Method of stretching an optical fiber preform with monitoring the diameter at two locations

A process for manufacturing an optical fiber preform, comprising:

• providing a heating furnace;
• providing a synthetic quartz ingot in the heating furnace;
• providing a variable-speed means for drawing the optical fiber preform from the synthetic quartz ingot in the heating furnace;
• heating the synthetic quartz ingot in the heating furnace to soften it;
• and
• drawing the optical fiber preform in the shape of a rod having a substantially constant, preset diameter using the variable-speed means for drawing,
• wherein the variable-speed means for drawing is controlled by a process comprising:
• measuring a diameter of the synthetic quartz ingot at a midway point along a decreasing diameter of the ingot caused by the drawing,
• comparing the measured diameter of the synthetic quartz ingot at the midway point with a value a, and
• adjusting the speed of the variable-speed means for drawing to match the diameter of the synthetic quartz ingot at the midway point with the value a,
• wherein the value a is initially preset, and thereafter adjusted by the process comprising:
• measuring a diameter of the synthetic quartz ingot at a point at which the ingot is almost completely drawn;
• comparing the measured diameter of the synthetic quartz ingot at the point at which the ingot is almost completely drawn with a preset value b;
• and adjusting the value a to bring the measured diameter of the synthetic quartz ingot at the point at which the ingot is almost completely drawn closer to preset value b;
• wherein a time between measuring the diameter of the synthetic quartz ingot at a point at which the ingot is almost completely drawn and adjusting the value a based on that measured diameter is controlled to be not less than the distance between the midway point and the point at which the ingot is almost completely drawn divided by an average drawing speed.