Optical fibre with enhanced security

An optical fibre has a light conducting core surrounded by first and second cladding layers of sequentially lower refractive index. A data-carrying signal can be propagated in the core and a monitoring signal--of a higher modal order than the data signal--can be propagated in the core and first cladding layer. Any attempt to rap the fibre and abstract data information results in aberration of the monitor signal, which can be detected. The monitor signal can be such that resolvable signals cannot be obtained from the data channel by a tap.

This invention relates to an optical fibre with enhanced security, and 
particularly to the improvement of security against unauthorized 
abstraction of information from the fibre. 
Fibre-optic transmission systems have several advantages, such as high 
information capacity, compactness, low attenuation, immunity from atomic 
and electrical radiation. This latter advantage, immunity from electrical 
radiation, is important for high electrical field environments, avoidance 
of interfibre cross talk and security. However an optical fibre can be 
tapped such that very little of the input data signal is extracted for 
amplification. If the perturbation on the signal is below the 
detectability of the receiver, then the tap will go unnoticed. 
Several tapping techniques exist. Firstly, the cable, if any, is entered 
and the fibre jacketing is locally removed from a fibre. The fibre may 
then be cut and an optical T inserted to extract some data signal. This 
relatively crude method will severely perturb the signal, especially at 
the time of the cutting, and the tap would be readily detected at the 
receiver. A partial diagonal cut may reflect out a sufficient strength of 
signal. Alternatively, the fibre cladding may be removed locally, for 
example chemically or mechanically, and some signal extracted as by a 
contacting prism. This may go undetected. In a further alternative, the 
cladding is not removed and the extracting element obtains either 
evanescent liquid or radiated light. This method is even less susceptible 
to detection. Also, a local fibre diameter reduction, as in a taper, will 
release some higher order modes. The above methods may be enhanced by 
local stressing and/or bending the fibre appropriately. 
Since some of the possible tapping methods may be undetected at the 
receiver it is desirable to improve the possibility of detection and/or 
prevent a useful signal being abstracted. Generally, the invention 
provides for the use of a monitoring signal separate from the data signal.

As illustrated in FIG. 1, an optical fibre has a core 10 of refractive 
index n.sub.1 surrounded by a cladding 11 of refractive index n.sub.2 in 
turn surrounded by a second cladding 12 of refractive index n.sub.3. The 
refractive indices n.sub.1, n.sub.2 and n.sub.3 are of reducing value, as 
indicated in FIG. 2. The data signal is carried in the core 10 as 
represented by the light rays 13, the light rays 13 reflecting off the 
interface 14 between core and cladding. The monitor signal is carried in 
both the core 10 and the inner cladding 11, as represented by the light 
rays 15 or 15a. The rays, 15 or 15a refract through the interface 14 and 
reflect off the interface 16 between the cladding layers 11 and 12. 
Since interface 16 surrounds interface 14, and since the rays 15, 15a are 
of higher mode order than the rays 13, the monitor signal is more loosely 
bound in the fibre than is the data signal. Hence the monitor signal is 
more sensitive than the data signal to any of the above referred tapping 
techniques. 
The data signal is injected as a beam of low order rays into the core 10 at 
one end, the signal being detected at the other end. The monitor signal 
can be injected as high order rays near the detector or output end of the 
fibre in the opposite direction to the data rays 13, as shown by rays 15. 
Alternatively the monitor signal can be injected as high order rays at the 
data input end of the fibre, as shown by rays 15a. 
An advantage of this system is that the high and low order mode groupings 
are kept separated by the index discontinuity at interface 16. This makes 
the directionability of the monitor signal relative to the data signal 
less important and also makes the fibre more suitable for long distances 
and levels. 
FIGS. 3 and 4 illustrate an alternative form of fibre in which the core 10 
has a graded refractive index, as particularly indicated by the index 
profile in FIG. 4. The graded index core substantially reduces modal 
differential time delays and thus narrows the data impulse response width. 
The low order light rays 13 follow quasi-sinusoidal paths rather than 
zig-zag paths as in FIG. 1. The profile of the refractive index of the 
core is typically close to parobolic. While in FIGS. 3 and 4 the cladding 
layer 11 is shown with a constant refractive index n.sub.2, this cladding 
may also have a graded index, the index decreasing as the radius 
increases. 
A further modification, illustrated in FIGS. 1 and 5, is to add a further 
cladding layer 17 indicated by dotted line 18 in FIG. 1, and giving a 
further interface 19 between cladding layers 12 and 18. The cladding 18 
has a lower refractive index than the other layers, as indicated in FIG. 
