2. Shanghai Cable Research Institute, Shanghai 200093, China

**Overview**: The fiber geometry of communication fibers and medical fibers are always important parameters to evaluate the quality of optical fibers. The fiber geometry mainly includes the diameter and ovality of claddings and cores, and the concentricity error of claddings and cores. The measurement of fiber geometry with gray scale method is one of the commonly used measurement methods proposed in the national standard GB15972.20-2008. In the gray scale method, the fiber geometry is obtained by two-step fittings that are the fitting of circular curve and the fitting of ellipse curve. However, the geometric centers of the two fitting curves may not necessarily coincide, causing the measurement inaccuracy of fiber geometry. Obviously, there is a defect of the measurement principle in the method. The measurement of fiber geometry with gray scale method has a high requirement for cutting effects and lighting conditions. When measurement conditions change, it often leads to the instability of the measured data and brings errors. This paper proposes a method for obtaining the optical fiber geometry with fitting the edge of optical fiber in an arbitrary ellipse curve (non-standard ellipse) which more coincide the real optical fiber end face.

The method is mainly divided into three steps: image preprocessing, edge extraction and ellipse curves fitting. The first step, image preprocessing, is to eliminate some of the noise errors in the image by median filtering the image, in order to make the subsequent edge extraction better. The second step, edge extraction, is to use the Canny operator to extract the image of the optical fiber end face. At this time, there are still some noise signals and false edges at the edge of the optical fiber. The third step, ellipse curves fitting, is to fit the edge data points with the arbitrary ellipse, and to set an appropriate threshold value at the edge of the fitting ellipse curve, then is to remove the data points beyond the threshold value as error data points. All optical fiber geometry can be calculated by arbitrary ellipse curve in one-step fitting, so eliminating the measurement error caused by the center inconsistency between the circle curve fitting and the ellipse curve fitting. At the same time, because the specific value of the image distribution gray scale is not required when calculating each parameter, the edge data of the optical fiber is used for fitting, thereby the measurement condition is effectively reduced. Taking fiber core data as an example, the data of diameter and ovality measured by the standard instrument are 8.420 μm and 0.670%, respectively. When cutting effect of fiber end face or lighting condition is poor, the instrument data become 9.436 μm and 2.016%, while the data measured in this paper are 8.804 μm and 0.553%, respectively. Experiment results show that the method can significantly improve the accuracy and precision of the measurement results of instruments.