Fiber Optic Connectors
We'll start our section on termination by considering connectors. Since fiber optic technology was introduced in the late 70s, numerous connector styles have been developed. Each new design was meant to offer better performance (less light loss and back reflection), easier and/or termination and lower cost. Of course, the marketplace determines which connectors are ultimately successful.
Fiber Optic Connectors and Splice Loss Mechanisms
Connector and splice loss is caused by a number of factors. Loss is minimized when the two fiber cores are identical and perfectly aligned, the connectors or splices are properly finished and no dirt is present. Only the light that is coupled into the receiving fiber's core will propagate, so all the rest of the light becomes the connector or splice loss.
End gaps cause two problems, insertion loss and return loss. The emerging cone of light from the connector will spill over the core of the receiving fiber and be lost. In addition, the air gap between the fibers causes a reflection when the light encounters the change n refractive index from the glass fiber to the air in the gap. This reflection (called fresnel reflection) amounts to about 5% in typical flat polished connectors, and means that no connector with an air gap can have less than 0.3 dB loss. This reflection is also referred to as back reflection or optical return loss, which can be a problem in laser based systems. Connectors use a number of polishing techniques to insure physical contact of the fiber ends to minimize back reflection. On mechanical splices, it is possible to reduce back reflection by using non-perpendicular cleaves, which cause back reflections to be absorbed in the cladding of the fiber.
The end finish of the fiber must be properly polished to minimize loss. A rough surface will scatter light and dirt can scatter and absorb light. Since the optical fiber is so small, typical airborne dirt can be a major source of loss. Whenever connectors are not terminated, they should be covered to protect the end of the ferrule from dirt. One should never touch the end of the ferrule, since the oils on one's skin causes the fiber to attract dirt. Before connection and testing, it is advisable to clean connectors with lint-free wipes moistened with isopropyl alcohol.
Two sources of loss are directional; numerical aperture (NA) and core diameter. Differences in these two will create connections that have different losses depending on the direction of light propagation. Light from a fiber with a larger NA will be more sensitive to angularity and end gap, so transmission from a fiber of larger NA to one of smaller NA will be higher loss than the reverse. Likewise, light from a larger fiber will have high loss coupled to a fiber of smaller diameter, while one can couple a small diameter fiber to a large diameter fiber with minimal loss, since it is much less sensitive to end gap or lateral offset.
These fiber mismatches occur for two reasons. The occasional need to interconnect two dissimilar fibers and production variances in fibers of the same nominal dimensions. With two multimode fibers in usage today and two others which have been used occasionally in the past and several types of singlemode fiber in use, it is possible to sometimes have to connect dissimilar fibers or use systems designed for one fiber on another. Some system manufacturers provide guidelines on using various fibers, some don't. If you connect a smaller fiber to a larger one, the coupling losses will be minimal, often only the fresnel loss (about 0.3 dB). But connecting larger fibers to smaller ones results in substantial losses, not only due to the smaller cores size, but also the smaller NA of most small core fibers.
Guide to Fiber Optic Connectors
Check out the "spotters guide" below and you will see the most common fiber optic connectors. (All the photos are to the same scale, so you can get an idea of the relative size of these connectors.)
ST (an AT&T Trademark) is the most popular connector for multimode networks, like most buildings and campuses. It has a bayonet mount and a long cylindrical ferrule to hold the fiber. Most ferrules are ceramic, but some are metal or plastic. And because they are spring-loaded, you have to make sure they are seated properly. If you have high loss, reconnect them to see if it makes a difference.
FC/PC has been one of the most popular singlemode connectors for many years. It screws on firmly, but make sure you have the key aligned in the slot properly before tightening. It's being replaced by SCs and LCs.
SC is a snap-in connector that is widely used in singlemode systems for it's excellent performance. It's a snap-in connector that latches with a simple push-pull motion. It is also available in a duplex configuration.
