Analysis Of High-energy Laser Atmospheric Transmission Thermal Blooming Effects

A series of linear and nonlinear effects can be produced when high-energy laser propagated in the atmosphere. Among them, the thermal blooming is one of the important nonlinear effects. Thermal blooming effect is a phenomenon that the atmospheric molecular and aerosol particles of atmosphere will absorb laser energy which accordingly cause heating expansion and the decrease of the local refractive index when high-energy laser propagates in atmosphere. Eventually, the energy of this high-energy laser beam will be reduced, the spot of this high-energy laser beam will be larger and the wave-front of this high-energy laser beam will be distorted.Take Gaussian beam as an example, the impact of various parameters on the thermal blooming by numerical simulation of the steady and transient state of the thermal blooming effect in detail was analyzed and how to reduce the impact of thermal blooming effect on high energy laser propagation was discussed in this dissertation. The main contents of the dissertation are as follows:1. The absorption and scattering properties of the different wavelengths of laser beam are absorbed by atmospheric molecules was introduced; and to simulate the transmission of carbon dioxide, water and ozone in the 1.1μm -11.0μm wavelength by using the software Modtran; and analyzed the reason why the transfer results by using a wavelength of 10. 6μm of better than by using 3.8μm .2. Thermal blooming equations, which is a second-order differential system, were derived from the scalar wave equation for paraxial beams and perturbation equation of the air density in the condition of approximation of constant pressure. And the power threshold of the thermal blooming effect was calculated.3. The numerical solution of the paraxial beam propagation equations was analyzed and the hydrodynamic equations under the condition of constant pressure approximation by the integration algorithm, differential algorithm and fast Fourier transform algorithm, and compared the advantages and disadvantages of each method. Simulated the thermal blooming effect of continuous high-energy laser beam transmitted in the atmosphere with Sinpson integration algorithm. And good agreement is obtained by comparing with the numerical simulation results and references results at home and abroad. According to the results of numerical simulation, the impacts of the power of laser, the wind speed of atmosphere horizontal, the diameter of laser and the distance of laser propagated on the thermal blooming were discussed.4. Take Gaussian beam as an example, the perturbation solutions of the transient state pulse effect was analyzed in this dissertation. The results of simulation for single pulse show that: The effect of thermal blooming is relatively larger when t pulse beam propagated in the atmosphere than t3 pulse which is only caused by linear absorption. For two long pulse laser beams with pulse repetition number in 0.7-2.0, which are propagated in atmosphere, its relative strength is largest on the pulse target surface.