Efficient Fabrication of Compound Refractive Lens for Hard X-ray Using High-Repetition-Rate Femtosecond Laser
Objective A compound refractive lens(CRL)is a kind of X-ray-focusing optical component comprising multiple concave lens units based on the refraction principle.It exhibits the advantages of compact geometric structure,adjustable focal length,and convenient collimation.This makes the CRL an important element for focusing hard X-ray beams delivered from X-ray free electron laser(XFEL)facilities.Given their excellent heat dissipation performance,diamond and silicon carbide(SiC)are considered as the preferred materials for CRL.However,manufacturing special surface contours(such as CRL with parabolic cylinder holes)on super-hard materials,while ensuring processing accuracy and efficiency,is an urgent problem that requires a solution.Compared to common processing methods,femtosecond laser processing utilizes several characteristics,including"ultra-narrow pulse duration to avoid or reduce thermal effect,""ultra-high peak power density exceeding the optical damage threshold of any solid material,"and"ultra-small focal spot size with Gaussian profile to achieve precision removal or modification of material,"to realize nearly"heat-free"micro-zone precision removal of materials.Therefore,with precise optimization of the processing strategy and controllable adjustment of laser parameters,a femtosecond laser can realize CRL fabrication on super-hard materials such as diamond and SiC.These materials are ideal for high-precision machining technologies for optical components,which are required for focusing hard XFEL beams.Methods Using a high-repetition-rate femtosecond laser(with the center wavelength of 1030 nm,pulse duration of 400 fs,maximum single pulse energy of 200 μJ,and repetition rate of 200 kHz)alongside a 3D programmable precision machining platform(with a 3D precision motion stroke of 100 mm× 100 mm×50 mm,absolute positioning accuracy of±0.3 μm,and repetitive positioning accuracy of±0.03 μm),a CRL array with parabolic cylinder holes is fabricated.This array features an opening diameter of 500 μm and a vertex curvature radius of 73.5 μm,and is fabricated on a SiC substrate with a thickness of 330 μm.Initially,a geometric structure for the SiC CRL with parabolic cylinder holes,which satisfies the specific requirements of the test experiment layout(Table 1),is designed.Following this,two machining strategies,focusing on contour bias and linear scanning,are proposed(Figs.4 and 5).An analysis of the advantages and disadvantages of these strategies leads to the recommendation of an optimal machining method(Fig.6).These techniques,in conjunction with predrilling,ultrasonic cleaning,and processing of front and back sides(Fig.7),are employed.To conclude,the morphology of the SiC CRL sample is examined using scanning electron microscope(SEM),and its focusing performance is evaluated through a test experiment conducted at the Shanghai Synchrotron Radiation Source.Results and Discussions Femtosecond laser processing based on internal linear scanning combined with edge contour bias improves the processing rate while satisfying contour processing accuracy.Furthermore,it avoids the problem of uneven processing removal caused by inconsistent processing rates.SEM photographs show that the surface profile of the CRL basically conforms to the structural design,the error of each feature dimension is less than 5 μm,and the parabolic profile is smooth and free of obvious cracks[Figs.8(a)-(c)].The parabolic sidewalls are not as smooth as the planar sidewalls[Fig.8(d)].Focusing performance testing experiment conducted at Shanghai Synchrotron Radiation Source shows that the femtosecond laser fabricated CRL can focus X-rays with photon energy of 12 keV and size of 170 μm×55 μm to a size of 170 μm×10 μm.However,there is a certain degree of inhomogeneity in the spot intensity distribution(Fig.9).Conclusions Using a high-repetition-rate femtosecond laser,a SiC CRL with parabolic cylinder holes is fabricated based on a composite path of internal linear scanning combined with edge contour bias.The face profile of the CRL conforms to the structural design.However,the smoothness and tapering of the sidewalls are insufficient,resulting in a certain degree of uneven intensity distribution of the X-ray spot focused through the CRL.These problems can be solved by further optimizing the machining process(e.g.,the precise alignment of the processing of front and back sides and precise regulation of the z-axis step)in subsequent research.This study provides a useful reference for subsequent development of 2D CRL processing.
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