The refractive index for matter in the hard X-ray range changes rather rapidly with increasing photon energy. In fact
away from absorption edges the refractive index decrement varies with the square of the photon energy. As the photon
energy dispersion depends on the refractive index decrement, prisms, as used in the visible spectral range, can also be
used in the X-ray range for monochromatising or dispersing an incident polychromatic X-ray beam. The dispersion in a
single prism is rather small and it is thus mandatory to use ensembles of many prisms for the dispersion process. A
particular prism assembly is the Clessidra X-ray lens. The name of the lens, which is Italian for hourglass, describes its
appearance as many tiny prisms form two larger prisms with opposing areas. In this form the device can focus an
incident plane X-ray wave by refraction. Then the dispersion will make the focusing suffer from chromatic aberrations.
The latter can be used in order to operate the structure as a monochromator in combination with an exit slit. For this
operation it was found to be advantageous to use a central obstruction in the lens. As the prism array focuses only onedimensionally
the object rotation around an axis, which is perpendicular to the incident X-ray beam and which lies in the
dispersion plane, is now an available degree of freedom. In a monochromator one can make good use of it, as it permits
us to tune the photon energy in a fixed slit. This contribution will discuss the limits for such an operation in terms of
achievable spectral resolution and in terms of rapid tuning.
The Clessidra prism array lens appears like an hourglass with two perfectly triangular opposing areas. Each of these halves is then formed of a multitude of smaller abutting triangular prisms. These devices focus an incident plane X-ray wave into a focus by refraction. Due to the wavelength dependence of the refractive index the focusing is suffering from chromatic aberrations. This can be used in order to operate the structure as a monochromator in combination with an exit slit. Photon energy tuning in a fixed exit slit is possible, when the structure is rotated around an axis, which is perpendicular with respect to the incident beam and perpendicular to the bases of the prisms. Such a rotation can be performed with rather high speed. In this study the advantages of this concept for the energy tuning are discussed as are the limitations.
At the Institute of Microstructure Technology (IMT) at Karlsruhe Institute of Technology (KIT), refractive X-ray optics are developed. These optics are proposed to be used as condenser optics in X-Ray spectroscopy and microscopy applications with an X-ray tube as a source. To produce the lenses, a thin structured foil with equidistant fins in triangular form is casted from a structured silicon wafer. The foil is then wound around a glass fibre core. Due to this fabrication method, it is possible to produce large-aperture lenses with low absorption in comparison to other types of refractive X-Ray optics, like X-ray lenses with continuous parabolic shape or prism lenses. The first are limited due to their absorption while the latter are limited due to their mechanical stability of the prism columns. The optimisation of the so called X-Ray rolled prism lenses (RXPL) is underway at the institute and involves several parameters. One important property of the lenses is the correct form of the wound foil layers. This determines the number of necessary refractive elements at a given radius, which in turn determines the refracted slope and focal position of the transmitted beam. The spatial extent of the x-ray source is also being accounted for in the lens design. Another important point is the diameter of the winding core, which should be as small as possible due to the fact that the winding core reduces the active area of the lens. The rolling process itself is also revised to produce lenses with the above-mentioned small diameter winding cores and bend foil layers while sustaining a tight- fitting foil bundle. The lenses are studied at different energies and types of X-Ray tubes, as well as synchrotron sources, to gain additional information of the internal structure of the lens after the winding process. In this paper the current status of the lens development and results at X-Ray tube sources for use in diffractometers is presented.
At the Institute of Microstructure Technology (IMT) at Karlsruhe Institute of Technology (KIT) X-ray refractive line
focus lenses have been developed. They consist of a large number of concave bi-parabolic lens elements made of SU8.
