Connectors are used to interconnect fiber sections. Fiber optic cable is delivered on reels of from 1 to 25 km long. For longer networks these fiber reels have to be interconnected. Either fiber connectors or splices can be used for this purpose.
But common sense says that we should use connectors at the ends of a link and splices for the middle sections. Splice is a more permanent solution and connectors are used where we need to have many connecting/disconnecting such as fiber optic patch panels or cross-connects.
Fiber optic connectors may be field-installable or factory installed.
The factory-installed connector is sometimes called the pigtail connector. A pigtail is a short length of fiber that is un-terminated on one end, and a factory installed connector on the other end as shown in the left picture.
The right side picture shows a field-installable connector with all its components separated apart.
The basic connector consists of three parts:
- Ferrule (there are 2.5mm diameter ferrules and 1.25mm diameter ferrules)
- Coupling receptacle
- Coupling nut
The connector has a shell that carry a concentric ceramic ferrule with a precision concentric bore. The bare fiber is inserted into the ferrule and retailed with a resin or hot-melt adhesive. The fiber end is then cleaved and polished to a flush, flat mirror finish.
The shells align and butt the ceramic ferrule under gentle pressure. For single mode fibers, alignment accuracies must be greater than 0.1µm and less than 5° angular misalignment. You need to check the insertion loss of the installed connector before putting it into service. You should also check its return loss which should be 40 dB or better.
All fiber connectors require two important steps.
First, the fiber is epoxied into the connector. The epoxy process is important to the long-term reliability of the connector. The epoxy keeps fiber movement over temperature at a minimum, allows polishing without fear of fracturing the fiber, and seals the fiber from effects of the environment. It also allows the fiber to be aggressively cleaned on the endface.
It is very important that the epoxy be present around the entire length of the bare fiber, around the buffer as the fiber enters the connector, and as a bead surrounding the fiber on the endface.
Second, the connector must be polished. A physical contact (PC) finish is done which means the fiber will be physically touching inside the connector adapter as they are held under compression. If it is not physically contacted, this will result in an airgap between the fibers and increased attenuation. This is shown on the bottom picture.
There are several polishing methods recommended, which are typically dependent on the ferrule material used. If a ferrule material is very hard – ceramic, for example – the ferrule is pre-radiused on the endface. Softer ferrule materials such as composite thermoplastic or glass-in-ceramic may be polished flat. These materials wear away at approximately the same rate as the fiber and can be polished aggressively and still maintain a PC finish.
Return reflection can be lowered even further with angle PC (APC) polish. APC polish has an 8° angle, this angle reflects the light into the cladding rather than back down the fiber core. Connector return loss should be at least 40 dB.
Another performance parameter is the number of matings. This refers to the number of times a connector can be mated (i.e. connected-disconnected) before its performance starts to deteriorate. The number of matings for connectors can vary from 200 times to 600 times.
ST connectors use a quick-release bayonet coupling, which requires only a quarter turn to engage or disengage. Built-in keying provides repeatable performance because the connector will always mate with a coupling bushing in the same way. The ST style connector is being superseded by the SC connector. The insertion loss of the ST-style connector is on the order of 0.5 dB.
The FC connector was originally developed in Japan by its national telephone company, Nippon Telegraph and Telephone Company. This connector is popular with single mode fiber. It has an insertion loss of 0.4 dB. The FC connector has a means of tunable keying such that the key can be adjusted to the point of lowest loss. Insertion loss can be reduced up to several tenths of a decibel by tuning. Once the lowest loss position is found, the key is locked into place. The connector is available in both single-mode and multimode versions.
The SC connector is a widely used connector for both single mode and multimode fibers. The term SC derives from “subscriber connector”. The term came from popular early applications in the subscriber plant. The SC connector is a general-purpose connector used in long-haul network applications and for premise cabling. It uses a push-pull mechanism for mating. The basic SC connector consists of a plug assembly containing a ferrule. These plugs mate in a connector housing that aligns the ferrules. One advantage of the SC connector is that several plugs can be joined together to form a multi-position connector. This multifiber capability is especially useful for building duplex (two-position) connectors. In this case, one fiber can carry information in one direction while the other fiber carries information in the other direction. The connectors are keyed to prevent mismating. The insertion loss of the SC connector is under 0.4 dB.
The FDDI (fiber distributed data interface) connector is a two-channel device using two ceramic ferrules and a side-latch mechanism. A rigid shroud protects the ferrules from accidental damage, while a floating interface ensures consistent mating without stubbing. Different keying arrangements are provided to key the connector to different FDDI requirements. Expect an insertion loss of around 0.3 dB for the single-mode application and 0.5 dB for the multimode application. FDDI connectors are not restricted to FDDI applications. FDDI is a 100 Mbps (125-megabaud) local area network arrangement governed by an ANSI standard.
D4 style connector is a popular connector used with single-mode fiber. It is similar in most respects to the FC connector but has a smaller ferrule, only 2.0mm in diameter. The insertion loss is about 0.4 dB.
LC connector is a miniature connector, also called small form-factor connector. It is about half the size of a standard connector counterparts (such as SC, FC, ST). LC connector has a 1.25mm diameter ferrule rather than a 2.5mm ferrule as with FC, SC, and ST. It allows denser cable packing typically at patch panels and dense rack mounting installations.
MT-RJ is a fiber-optic Cable Connector that is very popular for small form factor devices due to its small size. Housing two fibers and mating together with locating pins on the plug, the MT-RJ comes from the MT connector, which can contain up to 12 fibers. The MT-RJ is one of the newly emerging small form factor connectors that are becoming more common in the networking industry. The MT-RJ utilizes two fibers and integrates them into a single design that looks similar to a RJ45 connector. Alignment is completed through the use of two pins that mate with the connector. Transceiver jacks found on NICs and equipment typically have the pins built into them. The MT-RJ is commonly used for networking applications. Its size is slightly smaller than a standard phone jack and just as easy to connect and disconnect. It's half the size of the SC Connector it was designed to replace. The MT-RJ connector is a small form-factor Fiber optic connector which resembles the RJ-45 connector used in Ethernet networks. Compared to single-fiber terminations such as SC, the MT-RJ Connector offers lower Termination cost and greater density for both electronics and cable management hardware. The MT-RJ Connector is significantly lower in cost and smaller in size than the SC Duplex interface. The small MT-RJ Interface can be spaced the same as copper, effectively doubling the number of fiber ports. The net effect is a Drop in the overall price per fiber Port making fiber-to-the-desktop solutions more competitive with copper.