地基激光雷达对大气金属层的探测研究 下载: 669次特邀综述
The atmospheric detection of the mesosphere (about 80-110 km) is of scientific research and application significance. There are many important spatial features and phenomena in this region, including the coldest altitude of the Earth's atmosphere (~90 km), and special phenomena such as high-altitude noctilucent clouds, temperature inversion layer, and atmospheric metal layers also occur in this region. Gravity wave fragmentation makes the atmospheric disturbances in this region particularly intense, and the wind shear in this region becomes extremely intense. This region is also part of the atmospheric photochemical layer, and the atmospheric compositions have dramatic diurnal variations. With the development of aerospace, the influence of this region needs to be closely focused on. For example, suborbital flights (generally defined as 35 to 300 km to the Earth) involve this region, and these studies will lay a solid foundation for future suborbital commercial flights.
This region has long been relatively unknown to humans because of the limitations of traditional detection methods. Fortunately, there are atmospheric metal layers in this region. As the cross section of resonance fluorescence scattering of metal atoms and ions is much larger than those of Rayleigh scattering and Raman scattering, it can be employed to detect low-concentration atmospheric components. In the past half century, by adopting the transition spectra of atoms and ions at specific wavelengths, the metal layer of the atmosphere has been detected by lasers with specific wavelengths and laser remote sensing technology. These metal atoms and ions are excellent tracers of atmospheric fluctuations, and many parameters such as atomic number density, temperature, and wind have been obtained. In recent years, with the discovery of thermospheric metal layers, the height range of atmospheric metal layers has been expanded, and the study of metal layers has been paid great attention to.
Based on the research of our team and collaborators, we introduce the development of atmospheric metal layer lidar and the current situation and trend of atmospheric metal layer detection by lidar. First, the dye laser opens the door to the atmospheric metal layer. Second, the sum frequency of the dye laser and YAG laser increases the laser energy further. Third, with pulsed dye amplifier employing dye as the working substance, directly amplifies the single-mode continuous seed laser into a high-power pulse laser. This meets the dual characteristics of high spectral resolution and sound center frequency stability of wind and temperature detection in metal layers. Fourth, narrow-band filtering technology extends lidar detection from night to all time. Fifth, a dye laser needs to change dye frequently with low single pulse energy, and all-solid-state laser solves this problem. Sixth, OPO lasers have many advantages such as high integration degree, good pump light spot, high single pulse energy, and further improved detection ability of atmospheric compositions. Additionally, we list the parameters of sodium atom, calcium atom and ion, iron atom lidar, and potassium atom lidar in four tables respectively.
With the development of Q-switching, harmonic generation, tuning, high-power optical fiber devices, and other technologies, the pulse energy, stability, and operation convenience of lasers are constantly improving. In recent years, the simultaneous detection of multi-component density, temperature, and wind has become the trend. The multi-function lidar with high resolution and detection accuracy has been excavated and applied in China and abroad. In the future, the development and application of automated and intelligent lidar will promote satellite lidar, and in combination with ground-based lidar, more ion component detection will be possible to provide support for temperature and wind detection at higher altitudes. Finally, the cognition of chemical and physical processes in the upper atmosphere, and the coupling research on different regions of the ionosphere will be advanced.
