Ralf Steinkopf

Broadband antireflective surface-relief structure for THz optics

The requirements for a broadband antireflective structure in the THz spectral region are derived. Optimized structural parameters for a surface-relief grating adapted to the spectrum of an intended THz pulse are deduced. The effect of a structure fabricated into Topas((R)) by a single-point diamond-turning process is demonstrated.

Incoherent beam shaping with freeform mirror

Beam shaping of incoherent light sources (LEDs, halogen lamps) for arbitrary target light distribution is obtained by a single free-shape mirror. Special design algorithm ensures continuous profile without abrupt changes and shadowing regions. The mirror is manufactured by single point diamond turning combined with Fast-Tool-Servo (FTS) for simultaneous figuring of base surface and fine structure (for redistributing the light energy). Lateral and axial resolution of the fine structure is determined by FTS and considered during the design and data transfer. Directly turned surfaces can be used as replication tools for polymer or glass moulding and embossing.

3D phase-shifting fringe projection system on the basis of a tailored free-form mirror

Phase-shifting fringe projection is an effective method to perform 3D shape measurements. Conventionally, fringe projection systems utilize a digital projector that images fringes into the measurement plane. The performance of such systems is limited to the visible spectral range, as most projectors experience technical limitations in UV or IR spectral ranges. However, for certain applications these spectral ranges are of special interest. We present a wideband fringe projector that has been developed on the basis of a picture generating beamshaping mirror. This mirror generates a sinusoidal fringe pattern in the measurement plane without any additional optical elements. Phase shifting is realized without any mechanical movement by a multichip LED. As the system is based on a single mirror, it is wavelength-independent in a wide spectral range and therefore applicable in UV and IR spectral ranges. We present the design and a realized setup of this fringe projection system and the characterization of the generated intensity distribution. Experimental results of 3D shape measurements are presented.

Data handling and representation of freeform surfaces

Freeform surfaces enable innovative optics. They are not limited by axis symmetry and hence they are almost free in design. They are used to reduce the installation space and enhance the performance of optical elements. State of the art optical design tools are computing with powerful algorithms to simulate freeform surfaces. Even new mathematical approaches are under development /1/. In consequence, new optical designs /2/ are pushing the development of manufacturing processes consequently and novel types of datasets have to proceed through the process chain /3/. The complexity of these data is the huge challenge for the data handling. Because of the asymmetrical and 3-dimensional surfaces of freeforms, large data volumes have to be created, trimmed, extended and fitted. All these processes must be performed without losing the accuracy of the original design data. Additionally, manifold types of geometries results in different kinds of mathematical representations of freeform surfaces and furthermore the used CAD/CAM tools are dealing with a set of spatial transport formats. These are all reasons why manufacture-oriented approaches for the freeform data handling are not yet sufficiently developed. This paper suggests a classification of freeform surfaces based on the manufacturing methods which are offered by diamond machining. The different manufacturing technologies, ranging from servo-turning to shaping, require a differentiated approach for the data handling process. The usage of analytical descriptions in form of splines and polynomials as well as the application of discrete descriptions like point clouds is shown in relation to the previously made classification. Advantages and disadvantages of freeform representations are discussed. Aspects of the data handling in between different process steps are pointed out and suitable exchange formats for freeform data are proposed. The described approach offers the possibility for efficient data handling from optical design to systems in novel optics.

Fly-cutting and testing of freeform optics with sub-μm shape deviations

Optical designs in various applications profit from the increasing use of freeform elements. However, freeform optics always challenges the manufacturing process. The complexity of the fabrication derives from the missing symmetry in freeform surfaces. Ultra-precision machining is an appropriate method to realize complex optical freeforms. Surface deviations can be reduced in a deterministic process by a test and correction loop to achieve shapes with sub-μm deviations. But freeform elements do not only require the optical performance, they also depend on tight tolerances of the surface position with respect to reference structures. Due to the absence of rotation symmetry in freeform elements, all six degrees of freedom have to be constrained. Diamond machining allows to machine reference structures on the optical part. They can be used for alignment while testing or during the assembly processes. This paper shows a deterministic approach to manufacture optical freeform surfaces with sub-μm surface deviations by fly-cut-machining and servo assisted diamond turning. Reference structures are included at the edge of the element in order to support the following measurement and assembly processes. The reference structures are manufactured within the machining process of the optical surface. This procedure ensures tight tolerances between reference structures and optical surface /1/. The complex optical surface is measured with respect to the references with the tactile measurement system UA3P. The reference structures are used to locate the coordinate system of the element and hence to constrain the alignment parameter. After fitting the data, a revised tool-path is used to improve the shape deviation to sub-μm accuracies.

