Design and fabrication of thin-film spatial filter

Abstract

In the high-power laser system, the pinhole spatial filter is widely used for beam cleanup, its requisite focusing of the laser beam presents poor functionality [1]. The energy intensity at the focal plane of the pinhole filter is more than 10 12 W/cm 2. So the plasma closure may be probably caused and the material of the pinhole plate is easily burned [2,3]. As a hard-edge diaphragm, the low-frequency diffractive rings are likely to appear [4,5]. The spatial filter has the advantages of filtering out the rapid growing non-linear components of spatial frequency, and effectively improving the beam quality and so on [6-15]. Spatial filter is an important optical application of the thin-film technology. Non-focusing spatial filtering technique bases on multilayer dielectric film. By the matrix transfer method, the diffraction efficiencies, angular spectrum bandwidths, and the light spectrum bandwidths of multilayer dielectric film are analyzed. The combination thin-film has better spatial filtering performance than that of the combination of lens and pinhole. We design the spatial filtering is using three different methods and analyze their advantages and disadvantages. Finally, we find out the best optimized method and the film is deposited by ion-beam sputtering technique. The microstructure, composition, and optical properties have been investigated by scanning electron microscopy (SEM) and spectrophotometer. The result shows that the film has optically flat and uniform surface and the structure of the film is compact. The result also shows that the transmittance can be attenuated by the absorption of materials, thickness errors, and refractive index inhomogeneity in the process of the film depositing. At the same time, the course of manufacturing the film is needed to be controlled accurately in order to gain result which is consistent with the calculated results.