This PDF file contains the front matter associated with SPIE Proceedings Volume 10829, including the Title Page, Copyright information, Table of Contents, and Conference Committee listing.

Advanced figuring technology has enabled manufacturing of high accuracy optics for precision applications. The measurement technologies to verify them are largely based on Fizeau interferometry, which is limited in terms of accuracy because of external accessories such as reference flat. Lack of appropriate verification method is adversely affecting the manufacturing and optimization of precision optics. In this paper, we explore a fundamentally different interferometry arrangement, D7 produced by Difrotec. A phase shifting point diffraction interferometer (PSPDI) and present measurement results for concave spheres with an accuracy of λ/1000 PV, and compared this full-shot result with wavefront maps obtained by subaperture stitching (SAS) to verify stitching accuracy. We also describe measurement of asphere cavity using SAS, with higher accuracy, λ/500 RMS, discuss strategies to measure concave/convex spheres and aspheres with R-number ≥ 0.5 with nanometer accuracy, and conclude with perspectives on the future applications of PSPDI D7.

This research is focused on the link between manufacturing parameters and the resulting mid spatial frequency error in the manufacturing process of precision optics. This first publication focuses on the parameters of the grinding step. The Goal is to understand and avoid the appearance of the mid spatial frequency error and develop a simulation which is able to predict the resulting mid spatial frequency error for/of a manufacturing process.

Green light for the construction of the 39-m aperture giant Extremely Large Telescope (ELT) was given by the European Southern Observatory (ESO) council on Dec 4th, 2014. Procurement of the key elements, especially the optics, was immediately initiated by ESO team. Up today, Safran Reosc was awarded all the key optical polishing and testing contracts with:

2015-07: contract for the Adaptive Optics M4 mirror thin glass petals,

2016-07: contract for the 4-m M2 convex mirror,

2017-02: contract for the 4-m M3 mirror.

2017-05: contract for polishing and intergation of the 931 1.45-m hexagonal segments for the giant 39-m M1 mirror assembly

This paper is dedicated to highlighting the various challenges linked to these various optical fabrication projects and reporting about the progress of our work so far.

Optical fabrication relies on precision metrology over a wide range of lateral scales. Consequently, an important performance parameter for Fizeau interferometers is the instrument transfer function (ITF), which specifies the system response as a function of surface spatial frequency. Advances in test procedures, instruments and automated analysis techniques now enable reliable ITF characterization independent of many traditional sources of error. Results here show the ITF for a commercial 100-mm aperture interferometer with spatial frequency response ranging from 0 to 1500 cycles per aperture

In the past, steadily increasing demands on the imaging properties of optics have led more and more precise spherical apertures. For a long time, these optical components have been produced in a satisfying quality using classic polishing methods such as pitch polishing. The advance of computer-controlled subaperture (SA) polishing techniques improved the accuracy of spheres. However, this new machine technology also made it possible to produce new lens geometries, such as aspheres.

In contrast to classic polishing methods, the high determinism of SA polishing allows a very specific correction of the surface defect. The methods of magneto-rheological finishing (MRF) [1], [2] and ion beam figuring (IBF) [3], [4] stand out in particular because of the achievable shape accuracy. However, this leads to the fact that a principle of manufacturing "As exact as possible, as precise as necessary" [5] is often ignored. The optical surfaces often produced with unnecessary precision, result at least in increased processing times.

The increasing interconnection of the production machines and the linking with databases already enables a consistent database to be established. It is possible to store measurements, process characteristics or tolerances for the individual production steps in a structured way. The difficulty, however, lies in the reasonable evaluation of the measurement data.

This is where this publication comes in. The smart evaluation of the measurement data with the widespread Zernike polynomials should result in a classification, depending on the required manufacturing tolerance. In combination with the so-called ABC analysis, all surface defects can be categorized. In this way, an analytic breakdown of a - initially confusing - overall problem is made. With the aid of cost functions [6] an evaluation and consequently a deduction of actions is made possible. Thus, for example, the isolated processing of rotationally symmetrical errors in spiral mode, setup times and machining times can be reduced while avoiding mid spatial frequency errors (MSFE) at the same time.

