# 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
As mentioned above, different from fs-LIBS, FLIPS generally uses an fs pulse focused lens with a longer focal length, and the clamping intensity of the generated filament is about 10\({}^{13}\)\(\sim\)10\({}^{14}\) W/cm\({}^{2}\)[12; 13]. By contrast, the breakdown threshold of fs-LIBS is about 2\(\sim\)4 \(\times\) 10\({}^{14}\) W/cm\({}^{2}\) in air at \(\lambda\) = 800 nm [89; 111]. As a consequence, FLIPS does not suffer from interference of continuous background and contains substantial molecular bands, which can provide more information for spectral analysis. [PERSON] et al. [112] used FLIPS to achieve a stable and reliable gas sensing at elevated pressures. The results showed that the signal level of FLIPS increases with the increase of pressure while maintaining the stability in the pressure range of 1-40 bar. Our group [113] developed FLIPS for instantaneous one-dimensional equivalence ratio measurement in a free jet with non-reacting premixed CH\({}_{4}\)/air. By measuring the spatially resolved spectra of FLIPS, we found that the spectral peak area ratios of CH (431 nm)/N\({}_{2}\) (337 nm), CH (431 nm)/N\({}_{2}\) (357 nm), and CH (431 nm)/O (777 nm) can be utilized to achieve one-dimensional local equivalence ratio measurement. Among them, the CH peak at \(\sim\)431 nm and the O peak at \(\sim\)777 nm is strong enough to be used to achieve single-shot measurements, and a spatial resolution of 150 \(\upmu\)m was achieved, which is important for turbulent flow fields. Figure 6 indicates the results of FLIPS technique used in both laminar and turbulent flow fields for instantaneous one-dimensional equivalence ratio measurement. Figure 5: The photo of the flame on the alcohol burner array together with the femtosecond laser-induced filament [108]. (Reproduced from [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON]; [PERSON], and [PERSON], Lasing action induced by femtosecond laser filamentation in ethanol flame for combustion diagnosis, with the permission of AIP Publishing). Figure 6: Single-shot images of femtosecond laser-induced plasma spectroscopy (FLIPS) in laminar (**a**) and turbulent (**b**) flow field with two intensified charge-coupled device (ICCD) cameras and two filters [113]. (**c**) _2019 Optical Society of America_). The main advantages of FLIPS are as follows. Firstly, FLIPS can achieve instantaneous one-dimensional measurement with a simple experimental setup. Secondly, the spatial resolution of FLIPS is on the order of 100 microns, which is at least one order of magnitude lower than that of ns-LIBS. Thirdly, compared with traditional ns-LIBS, FLIPS technique is free from broadband bremsstrahlung interference. Fourthly, FLIPS technique also has feasibility under high-pressure conditions. Femtosecond laser-induced emissions can last for a while, and the filament offers a one-dimensional spatial resolution. Combining these two features, researches established another application to achieve velocity measurements in the flow field.