GB2303753A

GB2303753A - Optical fiber transmission with intrusion detection and location.

Info

Publication number: GB2303753A
Application number: GB9119976A
Authority: GB
Inventors: Jacques Auge; Jose Chesnoy; Philippe Darbon
Original assignee: Alcatel CIT SA
Current assignee: Alcatel CIT SA
Priority date: 1990-09-21
Filing date: 1991-09-19
Publication date: 1997-02-26
Anticipated expiration: 2011-09-19
Also published as: GB9119976D0; FR2738430A1; NL9101538A; SE9102563L; ITTO910710A0; IT1269223B; CA2051913A1; FR2738430B1; DE4131009C1; SE513651C2; ITTO910710A1; GB2303753B; CA2051913C

Abstract

In conventional manner, an information-carrying main wave and an auxiliary wave are injected into a link fiber. The auxiliary wave is chosen to couple are easily with the outside than the main wave. According to the invention, test modulation is applied to the auxiliary wave and the phase of the wave backscattered by the fiber is measured. The invention is applicable to transmitting confidential information.

Description

METHOD AND APPARATUS FOR TRANSMITTING INFORMATION OVER AN OPTICAL FIBER WITH INTRUSION BEING DETECTED AND/OR LOCATED The present invention relates to transmitting information over optical fibers. It is applicable when the information to be transmitted must be protected against attempts at tapping and/or alteration.

BACKGROUND OF THE INVENTION The transmission of such information is often performed by means of an optical link in which an information-carrying lightwave is guided over a considerable distance by an optical fiber. This fiber is referred to below as the "link" fiber.

Although compared with other transmission means it is easier to protect the information to be transmitted when transmission takes place via such a link, even under these circumstances there remain difficult problems for the person skilled in the art. It is not necessary to break the link fiber in order to take off a small fraction of the informationcarrying lightwave. Such "tapping" can be achieved, in particular, by curving the fiber, thereby causing the fraction of light that is to be tapped to escape. This fraction suffices to impart the information being transmitted, and this is achieved without necessarily altering the wave that is received at the fiber outlet in detectable manner.

This constitutes passive intrusion. However, it will be understood that active intrusion can be achieved in the same manner by injecting light into the core of the link fiber to alter the information being transmitted.

Since such intrusions cannot be prevented, attempts have been made to detect them and to locate them.

One particular proposal for performing such detection has been to inject an auxiliary wave into the link fiber in addition to a main lightwave that carries the information and that propagates along the core of the fiber. The auxiliary wave is caused to propagate in an annular waveguide formed in the fiber around its core. If intrusion is attempted by curving the fiber, attenuation can be detected in the received auxiliary wave before it is possible to tap or alter the information being carried by the main wave.

Such detection methods are described, in particular, in the following documents: EP-A-O 083 843 (Corning); US-A-4 134 642 (Kapron); and French patent application FR-A-2 635 876 and the corresponding US application US-SN 400 172 (huge).

The present invention makes it possible to detect an intrusion of the above kind quickly and reliably.

The present invention has the following objects, in particular: enabling such an intrusion to be located accurately and quickly; and enabling such detecting and/or locating to be performed by means of apparatus that is simple and cheap.

SUMMARY OF THE INVENTION The present invention provides a method of transmitting information over an optical fiber with an intrusion being detected and/or located, wherein an information-carrying main wave and an auxiliary wave are both injected into a link fiber, the auxiliary wave being selected so that it couples more easily with the outside than does the main wave, wherein at least one test modulation is applied to said auxiliary wave and the phase of the wave backscattered by said fiber is measured.

BRIEF DESCRIPTION OF THE DRAWING An embodiment of the invention is described by way of example with reference to the accompanying drawing, in which: Figure 1 is an overall view of an optical link implementing the method of the present invention.

Figure 2 is a graph showing three curves representing phase variation in a receive test signal plotted up the Y-axis as a function of intrusion distance plotted along the X-axis.

DETAIL LED DESCRIPTION As shown in Figure 1, apparatus for transmitting information over an optical fiber with intrusion detection includes the following items that are known for performing the functions specified: a main laser 2 for receiving an information signal to be transmitted SI and for responding thereto by emitting a main wave OP which is modulated by said information signal; an optical fiber 14 constituting a link fiber; a coupling system 10 for injecting the main wave OP into the link fiber; and a main receiver 16 for receiving the main wave after it has been guided by said link fiber.

