As a far-field secondary radiation light source, the air laser has shown broad application prospects in the fields of exploring molecular rotation coherence, atmospheric sensing, and the generation of optical frequency combs. In 2013, Zhang et al. found that the femtosecond intense laser induced N₂⁺ to produce a rotational wave packet, and the rotational coherence of the molecular wave packet could be reconstructed by the Fourier transform spectrum of the air laser when they were measuring the N₂⁺ forward laser signal generated by the external seeding. Then, Malevich et al. demonstrated the feasibility of using backward N₂ laser to detect greenhouse gases in the atmosphere. In the experiment, they used the excitation method of resonance transfer to generate a backward N₂ laser, which interacts with the forward Stokes light in the detection area during the backward-transmitting process to generate the backward-transmitting stimulated Raman signal. By analyzing the Raman signal, the type and concentration of the pollutants in the detection area can be obtained. Recently, the Cheng’s research group found that the CO₂ molecule collinearly excited by strong femtosecond laser pulse and N₂⁺ laser can generate multi-stage Raman scattering signals , and the Raman shift can reach 2000 cm^-1, forming a quasi-periodic Raman optical frequency comb. This phenomenon shows that the N₂⁺ air laser has a broad application prospect in the fields of ultrafast nonlinear spectrum and optical frequency comb.

Since it was first proposed in 2003, air lasers have become a hot spot in the current high-field laser physics research. This article, taking N₂ and N₂⁺ lasing behavior induced by ultraintense femtosecond laser filaments as an example, has reviewed the research progress of air laser from the aspects of phenomenon observation, mechanism exploration and potential applications, and the influence of laser polarization on N2+ coherent radiation is also emphasized. As far as the current research status is concerned, there are still some challenges in the field of air laser: 1) Deepen the understanding of the physical and chemical processes of particle number inversion caused by ionization and dissociation of atmospheric components under various atmospheric environments and strong laser field conditions; 2) Find new excitation and generation methods to improve the energy conversion efficiency of air lasers, and achieve long-distance, high-intensity air laser output; 3) Study the interaction process between air lasers and atmospheric pollution components, and expand the application of air lasers in the atmospheric environment. In short, the development of femtosecond laser technology, especially the generation of ultra-high power and long-wavelength femtosecond lasers, has provided more abundant tools for research in the field of air lasers. In addition, people have a deeper understanding of the physical properties of atoms and molecules under strong laser fields, which will also provide new ideas for the development of air lasers. With the development of various novel excitation methods, the output of high-intensity air lasers will be realized in the near future.