OPtO-MECHANICAL METHOD FOR SUB-MICRON MEASUREMENT OF FERRULE ROUNDNESS AND CONCENTRICITY
Townley-Smith, Paul A.
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The insertion loss of fiber-optic components is one of the most important parameters that determine the performance of fiber-optic transmission systems. Some of the most widely used components in fiber optics systems are fiber optic connectors. To achieve a low loss connection between two optical fibers, precise and accurate alignment of their cores is essential. To achieve good quality alignment in a typical connector, high accuracy and precision must be maintained with respect to all connector ferrule geometrical parameters such as concentricity error, outside diameter, micro-hole diameter and end radius. Having an accurate measurement method for these parameters is a fundamental need for the connector manufacturing industry to asses and control their manufacturing processes. Up to now, the industry was unable to directly measure ferrule concentricity because all current standard test methods measure a combination of ferrule outside diameter roundness error and concentricity error. However, it is important to distinguish between concentricity error and roundness error because the ferrules are inserted into a ceramic sleeve that mostly accommodates the roundness error of the ferrule. At the same time, ferrule roundness error is an important parameter of the ferrule and it should not be ignored. A clear separation between the measurement of ferrule concentricity and roundness error would improve the concentricity measurement accuracy, resulting in better process controls and a subsequent reduction of connector insertion loss in the field. The new method proposed in this thesis, allows the concentricity error to be separated from the roundness error. The mechanical configuration of the method is identical to the standard surface reference method but more sophisticated analysis is employed. In the proposed method, the concentricity and roundness errors are calculated using a discrete Fourier transform (DFT). Because the frequency components generated by the DFT are orthogonal, the roundness and concentricity errors are also orthogonal when calculated by this method, and thus are separate. The proposed method will yield a more accurate measurement of installed concentricity while providing an accurate roundness error measurement at the same time. This method will not only improve the industry's measurement capability but it will also serve as an invaluable tool for process control. Subsequently, the insertion loss of fiber optic connectors will improve. Thus the ultimate goal of the work described in this thesis. is to reduce the loss in optical fiber systems.