Tilo Pfeifer

Fiber-optical sensor with miniaturized probe head and nanometer accuracy based on spatially modulated low-coherence interferogram analysis

Fiber-optical sensors have some crucial advantages compared with rigid optical systems. They allow miniaturization and flexibility of system setups. Nevertheless, optical principles such as low-coherence interferometry can be realized by use of fiber optics. We developed and realized an approach for a fiber-optical sensor, which is based on the analysis of spatially modulated low-coherence interferograms. The system presented consists of three units, a miniaturized sensing probe, a broadband fiber-coupled light source, and an adapted Michelson interferometer, which is used as an optical receiver. Furthermore, the signal processing procedure, which was developed for the interferogram analysis in order to achieve nanometer measurement accuracy, is discussed. A system prototype has been validated thoroughly in different experiments. The results approve the accuracy of the sensor.

Interferometric measurement of injection nozzles using ultra-small f iber-optical probes

The measurement of boreholes with diameters smaller than 500 ?m is a demanding task that cannot be performed using state-of-the-art production metrology. In this letter, a miniaturized fiber probe with a diameter of 80 \mu m is presented. A probe is used for low-coherence interferometry to conduct highly precise measurements of form deviations of small boreholes. Measurements conducted in nozzles are also presented. The results prove the potential of the fiber-optical sensor for quality inspection of high-precision parts, such as injection nozzles, for common-rail diesel engines.

Model-based optimization of interferometers for testing aspherical surfaces

This paper presents an approach to increase the understanding of errors and alignments in interferometrical contouring. In order to achieve a fully determined interferometrical set-up with a controllable alignment status, real existing interferometers are simulated with the help of a software package called the Virtual Interferometer (VI). It consists of a raytracing module, a measurement simulator for phase shifting evaluation and a statistics module to simulate the environmental aspects of interferometrical contouring (e.g. misalignments, surface errors of the optical components, phase shift errors). Additionally, a further software package for controlling the corresponding real interferometrical systems and a module for addressing two separate positioning systems each having 5 degrees of freedom was developed. With the help of the VI, two closely related aspects of interferometrical shape testing were examined. First, work in the field of testing aspheric surfaces with the use of Multiple Wavelength Interferometry (MWI) will be presented. New experimental results are introduced and the effect of aberrations due to strong surface slopes is discussed. Second, nulltests are presented that allow the measurement of aspheric mirrors like paraboloids and hyperboloids. In this application, especially the alignment of the complex interferometrical setup with its up to ten independent parameters was the main point of investigation. Both applications were analyzed with the VI in order to optimize the measurement methods with regard to alignment optimization and correction of error inducing aberrations. Then the optimized methods were applied in the real interferometrical systems for measuring various aspheric surfaces. Finally, future application for the VI will be discussed shortly.

Manufacturing of optical molds using an integrated simulation and measurement interface

The manufacturing of optics is an important field of technology and will serve key-markets in the future. The research activities of the Transregional Collaborative Research Center ”Process Chains for the Replication of Complex Optical Elements” SFB/TR4 of the Universities of Aachen, Bremen and Stillwater (USA) have the objective to lay the scientific foundations for a deterministic and economic mass production of optical components with complex geometries, e.g. aspheric, non-rotational asymmetric or microstructured surfaces eventually superimposed on freeform geometries. The paper presents an approach for an integrated simulation and measurement interface for the analysis of manufacturing effects during the mold making as well as first results of its application on the basis of the manufacturing of mold inserts.

Strain/stress measurements using electronic speckle pattern interferometry

In its classical application Electronic Speckle-Pattern Interferometry (ESPI) is used to measure deformations with high resolution. Additionally, this methods also able to measure the 3D topography of technical surfaces even with discontinuities. If adequate set-ups are used, combined measurements of shape and deformation can also be carried out. Especially in production of machines and other metal parts, the determination of the stress and strain e.g. of welding points is a very important issue for evaluating about the quality of the specimen under test. Here, the use of wire resistance strain gauges is state-of-the-art for measuring length variations of parts under mechanical load. It is very time-consuming to prepare the corresponding measurement environment for strain gauges. Moreover, only very limited information about the strain can be measured by this means because all information is integrated over the whole area covered by the strain gauges without lateral resolution. In order to extend this kind of metrology to a matrix of some thousands points even including sensitivity in the out-of-plane-direction, ESPI-methods can be used. As described in this paper, it is therefore necessary to perform both,the shape and the deformation measurement to obtain the necessary information. Of course, the directions of sensitivity depend on the contour of the test specimen ad can be determined due to the previously measured topography. In this paper current work on the field of stress and strain measurements with ESPI is described. The experimental result of several technical applications is shown and it is compared to measurements with conventional strain gauges. Further possible technical applications are discussed and the prototype of an ESPI stress sensor is presented.

