Optical fiber geometry The fiber geometry includes core diameter, cladding diameter, core/clad concentricity, and out-of-roundness. 1. Core Diameter The core diameter is primarily a requirement for multimode fibers. According to ITU-T, the core diameter of a multimode fiber is 50±3μm. 2. Cladding Diameter The cladding diameter refers to the outer diameter of the optical fiber. ITU-T stipulates that the cladding diameter of multimode and single-mode optical fibers should be 125±3μm. At present, optical fiber manufacturers have increased the fiber outer diameter specification from 125.0±3μm to 125.0±1μm. 3. Core/clad concentricity and out-of-roundness Core/clad concentricity refers to the degree to which the core is centered within the fiber. Optical fiber manufacturers have now increased the core/cladding concentricity from a specification of ≤0.8 μm to a specification of ≤0.5 μm. Out-of-roundness includes roundness of the core diameter and out-of-roundness of the cladding. According to ITU-T, the roundness of the core diameter is ≤ 6% and the roundness of the cladding (including single mode) is less than 2%.