An optical assembly and a method for producing such

Abstract

The invention relates to an optical assembly for producing such. The invention also relates to the use of the optical assembly. Laser radiation received via a bundle of individual optical feed fibers is guided to a fiber laser fiber. Each feed fiber has a cladding layer surrounding the core of the fiber to provide total internal reflection in said core, and the cladding layers of the fibers are fused at least partially together to form a zone containing the cores of the feed fibers arranged in a cylindrical configuration inside said zone This configuration provides the shaping of an annular laser beam that can be fed into a fiber laser fiber having an annular light guiding zone and to present the annular laser beam e.g. to a workpiece.

Claims

1. An optical assembly for guiding laser radiation received via a bundle of individual optical feed fibers, each feed fiber having at least one cladding layer surrounding the core of the fiber to provide total internal reflection in said core, wherein the cladding layers of the fibers fused at least partially together in a cylindrical confinement to form a zone containing at least part of the cores of the feed fibers arranged in a cylindrical configuration in said zone to provide an annularly shaped light guide.

2. The optical assembly according to claim 1, wherein the bundle of individual optical feed fibers are fused to form an annular zone containing the cores of the feed fibers arranged in a cylindrical configuration in said zone.

3. The optical assembly according to claim 2, wherein the bundle of individual optical feed fibers further comprises a further optical fiber being fused in the center of said annular zone to provide a light guide in the center of said annularly shaped light guide.

4. A method for producing an optical assembly for guiding laser radiation received with a bundle of individual optical feed fibers, comprising the steps of: providing a cylindrically-shaped mold, fitting a plurality of optical feed fibers in said mold along the periphery of the cylinder, each fiber having a core and at least one cladding layer surrounding the core to provide total internal reflection in said core, and applying heat to and at least partially fusing together the cladding material of said fibers in said mold and forming a zone with at least part of the cores of said feed fibers arranged in a cylindrical configuration in the fused cladding material.

5. The method according to claim 4, further comprising fusing the cladding material of said fibers to form an annular zone containing the cores of said feed fibers arranged in a cylindrical configuration inside said zone.

6. The method according to claim 5, further comprising fusing a further optical fiber in the center of said annular zone.

7. The method according to claim 4, further comprising using as said cylindrically-shaped mold a tubular mold having a bore forming a waist section, and by applying heat to fuse said bundle of fibers at said waist section.

8. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG. 1 shows a cross-section of a bundle or preform of optical fibers;

[0029] FIG. 2 shows a fused bundle that forms an optical assembly according to one embodiment of the invention;

[0030] FIGS. 3a and 3b illustrates the structure and refractive properties of an annular fiber laser fiber;

[0031] FIG. 4a shows a tubular molding device according to one embodiment of the invention;

[0032] FIG. 4b shows a tubular molding device according to another embodiment of the invention;

[0033] FIG. 5 illustrates a fused bundle of input fibers according to one embodiment of the invention;

[0034] FIG. 6 illustrates a coupling zone between an inventive optical assembly and a fiber laser fiber.

DETAILED DESCRIPTION OF EMBODIMENTS

[0035] FIG. 1 shows a cross-section of a bundle 10 of optical fibers 11 that constitutes a preform for an optical element according to one embodiment of the invention. The bundle has N fibers (here N=4). Each fiber 11 has a core 12 and a cladding 13. The cladding is made of material having a lower refractive index than that of the cores 12. As is well known for those familiar with the art, light launched into the core of such a fiber (also called step-index fiber) will be guided by the refractive index step between the core and the cladding, and hence will remain inside the core according to the principle of total internal reflection.

[0036] The fibers in the bundle are according to the invention very thin in order to maximize the brightness of the optical pattern formed by the multiple cores. In particular, the diameter of the fibers may be as low as 40 μm, or even less. As the handling and bundling of such thin fibers is very challenging, the fusing of the bundle is preferably performed inside a supporting cylindrically-shaped mold for improved manufacturability.

[0037] FIG. 2 shows a fused bundle 20 that forms an optical assembly according to one embodiment of the invention. A fused bundle 20 is made by first forming a preform 10 of bundled fibers as shown in FIG. 1, and then puffing the preformed bundle through a heated cylindrical mold, that may consist of a capillary tube. When passing the mold, the cladding 23 of the fibers 21 are fused together in a controlled fashion. The spatial pattern formed by the cores 22 is determined by the preform and the mold. Here the fused bundle has N core regions 22 (here N=4). The cladding regions 23 and possibly also the core regions 22 are deformed from their initial generally round shape by the fusing process. The dashed lines in FIG. 2 illustrate the approximate deformed boundaries of the individual fibers 11 of the bundle of FIG. 1. Such physical interfaces may disappear in the fusing process.

