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Abstract
The differential absorption lidar (DIAL) has been proposed as an effective method for detecting polluted gases in the atmosphere. In this paper, we present a compact and movable ozone differential absorption (O3-DIAL) based on an all-solid-state and tuning-free laser source. For the first time, solid-state stimulated Raman scattering technology is used in the emitting source of the lidar for wavelength conversion. A high repetition frequency Innoslab laser is used for pumping SrWO4 crystals to get yellow lasers which can achieve up to 70% light-to-light conversion efficiency. Our results demonstrate that using the SrWO4 crystal as the Raman frequency-shifting media of the lidar laser source for obtaining the vertical profiles of tropospheric ozone in the Planetary Boundary Layer (PBL) is a suitable choice. As a compact movable lidar system, the results demonstrate the reliability and stability.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Ozone is one of the most important trace components in the earth’s atmosphere. High concentrations of ozone in the stratosphere absorb most of the ultraviolet rays from the sun to prevent the earth from being harmed. Tropospheric ozone is one of greenhouse gases and pollutant detrimental to human health and plant growth [1,2]. Early in 1991, the formation of tropospheric ozone was described in detail and it recommended increased air-quality measurement and modeling to systematically devise and check future regulatory strategies [3]. Nowadays zone pollution occurs in many countries around the world [4]. China’s megacities are even worse, with increasing emission of chemical precursors [5–7]. The State Council of China issued the Air Pollution Prevention and Control Action Plan (APPCAP) for reducing anthropogenic emissions which have led to severe region-wide air pollution [8]. So a long-term and regional monitoring of tropospheric ozone is required for emission control.
The differential absorption lidar (DIAL) technique [9,10] is well suited for the demands as its rapid and repetitive measurement of spatial distributions of ozone concentrations among various ozone measurement methods. UV lasers in the spectral region from 270 to 300 nm is generally used to measure ozone concentration in the troposphere and the spectral width of the lasing line of the SRS converter is determined by the spectral width of the Nd:YAG laser’s radiation and is less than one reciprocal centimeter, and this is quite adequate for probing ozone [11].There are many research on the DIAL systems for ozone measurements recently. Using a fourth harmonic of a Nd:YAG laser to pump a tens-of-centimeters-long Raman cell filled with CO2, H2, D2 or H2/D2 mixtures [12–14] is a conventional technique which can obtain efficient stimulated Raman shifting in the ultraviolet (UV) spectral region (266-316 nm) with pulse energy <10mJ. The low particle concentration leads to a large size of the device for increasing the conversion efficiency, and the low thermal conductivity of gases means that the lidar can only use pump lasers of low repetition rates. So it is difficult to achieve a small size and high time resolution by a DIAL based on this technique. An airborne DIAL system used Nd:YAG-pumped dye lasers for generating the DIAL UV wavelengths of 288nm-300 nm of 30 mJ per pulse for ozone measurements [15]. It need to supplement the dye frequently for continuous measurements. Beams with laser pulse energy >30 mJ at UV wavelengths of 289 and 300 nm were generated through a tripling of the frequencies of two frequency-doubled Nd:YAG-pumped Ti:sapphire lasers and were intended to be used in an airborne ozone DIAL system [16]. An airborne ozone DIAL system also leveraged advanced Nd:YAG and optical parametric oscillator (OPO) laser technologies for generating the wavelengths of 290 and 300 nm with pulse energy <10mJ [17,18]. A mobile ground-based ozone lidar system based on a cerium-doped lithium calcium aluminum fluoride (Ce:LiCAF) laser produced UV wavelengths of 286 and 291 nm with energy of approximately 0.2 mJ/pulse [19,20]. These tunable lasers are complex to be maintained and the long-term stability needs to be discussed.
