GB2283579A

GB2283579A - Optical fibre couplers using self-aligned core-extensions, and their manufacturing methods

Info

Publication number: GB2283579A
Application number: GB9401365A
Authority: GB
Inventors: Sang Keun Sheem; Susan Kim Sheem
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-01-25
Filing date: 1994-01-25
Publication date: 1995-05-10
Anticipated expiration: 2014-01-25
Also published as: FR2716011A1; AU5489994A; CN1106542A; DE4402431A1; GB9401365D0; CA2114899A1; GB2283579B; FR2716011B1

Description

2283579 I "OPTICIAL FIBEY COUPLE"RS USING CORE EXTENSIONS, AND THEIR

MANUFACTURING' METHODSle S PE, C1 FI CATI 0 N

BACKGROUND OFTHE INVENTION

The most common optical waveguide is the fiber with a roundshaped core supported by a round-shaped cladding. Guided light resides mostly inside [lie core. The sectional dimension of the fiber core is typically less than ten microns (0.01 mm) in single-mode fibers, and usually less Ilian 200 microns even in the multimode fibers. Accordingly, connecting and coupling (mixing) of light between two or more fibers present enormous technical challenge.

To make a fiber coupler, it is necessary to perform art extensive fabrication with strands of fiber. The first multiniode fiber star coupler or fight mixer available in early 1970's comprises one linear array of fibers bu(ted against the one side of a narrow and long tectangular ctivily, arid (lie offier linear array Outted against the opposite side of the cavi(y. Light from any of the fibers spreads inside the cavily while propagating the long lerigth, and uniformly illuminates (lie array of' the fibers on the opposite side. Advantage of this coupler is Ore simple construction and waveleng(h independence of the coupling ratio. The drawback of such a fiber star coupler is (hat the core occupies only a fraction of the sectional area due to its roundness and (lie existence of the cladding around each core. Accordingly (lie excess loss, (fire to the so-callcd packirig densi(y fac(or, is inliewnfly high even with intillimode fibers, and cannot be used at all for (lie single- mode fibers due to (lie extremely sinall core-to-cladding area ratio, which is abou( one to a hundred. I An alternative approach, so-called biconical fused coupler, was used from early 1970's for multimode fibers, in which fibers are twisted, thermally softened, and pulled very slowly, until the light guided in the core leaks out of the core into the cladding due to the size decrease in the pulled -and-fu sed area. Once the light resides in the cladding area it is freely spread into the claddings of other neighboring fibers, that have been fused together in the pulling process. The cladding modes return to the cores as the fiber sizes increase in the second half of the fused section. This process relies on the taper being gradual and smooth, ensuring an adiabatic mode transformation between the core modes and cladding modes.

In the early 1980's, the single-mode fiber displaced the multimode fibers almost completely in the fiber optic market. The industry kept improving the biconical fused coupler technique until it became good enough even for the single-mode fibers. A series of invention disclosures have been made along the way, for example in U.S. Patents 4,798,438, 4,842,359 of Imoto, et.al., and 4,961,617 of Shahidi, which use the fused-tapered technique in modified forms. However it has remained as a very delicate process, especially when the number of the input or output fibers exceeds two. The largest number of ports for single-mode fiber couplers available today have four inputs and four outputs, or so-called (4x4), and one input and seven outputs (W). To get a larger channel numbers, a number of (W) couplers are cascaded. This results in a extensive labor and high price.

As an alternative approach, light coupling may be performed by optical channel waveguides fabricated on a bulk optical substrate. Examples of such a coupler design and its variations include U.S. Patents 4,566,753 of L. Manscheke, 4,653,845 of Y. Tremblay and et. al., 4,904,042 of C. Dragone, 4,950,045 of T. Brichenno. This method of using planArized waveguides, being the most expensive way, is employed almost exclusively for fabricating single-mode fiber couplers with the number of input/output ports larger than (4x4) or (10). The production process involves the fabrication of the planarized channel waveguides, then cutting and polishing the end facets of the substrate to make them flat, smooth, and sharply cornered at the connecting interface, then aligning ten-micron fiber cores to the several-micron waveguides in an end-butt fashion with better than one or two micron accuracy, and then gluing down the fibers in the aligned positions, and making sure that the fibers do not 2 move more than one or two microns while the glue is being cured. The difficulty of the tedious process steps is reflected in the high price of such a single-mode fiber star coupler.

