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Abstract
On-line gas detection under strong impact such as combustion and explosion is of great significance for understanding the reaction processes. To realize simultaneous on-line detection of various gases under strong impact, an approach based on optical multiplexing for enhancing spontaneous Raman scattering is proposed. A single beam is transmitted several times using optical fibers through a specific measurement point in the reaction zone. Thus, the excitation light intensity at the measurement point is enhanced and the Raman signal intensity is substantially increased. Indeed, the signal intensity can be increased by a factor of ∼10, and the constituent gases in air can be detected with sub-second time resolution, under a 100 g impact.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Most violent chemical reactions, such as those occurring during combustion and explosions, are accompanied by the production of large quantities of gases. In order to understand the reaction processes, it is necessary to carry out on-line, possibly time-resolved monitoring of the compositions and concentrations of environmental gas mixtures. However, these violent reactions are often accompanied by fast changes, high temperatures and strong impact, which bring significant challenges to the on-line measurement of gas composition and concentration [1–4]. In recent years, with the development of spectroscopic technology, on-line measurement technology based on laser spectroscopy has matured. This development has been fueled by the particular capabilities of spectroscopy to deliver real-time, fast, qualitative, and quantitative analysis. Laser spectroscopy has been widely used in the detection and measurement of complex environments [5–9]. At present, commonly used spectroscopic gas detection techniques include infrared absorption spectroscopy [10,11], photoacoustic spectroscopy [12,13], and cavity decay spectroscopy [14,15]. Gas molecules typically poses a large absorption cross-section, so high sensitivity is possible for infrared absorption spectroscopy measurements. However, for symmetric homonuclear diatomic gas molecules such as N2, O2, and H2, infrared spectroscopy is not useful as these molecules have no absorption bands in the infrared region [16,17]. In addition, because different gas molecules have different energy level structures and therefore different absorption bands, it is difficult to detect multiple gases using a single laser by means of absorption spectroscopy. Raman spectroscopy can be used to observe the differences between molecular energy levels that produce different Raman frequency shifts, independent of the excitation wavelength. Therefore, a single-wavelength laser can be used to detect almost all gases, including homonuclear diatomic gases, and hence this spectroscopic method effectively overcomes the above-mentioned difficulties associated with the use of infrared absorption spectroscopy for detection [18–22].
Raman spectroscopy is a method that detects the inelastic scattering of photons by molecular vibrations. Raman spectroscopy has been widely used in the petrochemical industry as well as for environmental protection, food identification, combustion diagnostics [23–25] and so on because of its advantages. These include the fact that no special sample preparation is required, samples are not damaged or consumed during spectral acquisition, and the excellent repeatability of the Raman acquisitions. However, because the Raman scattering cross sections of gas molecules are small and the Raman scattering intensity is low, the sensitivity of Raman spectroscopy for the detection of trace gases needs to be improved [26]. At present, commonly used enhancement methods include surface enhancement [27,28], resonance enhancement [29,30], resonant cavity enhancement [31,32], cavity enhancement [33–35], and hollow-core fiber enhancement [36,37]. Although these methods have been proved to be effective, their enhancement principles are somewhat complex, and implementation requires high-precision set-up that are not robust. For example surface enhancement, its influencing factors are complex [38], and the substrate needs to be processed complex when used. And then Herriott cavity enhancement [34], the system is built with two concave mirrors to make the excitation light pass through the central focus multiple times to improve the power at the measurement point, but this method has high requirements for the orientation of the mirror and is difficult to adjust, and have small error tolerance space. In recent years, hollow core fiber reinforced Raman spectroscopy has been widely studied, especially in gas detection. By introducing gas into the hollow structure of the hollow core fiber, the excitation photon collides with gas molecules many times, and the Raman signal is enhanced. As mentioned above, gas needs to be introduced into the hollow fiber core, which is difficult to achieve for extremely fine fiber core and can hardly be applied to online measurement [18,35]. In particular, for measurements during combustion, explosion, and other violent reaction processes, these enhanced Raman methods cannot be used. For these well-known Raman enhancement methods, the enhancement effect coincides with a reduction in the impact robustness of the measurement