Volume 6, Issue 5 - Optical Surfaces / Guest Editor: Angela Duparré

Optical systems can benefit strongly from freeform surfaces; however, the choice of the right surface representation is not trivial and many aspects must be considered. In this work, we discuss the general approach classical globally defined representations, as well as the basic mathematics and properties of the most commonly used descriptions and present a new description developed by us for describing freeform surfaces.

Optical systems can benefit strongly from freeform surfaces; however, the choice of the right representation is not trivial, and many aspects must be considered. Many possibilities to formulate the surface equations in detail are available, but the experience with these newer representations is rather limited. Therefore, in this work, the focus is to investigate the performance of several classical descriptions as well as one extended freeform surface description in their performance in concrete design optimization tasks. There are different influencing factors characterizing the surface representations, the basic shape, the boundary function, the symmetry, a projection factor, as well as the deformation term describing higher order contributions. We discuss some possibilities and the consequences of describing and using these options with success. These surface representations were chosen to evaluate their impact on all these aspects in the design process. As criteria to distinguish the various options, the convergence over the polynomial orders, as well as the quality of the final solutions, is considered. As a result, recommendations for the right choice of freeform surface representations for practical issues in the optimization of optical systems can be given under restrictions of the benchmark assumptions.

The process of ductile mode grinding has been analyzed experimentally. Besides the traditional approach, controlling the depth of a cut on feed-controlled ultra-precision machines (UPMs), two alternative approaches have been tested. By applying fluid jet polishing (FJP) using a grinding slurry, a stable ductile mode grinding could be set up. Subsequently, loose abrasive grinding processes on load-controlled traditional spindle machines have been tested to be suited for ductile mode grinding identifying the dependence of crack initiation on the type of slurry fluid being applied. In that way, substantially improved levels of surface roughness could be achieved, although local brittle cracking within the generated clear apertures still remained. Furthermore, critical process parameters were identified determining the process window of feed-controlled ductile grinding applied on state-of-the-art UPM machineries. These have been analyzed experimentally, and it was found that the critical depth of a cut significantly depends on the set of critical process parameters being applied. Finally, the critical ductile grinding process parameters could be identified determining the generated level of surface roughness achieving 0.83 nm Ra on tungsten carbide.

The spectral laser-induced damage of optical components was measured using a new instrument based on combining a laser-induced damage threshold (LIDT) testing procedure with angle-resolved light scattering and using a tunable optical parametric oscillator laser source. Tests on aluminum mirrors revealed a significant drop of the LIDT around 800 nm, which is not predicted by simple scaling laws. For near-infrared edge filters, remarkable changes in the LIDT around the band edge were observed, which are linked to the spectral variation of the field distribution in the interference coating.

In most applications, contamination of optical thin film coatings is inevitable over time. State-of-the art approaches to tackle this problem are usually based on two strategies – avoiding contamination or removing already existing contamination. We demonstrate that the coating design can be tailored to reduce light scattering and stray light arising from particle contamination. This allows reducing the optical symptoms (light scattering) rather than trying to address the inevitable cause of the problem (contamination) itself. This new approach can consequently be easily combined with state-of-the-art approaches.

The accurate simulation of stray light is essential for the prediction of signal detection fidelity in an optical instrument with high contrast requirements. In a spectrometer, the scattering from reflective gratings is poorly understood and difficult to characterize, but contributes significantly to the overall system stray light and reduction in contrast. This paper describes the approaches that will be taken at OHB System AG to establish a sufficiently precise fitting of bidirectional scatter distribution functions (BSDFs) to existing data obtained from measurements of scattering from gratings. The work is being undertaken in preparation for the analysis and fitting of a BSDF to the scattering from the grating for the FLEX low-resolution spectrometer that will be measured in the second half of 2017.

Optical surfaces equipped with specific functional properties have attracted increasing importance over the last decades. In the light of cost reduction, hydrophobic self-cleaning behavior is aspired. On the other side, hydrophilic properties are interesting due to their anti-fog effect. It has become well known that such wetting states are significantly affected by the surface morphology. For optical surfaces, however, this fact poses a problem, as surface roughness can induce light scattering. The generation of optical surfaces with specific wetting properties, hence, requires a profound understanding of the relation between the wetting and the structural surface properties. Thus, our work concentrates on a reliable acquisition of roughness data over a wide spatial frequency range as well as on the comprehensive description of the wetting states, which is needed for the establishment of such correlations. We will present our advanced wetting analysis for nanorough optical surfaces, extended by a vibration-based procedure, which is mainly for understanding and tailoring the wetting behavior of various solid-liquid systems in research and industry. Utilizing the relationships between surface roughness and wetting, it will be demonstrated how different wetting states for hydrophobicity and hydrophilicity can be realized on optical surfaces with minimized scatter losses.

Methodologies for the assessment of technology’s maturity are widely used in industry and research. Probably the best known are technology readiness levels (TRLs), initially pioneered by the National Aeronautics and Space Administration (NASA). At the beginning, only descriptively defined TRLs existed, but over time, automated assessment techniques in the form of questionnaires emerged in order to determine TRLs. Originally TRLs targeted equipment for space applications, but the demands on industrial relevant equipment are partly different in terms of, for example, overall costs, product quantities, or the presence of competitors. Therefore, we present a commonly valid assessment methodology with the aim of assessing laser-based equipment for industrial use, in general. The assessment is carried out with the help of a questionnaire, which allows for a user-friendly and easy accessible way to monitor the progress from the lab-proven state to the application-ready product throughout the complete development period. The assessment result is presented in a multidimensional metric in order to reveal the current specific strengths and weaknesses of the equipment development process, which can be used to direct the remaining development process of the equipment in the right direction.

In human eyes, the maximum visual acuity correlates locally with the fovea, a shallow depression in the retina. Previous examinations have been reduced to simple geometrical fovea models derived from postmortem preparations and considering only a few superficial ray propagation aspects. In the current study, an extended and realistic analysis of ray-optical simulations for a comprehensive anatomical realistic eye model for the anterior part and realistic aspherical human foveal topographical profiles deduced from in vivo optical coherence tomography (OCT) are presented, and the refractive index step at the transition from vitreous to retinal tissue is taken into account. The optical effect of a commonly shaped (averaged) and an extraordinarily shaped foveal pit were both compared to the analysis of an assumed pure spherical boundary layer. The influence of the aperture size, wavelength, and incident angle on the spot size and shape, as well as the axial focal and lateral centroid position is investigated, and a lateral displacement of about 2 μm and an axial shift of the best focal position of less than 4 μm are found. These findings indicate only small optical effects that are laterally in the range of inter-receptor distances and axially less than the photoreceptor outer segment dimension.