doping profile and refractive index profile of optical fiber amplifier

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Does anyone know how to measure the doping profile and refractive
index profile of optical fiber amplifier?

Re: doping profile and refractive index profile of optical fiber amplifier



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Many people know.  It's in all the textbooks, having been worked out between
1975 and 1985, or so.  Do the research.  Here are some starting points.

For communications fibers, multimode is ~parabolic, while for single-mode it's
step index, with perhaps a few stepped rings.

For carriage of gross amounts of optical power, it's usually a larger
stepped-index fiber than for communications.

The difference in refractive index between core and cladding is 1% to 2%.

Joe Gwinn

Re: doping profile and refractive index profile of optical fiber amplifier



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I suspect you'll agree, however, that when you look at the actual (i.e.,
measured) index profiles of real fibers, they often display horrible
looking spikes and irregular small-scale variations about these
idealized parabolic or step index profiles -- irregularities that lead,
in most cases, to very little or practically no real difference in
actual mode propagation.

Re: doping profile and refractive index profile of optical fiber amplifier



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Absolutely.  In practice, the desired refractive-index (RI) profile is
approximated by some number of constant-RI layers.  The number of layers varies
between fiber makers, and I recall one boasting about using at least 100 layers,
to better approximate the desired profile.  

Now I remember.  It is Draka: <http://www.thor.edu/kvasir/2-draka.pdf .

It should be noted that many practical multimode optical fibers exhibit a
centerline RI dip or bump and that unless avoided this central defect has a
large effect on the bandwidth (MHz-Km product) of the fiber.  Avoidance of such
central defects was a major motivation for the donut-launch requirement in IEEE
802.3 (Ethernet).

Joe Gwinn

Re: doping profile and refractive index profile of optical fiber amplifier



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Joe, I've been looking at the analysis of fiber modes (or more often,
just TE planar waveguide modes), motivated by some fundamental questions
as to the effects of loss and especially gain on these modes.  (For
example, an elementary step-profile fiber with even a small amount of
uniform loss in the core has _no_ stable modes; all of its modes are
perturbation-unstable to any kind of random deviations in the fiber.  
With uniform gain in the core, however, the modes become stable.)

In the process, I've observed that most of the texts and published
literature tackle the problem of finding the modes in an optical
waveguide by specifying an index profile, then attempting to solve for
the modes that propagate in this waveguide -- which is often hard.

But if you turn this process around and start by specifying the modal
function (most any arbitrary modal function) that you want, then the
index profile that you need to produce or obtain this modal function
falls out trivially (I've come to refer to this as "designer waveguides"
or "designer fibers") -- though of course fabricating that desired index
profile may be a "whole 'nother problem".

Have you encountered this "designer fiber" concept in the published
literature anywhere?

Re: doping profile and refractive index profile of optical fiber amplifier


In article

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Why would uniform gain yield stable modes, while uniform loss would
yield unstable modes?

But this arrangement is very common, as almost all fibers have some
loss, so somehow it isn't a problem.  What immediately come to mind is
that the fiber is usually coiled up.  Will curvature break the symmetry
and allow stable modes to exist?

 
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I think that fiber designers do exactly this (design fiber for the
desired modes), but fiber manufacture is a very competitive industry.  A
good example would be designing fibers to well-match the donut launch
mentioned above.

Manufacture of such a designer RI profile would be no problem with
current computer-controlled profile generators.  And Draka's original
patents are long since expired.

 
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Not in the literature, but I have not looked very hard either.  All the
fiber design groups I knew of (from working in standards groups) had
company-developed proprietary full-fidelity simulation codes to predict
fiber performance from RI profile.  But the details were very closely
held, if for no other reason than that such codes are very expensive to
develop and validate, and would give much detailed process information
away.


Joe Gwinn

Re: doping profile and refractive index profile of optical fiber amplifier



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Short answer:  Higher-order modes have lower filling factors in the core
(i.e., they're 'larger', with more of their energy outside the core and
in the cladding) than do lower-order modes.  The core is lossy.  
Therefore, higher-order modes have _lower_ losses than higher-order
modes, and therefore survive longer (i.e., propagate further down the
waveguide) than do lower order modes.  

Any kind of random scattering along the waveguide scatters energy among
the modes; or any kind of deviation from the lowest-order mode in the
excitation at an input to the waveguide puts some energy into the higher
order modes -- and these modes last longer and eventually dominate.

Re: doping profile and refractive index profile of optical fiber amplifier



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So, the key is that because gain is by stimulated emission, the new
photons are going in the same direction as the original photons (and all
are phase coherent), so there is little mode mixing; while loss causes
scattering into other often higher-order modes, so there is considerable
mode mixing?  This seems to require loss by scattering versus loss by
absorption.

Joe Gwinn

Re: doping profile and refractive index profile of optical fiber amplifier


AES wrote:
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They're more vulnerable to microbending, though, because the guiding is
weaker.  That's particularly true at short wavelength--if you run UV
through a MMF, the high order modes are much lossier than the low order
ones.

And that business about lossy fibre being perturbation-unstable
intrigues me--in one way of speaking, lossy fibre has no stable modes at
all, simply because of the loss.  Since that's sort of trivial, I gather
you're talking about lossy fibres being more vulnerable to scattering
loss, is that right?    What's the physics there?


Cheers

Phil Hobbs

Re: doping profile and refractive index profile of optical fiber amplifier

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Not sure I ever answered this one.  Still looking for an answer?

   --AES

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