Long-term measurements at many geographic locations are required in order to monitor the trend of tropospheric ozone and figure out the source of ozone pollution. Routine observations needs an unattended system which can be a part of the regional air quality monitoring network. A reliable and stable laser source is needed for ground-based and movable systems. This paper will describe a compact and movable ozone lidar based on an all-solid-state tuning-free laser source for reliable and unattended continuous observation. A 532 nm Innoslab laser pumping two external Raman resonators were developed and operated at visible wavelengths and subsequently frequency doubled into the UV for ozone atmospheric measurements. For the first time, solid-state Raman technique is used as the key technology of ozone differential absorption lidar. The set-up of the system can be fixed-location and mobile and the size reduction makes it much lighter. Because of the specific frequency-shifting of Raman crystals, the laser source needs no tuning, which makes the center wavelength and the energy of the output pulse much more stable. The ozone lidar reduces complexity and maintenance problems and it has provided the spatial distribution of ozone concentration from ground to approximately 3 km through ground-based and mobile measurements.
2. Solid-state Raman-shifting system
2.1 Selection of Raman crystal
Stimulated Raman scattering (SRS) is a well-known and widely applied method for frequency conversion of laser radiation [21]. Since the 1980s, the solid-state Raman laser technique has been clearly progressing due to the discovery of solid-state materials with high Raman gain such as KGd(WO4)2, Ba(NO3)2 crystals [22]. Compared with gas and liquid Raman media, solid-state Raman media has advantages of higher Raman gain, better thermal conductivity and mechanical properties to integrate with solid-state pump lasers.
The response time of a crystalline material to a radiation field is a finite value. The stimulated Raman scattering of crystalline materials is divided into two types according to the response time. One is the steady-state stimulated Raman scattering, the pulse width ${\tau _p}$ of the pump laser is much longer than the relaxation time t2 of the molecular vibration which is ${\tau _p}$> t2. Generally, Raman crystals pumped by a nanosecond laser can lead to the steady-state stimulated Raman scattering. The other type is transient stimulated Raman scattering, for when ${\tau _p}$ is less than or equal to t2. For crystalline materials, t2 is on the order of picoseconds. Therefore, Raman crystals pumped by a picosecond laser can cause transient stimulated Raman scattering effect.
For steady-state stimulated Raman scattering of crystalline materials, the general expression of the Raman gain coefficient as follows [23]:
The steady-state Raman gain coefficient is proportional to the spontaneous Raman scattering cross-section ${{{d\sigma }} \!\mathord{\left/ {\vphantom {{d\sigma } {d{\Omega }}}}\right.}{{d{\Omega }}}}$. N is the number density of Raman-active molecules and the value of N is almost the same for different crystalline materials. So a higher steady-state scattering gain coefficient often depends on the narrower Raman linewidth and the lager scattering cross section. The peak intensity of the spontaneous Raman scattering spectrum $\sum peak \propto {\raise0.7ex\hbox{${d\sigma }$} \!\mathord{\left/ {\vphantom {{d\sigma } {d\Omega }}} \right.}\!\lower0.7ex\hbox{${d\Omega }$}}$($\Delta {v_s}$)-1 can be used to determine in advance which crystalline material may be suitable for the steady-state stimulated Raman scattering.
Some key spectral properties for common Raman crystals are shown in Table 1. Ba(NO3)2 (BN) was reported by A.S. Eremenko et al. [24], as a high-quality solid-state Raman medium in 1980. Since then his Raman crystal has been widely studied and applied. BN has the highest steady-state Raman gain coefficient and is considered to be the preferred Raman crystal for applications with nanosecond pulses. However, it is deliquescent easily which makes storage and transportation complicated. In addition, its low thermal conductivity and poor processing performance are not conducive for long-term running. We can obtain different frequency-shifting under different pumping configurations of KGd(WO4)2 (KGW) [25] for a rich output spectrum. But KGW has a high-integrated Raman scattering cross section which limits its use for steady-state SRS and a comparatively broad Raman linewidth which lowers the steady-state Raman gain. LiIO3 [26] is deliquescent and the laser damage threshold is very low. SrWO4 [27] has a high-integrated Raman scattering cross and peak cross sections and the relaxation time of the molecular vibration is about 3.55ps which is much less than the pulse width of nanosecond of a pump laser generally used in ozone DIAL systems. So it is a high-quality solid-state Raman medium for wavelength conversion of nanosecond lasers which can lead to the steady-state stimulated Raman scattering of high Raman gain coefficient. As such, we intend to use SrWO4 as a part of a nanosecond laser-pumped Solid-state Raman-shifting system.
