Significance Intense femtosecond laser pulses propagate far beyond the diffraction limit in air, producing high-intensity filaments and low-density plasma along with the radiation of supercontinuum white light. The properties of filamentation have attracted significant attention owing to their potential applications in many areas, such as lighting control, remote sensing of atmospheric pollution, terahertz emission, and rainmaking. To achieve these goals, a filament with a long-distance transmission is required. However, the variety of complicated environments, for example, cloud, fog, aerosol, and rain, has strong influence on the propagation of filamentation. The atmospheric turbulence and inhomogeneous energy distribution of the initial beam profile result in the generation of multiple filaments, which can shorten the filament length, reduce the spot quality of the beam, and limit various applications of the laser filamentation. Therefore, the generation and control of long-distance filamentation are crucial. In this study, the research progress on the long-distance propagation of femtosecond laser pulses for space-based remote sensing applications is summarized, including the basic research methods of filament propagation, producing long-distance filaments, and modulation of filament characteristics. Furthermore, the advantages of femtosecond laser filaments in atmospheric remote sensing applications and the fundamental science problems to be solved are summarized. Progress The propagation of laser filament in air relies on a dynamic balance between Kerr self-focusing, which causes laser intensity to be clamped at a level of 10(13)-10(14) W/cm(2), and plasma defocusing due to laser-induced ionization, with typical peak electron densities limited to 10(16)-10(17) W/cm(-3). The high intensity filament persists over many diffractions in this process, providing a great opportunity for various applications, particularly remote sensing. Currently, numerous methods have been developed to manipulate the filamentation. The filament lengths can be extended by simply increasing the input power. While the incident pulse exceeding the critical power by an order of magnitude will quickly lead to multifilamentation, which is unstable both in space and time, incorporating certain external conditions can optimize the characteristics of the optical filament. Using a phase plate, spatial light modulator, and axicon (Fig. 1) to reshape the phase of the laser beam, the formation of multiple filaments can be effectively suppressed, and the filament length can be extended. In addition, the onset and length of the filament and the intensity of the laser and plasma density can also be controlled by numerous other methods, such as using optical systems of certain lens combinations (Figs. 2 and 4), introducing an initial pulse chirp, changing the wavefront phase of the Gaussian beam to obtain the Bessel beam (Fig. 7), phase-nested beam (Fig. 8), and annular beam (Fig. 9), externally refuelling the energy of the filaments (Figs. 10 and 11), and adopting a technique with two-or multiple-pulse (Figs. 12 and 13). The studies conducted on the methods of long-distance propagation of filamentation provide a great opportunity for remote sensing applications. Since Braun et al. observed the self-guided propagation of intense femtosecond laser pulses in air, the generation of long-distance filaments has attracted much attention. Subsequently, an optical filament was transmitted over more than 50 m in the Laboratoire d'Optique Appliquee, and then La Fontaine et al. obtained a propagation distance of several hundred meters. In 2004, Mechain et al. showcased horizontal filamentation over a distance greater than 2 km. Then, the Teramobile group observed a filamentation that was generated by the vertical propagation of high-power femtosecond pulses and emitted in a supercontinuum from the ultraviolet to the infrared regions, which was detected from an altitude of more than 20 km (Fig. 14). Furthermore, the linear absorption spectra of some molecules, such as water (humidity) and ozone, were measured by filament-based LIDARs in an atmospheric environment from several km to tens of km. Moreover, the proof-of-concept of spaceborne laser filamentation for atmospheric remote sensing was presented by the European Space Agency (ESA) group. They numerically simulated the remote generation of filaments from an Earth-orbiting satellite, as well as a white light continuum extending from 350 nm to 1.1 mu m (Fig. 15). Spaceborne laser filamentation might offer promising applications for atmospheric science and chemistry studies. Recently, the characteristics of the filaments generated by propagating a femtosecond Gaussian beam in a 2 m long gas cell with continuously varying pressures at different focal distances have been numerically investigated. It was demonstrated that maintaining a large pressure (1 atm) and changing to a larger pressure (such as 0.3-1.0 atm) benefit filament propagation and spectral broadening (Fig. 16). Although a large refractive index gradient was present in our calculation, we predict that the similar impact induced by the pressure variations is also applicable to propagating a femtosecond laser pulse over a real atmosphere. In addition, we also numerically simulated the propagation of a femtosecond laser pulse from a 400-km altitude towards the ground (Fig. 17). It is crucial to improve the simulation precision and perfect the theoretical model in the future. Conclusions and Prospects Based on remote sensing applications, we review the major advances in the long-distance transmission of laser filament, including the basic research methods, the generation and modulation of the long-distance filaments, and the transmission of a femtosecond laser pulse over an ultra-long-distance. After more than 20 years of continuous exploration, research on femtosecond laser filament has made great progress in both theoretical mechanism and practical application. However, numerous scientific problems remain to be explored regarding the ultra-long distance transmission of filaments, such as the intensity of the filament and peak plasma density not being high enough, developing a laser technique with high power for complicated atmospheric conditions, and establishing a complete theoretical model for the atmospheric environment. Although laser filamentation and remote supercontinuum generation from orbital altitudes are in the theoretical proof-of-concept stage, an earth-orbiting white-light LIDAR might become a new remote sensing tool for atmospheric research.
