Re: [10GMMF] channel models based on measured DMD
Jon,
One further point for your consideration in support of my assertion that we
are in the regime of very weak mode-mixing: I draw your attention to slide 7
of balemarthy_1_0105 (Vancouver). It shows several DMD traces for long
length fiber (>5km) that show considerable structure and sharp features.
Robert
Robert Lingle, Jr, Manager
Fiber Design and Transmission Simulation
OFS R&D, Atlanta, GA
-----Original Message-----
From: Lingle, Robert L, JR (Robert)
Sent: Wednesday, April 06, 2005 11:27 AM
To: IEEE P802.3aq 10GBASE-LRM
Subject: RE: [10GMMF] channel models based on measured DMD
Jon,
I am on vacation and want to be able to spend enough time to carefully
respond to your comments.
In the meantime I need to clarify a significant misunderstanding you
indicate below.
SUMMARY
The data on my slides 8 and 9 do NOT indicate square-root scaling of BW with
length (strong mode-mixing regime). Instead, the actual value of gamma is
rather close to 1 (indicating non-zero, but weak mode-mixing). Several of my
colleagues also reminded me that a significant reason (aside from
mode-mixing) that gamma is less than 1.0 is differential mode attenuation,
which suppresses the effect of unruly high order modes (e.g. due to profile
defects near the cladding boundary) in longer length fibers. Again, the
gamma argument points to non-zero, but weak, mode mixing.
Before I respond to the remainder of your points below, I want to ask if
this clarification changes your arguments in any way? Please also
specifically comment on how your argument on the effect of mode-mixing on
OSL DMD squares with the OSL-BW data on slide 11.
DETAILS
As I stated in the text of the file posted in the public area, these data
are CONSISTENT WITH a value of the gamma exponent "between 0.8 and 1.0."
The ACTUAL value of gamma for the fibers shown is at the upper end of the
possible range between 0.5 and 1.0, being rather close to 1. I initially
gave an open range of "0.8 to 1.0" because various manufacturers over the
years may have found different values which I do not know. I used the
terminology "consistent with" because these data do NOT represent a proper
cutback study where one measures the same fibers at different lengths. What
is plotted is the average OFL-BW for the population. These are also reasons
why the histogram bars for different length bins do not uniformly change
with length.
This is simply a rough and ready indication that a population of fibers at
different lengths shows the approximate phenomenon of improving
length-normalized OFL-BW with length over this range of lengths.
The y-axes (OFL-BW in MHz-km) on slides 8 and 9 are not given, and the zero
point is far off the scale for both wavelength cases. Thus you will get an
incorrect rough and ready estimate of gamma if you assume that the bottom of
the y-axis is zero, and then apply the formula on my slide 6 to the data on
slides 8 and 9.
Robert
Robert Lingle, Jr, Manager
Fiber Design and Transmission Simulation
OFS R&D, Atlanta, GA
-----Original Message-----
From: Jonathan King [mailto:jking@BIGBEARNETWORKS.COM]
Sent: Friday, April 01, 2005 10:34 PM
To: STDS-802-3-10GMMF@LISTSERV.IEEE.ORG
Subject: Re: [10GMMF] channel models based on measured DMD
Hi Robert
thanks for your e-mail - here's my full and frank commentary on your
slides ' validation of using measured DMD from long fibre spools to
characterize the installed base'.
On your slide 2, you correctly summarize my question/comments as being
directed at the inadvisability of using data from very long (5km) fibres
to predict the PIE-D of short (300m) lengths. My primary concern being
that the effects of significant mode mixing/coupling in long fibres.
Your slide set attempts to justify the use of long fibres, but I would
argue that the results you show actually do just the opposite, and in
fact confirm the necessity of making measurements on short fibre lengths
(ie 300m). Let me try to explain myself.....
Firstly, commenting on your slide 4, temporal resolution requirements
alone do not require long fibre lengths: photo-detectors with
significantly higher bandwidths than 10GHz are easily available (I'd be
pleased to loan you one if that would help).
Secondly, on slide 6 - your comments are all true, but only for OFL
launches. LRM does not use OFL launches.
