Current research / employment

Short Abstract of IMPRS PhD Project

Aspheric surfaces are used in optical systems in order to improve the aberration correction and, often, without added weight and size. However, the major obstacle in using them lies in the difficulty in their fabrication and accurate testing. The main aim of the research work was the precise characterization of aspherical surfaces. The thesis discusses the following two major activities which have been conducted to achieve the above said goal.

• Frequency analysis of nano-scale roughness formation on single point diamond turned optical surfaces.

• Quasi-absolute measurement of aspheric surface deviations by using a diffractive optics based interferometric null method.

The first activity was performed at National Aspheric Facility, Central Scientific Instruments Organisation (CSIO), Chandigarh, India. The flat metal mirrors were fabricated and characterized by performing the frequency analysis (power spectral density and autocorrelation) of the surface roughness. The study focused on establishing a relationship between the manufacturing process and the surface roughness generation which is crucial for developing metal components with the optical quality finish.

The second activity was carried out at Max Planck Research Group; Institute of Optics, Information and Photonics (IOIP), University of Erlangen-Nuremberg, Germany. A three position quasi absolute test for rotationally symmetric aspherics by using combined-diffractive optical elements (Combo-DOEs) is developed. The effects of the DOE substrate errors on the proposed calibration procedure are investigated and a set of criteria for designing an optimized Combo-DOE has been established. It is demonstrated that the optimized design enhances the overall consistency of the procedure. Furthermore, the rotationally varying part of the surface deviations is compared with the rotationally varying deviations obtained by an N-position averaging procedure and both results have been found to be in good agreement. The capability of extracting the complete deviations of the specimen signifies the proposed technique as a promising tool for interferometric calibration of aspheric metrology.

Publications G. Khan, K. Mantel, I. Harder, N. Lindlein, J. Schwider, Design considerations for the absolute testing approach of aspherics using combined diffractive optical elements, Appl. Opt., 46, 7040-7048, (2007)

F. Simon, G. Khan, K. Mantel, N. Lindlein, J. Schwider, Quasi-absolute measurements of aspheres with a combined-diffractive optical element as reference, Appl. Opt., 45, 8606-8612, (2006)