# A Review of Femtosecond Laser-Induced Emission Techniques for Combustion and Flow Field Diagnostics
## 3 Femtosecond Laser-Induced Breakdown Spectroscopy (fs-LIBS)  
Laser-induced breakdown spectroscopy (LIBS) is broadly applied to combustion diagnostics, largely encouraged by its simplicity in terms of both experimental setup and data evaluation, which have been applied to many fields such as industry [69], chemistry [70], biology [71; 72], nuclear [73], earth sciences [74; 75; 76], and cultural heritage [77]. Different from LIF, LIBS technique involves non-resonant excitation, and an ns laser with a fixed wavelength meets the demand. For the current development of ns-LIBS, readers can refer to [78; 79]. In the field of combustion, LIBS is mainly used to measure the equivalence ratio. When a pulsed laser is focused at a premixed fuel/air mixture, a breakdown will be generated, which acts as a virtual measurement probe. When the assumptions of local thermal equilibrium (LTE) and optically thin plasma are satisfied [80; 81], the intensities of the spectral emission lines of the plasma can be used to determine the equivalence ratio quantitatively. In most of the previous studies, the ratios of certain spectral lines of LIBS could be used to correlate and quantify mixture fraction including H (656 nm)/O (777 nm) [82; 83; 84], H (656 nm)/N (568 nm) [85], C (833 nm)/N (744 nm) [86], C (833 nm)/O (777 nm) [86], C (711 nm)/(N (745 nm)+O (777 nm)) [87], and C\({}_{2}\) (516.5 nm)/CN (388 nm) [88].  
With the development of ultrafast lasers, LIBS with fs lasers as a light source (fs-LIBS) was introduced [89]. In recent years, fs-LIBS has been applied to combustion diagnostics, and it was also mainly used to measure equivalence ratio. Femtosecond-LIBS is similar to ns-LIBS. The fs laser is focused by a spherical lens with short focal length (e.g., 65 mm focal length [90]) to generate a breakdown (or plasma) in the flow field. By measuring and analyzing the emission spectrum of the plasma, the parameters, such as equivalence ratio, can be obtained. Femtosecond-LIBS has the potential to solve some technical defects faced by traditional ns-LIBS. For example, in ns-LIBS measurements, the intrinsic unstable character of avalanche ionization will lead to volatile signal and hence, result in the instability of the equivalence ratio measurements. The situation will be even worse under high pressure or with the presence of a high level of soot [91]. Femtosecond-LIBS, on the other hand, can avoid the problem to some extent [92].  
For the first time, [PERSON] et al. [90; 93] performed fs-LIBS technique in a premixed CH\({}_{4}\)/air flame. They used the spectral intensity of CN at 388 nm to calibrate equivalence ratio, as shown in Figure 3, and in situ equivalent ratio measurements were realized. In addition, fs-LIBS was also applied by [PERSON] et al. [94]. They used the intensity ratio of H (656 nm)/O (777 nm) and C\({}_{2}\) (516.5 nm)/CN (388 nm) to calibrate the fuel concentration in flames. [PERSON] et al. [95] developed a multivariate analysis to understand and mitigate the measurement instability by varying the pulse duration of the light source from ns to fs. The results indicated that LIBS measurements with fs pulse excitation could reduce the instability in equivalence ratio measurements under high pressure.  
Although its feasibility in equivalence ratio measurements has been confirmed, more works are yet to be done to fully exploit the advantages of fs-LIBS, e.g., its capability for one-dimensional or high spatial resolution measurements. In addition, similar to ns-LIBS, drastic breakdown happens in fs-LIBS based on tight focusing of the fs laser. Therefore, the spectrum of fs-LIBS is dominated by atomic emissions, which might miss some crucial information related to molecules. Furthermore, fs-LIBS has continuum background interference [90; 93], which may disturb the measurements. If using a spherical lens with long focal length, this issue will be reduced. Hence, many techniques depending on femtosecond laser-induced emission have been developed.