Multiple Core Optical Waveguide for Secure Transmission

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.

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.

FIG. 1 depicts a prior art optical waveguide and FIG. 1A shows the radial refractive index profile for this waveguide;

FIG. 2 shows a multiple core waveguide in accordance with this invention and FIG. 2A shows the radial refractive index profile for this waveguide;

FIG. 3 shows another embodiment of the invention and FIG. 3A shows the radial refractive index profile for this waveguide;

FIG. 4 shows another embodiment of the invention;

FIG. 5 shows a waveguide of this invention in a secure communication system; and

FIG. 6 shows the waveguide in a communication system in which different wavelengths of light are applied to the secondary cores.

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<p<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:

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.