Heat Dissipating Model Of Rough Surface And Its Application In Thermal Analysis Of Laser

This dissertation studys the problem of thermal effect in the laser-diodeend-pumped solid-state laser. In practice, two metal heat sinks are usually used to holdround rod crystal and dissipatie heat. The contact surfaces of crystal and heat sinks areboth rough. Due to the special structure of the crystal, the assembly pressure is unevenlydistributed in the side-face of crystal, which forms the nonaxisymmetric thermal contactconductance in the side face of crystal. In the case of the axisymmetry distribution ofGaussian pumping source, the uneven dissipation of the side face of crystal makes thespatial distribution of temperature in the crystal nonaxisymmetric, and makes thethermal effect change in the circumferential direction. The key contents of this study areto establish the heat-dissipating model of the crystal, to discuss the influences of theassembly force, the performance of the rough surface between the crystal and heat sinksto the thermal contact conductance and the spatial distribution of the temperature in thecrystal. Then discuss the influence of the assembly force, pumping power, and rudias ofthe pumping light to the nonaxisymmetriy of the end face temperature. Study therelationship between the assembly force, the pumping power and the wavefrontdistortion, the laser spot pattern, along with the factors of beam quality of the outputlaser. The main contents of this paper contact four parts:The first part:For the special structure of using two heat sinks with half-round groove holding theround rod crystal，the assembly pressure in the side face of the crystal changes in thecircumferential direction, which forms nonaxisymmetric thermal contact conductance inthe side face of crystal. Under the model of the truncated-Gaussian rough surface, thethermal contact conductance models of plastic-deformation and elastic-deformation areestablished. Under this circumstance, three kinds of thermal contact conductance modelare established, including not using thermal interface material, using thermal interfacematerial with its thickness equal to the average thickness of the gap and using thermalinterface material with its thickness much thicker than the thickness of the gap. And theconcept of the gap thermal conductanse is proposed to describe the contant performanceof the crystal and heat sinks. The influences of the assembly force, the equivalentroot-mean-square (RMS) roughness and the equivalent mean-absolute slope on thermalcontact conductance and the spatial distribution of the temperature are discussed. Theresults show that without using thermal interface material, the thermal contact conductance between the crystal and the heat sinks changes significantly in thecircumferential direction, which reaches a maximum point on the bottom of the grooveand a minimum point on the direction of the contact areas of the heat-sink couple usingthe thermal contact conductance models of plastic-deformation and elastic-deformation.With the assembly force increasing, the equivalent root-mean-square roughnessdecreasing or the equivalent mean-absolute surface slope increasing, the thermal contactconductance gets larger but more nonuniform, and the temperature of the crystal rodreduces. When the indium foil is used as thermal interface material, the thermal contactconductance gets larger and more uniform, the temperature of the crystal rod reduces aswell and its distribution becomes axisymmetric.The second part:For the LD end-pumped solid-state laser working with round-rod material holdedby two heat sinks, the thermal contant conductance in the side face of the crystalpresents nonaxisymmetric without using thermal interface material. It causes thegradient of the temperature in the crystal to change in the circumferential direction andthe thermal effect of the crystal to be elliptic distribution. In the case of end-pumping,the heat consumption is mainly near the pumped end-face of the crystal. Using themodels of the truncated-Gaussian rough surface and the thermal contact conductance ofplastic-deformation, the effect of assembly force, the pumping power, and the rudias ofthe pumping source to the nonaxisymmetriy of the end face temperature is studied. Inorder to improve the spatial uniformity of the contact between the crystal and the heatsinks in the case of using two heat sinks, the methods of using three or four heat sinks tohold crystal to dissipate heat is proposed. The results show that when the assembly forcegets larger，the thermal contact conductance between the crystal and heat sinks becomeslarger overall, and the uniformity of thermal contact conductance gets worse, whichcause the fluctuation of the temperature in the same concyclic in the end face of thecrystal to get larger. As the assembly force gets larger further, the value of the thermalcontact conductance gets much larger, and the distribution of the temperature in thecrystal tends to become axial symmetry. As the pumping radius becomes larger, thefluctuation of the temperature on the same concyclic firstly gets smaller, and then getslarger. When the pumping power increases, the fluctuation of the temperature on thesame concyclic gets larger. In the method of using three heat sinks, the uniformity of thepressure、the thermal contact conductance and the distribution of temperature inside thecrystal is greatly improved, and the central value of the end-face temperature is reducedat the same time. Using four heat sinks, the result is best and the temperature in the center of end-face becoems lowest.The third part:In the case of using two heat sinks to hold the crystal and dissipate heat, the spatialdistribution of heat-dissipating of the crystal is nonaxisymmetric. The temperature ofthe crystal changes in three directions, and the thermal effect in the crystal becomesvery complex. The finite element method is used to calculate the temperature whilethere is nonuniform thermal contact conductance between the crystal and the heat sinksin the side face of crystal. Under the theory of the diffraction integral, the modulation ofthe nonaxisymmetrical phase to the models of optical field is studied, and thedistribution of optical field is obtained. The laser spot pattern and the factors of beamquality are stuied. The results show that the distribution of the wavefront distortion isnonaxisymmetrical, which decreases as the the radius increase and have a maximumpoint in the center. The wavefront distortion changes in the circumferential direction,which reaches a maximum point on the bottom of the heat-sink groove and a minimumpoint in the direction of the contact areas of the heat-sink couple. As the assembly forceincreases, the wavefront distortion gets smaller and more uniform, and the partialroundness of the laser spot gets smaller. Otherwise, the factors of beam quality in twodirection of X and Y decrease at the same time. While the pumping power increases, thewavefront distortion gets larger and more nonuniform, and the partial roundness of thelaser spot gets larger. The factors of beam quality in two directions of X and Y decreaseat the same time. In the method of using three or four heat sinks to hold the crystal rod,the additional phase caused by thermal effect presents the uniform distribution in thecircumferential direction. Under this circumstance, the laser spot pattern is close to acircular spot, and the factors of beam quality in two directions of X and Y decreaseconsiderably. Through the experiment of using two heat sinks holding crystal in thecase of not using thermal interface material, the laser spot pattern and the factors ofbeam quality are stuied when there is nonuniform thermal contact conductance betweenthe crystal and the heat sinks. The experimental results in and theoretical calculationresults are basically the same.