Fibre-optic glossary

Short definitions of the terms used across this category, grouped the way they are met: the physics of light in glass, fibre and cable types, the passive components between two transceivers, connectors, installation, testing and operation. Each entry points to the material where the term is explained in depth. Transceiver-side vocabulary — form factors, DDM, memory maps, compliance codes — lives in the optics glossary.

Light and fibre physics

Term	Meaning	See
Core	the central glass region (9 µm single-mode, 50 or 62.5 µm multimode) with a higher refractive index that guides the light	How light propagates
Cladding	the 125 µm glass layer of lower index around the core; the light is confined by the index step	same
Coating	the dual acrylate layer (250 or 200 µm) that protects the glass; stripped before splicing	How fibre is made
Refractive index (n)	ratio of the speed of light in vacuum to that in the material; about 1.468 for silica	How light propagates
Group index (IOR)	the index that governs the speed of a pulse; the value entered into an OTDR (1.467–1.469)	Reading an OTDR trace
Numerical aperture (NA)	the sine of the maximum acceptance angle; 0.12–0.14 single-mode, 0.20 multimode	How light propagates
Total internal reflection	the effect that keeps light in the core when it hits the core–cladding boundary at a shallow angle	same
Mode	a stable field pattern the fibre can carry; single-mode fibre carries one above its cutoff wavelength	same
Mode field diameter (MFD)	the effective width of the single-mode light spot (9.2 µm at 1310 nm, 10.4 µm at 1550 nm); mismatch costs splice loss	same
Cutoff wavelength	below it a single-mode fibre carries more than one mode; cabled cutoff ≤ 1 260 nm	same
Effective area (A_eff)	the area over which the power is spread (≈ 80 µm² G.652); smaller means stronger non-linear effects	Non-linear effects
Attenuation	loss of power with distance in dB/km; 0.35 dB/km at 1310 nm, 0.2 dB/km at 1550 nm	Attenuation & windows
Rayleigh scattering	scattering from density fluctuations frozen into the glass; the dominant loss, falling with the fourth power of wavelength; source of the OTDR backscatter	same
Water peak	the OH absorption band around 1 383 nm; suppressed in low-water-peak (G.652.C/D) fibre	same
Transmission window	a wavelength region of low loss: 850, 1 310, 1 550 nm and the O–L bands	same
Macrobend loss	light lost at a visible bend below the minimum radius; grows with wavelength	Plant faults
Microbend loss	loss from microscopic axis deviations caused by cable pressure, ice or coating stress	same
Modal dispersion	pulse spreading in multimode fibre because modes travel at different speeds; sets the bandwidth–distance product	Dispersion & bandwidth
Chromatic dispersion (CD)	pulse spreading because different wavelengths travel at different speeds; D in ps/(nm·km), 17 at 1550 nm on G.652	same
Zero-dispersion wavelength (λ₀)	where chromatic dispersion crosses zero: 1 310 nm on G.652, 1 550 nm on G.653	same
Polarization mode dispersion (PMD)	spreading because the two polarization modes travel at different speeds; coefficient in ps/√km	same
Differential group delay (DGD)	the instantaneous delay between the two polarization modes, in ps; its mean is the PMD of the link	Fibre characterization
Modal bandwidth (EMB)	the bandwidth–distance product of multimode fibre in MHz·km (OM3 2 000, OM4 4 700)	Dispersion & bandwidth
Non-linear effects (SPM, XPM, FWM, SBS, SRS)	power-dependent distortions: self- and cross-phase modulation, four-wave mixing, Brillouin and Raman scattering	Non-linear effects
Fresnel reflection	the reflection at any index step, 3.6 % (−14.4 dB) at glass–air	Reflections & return loss
Reflectance	the reflected fraction of one event in negative dB (−55 dB for a clean UPC pair)	same
Optical return loss (ORL)	the total power returned by the whole link, in positive dB	same
Fibre fuse	a self-propagating destruction of the core above roughly 1 W, travelling toward the source	Reliability & ageing
Proof test	the tension (0.69 GPa, 1 % strain) every metre of fibre survives in the factory	same
Static fatigue	slow growth of surface flaws under stress and moisture; the reason for the 20 % strain rule	same

