| Summary: | 碩士 === 逢甲大學 === 光電科學與工程學系 === 107 === In this paper, demonstrated a method for measure the acetylene absorption photothermal spectrum and frequency stabilization using a quartz tuning fork (QTF).
The QTF playing the role of accurately measure ~32 kHz (3f) signal. And the laser has modulated with 1f signal (~10.9 kHz), and incident to the acetylene cell will generate the absorption signal (2f). Then the 2f signal can make the QTF resonant by harmonic to sense acetylene absorption photothermal spectrum （P(9)）. The sensor based on QTF has high Q value (~10,000 at normal pressure), it’s full width half maximum of responsivity only 2.6 Hz. So, it is conveniently removing noise and improving the SNR.
After stabilizing the frequency of DFB-LD, can detect the acetylene content using the beam with modulation signal. And the absorption signal be analyzed use fast fourier transform and observe the linear relation with 2f signal and pressure of acetylene. It can find that the 2f signal decreased with pressure. But the 2f signal gradually flattens under high pressure cause by pressure broadening.
Also measure the relation with 2f signal and concentration of acetylene by mixing nitrogen and acetylene. And measure the limit of concentration is 0.0756 ppm under 55 Torr work pressure. Unexpectedly, the 2f signal and concentration should be linearly related, but the result is exponentially related. This problem is presumably because the design flaws in the gas path are not effective in allowing the gas to mix, causing the acetylene to not be diluted to the desired concentration. Finally, the relation between high concentration and 2f signal is experiment with a method of single dilution. The result is linear correlation, and the result and signal-to-noise ratio (SNR) are used to estimate the NEC=3.29×10^(-6) and NNEA=6.02×10^(-11) cm^(-1) W√Hz.
Key words：Quartz tuning forks, Acetylene absorption spectrum, Photothermal effect, Gas sensing, Stabilized frequency of laser, Third-order derivative spectrum
|
|---|
