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Abstract
A high-sensitive photoacoustic spectroscopy (PAS) sensor, which is based on a multi-pass-retro-reflection-enhanced differential Helmholtz photoacoustic cell (DHPAC) and a high power diode laser amplified by erbium-doped fiber amplifier (EDFA), is presented in this work for the first time. In order to improve the interaction length between the light and target gas, the incident light was reflected four times through a multi-pass-retro-reflection-cell constructed by two right-angle prisms. A 1.53 µm distributed feedback (DFB) diode laser was selected to excite photoacoustic signal. Moreover, its power was amplified by an EDFA to 1000 mW to improve the amplitude of photoacoustic signal. Acetylene (C2H2) was chosen as the target analysis to verify the reported sensor performance. Compared to double channel without multiple reflections, the 2f signal of double channel with four reflections was improved by 3.71 times. In addition, when the output optical power of EDFA was 1000 mW, the 2f signal has a 70.57-fold improvement compared with the multi-pass-retro-reflection-cell without EDFA. An Allan deviation analysis was carried out to evaluate the long-term stability of such PAS sensor. When the averaging time was 400 s, the minimum detection limit (MDL) of such PAS sensor was 14 ppb.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Trace gas detection plays a significant role in many areas of fundamental and applied research, such as biological applications, industrial process control and environmental monitoring [1–7]. The rapid advancement of laser technology has led to significant progress in the field of trace gas detection [8–13]. Photoacoustic spectroscopy (PAS) is a form of indirect absorption spectroscopy that relies on the photoacoustic effect. This technique has proven to be highly sensitive and selective when it comes to detecting trace gases [14–20].
Photoacoustic cell (PAC) is one of the core components of the PAS system [21–24]. It can be divided into two types according to its working modes: resonant and non-resonant [25–27]. The sound waves in non-resonant PAC are plane wave mode and the internal sound pressure is evenly distributed [28]. The non-resonant PAC has a simple structure and low cost, but its detected signal intensity is weak and the sensor sensitivity is poor [29]. The resonant PAC means that the modulation frequency of the light source is consistent with the resonant frequency of the PAC [30]. Under this circumstances, the sound wave forms a standing wave inside the PAC, which has the effect of resonance amplification of the generated photoacoustic signal [31]. Therefore, it has excellent detection sensitivity and is widely used in PAS sensors [32]. Among resonant PACs, Helmholtz PAC based on the Helmholtz resonance principle can achieve low frequency resonance in a small volume [33,34]. Furthermore, due to its large optical aperture the requirement for the collimation of incident light is not strict [35].
Due to the differential characteristics, photoacoustic signal can be doubled and the incoherent noise can be suppressed when a differential PAC is used [36–38]. In order to increase the path length between light and gas sample, two mirrors based multiple light reflection in the PAC has attracted attentions [39]. But at present there are still many shortcomings, such as difficulty in adjusting the incident light angle. Furthermore, when a multiple reflection based on two-mirror structure is adopted, the system has poor anti-misalignment capability.
A novel high-sensitive PAS sensor, which is based on a multi-pass-retro-reflection-enhanced differential Helmholtz PAC (DHPAC) and a high power diode laser amplified by erbium-doped fiber amplifier (EDFA), is presented in this work. Notably, this is the first time such a sensor has been presented. The retro-reflection-cavity constituted by two right-angle prisms has the merits of easy incident light adjustment and excellent anti-misalignment capability. It can achieve four reflections of incident light. A theoretical model based on finite element analysis was established to optimize the design of DHPAC. A 1.53 µm distributed feedback (DFB) diode laser was selected for excite photoacoustic signal, and its power was amplified by an EDFA to 1000 mW to improve the acoustic wave strength. Acetylene (C2H2) was chosen as the target analysis to verify the reported sensor performance.
2. PAS sensor design
2.1 Design and simulation analysis of the DHPAC
A DHPAC with small volume and low resonant frequency was designed. The DHPAC consisted of two cavities and a tube. The dimensions of the cavity were 10 mm in diameter and 54 mm in length, while the tube had a diameter of 8 mm and a length of 10 mm. Excitation light was incident into one of the cavities to excite a photoacoustic signal. A DHPAC model was built to simulate the performance of the DHPAC. The acoustic field and frequency response of DHPAC are illustrated in Fig. 1. As shown in Fig. 1(a), the acoustic pressure distribution of the DHPAC at the resonant frequency is obtained. It can be seen from Fig. 1(a) that the two acoustic pressure signals had equal amplitude and opposite in direction, which confirms the differential characteristics. The maximum acoustic pressure was 2.33 × 10−5 Pa. Microphone 1 and microphone 2 were placed at both ends of the two cavities to achieve the maximum response of the photoacoustic signal. The frequency sweep result of microphone 1 and microphone 2 of DHPAC is illustrated in Fig. 1(b). The maximum value of DHPAC was about 1266 Hz. The volume of this designed DHPAC was only 9 mL to achieve a minimum gas consumption.
