Re: Dispersion in an optical fibre

On Feb 1, 9:08 am, Randy Poe <poespam-t...@xxxxxxxxx> wrote:
On Feb 1, 11:53 am, "Ken S. Tucker" <dynam...@xxxxxxxxxxxx> wrote:



Hi again Harry C.

On Jan 31, 3:16 pm, "hhc...@xxxxxxxxx" <hhc...@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 stretchs 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.

-- Peter Fairbrother

Ken

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 teltale
imedance discontinuity. Of course, this can and has been done but its
success depends on the presence of far more obious flaws in the
cable.

Dispersion in the fiber is a far more interesting topic, consider that
it is being driven by a more or less monochomatic light source -- a
laser. In generally, the dispersion generally relates to the fact that
no optical fiber is totally capable of propagating a totally singular
mode, even the best single mode fibers. Different optical modes
propogate at different speeds, resulting in dispersion. This has
little conntection with the fact that different optical wavelenghts
propagate at different velocities in transmission media other than
free space (e.g., the variation of the index of refraction of a
transmission medium with wavelength).

If you research how optical fibers and made, and the difference beween
a "single mode fiber" and a "multi-mode fiber", you'll realize that it
is physically impossible to create a totally "single mode" fiber,
although what is sold as single mode fiber comes damn close, but not
100%. The result is that the dispersion created tends to spread the
pulse, which in turn limits the transmission rate. Splices in the
cable add to this dispersion.

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. 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.

Now the disclaimer: Other than having a small amount of SONET
experience, I have no classified knowledge about this subject. I'm
simply speculating on how I believe an engineer would address the
problem.
Harry C.

I did some hands-on work in the field in 80's,

Heh. About the time the security people in the
classified environment I worked in, were telling
me that fibers were not tappable.

Yeah that's right it's a "black art".

As you know, this was at a time when the KGB
was extremely active and creative and people
worried a lot about bugging, how to do it and how
to detect it. I was told that if anyone tried to
tap a fiber the breach would be detectable.

I thinks it not classified now.

You might want to be careful with that.

Yeah, I got a head full of classified crap.

Tapping a working fiber is nerve wracking,
(I've spliced fibers, but not for tapping).
The fiber is stripped then acid is used to get
bare glass fiber. A second fiber is set parallel
to the main fiber and a quartz crystal cylinder
is clamped around the set-up.
The quartz is heated and, via infared, softens
the glass fibers to the extent where they fuse
very slightly.
This is all done under a very pricey twin eyed
microscope with a delicate clamping table.

Ah. So not being done by James Bond sneaking
into the utility tunnels of your local Soviet
embassy then.

LOL, it's more like an aptitude for tedium, some
enjoy repetition, that's who you want.

As you said Harry, you might get the fusion
done "in the clutter", so the reflected impedance
will go unnoticed, I imagine a skilled practioner
could do it.

- Randy

Regards
Ken



.

Quantcast