Nondestructive measurement and characterization of bi-paraboloidal x-ray refractive lenses fabricated by femtosecond laser

There is an urgent demand of novel X-ray optics for high heat load and radiation damage resistance for the new light source development. The compound refractive lens (CRL) is a good choice for the photon beam manipulation. Diamond, SiC and sapphire have been proved to be the preferred materials for CRL. However, on account of these materials are hard, brittle and difficult to process, conventional preparation prevents highly precise removal of materials. Meanwhile, CRL achieves X-ray focusing by stacking multiple lens units, which requires efficient and precise processing method to ensure that the error of each single lens is within an acceptable range. This paper uses scanning confocal laser microscopy and synchrotron X-ray computed tomography (CT) techniques to detect the manufacturing errors of bi-paraboloidal 2D focusing X-ray lenses of curvature radius <i>R</i>=1000 μm and aperture <i>D</i>=2000 μm produced via femtosecond laser ablation. Point cloud processing method is introduced to accurately characterize the relative position of two paraboloids deviation. The results showed that the local micro-roughness of lens surface is about 400 nm <i>S_q</i>. Relative transverse offsets of the front and back surfaces can be controlled within 35 μm and the axis deflection angle of the front and back paraboloid is between 20 and 40 mrad. Using the aforementioned methods, the manufacturing errors of bi-concave lenses can be effectively obtained, providing feedback for the optimization and improvement of process parameters.