Nowadays Ion Beam Figuring (IBF) is a well-known finishing technique for the production of ultra-precise optical surfaces. The diameter of optics can be in the range of 5 mm up to 2000 mm. Newest in-house developments extend the range down to 1 mm, which follows the upcoming market for micro optical systems. Besides IBF, ion beam etching technology (IBE) enables roughness improvement by different methods. Feature sizes from < 100 nm up to > 10 μm can be smoothed. However, operational parameters of IBF or IBE technology need to be adapted to the optical element. Beside the right choice of ion beam sizes (tool size) to remove equivalent feature sizes of the optics, also the shape (concave/convex) is of importance to consider side effects like re-sputtering or contamination originating from the ion beam source. This article will tabulate the state of the art of ion beam technology for ultra-precise optics manufacturing considering all parameters and side effects for efficient optics finishing.

Measuring large surfaces interferometrically is a straight forward established technology, as long as they are concave and spherical. The situation chnages completely if aspheres and freeforms have to be measured. The application of a Tilted Wave Interferometer opens up possibilities to measure large concave surfaces of any shape without compensation optics. For the investigation of large convex aspheres, it is necessary to make use of stitching methods. Due to the freeform capability of the Tilted Wave Interefrometer, it is possible to acquire larger subapertures compared to null interferometers. Therefore measurement and computation time are reduced.

Subsurface Damages (SSDs) can cause a wide variety of defects to optical lenses and other components. In addition to the adhesion and quality of coatings, the mechanical stability, the transmission quality and the laser-induced damage threshold (LIDT) of the products, is also affected. It is, therefore, attempted to get components as SSD-free as possible at the end of the production chain. Already during the individual production steps, it is important to know the depth of the SSDs in order to remove them in the following manufacturing steps. To design the manufacturing processes efficiently and avoid damage, it is important to be able to measure the depth and characteristics of SSDs as precisely as possible.

There are a several approaches and methods to determine SSDs known in literature. However, many of them inevitably lead to the destruction of the workpiece. Although others are non-destructive, but very complex in design and/or associated with large investments. Likewise, only a few are suitable for determining SSDs on ground rough surfaces.

Filled-Up Miicroscopy (FUM) is an alternative approach to approximating the depth of SSDs, even on rough surfaces without destroying them. At a first glance at the method, the procedure is described in detail and all necessary steps of preparing the samples are shown. A first comparison with the known Ball Dimpling Method confirms the functionality of the concept.

The effects, the extent and the importance of workpiece deformations, particularly lenses, caused by the weight of the workpiece itself, were examined in a previous paper¹ . The considered deformations are in the single-digit to two-digit nanometer range. The investigation was carried out by FEM calculations. The conclusion of the previous aper was that a full-surface support of a workpiece in the processing of one surface presumably produces the best results. Furthermore, it was found that if the second functional surface is not to be touched in the process, a full contact lens mounting on its circumference is advisable. An alternative method for fixing precision lenses is therefore desirable. This can be accomplished in two steps. As a first step, the lens must be gripped at its periphery so that none of the optically functional surfaces of the lens is compromised. However, the complete circumference has to be fixated gaplessly because a punctual fixation has the disadvantage of deforming the lens surface asymmetrically. As a second step, the freely hanging lens surface should be supported to minimize deformation. An approach had to be found that supports the surface like a solid bearing but at the same time does not touch it. Therefore, the usage of an incompressible fluid as a hydrostatic bearing for full-surface support is pursued. For this purpose, the bottom side of the lens has to be stored on water. The results of the FEM simulation showed that with a fluid bearing the resulting deformations can be drastically reduced in comparison to a freely hanging surface. Furthermore, under the right conditions, a resulting deformation comparable to a full surface solid support can be achieved. The content of this paper is a test series under laboratory conditions for a first validation of the theoretical results. Therefore, a prototype model to test a lens fixation with a fluid bearing was developed and manufactured. The resulting deformations were measured with an interferometer and the effects are discussed.

