Research On The Thermal Blooming Of Laser Propagation In The Inner Channel

As the laser propagates through the channel,the laser energy can be absorbed by the gas and the optical and structural elements,which leads to thermal blooming effects.Wave front errors can result from thermo-elastic distortions of optical elements such as mirrors and solid windows,and from uneven distribution of the gas density within the beam path.In this paper,a coupling analysis method which can be assess the quality of laser transmission in the inner channel is put forward to comprehensive analysis the thermal boundary layer,the flow and optical field.Based on fluid dynamic equations and optical transmission equation,this research use the CFD software FLUENT to solve the flow field and use the UDF(User Defined Function)to couple the optical to the flow field,and a simulation platform for analyzing the laser propagation and flow field is established.Thermal blooming is analyzed and compared when laser propagation in the three kinds of line pipes include normal pipe,periodic extended pipe with aspirating and doublelayer pipe with aspirating.The result shows that the thermal boundary layer is spread to the main optical path in the normal pipe and the maximum temperature rise is reached to 0.7K because of the heat accumulation in the main optical path.The heat accumulation and wave frond error are less compared to the normal pipe,the maximum temperature rise is 0.4K.But it is difficult to fabricate a periodic extended pipe with multiple inlet and outlet.The double-layer pipe with aspirating is an ideal structure which use the double suction structure to control the thermal boundary layer.The maximum temperature rise is 0.1K and the exit optical aberration is fairly compared to the periodic pipe.The thermal deformation of the mirror is analyzed by the finite element analysis of thermomechanical coupling.The temperature distribution and thermal distortion of mirror substrates with such material as silicon,sapphire,silicon carbide,and fused quartz are calculated and analyzed.The results show that when the power of input laser array with the same heat flux,the temperature rise and thermal deformation of silicon and silicon carbide is the smallest.The temperature rise and thermal deformation of silicon mirror are larger under the irradiation of array laser compared with the hollow laser beam,which puts forward higher requirement for aberration correction.The thermal blooming effect produced by the laser energy absorbed by the gas,tube wall and mirror is comprehensive simulated and analyzed.High quality structured grid of complex double layer flow field is generated by ICEM.The thermal blooming effect of different condition such as no ventilation,only gas absorb energy,and different gas flow is calculated.The results show that the phase PV of the outlet in the ventilation is decreased about an order of magnitude compared with the no ventilation condition.And the simulation results are agree well with the experimental results.Based on the verified simulation model,the quantity of the gas flow is optimized by calculating the thermal blooming effects in the condition of different inlet gas,which provides the theoretical analysis and data support for the thermal control of the inner channel project.