Sven Brinkmann

Interferometric testing of plane and cylindrical workpieces with computer-generated holograms

Increasing demands for accuracy and speed in manufacturing and international standards of quality control require faster and more precise measurement techniques. Surface inspection and shape control of technical workpieces is commonly done by tactile profilometers. A faster alternative to this mechanical tool can be realized interferometrically. Grazing incidence of laser light onto the technical surface reduces speckle noise significantly. In our setup computer-generated holograms (CGHs) are used both as references for the technical surfaces to be tested and as beamsplitter or recombining element. Each class of workpieces requires specific CGHs, e.g., phase gratings for plane surfaces or diffractive axicons for cylindrical and conical surfaces. An ideally shaped workpiece will result in a zero fringe field. Deviations from the ideal shape will be indicated by interference fringes and fringe distortions. The sensitivity of the interferometer can be adapted to technical needs. The surface deviations of the workpiece are superimposed by adjustment aberrations which can be described mathematically with sufficient accuracy and eliminated by a least squares fit. This measurement technique is demonstrated with workpieces of different shape.

Testing of rod objects by grazing incidence interferometry: theory

Interferometry at grazing incidence allows one to perform macroscopic shape tests of rough surfaces with submicrometer precision. The highest measurement accuracy is achieved with a null test: the object wave front is adapted to the ideal surface under test through the use of diffractive optical elements. Deviations between the ideal and the real surface shape result in characteristic phase distributions at the interferometer output. Rod objects with rather arbitrary cross-section profiles are especially suitable for this type of measurement. The design of appropriate test wave fronts can be carried out by means of geometrical considerations. Expressions describing aberrations that are due to misalignments of the work piece can easily be derived. Misalignment aberrations and genuine surface deviations are separated by least-squares fitting.

Testing of rod objects by grazing-incidence interferometry: experiment

A grazing-incidence interferometer for the testing of technical surfaces for macroscopic surface deviations is described. Computer-generated holograms serve as beam splitters and references for the workpieces tested. The sensitivity of the interferometer depends on the period of the computer-generated holograms. The method is demonstrated at a rod object of convex profile. Using phase-stepping techniques, the grazing-incidence interferometer provides fast measurements of the entire mantle surface of the test sample with submicrometer precision.

Axicon-type test interferometer for cylindrical surfaces: systematic error assessment

The principle and the alignment aberration functions are described for an axicon-type test interferometer for measuring cylindrical mantle surfaces. Additionally, we show that the derived systematic alignment functions fulfill for reasonably small misalignments the requirements for measurements in the range of approximately 1/100 of a fringe. We verify this with optical path-length calculations, using ray tracing.

Interferometric testing of technical surfaces with computer-generated holograms

Increasing demands for accuracy in manufacturing and international standards of quality control require faster and more precise measurement techniques. Surface inspection and shape control of technical workpieces is commonly done by tactile profilometers. A faster alternative to this mechanical tool can be realized interferometrically. Grazing incidence of laser light onto the technical surface reduces speckle-noise significantly. In our setup computer generated holograms are used both as references for the technical surfaces to be tested and as beam splitter or recombing element. Each class of workpieces requires specific computer generate holograms, e.g. phase-gratings for plane surfaces or diffractive axicons for cylindrical and conical surfaces An ideally shaped workpiece will result in a zero fringe field. Deviations from the ideal shape will be indicated by interference fringes and fringe distortions. The sensitivity of the interferometer can be adapted to technical needs. The surface deviations of the workpiece are superimposed by adjustment aberrations which can be described mathematically with sufficient accuracy and eliminated by a least square fit. We will demonstrate this measurement technique with workpieces of different shape.

Interferometric testing of technical surfaces by grazing incidence interferometry in the infrared region

Increasing demands for accuracy in manufacturing and international standards of quality control require faster and more precise measurement techniques. Surface inspection and shape control of technical workpieces is commonly done by tactile profilometers. Interferometric testing of optically rough surfaces is faster, but the interference pattern is usually disturbed by high contrast speckle noise. Grazing incidence interferometry is an appropriate method to increase the effective illuminating wavelength. This leads to dramatically reduced speckle noise. Increasing of the wavelength from the visible to the infrared region is another opportunity to diminish speckle. An interferometric set-up combining both methods is presented. Well collimated laser light is splitted into several diffraction orders by a computer generated hologram (CGH). The zero diffraction order passes through to a second CGH and is used as the reference wavefront. The first diffraction order hits the object and is reflected to the second CGH where it is recombined with the reference wave. In the ideal case only uniform intensity is observed. Deviations from the ideal shape and misalignments of the object in the set-up lead to interference fringes after the second CGH. The fringe pattern is evaluated by using phase shifting interferometry. Further data processing eliminates the misalignment errors and reconstructs the shape of the object. The sensitivity of the interferometer depends on the design of the CGHs and can be adapted in a wide range to technical needs. The use of infrared light expands the measurement range. Rough surfaces can be tested with a convenient resolution in the direction of the optical axis. The capability of the IR-interferometer will be shown with some measurements of cylindrical workpieces.

Production and control of refractive and diffractive microlenses

Refractive or diffractive microlenses have already been reported. Here we discuss two examples of microlenses where the generation process and the interferometric control are strongly interwoven. For refractive lenses we use lenses melted in photoresist and also reactive ion etched samples. The control is done with the help of a phase shifting interference microscope of the Mach-Zehnder type. We developed an evaluation software under Windows. The software allows for the evaluation of the wave aberrations and related functions as are psf and otf.

Grazing incidence interferometry with the help of diffractive masters

The test of rod-like technical objects can be carried out in grazing incident using diffractive optical elements as beam splitters and at the same time as generators for master wave fronts. The occurring interference patterns can be evaluated with the help of the well known phase shifting technique. Wave aberrations caused by the adjustment state of the test sample can be removed from the data set via least squares fitting a functional representing the wave aberrations due to misalignments.

Optical testing of cylindrical surfaces by grazing incidence interferometry in the infrared region

Interferometric testing of technical, optically rough surfaces is disturbed by high contrast speckle noise. Grazing incidence interferometry is an appropriate tool to increase the effective illuminating wavelength. This leads to dramatically reduced speckle noise. Increasing of the wavelength from the visible region to the infrared is another opportunity to diminish speckle. An IR-interferometric combining both methods is presented. The advantage of the IR-interferometer compared to the VIS-interferometer is the reduced anamorphotic distortion of the image, which increases the resolution in the z-direction of the cylinder and enables the measurement of rougher surfaces. The interferometric set-up consists of two diffractive axicons with the test piece in between. The axicons serve as references for the cylindrical test samples and as beam splitters and combiners. The plane wave of the 0- th diffraction order is the reference beam whereas the conical wave of the first order is deflected onto the object. After reflection from the cylindrical test sample the first order contains wavefront deformations due to deviations of the test sample from cylindricity. After recombination of the object and the reference beam by the second axicon a low frequency interference pattern is detected by a high resolution PtSi focal plane array. The surface deviations can be reconstructed with the methods of phase shift interferometry.