on). The optimization need to be done after we ensure we can see EIT at high OD(~100). If we do so, OD may increase many times.

並列關鍵字

High OD ； Rb atom ； Cold atom cloud

參考文獻

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