1. Optical Fibre

1.1. What is Optical Fibre?

Optical fibre is a filament, normally made of glass or polymer, which guides light in its "core".

1.2.   How Does It Work?

A physical phenomenon called total internal reflection, which occurs at the boundary of two dielectric

(non-conductive) materials means light can be guided in a "core", which is surrounded by a material of lower refractive index (called the "cladding").

1.3. What is it used for?

Communications, transmission of light energy, medical purposes, visual displays.

1.4.  Advantages/Disadvantages

1.4.1.    Advantages

•   Electrically isolated (immune to electrical interference)

• Very high bandwidth (communications capacity)

•   Lightweight

•   Long range

1.4.2.     Disadvantages

•  Glass fibre is mechanically brittle

•  Sensitive to excessive bending.

1.5.   Properties

1.5.1.     Fibre Nomenclature

Fibres are normally described as their core diameter in (microns) e.g. 50µm over their cladding diameter

e.g. 125µm (said "fifty, one, two, five"). Common descriptions are 9/125, 50/125, 62.5/125, 100/140.

Fibres, which have a core diameter of approximately 9 microns, are normally called "single-mode" because the light can only travel in one mode. Fibres with larger cores are called multi-mode because there are many modes in which the light can travel. Singe-mode fibres tend to be used in long distance and very high data rate applications, multimode fibres tend to be more forgiving on interconnections and are more suited to shorter links.

The central core can have a uniform index, in which case the fibre is "step index" or it can be graded (to increase bandwidth in multi-mode fibres normally) in which case it is called "graded index".

1.5.2.    Attenuation

Attenuation is the rate at which power is lost along a fibre, normally expressed in dB/km. (E.g. 3dB/km corresponds with a 50% power loss in 1km of fibre). Attenuation in pristine fibre is normally linear with distance, with some exceptions for short lengths of multi-mode fibre. Attenuation (with the exception of some absorption peaks) becomes lower at longer light wavelengths. Attenuation is often specified

for operational wavelengths of 850, 1300 and 1550nm (all infrared), which one(s) depending on the

fibre type. Special fibres may use other wavelengths, including those in the visible spectrum. Unlike signal attenuation in copper communication cables, fibre attenuation is independent of the signal frequency in normal operation.

1.5.3.      Bandwidth

Expressed in MHz-km bandwidth indicates the maximum frequency at which light can be modulated over 1km of fibre while retaining a discernible signal. An assumption of linear bandwidth reduction with distance is normally conservative (e.g. S00Mhz@ 1km, 250Mhz@ 2km etc). Some newer fibres are tuned and designed to work at very high frequency (e.g. 1GHz) over a specific maximum distance (e.g. 300m), mainly for high bandwidth LAN applications. Bandwidth is normally applied to multi-mode fibres only, and may vary with operational wavelength.

1.5.4.      Dispersion

Fibre dispersion, normally specified for single-mode fibres, indicates the degree of pulse spreading for light of different wavelengths. This is normally the parameter, which limits the bandwidth of single- mode fibres. Most single-mode fibres of the same type have similar dispersion characteristics, and the overall fibre bandwidth is therefore mostly controlled by the wavelength spread in the modulated light source used, rather than the fibre itself.

1.5.5.      Proof Test

Indicates the level of tensile load applied to the fibre during manufacture, which is used to screen out weak flaws in the fibre. This is mainly of importance if the fibre will be subject to mechanical strain, particularly long term strain. A load corresponding to an elongation of between 0.5 & 1% in the fibre is generally accepted as sufficient in most applications.

1.5.6.      Numerical Aperture (NA)

Indicates the angle of capture for light entering the fibre. This is a function of the difference in refractive index between the core and the cladding. A larger NA indicates more light can potentially be coupled into a fibre. Most standard fibre types, from different manufacturers, will have the same nominal NA, but slight variation in tolerances and manufacture can cause a slight "mismatch" at joints and connections. This means some measurements must be done in two directions to get a true loss reading, but excess loss due to this mismatch is normally negligible.

1.5.7.      Fibre Coating

To prevent brittle failure of glass fibre a complete pristine plastic coating on the glass surface is essential. This is applied as part of the fibre manufacturing process and normally consists of one or two very thin layers of polymer. Additional coatings may be applied later to increase the robustness and ease of handling of the fibre. Fibre with just its first thin coating (normally to approx. 250µm)

is generally called "primary coated fibre"; fibre with additional coating(s) is called "secondary coated fibre".

2. Optical Fibre Cables

2.1.Types of Cable

Glass optical fibre is relatively fragile, and to use it successfully normally requires incorporation into a protective cable. Many different cable designs exist but in all cases the main purpose of the cable is to prevent mechanical damage and excessive mechanical strain on the optical fibre, during installation and operation.

