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Abstract
In this paper we present a quartz-enhanced photoacoustic spectroscopy (QEPAS) of methane near 1651 nm. QEPAS is a high-sensitivity gas sensing method that relies on detecting acoustic waves generated by gas molecules. The sensor setup consists of a bismuth-doped fiber amplifier (BDFA) operating at 1651 nm that is used to enhance the amplitude of the QEPAS signal and increase the detection sensitivity. With the BDFA delivering ∼250 mW of optical power to the sample, the minimum detection limit of ∼11 ppb was achieved for the integration time of 150 s.
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1. Introduction
Optical gas sensors have become useful tools in various applications, including industrial process control or environmental monitoring [1,2]. Sensors based on near- or mid-infrared laser spectroscopy typically provide high sensitivity, high selectivity, and short response time. Most of these instruments use tunable diode laser absorption spectroscopy (TLDAS) to analyze molecular absorption lines directly [3], or with additional wavelength modulation and signal demodulation [4,5]. Several other methods have also been explored in past few decades. Among them, photoacoustic spectroscopy (PAS) has been particularly popular, especially with the use of a quartz tuning fork (QTF) for photoacoustic signal detection. So-called quartz-enhanced photoacoustic spectroscopy (QEPAS) has been introduced in [6]. The general principle of all PAS-based methods is that when modulated radiation is absorbed by gas particles, the absorbed energy is converted into heat. As a result of thermal expansion and contraction of gas particles, an acoustic wave (temporary changes of gas pressure) is created. In QEPAS, the modulated light beam is focused between the prongs of an U-shaped piezoelectric quartz tuning fork which is used as a detector, which converts an acoustic wave into an electrical signal.
QEPAS systems have demonstrated sensitivities down to ppb or even ppt levels for various species, primarily when mid-infrared sources were used to target the strongest, fundamental molecular transitions (some examples can be found in [7–11]). For near-infrared wavelengths (which typically allow the use of fiber-coupled sources and fiber-based components) the performance of QEPAS-based systems may be comparable to mid-infrared setups only when optical amplifiers are used. In this approach, one takes advantage of the fact that the photoacoustic signal is directly proportional to the absorbed optical power. Therefore, if an optical amplifier is available, a higher optical power can be used to compensate for the lower absorption coefficients in the near-infrared spectral region and still achieve ppb-level sensitivities. Some examples include QEPAS systems with erbium- or erbium/ytterbium-doped fiber amplifiers designed to detect ammonia [12–15], hydrogen sulfide [12,16], hydrogen cyanide [17] or acetylene [18]. Encouraged by these reports, in this work we have focused on QEPAS-based methane sensing.
Methane sensing has been successfully performed in the near-infrared spectral region using, e.g. tunable diode laser absorption spectroscopy with multipass gas cells, or cavity-enhanced techniques using molecular transitions located near 1.65 µm (e.g. [19–21]). Figure 1 shows the absorption spectrum of methane in this spectral region, generated using the HITRAN database. Furthermore, the absorption spectra of water vapor and carbon dioxide have also been shown to demonstrate that they do not overlap with the important methane transitions near 1651 nm. It is also worth mentioning that near-infrared QEPAS has several practical advantages comparing to the mid-infrared implementations. With the availability of low-cost fiber-optic components, near-infrared systems are much more flexible. For example, single laser source can be divided into several QEPAS sensing modules using standard, telecom-graded splitters [22]. It is also much simple to deliver the laser light from the source to the QEPAS-module using optical fibers [23]. And although mid-infrared optical fibers are also available, there still some issues with using them in spectroscopic systems (for example, more challenging splicing, higher transmission and bending losses, lack of other fiber-optic components such as isolators or splitters, and general small robustness of mid-infrared glasses). Additionally, the price of semiconductor laser sources in the near-infrared (especially in the 1.55 µm region but also near 1.65 µm) is still much lower than the price of interband or quantum cascade lasers. Unfortunately, wavelengths near 1.65 µm are beyond the gain bandwidths of rare-earth-doped fiber amplifiers. As a result, the performance of photoacoustic-based methane sensors in the 1.65 µm region (e.g. [6,24–27]) has never been comparable to mid-infrared systems at 3.3 µm (e.g. [28,29]) or 7.7 µm (e.g. [30]). One exception is the setup presented in [31] where the Raman amplifier was used. With more than 1 W of optical power at 1651 nm, the sub-ppm detection limit (110 ppb for 1 s integration and 17 ppb for 130 s integration) was obtained. However, the complexity of Raman amplifiers (comparing to that of rare-earth-doped amplifiers) is rather large. They require not only multi-watt pump sources but also multi-km-long optical fiber (which often generates problems with stimulated Brillouin scattering).
