A Novel QEPAS with Microresonator in the Open Environment

Abstract

An improved quartz-enhanced photoacoustic spectroscopy (QEPAS) sensing system for trace gas detection is proposed. The optical fiber Fabry–Perot (F–P) demodulation method is used to replace the conventional electrical one in the QEPAS system. The experimental QEPAS system, which has a microresonator consisting of two stainless steel tubes with a length of 2.3 mm and an inner diameter of 0.9 mm, is implemented to detect the absorption of water vapor in the open environment. The structure parameters of the quartz tuning fork (QTF) are optimized in order to make the sensing system work more stably and reliably. Demonstration experiments are carried out. The vibration signal of the QTF was picked up by the optical fiber F–P demodulator and the conventional electrical scheme at the same time. Normalized noise equivalent absorption coefficients of $4.05\times 10^{-7}\,\text{ cm}^{-1}~{\cdot }~\mathrm{W}~{\cdot }~{\text{ Hz}}^{-1/2}$ and $2.40\times 10^{-6}\,\text{ cm}^{-1}~{\cdot }~\mathrm{W}~{\cdot }~{\text{ Hz}}^{-1/2 }$ are obtained, respectively. The experimental result demonstrates that the sensitivity of the improved QEPAS sensing system with an optical fiber F–P demodulator is about 5.9 times higher than that of the conventional QEPAS system.

Appendix: Equations

The relationship between the vibration amplitude of QTF and the light intensity detected by F–P can be expressed as

$$\begin{aligned} \Delta L(t)=L-L(t)=L-\lambda \frac{\arccos \left({\frac{({1+R_1 R_2})I(t)-R_1 -R_2}{2\sqrt{R_1 R_2}({I(t)-1})}}\right)}{4\pi }, \end{aligned}$$

(1)

where L is the initial cavity length and $R_{1}, R_{2}$ are the reflectivities.

The maximum piezoelectric voltage $V(t)$ generated by the deflection of the QTF is

$$\begin{aligned} V_0(t)=R_{0}{I}_{0}(t)=\frac{R_{0}\beta \lambda ({R_1 +R_2 -R_1 R_2 -1}){I}^{\prime }({t_0})}{4\pi \sqrt{R_1 R_2}({I({t_0})-1})^{2}\sqrt{1-\left({\frac{({1+R_1 R_2})I({t_0})-R_1 -R_2}{2\sqrt{R_1 R_2}({I({t_0})-1})}}\right)^{2}}}, \end{aligned}$$

(2)

where $\beta$ is the effective piezoelectric coupling coefficient, $R_{0}$ is the resistance of the QTF, and $I^{\prime }(t_{0})$ is the differential of $I(t)$ at $t_{0}.$

The photoacoustic signal intensity to the optical absorption in QEPAS can be expressed by the following equation:

$$\begin{aligned} S=\kappa \alpha CP\xi , \end{aligned}$$

(3)

where $\kappa$ is a constant describing system parameters, $\alpha$ is the absorption coefficient per unit concentration of the target trace gas species, C is the concentration of the target trace gas species, P is the excitation laser power delivered to the QTF, and $\xi$ is the conversion efficiency from light to the acoustic signal.

$$\begin{aligned} \text{ NNEA}={\alpha _{\min }P}/{\sqrt{B}}, \end{aligned}$$

(4)

where $\alpha _{\min }$ is the minimum detectable absorption coefficient at SNR = 1, $P$ is the optical power, B is the measurement bandwidth, and

$$\begin{aligned} \alpha _{\min } =\rho _x NSg(\nu ) \end{aligned}$$

(5)

$$\begin{aligned} N = \frac{p}{RT}N_{\mathrm{a}}, \end{aligned}$$

(6)

where $\rho _{x}$ is the detectivity of gas x of interest, S is the line intensity, $g(\nu )$ is the line shape function, $N_\mathrm{a}$ is the Avogadro constant, p is the gas pressure, T is the absolute temperature, and R is the molar gas constant, $8.314\,\text{ J}~{\cdot }~\text{ mol}^{-1}~{\cdot }~\text{ K}^{-1}.$