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Connector types
The ST, or Straight Tip connector, is one of the oldest types still in use. Designed for a single fibre strand, it has a metal body screwed onto a thread on the socket to secure the connection.
The SC, or Subscriber Connector, sometimes called "Stab and Click" for the way it pushes into the socket with a latch that clicks to hold it in place. Can be single or dual — two clipped together for a fibre pair, giving rise to the terms simplex and duplex connector.
Two varieties of duplex SC (centre and right) together with a simplex SC. Note the two rubber stoppers fitted to the centre one — important for plugs & sockets when not in use, to prevent dust ingress.
The LC, or Lucent Connector (after the company that invented it), sometimes called the "Little Connector," as it's smaller than an SC. Its small size allows more ports in a 19" wide panel. Shown here in duplex; simplex versions are also used.
Blue v Green, UPC v APC
Blue UPC SC plug & socket on a Huawei DSLAM.
Some connectors are green and some are blue. This indicates the amount of back reflectance they cause. When you're sending digital signals down fibre — turning the light on and off like Morse code — the difference isn't critical, as digital signals are less susceptible to signal degradation.
But some fibre optic systems are actually used to send analogue signals — the light doesn't pulse on or off, but varies continuously, as if on a dimmer switch. This is sometimes used to deliver cable TV over fibre.
Traditional cable TV systems send analogue electrical signals over metallic coax cables. Long term, the future is clearly to migrate that to digital signals over fibre, but some interim systems actually modulate a light wave to represent the analogue electrical signal as light. Where this is done, it's more sensitive to back reflection altering what's received.
Green connectors cause less back reflection and are suitable for both analogue and digital signals. Blue connectors cause more back reflection and are used for pure digital signals. As digital signals predominate, so do blue connectors.
Green ones are called APC connectors, which stands for Angled Physical Contact; blue ones are called UPC connectors, or Ultra Physical Contact.
Cables
Cables are classed as single mode or multimode. Single mode has a narrower core, which prevents the light spreading out as it travels, allowing signals to travel further with less degradation. Cables are referred to by their core size and cladding size — for example, 9/125 indicates a 9 micron core with 125 micron cladding.
Single mode cables are typically coloured with a yellow outer jacket; multimode are typically coloured with an orange outer jacket. Exterior-grade cables are typically black, but often have a coloured stripe along them to indicate they're fibre, not copper.
| Standard name | Type |
|---|---|
| OM1 | 62.5/125 multimode, 850nm wavelength (USA) |
| OM2 | 50/125 multimode, 850nm wavelength (rest of world) |
| OM3 | 50/125 multimode, laser optimised, 850nm wavelength (rest of world) |
| OM4 | 50/125 multimode, laser optimised, 850nm wavelength (rest of world) |
| OS1 | 9/125 singlemode for 1310nm wavelength, 1dB/km attenuation (1.6dB/mile) |
| OS2 | 9/125 singlemode for 1550nm wavelength, 0.4dB/km attenuation (0.64dB/mile) |
Like copper telecom cables, fibres in a multi-stranded cable are colour coded for identification. The colour scheme generally adopted is shown below, though some cables don't follow this pattern. It's also worth noting that patch cables containing only one or two strands typically just use white and yellow.
| Fibre No. | Colour |
|---|---|
| 1 | Blue (dark blue) |
| 2 | Orange |
| 3 | Green |
| 4 | Brown |
| 5 | Slate (grey) |
| 6 | White |
| 7 | Red |
| 8 | Black |
| 9 | Yellow |
| 10 | Rose (pink) |
| 11 | Aqua (light blue) |
External-grade loose tube buffered cable. Note cladding colours on strands, and cladding stripped on some ends.
Up to 12 fibres are placed inside one "loose tube" buffer. Multiple loose tubes can be placed inside the cable. The loose tube buffer contains petroleum jelly to protect the cladding of the fibres inside from getting nicked as they rub against each other or against the tube.