The Development of Fibre

I shall consider the development of fibre in several sections rather than giving a general discussion of fibre properties.

Two types of material are used to manufacture fibres – glass and plastic. There are several properties of a material that dictate how useful it is as a fibre:

We must remember that there are many different types of fibre and applications for fibre. The different properties of fibres can be combined to provide a suitable fibre for a particular job – not all fibres will be applicable for every situation.

The purity of the fibre will be reflected in both its attenuation properties (consider the scattering effect of an impurity particle) and its refractive index. The original breakthrough in reducing fibre attenuation was achieved by purifying the glass used to make the fibres. There are other intrinsic and extrinsic factors which contribute to the attenuation, such as absorption by OH- ions, absorption of infra-red radiation leading to molecular vibrations, leakage from the core (can be caused by Rayleigh Scattering and fibre curvature) and leaky modes. Curvature is important in fibre specification; again a more detailed analysis of the propagation of light through fibres is required to fully explain this, but essentially a small amount of the light is radiated as the fibre bends. Leaky modes are modes slightly below the cut-off, but can be propagated for a short distance along the fibre; they can be initially avoided at the light source by restricting the angle at which light enters the fibre, but can be introduced along the fibre by microbending. Microbends are minute bends in the fibre which can be introduced during manufacture or cabling (see later for more detail on cabling); they can cause power to be transferred between modes, possibly to leaky modes and hence can result in power loss.

The level of attenuation must clearly be minimised to increase the distance the signal can travel along a fibre without amplification. An example plot of how attenuation varies with wavelength is given in figure 5. The two types of fibre noted (Single-Mode and Graded-Index) will be explained further through this essay. The units dB are logarithmic. It indicates how we choose the most appropriate wavelengths of light to use with a particular fibre.

Fibre Attenuation

Figure 5 - Wavelength Dependence of Fibre Attenuation (source: TestMark Laboratories)

The two attenuation minima (shown by the dashed lines) are approximately at lambda=1300nm and lambda=1550nm, and hence these two wavelengths are the most commonly used amongst modern fibre systems.

The refractive index of the material is crucial to total internal reflection. A single-mode fibre has essentially a discontinuity in refractive index at the core-cladding boundary (although a rigorous analysis of such a fibre should consider that during the manufacturing process there is some ‘blurring’ of the index over a very short distance (~10^-9 m)). As mentioned earlier, various modes can pass through an optical fibre; the number of modes that can pass through a fibre can be shown to be dependent on the diameter of the fibre. The diameter of the fibre can be reduced such that only one mode can exist within the fibre – this removes the problem of modal dispersion and hence allows significantly higher bandwidths than would otherwise be available.

The alternative type of fibre is multi-mode. This, as it sounds, allows the existence of various modes. This initially led to modal dispersion problems, and so was used for short distances (~500m) where dispersion was not relevant. An alternative solution to the problem was later found – graded-index fibre, where there is a gradual gradient of refractive index through the core which peaks in the centre (in reality this is a series of gradually changing refractive indices – typically with around 200 different values). This causes the paths of the various modes to be sinusoidal and can be used to ensure that the different modes travel along the fibre with a similar effective velocity. This considerably reduces the modal dispersion to values of the order of 1ns per km (the pulse width increases by 1ns every km) (TestMark Laboratories, 1999).

There are other types of dispersion:

There has also been development of polarisation-sensitive fibres, of which there are two principle types. A polarisation-sensitive fibre will transmit light of one linear polarisation but not the other. A polarisation maintaining fibre is birefringent; it effectively isolates the two components of the light and transmits them individually (due to different properties of the fibre for different linear p olarisations); thus the emerging light has the same polarisation as the incident light.

With the constantly increasing demand for data bandwidth, fibre designers are continually looking for ways to increase the bandwidth of a single fibre. All high-bandwidth long-distance fibres are single-mode, and so there is no modal dispersion to reduce bandwidth. Further developments are always sought though – the most recent of these is DWDM (dense wave division multiplexing). This allows multiple light sources (each operating at a different wavelength) to transmit data down a single fibre. Currently 4, 8, 16 or 32 wavelengths can be multiplexed on a fibre , with 64- and 128-channel systems in development. It is also reported that multiplexing of over 200 wavelengths on one fibre has been demonstrated by manufacturers.