1 引 言
中层顶区域(80~110 km)有许多非常重要的空间特征与现象,高空夜光云、中层顶逆温、大气金属层等特殊现象也都发生在该区域,因此对该区域大气的探测具有重要的科研和应用意义。重力波破碎使得中层顶区域的大气扰动尤为剧烈,导致该区域的风场、风切变等极为强烈,而该区域也是大气光化学反应的重要组成部分,大气成分具有显著的昼夜变化特征。随着航空航天技术的发展,这一区域产生的影响受到了密切的关注。例如亚轨道飞行(通常指距离地球35~300 km的范围)正好涉及这一区域,这些研究工作将为未来亚轨道商业飞行打下坚实基础。由于大气密度过低,激光的瑞利散射信号在80 km以上已经非常微弱,难以开展有效的瑞利激光雷达探测。
大气金属层是以原子、离子、化合物等形式稳定存在于80~105 km高空大气的金属蒸气,一般认为每天大约有44 t金属蒸气在流星消融的过程中注入地球大气,包含Fe、Mg、Na、Ca、Ni、K等元素的蒸气[1]。由于金属原子、离子共振荧光散射的散射截面比瑞利散射、拉曼散射大得多,采用大规模的望远镜、激光器配置,可以得到较强的荧光回波信号。荧光激光雷达作为一种主动光学地基探测手段,利用原子、离子特定波长的跃迁光谱,向高空大气发射特定波长的激光,金属原子、离子吸收激光能量后产生共振荧光,被望远镜接收后可以实现高空大气的特定金属成分高时空分辨率、高探测精度的密度探测,还可以通过探测原子超精细结构谱线的展宽与频移,实现中高层大气风场与温度的激光雷达探测。随着调Q、调谐、倍频、非线性光学、光电检测等技术的发展,激光雷达的探测参数、范围、精度都在不断提升。自20世纪60年代Bowman等[2]首次报道了地基激光雷达对大气钠原子层的观测以来,全球范围已有数十个地基激光雷达台站,目前积累了钠(589 nm)、铁(372 nm;374 nm)、钾(770 nm)、钙(原子:423 nm;离子:393 nm)、镍(337 nm;341 nm)等金属原子、离子的观测数据。由于金属原子、离子共振荧光波长都不是常用固体激光器的本征波长,因此国际上主要采用有机染料作为增益物质来获得上述波长的激光,用于激发金属原子、离子的共振荧光散射。大气金属层中钠原子的浓度较高、后向散射截面较大,加上钠染料激光器发展最为成熟,大气钠原子成为探测最为广泛的成分,钠层风温激光雷达也成为当今临近空间探测的主流手段,受到各个国家的高度重视。近年来,研究人员纷纷报道了不同地区、高度在110~200 km的“热层金属层”,拓展了大气金属层的高度范围,使得该区域空间环境的激光雷达探测成为可能,因此大气金属层的研究又受到人们的广泛关注。最近,随着固体OPO激光器的发展,单脉冲能量进一步增强,十分有效地提高了激光雷达的探测灵敏度,发现了近300 km的金属离子层,在极大程度上拓展了金属层激光雷达的探测范围。目前,利用钠原子层后向散射荧光的钠导引星已经成为地基光学望远镜自适应光学系统的理想信标光,并且在空间碎片跟踪、星-地激光通信、中间层磁力计等方面开展了广泛应用[3-6]。对大气金属层长期、深入的观测,并以金属原子、离子为示踪物,获取金属成分密度以及相应的风场与温度,对于研究中高层大气的结构和变化特征、理解发生在这个区域的基本物理、化学过程,保障航天器和航天活动具有重要意义,对于研究宇宙起源与演化、保障深空探测也将有更为深远的影响。
本文以作者团队及合作者的工作为基础,以大气金属层激光雷达采用的激光器发展历程为主线,介绍了激光雷达对大气金属层探测研究的发展过程以及国内外现状与趋势。
2 染料激光器:开启大气金属层的探测之门
地球大气上空存在一层Na、K、Fe、Ca、Ni等金属原子/离子,为获得该金属层的相关信息,激光雷达发射激光波长必须精确对准被测金属原子的能级跃迁特征谱线,且金属原子/离子的特征谱线各不相同。通常红宝石和钇铝石榴石等材料作为工作物质能产生大功率脉冲激光,这些晶体材料的增益范围通常只有几nm。荧光染料具有较宽的荧光光谱,分布在紫外到近红外的光谱范围(
图 1. 一些染料物质的激发效率与光谱分布图