Fig. 1. (a) Acoustic field of DHPAC at the resonant frequency; (b) Frequency sweep result of microphone 1 and microphone 2 of DHPAC.
2.2 PAS experimental setup
A setup of PAS sensor consisting of multi-pass-retro-reflection-enhanced DHPAC and EDFA amplified diode laser is depicted in Fig. 2. A central wavelength of 1.53 µm, output power of 16 mW DFB diode laser was selected as the seed laser. The laser beam, which amplifies the output optical power of the seed laser through an EDFA, was collimated via a fiber collimator. It was fed into the DHPAC after collimation. Two CaF2 windows were equipped on the DHPAC. A multi-pass-retro-reflection-enhanced DHPAC was designed as a place to generate photoacoustic effects, which could achieve four reflections of incident light. The retro-reflection cell was made of two right-angle prisms. The length of the right-angle side of the two prisms was 14 mm and 20 mm, respectively. Microphone 1 and microphone 2 were placed at both ends of the two cavities of the DHPAC to measure photoacoustic signals. As per the latest HITRAN 2020 database, the absorption line of C2H2 at a wavenumber of 6534.37 cm-1 (corresponding to a wavelength of 1530.37 nm) has a strong absorption. A bottle of 2% C2H2 standard gas and a bottle of pure nitrogen (N2) were diluted to produce different concentrations C2H2 gas, and two mass flow controllers were used to control the flow rate. The system was calibrated using certificated gas with different concentrations. Wavelength modulation spectroscopy (WMS) with the second harmonic (2f) detection was employed in the investigation of the PAS sensor. A superimposed signal composed of a low-frequency sawtooth wave generated by a dual-channel function generator and a high-frequency sine wave generated by a lock-in amplifier were sent to the laser controller to scan and modulate the laser wavelength. A differential operation of the two microphones was performed through a differential amplifier. Ultimately, the produced signal was transmitted into the lock-in amplifier for extracting the gas concentration information. The detection bandwidth of the used lock-in amplifier was set to 1.014 Hz.
Fig. 2. Setup of PAS sensor with multi-pass-retro-reflection-enhanced DHPAC and EDFA amplified diode laser.
3. Results and discussions
Parameters such as resonant frequency (f0) and quality factor (Q) are several important indicators for DHPAC, which must be optimized for the purpose of obtaining the optimal experimental results. The frequency response of DHPAC under optical excitation method is described in Fig. 3. f0 was determined as 1250.74 Hz, which had a slight deviation from the simulation results. This deviation may be due to machining processing errors. The detection bandwidth (Δf) was determined as Δf = 113 Hz. Therefore, Q = f0/Δf of the DHPAC was calculated as 11.07. In the following experiments, the value of modulation frequency was configured to be one-half of f0.
To obtain the strongest photoacoustic signal, the laser wavelength modulation depth of the PAS sensor was investigated at C2H2 concentration of 2%. The results, as illustrated in Fig. 4, indicated that the 2f signal of C2H2-PAS sensor exhibited an initial increase followed by a subsequent decrease with the modulation depth. The 2f signal had a maximum value at the modulation depth reached 0.24 cm-1. Consequently, a modulation depth of 0.24 cm-1 was identified as the optimal choice for subsequent experiments.
The feasibility of differential characteristics for the multiple reflections DHPAC was verified experimentally. 2f signals of C2H2-PAS sensor for channel 1, channel 2, double channel without multi-pass reflections and double channel with four reflections were measured, respectively. 2f signals for channel 1 and channel 2 are shown in Fig. 5(a). The collection of 2f signals of for channel 1, channel 2, double channel without multi-pass reflections and double channel with four reflections are displaying in Fig. 5(b). The corresponding noise are depicted in Fig. 5(c). The measured 2f signal amplitudes of C2H2-PAS sensor under channel 1, channel 2, double channel without multi-pass reflections and double channel with four reflections were 251.61 µV, 251.09 µV, 498.13 µV and 1.85 mV. It can be found that before differential operation, the amplitudes of the two signals (channel 1 and channel 2) are almost the same. Therefore, the designed DHPAC basically conforms to the differential characteristics. The 2f signal amplitude of double channel with four reflections was improved by 3.71 times when compared to double channel without multi-pass reflections. The reason for the difference between 3.71 times and four times is the loss of multiple reflections of the laser beam and the two CaF2 windows.