Together these items 2, 14, 10 and 16 constitute an optical link. It is known that surveillance means can be associated with this link for detecting and locating possible intrusion.

According to the present invention, these surveillance means comprise the following items, for example: a test generator 4 for providing a sinewave send test signal SM at a test frequency lying in the low frequency range; an auxiliary laser 6 for receiving said send test signal and for responding thereto by emitting and auxiliary wave OA carrying modulation constituted by said test signal on a wavelength that is longer than the wavelength of the main wave OP; an auxiliary receiver 8 for receiving a lightwave at the same wavelength as said auxiliary wave and for providing a test receive signal SR representative of modulation of said lightwave at said test frequency; said coupling system 10 for coupling not only the main laser 2 but also the auxiliary laser 6 and the auxiliary receiver 16 to one of the ends 13 of the link fiber 14; and a phase measuring device 12 for measuring the phase of the receive test signal relative to the send test signal, said device being advantageously constituted by means of a synchronous detector.

The fiber 14 is preferably a monomode fiber at both of the wavelengths used.

In general, the method implemented by the above apparatus includes the following operations that are known per se: (1) Injecting a main wave OP into a first end of the link fiber 14 so that said wave is guided to a second end 15 of said fiber. This main wave carries modulation SI representative of information, thereby implementing an optical link that transmits said information between said two ends.

(2) Injecting an auxiliary wave OA into an auxiliary injection end 13 which is preferably the first one of the two ends of the link fiber, as in the example shown. This wave is guided by the fiber to the other end 15 thereof. It is constituted by auxiliary light that has a "coupling characteristic" that is different from the corresponding "coupling characteristic" of the main wave OP. The meaning of this term is as follows: such "coupling characteristics" represent the ability of each of these waves to couple with the outside of the link fiber in the event of the fiber 14 being interfered with in a manner that runs the risk of causing such coupling to appear at an interference point 17.As described below, these characteristics are chosen, for example, so as to obtain the following effects: in a first case where it is merely detection of an intrusion that is required, the effect to be obtained is that any interference to be detected prevents a significant fraction of the auxiliary propagating, and preferably more than 20% of its energy. In a second case, where any intrusion is also to be located, the effect to be obtained is that any interference which affects the fiber sufficiently to enable the main wave OP to be coupled with an external system 18 that is not part of the optical link is certain to prevent the auxiliary wave OA propagating along the link fiber 14 beyond the interference point 17. It will naturally be understood that such interference would constitute an intrusion.

(3) Monitoring a fraction of the auxiliary light reaching one of the ends of the link fiber 14 to detect and possibly to locate such an intrusion. The intrusion may be located by measuring an unknown interference distance x which is the distance between the interference point 17 and the auxiliary injection end 13.

According to the present invention, during injection, the auxiliary wave OA is injected into the link fiber 14 in a manner that is prolonged and it carries modulation. The modulation is constituted at least by a send test signal SM which is a periodic signal at a test frequency. Monitoring includes receiving a backscattered wave OR which is constituted by a fraction of the auxiliary light that has been scattered by the link fiber 14 back towards the auxiliary injection end 13.

It also includes measuring the phase A of a re receive test signal SR modulating the backscattered wave at the test frequency.

This phase is measured relative to the phase of the send test signal SM.

In the absence of intrusion, the value of this phase is substantially constant or it varies very slowly. In the event of an intrusion, it drops much more quickly with increasing curvature or equivalent action being applied to the fiber.

Observing this drop constitutes detecting an intrusion.

The final value to which phase drops decreases with increasing distance to the intrusion. Measuring the final value thus serves to locate the intrusion. This possibility of locating an intrusion constitutes an essential advantage of the present invention, since merely detecting an intrusion can be performed by conventional means.

In the context of this method, it is preferable to adopt the following dispositions as well: (1) Said different coupling characteristics of the main wave OP and of the auxiliary wave OA are their optical wavelengths which respectively constitute a main wavelength and an auxiliary wavelength. The auxiliary wavelength is selected so that its product 1.Q is the smaller of the two. In this product, Q represents the length of the link fiber 14, or at least the maximum expected value for the interference distance x. k represents the linear attenuation coefficient of light having said wavelength in said fiber. It should nevertheless be understood that in a variant, coupling characteristics of a different kind could be constituted by different propagation modes for the two waves.The waves could then have the same wavelength, with the auxiliary wave being suitable, for example, for propagating along an annular waveguide formed in the link fiber around and at a distance from an axial waveguide followed by the main wave.