system. Therefore, there is a requirement for robust systems that detect enhanced Raman signals. The approach we present in this paper not only produces greater signal intensity, but it also ensures the impact robustness of the measurement system, extending the potential application scope of Raman spectroscopy. Based on the principle of spontaneous Raman scattering, the incident excited light is transmitted by multiple optical fibers in turn to realize optical fiber transmission and free-space transmission alternately. In terms of the system setup, a metal structure like a hollow cylinder is first made, and the fiber collimator and fiber coupler are installed around the hollow cylinder in order to improve the impact robustness of the system. The excitation light is converged at the measurement point through the fiber collimator. And after passing the measurement point, the excitation light is coupled into the fiber through the fiber coupler. And the excitation light transmitted in fiber changes the direction and enters the next fiber collimator. So it passes through the measurement point again, and the energy there improved. Each instance of free-space transmission corresponds to the beam passing through the reaction region at a single measuring point. Thus, the excitation intensity at the measuring point is effectively increased, and the Raman signal is enhanced. This method requires neither a complex design, a large gas chamber, nor an expensive laser. It is expected that the on-line measurement of gas components in complex environments can be realized using this simple method based on the concept of optical multiplexing enhancement. The design and construction of the experimental measurement system is presented in this paper, alongside details of the enhancement effect and key aspects of the optical multiplexing method. In addition, gas detection during impact using the system is reported.
2. Principles
Raman scattering is the inelastic scattering of photons by molecular vibrations. The basic process is shown in Fig. 1. When the molecules in the ground state or a low-lying excited state collide with photons whose energy is E0 = hυ0, the molecules absorb the energy of the photons and are excited to a virtual state. Because the virtual states are inherently unstable, the molecules will eventually return to the ground or low-lying excited state, with the release of a photon of energy E1 = hυ1. According to the relationship between E1 and E0, the process is divided into Rayleigh scattering (E1 ≠ E0), Stokes Raman scattering (E1 > E0,) and anti-Stokes Raman scattering (E1 < E0).
The Raman scattering intensity IR can be expressed as
3. Experiment
In order to verify the feasibility of our proposed approach, an experimental multiplexing measurement system was constructed using mirrors; the optical path of in this system is shown schematically in Fig. 2. A cw laser with a central wavelength of 532 nm (line width, 0.02 nm; power tunable up to 2000mW) was used as the optical source. The laser output is converged by a lens at the measurement point and then continues to propagate. A matching lens then re-collimates the light. The lens used here is plano-convex lens made of quartz, with a diameter of 25.4 mm and a focal length of 150 mm and the anti-reflection film are coated on both surfaces. After two reflections from mirrors, the lens once again converges the light at the measurement point; this is repeated seven times such that the light emitted by the laser passes through the measurement point seven times. The mirror used here is a flat mirror made of quartz with a diameter of 25.4 mm and both surfaces are coated with reflection-enhancing film. This effectively increases the optical power at the measurement point. The Raman signals are collected laterally using a camera lens with a diameter of 50 mm and a focal length of 50 mm. In the measurement, the mirrors used are flat mirrors and have no convergence ability, if it is collected from forward or backward, due to the divergent transmission, the signal will be seriously lost, and the efficiency of collection is greatly limited. Besides, this is also beneficial for reducing the effect of the excitation light as a background signal. The signal at the measurement point is directed into the end of the transmission fibers. In order to increase the signal collection, a concave mirror with a diameter of 50 mm and a focal length of 50 mm is positioned on the other side of the measurement point. The signal is transmitted to the slit (200 µm) at the front of the spectrometer by optical fibers, the background light is filtered by a long-pass filter (produced by Thorlabs; the cut-off wavelength, 550 nm), and then the signal is recorded by means of spectroscopic imaging using an intensified ICCD camera (DH734). There are 500 grooves per millimeter on the optical grating in the spectrometer used here, and the central wavelength of the spectral window is 600 nm, and the resolution is 0.2 nm. The transmission fibers consist of bundles of 37 fibers, with each fiber being a multi-mode fiber (200/220 µm, NA 0.22). The fibers at the end of the bundle that collects the signal are closely arranged to fill a circular cross-section, which ensures that signal collection efficiency is preferable than a single multi-mode fiber, whereas at the other end the fibers are linearly arranged (Fig. 3).