Fig. 1. Excitation efficiency and spectral distribution of some dye substances
表 1. 钠原子激光雷达参数
Table 1. Sodium atom lidar parameters
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表 2. 钙原子与钙离子激光雷达参数
Table 2. Calcium atom and ion lidar parameters
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染料激光器是以某种有机染料溶解于一定溶剂(甲醇、乙醇、水等)中作为激活介质的激光器,有机染料在光谱的可见光范围及其邻近的近紫外和近红外区域对光产生强烈吸收,可以作为激光工作物质且有很宽的调谐范围[7]。1966年,Sorokin和Lankard[8]用红宝石激光器泵浦花菁类染料,首次获得激光辐射,染料激光器开始迅速发展。由于液体染料激光器具有在很宽的光谱范围内可调谐、谱线带宽窄、光束发散角小、价格相对低廉的特点,1969年Bowman等[2]用闪光灯泵浦可调谐染料激光器,获得了589 nm的激光,首次探测到高空大气中的钠原子,开启了大气金属层的探测之门。1970 年,Peterson等[9]实现了连续波染料激光器的运转,染料激光器的可调谐范围广、输出功率高、吸收和增益容易控制(因为染料浓度容易控制)、可产生连续波、Q开关及超短脉冲输出等优点逐个得到开发,并且得到广泛的应用,从而提升了大气金属层成分密度的探测能力[7,10]。
以子午工程延庆台站双波长探测激光雷达系统为例,其基本组成包括激光发射、信号接收、信号处理及控制三大部分。
图 2. 延庆台站双波长激光雷达方案及实物图[14]。(a)设计方案;(b)实物图
Fig. 2. Dual-wavelength lidar scheme and object picture in Yanqing station[14]. (a) Design scheme; (b) object picture
信号接收部分包括大口径Cassegrain接收镀膜望远镜(直径为1 m,2018年10月28日更换为1.23 m)、分光镜、窄带滤波片、光电倍增管(PMT)等。接收的光信号经过分光镜后得到两束光,它们分别通过窄带滤光片,保留589 nm和770 nm的有效回波光信号,再通过高灵敏光电倍增管将光信号转化为电信号。
信号采集及控制部分包括前置放大器、光子计数卡(MCS-pci)、延时触发器等。先使用前置放大器对电信号进行匹配滤波、消噪等预处理,得到被放大到一定功率的电信号;再利用光子计数卡把光电子脉冲记录下来,并用回波光子计数值表示信号的大小。可见,信号采集及控制部分控制整个激光雷达系统的时序,保证发射部分、接收部分、采集部分协调运行[11-14]。
对于Nd∶YAG泵浦染料方案,由于染料激光的可调谐范围广(可调谐到钠原子共振吸收谱线)、光谱线宽窄,再加上高空大气中钠原子密度相对较高,后向散射截面较大,因此钠原子探测最先实现,钠层的观测研究范围也最为广泛。具有代表性的一些研究团队的钠原子密度激光雷达的具体参数[2,15-30]如
根据采集的回波信号,可以通过激光雷达方程反演得到高空大气的金属原子、离子密度。激光雷达方程包括发射项、散射项、接收项、传输项、噪声项5个部分,其中发射项是指单位时间内激光器发射的总光子数,散射项是指单位立体角内光子被后向散射的概率,接收项是指散射的光子被望远镜接收的概率,传输项是指激光在发射到接收过程的衰减,噪声项是指积分时间内的背景噪声。激光雷达方程的一般形式为
式中:
