Multiple core optical waveguide for secure transmission

A glass optical fiber has multiple cores and a cladding. The index of refraction of all cores is greater than the index of refraction of the cladding and the index of refraction of a first core is greater then the index of refraction of a second core. The change in the ratio of light loss from the first and second cores is detected to identify perturbations of the optical fiber before it reaches a level sufficient for a secure signal to be tapped.

BACKGROUND OF THE INVENTION 
This invention relates to optical waveguides and more particularly, to a 
multiple core optical fiber for use in a secure communication system. 
The increased burden on communication systems has fostered the development 
of high capacity systems using optical waveguides. These optical 
waveguides are constructed of a transparent dielectric material such as 
glass. They consist of a central core surrounded by a cladding having an 
index of refraction less than that of the core. Light propagates along the 
waveguide. The theory of optical waveguides is contained in U.S. Pat. No. 
3,157,726--Hicks et al and in a publication "Cylindrical Dielectric 
Waveguide Mode", by E. Snitzer, Journal of the Optical Society of America, 
Vol. 51, No. 5, pp. 491-498, May 1961. 
Recently, however, optical waveguides having very low attenuation per unit 
length have been developed. For example, the Maurer and Schultz, U.S. Pat. 
No. 3,659,915, "Glass Optical Waveguide", describes an optical waveguide 
comprising a cladding layer of pure fused silica or doped fused silica and 
a core formed of doped fused silica. Germania and silica glass waveguides 
can also be used in accordance with U.S. Pat. No. 3,884,550. Single mode 
waveguides fabricated in accordance with Keck and Schultz, U.S. Pat. No. 
3,711,262, are also suitable for use. 
Optical waveguides having multiple cores are also known, one example being 
shown in U.S. Pat. No. 3,930,714. 
In many communication systems, it is essential that the sender be able to 
recognize when his communications have been compromised. Normally, 
quiescent emanations from an optical waveguide are not detectable by an 
intruder. It is necessary for the intruder to perturb the waveguide, for 
example bend it, to enhance emanations from the waveguide so that the 
signal can be detected. 
SUMMARY OF THE INVENTION 
In accordance with this invention, a multiple core optical waveguide having 
differences in the indices of refraction between at least two of the cores 
is provided. 
The multiple core waveguide of this invention is particularly suitable for 
use in a secure communication system where perturbation of the waveguide 
is detected before it reaches a level which is sufficient for the intruder 
to tap the secure signal. 
The ratio of light loss from two cores of different indices of refraction, 
core diameters, and/or propagation wavelength is detected. A relative 
change in the light loss from the two cores identifies a perturbation of 
the optical fiber. 
In one embodiment, the multiple cores are coaxial. In the preferred 
embodiment, one core is coaxial with the cladding and the remaining cores 
are arranged around the axis and within the cladding. This embodiment has 
particular advantages in a secure communication system because the cores 
arranged around the axis will detect a perturbation from any angle. Since 
the primary core, which is the coaxial core, is surrounded by the 
remaining cores, which are the secondary, or alarm cores, a secondary core 
will always be perturbed in order to obtain perturbation of the primary 
core. This arrangement has other significant advantages in that different 
wavelengths of light can be applied to the secondary cores. Often, a false 
alarm is signalled by a perturbation at only one wavelength. However, by 
applying different wavelengths of light to the different alarm cores, a 
perturbation at all of the different wavelengths will signify a true 
intrusion upon the optical waveguide. 
In one embodiment, the optical fiber has a decreasing refractive index 
profile with distance from the axis of the fiber. Alternatively, the cores 
are concentric rings, and rings of high refractive index may be separated 
by rings of lower refractive index. 
In accordance with another aspect of the invention, the presence of a bend 
is detected because the relative light loss from a second core is very 
much greater than the light loss from a direct core. This is made possible 
because the relative index difference between the second core and the 
cladding, 
##EQU1## 
is substantially less than the relative index difference between the first 
core and its cladding, 
##EQU2## 
where n.sub.1 is the index of refraction of the first core, n.sub.2 is the 
index of refraction of the second core and n.sub.3 is the index of 
refraction of the cladding. 
The foregoing and other objects, features and advantages of the invention 
will be better understood from the following more detailed description and 
appended claims.

