无人机载污染气体激光监测技术的研究进展 
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In China's "ground-air-space" integrated environmental monitoring platform, satellites are positioned in space to observe both atmospheric and terrestrial activities. Meanwhile, numerous air quality monitoring stations are established on the ground to complement the satellite observations. However, conventional monitoring platforms are insufficient in effectively covering the surface boundary layer, which ranges from 0.1 to 1 km above the ground. To this end, unmanned aerial vehicles (UAVs) with high mobility, moderate flight altitude, and easy deployment are employed for gas monitoring in the surface boundary layer when combined with gas sensors. One critical issue with the utilization of UAVs for gas sensing is their endurance, which poses a challenge for onboard gas sensors. These sensors should be small and lightweight with low power consumption to be carried on the UAVs. Currently, gas sensors employed for onboard applications are classified into four categories of electrochemical type, photoionization, catalytic combustion, and infrared sensing. Among them, the first three types of sensors are small, lightweight, and low-power, making them suitable for UAV payloads. However, they are generally less selective, which leads to difficulty in distinguishing target gases. In contrast, infrared sensors rely on the unique spectral fingerprint of gases to identify and detect target gases. When adopted with lasers, they exhibit high sensitivity, resolution, and selectivity, thus reducing measurement errors. As a result, infrared sensors are preferred for onboard gas sensing in UAVs, providing accurate and reliable measurements for monitoring air quality in the surface boundary layer.
The miniaturized sensing module based on laser absorption spectroscopy technology is highly suitable for UAV platforms. With the development of microelectronics technology, the lasers' current driving board, temperature control board, and signal processing circuit can realize the small size and high accuracy. This progress further promotes the miniaturization of UAV-based laser monitoring platforms for pollutant gases. UAV laser monitoring platforms can measure gases in the atmospheric boundary layer, thereby enhancing China's "ground-air-space" integrated monitoring platform. Consequently, a positive effect is produced on China's efforts to build a beautiful world and a community with a shared future for mankind.
The UAV pollution gas laser monitoring platform comprises the UAV platform and the onboard laser sensor. Technological advancements have resulted in various UAV types with varying performance capabilities, which can cater to different task requirements (Fig. 1, Table 1). Pre-deployment tasks such as the availability of multiple UAV simulators, onboard computers, and open-source ground stations guarantee the flight safety of the UAV platform during task execution (Tables 3-4). Gas sensors suitable for UAVs must be small and lightweight, with minimal power consumption. In trace gas monitoring, several miniature optical sensing modules based on laser absorption spectroscopy technology have been experimentally verified (Figs. 2-6, Table 2). To minimize the flow field interference created by high-speed propeller rotation and obtain the most accurate onboard sensor system for the required sensing data, we employ fluid dynamics simulation to optimize and analyze the turbulent distribution around the UAV. The combination of miniaturized laser gas sensors with UAVs has been applied in atmospheric air quality monitoring and natural gas leakage monitoring (Figs. 8-11). Such a combination can achieve three-dimensional measurements of time, space, and spectrum.
Our study provides an overview of current UAV platforms, including their types, advantages, disadvantages, and applications. It also highlights several principles and applications of laser spectroscopy sensing technologies suitable for UAVs. Additionally, we compare various open-source UAV simulators, onboard computers, and ground stations, and examine the challenges and solutions involved in integrating sensors with UAVs. The significant potential and value of employing small-scale laser sensors with UAVs in gas monitoring are also discussed. Finally, we emphasize the development direction for UAV-based pollution gas laser monitoring platforms, which is towards miniaturization and micro scale.
