Issue link: https://www.e-digitaleditions.com/i/1327492
12 I ICT TODAY Cabling can comprise 30 to 60 percent of POL's installation costs. An initial investment, such as this, may act as a strong disincentive, especially for renovating and upgrading an existing network. Clearly, not being able to reuse the existing cabling infrastructure is a monumental obstacle to adopting a POL. Have there been technology advancements that now make it possible to implement the POL architecture without having to replace the MMF optical cabling infrastructure? The answer is yes! THE MODAL ADAPTING PASSIVE TECHNIQUE TO RESCUE LEGACY CABLING INFRASTRUCTURES One way to make MMF and SMF compatible and to overcome modal dispersion is to carry the information in just one mode over MMFs. By properly exciting a single MMF spatial mode, the coupling of the signal to different group modes and propagation speeds is negligible. Consequently, the spatial mode can then be used as an independent high-speed transmission channel with the same transmission properties as SMF. The solution lies with MPLC technology. Multi-plane light conversion is the best way to perform a specific light launching condition. This technology enables flexible and complete light shaping with very little loss through a succession of transverse phase profiles, which are similar to very complex optical lenses. By playing precisely with the phase of the light, it is possible to modify its field intensity profile and, thus, the spatial shape of the light. In practice, this technology shapes a laser beam (light from a transceiver) in such a way that each mode within the MMF can be independently and precisely excited. Because of this capability, MPLC technology is used to increase the transmission capacity of existing networks by performing perfect center launching on legacy MMFs. Legacy MMF cables that are upgraded with an MPLC perfect center launching modal adapter support 1 GbE to 100 GbE over very long link length. They are compatible with WDM technologies, which makes it possible The huge amount of MMF, comprising the existing campus backbone and distribution fiber links, is incom- patible with singlemode operation. Therefore, all MMF links in LANs are incompatible with GPON equipment. (Figure 5). FIGURE 5: Multimode fiber's incompatibility with POL. If singlemode and multimode fibers are directly connected through standard connectors or by splicing, the unique spatial mode from the SMF can divide several spatial modes of the MMF. This results in modal dispersion, limiting the reach for a given data rate and in multipath interferences, creating power fluctuations up to 12 dB. These phenomena strongly deteriorate the signal. For years, backbone/vertical fiber recabling was the only logical solution, and it was most likely a deterrent to POL adoption because of its costs and complexities. Redeploying the fiber cabling is frequently a difficult task. It depends upon the distances involved and the difficulties in pulling the optical fiber cables. An audit of the cabling infrastructure is often necessary in order to verify the availability of the cable duct as well as its condition. Depending on the project, civil engineering may be required to deploy new cable sheaths. For these reasons, the complexity and costs of redeploying new cables can be exorbitant. Following this approach may result in a complex installation with a significant adverse impact on the enterprise and its budget. That is why the impact of capital cabling expenditures can be more sensitive in existing infrastructure upgrades where old cables need to be removed before installing the new.