Fig. 5. (a) 2f signals of C2H2-PAS sensor for channel 1 and channel 2; (b) 2f signals of C2H2-PAS sensor for channel 1, channel 2, double channel without multi-pass reflections and double channel with four reflections, respectively; (c) The noise of C2H2-PAS sensor for channel 1, channel 2, double channel without multi-pass reflections and double channel with four reflections, respectively.
To further increase the photoacoustic signal, thereby improving the performance of designed C2H2-PAS sensor, an EDFA was employed to enhance the seed laser output power. The 2f signal from the designed C2H2-PAS sensor at output optical power ranging from 300 mW to 1000 mW was investigated and is shown in Fig. 6(a). When the EDFA output optical power was 1000 mW, the signal peak value was 130.56 mV. Compared to the multi-pass-retro-reflection-cell based PAS sensor without EDFA, there was a 70.57-fold improvement with EDFA. The 2f signal peak values of C2H2-PAS sensor at output optical power ranging from 300 mW to 1000 mW is drawn in Fig. 6(b), which has an excellent linearity. The minimum detection limit (MDL) for this designed PAS sensor was determined to be 906 ppb when EDFA reached its maximum output power of 1000 mW.
Fig. 6. (a) The 2f signals at different output power; (b) The 2f signal peak values at different output optical power.
Concentration response investigation for this PAS sensor was performed at output power of laser source was set to 1000 mW. N2 was used to generate gas samples with varying concentrations ranging from 1000 ppm to 20000 ppm by diluting the 2% C2H2:N2 standard gas. The 2f signal waveforms are depicted in Fig. 7(a). To investigate the linear response of designed C2H2-PAS sensor, the 2f signal peak value at different C2H2 gas concentration is described in Fig. 7(b). Multiple measurements were conducted on the 2f signal of each concentration, and the repeatability fluctuation did not exceed 1%, indicating an excellent repeatability of the system. Based on the R-square was 0.99522 after linear fitting, it can be found that there is a good linear response between the signal peak value and C2H2 concentration.
Fig. 7. (a) The 2f signals at different C2H2 concentration ranging from 1000 ppm to 20000 ppm; (b) The 2f signal peak values at different C2H2 concentration ranging from 1000 ppm to 20000 ppm.
In order to investigate the long-term stability of the C2H2-PAS sensor, pure N2 was fed into DHPAC to continuously measure the background noise for 3.5 hours. As shown in Fig. 8, the MDL can reach 14 ppb with the average time of 400 s.
4. Conclusions
In conclusion, a high-sensitive C2H2-PAS sensor is presented in this work. Notably, this is the first time such a sensor has been presented. The DHPAC with the virtues of small volume and low resonant frequency was designed based on a theoretical finite element model. The DHPAC consisted of two cavities and a tube. The dimensions of the cavity were 10 mm in diameter and 54 mm in length, while the tube had a diameter of 8 mm and a length of 10 mm. Resonant frequency and Q factor of designed DHPAC were determined as 1250.74 Hz and 11.07, respectively. In order to increase the interaction length between the light and target gas, the incident light was reflected four times through a multi-pass-retro-reflection-cell constructed by two right-angle prisms. It had the many advantageous, such as simple optical path adjustment, excellent anti-misalignment capability. A central wavelength of 1.53 µm, output power of 16 mW DFB diode laser was selected as excitation source. Moreover, its power was amplified by an EDFA to 1000 mW to improve the excitation intensity. According to the experimental results, the designed DHPAC had excellent differential characteristics. The 2f signal level for the designed multiple reflections DHPAC was 1.85 mV, which had 3.71 times improvement when compared with the sensor without four times reflection enhancement. Furthermore, in the case where the amplified diode laser by EDFA was operating at an output power of 1000 mW, the 2f signal increased significantly by a 70.57-fold improvement. An Allan deviation analysis was carried out to investigate the long-term stability of such PAS sensor. The MDL can be improved to 14 ppb with the average time was 400 s.
Funding
National Natural Science Foundation of China (62335006, 62022032, 62275065, 61875047); Key Laboratory of Opto-Electronic Information Acquisition and Manipulation (Anhui University), Ministry of Education (OEIAM202202); Fundamental Research Funds for the Central Universities (HIT.OCEF.2023011).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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