RE: Estimating the magnitude of PMD
Vipul,
One of the people who "does PMD for a living" is Fred Heismann
at Lucent Bell Labs. In his 1998 paper at ECOC[1], he writes
"While modern single-mode fibers exhibit negligible PMD, with
average Differential Group Delay (DGD) of the order of 0.1 ps/
sqrt(km), some of the older fiber cables - in particular those
embedded in terrestrial networks - can show large PMD effects
with average DGD's up to 2 ps/sqrt(km). Moreover, the instant-
aneous DGD in such high-PMD fibers generally fluctuates randomly
with time and hence, can temporarily exceed values of more than
100 ps for transmission distances of only a few hundred kilo-
meters, which may lead to a complete eye closure in a 10-Gb/s
signal."
Researchers from Royal PTT Research (Netherlands) published their
theory about the cause of high PMD in older cables[2]:
"The variety in PMD's of cables might be related to the cable
design, which is often not specified, unfortunately....In a heli-
cally wound loose-tube filled with gel, as in our cable, the fiber
can be guided without any mechanical contact with the walls of the
loose tube. Therefore it is plausible that fibers in a stranded
loose tube cable can have very long polarization mode-coupling
lengths...the long polarization mode-coupling has been shown to be
the main reason for a high PMD in our cable...From the viewpoint
of PMD, these cables may be actually too kind to the fiber, causing
unnecessarily large PMD's."
Simulations of PMD in 10-Gb/s links, again by Lucent[3], show the
counter-intuitive (for me at least) result that even 40 ps of DGD
adds negligible deterministic jitter while their measured eye
diagrams imply little added random jitter. This gives support to the
idea of treating PMD as an impairment to the center-of-eye opening
similar to the treatment of ISI. Plots of Power Penalty vs. DGD
are published in various places[cf. 3]. Henning Bulow wrote a paper
about measurements on one installed cable (52 km) where he found the
random PMD fluctuations occurred in 5-50 milliseconds[4] but said
they were more likely due to vibrations/movements at the fiber ends
than due to temperature/strain variations in the buried span.
Hope this helps.
Chris
P.S. The tutorial by Heismann[3] is by far the best introduction to
PMD that I have seen and should be required reading for those trying
to get up to speed.
[1] Heismann, F., et. al., "Automatic Compensation of First-order
Polarization Mode Dispersion in a 10 Gb/s Transmission System", ECOC
1998, Madrid, Spain, 329-30.
[2] de Lignie, M. C., et. al., "Large Polarization Mode Dispersion in
Fiber Optic Cables", Journal of Lightwave Technology, v. 12, no. 8,
p. 1325-9, 1994.
[3] Heismann, F., "Polarization Mode Dispersion: Fundamentals and
Impact on Optical Communications Systems (Tutorial)", ECOC 1998,
Madrid, Spain, p. 51-79.
[4] Bulow, H., et. al., "Measurements of the Maximum Speed of PMD
Fluctuation in Installed Field Fiber", OFC/IOOC 1999, San Diego, CA,
p. 83-5.
*******************************************************************
Christopher J. Madden, Ph.D. 3900 Freedom Circle, Suite 200
Principal Engineer/Scientist Santa Clara, CA 95054
Fiber-Optic Data Communications Phone: 408.565.6857
Tripath Technology Inc. Fax: 408.565.6850
http://www.tripath.com cmadden@xxxxxxxxxxx
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-----Original Message-----
From: owner-stds-802-3-hssg-equal@xxxxxxxx
[mailto:owner-stds-802-3-hssg-equal@xxxxxxxx]On Behalf Of Gair Brown
Sent: Friday, September 29, 2000 5:01 AM
To: vipul.bhatt@xxxxxxxxxxx
Cc: stds-802-3-hssg-equal@xxxxxxxx
Subject: Re: Estimating the magnitude of PMD
Vipul,
My experience with PMD is limited and I rarely work with it. Your
question on the magnitude would be best answered by some of the folks
who do PMD for a living.
A note on Johathon's comment. I don't believe that PMD should be
accounted for like DCD. The PMD will be convolved with the rest of the
link distortion and will simply (if anything about PMD is simple!) lead
to additional broadening. It will add into the ISI rather than decrease
the bit interval.
Gair
Vipul Bhatt wrote:
>
> Gair,
>
> And if there is no multiwavelength averaging, would you consider the
> pulse spreading values (7 ps and 17.5 ps) described in my earlier email a
> reasonable approximation?
>
> Thanks,
> Vipul
>
> ===============
>
> > -----Original Message-----
> > From: owner-stds-802-3-hssg-equal@xxxxxxxx
> > [mailto:owner-stds-802-3-hssg-equal@xxxxxxxx]On Behalf Of Gair Brown
> > Sent: Thursday, September 28, 2000 12:01 PM
> > To: vipul.bhatt@xxxxxxxxxxx
> > Cc: stds-802-3-hssg-equal@xxxxxxxx; HACKERTMJ@xxxxxxxxxxx;
> > SwansonSE@xxxxxxxxxxx; piers_dawe@xxxxxxxxxxx
> > Subject: Re: Estimating the magnitude of PMD
> >
> >
> >
> > Vipul,
> >
> > I'm not sure, but I believe you may still have an error in your
> > argument. The spectral width of typical long haul sources at 1550 is
> > extremely small. I don't believe the PMD response of this
> > small band of
> > wavelengths is really independent. So there will be no multiwavelength
> > averaging.
> >
> > Gair
> >
> >
> >
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