Besides the SC Duplex, you may occasionally see the FDDI and ESCON* duplex connectors which mate to their specific networks. They are generally used to connect to the equipment from a wall outlet, but the rest of the network will have ST or SC connectors. *ESCON is an IBM trademark
Below are some of the new Small Form Factor (SFF) connectors:
LC is a new connector that uses a 1.25 mm ferrule, half the size of the ST. Otherwise, it's a standard ceramic ferrule connector, easily terminated with any adhesive. Good performance, highly favored for singlemode.
MT-RJ is a duplex connector with both fibers in a single polymer ferrule. It uses pins for alignment and has male and female versions. Multimode only, field terminated only by prepolished/splice method.
Opti-Jack is a neat, rugged duplex connector cleverly designed aournd two ST-type ferrules in a package the size of a RJ-45. It has male and female (plug and jack) versions.
Volition is a slick, inexpensive duplex connector that uses no ferrule at all. It aligns fibers in a V-groove like a splice. Plug and jack versions, but field terminate jacks only.
E2000/LX-5 is like a LC but with a shutter over the end of the fiber.
MU looks a miniature SC with a 1.25 mm ferrule. It's more popular in Japan.
MT is a 12 fiber connector for ribbon cable. It's main use is for preterminated cable assemblies.
The ST/SC/FC/FDDI/ESON connectors have the same ferrule size - 2.5 mm or about 0.1 inch - so they can be mixed and matched to each other using hybrid mating adapters. This makes it convenient to test, since you can have a set of multimode reference test cables with ST connectors and adapt to all these connectors. Likewise, the LC, MU and E2000/LX-5 use the same ferrule but cross-mating adapters are not easy to find.
Connector Types
The ST is still the most popular multimode connector because it is cheap and easy to install. The SC connector was specified as a standard by the old EIA/TIA 568A specification, but its higher cost and difficulty of installation (until recently) has limited its popularity. However, newer SCs are much better in both cost and installation ease, so it has been growing in use. The duplex FDDI, ESCON and SC connectors are used for patchcords to equipment and can be mated to ST or SC connectors at wall outlets. Singlemode networks use FC or SC connectors in about the same proportion as ST and SC in multimode installations. There are some D4s out there too.
EIA/TIA 568 B allows any fiber optic connector as long as it has a FOCIS (Fiber Optic Connector Intermateability Standard) document behind it. This opened the way to the use of several new connectors, which we call the "Small Form Factor" (SFF) connectors, including AT&T LC, the MT-RJ, the Panduit "Opti-Jack," 3M's Volition, the E2000/LX-5 and MU. The LC has been particularly successful in the US.
Connector Ferrule Shapes & Polishes
Fiber optic connectors can have several different ferrule shapes or finishes, usually referred to as polishes. early connectors, because they did not have keyed ferrules and could rotate in mating adapters, always had an air gap between the connectors to prevent them rotating and grinding scratches into the ends of the fibers.
Beginning with the ST and FC which had keyed ferrules, the connectors were designed to contact tightly, what we now call physical contact (PC) connectors. Reducing the air gap reduced the loss and back reflection (very important to laser-based singlemode systems ), since light has a loss of about 5% (~0.25 dB) at each air gap and light is reflected back up the fiber. While air gap connectors usually had losses of 0.5 dB or more and return loss of 20 dB, PC connectors had typical losses of 0.3 dB and a return loss of 30 to 40 dB.
Soon thereafter, it was determined that making the connector ferrules convex would produce an even better connection. The convex ferrule guaranteed the fiber cores were in contact. Losses were under 0.3dB and return loss 40 dB or better. The final solution for singlemode systems extremely sensitive to reflections, like CATV or high bitrate telco links, was to angle the end of the ferrule 8 degrees to create what we call an APC or angled PC connector. Then any reflected light is at an angle that is absorbed in the cladding of the fiber.
Related Fiber Optic Components Links
•Types of Fiber Optic Cables
•Fiber Optic Cable Plant Link Loss Budget Analysis
•Fiber Optic Loss Budget Standard
•Troubleshooting Fiber Optic Transmitters & Receivers
•Fiber Optic Couplers & Splitters
•Fiber Optical Amplifiers