Table 1. Material properties of common Raman-active crystals.
2.2 Comparison of DIAL detection capability
Based on a 532 nm Nd:YAG laser pumping a SrWO4 crystal with a frequency-shifting of 921 cm-1, we can obtain visible wavelengths of 560 and 590 nm. Then the system achieves UV laser output at 280 and 295 nm using a second frequency doubling. So we can know that if SrWO4 crystals are used as a solid-state Raman medium of an ozone DIAL, the emission wavelengths are 280 nm and 295 nm. As an important indicator of a DIAL, the minimum detection concentration can be evaluated by the following equation [28,29]:
where NEP is the noise equivalent power, z is the detection range, K is the receiver efficiency, $\rho $ is the reflectivity of the topographic target, Pt is the output power of the laser beam, $\sigma $ is the difference in the absorption coefficients of the pollutant at the two wavelengths, and exp (-2$\sigma ^{\prime}$z) is the atmospheric attenuation.Three different DIAL systems are mentioned here: Our DIAL established before, based on the SRS effect of Raman-active gases, emitting three wavelengths of 266 nm, 289 nm and 316 nm which is the same as [30]. The LMOL lidar in Ref. [20], emitting 286 nm and 291 nm lasers for ozone measurements, is represented as a current advance DIAL system. Our new lidar based on the solid-state Raman shifting system emits two wavelengths of 280 and 295 nm. The relative comparison of the detection capability of these three DIAL systems is given in Fig. 1. Taking the pair of 280 nm and 295 nm as the standard, each curve indicates the ratio of the minimum detection concentration between two pairs of output wavelengths. A value more than 1 indicates that the relative detection capability of the output wavelengths is weaker than that of 280-295 nm output wavelengths. It can be seen from the figure that the pair of 280 nm and 295 nm has almost the strongest ability to detect a lower limit. So for the detection of ozone concentration of the troposphere, SrWO4 is a suitable Raman medium for the DIAL.
2.3 Design of external-cavity Raman resonator
SRS is an inelastic scattering process, providing new coherent light sources and extending the number of emission wavelengths of available solid state lasers over a fixed frequency-shifting. SRS in crystals is employed in developing powerful and compact solid-state lasers due to high conversion efficiency, no phase matching necessity, and easier handing comparing to gaseous and liquid Raman cells.
Recently compact laser sources were created based on an external-cavity solid-state Raman laser intending for diagnosing tropospheric ozone [31–33]. An external-cavity Raman laser system is shown in Fig. 2. External-cavity solid-state Raman lasers provide lower threshold of SRS, better beam quality and the output wavelength can be selected by the theoretical calculation of cavity mirrors.
The use of the rate equation to analyze the characteristics of resonators is usually applied in general lasers. The SRS term is added to the differential equation of the laser. According to the cascade effect of SRS and considering the third Stokes, in order to make the rate equation of the external Raman laser general, a normalized theoretical model is given by the following equation [34]:
The main parameters in the normalized theoretical model are the normalized Raman gain coefficient GL and the normalized pulse width NL, where NL = tL/tRT (tL is the pulse width of the pump laser). Physical quantities such as the pump energy, pulse width, Raman gain coefficient and length of a crystal are included in these two parameters. By controlling the values of them, the maximum conversion efficiency of the target output Stokes can be obtained.
By solving the rate equation we can know the output states of each order Stokes light, as shown in the figure below. Figure 3(a) shows the pulse waveform of the first and second Stokes obtained by the rate equation. When the pump laser intensity reaches the pumping threshold of the first Stokes, the intensity of the first Stokes increases rapidly. The second Stokes is generated because of the cascade effect of SRS similarly. That is, each order Stokes light can be considered as the pump light of the next order Stokes light. It can also be seen from the figure that the pulse width compression effect with the order of the Stokes becomes higher. The pulse waveforms, respectively obtained from experiments using a Tektronix TDS 3014c oscilloscope are shown in Fig. 3(b), which also confirms the effect.
