Optical Services Ltd

This page is designed as a brief tutorial on fibre optics to highlight the areas that Lucid's optical fibre training courses cover. This overview is not designed as a web-based training resource, since we believe in high-quality interactive training with expert human trainers who can explain technical concepts and issues in ways you can relate to. Our business is based on charging for this training as it should benefit your career, and we need to make a living. Our aim therefore, is to give excellent technical support and career support to all of our trainees, but not to just anyone who visits our website - our competitors have asked us questions on technical problems they have at public meetings and we know they visit our website. As expert professional trainers and consultants, we normally charge for our expertise, but whilst we may not want to give too much away on our website, we do offer free technical support to our trainees for life.

Optical Fibre Materials

The vast majority of optical fibres currently manufactured are 'all-glass' fibres made from silica glass. The light travels mostly in the inner 'core' area of the glass fibre with the outer cladding glass typically being 125 um (1/8 mm). The optical fibre you see from the fibre factory, or in the cable, is coated with layers of an acrylic plastic for protection. These plastic coatings typically make the standard fibre 250 um (1/4 mm) in diameter, allow the fibres to be coloured for identification, but need to be stripped-off in order to join optical fibres together.

There are now several companies working on developing all-plastic optical fibres (sometimes abbreviated to the acronym POF), and these are becoming popular for their benefits in the automotive and other specialist industries. Compared with all glass fibres they are high loss and hence are normally only used for short distances of the order of metres or tens of metres. There are still several issues to be addressed with plastic fibres for some other applications, but the manufacturers claim to be getting closer every year and their uptake in some specialist industries is now significant rather than just a novelty.

Types of optical fibre

There are two basic types of optical fibre, multimode and single-mode (previously sometimes referred to as monomode).

Multimode optical fibres have larger cores with the advantage of being easier to install and launch light into, but with the disadvantage that a large core results in multiple modes of light in the fibre all with different transit times through the fibre. The result of this is that multimode fibres suffer from an effect known as modal dispersion that limits their bandwidth or information carrying capacity.

Single-mode fibres have a much smaller core than multimode fibres, and this results in only one mode of light existing in the core of the optical fibre. This eliminates modal dispersion and allows single-mode fibres to have a much higher bandwidth or capacity. The bit-rates achievable in these single-moded fibres are then limited by chromatic dispersion and polarisation mode dispersion (PMD).

Standards in Fibre Optics

There are many standards that are either directly or indirectly relevant to fibre optics, and our aim here is merely to highlight some of the more important standards. We now have a more details on a separate page as we felt this was a topic worthy of more space, but below is a brief summary.

The ITU G650 series (includes ITU G.651, G.652, G.653, G.654, G.655. G.656 & G.657 recommendations) of standards deal with recommended specifications for various optical fibre types, with G.650 itself dealing with the optical test methods used to verify the fibre specifications. These standards are developed by the International Telecommunications Union (ITU), and are truly international recommendations in that they are referred to all over the world.

The BS 7718 standard, now withdrawn, is the Code of Practice for the installation of fibre optic cabling. This was a very significant document in that it stated how fibre optic cables should be safely installed and tested. Although now withdrawn so as not to conflict with other newer standards, there are elements of BS 7718 that have not yet been released in any other standards, and so some companies and organsiations have chosen to still reference this standard. A British standard developed from some FIA documents by the BSI, this standard found widespread appeal outside of the UK due to the lack of an equivalent international or European standards for many years.

IEC 11801 is the international standard developed jointly by ISO (International Standards Organisation) and the IEC (International Electrotechnical Commission) and is more properly therefore ISO/IEC 11801. The standard is entitled "Information technology - Generic cabling for customer premises" and deals with both copper and fibre optic cabling in buildings. Effectively this is a local area network (LAN) structured cabling standard, although it is important in that it is one of very few standards that detail installation specifications. A closely related standard is the BS EN 50173 standard.

