Optical fibre fundamentals: modes, wavelengths and loss

Harbourview needs fibre in three places: a backbone from the main equipment room on the ground floor to the telecommunications room on each upper floor, a campus link to the existing college building 400 m away, and short connections inside the data room. The owner's network team has asked for 'whatever fibre we will not regret in fifteen years'. Daniel has terminated plenty of fibre, but he has always been told which type to use. Now he has to choose, and to justify the choice. Roz starts with a question: 'Why can light go round a corner inside a strand of glass at all?'

Optical fibre carries information as pulses of light through a strand of extremely pure glass. It offers far more bandwidth than copper over far longer distances, is immune to electromagnetic interference, carries no electrical current between buildings and is lighter and thinner for the capacity it provides. Its weaknesses are mechanical and practical: it can be damaged by tight bends and crushing, its connectors must be kept clean, and the electronics that drive it cost more than copper transceivers at short distances. This lesson explains how fibre guides light, the types designers choose between, and where its loss comes from.

How fibre guides light

A fibre has two layers of glass. The core, at the centre, carries the light. The cladding around it has a slightly lower refractive index, which is a measure of how much the glass slows light down compared with a vacuum. A protective coating, usually acrylate, is applied over the cladding to 250 micrometres or 200 micrometres in diameter, and cables add buffers, strength members and jackets on top of that.

When light inside the core hits the boundary with the cladding at a shallow enough angle, it is reflected back into the core completely. This is total internal reflection. It only happens if the angle of incidence, measured from a line perpendicular to the boundary, is greater than the critical angle, which depends on the two refractive indices: \(\sin\theta_c = n_2/n_1\). Light entering the fibre within a cone of acceptance is guided; light entering at a steeper angle escapes into the cladding and is lost. The size of the acceptance cone is described by the numerical aperture.

Because the guiding depends on angles, bending the fibre too tightly changes the angle at which light meets the boundary. Some light then falls below the critical angle and escapes. This is bend loss, and it explains why fibre cables have minimum bend radius limits and why bend-insensitive fibres, which use a modified index profile to hold the light in more tightly, are popular in dense installations.

Modes: multimode and singlemode

Light in a fibre travels in a limited number of distinct paths called modes. The number of modes depends on the core diameter, the numerical aperture and the wavelength. A large core supports many modes, so it is called multimode fibre. A very small core, around 8 to 9 micrometres, supports only one mode at the wavelengths used, so it is called singlemode fibre. Both types have the same 125 micrometre cladding diameter, so they look alike to the naked eye and use similar connector ferrules.

Different modes travel different distances. In a step-index multimode fibre, where the core has a uniform refractive index, a mode that zigzags steeply takes much longer to reach the far end than one travelling straight down the axis. A short pulse at the input therefore spreads out in time as it travels. This is modal dispersion. If pulses spread too far, they overlap and the receiver cannot tell them apart, which limits data rate and distance.

Modern multimode fibre reduces modal dispersion by using a graded-index core. The refractive index is highest at the centre and falls gradually toward the cladding. Light travelling the longer, outer paths moves through lower-index glass, where it travels faster, so it catches up with light on the shorter central path. Careful control of this profile is what distinguishes the higher grades of multimode fibre.

Singlemode fibre has no modal dispersion because it carries only one mode. Its bandwidth is limited instead by chromatic dispersion: different wavelengths in the light source travel at slightly different speeds. With narrow-linewidth lasers, singlemode fibre supports very high data rates over tens of kilometres, which is why it is the standard choice for campus, metropolitan and long-haul networks, and increasingly inside data centres.

Fibre types designers specify

Multimode fibres are graded by the OM classes in ISO/IEC 11801 and the matching TIA designations. They are rated by effective modal bandwidth (EMB), measured in megahertz-kilometres at a given wavelength: the product of the bandwidth and the length that a fibre can support.

Laser-optimized multimode fibres, OM3 and newer, are designed for vertical-cavity surface-emitting lasers (VCSELs), the inexpensive light sources used by short-reach multimode transceivers at 850 nm. The higher the grade, the further a given application can reach. For example, a common 10 gigabit multimode application reaches about 300 m on OM3 and about 400 m on OM4. Higher-speed multimode applications have much shorter reaches, often 100 m or less, which is why many large data centres now use singlemode even for short links.

