Complex diffractive surface patterns on metals by UV-ps laser ablation.

Diffractive surface patterns with complex textures are generated on metal surfaces by picosecond UV laser ablation using an interference setup. Two diffraction gratings with variable distance and rotation angle provide a huge variety of interference patterns and thus resulting surface topographies. This variety can be further enhanced by selecting or blocking particular beams. A correlation analysis of the complex diffraction patterns generated by reflection of visible laser light at these surface topographies demonstrates that patterns with slightly differing fabrication parameters (variation of 0.5 mm in distance or 1° in rotation) can be clearly distinguished.

Hybrid micro-optical elements by laser-based fabrication of Fresnel lenses on the end face of gradient index lenses.

Fresnel lenses are fabricated directly upon the end face of gradient index (GRIN) lenses by F2-laser machining at 157 nm wavelength. The employed laser processing technique combines a mask projection configuration at 25-x demagnification with a rotation of the structured lens. The ablation characteristics of the GRIN materials require very high pulse fluences with typical values above 7 J/cm2. Topography measurements on the Fresnel lenses reveal a good contour accuracy with residual deviations from the design profile well below 100 nm. Such hybrid optical elements, combining GRIN lenses with diffractive lenses in one element, can serve as the basis for high-performance micro-optical imaging systems with diameters up to 2 mm. Examples of possible applications include imaging sensors like proximity sensors or color-corrected microscope objectives with high numerical aperture for endoscopy applications.

Speckle interferometry: three-dimensional deformation field measurement with a single interferogram.

An electronic speckle interferometer, arranged for out-of-plane sensitivity and with an off-axis reference beam to produce spatial phase bias, is used for three-dimensional deformation field measurements. The complex amplitude of the object wave is calculated by application of a Fourier-transform method to a single interferogram. The change in phase after object deformation yields the out-of-plane component of the displacement field. The two in-plane components are obtained by cross correlation of subimages of the reconstructed object wave's intensity, a method that is also referred to as digital speckle photography. The Fourier-transform algorithm is extended and modified, leading to random measurement errors that are below widely accepted theoretical limits and also to an extended measuring range. These properties and the mutually combined information improve the accuracy of both methods compared with their usual single implementation. The performance is evaluated in experiments with pure out-of-plane, pure in-plane, and combined deformations and compared with theoretical values. An example of a practical application is given.

Corrosion monitoring with speckle correlation.

The changes in the microtopography of a metal surface during a corrosion process are measured by decorrelation of the scattered speckle fields under coherent illumination. For that purpose a quantitative relation between the decorrelation of the scattered light fields and the rate of corrosion is established in a theoretical model, based on the statistics of phase and reflectivity changes of point scatterers at the surface. The speckle fields are recorded by a CCD camera and processed numerically in a computer, yielding the standard deviation of the topography changes with nanometer sensitivity. From the analysis of a series of images taken at equal time intervals during the corrosion process, the degree of interrelation among subsequent topography changes is calculated.