GB2157523A - Secure optical communication system - Google Patents

Abstract

A secure optical communications system is provided in which injected optical signals are coded to have a substantially constant mean intensity. The mean intensity of the optical signals is monitored at a receiving station 4 and fluctuations outside predetermined rate and/or amplitude limits are taken to indicate breakage or unauthorised tapping of the communications link. <IMAGE>

Description

SPECIFICATION Optical communication security The present invention relates to the security of communication along optical fibres.

When transmitting information down optical fibres it is often desirable to know whether the information is being lost or extracted from the fibre at some point along its length between the transmitter and receiver of the information.

An object of the present invention is to detect light loss from an optical fibre communication system merely by monitoring the transmitted light at the receiving station.

According to one aspect of the present invention, in a method of detecting abnormal light loss between first and second remote locations in an optical fibre communication system, optical communication signals injected at said first location into said system are coded so as to have a substantially constant mean intensity, fluctuations of said mean intensity are detected at said second location and an indication of abnormal light loss is derived from said fluctuations independently of the instantaneous optical intensity injected at said first location.

Since no comparison is needed of the optical intensities at said first and second locations no signal path is required between them except the optical fibre communication system itself.

By a substantially constant mean intensity level is meant an intensity level which when averaged over a time which is large in comparison with the period of a single character, varies to an extent or at a rate which is sufficiently small to enable fluctuations caused by abnormal light loss to be distinguished from fluctuations due to variations in the injected intensity level.

The injected communication signals may be superimposed on an optical signal of substantially constant light intensity injected into the communication system at said first location. If the intensity of this signal is sufficiently large in comparison with the amplitude of the optical communication signals, the mere superimposition of the two signals may effectively "code" the optical communication signals (which may be either analogue or digital) to generate a substantially constant mean intensity within the scope of the present invention. However in order to maintain a high signal:noise ratio it will generally be preferable to employ an optical communication signal which is modulated so that its mean intensity level is equal to or slightly greater than half its peak amplitude.An encoding format of this type, suitable for digital signals, is described in the appendix to the CCITT recommendation G.703 for HDB3 code, which is hereby incorporated by reference. An equivalent analogue signal may be generated by adding an analogue A.C. electrical signal of zero mean value to a D.C. electrical signal having a value greater than the peak excursion of the A.C. signal, and generating an optical signal corresponding to the resulting sum signal.

However it will be appreciated that other coding formats may be employed-for example a digital optical communication signal comprising a stream of square pulses with a constant mark-to-space ratio but variable pulse width may be employed.

The "abnormal light loss" detected by the invention might be caused, for example, by unauthorised tapping of an optical cable or an accidental partial breakage of an optical cable.

According to another aspect of the invention, apparatus for detecting abnormal light loss from an optical communication system comprises an optical receiver arranged to monitor the light intensity level of optical communication signals in said system, means for averaging said monitored intensity level over a period which is large in comparison with the period of one optical character, and means for indicating abnormal fluctuations of the average monitored intensity level.

Preferably said apparatus includes a differentiator arranged to detect an abnormal rate of change of said average monitored intensity level but to disregard slow changes (such as might be caused by ageing of the optical cable oropto-electronic components or changes in ambient temperature, for example).

Said apparatus may alternatively or additionally incorporate a threshold detector which is arranged to detect excursions of said average monitored level outside predetermined limits.

Typically, the period over which the intensity level of the optical signals is averaged may be 1 millisecond.

One embodiment of the invention will now be described by way of example with reference to Figures 1 to 3 of the accompanying drawings, of which: Figure lisa schematic diagram of an optical communication link incorporating the invention; Figure 2 shows a waveform of an analogue signal suitable for use in the system of Figure 1, and Figure 3 is a waveform of a digital signal suitable for use in the system of Figure 1.

Referring to Figure 1, the input information is presented to a modulator or coding device 2which provides an electrical output to drive a light producing transmitter 3. This transmitter provides the light input to a fibre optic cable 1. This light output is arranged such that its mean level averaged over anytime interval of some fixed duration (typically 1 millisecond) is constant within some defined limits +E (typically +0.1%).

