Optical loss at connectors can originate from dust, dirt and improper polish. When using OMPlex devices the centre of the core is excited with the fundamental mode such that dust and dirt on the surface of the connector affects the performance in a similar way to standard single-mode fibre links of over 25 Gbps.
Figure 1: Examples of a clean (left) and dirty (right) connector.
If a connector is clean and the fibre core is free of any damages, the main source of optical loss is from a misalignment of the two fibre cores. The misalignment can be a transverse offset of the fibre cores as well as an angular offset. These sources of misalignment result in the unwanted excitation of higher order modes when used with OMPlex, manifesting itself in the occurrence of modal dispersion. Most common are transverse offsets that are introduced by the tolerance inherent to the fibre geometry, i.e., cladding diameter and core to cladding concentricity as well as the tolerance in the concentricity of the ferrule of the fibre connector and bore diameter. Standard multimode fibre connectors a specified with a ferrule concentricity tolerance of ±4 µm. Multimode optical fibre such as OM2 (50 µm/125 µm) typically has a core to cladding concentricity ≤1.5 µm and a cladding diameter tolerance ±1 µm. In comparison, single-mode optical fibres exhibit much tighter tolerances for the fibre geometry as well as connectors ferrules. Table 1 and Table 2 summarize the typical geometrical tolerances of multimode and single-mode optical fibres as well as the corresponding connector ferrules.
Table 1: Fibre geometry tolerances for MMF and SMF.
| Multimode OM1 | Multimode OM2/3/4 | Single-Mode | |
| Core Diameter | 62.5±2.5 µm | 50±2.5 µm | 8.2 µm |
| Cladding Diameter | 125±2 µm | 125±1 µm | 125±0.7 µm |
| Core-Cladding Concentricity | ≤1.5 µm | ≤1.5 µm | ≤0.5 µm |
Table 2: Geometrical tolerances for ferrules used for multimode and single-mode connectors.
| Multimode | Single-Mode | |
| Bore Diameter | 127 µm | 126 µm |
| Bore Tolerance | +5/-0 µm | +1/-0 µm |
| Concentricity | ±4 µm | ±1 µm |
| Outside diameter (FC, SC, ST) | 2.498±0.002 mm | 2.499±0.0005 mm |
| Outside diameter (LC) | 1.249+0.0007/-0.0023 mm | 1.249±0.0007 mm |
By using single-mode grade ferrules for multimode optical fibre connectors, vastly improved performance in conjunction with Modular Photonics’ OMPlex modules can be achieved. OMPlex modules are factory equipped with low tolerance, low loss, high performance multimode connectors.