Optical Power Budget

The optical power budget is the difference between the transmitter's minimum output power and the receiver's sensitivity, expressed in decibels (dB). A system with +1.5 dBm output and −27 dBm sensitivity, for example, has a 28.5 dB budget. The sum of all link losses plus a safety margin must stay below this figure. PON standards express the budget as optical classes. For GPON, B+ is 28 dB and C+ is 32 dB. For XGS-PON, N1 is 29 dB and E1 is 33 dB.

The loss calculation adds up every element of the link. Single-mode fiber attenuation is about 0.35 dB/km at 1310 nm, 0.25 dB/km at 1490 nm and 0.20 dB/km at 1550 nm. Each connector adds 0.3–0.5 dB and each fusion splice 0.05–0.1 dB, plus splitter insertion loss (about 17 dB for 1:32). A margin of at least 3 dB is usually reserved for repairs, ageing and measurement uncertainty.

Example: 10 km of fiber (3.5 dB at 1310 nm) + 1:32 splitter (17.1 dB) + 4 connectors (2 dB) + 8 fusion splices (0.8 dB) = 23.4 dB. On a class B+ GPON system with a 28 dB budget, about 4.6 dB of margin remains and the design is acceptable. With a 1:64 splitter on the same route, loss would rise to about 26.8 dB and the margin would shrink to 1.2 dB. That calls for a class C+ optic or a shorter route.

The budget has a lower bound too. On very short links, high-power optics can saturate the receiver, requiring an optical attenuator. After installation, real values should be verified with an optical power meter and an OTDR, and the results recorded in project documentation together with splice and connector locations.