More importantly, we can determine how to control the output mirror of the resonant cavity in order to obtain the maximum conversion efficiency of the target output Stokes with different parameters of pump lasers and the Raman resonator. The simulation parameters are shown in Table 2. We assume that the pump laser is completely transmitted through the input mirror and output mirror has a complete reflectivity at the pump laser, which means the pump laser is double-passed through the Raman medium. The double-pass pumping can reduce the threshold of the Stokes and improve the conversion efficiency. The design concept is such that when the first Stokes needs to be output, the input mirror has high reflectivity at the first Stokes and the two mirrors both need to be highly transparent to other orders of Stokes light to suppress their generation. It is necessary to optimize the reflectivity of the output mirror at the first Stokes to obtain high conversion efficiency. When the second Stokes needs to be output, the two mirrors have highly reflective at the first Stokes and the input mirror also has high reflectivity at the second Stokes. We can optimize the reflectivity of the output mirror at the second Stokes to obtain high conversion efficiency.
Table 2. Simulation parameters.
It can be obtained that different normalized pulse widths and Raman gains have corresponding maximum conversion efficiencies and optimal reflectivities in Fig. 4. The simulation result of the first Stokes output is shown in Fig. 4(a). For a certain normalized pulse width, the first Stokes is not generated because of the low normalized Raman gain. As the normalized Raman gain increases, first Stokes is generated. The maximum conversion efficiency increases and the corresponding optimal reflectivity decreases. The existence of the reflectivity of the output mirror at first Stokes is actually to allow a little energy of first Stokes to pass through the crystal again. This is in order to generate stimulated amplification as a seed light and obtain higher conversion efficiency. But when the normalized Raman gain is gradually increased, the intensity of the first Stokes gets higher and higher. Then the increase of the intensity caused by this effect is much lower than when the normalized gain is low. So the transmittance of the output mirror needs to be increased to improve the conversion efficiency of the first Stokes. It can be seen that the conversion efficiency increases and the optimal reflectivity decreases. When the normalized Raman gain is a certain value, the higher NL, the higher maximum conversion efficiency and it give a smaller optimal reflectivity. This shows that the normalized pulse width has little effect on the optimal reflectivity. The effect of NL on the maximum conversion efficiency can be summarized as follows: it can be known that the pump light intensity is constant when the normalized Raman gain is a certain value. The larger the pulse width of the pump light, the longer time the scattered light is amplified in the Raman medium. The intensity of the SRS light is also higher. This shows that shortening the Raman cavity length as much as possible is conducive to improving the conversion efficiency. Figure 4(b) shows the simulation result of optimization of the second Stokes output. The analysis is similar to that of the first Stokes.
Fig. 4. Maximum conversion efficiency of the target output Stokes and the corresponding reflectivity of the output mirror at the output Stokes under different configurations (a: first Stokes b: second Stokes)
2.4 Establishment of external-cavity Raman shifting system for DIAL
Based on the simulation, we establish an external-cavity Raman shifting system using two Raman resonators to obtain first and second Stokes, respectively. The parameters is shown in Table 3.
Table 3. Parameters of external-cavity Raman shifting system.
We can use this system by adjusting GL and NL to get the conversion efficiency of the first and second Stokes for verifying the significance of the theoretical model to the experimental results. Here the NL is changed by adjusting the length of the Raman resonator (NL = 6.08, 10.86 corresponds to the length of 125 mm, 60 mm, respectively) and GL is changed by adjusting the intensity of pump light (GL = 2.79, 4.27, 5.09, 7.06 corresponds to the pump intensity of 46.5MW/cm2, 71.1MW/cm2, 84.8MW/cm2, and 117.7MW/cm2). The simulation results are compared with the experimental results in Fig. 5. The light-to-light conversion efficiencies related to different pump intensities at each resonator length of first and second Stokes are shown in Figs. 5(a) and 5(b), respectively. It can be seen that the shorter the length of the resonator, the higher the conversion efficiency. The conversion efficiency has a maximum value when the pump intensity is appropriate. Once the pump intensity continues to be increased, the conversion efficiency will be reduced because of the energy is converted to higher-order Stokes. From the figures, it can be known that the theoretical model has a certain guide on the control of experimental parameters. More importantly, we have achieved a light-to-light conversion efficiency (the ratio of input to output energy in the Raman resonator) up to 70% using the solid-state Raman technique, which is greatly improved compared with that of previous frequency-shifting techniques.
