In OptiGrating 4.2, the index profile of the fiber or slab waveguide can be complex.
The imaginary part of the modal effective index can be calculated by the perturbation
method based on the coupled mode theory as:

Optical Grating - Equation 13

where N is the number of layers of the waveguide index profile, Ki is the imaginary
part of the refractive index of ith layer material, and Γi is the confinement factor of
the mode in the ith layer. The above equation is based on the perturbation method,
and it is only valid when Keff « 1 .