Sometimes the simple things get ignored, or lost, or forgotten. With all the technical questions we get at the Fiber Optics Association, we are often reminded that these details are sometimes very important and are worthy of bringing to everyone’s attention again.
One example of this is the fiber optic connector. Almost everyone is aware of the four most common connectors—the standard connector (SC), lucent connector (LC), straight tip (ST) and multifiber push-on (MPO)—and the different ways they can be terminated. Many know the SC and ST share a common ferrule design, which is made from ceramic and is 2.5 mm in diameter. LCs also have ceramic ferules, but are only 1.25 mm in diameter. MPOs are the multifiber connectors—plastic plugs with 12, 16, 24 or 32 fibers.
Beyond this, the devil is in the details. We often get questions on why some connectors are beige, black, aqua, lime green, blue or green. Many times, I’ve been asked by users who figured the colors were just for identification if the connections between different colors are bad.
Some people didn’t understand that fiber optic connectors are color-coded to indicate the fiber types and connector specifications. Beige connectors are multimode for OM1 (62.5/125 micron fiber), black is OM2 (50/125), aqua is OM3 or OM4 (laser-optimized 50/125) and lime green is OM5 (50/125 for wavelength-division multiplexing). Blue connectors are single-mode with a physical contact (PC) ferrule polish and green connectors are single-mode with angled physical contact (APC) ferrules.
Multimode connectors help identify the types of fibers used in premises cabling and warn you not to connect beige connectors with 62.5/125 fiber to other multimode connectors with 50/125 fiber, which can cause up to 4 decibels (dB) of loss in one direction (62.5–50 micron fiber cores). For OM2, OM3, OM4 and OM5 fibers, it generally just keeps techs from mixing up patch cords.
The difference between blue and green single-mode connectors is more serious and leads to a discussion of connector ferrules. Early fiber optic connectors had ferrules of metal or plastic that were designed to have a minute air gap between them when they were mated. The gap was to ensure the connectors would not scuff the mated ferrules as they rotated or vibrated. That air gap, however, created reflections that caused loss and limited these connectors to losses greater than about 0.3 dB. The air gap reflections could also affect laser system performance at high speeds.
With the advent of keyed connectors that did not rotate when mated and ceramic ferrules that were hard enough to prevent scuffing, a new type of connector called PC became the norm. The end of the ceramic ferrule was molded into a convex surface, a dome with the fiber end at the center. When two ferrules were mated, the fibers actually contacted. By eliminating the air gap, these connectors had significantly lower mated loss and less reflectance. Typical connection losses dropped to 0.1 to 0.3 dB and reflection decreased to about –40 to –50 dB. In addition, the ceramic ferrules were easier to terminate and more reliable.
The ST, SC and PC connectors using these 2.5-mm ceramic ferrules became the most popular connectors. The ferrules were also adopted into several duplex connectors, including the ones designed specifically for two networks, fiber distributed data interface and enterprise systems connection. Since all these connectors used the same ferrule, mating adapters allowed connections between any of them, which is a convenience for patch panels and test equipment.
As technology often does, even these connectors needed improvement to keep up with networks. Reflectance became a greater problem as speeds increased, lasers became more powerful and fiber amplifiers were used for very long links. This led to another development in ceramic ferrules: the APC connector. By angling the convex end of the single-mode connector ferrule at an 8-degree angle, any reflected light was absorbed in the cladding of the fiber instead of being transmitted back up the fiber core. Reflectance dropped to more than –60 dB, almost unmeasurable.
However, the regular PC connectors and the APC connectors were not compatible; they could only mate to a similar connector to prevent poor connections and potential damage to the ferrule. To prevent mismatch, regular PC single-mode connectors were standardized to blue and APC connectors became green.
Techs in the know carry single-mode patch cords with a PC connector on one end and an APC on the other to ensure they can match the connectors that they find in the field. But at the Fiber Optic Association, we still get questions regularly about whether the different connectors can be mated or even the more basic “Why do these connectors come in blue and green?”