X-ray focusing lens: Compound refractive lens, CRL

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The X-ray Compound Refractive Lens, CRL, developed by Karlsruhe Institute of Technology (KIT) is a refractive lens focusing monochromatic hard X-ray to a line or point focus.

Compound refractive lens (CRL)

As the refractive index of X-ray in materials is slightly smaller than 1, the X-ray lens has a biconcave lens element and each focusing element has a very small focal length. By aligning many pieces of the elements linearly, the focal length can be significantly reduced down to a several cm range.

Compound refractive lens focusing concenpt With more lens elements, the focal length becomes shorter and beam size becomes smaller. This schematic illustrates one directional (line) focusing.

Compound refractive lens for 2D focusing Two-directional (point) focusing is achieved by arranging a pair of one-directional focusing lens rows crossed under 90°. Fresnel lens type also available.

Standard CRL specifications

Standard CRL focusing example

Here is a focusing example at BL10XU, SPring-8, Japan. At first, a long-working-distance type CRL focuses the incident beam from the undulator and monochromator, and then the second CRL, a short-working-distance type CRL, focuses the beam coming from the virtual source made by a gold pin-hole aperture (200 µm depth, 10 µm diameter) manufactured by Microworks GmbH.

Focusing example at BL10XU, SPring-8, Japan
Lens type: #1357 for long working distance and #1405 for short working distance. Both types are used for 2D focusing. Virtual source: 10 µm diameter pin-hole structure on 200 µm thick gold, manufactured by Microworks GmbH. Data courtesy of Dr Naohisa Hirao (JASRI / SPring-8) and Dr Yasuo Ohishi (JASRI / SPring-8).

R&D products: X-ray prism lens, XPL

X-ray refractive prism lenses (XPL) realise the largest lens apertures with the highest average transparency. These lenses consist of tens of thousands of triangular prisms. The prisms are arranged in order to achieve the maximum throughput. At each prism the direction of the rays is redirected by a small angle resulting in an approximately circular path of the rays in the areas filled with prisms. Each prism has to be positioned exactly on the curved path of the light through the lens.
With this lens concept one can realise a magnifying lens. [1]

[1] O Márkus et al., "Optimizing illumination for full field imaging at high brilliance hard X-ray synchrotron sources", Vol. 26, No. 23 | 12 Nov 2018 | OPTICS EXPRESS 30435

Intensity simulation with incident X-ray into the prism lens This figure shows the simulated intensity distribution in different distances behind XPL prisms. The width of the high intensity focal line (red) in the 5th plane behind the lens corresponds approximately to the size of a single prism.

Line-focusing prism lens with 20 µm prisms and 45° supporting planes Left half of the image with supporting planes and nicely arranged prisms, right half without planes showing misaligned prisms.

To realise a point focusing prism lens, two lithographically processed line focus prism lenses are tilted by 90° around the optical axis and positioned one behind the other.

Comparison with other focusing elements

In principal, you can expect to reduce the cost and set-up time compared to other X-ray optical elements.

Comparison with FZP
- Using refractive effect, no diffraction different order light is expected (even in Fresnel type lenses).
- Large aperture lenses available (up to 2,000 µm, depending on the design)

Comparison with KB mirrors
- Optic axis does not change with the lens.
- You can expect a far shorter set-up time. See the X-ray optic products brochure for the required alignment precision of the lens stage.
- You only need to prepare one location for the lens plate to make a point focus.

Unique strengths of CRL
- Multiple lens rows with different arrangements for different optical set-ups on single substrate.
- The lens is delivered after the optimisation for your set-up, depending on e.g. source distance, photon energy, requested focal length and incident beam size.

Manufacturer

Karlsruhe Institute of Technology (KIT) / IMT has a long experience in the LIGA micro fabrication technology and has applied it to manufacture X-ray optic products. ASICON Tokyo Ltd. serves as its local distributor in Japan, South Korea and Taiwan.