In-situ measurements of complex surfaces during the polishing process is a challenge for the production of aspheric surfaces or freeforms. We are providing a new attempt by using a scanning deflectometric device based on our recently published DaOS [1] principle, which allows in-situ measurements of large optical surfaces in realistic production environments and offers the conditions for direct intervention and correction in the polishing process. The results of insitu surface measurements after three polishing steps of a large glass substrate (320 mm in diameter) in a lever-polishing machine (NLP500 from Stock Konstruktion GmbH) are shown and critically compared with interferometric measurements on a SSI-A Interferometer. In this paper, the technical setup consisting of a highly precise scanning penta prism device and a Vignetting Field Stop (VFS) Sensor is explained. Secondly, we are discussing the mathematical algorithm to reconstruct the complete surface from angle measurements from a given number of cross-sectional cuts. The data of the surface reconstruction are transformed into a XYZ-file format to be analyzed with MetroPro®. The results are shown and discussed in terms of accuracy and reproducibility. Finally, a comparison with interferometric measurements on an SSI-A (QED) at TH Deggendorf (THD), Technology Campus Teisnach is shown to proof the degree of accuracy and applicability of our new, fast and reliable device for in-situ measurements of complex surfaces.

We present a new light source capable of locating interference fringes at an adjustable distance from the interferometer. The spectrum is electronically controlled in such a way that the fringes are limited to only one of the surfaces of the optics under test. With the new source it is straightforward, for example, to measure the parallel surfaces of thin glass plates and multiple surface cavities. Existing interferometers, as well as older systems, can be upgraded with this source.

Traditional methods of interferometry are widely used and accepted for simple measurement configurations, but measurement accuracy can decrease rapidly with increasing measurement complexity. For example, coherent interferometry struggles to achieve accurate and repeatable results with the presence of any additional feedback surface in the measurement cavity due to temporally coherent back reflections. Conversely, incoherent interferometers can isolate single surfaces for measurement but require more complex interferometer system designs. As a result, many of these systems are limited in their dynamic range of measurable cavity sizes and present considerable difficulties in the alignment process, increasing total measurement time. Both methods are inherently restricted by the intrinsic properties of their respective source.

Spectrally controlled interferometry (SCI) is a source driven method which inherits many advantages from both coherent and incoherent interferometry while evading typical limitations. The sources spectral properties are manipulated to produce a tunable coherence function in measurement space which allows control over the coherence envelope width, the fringe location, and the fringe phase. With this source realization, a host of measurement advantages which simplify measurement complexity and reduce total measurement time becomes available. One major application is the extinction of extraneous surface back reflections. Without any mechanical translation, realignment, or traditional piezoelectric transducers, front and back surfaces of planar optics can be isolated independently and complete phase shifting interferometric (PSI) measurements can be taken. Furthermore, because all control parameters are implemented at the source level, the spectrally controlled source is a good candidate for upgrading existing interferometer systems.

In this paper, we present the theoretical background for this source and the implications of the method. Additionally, a multiple surface cavity measurement is provided as a means of demonstrating the spectrally controlled sources capability to isolate individual cavities from detrimental back reflections across a large dynamic range of measurable cavity sizes without mechanical realignment. A discussion of the implementation benefits and practical details will be included. Limitations and comparisons to alternative methods will be addressed, as well.

