An estimation of the effective index of the structure must be provided. Since the structure
can vary as a function of z-distance, the estimated effective index can also vary as a function
of distance. Consider the following example:
2.2 100.0 2.0
This command will assign an effective index of 2.2 for 0.0 < z < 100.0 and 2.0 for z>100.0.
A single number, such as
2.2
will assign 2.2 as the estimated effective index for all z.