散射项的后向散射系数β中包含了散射粒子的密度(30~80 km高空大气瑞利散射中该项表示大气密度,80~400 km共振荧光散射中该项表示金属原子密度,随着激光雷达技术的发展,探测上限可能更高)。由于光学效率η、大气透射率T等的测量值每天都有差异,因此直接求解共振荧光散射的金属原子密度非常困难。对于同一台激光雷达接收的不同高度的光子数,受到发射项、接收项的影响基本相同,对于30 km以上的高度,气溶胶很少,受到传输项的影响几乎相同。因此,通常采用30 km作为参考高度zR,将共振荧光散射激光雷达和瑞利散射激光雷达进行比较,消掉不便测量的参数,得到只需考虑散射项的方程。对于散射项,有
式中:
对高空钙原子/离子、镍原子的探测也主要采用染料激光雷达。1985年,Granier等[31]首次报道了法国Observatoire de Haute Provence(44°N,6°E)台站1983—1984年14个夜晚的钙原子和钙离子的同时观测结果,钙原子柱丰度没有呈现系统的季节变化,而钙离子并不是每次都能探测到。2000年,Gerding等[32]报道了德国Kühlungsborn(54°N,12°E)台站1996年12月至1998年12 月钙原子在112个夜晚和钙离子在58个夜晚的观测结果,钙原子的柱丰度为2.1×107 cm-2,平均峰值密度为22 cm-3,在90 km以上,钙离子占主导地位,平均柱丰度为 4.9×107 cm-2,钙原子数密度呈现10月最大、6月次大的显著特征,钙离子每晚几乎都有密度较大的突发层,季节变化的时空尺度较小,9—11月在90~95 km出现峰值,但5—6月和 7—8月在100 km高度以上的两个奇特的密度极大值,可能受到8月的英仙座(进入速度为60 km/s)、10月的猎户座(进入速度为66 km/s)和11月的狮子座(进入速度为71 km/s)流星雨的影响。部分代表性研究团队测量的钙原(离)子激光雷达的具体参数[31-40]如
镍原子数密度低、散射截面小,其信号比钠信号低约4个量级,因此镍原子是大气金属层里比较难探测的成分。2015年,Collins等[41]率先报道了镍原子的探测结果,但是他们探测的镍原子数密度比大气金属层模型的估计结果高了2个量级,因此该探测结果对大气金属层的基本物理过程提出了挑战。2018年,德国Gerding等[42]进行了数次镍原子探测实验,发现镍原子数密度为280~450 cm-3,与大气金属层模型的模拟结果接近。在Plane等的建议下,延庆台站利用染料激光器搭建了镍激光雷达系统,于2018年开始镍原子的探测研究,发现镍原子数密度的总体平均值为(258±115)cm-3,与模型的模拟结果基本一致[43]。后来又给出了镍原子数密度随季节变化的探测结果,并与大气金属层模型的模拟结果进行对比,发现镍原子层的最大原子数密度(460 cm-3)出现在冬季,最小原子数密度出现在夏季(98 cm-3)[44]。
3 和频方案:进一步提升激光发射能量
采用染料激光与YAG激光和频,进一步拓展了染料激光器的输出波长范围,实现更宽波段、更高效率的激光输出。大气金属成分的荧光波长大多在紫外波段,相比于通常采用的染料激光器先输出红外激光再倍频到紫外波段,和频方案由于与大功率的YAG激光和频,有着能量更高、光斑更好的优点,进一步提升了金属层激光雷达的探测灵敏度。
1989年,Granier等[45]首次报道了将Nd∶YAG激光器输出激光倍频后(532 nm)和染料激光器输出的572 nm激光通过KDP晶体和频得到372 nm的激光,在法国Observatoire de Haute Provence(44°N,6°E)台站观测到铁原子层。随后,美国Kane和Gardner[46]在Urbana(40°N)、Raizada等[47]在 Arecibo(18°N)、Gardner等[48]在South Pole(90°S)分别给出了铁原子数密度的季节变化。Gardner 等[49]利用南极Rothera(67.5°S,68.0°W)台站2002年至2005年3月的观测数据,总结出铁原子数密度在4月极大、10月次之,其他月份较小的变化规律,并基于其他台站不同的季节变化特征,通过改进流星输入函数、环境温度的季节变化特征、引力波的垂直输运,以及对PMC冰晶、烟尘的不均匀吸收,建立了和观测结果相吻合的模型。
