# A Review of Femtosecond Laser-Induced Emission Techniques for Combustion and Flow Field Diagnostics
## 5 Femtosecond Lasers Guided High Voltage Electric Discharges  
In addition to the above-mentioned fs-laser-based techniques, fs lasers can also be used to guide high voltage electric discharges, which can be used for flow field measurements and plasma applications. Plasma is widely used in the fields of industry [167], environment [168], and medical science [169; 170] due to its conductive and germicidal properties. High voltage discharges are the most commonly used methods to generate plasma, such as corona discharge [171; 172], spark discharge [173], and dielectric barrier discharge [174; 175]. Under atmospheric or high pressures, electric discharge in the air can form a long thin filamentary volume of ionized plasma [176]. However, no matter the kind of discharges, they are all based on a strong electrical field, which suffers the problem of random discharges in both time and space. Controlling the time and space of discharges simultaneously and accurately is so hard that it brings many limitations to its practical applications. In the 1970s-1990s, researches on controlling high voltage discharges using pulse lasers have been reported [177]. The high voltage discharge can be well controlled in the air through nanosecond  
Figure 9: Following the movement of femtosecond laser-induced cyano chemiluminescence (FLICC) signal using a single lens reflex camera (**left**) and an ICCD camera in the on-chip multi-exposure mode (**right**). laser guiding [178]. Due to the limitation of the Rayleigh range of the focused laser beam, discharge guidance over the range of about 1 m was achieved. The fs laser-induced filament appears to be ideally suitable for guiding a long-range atmospheric discharge. Femtosecond laser-induced filament is the result of a dynamical competition between the Kerr effect and plasma defocus effect. The length of the filament depends on the femtosecond laser pulse energy and can be extended to several kilometers [13] when the laser energy is high enough. The long-distance transmission of the filament offers many potential applications. [PERSON] et al. [179] reported a free space laser telecommunication through the fog. They use a filament-induced shock wave to radially expel the droplets out of the beam from the air it sweeps, then providing a clean channel for telecom laser transmission, which has the potential to be applied to Earth-satellite free-space optical communications and secure ground-based optical communications. The fs laser-induced filament is a weakly ionized plasma channel with a diameter of \(\sim\)100 \(\upmu\)m and an electron density of \(\sim\)10\({}^{16}\) cm\({}^{-3}\)[180]. This plasma channel is weakly conductive and can be used for guiding high voltage discharges. The most practical application of fs laser-induced filament is laser-triggered switching. Many applications rely on the initiation of a well-controlled discharge between two charged electrodes, especially for high current equipment, where a gas-filled trigger is necessary. Since the first laser-triggered switching experiment in the early 1960s, a wide variety of spark gap geometries and laser types have been investigated. Using ns lasers to trigger discharges can achieve an excellent temporal stability of switched pulse with a jitter time of one nanosecond [181]. Better temporal stability, within picoseconds, was achieved by using an fs laser to trigger discharges [182; 183], and the discharge path is strictly defined along the filament. Femtosecond laser-induced filament guiding discharge provides a safe, remote, and highly accurate controllable switch for high current equipment [184], which perfectly solves the problem of random discharges in both time and space. Since the time and space of discharges can be accurately controlled, many applications based on laser-induced filament were developed [185; 186]. [PERSON] et al. [187] reported a multi-pulse scheme for laser-guided electrical breakdown of air, which is shown to be suitable for guiding discharges propagating in either direction or along curved paths.