Fig. 1. Absorption spectrum of methane, water vapor and carbon dioxide near 1651 nm simulated using the HITRAN database (https://hitran.iao.ru/) assuming the path length of 1 cm and the pressure of 1 atm.
In this paper we demonstrate that methane sensing using near-infrared photoacoustic spectroscopy with the ppb-level detection limit is possible using significantly less complex optical setup than presented in [31]. The sensor presented here relies on QEPAS and uses bismuth-doped fiber amplifier (BDFA) for the signal enhancement. With approximately 250 mW of optical power at 1651 nm delivered to the quartz tuning fork, the detection limit down to 11 ppb (for integration time of ∼150 s) was obtained.
2. Experimental setup
Figure 2 show the schematic diagram of the sensor. A distributed feedback (DFB) laser diode emitting at 1651 nm is used as the source. Wavelength modulation was achieved by applying sinusoidal modulation of the diode driving current at 6.2314 kHz (half of the QTF resonance frequency). An additional low-frequency sawtooth signal was added to the diode current for wavelength tuning. The light from the laser diode was amplified in a homemade bismuth doped fiber amplifier (BDFA). Its main part was a 90 m long germanosilicate bismuth-doped fiber (BDF) [32,33]. The core and outer diameter of the BDF were 2.2 µm, and 125 µm, respectively, and the concentration of GeO2 was ∼50 mol.%. The fiber was pumped at 1540 nm from both directions through commercially available wavelength multiplexers (from Haphit). The pump source consisted of a fiber laser emitting at 1540 nm and two erbium-ytterbium-doped fiber amplifiers (EYDFA, from BKtel, model GOA-S320), each providing up to 32 dBm of pumping power. Figure 3 shows the output power of the BDFA as a function of the input power. For the highest input power (i.e. when the 1651 nm laser diode was driven at 110 mA) the output power of ∼334 mW was obtained. However, for the laser current for which the operating wavelength of the DFB laser diode matched the target methane transition (IBIAS ≈ 75 mA), the optical power at the output of the BDFA was ∼296 mW. After exiting the BDFA, 1651 nm light passed through a fiber optic GRIN collimator (Thorlabs, model 50-1550A) and a focusing lens (fused silica, 50 mm focal length) before entering the QEPAS module. The QEPAS module (ADM01 from Thorlabs) consisted of a QTF and two microresonator tubes placed between wedged BaF2 windows (according to the specifications, the transmission of each window was approximately 93%). With each element causing some signal power loss, the actual optical power inside the QEPAS module was approximately 246 mW. The signal from the QEPAS module was demodulated at ∼12.4628 kHz (resonance frequency of the QTF) using a lock-in amplifier (LIA-MVD-200-L from Femto) to retrieve a 2f-QEPAS spectrum, which was recorded with a data acquisition card (at certain conditions the amplitude of the 2f-QEPAS signal can be assumed to be proportional to the molecular concentration)
Fig. 2. Schematic diagram of the experimental setup, containing the QEPAS module filled with a gas sample, bismuth-doped fiber amplifier (BDFA), the laser diode (LD) operating near 1651 nm, and lock-in amplifier demodulating the QEPAS signal (C – collimator, L – lens, ISO – optical isolator, BDF – bismuth doped fiber)
Fig. 3. Bismuth-doped fiber amplifier output power as a function of input power at ∼1651 nm. The input power was varied by changing the laser current of the DFB laser diode. For IBIAS ≈ 75 mA the wavelength of the DFB laser diode matches the center of the target methane absorption line.
In order to obtain the highest level of QEPAS signal one needs to select optimal wavelength modulation parameters. This includes choosing optimal modulation frequency (which should match half of the QTF resonant frequency) and modulation amplitude (which is related to the width of the target molecular transition). Both parameters have been determined experimentally, using light directly from the laser diode (i.e. without signal enhancement with BDFA). Figure 4 shows the normalized 2f-QEPAS amplitude measured for different modulation/demodulation frequencies. The maximum value was achieved for the modulation frequency of 6.2314 kHz (demodulation at 12.4628 kHz), which was used in all other experiments presented in this paper. Figure 5(a) shows the 2f-QEPAS spectra recorded for different current modulation depths. As shown in Fig. 5(b), the current modulation amplitude that results in the largest signal amplitude is ∼27.5 mA (peak-peak).