2009年,Yi等[50]也基于染料和频方案搭建了国内首台铁原子玻尔兹曼激光雷达,实现了铁原子数密度和中层顶区域温度的探测。具有代表性的一些研究团队的铁原子激光雷达的具体参数[22,45,47-48,50]如
表 3. 铁原子激光雷达参数
Table 3. Iron atom lidar parameters
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4 染料脉冲放大方案:揭开中高层大气风温面纱
脉冲染料激光器(PDL)在大气金属层原子数密度探测中发挥了极为重要的作用,然而,脉冲染料激光器由单/双光栅组成振荡腔,输出激光为多模宽线宽(GHz)激光,其中心频率的抖动较大,难以满足金属层风温探测对高光谱分辨激光的要求。脉冲染料放大器(PDA)利用染料作为工作物质,直接将单模连续光种子激光放大成高功率脉冲激光,满足了金属层风温探测光谱分辨率高和中心频率稳定性好的双重要求。
早在1967年,Soffer等[51]发现利用衍射光栅作为腔反射器,既可以压窄输出激光线宽,还能提高转换效率。1970年,IBM公司的Huth[52]将罗丹明6G染料溶于乙醇溶液中,在21.5 J的泵浦能量下获得了55 mJ的输出激光,证实了脉冲染料放大的可行性。此后,法国巴黎综合理工学院的Flamant等[53]采用六级闪光灯进行泵浦,获得了700倍放大增益,并实现了波长可调谐激光输出。1972年,Hänsch[54]利用高光谱分辨率光栅、长腔镜、光束准直镜以及腔内标准具等线宽压窄措施,获得了线宽小于0.4 pm的窄线宽脉冲染料激光。1974年,斯坦福大学的Wallenstein等[55]采用氮激光器作为泵浦光源,利用腔内标准具以及衍射光栅等波长精密选择器件实现了约25 MHz超窄线宽的脉冲激光输出。此后,美国Bölger等[56]、意大利 Burlamacchi等[57]和法国 Pinard等[58]利用闪光灯泵浦激光器泵浦连续光种子激光光源,实现了对连续光的脉冲放大。从此,脉冲染料放大激光器产生的窄线宽激光在原子分子光谱、激光医疗、激光雷达等方面得到了极为广泛的应用。
1985年,Fricke 等[59]利用脉冲染料放大器获得高峰值功率窄线宽589 nm脉冲激光,激发金属层钠原子D2线超精细结构,通过扫描钠原子光谱线实现了钠原子层大气温度的探测。1990年,美国科罗拉多大学的She等[60]通过将连续光种子激光锁频到钠原子共振谱线,并对其脉冲染料进行放大,获得高频率稳定度、窄线宽的589 nm激光,并通过双频机制实现了钠层绝对温度的测量,温度精度优于3 K。1994年,She等[61]再次利用三频激发原子共振谱线的方式,实现了80~110 km大气风场和温度的同时快速准确探测,为钠层风温探测提供了高效可行的新思路。1996年,She课题组[62]采用钠原子滤光器实现了钠层荧光的全天时探测。She课题组的三频钠风温同时探测方案被国际同行广泛采用,此外美国 Illinois大学、Utah State 大学等,以及挪威ALMOR、南极McMurdo等台站先后建设了钠风温激光雷达,在中层顶区域的化学与动力学过程等研究方面取得了非常多的研究成果。国内的中国科学院国家空间科学中心、中国科学技术大学、中国极地研究中心等单位采用上述三频切换染料放大技术,相继实现了钠风温的同时探测[63-67]。中国极地研究中心牵头建设的南极首台钠风温激光雷达获得了大量高质量、高价值的数据。中国科学院精密测量科学与技术创新研究院结合光纤AOM、光线开关、光纤拉曼放大等技术,实现了更加鲁棒的钠风温激光雷达系统。子午工程二期兰州和乌鲁木齐两个台站的中高层风温激光雷达系统方案如
图 3. 子午工程中高层风温激光雷达设计方案及实物图。(a)设计方案;(b)实物图
Fig. 3. Design scheme and object picture of wind and temperature simultaneous measurement lidar in the middle and upper level for Meridian Project. (a) Design scheme; (b) object picture