1 引 言
随着工业的快速发展以及人民生活水平的不断提升，我国许多地区面临着大气环境质量改善的巨大压力。目前，我国正在积极建设“天地空”一体化监测体系，以满足环境综合监测等方面的迫切需求。“天”上有大气和陆地综合观测卫星，“地”面布控大量的空气质量监测站，对“空”的监测往往借助于飞机或热气球。然而，对于距离地面0.1~1 km的地表边界层，传统的监控平台很难覆盖。这是因为飞机由于安全原因无法在人口或工业密集区进行低空飞行，卫星数据无法提供污染气体的垂直高度信息，而且在污染气体溯源上也无法满足横向空间分辨率要求，而热气球仅仅能够提供单点位置的垂直数据信息。因此，在环境监控方面，我国亟需发展对“空”监控的新技术、新装备。
近年来，全国化工企业重大安全事故频发，天津港、四川江安、河北张家口、江苏响水、河南义马等地的多起化工厂爆炸事故，使人民的生命财产遭受了巨大损失。这些化工企业经常建在偏远的地区，受基础设施的限制，在厂区附近建设地面监测站的成本较高，但效果不佳。车载的移动式激光遥测监控往往会受到厂区内建筑物或生产设备的阻挡。因此，化工厂的安全检查除了法规的完善，更离不开具有高科技含量的监测监管手段。
无人机以其机动性强、飞行高度适中等特点，很容易实现对地表边界层的监测，完善“天地空”一体化监测体系。最初研制无人机大多是基于**用途，或是为了代替航空人员执行一些危险性高的任务，后来逐渐用于民生和科研领域，例如地球科学考察、大气取样、农田植被监测、地图测绘等，并展现出巨大的潜能［1-6］。至今，各种无人机平台出现在人们的视野，包括固定翼无人机［7-9］、直升无人机［10-12］、伞翼无人机［13-15］、垂直升降无人机［16-18］等。然而，影响无人机现场应用的最主要因素是无人机续航能力不足、单一无人机效率低下，因此利用多架无人机协同工作完成预期的任务是当前无人机的主要发展方向［19］。
由于无人机的续航时间较短，机载气体传感器在实现高灵敏度、高选择性的同时，必须满足轻质量、小体积、低功耗的要求。当前用于无人机平台的痕量气体传感器类型主要分为电化学型、光致电离型、催化燃烧型和红外传感型。前3种类型的质量轻、体积小、功耗低，优势明显，符合无人机对载荷的要求，但气体选择性差的缺点也很突出。红外传感器依靠气体独特的指纹光谱来区分和探测目标气体，与窄线宽的激光器连用将同时具有高灵敏度、高分辨率和高选择性特征，显著减少对测量的误判，满足无人机平台对污染气体监测以及污染源定位的要求［20-25］，但是需要对其结构进行改造，以满足无人机平台对其体积、质量和功耗的要求。随着激光技术的发展，各种类型的激光器应运而生，极大地推动了频谱技术的发展。目前，已经公开报道了许多基于激光传感技术的小型传感器，尤其是微纳加工技术的不断改进，使得光学传感器核心部件的体积越来越小、功耗越来越低，而测量精度越来越高，从整体上进一步缩小了激光气体传感器的体积。
本文从无人机平台和激光气体传感器出发，分别介绍当前无人机平台的类型以及适用于无人机装载的几种激光传感技术的原理，阐明不同类型无人机的优劣以及无人机载激光传感技术的应用，并讨论无人机载污染气体激光监测平台在气体监测领域的应用潜能。
2 无人机平台类型
随着科技的发展，市场上出现了各式各样性能不同的无人机，以满足不同种类的任务需求。无人机种类划分有着不同的标准，
图 1. 无人机按空气动力学原理、升降方式和尺寸性能的分类结果［26-36］
Fig. 1. Classification results of UAVs based on aerodynamics, landing, size, and performance [26-36]
表 1. 基于尺寸性能的无人机分类［36-39］
Table 1. Classification of UAVs based on size and performance[36-39]
