How to Prevent Losses in Fibre Optic Transmission

When building and maintaining a network infrastructure, efficiency and performance are always key. Every business is always looking for the best and most cost-effective ways of creating a fast, responsive network that can meet the needs of their organisation.


Fibrestorm Optical Cable

Author: Mike Peacey

Fibre optic cabling has emerged as one of the best ways of achieving these goals. Lighter, smaller and more flexible than copper, fibre optic components are known for transmitting fast signals over much greater distances than would be possible with older technology. Most modern networks are based heavily on the performance that optical cables and transceivers can provide.

However, it is also vital to remember that many factors can influence and degrade the performance of fibre optic transmission, resulting in losses of speed, signal and reliability. As such, those responsible for managing and maintaining optical networks need to be aware of these factors, and take the necessary steps to prevent optical losses on a link.

What causes losses in optical fibre?

The potential causes of fibre loss - also known as signal loss or fibre attenuation - are numerous, but can generally be categorised either as a result of intrinsic or extrinsic factors. It is important to be able to differentiate between these, as the root cause of the problem will naturally affect the solution required to fix it.

Intrinsic optical fibre losses
When problems arise due to issues with the intrinsic properties of an optical fibre, it can cause serious performance drop-offs for your network. Potential examples include:

  • Absorption losses, which occur when photons interact with the components of the glass, electrons or metal ions in the fibre, causing the light power to be absorbed and transferred into other forms of energy, such as heat. This can be caused by molecular resonance and wavelength impurities, and represents the biggest cause of losses during transmission
  • Dispersion losses, resulting from distortion of the optical signal while travelling along the fibre. Intermodal dispersion occurs when the pulse broadens due to propagation delay differences between modes in multimode fibre. Intramodal dispersion happens when the pulse spreads in single-mode fibre, due to wavelength variances in the refractive index or the propagation constant
  • Scattering losses, due to microscopic variations in material density, compositional fluctuations, structural inhomogeneities and manufacturing defects

Extrinsic optical fibre losses
Extrinsic losses are caused by factors beyond the fibre components themselves, and therefore need to be managed in a different way. Examples include:

  • Fibre splicing, the method used to connect two optical fibres joined end-to-end. The method aims to ensure that any light passing through the spliced junction is almost as strong as it would be for pure fibre, but some degree of optical loss is inevitable.
  • Connector losses or insertion losses, which are losses of light power caused by the insertion of a device into a transmission line or optical fibre
  • Bending of the fibre, a common problem when installing equipment within a tight space. There are two main types, micro and macro bending, with macro bending referring to a large bend in the fibre with a radius of greater than 2mm

Measuring optical fibre losses

Measuring the total impact of these optical fibre losses on your network is essential in calculating an overall "link budget". This accounts for all of the gains and losses experienced by your network communication signals.

In order to get an accurate read on this, all types of losses must be considered, including the light power budget margin, which takes into account other factors such as the ageing of the fibre, or incidental bends and twisting. Calculation of losses in an optical fibre span should be based on the following formula:

Link budget = [fibre length (km) x fibre attenuation per km] + [splice loss x number of splices] + [connector loss x number of connectors] + [safety margin]

According to EIA/TIA-568 standards, the average losses for different fibre types are as following:

FIbre Type Wavelength Fibre Loss
Multimode 50/125µm (OM2/OM3/OM4) 850nm 3.5dB/km
Multimode 62.5/125µm (OM1) 850nm 3.5dB/km
Single-mode 9µm 1,310nm 0.4dB/km
Single-mode 9µm 1,550nm 0.3dB/km

Below are some average estimates of the losses associated with the most common forms of fibre attenuation:

  • Fusion splicing losses for multimode fibre are usually between 0.1dB and 0.5dB, with 0.3dB being a good average value
  • Fusion splicing losses for single-mode fibre is typically less than 0.05dB
  • Multimode connector losses are usually between 0.2dB and 0.5dB, with 0.3dB seen as typical
  • Connector losses for factory-made single-mode connectors are between 0.1dB and 0.2dB on average
  • Connector losses for field-terminated single-mode connectors can be as high as 0.5dB to 1.0dB, with 0.75dB set as the TIA-568 acceptable maximum

With all of these factors taken into account, most system designers will add a loss budget margin of 3dB to 10dB when setting up a network. However, the true figure can be kept to a minimum through a sensible, efficient approach to network design and management.

How can losses in fibre optic transmission be avoided?

Achieving peak network efficiency means taking action to address the causes of optical fibre loss. By doing so, you will be able to ensure that the network's output power is within the sensitivity of the receiver, while leaving enough margin for any unavoidable performance degradation.

Here are some of the most common approaches to fibre link design and installation that can help minimise potential losses:

  • When purchasing components and cables, ensure the quality of the fibre is as high as possible
  • Select high-quality connectors, with an insertion loss lower than 0.3dB and additional loss below 0.2dB
  • Create a qualified construction team to oversee the installation process, paying particular attention to choosing the best route and methods to lay the fibre cables during the design phase to avoid bending and twisting the cables
  • Follow the processing and environment requirements strictly when splicing to avoid introducing any inefficiencies
  • Ensure excellent patching and closed coupling when using connecting joints to prevent excess light attenuation
  • Strengthen the protection work for all components, especially lightning, electrical, anti-corrosion and anti-mechanical damage, and use high-quality heat-shrinkable tubing to keep your setup insulated
  • Maintain the cleanliness of your connectors at all times

By taking sensible precautions and common-sense steps in designing and maintaining your network, you can make sure that your fibre optic network components are able to deliver optimal performance in the long term, avoiding costly disruptions and maximising your return on investment.

If you'd like more advice on how to prevent losses in your fibre optic network, get in touch with us today. Our in-house team of technical experts can answer any questions you might have about designing an efficient network, including offering recommendations on the products you need for your setup.