EN 50173 is the European standard broadly equivalent to IEC 11801. In the UK this standard is publishes by BSi as BS EN 50173 and is entitled "Information technology - Generic cabling systems". Different parts of this standard are purchased separately and deal with different installation environments.

EN 50174 is the European standard now dealing with the installation aspect of IT cabling for fibre optic and copper datacoms cables. Released in the UK by BSI as BS EN 50174, this standard comes in three parts identified on the cabling standards page.

EN 50346 is the European standard dealing with the testing of installed fibre optics cabling for IT systems. The BSI version of this standard is BS EN 50346 "Information technology - Cabling installation - Testing of installed cabling".

ISO / IEC 14763-3 is a standard dealing with the testing of datacoms networks. It is complex for the uninitiated to undertstand, and takes a different approach to earlier datacoms testing standards, but makes sence given the growing trend for duplex fibre networks and different connector styles at either end of a fibre.

IEC 60825-2 and BS EN 60825-2 are the international and European standards dealing with laser safety in optical fibre communications systems. With the advent of more powerful laser transmitters and in particular optical amplifiers with laser power levels as high as those produced by Raman amplifier system lasers, this standard is becoming more and more important.

Fibre optic cable installation environments

Optical fibres and optical fibre cables are often much more robust than many people think, none-the-less however, it is important that fibre optic cables are installed with care and with suitable supporting infrastructure. The cables can be installed into relatively small spaces, and some can accommodate quite tight bends, but some can't, and generally fibre cables should be laid as straight as possible and without any tight restrictions. Although optical telecommunications cables can be installed overhead, or on bearers or in a trough alongside other infrastructures (for example pipelines or railways), the majority are installed below ground. Underground telecommunications cables can be directly buried, but for flexibility and future maintenance it is most popular and beneficial to install the cables in pipes or cable ducts. With optical fibre cables it is preferable to use sub-ducts, whilst for copper cables this was not necessary.

Cable installation techniques

The most common cable installation technique is to pull the fibre cable into place with a rope that has been rodded or blown through the duct. Where the cable span is short it is often possible to hand-haul light-weight fibre optic cables through a duct, but for heavier cables or longer runs a cabling winch with a capstan pulley wheel is normally used.

A more modern method for installing fibre optic cables is to blow them into place with special cable blowing equipment. In this case it is generally preferable to use a cable designed for cable blowing rather than one designed for pulling, however even many cables designed for pulling can be blown a certain distance into the duct.

Cable blowing should not be confused with fibre blowing, or blown fibre methods. These blowing techniques are obviously related and the question then arises of when does an optical fibre, or an optical fibre bundle, become a cable? Strangely this is a very controversial topic in the fibre-optics world, that arose with the development of fibre bundles and ribbon fibres that are more than just bare fibre, but obviously nowhere near as robust as a typical cabled fibres.

Blown fibre installation

Fibre blowing is a technique pioneered by BT for blowing small bundles of fibres into small tubes that can be placed around a building. Fibres can be easily blown into the tubes, and since these fibres can at any time be quickly blown out of the tubes if required, then upgrading from for example an unknown multimode fibre to an OM3 multimode or an OS1 single-mode fibre is simple. This flexibility and opportunity for easy upgrading has made blown fibre installation the method of choice for many prestigious buildings. Blown fibre is also attractive for fibre-to-the-home, or FTTH, projects where the tube can be installed and allow easy fibre installation with minimal disruption at a later date.

Fibre Splicing and Termination

Once the fibre cables are installed in the duct, they will need to either be spliced together to make one concatenated link, or terminated to allow equipment or patch cables to be plugged into the ends. Fibre splicing is normally achieved by stripping a section of coating of the end of the fibre, cleaving the fibre to produce a flat end face, and putting the fibre into a fusion splicing machine. The same procedure is repeated for the other fibre end, and the fusion splicing process then heats both fibre ends and pushes them together. The welded, or spliced, fibre join is then protected with a heatshrink splice protection sleeve.