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基于空气动力学原理，无人机可分为固定翼（fixed wing）无人机、旋翼（multirotor）无人机、扑翼（flapping wing）无人机。固定翼无人机的起飞方式为手抛起飞或者弹射起飞，这两种方式都需要借助外力，所以这类无人机的缺点是无法实现悬停姿态且对起飞环境有较高要求，优点是拥有较强的续航能力和高速飞行能力，可用于执行大范围、高速的侦察和监测任务［27-28］。直升无人机是固定翼无人机和旋翼无人机的结合，采用旋翼提供升力，具有垂直起降能力，能够在狭小的空间内起降，适合执行低空近距离侦察和监测任务，但是续航时间较短，飞行速度较慢［10-12］。扑翼无人机的设计灵感主要来自小型蜂鸟和大型蜻蜓等昆虫，用扑翼模拟鸟类和昆虫的翅膀，通过振动扑翼产生无人机所需的升力［29-31］。目前扑翼无人机的相关技术还非常不成熟，几乎没有进入实用市场。旋翼无人机的优势是具有垂直升降的能力，可以在相对苛刻的起飞条件下起飞，可以实现在某个位置定高、悬停等飞行姿态，有助于采集数据和提高时空分辨率，这就使得它们成为环境监测的理想无人机［32-33］。旋翼无人机的缺点是采用锂电池供电，这就使得该类型无人机的负载能力和续航能力有限。
按平台升降方式，无人机可分为水平升降（HTOL）无人机和垂直升降（VTOL）无人机。水平升降无人机的典型代表是固定翼无人机，它们的巡航速度快、着陆平稳，但不能实现悬停滞留等飞行姿态，且需要跑道来起飞和降落，成本相对高；垂直升降无人机的典型代表是旋翼无人机，优点是其布控便携，无需跑道辅助起飞，能实现垂直飞行、降落和悬停等飞行姿态，且可用于远程操控和复杂地形勘测，成本相对较低［34-35］。
按平台尺寸性能（质量、飞行高度和续航时间），无人机可划分为纳米型（nano）无人机、微型（micro）无人机、轻型（light）无人机、小型（small）无人机、中空长续航（MALE）无人机以及高空长续航（HALE）无人机［26，36］。
综上所述，在无人机载污染气体监测中：如果考虑低成本，选用微型无人机是最合适的，且微型无人机包括固定翼无人机和旋翼无人机；考虑到污染气体监测区域无人机的起飞环境以及数据采集监测和时空分辨率的关系，选用具有悬停和低速飞行性能的旋翼无人机比较合适，当然也可选用固定翼式垂直升降无人机，但该类型无人机的成本较高。旋翼无人机又分为单旋翼、三旋翼、四旋翼、六旋翼和八旋翼无人机，其中考虑到多无人机协调合作效率以及“炸机”风险，选用四旋翼或者六旋翼无人机比较合适。由于旋翼无人机普遍采用可充电电池，因此其缺点是续航时间短，这样在无人机污染气体激光监测平台中气体传感器就需要满足小体积、轻质量、低功耗和高灵敏度的要求。
3 无人机载激光传感技术
无人机的续航问题对机载气体传感器提出了小体积、轻质量、低功耗的要求。红外光学气体传感器依靠气体独特的指纹光谱来区分和探测目标气体，如果和窄线宽的激光器连用，将同时具有高灵敏度、高分辨率和高选择性特征，缺点是体积大、质量重。随着现代微加工技术的发展，许多光学驱动元件以及电学元件可以做到很小的尺寸，激光器种类的多样性也极大地推进了无人机载激光气体传感器发展，许多微型光学传感模块的有效性在痕量气体监测领域已被实验验证［40-45］。本节将介绍适用于无人机的激光气体传感技术原理与应用，涉及直接吸收光谱技术和间接吸收光谱技术两大类。
3.1 直接吸收光谱技术
直接吸收光谱技术是通过分别测得初始光强信号和经过待测气体的光强信号来反演气体分子浓度信息的技术［20-22，46-48］。直接吸收光谱技术的原理为朗伯-比尔定律，当一束光通过待测气体时，一部分能量被气体吸收，在不考虑散射的情况下，入射光的能量等于出射光的能量与被测气体吸收能量之和。初始强度为
式中：
基于可调谐二极管激光吸收光谱技术的痕量气体检测方法具有分辨率高和响应速度快的优点，可用于目标气体分子浓度在线监测。它经常和波长调制光谱（WMS）技术相结合来降低噪声、提高信噪比。波长调制光谱技术是用相对较高的频率（kHz）调制激光的波长，并在调制频率或该频率的整数倍处解调吸收谱信号，并进行评估和分析，从而得到气体分子浓度。由于粉红噪声即1/f噪声的影响主要存在于低频段，而波长调制光谱技术将吸收光谱的检测转移到具有低噪声本底的高频段，因此提高了在嘈杂环境下可调谐二极管激光吸收光谱的信噪比。由
