Ralf Dorn

Reduction of the spot size by using a radially polarized laser beam

We present numerical calculations on the field distribution in the focus of an optical system with high numerical aperture. It is shown that a radially polarized doughnut mode fits the symmetry of the optical system and focuses down to spot sizes significantly smaller as compared to the case of linear polarization. An experimental setup to generate a radially polarized beam is presented along with a setup which is based on the knife edge method and tomographic reconstruction to measure the 3D intensity distribution in the focal region.

Radial polarization minimizes focal spot size

We experimentally demonstrate that a radially polarized beam can be focused to a spot size significantly smaller than for linear polarization and that the longitudinal field component arising at focus best coupled to small particles

Polarization-dependent effects on phase singularities in the vicinity of sub-lambda structures

We present theoretical and experimental studies of the reflected field in the vicinity of sub λ structures. Rigorous numerical calculations and measurements were performed to get high-precision information of certain object parameters which go beyond the limits of classical microscopy. We show that the polarization of the illumination plays a key role for the field distribution, which is reflected from the examined objects. Furthermore, we present a simplified model, which is able to qualitatively predict the behavior of the phase singularities correctly.

Untersuchung von Phasensingularitäten in der Umgebung kleiner Strukturen mit einem Linnik-Interferenzmikroskop (Investigation of Phase Singularities in the Vicinity of Small Structures Using a Linnik Interference Microscope)

Wir präsentieren Untersuchungen zur Feldverteilung in der Umgebung von Sub-λ-Strukturen. Das Auftreten von Singularitäten in der Phasenverteilung des reflektierten Feldes wird theoretisch und experimentell in Hinsicht auf die Möglichkeit der lateralen Überauflösung untersucht. Ein theoretisches Modell für die Berechnungen des elektrischen Feldes, welches auf dem Greenfunktion-Formalismus basiert, wird diskutiert. Messungen an zweidimensionalen Objekten wurden mit Hilfe eines Linnik-Interferenz-Mikroskopes durchgeführt.

From optics testing to micro optics testing

Testing micro optics, i.e. lenses with dimensions down to 0.1mm and less, with high precision requires a dedicated design of the testing device, taking into account propagation and wave-optical effects. In this paper, we discuss testing methods based on Shack-Hartmann wavefront technology for functional testing in transmission and for the measurement of surface shape in reflection. As a first example of more conventional optics testing, i.e. optics in the millimeter range, we present the measurement of binoculars in transmission, and discuss the measured wave aberrations and imaging quality. By repeating the measurement at different wavelengths, information on chromatic effects is retrieved. A task that is often tackled using Shack-Hartman wavefront sensors is the alignment of collimation optics in front of a light source. In case of a micro-optical collimation unit with a 1/e² beam diameter of ca. 1mm, we need adapted relay optics for suitable beam expansion and well-defined imaging conditions. In this example, we will discuss the alignment process and effects of the relay optics magnification, as well as typical performance data. Oftentimes, micro optics are fabricated not as single pieces, but as mass optics, e.g. by lithographic processes. Thus, in order to reduce tooling and alignment time, an automated test procedure is necessary. We present an approach for the automated testing of wafer- or tray-based micro optics, and discuss transmission and reflection measurement capabilities. Exemplary performance data is shown for a sample type with 30 microns in diameter, where typical repeatabilities of a few nanometers (rms) are reached.

Testing micro optics with micro optics

An increasing part of the optic industry’s added value consists of micro optical components. This increases the demand for effective test methods for micro optics. When conventional test methods are transferred to micro optics, special attention should to be paid to diffraction effects, wave front propagation and precise sample imaging. We show how a well-established tool – the Shack-Hartmann wave front sensor (SHWFS) – which uses a micro lens array as a key element, can be used for testing micro lenses. Different measurement configurations for transmitted light and reflected light testing are discussed. A system which takes advantage of a combination of these test configurations to retrieve information on surface quality, wave front performance, focal length, and defects is presented. Measurement results are shown to demonstrate the system performance.