Thus, there is a keen need to devise an embodiment for manufacturing multi-port fiber couplers, especially for the singlemode, that does not require these tedious fabrication and piece wise assembly steps.

Optical fiber connector is another key component in fiber optics, especially in the optical fi6er communication. The cost becomes the critical issue when the applications come close to endusers, such as interconnecting computer networks. The existing optical fiber connectors are very expensive and intricate for such applications. Also such applications will require multi-fiber array connectors, the counterpart of multipin connectors for electronic cables. Array connectors minimize connector space, per-connection cost, and overall connection time. Technology for such multi-fiber array connectors are in its infancy at best at the present time, and the price is impractically high.

In general, the connection between fibers becomes easier when light beam is enlarged in size in the mating plane. When the beam is enlarged, the alignment tolerance becomes relaxed, while the angular tolerance becomes more stringent. For example, Wasserman and Gibolar show in US Patent 5, 097,524 a connector embodiment that employs lenses to expand light beam. Moslehi et. al. describes in Optics Letters, Volume 14, Number 23, on page 1327, a fiber optic connection based on expanded-beam optics. Hussey and Payne describes in Electronics Letters, Volume 24, Number 1, on page 14, a fiber-horn beam expander. However these techniques still require critical alignment between fibers and the beam-expanding elements. Also, these prior arts are for single fiber connection, and do not lend themselves to array connectors.

Another important fiber optic technology is connection between an optical fiber and a channel waveguide. Currently, channel waveguides are patterned on or near the flat top surface of bulk optical substrate using photo-lithography or other advanced techniques such as electron-beam or laser-beam writing. In most of the applications, a channel waveguide needs to be connected to an optical fiber in one-to-one, end-butt fashion. To make this 3 connection, the end of the channel waveguide should be cut flat and right- angled with respect to the waveguide plane, and then polished with the fabrication tolerance in the order of a fraction of the optical wavelength while maintaining the edge sharply right-angled within one or two microns from the substrate surface. Then an optical fiber with a cleaved facet is brought against the end facet of the channel waveguide. The lateral alignment between the optical fiber core and the channel waveguide should be made within a few microns or less. Then a cementing material is applied to the butted region. The alignment often deteriorates while the cement is being cured due to the volume change and shift, causing connector loss. Even with the perfect alignment, the geometrical mismatch between the round fiber core and the largely square- shaped channel waveguide causes substantial connector loss. Overall, a fiber-to-cliannel connection is a very expensive fabrication step. This is another reason why the fiber optics has not been able to penetrate into the wider consumer market despite of the enormous potential benefits.

SUMMARY OF THE PRESENT INVENT1ON

Accordingly, it is the primary objective of the present invention to devise a novel optical interface embodiment that resolves the technical difficulties in manufacturing optical fiber couplers and connectors.

It is an accompanying objective of the present invention to make the cost of optical fiber couplers and connectors low enough even for the lowdensity, low-end optical fiber communication applications.

The basic element of the present invention comprises an optical fiber having -a core and a cladding, and a core-extension, wherein the coreextension is built upon the core end facet in a shape of the diverging horn-like structure. The core-extension extends the waveguide effect substantially beyond the end facet of the core, with its sectional area increasing gradually as it extends further from the end facet until the sectional diameter is substantially larger than the core diameter of the optical fiber. In some of the embodiments of this invention disclosure is added an additional condition that the maximum diameter of the coreextension be larger than the cladding diameter, because in those embodiments the core-extension has to reach and touch either the coreextensions of neighboring optical 4 fibers, or the inside walls of optical channels. This point will be clarified further below in the detailed description. Further, the index of refraction of the core-extension is larger than that surrounding tile core-extension so that the light entering the core-extension with a proper input angle is confined within the core-extension while propagating.

