Coupling Ratio/Loss (CR, CL)
The ratio/loss of optical power from one output port to the total output power, expressed as a percent. For a 1 x 2 WDM or coupler with output powers O1 and O2, and Oi representing both output powers:
CR(%) = (Oi/(O1 + O2)) x 100% and
CR(%) = -10·Log10 (Oi/(O1 + O2)).