5. A second monitor signal may be injected into the cladding 12 and 
confined by the interface 19. The two monitor signals may be the same or 
different and may both travel in the same direction or in opposite 
directions. This may be extended to multiple data signals and multiple 
monitoring signals. 
FIG. 6 illustrates a form of optical fibre having two cores 20 and 21 
surrounded by common cladding 22 and an outer cladding 23. The cores 20 
and 21 may carry independent data signals while the monitor signal is 
carried throughout both the cores 20 and 21 and the cladding 22. This can 
be applied to a cable concept, as illustrated in FIG. 7. There are three 
cores 30 each independently clad by a layer of cladding material 31. The 
three clad fibres are surrounded by a low index cladding material 32, 
which can be a cushioning transparent material. The monitor signal is 
carried in the fibres 30, claddings 31 and clading 32. A further layer 33 
acts as an outer cladding layer, with a lower refractive index than 
cladding 32. A further layer 34 is provided for protection and strength. 
Graded refractive indices can be applied to the embodiment illustrated in 
FIGS. 6 and 7, both for the cores, and for the cladding. Additional 
cladding layers can also be provided to make additional monitor signals to 
be provided. 
With codirectional signalling, that is data and monitor signals travelling 
in the same direction, data cannot easily be extracted without affecting 
the monitor signal. Contradirectional propagation is thus less suitable 
for those tapping methods that are sensitive to direction. 
Codirectional data and monitor transmissions may be preferred over the 
contradirectional type because it will generally result in more monitor 
signal being tapped out. This means a larger monitor noise-to-data ratio 
inflicted upon the intruder and a greater detection sensitivity for the 
operator. However, contradirectional signalling has the advantage of 
proximity of the monitor receiver to the data transmitter. This allows for 
convenient alarming for transmitter shutdown. In the codirectional case an 
alarming channel, probably electrical and itself subject to sabotage, is 
necessary. To utilize the advantages of both types of signalling, 
launching is codirectional, with the monitor signal retro-reflected at the 
data output end and received back at the input end. It is also commented 
that a single tap with give rise to two perturbations in the monitor 
signal. 
Retro-reflection is obtained by providing a reflective surface at the data 
output end for the monitor-signal-only layer or layers. FIGS. 8 and 9 are 
end views on a fibre, for example as in FIGS. 1 and 3, the same reference 
numerals applied, FIG. 8 illustrating--in the hatched area--a mirrored end 
formed by any method well known in the optical arts--for example 
metallization or dielectric layers--extending over the whole end area of 
the inner cladding layer 11. FIG. 9 varies in that the mirrored end avoids 
the interface 14 and overalps part of the outer cladding 12. This also 
avoids possible retro-reflection of data rays. In both examples some 
monitor signal will pass out through the central uncoated region, so that 
retro-reflectivity will be less than about half. The retro-reflectivity 
can be increased by angle-selective reflecting layers covering the whole 
end face so that high modes are reflected while low modes pass through. 
The more loosely bound monitor signal can be made much stronger than the 
data signal. The tapped signal mix is then composed predominantly of the 
monitor signal so that the ratio of data signal-to-monitor noise is very 
small. It is possible to make the ratio so small that the data signal 
portion of the total mixed signal is below noise level and is therefore 
unresolvable. 
Monitor noise can be further enhanced by use of a noisy source with a 
bandwidth exceeding that of the data signal. 
Alternatively, in the present invention, the monitor channel can itself add 
noise to the monitor signal via the introduction of fibre fluctuations 
into the monitor-signal-only region. For example, in the embodiments of 
FIGS. 1 and 3 the monitor signal can be scrambled by making the interface 
16 in FIG. 1 and FIG. 3 such that they randomly scatter the monitor signal 
rays 15 and 15a. This may be done by refractive index and/or boundary 
fluctuations at the interface. A typical example is to roughen the surface 
of layer 11 before forming layer 12. In the modified form of FIG. 1, the 
surface of layer 11 is roughened before forming layer 12 and then the 
surface 19 of layer 12 roughened before forming layer 17. Another method 
is to distribute scattering fluctuations throughout the layer 11 in FIG. 1 
and FIG. 3 and in layer 12 in the modified form of FIG. 1, with layer 17, 
via tiny index or reflectivity centres. This is seen in FIG. 10 which 
illustrates in cross-section part of layer 11 and adjacent parts of layers 
12 and core 10 only with index or reflective centres indicated at 40. 
