本實驗使用波長為2μm的分佈反饋二極體雷射(DFB diode laser)，入射至充滿二氧化碳氣體的光聲氣室內，經由光聲效應的作用，將光吸收的變化以聲波的型式讀取出來。我們以石英音叉取代傳統的麥克風，當作接收訊號的換能器，可達到3.23×10-8 的靈敏度。本系統擁有體積小、簡單、堅固耐用…等優點，只要搭配合適的光源，就能充份地利用光聲氣室。這樣的架構將是可攜帶式氣體分析儀的基礎，可應用在各種領域上，例如: 環境上的即時監控、工業製程的控制和醫學上的診斷。

本實驗所利用的理論就是光聲效應: 當氣體分子吸收雷射光的能量後，被激發到較高的電子、振動或轉動的量子能階上，隨後會經由發射螢光或碰撞的方式，再衰減到較低的能階。若是經由碰撞的過程，會將能量轉換成平移的動能，而使得氣體溫度增加。以特定的頻率調制雷射光源，使得溫度會有周期性的改變，進而引起壓力周期性的變化，產生聲波訊號，再以音叉來偵測這個效應產生的聲波訊號。

實驗中意外發現光熱訊號的產生，此訊號不同於原本預期的光聲訊號，主要是雷射光打在音叉上所造成的結果。我們也將在最後討論二者之間的差異及優缺點，並檢討之後須改進的地方，以及談談未來的展望。

In this thesis, the DFB diode laser of 2μm sends into the photoacoustic cell which is full of carbon dioxide. Because of the photoacoustic effect, the variation of absorption is read in the form of acoustic wave. We use the quartz tuning fork instead of the traditional microphone as a transducer to accept the signal and the sensitivity of this system is 3.23×10-8 . The outstanding features of the PA cell, most importantly its small size, simplicity, and robustness, can be fully exploited when it is combined with a suitable laser source. The sensor architecture can be the basis for a portable gas analyzer, and apply in diverse areas such as environment real-time monitoring, industrial process control, and medical diagnostics.

The photoacoustic effect is a theory utilized in this experiment. A gaseous molecule that absorbs laser radiation is excited to a higher electronic, vibrational or rotational quantum state. Generally, depopulation of this quantum state to lower lying states occurs either via fluorescence or collisions, the latter giving rise to a temperature increase of the gas due to energy transfer to translation. By modulating the radiation source at an acoustic frequency, the temperature changes periodically, giving rise to a periodical pressure change which can be observed as an acoustic signal. In the gas phase, the effect can be detected with a sensitive tuning fork.

From our preliminary result, we infer that the observed signal is resulted from photothermal effect not the photoacoustic effect, because of the heating process on the surface of a tuning fork by the laser field. We will discuss further the discrepancies between the photoacoustic and photothermal signal in chapter 4. The further improvements and future works will be discussed in the last chapter.

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