Dielectric pinholes for laser-beam filtering

The spatial-frequency filtering of high power laser beams with a high divergence requires pinholes with small hole diameters, small thicknesses and a high laser damage threshold. Conventional pinholes, based on absorption or reflec-tion are difficult to realize under these conditions. We present a new method for the filtering of laser beams by using dielectric pinholes, which do not absorb or reflect but rather deflect the high spatial-frequency parts of the beam. Ad-vantages of this new kind of pinholes are a significantly increased resistance to high laser energies, which is comparable to the damage threshold of the bulk form of the dielectric material, and a simplified handling because of the permanent visibility of the beam. To demonstrate the principle, we fabricated diffractive pinholes in fused silica by use of electron beam lithography with subsequent reactive ion beam etching. Finally some measurements of the filtering effect are presented.