Difference in RF dB levels and Fiber dB levels?

CATV RF systems typically measure signal levels in dBmV units and it expresses a signal level relative to 1 millivolt across a 75-ohm system.

Examples:

• 0 dBmV = 1 mV

• +20 dBmV = 10 mV

• −20 dBmV = 0.1 mV

Fiber optic systems measure optical power using dBm units and this represents power relative to 1mW. 

Examples:

• 0 dBm = 1 mW

• -2 dBm = ~ 0.63 mW

• 14 dBm = ~ 25mW

In both domains, a +3 dB change means doubling the power, and a -3 dB change means cutting the power in half.