Fig. 5. Conversion efficiency of Stokes as a function of GL under different NL (a: first Stokes b: second Stokes)
Using the external-cavity Raman shifting pumped by the frequency-doubling Innoslab laser makes it possible to obtain the first Stokes (560 nm) radiation with an energy up to 3.3 mJ and the second Stokes (590 nm) radiation with an energy up to 2.9 mJ. The near field spatial profiles of the two Stokes beams are shown in Fig. 6. Based on the Stokes radiations with the clean spatial profiles, we can do the next-step work of constructing the transmitter of an ozone DIAL.
Fig. 6. Spatial profiles of Stokes radiations (a: first Stokes, about 3 mm b: second Stokes, about 3 mm)
3. Ozone DIAL system
3.1 DIAL system device
The ozone DIAL system (SSRL-lidar, solid-state-Raman-laser-DIAL) is shown in Fig. 7. The transmitter of the DIAL is established based on the external-cavity Raman shifting system. The Innoslab laser lases at 532 nm which provides an energy about 14 mJ at a repetition rate of 1 kHz. Visible wavelengths of 3.3 mJ at 560 and 2.9 mJ at 590 nm are obtained through the external-cavity Raman shifting system pumped by the Innoslab laser. Then the lidar system achieves UV laser output at 280 and 295 nm by a second harmonic generator (SHG). The pulse energy at 280 nm is 0.4 mJ and 290 nm is 0.2 mJ. The conversion efficiencies are about 14% and 9% considering the input and output energy in the SHG process. Before transmission into the atmosphere, UV beams pass through a beam expander and then they have a divergence of 0.3 mrad. The receiving system is a Cassegrain telescope of 200 mm caliber and the field of view is 1.5 mrad. The telescope collects the backscattering light and a dichroic mirror reflects the UV light of 280 nm to a narrowband interference filter with an FWHM bandwidth of 2.0 nm with a transmittance over 85%. Simultaneously, the UV light of 295 nm passes through the dichroic mirror and a narrowband interference filter. Both the lights are then focused by a lens onto a photomultiplier tube (Hamamatsu R9800), respectively. Analog digitizer and photon counting are used as a combination in data processing. In addition, a GPS is placed in the device to record the coordinate position during mobile detection. The temperature controller box is used to radiate the Raman medium and keep the temperature of frequency-doubling crystals steady.
Table 4 shows the main parameters of the SSRL-DIAL system.
Table 4. Main parameters of the SSRL-DIAL.
3.2 Ground-based measurements and results
The measurement data of SSRL-DIAL is validated by comparing the results with a lidar system which is based on a Nd:YAG laser emitting at 266 nm. A high-pressure Raman cell filled with D2 is used to generate UV wavelengths (289, 316 nm) for ozone measurements (GRL-DIAL, gases-Raman-laser-DIAL). Measurements results of the GRL-DIAL was validated by comparing with data of a national monitoring and control station and ozonesonde. Besides, the lidar is working stably and well all along. The measurements during the same period by the two DIAL systems can be compared to prove the reliability and accuracy of ground-based observation of the SSRL-DIAL. Figure 8 shows the results of ozone profiles taken by two lidars at approximately the same time are almost in good consistency. The difference occurred below 0.4 km due to the caliber of the GRL-DIAL’s telescope is larger than that of the SSRL-DIAL which led a higher value of the overlap between the receiver and transmitter optics.
In order to prove the stability and accuracy of long-term measurements, a continuous observation was carried out on the campus of the Anhui Institute of Optics and Fine Mechanics of Chinese Academy of Sciences (31.908$^\circ $N, 117.182$^\circ $E). The experiment started at 18:00 on the 22nd of January and ended at 9:00 on the 23rd January 2019. Figure 9 shows the comparison of the spatial-temporal distribution of ozone concentration measured by the SSRL-DIAL system and the GRL-DIAL system. From 0.3 km to 3 km, a general trend of ozone concentration between two results is acquired. The result illustrates that the SSRL-DIAL has a significant improvements in spatial and temporal resolution, and signal-to-noise ratio.
