Fiber optics is the technology of transmitting data as pulses of light through thin strands of glass or plastic. In networking and network hardware, it refers to the optical cabling, connectors and transceivers that link switches, routers, servers and buildings at speeds from 1 Gbit/s to 400 Gbit/s and beyond.
Compared with copper cabling such as CAT6, optical fiber offers far greater reach, higher bandwidth and complete immunity to electromagnetic interference. It is the standard medium for data center uplinks, campus backbones and every long-distance network link.
How light travels through an optical fiber
An optical fiber consists of a core surrounded by cladding with a slightly lower refractive index. Light entering the core at a suitable angle is kept inside by total internal reflection at the boundary. A protective coating and jacket surround the glass, which itself is about 125 micrometers in diameter, close to a human hair.
A transmitter, usually a laser or LED inside a transceiver such as an SFP module, converts electrical signals to light. A photodiode at the other end converts the light back. Each direction normally uses its own fiber, so a typical link needs a duplex pair, although bidirectional optics can share one strand by using two wavelengths.
Signal quality is limited by two effects. Attenuation reduces the light power over distance, and dispersion spreads each pulse in time until neighboring pulses overlap.
One fiber can also carry many signals at once on different wavelengths. This technique, called wavelength division multiplexing, comes in a coarse form with up to 18 channels spaced 20 nm apart and a dense form with dozens of closely spaced channels. It lets operators multiply the capacity of existing cables by changing only the optics at each end, which is why installed fiber often outlives several generations of network equipment.
Single-mode and multimode fiber
The most important choice in fiber optics is between single-mode and multimode fiber. The core diameter decides how many light paths, or modes, can travel at once, and this in turn limits distance.
| Property | Multimode fiber | Single-mode fiber |
|---|---|---|
| Core diameter | 50 or 62.5 micrometers | About 9 micrometers |
| Typical wavelength | 850 nm | 1310 nm and 1550 nm |
| Light source | VCSEL lasers or LEDs | Narrow-line lasers |
| Reach at 10 Gbit/s | About 300 to 400 m on OM3 and OM4 | 10 km to 80 km and more |
| Transceiver cost | Lower | Higher, though the gap has narrowed |
| Typical jacket color | Aqua or violet | Yellow |
Multimode fiber suffers from modal dispersion: different light paths arrive at slightly different times. This is why its reach shrinks as speed rises. Single-mode fiber avoids the problem and is the safe choice for new backbones that must support future speeds.
Fiber categories, connectors and reach
Multimode fiber is graded in OM categories, and single-mode fiber in OS categories. The category defines bandwidth and therefore the reach of each Ethernet standard.
| Fiber category | 1 GbE | 10 GbE | 100 GbE |
|---|---|---|---|
| OM1 | About 275 m | About 33 m | Not supported |
| OM3 | About 1 km | About 300 m | About 70 m with SR4 |
| OM4 | About 1 km | About 400 m | About 100 m with SR4 |
| OS2 | 10 km and more | 10 km and more | 10 km and more with LR4 |
Connectors also differ. LC connectors dominate in switches and SFP modules, SC connectors remain common in older installations and fiber-to-the-home, and MPO connectors carry 8, 12 or more fibers for parallel optics at 40 and 100 Gbit/s. End faces are polished as UPC or as angled APC, marked green, and the two must never be mated.
Fiber optics compared with copper cabling
Fiber and copper complement each other in modern networks. Copper with RJ45 connectors wins at the desk and for PoE devices, while fiber wins on distance, speed and electrical isolation.
| Factor | Fiber optics | Copper twisted pair |
|---|---|---|
| Maximum reach | Hundreds of meters to many kilometers | 100 m per segment |
| Speeds in common use | 1 to 400 Gbit/s | 1 to 10 Gbit/s |
| Interference | Immune to electromagnetic noise | Sensitive to noise and crosstalk |
| Power delivery | None | PoE up to about 90 W |
| Grounding issues between buildings | None, glass is non-conductive | Risk of ground loops and surges |
| Handling | Needs care with bending and cleaning | Robust and simple to terminate |
For short links inside one rack, direct attach copper cables with SFP+ ends are often cheaper than optics, while active optical cables bridge the gap for a few tens of meters.
Installation pitfalls and link budget
A fiber link works only if the received power stays within the receiver range. The optical link budget subtracts losses from fiber length, connectors and splices from the transmitter power. Typical planning values are about 0.3 to 0.5 dB per mated connector pair and about 0.35 dB per kilometer for single-mode fiber at 1310 nm.
- Dirty connectors are the most common cause of fiber faults; inspect and clean every end face before mating.
- Exceeding the bend radius, often about ten times the cable diameter, causes loss or permanent damage.
- Mixing fiber types, such as single-mode optics on multimode patch cords, leads to unstable or failing links.
- Too much power on short single-mode links can saturate a long-range receiver; an attenuator solves it.
- Never look into an active fiber, because infrared laser light is invisible and can damage the eye.
Most SFP modules report digital optical monitoring values, including transmit and receive power in dBm. Comparing these with the module specification is the fastest way to tell a weak link from a dead one.
In plain words
Fiber optics sends information as flashes of light inside a very thin glass thread. The glass is designed so the light bounces along inside it and cannot escape, even around gentle curves.
Light does not care about nearby power cables or lightning. It also fades very slowly in pure glass. That is why fiber can carry huge amounts of data across long distances, while ordinary network cables stop working after about one hundred meters.
A real-world example
A manufacturing company connects its office building to a production hall 350 meters away. Copper is not an option because of the distance and because heavy machinery creates strong electrical noise. The team also wants to avoid ground potential differences between the two buildings.
The installer pulls a 12-fiber OS2 single-mode cable with LC connectors on both ends. Two fibers carry a 10 Gbit/s link with LR SFP+ modules, and the remaining ten are reserved for future growth. The measured loss of the whole link is under 2 dB, far inside the budget of the optics. The result is a stable backbone that is immune to machine noise and ready for 25 or 100 Gbit/s later simply by changing transceivers.