Research On Heat Dissipation And Temperature Uniformity Control Strategy Of Gained Fiber In Packaging Structure

Single-mode ytterbium-doped double-clad fiber(YDDCF)laser is a compact diffraction limited device,whose output power can reach over 100k W by laser diode pumping.In practical applications,the power growth of high power laser is severely limited by thermal effect.The main reason is that the absorbed pump light deposited in the quartz matrix in the form of heat due to the energy difference between the pump light and the laser in the process of laser particle conversion from absorption band to upper energy level.The heat generation and cooling process of the fiber under steady state affects its temperature distribution uniformity and forms a temperature gradient between the fiber core and the claddings.When the pump power is too high,this temperature gradient will bring many adverse impacts.On the one hand,the quantum efficiency of laser is reduced,on the other hand,the elastic-optic effect occurs in the core,and the thermal stress is too large,which leads to the fracture of the fiber and further deteriorates the performance of the fiber laser.Based on the thermal mechanism of gained fiber in YDDCF amplifier,the thermal resistance models of fiber in a variety of packaging structure are proposed,and the thermal performance test platform of amplifier is established.The heat dissipation and temperature uniformity of fiber are analyzed,and topology optimization of the micro-channel heat sink is carried out.The specific work contents are as follows:(1)Aiming at the thermal effect of gained fiber from 4.40W to 31.07W pump powers,the thermal resistance model of fiber with single-layer substrate is established.Firstly,the temperature distribution of fiber is obtained.Then the model is verified by simulations and experiments respectively.Finally,the influence of fiber placement on the fiber temperature is analyzed.It is shown that the thermal resistance model can accurately predict the temperature distribution of fiber in this packaging structure.It is necessary to optimize fiber placement to increase the temperature uniformity of fiber at the early stage of fiber laser thermal design.(2)Aiming at the thermal effect of gained fiber from 26.3W to 212.0W pump powers,a substrate with internal water channel is adopted to cool the high power fiber laser.Firstly,the thermal resistance models before and after adding convection heat transfer to the upper surface of substrate are presented respectively.And then,the temperature values of two measurements on the fiber are verified experimentally when there is forced convection heat transfer inside the substrate.Next,simulations are also used to verify the temperature change of two measurements with the effective convection heat transfer coefficient of lower surface when the convection heat transfer coefficient of upper surface is 1,5 and 15W/m~2·K,respectively.Finally,the axial temperature distribution of fiber is obtained by these thermal resistance models.The results show that the models can accurately predict the temperature distribution after the fiber packaged in the substrate with water channel.It is necessary to optimize the substrate structure by combining the fiber placement to avoid the area where the peak temperatures of fiber are concentrated in the thermal design of fiber laser.(3)Aiming at the thermal effect of gained fiber at 3000W,a double-layer substrate is designed to package the high power fiber laser.Firstly,the thermal resistance model of fiber with double-layer substrate is presented when the contact between the two layers is ideal,and the analytical results are verified by numerical simulations.The influence of thickness,thermal conductivity and position of the layer on the fiber temperature are predicted respectively.Secondly,the thermal resistance model is also presented when the contact between the two layers is non-ideal,and the analytical results are verified by simulations.When the two layers are made of isotropic and orthotropic materials,the influence of thickness and interfacial heat transfer of the layers on the fiber temperature are also predicted by using the non-ideal thermal contact resistance model.The results show that the composite systems are beneficial to the temperature uniformity of fiber,and the thermal resistance models can be used to analyze more complex fiber packaging structure.(4)The heat sink model to be optimized is introduced for the heat dissipation and temperature uniformity of gained fiber.It is assumed that the fluid flows in the plane,and the three-dimensional heat sink is simplified into a two-dimensional plane structure.The governing equations in the heat sink optimization process are given.And then,based on the material interpolation model of density method,the relationships between material properties and design variable are established,and the dual objective function of heat dissipation maximization and pressure loss minimization in the topology optimization is constructed.Next,the function is solved by moving asymptote method,and the influence of four parameters on the optimization structure is discussed.Finally,the performance of the topologic heat sink is analyzed by numerical simulation.It is shown that the flow heat transfer performance and temperature uniformity of the heat sink are excellent.The downstream convective heat transfer conditions of the heat sink are better than the upstream.The area where the peak temperatures are concentrated can be arranged at the downstream of the heat sink,which will help to reduce the temperature difference between the fibers and finally achieve the temperature uniformity of gained fiber.