Insertion loss, or the loss of signal that happens along the length of a fiber optic link, is expressed in dBs and should always be a positive number. But it can be a negative number (which isn't a good thing). ” Optical loss is measured in “dB” which is a relative measurement, while absolute optical power is measured in “dBm,”. The dB values can be positive or negative; they are positive when and negative when. Since dB is a ratio, it does not provide an absolute value of power. So from 1mw to 10mw, we see a gain from 0dBm to +10dBm or 10dB, a positive change.