Other considerations may be the performance of the cable during exceptional events such as fires or attack by rodents. The choice of the correct cable is essential to ensure long term system reliability. Most optical fibre cables fall into one of two categories:

2.1.1.       Loose Buffer Construction

Loose buffer construction cables normally use primary coated fibres. These are loosely housed in

bundles within tubes or grooves. The fact they are "loose" means there is a small amount of de-coupling between mechanical stress in the cable and stresses on the fibres, which the primary fibres require to work satisfactorily. This type of cable is often the most cost effective for high fibre counts, and in this case offers the most compact design. Its main disadvantage is that the primary fibres used are quite fragile so normally need to be connected to more robust fibre or small cables before optical connectors can be installed, and before final connection to transmission equipment.

2.1.2.     Tight Buffer Construction

In tight construction cables secondary coated fibres are normally used. The fibres are more robust and therefore, with the correct cable construction, do not require the "looseness" protection required by the primary coated fibres. The easy-to-handle robust fibres can be directly terminated with connectors, which can remove the need for intermediate joints. Tight buffer constructions will often be easier to install because of the reduced fibre fragility.

Both these constructions may be supplied with parallel or stranded components/fibres. Parallel components mean that when a cable is bent the outer components tend to be stretched where as the inner ones are buckled. With stranded cables this does not happen, the bend is taken by slight twisting or untwisting of the cabled bundle. In summary stranded cables offer superior bending performance,

and better defined long term temperature performance, where parallel cables offer less expensive cables because they do not require the relatively slow stranding process for their manufacture.

2.2.   Optical Fibre Cables, the main parameters to consider or specify:

2.2.1.       Mechanical Parameters

• Tensile strength

•   Bend radius

•  Crush resistance

•   Kink resistance

•   Repeated bending/flexibility

•   Diameter and weight

• Type of armouring required (if any)

2.2.2.       Environmental Parameters

•   Indoor or outdoor use

•   UV resistance

• Water/chemical resistance

• Temperature range before, during and after installation

•   Performance in the event of a fire.

2.2.3.      Optical Parameters

• According to fibre parameters above. (The only parameter subject to possible significant change due to the cable manufacture process is the fibre attenuation, but in most cases this is negligible or nothing.

•   Numbers of fibres, type, colour coding, and distribution in the cable.

2.2.4.       Electrical Parameters

•   Maximum voltage (Operating above 1000V requires special attention (see section 6)

•   Earthing requirements for cables with metallic components.

2.2.5.       Miscellaneous

•  Sheath marking

•  Sheath colour

•   Delivered lengths/drum dimensions/drum marking

3. Interconnection

3.1.   Different Interconnection Techniques

3.1.1.       Fusion Splicing

Fusion splicing, as the names suggests, uses an electric arc to the heat and fuse the ends of two optical fibres together. The uncoated fibre near the joint normally is protected with a reinforced heat shrinkable sleeve. Fusion splicing is generally the fastest, most reliable and cost effective way of joining large numbers of fibres. Average excess losses with modern automatic machines are normally <0.1 dB.

The main disadvantage of fusion splicing is that the joints are not "de-mountable" and must be remade to alter or change fibre connections.

3.1.2.      Connectors

Optical connectors are normally used to connect to terminal equipment. They can also be used for

"in-line" joints and are particularly useful if frequent "patching" (i.e. changing fibre connection routes)

is required. A connector joint will normally use two identical connectors (one on each fibre to be joined) and in-line bulkhead adapter.

Many different connector types exist, and in some cases different types can be joined with special adapters. Insertion losses are typically higher than fusion splices. Another important difference from fusion joints is that there is often a back reflection of light from the joint because of a glass to air interface. This reflected light might affect some transmission equipment lasers or LED's. To reduce this reflection the surface of the connector can have a "convex" or "angle" polish and care must be taken in using compatible types of connectors and polish types in this case. The amount of reflected light is indicated by the connector "return loss" in dB, the more highly negative this value, the lower the amount of reflected light.

In high volumes connector-connector joints will be more expensive than fusion jointing, more time consuming and less reliable, however in low volumes and where a high level of interconnectivity is required they may be preferred.

3.1.3.       Mechanical Splices

Mechanical splices are small stand-alone devices for mechanically aligning and retaining optical fibres. In some cases they resemble connector-connectors joints, and are de-mountable, although unlike connectors they are not suitable for many repeated connect/disconnect operations. Other joints are a one off device where once the fibres are secured they are designed to remain in place for life. As with connectors they are normally only more cost effective, and preferred over fusion splicing, in low volumes. Again reflected light may be an issue but this can often be reduced by use of an

"index-matching" material.

4. Testing

4.1.   Optical Fibre Tests

Optical fibre can be subject to many different types of test. The fibre manufacturer will normally carry out a range of test to assure the quality of the manufactured fibre. This will include all the fibre geometric parameters and optical parameters such as attenuation at different wavelengths, bandwidth or dispersion and numerical aperture. The only parameter, which may be subject to significant change after fibre manufacture, is attenuation, as excessive large or small scale bending (macro/microbending) of the fibre, can increase fibre attenuation. This means a cable manufacturer and fibre installer will normally measure fibre attenuation.