Fusion splicing machines are now mostly automatic, or at least semi-automatic, these days, but the old machines of the past required the user to align the fibres and do a much more manual fusion splicing operation. For ribbon fibres there are special ribbon fibre splicing machines that perform a mass-fusion splice on up to 12 fibres at a time. When ribbon fibres were new and ribbon fibre splicing was a new technology, the performance of optical fibre ribbon splices was poor, but now with modern machines and much higher quality fibre ribbons, very good splicing results can be achieved.

Fibre optic network testing

The testing of fibre optic networks can be an area of controversy, especially because most fibre optics testing is not conducted according to best practice, and in many cases can yield incorrect or misleading test results. The optical fibre test measurements made on field installed systems are:

• ILM - Insertion Loss Measurement: this is a measurement of the loss, or attenuation, of a fibre optic component or system. This is normally measured with a light source and an optical power meter, or alternatively a loss test set that incorporates both a light source and power meter. The principle is to find the difference between the input and output power levels to yield the loss between these two measurement points. This sounds straightforward, and yet whilst ILM testing can be easy, there are also many mistakes that can be made and result in variable measurements and plain incorrect measurements. Some of the errors that can occur in ILM measurement methods are obvious, whilst other are more obscure and not widely known by testers who have not been trained by fibre optics testing experts. This is sometimes also abbreviated to LSPM for Light Source and Power Meter.

• OTDR testing: the Optical Time Domain Reflectometer is a powerful fibre optic test instrument in that it acts like an optical radar in locating sources of error with the fibre system. The OTDR yields a plot, or trace, indicating the loss versus distance. This information allows events in the optical fibre system such as fibre splices and connections to be located and their loss measured. The OTDR is a complex instrument and so detailed interpretation of an OTDR trace is a job for an experienced or expert fibre tester. The OTDR is not the reference test method for fibre attenuation, this being an ILM technique, and indeed the OTDR has some short-comings as an accurate loss measurement instrument. Where the OTDR does dominate is in the spatial information it provides on any fibres with high attenuation, or any poor quality splicing. The other advantage of the OTDR technique is that the stored trace shows exactly how the measurement was conducted, something the ILM technique does not provide, and this allows an expert OTDR trace analyser to know something of the quality of the measurement results provided.

• Modal Dispersion - Bandwidth testing. This is a multimode measurement to establish the capacity of installed multimode optical fibre. This is not a normal field measurement, and there is much controversy in the industry over whether this is a useful field measurement or just an expensive test that may not be representative of the bandwidth actually achievable in the fibre.

• Chromatic Dispersion testing: the field measurement of the chromatic dispersion of single-mode optical fibres is now becoming standard for telecommunications systems. This test is designed to see the change in group delay (effectively the spreading or dispersion) of optical signals of different wavelengths (chromatic infers different colours or wavelengths of light). This chromatic dispersion can be the limiting factor in achieving high bit-rates through the fibre system. Although a common laboratory measurement for fibre factories, the need for this measurement in the field comes from ever increasing bit rates and optical amplification leading to long optical spans between transmitters and receivers. The measurement method used is normally double ended and involves measuring the delay of a wavelength of light by reference to the phase of a high-frequency signal. For field measurements the differential phase shift technique (sometimes called the double demodulation technique) is the most common method employed, but there are other techniques including a single-ended OTDR time-of-flight technique.

• Polarisation Mode Dispersion - PMD testing: the field measurement of PMD is an important measurement for single-mode telecommunications systems since the PMD can change with cabling and installation. This is a strange parameter in that it's behaviour is not regular, or deterministic, and hence the analysis of PMD is statistical. Poor PMD will lead to poor system performance, particularly at high bit rates since this is a form of dispersion or pulse spreading. The most common technique for making field measurements of fibre PMD is using white light interferometry, a Michelson interferometer. This advanced instrument yields a plot, or interferogram, and from this data calculates the fibre PMD. Many people have said that the PMD measurement plot is meaningless or useless, and yet both statements are often false. Expert testers know the value of recognising various features on a PMD plot when they occur, and can advise their clients accordingly.