Fibre and cable types

Term	Meaning	See
G.652.D	standard single-mode fibre with low water peak; the default fibre of the world	Fibre types
G.655	non-zero dispersion-shifted fibre for 2000s DWDM long haul	same
G.657	bend-insensitive single-mode fibre (A1: 10 mm, A2/B2: 7.5 mm, B3: 5 mm radius)	same
G.654	cut-off shifted pure-silica-core fibre with 0.15–0.17 dB/km for submarine and long-haul coherent	same
OM1–OM5	multimode grades: 62.5 µm OM1; 50 µm OM2, laser-optimised OM3/OM4, wideband OM5	same
OS1a, OS2	single-mode cabled fibre categories of ISO/IEC 11801: 1.0 dB/km indoor, 0.4 dB/km outdoor	Standards map
Dispersion-compensating fibre (DCF)	fibre with large negative dispersion used in modules to cancel the line's dispersion	Dispersion & bandwidth
Polarization-maintaining fibre (PM)	fibre with built-in stress that keeps a launched polarization; inside transceivers and instruments	Fibre types
Loose tube	cable design where 12 or 24 fibres float in a gel- or dry-filled tube, decoupled from cable strain	Cable construction
Tight buffer	a 900 µm plastic layer on each fibre; indoor cables and pigtails	same
Ribbon, rollable ribbon	12 fibres bonded side by side (or intermittently) for mass splicing and density	same
Micro-cable, microduct	small cables (200 µm fibres) blown into 5–16 mm ducts	same
ADSS	all-dielectric self-supporting aerial cable for power-line corridors	same
OPGW	optical ground wire: fibres inside the lightning shield wire of a high-voltage line	same
Strength member	fibreglass rod, aramid yarn or steel that carries the tension instead of the fibres	same
Armour	corrugated steel tape, steel wires or dielectric yarns against crush and rodents	same
Water blocking	gel or dry swellable tapes and yarns stopping water from travelling along the cable	same
LSZH, CPR Euroclass, OFNP/OFNR	fire ratings of indoor cable jackets in IEC, EU and NEC regimes	same
Colour code (TIA-598)	blue, orange, green, brown, slate, white, red, black, yellow, violet, rose, aqua for fibres 1–12 and tubes	same

Passive components

Term	Meaning	See
Passive plant	everything between two transceivers that carries light without electronics: cables, panels, splices, connectors, splitters, filters	Passive plant
Patch panel, ODF	the rack unit or frame where cable fibres end in adapters for patch cords	same
Adapter (coupler)	the bulkhead sleeve that aligns two connector ferrules	same
Pigtail	a short fibre with a connector on one end, spliced to a cable fibre on the other	same
Patch cord (jumper)	a connectorised fibre for panel-to-equipment or panel-to-panel connections	same
Splice closure	the sealed outdoor enclosure where cables are opened and fibres spliced on trays	same
Splice tray	the tray in a closure or panel holding splice protectors and slack	Splicing & termination
Splice protector	the heat-shrink sleeve with a steel rod that protects a fusion splice	same
Fibre distribution hub (FDH)	the street cabinet of a PON with splitters and cross-connect	Fibre network topologies
Splitter	a passive divider of power 1:2 … 1:128 for PON; 3.5 dB per halving plus excess loss	Passive plant
WDM filter, mux/demux, OADM	components that combine or separate wavelengths on one fibre	same
Attenuator	a fixed or variable element inserting loss to protect a receiver from overload	Link budget
Terminator	a non-reflective end (APC or absorptive) for an unused fibre	Reflections & return loss
Cassette, MPO trunk, breakout	data-centre modular cabling: MPO trunk cables, cassettes fanning out to LC	Fibre network topologies
Polarity (methods A/B/C)	the scheme that keeps transmit and receive fibres paired through MPO trunks	Breakout & MPO cabling
Launch cord, receive cord	150–1 000 m fibres before and after the link under test so the OTDR can see its first and last connectors	Reading an OTDR trace

Connectors

Term	Meaning	See
Ferrule	the ceramic cylinder (2.5 mm SC/FC/ST, 1.25 mm LC) that holds and aligns the fibre end	Connectors & fibre types
Endface	the polished end of the ferrule and fibre; inspected before every mating	Endface inspection & cleaning
LC, SC, FC, ST, MPO/MTP	connector families: small duplex LC (transceivers), SC (PON, panels), threaded FC (instruments), bayonet ST (legacy), 12–24 fibre MPO (parallel optics)	Connectors & fibre types
PC, UPC, APC	polish types: physical contact, ultra-polished (blue, −50 dB), 8° angled (green, −60 dB, never mated to UPC)	same
Insertion loss	the loss added by a connector pair or splice; 0.3 dB typical, 0.75 dB maximum per pair in structured cabling	Link budget
Mating	connecting two connectors through an adapter; 500–1 000 matings per connector life	Reliability & ageing
Key, guide pins	the features that fix the rotational position (APC angle, MPO fibre order)	Connectors & fibre types

