How optical fibre is made: preforms, drawing, coating, proof test

A telecom fibre is a glass thread 125 µm across, drawn from a preform the size of a baseball bat at tens of metres per second, coated within a fraction of a second and stressed to 1 % strain along its whole length before it is sold. The manufacturing route explains most of the numbers an engineer meets later: why mode field diameter is specified to ±0.4 µm, why splices between vendors lose a little more, why installation strain is capped at a fraction of the proof test, why modern fibre is hydrogen-safe and why 200 µm fibres need different splicer settings. This page follows the process from raw chemicals to the spool and ends with what it means in the field.

Materials

Material	Role
SiCl₄ (silicon tetrachloride)	ultra-pure silica source; oxidised or hydrolysed in a flame or furnace to SiO₂ soot
GeCl₄ → GeO₂	raises the refractive index — the core dopant of almost all telecom fibres; slightly increases Rayleigh scattering
Fluorine (SiF₄, C₂F₆, SF₆)	lowers the index — depressed claddings, the trench of bend-insensitive G.657 fibre, the cladding of pure-silica-core fibres
POCl₃ → P₂O₅	lowers viscosity, used in MCVD claddings; makes glass hydrogen-sensitive — a reason old fibres age badly
Al₂O₃, Er, Yb, Tm	active fibres for amplifiers and lasers, added by solution doping of the porous soot
Cl₂ and He	drying and consolidation: chlorine removes OH to below 1 ppb (low water peak), helium purges the porous body
UV-curable acrylates	dual coating: a soft primary (modulus ~1 MPa) cushions the glass, a hard secondary (~1 GPa) protects it
Colour inks, ribbon matrix	identification (12 colours) and ribbon assembly

Preform processes

Process	How it works	Who uses it	Strengths
MCVD (modified chemical vapour deposition, Bell Labs 1974)	vapours flow inside a rotating silica tube heated by an external torch; soot deposits layer by layer (cladding, then core); the tube is collapsed into a solid rod	most manufacturers, all specialty fibre makers	flexible: erbium-doped, polarization-maintaining, dispersion-compensating fibres; moderate preform size
PCVD (plasma-activated CVD)	like MCVD, but a microwave plasma replaces the torch; thousands of very thin layers	graded-index multimode makers	precise index profiles — OM3/OM4/OM5
OVD (outside vapour deposition)	soot from a flame is deposited on a rotating target rod, core layers first; the rod is removed, the porous body is dried in Cl₂ and sintered to clear glass	large-volume single-mode and multimode production	very low OH, large preforms, core and cladding in one process
VAD (vapour-phase axial deposition)	soot grows on the end of a rotating rod that is pulled upward; continuous process	large-volume single-mode production	very long preforms, low OH, high throughput
Overcladding (RIC — rod in cylinder; soot overclad; sleeving)	the core rod, which carries the whole index profile, is surrounded by cheap synthetic silica — a cylinder fused on during draw, or extra soot	everyone	the expensive doped glass is only a few percent of the volume

A finished preform is 100–200 mm in diameter and 1–3 m long. A 150 mm × 2 m preform draws into roughly 2 500–3 000 km of 125 µm fibre; the largest modern preforms yield more than 5 000 km.

Drawing

Step	What happens	Numbers
Furnace	the preform tip softens in a graphite or zirconia induction furnace	1 900–2 100 °C
Neck-down and draw	a thread is pulled from the molten tip; speed sets the diameter	20–40 m/s (1 200–2 500 m/min), modern towers faster
Diameter control	a laser gauge measures the bare fibre and feeds back to the capstan speed	125 ± 0.7 µm specified, ±0.1 µm achieved
Cooling	helium-filled tube cools the glass enough to be coated	a few hundred °C in less than a metre
Primary coating	soft acrylate applied through a pressurised die, cured by UV lamps	to ~190 µm
Secondary coating	hard acrylate, cured	245 ± 10 µm standard; 200 ± 10 µm for high-density cables
Spinning	the fibre is twisted alternately clockwise and counter-clockwise during draw, mixing the birefringence axes	PMD coefficient < 0.1 ps/√km instead of 0.5–2 ps/√km on 1980s fibre
Capstan and winding	tension-controlled winding on spools	25–50 km per spool; towers are 20–40 m tall

Colouring adds an ink layer (or a coloured secondary); ribbons bond 4–12 coloured fibres in a matrix, rollable ribbons bond them intermittently so they can be rolled into a tube.

