A METHOD FOR MEASURING THE DIAMETER AND/OR DIAMETER CHANGE OF A GLASS FIBER ON THE BASIS OF FABRY-PEROT RESONANCES AND APPARATUS FOR CARRYING OUT THE METHOD

摘要

Method for measuring the diameter and/or diameter change of a glass fiber on the basis of Fabry-Perot resonances, in the course of which the glass fiber is illuminated by a laser of constant wavelength from side direction making smnall angle with the direction being orthogonal to the fiber axis, the far-field intensity of light rays backscattered in the direction making the same but opposite angle with the surface normal is detected and the path length inside the glass fiber of light rays to be detected and backscattered from the inner back surface of the glass fiber is changed by changing the said small angle or by displacement of the glass fiber in axial direction. The apparatus capable to carry out the method comprises a laser light source illuminating the glass fiber from side direction and a photodetector sensing the far-field intensity of a ray boundle backscattered in a determined direction. At a first variant of the apparatus a direction changing device (beam deflector) is arranged between the laser and the glass fiber and a slit is placed between the photo detector and the glass fiber and a slit is placed between the photodetector and the glass fiber. At a second variant, the output beam of the laser light source illuminates the glass fiber at two different incidence angles through a beam splitter on one hand and through the beam splitter and a mirror on the other hand, and separate photodetectors are used for sensing the backscattered light rays arising from each of the two illuminating beams, and 90 degrees phase difference is produced between the photodetector signals. s