Search

Modern optical sensors and measurement systems usually are a powerful combination of optical elements, active hardware components like actuators or sensing devices as well as a sophisticated control software and data evaluation algorithms. In order to develop and operate such systems, it is necessary to have a flexible, intuitive, and fast underlying software framework that also allows for rapid prototyping of a sensor in a dynamic lab environment. This software must be able to control and communicate with all necessary hardware devices and has to provide all the highly performant evaluation, data, and image processing algorithms required. In this publication, we want to present the open source measurement and data evaluation software suite itom, which has been designed considering the denoted requirements and whose development began in 2011.

Confocal gating is a very efficient tool to restrict light reaching the detector of an optical sensor to a certain volume. In fact, it was invented to facilitate imaging inside biological, strongly scattering media [1]. From there on a number of different sensor implementations relying on confocal gating were developed and so called confocal microscopes have been used extensively in many kinds of applications [2]. Meanwhile their popularity is so high, that confocal microscopy has been put on Natures list of milestones in light microscopy [3]. In classic confocal sensors, imaging is achieved by mechanically scanning the measurement object in axial direction relative to one or several optical measurement spots in parallel [4, 5]. This mechanical, axial scan has been replaced later by a spectral distribution of the measurement spot in axial direction both for point sensors [6] and areal sensor configurations [7]. More recently this chromatic confocal measurement principle has been combined with spectral interferometry in order to achieve a more constant lateral resolution over the measurement range while retaining an interferometric axial resolution [8]. Also it was realized recently that even in measurement systems, that utilize coherence gating [9], confocal gating is inherent and has to be taken into account [10]. Furthermore, it was shown that by properly aligning the properties of the coherence and the confocal gating, resolving power of coherence gating sensors can be improved [11].

Microoptical components play an increasing role in different technology fields such as medical engineering, materials and information processing, imaging and metrology. But their realization needs the combination of modern design concepts with sophisticated processing technologies, new materials and design tools. Furthermore, the introduction of ambitious processing technologies must be accompanied by effective metrology and inspection tools. Therefore, this paper reports about the technologies for making microoptics at ITO. Because sophisticated measurement tools are an indispensable part of the fabrication process, the paper describes our multi-scale inspection approach for the testing of microstructures on wafer-scale level. Finally, some representative applications of microoptical components for advanced measurement and imaging are explained.

The hybrid measurement principle Chromatic Confocal Spectral Interferometry combines Spectral Interferometry with Chromatic Confocal Microscopy and therefore benefits from their respective advantages. Our actual demonstrator setup enables an axial measurement range up to 100 μm with resolution up to 5 nm depending on the employed evaluation method and the characteristics of the object’s surface. On structured surfaces, lateral features down to 1 μm can be measured. As the sensor raw signal consists of a Spectral Interferometry type wavelet modulated by a confocal envelope, two classes of evaluation methods working on the phasing or the position of the envelope are employed. Even though both of these information channels are subject to their respective problems, we show that a proper combination of the individual methods leads to a robust signal evaluation. In particular, we show that typical artifacts on curved surfaces, that are known from Chromatic Confocal Microscopy, are minimized or completely removed by taking the phasing of the Spectral Interferometry wavelet into consideration. At the same time the problem of determining the right fringe order of the Spectral Interferometry signal at surface discontinuities can be solved by evaluation of the confocal envelope. We present here a first approach using a contrast threshold on the signal and a median referencing for trusted sections of the analysed topography, which yields a reduction of artifacts in a submicron range on steep gradients, discontinuous specimen or curved mirror-like surfaces.

In this paper, we demonstrate how short coherence digital holography with a pulsed fiber laser frequency comb may be used for multi-level optical sectioning. For the proof of the principle, a conic object having a size of few centimeters is used. The object shape is obtained by digitally reconstructing and processing a sequence of holograms recorded during stepwise shifting of a spherical mirror in the reference arm of the holographic set-up. First experimental results are presented.

An automatic method for the positioning of a test surface in a non-null interferometer is presented. If the test surface is positioned incorrectly with respect to the test beam this leads to aberrations, which distort the measurement of the surface. A central issue in the interferometric characterization of surfaces is to avoid aberrations due to an incorrect placement of the test surface. In case of spherical and plane surfaces these errors can usually be distinguished from the surface figure errors and are eliminated in post processing. For aspheric and free-form surfaces this task is no longer trivial. Therefore it is important to minimize the alignment error of the surface. In this work the effect on the measured phase due to lateral and axial displacements of the aspheric surface is calculated and an adjustment method for the positioning of the surface at a predefined measurement location is presented. Experimental results showing the feasibility of the proposed procedure are presented.

Simulation of grating spectrometers constitutes the problem of propagating a spectrally broad light field through a macroscopic optical system that contains a nanostructured grating surface. The interest of the simulation is to quantify and optimize the stray light behaviour, which is the limiting factor in modern high end spectrometers. In order to accomplish this we present a simulation scheme that combines a RCWA (rigorous coupled wave analysis) simulation of the grating surface with a selfmade GPU (graphics processor unit) accelerated nonsequential raytracer. Using this, we are able to represent the broad spectrum of the light field as a superposition of many monochromatic raysets and handle the huge raynumber in reasonable time.

