Insertion Loss of Optical Passive Component

At the first we should know that the optical power unit was used to be defined:
10log(P/1mW) and named dBm. P is the power measured by power meter or OSA.
Example:
If P is 30mW then we say it is 14.77dBm (10log(30mW/1mW)).

The second, we cared about the power was decreased when it passed through component so we defined the insertion loss as -10log(Pout/Pin) and called it dB. In here the negative symbol used was because the number, 10log(Pout/Pin), was always negative.
Example:
If Pin is 30mW (or 14.77dBm) and Pout is 25mW (13.98dBm) then we can calculate the insertion loss is:
– (13.98-14.77) = 0.79 (dB) or – (10log(25/30)) = 0.79 (dB).
It is natural that we should calculate it by the front, it is easier.