This PDF file contains the front matter associated with SPIE Proceedings Volume 13221, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

The glass ceramic ZERODUR® with its extremely low thermal expansion finds widespread application in precision optics and astronomical instruments. It has been used for several very large telescope mirrors, as its unique properties make it indispensable in environments demanding stringent dimensional stability. As for all brittle materials, mechanical grinding processes involved in manufacturing introduce subsurface damage (SSD), which compromises the optical and mechanical performance of workpieces. Beyond its implications for quality, SSD is often a significant cost driver, necessitating expensive finishing processes. Characterizing SSD and determining its depth at various manufacturing stages from pre-grinding to final polishing are critical for ensuring the quality and reliability of optical components. Conventional measurement techniques predominantly involve destructive methods such as taper polishing or cleaving the surface, posing limitations in assessing SSD without compromising the integrity of the workpiece. Moreover, these methods are often time-consuming and labor-intensive. In a collaborative study, we conducted a round-robin test to establish and validate the efficacy of nondestructive optical coherence tomography (OCT) as a new technique for characterizing SSD depths in ground ZERODUR®. The aim is to provide a first comprehensive assessment of the capability of OCT in nondestructively quantifying SSD in glass ceramics. The methodology involves comparing measurements obtained from two distinct high-resolution OCT systems applied to identical ZERODUR® samples machined with selected process parameters. Furthermore, the study investigates the influence of different grain sizes on the SSD characteristics by comparing measurements across samples processed with varying grain sizes. By conducting the round-robin test, we seek to demonstrate the applicability of OCT as a non-destructive alternative for SSD characterization in ground ZERODUR®. Successful validation of OCT extended to a variety of relevant optic materials such as ZERODUR® would enhance overall quality control processes, offering a rapid and non-invasive means of assessing SSD depths and ensuring the integrity of critical optical components.

Autocollimators (AC) are used for different metrological measurement tasks. These instruments are compact and versatile and easy to use. Angle deviations of polygons, local slopes, fiducials, e.g., are measured with autocollimators. If calibrated electronic autocollimators are used, then the measurements can be performed with uncertainties down to 0.01 arc sec. The uncertainty might be increased further by one order of magnitude or more if surfaces with small apertures, aberrant surfaces or surfaces placed at long working distances are measured. To define the measuring window, respectively the limiting parameters, a model of the optical system was implemented. Aberrations, diffraction, temporal and spatial coherence are used. To increase the measurement range, a phase-shifting reticle can be used. Simulations made for small apertures show, that a significant reduction of the uncertainty can only be realized for a limited parameter range of the components used. For example, incoherent and coherent illumination are rather exceptions than standards. Partial coherence is used in a plurality of applications, e.g. for interferometry and space-bandwidth-limited wave front reconstruction, just to name a few. The system and the coherence function Γ must be tailored to provide specific operation. Almost all embodiments come with little changes of the light source’s coherence properties only. For example, to use a spectral bandpass filter or to limit the size of the light source seems to be the standard solution for almost everything. Some error sources related to reticle imaging within an AC are identified. And the acceptable parameter range will be described.

Atmospheric pressure plasma jet (APPJ) etching of Zerodur® with fluorine containing process gases leads to the generation of non-volatile metal fluorides which remain on the surface and form a residual layer. This results in masking effects and therefore an increasing roughness and altered etching behavior. The removal of the residual layer by laser irradiation has already been demonstrated and the aim of the present work is to further develop this process combination by reducing the residual layer thickness. For this purpose, etchings were performed with APPJ and the resulting residues were subsequently irradiated with an excimer laser (λ = 248 nm) with varying fluence. The samples were analyzed by white light interferometry, scanning electron microscopy, and energy dispersive X-ray spectroscopy. In the fluence range of 0.3-0.6 J·cm^-2 the residual layer can be removed without damaging the glass ceramic surface. EDX measurements show similar results for the laser-cleaned regions and an untreated Zerodur reference indicating that the residual layer can completely be removed.