近年来，小型化多通池设计受到广泛关注。2016年，Dong等［47］报道了一种物理尺寸为17 cm×6.5 cm×5.5 cm的密集光斑模式多通池（MPC），其有效光程为54.6 m，把它和一个带间级联（ICL）激光器联用，可实现最小检测限为5×10-9的CH4检测，整机功率仅为3.7 W。2018年，Cui等［48］采用波长为2.33 μm的分布式反馈（DFB）二极管激光器和光学路径长度为14.5 m的赫里奥特型多通池，实现了10-6量级的CO检测。2020年，Cui等［42］设计了一种基于三维（3D）打印光纤耦合且免校准的迷你型多通池，其物理尺寸为4 cm×4 cm×6 cm，容积为20 mL，有效光程为4.2 m，实物如
图 2. 三维打印的迷你多通池实物图［42］。（a）七个不相交圆圈密集光斑模式；（b）两个球面镜间光线轨迹；（c）带有进气口和出气口的迷你多通池；（d）迷你多通池俯视图
Fig. 2. Photographs of 3D printed mini-MPC[42]. (a) Seven nonintersecting-circle spot pattern; (b) ray trajectories between two spherical mirrors; (c) mini-MPC with a gas inlet and an outlet; (d) top view of the mini-MPC
对于可调谐二极管激光吸收光谱技术来说，光电探测器是必不可少的元件，它的波长工作范围和响应带宽受到技术、工艺、设计、材料的限制。最近，利用音叉式石英晶振的压电效应设计光热探测器的研究受到广泛关注，目前已经证明了石英音叉可以作为窄带宽（1 Hz）、快速响应（几十kHz）、宽波长工作范围的红外光热探测器，适用于可调谐二极管激光吸收光谱技术［49-50］。如
非色散红外技术也是直接吸收光谱技术的一种，它利用气体分子吸收特定波长的红外辐射后产生的热效应变化，将这种热信号通过热释电探测器转化为可测量的电流信号，以此测定该气体的分子浓度。该方法常用于分析对红外辐射有较强吸收性能的气态物质，如一氧化碳、二氧化碳、甲烷、氨气等［51］。它采用的光源通常是廉价的非相干光源，比如灯泡，其光源发出的光经过气体池后入射到滤光片上进行分光，最后入射到热释电探测器，得到所需波段的光吸收信息，从而反推出气体的分子浓度。由于从光源发出的、经过气体池的光线是复合的、非分散的，因此这种测量方式被称为非分散红外技术。这种技术由于微型设备的发展而得到了广泛的应用，尤其是被应用到某些常见气体的大批量、低成本测量中，例如二氧化碳，目前已设计出各种各样的气体池［52-53］。
3.2 间接吸收光谱技术
间接吸收光谱技术是通过测量目标气体与激光相互作用后产生的热、声、光等信号来反演气体分子浓度信息。光声光谱技术是间接吸收光谱技术的一种。在光声光谱技术中，激光通过被测气体后，气体分子吸收激光能量跃迁至高能级，通过无辐射跃迁回基态来释放能量，调制的激光会使气体分子周期性地吸收、释放热量，导致分子局部温度升降，进而产生声波。传统的光声光谱技术采用宽带麦克风来探测声音信号，这就导致探测带宽内过多的无用噪声被探测到［54-55］。石英增强光声光谱（QEPAS）技术是近几年才发展起来的一种新型光声光谱技术，其显著特点是使用音叉式石英晶振代替传统光声光谱中的宽带麦克风来实现对微弱光声信号的探测，在石英增强光声光谱技术中，音叉式石英晶振是关键部件，它利用石英材料的压电效应将声波引起的机械振动转化为电信号，然后使用跨阻抗前置放大器进行放大。这种小尺寸的音叉式石英晶振的典型谐振频率为32.768 kHz，品质因子很高，在真空封装中品质因子高达100000，在常压下为10000左右，常被用于石英表的频率标准［56-57］。该技术不但保留了传统光声光谱技术所具有的零背景、无波长选择性、信号幅值正比于激励光功率的特点，而且拥有品质因数高、响应带宽窄、对环境噪声免疫的优点，以及体积小、结构紧凑以及成本低的优势［56-60］。
光声信号的强度表达式为
式中：
图 5. 光纤耦合式光声探测模块［64］。（a）光声探测模块的CAD图；（b）光声探测模块实物图