As the first application of the novel core-extension summarized above, an optical fiber coupler is devised that comprises a plurality of cores, each core having an above-mentioned core-extension, that are laid in proximity and substantially in parallel so that the neighboring core- extensions gradually merge together at a distance, forming one common core-extension. The overlap region then works as light mixing area for light coupling and splitting. Tubings, rods, lenses, mirrors and other optical components may be added to the coupler embodiment to enhance the light mixing function.

As the second application of the novel core-extension scheme, a connector is devised that comprises a single optical fiber having a core and a cladding, a core-extension, and a channel waveguide with a hollow endsection in which the core-extension is placed. The coreextension is built upon the core end facet of the fiber in a shape of the diverging hornlike structure, filling up the inside space of the hollow end-section of the channel, thus connecting the fiber core to the channel waveguide. Accordingly, connection between the fiber and the channel waveguide is self-aligned. The channel waveguide may performs one of the various functions, such as modulation, wavelength multiplexing, switching, coupling, and connection. In an embodiment for fiber-to-fiber connection, the channel waveguide is tapered along the length to have a substantially large sectional area.

In order to achieve the objective of the price affordability, the fabrication should be easy and simple. The present invention discloses simple fabrication methods. In one method, the coreextension is fabricated by immersing the end facet of a fiber waveguide in a photo- reactive material, such as light-curable or light-polimerizable ones, and then injecting a light of a proper wavelength and intensity into the opposite end facet of the waveguide so that the light radiating out of the end facet exposes photo-reactive material within the envelope of the diverging radiation pattern, forming the desired core-extension.

BRIEF DESCRIPTION OF. THE DRAWINGS

FIG. I shows a perspective view of an optical fiber having a core and an individual core-extension attached to the one end of the core.

FIG. 2 shows the plan view of the embodiment of FIG. 1.

FIG. 3 shows the plan view of two fibers with the individual coreextensions that merge and form a common core-extension for light mixing and coupling.

FIG. 4 shows two of the embodiments shown in FIG. 3 mated at the far end of the common core-extensions in a face-to-face fashion so as to allow the light from the fibers on one side is coupled to those on the other side.

FIG. 5 shows the same as that in FIG. 4, except that the number of fibers is larger than two on each side of the coupler.

FIG. 6 shows the sectional view of the embodiment of FIG. 5 along X-X' for a case in which the fibers are arranged in an one- dimensional linear array.

FIG. 7 shows the sectional view of the embodiment of FIG. 5 along Y-Y for the case of one dimensional array of FIG. 6.

FIG. 8 shows one sectional view of the embodiment of FIG. 5 along X-X' for a case in which the fibers are arranged in a two- dimensional arrangement.

FIG. 9 shows one sectional view of the embodiment of FIG. 5 along Y-Y for the case of two-dimensional arrangement of FIG. 8.

FIG. 10 shows the same as in FIG. 4, except that the common coreextension is housed inside a tubing.

FIG. 11 shows the same as that of FIG. 4, except that a solid block is inserted in the middle of the core-extension.

FIG. 12 shows the same as that of FIG. 4, except that a lens is inserted in the middle of the core-extension.

6 1 FIG. 13 shows the self-aligning fabrication method for the embodiment shown in FIG. 1, in which the end facet of the fiber is immersed in a photo-reactive material, emitting a light to expose and transform the material's characteristics within the diverging radiation envelop.

FIG. 14 shows the self-aligning fabrication method for the embodiment shown in FIG. 3, in which the end facets of the fibers are immersed in a photo-reactive material, emitting lights to expose and transform the material's characteristics within the diverging radiation envelops.

DETAILED DESCRIPTION

In FIG. I is shown an optical fiber, with a core 1, a cladding 2, two end facets 3 and 4, onto one end of which 3 is built a diverging, horn-like structure 9, which will be called "core-extension" in the present invention. The core-extension 9 of the present invention is about as narrow as the core I at the interface on the end facet 3, and diverges out as it 9 extends away from the end facet 3. FIG. 2 shows the side view of the embodiment shown in FIG. 1. The maximum diameter of the core extension 9 is larger than the cladding diameter, as indicated in FIG. 2. This specification is necessary for coupling and connecting functions in the present invention, as will be clarified below.

