Fiber technology uses light to transmit data – and is the fastest transmission solution for long distance lines. A fiber contains a glass core through which light travels, surrounded by another layer of glass ‘cladding’ and is protected by aramid yarns to keep environmental elements out. Connectors are used to get the light into and out of the fiber, providing flexible interconnection to transmission and receiver equipment, interconnection to passive optical devices, and cross-connect functions between different fibers from other cables.
The current generation of connectors is based on the physical contact between two well-polished fibers in a ferrule – maximizing the transmission of optical power while minimizing optical reflections. Fibers within a ferrule are aligned using a slitted alignment sleeve in the adaptor and all connectors have a keying mechanism to prevent the connector ferrule from rotating along its longitudinal axis inside the adapter sleeve. There are four main types of connectors: Ferrule connectors, Subscriber connectors, Multi-Fiber Push-On Connectors, and Local Connectors.
Ferrule-Based Connectors (FC) are commonly used and will undergo a sophisticated polishing process when terminated to optical fibers. This takes place in the factory, where the optical performance (attenuation and return loss) is checked. The FC is a “screw-type” connector with a nickel-plated or stainless steel body, making it a practical choice in vibration-heavy environments.
Subscriber Connectors (SC) have a polymer square body. Equipped with a spring-loaded ceramic ferrule and push-pull coupling mechanism, these were originally used for Gigabit Ethernet. As its cost dropped, it became more popular and was long considered the standard connector.
Multi-Fiber Push-On (MPO) Connectors are increasingly being used in data centers with the rise of 40/100 Gigabit Ethernet. Rectangular ferrules with 12 and 24 fibers allow multi-fiber connectivity, with up to 12 to 24 times the density of the standard single fiber connector. It supports higher bandwidths per connected cable and saves space and cost. MPO cable assembly often terminates in several smaller cables with a ‘fan out’, making it possible to change the order of the individual fibers by hand after a connection has been made.
Local Connectors (LC) have largely replaced the SC as the standard connector, although the SC is still widely used at the locations of FTTH networks. It is a push-pull type like the SC nut with a small form factor – but has a different connection mechanism and a smaller diameter ferrule. Its small size makes it practical for current transmission equipment, which features large numbers of connections in a limited space.
The strength of an optical signal is always higher at its point of origin than at the receiving end of a line as the light always suffers some degradation over the length of the network connection – therefore, fiber cables need to connect seamlessly to other devices and cables in a network. Return loss occurs whenever two fibers are joined and dirt or scratches result in portions of light becoming diffused or reflected back to the source. Attenuation or insertion loss occurs at the point of connection. Measuring the power of the light in the fiber core and after the point of connection usually shows a value of 0.1 dB to 0.5 dB. The lower the signal loss, the smaller the value in dB.
Fibers are often ‘fusion spliced’ which means that they are connected directly together by melting the glass (which will introduce some optical loss). These connections are permanent which means that connectors are used instead of fusing the cable together if connections need to be changed in the future. The fiber cable is fitted with a connector plug, which goes into an adapter or socket on an active device to establish a connection. The core of an optical fiber is far smaller than a speck of dust so enormous precision is necessary to align the fibers accurately. Transported optical information can ‘leak’ if the light is lost as it transfers from one fiber to the other.
Fiber misalignment, an air gap between fibers, unclean or damaged fiber ends or mismatched core sizes can all cause light loss. To minimize this loss, it’s best to avoid tight bends, small coils and any pulling or stretching of the fiber. Connectors must be clean, and only the correct cleaning products and inspection tools should be used. Furthermore, it’s best to limit the number of connections and splices in the network while ensuring that these have very low connection losses.
Calculating the link budget
The link loss budget can be thought of as the ‘worst case scenario’ to a data transmission path. This is used during cabling design to predict the amount of light needed to guarantee an uninterrupted communications link. The total link budget is calculated when multiplexing and demultiplexing losses, fiber losses, splice losses and patch panel and connector losses are subtracted from the transceiver power budget. Fiber connectors play a crucial role in minimizing loss. The link loss budget is essential both before and after installation and it must be tested to ensure that a link will operate as intended.
You can learn more about FTTX considerations here.