Fibre cable jackets are often colour-coded by type in premises cabling: aqua is common for OM3 and OM4, sometimes violet for OM4, lime green for OM5 and yellow for singlemode. Colour coding is a convention, not a guarantee. The printed legend on the jacket and the manufacturer's documentation identify the fibre.

Wavelengths and the attenuation curve

Glass absorbs and scatters light differently at different wavelengths. Premises networks use a few well-established operating windows where loss is low and practical light sources exist: 850 nm and 1300 nm for multimode, and 1310 nm and 1550 nm for singlemode, with other wavelengths used for multiplexing.

Two main mechanisms cause loss inside the glass. Rayleigh scattering comes from tiny variations in the glass structure that scatter light in all directions. It falls sharply with wavelength, roughly as \(1/\lambda^{4}\), which is why loss is much lower at 1550 nm than at 850 nm. Absorption comes from impurities and from the glass itself. Older fibres had a pronounced absorption peak near 1383 nm caused by water molecules trapped in the glass; low-water-peak fibres such as those meeting ITU-T G.652.D remove most of it.

The attenuation coefficient of a fibre is quoted in decibels per kilometre. Typical cabled values are a few dB per kilometre for multimode at 850 nm and well under 1 dB per kilometre for singlemode. Cabling standards set maximum values for design, which are higher than the typical values in data sheets. Lesson 7 uses those maximum values to build loss budgets.

Loss outside the glass

In premises networks, the fibre itself is rarely the largest source of loss. Most loss comes from connections, splices and installation damage. A mated connector pair loses light because the two cores are never perfectly aligned, the end faces are never perfectly clean or flat, and some light reflects at the gap. Splices, which join two fibres permanently, lose less: a good fusion splice typically loses well under a tenth of a decibel. Bends cause two kinds of loss. Macrobends are visible curves tighter than the minimum bend radius. Microbends are tiny distortions caused by pressure on the fibre, for example from over-tightened ties or a crushed cable, and they can cause significant loss even when the cable looks undamaged.

Contamination is the leading cause of fibre link failures. A speck of dust a few micrometres across can block a large part of a singlemode core. Inspection and cleaning before every mating, using a fibre inspection scope, is the single most effective practice for keeping fibre links healthy, and current standards and test procedures expect it.

Choosing fibre at Harbourview

Roz and Daniel work through the three uses. Inside the data room, links between switches and servers are short, but the network team expects to move to higher speeds that have shorter multimode reaches. Inside the building, the backbone runs are under 100 m. For the campus link of 400 m, multimode would limit future speeds. Roz's recommendation is a hybrid backbone with singlemode for the campus link and the riser backbone, plus OM4 multimode in the riser for any existing equipment that needs it, and singlemode-ready pathways in the data room. 'Singlemode glass is cheaper than multimode,' she points out. 'The optics cost more today, but the glass will outlive several generations of optics.' The decision will be confirmed with the network team, and the loss budgets in Lesson 7 will check that each link works.

Exam focus: choosing between fibre types

Questions about fibre types usually describe a distance, a speed and sometimes an expected upgrade, and ask which fibre to choose. Work through them in order. First, how far must the link reach? Beyond a few hundred metres, singlemode is almost always the answer. Second, how fast must it go, now and later? Higher-speed multimode applications have shorter reaches, so a link that works for 10 gigabits on OM3 may not work for 100 gigabits. Third, what equipment exists? Legacy equipment may need OM1 or OM2 compatible links, which is a reason to keep some multimode in a backbone. Fourth, what is the cost balance? Multimode transceivers have traditionally been cheaper at short distances, but that gap has narrowed.

Be careful with mixed-type questions. A 62.5 micrometre fibre should never be connected to a 50 micrometre fibre in a link, and multimode should never be connected to singlemode. The core mismatch causes large loss, especially from the larger core into the smaller one. Where both types are installed, colour coding, labelling and different connector or adapter styles help prevent mistakes.