(2) When both waves propagate in the same mode, the auxiliary wavelength is longer than the main wavelength. For example, for use with the monanode fibers that are in general use at present, these wavelengths may be close to 1500 nm and close to 1300 nm.

(3) A test frequency lies in the range 20% to 90% of a limit frequency given by V/4.Q, where V is the propagation velocity of the auxiliary light in the link fiber 14 and Q is the length of said fiber. This test frequency is the same as the frequency of said send test signal if there is only one send test signal. If a plurality of send test signals are used, then it is the lowest test frequency that lies in the above range.

(4) Said id phase measurement is performed by synchronously detecting the receive test signal SR in the optical intensity of the backscattered wave.

(5) Injection of the auxiliary wave OA and of the send test signal OM modulating said wave is prolonged for an injection time of not less than one-hundredth of a second, it being understood that longer injection times and even continuous injection are generally preferred.

The method described above in terms of its main dispositions makes use of the Rayleigh back scattering that accompanies the propagation of the auxiliary wave OA and which creates a backscattered wave OR. The backscattered wave is made up of a superposition of elementary waves which are backscattered by each of the successive portions of the link fiber. Relative to the elementary wave backscattered by a portion adjacent to the auxiliary injection end 13, the elementary wave backscattered by a portion of fiber having the same length and situated at a distance z fran said end is attenuated by a factor e-2kz and is retarded by a time delay of 2z/V, where V is the propagation velocity of the auxiliary light in the link fiber.If the send test signal is of the form cos(wt), then the re receive test signal will be proportional to cos(wt + A) where A is the phase to be measured.

When the link fiber 14 is subjected to a high degree of curvature at an interference point 17 situated at a distance x from the end 13, the curvature causes the auxiliary light to escape to the outside of the fiber. It is then no longer possible for any of this light to be backscattered from portions of the fiber situated beyond the intrusion point.

Ignoring a scale factor, the receive test signal SR is given by the following equation:

The tangent tan A of the phase A of this signal is plotted up the Y-axis in Figure 2 as a function of the interference distance x to be determined which is plotted along the X-axis in kilometers. Curves 20, 22, and 24 in Figure 2 are obtained by calculation for the following values respectively of the coefficient k: 0.5 dB/km; 0.3 db/km, and 0.2 dB/km.

Further information concerning the location of the intrusion can be obtained by measuring the intensity of the backscattering. This can only be done once an intrusion has been detected or is suspected by other means.

The main parameter governing the accuracy with which intrusion is located is the ratio of measurement range, i.e. in general the length Q of the link fiber 14, to the attenuation length l/k of said fiber for the auxiliary light. Beyond a limit length 1 = 2/k, phase A is substantially insensitive to intrusion and measuring phase therefore cannot locate intrusion.

For attenuation of 0.5 dB/km, this limiting length is 18 km, and it rises to 45 km for attenuation of 0.2 dB/km.

In addition, locating accuracy increases with w2, i.e.

with the square of the test frequency, thus causing a high frequency to be selected.

However, locating becomes ambiguous if the frequency exceeds a limiting value equal to V/4. . That is why it seems preferable for said frequency to lie in the range about 50% to 75% of the limiting frequency, unless an additional measurement can be used to remove the ambiguity. In other words, the modulation wavelength formed by the send test signal in the link fiber should be close to six to eight times the length of the fiber.

The intensity of the backscattered wave is about onethousandth of the intensity of the auxiliary wave. That is why the auxiliary laser 6 and the coupling system 10 are designed to emit high power and to minimize coupling losses.

Additional distance measurements may be performed by modulating the auxiliary wave with a plurality of send test signals either simultaneously or else in succession, said signals constituting a small number of different test frequencies. It appears that this number should be limited to three. Such additional measurements may serve to locate an intrusion even when the auxiliary wave continues to propagate, albeit much attenuated, beyond the intrusion point.

On this topic, the following document may be mentioned: H. Ghafoori, Shira and T. Okoschi, "Optical frequency domain reflectometry" Optical and Quantum Electronics, 18, p.

265 (1986).

