Research On Thermal Effects And Gain Characteristics Of Grazing Incidence Slab Geometry

Laser diode(Laser Diode)pumped solid-state lasers have the advantages of compact structure,stable performance,long life,low cost and so on.They are widely used in material processing,medical treatment,scientific research and military fields.In general,due to the very low output power of the seed source,a high power and high beam quality laser output is required using a Matser Oscillator Power Amplifier(MOPA).Seed source power is generally in milliwatts of magnitude,you can use multi-stage optical pre-amplification,upgrade to the watt-order,and then into the side pump or end pump structure to enlarge.However,the multi-stage fiber preamplifier structure is complicated,and it is difficult to obtain the pulse output with high peak power due to the non-linear effect in the fiber and the damage of the fiber itself.Regeneration amplification technology is also an effective way to achieve high gain amplification,but its structure is complex and expensive,not practical.In recent years,the technology of direct amplification of low-power seed sources based on high-gain solid media has attracted more international attention.In this dissertation,the technology of direct amplification of low power seed source using Nd:YVO4 slab crystal was studied in order to obtain high brightness and high gain all-solid-state laser amplifier with simple structure and low cost.Compared with the plank structure to amplify the high-power seed light,the slat crystal can directly amplify the small-signal seed light to obtain a higher gain and directly raise the milliwatt-magnitude seed light to the watt-order.However,due to the low extraction efficiency when the small-signal seed light is incident,the thermal conversion coefficient will be greatly increased compared with that when large-signal light is extracted.This makes the thermal effect in the gain medium serious,affecting the gain of the Nd:YVO4 crystal,while thermal lensing and thermal distortion also change the transmission characteristics of the seed light,resulting in degraded output laser beam quality.In this paper,the temperature gradient distribution in the slab crystal is analyzed.A new method to measure the thermal conversion coefficient of the slab crystal is proposed by using the axial prism structure caused by the linear temperature gradient in the slab pumping surface.The finite element analysis software ANSYS simulates the temperature distribution of the slab,as well as through the actual thermal imaging camera to measure the slab temperature rise,to verify the accuracy of the axial prism model.The experimental results show that the heat transfer coefficients of 0.1 mW seed light and 1W seed light are 0.37 and 0.28,respectively.When the pump power is 55W,the maximum temperature difference of slab is 16K in both cases.In the case of small signal extraction,when the pump power is too high,the temperature rise in the slab is high,causing the gain of the Nd:YVO4 crystal to decrease,while the Amplified Spontaneous Emission(ASE)is severe and the horizontal and vertical heat Degradation As the pump power increases linearly,the output laser beam quality is affected.Therefore,Nd:YVO4 slab crystals directly amplify small signal seed light,it is not appropriate to use excessive pump power.Based on the direct amplification of small signal seed light of Nd:YVO4 slab crystal with the seed beam quality of M2 = 1.2,the output of single-pass amplification is obtained with a pumping power of 50W and a seed light power of 0.1 mW.The output of the laser is 1.8W with gain of 43dB and beam quality of Mx2 =1.30 and My2 = 1.28,at the pump power of 55W,the 0.1 mW seed light is double-pass amplified to obtain a 4.39W laser output with a gain of 46dB,a beam quality of Mx2 = 1.42 and My2 = 1.38.The 0.1 mW seed light tee was amplified and a laser output of 9.26W was obtained with a gain of 50dB with a beam quality of Mx2 = 1.50 and My2 = 1.48.