While only illustrated two dimensionally, it will be appreciated that the 
effect is three dimensionally in layer 11, and also similary in layer 17 
for the modified form of FIG. 1. 
Either method has the effect of inducing coupling amongsthigh order monitor 
modes. Hence rays 15 and 15a will experience randomly different path 
lengths down the fibre and any regularity in the monitor signal will be 
scrambled, or a noisy injected monitor signal made more noisy. An intruder 
will then find it more difficult to separate the tapped noisy monitor 
signal from the weaker tapped data signal. 
The tapped signal cannot then be resolved to yield the data; a security 
alarm may not be necessary. With a noisy monitor, an alarm would respond 
to the noise d.c. level. In systems in which the tapped signal would be 
resolved to obtain a data signal, a detector can be provided to detect the 
relatively large perturbations of the monitor signal which would be 
occasioned by a tap. A detector could merely indicate that a fibre is 
tapped or could shut the system down. 
Relatively large perturbations of the monitor signal will occur because a 
region in which only the monitor signal propagates must be crossed before 
access is obtained to the data signal region. 
Launching or receiving may be done through the fibre end faces or through 
the side of a tapered fibre. In both cases it must be ensured that the 
data rays 13 of FIG. 1 make an angle .theta..sub.D with the axis inside 
the fibre satisfying, 
EQU O .ltoreq. .theta..sub.D .ltoreq. .theta..sub.1 = cos .sup.-1 (n.sub.2 
/n.sub.1). (1) 
For the monitor rays 15, 
EQU .theta..sub.1 .ltoreq. .theta..sub.M .ltoreq. .theta..sub.2 = cos .sup.-1 
(n.sub.3 /N.sub.1). (2) 
for minimal crosstalk the ranges of .theta..sub.D and .theta..sub.M should 
be well separated. 
For end-launching as shown in FIG. 11, the data source must illuminate only 
the core up to a certain angle (.phi..sub.1) measured with respect to the 
end-face normal. The monitor source can illuminate the core at large 
angles (up to .phi..sub.2 beyond which the rays 50 are not trapped) and/or 
it can illuminate the first cladding up to a certain angle (.phi..sub.3, 
beyond which rays 51 are untrapped). The required angular and spatial 
tolerance can be achieved by those skilled in the optical arts. The angles 
are given by: 
EQU n.sub.o sin .phi..sub.1 = (n.sub.1.sup.2 - n.sub.2.sup.2).sup.1/2 
EQU n.sub.o sin .phi..sub.2 = (n.sub.1.sup.2 - n.sub.3.sup.2).sup.1/2 (3) 
EQU n.sub.o sin .phi..sub.3 = (n.sub.2 /n.sub.1)(n.sub.1.sup.2 - 
n.sub.3.sup.2).sup.1/2 
where n.sub.o is the index of the medium surrounding the fibre. 
For end-detection the situation is analagous and optical techniques 
providing angular and spatial resolution can be used. Alternatively, and 
more simply, as shown in FIG. 12, a detector 60 covering essentially the 
core 10 will intercept the signal D + fM, where f is some fraction of the 
monitor signal. A separate detector 61 covering essentially the first 
cladding 11 will intercept the remaining monitor (f-1)M. An electronic 
circuit can simply perform a subtraction to give D and M separately. 
For side-launching, as shown in FIG. 13, the fibre is up-tapered and 
surrounded by a higher index medium n.sub.o. The data source rays must 
illuminate between a range of angles measured with respect to the side 
normal (X.sub.1 to X.sub.2, above which the rays 52 do not enter the 
fibre). Similarly the monitor rays must illuminate an angular range 
(X.sub.2 to X.sub.3, below which rays 53 pass through the fibre). The 
angles are given by: 
EQU n.sub.o sin X.sub.1 = n.sub.3 
EQU n.sub.o sin X.sub.2 = n.sub.1 sin [sin.sup.-1 (n.sub.2 /n.sub.1) - 
2.epsilon.] (4) 
EQU n.sub.o sin X.sub.3 = n.sub.1 sin [sin.sup.-1 (n.sub.3 /n.sub.1) - 
2.epsilon.] 
where .epsilon. is the taper half-angle. These equations for side-launching 
correspond to equations (3) for end launching. 
For side-detection the situation is analogous and optical techniques 
providing angular and spatial resolution can be used. The fibre is 
down-tapered. 
In a hybrid scheme it may be simpler, for example, to end-launch the data 
and side launch the monitor. At the other terminal the data may be 
end-detected and the monitor side-detected. Various permutations of this 
arrangement are possible. These may be adapted to codirectional or 
contradirectional monitoring