Practical Industrial Data Networks:Design, Installation and ...

6.6.3 ConnectorsFiber optics overview 83precise alignment. Fusion splices have consistently very low losses and arethe preferred method for joining fibers, particularly for single-mode systems.Modern single-mode fusion splicers utilize core alignment systems to ensurethe cores of the two fibers are precisely aligned before splicing.• Mechanical splicingMechanical splicing involves many different approaches for bringing the twoends of the fibers into alignment and then clamping them within a jointingstructure or gluing them together. Mechanical splices generally rely onaligning the outer diameters of the fiber cladding and assumes that the coresare concentric with the outside of the cladding. This is not the case always,particularly with monomode fibers. Various mechanical structures are usedto align the fibers, including V-grooves, sleeves, 3-rods and variousproprietary clamping structures.Connectors are used to make flexible interconnections between optical devices.Connectors have significantly greater losses than splices since it is much more difficult torepeatedly align the fibers with the required degree of precision. Active alignment, as wasused to minimize some splice losses, is not possible. Axial misalignment of the fiberscontributes most of the losses at any connection, consequently connector loss can beexpected to be in the range from 0.2 to over 3 dB.Most of the connector designs produce a butt joint with the fiber ends as close togetheras possible. The fiber is mounted in a ferrule with a central hole sized to closely matchthe diameter of the fiber cladding. The ferrule is typically made of metal or ceramic andits purpose is to center and align the fiber as well as to provide mechanical protection tothe end of the fiber. The fiber is normally glued into the ferrule then the end is cut andpolished to be flush with the face of the ferrule. The two most common connectors are theSC and ST, as detailed below. Although, many new proprietary connectors are nowavailable for different types of equipment.• SC connectorThis is built with a cylindrical ceramic ferrule, which mates with a couplingreceptacle. The connector has a square cross-section for high packing densityon equipment and has a push–pull latching mechanism. The ISO and TIAhave adopted a polarized duplex version as standard and this is now beingused as a low-cost FDDI connector. The SC connector has a specified loss ofless than 0.6 dB (typically 0.3 dB) for both monomode and multimode fibersand a typical return loss of 45 dB.• ST connectorThe ST connector is shown in Figure 6.5. This is an older standard used fordata communications. This is also built with a cylindrical ceramic ferrulewhich mates with a coupling receptacle. The connector has a round crosssectionand is secured by twisting to engage it in the spring-loaded bayonetcoupling. Since it relies on the internal spring to hold the ferrules together,optical contact can be lost if a force greater than that of about one kilogramis applied to the connector.