[0038] The final physical dimensions and spatial separations of the cores of the fused bundle 20 are determined by the fiber dimensions and the degree of fusing of the cladding. The outer layer 24 may consist of the tubular mold. Thus the capillary tube has been fused together with the fiber to form a solid section of glass. This provides a fused fiber bundle with improved mechanical robustness, forming a strong solid piece of glass as the fiber bundle and the capillary tube are fused together. Alternatively, if the mold is not part of the structure, any suitable cladding may be formed on the fused fiber. The formed fiber bundle 20 can be polished or cleaved with conventional methods to form a flat end or interface surface, and common methods of fiber optics can be used to further process the resulting fiber, such adding an outer protective polymer coating, stripping off the coating, etc.

[0039] FIG. 3a shows a fiber laser fiber 30 with an annularly formed light guide (doughnut fiber) that receives the laser beam that is output by the fused fiber bundle of FIG. 2. The doughnut fiber 30 has a central cladding 34, an annular light guide or core 31, a primary cladding 32 and a secondary cladding 33. The doughnut fiber 30 can be polished or cleaved to form a flat plane to it by using well-known methods of fiber optics.

[0040] The fused fiber bundle 20 and the doughnut fiber 30 may be optically coupled together either by splicing them together or by using free-space optics (lenses etc.) between them. The laser radiation coupled from the cores of the feed fibers 21 into the core 31 of the doughnut fiber forms a spatial intensity distribution that can be approximated by a doughnut shape at the exit face of the doughnut fiber. This spatial intensity pattern can be further imaged with processing optics onto the workpiece.

[0041] FIG. 3b shows an example of a possible refractive index profile of the doughnut fiber 30 of FIG. 3a. The central cladding 34 has an index of n.sub.4 and the primary cladding 32 has an index of n.sub.2. The core 31 has an index of n.sub.1, where n.sub.1>n.sub.2 and n.sub.1>n.sub.4 in order for light to be and remain guided in the core 31. The index n.sub.3 of the secondary cladding has no definite restriction in terms of its magnitude, but since in practice this region is generally made of pure fused silica, n.sub.3 can be about 1.45. The refractive index of fused silica can be tailored by doping it with impurities. For instance, doping fused silica with Germanium results in an increase of the refractive index, while doping it with Fluorine results in reduction of the refractive index. Therefore the core 31 of the doughnut fiber may be made of Ge-doped fused silica and the primary cladding 32 of F-doped fused silica. The central cladding 34 and secondary cladding 33 may be made of un-doped fused silica.

[0042] Obviously, other material choices exist that satisfy the requirements for the refractive index values of the different regions of the fiber 30. As some light may also be launched into the central cladding 34, the index n.sub.2 of the primary cladding may be smaller than the index n.sub.4 of the central cladding to ensure that light launched into the central cladding 34 will not propagate through the primary cladding 32.

[0043] With reference to FIG. 4a, according to one embodiment, the tubular molding device comprises a capillary tube 42 (e.g. fused silica, quartz, doped quartz etc.) that has been tapered by a glass drawing method in order to obtain a waist portion 43 of some suitable length (e.g. 1 mm-5 cm, preferably 3 mm-3 cm) of a substantially constant diameter. A bundle 40 of feed fibers 41 is fitted within the capillary tube 42. The inner diameter of the capillary tube 42 at the waist portion 43 is designed to be slightly larger than the outer dimension of the bundle 40 of feed fibers 41, for example about 1 μm larger. The bundle 40 may be organized into a close-packed configuration by a suitable bundling aid tool and bundle geometry may be fixed or secured by the feed fibers having an adhesive coating (not shown) or alike.

[0044] Within the waist portion 43 of the capillary tube, the bundle of feed fibers 41 becomes fused with the wail of the capillary tube 42 e.g. by applying heat at a heating zone 44, preferably to achieve adiabatic (gradual) fusing of the fibers. The result is the fused fiber bundle 45.