To form a point focus two of these lens stacks, tilted by 90° with respect to each other around the optical axis, need to be
arranged in the optical path. To increase their transmission, the Fresnel principle can be applied to the lenses to provide
higher ratios of refractive power to absorption. The lenses are fabricated by deep X-ray lithography which allows to
pattern high aspect ratio structures and gives the possibility to fabricate the lens elements tilted by 90° with respect to
each other on a single substrate by tilted double exposure. Nevertheless, the aspect ratio is limited, due to the fact that the
columns tend to collapse from capillary forces during fabrication if they exceed a certain height. To overcome this issue
and to simplify the fabrication process a new type of lenses as well as a method to fabricate refractive large aperture
lenses has been developed recently at IMT. These lenses are fabricated out of a structured polyimide film which is cut
into a calculated shape and rolled around a glass-fiber core. The structure on the film itself consists out of triangular
shaped ribs. The lenses provide the advantage of Fresnel lenses and also provide a point focus through their approximate
rotational symmetry. The full width at half maximum (FWHM) of the focal spot of such lenses is mainly determined by
the height of the triangular ribs. Such X-ray optical elements are well suited to be used as condenser lenses, because they
provide efficient illumination of an area in the exit working distance. To increase the lens performance, the lens
fabrication process has been optimized. In the paper we provide information on how the lenses where improved and
present results from tests with X-ray tube sources.
The unique beam characteristics of PETRA III at DESY promote novel applications for many scientific fields,
including imaging applications. For tomography these are techniques like high-speed and in-situ measurements
marked by highest density resolutions and spatial resolutions down to the nanometer range. Furthermore, the
high coherence enables phase contrast applications in an exceptional way. Therefore, the Imaging Beamline IBL
is equipped with two dedicated endstations, one for micro and one for nano tomography. In addition, a very
flexible X-ray and light optics concept is implemented. The micro tomography endstation is designed for samples
requiring (sub-) micrometer resolution. The technical specifications of the nano tomography endstation aim for
a spatial resolution of below 100 nm. The nanometer resolution will be achieved by using different combinations
of compound refractive lenses as X-ray optics. The overall setup is designed to be very flexible, which allows
also the implementation of other optical elements as well as the application of different magnifying techniques.
Refractive X-ray lenses can be used effectively, to focus or collimate X-rays with photon energies clearly above 10 keV.
On the one hand parabolic Compound Refractive Lenses (CRLs) are suitable as imaging optics in high resolution X-ray
microscopy. The most recent developments are nanofocusing refractive X-ray lenses (NFLs). These show focal spot
sizes of less below 100 nm. On the other hand refractive X-ray lenses can provide a high photon flux when used as large
aperture condenser optics. Two types of refractive condenser optics made out of structures with triangular profile have
been developed at the Institute for Microstructure Technology (IMT) at the Karlsruhe Institute of Technology (KIT) and
have been tested at synchrotron sources in recent years. One type of special interest is the Rolled X-ray Prism Lens
(RXPL). These lenses are made of a rolled polymer foil structured with micro grooves with triangular profile. The
combination of such condenser optics and NFLs provides a basis for future hard X-ray microscopes.
X-ray prism lenses have been defined with the aim to collimate X-ray radiation emitted from an X-ray tube working as a
condenser lenses. Such a lens must have a large aperture as low absorption as possible. X-ray prism lenses combine low
absorption and large apertures. They are made up of a large array of equilateral triangular prismatic microstructures. The
intent by using these structures is to obtain as many refracting surfaces as possible in the smallest volume. The higher
surface-volume-ratio in comparison to standard lenses reduces absorption significantly at the expense of focus quality.
A first lens has been fabricated by X-ray lithography out of PMMA, with a designed aperture of up to 1.4 mm working
distances of 325 mm to the point source and X-ray energy of 9 keV. The edge-length of the prismatic microstructures is
10 μm. The lenses have been tested at the ESRF in (Grenoble, France) and at ANKA (Karlsruhe, Germany). The results
show an influence of the imperfections of the lens structures (bended prismatic microstructures) on the focal spot along
the focal line. The measured gain was 28 at a focal width of 8 μm at full width at half maximum. Due to these
imperfections the relevant aperture is currently limited to 500 μm.
We develop a new type of X-ray lens system which is achromatic in a limited energy range. For such achromats
we combine different types of refractive and diffractive elements. For example, Fresnel zone plates and planar
parabolic concave SU-8 lenses are combined with lenses with a biconvex parabolic shape and with Fresnel lenses,
respectively. We present numerical results from a theoretical study of such optical systems. We determine the
focal spot size for an energy range of about E ± ΔE with ΔE/E ≈ 17%. Amongst other results we find that, compared with conventional lens systems, the spot size can be reduced by several tens of percent by using such achromatic lens systems.
