共振腔振盪衰減是一種測量弱吸收光譜極敏感、準確的方法。藉由測量通過充滿氣體之高精微值腔體雷射光的的振盪衰減時間，我們可以得到待測氣體的吸收係數。我們利用這種方量測二氧化碳在1064 nm附近的吸收光譜。
在這篇論文裡，我們使用兩種方法擬合以求得振盪衰減時間。第一種方法是以理論振盪衰減曲線公式擬合，得到的c2值約為10-9。第二種方法是使用指數衰減公式擬合，得到的c2值約為10-8。第一種方法得到的振盪衰減時間比第二種方法約大了15%。造成此種差異的原因目前並不清楚。利用振盪衰減公式擬合的結果求得吸收中心為9394.00513 cm-1，不準度為2.4’10-4 cm-1。

我們也研究了用理論的共振腔振盪衰減曲線作為實驗數據來作指數衰減公式擬合的有效條件。藉由捨去某個「切斷時間」之前的數據，我們可以使用指數衰減公式擬合來得到正確的振盪衰減時間。

未來我們將把更換腔體的反射鏡為更高反射率的以增加精微度，並利用新的腔體測量氘氫分子在1 mm附近的吸收光譜。

Cavity ringdown spectroscopy (CRDS) is a sensitive, accurate method to acquire weak optical absorption spectra. By measuring the ringdown time of light in a high finesse cavity filled with gas we can get the absorption coefficient of the gas. We use this method to measure the weak absorption spectrum of CO2 near 1064 nm.
In this thesis, we use two methods to fit the experimental ringdown curve to obtain the ringdown time. The first method is fitting the experimental ringdown curve with the theoretical ringdown curve formula (RCF), and the reduced c2 we get is about 10-9. The second method is with the exponential decay formula (EDF), and the reduced c2 we get is about 10-8.The ringdown time obtained by the first method is about 15% larger than the second method. The reason for this difference is not clear now. Using the fitting results of RCF the absorption center of CO2 (2003←0000) R(6) transition we obtained is 9394.00513 cm-1 with an uncertainty of 2.4’10-4 cm-1.

We also study the valid condition of the EDF fitting using the theoretical cavity ringdown curve as the experimental data. By neglecting the data before a "cut time", we can obtain the correct ringdown time using the EDF fitting.

In the future, we will replace the reflective mirrors of the cavity to higher reflection ones to increase the finesse, and use the new cavity to measure the overtone spectrum of HD near 1 mm.

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