What is Coupling factor in terms of frequency

How is C defined in terms fo frequency?

So the power gain of a resonator ring is : $$text{Power Gain}=frac{C^2}{|1-Tsqrt{1-C^2}|^2}$$

$C =$ coupling factor,
$T =$ attenuation.

How do I modify the formula in order to get power gain vs frequency with different attenuation values?