5 发展窄带滤光技术:将激光雷达探测从夜间拓展至全天时
由于白天的强烈太阳光对激光雷达是一种严重的背景光干扰,白天天空的背景光噪声比夜间高4~5个数量级,因此传统大气金属层激光雷达主要是在晴朗的夜空探测的,国际上的全天时大气金属层激光雷达探测结果很少。要实现白天强背景光噪声下的共振荧光激光雷达微弱回波信号的提取,必须采用小收发视场配合超窄带宽滤光方式来抑制回波信号的噪声。对于白天观测激光雷达而言,目前常用的超窄带宽滤光方式以标准具(或干涉仪)滤光和原子滤光两种方式为主。标准具滤光具有滤光波长和带宽均可调的优势,已经在很多激光雷达台站得到应用。原子滤光是基于原子的共振跃迁与磁致旋光实现对波长的精准选择,具有中心频率自稳定的特点,因此原子滤光中心波长与原子共振波长完全匹配,共振跃迁波长外的波长完全被滤掉,波长选择有限。
1982年,Granier和Megie[71]借助高分辨光谱仪实现了钠层激光雷达的白天观测。1987年,美国Illinois 大学的Kwon和Gardner等[72]借助法布里-珀罗(F-P)标准具实现了钠层激光雷达的全天时观测,并利用钠层密度数据研究了中层顶大气潮汐对钠层密度的调制作用。2001年,Franke等[73]又研制了基于窄带激光技术的白天钠层测风激光雷达。1996年,美国Colorado 大学的Chen等[67]采用法拉第滤光器技术成功研制出可以在白天观测的钠层测温激光雷达。2003年,美国Colorado大学的She等[74]报道了长达80 h的钠层温度和风场连续观测结果,分析了中层顶大气潮汐频率扰动结果,并和大气模式的研究结果进行了对比。此外,Diettrich等[75-77]研发的在南极McMurdo台站(77.8°S,166.7°E)的铁原子玻尔兹曼测温激光雷达,也实现了白天探测的功能。
在国内,中国科学院精密测量科学与技术创新研究院(原武汉物理数学所)激光雷达团队利用自主研发的原子滤光技术,于2006年首次在国内实现了钠层密度的白天探测[78];应用该原子滤光技术,于2014年在延庆台站也实现了钠层密度的白天探测,获得了信噪比非常好的白天钠层信号,并发现了高度低达75 km的白天钠层[79]。2019年,该团队进一步研制出全天时探测所需的组合型多峰原子滤光器,并应用到南极的钠层风温激光雷达,制备出南极首台钠风温同时探测激光雷达;在国家自然科学基金国家重大科研仪器研制项目的资助下,本团队在延庆台站建成了基于激光和频方案的全固态大功率589 nm激光发射系统,实现了高空钠层密度、温度、风场的激光雷达全天时同时探测,这也是目前唯一一台能在白天进行大气金属层风温密探测的固体激光雷达,实物如
图 4. 全固态风温激光雷达全天时观测实物图
Fig. 4. Object pictures of all solid-state wind-temperature lidar realizing all-sky observation
6 固体激光发射方案:大气金属层激光雷达探测的未来发展方向
目前国际上大气金属层激光雷达的主流方案仍然是染料激光发射方案,但是除了589 nm激光染料的效率比较高之外,其他大气金属成分对应的波长都存在染料效率较低、寿命有限、流速不均等问题,影响光斑的形状、光束质量。此外,染料激光器普遍存在需要频繁更换染料、难以实现很高的单脉冲能量等缺点,这大大限制了对金属层微弱成分的探测能力。固体激光发射技术近年来发展迅速,具有能量高、稳定可靠、光束质量好等优点,在大气金属层激光雷达探测领域的应用越来越多。