The index of refraction is higher inside the core-extension 9 than outside so as to confine the light within the core-extension 9.

When light propagates in a tapered section with decreasing diameter such as found in the core-extension 9, the incident angle becomes smaller. When the taper length is too long and the taper angle too large, the incident angle could become too small to experience the total internal reflection at some point along the taper. When this happens, the light could escape from the guiding structure. This effect can be reduced or even completely eliminated by making the taper angle small and the taper length shorter. When the taper angle is small enough, the so-called adiabatic process is achieved in which light propagates without experiencing any conversion in local eigenmodes while propagating along the taper. Thus it is preferred to keep the taper angle of the coreextension 9 small. It helps to reduce 7 the thickness of the fiber cladding 6 by etching or thermal tapering so that the core-extension 9 does not have to expand too much to perform its intended functions: this point will be clarified below as the desired embodiments are described in detail. This teaching is very important because for many applications the core-extension as revealed here would be too lossy to be useful unless its taper angle is limited to a small value such as a few degrees for multimode fibers, and one or two degrees for single-mode fibers.

FIG. 3 shows the basic embodiment of the coupler of the present invention. As mentioned above, the maximum diameter of the core extension 9 is larger than the diameter of the cladding 2, as indicated in FIG. 2. This specification is necessary for the coupler -embodiment of the present invention because one core extension, as shown in FIG. 3, should be able to overlap with the neighboring coreextension and form a light mixing region 10. In FIG. 3 two fibers are closely laid side-by-side, and the individual core-extensions start at the core end facets as two separate bodies, but merge at a distance to form a common core-extension 10. Accordingly, a light 11 entering the core I will emerge from the coupling region 10.

It is worthwhile to note that the light mixer embodiment shown in FIG. 3 works also as a light power divider when a light enters from the coreextension side: The light, for example from a laser, will be coupled to the two cores 1 and 5.

The core-extension of the present invention is built on the fiber core end facet as a built-in structure. Most conveniently, the coreextensions may be made of a photo-reactive material the physical characteris tics of which is influenced and transformed by a light exposure. Then the coreextensions can be fabricated by exposing the material with lights being emitted from the core end facets that diverge into a horn-like shape. In other words, the core-extensions occupy the space defined by the diverging radiation patterns of the light being emitted from the end facets of the cores. The individual radiation patterns extend beyond the core end facets over a sufficient distance, thus inerging together to form a overlapping region, namely the common core-extension for light mixing. This aspect and variations will be elaborated and clarified further when the method inventions are disclosed below.

A complete fiber-to-fiber coupler embodiment is constructed 8 by mating the light mixing element shown in FIG. 3 with its own mirror- image duplicate, as shown in FIG. 4, at the far end of the common coreextension 10. The common core-extension 20 works as a light mixing or coupling region for the cores 1, 5, 15, and 17: A light entering the core 1 for example will reach and spread across the core-extension 20, and then coupled to the core 15 and 17. If the common core-extension 20 is wide and long enough, the output light distribution between the two cores 15 and 17 will be equal. The light split ratio will be independent of the wavelength and the polarization, which is advantageous aspects. A part or the whole body of the core-extension 20 may be surrounded by an air. However, a liquid or solid material may be added to the outside of the core-extension body 20, so long as the material is transparent and has the index of refraction lower that of the core-extension 20.

In FIG. 4 the fibers may be slightly tilted inwardly toward the common coupling region 10 so as to increase the light overlap in the common coreextension 20. Throughout the present invention description, it will be understood that the fibers may be tilted slightly without changing the basic coupling mechanism.

A straightforward extension from the two-by-two (2x2, meaning two input fibers and two output fibers) coupler structure of FIG. 4 produces a multi-port coupler, as shown in FIG. 5, six-by-six in this particular example. Any combinations such as W, 8xl6, 16xI6, lxl6 are possible.

