Objective  When a high-energy laser passes through the target airflow boundary layer, its optical path transmission path will change, and its energy density will also decay. It is the key to study the influence mechanism of the target gas boundary layer on the laser transmission performance for the application of laser as energy carrier in military field. The laser passing through the high-speed flow field to reach the target material is a nonlinear dynamic problem involving the coupling of electric fields, magnetic fields, temperature fields, and flow fields. In order to study the transmission and attenuation characteristics of laser, it is necessary to accurately describe and characterize the interaction between laser and flow field.

  Methods  Based on the classical electrodynamics theory, a theoretical model describing the laser propagation behavior in the atmosphere is constructed. At the same time, the flow field distribution characteristics of different Mach numbers are calculated based on the flow field governing equation. On this basis, considering the influence of flow field density, temperature, velocity and pressure, the nonlinear refractive index model is established, the influence of each factor on laser refractive index is studied. Finally, based on the laser transmission and attenuation characteristics, the influence of the gas boundary layer on the laser-to-target parameters is revealed.

  Results and Discussions   The radius of the laser spot gradually increases with the increase of transmission distance, which indicates that the flow field and glass have a divergent effect on the beam (Fig.2-4). Under the action of glass and flow field, the power to target is in good agreement with the experimental data. The electric field intensity decreases gradually with the increase of transmission distance, which indicates that the flow field and the glass have a weakening effect on the beam energy. The loss of laser transmission power is directly proportional to density and pressure, and inversely proportional to temperature (Fig.19). At the same Mach number, the influence of the flow field on the laser mainly comes from the refractive index of the flow field, which is closely related to the flow field density. The smaller the flow field density is, the greater the attenuation rate of the laser passing through the flow field is. The smaller the pressure in the flow field is, the thinner the air is, which affects the refractive index of the gas.

  Conclusions  The distribution characteristics of laser cross section always follow the middle electric field value to the maximum, and then gradually decrease to the surrounding, which shows that laser is a typical nonlinear light. Different Mach numbers correspond to different gas densities. From air to high-speed flow field, there will be a sudden change in gas density. Under the same transmission distance, the power at high Mach number is greater than that at low Mach number. When the Mach number is fixed, the corresponding power decreases approximately linearly with the increase of transmission distance.