Method for detecting external influence on an optical cable
First Claim
1. A method for detecting external influence on an optical fiber in an optical cable, the method comprising the steps of:
- transmitting coherent radiation for surveillance purposes in multimode through the optical fiber;
receiving the coherent radiation on a receiver end of the optical cable;
splitting the received radiation into two beams, each beam including an entire interference pattern created by the transmission in the fiber;
plane-polarizing the two beams in two different directions;
detecting amplitude of each beam and generating measurement signals respectively representative thereof;
comparing the measurement signals of each beam to generate an output signal representing the relationship therebetween; and
triggering an alarm when the output signal exceeds a predetermined threshold.
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Abstract
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 method means that coherent radiation for surveillance purposes is transmitted in multimode in an optical fibre. On the receiver side the surveillance radiation is then split into two beams, that are plane-polarized in two different directions. Then the amplitude and/or frequency content of each beam is detected. A signal which is a measure of their amplitude and/or frequency relation is formed from this. Finally an alarm function is initiated when the signal exceeds predetermined values in time and/or level.
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Citations
14 Claims
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1. A method for detecting external influence on an optical fiber in an optical cable, the method comprising the steps of:
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transmitting coherent radiation for surveillance purposes in multimode through the optical fiber; receiving the coherent radiation on a receiver end of the optical cable; splitting the received radiation into two beams, each beam including an entire interference pattern created by the transmission in the fiber; plane-polarizing the two beams in two different directions; detecting amplitude of each beam and generating measurement signals respectively representative thereof; comparing the measurement signals of each beam to generate an output signal representing the relationship therebetween; and triggering an alarm when the output signal exceeds a predetermined threshold. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11)
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12. A method for detecting external influence on an optical fiber in an optical cable, the method comprising the steps of:
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transmitting coherent radiation for surveillance purposes in multimode through the optical fiber; receiving the coherent radiation on a receiver end of the optical cable; splitting the received radiation into two beams, each beam including an entire interference pattern created by the transmission in the fiber; plane-polarizing the two beams in two different directions; detecting a frequency distribution of each beam and generating measurement signals respectively representative thereof; comparing the measurement signals of each beam to generate an output signal representing the relationship therebetween; and triggering an alarm when the output signal exceeds a predetermined threshold.
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13. A method for detecting external influence on an optical fiber in an optical cable, the method comprising the steps of:
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transmitting coherent radiation for surveillance purposes in multimode through the optical fiber; receiving the coherent radiation on a receiver end of the optical cable; splitting the received radiation into two beams, each beam including an entire interference pattern created by the transmission in the fiber; plane-polarizing the two beams in two different directions; detecting a frequency distribution of each beam and amplitude of each, and generating frequency distribution measurement signals and amplitude measurement signals, respectively representative thereof; comparing the frequency distribution measurement signals of each beam and amplitude measurement signals of each beam to generate output signals representing the relationships therebetween; and triggering an alarm when the output signals exceed a predetermined threshold. - View Dependent Claims (14)
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Specification