Calculating - Fibre Optic Power Budget

Calculating the Optical Power Budget for a fibre optic cable link is really simple. Firstly, we need to start with some known facts. The losses quoted below are the norm but you can obtain the cable and connector losses from your local cable manufacturer for a more accurate calculation.

Let’s look at the following example:

1. Calculate the Cable Loss
Cable loss = Cable Distance x Cable Loss per km
Cable loss = 2km x -2.5dB/km
Cable loss = -5dB

2. Calculate the Connector Loss
Connector Loss = Connector Quantity x Connector Loss
Connector Loss = 2 x -0.5dB
Connector Loss = -1dB

3. Calculate the Splice Loss
Splice Loss = Splice Quantity x Loss per Splice
Splice Loss = 2 x -0.2dB
Splice Loss = -0.4dB

4. Calculate the Total Link Losses
Total Link Losses = Cable Loss + Connector Loss + Splice Loss
Total Link Losses = -5dB + -1dB + -0.4dB
Total Link Losses = -6.4dB

We have now successfully completed the first part of the calculation and we have established that the total loss on this fibre optic link is -6.4dB. The second part of the calculation is even simpler because most manufacturers of fibre optic transmission equipment quote the Optical Power Budget in their spec sheets. For example the Optical Power Budget for the Fibre Lite Range of products is -12dB. Alternatively, we can calculate the Optical Power Budget:

Transmitter Output Power (LVTXD-010-SMM Fibre Lite) = -14dBm (quoted by the manufacturer)
Receiver Sensitivity (LVRXD-010-SMM Fibre Lite) = -26dBm (quoted by the manufacturer)

Optical Power Budget = Output Power - Receiver Sensitivity
Optical Power Budget = -14dBm - (-26dBm)
Optical Power Budget = 12dB

Power Margin = Optical Power Budget – Total Link Losses
Power Margin = 12dB + (-6.4dB)
Power Margin = 5.6dB

It is good practice to allow for a Power Margin of 3 dB. This allows for cable degradation and should you ever have the misfortune of having the cable damaged or cut you still have enough Budget to re-splice the cable.


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