DESCRIPTION OF TICULAR EMBODIMENTS 
Referring to FIG. 1, a normal optical waveguide includes a core 10 having 
an index of refraction n.sub.1 surrounded by a cladding layer 11 having a 
lower index of refraction n.sub.2. A radial refractive index profile for 
this waveguide is shown in FIG. 1A which depicts refractive index as 
abscissa and radial distance from the axis of the waveguide as ordinate. 
It has been shown that, in the presence of random bends, the light lost 
from the waveguide is inversely proportional to a power of the relative 
core cladding index difference, 
##EQU3## 
The loss coefficient for multiple mode waveguides is given by: 
##EQU4## 
where a is core radius and p is a quantity dependent upon the nature of 
the bend. Typically it has a value, 0&lt;p&lt;2. See R. Olshansky, App. Opt. 14, 
No. 4, 935 (1975). 
In accordance with one embodiment of this invention, a multiple core 
waveguide is constructed as depicted in FIG. 2. It includes a first core 
12, at least one second core 13 and a cladding 14. Light is propagated in 
at least cores 12 and 13. The refractive index profile is shown in FIG. 
2A. The refractive index n.sub.1 of the first core is greater than the 
refractive index n.sub.2 of the second core. The indices of refraction of 
both cores are greater than the index of refraction of the cladding 
n.sub.3. In the presence of a bend, the relative loss from the second core 
13 is much greater than the loss from the first core 12 if .DELTA..sub.23 
is substantially less than .DELTA..sub.12. 
In accordance with this invention, intrusion can be detected by monitoring 
the signals in the cores 12 and 13. If the ratio of light loss between the 
two cores changes, this indicates an intrusion. 
Other multiple core waveguides will similarly exhibit a change in the ratio 
of light loss in the presence of perturbations. 
FIG. 3 shows a ring type guide surrounding a conventional guide. The 
conventional guide includes a core 15 and a cladding 16. The ring guide 
includes a second core 17 and a cladding 18. The refractive index profile 
is shown in FIG. 3A. 
FIG. 4 shows the preferred embodiment wherein the first core 19 is coaxial 
with the cladding 20 and wherein a plurality of second cores 21-24 are 
disposed around the first core 19. The index of refraction of core 19 is 
greater than the index of refraction of the cladding 20. Core 19 forms a 
primary channel. The surrounding cores 21-24 have an index of refraction 
which is also greater than that of the cladding 20. By proper choice of 
the core diameters, the refractive index difference and the propagating 
light wavelength for the primary and secondary channels, the secondary 
channels 21-24 can be made more susceptible to distortion loss. This can 
readily be illustrated for the case of the channels being single mode 
waveguides. Then the radial power distribution in the region surrounding 
the waveguide core is shown by Marcuse in "Theory of Dielectric Optical 
Waveguides," Academic Press 1974, to be: 
EQU P(r)=P(a)e.sup.-.gamma.r 
where P(a) is the power at the core-cladding interface. The quantity 
.gamma. depends on the V-parameter of the waveguide 
##EQU5## 
where V.sub.c is the mode cutoff value, (V.sub.c =2.405 for single mode 
operation) and for a step index profile, 
##EQU6## 
By proper choice of the parameters a, n, and .gamma. , V may be adjusted 
to make P(r) comparatively large for large V in the secondary channels. 
This in turn makes the channels including cores 21-24 much more 
susceptible to distortion losses. By detecting the ratio of the loss in 
any one of the cores 21-24 to the loss in the primary channel 19, a 
perturbation in the fiber can be identified. A similar analysis shows that 
multiple mode waveguides exhibit a loss ratio when perturbed. 
In accordance with the present invention, the relative index difference for 
the primary channel is substantially greater than the index difference for 
the secondary channels. Alternatively, for multimode waveguides, the 
diameter of the primary core 19 is substantially less than the diameter of 
the secondary cores 21-24. Alternatively, the waveguide of light 
transmitted in the primary core 19 is substantially lower than the 
wavelength of light transmitted in the secondary cores 21-24. As used 
herein, the word "substantially" means a difference in waveguide 
parameters such that the loss difference between the primary and secondary 
core is at least an order of magnitude of difference. In the case of index 
differences, the index difference for the primary cores is at least twice 
the index difference for the secondary cores. 
