Fibre Access Network
Planning and Design

Fibre optics is the technology of sending information as pulses of light through a thin strand of glass, instead of as electrical signals through copper wire. It is the backbone of modern communications — the internet, phone calls and television all travel as light through fibre for most of their journey.

An optical fibre is a strand of extremely pure glass, roughly the thickness of a human hair. Information is sent by switching a light source on and off very fast — billions of times per second — so each flash and gap becomes the 1s and 0s of digital data. A light detector at the far end reads those flashes back into the original information. Because light travels enormous distances through clear glass with very little weakening, one fibre can carry vastly more information, much further, than a copper wire of the same size.

The clever part is how the light is kept inside the glass over many kilometres, even around bends. A fibre has two layers: an inner core and an outer cladding, made of glass with slightly different properties. When light tries to escape from the core into the cladding, it instead bounces back in — a phenomenon called total internal reflection. The light effectively ricochets down the core, staying trapped all the way to the far end.

However complex a network looks, every fibre connection has the same three basic parts. Hold these in mind — the whole course is about getting light successfully from the first to the third.

Everything in fibre design comes down to one question: does enough light arrive at the receiver for it to read the signal clearly? If too much light is lost along the way, the receiver "can't hear" and the connection fails. To work that out, we first need a way to measure how much light there is and how much is lost — and that is what decibels give us.

Optical design is bookkeeping in decibels. Get the units right and everything else follows; confuse them and every later calculation is wrong.

dBm is an absolute optical power, referenced to 1 milliwatt: 0 dBm = 1 mW, +3 dBm ≈ 2 mW, −3 dBm ≈ 0.5 mW. dB is a relative change — a ratio, i.e. a loss or gain with no absolute meaning on its own. The arithmetic that makes the decibel worth using: because it is logarithmic, you add and subtract dB values instead of multiplying ratios. A chain of losses is just a sum.

The 3 dB rule is worth committing to memory: every 3 dB of loss halves the optical power, and every doubling of a splitter's split ratio adds ~3 dB. That single fact lets you estimate a budget in your head before you ever open a calculator.

Light weakens in fibre through several distinct physical effects. A designer must know which dominate where.

Attenuation vs dispersion — two different limits

Attenuation limits how far light reaches before it is too faint to detect. Dispersion limits how fast you can signal: pulses spread out (chromatic dispersion from different wavelengths travelling at slightly different speeds; modal in multimode only) and eventually blur into each other. For PON access at GPON/XGS rates over ≤20 km, attenuation is the binding constraint. Dispersion becomes a real consideration on long, high-bit-rate transmission spans — which is why it matters more on a DWDM backbone than on a 12 km access PON.

Plot loss against wavelength and three usable low-loss "windows" emerge. PON deliberately places its signals in them. Note the numbers — you will use these exact coefficients in Module 8.