高效率多通泵浦平面波导激光放大器

High Efficiency Multipass Pumped Planar Waveguide Laser Amplifier

设计了一种高效率、结构紧凑的高功率平面波导激光放大器。放大器使用新型平面波导结构,可以实现对芯层的多通泵浦,从而大幅度增加泵浦光的吸收长度,提高低掺杂Yb∶YAG芯层的泵浦吸收率。基于Yb3+的激光动力学方程,结合光学追迹法与有限元方法构建了三维激光放大模型,并分别对双包层平面波导与新型平面波导的高功率激光放大器进行仿真。仿真结果表明,当泵浦总功率为10 kW和注入种子光的功率为200 W时,新型平面波导放大器具有更高的泵浦吸收率、更好的泵浦均匀性、更低的热损伤风险及更高的光-光转化效率。

Objective Planar waveguide amplifiers have the advantages of slab and fiber ones and become an essential branch of the high average power solid-state lasers.Yd∶YAG planar waveguide amplifiers have the potential for higher output than Nd∶YAG due to the less generated heat and higher extraction efficiency under the same absorption pump power.The absorption cross-section of Yd∶YAG is smaller than Nd∶YAG;a higher doping concentration of the core and end-pumping must fully absorb the pump power.To maximize extraction efficiency,the doping concentration is significantly reduced due to self-absorption.To ensure that the pump power can be fully absorbed,extending the length of the core is necessary.However,the size of the core is limited by the process conditions.If the doping concentration is sufficiently small,the end-pumping cannot meet the absorption of pump power requirements.In this study,we design a multipass pumped planar waveguide with the low-doped Yd∶YAG core to prolong the pump absorption length.The amplifier using a multipass pumped planar waveguide shows higher pump absorption,better absorption uniformity,and higher optical-optical efficiency than the double-clad planar waveguide amplifier.We hope that the new structure can provide methods and ideas for designing and optimizing planar waveguide lasers.Methods Based on the theory of single-mode transmission and laser mode competition in the planar waveguide,the core and inner cladding of the planar waveguide are Yb∶YAG with a thickness of 0.2 mm and a doping concentration of 1%and Er∶YAG with a thickness of 0.5 mm and a doping concentration of 0.5%,respectively.The core and inner-claddings form the core area,which is 80.0 mm×16.0 mm×1.6 mm.YAG is bonded around the core area with a thickness of 1.2 mm.The outer-claddings of 1 mm cover the core and expansion areas,forming a double-cladding waveguide with the core area and a single-cladding waveguide with the expansion area.A multipass pumping ray path in the waveguide is formed through internal reflections on the waveguide surfaces.Symmetrical double-end pumped and single-pass power extraction configurations are adopted for laser amplification.Based on the Yb3+laser kinetics model,we develop a three-dimensional(3 D)laser amplification model using ray tracing and finite element methods.We use the model to simulate double-clad planar waveguide and multipass pumped planar waveguide amplifiers.Besides,we compare their pump absorption and amplification extraction characteristics under 10 kW pump power and 200 W-injected seed power.The temperature distribution is simulated based on the results from the laser magnification model.Results and Discussions Compared with a double-clad planar waveguide amplifier,a multipass pumped planar waveguide amplifier exhibits higher output power and optical-optical efficiency(Fig.9).The simulation results using the laser amplification model show that the planar waveguide core absorption coefficient is 0.24 cm-1,which is 77.3%lower than the passive absorption coefficient,strongly affected by nonlinear absorption(Fig.11).The pump absorption decreases with a decrease in the absorption coefficient.The multipass pump absorption efficiency is still above 90%due to an increase in the absorption length(Table 2).The absorption power density distribution of the core simulated by the laser amplification model is different from that of the passive absorption coefficient model,which is caused by considering the effects of the laser and pump intensities on pump absorption(Fig.12).Because of better pump absorption uniformity,although the absorbed power increases,the maximum temperature does not increase significantly.However,the temperature distribution in the width direction is asymmetric,which may cause low actual beam quality(Fig.13).The maximum thermal stress of core is only 18.5%of the safety limit,which is lower than that in the double-clad planar waveguide amplifier(Table.3).The extraction efficiency of the multi-channel pump is higher(Fig.16)since the high pump intensity is preferred to extract the absorbed power(Fig.15).Conclusions A noble planar waveguide is designed for a high-power planar waveguide laser amplifier with the low-doped Yd∶YAG core.multipass pumping of the core is achieved through bonding YAG around the core area and high reflection coating on reflective surfaces.The geometric of the waveguide is reshaped and optimized to improve pump absorption and uniformity.The double-clad planar waveguide amplifier and multipass pumped planar waveguide amplifier are simulated using a 3 Dlaser amplification simulation model combining the ray tracing and finite element methods.When the pump power is 10 kW,the opticaloptical efficiency of the multipass pumped planar waveguide and double-clad planar waveguide amplifier becomes saturated with 200 Wof injected seed.When the injected seed is 200 W,the multipass pumped planar waveguide threshold pump power becomes 500 W,which is almost the same as the double-clad planar waveguide amplifier.However,the slope and optical-optical efficiencies are higher than those of the double-clad planar waveguide amplifier.When the injected seed is 200 W,and the pump power is 10 kW,the absorption power density distribution simulated using the laser amplification model significantly differs from that of the passive absorption coefficient model.The simulation results using the laser amplification model showed that in the multipass pumped planar waveguide amplifier,the pump absorption efficiency is93.3%,the output power is 7311 W,and the optical-optical efficiency is 71.1%.It is significantly higher than that of the double-clad planar waveguide amplifier.The multipass pumped planar waveguide amplifier shows better pump absorption uniformity and a smaller risk of thermal damage.