Fig. 4. Normalized 2f-QEPAS signal amplitude as a function of the demodulation frequency shows the resonance profile of the QEPAS module at ambient pressure.
Fig. 5. (a) QEPAS spectra of methane transition for different current modulation amplitudes (for this measurement QEPAS module was filled with 103 ppm methane mixture at 760 torr); (b) normalized 2f-QEPAS amplitude as a function of current modulation amplitude show the optimum modulation amplitude of ∼27.5 mA.
3. QEPAS sensor performance
All measurements presented in this paper were performed in room temperature and at atmospheric pressure. The impact of optical power on QEPAS signals is shown in Fig. 6. For this measurements the QEPAS module was filled with a gas mixture containing 103 ppm of methane (in synthetic air, 21% of oxygen and 79% of nitrogen) at 760 Torr. The gas was introduced into the QEPAS module from cylinders. During the measurement the inlet and outlet of the QEPAS module were closed so that there was no gas flow through the module. The optical power level was varied by changing the BDFA’s pump power. As shown in Fig. 6, the amplitude of the QEPAS signal (measured at the center of the methane transition near 1650.96 nm) is directly proportional to optical power. At the same time, noise level (calculated as standard deviation of the signal away from the absorption line) stays at the same level (as shown in Fig. 6(b)).
Fig. 6. (a) 2f-QEPAS spectra measured for different optical power levels; (b) 2f-QEPAS amplitude vs. optical power.
Furthermore, the linear response of the sensor to various methane concentrations has been analyzed. Figure 7(a) shows the 2f-QEPAS spectra recorded for four different gas samples, including calibrated methane in synthetic air mixtures (methane concentrations of 5.8, 23.9 and 103 ppm) and nitrogen. As expected (based on previous reports on amplifier-enhanced QEPAS), a linearity between the amplitude of the QEPAS signal and the molecular concentration has been obtained (as shown in Fig. 7(b)).
Fig. 7. (a) 2f-QEPAS spectra measured for the optical power of 246 mW and the gas samples with different methane concentration; (b) 2f QEPAS amplitude vs. methane concentration.
In order to characterize the long-term stability of the system and estimate the detection limit, the laser diode current was not ramped but adjusted so that the signal at the center of the 2f-QEPAS spectrum was measured continuously. The QEPAS module was filled with a gas sample of 5.8 ppm of methane and the lock-in time constant was set to 300 ms (resulting in the demodulation bandwidth of 0.83 Hz). A 5-hour measurement series was recorded with a sampling rate of 2 Hz (shown in Fig. 8(a)). This time series was used to calculate the Allan-Werle variance shown in Fig. 8(b). A drift visible in data in Fig. 8(a) is due to drift of the laser wavelength from the transition center and from small leaks of the QEPAS module. From the Allan-Werle variance the detection limit of 94 ppb is estimated for 1-s integration time. This corresponds to noise equivalent absorption coefficient of 9.52 × 10−9 cm−1W/√Hz (assuming absorption coefficient of 0.375 cm−1, based on HITRAN database). With longer averaging noise was reduced and the minimum detection limit of 11 ppb was obtained (for integration time of ∼150 s).
Fig. 8. (a) A time series of 2f QEPAS amplitude recorded with a sampling rate of 2 Hz for the gas sample containing 5.8 ppm of methane; (b) calculated Allan-Werle deviation.
4. Discussion and conclusions
In this paper we have demonstrated that using a novel fiber-based optical amplifier, near-infrared QEPAS-based methane sensors can obtain detection limits similar to those of mid-infrared systems (e.g. [28–30]). To the best of our knowledge, only one near-infrared PAS-based methane sensor with optical amplifier for signal enhancement has been demonstrated so far. In [31] the detection limit of 110 ppb was estimated using Allan-Werle deviation analysis and for 1 s integration time. In our work a better detection limit (94 ppb) was obtained using much lower optical power (246 mW vs. 1.1 W in [31]). It is also worth mentioning that the design of the bismuth-doped fiber amplifier used in this work is very similar to typical rare-earth-doped fiber amplifiers and is much simpler than the Raman amplifier presented in [31]).
