Re: Dispersion in an optical fibre

hhc314@xxxxxxxxx wrote:
On Jan 31, 8:41 am, "Ken S. Tucker" <dynam...@xxxxxxxxxxxx> wrote:
On Jan 31, 5:22 am, Peter Fairbrother <zenadsl6...@xxxxxxxxx> wrote:

Ken S. Tucker wrote:
On Jan 31, 4:56 am, Peter Fairbrother wrote:
Ken S. Tucker wrote:
My "BLACK" assignment was to tap a fiber optic,
without detection, that's regarded as a difficult
assignment.
Bend it, bend it, just a little bit ... :)
No it's more complicated than that. The sent pulse
will reflect back any anomaly in the fiber, and secure
systems use that to find breaks and stretches in the
optic fiber.
So how do you do it then?
Well, my point is, it's very difficult to tap a fiber.
That's why it's a BLACK art.

It's far easier to explain why tapping a fiber is so difficult, than
how to accomplish the task without detection.

Pretty much any tap on the fiber will created an impedance
discontinuity, in turn creating a reflection which is readily
detectable by use of a simple instrument which is known as an OTDR
(Optical Time Domain Reflectometer).. Today, this is a rather common
piece of equipment that is most often employed to identify the
location of breaks or poor joints in the fiber transmission line. It's
damn hard to put a tap on a fiber cable without producing this telltale
impedance discontinuity. Of course, this can and has been done but its
success depends on the presence of far more obvious flaws in the
cable.

[..]

I have no idea how the professionals at NSA or other facility hide
their optical cable taps, but I can make an uneducated guess. That
would be that you would slightly abrade the out part of the fiber to
allow taping into the multi-mode component.

Or bend it ...

NSA also do it underwater, through a copper sheathing which carries 10,000 Volts or so, from the USS Jimmy Carter nuclear submarine (which cost something like $3.2 billion). Ouch!

Done very, very carefully,
the OTDR would see this a just one among many marginal spices on the
transmission line. Still, enough signal would leak out into the tap to
be provide potentially very useful SCI type information. You can't
make the tap totally invisible, but with careful work you can hide it
in the clutter. Were it me, I'd look for a splice location and go from
there.



-- Peter Fairbrother
.

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