Note: Descriptions are shown in the official language in which they were submitted.
104783~0
This invention relates to an optical fibre with
increased security, particularly against unauthorized abstract
of information from the fibre.
Several well known advantages of fibre-optic
transmission systems include high information capacity, compactness,
low attenuation, and immunity from atomic and electrical radiation.
This last advantage is important for high electrical field
environments, prevention of fibre 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.
There are several techniques for abstracting
information from an optical fibre. Firstly, the cable, if such,
is entered and the fibre jacketing is removed locally from a
fibre. The fibre may 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 cut, 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, chemically or
mechanically, and some signal extracted, for example with a contact-
ing prism. This may go undetected. In a further alternative,
the cladding is not removed and the extracting element obtains
either evanescent light or radiated light. This latter 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 techniques are 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
~ )478~)
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.
Thus a secure optical comprises a light conducting
core having a predetermined refractive index, a first cladding
layer surrounding the core and having a refractive index lower
than that of the core, a light conducting layer over the first
cladding layer and having a refractive index higher than that of
the first cladding layer, and a second cladding layer over the
light conducting layer and having a refractive index lower than
the light conducting layer.
Signal launching means can be positioned at one
end of the core and detecting means at the other end of the core.
Further signal launching means can be positioned at one end of
the light conducting layer and a further detecting means can be
positioned either at the same end of the light conducting layer
at the launching means, or at the other end. If signal launching
means and detecting means are at the same end, then a reflecting
surface is formed on the other end of the light conducting layer.
The invention will be readily understood by the
following description of certain embodiments by way of example,
in conjunction with the accompanying drawings, in which:-
Figure 1 is a diagrammatic illustration of one
form of optical fibre, in accordance with the present inventioni
Figures 2, 3, 4 and 5 are curves illustrating
various alternative refractive index (n) profiles across the
radius (r) of a fibre as in Figure l;
Figure 6 is a curve illustrating the refractive
index (n) profile of a further form of fibre,
Figures 7 and 8 are diagrammatic cross-sections of
alternative fibre formations embodying the invention;
1~4781C~
Figure 9 is a diagrammatic end view of a fibre,
illustrating the provision of an end mirrori
Figure 10 is a diagrammatic longitudinal cross-
section through part of a fibre core and cladding illustrating
index or reflecting centres in the cladding;
Figures 11 and 12 illustrate diagrammatically one
form of signal launching and detecting device, Figure 12 being
a cross section on the line XII-XII of Figure 11 to a smaller
scale.
As illustrated in Figure 1, an optical fibre has
a core 10 and successive layers 11, 12 and 13. The core 10 and
layer 12 have a higher refractive index than the layers 11 and 13,
which act as cladding layers. The thickness of the layer 11 is
large enough to ensure optical isolation of the core 10 and layer
12 from each other. The core 10 and layer 12 serve as the guiding
regions for two optical channels without crosstalk. The data
signal is carried by the light rays 14 in the core 10 and the
monitor signal is carried by the light rays 15 in the annular
layer 12. Since the annular layer 12 surrounds the core 10 and
since rays 15 are more sensitive to tapping techniques than are
rays 14, a monitored secure fibre is established.
The invention provides a secure optical fibre
transmission channel in which the data and monitor signals are
spatially separated. Hence mode spreading and backscattering
effects are avoided and the data and monitor discrimination
sensitivity can be high. This allows long distances, bends, and
codirectional or contradirectional data and monitor signals
transmission. Launching and receiving are straightforward. In
a codirectional systems, that is with the data and monitor signals
travelling in the same direction along the fibre, the transmitter
could be a dot - ring light emitting diode or laser; the dot would
~.~4781~
inject the data signal into the core 10 while the ring would
inject the monitor signal into the layer 12. The receiver could
be a dot ring PIN or APD device. In the contradirectional system,
that is with the monitor signal travelling in the opposite direction
to the data signal, there could be a dot source and a ring detector
at one end and a ring source and a dot detector at the other end
of the fibre. Such arrangements are described in more detail
later.
The particular form of the fibre~ as in Figure 1,
can be varied. Thus, as illustrated in Figure 2, the core 10
and each layer 11, 12 and 13 can have a constant refractive index,
the index for core 10 and layer 12 the same, and the index for
the layers 11 and 13 the same. As illustrated in Figure 3~ the
refractive indexes for core and all layers are all different, but
meeting the requirement that the indices for the core 10 and layer
12 are higher than the indices for the layers 11 and 13.
Figure 4 illustrates a refractive index profile
with the index for the core 10 higher than the index for the
layer 12, while the indices for the layers 11 and 13 are the same,
and lower than those for the core 10 and layer 12. This has the
advantage of making the monitor signal more loosely bound than
the data signal, thereby enhancing tap detectability. The core 10
may have a graded refractive index, to reduce modal dispersion,
as illustrated in Figure 5. The data light rays now follow quasi-
sinusoidal paths rather than the zig-zag paths 14 in Figure 1.
