Fibre-optic links: how light carries data

Every material in this documentation ends at the same place — a glass thread a tenth of a millimetre thick carrying pulses of infrared light. A fibre-optic communication link is the transmitter that makes the pulses, the fibre plant that carries them and the receiver that turns them back into bits, plus everything in between: connectors, patch panels, splices, splitters and filters. This section explains the physics and engineering behind that chain in the depth needed to design, test and troubleshoot a link — why light stays in the fibre, what weakens and smears it, how far a given optic can go, what the passive plant is made of, how it is measured and how it fails.

Why fibre

The chain

A link is a series of elements, each with a loss and some with a distortion:

1. Transmitter — laser or VCSEL in the transceiver, launching −8 … +4 dBm of light at 850, 1310 or 1550 nm (Lasers).
2. Patch cord — 1–5 m jumper from the module to a patch panel; two connector endfaces.
3. Patch panel / ODF — adapter where the jumper meets a pigtail spliced to the cable.
4. Cable — indoor, outdoor or hybrid, metres to tens of kilometres, one or hundreds of fibres (Passive plant).
5. Splices, splitters, filters — fusion splices along the route; PON splitters or WDM muxes where the topology needs them.
6. Far-end panel and patch cord — the mirror image.
7. Receiver — photodiode in the far transceiver, needing −12 … −28 dBm depending on type (Receivers).

The link budget is the arithmetic that adds the losses of 2–6, compares them with the difference between 1 and 7, and leaves a margin (Link budget engineering).

The four things that limit a link

Fibre in one paragraph

Light stays in the core because the core's refractive index is slightly higher than the cladding's — total internal reflection. A single-mode fibre (9 µm core) carries one path of light and reaches tens of kilometres; a multi-mode fibre (50 µm core) carries hundreds of paths that arrive at slightly different times, which limits it to hundreds of metres but lets it work with cheap 850 nm VCSELs. The details — modes, numerical aperture, bend loss, fibre grades OM1–OM5 and OS2 — are in How light propagates and Connectors & fibre types.

Generations of fibre links

The transceiver side of this history: What is a transceiver.

Further reading

• How light propagates in fibre — refractive index and total internal reflection, numerical aperture, modes, single- vs multi-mode, mode field diameter, cutoff wavelength, bend loss, fibre grades.
• Attenuation and transmission windows — Rayleigh scattering and absorption, the water peak, 850/1310/1550 nm windows, dB/km by fibre and wavelength, connector and splice losses.
• Dispersion and bandwidth — modal, chromatic and polarization-mode dispersion, bandwidth–distance product, dispersion-limited reach, compensation.
• Fibre types — ITU-T G.652–G.657 single-mode, OM1–OM5 multi-mode grades, specialty fibres, dissimilar-fibre joint losses, identifying installed fibre.
• Non-linear effects — SPM, XPM, FWM, Brillouin and Raman scattering, where they matter, optimum launch power, mitigation.
• Reflections & return loss — Fresnel reflection, reflectance versus ORL, typical values per component, what standards require, measurement, keeping reflections low.
• How fibre is made — materials, MCVD/OVD/VAD/PCVD preforms, drawing and coating, proof test and QA, specialty fibres, what it means in the field.
• Reliability & ageing — strength and static fatigue, bend and strain rules, ageing mechanisms, service life of each part, fibre by era, designing for ageing.
• Link budget engineering — the equation, loss allowances, penalties, margin policy, worked examples, receiver overload and attenuators.
• Passive plant — cables, patch panels, pigtails, splices, attenuators, splitters, WDM filters, enclosures and their loss allowances.
• Cable construction — layers, cable families from loose tube to OPGW and rollable ribbon, fire ratings, colour codes, reading a cable data sheet, choosing the cable.
• Fibre network topologies — point-to-point, rings, meshes, PON trees, WDM; structured cabling, data-centre patterns, FTTx, fibre-count planning, protection.
• Cable routes and installation — ducts, direct burial, aerial, indoor pathways; bend and tension limits, route planning, closures, labelling, acceptance.
• Documentation & labelling — the record set, identifier schemes, labels, standards, tools, the process and its common failures.
• Active optical infrastructure — passive versus active, the map of active elements, where the decibels go, ownership and demarcation, lifecycle.
• Transceivers in the link — matching the module to fibre and distance, the budget from the module's side, connectors and polarity, compatibility, diagnostics, common mismatches.
• Amplifiers & regeneration — when the budget ends, EDFA/Raman/SOA/OEO, placement, the OSNR budget, operating an amplified line, amplify or regenerate or go coherent.
• Transport & access equipment — media converters, transponders, ROADMs, OLT/ONU, protection switches, power and sites, management, pitfalls.
• Monitoring & management — layers of visibility, alarms and their meaning, baselines and trends, protocols and tools, correlating module and fibre data.
• Splicing and termination — the fusion splicing process, mechanical splices, pigtails and field connectors, acceptance criteria and defects.
• Testing and measurement — light source and power meter, OTDR, return loss, endface inspection, certification, using DDM as a meter.
• Reading an OTDR trace — setup parameters, the event catalogue, dead zones and dynamic range, bidirectional averaging, distance to route, misreadings, live fibre and PON.
• Endface inspection & cleaning — what contamination does, IEC 61300-3-35 zones and criteria, equipment and technique, contamination types, cleaning methods, the procedure.
• Fibre characterization — when it is needed, the tests, limits per service, reading the results for fibre identity and age, procedure and reporting.
• Fibre plant faults — bends, dirt, mismatches, bad splices, reflections, water: symptoms, signatures, fixes.
• Maintenance and restoration — preventive cycle, monitoring, spares, the restoration process, root causes of outages, plant lifecycle.
• Safety and handling — laser classes and hazard levels, live-fibre rules, glass and chemicals, site hazards, module and cord handling.
• Standards map — ITU-T, IEC, TIA/ISO, Telcordia, IEEE/OIF and national documents by body and by task, designations side by side.
• Fibre-optic glossary — the terms of light, fibre and cable types, passive components, connectors, installation, testing and operation.
• Typical values — attenuation, loss allowances, reflectance, geometry, dispersion, mechanical limits, wavelengths, OTDR figures, interface budgets, power and safety.
• Formulas & calculations — decibels, attenuation, guidance parameters, dispersion, link budget, reflections, OTDR, latency, mechanics and planning counts, each with a worked example.

In CodingBox

The transceiver's DDM is the link's built-in power meter: launch power on one end, received power on the other, and the difference is the plant loss — which CodingBox reads on the bench and the switch reads in service (DDM in the app, Rx power & link budget).