Installation and routes

Term	Meaning	See
Duct, sub-duct, microduct	the pipes fibre cables are pulled or blown into	Installation
Blowing (jetting)	installing cable with compressed air along a duct; kilometres per shot	same
Pulling tension	the force on the cable during installation, capped by the cable's rated tensile strength	same
Minimum bend radius	20 × cable diameter under tension, 10 × installed; 30 mm for bare fibre	same
Direct burial	cable in a trench or ploughed in without a duct; armoured	same
Aerial	cable on poles or towers: lashed, figure-8, ADSS, OPGW	same
Sag	the vertical droop of an aerial span; sets tension and clearance	same
Slack, storage loop	extra cable coiled at closures and poles for future splicing and repair	same
Horizontal directional drilling (HDD)	trenchless installation of ducts under roads and rivers	same
Marker post, warning tape	above-ground and in-trench indicators of a buried route	same
As-built	the documentation of what was actually installed, not what was designed	Documentation & labelling

Testing and measurement

Term	Meaning	See
Light source and power meter (LSPM, OLTS)	the reference loss test: launch a known power, measure what arrives; Tier 1	Testing & measurement
Reference method (one-jumper)	the way the meter is zeroed so that the link's own connectors are counted	same
Encircled flux	the standardized launch condition for multimode loss tests (TIA-526-14, IEC 61280-4-1)	same
OTDR	optical time-domain reflectometer: sends pulses and plots backscatter and reflections versus distance	Reading an OTDR trace
Pulse width	the OTDR pulse duration (5 ns–20 µs) trading resolution against range	same
Dead zone (event, attenuation)	the distances after a reflection in which events cannot be separated or measured	same
Dynamic range	the OTDR's usable dB between initial backscatter and noise	same
Backscatter coefficient	the fraction of pulse power the fibre scatters back, used to compute reflectance	same
Ghost	a false reflective event caused by multiple reflections	same
Gainer	an apparent negative splice loss caused by a backscatter mismatch; averaged out bidirectionally	same
Bidirectional measurement	OTDR from both ends, averaged, required for true splice and connector losses	same
.sor file	the standard OTDR trace file (Telcordia SR-4731)	same
ORL meter (OCWR)	instrument measuring the total return loss of a link	Reflections & return loss
Visual fault locator (VFL)	a red laser showing breaks and tight bends in cords and trays	Testing & measurement
Fibre identifier	a clamp-on tool detecting traffic and its direction in a bent fibre without disconnecting it	same
Inspection scope	a video microscope grading endfaces against IEC 61300-3-35	Endface inspection & cleaning
Tier 1, Tier 2	TIA-568.3-D test levels: loss/length/polarity pass-fail; plus OTDR characterization	Testing & measurement
Fibre characterization	CD, PMD, spectral attenuation, ORL and latency measurements before high-speed or DWDM services	Fibre characterization
Link budget	transmitter power minus receiver sensitivity, compared with the sum of losses and penalties	Link budget
Margin	the decibels left in the budget after all losses and penalties; 3 dB is the usual design target	same
Link passport, baseline	the recorded commissioning values (OTDR, loss, DDM) every later measurement is compared with	Documentation & labelling

Operation and safety

Term	Meaning	See
Dark fibre	an installed fibre without equipment, leased or reserved	Fibre network topologies
Remote fibre test system (RFTS)	an OTDR permanently connected to live fibres via 1625/1650 nm couplers, alarming on new events	Maintenance & restoration
Distributed acoustic sensing (DAS)	using the fibre as a vibration sensor to detect digging near the cable	same
Restoration	the process from alarm to temporary and then permanent repair of a cut	same
Automatic power reduction (APR, ALS)	amplifier shutdown on loss of signal so that a broken fibre is not hazardous	Safety & handling
Laser class (IEC 60825-1)	1, 1M, 2, 3R, 3B, 4 — the hazard class of a source	same
Hazard level (IEC 60825-2)	the class of power accessible at a location in a fibre system	same
DDM / DOM	the transceiver's digital diagnostics: Tx and Rx power, temperature, bias — the link's built-in power meter	DDM basics

In CodingBox

The bench side of this vocabulary — what the module reports about its wavelength, connector code, fibre length ratings and DDM — is what CodingBox reads and decodes, so the terms above meet the transceiver's own words on the Check screen (Check transceiver, Optics glossary).