本論文中，我們利用鈦藍寶石雷射(Ti:Sapphire laser)與Nd:YAG雷射泵浦的中紅外PPLN (Periodically poled lithium niobate)差頻光源，測量N2O基頻帶(0001←0000, fundamental band)中R(10)譜線的飽和光譜，並以飛秒光頻梳(femtosecond optical frequency comb)測量其絕對頻率。
我們探討兩種穩住差頻光源頻率的方法，並做比較。方法一：Ti:S scan為將Nd:YAG雷射的倍頻光頻率穩在碘分子127I2 R(56) 32-0, a10標準譜線上，並將此頻率視為已知；藉由改變鈦藍寶石雷射頻率掃描差頻光源頻率，可以得到待測譜線的飽和吸收訊號，將此訊號當作誤差訊號送至鈦藍寶石雷射，使差頻光源頻率穩在譜線中心。而方法二：YAG scan為將鈦藍寶石雷射頻率穩至一熱絕緣良好的Fabry-Perot共振腔；相對於Ti:S scan，我們藉由改變Nd:YAG雷射頻率掃描差頻光源頻率。另外將誤差訊號送至Nd:YAG雷射，使差頻光源頻率穩在譜線中心。譜線的絕對頻率即兩台穩頻雷射的差值，可利用飛秒光頻梳系統決定，Ti:S scan假設Nd:YAG雷射頻率已知，因此僅測量鈦藍寶石雷射頻率；YAG scan，則需同時測量鈦藍寶石雷射與Nd:YAG雷射的頻率。
然而Ti:S scan中Nd:YAG雷射頻率實際上並非定值，在不同的實驗條件下會得到不同的頻率。經過實際測量與修正，兩種方法測量R(10)譜線的頻率僅差異約1 kHz。因為Nd:YAG雷射共振腔結構為monolithic non-planar ring oscillator，頻率穩定度較鈦藍寶石雷射佳，因此利用YAG scan方法所得到的譜線相較於Ti:S scan中有較佳的頻譜解析度。此外，在本實驗中，YAG scan的回授系統仍需加以改善以增進測量的精確度。

In this thesis, we observe the saturation spectrum of N2O 0001←0000 fundamental band R(10) transition by a CW mid-infrared MgO:PPLN-based difference frequency generation (DFG) source pumped by a Ti:Sapphire laser and a Nd:YAG laser. The absolute frequency of R(10) transition is also determined by the femtosecond optical frequency comb (OFC).
We present two methods to stabilize the frequency of DFG source. Method 1: Ti:S scan in which we stabilize the frequency of Nd:YAG laser on the a10 component, R(56) 32-0 transition iodine molecule 127I2, and the frequency of DFG source is scanned and locked by tuning the frequency of Ti:Sapphire laser. Method 2: YAG scan in which we stabilize the frequency of Ti: Sapphire laser using a thermally isolated Fabry-Perot cavity, and the frequency of the DFG source is scanned and locked by tuning the frequency of the Nd:YAG laser.
The absolute frequency of N2O transition is equal to the difference of two laser frequencies. In Ti:S scan method, we assume that the frequency of Nd:YAG laser is a known value, so we only measure the frequency of Ti:Sapphire laser by the OFC. And in YAG scan method, we have to measure the frequencies of Ti:Sapphire laser and Nd:YAG laser simultaneously. However, in Ti:S scan, the frequency of Nd:YAG laser isn’t a fixed value actually and it is different for different frequency locking condition.
After measuring the Nd:YAG laser, the frequency difference of the absolute frequency measurements using Ti:S scan and YAG scan is approximate 1 kHz. Due to the monolithic non-planar ring oscillator structure of Nd:YAG laser, its frequency stability is better than Ti:Sapphire laser. Our system has better resolution by adopting the YAG scan method. In addition, we have to improve the feedback system to increase the accuracy in measuring the absolute frequency.

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