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At CERN we use mainly 2 fibre types: Graded index 50/125 µ m (ITU G.651) Single-mode 9/125 µ m (ITU G.652) ITU = International Telecom Union Head office Geneva/CH. d= 50 or 9 µ m. coating. cladding. core. d= 125 µ m. d= 250 µ m. WEB Client.

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More than a simple piece of glass.

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At CERN we use mainly 2 fibre types:

  • Graded index 50/125 µm (ITU G.651)
  • Single-mode 9/125 µm (ITU G.652)
  • ITU = International Telecom Union
  • Head office Geneva/CH

d= 50 or 9 µm




d= 125 µm

d= 250 µm

WEB Client

Unleashing the Light around the CERN Accelerators


Luit Koert de Jonge.


Unleashing the light around the accelerators is may be a somewhat misleading title, but hundreds of laser light sources will transmit vital data for the operation of LHC at 2/3 of the light speed (200’000 km/s) all across the CERN surface and underground sites.

Optical fibres play today a vital role in communications, machine controls, instrumentation and safety systems. CERN will count at LHC commissioning over 25’000 installed fibre kilometres and more than 40’000 optical terminations. The optical fibre network for LHC must be extremely reliable and redundant loops shall exist to avoid single points of failure. Laser power used at the transmitter rarely exceeds 1 mW and optical connections must therefore have a high quality standard in order to keep optical reflections and attenuation within acceptable limits. The long distance surface optical fibres (in ducts) may suffer from mechanical stress and will therefore be permanently monitored via an autonomous monitoring system using optical time domain reflectometry. The optical fibres in the LHC tunnel will be subject to irradiation and will therefore darken, thus increasing their attenuation. This process will be closely monitored as well, in order to set off a replacement of these fibres in time.

More than a simple piece of glass.

Optical glass fibre production can be considered as a wonder of modern engineering.

A preform of ultra pure silica is produced, usually by chemical vapour deposition technology and by adding concentrations of dopants (germanium oxide) to obtain the required refractive index profile across the diameter.

The optical fibre is drawn from the preform in a large tower between 10 and 20m high, where the preform is heated to form a got that drops by gravity. An operator picks it up at the bottom of the tower.

The thickness of the fibre is adjusted to 125 µm, then it is covered with several layers a of flexible polymer coating, before it is being spooled on a drum at a speed of about 20 m/s.

One can produce today up to 1000 km of optical fibre in one length, but normal production lengths are 80 to 200 km.

Preform Feed


How are optical fibres terminated?


Blowing of fibre cables is that possible?

Laser Micrometer

Optical fibres are usually terminated with an optical connector. The connector mounting is a very precise job and is usually done at the factory.

We buy so called “pigtails”. This is a 2m long buffered fibre with the appropriate connector type.

The pigtail is then fusion spliced to the fibre.

The connectors must be very precisely polished to avoid reflections as much as possible.

Optical connectors are always “male” connectors. They connect to each other via an adapter.

  • Reflections occur on fibre surfaces at the exit as well as entry connector endfaces
  • Defects on the endface and poorpolishing quality, as well as air gaps between the fibres are responsible for reflections
  • Reflections reduce optical trans-mission performance

Coating Cup 1

Today we blow the optical cables into the cable ducts which lay along the HV cables between the CERN sites. The laying speed varies from 40 to 100 m/min. if all goes well. The air compressor must deliver 10m3/min. at a pressure of 10 bar.

At the pictures we see the blowing of a 216 fibre cable between PA8 and Bld. 513.

UV Curing Oven 1

Coating Cup 2

UV Curing Oven 2

Tractor & Drum

To reduce reflections we can use Angled Polished Connectors with end faces having an angle of 8°.

These connectors are only used with single-mode fibres.


9/50/62.5 µm

Non colored fibre.

Coloring of fibre.

Assembling of colored fibres into a jelly filled loose tube.

The optical core alignment must be optimal for low attenuation.

In CERN’s optical access network we use following 3 connectors.

Optical fibre splice.

E2000-APC single-mode 9/125 µm

Buffered optical fibre Ø 0.9 mm.

FC-PC single-mode 9/125 µm

ST multimode 50/125 µm

E2000 adaptor & connector

For the LHC surface installations, the shafts and underground machine areas, we intend to use a new cabling concept called JETnet. It is based on tube technology and was developed by the company Draka Comteq NKF. It is a complete new solution for the laying of optical fibre access networks with parallel and serial upgradeability. It requires lower initial cost and the network can grow with demand.

A fibre has a diameter of 125 µm. A hair measures 80µm.

Basic structure of a fibre.

Laser light protection cover

The system is based on individual guide tubes running through protective ducts. In these guide tubes small, but ”outside plant resistant” mini optical cables can be installed without any splice.

Latest developments in optical transmission.




Blowing of guide tubes with a Superjet.

(C = speed of light)

Wavelength versus frequency

Dense Wavelength Division Multiplexing

Redundant loops can be realized all over the network.

Remote trouble shooting and monitoring.

The CERN optical access network is very dense and can be compared with a Metropolitan optical fibre network.

Theoretical bandwidth of single-mode optical fibre is extremely high (75 THz) and instead of installing many fibres, telecom operators prefer investing in DWDM.

DWDM technology is still very expensive but allows today typically 100 wavelengths @ 10 Gb/s TDM channel rate on a single fibre. This equals 15.625.000 simultaneous telephone conversations and still even many more wavelengths are possible………

With so many channels on one single fibre, a network must be protected via redundant automatically restoring loops and one must avoid single points of failure.

At CERN, with maximum distances not exceeding 15 km, one does not intend to use DWDM at the moment. It seems more economical to install many fibres.

The installation however, must have redundant loops forSafetyandReliability.

Transmission technology trends

to other

cables under



SNMP Agent




Optical Time Domain Reflecto meter

Optical Test Access Unit

RTU Remote Test Unit




Network terminal

Cable under test

Time Division Multiplexing

System Management

1st Optical window 850 nm

2nd Optical window 1300 nm

3rd Optical window 1550 nm

single mode Laser

multi mode Laser


wavelength nm


CERN intranet




1800 1600 1400 1200 1000 800 600 400 200







Frequency Hz


fibres in a



A fibre monitoring system has been installed, for the surveillance of all main optical trunks and in the future the optical network in the LHC tunnel, as the fibres will be subject to radiation damage.

The system has 24 optical ports. Each port can test an optical fibre link with a maximum length of 200 km.

The heart of the system is a powerful optical time domain reflecto meter at =1550 nm.












Dense Wavelength Division Multiplexing

5th ST Workshop, 28 January 2002 Echenevex France