Optical power budget and Link loss


Link power budget

Amount of light required to transmit signals successfully through a fiber cable from source to destination.

Link power budget calculation is used to ensure that power reach at the receiver end is greater than average power required by the receiver. In order to estimate the maximum fiber cable length we should specify the output power of the transmitter, receiver sensitivity and system margin.

Power budget = Transmitted Power – Receiver sensitivity

Suppose we have a system with  -4 db  output power and receiver with -10 db input sensitivity

Power budget = -4 – (-16)  =  12 db

Now we have optical power margin of 12 db power loss during fiber cable installation.

Link loss budget

Loss budget calculate the estimate losses in  fiber optic cabling system.

The easiest way to calculate link losses is to perform an OTDR test of the actual fiber link. This will give you the actual loss values for all events in the link.Also known as cable plant loss budget.

Link loss = (Fiber attenuation * distance) + ( Splice loss * Number of splices) + (Connector loss * Number of connectors )

Factors affecting Link loss

  • Fiber Loss : Fiber manufacturer provides a loss factor in terms of dB per kilo meter. A total fiber loss calculation is made based on the distance x the loss factor.
  • Connector loss : Multi mode connectors have more losses than Single mode connectors.
  • Splice loss : There are two types of splices. Mechanical splice : loss is generally in a range of 0.7 to 1.5 dB per connector. Fusion splices : loss is generally in a range of 0.1 and 0.5 dB per splice.
  • Margin : System designers add a margin of 3 dB to 10 dB. This margin is due to the aging of the fiber, aging of the transmitter and receiver components, the additional events along the cable path.

Power Margin

power margin help us determine the sufficient power to operate the receiver.

Power margin = Power budget – Loss budget

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