Also, the layers 11 and/or 12 can have graded refractive indices,
if desired.
Figure 6 illustrates the refractive index (n)
profile across the radius of an optical fibre, similar to that
of Figure 1, but having two additional layers, indicated at 16 and
17 in Figure 6. The layers 12 and 16 carry monitor signals which
~047~1~
may be the same or different. The two monitor si~nals may travel
in the same direction or in opposite directions. This can be
extended to mu1tiple data signals and multiple monitoring signals.
In Figure 7, two cores 20 and 21 carry independent
data signals and are surrounded by a common cladding 22, a
protective monitoring layer 23 and outer cladding 24. This can
be applied to a cable concept, as illustrated in Figure 8 in which
three fibre cores 30, each with their own individual cladding layer
31, and with each core carrying a data signal, are surrounded by
10 a cushioning material 32. A monitor layer 33, which can be a
composite of layers, carries the monitor signal, and is surrounded
by a further layer 34 of protection and strength.
The various refractive indices of the cores 20,
21 and 30, and the various layers 23, 31 and 33 can be constant
or graded, and the number of layers can be increased if desired.
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
20 tapping methods that are sensitive to direction.
Codirectional data and monitor transmission 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
30 itself subject to sabotage, is necessary. To utilize the
advantages of both types of signalling, launching is codirectional
.
~4783~0
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 will 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. Figure 9 is an end view on a fibre, for
example as in Figures 1 and 3, the same reference numerals
applied, Figure 9 illustrating, in the hatched area 38, 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 high index layer 12, and also extending
slightly over the adjacent layers 11 and 13 to accommodate
evanescent light. Reflectivity should be near unity.
The more loosely bound monitor signal can be made
much stronger than the data signal. The tapped signal mix is then
composed predominantly of 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 furtherenhanced by use of a
noisy source with a bandwidth exceeding thatof 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 embodiment of Figure 1, the
monitor signal can be scrambled by making the interfaces between
layer 12 and layer 11 and/or between layer 12 and layer 13 such
that they randomly scatter the monitor signal rays 15. This may
3~ be done by refractive index and/or boundary fluctuations at the
interface. A typical example is to roughen the surface of layer 11
1047B10
before forming layer 12 and then roughening the surface of layer
12 before forming layer 13.
Another method is to distribute scattering
,luctuations throughout the layer 12 in Figure 1, via tiny index
or reflectivity centres. This is seen in Figure 10 which
illustrates in cross-section part of layer 12 and adjacent parts
of layers 11 and 13 only, with index or reflective centres
indicated at 39. While only illustrated two dimensionally, it will
be appreciated that the effect is three dimensional and scattering
occurs three dimensionally in layer 12.
Either method has the effect of inducing coupling
amongst high order monitor modes. Hence rays 15 will experience
randomly different path length 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 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 response to the monitor DC 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.
Figures 11 and 12 illustrate one particular
arragement for launching and/or detecting signals in the data and
, _ .
1~)478~0
monitor channels. Figure 11 is a cross-section on a plane
parallel to and coincident with the axis of a fibre, illustrating
a diode structure on the fibre end, and Figure 12 is a cross-
section on the line XII-XII of Figure 11. Considering the
arrangement as a transmitter-receiver, it comprises a circular p-n
junction 40 and an annular junction 41 respectively aligned with
the high index data region 10 and monitor region 12 respectively.
The junctions consist of a "transparent" n-type GaAlAs substrate
42 and layer 43; a p - or n - type ~aAs active layer 44; and a
10 "transparent" p-type GaAlAs layer 45. Proton-bombarded annular
regions 46 and 47 are of high electrical resistance and optical
absorption so that electrical and optical isolation between
junctions 40 and 41 is achieved. Insulating oxide annular areas
48 and 49 cover the bombarded regions 46 and 47 and metaYlic
circular and annular areas 50 and 51 provide independent p-side
contacts. The common n-side contact ring 52 is on the opposite
of the device to p-side contacts, in the present example outside
the fibre diameter to assist in aligning.
Alternatively the p-side contact is positioned
20 opposite a low index region of the fibre, that is opposite 10
- or 12, with which light is not coupled. A transparent epoxy 53
binds the fibre and device into a unit.
In operation, the junctions can emit under forward
bias or detect under reverse bias. For example, if the p-contact
50 is made positive with respect to the n-contact 52, the circular
junction 40 may be modulated to emit a data signal into the core 10
of the fibre. If the p-contact 51 is similarly biased, the annular
junction 41 will emit a monitor signal into the region 12 of the
fibre. This is suitable for the input end of a link with
30 codirectional signalling. However, if p-contact 51 is made negative
with respect to the n-contact 52, then the annular junction 41 can
-- 8 --
1047B10
detect a monitor signal leaving the region 12 - this is the data
input end of a link with contradirectional si~nallin~. nbvious
reversals of biases in both the above cases provides suitable
operation for the other end of the link.
