Abstract
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.