All laser-based fabrication of optical elements

We report on an all-laser based fabrication process for optical elements made of glass. Two laser systems, namely a 1030 nm ultrashort pulsed and a CO₂ laser are applied. Firstly, a femtosecond laser is used to precisely ablate the glass substrate layer-wise, forming the designed geometry. This ablation process is investigated in detail, focusing on the influence of the pulse distance as well as the laser fluence on the ablation depth, ablation efficiency and the surface roughness. It is found that the ablation depth decrease with increasing pulse distance while the ablation efficiency shows a maximum in the middle of the pulse distance regime for all investigated fluences. Contrary to these results, no significant influence on the surface roughness is observed. The well developed ultrashort pulsed laser ablation process is demonstrated for the fabrication of optical preforms such as cone-shaped (axicon), spherical and cylindrical lenses. In order to meet high surface quality requirements, inevitable stipulated for optical use, the surface roughness of the generated elements has to be reduced by CO₂ laser polishing. To demonstrate the subsequent surface finishing process, a complex optic geometry i.e. an axicon array consisting of 37 individual axicons is fabricated within 23 minutes while the polishing shows a reduction of the surface roughness from 0.36 μm to 48 nm. For a detailed investigation of the fabricated optic, the axicon array is mounted into the ultrashort pulsed laser machine. Several sub-Bessel beams exhibiting the typical zeroth-order Bessel beam intensity distribution are observed, in turn confirming the applied manufacturing process to be well applicable for the fabrication of complex optic geometries. Cross-sections of the quasi-Bessel beam at the axicon in the middle of the array in both, x- and y-direction, show an almost identical intensity profile, indicating the high contour accuracy of the axicon. The diameter of the sub-beam is measured to be 9.5 μm (FWHM) and the Bessel range in propagation direction amounts to 8.0 mm (FWHM).