Optical transmitting element, esp. photoconducting cable, comprises a photoconducting core (LW), a bolster layer (PO) typically of silicone rubber, around this, and a layer of spun optical fibres (GE1), embedded in a binding agent, esp. epoxy resin, surrounding the bolster; this in turn is surrounded by a protective sheath (SH), esp. of polyurethane or polyvinyl alcohol elastomer, followed then by a further structural layer of tension-resistant fibres (GE2) embedded in a binder. This is also pref. surrounded by an outer casing (MA). The effect of the closed ring of photoconducting fibres (GE1) around the photoconducting cable is that the cable cannot be punctured or tapped by unauthorised persons without this fact being revealed by the rupture of the external casing of optical fibres. If the cable is bent sharply in an attempt to obtain stray leakage transmissions from it, these outer optical fibres will rupture.
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