At the receiver 4 the light input is converted to an electrical output. This electrical output is demodulated or decoded by a demodulator 5 to extract the received information. In addition the electrical output is passed to an integrating device 6 which determines the mean level of the signal averaged over time interval of fixed duration T (typically 1 millisecond). A signal representing this mean level is passed to a change detector7 which detects changes in this level which are greater than some fixed limits ti5 (typically 0.2%) related to +E.

This change detector may also include a rate of change detector which signals if the mean level changes at a rate greater than a predefined threshold. Note that an interactive combination of limit and rate may also be employed. This change detector therefore produces a signal to indicate that a change in mean level has occurred greater than some allowable limit.

The security of information on the link is therefore facilitated for a light loss from the fibre will be reflected in the change in mean light level. The change detector 7 incorporates the ability to disregard changes due to ageing of the link which provide a slow change of mean level.

Figure 2 shows a typical analogue waveform (having a mean value M) which might be generated by the transmitter 3 in response to the output of coder 2. It may be generated by simply adding an appropriate D.C. level M to an inputA.C. waveform (of zero mean value) in coder 2, and generating a corresponding optical output from transmitter 4. M must be greater (but is preferably only slightly greater) than the peak excursion P ofthewaveform from its mean value M.

Figure 3 shows an alternative (digital) waveform which may be generated by the transmitter, which also has a constant mean value M. This waveform is encoded in accordance with the appendix to CCITT recommendation G.703 for HDB3 code.

Claims (15)

1. A method of detecting abnormal light loss between first and second remote locations in an optical fibre communication system, wherein optical communication signals injected into said system at said first location are coded so as to have a substantially constant mean intensity, fluctuations of said mean intensity are detected at said second location and an indication of abnormal light loss is derived from said fluctuations independently of the instantaneous optical intensity injected at said first location.

2. A method according to Claim 1 wherein said indication of abnormal light loss is derived from the amplitude of said fluctuations at said second location.

3. A method according to Claim 2 wherein said indication of abnormal light loss is derived from the rate of change of light intensity at said second location due to said fluctuations.

4. A method according to any preceding Claim wherein the waveform of said optical communication signals is amplitude modulated such that its mean intensity level is equal to or slightly greater than half its peak amplitude.

5. A method according to any of Claims 1 to 3 wherein said optical communication signals are coded by superimposing them on an optical signal whose intensity is substantially constant and is large in comparison with their peak amplitude.

6. A method according to any preceding Claim wherein said optical communication signals are digital signals.

7. A method according to Claim 6 as dependent on any of Claims 1 to 3 wherein said optical signals are pulse-width modulated so as to maintain a constant mark-to-space ratio.

8. Apparatus for detecting abnormal light loss from an optical communication system, comprising an optical receiver arranged to monitor the light intensity level of optical communication signals in said system, means for averaging said monitored intensity level over a period which is large in comparison with the period of one optical character, and indicating means responsive to abnormal fluctuations of the average monitored intensity level.

9. Apparatus according to Claim 8, including a differentiator arranged to detect an abnormal rate of change of said average monitored intensity level but to disregard slow changes in said intensity level.

10. Apparatus according to Claim 8 or Claim 9 including a threshold detectorwhich is arranged to detect excursions of said average monitored level outside predetermined limits.

11. Apparatus according to Claim 10 wherein the values of said limits are set within the range + 1% of the average monitored intensity level.

12. Apparatus according to any of Claims 8 to 11, incorporating means for integrating said optical communication signals over a period of between 0.1 and 10 milliseconds.

13. Apparatus according to any of Claims 8 to 12 wherein means are provided for accommodating gradual variations of the mean detected intensity of said optical communication signals due to ageing.

14. An optical communication system incorporating apparatus as claimed in any of Claims 8 to 13.

15. An optical communication system substantially as described hereinabove with reference to Figures 1 and 3 of the accompanying drawings.