# A Review of Femtosecond Laser-Induced Emission Techniques for Combustion and Flow Field Diagnostics
## 4 Femtosecond Laser-Induced Filament Emission
### Tagging Velocimetry Based on Femtosecond Laser-Induced Emission  
Velocity measurement in the flow field is another important application of fs laser-induced filament emission. Velocity is one of the most significant parameters in the combustion flow field. Techniques that can be used for velocity measurements include invasive techniques, such as pitot tube [2] and hot wire anemometer [114], and non-invasive techniques, such as particle imaging velocimetry (PIV) [115; 116], laser Doppler velocimetry (LDV) [117], and molecular tagging velocimetry (MTV) [118]. As a cutting-edge technique, MTV has developed rapidly in recent decades, and it has been extensively studied in the combustion flow field. MTV measures flow field velocity by time-of-flight analysis. The molecules in the flow field are tagged through a \"write\" process, which is followed by a \"read\" process in order to probe the tagged molecules at a known delay time. Consequently, the movement of the tagged molecules gives the velocity. MTV techniques can be categorized by whether the atomic/molecular tracer is \"seeded\" or \"unseeded\". In \"seeded\" MTV methods, the flow is seeded with molecules (or atoms) such as ketone [119; 120], ester [121], nitric oxide [122; 123], and metal atoms [124; 125]. Different atomic/molecular tracers are seeded in terms of different flow fields. Therefore, these methods have a wide scope of applications and high accuracy. However, seeding of these atomic/molecular tracers is often undesirable due to expense, toxicity, corrosivity, and so on. Furthermore, when these methods are applied in the combustion flow field, it is necessary to consider whether the additional tracers have effects on the combustion reactions. Various \"unseeded\" MTV methods were developed as well, e.g., Raman excitation plus laser-induced electronic fluorescence (RELIEF) with O\({}_{2}\) as tracer [126; 127], air photolysis and recombination tracking (APART) with NO as tracer [128], ozone tagging velocimetry (OTV) with O\({}_{3}\) as tracer [129; 130], and hydroxyl tagging velocimetry (HTV) with OH as tracer [131; 132; 133]. For detailed information about MTV, readers can refer to a review [118]. Traditional MTV techniques use ns lasers as the light source. With the evolution of fs lasers and their gradual applications to the field of combustion, some velocity measurement techniques based on fs lasers have been developed, including femtosecond laser electronic excitation tagging (FLEET) and femtosecond laser-induced cyano chemiluminescence (FLICC). FLEET is a recently developed fs-laser-based tagging velocimetry in nitrogen-containing flow field, which was proposed by [PERSON] et al. [134]. It is the first attempt of an fs laser in gaseous flow field velocity measurement. The FLEET nitrogen tagging mechanism is shown in Figure 7. When an fs laser with a wavelength of 800 nm was focused at the nitrogen-containing flow field, the laser pulse ionized and dissociated molecular nitrogen into atomic nitrogen, which produced long-lived fluorescence as the nitrogen atoms recombined into excited electronic states of molecular nitrogen. Through imaging N\({}_{2}\) fluorescence of first positive band (\(B^{3}\Pi_{g}\to A^{3}\Sigma_{u}^{+}\)) at different delays and analyzing, the flow field velocity can be obtained. However, FLEET is a non-resonant process, as shown by the red transition of Figure 8. FLEET process requires absorption of at least eight photons to overcome the 9.8 eV [135] dissociation energy of N\({}_{2}\). As a result, in order to induce multi-photon dissociation, the femtosecond laser with an energy of several mJ is needed, which can photo-dissociate many other species, heat the probing volume, or alter local chemistry. One way to avoid these problems is to use low laser pulse energy. Therefore, selective two-photon absorptive resonance FLEET (STARFLEET) approach was developed by [PERSON] et al. [135; 136; 137; 138], which significantly reduces the per-pulse energy to 30 \(\upmu\)J.