Field Guide · concept

Also known as: optical fiber, fibre

Fiber-optic communication sends data as pulses of light through thin strands of glass, delivering very high bandwidth over long distances with low loss.1

laser det. core (high index) cladding (lower index) — reflects the ray back in total internal reflection guides the light for kilometres with little loss
Light launched into the fiber's dense glass core strikes the boundary with the lower-index cladding at a shallow angle and reflects entirely back inward — total internal reflection — so the pulse zig-zags down the strand for kilometres before a detector recovers it.

Overview

A fiber carries a modulated beam of light by total internal reflection: a dense glass core is wrapped in a lower-index cladding, and light striking that boundary at a shallow angle reflects entirely back inward rather than escaping. So the signal travels for kilometres with little attenuation and — being optical — is immune to the electromagnetic interference that plagues copper.

Single-mode fiber uses a tiny core that supports just one light path, giving the longest reach and highest rates for long-haul links; multimode fiber has a wider core that admits several paths at once, which is cheaper to couple into but limits distance because the paths spread the pulse. Fiber underpins long-distance internet backbones, data-center interconnects, and high-speed Ethernet, where pluggable transceivers (SFP modules) convert light to and from electrical signals at a switch.

Compared with coaxial cable, fiber offers vastly more capacity and reach and does not radiate or pick up noise, which is why carriers have pushed it ever closer to the home.

Single-mode vs multimode

The two families trade cost against reach:

Trait Single-mode Multimode
Core diameter ~9 µm (one path) ~50–62.5 µm (many paths)
Reach Kilometres to hundreds of km Up to a few hundred metres
Source Laser LED or VCSEL
Cost Higher (precise coupling) Lower
Typical use Backbones, long-haul Within a building or rack

Where it fits

Fiber’s bandwidth and noise immunity come at the cost of more delicate cabling and pricier transceivers, so for most short LAN runs copper Ethernet is simpler; fiber earns its place over distance or in electrically noisy sites. For a GopherTrunk rooftop or tower installation, a fiber backhaul can carry the decoded stream down a long run without picking up the electrical noise a copper cable would — and its immunity to interference keeps it well clear of the RF the antenna is trying to hear. The underlying idea — modulating light instead of an RF carrier — is a close cousin of the modulation an SDR performs on radio waves.

Sources

  1. Optical fiber — Wikipedia, on fiber-optic transmission, total internal reflection, and single-mode versus multimode fiber. ↩

See also