Microcracks occur during the conventional manufacturing chain (grinding, polishing), but also as a result of ultra-short pulse laser processing of hard-brittle materials, such as those frequently used in the optical industry. Non-destructive methods, such as optical coherence tomography, are being discussed in order to understand the development of nearsurface damage and to enable process control without sample consumption. Destructive testing methods will be necessary to validate those methods. Beam-based etching applied to Fused Silica by plasma jet or ion beam is presented. The application of reactive plasma jet etching causes process-induced roughness. Nevertheless, significant holes and defects can be detected. No additional roughening occurs with ion beam etching. Both applied beam-based etching processes ensure that small defects are enlarged, allowing these microcracks to be detected even if they are not directly visible on the initial surface. For an accurate depth determination of prominent, sharp-edged defects, white-light interferometry is limited. Confocal laser scanning fluorescence microscopy is shown as an alternative measurement technique to determine and visualize the development of the extent of defects with increasing etching attack.

Joining processes that do not require joining materials are increasingly needed as an alternative to classic fine cementing in optical production to meet the growing quality requirements. In order to satisfy this demand, processes such as direct bonding (e.g. silicone on insulators) have been adopted successfully from the wafer industry and transferred to fused silica in optical industry after further development [1]. So far, optical contact bonding has only been tested on pure fused silica or coated fused silica [2, 3] where the underlying bonding mechanism is known very well and described in detail [4]. As part of own previous work [5], a process based on a dielectric barrier discharge plasma at atmospheric pressure was developed so that fused silica optics with very sensitive functional optical layers could also be successfully bonded for the first time. In this contribution, the further development of this approach for bonding of Schott’s N-BK7 as one of the most common and used optical glasses is introduced. It differs from fused silica in terms of chemical composition, manufacturing qualities and optical properties. It was successfully bonded with the developed process and mentionable high bond strengths were achieved. Possible bonding mechanisms leading to the different bond strengths are discussed here.

In recent years, there has been a rising demand for highly precise glass optics, especially in sectors like lighting, automotive, laser technology, and renewable energies. This increase represents a significant challenge to the industry due to the growing complexity of optical systems and escalating production costs. Precision glass molding (PGM) is an established technology to manufacture precise and highly complex shaped glass optics by pressing a glass preform between two precisely manufactured molding tools into the desired final shape. Costs in process development can be reduced by using Finite Element Method (FEM) simulation to predict the glass flow and derive the optimal mold geometry. However, this is still limited to manufacture a specific lens design with a dedicated set of molding tools and parameters. In this investigation, FEM simulation tools were employed to analyze the possibility to manufacture different lens geometries with one mold insert pair. The study involved varying the process configuration, including molding force and temperature, in various combinations, followed by simulating the processes. The objective is to assess how alterations in process parameters influenced the shape of the molded lenses, aiming to determine the flexibility of the PGM process and optimize its efficiency.

CNC-controlled machining processes have become essential in modern optics production, driven by enhanced precision and reproducibility. However, escalating demands for component quality necessitate ongoing optimization of process stability and efficiency. The selection of parameters crucial for high-quality outcomes still relies heavily on the expertise of machine operators. This study focuses on the real-time investigation and recording of process vibrations during CNC grinding, combined with an objective analysis and control of their influence on the surface quality of optical components. Using Polytec's high-resolution optical measurement technology, inline vibrations were measured with Laser Doppler Vibrometry, while two coherence scanning interferometers were used for areal non-contact characterization of the surface topography. The primary objective was to detect process vibrations and their dependence on different grinding parameters to draw conclusions about resulting surface qualities. Extensive process and component data were collected, incorporating surface metrology parameters (Ra, Rq) and applying the power spectral density (PSD) function for surface quality characterization. Insights gained into vibration development within the CNC processing machine revealed direct correlations with resulting component qualities. The machine's capability for ultrasonic-supported machining exposed critical correlations between the set US frequency and spindle speed. Investigations also covered mid-spatial frequency analysis and periodic surface errors. At the same time, a machine learning model was developed, which enables a prediction of surface qualities depending on the grinding parameter selection even on the basis of a small database. By analyzing the frequencies recorded through vibrometry in the process, additional correlations with the formation of sub-surface damage could be assumed.

