The forward scattering of light by an optical fiber produces an interference fringe pattern, and the fringe period is inversely proportional to the fiber diameter. An electrooptic system has been developed to produce and detect this scattering pattern to provide an instrument which will measure fiber diameter during the drawing operation. The system measures the fiber diameter at a 1-kHz rate with a precision of 0.25 microm and an accuracy of +/-0.25 microm over a range of 50-150-microm diams. The instrument allows the fiber to move laterally in a 1-cm diam window maintaining the above accuracy. The system can be calibrated optically and does not need a standard fiber for this procedure. The instrument has been used for months without the need for recalibration. In addition to the digital diameter output, the system employs a microprocessor to compute mean and standard deviation values for various sample lengths and provides suitable signals for feedback control of fiber diameter.

The optical fiber drawing process is examined and a feedback control loop identified. The incremental dynamic response of each loop component is determined, and the sensitivity of loop response to system parameters is examined. The control loop is optimized, based upon a mean square error criterion with constraints imposed for periodic disturbances. An expression is derived for the effectiveness of the control loop with respect to sources of system disturbance and found to correlate well with experimental results.

A technique has been implemented to measure and control the eccentricity of lightguide fiber in transparent polymer coating materials. It is based upon a model which describes the characteristics of a forward‐scattered light pattern generated by transversely illuminating coated fiber with a laser beam. The model predicts the behavior of the principal characteristics of the pattern as a function of fiber eccentricity within the coating. The implementation automatically detects and controls the position of the dominant pattern feature to maintain an average fiber‐coating concentricity within 2 μm over multikilometer lengths of fiber.

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A ray trace model is developed which describes the intensity and phase characteristics of a forward-scattered pattern generated by transversely illuminating a silicone-coated fused-silica fiber with a laser beam. The dominant characteristic of this pattern is a region of interference fringes which result from the interference of rays traveling through the fibers and the coating and rays traveling through the coating only. The location of the fringe region and the fringe period depends on fiber and coating parameters and the fiber eccentricity within the coating. The parameter sensitivities are examined, and the model results compared to experimentally measured'characteristics of the light scattering pattern.