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

摘要

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 [9]. 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 avalaible with edge lengths smaller than 500 urn. 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 singlemode and GRIN-fibers. Special large-diameter fibers (LDF) with core diameters of up to 400 urn 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 urn, special reduced cladding (RC) fibers or tapered fibers with outer diameters of 80 urn and below can be used, though beam shaping with such fibers will hardly result in high NA.