The above-described system can produce enhanced signal intensities using multiple mirrors, but its tolerance to impact is very poor, which means the impact robustness is poor. In order to improve the impact robustness of the system, an optical fiber, optical fiber collimator, optical fiber coupler, and specially machined support structure were used in place of the mirrors in the system to build the fiber-based multiplexing measurement system. The optical path of this improved system is shown in Fig. 4. The optical fibers used here are multi-mode fibers. The core/cladding size is 200/220 µm, and the numerical aperture is 0.22, and the length of which represented by number 3 in Fig. 4 is 1 m, and of which represented by number 6 is 0.3 m. The optical fiber collimators used here (represented by number 4 in Fig. 4) are purchased from Shaanxi Yuanxun Co., Ltd., and is made from multiple lenses, which can converge the excitation light transmitted in space to the measuring point. The optical fiber couplers used here (represented by number 2 and number 5 in Fig. 4) are purchased from Shaanxi Yuanxun Co., Ltd., and is made from multiple lenses, too. The incident aperture is 10 mm and connected with the fiber through a FC/ PC interface. The specially machined support structure used here (represented by number 7 in Fig. 4) is a hollow cylindrical structure with a diameter of 300 mm, which is machined from aluminum alloy. There are two lids at the two ends of the support structure, which can realize the seal of the device, and the air pressure inside can be adjusted by pumping. Through this set-up, the laser passes through the measurement point seven times and the light emitted.
Fig. 4. Optical path of the measurement system. key: 1—laser, 2—fiber coupler, 3—fiber, 4—fiber collimator, 5—fiber coupler, 6—fiber, 7—structural body, 8—concave mirror, 9—collection lens, 10—fiber bundle, 11—filter, 12—slit, 13—spectrograph, 14—ICCD, 15—PC, 16—measurement point.
4. Results and discussion
First, the enhancement effect of the mirror-based multiplexing measurement system was investigated experimentally. Figure 5 shows Raman spectra of the main constituent gases of air acquired using the mirror-based multiplexing measurement system built in the laboratory. Figure 5(a) shows the Raman bands of N2 (2330.7 cm−1) and O2 (1556 cm−1) in the air which measured with single pass configuration when the incident light power was 200 mW; Fig. 5(b) shows the Raman signals of N2 and O2 in the air as measured using the optical multiplexing experimental system and the same incident light power; the integration time was 1s in each case. It is apparent that the signal intensity for each substance was enhanced when the optical multiplexing structure was used, which indicates that the method can realize effective measurement of the principal gases in the air. Figure 5(c) shows an air spectrum acquired using an incident optical power of 500 mW and an integral time of 10 s. Two Raman bands assigned to CO2 (1285 cm−1 and 1388 cm−1) in the air can be seen in the spectrum. However, because of the interference of O2 signal, the CO2 bands need to be extracted. Using numerical fitting, the baseline and O2 signal interference can be removed to extract the CO2 signal [Fig. 5(d)]. The normal CO2 content in the air is 400 ppm. According to the above results, it can be found qualitatively that the detection limit of the mirror-based multiplexing measurement system is better than 400 ppm, when the incident light power is 500 mW and the integral time 10 s. These results, as summarized in Fig. 5, demonstrate that this optical multiplexing method can effectively increase the signal intensity of gas.
Fig. 5. Raman spectra of air acquired using our experimental system. (a) N2 and O2 Raman bands measured with single pass configuration (incident light power: 200 mW, integration time, 1 s). (b) N2 and O2 Raman bands measured using optical multiplexing (incident light power: 200 mW, integration time, 1 s). (c) Air Raman spectrum measured using optical multiplexing with weak CO2 bands just visible at 1285 cm−1 and 1388 cm−1 (incident light power: 500 mW, integral time, 10 s). (d) Spectrum shown in panel (c) after subtraction of the background and O2 band.