For the (CNC) polishing of aspheres, generally a compliant, sub-aperture tool is applied, which may cause mid- spatial frequency errors on the surface of the workpiece. The tolerance on surface figure is commonly given in peak-to-valley (PV) or root-mean-square (RMS). Even if a surface is fabricated within specified tolerances according to one of the mentioned metrics, the optical performance may be inadequate for the desired application. For the specification of the tolerance on mid-spatial frequency errors, several other characteristics have been proposed, e.g. power spectral density (PSD) or surface slope error. This paper presents an investigation into the mid-spatial frequency form error of mass-produced aspheres, discusses the results and draws relevant conclusions.

Ultra-precision molded polymer optics range from high precision imaging objectives to tiny lenses like those used in camera modules for cell phones, where centration tolerances and filling of small features is a challenge. We propose a manufacturing process termed Compression Molded Polymer Aspheres (COMPAS). Polymer preforms are inserted into mold cavities, and isothermally heated above glass point. Novel tooling has been developed to produce high volumes of COMPAS optics at reasonable cost and cycle time, using large scale parallelization of mold cavities. First results of the COMPAS process are very encouraging: shape accuracy (<500 nm peak-to-valley), surface centration (<5 μm), and birefringence (<20 nm/cm) are well below values typically measured for injection molded lenses. COMPAS lenses are also gate free. We describe details of the on-axis turning of arrays and multi-cavities (DPI) and the COMPAS precision polymer molding process. We describe the metaphysical background of disruptive engineering based on physical principles, which is the reason behind developing DPI and COMPAS.

The aim of our research is to study middle spatial frequency errors (MSFE) on optical surfaces. We investigate the surfaces after all manufacturing processes to find out the main affecting factors and to choose the proper processing parameters to minimize the size of the errors. In this paper we describe some middle spatial frequency errors, which occur during grinding. As there are limited possibilities to measure ground surfaces, their analysis from the point of measurement is most difficult. Therefore, it is of utmost importance to optimally organize the measurement guaranteeing sufficient data for the reconstruction of the toolpath and avoidance of aliasing effects. In the paper discuss possible classifications and some difficulties during measuring of grinded surfaces.

Increasing demands for single lenses and lens systems influence in particular their production technology. It has become unfeasible – both technologically as well as financially – to manually adjust lenses in pre-assembled objective lenses. In recent years, it has thus become desirable to automate many steps of the process to chain. This enables new assembly strategies that allow for tilt and air gap accuracies in the micron range and drastically reduce the time and labor of manually correcting for astigmatism and coma at the same time.

The Grinding Process Validation Approach (gPVA) presented in 2017 enables the determination of suitable parameter windows for grinding tools. The abrasion properties of grinding tools are determined experimentally. The collected data can be used to derive optimum parameters for defined grinding tasks so that service life, process stability and productivity can be maximized. In this publication, the gPVA method is used to compare different grinding tools. Differences in stock removal performance with identical specified tools from different manufacturers are investigated. In addition to that, recommended tools for fine grinding of fused silica are examined also.

The manufacturing of optical lenses has various steps. Generally, the manufacturing can be split up into the following steps: the workpiece is pre-ground with a coarse tool; it is then fine-ground with a finer tool. As the final polishing is a demanding and time-consuming process that cannot manage large removal rations not can it equalise rough shape errors, the starting quality and surface quality needs to be as high as possible. According to the current state of technology, ground lenses must be measured with tactile measuring techniques in order to detect shape errors. This is timeconsuming and expensive, and only two dimensional profiles can be measured. DefGO’s project objective is to introduce deflectometry as a new, three dimensional lens measuring standard. A problem with the application of deflectometry is that the object to be measured has to reflect enough light, which is not the case for ground glass with rough surfaces. DefGO’s solution is to wet the lens with a fluid to create a closed reflecting surface.

Ductile mode machining is usually applied for the optical finishing operation of e.g. tungsten carbide molds. One request for this mode is not to exceed the critical depth of cut hcu,crit characterized by the transition point from ductile to brittle material removal.

