Student Projects

Project: Tabletop White Light Interferometry

White light interferometry (WLI) exploits the fact that for white light constructive interference only occurs when the two beam paths are of the exact same length. A Michelson interferometer setup is used. The light of a collimated white LED is split with a beam splitting cube in two perpendicular beams. One beam is reflected by a reference mirror, the other one by the sample surface. The beams are recombined in the cube and the intensity is detected spatially resolved with a 2D CCD sensor. To record the interference, the reference mirror is moved by a piezo actuator with nanometre (1nm = 10^-9m) resolution.

The interferometer can compare the surface unevenness of two reflecting surfaces with very high accuracy. Using an extremely flat reference mirror the surface topology of a sample can be analysed. The achievable resolution was determined by comparing the surface of two reference mirrors. It was possible to detect few nm height differences in a surface of several square millimetres. The result of such a measurement is shown in the image.

The resolution is limited by several factors. The used beam splitting cube was the least precise part with a surface flatness of lambda/8. The reference mirrors have a sub nm unevenness, but they are bent by the adhesive bonding to the mirror holder. The accuracy of the data processing is limited. To achieve fast results a simplified fit function was used for analysis. This causes some fit errors with one wavelength offset, which can be recognized as black pixels in the image.

Important aspects of the WLI setup are stability and mobility. Mobility is important, to investigate not only samples on a optical table but also on-site in an existing experiment. Therefore solid components are used and mounted on a single breadboard, so that only the sample needs to be adjusted for measurement.

The above set up was assembled by Christoph Thiele in May 2015 who attended the group as a student assistant in Winter/Spring 2015.

Project: Schlieren Imaging

Schlieren photography of a white light source in original colours.

The monochrome movie (below) shows the capabilities of schlieren technique: imaged striae in heated air ascending from a hot soldering tip. The varying temperature creates changes in the refractive index of air. And these changes are captured in the schlieren images. The technique is primarily used to investigate flow dynamics, and was developed to characterize the quality of optical instruments like lenses [E. Hecht: Optics]. A trainee set the optical assembly up and recorded the images in February 2015.
The technique beautifully comprises the fundamental principles of imaging – the basis of our everyday work.