Monitor unit for monitoring an optical waveguide

Claim 2

A monitor unit for monitoring an optical signal carried by a waveguide to be monitored and having a plurality of propagation states, said monitor unit comprising: a first waveguide optically coupled to said waveguide to be monitored; a photodetector; coupling means optically coupled to said first waveguide and to said photodetector for coupling a portion of the optical signal from the waveguide to be monitored to the first waveguide and the photodetector, the coupling means and photodetector including means for detecting at least one but less than all propagation states of the optical signal; and monitoring means responsive to the optical energy detected by the photodetector to provide an indication when a change in said optical energy greater than a predetermined amount is detected thereby detecting the presence of physical tampering with said waveguide to be monitored at some point between an optical signal transmitter and an optical signal receiver coupled to opposite ends of the waveguide to be monitored.

Claim 3

A monitor unit as in claim 1 wherein: the waveguide to be monitored is an optical fibre and the coupling means comprises a curved optical waveguide and clamping means for clamping the waveguide to be monitored into a curved position whereat the waveguide to be monitored at least substantially conforms to the inner side of the curved waveguide, the respective refractive indices of the waveguide to be monitored and the curved waveguide, and the radius of curvature of the inner surface of the curved waveguide being such that in use, optical radiation is leaked from the waveguide to be monitored coupled at the curved waveguide into the first waveguide.

Claim 4

A monitor unit as in claim 1 wherein the coupling means comprises a second waveguide coupled in series to the waveguide to be monitored, and transversely coupled to the first waveguide at a coupling region.

Claim 5

A monitor unit as in claim 1 wherein the first waveguide is a multimode waveguide and the propagation states comprise different order modes of the optical signal, the photodetector being positioned to detect a high order mode of the optical signal and positioned not to detect the lowest order mode of the optical signal.

Claim 6

A monitor unit as in claim 1 wherein the propagation states comprise different order modes of the optical signal and a filter is positioned to filter out lower order modes of the optical signal.

Claim 7

A monitor unit as in claim 5 wherein the filter has a substantially circular cross-section, and is positioned coaxially with the first waveguide.

Claim 8

A monitor unit as in claim 5 wherein the filter comprises a knife edge and is positioned off-axis relative to the first waveguide.

Claim 9

A monitor unit as in claim 5 wherein the filter comprises the coupling region.

Claim 10

A communications system comprising a communications optical waveguide, a transmitter capable of transmitting an optical signal along the communications optical waveguide, a receiver capable of receiving the optical signal and a monitor unit as in claim 5 wherein the communications optical waveguide comprises a multimode optical fibre substantially identical to the first optical waveguide.

Claim 11

A monitor unit as in claim 1 wherein the first waveguide and said coupling means are each polarisation independent single mode waveguides and the propagation states comprise different polarisation states, the monitor unit further comprising a filter capable of filtering out one polarisation state to allow one selected polarisation state to pass to the photodetector.

Claim 12

A monitor unit as in claim 10 wherein the first waveguide is substantially identical to the waveguide to be monitored.

Claim 13

A communications system a communications optical waveguide, a transmitter capable of transmitting an optical signal along the communications optical waveguide, a receiver capable of receiving the optical signal and a monitor unit as in claim 10 wherein the communications optical waveguide comprises a single mode optical fibre substantially identical to the first optical waveguide.

Claim 14

A monitor unit as in claim 1 wherein each of the waveguides is an optical fibre.

Claim 15

A communications system comprising a communications optical waveguide, a transmitter capable of transmitting an optical signal along the communications of optical waveguide, a receiver capable of receiving the optical signal, and a monitor unit as in claim 1 coupled to the communications optical waveguide.

Claim 16

A communications system according to claim 12 wherein the first optical waveguide is coupled to the communications optical waveguide such that the coupler is positioned between the transmitter and the receiver.

Claim 17

A communications system as claimed in claim 12 wherein 10% of the optical signal is coupled into the coupler.

Claim 18

An optical communications line security monitor for detecting the presence of physical tampering with an optical communications channel which causes changes in the relative power distribution between different modes of the optical signal being carried by the optical communications channel at some point between an optical signal transmitter and an optical signal receiver coupled to opposite ends of said optical communications channel, said monitor comprising: a selective optical signal mode coupler disposed to extract from said optical communications channel at least one mode but not all modes of optical signal propagation being carried by the optical communications channel; an optical-to-electrical transducer connected to said mode coupler for converting said selectively coupled mode(s) of optical signals to an electrical signal; and a monitor circuit connected to said transducer to monitor said electrical signal and to detect changes therein greater than a predetermined amount thereby detecting the presence of physical tampering with the optical communications channel at some point between an optical signal transmitter and an optical signal receiver coupled to opposite ends of the communications channel.

Claim 19

An optical communications line security monitor for detecting the presence of physical tampering with an optical communications channel which causes changes in the relative power distribution between different modes of the optical signal being carried by the optical communications channel at some point between an optical signal transmitter and an optical signal receiver coupled to opposite ends of said optical communications channel, said monitor comprising: a selective optical signal mode coupler disposed to extract from said optical communications channel at least one mode but not all modes of optical signal propagation being carried by the optical communications channel; an optical-to-electrical transducer connected to said mode coupler for converting said selectively coupled mode(s) of optical signals to an electrical signal; and a monitor circuit connected to said transducer to monitor said electrical signal and to detect changes therein greater than a predetermined amount thereby detecting the presence of physical tampering with the optical communications channel at some point between an optical signal transmitter and an optical signal receiver coupled to opposite ends of the communications channel.