Maybe it could be possible to build a dedicated TDR system intended to beconnected to installed cablings, periodically test the cables by sending pulses along them and watch what returns, compare the result with long-term average, and report differences.
This is already done in some networks. Actually, the OTDRs are set up on quasi-permanently to observe how a certain subset of a network changes over time. This is done particularly in extreme environmental conditions. (For instance, in the late 90s some of the service providers started to see occasional blackouts associated with very low temperatures. OTDRs deployed revealed it to be "Temperature Induced Cable Loss"...the sheath on a fiber bundle was shrinking in the cold, constraining some of the fibers within the sheath and causing high attenuation.)
Of course, fiber optic networks change regularly over time as a function of temperature. (Polarization Mode Dispersion can change drastically in older fibers when comparing day and night).
BUT, this is often overkill...
Could possibly also help with
early discoveries of various "natural" damages, not only intrusions.
Opinions, comments, construction hints, ideas about better and/or simpler approaches?
There'are already "smart structures" such as bridges that use VERY cheap versions of "OTDRs" to look at much less detailed information than a true OTDR. These incoporate FBGs (fiber bragg gratings) into the concrete, and the reflected energy through the grating is a strong function of the stress and strain on the structural elements.
As for looking for spooks and terrorists, it's been known for a long time that NSA has its own sub that makes undersea taps, for monitoring intercontinental traffic. I've thought about how you'd detect such a splice, and I believe it would be difficult but do-able. Difficult because there's going to be a mandatory few dB of loss associated with the split, but that kind of thing can easily happen to fibers....maybe a killer dolphin chewed on the cable or something (and of course they'll use an isolator in order to hide whatever's on their side of the tap).
But that kind of splice might have a characteristic signature that will look different from other random kinks or attenuation, particularly when combined with certain databases. (I'd say looking at it over time would help, but its probably too late for the undersea fibers.)
-TD
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