The multiple fibers in FIG. 5 may be arranged either as an linear array or in two-dimensional space. The sectional view across X-X' of FIG. 5 is shown in FIG. 6, and that across Y-Y' in FIG. 7, for the linear array arrangement. For the case of a two-dimensional arrangement, the sectional view across X-X' of FIG. 5 is shown in FIG. 8, and that across Y-Y' in FIG. 9. In either arrangement, there is no spatial overlap among the small fiber cores 21 through 26, and if a broad light beam impinges upon the sectional area X-X', the portion of the light failing on the fiber cores 21 through 26 is very small. On the other hand, the overlap is substantial in the sectional area Y-Y', ensuring a good coupling with low loss.

Some variations of the basic coupler embodiment of FIG. 4 are shown in FIG. 10 through FIG. 14: FIG. 10 shows that a hollow 9 channel 51 is added to the basic coupler embodiment of FIG. 4. The material of the tubing 51 should have an index of refraction lower than that of the core-extension 20. Note that the inner dimension of the tubing 51 is narrower than the possible maximum diameter that the common- extension 51 would possess in the absence of the tubing 51. Thus the tubing 51 truncates the common-extension 20 along the inside wall. The cross-section of the tubing 51 may be of a circular, square, rectangular, or any other shape.

FIG. 11 shows that a solid, transparent block 52 is inserted inside the common core-extension 20 to make the coupling length longer.

FIG. 12 shows that a focusing lens 53 is inserted inside the common coreextension 20 so as to increase the light overlap.

A reflective surface may mounted on the end of the common core-extension 10 of FIG. 3 so that light entering any of the two fiber cores I or 5 is mixed in the common core-extension 10, and then reflected back to be coupled to the both fiber cores I and 5, after a further light mixing in the common core-extension 10.

The core-extension embodiments presented above may be fabricated by precision molding techniques. A matched molded part may be fabricated to position the fibers so as to ensure proper alignment between the fiber end facets and the split input ports of the core-extensions. This fabrication technique will be especially useful for large-quanity production. The initial tooling cost will be high, but the unit- manufacturing cost will be low.

Another fabrication method is disclosed below. This technique possesses an -tremendously advantageous feature, namely a perfect self-alignment between a fiber core end and a starting end of individual core-extension. To be low loss, the alignment requires better than one or two micron accuracy for the single-mode fiber case. Thus, this self-alignment technique will be especially useful for fabricating single-mode fiber couplers, which represents more than 90% of the coupler market volume. This fabrication technique can be readily practiced, with a low initial investment, to fabricate a lowcost single-mode fiber couplers in a simple and easy way. This 10 method of constructing the couplers will be described herein as an accompanying "Method invention disclosure", using FIG. 13 and FIG. 14:

FIG. 13 shows the fiber core I and cladding 2 of FIG. I is immersed in a photo-reactive material 55. In this invention disclosure, "photo-reactive material" is defined as a substance tile physical characteristics of which is influenced and transformed by a light exposure in such a way that the light exposure may be used to form the physical shape of the material body. Examples of such materials include a photoresist material that remains solid only when exposed by UV (ultraviolet) light and is dissolved otherwise by a solvent called photoresist developer; and an UV- cure polymer that transforms from liquid to solid only in the region exposed by a light composed of certain wavelength components near 0.3 and 0.4 microns; a special glass raw material that solidifies when exposed by light; and an organic material the index of refraction of which changes upon light exposure.

In FIG. 13 a light 56 with a proper wavelength contents, usually UV light, enters the fiber core I from the input end 4 of the fiber to radiate out from the output end 3 with a certain divergence angle to expose the photoreactive material 55 to a level enough for changing the material characteristics. The depth of the photoreactive material 55 should be deep enough to ensure that the maximum width of the core-extension 9 being formed by the light exposure is larger than the fiber diameter, as specified earlier. This fabrication procedure will results in the embodiment shown in FIG. 1. The output end 3 does not have to be prepared flat: It may be modified to have a concave or convex surface to control the solid angle of the light cone 9. Earlier, it was mentioned that the smaller the taper angle of the core-extension, the smaller the loss. This can be achieved, while fabricating the core-extension, by limiting the UV light entering the fiber core 7 to the lower order modes of the fiber.

