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RE: 2.5 GBS vs 10 GBS on same fiber


All materials fiber included are non linear to a small degree.
Glass has no 2nd-order nonlinearity because of symmetry reasons.
However there is a minute dependence of the refractive index as a 
function of optical power, such that n = no + (3/8no)*RE[Chi3]*I

Where no is the linear refractive index, and RE[Chi3] is the
real part of the 3rd-order nonlinear susceptability tensor.
This effect is called the optical Kerr effect (OKE).  This should 
not be confused with the "electro-optic Kerr effect" which is the
DC-Term so to speak of the OKE.

The real part of the nonlinear coefficient [Chi3] of SMF 
is in the range of 10^-19 to 10^-20 inverse Watts, according 
to my reference. For a 1 mW pulse, and no = 1.5, the Kerr 
effect would be a shift of the refractive index by a few
parts in 10^-23 upward. From 1.5 to 1.500000000000000000000015

Mind you, the Kerr effect is very fast, it tracks the optical
power instantaneously at the _optical_ frequency, which is
about 200 THz for 1550 nm light.

While I am not an expert in impairments on communication
systems, these temporal perturbations in refractive index
will manifest themselves in all kinds of curious ways under

  Self-Focussing of light beams. This should not be an issue in 
  guiding media like fiber - only in bulk media. This is a major
  issue in laser fusion and ultrafast laser research.

  Self-Phase modulation (SPM) is when the phase gets diddled from
  an intensity-modulated optical field. Under the right conditions,
  SPM can induce chirp, that will cause pulse boadening like
  the chromatic dispersion. There can be some interplay between
  SPM and chromatic dispersion with very strange effects at high
  powers. Dont know about Kerr + DMD    :-0

  Cross-Phase modulation (XPM) is similar to SPM, but it involves
  more than one optical signal in the same medium of different
  wavelengths. If two or more pulses are simultaneous, intensity
  of one can perturb the refractive index seen by the pulses.
  This has ramifications in WDM systems.

  Four-Wave Mixing will actully generate signals at new wavelengths
  which correspond to the various permutations of the sums and 
  differences of the optical frequencies preesnt. My impression is 
  that this is one the main impairments of DWDM systems. FWM and XPM
  are not issues in serial or parallel optics.

  Brillouin effect. This is a interaction of the optical wave, an
  optically-induced acoustic wave (dont ask), and the backscattered
  "Stokes" wave. This causes the backscattered light to have an 
  optical frequency downshifted by about 23 MHz at 1550 nm. This 
  only occurs at very high powers.

  Raman Effect. This is an interaction between light and nuclei
  which have been excited to a vibrational state. The forward-
  scattered Raman-shifted light is extremely broadband and downshifted 
  by about 12 THz. You can take advantage of the Raman gain and
  make an amplifier or laser without Erbium or rare-earth doping!

As to the bottom line, how much "OKE penalty" we will have?
For single-mode fiber, these effects come into play after many km
of fiber, and at powers well above the power levels in our draft 
standard. However at what point we will see additional eye closure 
for the different PHYs I cannot say at this time. There is an expert
at Lucent by the name of Dr. Andrew Chraplyvy at Holmdel who would 
be able to talk to the specifics of our situation (low channel count,
low powers, etc) if Agilent, Blaze, and others havent already addressed
them for the CWDM case. Maybe a tutorial or presnetation might be in
order - Paul could you talk to Andrew i think he has a tutorial.

I would guess for MMF these effects are further reduced, due to the 
low optical power density in the core region.

See texts by Iannone "Nonlinear Optical Communication Networks" and
Agrawal "Nonlinear Fiber Optics."


Robert Dahlgren, President, Silicon Valley Photonics
PO Box 60638,  Sunnyvale, CA 94088  USA
+1-408-437-9292                  bob@xxxxxxxxxxxxxxx

> -----Original Message-----
> From: owner-stds-802-3-hssg@xxxxxxxx
> [mailto:owner-stds-802-3-hssg@xxxxxxxx]On Behalf Of Ron Miller
> Sent: Sunday, December 10, 2000 9:30 PM
> To: Robert Dahlgren
> Cc: 'johnathan.thatcher@xxxxxxxxxxxxxxxxxxxx';
> 'stds-802-3-hssg@xxxxxxxx'
> Subject: 2.5 GBS vs 10 GBS on same fiber
> Hi Bob
> At the standards meeting ieee802.3 met and I volunteered to ask
> you, our expert in the optical field the following question.
> Please give it a whirl.
> Thanks
> Ron
> +++++++++++++++++
> Durring the final session IEEE session on Friday there was a 
> duscussion on
> Single mode Fibre for 2.5 GBS and 10 GBS usage.   Jonathan 
> Thatcher Was trying to get 
> some concurrence in the discussion but many red flags were being
> Thrown up with people looking at the fibre as being a 
> non-linear medium.
> I refuted them and explained that the edge rate was the only 
> differentiator.  
> Also that the amplitude and phase for a 1 GBS or 10 GBS would 
> be the same
> Because the edge rate was what could make a difference.  
> However, I also 
> volunteered to get Bob Dalhgren our Consultant to fill in 
> further details.
> ++++++++++++++++++++++
> Ron Miller, Signal Integrity Engineer,
> Brocade Communicaitons
> 408-487-8017, page 888-354-0690