光纤激光诱导背面干法刻蚀制备二元衍射光学元件

为了降低激光直接辐照透明介电材料的表面加工粗糙度和激光能量密度刻蚀阈值,提高微光学元件的产出率,介绍了一种用固体介质作吸收层,激光直接作用在透明光学材料上进行微纳加工的激光诱导背面干法刻蚀工艺。首先,选用95氧化铝陶瓷作固体材料辅助吸收层,应用中心波长为1 064 nm的掺镱光纤激光器,在3.2 mm厚的熔融石英玻璃表面刻蚀了亚微米尺度的二维周期性光栅结构。然后,对刻蚀参数进行拟合并探讨了激光能量密度对刻蚀参数的影响。最后,观察该二元光学元件的衍射花样图形并讨论其衍射特性。实验制备了槽深为4.2 μm,槽底均方根粗糙度小于40 nm,光栅常数为25 μm的二维微透射光栅,其刻蚀阈值低于7.66 J/cm²。结果表明,应用该工艺制备二维透射光栅,降低了激光刻蚀透明材料的密度阈值及加工结构的表面粗糙度。

Fabrication of binary diffractive optical element by fiber laser induced backside dry etching

With the aim to lower the surface roughnesses of etched samples and the etching threshold fluence of a direct laser irradiation source and to raise the yield of micro-optical elements, a laser induced backside dry etching technique with the assistance of a solid medium as the absorbed layer was proposed to fabricate transparent dielectric materials by direct laser irradiation. By using an alumina ceramic wafer (Al₂O₃ in 95%, surface roughness less than 500 nm) as the absorber, a 2D transmittance grating in a micro size binary Diffractive Optical Element (DOE) was fabricated on the fused silica with a thickness of 3.2 mm by the 1 064 nm Ytterbium Doped Fiber (YDF) laser. And then, the etching parameter curves were fitted and the effect of laser energy density on the parameters was discussed. Finally, the diffraction patterns of micro structure were observed to examine the features of binary DOE. According to the measured results, the binary transmittance grating shows its grating constant in 25 μm,the depth of trench in 4.2 μm, and the RMS roughness of the trench bottom below 40 nm. Furthermore, the etching threshold influence is estimated below 7.66 J/cm². These results are much lower than those of the etching without any absorbers.