The enhancement effect of the measurement system incorporating the new optical excitation and detection setups is now discussed. In the experiments, it is found that the background value of the fiber-based measurement system is basically the same as that of the mirror-based measurement system, which means the introduction of multiplexing fibers does not introduce additional background. Therefore, the following discussion directly based on the peak band intensity. Figure 6(a) indicates that the Raman signal intensity of the N2 in the air as measured using our experimental setups varies with the incident light power. The black squares in the plot show the results acquired using mirror-based multiplexing measurement system, and the red diamonds show the results obtained from fiber-based multiplexing measurement system. It is apparent that the N2 Raman signal intensity increases with the incident light power for both measurement systems, as predicted by Eq. (1). Comparing the two datasets, it can be observed that the signal intensity of the fiber-based multiplexing measurement system was weaker than that of the mirror-based system when the incident light power was the same. This is because when the line polarized incident light passes through a multi-mode optical fiber, its polarization state becomes very confused, and the measurement system can only collect the Raman signal excited by some of the polarized light whose polarization direction is perpendicular to that of Raman signal collection; hence, the excitation light energy is not fully utilized when the Raman scattering signal is collected over a specific limited range of angles, which means that the effective excitation energy is reduced. In the fiber-based measurement system, although the use of multi-mode fiber leads to the confusion of polarization state and the decrease of excitation effect, its large core diameter can guarantee the coupling efficiency of excitation light, especially in the case of coupling many times. It is superior to single-mode fiber or polarization maintaining fiber which leads to high energy attenuation due to the small core. However, the reason for this lower signal could also be that the excitation light is coupled to optical fibers many times, and some energy is lost each time this coupling occurs, which is approximately 5% according measurement. As a result, the signal intensity was lower when the fiber was used. Figure 6(b) shows the N2 signal gain factor of the two types of multiplexed Raman enhancement measurement system versus the incident light power. An enhancement effect is apparent for both of the two measurement systems. Moreover, there was no significant change of the gain factor as a function of the excitation light power. The average gain factor of the mirror-based multiplexing measurement system was 15.02, and the average gain factor was 10.10 for the fiber-based multiplexing measurement system. For the gain factor of the mirror-based system, it is greater the 14, which may be the theoretical limit of the experimental set-up. Maybe it has something to do with the following two reasons. One is the collection direction. Each excitation light will excite the Raman signal, but the angle between each excitation light and the collection direction is different, and not all of them are 90°or other degree. In the case of single excitation light, the angle between the collection direction and the excitation light is the smallest among all of the seven light, which may not be as efficient as that of one or several other excitation lights. As a result, the signal intensity is small, and the actual ratio is larger than the theoretical ratio. The other is the background. In the presence of multiplexing excitation light, the scattering is stronger, which will produce greater background noise. However, the comparison using peak value here will make the result larger, and eventually lead to the actual ratio becoming larger.
Fig. 6. Comparison of enhancement effects of the multiplexing measurement systems based on mirrors and optical fibers. (a) Raman signal of N2 in air versus incident light power. (b) N2 Raman signal gain factor versus incident light power.
Figure 7(a) presents the Raman signal of N2 in the air as measured by the fiber-based measurement system versus the air pressure in the support structure (represented by number 7 in Fig. 4) under different incident optical power conditions. The N2 Raman signal increased with the air pressure at each incident light power, with a higher gradient for the intensity enhancement for higher optical powers, which is consistent with Eq. (1). Figure 7(b) shows the experimental data and linear fit results for the N2 Raman signal intensity (peak intensity value for the N2 band) versus the N2 content (the total pressure,101 kPa; the rest component, O2) when the incident light power was about 500 mW and the integral time is 1 s. The fit result was peak band intensity I = 310188.11C + 877.94, with Pearson’s r = 0.994 and good linearity. These findings demonstrate that this fiber-based multiplexing measurement system can be used to characterize the concentration of the gas. Based on the linear fitting results, it can be found that when the power of excitation light and integral time is set which is 500 mw and 1s, when the N2 content is 0, the signal value is 877.94, which indicates the value of the background noise. It is believed that a reliable signal can be measured when the signal value is greater than three times the background, that is, the signal value is required to be 2633.82, and plug into the result of the linear fit, and the detection limit of N2 here is calculated to be 849 ppm.
