Re: [TSCM-L] Fiber Optic Spying -

From: <Alway..._at_aol.com>
Date: Thu, 14 Dec 2006 01:07:38 EST

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From: reginal..._at_hotmail.com
To: "TSCM-L Professionals List" <TSCM-..._at_googlegroups.com>
Subject: Fiber Optic Spying -
Date: Wed, 13 Dec 2006 10:12:10 -0800
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If my memory is correct, about a year ago there was some discussion as
to how the information flow in a fiber optic system could be
intercepted. The following is from this week's issue (Dec. 12/2006 -
Volume 6 Issue 50A) of Techs & Specs - The Engineering Newsletter that
is found on GLOBAL SPEC - The Engineering Search Engine. This is
mostly a product bulletin from manufactures in the science and
engineering fields. However, one of the features is a question relating
to an issue relating to these fields. The next issue contains the
'correct' answer to the question and poses a new question. Some are
quite interesting.

Anyway, last weeks question was:

   "Your devious friend wants to listen in on your next-door neighbor's
phone conversation. Your neighbor's phone line is running on optic
fiber, and your friend claims he knows of a way to listen in without
cutting the fiber. How?"

Answer:

"A fiber optic cable basically consists of a core, cladding, and
jacket. The core is thin glass fiber. The cladding is a material of a
lower index of reflection than the core and is used to reflect the
light signal back into the core. The jacket is for protection. Fiber
optic cable is used to transmit light signals from a source to a
detector; the light signal stays in the core due to total internal
reflection. Total internal reflection works because there is an angle
at which a light signal in the core can hit the cladding and will
result in no transmission, only internal reflection. If the light
signal were to approach the cladding at a sharp angle, some of the
signal is transmitted instead of reflected. The sharper the angle, the
more signal is transmitted. It is possible, by bending the fiber optic
cable, to create a sharp enough angle to result in signal loss. This
signal could then be detected and read while the weaker, internally
reflected signal, continues down the fiber. This microbending technique
would allow you to listen in on the conversation without breaking the
fiber optic cable.

"Another method to listen in would involve macrobending to exploit the
evanescent field of the fiber. As a light signal travels down a fiber
optic cable, not all of its energy is confined to the core of the
fiber. Light, as an electromagnetic wave, has an electromagnetic field
that varies over time. Some of this field penetrates into the cladding
of the cable, espectially at the point of reflection when the light is
closest to the cladding. The electromagnetic field in the cladding is
referred to as the evanescent field. If you were to place a fiber with
a core material similar to the core of the original fiber within the
evanescent field, a signal similar to the original signal would form
with an intensity dependent on the location of the fiber in the
evanescent field. (The evanescent field decays exponentially the
further you get from the core-cladding interface). By introducing a
bend of large radius, you can intensify the evanescent field on the
outside of the bend. (The light travelling down the core is now off
center, intensifying the evanescent field in the cladding on one side).
This results in an intensified evanescent field, which is now easier to
tap.

"It should be noted, though, that for either of these methods to work,
the jacket would have to be partially removed."

THE END

Reg Curtis/VE9RWC
Received on Sat Mar 02 2024 - 00:57:26 CST