[0045] In FIG. 4b is shown an alternative embodiment where the tubular mold 46, having a waist portion 47, does not form part of the fused fiber bundle 48. Both embodiments of FIGS. 41 and 4b illustrate an important feature of the present invention, i.e. the fiber bundle to be fused is made subject to very gentle manufacturing steps to preserve adiabatic light guidance through the optical assembly. In practice this means deformation, bending and disruption of the fiber cores are avoided to the extent possible.

[0046] It should be noticed that usually, due to the geometries involved, the cores of the fibers undergo in cross-section a change of shape from generally circular shape to non-circular shape as the fibers of the bundle and capillary fuse together and air pockets between fibers and between them and the inner wall of the cylindrical mold vanish due to the reflow of glass during fusing. The change of fiber shape must be done in a gradual fashion (adiabatically) along the length of the fused region. The gradual shape change can be accomplished by controlling the heating power in a heating zone like the zone 44 in FIGS. 4a and 4b, as the fiber moves along the length of an elongated fusing region with constant velocity, or by increasing the velocity of the heat source with constant heating power, or both in combination. The minimum heating power should be such that the cores of the feed fibers 41 remain in their original shape and that the capillary (or mold) is not substantially collapsed. A gradual change of core shape is essential for achieving low losses and low degradation in the brightness of the laser radiation.

[0047] FIG. 5 shows a cross-section of an embodiment of the invention with a fused fiber bundle 50 with seven feed fibers. In this close-packed configuration of feed fibers one of the fibers is located in the center of the bundle, while the remaining six fibers are located in a cylindrical fashion and appear in the cross-section arranged in a circle. The peripheral fibers have cores 51 and the central fiber has a core 52. The solid glass matrix 53 consists of the claddings of the 7 individual feed fibers, a capillary mold tube and/or other claddings applied around the original fiber bundle.

[0048] FIG. 6 shows the optical interface between the fused bundle 50 of FIG. 5 and the doughnut fiber 30 of FIG. 3a. For clarity, dashed lines from reference numbers are pointing to structures represented by dashed lines. The cores 51 of the annularly arranged and now fused feed fibers are aligned to launch optical power into the core 31 of the doughnut fiber 30. Correspondingly, the core of the central fiber 52 of the fused bundle 50 is designed to launch optical power into the central cladding 34 of the doughnut fiber 30. The optical intensity at the center of the doughnut fiber 30 will thus not be zero if optical power is launched into all of the fibers of the fused bundle 50.

[0049] The dimensions of the fused bundle 50 and the doughnut fiber 30 may also be chosen so that the peripheral fibers 51 have an overlap with the central cladding 34 of the doughnut fiber 30, as it in some cases may be preferred that some optical power from the cores 51 also enters the central cladding 34. Any optical power launched into the central cladding 34 will not remain constrained to the cladding, since its refractive index n.sub.4 is smaller than the index n.sub.1 of the core 31.

[0050] If on the other hand the overlap between the cores is 100%, that is, all cores 51 of the fiber bundle 50 fit inside the core 31 of doughnut fiber 30, and the core 52 is kept essentially dark, no optical power will be launched into the central cladding 34. Thus, the central cladding 34 will also appear dark, i.e. it will have practically zero intensity.

[0051] Thus, the spatial configuration and dimensions of the core regions of the optical elements 30 and 50 define the total overlap of cores. In most cases it is preferable not to launch any power into the primary cladding 32, as this light will not be contained in the core 31 and the central cladding 34, and thus could be regarded as undesirable losses to the component. This would be especially true for the important practical case of n.sub.3>n.sub.2, in which case any light launched into the primary cladding 32 would also leak into the secondary cladding 33.

[0052] It is to be understood that the embodiments of the invention disclosed are not limited to the particular structures, process steps or materials disclosed herein, but are extended to equivalents thereof as would be recognized by those ordinarily skilled in the relevant arts. It should also be understood that terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting.

[0053] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment.

[0054] Various embodiments and example of the present invention may be referred to herein along with alternatives for the various components thereof. It is understood that such embodiments, examples, and alternatives are not to be construed as de facto equivalents of one another, but are to be considered as separate and autonomous representations of the present invention.

[0055] Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the description numerous specific details are provided, such as examples of lengths, widths, shapes, etc., to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention.

[0056] While the forgoing examples are illustrative of the principles of the present invention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the darns set forth below.