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Losses in Fibre Optic Transmission

Losses in Fibre Optic Transmission

24th April 2020 | In Optical Transceivers
 
Optical Fibre Loss / Attenuation

Fibre optic cable is lighter, smaller and more flexible than copper and can transmit fast signals over much greater distances. However, many factors can influence the performance of fibre optic transmission. One of the top priorities of optical network engineers is to figure out and work with the optical losses on a link.

Different Types of Losses in Optical Fibre

Losses in optical fibre are a result of either intrinsic or extrinsic factors. Fibre attenuation, is also known as signal loss or fibre loss, is the consequence of the intrinsic properties of an optical fibre. Other types of losses in the optical fibre that contribute to the link loss, such as splicing, patch connections and bending, are examples of extrinsic losses.

Intrinsic Optical Fibre Losses

Absorption losses in optical fibre is the biggest cause of loss during transmission. When the photon interacts with the components of the glass, an electron or metal ions, the light power is absorbed and transferred into other forms of energy like heat. This is due to molecular resonance and wavelength impurities.

Dispersion losses are the results of the distortion of the optical signal when travelling along the fibre and can be intermodal or intramodal. Intermodal dispersion is the pulse broadening due to the propagation delay differences between modes in multimode fibre. Intramodal dispersion is the pulse spreading in single mode fibre, because the refractive index or the propagation constant varies with wavelength.

Scattering losses are due to microscopic variations in the material density, compositional fluctuations, structural inhomogeneities and manufacturing defects.

Each of these factors are intrinsic attenuation losses in optical fibre. According to EIA/TIA-568 standards, the fibre losses for different fibre types are as following:

Fibre Type

Wavelength

Fibre Loss

Multimode 50/125 µm (OM2/OM3/OM4)

850 nm

3.5 dB/km

Multimode 62.5/125 µm (OM1)

850 nm

3.5 dB/km

Single-mode 9 µm

1310 nm

0.4 dB/km

Single-mode 9 µm

1550 nm

0.3 dB/km



Extrinsic Optical Fibre Losses

Fibre splicing is another cause of loss in optical fibre links. By joining two optical fibres end-to-end, splicing aims to ensure that the light passing through it is almost as strong as the pure fibre itself. But no matter how good the splicing is, the optical loss is inevitable. Fusion splicing losses of multimode fibre are 0.1-0.5 dB, 0.3 dB is a good average value. For single-mode fibre, the fusion splicing loss is typically less than 0.05 dB.

Connector losses or insertion losses in optical fibre are the losses of light power resulting from the insertion of a device in a transmission line or optical fibre. Multimode connectors will have losses between 0.2-0.5 dB (0.3 typical). Factory made single-mode connectors will have losses of 0.1-0.2 dB and field terminated single-mode connectors may have losses as high as 0.5-1.0 dB (0.75 dB, TIA-568 max acceptable).

The bending of a fibre is a common problem that can cause optical fibre losses. There are two main types; micro and macro bending. Macro bending refers to a large bend in the fibre (with more than a 2mm radius).

Optical Fibre Loss Measurement

When measuring the total losses in optical fibre, also used to calculate the “link budget”, the types of losses must all be considered. The light power budget margin (due to the ageing of the fibre, incidental bend and twisting, etc.) is equally important. Most system designers will add a loss budget margin of 3-10 dB. Calculation of losses in an optical fibre span should be as follows:

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

How to Reduce Losses in Optical Fibre

To ensure the output power can be within the sensitivity of the receiver and leave enough margin for the performance degradation, it is essential to reduce the losses in an optical fibre.

Here are some common approaches to fibre link design and installation:

  • Ensuring the quality of the fibre is as high as possible
  • Choosing high-quality connectors, ensuring the insertion loss is lower than 0.3dB and the additional loss lower than 0.2dB
  • During splicing, strictly follow the processing and environment requirements
  • The connecting joints must have excellent patching and closed coupling to prevent excess light attenuation
  • Ensuring the cleanliness of the connectors
  • Choose the best route and methods to lay the fibre cables during the design phase
  • Select and form a qualified construction team to guarantee the quality of the installation
  • Strengthen the protection work, especially lightning, electrical, anti-corrosion and anti-mechanical damage
  • Use high-quality heat-shrinkable tube
Mike Peacey, Product Manager, Hardware
This should be enough to get you started, if you have any further questions please don’t hesitate to Get in touch with our team of experts.

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