The various factors mentioned above (attenuation and various forms of dispersion) require the use of repeaters. This term, although widely used, is unclear. It is used to describe both optical repeaters and regenerators. Optical repeaters are purely optical devices that are used simply to combat attenuation in the fibre; typically spans of 80km upwards are now possible. The recent introduction of soliton transmission methods has increased the allowed distance between repeaters and systems spanning 130km without a repeater are now possible. Regenerators are devices consisting of both electronic and optical components to provide ‘3R’ regeneration – Retiming, Reshaping, Regeneration. Retiming and reshaping detect the digital signal which will be distorted and noisy (partly due to the optical repeaters), and recreate it as a clean signal (see figure 6). This clean signal is then regenerated (optically amplified) to be sent on. It should be noted that repeaters are purely optical devices whereas regenerators require optical-to-electrical (O/E) conversion and electrical-to-optical (E/O) conversion. The ultimate aim of many fibre system researchers is to create a purely optical network without electronics, which would maximise efficiency and performance. Many aspects of such a system are in place, but some still require the O/E and E/O conversion.

Regeneration

Figure 6 - A digital signal before (noisy and attenuated) and after regeneration

The most common optical amplifier currently in use is the EDFA (Erbium Doped Fibre Amplifier). These consists of a coil of fibre doped with the rare earth metal erbium. A laser diode (see later) pumps the erbium atoms to a high energy state; when the signal reaches the doped fibre the energy of the erbium atoms is transferred to the signal, thus amplifying it.

Cabling is the procedure of grouping fibres together with strength members (see below) and possibly other materials to produce a product that is ready for field use. The type of cable is dependent on the environment in which it will be used. It would not be possible within this short essay to comprehensively describe the types of cable applicable to every situation, but I shall endeavour to cover the majority of general environment categories.

First, we must analyse the purpose of the various elements in a cable. Of course there is fibre to carry the data; there may be many fibres within an individual cable and a considerable amount of research is devoted to the finding the strongest and most efficient systems of organisation of various numbers of fibre within a cable. Strength members are used to add tensile strength to fibres; although fibre has a high tensile strength of its own, this has to be increased for many applications (e.g. submarine cables). One of the most common strength members is Kevlar® which is a specialised material (manufactured by DuPont) consisting of long molecular chains produced from paraphenylene terephthalamide. It has a tensile strength significantly greater than high-tensile steel and has the advantage of being lighter than steel. Armour is used to protect the cable from pressure and rodents (most fibre literature, being US-based, mentions gophers as a primary cause of fibre failure!).

The main properties that are required of fibre cables in various environments are:

The following table describes the properties of cables for use in different environments:

Environment

Cable Properties

Intra-Device (e.g. inside a computer or switching system)

Should be small with little protection as this is given by the unit in which it is housed.

Intra-Office (e.g. server backbones of computer networks, and recently directly to workstations)

Usually contain one or two fibres. They should be small if being routed through an office, but should be durable enough to withstand being moved by office staff. Under-carpet cables are available which have an extremely low profile and high crush-resistance. These cables must also be compatible with the kind of connectors used for network connections (which I shall not discuss here – it would provide enough material for another essay!)

Intra-Building (i.e. between offices)

Similar to intra-office cables, but possibly need to be more robust (added strength members and a tougher outer coating) to survive installation. A larger number of fibres may be required within a single cable for vertical network backbones (in a topological rather than physical sense). Cables for use in factories will often be susceptible to added physical stress and so must include added rigidity and possibly a form of light armour.

Breakout Cables (i.e. cables that are split up before termination)

Again similar to above, but must be suitable for splitting into individual fibres for data distribution. The sub-cables must therefore be suitably protected as well as having an outer shielding.

