1. Field of the Invention
This invention relates to methods of monitoring successive axial portions of an optical fiber and, more particularly, to methods of measuring the diameters of successive axial portions of an optical fiber during the process of drawing the fiber, and for controlling the fiber drawing process in response to the diameter measurements taken.
2. Description of the Prior Art
In copending application Ser. No. 482,707, filed on June 21, 1974 by L. S. Watkins and entitled, "Method for Measuring the Parameters of Optical Fibers", which issued on Sept. 28, 1976 as U.S. Pat. No. 3,982,816 there is disclosed a technique for determining the diameters of successive axial portions of a clad or unclad, optical fiber. Such technique, which is typically performed upon an axially advancing fiber during the fiber drawing process, involves the illumination of a newly drawn, axially advancing fiber with a radially directed beam of coherent, monochromatic radiation, thereby creating a forward scattering pattern of interference fringes. The pattern is examined over a predetermined range of scattering angles in order to determine the number of fringes present in the predetermined range. A succession of counts of such fringes provides information as to the diameters of successive axial portions of the fiber advancing past the beam of radiation.
It has been discovered, however, that anomalous diameter readings, or "dropouts", will occur from time to time in practicing the technique of the copending Watkins application. These dropouts involve outputs which are in no way related to the diameter of the fiber. The dropouts are generally caused by local inhomogenieties in the advancing fiber, such as trapped air bubbles or surface imperfections, which greatly distort or completely break up the interference fringes in the forward scattering pattern being examined. Such dropouts can be of major significance, in that the instantaneous diameter readings caused by dropouts may vary to a very great degree from true diameter values. For example, dropouts indicating 50 micrometers to 100 micrometers apparent diameter changes are typical in examining fibers of approximately 100 micrometers true diameter. As a result, the use of the Watkins technique in controlling the diameter of an optical fiber during the drawing of the fiber, by providing process-controlling diameter indications, is severely affected by the occasional occurrence of dropouts. Since it may not be feasible to improve the fiber drawing process to such an extent as to eliminate the presence of occasional local inhomogenieties, it would clearly be advantageous to provide an improved technique for measuring the diameter of an advancing fiber, unaffected by occasional dropouts, and for controlling the fiber drawing process, also unaffected by occasional dropouts.