Fibre optics: The lowdown

US-based cable specialist Corning looks at some of the technical issues that need to be considered when fibre optic cable projects are undertaken.

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By  Guest contribution Published  December 28, 2009 Communications Middle East & Africa Logo

W ith the arrival of several submarine fibre optic cables on the African coasts, a record number of terrestrial long-haul networks are being deployed.

Since the main capital cost of deploying a terrestrial fibre-optic network is in the civil works, and the fundamental bandwidth capability is defined by the optical fibre itself, it is imperative to select the appropriate optical fibre type to extend the cable’s capacity to meet continuously increasing bandwidth requirements and avoid a premature need to redeploy new cable. The first step in selecting the appropriate fibre is to understand the importance of the following attributes which strongly influence the maximum achievable span, the overall link length and the bandwidth capability.

Attenuation

Attenuation refers to the loss of power that occurs in an optical signal as it travels along an optical fibre. Attenuation varies with wavelength: typical fibre attenuation is lowest at 1550nm, making the “C-Band” (between 1530nm and 1565nm) the most suitable for long-haul transmission. After some distance, attenuation will render the optical signal to noise ratio (OSNR) too low for the signal to be deciphered at the receiver.

Chromatic and polarisation mode dispersion

An optical signal pulse is comprised of a narrow spread of wavelengths. Due to chromatic dispersion, each individual wavelength will travel at a different speed along an optical fibre, resulting in pulse broadening which increases with distance. At high data rates (≥10Gb/s) the signal pulses are short and so are more prone to dispersion induced overlap, known as inter symbol interference (ISI), therefore, particularly for high dispersion fibres, dispersion compensation must be used.

Polarisation mode dispersion (PMD) also causes ISI and can severely limit high data rate transmission (≥10Gb/s), but PMD compensation is not easily achieved and so a fibre that operates at 2.5Gb/s today may not work at 10Gb/s or 40Gb/s because of high PMD. Fibres with low PMD must therefore be chosen.

Non-linear effects

Non-linear effects occur due to high light intensities, caused by high optical power and or small fibre effective area. Non-linear effects cause significant concern in WDM (wavelength division multiplexed) systems, where the power is high due to simultaneous transmission of multiple channels: the result is increased system noise and reduced OSNR. A large fibre effective area will reduce all non-linear effects. A moderate amount of fibre dispersion (non-zero) will also assist in reducing non-linear noise.

In the typical WDM system several optical signals emitted by transmitters at different wavelengths are combined by a multiplexer onto one single fibre, transmitted across the link and then demultiplexed before reaching the receivers at the other end.

To combat attenuation the signals are amplified after a span of circa 80km, but accumulated amplifier noise (ASE) ultimately limits the maximum link reach by increasing the noise level (reducing the OSNR).

The fibre types typically used for long-haul networks are G.655, G.652 and to a much lesser extent G.656. More recently, next generation ultra low loss G.652 fibre has also been deployed.

Non-zero dispersion shifted fibre

G.655 and G.656 fibres are also known as non-zero dispersion shifted (NZDS) fibres. The original and most common NZDS fibre is the G.655 type. Lower dispersion G.655 fibre, such as Corning LEAF fibre has about 25% (at 1550nm) of the dispersion of standard G.652 single-mode fibre and this optimised dispersion enables 4 times more reach before dispersion compensation is required. The reduced number of DCMs enables the use of cheaper single-stage amplifiers, having the combined effect of significant reductions in the equipment CAPEX of a network.

Ultra Low Attenuation Fibre

In Africa, where there are power provision and site security challenges, it can be particularly important to design networks with low attenuation fibre to avoid the need for frequent amplification sites that require electric power. 

System simplicity leading to low first-installed cost, and network upgradeability enabling changes in electronics without installing new fibre are considerations for long-haul network designers looking to reduce both first installed costs and total cost of ownership. Both can be achieved by selecting the most appropriate fibre for your network either with low dispersion to reduce the number of DCMs and associated dual stage amplifiers or very low attenuation fibre to improve the OSNR.

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