Existing refractive X-ray lenses are characterized by either small apertures due to high absorption in the border areas. They can only be used with synchrotron sources, offering high brilliance. By increasing transparency and aperture the range of applications will expand, common X-ray tubes might turn out to be reasonable X-ray sources in an application with X-ray lenses. A basic concept that meets the demands is an X-ray Fresnel lens. But, Fresnel X-ray lenses are hard to fabricate, since the smaller lens structures need to be produced with extremely high aspect ratios. As an alternative, the
Fresnel structures can be replaced by an array of prism-shaped structures. In particular equilateral triangular structures are easier to fabricate and additionally give a higher
surface-volume-ratio, increasing transparency. At the Institute for
Microstructure Technology the development of such prism lenses is under way. Due to the physical properties of X-rays, several thousands of precisely arranged prisms with large aspect ratio and smooth sidewalls are needed for a single X-ray lens. Therefore, direct X-ray lithography is used to fabricate the SU-8 microstructures. The length of one single prism edge is of the order of 10 μm. One single prismatic X-ray lens consists of up to 60.000 prisms. With the appropriate X-ray mask, refractive X-ray lenses with an aperture of up to 2 mm, for a source distance of 350 mm and a working distance of 350 mm are being produced, assuming a point-shaped source. These X-ray prism lenses are not optimized for
smallest focal diameter, but designed to illuminate samples in X-ray optical systems. Most important in this application is an as high transparency as possible.
Over the last decade refractive lenses for monochromatic X-ray radiation have been realized for many different materials by microfabrication technology. All these lens systems are successfully working only for one discrete energy, i.e. the lenses are chromatic. Thus each discrete energy within a certain energy range has a different focal length. While the focal spot size is smaller than a micron for a particular energy at the corresponding focal
distance, it increases up to several tens of microns for a larger energy range. We present results of numerical simulations for a new type of lens system which addresses this problem. We are developing achromats by combining different refractive elements of different materials. Via ray-tracing we determine the parameters of
the lenses by minimizing the focal spot size for an energy range of about E ± ΔE with ΔE = 15%. Thus the spot size of an energy range can be noticeably reduced compared with conventional refractive (chromatic) lens systems.
An important challenge that remains to date in board level optical interconnects is the coupling between the optical
waveguides on printed wiring boards and the packaged optoelectronics chips, which are preferably surface mountable on
the boards. One possible solution is the use of Ball Grid Array (BGA) packages. This approach offers a reliable
attachment despite the large CTE mismatch between the organic FR4 board and the semiconductor materials.
Collimation via micro-lenses is here typically deployed to couple the light vertically from the waveguide substrate to the
optoelectronics while allowing for a small misalignment between board and package. In this work, we explore the
fabrication issues of an alternative approach in which the vertical photonic connection between board and package is
governed by a micro-optical pillar which is attached both to the board substrate and to the optoelectronic chips. Such an
approach allows for high density connections and small, high-speed detector footprints while maintaining an acceptable
tolerance between board and package. The pillar should exhibit some flexibility and thus a high-aspect ratio is preferred.
This work presents and compares different fabrication methods and applies different materials for such high-aspect ratio
pillars. The different fabrication methods are: photolithography, direct laser writing and deep proton writing. The
selection of optical materials that was investigated is: SU8, Ormocers, PU and a multifunctional acrylate polymer. The
resulting optical pillars have diameters ranging from 20um up to 80um, with total heights ranging between 30um and
100um (symbol for micron). The aspect-ratio of the fabricated structures ranges from 1.5 to 5.
We discuss on-going reliability studies of micro-optical components and assemblies as conducted in the EU FP6 Network of Excellence on Micro-Optics "NEMO". We focus on three case studies including first biaxial fatigue testing of micro-optical components, second reliability testing and quality control of MEMS and third micro-interferometric tomography for measuring optical fibre refractive index changes. For each of these case studies we discuss the dedicated measurement and characterization methods as well as first results and the perspectives for future research.
