Every engineer who working with DWDM equipment is know that total optical power in DWDM line can be calculated by this formula:
P = P + 10*lg(N),
Where
Psingle(dB) – optical power of single channel,
N – Quantity of optical channels inDWDM line.
But why it’s can be calculated by this formula? Why total optical power is can be calculated just by channels quantity and optical power for single channel? Let’s try to derive this formula.
Like we know, in best case, in DWDM line all optical channels should have identical optical power P; i. e. optical power should be flat – see picture #1
If we measure optical power in watts we can found total optical power by summing power from all channels:
P = P+ P+Psingle2 +… + P,
Where
P - optical power of single channel in watts.
And because all channels have same optical power (P= P= PsingleN) we can write:
Ptotal = N * Psingle
Where
N – Quantity of optical channels.
To translate optical power in watts to decibels we can write:
So we finished derive final formula to calculate total optical power in DWDM line:
Ptotal(dB) = Psingle(dB) + 10*lg(N)
Please do not forget that this formula can be used only if total channels quantity is more than 8 (eight). Instead need add some additional power N which value is depend on EDFA amplifier quantity which already pass by signal.
