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使用线栅偏振片测试光源的偏振态,其本质上就相当于一个检偏器。当入射光的偏振态与线栅平行时,偏振光的透过率最大,对应功率计的示数最大。因此,通过观察功率计示数的变化可以反推出光源的偏振方向。
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根据文中所示的数据,该实验使用4.05 μm QCL的阈值电流为220 mA,如表1和图1所示,随着电流的增加,其功率曲线呈线性增加。为了保护QCL芯片,该实验将最大电流控制在350 mA,其最大功率为0.241 W。
图 1 线栅振片测试偏振态实验光路示意图
Figure 1. Schematic diagram of the experimental optical path of the polarization state of the wire grid vibrator
表 1 不同电流下QCL功率
Table 1. QCL power at different current
I/mA 220 230 240 250 260 270 280 290 300 310 320 330 340 350 P/W 0.031 0.050 0.068 0.087 0.104 0.122 0.140 0.156 0.173 0.187 0.202 0.215 0.228 0.241 -
(1) 半波片对4.05 μm中波红外透过率测试
该实验使用由索莱博公司生产的中波4 μm半波片,测试该半波片对4.05 μm激光的透过率,为后续偏振合束效率分析提供参考。
根据实验测试数据,半波片对4.05 μm的透过率很高,通过计算其平均值,其透过率高达97%。图2中,两者的功率曲线基本吻合,半波片对激光能量的损耗很小且没有影响功率电流曲线的线性特征。
(2) 线栅偏振片对4.05 μm中波红外透过率与入射角的关系
对线栅偏振片透过率的测试,考虑到偏振合束的实际情况,其入射光束与偏振片应有一定的夹角,因此该实验测试了不同入射角度下的透过率,通过与最初QCL的功率电流曲线对比,计算出不同角度下线栅偏振片的透过率,从而得出最佳的入射角度,见图3。
图 3 不同入射角度下通过线栅偏振片的功率和电流对比曲线
Figure 3. Comparison curves of power and current passing through the wire grid polarizer at different incident angles
根据以上测试数据,在不同入射角度下,线栅偏振片的透过率差别较大,分别测试了30°和45°入射情况,实验数据表明线栅偏振片对偏振方向平行于栅格方向的光具有较高的透过率,但也存在一定损耗。当入射光与偏振片的夹角为45°时,其透过率平均值约为75%,当入射光与偏振片夹角为30°时,其透过率平均值约为81%。实验结果表明入射光与偏振片夹角为30°时,具有较高透过率是比较理想的入射角度。
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在偏振合束实验中,利用偏振片以及光源的偏振特性,当入射光的偏振态垂直于偏振片栅格方向时,其透过率基本为零,此时入射光的反射率很高,利用该特性,通过改变光源的偏振态,将光源的偏振态旋转90°,从而实现偏振片对反射路光束的高反射率。在该实验中,测试不同角度下偏振片对与栅格方向垂直光束的反射率,如图4所示。
图 4 不同入射角度下线栅偏振片反射的功率和电流对比曲线
Figure 4. Comparison curve of power and current reflected by wire grid polarizer at different incident angles
在该实验中,分别测试了30°和45°入射的情况,根据以上实验数据,在不同入射角度下,线栅偏振片对入射光的反射率差别比较大。当入射角为45°时,其反射率平均值约为81%,当入射角为30°时,其反射率平均值约为91%。从实验数据可知,相同角度入射下,反射率要高于透射率。透过率较低的原因主要是由于光在通过偏振片时,光波的电场和磁场对偏振片内部原子的电荷分布产生影响,进而对自身电磁场的能量产生损耗,而对于反射光而言,偏振片对入射光能量的损耗小,因此反射能量相对更高。结合上面的实验数据,当入射光和反射光与偏振片的夹角为30°时,反射和透射都具有相对较高的效率,因此,对于该实验所使用的偏振片,在偏振合束实验中,使入射光和反射光与偏振片的夹角为30°。
Study on polarization beam combining experimental of mid-infrared quantum cascade laser
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摘要: 合束是实现量子级联激光器高功率输出的关键技术,基于激光的偏振特性,研究了偏振合束的实验原理及实验方法。使用线栅偏振片和中波半波片组成偏振合束装置,对两路4.05 μm量子级联激光器进行偏振合束,测试了中波半波片对4.05 μm激光的透过率以及中波线栅偏振片对4.05 μm激光透射率和反射率与入射角的关系,通过实验研究,当透射路光束和反射路光束与线栅偏振片的夹角为30°时,透射路的透射率为81%,反射路反射率为91%,其光束合束效率达到约86%,并使用光束质量分析仪对合束之后的光束质量进行测试分析。结果表明:两路光束通过该合束装置合束之后,在保证合束效率的条件下,具有较好的光束质量。Abstract: Beam combining is a key technology to achieve high-power output of quantum cascade lasers. Based on the polarization characteristics of lasers, the experimental principles and methods of polarization beam combining were studied. The polarization beam combining device was composed of a wire grid polarizer and a mid-wave half-wave plate to perform polarization combining of two 4.05 μm quantum cascade lasers. The transmission rate of the mid-wave half-wave plate to 4.05 μm laser and the relationship between the transmission rate and reflectance of the wire grid polarizer to 4.05 μm laser and the angle of incidence were tested. When the angle of the wire grid polarizer was 30°, the transmittance was 81%, the reflectivity was 91%, and its beam combining efficiency reached about 86%. The beam quality after combining was tested and analyzed by a beam quality analyzer. The results show that the beam after beam combination has better beam quality under the condition of ensuring beam combination efficiency.
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表 1 不同电流下QCL功率
Table 1. QCL power at different current
I/mA 220 230 240 250 260 270 280 290 300 310 320 330 340 350 P/W 0.031 0.050 0.068 0.087 0.104 0.122 0.140 0.156 0.173 0.187 0.202 0.215 0.228 0.241 -
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