A wireless displacement sensor is proposed which is composed of a fixed active head and a passive head which moves relatively to a grating scale. The passive head comprises a grating and two mirrors. It can be so small as to be inserted non-obtrusively where the displacement must be measured without spoiling the system optimum by undesirable size and structural compromises. It is insensitive to electromagnetic perturbations. The key characteristics of the sensor are high resolution and a very small size. The encoder uses a fixed grating scale as in standard designs, a passive-optical read
head on the moving element and a stationary, opto-electronic source/detector module. Communication between the moving head and the detector module is done without cables using a free-space optical interconnection.
A sinusoidally weakly undulated continuous thin gold film embedded between a polymer substrate and a thin cover of the same polymer, the metal film thickness, the period and the wavelength being such that a normally incident wave excites the long range plasmon mode of the metal film, is shown to exhibit strong resonant transmission for the local TM polarization and strong reflection of the TE polarization. Such structure represents a very simple, average performance polarization beam splitter for white light processing.
The device presented in this paper is designed for coupling a free space optical wave under quasi-normal incidence in and out of a highly multimode waveguide with high efficiency. It uses two resonant diffraction gratings at the substrate-waveguide interface that are made of a shallow metal grating, covered with a high refractive index layer. It is shown that the resonant structure can theoretically diffract up to 90% of the incident energy in and out of the waveguiding layer. The geometrical parameters of the structure and the tolerances can easily be achieved by conventional technology means.
The aim of the presented LIGA-microspectrometer design is, to improve the spectral resolution and to achieve a high sensitivity covering at the same time a large spectral range. The footprint of the microspectrometer had to be increased to achieve these goals. To limit the increasing of the size of the system, the internal optical path was folded by introducing a mirror. The spectrometer is a grating spectrometer where the light is guided in a hollow waveguide. To improve the sensitivity of the spectrometer, the losses in the hollow waveguide had to be limited. As these losses increase with the number of reflections in the waveguide, a collimator lens in front of the entrance slit was introduced to realize a quasi free space propagation of the light in the waveguide. The concept of this microspectrometer, its characteristics, dimensions and key elements, such as entrance slit, collimator lens, hollow waveguide, optical path folding and decoupling mirror, are explained. Also the result of the photographic characterization of the microspectrometer is shown.
In this paper, the realization and characterization of a microoptical sensor using the chromatic confocal principle is presented. The sensor head is designed for distance gauging applications in high aspect ratio cavities with a diameter of about 2 mm. The first part of this paper focuses on the design and fabrication process of the hybrid optical benches, which combines refractive and diffractive micro optical components. Very tight tolerances of the optical path are required for the functionality of the sensor. Therefore the alignment structures and mounts between the different optical elements are produced from PMMA using deep X-ray lithography, the first step of the LIGA process.
In the second part of this paper the characterization of first prototypes using different light sources are described and results presented.
Increasing demands for the monitoring of tolerances of small mechanical and optical precision components require improved measurement techniques. In this paper the basic concept and different optical designs of a confocal microoptical distance-sensor are presented. The sensors use the chromatic-confocal measurement principle which does not require a mechanical depth scan. Therefore, a chromatic-confocal point sensor can be designed without any moving parts. This fact is used to design a miniaturized sensor head with an outer diameter smaller than two millimetres. A special feature of the sensor head is its capability to measure sideways. This enables e.g. to measure surfaces in small drilling holes.
The need for low cost micro optical devices is increasing thru all application fields like tele- and data-communication, industrial automation, displays, automobile, sensor applications etc. Polymer technologies can follow this demand due to the possibility of mass fabrication by replication techniques. Various technologies have been developed in the past to fulfill the demanding requests given by the use of micro structures in optical applications. Part of them are already used in industrial manufacturing. Also demanding products are introduced into the market. In the paper we will give an overview of the relevant techniques and demonstrate their possibilities by a few product examples.
Increasing demands for controlling tolerances of small mechanical and optical components require improved measurement techniques. In particular, components with a complex geometry such as small holes or channels are difficult to access by classical tactile measurement systems. These systems are also limited in their measurement speed. Optical distance sensors do not have many of these disadvantages, but the sensor heads are normally too large to access e.g. small holes. Presented in this paper is a novel microoptical sensor concept using the chromatic confocal principle for distance gauging applications. This is used in high aspect ratio cavities with a diameter of about 2 mm. The distance resolution of the sensor is aimed to be in the sub-micrometer range.