Objective Quartz-enhanced photoacoustic spectroscopy (QEPAS), as a relatively mature trace gas detection technology, has promising application prospects. QEPAS has several advantages such as high-quality factor, narrow response bandwidth, strong anti-interference, the small size of detection module, and low cost. However, because the output power of the laser excitation source limits the QEPAS system’s performance, passing the laser beam through the acoustic detection module multiple times can effectively improve the optical power of the gas absorption. Conventional double-pass QEPAS systems, in which the laser beam passes through the acoustic detection module twice, are not visible for photoacoustic signals enhancement. This paper proposes an off-beam QEPAS system based on a lens-reflector combination (LR-QEPAS) in which the laser beam passes through the acoustic detection module four times to achieve a stronger photoacoustic signal. The lens-reflector combination structure comprising a convex lens, concave reflector, and plane reflector is applied to improve the detection limit of the QEPAS system. We hope that the results of this study can be applied to different types of QEPAS systems, allowing us to further improve the detection limit of trace gases.
Methods A fiber-coupled near-infrared distributed feedback laser (NLK1E5EAAA, NEL) was used as a laser source; the current and temperature of the laser were controlled by a commercial diode laser controller (ILX Lightwave LDC-3724C). The current of the laser controller was tuned to achieve coarse and fine-tuning of the laser wavelength, respectively. The second harmonic detection was used to enhance the sensitivity of the off-beam LR-QEPAS. A sine wave at half of the quartz tuning fork (QTF) resonant frequency provided by the function generator was used to modulate the laser source and input the laser controller. The reference sine wave at half of the QTF resonant frequency was an input into lock-in amplifier. The piezoelectric signal generated by the QTF was amplified by a low noise trans-impedance amplifier and converted into a voltage signal. A lock-in amplifier (Stanford Research SR850) at the QTF resonant frequency demodulated the amplified signal. The acoustic detection module of the off-beam LR-QEPAS comprises a plane reflector, concave reflector, convex lens, and microresonator. An infrared sensor plate (HCP-IR-1201) and laser beam analyzer S-WCD-QD-1550 (Dataray Inc.) were used to fine-tune the optical path.
Results and Discussions The photoacoustic signal of the off-beam LR-QEPAS system increases nonlinearly with the increases of times that the laser beam passes through the miroresonator (Fig. 4) for the laser beam is scattered in the air. The modulation depth of the off-beam LR-QEPAS is compared with the optimized off-beam QEPAS, P-QEPAS, and C-QEPAS systems, and the results show that the systems have the optimal modulation depth of 0.34 cm -1 (Fig. 5). The measurement of the second harmonic signal shows that the off-beam LR-QEPAS system has the highest signal amplitude because the lens-reflector combination effectively increases the power that the laser passes through the miroresonator. The normalized noise equivalent absorption coefficient is 3.46×10 -9 cm -1 ·W·Hz -1/2 (Fig. 6). The Allan variance analysis results show that the lowest detection limit of the off-beam LR-QEPAS system is 7.54×10 -8 with an integration time of 64 s (Fig. 7). The application of the lens-reflector combined structure greatly improves the detection limit.
Conclusions The off-beam LR-QEPAS system based on a lens-reflector combination was proposed. The lens-reflector combination structures combined with the second harmonic detection technique were used to measure the water vapor with volume fraction of 1.2% at 1392.58 nm for evaluating the system performance. In the experiment, the off-beam QEPAS and two dual-pass QEPAS systems were used as a reference to determine the optimal modulation depth of the system and verify that the detection sensitivity of the system increases as the number of times the laser beam passes through the miroresonator. The Allan variance analysis shows that the minimum detection limit of the off-beam LR-QEPAS system is 7.54×10 -8 with an integration time of 64 s, and the NNEA coefficient is 3.46×10 -9cm -1·W·Hz -1/2. The results show that the performance of the off-beam LR-QEPAS system is better than that of the off-beam QEPAS, P-QEPAS, and C-QEPAS, and it meets the accuracy requirements in atmospheric trace gases detection. The lens-reflector combined structure significantly improves the sensitivity of the system and can be used in combination with different types of microresonator structures to achieve higher detection sensitivity. The next step will focus on realizing the multipass of the laser beam in the miroresonator, which can be expected to achieve sub-ppbv and applied to the other trace gas detection.
