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DFG project: Optical surface metrology with spatially and temporally partially coherent light wave fields (OPAL)

Within the framework of an ongoing joint project between the Bremer Institut für angewandte Strahltechnik (BIAS) and the Physikalisch-Technische Bundesanstalt (PTB), new procedures to determine the form of optical surfaces are being investigated and developed. The activities are funded by the Deutsche Forschungsgemeinschaft (DFG).

The project focuses on developing a shearing interferometer with multiple-source lighting to measure aspherical and free-form surfaces. Using a spatial light modulator (SLM) as a shearing element provides great advantages: nearly equal paths of propagation of the interference fields needs very low requirements on the coherence of the light, so that it is possible to use LEDs. This enables, in particular, the simultaneous illumination of the measuring area with several light-emitting diodes without disturbing interferences occurring in the overlapping areas. Due to the individual positioning of the light-emitting diodes, it is possible to acquire data for areas which are difficult to measure (such as, for example, surfaces which exhibit asymmetries or large differences in the edge steepness) without having to use special lens systems or computer-generated holograms (CGH). The known advantages of shearing interferometry, which reside in its compact design and high robustness against vibrations, remain unchanged.

Figure 1:
Measurement principle of partially coherent shearing interferometry with multiple-source lighting

In order to investigate the measuring system in more detail, a ray-tracing model is being developed. It can be not only used to estimate the measurement accuracy, it can also be an elementary part of the validation procedure.

Within the scope of the project, it was possible to show that multiple-source lighting (see Fig. 2a), consisting of several partially coherent light sources, yields a coherent interferogram (see Fig. 2b) when the entire measuring area is illuminated at the same time.

Since industry increasingly demands measurements in reflection, it is planned to extend the measurement procedure to measurements in reflection.

Figure 2:
a) Realization of flexible 7-fold multiple-source lighting
(in the foreground: the specimen),

b) shearing interferograms recorded; due to the partial coherence of the individual light sources, the different light sources cannot interfere with each other.

Reference

[Fal14]   
Falldorf, C., Simic, A., Ehret, G., Schulz, M., v. Kopylow, C., Bergmann, R. B.: Precise optical metrology using computational shear interferometry and an LCD monitor as light source. Fringe 2013: 7th International Workshop on Advanced Optical Imaging and Metrology: 729 -734, 2014.
[Hag16]
Hagemann, J.-H., Ehret, G., Bergmann, R. B., Falldorf, C.: Realization of a shearing interferometer with LED multipoint illumination for form characterization of optics, DGaO Proceedings 2016 – www.dgao-proceedings.de – ISSN: 1614-8436 - urn:nbn:de:0287-2016-P020-3
[Fal17]   
Falldorf, C., Hagemann, J.-H., Ehret, G., Bergmann, R. B.: Sparse light fields in coherent optical metrology, Applied Optics 56 (13), pp. F14-F19, 2017
[Hag17]   
Hagemann, J.-H., Falldorf, C., Ehret, G., Bergmann, R. B.: Error influences of the shear element in interferometry for the form characterization of optics, Proc. SPIE 10329, Optical Measurement Systems for Industrial Inspection X, 103291T (2017/06/26); doi: 10.1117/12.2269932
[Hag18]
Hagemann, J.-H., Falldorf, C., Ehret, G., Bergmann, R.-B., Form metrology of optical surfaces based on partial coherent shearing interferometry in reflection, Proc. of EUSPEN, 2018,
https://www.euspen.eu/euspen-knowledge-base/proceedings/
[Fal18]
Müller, A., Falldorf, C., Agour, M., Hagemann, J.-H., Ehret, G., Bergmann, R.-B., Lens inspection using Multiple Aperture Shear Interferometry: Comparison to Wave Front Sensing Methods, DGaO Proceedings 2018,
https://www.dgao-proceedings.de/download/119/119_a12.pdf

Contact

Dr.-Ing. Gerd Ehret
Phone: +49 (0)531 592-4220
E-mail: Gerd.Ehret(at)ptb.de