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Abstract
Raman signal with a high signal-to-noise ratio (SNR) under a strong sunlight environment of 30000∼60000 Lux is very hard to obtain. A compact remote Raman spectrometer (CRS) has been developed for the purpose of detecting diverse types of lunar and earth minerals. The system comprises a spectrometer that is equipped with an ICCD detector, a 30 mm entry pupil diameter beam expander, and a 532 nm Nd: YAG Q-switched laser serving as the source of Raman scattering. The implementation of synchronous trigger and gating technology effectively overcomes the impact of strong sunlight. We obtained Raman spectra using a shorter integration time than in previous studies. The experimental results demonstrate that the detection of remote Raman spectroscopy under intense sunlight conditions facilitates the identification of silicates that have been discovered on the moon.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Raman spectroscopy has the advantages of non-destructive, non-contact and no sample pretreatment. It can effectively conduct qualitative and quantitative analysis in-situ. Raman spectrometer is an ideal tool to measure the content of unknown minerals on the surface and interior of Mars and moon, and can quickly survey the geological environment characteristics. Raman spectroscopy can provide accurate and detailed molecular and structural information of satellite and planetary material. Many Raman spectroscopic studies of extraterrestrial sample returns [1], archaeology [2], gem identification [3], planetary material analysis [4], space exploration [5], biology [6,7], cultural heritage and mineralogy [8] have been reported due to its advantages of fast nondestructive analysis, clear phase identification, and precise identification of chemical bonds for rock, planetary materials and organics.
To date, several in-situ or remote Raman systems have been proposed for lander or rover exploration missions landing on the Moon, Mars, Venus, asteroids, etc [9,10]. Remote Raman systems for planetary exploration have been established at home and abroad, in particular the University of Hawaii has indoor remote Raman instruments could work under daytime conditions from standoff distances of several meters [11,12]. Genesis et al. proposed a compact system named CRRS, demonstrates remote Raman spectra of natural rocks at a distance of 5 meters in-house using 30 s integration time [13].
In the early 1960s, remote Raman was developed and used for gas detection [14,15]. The application research of Raman spectrometer under sunlight environment is even less. Zhizhong Sheng et al. used Raman Mi lidar to study the vertical distribution of aerosol in the solar environment of Beijing, analyzed the source and propagation path of pollution [16]. Hongkun Qu et al. of Shandong University, based on a telescope with a diameter of 150 mm and a small 0-200 mJ/pulsed 1064 nm Nd:YAG Q-switched laser, achieved indoor remote Raman spectroscopy at a distance of 4 meters, enabling the identification of samples that may exist on extraterrestrial planets [17].
Wiens et al. detailed the SuperCam Remote Raman spectrometer instrument suite in terms of optics, mechanics, electronics, software, operations etc [18]. What's more, the SuperCam on board the Mars2020/NASA Perseverance rover observed the Raman spectrum of selenite gypsum powder particles compression by integrating 100 laser pulses at a distance of 2.25 meters [19,20]. All of these shows that Raman spectroscopy is a mature and reliable technique that can provide highly accurate analytical results and can be applied to planetary exploration. However, the application of Raman spectrometer under strong sunlight environment is rarely studied at home and abroad.
2. Methods
For Raman spectrometer, Raman scattered light is signal light. Rayleigh scattered light, sunlight and fluorescence are all stray light. The outdoor experiment was conducted under strong sunlight illumination (light scattering) environment. We use high synchronous trigger and gate control circuit technology to achieve stray light suppression, as shown in Fig. 1. In order to make the Raman spectra collected on the CCD stronger and to minimize the fluorescence signal and strong sunlight signal reaching the CCD as much as possible, the acquisition gate width is not only larger than the width of the laser pulse, but also can shut out the strong fluorescence peak signal and reduce the accumulation of strong sunlight signal. The pulse laser and the acquisition gate are both electronically controlled and triggered synchronously. When the trigger signal is received, the pulse laser emits light at 10 nanoseconds. The laser emits a laser pulse every 0.2 ms, at the same time, because the two are triggered synchronously, ICCD collects the signal every 0.2 ms, and repeats the integration and accumulation until the required integration time is met. Light travels at a speed of v = 2.99710 × 108 meters per second in air. For a detection range of 2.3 m, according to
The time for the laser to reach the sample surface plus the time for the Raman scattered signal light to reach the CCD requires a total of at least 15 nanoseconds. Long time exposure acquisition will lead to strong background light, so we set the gate width to 25 ns to collect Raman scattering signals.