Fig. 5. Optical fiber coupled photoacoustic detection module[64]. (a) CAD image of photoacoustic detection module; (b) photo of photoacoustic detection module
石英增强光声光谱中石英音叉的高品质因数特性使得其响应时间达到约100 ms，加上信号处理时间，系统完成单次数据点测量的时间长达400 ms，因此无法实现痕量气体的快速检测，而且需要经常对音叉式石英晶振进行频率校准［56-64］。针对上述问题，Wu等［65］报道了拍频石英增强光声光谱（BF-QEPAS）技术，如
图 6. 传统石英增强光声光谱和拍频石英增强光声光谱的比较［66］
Fig. 6. A side-by-side comparison of conventional QEPAS and BF-QEPAS techniques[66]
BF-QEPAS技术可实现对目标气体分子浓度C、石英音叉共振频率f0及其品质因数Q的同步探测，从而在快速测量目标气体分子浓度的同时获取校准系统所需的电学参数，并自动完成相关实验参数的校准，真正实现了对痕量气体分子浓度的快速、连续监测。Li等［67］报道了基于BF-QEPAS的应用，采用10.359 μm的中红外QCL激光器和BF-QEPAS实现了人体呼出氨气10-9量级的检测。基于BF-QEPAS的传感器满足机载激光传感器的小体积、轻质量、低功耗、快速响应、免校准的要求，但需要在近红外波段实现痕量气体高灵敏度探测以及强振动环境下的稳定连续运行。
表 2. 机载激光监测技术指标［40-67］
Table 2. Indicators of airborne laser monitoring technologies [40-67]
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4 无人机载激光传感系统的应用
无人机在执行任务时，由于极易出现炸机事故，实验成本很高，因此对无人机路径规划代码的模拟验证和起飞前状态参数的检查十分重要。选择适合任务的无人机模拟器、机载计算机和地面控制站可以明显降低执行任务时的容错率。无人机模拟器能为无人机创建一个模拟真实飞行情况的环境，多个基于Ubuntu系统的免费开源模拟器JMavSim、Gazebo、FlightGear、X-Plane、Air Sim可进行执行任务所需路径规划代码的模拟验证，确保真实飞行时的安全性。
表 3. 可用的无人机模拟器的比较［66，68］
Table 3. Comparison of available UAV simulators [66,68]
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表 4. 地面控制站软件的比较［73-75］
Table 4. Comparison between GCS software [73-75]
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无人机螺旋桨在转动的过程中产生的湍流是一种高度复杂的三维非稳态、带旋转的不规则流动。在湍流中，空气的各种物理参数（如流动速度、压力、温度等）随时间和空间发生随机变化，从而导致激光气体传感器输出信号产生波动。为了抑制湍流扰动对无人机周围传感器的影响，需要分析无人机周围的湍流分布，并寻找最优的传感器安装位置。这是无人机飞行时面临的一个复杂问题。Li等［76］使用ANSYS FLUENT 18.0软件对无人机在悬停模式下的螺旋桨气流进行了气动流动模拟和实验验证。Marturano等［77］使用COMSOL软件中的流体动力学数值模拟来分析挂载在无人机上的探测器的最佳位置，研究无人机螺旋桨流的流体动力学和传感器位置对数据采集和调节的影响，以最大限度地提高无人机探测气体的能力。Ma等［78］结合实验，分析和研究了不同的无人机航速、转子组件结构、泄漏量和采样条件下无人机周围流场对气体采样的影响，通过仿真和实验确定了优化后的无人机装配结构和气敏传感器安装位置，并进行了验证。Roldán等［69］在四旋翼飞行器的仿真模型中增加了螺旋桨附近的细节设计，从而提高了仿真结果的精度，并减少了四旋翼飞行器其他部分的复杂性，进而降低了仿真计算成本。他们使用模拟软件Autodesk Simulation CFD 2014来确定四旋翼的空气动力学和转子的气流演变，并用一架四旋翼飞行器进行实验，验证了CFD模拟结果。流体动力学模拟的主要目的是优化无人机平台上气体传感器的位置，以最大限度地减少螺旋桨高速旋转产生的流场干扰，从而获得一个能够精确获取所需传感数据的最佳机载传感器系统。
