[10GMMF] Cambridge Rel 3.0 scaled profiles
I brought up an observation on the channel
modeling teleconference earlier today regarding the treatment of fiber
41 in the Release 3.0 package some of you may have received recently that
contains scaled index profiles. Richard Penty asked that I capture
my thoughts in an email to the reflector, so here they are.
First I would like to say that the modifications
to the profile scaling process that modify the radial width of the center
defect and the onset of the edge perturbation are probably good adjustments.
But one issue has crept in.
The fiber in question started out as
one devoid of intentional perturbations. The DMD scaling process magnifies
the small alpha non-idealities to the point where they reach a limit of
500 MHz-km OFL BW. This apparently occurs at about 1 ns/km DMD. The
scale factor for this fiber turns out to be about 10, which is the largest
of any fiber in the set by roughly a factor of 3. The release note
expressed concern that the profile scaling process could not be applied
to a fiber with a perfect profile, and therefore the profile supplied was
not scaled (although a DMD scaling factor is provided). In examining
the DMD structure of the scaled fiber, it looks realistic. So the
result of the DMD scaling did not produce a nonphysical result. In
addition, the structure appears to be of a type that is not commonly found
in the other fibers of the set and therefore its presence could be of additional
value to the simulations. So I would recommend that this scaled fiber
be retained in the analysis.
I think the issue is how to scale a
perfect fiber using the profile scaling algorithm established for the other
fibers of the set. There is no center or edge perturbation to adjust
and the alpha error is zero. So for at least this one case (and more
rigorously for all cases) I believe the correct approach may be to scale
relative to the profile of a fiber that generates zero DMD. Such
a fiber will nominally have a constant alpha, but with some small variations
that produce zero DMD. These variations provide the basis for scaling
the "perfect" fiber (and all others, but the added value is likely
small in most cases).
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