Fig. 1. Time sequence diagram of Raman spectrum signal acquisition based on synchronous triggering and gating technology
For the filtering of Rayleigh scattered light, we set filters before and after the collimating mirror of the spectrometer channel to suppress the 532 nm laser energy, and use two OD6 filters to attenuate the laser energy to less than 10−6 of the incident energy. For ambient light, sunlight is a polychromatic light, which is also natural light. The band below 536.4 nm is filtered twice by the filter light sheet. Bands higher than 536.4 nm enter the system and reach the detector become background light. For the strong sunlight environment of 60000 Lux, the ambient illuminance of the ground is E = 60000lm/m2. According to the spectral light efficiency function, the visual response function, the luminous flux or the visible light brightness is calculated by the following formula:
Then, according to the definition of illuminance,
Since the laser spot diameter on the sample surface is about 2 mm, calculate dΦ = 0.06П lm. Then, total luminous flux of 30898.192 lm in the whole visible spectrum is integrated by Eq. (2).Then, according to I=Φ×t, multiplied by the gated time of 25 nanoseconds. After calculation, the intensity of sunlight is equal to 0.772 µJ.
For fluorescence signal, fluorescence will not be generated immediately, but a delay of several nanoseconds or even longer than Raman light when the laser hits on samples. The Raman-scattered light lifetime is on the order of ps (about 0.01 ps ∼ 10 ps), which can be considered that Raman-scattered light and laser light appear and disappear almost simultaneously. Due to the mechanism of internal energy exchange, fluorescence lifetime is generally in the order of nanosecond. It lags the generation of ms at the moment of laser irradiation (about several ns to several ms), and the fluorescence decays exponentially after peaking. As an exponential decay of the form $\textrm{y}({{\; \textrm{t}\; }} )= \textrm{a exp}\,({ - {\; \textrm{t}}/\mathrm{\tau}} )$.
According to the temporal characteristics of Raman spectroscopy, and the successive relationship between Raman light and fluorescence in generation time, remote Raman spectroscopy technology uses pulsed laser and gating acquisition technology to realize synchronously trigger spectrometer module reception. It uses ICCD gating technology to suppress background stray light in the time domain to realize the detection of Raman-scattered light signal.
Therefore, the technique of ICCD detector with electronic shutter and multiple integral accumulation detection of short-wavelength ultrashort pulse laser can extract the Raman spectral signal from the strong fluorescence background. After using integral accumulation counting and image intensifier amplification, the energy of the Raman scattering signal exceeds the background energy of strong sunlight. The strong sunlight signal lifts the base of the Raman signal. Thereby we realizing the weak light measurement of Raman signal and stray light suppression.
2.1. Experimental setup and samples
The schematic diagram of the compact remote Raman spectrometer system is shown in Fig. 2, the Raman spectrometer focus laser energy on sample, a splits part of the beam to the expansion optical collector through beam splitter to achieve remote imaging, divides the Raman channel light to the weak light spectrometer module, the weak light spectrometer adopts grating spectrometer reception.