Plenum Cables (which are routed through ducts already present in a building such as heating ducts)

These must be fire-retardant; originally fluoropolymers were used, although alternatives are now being developed. The materials used can be expensive and so cable size must be kept to a minimum.

Temporary Cables – Light-Duty (e.g. for a video link)

These should have similar properties to intra-office cables, but with added rigidity to survive constantly being moved.

Temporary Cables – Heavy-Duty (e.g. for military field use)

These should have extreme crush-resistance, very high tensile strength, be waterproof and be resistant to impulse damage and damage from bending.

Aerial Cables

These are for outdoor use, to be strung between supports. If directly strung then the tension on the cable is extremely high; it is more usual to lash the fibre cable to a supporting cable, so that considerably less stress is applied to the fibre cable. The cables should have strength members and structures to isolate the fibre itself from any of these stresses. The cable must also be waterproof and resistant to temperature changes.

Ducted Cables & Direct Burial Cables (buried in the ground directly with no duct or other protection)

Similar to aerial cables but with added armour to protect the fibre from rodents. Polymer-coated galvanised steel is common as armour since it is strong and resistant to corrosion by oxidation.

Submarine Cables

Great crush resistance is required to withstand the pressure of several kilometres of water. High tensile strength is required because of the strain put on the cable whilst laying it. Even higher strains are put on cables if they are retrieved and relayed for maintenance work. If the cable is to be placed in shallow water then extra armour is essential to protect it from activities such as fishing.

Power Line Cables

Some fibres are run along with overhead power lines – usually the single earth cable. Two systems are used; (i) the fibre being wrapped around the conducting cable and (ii) OPGW (Optical Ground Wire) where the conductor contains the fibres in tubes at the centre of the conductor construction.

We should also consider fibre housing – the structure that surrounds the fibre within the cable. There are two main designs – loose-tube and tightly buffered. Loose-tube cable is where one or more fibres run in a loose helix through a tube allowing the fibres to move freely within the tube and protecting them from stresses that could cause microbending. There are different types and shapes of loose-tube cable, all of which isolate the fibre from external forces. For outdoor cables a gel is added which lubricates the fibre to aid it to move within the tube and to protect it from moisture. Tightly buffered fibre is directly encased within one or more layers of casing; the innermost layer is usually a soft plastic that will reduce the forces transferred to the fibre through the casing, whereas the outer layers are made from harder plastics to give added physical protection to the fibre.

Loose-tube cables provide better physical protection and so are useful for preventing microbending loss. Tightly buffered fibres provide better crush-resistance and allow for more densely packed cables. It also makes automated joining easier for multi-fibre cables since the fibre positions are predictable with respect to the cable.

Joining Fibres

This has always been seen as a ‘black art’; in the past there have only been specialised technicians who have been able to reliably join fibres. Nowadays, with fibres manufactured to higher quality, there is more tolerance in the joining of fibres although it is still a precision technique. When joining fibres, alignment is critical.

The two methods of joining fibres are splicing and using connectors.

Splices

Splicing involves forming a permanent join between two lengths of fibre. The fibres are only of the order of micrometers in diameter (although multimode fibres do have a slightly larger diameter), and so must be joined with a lateral alignment precision of ~10^-7m to avoid attenuation. This requires the use of specialist equipment including some kind of magnification.

There are two types of splice – mechanical and fusion. A mechanical splice is performed using an epoxy substance to match the refractive indices of the materials and the cables are held together using a clamp. The epoxy material reduces Fresnel reflection (which is caused when light leaves a material of one refractive index and enters a material of another). A fusion splice fuses the two cables together using a short intense blast of heat. These splices are reportedly at least as strong as the fibre itself, if not stronger.

Connectors

These are permanently connected to the end of individual cables so that they can be joined simply at a later stage (i.e. during installation) and can even be moved. Again, lateral alignment is a crucial issue in the design of connectors – when two connectors mate, the cables must be aligned precisely. This is easier with multimode fibre (the type of fibre generally used for topologically horizontal network connections and so the type that is most likely to need to be moved / reconnected during its lifetime) because of its greater diameter.