1996年,von Zahn等[80]研制出一台全固态钾层测温激光雷达,基于紫翠宝石环形激光器,由激光二极管注入种子获得770 nm的激光,探测了钾层密度和大气温度的初步观测结果;1998年,Eska等[81]利用上述钾层激光雷达对德国Kühlungsborn(54°N,12°E)上空的钾层进行了持续一年的观测,得出钾层的年平均柱密度为4.4×107 cm-2,并呈现冬夏大、春秋小的季节变化特征。由于这台全固态激光雷达体积小、便于运输,他们将其安装到德国的科考船Polarstem上,随船对54°N和71°S之间的钾层进行观测,发现钾原子数密度随着纬度的增加逐渐降低,并建立了第一个钾层模型[82]。此后,Fricke-Begemann和Höffner[83]分别对低纬地区Tenerife(28°N)和高纬地区Spitsbergen(78°N)上空的钾原子层和大气温度进行了详细的报道。2002年,Friedman等[84]同样利用种子注入的紫翠宝石激光器观测了 Arecibo Observatory(18.35°N,66.75°W)的季节变化特征。采用前文介绍的双波长方案,本团队在北京延庆台站和巴西台站搭建了钾原子激光雷达,采用染料激光器获得770 nm的激光。Wang等[85]基于北京延庆台站两年共220个夜间的观测数据,得出钾原子数密度呈现冬季极大、夏季次大、春秋较小的季节变化特征。Andrioli等[86]利用巴西钾激光雷达的数据,获得巴西低纬地区的钾原子数密度随季节变化的平均特性、突发特征等。具有代表性的一些研究团队的钾原子激光雷达的具体参数[30,80,82,84,87]如
Chu等[77]在南极McMurdo台站(77.8°S,166.7°E)建设了首台铁原子玻尔兹曼激光雷达,成功实现了铁原子数密度及温度的探测(具体参数见
近年来,全固态钠原子测风测温激光雷达方案的应用,进一步提升了激光雷达的探测能力
表 4. 钾原子激光雷达参数
Table 4. Potassium atom lidar parameters
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图 5. 延庆台站风温密同时探测激光雷达的设计方案及实物图[95]。(a)设计方案;(b)实物图
Fig. 5. Design scheme and object picture of wind, temperature, and density simultaneous detection laser at Yanqing station[95]. (a) Design scheme; (b) object picture
钠原子随着温度变化会出现共振荧光谱线的多普勒增宽,钠原子随大气风场的定向移动会引起钠共振荧光的多普勒频移[96]。因此,用中心频率为ν0、ν0+585 MHz和ν0-585 MHz的3束激光同时探测钠原子层,接收的信号变化记录了光谱增宽和多普勒频移量的变化,从而可以得到大气温度和风场的信息。温度比和风速比可表示为
式中:
图 6. 温度风场探测原理及反演方法[13]。(a)(b)钠荧光光谱随温度和风速的变化;(c)温度比、风速比的二维校正曲线
Fig. 6. Temperature and wind detection principle and inversion method[13]. (a)(b) Variation of sodium fluorescence spectra with temperature and wind speed; (c) two dimensional correction curves of temperature ratio and wind speed ratio
近年来,本团队与南京先进激光技术研究院、中国科学院上海光学精密机械研究所合作,研发了基于半导体泵浦的全固态589 nm激光发射系统。该系统采用无谐振腔YAG直接放大技术与Ram and fire技术,有效克服了钠层风温激光雷达系统中普遍存在的chirp效应。
2021年,Li等[97]首次给出了电光调制的伪随机调制连续波(PMCW)钠测温测风激光雷达的设计方案及探测结果,窄带589 nm连续波激光器PMCW激光雷达传输系统更简单、更稳定,实现了超窄带宽、全固态、体积紧凑等功能。
7 OPO激光器:延伸高空大气探测上限