The experimental equipment is a compact remote Raman spectrometer (see Fig. 3). In the experiment, the light source is a 532 nm pulsed laser (The Spectra-Physics Explorer One CA 95054, USA), the repetition frequency is set to 5000 Hz, the pulse width is 2 ns, and the single-pulse laser energy is adjustable up to 201 µJ. After the laser passes through a semi-permeable semi-reflective beam splitter, the reflected light passes through a beam expander and travels a certain distance in the air before being incident vertically on the surface of the sample to be measured. The excited Raman spectral signal passes through the 5x beam expander (THORLABS), beam splitter, 532 nm long-pass edge filter (RazorEdge ultrasteep long-pass edge filter), and focusing objective lens in the opposite direction to the incident laser before being collected into the spectrometer system (Wasatch Photonics Ultimate Diffraction Gratings VOLUME PHASE HOLOGRAPHIC TRANSMISSION GRATINGS 1800 I/mm at 532 nm). The beam expander has a pupil aperture of 30 mm. The spectrometer detection range is 150∼2900 cm−1, and the aperture of image intensifier (Ciic, 2DSPC single photon camera) is 18 mm. Figure 3 is the experimental physical diagram in the outdoor lighting environment, where (a) is the appearance of the covered instrument, and (b) is the internal structure diagram (40 × 30 × 8 cm3) after removing the outer cover. The test distance is 2.3∼2.5 m. The outdoor sunlight illuminance ranges from 34432.0∼59395.2 Lux, the temperature range is 34.3-47 °C, and the relative humidity range is 10%∼33%. We use the TES-1334A illuminometer to measure daylight intensity. Its measuring range is: 0.01 Lux/fc∼20000 Lux/fc, and Accuracy: ± 3%rdg ±0.5%f.s.
Fig. 3. Physical diagram of Raman spectrometer experimental system (a) with cover appearance; (b) the internal structure below the cover.
The laser power is an adjustable parameter in the traditional Raman spectrometer. The Raman signal with good SNR can be obtained by using appropriate laser power, accumulation times and integration time, and the samples can also be protected from the damage of high-energy laser. Typically, if the Raman scattering is excited by higher laser energy and longer integration time, the surface of the sample will be destroyed by ablation.
Selection of minerals common on the lunar surface, such as silicon wafer, quartz of different forms and so on as test samples. In order to test the detection ability of the experimental system for different types of samples, a variety of samples are selected as test samples. Samples can be categorised into these form: block, crushed stones, slice, powder particles, etc.
2.2. Strong sunlight working environment of the instrument
In this work, some experiments are carried out under strong sunlight and demonstrate the stray light suppression performance of the Raman spectrometer. In the strong sunlight suppression experiment, the Raman spectrum of pre-selected samples is tested and evaluated in the design working scenario by using our experimental system. The strong sunlight suppression and spectral performance evaluation of Raman spectroscopy are realized through experiments.
During the outdoor experiment, the system was in an strong sunlight environment, and the illuminance variation range was 46052.8∼59395.2 Lux (50.41∼86.96 W/m2 @555 nm, time 11:00∼15:50 of July summer), the specific variation trend is shown in Fig. 4, and the working scene is shown in Fig. 5.
3. Results
The experiment was carried out in outdoor sunlight environment, and the laser incidence angle is 90°, the laser energy is 52.1 µJ, the image intensifier gain is 3200, the gate width is 25 ns.
3.1 Instrument wavelength calibration
In order to test the ability of CRS to inhibit strong sunlight, we tested the samples under sunlight environment. To better obtain a back-scattered Raman spectrum, the incident angle of laser towards sample surfaces is typically set as 90°. We use silicon wafer to calibrate the obtained spectral data. The 523.04 cm−1 Raman peak of the silicon wafer is shifted by about ±3 cm-1 from the standard spectrum (520.50 cm−1).