This study investigates the cutting behavior and surface defects in ultra-precision grinding of glassy carbon, an alternative material for mold manufacturing. The aim was to gain a comprehensive understanding of the cutting behavior to determine the suitability of ultra-precision grinding for mold manufacturing. Therefore, glassy carbon samples were ground flat and the process parameter cutting speed, cutting depth, and diamond grain size of the tool were altered. The machined surface was measured by interferometry and a scanning electron microscope to analyze the surface roughness and topography. The investigations show that ultra-precision grinding is capable of creating nearly defect-free surfaces with resulting surface roughness Sq lower than 5 nm. Additionally, it is analyzed, that the dominant cutting behavior is brittle, and the surface topography created is probably based on the fullerene-like structure from glassy carbon itself. In the case of occurring defects, the dominant breakouts are clods and resulting gaps in the surface topography. Further investigations are required to investigate the subsurface after grinding and its influence on the glass molding process.

3D machining in glass has experienced a significant boost in the last few years and has established itself in research as a manufacturing process for micro-components. We present a novel optical fabrication chain which enables the realization of complex mini-optics by using lasers as a shaping and polishing tool. In a first step by using selective laser induced etching (SLE) the outer shape and the figure of optical surface is formed. The SLE process leaves a RMS roughness of about 0.2 to 2 µm. That’s why, for optical surfaces subsequent polishing is needed. In our case a novel CO2 laser polishing approach is being used which has recently been developed at IMP institute and is called the “one-shot laser polishing”. By using this fabrication chain on one glass substrate (wafer) several mini optics can be placed which are connected to the wafer by small connections, which later can be removed or separated. This novel concept enables a wafer-level production which opens the possibility for better compatibility with other wafer-based production methods. In addition the novel SLE wafer-level approach where the lenses are mechanically separated from the wafer material, enables an optimization of the heat flow during the subsequent one-shot laser polishing.

Laser polishing of glass can achieve a surface roughness comparable to conventional polishing methods in a fraction of the process time. However, the thermal processing with rapid temperature changes of the glass substrate induces thermal stress, resulting in a deviation from the initial shape. The exception being fused silica, were after laser polishing the deviation is often less than a tenth of the value induced in other glasses. A possible cause for this difference is the necessity for preheating of most glasses during laser polishing, whereas fused silica can be polished at room temperature. Therefore, in this study the results of a thorough investigation of the impact of the preheating conditions on the form deviation during laser polishing of N-BK7 are presented. By modifying the preheating conditions to a lower temperature and reducing the heating and cooling rates, we achieved a reduction in form deviation for N-BK7 substrates (diameter 30 mm, thickness 5 mm). Compared to state-of-the-art process parameters resulting in a form deviation of PV ≈ 32 μm, a value of ~6.8 μm could be obtained. The form irregularity RMSi simultaneously has been reduced from RMSi ≈ 4 μm to ~0.3 μm, indicating a significant shift in the form deviation from free-form errors towards spherical errors. Simultaneously, the achieved roughness Sq after laser polishing is maintained at ~1.7 nm.

In recent scientific studies it has been demonstrated that specialised spectacle lenses with incorporated microlenslets can help to correct the vision of children, suffering from myopia.1 While these lenses can be manufactured in large quantities via molding techniques, individualized customization presents significant challenges, such as high costs and production lead times. A solution for this issue would be a setup capable to produce those lenses by printing custom patterns of lenslets on various different lens surfaces. The idea is a multistep process in which you would print a liquid, UV curable resin on a curved surface. After which those droplets will be partly cured to guarantee stability while further deformation is possible. Our contribution is a novel system, designed to print in 6 Degrees of Freedom (6-DOF) on non-planar surfaces, that also can partly cure the material. For the purposes of this paper we will have a look at the setup and printing process of this system. This paper outlines our experimental setup utilized to print lenslets with a diameter of 140 µm to 1000 µm. Our in-house developed system is based on two industrial robot arms to maneuver the printhead and the UV structuring unit towards the substrate. A hexapod platform is used to precisely orientate the substrate under the printhead during the printing process. The presented results show the capability of the system to print arbitrary micro-lens arrays on 3D surfaces. This shows a possible solution for customized myopia correction on a commercial scale.

