Contactless inspection of complex optical components

Test objects:
Novel quartz and diamond optics with complex geometries, such as aspheric lenses and array structures manufactured using a precision molding technique.

Relevant parameters:
Shape and roughness

Metrological challenge:
Fast, contactless measurement in very tight spaces (small, densely packed arrays, concave test specimens, complex free-form surfaces), tolerances in the submicron range

Solution:
Precision fiber-optic measuring system with high-accuracy sensor and custom-made miniature probe (Ø 1.4 mm)

In order to improve beam guidance systems, Aixtooling GmbH is developing novel quartz and diamond optics with complex geometries, such as aspheric lenses and array structures using a precision molding technique. Due to the high pressures and temperatures involved during manufacture, micro-cracks and adhesions can result that impair the quality of the lenses. Because of this, continuous metrological quality control is necessary.

The parameters that affect the function and performance of the optical components are shape and roughness. Tolerable deviations lie in the sub-micron range. What was needed was a fast, production-coupled, and above all, contactless inspection process. Tactile measuring devices are thus of only limited interest for inspecting optics. The limitations of optical measuring systems, on the other hand, are governed by the shape and size of the measuring apparatus. When it comes to very small, densely packed arrays, concave-shaped test specimens and complex free-form surfaces, standard sensor heads are likely to hit the flanks of adjacent components.

The measuring solution from fionec comprises a fiber-optic distance measuring system with specially adapted miniature probes with a diameter of 1.4 mm. The connection between probe and evaluation unit can be of almost any length. It works using low-coherence interferometry as the measurement principle. In this way, the system obtains absolute distance values with an accuracy of just a few nanometers. The comparatively large numerical aperture of the probes means they can measure flank angles of up to 10° without impinging on signal quality. Larger flank angles can be measured by adjusting the position of the probe.

The sensor is integrated into a precision-guided, multiaxial kinematic unit being developed by the Fraunhofer Institute for Production Technology (IPT). In combination with the kinematic unit, a full-surface scan of the optical components is performed using freely programmable scan paths and individually configurable point densities. A straightforward nominal/actual comparison can be made using the supplied software. The evaluation of all roughness parameters (R or S parameters) is performed based on the relevant standards for roughness analysis.

The integrated fiber-optic measuring system offers a reliable, and above all, non-contact alternative to tactile methods for measuring complex optics. The new technology will make it possible to perform fast areal scans of free-form optical surfaces using custom-programmable scanning paths for the first time and analyze topographical deviations and roughness parameters in a precise and non-destructive way.

To the MaGeoOptik DPP research project >>