The results presented in this paper were obtained using dry methane/synthetic air mixtures at ambient pressure. This means that there is still some room to improve the performance of the system. For example, because the transfer of vibrational energy into the kinetic energy (so called V-T relaxation) depends on gas sample composition, it is often possible to increase QEPAS signal by adding water vapor to the sample. This effect was previously studied in [25,34]. Figure 9 shows two QEPAS spectra recorded in our setup, one for dry sample with 5.8 ppm methane, and second for sample to which water vapor was added. In this measurement, the wet sample was obtained by sending the gas from the cylinder through the bubbler gas humidifier filled with water at room temperature (which results in water vapor concentration of approximately 2%). By adding water vapor to the sample, the amplitude of the QEPAS signal has been increased by more than 40%.
Fig. 9. 2f-QEPAS spectra measured for a dry and humified sample of 5.8 ppm of methane at 760 torr. The sample was moistened by passing it through the bubbler humidifier.
In conclusion, a near-infrared QEPAS-based methane sensor has been demonstrated. With a novel bismuth-doped fiber amplifier a minimum detection limit of 94 ppb (for integration time of 1 s) has been achieved. The simple and fiber-based design of the sensor makes it particularly suitable for sensitive QEPAS-based methane detection in distant/remote locations.
Funding
Narodowe Centrum Nauki (UMO-2018/29/B/ST7/01730).
Acknowledgments
Sergei Firstov from Dianov Fiber Optics Research Center, Moscow, for designing, fabricating and providing Bi-doped fiber used in this work. The Department of Optics and Photonics (Wroclaw University of Science and Technology) acknowledges the financial support within the National Laboratory for Photonics and Quantum Technologies (NLPQT) infrastructural project (POIR.04.02.00-00-B003/18), co-financed by the European Regional Development Fund (ERDF).
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.
References
1. A. Farooq, A. B. S. Alquaity, M. Raza, E. F. Nasir, S. Yao, and W. Ren, “Laser sensors for energy systems and process industries: Perspectives and directions,” Prog. Energy Combust. Sci. 91, 100997 (2022). [CrossRef]
2. J. Xia, F. Zhu, J. Bounds, E. Aluauee, A. Kolomenskii, Q. Dong, J. He, C. Meadows, S. Zhang, and H. Schuessler, “Spectroscopic trace gas detection in air-based gas mixtures: Some methods and applications for breath analysis and environmental monitoring,” J. Appl. Phys. 131(22), 220901 (2022). [CrossRef]
3. J. B. McManus, D. D. Nelson, and M. S. Zahniser, “Design and performance of a dual-laser instrument for multiple isotopologues of carbon dioxide and water,” Opt. Express 23(5), 6569–6586 (2015). [CrossRef]
4. P. Kluczynski and O. Axner, “Theoretical description based on Fourier analysis of wavelength-modulation spectrometry in terms of analytical and background signals,” Appl. Opt. 38(27), 5803–5815 (1999). [CrossRef]
5. G. B. Rieker, J. B. Jeffries, and R. K. Hanson, “Calibration-free wavelength-modulation spectroscopy for measurements of gas temperature and concentration in harsh environments,” Appl. Opt. 48(29), 5546–5560 (2009). [CrossRef]
6. A. A. Kosterev, Y. A. Bakhirkin, R. F. Curl, and F. K. Tittel, “Quartz-enhanced photoacoustic spectroscopy,” Opt. Lett. 27(21), 1902–1904 (2002). [CrossRef]
