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DFG project: Form measurement of precision components with a dynamically tracked interferometric line sensor

Today's optical and mechanical precision components require sub-micrometer precision when it comes to form tolerances. Optical procedures are often used to measure the form of the surface.

Within the scope of a joint DFG project, a new optical measurement technique is being developed, set up and tested in cooperation with the University of Kassel (Germany) in order to measure rotationally symmetric surfaces. Compared to procedures based on areal or pointwise measurements, this sensor exhibits particularities. The sensor used here is an interferometric line sensor with an oscillating reference mirror. This procedure, which is based on the modulation of the optical path, was first introduced by Sasaki [1, 2] and later further developed [3, 4]. Within the scope of this DFG project, it was possible to considerably increase the precision of this procedure by synchronizing the recording time of the line scan camera with the oscillating reference mirror displacement [5, 7].

Another development allows the 3D measurement of more strongly curved surfaces: ring-shaped sub-apertures are combined to obtain a global topography [8] (see Fig. 1).

Figure 1: (a)
Schematic diagram of the measurement sequence: individual circular rings are measured with the interferometric line scan sensor; by adjusting them to each other, a global topography of the specimen is obtained within a certain overlapping area.
(b)
Measuring system in its set-up arrangement with the sensor, the specimen and different displacement axes.

To measure spherical and aspherical surfaces, the displacement system is equipped with a tilting axis and with a linear axis in order to track the contour of the specimen. To determine the absolute spacing between the sensor and the specimen, a calibration strategy was elaborated and tested [9]. A detailed model of the entire measurement and assessment procedure has been developed. In the future, this will allow the measurement parameters to be optimized in order to obtain the smallest possible measurement uncertainty [6, 10].

The measuring arrangement, initial 3D measurement results, a comparison with measurements realized with an optical pointwise sensor, and initial deviations of design and best-fit data have been published [11-14]. In the meantime, the interferometric sensor has been developed in such a way that it can also be operated as a depth-scanning white-light interferometer and that it can perform absolute spacing measurements which are used to calibrate the system.

Literature

[1]   
O. Sasaki and H. Okazaki, “Sinusoidal phase modulating interferometry for surface profile measurement”, Applied Optics 25, 3137-3140, 1986, doi: 10.1364/AO.25.003137
[2]   
O. Sasaki and H. Okazaki, “Analysis of measurement accuracy in sinusoidal phase modulating interferometry”, Applied Optics 25, 3152-3158, 1986, doi: 10.1364/AO.25.003152
[3]   
U. Minoni, E. Sardini, E. Gelmini, F. Docchio and D. Marioli, “A high-frequency sinusoidal phase-modulation interferometer using an electro-optic modulator: Development and evaluation”, Review of Scientific Instruments 62(11), 2579-2583, 1991, doi: 10.1063/1.1142233
[4]   
P. Lehmann, M. Schulz and J. Niehues, “Fiber optic interferometric sensor based on mechanical oscillation”, SPIE Proceedings 7389, 738915, 2009, doi: 10.1117/12.827510
[5]   
H. Knell, P. Lehmann, „High speed measurement of specular surfaces on carrier fringe patterns in a line scan Michelson interferometer setup”, Proceedings of SPIE 8788, 87880R, 2013, doi: 10.1117/12.2020121
[6]
H. Knell, S. Laubach, G. Ehret, P. Lehmann, „Continuous measurement of optical surfaces using a line-scan interferometer with sinusoidal path length modulation”, Optics Express 22, 29787-29798, 2014, doi: 10.1364/OE.22.029787
[7]
H. Knell, M. Schake, M. Schulz, P. Lehmann, „Interferometric sensors based on sinusoidal optical path length modulation”, Proceedings of SPIE 9132, 91320l, 2014, doi:10.1117/12.2051508
[8]
S. Laubach, G. Ehret, H. Knell, P. Kühnhold, P. Lehmann, „Stitching streifenförmiger Subaperturen zur Formmessung”, DGaO Proceedings - www.dgao-proceedings.de/download/115/115_a20.pdf - ISSN: 1614-8436 - urn:nbn:de:0287-2014-A020-8, 2014
[9]
S. Laubach, G. Ehret, H. Knell, P. Kühnhold, P. Lehmann, „Calibration strategies for a new fast line-based form measuring system“, DGaO Proceedings – www.dgao-proceedings.de/download/116/116_p5.pdf – ISSN: 1614-8436 – urn:nbn:de:0287-2015-P005-3, 2015
[10]
S. Laubach, G. Ehret, J. Riebeling, P. Lehmann, „Error analysis of an interferometric line-based form measuring system“, DGaO Proceedings – www.dgao-proceedings.de/download/117/117_p66.pdf – ISSN: 1614-8436 – urn:nbn:de:0287-2016-P066-6, 2016
[11]
G. Ehret, S. Laubach, A. Straub, M. Schulz „Form measurement of large optics with deflectometric and interferometric procedures at PTB“, Third European Seminar on Precision Optics Manufacturing, Proceedings of SPIE 10009, 100090B, 2016, doi: 10.1117/12.2235474
[12]
S. Laubach, G. Ehret und P. Lehmann, „Interferometrischer Liniensensor zur Formmessung von optischen Oberflächen“, Tagungsband AHMT 2016 - Symposium des Arbeitskreises der Hochschullehrer für Messtechnik, doi: 10.1515/9783110494297-017, 2016
[13]
S. Laubach, G. Ehret, J. Riebeling, P. Lehmann, „A new form measurement system based on sub-aperture stitching with a line-scanning interferometer”, Advanced Optical Technologies, doi: 10.1515/aot-2016-0039, 2016
[14]
S. Laubach, G. Ehret, J. Riebeling, P. Lehmann, „Interferometrischer Liniensensor zur Formmessung von rotationssymmetrischen optisch glatten Oberflächen”, Technisches Messen 84, 166-173, 2017, doi: 10.1515/teme-2016-0067