Study on multi-beam superposition using complementary polarization control plates

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.