An error analysis of the long-term observation was shown in Fig. 10. From the linear fitting (fix intercept at 0) of measurement values during the period at different heights we can obtain the factors of slope are 0.90 (500 m), 0.92(600 m) and 0.92 (700 m). It can be prove that the measurements of the SSRL-DIAL is reliable. Therefore, the lidar can be used to obtain the spatial and temporal distribution of tropospheric ozone concentration on a fixed platform. Moreover, we can also apply the lidar to the mobile monitoring based on reliable data obtained from ground-based monitoring.
3.3 Mobile vehicle DIAL for ozone monitoring
The DIAL is convenient for mobile platform observation due to the appropriate size and weight. Figure 11 shows the appearances of the entire DIAL and the mobile platform. The ozone DIAL is mounted inside this vehicle, which can carry out a continuous mobile monitoring of the spatial and temporal distribution of ozone concentration in a specific place.
We conducted a mobile monitoring at Hengshui city of Hebei Province. The monitoring started at 14:22 and ended at 18:56 on the 4th July 2019 (local time). Figure 12(a) presents the spatial distribution of the ozone concentration profile on a map. The valid data during the period after 16:00 due to the refuel in a station. It can be seen from the Fig. 12(b) that during the time of the detection, ozone concentration was high in the near-ground.
Through the long distance of vehicle-borne detection, the regional ozone concentration distribution profile can be quickly obtained. In combination with other atmospheric factors, the source of regional ozone pollution can be considered in-depth.
4. Conclusion
A compact and mobile ozone DIAL based on an all-solid-state, tuning-free laser source applied the solid-state Raman technique in the transmitter unit of the lidar for the first time. Compared with the previous ozone DIAL, the advantages are obvious: the smaller total volume of the system makes it convenient for multi-platform monitoring. The source is tuning-free, which keeps the lidar more stable and reliable. During the observation period, the system was operating normally and the monitoring data were well. It showed a good adaptability to complex situations such as vibration, and changeable temperature. We have carried out the airborne monitoring of ozone profiles recently and obtained good results. In terms of the prospect, the solid-state Raman technique may be applied to the spaceborne ozone DIAL to obtain the global profiles and spatial distribution of ozone concentration. This can be utilized for solving the technical problems of spaceborne proactive monitoring of ozone.
Funding
National Key Project of MOST (2016YFC0200401, 2017YFC0209603, 2018YFC0213101); National Natural Science Foundation of China (41605020); National Research Program for Key Issues in Air Pollution Control (DQGG0102); Natural Science Foundation of Anhui Province (1908085QD160, 1908085QD170).
Disclosures
The authors declare no conflicts of interest.
References
1. J. Lelieveld, J. S. Evans, M. Fnais, D. Giannadaki, and A. Pozzer, “The contribution of outdoor air pollution sources to premature mortality on a global scale,” Nature 525(7569), 367–371 (2015). [CrossRef]
2. T.F. Stocker, D. Qin, G.-K. Plattner, M. Tignor, S. K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex, and P. M. Midgley, Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA., 2013).
3. N. R. Council, Rethinking the ozone problem in urban and regional air pollution by committee on tropospheric ozone formation (National Academies Press, 1992).