That document describes amplitude and phase measurement of the backscatter fran a modulated optical signal by the optical frequency domain reflectometry (OFDR) method. In that method, the local transmission characteristic of an optical fiber can be obtained by analyzing the back scattered signal over a large frequency range by means of the Fourier transform. The apparatus used for that purpose can be implemented in the laboratory but it is much too complex to be implemented in an industrial context. The method of locating curvature according to the present invention also makes use of Rayleigh backscattering. However, it includes measuring backscatter at a single test frequency or at a small number of such frequencies. As a result, it can be implemented using apparatus that is much more simple.

Claims

1/ A method of transmitting information over an optical fiber with an intrusion being detected and/or located, wherein an information-carrying main wave and an auxiliary wave are both injected into a link fiber, the auxiliary wave being selected so that it couples more easily with the outside than does the main wave, wherein at least one test modulation is applied to said auxiliary wave and the phase of the wave backscattered by said fiber is measured.

2/ A method according to claim 1, the method comprising the following steps: injecting a main wave into a first end of an optical fiber constituting a link fiber so that said wave is guided by said fiber to a second end thereof, said main wave carrying modulation representative of information, thereby providing an optical link transmitting said information between said two ends;; injecting an auxiliary wave into an auxiliary injection end constituted by one or other of said two ends of said link fiber so that said wave is guided by said fiber to the other end thereof, said auxiliary wave being constituted by auxiliary light and having a coupling characteristic that is different from a corresponding coupling characteristic of said main wave, said coupling characteristics being selected so that interference constituting an intrusion on said link prevents, at least partially, said link fiber from guiding said auxiliary wave beyond an interference point; and monitoring a fraction of said auxiliary light reaching one of said ends of said link fiber to detect and/or possibly locate such an intrusion; said method being wherein in said step of injecting said auxiliary wave, said wave is injected into said link fiber in a manner that is prolonged and it carries modulation constituted at least by a send test signal which is periodic at a test frequency; said monitoring step including receiving a backscattered wave which is constituted by a fraction of said auxiliary light that has been scattered by said link fiber back towards said auxiliary injection end, said step further including measuring the phase of a re receive test signal that modulates said backscattered wave at said test frequency, said phase being measured relative to said send test signal.

3/ A method according to claim 2, in which said different coupling characteristics of said main wave and of said auxiliary wave are their optical wavelengths which respectively constitute a main wavelength and an auxiliary wavelength, said auxiliary wavelength being such that its product 1.Q is the smaller of the two, where Q is a monitored length of said link fiber, and k is the linear attenuation coefficient of light at said wavelength in said fiber.

4/ A method according to claim 3, wherein said auxiliary wavelength is longer than said main wavelength.

5/ A method according to claim 2, wherein said test frequency lies in the range 20% to 90% of a limit frequency equal to V/4.Q, where V is the propagation velocity of said auxiliary light in said link fiber, and Q is a monitor length of said fiber.

6/ A method according to claim 2, in which said phase measurement is performed by synchronously detecting said receive test signal.

7/ A method according to claim 2, wherein said injection of said auxiliary wave and said send test signal modulating said wave are continued for an injection time of at least onehundredth of a second.

8/ A method according to claim 2, wherein said auxiliary injection end of said link fiber is constituted by its said first end.

9/ A method according to claim 2, wherein said auxiliary wave is modulated by not more than three of said send test signals.

10/ Apparatus for transmitting information over an optical fiber with detection and/or localization of an intrusion, the apparatus comprising: a main laser for receiving an information signal to be transmitted and for responding thereto by emitting a main wave modulated by said information signal; an optical fiber for constituting a link fiber; a coupling system for injecting said main wave into said link fiber; a main receiver for receiving said main wave at the outlet fran said link fiber, thereby constituting an optical link; and surveillance means for detecting and/or locating a possible intrusion on said optical link; wherein said surveillance means comprise: a test generator for providing at least one periodic send test signal at a test frequency lying in the low frequency range; an auxiliary laser for receiving said send test signal and for emitting an auxiliary wave carrying modulation constituted by said send test signal; an auxiliary receiver for re receiving a lightwave at the same wavelength as said auxiliary wave and for providing a receive test signal representative of said lightwave being modulated at said test frequency; said coupling system for coupling said main laser, said auxiliary laser, and said auxiliary re receiver to one end of said link fiber; and a phase measuring device for measuring the phase of said receive z ive test signal relative to said send test signal.

