Process of making multi-core fiber preform by integrating core rods and cladding cylinder

A process for manufacturing a multicore optical fiber preform having a center longitudinal axis, a plurality of core rods each being positioned in a respective peripheral core hole and extending along the longitudinal axis, and a common cladding covering each of the plurality of core rods, the process comprising the steps of:

• providing a cylinder which will form the cladding of the preform, the cylinder having a center core hole;
• creating a plurality of peripheral core holes in the cylinder extending along the longitudinal axis;
• inserting each of a plurality of core rods into a respective peripheral core hole of the cylinder and a center core rod into the center core hole;
• heating the cylinder with the plurality of core rods inserted in the respective peripheral core holes by exposing the cylinder and core rods to a heating element, thereby integrating the plurality of core rods and the cylinder and forming the preform;
• and
• before the step of creating the plurality of peripheral core holes, a step of defining positions of the plurality of peripheral core holes in the cylinder by considering a positional variation of the plurality of peripheral core holes before and after the step of heating, and wherein the step of creating the plurality of peripheral core holes includes creating the plurality of peripheral core holes at the defined positions,
• wherein a position error of the plurality of peripheral core holes with respect to a diameter of the preform is no larger than 0.6%,
• wherein the step of heating the cylinder is performed as part of an upward draw process in which the cylinder is collapsed onto the core rods in the peripheral core holes, the upward draw process including:
• supporting from below both the cylinder and the core rods so that the weight of the cylinder and the core rods is completely supported from below and the core rods do not move longitudinally relative to the cylinder as the cylinder collapses onto the core rods, and
• moving the cylinder and the core rods upward with respect to the heating element so the cylinder is continuously collapsed onto the core rods while the cylinder and core rods move upward,
• wherein a differential flow of the cylinder and the core rods along the longitudinal axis due to gravity is minimized,
• wherein the upward draw process includes simultaneously stretching the cylinder and core rods while collapsing the cylinder onto the core rods, and
• wherein the step of defining positions of the plurality of peripheral core holes in the cylinder by considering a positional variation comprises:
• a structure parameter after the heating step, β is defined as a final diameter of the preform,
• structure parameters before the heating step, a is defined as an original diameter of the cylinder, r is defined as an original radius of one of the plurality of core rods, R is defined as an original radius of one of the plurality of peripheral core holes into which the respective one of the core rods is to be inserted, and D is defined as a distance between a center of the respective one of the peripheral core holes and the central longitudinal axis of the cylinder, and
• the defined positions are defined such that the distance (d″) from each peripheral core hole center to a center of the preform after collapsing and stretching satisfies the following relation:
• d″≥(β/α)×[D−(R−r)×(1+R/D)].