4. D. Tarasick, I. E. Galbally, O. R. Cooper, O. García, G. Foret, I. Petropavlovskikh, K.-L. Chang, M. G. Schultz, C. Vigouroux, G. Ancellet, T. Leblanc, T. J. Wallington, J. Ziemke, X. Liu, M. Steinbacher, J. Staehelin, P. Zanis, E. Weatherhead, A. Gaude, A. M. Thompson, S. J. Oltmans, J. Cuesta, V. Thouret, B. Hassler, J. W. Hannigan, H. Worden, M. Osman, J. Liu, M. Lin, M. Granados-Muñoz, G. Dufour, T. Trickl, and J. L. Neu, “Tropospheric Ozone Assessment Report: Tropospheric ozone from 1877 to 2016, observed levels, trends and uncertainties,” Elem. Sci. Anth. 7(1), 39 (2019). [CrossRef]
5. M. Kulmala, “Atmospheric chemistry: China’s choking cocktail,” Nature 526(7574), 497–499 (2015). [CrossRef]
6. T. Wang, L. Xue, P. Brimblecombe, Y. F. Lam, L. Li, and L. Zhang, “Ozone pollution in China: A review of concentrations, meteorological influences, chemical precursors, and effects,” Sci. Total Environ. 575, 1582–1596 (2017). [CrossRef]
7. B. Zheng, D. Tong, M. Li, F. Liu, C. Hong, G. Geng, H. Li, X. Li, L. Peng, J. Qi, L. Yan, Y. Zhang, H. Zhao, Y. Zheng, K. He, and Q. Zhang, “Trends in China's anthropogenic emissions since 2010 as the consequence of clean air actions,” Atmos. Chem. Phys. 18(19), 14095–14111 (2018). [CrossRef]
8. L. Feng and W. Liao, “Legislation, plans, and policies for prevention and control of air pollution in China: achievements, challenges, and improvements,” J. Cleaner Prod. 112, 1549–1558 (2016). [CrossRef]
9. M. H. Proffitt and A. O. Langford, “Ground-based differential absorption lidar system for day or night measurements of ozone throughout the free troposphere,” Appl. Opt. 36(12), 2568–2585 (1997). [CrossRef]
10. G. G. Gimmestad, “Differential-absorption lidar for ozone and industrial emissions,” in Lidar (Springer, New York, 2005), pp. 187–212.
11. A. Fix, M. Wirth, A. Meister, G. Ehret, M. Pesch, and D. Weidauer, “Tunable ultraviolet optical parametric oscillator for differential absorption lidar measurements of tropospheric ozone,” Appl. Phys. B 75(2-3), 153–163 (2002). [CrossRef]
12. G. R. Ancellet and F. Ravetta, “Compact airborne lidar for tropospheric ozone: description and field measurements,” Appl. Opt. 37(24), 5509–5521 (1998). [CrossRef]
13. S. Tzortzakis, G. Tsaknakis, A. Papayannisu, and A. A. serafetinides, “Investigation of the spatial profile of stimulated Raman scattering beams in D2 and H2 gases using a pulsed Nd:YAG laser at 266 nm,” Appl. Phys. B 79(1), 71–75 (2004). [CrossRef]
14. K. B. Strawbridge, M. S. Travis, B. J. Firanski, J. R. Brook, R. Staebler, and T. Leblanc, “A fully autonomous ozone, aerosol and night time water vapor LIDAR: a synergistic approach to profiling the atmosphere in the Canadian oil sands region,” Atmos. Meas. Tech. 11(12), 6735–6759 (2018). [CrossRef]
15. E. V. Browell, S. Ismail, and W. B. Grant, “Differential absorption lidar (DIAL) measurements from air and space,” Appl. Phys. B: Lasers Opt. 67(4), 399–410 (1998). [CrossRef]
16. K. A. Elsayed, S. Chen, L. B. Petway, B. L. Meadows, W. D. Marsh, W. C. Edwards, J. C. Barnes, and R. J. DeYoung, “High-energy, efficient, 30-Hz ultraviolet laser sources for airborne ozone-lidar systems,” Appl. Opt. 41(15), 2734–2739 (2002). [CrossRef]
17. J. W. Hair, E. V. Browell, T. McGee, C. Butler, M. Fenn, S. Ismail, A. Notari, J. Collins, C. Cleckner, and C. Hostetler, “Development of the global ozone lidar demonstrator (GOLD) instrument for deployment on the NASA Global Hawk,” (2010).