3.2 Raman spectroscopic analysis of silicate minerals
The Raman spectra of and quartz in sunlight and darkroom environments were tested. Figure 6(a) shows remote Raman spectra of quartz. The integration time is 6 s, and the detection distance is 2.5 m. The illuminance of the outdoor sunlight scattering environment is ∼50249.2 Lux. The test sample morphology is quartz powder particles compression. Within 6 s of acquisition, it can be considered that the intensity of sunlight remains unchanged, and the total number of photons entering the system and reaching the detector is also a constant value. In the early stage of Raman signal acquisition, the number of signal photons is far less than the number of ambient photons. After continuous integration and accumulation, the number of photons of Raman scattered signal reaching the detector exceeds the number of photons of ambient light. Therefore, we can observe Raman peaks in the background of strong sunlight. It can be seen from Fig. 6(a) that CRS can accurately identify quartz's strong peaks 202, 465 cm−1, weak peaks 398, 355, 1075, 1173 cm−1. Raman spectrum is quite broad and Raman bands are observed at 1075 cm−1, 1173 cm−1 are assigned to the Si–O stretching vibrations. The series of Raman bands at 355, 398 cm−1 are assigned to metal–oxygen stretching vibrational modes (Na–O, Fe–O, Al–O). These bands are attributed to metal oxygen stretching vibrations (MO) [21]. Intense Raman bands are observed at 465 cm−1 are assigned to O–Si–O bending vibrations. Bands of lesser intensity are observed at 202 cm−1 may be attributed to Si–O molecular rotation vibrations. The above demonstrates the ability of our instrument to detect weak Raman signal peaks.
Fig. 6. Raman spectra of original quartz and plagioclase sample signals in outdoor sunlight scattering and indoor environments
Raman spectra related to quartz have been investigated in detail under different pressures and temperatures in previous studies [22–27]. The Raman spectra of plagioclase sample is shown in Fig. 6(b). The integration time is 20 s, and the detection distance is 2.5 m. The illuminance of the outdoor sunlight scattering environment is ∼57458.4 Lux. The test sample morphology is plagioclase crushed stone, with particle size of about a few mm. It can be seen from Fig. 6(b) that the system can identify the strong peaks of plagioclase 284, 508 cm−1, and the weak peaks of plagioclase are 332, 408 cm−1 [28–30]. From the comparative analysis of characteristic peak intensity, we can uncover that for ∼57458.4 Lux sunlight and darkroom environment, the former intensity is higher than the latter by about 1000DN.
It can be concluded from Fig. 6, under the test conditions of the experiments, the sunlight environment mainly affects the intensity of the background continuum, compared with the darkroom environment, strong sunlight will increase the Raman spectral background, but it has almost no effect on the Raman signal peak intensity.
3.3 Instrument SNR
To verify the Raman spectrometer's ability to suppress strong sunlight and stray light interference, quartz samples in different states (block, powder particles) were tested. Figure 7 is the Raman spectrum of quartz samples in the light scattering environment and the evaluated spectral signal-to-noise ratio (465.85 cm-1), the experimental parameters used are shown in Table 1, the illuminance range under light scattering is 46052∼52930 Lux.
Fig. 7. Raman spectra of quartz (465.85 cm-1) in indoor and sunlight environment and the evaluated spectral SNR
Table 1. Test conditions of quartz samples under sunlight environment
The root mean square (RMS) SNR values shown in Fig. 7 were calculated using formula below [31]:
Figure 7 shows the spectral diagram and SNR pair diagram. The black spectral lines of the block sample were obtained under direct sunlight shot. We can see from Fig. 7(a) that the background under direct sunlight is higher. The results show that the background value of Raman spectrum is increased and the signal-to-noise ratio is slightly decreased in the stray light interference environment. However, the spectral SNR is still perfect in sunlight environment, which indicates that our instrument has the ability to suppress stray light interference by using synchronous trigger and gating technology. Combined with the results of the quartz samples, CRS has the ability to identify basic silicate minerals under strong sunlight.
4. Discussion
In summary, we have clearly demonstrated the design of a compact remote Raman spectrometer (CRS). In addition, some international scholars have qualitatively proposed the study of remote Raman system working in daytime condition, we have quantitatively revealed its applications under strong sunlight environment, and completely overcome the influence of sunlight on the weak Raman peak, which can further contribute to novel Raman spectrometer development for spectral signal detection in harsh space environments. It will be helpful for future space planetary material detection, which not only small and lightweight, but also high efficiency. We speculate that more practical and powerful space Raman instruments will be developed in the future.
