I am not sure what is the device configuration? usually we do not extract the propagation constant in FDTD as no monitor can directly monitor it. If really necessary, you can try this method:
Equi-frequency contours of photonic crystal – Ansys Optics
and " kappa_uu" is the coupling coefficient? power or amplitude? for power coupling coefficient, it is relatively easy, as the transmission into a defined channel is this coefficient. for the amplitude coupling coefficient, since it is usually a complex number ,it can be difficult as the phase is very sensitive to the source and the monitor locations, as well as the coupling region. A careful, quantifiable definition is required.