# A Review of Femtosecond Laser-Induced Emission Techniques for Combustion and Flow Field Diagnostics
## 4 Femtosecond Laser-Induced Filament Emission
### Spectroscopy Techniques Based on Femtosecond Laser-Induced Filament Emission  
Filament-induced nonlinear spectroscopy (FINS) is a non-resonant spectroscopy technique, which utilizes the short-lifetime signal of the fs laser-induced filament. FINS technique has already demonstrated its potential application prospect in many fields [97; 98], such as remote sensing of greenhouse gases [99], pollutants measurements in atmosphere [100], humidity monitoring [101], and heavy-metal contaminants detection in water [102]. The applications of FINS in combustion diagnostics have emerged in recent years. In 2013, [PERSON] et. al. [103] demonstrated the proof-of-concept for FINS in combustion diagnostics, and simultaneous monitoring of multiple combustion intermediates was realized in ethanol/air flame. They compared the emission spectra obtained by FINS, ns-LIBS and spontaneous emission of ethanol/air flame [104], as shown in Figure 4. They found that the signal of FINS in flame mainly results from the interaction between fs laser pulses and the combustion intermediates, such as OH, CH, and C\({}_{2}\). They expanded the application of FINS into different alkanol/air flames [105], i.e., methanol (CH\({}_{3}\)OH), ethanol (CH\({}_{3}\)CH\({}_{2}\)OH), n-propanol (CH\({}_{3}\)(CH\({}_{2}\))\({}_{2}\)OH), n-butanol (CH\({}_{3}\)(CH\({}_{2}\))\({}_{3}\)OH), and n-pentanol (CH\({}_{3}\)(CH\({}_{2}\))\({}_{4}\)OH). [PERSON] et al. [106] also demonstrated that FINS could be utilized to sense the combustion intermediates by measuring the backward fluorescence spectra of filament emission in the flame. [PERSON] et al. [107] also obtained critical power and clamping intensity of filament in the combustion flow field. The results indicate that the critical power in the flame is 4-5 times smaller than the usually quoted one in air, and the clamping intensity inside the filament is roughly half of that in the air  
Figure 4: A comparison of emission spectra obtained by (**a**) femtosecond filament excitation, (**b**) nanosecond laser-induced breakdown excitation, and (**c**) without any laser excitation [104]. (Reprinted from Sensors and Actuators: B Chemical, 203, [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON], femtosecond laser filamentation for sensing combustion intermediates: A comparative study, 887, 2014, with permission from Elsevier; Reprinted from The Lancet, 203, [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON], femtosecond laser filamentation for sensing combustion intermediates: A comparative study, 887, 2014, with permission from Elsevier. They also found the lasing action [108] and third-harmonic generation [109] in flame. These results and observations can provide insights into the understanding of FINS for practical applications in combustion diagnostics. There are some advantages of FINS in the application of combustion diagnostics. First of all, FINS can measure multiple combustion intermediates simultaneously with a simple optical setup, which is based on the unique nonlinear optical phenomenon of the filament. Compared with the ns-LIBS technique, the spectra of FINS consist of not only atoms but also molecules, which provide more valuable information. Consequently, FINS can reflect the composition of the measured flow field more realistically. However, the current exploitation of FINS is not complete. For example, previous works have focused on FINS in an alcohol burner array, as shown in Figure 5, which lacks standard data such as temperature or component concentration for comparison. It is thus more meaningful to perform FINS in a standard burner, such as a McKenna burner [110]. Besides FINS, another technique called femtosecond laser-induced plasma spectroscopy (FLIPS) should also fall in this category. FLIPS can acquire component information based on quantitative spectral analysis of the plasma emission.