11/ Apparatus according to claim 10, wherein the wavelength of said auxiliary wave is longer than the wavelength of said main wave.

12/ Apparatus according to claim 10, wherein said phase measuring device is a synchronous detector.

13/ A method of transmitting information over an optical fiber with an intrusion being detected and/or located, substantially as herein described with reference to the accompanying drawing.

14/ Apparatus for transmitting information over an optical fiber with detection and/or localization of an intrusion, substantially as herein described with reference to the accompanying drawing.

Amendments to the claims have been filed as follows 1. A method of transmitting information over a fibre-optic link with intrusion being detected and located including: injecting main and auxiliary waves into first and auxiliary ends respectively of the fibre link, the two waves each being modulated to transfer information and test signals to second and further ends respectively of the link, and monitoring the phase of the back-scattered test signals relative to the phase of the injected test signal, wherein the auxiliary wave is selected so that it couples more easily with the outside of the fibre than does the main wave, and the frequency of the test signal is such that, when the phase change in the test signal is characteristic of an intrusion, the phase of the back-scattered test signal corresponds to an abscissa of the intrusion point measured from an origin point along the fibre, the back-scattered test signal comprising substantially the sum of back-scattered test signals returned from between the auxiliary end and the intrusion point.

2. A method as claimed in claim 1, wherein the test signal is periodic at a test frequency.

3. A method as claimed in claims 1 or 2, wherein the test frequency is between 0.2 to 0.9 of the limit frequency for the link, given by V/4L where V is the propagation velocity of the auxiliary wave and L is the fibre length.

4. A method as claimed in any preceding claim, wherein the auxiliary wave has a lower product K.L than the main wave; K being the linear attenuation coefficient of light at the respective optical frequency and L being the fibre length.

5. A method as claimed in claim 4, wherein the auxiliary wave has a wavelength greater than that of the main wave.

6. A method as claimed in any preceding claim, wherein the auxiliary wave and the test signal are injected for at least 1/100 second.

7. A method as claimed in any preceding claim, wherein the test signal has no more than three test frequencies.

8. A method as claimed in any preceding claim, wherein the phase measurement is performed by synchronously detecting the back-scattered test signal.

9. A method as claimed in any preceding claim, wherein the auxiliary injection end is the first end.

10. Apparatus for transmitting information over a fibre-optic link with intrusion being detected in use, including: main and auxiliary laser means for injecting main and auxiliary waves into first and auxiliary ends respectively of the fibre link, information supply and test signal generation means to modulate in use the two laser means to give information and periodic low frequency test signals to transfer information and test signals to second and further ends respectively of the link, information and test signal receiving means provided at second and auxiliary ends respectively, the test signal receiving means having phase measuring means to monitor in use the back-scattered test signals relative to the phase of the injected test signal, wherein the auxiliary laser means is arranged to inject an auxiliary wave which couples more easily with the outside of the fibre than does the main wave, and the test signal generation means is arranged to modulate the auxiliary wave such that, when the phase change in the test signal is characteristic of an intrusion, the phase of the back-scattered test signal corresponds to an abscissa of the intrusion point measured from an origin point along the fibre, the back-scattered test signal comprising substantially the sum of back-scattered test signals returned from between the auxiliary end and the intrusion point.

11. Apparatus as claimed in claim 10, wherein the test signal generation means provides in use, a test frequency between 0.2 to 0.9 of the limit frequency of the link, given by V/4L where V is the propagation velocity of the auxiliary wave and L is the fibre length.

12. Apparatus as claimed in claim 10 or 11, wherein the test signal generation means provides, in use, a test signal having no more than three test frequencies.

13. Apparatus as claimed in claims 10 to 12, wherein the auxiliary laser means has in use a wavelength greater than that of the main laser means.

14. Apparatus as claimed in claims 10 to 13, wherein the phase measuring means is a synchronous detector.

15. Apparatus as claimed in claims 10 to 14, wherein the auxiliary wave has a lower product K.L than the main wave; K being the linear attenuation coefficient of light at the respective optical frequency and L being the fibre length.

16. A method of transmitting information over a fibre optic link with intrusion being detected and located substantially as hereinbefore described with reference to the accompanying drawings.

17. Apparatus for transmitting information over an optical fibre substantially as hereinbefore described with reference to the accompanying drawings.