| 摘要 |
In an optical fibre data transmission system, detection of an unauthorised tap is achieved by applying to a fibre conveying the data, a standing optical signal whose amplitude P is considerably greater than the amplitude PS of the data. This depresses the sensitivity of the system receiver to an extent at which the receiver noise has as its dominant component the shot noise in the signal. Thus the reduction in received power due to a tap is readily detectable. <IMAGE>
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