FIG. 14 illustrates in a schematic fashion a method of fabricating the basic light mixer embodiment of the present invention shown in FIG. 3. The fiber 2 shown in FIG. 13 is accompanied by a neighboring fiber 6 located in a close proximity, and the both fiber cores I and 5 receive the exposure light 56. The lights emerging from the output ends 3 and 7 overlap to form a common core-extension 10, resulting in the light mixer embodiment I I shown in FIG. 3.

Obviously many applications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

- [I-.-

Claims

CLAIMS 1. An optical coupling structure for extending an optical fibre

comprising; a light guiding core with an end facet perpendicular to the light propagation direction; a cladding surrounding the core; and a coreextension for the light guiding core; wherein the core-extension is made of a photo-reactive material the physical characteristics of which can be altered and shaped by a photo-exposure, and is built on the end facet of the core following the shape of the diverging radiation pattern of light that stretches over a distance far enough to make the maximum diameter at the farthest end substantially larger than the core diameter, and has an index of refraction greater than that of the surrounding medium so as to possess light confining function.
2. A structure according to claim 1, wherein the largest diameter of the core-extension is greater than the diameter of the cladding of the optical fibre.
3. A structure according to either of claims 1 or 2, wherein the thickness of the cladding is substantially reduced.
4. An optical fibre mixer comprising; a plurality of light guiding cores with end facets perpendicular to the light propagation direction, being laid substantially in the parallel orientation in a close proximity; a plurality of core-extensions for the light guiding cores, each of the light guiding cores having one coreextension; wherein the individual coreextensions are built onto the core end facets in a shape of the diverging horn-like structure, extending the waveguide ef f ect substantially beyond the end facets of the cores, with the sectional area increasing gradually as the individual core-extension extend 13 further from the core end facets over a sufficient distance so as to make the maximum diameter of the each horn-like structure at the farthest end larger than the distance between two neighbouring cores, thus allowing the individual coreextensions from the individual cores to merge together to form a common core-extension for light mixing, and have an index of refraction greater than that of the surrounding medium so as to possess light confining function.

5. An optical fibre mixer according to claim 4 wherein the cores are arranged in a linear array.

6. An optical fibre mixer according to either of claims 4 or 5 wherein the cores are arranged in a two-dimensional space.

7. An optical fibre mixer according to any of claims 4 to 6 wherein the common core-extension is housed in a transparent tubing the inner dimension of which is narrower than the maximum possible diameter that the common core-extension would have in the absence of the tubing, and the refractive index of which is lower than that of the core-extension.

8. An optical fibre mixer according to any of claims 4 to 7 wherein the individual core-extensions and the common coreextension are made of molded parts.

9. An optical fibre mixer according to any of claims 4 to 8 wherein the individual core-extensions are made of photreactive material the physical characteristics of which can be altered and shaped by a photo-exposure.

10. An optical fibre mixer according to claim 9, wherein the photoreactive material is an UV-cure polymer.

11. An optical fibre mixer according to claim 9, wherein the photoreactive material is a photoresist material.

14 12. An optical f ibre mixer according to claim 9, wherein the photoreactive material is an organic material the refractive index of which is altered by a light exposure.

13. An optical fibre mixer according to claim 9, wherein the photoreactive material is a glass raw material that alters its physical characteristics by a light exposure.

14. An optical fibre coupler comprising; a first set of a plurality of light guiding cores with end facets perpendicular to the light propagation direction, being laid substantially in a parallel orientation in close proximity and having a plurality of core-extensions for the light guiding cores, each of the light guiding cores having one core-extension; a second set of a plurality of light guiding cores with end facets perpendicular to the light propagation direction, being laid substantially in a parallel orientation in close proximity; and having a plurality of core-extensions for the light guiding cores, each of the light guiding cores having one core-extension; wherein the f irst set of light guiding cores and the second set of light guiding cores are positioned on a common axis in a face-to-face fashion, with the core-extensions of the f irst set and the core-extensions of the second set located between the f irst and second sets of light guiding cores; and within each of the first and second sets of light guiding cores the individual core-extensions are built onto the core end facets in a shape of a diverging horn-like structure extending the waveguide ef f ect substantially beyond the end facets of the cores, with the sectional area increasing gradually as the individual core-extensions extend further from the core end facets over a sufficient distance so as to make the maximum diameter of the each horn-like structure at the furthest end larger than the distance between two neighbouring cores, thus allowing the individual core- extensions from the individual cores to merge together to form a common core-extension for light mixing, and have an index of refraction greater than that of the surrounding medium so as to possess light confining function; and the common coreextension of the f irst set of light guiding cores and the common core-extension of the second set of light guiding cores face each other.