The installer will also be interested in the loss at joints in the link. There are two principle ways to measure fibre attenuation.

4.1.1.      OTDR (Optical Time Domain Reflectometer)

This machine is a probably the most important diagnostic tool for the general optical fibre user.

The machine launches a series of high power laser pulses into the fibre and measures light scattered back to the launch (backscatter). The backscatter, displayed as a trace on a screen normally, shows the signal reduction as a function of length, and can therefore be manipulated to measure fibre attenuation, splice losses and any high loss areas in a fibre link. Modern machines with powerful software tools can quickly and automatically evaluate a fibre link and give the position and loss of joints or problem areas. True evaluation of a link requires a two-way average OTDR measurement because slight variations in fibre properties such as NA can cause apparent gains and excess losses at joints.

The range and resolution of the OTDR will depend principally on the pulse width(s) that the laser uses. A short pulse width will yield high resolution so problems close together may be easily distinguished, whereas a long pulse width will provide a lot of energy for long range measurements. The backscatter signals are relatively weak and therefore most OTDR's need to perform some level of averaging to improve the signal to noise ratio on the displayed trace. The amount of averaging used will depend on the application and the signal noise ratio required. Most machines give a minimum average "real time" display, which allows the machine to be connected and optimised with the test fibre.

All OTDR's have a "dead-zone" which is a distance into the test fibre where the OTDR cannot measure properly. This is related primarily to the pulse width, but in some cases may be reduced slightly by use of a length of fibre between the OTDR and test fibre, or special masking features available on

some machines.

OTDR's can operate at various wavelengths, however it is important that single-mode machines test single-mode fibres, and likewise for multi-mode machines, where care must be taken in matching the OTDR to the fibre type. Single-mode machines operating at 1300 and 1550nm are particularly useful as a bending loss is much more sensitive at 1550nm than 1300nm in single-mode fibres and comparing traces at two wavelengths can help with fault diagnosis.

4.1.2.       Power Meter Set (Transmitter and receiver)

Power meters will simply measure the amount of power lost over the fibre link. They are useful to use in addition to an OTDR as they measure the entire fibre link, including the sections at each end that the OTDR cannot "see".

The critical parameter when using a power meter is determining the input power to the fibre link. To do this, a short known length of fibre, with connectors identical to link, is installed between transmitter and receiver, and the receiver light power is recorded, or "zeroed". The optical link is then connected in place of the short fibre and the new power recorded. The reduction in power is then the loss of the fibre link. This will normally correspond reasonably well with the OTDR overall attenuation, and any major discrepancies should be investigated.

In the case of multi-mode fibre an excess power meter loss of 1-2dB can be expected unless special precautions to provide stable light transmission in the short length fibre and OTDR measurements, because of non-linear attenuation in short lengths of multi-mode fibre.

In the absence of an OTDR a power meter can give a broad indication as to that state of a link, and may give over a period of time, any degradation, but it cannot show up small localised problems and assist in finding any problems.

4.2.   Optical Fibre Cable Tests

As described above the fibres in optical fibre cables are checked for fibre attenuation after the cable has been manufactured. Other routine checks such as dimensional and constructional details should also be carried out. In some cases type tests may be required to prove that a cable is suitable for a particular purpose, and will be done to confirm the cable meets the required performance criteria based on the parameters as described in section 2 above. Type tests can be quite lengthy and expensive, and many projects do not justify this amount of effort. One solution is to use cables based on "Generic" designs, on which testing has been carried out,

or where the supplier has a great deal of service experience. In this way the customer and supplier can have a high confidence level that the cable is fit for purpose, without performing a set of special tests.

5. Cable Installation

5.1.   Installation Parameters to Consider

Correct and careful installation of optical fibre cables is important to their successful long-term operation and to prevent problems appearing during commissioning. The main parameters to consider and observe are:

•   Ensure the maximum recommended tensile loading is observed (e.g. by using a mechanical fuse)

•   Ensure the gripping method (cable sock etc) actually transmits load to the strength members of the cable.

•  Check twist of cable during installation. (Some unbalanced cables may twist during installation).

•   Ensure any fittings (cleats, clamps, splice boxes etc) are compatible with the cable being used.

•   Ensure the minimum loaded/unload bend radii are observed (e.g. using installation pulleys)

•   Ensure there a no sharp edges over which cable may be laid and then crushed against.

• Take care not to damage the outer sheath of the cable.

6. Optical Fibres at High Voltage

6.1.   Important Parameters to Consider at High Voltage

Glass is inherently a superb insulating material and is therefore an obvious choice for data transmission between equipment at high voltage and the ground. How this fibre is used is however critical to its long term performance. High voltages will quickly, or some times over a period of several years, reveal weaknesses in fibre links. Normal fibre coating materials will breakdown in the presence of moisture and pollution. Also, in a typical high voltage stress environment, fibre optic cables that are used in normal data applications will breakdown and fail due to their construction and materials. It is for this reason special fibre optic links and cables with tried and tested materials/constructions are now normally used for these types of applications.