Development of SiO_2 multistep diffractive optical element for beam-homogenizing

摘要

Diffractive Optical Element (DOE) is an advanced optics which utilizes the optical diffraction phenomena by a microstructure on its surface and can realize various applications such as homogenizing, shaping and splitting in laser material processing. The optical property of DOE is influenced by the accuracy of its microstructure formed by photolithography and the dry etching technique. The development of the reproducible dry etching technique of fused silica to obtain high depth precision in a microstructure by inductively coupled plasma reactive ion etching (ICP-RIE) is described. In ICP-RIE depth is controlled by the time determined by the etching rate of the previous batch. To stabilize the etching rate which is determined by ion energy, the ICP power is reduced to decrease the range of ion energy distribution and the thickness of grease for cooling the substrate is controlled between each batch. Depth precision of less than 10nm has been obtained. Good depth uniformity of less than 30nm P-V at the 1183nm target depth in the 50mm diameter area is also obtained using gas flow simulation. With these improvements a beam-homogenizer DOE with a 16-step microstructure for 532nm YAG-SHG is produced. Intensity is changed by this DOE from the Gaussian beam to the flat top beam whose optical intensity uniformity is less than 10% in the 1.0 X 0.5 mm region.