Quick answer: A fiber network sends light instead of current. A transmitter, usually a laser at 1310 or 1550 nm, launches a modulated beam into a glass core about 9 um wide in single mode fiber, where it stays guided by total internal reflection. Splitters divide the light passively, connectors join strands, and a photodiode at the far end turns the light back into an electrical signal. Because splitters and fiber only attenuate, the design is a loss budget: a typical access network allows about 28 dB between transmitter and receiver.
The absence of amplifiers is what makes fiber economics work. Copper needs a powered switch every 100 m; a single mode strand reaches 20 km and then serves 32 subscribers through a passive splitter in a street cabinet. Everything in that path is a fixed loss, measured in decibels, and the network either fits inside the budget or it does not.
Light in glass, not current in copper
Total internal reflection
The core has a slightly higher refractive index than the cladding around it. Light hitting the boundary at a shallow angle reflects back into the core rather than escaping, so it zigzags along the fiber with very little loss. Attenuation in single mode fiber at 1310 nm is about 0.35 dB per kilometre, and at 1550 nm about 0.2 dB, which is why long haul links use the longer wavelength.
Single mode and multimode
A 9 um core carries one propagation path, so pulses stay sharp over tens of kilometres. A 50 um multimode core carries hundreds of paths that arrive at slightly different times, which limits 10G links to about 300 m on OM3. Buildings use multimode with cheap 850 nm optics; access and metro networks use single mode.
One passive optical network, end to end
OLT, splitter, ONT
At the exchange an optical line terminal transmits downstream on 1490 nm and receives upstream on 1310 nm. A splitter in a cabinet divides that one strand into 32, and at each home an optical network terminal converts light back to Ethernet and copper pairs for the subscriber. Because every subscriber shares the same downstream wavelength, the OLT tags each frame and the ONT filters for its own identifier.
Where the losses sit
Fiber loss is the smallest item on a short access link. Connectors cost about 0.3 dB each, splices 0.1 dB, and the splitter dominates: a 1:32 splitter takes about 17 dB before any cable is counted. That is why access networks budget generously and why a dirty connector at the cabinet can take a whole estate offline.
The loss budget in numbers
| Element | Typical loss | Count in a 32-way PON | Total |
|---|
| 1:32 splitter | 17 dB | 1 | 17.0 dB |
| Connector pair | 0.3 dB | 4 | 1.2 dB |
| Splice | 0.1 dB | 6 | 0.6 dB |
| Fiber, 5 km at 1310 nm | 0.35 dB/km | 5 km | 1.8 dB |
| Margin | 3 dB | 1 | 3.0 dB |
The example totals 23.6 dB against a 28 dB class budget, leaving room for ageing and for one extra connector after a repair. Once the budget is used up, the options are a smaller splitter ratio, a shorter reach, or moving the splitter closer to the subscriber.
Where light becomes copper again
Inside a building the fiber usually stops at a media converter or a switch with an SFP port, and copper takes over for the last few metres. A fiber to RJ45 converter pair is the simplest way to cross a long distance between two copper switches, and it also breaks any earth potential difference between buildings.
Test the physical layer before blaming the optics. An optical power meter at the receiver tells you whether the light arriving is inside the receiver’s sensitivity window; a reading 5 dB low points to a dirty connector or a bend, not to a failed laser. The fiber network cable overview covers cable construction for indoor and outdoor runs, and the single mode patch cables page covers the cordage that connects the panels.
Practical scenarios
- Factory connecting two buildings 800 m apart: single mode between two media converters, one connector pair at each end, and a spare fiber in the duct for the day a digger finds the first one.
- Apartment block with 24 units: one 1:32 splitter in the basement, drop cables to each unit, and a documented loss budget so a future 1:64 upgrade is a known trade-off.
- Data centre spine: multimode OM4 for 100G inside the hall, single mode for anything leaving the building, and never mix the two in one link without a converter.
- Rural link beyond 20 km: plan for 1550 nm optics and a larger budget, and test the span with an OTDR before accepting the route from the contractor.
FAQ
Does fiber carry power?
No. Glass carries light only, so every device at the far end needs its own supply or a separate copper pair. Power over Ethernet cannot run on fiber, which is why fiber-to-copper converters usually have their own power input at the remote end.
Why is a splitter loss so high?
Splitting one beam into 32 paths divides the optical power by 32, which is 15 dB, plus a small excess loss in the waveguide, giving about 17 dB. The loss is fixed and passive, and it is the reason access networks are designed around a 28 dB budget rather than around distance alone.
Can I join two fiber cables myself?
With a fusion splicer, yes, and a good splice loses about 0.1 dB. Mechanical splices and field-installable connectors are acceptable for repairs at slightly higher loss. What is not acceptable is a bare butt joint or tape, because the air gap reflects light and the link becomes unstable with temperature.