The overarm polishing process chain consists of polishing, cleaning and interferometric metrology of the workpiece. At the TC Teisnach Optics, we strive to automize this entire process chain using an industrial robot. The time proven conventional system requires a skilled operator adjusting the process parameters after each measurement in order to achieve the desired result. These process parameters depend on various conditions and are generally not intuitive. Therefore, a profound knowledge acquired within several years of experience is required. In order to automate the process- chain a robot based polishing cell has been set up, that realizes the conventional overarm polishing process chain. Subsequently this system will be enabled to adjust the process parameters to achieve proper processing of the surface. This can be achieved by an algorithm using a mathematic removal model. Thus, at TC Teisnach Optics we convert the overarm polishing process from manual and highly operator dependent to automated and reproducible. In this contribution we report on the first step towards the automated overarm polishing cell: The exact transfer from the conventional overarm polishing system to the robot cell. To realize this, we built a lens holder for the robot which has the same features as the holder in the conventional polishing machine. We took all adjustable polishing parameters as well as all polishing media on the conventional machine and transferred them to the robot system. Subsequently several tests were conducted using the same parameters first at the conventional machine then on the robot polishing system and the differences between the systems were investigated.

Temperature is an important process factor in grinding. Cryogenic cooling has proven to be very successful in metal machining, for example, as oxidation is prevented by lowering the temperature level. This enables significantly higher cutting speeds and longer tool life. In our research, we aim to determine whether similar effects occur when grinding brittle-hard materials, which is why the temperature in the contact zone is explicitly examined here. This article presents a very simple and extremely cost-effective approach using a germanium window and an infrared temperature sensor. It explains how the system is constructed, what results can be achieved with it and how the system compares to other commercially available approaches.

In this article, a concept for the additive manufacturing (AM) of ultra-fine grinding tools is presented, which enables a flexible and efficient production with defined surface properties (e.g. grain size, concentration, integrated cooling channels). The basis of this innovative concept is a newly developed filament consisting of a compound of polyamide 12 (PA12), zirconium oxide particles (ZrO₂) and integrated diamond grains with an average grain size of 15 μm. The overall aim of the investigations is to examine the behavior and properties of these tools during a CNC machining process in relation to conventional resin-bonded tools so that quality, process efficiency and cost-effectiveness can be optimized. Using different additive manufacturing parameters, tools are produced by fused layer modelling (FLM) and tested in a fine grinding process on planar fused silica samples. The grinding tests show that it is possible to produce reflective, transparent surfaces with high surface qualities. In addition, reproducible surface roughness (Rq = 10.5 - 13.5 nm) can be generated, which, compared to the initial ground surface of the samples, results in a reduction in surface roughness of 97 - 98 %. Furthermore, partially better-quality results are achieved with the new tools compared to a conventional resin-bonded tool, which demonstrates the promising potential of additively manufactured fine grinding tools.

In the paper methods and results of testing optical surface form with the use of the point diffraction interferometer (PDI) D7 are presented. This interferometer can produce much higher accuracy of testing optics of various kinds than traditional interferometers, and validation of its industrial applicability is the scope of the paper. An overview and analysis of techniques making using D7 in manufacturing process easier and faster are given, the accuracy which can be achieved with common requirements of optical industry is evaluated at level of λ/150 – λ/250 PV and λ/600 – λ/1000 RMS. Measurements by subapertures for consequent stitching of subaperture wavefronts with desired overlay are considered. Conclusions followed by further perspectives of the described instrument are given.