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There are two additional known sources of
polarization effects I would like to mention, in addition to the possibility of
the launch-related effect which Stefano has raised.
1. The first is
well-known and is that slight core ellipticity will result in a polarization
effect. The specification for the FDDI installed base is that the core
ovality be less than 6%, which is defined as Ovality = 2*(a-b)/(a+b) * 100
where a & b are the largest and smallest "core radii" corresponding to the
major & minor axes for an ellipse. Chapter 16 of Snyder & Love on
"noncircular" waveguides addresses this topic (also 13-9 p.289). I will
call this the "elliptical core effect".
2. The second is less
well-known but is mentioned in the Snyder & Love references David provided,
for example p.286 and pp.628-629. There is a natural birefringence because the
vector equations are not perfectly symmetric, and in deriving the scalar wave
equation a term proportional to (d/dr)(n(r)) is neglected.
On page 281
Snyder and Love define "weakly guiding" as Delta <<1 and discuss
that the term is somewhat ambiguous. It is often assumed that this
would imply that dn/dr <<1 as well. However in the manufacture
of optical fibers a significant layer structure in the optical fiber is
sometimes seen, particularly in pre-MBI fibers made with the MCVD method.
This can be seen in some of the profiles in the report on the 12/96-06/97 TIA
/NIST fibers. Because the index change occurs over a small distance dr,
the dn/dr term is not necessarily negligible. This effect was analyzed in
detail in an OFC paper TUQ6 in 1985 by Carnevale/Potkay/Paek from ATT.
This
"layer structure effect" is interesting because it will cause a
polarization effect even if the core is perfectly
symmetric.
-----Original
Message-----
From: Bottacchi.external@INFINEON.COM [mailto:Bottacchi.external@INFINEON.COM]
Sent:
Monday, October 18, 2004 6:35 AM
To:
STDS-802-3-10GMMF@listserv.ieee.org
Subject: Re: [10GMMF] Polarization
effects for 10GBASE-LRM
Importance: High
David,
please find
my comments in blue.
> -----Original Message-----
>
From: CUNNINGHAM,DAVID
(A-England,ex1)
> [mailto:david_cunningham@agilent.com]
>
Sent: Dienstag, 12. Oktober 2004 22:45
> To: Bottacchi Stefano
(IFFO MOD CE external);
> david_cunningham@agilent.com
>
Cc: STDS-802-3-10GMMF@listserv.ieee.org
>
Subject: RE: Polarization effects for
10GBASE-LRM
>
> Stefano,
>
> RE: I appreciated your
comments and I understand you disagree on
> almost any
polarization-dependent effect in MMF transmission, at least
> assuming
circular symmetric refractive index without any
>
birefringence.
>
>