高解析中紅外一氧化二氮光譜可行性研究

Feasibility study on high resolution mid-IR spectroscopy of nitrous oxide

張俊宏(Chin-Hung Chang)

指導教授 ： 施宙聰

國立清華大學/理學院/物理學系/碩士(2012年)

https://doi.org/10.6843/NTHU.2012.00391

摘要

本論文主要是利用鈦藍寶石雷射 (Ti:Sapphire laser) 與Nd:YAG雷射透過中紅外PPLN （Periodically poled lithium niobate) 產生的中紅外差頻光源 (DFG)，嘗試利用已知的技術將一氧化二氮 (N2O) 的基頻帶 (0001←0000)中的R(15) 線寬縮減至200 kHz左右，希望可以解析基頻帶中的R(1)的電四極超精細結構 (Electric quadrupole hyperfine structure)，並重新計算電四極耦合常數 (Electric quadrupole coupling constants)。

根據線寬增寬的原理，N2O分子的線寬受到都普勒線寬增寬、碰撞增寬、穿越時間增寬和功率增寬的影響而增寬。為了將N2O分子的線寬縮減至200 kHz以下，我們利用飽和吸收光譜法，並取三階微分飽和訊號，消除都普勒線寬增寬的影響。利用渦輪幫浦 (Turbo pump) 將N2O氣室的壓力降至6 ~ 8 mtorr，降低碰撞增寬的影響。利用擴束鏡組增大DFG的束徑，降低穿越時間增寬的效應。在本實驗中，我們嘗試利用三種擴束鏡組，將DFG束徑增至0.37 mm、2 mm和4.17 mm，並在每一種束徑下，量測線寬與壓力變化的關係。線寬可以利用偵測飽和訊號三階微分的大小隨調制寬度的改變，並透過擬合的方式決定。根據我們的實驗結果，線寬與壓力和束徑的關係符合預期：當束徑越大的時候，線寬就越窄 ; 當壓力越小的時候，線寬也越窄。

考慮基頻帶R(15)的超精細結構170 kHz，加上DFG雷射線寬為300 kHz，此為R(15)的線寬解析極限。然而，我們量到最窄的線寬為625 kHz，顯示線寬可以再進一步窄化。未來可以嘗試利用擴束比例較大的擴束鏡組，將線寬再進一步窄化。

關鍵字

差頻光 ； 中紅外 ； 飽和吸收光譜 ； 線寬縮減 ； 一氧化二氮

並列摘要

In this thesis, we try to narrow the linewidth of N2O 0001←0000 fundamental band R(15) transition down to 200 kHz by a CW mid-infrared MgO:PPLN-based difference frequency generation (DFG) source pumped by a Ti:Sapphire laser and power boosted Nd:YAG laser. We expect that the electric quadrupole hyperfine structure of N2O 0001←0000 fundamental band R(1) transition can be resolved and could calculate the electric quadrupole constant.

According to the mechanism of line broadening, the linewidth of N2O molecular is broadened affected by the Doppler line broadening, collisional line broadening, transit-time broadening and power broadening. Hence, in order to narrow the linewidth down to 200 kHz, we utilize the saturation spectroscopy to remove the Doppler broadening and produce the 3rd derivative of Lamb-dip signal. In order to eliminate the pressure broadening, we utilize a turbo pump to lower the pressure of N2O gas cell to near 6 ~ 8 mtorr. We use beam expander to expand the DFG to reduce the transit-time broadening. In this experiment, we use three kinds of beam expanders to expand the DFG with spot size 0.37 mm, 2.0 mm and 4.17 mm, and we measure the dependence between linewidth and pressure in each spot size. The linewidth can be determined by fitting the 3rd derivative saturation signal in response to the different modulation width. According to the result of experiment, the relation between linewidth versus pressure and spot size are the same as expectation respectively: the lower the pressure is, the narrower the linewidth is; the larger the spot size is, the narrower the linewidth is.

Adding the span of the hyperfine structure of fundamental band R(15) 170 kHz and the DFG linewidth is near 300 kHz, this is the limitation of linewidth resolving on R(15). Because the narrowest linewidth we measured is 625 kHz, this means the linewidth can be narrowed further more. We can apply another beam expander with larger beam expanding ratio to investigate the linewidth in the future.

並列關鍵字

difference frequency generation ； DFG ； mid-infrared ； Saturation spectroscopy ； linewidth reducing ； N2O ； nitrous oxide

參考文獻

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