Sub-aperture EUV collector with dual-wavelength spectral purity filter

The combination of a 10.6 μm main pulse CO2 laser and a 1064 nm pre-pulse Nd:YAG laser in EUV source concepts for HVM would require collector mirrors with an integrated spectral purity filter that suppresses both laser wavelengths. This paper discusses a new approach of a dual-wavelength spectral purity filter to suppress 10.6 μm and 1064 nm IR radiation at the same time. The dual-wavelength spectral purity filter combines two binary phase gratings that are optimized for 10.6 μm and 1064 nm, respectively. The dual phase grating structure has been realized on spherical sub-aperture EUV collector mirrors having an outer diameter of 150 mm. IR suppression factors of 260 at 10.6 μm and 620 at 1064 nm have been measured on the sub-aperture EUV collector while its EUV reflectance exceeded 64 % at 13.5 nm.

Design of freeform optics for an ophthalmological application

Optical freeform surfaces are gaining importance in different optical applications. A huge demand arises e.g. in the fields of automotive and medical engineering. Innovative systems often need high-quality and high-volume optics. Injectionmoulded polymer optics represents a cost-efficient solution. However, it has to be ensured that the tight requirements with respect to the system’s performance are met by the replicated freeform optics. To reach this goal, it is not sufficient to only characterise the manufactured optics by peak-to-valley or rms data describing a deviation from the nominal surface. Instead, optical performance of the manufactured freeform optics has to be analysed and compared with the performance of the nominal surface. This can be done by integrating the measured surface data of the manufactured freeform optics into the optical simulation model. The feedback of the measured surface data into the model allows for a simulation of the optical performance of the optical subsystem containing the real freeform optics manufactured. Hence, conclusions can be drawn as to whether the specifications with respect to e.g. imaging quality are met by the real manufactured optics. This approach will be presented using an Alvarez-Humphrey optics as an example of a tuneable optics of an ophthalmological application. The focus of this article will be on design for manufacturing the freeform optics, the integration of the measured surface data into the optical simulation model, simulation of the optical performance, and analysis in comparison to the nominal surface.

MERTIS: optics manufacturing and verification

The MERTIS reflective infrared optics can be beneficial implemented as diamond turned aluminium mirrors coated with a thin gold layer. The cutting processes allow the manufacturing of both, the optical surface and mechanical interfaces, in tight tolerances. This is one of the major advantages of metal optics and was consequently used for the MERTIS sensor head optics. This paper describes the entire process chain of the MERTIS spectrometer optics including the manufacturing methods for the mirrors and for the spherical grating, the coating with sputtered gold for infrared reflectivity as well as the alignment and the verification of the spectrometer optics.

Optical performance simulation of free-form optics for an eye implant based on a measurement data enhanced model

This paper describes the application of a modeling approach for precise optical performance prediction of free-form optics-based subsystems on a demonstration model of an eye implant. The simulation model is enhanced by surface data measured on the free-form lens parts. The manufacturing of the free-form lens parts is realized by two different manufacturing processes: ultraprecision diamond machining and microinjection molding. Evaluation of both processes is conducted by a simulation of the optical performance on the basis of their surface measurement comparisons with the nominal geometry. The simulation results indicate that improvements from the process optimization of microinjection molding were obtained for the best manufacturing accuracy.

Wide-band phase-shifting fringe projector on the basis of a tailored free-form mirror

Abstract. Phase-shifting fringe projection is an effective method for three-dimensional shape measurements. Conventional fringe projection systems utilize a digital projector that images fringes into the measurement plane. The performance of such systems is limited to the visible spectral range (VIS), as most projectors experience technical limitations in ultraviolet (UV) or infrared (IR) spectral ranges. However, for certain applications these spectral ranges are of special interest. A novel fringe projector was developed on the basis of a single-tailored free-form mirror. The free-form mirror generates a sinusoidal fringe pattern by redistribution of light. Phase shifting can be realized by a slight rotation of the free-form mirror. In this system, the fringe pattern is generated by illuminating the free-form surface and not by the classical imaging technique. As the system is based on a single mirror, it is wavelength-independent in a wide spectral range and therefore applicable in UV and IR spectral ranges. Additionally it does not feature any chromatic aberrations. We present the design and an experimental setup of this novel fringe projection system. Fringe projection is realized in different wavelength ranges (VIS and UV) and the advantage of fringe projection in the UV spectral range can be shown for certain materials.