Based on experimental investigations a formula for the critical depth of cut, relating the material specific properties Young’s-Modulus E, material hardness H and fracture toughness KC was developed by Bifano et. all [1]. Even when the influence of cutting conditions, like tool or process characteristics, are neglected the formula is widely used for setting up UPM machines ever since. However, previous investigations have shown that hcu,crit strongly depends on coolant fluid characteristic as well as on the compressive stress applied into the cutting zone by the use of tools with e.g. negative rank angles [2].

In this paper, we report on a ductile process analysis applying a recently developed method for process optimization in optics fabrication [3]. Following that trail, critical process parameters have been identified and their influences on the critical depth of cut hcu,crit have been tested experimentally in fundamental ruling tests.

Among others, following parameters were identified and tested: (a) characteristics of the coolant used, (b) the pH value of the coolant, (c) the tool specifications of the applied diamond and (d) whether ultrasonic assistance (US) is being switched on or off. Depending on the applied set of process parameters and for the experimental data collected, maximum ductile mode material removal rates could be achieved with dcmax = 1600 nm.

That way, a new formula was developed, which allows the prediction of the critical depth of cut depending on critical process parameters while machining binderless nanocrystalline tungsten carbide. The formula was set up based on experimental results and is one step towards extending Bifanos formula taking the influences of critical process parameters into account.

The contents of this work are based on [1], [2] and [3]. Using the three wagons approach, critical parameters were identified and the process window of ductile machining was considerably enlarged. This made it possible to increase the critical depth of cut, which is ten times greater than predicted by the Bifano formula. A new formula to describe the machining process was developed and verified experimentally. In addition, the level of surface roughness (Sq) generated in ductile mode was analyzed and a formula was generated that allows roughness prediction depending on the critical process parameters. Finally, both formulas were used to create optimized sets of process parameters that produce a "first light" in ductile machining for a) single point diamond turning (SPDT) on ultra-precision machines (UPM) of binder-free carbide form and b) non-UPM, standard CNC ductile grinding of WC and glass.

A challenge of coaxial - measurement cavity based - interferometer is to realize an interference contrast in the vicinity of one and to realize a complete elimination of the parasitic reflections. Another challenge, which also exists in non-coaxial setups, is the phase transfer function of extended measurement cavities. Ideally, the surface under test (SUT) and the reference surface (REF) are both exactly imaged onto the detector plane. In practice, SUT and REF have to be placed within the depth of field (DOF), which refers to the object space. The term depth of focus refers to the image space. To avoid confusion, the depth of field might be referred to as DOOF (depth of object field) and the depth of focus might be referred to as DOIF (depth of image field).

However, in many measurement situations, the REF is not placed within the DOOF, which is the small z-range, which is imaged onto the detector plane. Furthermore, the phase transfer function (PTF) of the REF and the image distortion of the REF are both dependent on the focal plane used to image the SUT onto the detector plane. Effects as phase deformation, image distortion and image blurring have to be taken into account when using extended measurement cavities. This can be done by using a look up table (LUT), which contains simulated and/or calibrated data. Thus, the related system error can be subtracted. A remaining challenge is an unknown object under test (OUT), which is measured by using a double path arrangement. The measured wave front depends on the two surfaces of the OUT and the position of the return mirror. For simplicity, a homogeneous substrate and a perfect return mirror might be presumed. The simulation of waves propagating within extended measurement cavities, as well as measurement results, will be discussed. In addition, the influence on the power spectral density (PSD) will be described. This is important for high end correction techniques as e.g. magneto rheological figuring (MRF) and ion beam figuring (IBF).

A novel fabrication parameter controlling method for laser polishing processes called CLasso (Control of LASer Surface Optimization) is presented, monitoring within the footprint the smoothening process as well as the removal of ssd in situ. Therefore, it is possible to determine and control the optimum dwell time a footprint needs to stay at a certain point before moving further enabling a more stable and cost optimized polishing.