15. A coupler according to claim 14, wherein a light transmitting medium is inserted in the common core-extension area.

16. A coupler according to either of claims 14 or 15 wherein the light transmitting medium is a lens.

17. A method of manufacturing a core-extension of an optical f ibre as def ined in claim 1 wherein the f irst end f acet of the core is immersed in the photo-reactive material the physical characteristics of which can be altered and shaped by a photoexposure, and the light for exposing the photo-reactive material enters the second end of the core and then radiates from the first end of the core with the characteristic divergence angle, then forming the shape of the core-extension as defined.

18. A method according to claim 17 wherein the cone angle of the light radiating from the first end of the core is made small by selecting only the light rays with small radiation angles.

19. An optical coupling structure substantially as hereinbefore set forth with reference to, and/or as illustrated in the accompanying drawings.

2 0. An optical f ibre mixer substantially as hereinbef ore set forth with reference to, and/or as illustrated in the accompanying drawings.

16 21. An optical fibre coupler substantially as hereinbefore set forth with reference to, and/or as illustrated in the accompanying drawings.

17 Amendments to the claims have been filed as follows CLAIMS:

1. An optical fibre mixer comprising; a plurality of light guiding cores with end facets perpendicular to the light propagation direction, being laid substantially in a parallel orientation in close proximity; a plurality of core-extensions for the light guiding 10 cores, each of the light guiding cores having one core extension; wherein the individual core-extensions are made from a photo-reactive material, the physical characteristics of which can be altered and shaped by photo-exposure and are built onto the core end facets in a shape of a diverging horn-like structure, extending a waveguide effect substantially beyond the end facets of the cores, a sectional area of the horn-like structures increasing gradually as the individual coreextensions extend further from the core end facets over a sufficient distance so as to make the maximum diameter of the each horn- like structure at a farthest end larger than the distance between two neighbouring cores, thus allowing the individual core-extensions from the individual cores to merge together gradually with neighbouring core- extensions along their sides to form a common core-extension for light mixing, the core extensions being self-aligned with the cores by formation of the core- extensions by said radiation pattern of the light and further having an index of refraction greater than that of the surrounding medium so as to possess a light confining function.

