本論文建立了一個Z掃描量測系統來探討銫原子的非線性特性(折射率與吸收係數與入射光強度有關)，藉由量測雷射光在與樣本作用後通過光圈的穿透率來觀察銫原子的非線性效應。
在二能階系統中，原子的非線性效應是與雷射頻率與原子的密度的有關，所以本實驗選用銫原子在 譜線中 三個能階的等效中心頻率作為基準，並調整雷射頻率在此躍遷頻率附近來觀察調變量對於實驗的結果變化。實驗中控制銫原子溫度在攝氏80度左右以增加原子密度，藉由聲光調變晶體讓雷射頻率控制在離躍遷頻率400MHz甚至更遠來降低都卜勒加寬的影響。
由實驗的數據顯示，在選擇雷射頻率對於原子能階為紅調變與藍調變時能分別觀察到原子的負透鏡效應與正透鏡效應，而其透鏡效應的大小也與調變量成負相關，但與雷射光強度則是正相關，至於非線性吸收對於Z掃描過程造成的影響亦於本論文中討論與分析，而程式模擬的數值可讓我們得到在溫度80 且入射光為+496MHz藍調變下的 。

In this thesis, a typical Z-Scan system was constructed to measure the self-lensing effect in Cs vapor. The refractive nonlinearity (self-focusing or self-defocusing) can be observed by detecting the transmittance of the laser power that goes through an aperture.
In a two-level system, the nonlinearity is related to the detuning and the vapor density of the Cs atoms. In this experiment, the temperature of the Cs cell is thermal stabilized at 80 and the frequency of external cavity diode laser was locked on the Cs hyperfine transition ( ). The frequency of the laser beam to the Cs cell can be tuned by Acousto-Optic Madulator to around 400MHz from the resonance.
The phenomenon of self-focusing effect was observed when the frequency of the laser light was blue-detuned from the levels, and the red-detuning light gave the result of self-defocusing effect. The strength of the nonlinearity is proportional to the intensity of the incident light. The value of the second order of index of refraction id derived from the observed data to be under the condition of Cs cell at 80 and laser frequency at +496MHz blue detuning from the Cs line.

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