7. C. Shi, D. Wang, Z. Wang, L. Ma, Q. Wang, K. Xu, S.-C. Chen, and W. Ren, “A Mid-Infrared Fiber-Coupled QEPAS Nitric Oxide Sensor for Real-Time Engine Exhaust Monitoring,” IEEE Sensors J. 17(22), 7418–7424 (2017). [CrossRef]
8. Y. Ma, R. Lewicki, M. Razeghi, and F. K. Tittel, “QEPAS based ppb-level detection of CO and N2O using a high power CW DFB-QCL,” Opt. Express 21(1), 1008–1019 (2013). [CrossRef]
9. V. Spagnolo, P. Patimisco, S. Borri, G. Scamarcio, B. E. Bernacki, and J. Kriesel, “Mid-infrared fiber-coupled QCL-QEPAS sensor,” Appl. Phys. B 112(1), 25–33 (2013). [CrossRef]
10. B. Sun, A. Zifarelli, H. Wu, S. dello Russo, S. Li, P. Patimisco, L. Dong, and V. Spagnolo, “Mid-Infrared Quartz-Enhanced Photoacoustic Sensor for ppb-Level CO Detection in a SF6 Gas Matrix Exploiting a T-Grooved Quartz Tuning Fork,” Anal. Chem. 92(20), 13922–13929 (2020). [CrossRef]
11. J. P. Waclawek, H. Moser, and B. Lendl, “Compact quantum cascade laser based quartz-enhanced photoacoustic spectroscopy sensor system for detection of carbon disulfide,” Opt. Express 24(6), 6559–6571 (2016). [CrossRef]
12. H. Wu, L. Dong, X. Liu, H. Zheng, X. Yin, W. Ma, L. Zhang, W. Yin, and S. Jia, “Fiber-Amplifier-Enhanced QEPAS Sensor for Simultaneous Trace Gas Detection of NH3 and H2S,” Sensors 15(10), 26743–26755 (2015). [CrossRef]
13. Y. Ma, Y. He, Y. Tong, X. Yu, and F. K. Tittel, “Ppb-level detection of ammonia based on QEPAS using a power amplified laser and a low resonance frequency quartz tuning fork,” Opt. Express 25(23), 29356–29364 (2017). [CrossRef]
14. M. E. Webber, M. Pushkarsky, and C. K. N. Patel, “Fiber-amplifier-enhanced photoacoustic spectroscopy with near-infrared tunable diode lasers,” Appl. Opt. 42(12), 2119–2126 (2003). [CrossRef]
15. Z. Shang, S. Li, B. Li, H. Wu, A. Sampaolo, P. Patimisco, V. Spagnolo, and L. Dong, “Quartz-enhanced photoacoustic NH3 sensor exploiting a large-prong-spacing quartz tuning fork and an optical fiber amplifier for biomedical applications,” Photoacoustics 26, 100363 (2022). [CrossRef]
16. H. Wu, A. Sampaolo, L. Dong, P. Patimisco, X. Liu, H. Zheng, X. Yin, W. Ma, L. Zhang, W. Yin, V. Spagnolo, S. Jia, and F. K. Tittel, “Quartz enhanced photoacoustic H2S gas sensor based on a fiber-amplifier source and a custom tuning fork with large prong spacing,” Appl. Phys. Lett. 107(11), 111104 (2015). [CrossRef]
17. Y. He, Y. Ma, Y. Tong, X. Yu, and F. K. Tittel, “HCN ppt-level detection based on a QEPAS sensor with amplified laser and a miniaturized 3D-printed photoacoustic detection channel,” Opt. Express 26(8), 9666–9675 (2018). [CrossRef]
18. Z. Yang, H. Lin, B. A. Z. Montano, W. Zhu, Y. Zhong, B. Yuan, J. Yu, R. Kan, M. Shao, and H. Zheng, “High-power near-infrared QEPAS sensor for ppb-level acetylene detection using a 28 kHz quartz tuning fork and 10 W EDFA,” Opt. Express 30(4), 6320–6331 (2022). [CrossRef]
19. O. Peltola, I. Mammarella, S. Haapanala, G. Burba, and T. Vesala, “Field intercomparison of four methane gas analyzers suitable for eddy covariance flux measurements,” Biogeosciences 10(6), 3749–3765 (2013). [CrossRef]
20. K. Zheng, C. Zheng, D. Yao, L. Hu, Z. Liu, J. Li, Y. Zhang, Y. Wang, and F. K. Tittel, “A near-infrared C2H2/CH4 dual-gas sensor system combining off-axis integrated-cavity output spectroscopy and frequency-division-multiplexing-based wavelength modulation spectroscopy,” Analyst 144(6), 2003–2010 (2019). [CrossRef]
