Method for monitoring fiber tension by measuring fiber vibration frequency

A method for monitoring the tension in a moving fiber comprising the steps of:

• heating an optical fiber preform in a draw furnace,
• pulling an optical fiber from said preform,
• passing said fiber through coating means where a coating is applied to said optical fiber, and
• sensing the motion of said fiber in a direction transverse to the direction in which the fiber is moving, said motion being sensed at a location between said preform and said coating means;
• analyzing the sensed motion to determine a plurality of frequency components thereof, each frequency component having a magnitude, and selecting one of the frequency components, f.sub.m1, as constituting the maximum magnitude component;
• doubling the frequency of the maximum magnitude component to obtain a doubled frequency, 2(f.sub.m1);
• ascertaining the presence of a frequency component near said doubled frequency, 2(f.sub.m1), to verify that the maximum magnitude component is the principal harmonic frequency of motion of the fiber;
• determining the tension in said fiber from the verified principal harmonic frequency component;
• and controlling the temperature of said draw furnace in response to the ascertained principal harmonic frequency of motion of said fiber.