# A Review of Femtosecond Laser-Induced Emission Techniques for Combustion and Flow Field Diagnostics
); [EMAIL_ADDRESS] (D.Z. ); [EMAIL_ADDRESS] (J.L. ); [EMAIL_ADDRESS] (Y.T. ); [EMAIL_ADDRESS] (Z.L.) [PERSON]  
1 State Key Laboratory of Engines, Tianjin University, Tianjin 300072, China; [EMAIL_ADDRESS] (B.L. ); [EMAIL_ADDRESS] (D.Z. ); [EMAIL_ADDRESS] (J.L. ); [EMAIL_ADDRESS] (Y.T. ); [EMAIL_ADDRESS] (Z.L.) Zhongshan Li  
1 State Key Laboratory of Engines, Tianjin University, Tianjin 300072, China; [EMAIL_ADDRESS] (B.L. ); [EMAIL_ADDRESS] (D.Z. ); [EMAIL_ADDRESS] (J.L. ); [EMAIL_ADDRESS] (Y.T. ); [EMAIL_ADDRESS] (Z.L.) Zhongshan Li  
1 State Key Laboratory of Engines, Tianjin University, Tianjin 300072, China; [EMAIL_ADDRESS] (B.L. ); [EMAIL_ADDRESS] (D.Z. ); [EMAIL_ADDRESS] (J.L. ); [EMAIL_ADDRESS] (Y.T. ); [EMAIL_ADDRESS] (Z.L.) [PERSON]  
1 State Key Laboratory of Engines, Tianjin University, Tianjin 300072, China; [EMAIL_ADDRESS] (B.L. ); [EMAIL_ADDRESS] (D.Z. ); [EMAIL_ADDRESS]. that laser-based diagnostic technique \"is the only way to'spy' on the fundamentals of combustion inside\". The light source is the core component of laser diagnostic techniques, a typical example of which is nanosecond (ns) lasers. Over recent decades, ns-laser diagnostics have been widely adopted in characterizing many aspects of combustion and flow fields, e.g., flow and flame structure visualization [6], species measurement [7], temperature measurement [8], velocity measurement [9], and soot characteristics analysis [10]. Various ns-laser diagnostics, however, all have their own limitations. For example, in two-photon laser-induced fluorescence (TPLIF), it is hard to find the balance between high laser fluence, which is needed to compensate for the reduced absorption cross-section, and the laser-induced photolysis, which is proportional to laser fluence and is something we would like to avoid. Other challenges faced by ns-laser techniques include measuring in the near-wall region and in the sooty environment, and so on. Hence, it is necessary to develop new techniques to solve the problems mentioned above. The advent of femtosecond (fs) lasers [11] has significantly enriched the laser-based diagnostic techniques. At present, there are many commercial fs lasers, and the average power of the fs laser pulse could reach the order of tens of watts, and the repetition rate can reach the order of GHz. Among them, the most commonly used fs laser for diagnostics in combustion and flow field is the Ti:sapphire laser system, whose wavelength centered at \(\sim\)800 nm with a pulse duration of \(\sim\)45 fs, a pulse energy of 8 mJ, and a repetition rate of 1 kHz. Compared with ns lasers, fs lasers have the characteristics of a narrower pulse duration, a higher peak power, and a broader line width. Hence, when fs lasers interact with gases, different behaviors and phenomena will be observed, such as fs laser filamentation [12; 13]. To understand these behaviors and phenomena, a direct and convenient way is to monitor the emission that arises from the volume of the fs laser-gas interaction. By doing so, we can extract information from the gas field of interest, which will not be stimulated by ns lasers. Hopefully, this will shed light on solving the difficulties faced by ns-laser diagnostic techniques. For instance, femtosecond multiphoton laser-induced fluorescence (fs-MPLIF) has the advantages of high multiphoton excitation efficiency and, at the same time, low photolysis interference. Hence, the primary purpose of this paper is to discuss in detail these advantages along with the emerging diagnostic techniques based on them and to introduce the applications of fs laser-induced emission techniques for combustion and flow field diagnostics. In this paper, we put these incoherent techniques based on fs laser-induced emission into four categories. In Section 2, we focus on a resonant excitation technique: Femtosecond two-photon laser-induced fluorescence (fs-TPLIF). In Section 3, we introduce a non-resonant excitation technique: Femtosecond laser-induced breakdown spectroscopy (fs-LIBS).