For a typical SiO.sub.2 -GeO.sub.2 -P.sub.2 O.sub.5 multimode waveguide, 
the index difference .DELTA..sub.13 is 0.01 whereas waveguides with an 
index difference .DELTA..sub.23 less than about 0.005 have been observed 
to give rise to an order of magnitude greater relative loss for the core 
23 than the core 13 in the presence of random bends. Of course these 
values are not unique as suggested by the equation on page 4. The ratio of 
the core and cladding .DELTA.-value must be on this order, however. 
In the case of core diameters the typical diameter of a multimode core is 
50 .mu.m. If this value is chosen for the primary core, the secondary core 
diameters must be greater than approximately 90 .mu.m in order to exhibit 
an order of magnitude loss difference when subjected to a perturbation 
with p=1.85 which is quite typical. 
In the case of single mode waveguides there exists an inseparable 
relationship between .alpha., .DELTA. and .lambda.. Typical values for a 
single mode waveguide might be .DELTA..sub.13 =0.005 and .DELTA..sub.23 
.ltoreq.0.004 with a common core diameter of 2a=10 .mu.m and corresponding 
cutoff wavelengths of .lambda..sub.c.sbsb.13 =1300 nm and 
.lambda..sub.c.sbsb.23 .ltorsim.1170 nm respectively. These values will 
produce approximately an order of magnitude greater relative loss when 
both guides are operated at .lambda..congruent.1750 nm. 
Alternatively, one could consider identical single mode primary and 
secondary waveguides with .DELTA..sub.13 =.DELTA..sub.23 =0.004 and 2a=10 
.mu.m, but propagating different wavelengths. If for example, 
.lambda..sub.13 .ltorsim.1730 nm and .sup..lambda..sub.23 .gtorsim.1840 
nm, an order of magnitude greater loss can exist for the same 
perturbation. Generally speaking, the exact transmission wavelength can be 
altered for different values of the waveguide parameters .DELTA..sub.13, 
.DELTA..sub.23, 2a.sub.13, and 2a.sub.23 but in all cases, the wavelength 
for the second core will be greater than the primary core. For the 
wavelengths typically used in telecommunications systems this difference 
may be on the order of 100 nm or more although as is known to one skilled 
in the art in proper design of the individual parameters the wavelength 
difference may be made arbitrarily small. 
It should be understood that although the three single mode cases 
considered above involved holding all waveguide parameters, .DELTA., 2a 
and .lambda., fixed but one (e.g., .DELTA..sub.13 .noteq..DELTA..sub.23), 
there is complete freedom to choose different values for all parameters 
and still arrive at a substantially different loss characteristic of the 
two cores when subjected to a common bend. 
FIG. 5 shows a typical waveguide 25 of this invention in a secure 
communication system. The input end 26 of the waveguide is optically 
coupled to a variable intensity light source 27 which propagates light 
signals along the waveguide. Although not shown, a number of light sources 
can be modulated to propagate signals along the multiple cores of the 
waveguide. The output end 28 of the waveguide is optically coupled to 
light detectors 29 and 30. These detectors respectively detect light 
signals traveling along the first and second cores of the waveguide shown 
in FIG. 2, as an example. In order to detect the perturbation of the 
waveguide 25 the comparator 30 compares the relative signals from 
detectors 29 and 30. When the ratio of these signals changes 
significantly, an alarm is indicated. 
In FIG. 6, the preferred multi-core parallel axis waveguide of FIG. 4 is 
shown in a communication system. Light sources emitting light of 
wavelengths, .lambda..sub.1, .lambda..sub.2, .lambda..sub.3 . . . 
.lambda..sub.5 are applied to the primary channel 19 and the secondary 
channels 21-24, respectively. Light on these channels is conducted over 
the waveguide 25 to the far end at which the detectors 32-36 receive the 
transmitted light. The comparator 37 compares the output of all of the 
secondary cores to the output of the primary core. Only if a significant 
change in transmitted light is shown at all of the transmitted wavelengths 
is an error indicated. In this manner, false alarms, occasioned for 
example by a perturbation at only one wavelength, will be avoided. 
While step index waveguides have been shown in the drawings, gradient index 
waveguides such as described in U.S. Pat. No. 3,823,995--Carpenter can be 
used. 
While particular embodiments of the invention have been shown and 
described, various modifications are within the true spirit and scope of 
the invention. The appended claims are, therefore, intended to cover all 
such modifications.