New method for confocal microscopy and its endoscopic application

Today, confocal microscopy is widely used in biomedical applications, since its technology allows e.g. in cell biology to resolve sub- and intercelluar details in three-dimensions. Moreover, the recent miniaturization process of opto-electronical components even indicates a possible endoscopic application - to intraoperative examinations without need for a surgical biopsy. The current development of such a confocal endomicroscope, based on the concept of miniaturized fiberoptical components is described. The technical setup is explained in the context of an alternative method for confocal microscopy employing a Digital Micromirror Device (DMD). Moreover, in-vivo tissue staining techniques suitable for confocal endoscopic imaging are analyzed.

Object digitalization using a scanning fringe projection system

The advantage of optical 3D measurement systems like triangulation based fringe projection systems compared with tactile working systems is their contactless and fast 3D surface data acquisition. The results in the form of point clouds can be processed regarding a mathematical surface description for CAD data generation. In many cases the optical 3D surface data acquisition of complex workpieces cannot be achieved in one measuring process so that it is necessary to merge different measuring fields. This approach is essential when the object is larger than the measuring range or the object cannot be measured in one measuring process in consequence of reflections or shadings. According to these problems a fringe projection system was integrated in a coordinate measuring machine with 5 axes (3 linear axes and 2 rotation axes). By using these axes, every region of the workpiece can be reached with the optical 3D sensor for digitalization. To get the complete point cloud of the object surface the different measuring fields must be transformed in one coordinate system by using the relative axes positions of the coordinate measuring machine. Results concerning Reverse Engineering applications and considerations of the measurement uncertainty of the scanning fringe projection system are presented.

Fiber optic interferometer for absolute distance measurements with high measuring frequency

White light interferometry or coherence radar is a well known and established measurement technique for years. But especially the field of production technology and medical technology need measurement systems with continuously increasing performance. This paper demonstrates, how the use of special optical components for higher measurement frequencies and the miniaturizing ofthe sensor tip can enlarge the application fields for this system. First, the fundamental aspects of white light interferometry are presented and lead to the new concept of a distance measurement system. It is shown how the inset of a special stepped mirror in the detection interferometer increases the measurement frequency and how the sensor is miniaturized using a Fabry-Perot interferometer.

New measurement system for microscopic formtesting of microstructures by means of multiple-wavelength interferometry

Nowadays large area microstructured surfaces can be produced by innovative production technologies. The task for the measurement technologies is to cover the process chain during the production of these 3D-nano- and microstructures. At the Fraunhofer IPT the form of the structures is investigated because it determines the functionality of the components most significantly. Fast interferometrical measurement concepts are developed for the inspection of the surface. The possibilities of interferometrical formtesting of prisms, spherical lens arrays or gratings are adapted to the microscopic range. Especially for the characterization of gratings, the measurement range has been extended by using multiple wavelength interferometry.

Comparison of strain/stress measurements on free form surfaces using ESPI and strain gauge technique

ESPI is classically used in high resolution measurements of deformation, but ESPI can also be sued for contouring of surface even with discontinuities. Furthermore, measurements of both shape and deformation had been carried out. It has been shown that performing both measurements is necessary to obtain the actual strain information at the specimen topography, and the development of such techniques was presented as an alternative to other strain measurement methods, among who wire resistance strain gauges is the most popular one. But, despite its reliability and popularity, the strain gauges technique has some disadvantages. Its set- up is very time-consuming: in order to assure tight contact between gauge and measured surface, one has to take care about the specimen surface cleanliness and roughness and the gluing step takes hours; mounting the electric circuit also request for time and work due to the small voltage signs, demanding high sensible signal amplifiers and bridge-circuit units able to perform compensation of temperature variation and protection from noise. Beside s that once the given strain information result from the integration over the whole area covered by the gauges, this technique gives no lateral resolution.