Cheap TDR for fibers?

[Tyler Durden]

Telecom-grade laser packages (and the lasers inside them) not only do not 
have a monitoring diode, they are designed very carefully to prevent the 
kind of feedback you're talking about (it destabilizes the laser and causes 
a power penalty).

However, there's no real reason not to be able just to splice into the 
fiber. Hell, you don't even need a splice if you have access to the FDF 
(Fiber Distributing Frame, or fiber patch panel).

-TD


>From: Thomas Shaddack <shaddack at ns.arachne.cz>
>To: Cypherpunks <cypherpunks at al-qaeda.net>
>Subject: Cheap TDR for fibers?
>Date: Mon, 19 Jul 2004 05:25:35 +0200 (CEST)
>
>
>The laser diodes used in eg. CD players have a feedback photodiode,
>sensing the laser's optical output.
>
>If the lasers used for optical fibers have similar mechanism too, and if
>the diode is sensitive to the light coming to it not only from the chip
>but also from the fiber itself, and can react quickly enough with high
>enough sensitivity, maybe it could be exploited.
>
>In chosen moments, we could then send a short pulse of laser light into
>the fiber, then watch the signal from the feedback diode, what gets
>reflected back from nonhomogenities on the fiber. This would give us the
>distances of all the splices and connectors, and let us know immediately
>(if the test is performed eg. once per 5 seconds or with similar short
>period) that there is an attempt to compromise the line underway.
>Comparison of snapshots from longer periods apart could also serve to find
>deterioration of the signal path before it results in failure.
>
>The advantage of this approach, if possible, is the ability to add the
>functionality without having to modify the optical transceivers
>themselves.
>
>
>It sounds too good to be true, so it probably won't work, but I may be
>wrong...
>

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