Proof test and quality assurance

Test	Method (IEC 60793-1-xx)	Specification for G.652.D
Proof test	every metre runs through a tension of 0.69 GPa (100 kpsi, ≈ 1 % strain) for ≥ 1 s (part 1-30); submarine and special fibres 1.38 GPa	any flaw that would break below that stress breaks here, not in the ground
Geometry	cladding diameter, non-circularity, core–cladding concentricity (part 1-20)	125 ± 0.7 µm; ≤ 1 %; ≤ 0.6 µm — connector and splice alignment depend on it
Attenuation	cutback (part 1-40) at 1310/1550/1625 nm plus a spectral scan; OTDR on every spool for uniformity and point defects	≤ 0.35 / 0.21 / 0.23 dB/km typical; point defects ≤ 0.05 dB
Mode field diameter	far-field scan (part 1-45)	9.2 ± 0.4 µm at 1310 nm
Cutoff wavelength	part 1-44	cabled cutoff ≤ 1 260 nm
Chromatic dispersion	phase-shift (part 1-42)	λ₀ = 1 300–1 324 nm, slope ≤ 0.092 ps/(nm²·km)
PMD	part 1-48	link design value ≤ 0.20 ps/√km
Macrobend loss	part 1-47	≤ 0.1 dB for 100 turns at 30 mm radius (1625 nm)
Coating	strip force 1.3–8.9 N (part 1-32), coating geometry, curl	consistent stripping in the field
Hydrogen ageing	IEC 60793-2-50	≤ 0.01 dB/km increase after exposure — modern fibre is hydrogen-safe

Every spool ships with a data sheet of measured values. Cable makers keep the mapping spool → cable → fibre number; ask for those fibre data sheets during acceptance — they are the first baseline of your plant (Documentation & labelling).

Special fibres and what is different in making them

Fibre	Manufacturing difference	Use
Bend-insensitive G.657	a fluorine-doped trench ring deposited around the core (MCVD/PCVD or OVD)	FTTH, high-density panels, 200 µm cables
Graded-index multimode OM3–OM5	thousands of layers with a parabolic (α ≈ 2) index profile; differential mode delay measured on every preform	short-reach VCSEL links
Erbium-doped (EDF)	porous core soot soaked in Er/Al solution before sintering; short lengths (metres) per amplifier	EDFAs
Polarization-maintaining (PANDA, bow-tie)	holes drilled in the preform and boron-doped stress rods inserted, or shaped stress regions	coherent transceivers internally, sensors, pump combiners
Dispersion-compensating (DCF)	small core with high index step giving large negative dispersion	legacy 10G/40G compensation
Pure-silica-core G.654	undoped core, fluorine-doped cladding; large effective area variants (G.654.E)	submarine and long-haul coherent: 0.15–0.17 dB/km
Hollow-core (NANF and relatives)	stack-and-draw of silica capillaries forming an air core; light travels 1.46 × faster and almost without non-linearity	latency-critical links, research; losses below 0.2 dB/km reported in the 2020s
Multicore	4–7 cores in a 125–200 µm cladding, drawn from a stacked preform	space-division multiplexing, submarine systems since the 2020s
Radiation-hard	fluorine-doped core and cladding, no germanium	nuclear, space, accelerators
Carbon or metal coated	hermetic carbon layer applied in the draw tower before the acrylate	oil and gas, hydrogen-rich and high-temperature environments

Why a network engineer cares

Manufacturing fact	Field consequence
MFD specified to ±0.4 µm	splices between fibres from different vendors or eras can lose 0.02–0.05 dB more; gainers and losers on the OTDR (Splicing & termination)
Proof test at 1 % strain	long-term installed strain is limited to 20 % of proof (0.2 %), installation to 60 % (Reliability & ageing)
Concentricity ≤ 0.6 µm	connector loss is dominated by ferrule and polish, not by the glass
Spinning since the late 1990s	100G coherent runs on any fibre made after about 2000; pre-1995 fibre needs a PMD test (Fibre characterization)
Low-OH drying	the 1383 nm water peak is gone: E-band CWDM and 1625/1650 nm monitoring are usable (Attenuation & windows)
200 µm coating	smaller cables, but holders, stripper settings and cleaver clamps must match; mixed 200/250 µm splicing needs the right holders
Cabled attenuation > fibre attenuation	design with the cabled value from the cable data sheet, not the fibre value (Cable construction)
Ribbons	mass fusion splicers, 12 fibres per splice; identification by ribbon position

In CodingBox

A transceiver knows nothing about how the fibre was made, but its rated lengths for SMF, OM3 and OM4 assume fibre made to these tolerances — G.652.D or the stated OM grade. CodingBox shows those length fields; on unusual fibre (G.654, G.653, hollow-core) the rated distances do not apply and the link budget has to be built from the fibre's own data sheet (Check transceiver, Fibre types).