Characterization of highly divergent optics (DOC)

Project focus

  • Wavefront Measurement Device for Highly Divergent Optics
  • Wavefront Stitching and Reconstruction from Segmented Measurements
  • Automatic Characterization of an Optical Device

Description

Opto-mechatronic devices such as triangulation sensors or polychromatic confocal sensors project focused light beams onto the surface of the measuring object. Assessing the properties of the focused beam is essential as they are directly related to the achievable measurement resolution and precision of the opto-mechatronic device. Such properties can be evaluated by measuring the optical aberrations of the focused beam, since they directly deliver information about the defects in the optical system. To characterize or to improve the optical system, an exact knowledge of the involved optical aberrations is therefore crucial.

The focused spot produced by the mechatronic device can be examined by analyzing the resulting spherical wavefront.

The optical system of an opto-mechatronic device involves different components:

  • Light source (e.g. fiber coupled light source),
  • mirrors and
  • a lens system for manipulating the optical path.

The alignment (relative position and orientation) of the components and their geometrical properties are mainly determining the optical aberrations. One approach to derive the optical aberrations is to measure the surface geometry of single components by surface profilers such as deflectometers, interferometers and tactile or non-tactile measuring machines, determining their relative alignment within the optical path and make use of simulation tools to obtain the aberrations of the entire optical system. With an increasing number of involved optical components, this approach becomes complex, time-consuming and does not allow a direct characterization of the optical quality after assembling the entire optical system.

Instead, measuring the wavefront of the focused beam emitted by the optical system allows to directly characterize the optical aberrations of the entire system, as depicted in Fig. 1. A wavefront describes a surface composed of points of equal phase of the optical field within an optical wave. Each component inside the optical path contributes to the shape of the wavefront that can be measured by wavefront sensing devices. To determine the optical aberrations of the system, the measured wavefront is then compared to an ideal, aberration-free wavefront. Wavefront sensing is currently used in a whole variety of applications such as:

  • optical shop testing to determine the quality of lenses,
  • measuring the dynamic deformation of surfaces such as scanning mirrors,
  • in production lines to characterize the surface topology of wafers, flat panels and glass sheets,
  • in astronomy as part of a closed loop system to improve image quality and in medical imaging for determining eye aberrations.

To characterize the entire optical system with a wavefront measurement, the following challenges arise:

  • The measurement of wavefronts with a large curvature at high sensitivity,
  • achieving high spatial resolution within the entire measurement area for high accuracy measurements,
  • high flexibility to support measurement of a wide variety of optical assemblies, as each assembly has its own wavefront characteristic (e.g. curvature),
  • usage as an in-line measurement tool in an industrial environment and
  • short measurement time.

The major objective of this research area is the development of a mechatronic system that allows a precise measurement of the wavefront emitted by an opto-mechatronic device to evaluate its optical characteristics.

Scientific Imaging and Metrology Systems

The research in this area deals with a fundamental apect of optics: The problem of directly assessing the shape of a light wave. The available tools – optical devices themselves – are not yet fit to deal with curved wavefronts. We plan to overcome the limitations of the available sensor types by integrating them with a highly precise mechatronic positioning system. From such a setup, not only wavefront data, but also positioning and alignment data has to be acquired and subsequently merged. The development of suitable reconstruction and stitching methods forms another central challenge of this research project.

Applications

  • Wavefront sensing
  • Automated characterization of optical components
  • In-line quality assurance

Related publications

  • M. Fuerst, S. Unger, S. Ito, and G. Schitter, Wavefront measurement based feedback control for automatic alignment of a high-NA optical system, in XXII World Congress of the International Measurement Confederation (IMEKO), 2018.
    [BibTex]
    @Conference{TUW-2208181,
    Title = {Wavefront measurement based feedback control for automatic alignment of a high-NA optical system},
    Author = {Fuerst, M. and Unger, S. and Ito, S. and Schitter, G.},
    Booktitle = {XXII World Congress of the International Measurement Confederation (IMEKO)},
    Year = {2018},
    }

Project Partners

Funding