Originally the individual fibre ends were prepared such that they were completely flat, whereas the latest technique is to curve them. This reduces the attenuation to a minimum and also decreases the amount of back-reflection (reflection of the signal back along the fibre).

Other factors are important in reducing attenuation. The numerical aperture of the two fibres to be joined must be as similar as possible. Consider two fibres where the receiving fibre has a smaller numerical aperture than the larger fibre – some of the modes in the delivering fibre will not be able to enter or will not be internally reflected within the receiving fibre (obviously this is not applicable to single-mode fibres). If there is a gap between the two fibre ends then some light will be incident on the cladding of the receiving fibre rather than on the core (since as light leaves a fibre it spreads out). This loss increases as the distance between the fibres increases and as the numerical aperture of the fibres increases. Gaps between the fibre ends also increase the amount of Fresnel reflection (see above). Any impurities in the connector (such as dust) or an unpolished fibre end can cause the attenuation of a connector to increase appreciably.

Light Sources

Two types of light source are used with fibres, LEDs and Laser Diodes. LEDs can operate in the near-infrared (the main wavelengths used in fibres are 1300nm and 1550nm, along with 850nm for some applications); they can emit light at 850nm and 1300nm. They also have the advantages of long lifetimes and being cheap. Unfortunately they are large compared to the cross-section of a fibre and so a large amount of light is lost in the coupling of an LED with a fibre. This also reduces the amount of modal control designers have over incident light. Laser diodes can be made to emit light at either 1300nm or 1550 nm, and also over a small spectral width (unlike LEDs) which reduces chromatic dispersion. Their emitting areas are extremely small (~0.4 micrometers x 1 mmicrometers) and so the angle of incidence of light on a fibre can be accurately controlled such that <5% of the possible modes within a multimode fibre will be initially used (although microbending can shift energy between modes and so the number of modes used may increase along a length of fibre). They are more efficient than LEDs in terms of coupling of light into the fibre, although they have shorter lifetimes than and are more expensive than LEDs. One crucial advantage of lasers over LEDs in today’s world of digital communications is their high switching speed and small rise times (rise time is the time taken for power to rise from 10% to 90% of the maximum power of the pulse, fall time is the opposite), leading to increased bandwidth.

Detecting the Signal

The most efficient detectors are reverse-bias photodetectors. A full analysis of these requires an in-depth understanding of semiconductor physics and so is not appropriate here. They essentially cause a current to flow when light is incident on them. The choice of semiconductor that is used to fabricate the detector is dependent on the wavelength sensitivity and the responsivity (effectively proportional to quantum efficiency) that are required. Bandwidth considerations are also important (determined by the rise time and fall time of a detector); in detectors the fall time is often appreciably greater than the rise time and so this must be used to calculate the bandwidth of a detector.

There are many further complications in detectors, including noise equivalent power which indicates how ‘clean’ a signal from a detector is. An analysis of how analogue and digital signals are processed after the initial detector is also interesting, but not within the scope of this essay.

What’s Happening Now?

The best way to describe the current technology being deployed is to provide a brief case study of a company that extensively uses optical fibres. I have chosen Energis PLC, a British Company that is currently expanding its network throughout Europe.

Their network exclusively uses single-mode optical fibres. Partly due to their commercial history a large number of their fibre cables are installed on lines belonging to the National Grid Company and to some regional electricity companies. They also use cables buried in ducts under roads and pavements.

They use DWDM (some uni-directional and some bi-directional) with 10Gbit/sec on each of 4 wavelengths. They are currently introducing 32 wavelength systems, and will inevitably introduce 40 Gbit/sec-per-wavelength systems in the near future.

Their optical repeaters (for which they use EDFAs) are typically spaced at around 80km and their regenerators can be up to 600km apart, although this is partially dependent on previous repeater locations. New technologies that are emerging will theoretically increase the regenerator spacing to 5000km (and even further with the introduction of soliton transmission), although DWDM does reduce the allowed distance between amplifiers and regenerators.