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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.
From: Bottacchi.external@INFINEON.COM [mailto:Bottacchi.external@INFINEON.COM]
Sent: Monday, October 18, 2004 6:35 AM
Subject: Re: [10GMMF] Polarization effects for 10GBASE-LRM
please find my comments in blue.
> -----Original Message-----
> From: CUNNINGHAM,DAVID (A-England,ex1)
> Sent: Dienstag, 12. Oktober 2004 22:45
> To: Bottacchi Stefano (IFFO MOD CE external);
> Cc: STDS-802-3-10GMMF@listserv.ieee.org
> Subject: RE: Polarization effects for 10GBASE-LRM
> 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