2. An optical fibre mixer according to claim 1 wherein the cores are arranged in a linear array.

3. An optical fibre mixer according to claim 1 wherein the cores are arranged in a two-dimensional space.

is 4. An optical fibre mixer according to any of claims 1 to 3, wherein the common core-extension is housed in a tubing, an inner dimension of which is narrower than the maximum possible diameter that the common coreextension would have in the absence of the tubing so as to allow the common coreextension to merge gradually with the inner walls of the tubing, the tubing having a refractive index which is lower than that of the core-extension.
5. An optical fibre mixer according to any preceding claim, wherein the photo-reactive material is an UV-cure polymer.
6. An optical fibre mixer according to any of claims 1 to 4 wherein the photo-reactive material is a photoresist 15 material.
7. An optical fibre mixer according to any of claims 1 to 4 wherein the photoreactive material is a glass raw material that alters its physical characteristics by a light exposure.
8. An optical fibre mixer according to any preceding claim wherein each of the light guiding cores is surrounded by a cladding, the thickness of which is reduced so as to minimize the distance between two neighbouring cores.
9. A method of manufacturing a core-extension for an optical fibre mixer having a plurality of light guiding cores comprising the steps; immersing a f irst end facet of said cores in a photo- reactive material, the physical characteristics of which can be altered and shaped by photo-exposure so as to result in an increase in its refractive index; exposing the photo-reactive material to light within a space defined for individual core extensions, such individual core extensions forming as horn-like structure on each core, continuing the exposure until the individual core-extensions extend from the core end facets over a sufficient distance so M as to make the maximum diameter of the each horn-like structure at a farthest end larger than the distance between two neighbouring cores, thus allowing the core-extensions from the individual cores to merge together gradually along their sides with neighbouring core-extensions to form a common coreextension for light mixing.
10. A method according to claim 9, in which light f or exposing the photoreactive material is introduced to one end of each core and then radiates from the other end of each core with a characteristic divergence angle.
11. A method according to claim 10, wherein a cone angle of light radiating from each core is made small by selecting only light rays with small radiation angles.
12. An optical coupling structure for extending an optical fibre comprising; a light guiding core with an end facet perpendicular to the light propagation direction; a cladding surrounding the core; and a core-extension for the light guiding core; wherein the core-extension is made from a photo-reactive material, the physical characteristics of which can be altered and shaped by photo-exposure, the core-extension being formed on the end facet of the core with the shape of a diverging radiation pattern of light, the core-extension extending far enough to make the maximum diameter at its farthest end greater than the cladding diameter and further having an index of refraction greater than that of the surrounding medium so as to possess a light confining function.
13. An optical fibre coupler comprising; a first set of plurality of light guiding cores whose end 35 facets are arranged perpendicular to the light propagation direction, the cores being laid substantially in a parallel orientation in close proximity and having a plurality of core- o extensions, each of the light guiding cores having one coreextension; a second set of a plurality of light guiding cores whose end facets are arranged perpendicular to the light propagation direction, the cores being laid substantially in a parallel orientation in close proximity and having a plurality of coreextensions, each of the light guiding cores having one coreextension; wherein the first set of light guiding cores and the 10 second set of light guiding cores are made of a photo-reactive material, the physical characteristics can be altered and shaped by a photo-exposure, and wherein the individual coreextensions of each of the first and second sets of light guiding cores are formed onto the core end facets in a shape of a diverging horn-like structure, extending a waveguide effect substantially beyond the end facets of the cores, with the sectional area of the horn-like structures increasing gradually as the individual core- extensions extend further from the core end facets over a sufficient distance so as to make the maximum diameter of the each horn-like structure at a farthest end larger than the distance between two neighbouring cores, thus allowing the core-extensions from the individual cores to merge together gradually with neighbouring core-extensions along their sides to form a common core- extension for light mixing, the core extensions being selfaligned with the cores by formation of the core-extensions by said radiation pattern of the light, and further having an lindex of refraction greater than that of the surrounding medium so as to possess a light confining function; the common core-extension of the first set of light guiding cores and the common core-extension of the second set of light guiding cores being laid on a common axis.
14. An optical fibre coupler according to claim 13, wherein 35 the first and second sets of light guiding cores are positioned on a common axis in a face-to-face fashion, with the core-extensions of the first set and the core-extensions 9A of the second set located in the middle between the first and second sets of light guiding cores.
15. An optical fibre coupler according to claim 13, wherein 5 the coreextensions of the first set of light guiding cores are positioned on one side of a light transmitting medium in the f orm. of a solid transparent block and the core-extensions of the second set of the light guiding cores are positioned on the other side of the light transparent block.
16. The invention according to claim 13 wherein the core extensions of the first set of light guiding cores are positioned on one side of a light transmitting medium in the form of a lens and the core extensions of the second set of light guiding cores are positioned on the other side of the lens.
17. An optical fibre mixer substantially as hereinbef ore set forth with reference to and as illustrated in the accompanying 20 drawings.
18. A method of manufacturing a core-extension for an optical f ibre mixer substantially as hereinbef ore set forth with reference to and as illustrated in the accompanying drawings.
19. An optical coupling structure substantially as hereinbefore set forth with reference to and as illustrated in the accompanying drawings.
20. An optical fibre coupler substantially as hereinbefore set forth with reference to and as illustrated in the accompanying drawings.

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