目前公开报道了多种结合基于可调谐二极管激光吸收光谱技术的无人机天然气泄漏遥感探测仪［79-81］。如
图 9. 直升无人机气体传感平台［83］。（a）垂直腔面发射激光传感器的多通池；（b）机载VCSEL传感器的直升无人机
Fig. 9. Photographs of the gas sensing platform based on a helicopter UAV[83]. (a) Multi-pass cell in the vertical cavity surface emitting lasers sensor; (b) helicopter UAV with VCSEL sensor onboard
Liu等［84］提出一种低成本的基于非色散红外技术的CO2传感器，该传感器包括电池、气泵和干燥器，总质量为1058 g，尺寸为15 cm×9.5 cm×11 cm。将其装载在四旋翼无人机上，实现了大气中CO2的测量，如
图 10. 基于无人机的非分散红外气体传感器照片［84］。（a）便携式CO2传感器系统的组成设备；（b）装载传感器的多旋翼无人机
Fig. 10. Photographs of non-dispersive infrared gas sensor based on UAV[84]. (a) Components of the portable CO2 sensor system setup; (b) selected multirotor UAV
图 11. 无人机载非分散红外气体监测平台的照片以及原理图［85］
Fig. 11. Photograph and schematic of non-dispersive infrared gas monitoring platform based on UAV[85]
5 结束语
从空气动力学原理、升降方式、尺寸性能3种分类方式对各类无人机平台展开了讨论，并对比了它们的性能。考虑到污染气体监测区域无人机的起飞环境，以及数据采集监测和时空分辨率的关系，将具有悬停和低速飞行性能的旋翼无人机用于激光气体监测是最优选择。旋翼无人机的续航时间短，这就需要无人机载激光传感器具有小体积、轻质量、低功耗的特点。针对这些特点，介绍了4种激光气体传感技术的基本原理和应用，它们都具有小型的传感器尺寸，很适合应用于无人机平台。另外，随着微电子技术的发展，激光的电流驱动板、温度控制板和信号处理电路也能够做到小体积和高精度，这就进一步推动了无人机载污染气体激光监测平台向小型化和微型化发展。无人机监测技术的发展有利于完善中国“天地空”一体化立体监测体系，这对于中国参与全球环境治理和在应对气候变化国际合作领域发挥引领作用提供技术支撑，对中国建设美丽世界以及构建人类命运共同体具有积极的作用。
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王刚, 武红鹏, 廖洁麟, 魏永峰, 乔建波, 董磊. 无人机载污染气体激光监测技术的研究进展[J]. 光学学报, 2023, 43(18): 1899912. Gang Wang, Hongpeng Wu, Jielin Liao, Yongfeng Wei, Jianbo Qiao, Lei Dong. Advances in UAV Laser Monitoring Technology for Pollutant Gases[J]. Acta Optica Sinica, 2023, 43(18): 1899912.
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