可调谐激光器除了应用广泛的染料激光器、和频激光器,还有OPO激光器。OPO激光器集成化程度高、泵浦光光斑好、单脉冲能量高,对大气成分的探测能力得到进一步提升,近期已观测到高达300 km的钙离子层。
OPO激光器是利用光参量振荡与放大技术,将固定波长的光转化为新波长的可调谐固态光源。本团队与中国科学院大连化学物理研究所合作,建成首台采用OPO激光技术的金属层激光雷达,实现了金属钙原子、离子的同时探测。相较于国际上普遍采用的染料激光器,OPO激光器具有能量高、线宽窄、光束质量好等优点,极大地提高了激光雷达的探测能力。该系统的探测阈值远远优于国际上的同类设备,利用该系统发现了被前人忽略的背景钙离子层,并且用激光雷达首次探测到高达300 km的钙离子层[40]。观测到的热层-电离层Ca+(TICa+层)的剖面可达300 km左右,达到F区峰值以上。激光雷达记录了TICa+层从开始到结束的完整演化过程:Ca+在午夜前或午夜从位于约110 km的强突发Ca+层中上升,并进入F区,形成约300 km的TICa+层,TICa+层以不同的垂直传输速度上下移动;午夜后2~3 h,TICa+层迅速下降,最终在约140 km处与另一个突发E层合并。这样的激光雷达观测直接延伸了高空大气探测上限,为探索离子传输、E区和F区耦合规律与中纬度电离层的不规则性打开了一扇新窗口。
近年来,国内的OPO激光器技术发展迅速,纷纷被用于多台中高层大气激光雷达。由中国科学技术大学负责建设的子午二期海南激光雷达采用OPO技术来建设激光发射系统,将实现高达1000 km氦原子探测;本团队正在建设的子午二期漠河激光雷达,也包含多台OPO激光器。其中已经建成的423 nm激光器(用于钙原子探测)的单脉冲能量达到120 mJ。以上固体激光技术的发展,为未来金属层星载激光雷达探测奠定了坚实的基础。
8 结 语
大气金属层所处的中间层和低热层(MLT)区域对下方向上传播的大气波动和动力驱动以及上方的太阳辐射和高能粒子沉降都很敏感,对于人类活动引起的气候变化的响应也受到越来越多的关注。中高层大气的冷却将引起重力波动、行星波动和潮汐的长期变化,从而影响整个大气环流。1989年Roble和Dickinson[98]通过模型预测了CO2含量翻倍将引起全球中间层平均温度下降10 K,增加的温室气体在平流层增温,在中间层会作为辐射制冷器。其他的驱动包括平流层臭氧的消耗、大气动力学的变化、太阳和地磁活动的变化以及地球磁场的变化等。太阳活动是另一个引起MLT区域变化的主要源,因此,开展大气金属层长期、稳定的观测与研究,一方面可以表征中高层大气对于太阳活动的响应,另一方面可以表征中高层大气对于低层大气的响应。
本文以激光器发展历程为主线,重点介绍了激光雷达技术的发展带来的大气金属层地基探测研究进展。对于金属层的焦点科学问题(如金属层突发、重力波活动、热层金属层起源、金属层模拟研究等)并未开展深入研究。
随着调Q、倍频、调谐、高功率光纤器件等技术的发展,激光器的脉冲能量、稳定性、操作方便性都在不断提升。近年来,从单一大气成分密度探测到多成分密度同时探测,再到大气密度、温度、风场同时探测,国内外多功能、高分辨率、高探测精度的激光雷达不断被研究、被应用。未来,自动化、智能化的激光雷达研制与应用将促进星载激光雷达的发展,其与地基激光雷达相配合,更多成分离子的探测将成为可能,为更高范围的温度和风场探测提供支撑,将推动对高空大气化学物理过程的认知及电离层不同区域的耦合研究。
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Article Outline
荀宇畅, 程学武, 杨国韬. 地基激光雷达对大气金属层的探测研究[J]. 光学学报, 2023, 43(18): 1899903. Yuchang Xun, Xuewu Cheng, Guotao Yang. Detection of Atmospheric Metal Layer by Ground-Based Lidar[J]. Acta Optica Sinica, 2023, 43(18): 1899903.