Re: [10GMMF] Polarization effects for 10GBASE-LRM
RESEND of my note which was corrupted so it missed discussion of the Laguerre-Gauss vs Gauss-Hermite modes. I don't understand why there would be a difference in the mode delays which would show up as the polarization is changed, if the fiber is circularly symmetric and there is no tilt in the launch optics. I need to replot David's figure 3 for the Laguerre-Gauss modes & show what they look like in that equivalent system.
From: Abbott, John S Dr
Sent: Monday, October 25, 2004 10:59 AM
To: 'IEEE P802.3aq 10GBASE-LRM'; STDS-802-3-10GMMF@listserv.ieee.org
Subject: RE: [10GMMF] Polarization effects for 10GBASE-LRM
It is my impression from reading David's note and from rereading these older references that if one looks at the modes in the Laguerre-Gauss framework as we usually do, that the individual modes within a mode group vary in two distinct ways. There is a radial component and in fact a single axisymmetric mode in every other mode group, and there are the spiral or azimuthal modes in every group after the fundamental mode. In the Laguerre-Gauss framework the radially symmetric modes are not affected by polarization or the position of the input spot, while the azimuthal modes are actually "two" separate modes with a sine or cosine dependence. The Infineon calculations and also David's note show that the relative power in the sine mode or the cosine mode depends on either the polarization or the location of the input spot azimuthally around the fiber, and the intensity distribution in a fixed lab coordinate system will vary as either is changed. I disagree with David's comment w!
hich Stefano asks about in (2) be
Within the Laguerre-Gaussian formulation the variation in mode delays between individual modes in the mode group has to do with different radial distributions of the individual modes. The Gaussian-Hermite basis functions are equivalent but I think they do not add clarity to this specific feature.
From: Bottacchi.external@INFINEON.COM [mailto:Bottacchi.external@INFINEON.COM]
Sent: Monday, October 25, 2004 9:38 AM
Subject: Re: [10GMMF] Polarization effects for 10GBASE-LRM
I appreciate your critical analysis and I am quite happy you agree on my explanation for the observed polarization effects. I went through you detailed calculations and I have a couple of questions:
(1) You conclude the MGP is constant respect relative angle between
the polarization and the offset direction. On the other hand, changing the linear polarization you wrote the "coupled power is not equi-partitioned between the individual modes within a group". As a consequence, you concluded the measured variation of the impulse response versus polarization orientation after 200~300 meters of MMF were due to "delay differences between modes within same mode group". My concern is that since a mode group is specified (within WGA) for having the same propagation constant (asymmetries in refractive index circularities are excluded), how do you support the measured pulse distortion in terms of higher order beta variations? DMD should not be included in your conclusion since DMD refers to different mode groups.
(2) The second question is more fundamental and it coincides with my
early concern on the validity of the "natural orientation" assumption when linear polarization is included in the picture. I agree on this picture. The basis function will be naturally oriented along the polarization axis. If so, the mathematics will follow, but this is only an assumption and it should be proved or at least well justified. I was proposing first the "natural orientation assumption of the basis set", but I am still thinking to some physical justification about it.
Thank you one more time for your excellent work.
> -----Original Message-----
> From: CUNNINGHAM,DAVID (A-England,ex1)
> Sent: Donnerstag, 21. Oktober 2004 20:55
> To: Bottacchi Stefano (IFFO MOD CE external); email@example.com
> Cc: STDS-802-3-10GMMF@listserv.ieee.org
> Subject: RE: Polarization effects for 10GBASE-LRM
> Dear Stefano and Yu,
> I attach a PDF file which documents an analysis of SMF offset launch
> as a function of polarisation direction.