Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1991-09-09
1994-09-20
Chilcot, Jr., Richard E.
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359110, 359112, 359173, 340555, 340556, 25022714, 385 12, 385100, H04B 1008, H04J 1402
Patent
active
053494580
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a method for detecting external influence on an optical cable. The invention can be used as a sensor in an intrusion alarm, in which case the cable is buried and an alarm is triggered when someone treads on it. The invention can also be used as a surveillance system for an optical cable transmitting information. The present and more and more accelerating computerization presupposes in order to be effective that the computers are directly connected to each other. The demand for transmission capasity and quality makes cables having optical fibres well suited as transmission medium. Concurrently with the increased demand for capasity the optical cables will dominate completely.
The demand on an optical fibre connection handling sensitive information will be, that some sort of security system is installed. As the only possible future communication medium is optical fibres, the security systems will also be optical. Optical fibres is a medium which itself has a very great resistance against information tapping. It is very difficult to tap information from an optical fibre, but it is not impossible.
Today there exist some systems for discovering unauthorized influence on an optical fibre, all of which are based upon surveillance of the optical effect that is transmitted in the fibre core. The principle is in short that the external influence will lower the energy level in the fibre, at which an alarm is trigged. The method can be compared to an intensity modulated sensor, which is the least sensitive sensor type. The systems have considerable disadvantages and can not with an accaptable degree of security discover unauthorized dealing with the optical fibre or cable.
SUMMARY OF THE INVENTION
With great probability the next generation of security systems has to work according to a principle, that gives greater possibility to detect a manipulation of the optical fibre. Such a principle can be based upon the fact that every external influence on the fibre will modulate the phase of the light, which the receiver can detect. This is the sensor principle that gives the greatest sensitivity. The invention uses this system to solve the present problem by being designed in the way that is evident from the following claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a laser, an optical multimode fibre and the appearance of the interference pattern after the fibre,
FIG. 2 shows an interference detector according to the invention,
FIG. 3 shows a transmission system having the information signal and the surveillance signal wavelength multiplexed in one multimode fibre and
FIG. 4 shows a transmission system, in which the information signal is caused to digitally modulate the surveillance signal.
DETAILED DESCRIPTION OF THE DRAWINGS
In the present invention the surveillance signal is transmitted in multimode in an optical fibre. It is then possible to use multimode fibres, which can be advantageous, as one then uses cables and a technique, that are well tested and frequent in the market. Further, already installed multimode connections can afterwards be supplemented with safety systems according to the invention, without changing the cables. By the word multimode fibre is in this application ment both gradient index fibres as well as step index fibres. In the case with a single-mode fibre for transmission of information the method according to the invention could still be used. The surveillance signal is then transmitted at a wavelength below the cut off wavelength of the single-mode fibre, which causes the surveillance radiation to propagate in several modes. The single-mode fibre will then act as a multimode fibre for this radiation.
In a multimode fibre there exists many directions of propagation due to the angle of incidence to the fibre that the incident "light rays" have. These directions of propagation are called modes. If coherent light, as in FIG. 1, from a laser 1 is transmitted in the fibre 2 and the emerging light illuminates for example a w
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Bacares Rafael
Chilcot Jr. Richard E.
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