Automatic geometric dimension measurements of single-mode fiber preforms using a novel shadow method

A method is described in which a test preform is immersed in an index-matching liquid so as to detect optical signals from the core/cladding and cladding/index-matching liquid interfaces simultaneously. A light-beam sweeper with a mirror is used to sweep a He-Ne laser beam across a fiber preform, and the time intervals during which the preform intercepts the beam and casts a composite shadow on a photodetector are measured. The method is simple and accurate and has sufficient spatial resolution (better than 10 μm) to obtain detailed measurements of core eccentricity, core and cladding diameters and their ratio, and ellipticity in a step-index single-mode fiber preform for a subscriber line. On the basis of this principle, an automated geometry measurement system has been developed. The system is attractive for quality control applications in mass-production lines of single-mode fibers