Methods and apparatus for determining geometric properties of optical fiber preforms

A method comprising:

• providing an optical fiber preform having a longitudinal axis, an outer diameter, a circumference, and comprising at least two cylindrical layers of glass having different indices of refraction;
• providing a two-dimensional pattern having a length parallel to the longitudinal axis of the optical fiber preform and a width greater than the outer diameter of the optical fiber preform, wherein the two-dimensional pattern comprises alternating dark and bright straight lines, arranged periodically, and positioned at a non-perpendicular angle relative to the longitudinal axis of the optical fiber preform, wherein the two-dimensional pattern is illuminated, wherein the alternating dark and bright lines each have a thickness;
• providing a digital camera disposed such that the optical fiber preform is aligned between the two-dimensional pattern and the digital camera, wherein the thickness of each line of the two-dimensional pattern is at least equal to a width of two pixels of the digital camera, the alternating dark and bright straight lines extending across and beyond the outer diameter of the optical fiber preform in each direction when viewed from the digital camera;
• rotating the optical fiber preform about its longitudinal axis and acquiring a first plurality of images of the two-dimensional pattern viewed through the optical fiber preform at at least ten different points along the circumference of the optical fiber preform;
• moving the optical fiber preform in the direction of its longitudinal axis, and rotating the optical fiber preform about its longitudinal axis and acquiring a second plurality of images of the two-dimensional pattern viewed through the optical fiber preform at at least ten different points along the circumference of the optical fiber preform at a different location along its length than the first plurality of images;
• and
• determining the diameter, ovality and eccentricity of each of the at least two cylindrical layers of the optical fiber preform from the first plurality and second plurality of images, wherein the determination comprises evaluating perturbations of the two-dimensional pattern in the first plurality of images and second plurality of images to determine relative spatial position of an edge between the at least two cylindrical layers at two different positions along the longitudinal axis of the optical fiber preform;
• and wherein evaluating the perturbations of the two-dimensional pattern in each of the first plurality of images and the second plurality of images to determine the relative spatial position of the edge comprises an analysis of image pixels comprising image erosion noise reduction, standard deviation filtering, image projection averaging and magnification compensation.

An apparatus for determining a geometric property of an optical fiber preform, the optical fiber preform having a longitudinal axis and an outer diameter, the optical fiber preform comprising at least two cylindrical layers of glass having different indices of refraction, the apparatus comprising:

• an image capturing device;
• a two-dimensional pattern having a length parallel to the longitudinal axis of the optical fiber preform and a width greater than the outer diameter of the optical fiber preform, the two-dimensional pattern comprising a plurality of straight lines;
• a support configured to position the optical fiber preform in alignment between the image capturing device and the two-dimensional pattern, such that the two dimensional pattern extends length-wise in the direction of the longitudinal axis of the optical fiber preform and the plurality of straight lines extend across and beyond the outer diameter of the optical fiber preform in each direction when viewed from the image capturing device;
• and
• a driver configured to rotate the optical fiber preform about its longitudinal axis in the support.