18. A. Fix, F. Steinebach, M. Wirth, A. Schafler, and G. Ehret, “Development and application of an airborne differential absorption lidar for the simultaneous measurement of ozone and water vapor profiles in the tropopause region,” Appl. Opt. 58(22), 5892–5900 (2019). [CrossRef]
19. R. J. Alvarez, C. J. Senff, A. O. Langford, A. M. Weickmann, D. C. Law, J. L. Machol, D. A. Merritt, R. D. Marchbanks, S. P. Sandberg, W. A. Brewer, R. M. Hardesty, and R. M. Banta, “Development and application of a compact, tunable, solid-state airborne ozone lidar system for boundary layer profiling,” J. Atmos. Oceanic Technol. 28(10), 1258–1272 (2011). [CrossRef]
20. R. D. Young, W. Carrion, R. Ganoe, D. Pliutau, G. Gronoff, T. Berkoff, and S. Kuang, “Langley mobile ozone lidar: ozone and aerosol atmospheric profiling for air quality research,” Appl. Opt. 56(3), 721–730 (2017). [CrossRef]
21. W. Lu and R. G. Harrison, “Nonlinear dynamics of Raman lasers,” Phys. Rev. A 43(11), 6358–6367 (1991). [CrossRef]
22. T. T. Basiev and V. V. Osiko, “New materials for SRS lasers,” Russ. Chem. Rev. 75(10), 847–862 (2006). [CrossRef]
23. J. A. Piper and H. M. Pask, “Crystalline Raman Lasers,” IEEE J. Sel. Top. Quantum Electron. 13(3), 692–704 (2007). [CrossRef]
24. A. S. Eremenko, S. N. Karpukhin, and A. I. Stepanov, “stimulated Raman scattering of the second harmonic of a neodymium laser in nitrate crystals,” Sov. J. Quantum Electron. 10(1), 113–114 (1980). [CrossRef]
25. R. P. Mildren, M. Convery, H. M. Pask, and J. A. Piper, “Efficient, all-solid-state, Raman laser in the yellow, orange and red.pdf,” Opt. Express 12(5), 785–790 (2004). [CrossRef]
26. H. M. Pask and J. A. Piper, “Diode-Pumped LiIO3 Intracavity Raman Lasers,” IEEE J. Quantum Electron. 36(8), 949–955 (2000). [CrossRef]
27. P. Farinello, F. Pirzio, X. Zhang, V. Petrov, and A. Agnesi, “Efficient picosecond traveling-wave Raman conversion in a SrWO4 crystal pumped by multi-Watt MOPA lasers at 1064 nm,” Appl. Phys. B 120(4), 731–735 (2015). [CrossRef]
28. D. K. Killinger and N. Menyuk, “Remote probing of the atmosphere using a CO2 DIAL system,” IEEE J. Quantum Electron. 17(9), 1917–1929 (1981). [CrossRef]
29. N. Menyuk, D. K. Killinger, and W. E. DeFeo, “Laser remote sensing of hydrazine, MMH, and UDMH using a differential-absorption CO2 lidar,” Appl. Opt. 21(12), 2275–2286 (1982). [CrossRef]
30. M. Mytilinaios, A. Papayannis, and G. Tsaknakis, “Lower-free tropospheric ozone dial measurements over Athens, Greece,” EPJ Web Conf. 176, 05025 (2018). [CrossRef]
31. C. He and T. H. Chyba, “Solid-State barium nitrate Raman laser in the visible region,” Opt. Commun. 135(4-6), 273–278 (1997). [CrossRef]
32. V. V. Ermolenkov, V. A. Lisinetski, Y. I. Mishkel, A. S. Grabchikov, A. P. Cha kovski, and V. A. Orlovich, “A radiation source based on a solid-state Raman laser for diagnosing tropospheric ozone,” J. Opt. Technol. 72(1), 32–36 (2005). [CrossRef]
33. V. A. Lisinetskii, P. V. Shpak, S. Schrader, A. P. Chaikovski, and V. A. Orlovich, “Efficient frequency doubled Raman laser at 282 nm wavelength suitable for tropospheric ozone sounding,” Laser Phys. Lett. 10(7), 075405 (2013). [CrossRef]
34. S. Ding, X. Zhang, Q. Wang, F. Su, S. Li, S. Fan, Z. Liu, J. Chang, S. Zhang, S. Wang, and Y. Liu, “Theoretical models for the extracavity Raman laser with crystalline Raman medium,” Appl. Phys. B 85(1), 89–95 (2006). [CrossRef]
