Apparatus for drawing an optical fiber and method for controlling feed speed of an optical fiber preform

A method of controlling a feed speed of an optical fiber preform, comprising the steps of:

• (a) storing data representing a drawing speed of an optical fiber at intervals of a sampling period;
• (b)checking a result as to whether the present drawing speed is in one of (i) a stable drawing speed range and (ii) an unstable drawing-speed range;
• (c) beginning an automatic control of a preform feed speed when the check result indicates that the perform speed is in the unstable drawing-speed range;
• (d) obtaining a recent drawing-speed change tendency within a predetermined period of time based on the stored drawing speed data;
• (e) obtaining an expected deviation of the drawing speed of a subsequent arbitrary time based on the recent drawing-speed change tendency;
• (f) obtaining a compensation value of the preform feed speed based on the expected value;
• (g) obtaining a modification value of the preform feed speed by modifying the compensation value so as to accelerate the drawing speed toward the stable drawing-speed range;
• and
• (h) adding or subtracting the modification value of the preform feed speed to or from a target speed.