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Roundness measurements of boreholes with small diameters using a fiber-optic sensor

Rundheitsmessungen an kleinen Bohrungen mit faseroptischen Sensoren
: Schmitt, R.; König, N.; Depiereux, F.

Sensor + test 2008. Proceedings : OPTO 2008, Photonic Metrology, 8th International Conference on Optical Technologies for Sensing and Measurment; IRS2 2008, 10th International Conference on Infrared Sensors & Systems; Nürnberg, Germany, 6 - 8 May 2008
Wunstorf: AMA Service, 2008
ISBN: 978-3-9810993-3-1
International Conference on Optical Technologies for Sensing and Measurement <8, 2008, Nürnberg>
International Conference on Infrared Sensors & Systems (IRS2) <10, 2008, Nürnberg>
Conference Paper
Fraunhofer IPT ()
Bohrung; Bohrlochmessung; Meßgröße; optisches Meßverfahren; faseroptischer Sensor; Fizeau-Interferometer; medizinische Meßtechnik; Endoskop=medizinisch; konfokale Mikroskopie; berührungsloses Messverfahren

Within this work, a roundness measurement system has been described, which is based on low-coherence interferometry. The highly accurate, non-tactile probing and the miniaturized fiber-optic probe tip enables the system for the measurement inside small cavities like boreholes. Today, boreholes with diameters down to 1 mm can be inspected. Especially the measurement of injection nozzles with roundness tolerances smaller than 1 micron is highly demanded. A further field of application is optical coherence tomography (OCT). This technique is similar to low-coherence interferometry and enables the inspection of tissue e.g. human skin. The first fiber-optic probes for OCT are commercially available. A typical application is the endoscopic imaging of human veins. All proposed approaches for the deflection of the measuring beam require the use of right-angle micro-prisms. In terms of miniaturization this can be a limiting factor, because the prisms are commercially not available with edge lengths smaller than 500 micron. Furthermore, the assembly of prisms on fiber end surfaces is not easy to handle. Newly-available fiber lensing machines are a promising solution for this problem. Beam deflection can possibly be reached by polishing the GRIN-fiber with a defined angle. For the measurement on rough surfaces, a high numerical aperture (NA) of the fiber-optic probe is needed. This requirement cannot be fulfilled with standard single mode and GRIN-fibers. Special large-diameter fibers (LDF) with core diameters of up to 400 micron and high NA can be used to build such high-NA-probes, but also require a sophisticated splicing technique. For measurements inside boreholes with diameters lower than 100 micron, special reduced cladding (RC) fibers or tapered fibers with outer diameters of 80 micron and below can be used, though beam shaping with such fibers will hardly result in high NA.
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