光学诊断技术为流体力学的研究提供了大量可靠的数据。目前，对于湍流燃烧、高焓气流等复杂流场，仍然无法完全依靠计算流体力学（CFD，computational fluid dynamics）进行仿真。激光诱导荧光（laser-induced fluorescence, LIF）技术是一种激光光谱技术。它具有高组分分辨能力和高时空分辨率，是研究流场结构的理想工具。

  本文对LIF的背景与理论进行了阐述。介绍了LIF技术的发展，阐述了这项技术在湍流燃烧、高焓气流这两种复杂流场中的应用，指出现有技术值得改进的方面。然后解释了荧光过程和LIF的定量测量原理，并提出了对向LIF数据的递归处理方法，展示了通过该方法对激光吸收率的有效计算。最后介绍了LIF系统的重要组成设备。

  在湍流火焰的研究工作中，首先展示了利用平面激光诱导荧光（PLIF）技术对旋流火焰全方位的诊断。本文通过PLIF对OH、CH₂O指示物的分布进行了定性测量，有效地获得了火焰的结构信息。对比了OH PLIF与OH* 化学发光（chemiluminescence， CL）的信号分布，讨论了两种测量方法所表征现象的异同。另一方面，利用了OH PLIF的双线测温法（two-line thermometry, TLT）测量了火焰的温度分布，利用了对向OH PLIF首次获得了湍流火焰中OH浓度的瞬态分布。此外，利用以上数据，分析了在不同工况下火焰的特性转变。

  随后对火焰的动态特征进行了分析。介绍了本征正交分解（proper orthogonal decomposition，POD）方法，展示了通过POD方法对火焰脉动模态的分析。OH PLIF与OH* CL的POD模态表现出进动涡核（PVC，precessing vortex core）、热声振荡等特征。扩展本征正交分解（Extended POD）展示出OH分布与CH₂O、OH*的分布具有极强的相关性。

  最后对大流量贫燃火焰的结构与动态特性进行了研究。使用了同步的OH、CH₂O PLIF技术对火焰的反应区与预热区进行了可视化，并提取了OH的POD与CH₂O的EPOD模态。实验结果表明，利用OH、CH₂O PLIF可以观测到火焰结构与脉动模式随流量的转变。

  在高焓气流的研究工作中，搭建了应用于JF-10激波风洞的LIF诊断系统。对本系统的关键技术：时序控制和波长监测技术进行了详细的叙述。应用NO LIF的TLT，测量出了激波层内外的温度，然后对激波层外的自由流进行了有针对性的温度测量，对各种可能的误差来源进行了深入探讨，获得了精确的结果。最后，展示了利用分子标记测速法（molecular tagging velocimetry, MTV）对JF-10自由流的速度测量的结果。

     Optical diagnostics provide huge amount of data to facilitate the study in fluid mechanics. Nowadays, computational fluid dynamics (CFD) is still lack of the abilities to fully simulate the phenomena in complex flow, e.g. turbulent combustion flow and high enthalpy flow. Laser-induced fluorescence (LIF) is a laser spectral technique. As it is capable of distinguishing different species and providing temporally and spatially resolved results, it becomes the ideal methods for the investigation of the structure of the flow.

     In this paper, the background and theory of LIF is explained. The developments and the and the applications of the technique is introduced. The principle of LIF process and the theory for LIF based quantitative measurements are discussed. An algorithm for calculating laser absorption coefficient based on bi-directional LIF measurements is developed in this paper. The key components of LIF system is also introduced.

     In the investigation in turbulent flame, the utilizations of PLIF to achieve throughout measurement of the swirling flame is shown at the beginning. The structure of the flame is characterized by mapping the distribution of flame markers: OH and CH₂O. The signal distributions of OH PLIF and OH * chemiluminescence (CL) has also been compared. Meanwhile, transient temperature distribution measured by the two-line thermometry (TLT) of OH PLIF is discussed. The transient number density of OH radical is measured in turbulent flame for the first time. In addition, the above measurements under different operations indicate a transition of the flame character.

     The LIF measurements are followed by the investigation in the dynamic characters of the flame. The proper orthogonal decomposition (POD) method is explained, and the POD analysis of the flame is shown. The POD modes of OH PLIF and OH* CL exhibits the precessing vortex core (PVC) oscillation and thermal-acoustic oscillation. The extended POD analysis reflects the strong correlation between the distributions of OH radical and CH₂O/OH*. 

     In the end, the flame structure and dynamic character of swirl flame under fuel-lean and high flow rate conditions is studied. The flame visualization is achieved by simultaneous OH and CH₂O PLIF technique. The POD and EPOD modes of the flames are also extracted from the PLIF data. It is shown that such flame is well-characterized by OH CH₂O PLIF.

     In the investigation in high-enthalpy flow, the LIF diagnostic system for JF-10 shock-tunnel is established. The key technique of timing control and laser wavelength monitoring is explained in detail. The temperature distribution, both in the free stream and behind the shock wave, of the hypersonic flow is measured by NO LIF TLT. An experiment aimed at high accuracy measurement in the free stream is also carried out with well-discussed of source of error. The result agrees well with the previous measurements. The application of molecular tagging velocimetry (MTV) is also shown to provide the flow speed in the free stream.