5. Conclusions
We have developed a compact remote Raman system and completed its application research in the strong sunlight environment of 34432.0∼59395.2 Lux (50.41∼86.96 W/m2). The Raman spectra obtained from our remote Raman system at a distance of 2.3∼2.5 m demonstrate the capability of remote Raman detection of main components of the lunar highlands. Moreover, the peak intensity and background continuum analysis of quartz spectra under sunlight and darkroom conditions show that the instrument has the ability to suppress stray light interference by synchronous triggering and gating technology, and our compact remote Raman system has the ability to characterize minerals by Raman spectroscopy.
Acknowledgments
We would like to thank the editors and anonymous reviewers for their constructive suggestion, which significantly improve the quality of this work. We thank the funding from the Pre-research project on Civil Aerospace Technologies no. D020102 funded by China National Space Administration (CNSA) .
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.
Reference
1. H. Zhang, X. Zhang, and G. Zhang, “Size, morphology, and composition of lunar samples returned by Chang’E-5 mission,” Sci. China Phys. Mech. Astron. 65(2), 229511 (2022). [CrossRef]
2. K. M. M Shameem, “Echelle LIBS-Raman system: a versatile tool for mineralogical and archaeological applications,” Talanta 208, 120482 (2020). [CrossRef]
3. C.-Q. Liu, Y.-Y. Liu, and X.-H. Cai, “Application of Raman spectroscopy to gem identification,” Journal of Hunan University of Arts and Science 2011(1), 41–44 (2011).
4. A. Wang, B.L. Jolliff, and L.A Haskin, “Raman spectroscopy as a method for mineral identification on lunar robotic exploration missions,” J. Geophys. Res. 100(E10), 21189–21199 (1995). [CrossRef]
5. F Hanke, B Mooij, and F Ariese, “The evaluation of time-resolved Raman spectroscopy for the suppression of background fluorescence from space-relevant samples,” J. Raman Spectrosc. 50, 969–982 (2019). [CrossRef]
6. A Lewis and J Spoonhower, “Chapter 11 – tunable laser resonance Raman spectroscopy in biology,” J. Raman Spectrosc. 71(11), 347–376 (1974). [CrossRef]
7. Z Samuel A, S Horii, and T Nakashima, “Raman microspectroscopy imaging analysis of extracellular vesicles biogenesis by filamentous fungus penicilium chrysogenum,” Adv. Biol. 6, 2101322 (2022). [CrossRef]
8. B. Wopenka and S Sandford, “Laser Raman microprobe study of mineral phases in meteorites,” Meteoritics 19, 340 (1984).
9. I.B. Hutchinson, R. Ingley, H.G.M. Edwards, L. Harris, M. McHugh, C. Malherbe, and J Parnell, “Raman spectroscopy on Mars: Identification of geological and bio-geological signatures in Martian analogues using miniaturized Raman spectrometers,” Phil. Trans. R. Soc. A. 372(2030), 20140204 (2014). [CrossRef]
10. S.K. Sharma, A.K. Misra, S. Clegg, J. Barefield, R.C. Wiens, and T Acosta, “Time-resolved remote Raman study of minerals under supercritical CO2 and high temperatures relevant to Venus exploration,” Phil. Trans. R. Soc. A. 368(1922), 3167–3191 (2010). [CrossRef]
11. M.W. Sandford, A.K. Misra, T.E. Acosta-Maeda, S.K. Sharma, J.N. Porter, M.J. Egan, and M.N Abedin, “Detecting minerals and organics relevant to planetary exploration using a compact portable remote Raman system at 122 meters,” Appl. Spectrosc. 75(3), 299–306 (2021). [CrossRef]
12. A.K. Misra, S.K. Sharma, and P .G Lucey, “Remote Raman spectroscopic detection of minerals and organics under illuminated conditions from a distance of 10 m using a single 532 nm laser pulse,” Appl. Spectrosc. 60(2), 223–228 (2006). [CrossRef]
13. G Berlanga, E Acosta-Maeda T, and K Sharma S, “Remote Raman spectroscopy of natural rocks,” Appl. Opt. 58(32), 8971 (2019). [CrossRef]
14. T. Kobayasi and H Inaba, “Spectroscopic detection of SO2 and CO2 molecules in polluted atmosphere by laser-Raman radar technique,” Appl. Phys. Lett. 17(4), 139–141 (1970). [CrossRef]
15. S.K. Sharma, A.K. Misra, S.M. Clegg, J.E. Barefield, R.C. Wiens, T.E. Acosta, and D.E Bates, “Remote-Raman spectroscopic study of minerals under supercritical CO2 relevant to Venus exploration,” Spectrochim. Acta, Part A 80(1), 75–81 (2011). [CrossRef]
16. Zhizhong Sheng and Huizheng Che, “Aerosol vertical distribution and optical properties of different pollution events in Beijing in autumn,” (2017).
