QUICKSTART GUIDE
8 Optical Power Budget
Inevitably the question that arises is “What is the maximum practical communication distance when using a fiber optic cable?”. The answer isn’t straightforward, but must be calculated as follows:
•First the "Optical Power Budget" is determined by subtracting the receiver’s rated sensitivity from the transmitter’s rated power, both of which are defined in decibels of light intensity. For example if a particular transmitter is rated at minus15 db and the receiver’s sensitivity is rated at minus 31 db, the difference of 16 db is the "Optical Power Budget."
Xmt Output Pwr - Rcv Sens = OPB: Optical power budget
Magnum: (
The Optical Power Budget can be thought of as the maximum permitted attenuation of the light signal as it travels from the transmitter to the receiver, while still permitting reliable communication.
•The next step is to calculate the worst case Optical Power Budget by subtracting from the Optical Power Budget, 1 dB for LED aging and 1 dB for each pair of connectors (referred to as "insertion loss").
Worst Case OPB = OPB - 1dB (for LED aging) – 1dB (insertion loss for each pair of connectors) x number of pairs
The final step is to divide the calculated result by the rated cable loss per kilometer to determine the maximum distance.
For costly installations it is recommended that you always measure the actual cable loss before and immediately after the installation to verify that the cable was installed correctly. To avoid damaging the receiver, ensure that the maximum optical input power of the receiver isn’t exceeded.
•Worst case distance = {Worst case OPB, in dB} / [Cable Loss, in dB/Km] where the “typical cable loss” for:
•62.5/125 and 50/125ìm (M.m) is 2.8 dB/km
•100/140
•9/125
•9/125
•9/125
•9/125
•9/125
These are typical cable losses. There will be deviations depending on the manufacturer.
Always measure the loss before installation.
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