| 摘要 |
PCT No. PCT/SE89/00116 Sec. 371 Date Sep. 9, 1991 Sec. 102(e) Date Sep. 9, 1991 PCT Filed Mar. 9, 1989 PCT Pub. No. WO90/10921 PCT Pub. Date Sep. 20, 1990.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. |