17. H. Qu, Z. Ling, X. Qi, Y. Xin, C. Liu, and H. Cao, “A remote Raman system and its applications for planetary material studies,” Sensors 21(21), 6973 (2021). [CrossRef]
18. T Nelson, R Wiens, and S Clegg, “The SuperCam instrument for the Mars 2020 rover,” 2020 IEEE Aerospace Conference. IEEE, 2020.
19. G. J. Sharma and Taylor, “Next generation laser-based standoff spectroscopy techniques for Mars exploration,” Appl. Spectrosc. 69(2), 173–192 (2015). [CrossRef]
20. A. K. Misra, T. E. Acosta-Maeda, J. N. Porter, G. Berlanga, D. Muchow, S. K. Sharma, and B. Chee, “A two components approach for long range remote Raman and laser-induced breakdown (LIBS) spectroscopy using low laser pulse energy,” Appl. Spectrosc. 73(3), 320–328 (2019). [CrossRef]
21. T. Kobayashi, T. Hirajima, Y. Hiroi, and M. Svojtka, “Determination of SiO2 Raman spectrum indicating the transformation from coesite to quartz in Gfohl migmatitic gneisses in the Moldanubian Zone, Czech Republic,” Journal of Mineralogical and Petrological Sciences 103(2), 105–111 (2008). [CrossRef]
22. D Krishnamurti, “The Raman spectrum of quartz and its interpretation,” Proc. Indian Acad. Sci. 47(5), 276–291 (1958). [CrossRef]
23. R.S Krishnan, “Raman spectrum of quartz,” Nature 155(3937), 452 (1945). [CrossRef]
24. Anupam K. Misra, “Pulsed remote Raman system for daytime measurements of mineral spectra,” Spectrochim. Acta, Part A 61(10), 2281–2287 (2005). [CrossRef]
25. Samuel M. Clegg, “Planetary geochemical investigations using Raman and laser-induced breakdown spectroscopy,” Appl. Spectrosc. 68(9), 925–936 (2014). [CrossRef]
26. Angel and S. Michael, “Remote Raman spectroscopy for planetary exploration: a review,” Appl. Spectrosc. 66(2), 137–150 (2012). [CrossRef]
27. Rohit Bhartia, “Perseverance’s scanning habitable environments with Raman and luminescence for organics and chemicals (SHERLOC) investigation,” Space Sci. Rev. 217(4), 58 (2021). [CrossRef]
28. B. Gullu and K. Deniz, “Nature and crystallization stages of spherulites within the obsidian: Acıgöl (Cappadocia-Nevşehir, Turkey),” Turk. J. Earth Sci. 31(6), 545–562 (2022). [CrossRef]
29. S. Y. Park and C. Park, “Spatially-resolved mineral identification and depth profiling on chondrules from the primitive chondrite Elephant Moraine 14017 with confocal Raman spectroscopy,” Spectrochim. Acta, Part A 207, 46–53 (2019). [CrossRef]
30. R. Li, L. Duan, B. Chen, B. Xia, and J. Li, “Albitization and hydrothermal diagenesis of Yanchang oil sandstone reservoir, Ordos Basin,” Petroleum Exploration and Development 42(1), 56–65 (2015). [CrossRef]
31. HORIBA, Available online: https://www.horiba.com/en_en/technology/spectroscopy/fluorescence-spectroscopy/how-to-calculate-signal-to-noise-ratio/ (2021).
