基于互补型偏振控制板的多光束叠加特性研究

Study on multi-beam superposition using complementary polarization control plates

为满足惯性约束聚变对靶面光场辐照特性的要求,提出利用互补型偏振控制板改变光束内部偏振态,实现多光束消偏振叠加,进而改善聚焦光场均匀性的方案.建立了光束通过互补型偏振控制板进行变换的物理模型,理论分析了互补型偏振控制板影响聚焦光斑偏振特性的原因,比较了互补型偏振控制板与非互补型偏振控制板对聚焦光斑偏振特性及均匀性的影响,并进一步讨论了偏振控制板单元数的选取问题.结果表明:多光束通过互补型偏振控制板后,聚焦光场不再是单一偏振光,而是各类部分偏振光的随机混合;与其他类型双块偏振控制板相比,互补型偏振控制板能实现聚焦光斑的消偏振叠加且效果最佳,使光斑偏振度下降至0.2以下,并能有效地改善光斑的强度均匀性.采用互补型偏振控制板时,单元数对聚焦光斑偏振特性影响不大,但对强度均匀性则存在一定程度的影响,因而在实际工作中应根据应用需求合理选取单元数.

In order to meet the requirement for uniform irradiation on the target in inertial confinement fusion, a schemie is proposed for achieving the depolarized superposition of multi-beams and further improving the uniformity of the focus spot by using complementary polarization control plates to change the polarization state of the beam. A physical model of multi-beam transformation by complementary polarization control plates is built up, and the mechanism how complementary polarization control plates affect the polarization characteristics of the focus spot is analyzed theoretically. The polarization properties and uniformity of the focus spot are compared for the two cases： viz the complementary polarization control plates and the un-complementary polarization control plates. Furthermore, the selection of elements of polarization control plates is discussed. Results show that after beams propagating through the complementary polarization control plates, the focal spot is not completely polarized but mixed randomly with various kinds of partially polarized states. Compared with the un-complementary polarization control plates, the complementary polarization control plates can achieve better depolarization effect on focus plane, resulting in the degree of polarization dropping to below 0.2, and thus achieving the effective improvement of intensity uniformity. When the complementary polarization control plates are utilized, the number of elements of polarization control plates has little effect on the polarization characteristics of the spot, but it can influence the intensity uniformity to some extent. Consequently, the number of elements of polarization control plates should be chosen appropriately in practical application.