Fig. 7. Variation of the Raman signal of N2 in air with the air pressure as obtained using the fiber-based multiplexing measurement system. (a) Raman signal intensity versus air pressure at different optical powers. (b) Raman signal intensity versus N2 content (the pressure is 101 kPa, and the rest component is O2) with linear fit when the incident light power was about 500 mW. the integral time was 1 s
In order to verify the impact robustness of the multiplexing measurement system, the impact acceleration response experiments of the mirror-based measurement system and the fiber-based measurement system were carried out respectively. In the impact experiment of the mirror-based measurement system, under the impact of 20.16 g, the system has an irreversible deviation, mainly realized that the reflected excitation light no longer intersects at the same measurement point, and the signal intensity has decreased significantly. A similar impact experiment was performed on a fiber-based measurement system, the results of which are shown in Fig. 8. Figure 8(a) shows a typical horizontal impact wave with a maximum impact acceleration of 48.27 g, which was applied by hitting the support structure of the measurement system with a drop hammer; the inset illustrates the impact direction, which is parallel to the plane of the excitation light. Figure 8(b) shows the variation of the gain factor of the experimental system when different horizontal shocks are applied; Each data point in this graph is the average of the three measurements, with the impact acceleration being the average and the gain factor being the average. It was found that there was no obvious variation of the gain of the system over a range of impact accelerations. Figure 8(c) displays a typical vertical impact wave with a maximum impact acceleration of 58.26 g, which was also applied using a drop hammer to hit the support structure; the inset shows that the impact direction was perpendicular to the plane in of the excitation light. Figure 8(d) shows the variation of the gain factor of the experimental system with the vertical shock impact acceleration, and as for the horizontal shock, the gain factor of the measurement system is not obviously affected by the impact acceleration within a certain range. The data point is also the average of the three measurements. These results indicate that the measurement system had good impact resistance, and the impact robustness of the system was significantly improved alongside effective enhancement of the Raman signal. Thus, our method is an effective technique for the on-line measurement of gas in combustion and explosion environments. We suggest that second-order time-resolved measurements of gas composition and concentration can be achieved in certain strong-impact environments, and the gain can be maintained during impact. Thus, this method has good application prospects for gas monitoring in intense combustion and explosion environments.
Fig. 8. Impact wave shapes and gain factors versus impact acceleration of the fiber-based multiplexing measurement system. (a) Horizontal impact wave shape. (b) Gain factor versus impact acceleration for the horizontal wave shown in (a). (c) Vertical impact wave shape. (d) Gain factor versus impact acceleration for the vertical wave shown in (c). The insets in (a) and (c) illustrate the direction of the impact with respect to the plane of the measurement system.
5. Conclusions
In this study, a robust on-line Raman measurement system based on optical fiber transmission offering multi-component detection was designed and developed. A multiplexing approach was adopted for the generation of enhanced Raman scattering spectroscopic signals. By passing the incident light through the sample multiple times, the excitation light energy at the measurement point was effectively increased, and more than ten-fold signal enhancement was achieved. The gain factor for the measurement system was 9.01 under a 102.70 g impact—that is, the Raman signal was enhanced by a factor of 9.01—in the horizontal direction. When a vertical impact of 77.31 g was applied, the gain factor of the measurement system was 9.19. Thus, the system has good tolerance for both horizontal and vertical impacts. The experimental results demonstrate that the multiplexed enhanced Raman measurement system proposed in this paper can effectively detect component gases in gas mixtures during impact; moreover, the findings prove its feasibility as a new method for gas detection in complex environments.
Funding
National Natural Science Foundation of China (52106222, 9184133); State Key Laboratory of Laser Interaction with Matter (SKLIM2021-11).
Acknowledgments
The authors thank the Natural Science Foundation and the State Key Laboratory of Laser Interaction with Matter for financial support.
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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