Durch den Einsatz von optischer Topografiemesstechnik mit sehr hoher axialer Auflösung auf der neu entwickelten Diamantbearbeitungsmaschine ist es möglich, die geometrischen Begrenzungen bei der Endbearbeitung von hochgenauen Diamantwerkzeugen zu überwinden. Im Vergleich zu der oftmals eingesetzten digitalen Bildverarbeitung wird das Diamantwerkzeug bei der Weißlichtinterferometrie senkrecht angemessen. Nur so kann die hohe axiale Auflösung der Interferometrie zur Bestimmung der Schneidengeometrie genutzt werden. Erste Messungen zeigen die Vorteile der Integration der optischen Messtechnik auf der Maschine. Durch eine Zusatzbeleuchtung für das Diamantwerkzeug konnten darüber hinaus die Vorteile der Bildverarbeitung für die Formmessung nutzbar gemacht werden, so dass ein hybrides Messsystem zur Verfügung steht. Die mit dem neu entwickelten Maschinensystem gefertigten und mit der maschinenintegrierten Messtechnik geprüften Werkstücke müssen in einer weiteren Etappe durch hochauflösende Referenzmesstechnik im Labor verifiziert werden. Dazu sollen ein stationäres Weißlichtinterferometer hoher lateraler Auflösung und ein Rasterkraftmikroskop (AFM) eingesetzt werden.

To increase the quality of future products and decrease the manufacturing cost at the same time a systematic control of the fabricated objects is necessary. A promising approach for inline quality control of surface and form parameters is the use of optical measurement systems. This is due to the non-destructive nature of the optical measurement techniques. But in the production environment there are many challenges to overcome for optical sensors. Examples are temperature fluctuation, vibrations, fluids on the object surface and rough surfaces. Therefore, it is likely that not all optical measurement methods are suitable for that task. Hence, a classification of the different principles is necessary with the objective to identify the most appropriate measurement approach for a particular inspection task. In this contribution we start with a systematic approach for a review of sensors within production systems. Then we concentrate on the most robust class of optical sensors, the point sensors. In order to minimize the effect of mechanical vibrations it is desirable to employ measurement techniques that are able to measure the height of an object point in a very short time. Therefore, we focus in this work on chromatic-confocal microscopy and spectral interferometry. The aim is to compare these measurement methods for their ability to cope with the challenges given by the production environment in general. To this end we will develop simulation models for the mentioned techniques and compare two exemplarily sensors for their capability to be used for process control.

Chromatic confocal spectral interferometry (CCSI) is a hybrid method for fast topography measurement, which combines the advantages of the interferometric gain and accuracy with the robustness of confocal microscopy. The CCSI-principle provides a single shot measurement of depth while offering a higher lateral resolution than commonly used spectral interferometers. This contribution is focused on the modeling and simulation of a CCSI-sensor for measuring rough surfaces, based on sequential and non-sequential ray-tracing. With the simulation, the influence of surface roughness, surface reflectivity, and surface contamination on reliability of the sensor can be estimated.

Chromatic confocal microscopy (CCM) and spectral interferometry (SI) are established and robust sensor principles. CCM is a focus-based measurement principle, whose lateral and axial resolutions depend on the sensor's numerical aperture (NA), while the measurement range is given by the spectral bandwidth and the chromatic dispersion in the axial direction. Although CCM is a robust principle, its accuracy can be reduced by self-imaging effects or asymmetric illumination of the sensor pupil. Interferometric principles based on the evaluation of the optical path difference, e.g., SI, have proven to be robust against self-imaging. The disadvantage of SI is its measurement range, which is limited by the depth of focus. Hence, the usable NA and the lateral resolution are restricted. Chromatic-confocal spectral interferometry (CCSI) is a combination of SI and CCM, which overcomes these restrictions. The increase of robustness of CCSI compared to CCM due to the interferometric gain has been demonstrated before. In this contribution the advantages of CCSI in comparison to CCM concerning self-imaging artifacts will be demonstrated. Therefore, a new phase-evaluation algorithm with higher resolution concerning classical SI-based evaluation algorithms is presented. For the comparison of different sensor systems, a chirp comparison standard is used.

We present a single-shot line sensor based on spectral interferometry. Light of a broadband laser source is chromatically dispersed by a grating and focused onto a line on the surface such that each focal point on this line is formed by another wavelength. The entire height profile is obtained by applying a phase evaluation algorithm to the registered interference signal, followed by a model-based approach. The sensor concept is finally verified by experimental results.

An automatic method for the positioning of the test surface in a non-null interferometer is presented. A major task in the interferometric testing of surfaces is to avoid the introduction of surface aberrations due to an incorrect placement of the test object in the interferometer cavity. In the case of plane and spherical surfaces, adjustment errors can usually be distinguished from surface figure errors and therefore removed, but in the case of aspherical surfaces this task becomes nontrivial. In this work, the effect on the measured phase due to lateral and axial displacements of the aspherical surface is calculated, and an adjustment method for the positioning of the surface at a predefined measurement location presented. Experimental results showing the feasibility of the proposed procedure are shown.