• Spectral attenuation measurements: there are now a growing number of instruments on the market to perform spectral attenuation measurements that show the loss (attenuation) of the fibre at different light wavelengths. This information is increasingly important for DWDM and CWDM systems that multiplex different wavelengths of light, that is launch several signals at different wavelengths down the same fibre. In the past this spectral attenuation measurement was only performed on occasions when chromatic dispersion measurements were being made, and then simply because a chromatic dispersion test set can perform a limited spectral attenuation scan.

• Optical Spectrum Analyser - OSA measurements: field measurements with an optical spectrum analyser are performed to check the wavelengths present on DWDM and CWDM systems. The OSA is also able to check exact wavelengths and power in a wavelength channel of a CWDM or DWDM system.

• Return Loss Measurements - RLM testing: this is a measurement most commonly performed in the field using an OTDR, but there are some advantages in using a Return Loss Test Set (RLTS) on some single-mode fibre systems. A confusing measurement for many, it is effectively a measure of the reflectance of a fibre systems (a bulk measurement of the combined reflectance of connectors, fibre and other events) - although technically it is the opposite of reflectance! The return loss is is a measure of increasing importance, particularly for single-mode optical fibre systems, since it can cause channel drift and hence drop-out on DWDM systems. It is an important parameter for analog cable TV systems operating over optical fibres. The debate is over whether it is better to rely on the OTDR or do a bulk return loss measurement using a suitable test set.

Fibre cable and optical component testing

The testing of fibre optic cables at cable factories and optical components in laboratories is a quite different prospect to testing field installed cable and components, and involves some quite different tests. The advantages of a laboratory test environment are stable temperatures and cleanliness, plus the fact that both ends of the fibre cable or components are close together rather than being 100 km apart.

The tests performed include most of the above field tests (for example ILM, RLM and OTDR), though often using laboratory style equipment, plus tests to establish the product quality. Some of these tests can be destructive tests to short samples, or environmental tests to a long cable length and requiring temperature cycling in an environmental chamber. Some of these tests are very quick to perform whilst others can require a large cable to stabilise at alternating high and low temperatures, and this time to reach equilibrium can take from hours to days depending on the cable.

We are not going to detail all of the optical, mechanical and environmental testing and accelerated lifetime testing here, but contact us if this is an area you would like help or training with as our trainers have experience in many of these fields.

Fibre optic sensors - discrete and distributed

There are many types of fibre optics sensor, bit these can generally be placed into two main groups: discrete fibre optic sensors, and; distributed fibre optic sensors.

A discrete sensor is one that measures at one point only, often at the far end of the fibre, though this is now changing with the advent of an increasing number of Bragg grating based sensors. The alternative distributed fibre optic sensor provides measurements along the length of a fibre equivalent to having multiple discrete sensors.

What sort of measurements can be made with fibre optic sensors?

The main measurements are:

• strain - a bulk measurement on a length of fibre
• strain - discrete measurements sometimes using Bragg gratings
• strain - distributed technique using Brillouin scattering and a B-OTDR
• pressure - discrete methods or techniques to transfer a pressure to strain in a fibre
• temperature - various discrete methods or distributed using the Raman Stokes and anti-Stokes scattering in a Distributed Temperature Sensor (DTS)
• spectroscopy for chemical or other analysis of gases and liquids remotely
• fibre optic gyroscope for measuring rotational movement
• vibration, velocity, acceleration and distance
• flow rates in liquids and gases
• current flow in conductors
• other remote optical sensing or measurement requirements

The reasons for using fibre optics sensors are numerous, but advantages are often either based on the safety aspect of optical fibres not carrying electrical current and so are electrically safe, or on the fibre allowing a better optical measurement. With distributed fibre sensors comes the added advantage of an equivalent number of discrete sensors often being too expensive or logistically difficult to manage.

To find out more about optical fibres or fibre optics technology please check our list of training courses.