400mm口径4×2组合式片状放大器动态漂移

Dynamic Drift of Laser in 4 × 2 Assembled Slab Amplifier with 400 mm Aperture

在大口径片状放大器系统中,非对称泵浦导致经过增益介质的激光束发生动态漂移。研究了泵浦场对称性(用钕玻璃长度方向中心左右两侧的泵浦能量比值表征)对光束动态漂移的影响,建立了理论模型并开展了实验研究。实验结果表明,当增益介质泵浦场的对称性约为1.038∶1时,泵浦致光束的动态波前倾斜值约为0.98λ(波长为1053nm的激光经过六张钕玻璃片后的累积波前畸变量,λ=1053nm),375mm光束口径下激光单次经过单钕玻璃片的动态漂移角度约为0.32μrad,与理论模型吻合较好。激光装置全链路的分析结果表明,在基于反转器的四程放大构型中,累积的动态波前倾斜峰谷值约为2.78λ,漂移比例(漂移量与30倍衍射极限下的小孔直径的比值)约为2.14%。

Objective The research of inertial confinement fusion(ICF)has been a hot spot in international research in recent decades.The slap amplifier provides more than 99%of the energy for the entire laser device and is the heart.However,during laser amplification,asymmetric pumping causes uneven heating of the gain medium,resulting in a drift in the laser propagation direction through the neodymium glass sheet,that is,an angular drift of the laser.The accurate evaluation of the angular drift influences the multi-pass design of the amplifier,which ensures the main laser passes through the spatial filter aperture smoothly and provides a design basis for the filter aperture size and the working mode of the deformable mirror compensation.Starting from the influence of the pump on the medium,we have independently developed a set of design models,hoping to design the amplifier more accurately based on domestic components and obtain a smaller and more accurate angular drift,which is helpful to the better design of chip amplifiers.Methods A theoretical model is established as shown in Fig.2.We first use the pump configuration as the input condition to obtain the pump field distribution on the gain medium through three-dimensional ray tracing,and then obtain the heat distribution on the gain medium.On the one hand,the thermal distribution is first combined with the boundary conditions of the clamping method to obtain the stress distribution on the gain medium,and then the surface shape distribution is obtained.On the other hand,the heat distribution and stress distribution jointly determine the distribution of refractive index changes on the gain medium.These two together determine the angular drift of the transmitted laser beam.This article uses the model to design and manufacture the amplifier.A set of three pieces of 400 mm diameter 4x2 combined chip amplifiers is developed.In the height direction of the amplifier,the stack of four neodymium glass plates forms four laser apertures,and there are two laser apertures in the width direction,forming a total of 4×2 light-through aperture.The xenon lamp pumps the neodymium glass sheet from both sides.The main laser physical parameters of neodymium glass used in the design and experiment are shown in Table 1,and the basic technical parameters of the pumping scheme are shown in Table 2.Results and Discussions Using the model to calculate the device shown in Table 1,the obtained pump field distribution is shown in Fig.4 and the comparison of the pump field characteristics of the two schemes is shown in Fig.5.The device scheme has a better gain uniformity distribution under high gain conditions,but the beam dynamic drift is relatively large.There are two typical schemes of high-energy laser devices at home and abroad.One is the multi-path optical path based on the inverter as shown in Fig.9.The dynamic wavefront tilt distortion value accumulated in the entire link is given by Eq.(7),and it is about 2.78A(the detection wavelength A is 1053 nm).The dynamic drift angle of the output laser in the last pass is about 3.6 prad,the drift amount is about 112.7 pm,and the drift ratio(ratio of drift amount to aperture diameter at 30 times diffraction limit)is about 2.14%.The other is the full four-pass multi-pass amplification optical path in the cavity,as shown in Fig.11,which is used to check the calculation results of the model.In this configuration,the peak valley(PV)value of the dynamic wavefront tilt component of the laser beam passing through the single neodymium glass plate is about 0.028A,and the cumulative dynamic wavefront tilt of the equivalent 54 pieces chip amplifiers is about 1.51A.Then the predicted dynamic drift is about 4.2 prad(the beam diameter is calculated as 360 mm × 360 mm),which is consistent with the prediction result of the National Ignition Facility device(about 4 prad).Conclusions In this article,the influence of pump symmetry on the dynamic drift of the beam in a large-aperture chip amplifier is introduced.The beam dynamic drift characteristics of two large-aperture slap amplifiers and amplification configurations are compared by the numerical simulation calculations,and a large-aperture high-profile pump based on a pulsed xenon lamp with a diameter of 37 mm and asymmetrical arrangement of multiple lamps is studied experimentally.The dynamic drift characteristics of the beam caused by the pump of the gain slap amplifier are studied.When the symmetry of the pump field of the neodymium glass sheet is about 1.038:1,the pump-induced beam dynamic wavefront tilt PV value is about 0.98A(6 sheets accumulation),and the dynamic drift angle of a single laser passing through a single neodymium glass plate is about 0.32 jurad in 375 mm beam aperture,which is well consistent with the result of the theoretical model.This can provide reference for the design of slap amplifiers with different pump configurations as well as the design of multi-path optical paths of laser devices and the design of spatial filter apertures.