On Slide 7, I agree that the length normalized OFL Bw would apparently
improve with longer fibres as a result of mode mixing. OFL bandwidths
are (of course) measured with an over filled launch at the input to the
fibre - all modes are excited right from the start; as light propagates
down the fibre, at first little mode scrambling takes place, and the
full range of differential mode delay would be seen; as propagation
continues down the fibre more mode-mixing takes place, the effect of
which is that launched light spends only a fraction of the time in a
particular mode or mode group; for sufficiently long fibres the effect
would manifest itself as a root-length dependence of OFL Bw, which is
nicely shown in your slide 8. Slide 8 also provides confirmation that
mode mixing effects are significant in fibre lengths >1km. I do not
agree with your statement that the root length dependence of OFL Bw
shown in slide 8 leads to a conclusion of long fibre lengths giving an
optimistic PIE-D result, because the LRM application doesn't use an over
filled launch.
In contrast, LRM specifies a choice of 2 input conditions, each of
which excites selective mode-groups (i.e. very different to OFL), and
the choice of which results in the lowest PIE-D. My heuristic
understanding is LRM deliberately avoids exciting mode-groups which
would result in high DMD / high PIE-D. In this situation, mode mixing
effects would cause power to be coupled out of the initially selected
'benign' mode-groups into 'worse' mode groups - consequently, the
normalized DMD gets worse with fibre length, and the benefit of the
precise launch definition is lost. Your slide 8 shows that mode-mixing
effects are significant for fibre lengths>1km (root length dependence of
OFL Bw is maintained down to 1km)- i.e. DMD measurements on fibres
longer than 1km cannot be relevant to LRM.
On Slide 10, 1st bullet: linear scaling down to 300m is not justified
from your data because the mode mixing effects (which you clearly show
to be significant in slides 7 and 8) means you don't have the equivalent
of an OSL mode power distribution for a fibre length >1km - ie you
aren't measuring the DMD of an OSL.
Slide 10, 3rd and 4th bullets: I disagree with your statement that
linear scaling underestimates DMD for the 300m case: you haven't
measured DMD for an OSL, because mode-mixing destroys the OSL launch
MPD; If it didn't, you wouldn't see root length OFL Bw dependence
A comment on your last slide: I believe your fibre measurements to be a
small subset of those used to steer the development of the GEN67 Monte
Carlo model, and a vanishingly small sample of the installed base; as
such I think it would be wrong to place a greater significance on them
than I would the results of GEN67.
In conclusion, in my opinion the 99% PIE-D figures in balemarthy-1-0105
are actually quite seriously misleading, since they are derived from
measurements on long fibre lengths which actually conceal the benefits
of the precise launch conditions defined in the LRM draft.
best wishes
Jonathan
tel: 1 408 524 5110
e-mail: jking@bigbearnetworks.com
fax: 1 408 739 0568
Jonathan King
Director, Optical Systems
BigBear Networks
345 Potrero Avenue
Sunnyvale, CA 94085
-----Original Message-----
From: owner-stds-802-3-10gmmf@IEEE.ORG
[mailto:owner-stds-802-3-10gmmf@IEEE.ORG] On Behalf Of Lingle, Jr,
Robert (Robert)
Sent: Friday, April 01, 2005 6:43 AM
To: STDS-802-3-10GMMF@LISTSERV.IEEE.ORG
Subject: [10GMMF] channel models based on measured DMD
All,
Several questions were raised in Vancouver about possible issues with
using
measured fiber DMD data to model the installed based as presented in
balemarthy_1_0105. I had to miss Wednesday in Atlanta, but I also
understand that similar questions were raised during John Abbott's
presentation.
During the Atlanta meeting, Piers uploaded a presentation addressing
questions raised by Nick Weiner and Jon King in Vancouver on this
subject
at:
http://grouper.ieee.org/groups/802/3/aq/public/upload/Validate1998OFSfib
erse
t.pdf
I would appreciate comments and feedback on that.
Robert
Robert Lingle, Jr, Manager
Fiber Design and Transmission Simulation
OFS R&D, Atlanta, GA