Development of Refractive Optics for X-ray Wavefront Correction

Lead Research Organisation: University of Oxford
Department Name: Engineering Science


This project is primarily aimed at developing refractive optical elements that can be used to apply known perturb actions to an X-ray wavefront. This will allow correction of errors on the wavefront which, when propagated to a focus plane, cause a broadening of the focus profile. These wavefront errors are typically caused by the manufacturing errors in a X-ray mirror reflecting surface. Refractive optics are well known as X-ray focusing elements utilising the small deviation of the real part of the refractive index for X-rays from unity. This allows weak focusing to be obtained from single lenses. The weak refraction is actually an advantage for refractive corrective optics as correction of the wavefront to an accuracy of lamba/100. Requires accuracy of the optics thickness of only 100nm which is achievable using micro fabrication techniques. In a recent breakthrough the optics group at Diamond Light Source made an accurate surface measurement of an elliptical mirror and then fabricated a refractive structure which, when inserted into the X-ray beam before this mirror resulted in a reduction in the size of the focus profile by a factor of 10. The refractive structure was made from the polymer SU-8 using the LIGA micro-manufacturing process.

The manufacture of the refractive structures requires firstly the accurate determination of the mirror surface either off-line or in-situ using X-ray methods. The surface profile is then projected to the position of the corrective optics in order to find the wavefront error. This error is then used to determine the thickness profile of the refractive optic. The thickness profile is then translated into a design which is then passed to the LIGA manufacturer for production of the device. The device is then installed with the mirror on an X-ray beamline for sensitive measurements, to characterise the device performance. The project is an opportunity to participate in developing the important new technology at Diamond Light Source.

Training will be provided by members of the Optics Group at Diamond Light Source in the following areas;
X-ray optics basic principles - reflection, refraction, diffraction of electromagnetic waves
Use of computer simulations for modelling X-ray sources and optical elements - geometric optics (ray-tracing) and physical optics (wave propagation)
Methods for measuring X-ray wavefront aberrations (at wavelength methods) and metrology methods for X-ray optics
The design and use of refractive optical elements and the specification of refractive structures for compensation of X-ray wavefronts
Practical skills required for performing experiments on the test beamline (B16) and on other headlines at Diamond Light Source
Processing of the experimental data and interpretation of the results. Presentation of the results including writing scientific publications

In addition, training in appropriate soft skills and training on health and safety will be organised as required. Research techniques will include at wavelength metrology and imaging.

Beamline measurement data on test mirrors
Development of a refined procedure for simulating the X-ray wavefront and converting the measured mirror data into a design for refractive correction optics
Development of an efficient procedure for producing a layout drawing for using the LIGA micro manufacturing process
Successful demonstration of the correction of real optics achieving measurable improvements in focusing performance
Development of practical goniometry for incorporation into X-ray beamlines at Diamond
Exploration of the application particularly for engineering experiments. Participation in experiments on B16
Evaluation of the use of other focusing optics such as multilayers and bent perfect crystals

First measurements on a real X-ray mirror and accurate determination of the mirror figure error
Conversion of measurement into a refractive corrector design
Testing to fabricated optics


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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/N509711/1 01/10/2016 30/09/2021
1941824 Studentship EP/N509711/1 01/10/2017 30/09/2021 Thomas Moxham
Description Ptychography is a coherent diffraction imaging technique that uses a phase retrieval algorithm to recover information about the sample and probe incident on the sample, and usually requires the X-ray source be highly coherent. The requirement to measure wavefront errors caused by aberrated optics in 2D has led to the development and implementation of ptychography metrology using a partial coherent source.
A key observation was also discovered that the wavefront error caused by a large majority of aberrated optics can be decomposed sinusoidally, this has led to the development of a set of general refractive structures which can be combined to correct optics without the need for custom fabrication. The project has developed further by also correcting aberrated optics in 2D with the use of 3D printed structures and the ptychographic method outlined previously.
Exploitation Route All synchrotron beamlines which require the use of a nano-probe will benefit most from the corrective refractive structures, a smaller focal size allows the possibility of obtainer higher spatial resolutions for a range of techniques including imaging, x-ray diffraction and fluorescence to name a few. The
Sectors Aerospace, Defence and Marine,Chemicals,Manufacturing, including Industrial Biotechology,Pharmaceuticals and Medical Biotechnology,Other