21. Z. J. DeBruyn, C. Wagner-Riddle, and A. VanderZaag, “Assessment of Open-path Spectrometer Accuracy at Low Path-integrated Methane Concentrations,” Atmosphere 11(2), 184 (2020). [CrossRef]
22. Y. Ma, Y. He, X. Yu, J. Zhang, R. Sun, and F. K. Tittel, “Compact all-fiber quartz-enhanced photoacoustic spectroscopy sensor with a 30.72 kHz quartz tuning fork and spatially resolved trace gas detection,” Appl. Phys. Lett. 108(9), 091115 (2016). [CrossRef]
23. Y. He, Y. Ma, Y. Tong, X. Yu, Z. Peng, J. Gao, and F. K. Tittel, “Long distance, distributed gas sensing based on micro-nano fiber evanescent wave quartz-enhanced photoacoustic spectroscopy,” Appl. Phys. Lett. 111(24), 241102 (2017). [CrossRef]
24. G. Menduni, F. Sgobba, S. dello Russo, A. C. Ranieri, A. Sampaolo, P. Patimisco, M. Giglio, V. M. N. Passaro, S. Csutak, D. Assante, E. Ranieri, E. Geoffrion, and V. Spagnolo, “Fiber-Coupled Quartz-Enhanced Photoacoustic Spectroscopy System for Methane and Ethane Monitoring in the Near-Infrared Spectral Range,” Molecules 25(23), 5607 (2020). [CrossRef]
25. A. A. Kosterev, Y. A. Bakhirkin, F. K. Tittel, S. McWhorter, and B. Ashcraft, “QEPAS methane sensor performance for humidified gases,” Appl. Phys. B 92(1), 103–109 (2008). [CrossRef]
26. L. Hu, C. Zheng, M. Zhang, D. Yao, J. Zheng, Y. Zhang, Y. Wang, and F. K. Tittel, “Quartz-enhanced photoacoustic spectroscopic methane sensor system using a quartz tuning fork-embedded, double-pass and off-beam configuration,” Photoacoustics 18, 100174 (2020). [CrossRef]
27. T. Milde, M. Hoppe, H. Tatenguem, H. Rohling, S. Schmidtmann, M. Honsberg, W. Schade, and J. Sacher, “QEPAS sensor in a butterfly package and its application,” Appl. Opt. 60(15), C55–C59 (2021). [CrossRef]
28. H. Wu, L. Dong, X. Yin, A. Sampaolo, P. Patimisco, W. Ma, L. Zhang, W. Yin, L. Xiao, V. Spagnolo, and S. Jia, “Atmospheric CH4 measurement near a landfill using an ICL-based QEPAS sensor with V-T relaxation self-calibration,” Sens. Actuators, B 297, 126753 (2019). [CrossRef]
29. A. Sampaolo, S. Csutak, P. Patimisco, M. Giglio, G. Menduni, V. Passaro, F. K. Tittel, M. Deffenbaugh, and V. Spagnolo, “Methane, ethane and propane detection using a compact quartz enhanced photoacoustic sensor and a single interband cascade laser,” Sens. Actuators, B 282, 952–960 (2019). [CrossRef]
30. M. Jahjah, W. Ren, P. Stefański, R. Lewicki, J. Zhang, W. Jiang, J. Tarka, and F. K. Tittel, “A compact QCL based methane and nitrous oxide sensor for environmental and medical applications,” Analyst 139(9), 2065–2069 (2014). [CrossRef]
31. R. Bauer, T. Legg, D. Mitchell, G. M. H. Flockhart, G. Stewart, W. Johnstone, and M. Lengden, “Miniaturized Photoacoustic Trace Gas Sensing Using a Raman Fiber Amplifier,” J. Lightwave Technol. 33(18), 3773–3780 (2015). [CrossRef]
32. S. Firstov, S. Alyshev, K. Riumkin, A. Khegai, A. Kharakhordin, M. Melkumov, and E. Dianov, “Laser-Active Fibers Doped With Bismuth for a Wavelength Region of 1.6–1.8 µm,” IEEE J. Select. Topics Quantum Electron. 24(5), 1–15 (2018). [CrossRef]
33. I. Bufetov, M. Melkumov, S. Firstov, K. Riumkin, A. Shubin, V. Khopin, A. Guryanov, and E. Dianov, “Bi-Doped Optical Fibers and Fiber Lasers,” IEEE J. Sel. Top. Quantum Electron. 20(5), 111–125 (2014). [CrossRef]
34. S. Schilt, J.-P. Besson, and L. Thévenaz, “Near-infrared laser photoacoustic detection of methane: the impact of molecular relaxation,” Appl. Phys. B 82(2), 319–328 (2006). [CrossRef]
