Preform, manufacturing method of the preform, and manufacturing method of optical fiber

Abstract

A new class of optical fiber, the fabrication of preforms, and a new type of preform that, when drawn, yields an improved speckle-free output optical fiber, are presented. Useful fibers that achieve speckle-free, smooth output with flat-top transmission of light from Gaussian or few-mode sources are produced from the preforms introduced herein. The unique fabrication of these improved preforms is also presented. The preforms, and thus fibers produced with a variety of core dimensions from about 100 μm to over 1000 μm, are based on a structured silica section with a mode-mixing region adjacent to the inner core or, in the case of noncircular cores, within the core. A plasma vapor deposition process is tailored to achieve the structured section with a well-controlled morphology. The structured section is composed of a number of layer pairs, alternating thin down-doped layers with much thicker layers of core material. The ratio of the core layer thickness to the down-doped layer thickness is about 3 to 25. The number of paired layers is typically between about 8 and 30 layer pairs. The effective NA of the structured section depends on the details of the structured silica section and the individual down-doped layers. Both circular and non-circular inner core examples are possible and are discussed herein.

Description

[Technical Field]

[0001] In many applications using lasers and fiber lasers, it is desirable to have a speckle-free output at the distal output end of the fiber system, instead of the typical Gaussian distribution obtained from a laser source. Such an output is often referred to as a top-hat or flat-top distribution. Generally, most systems / applications require an efficient mode-mixing fiber section to function well. Other applications require a speckle-free output, for example, for laser cleaning or to achieve speckle-free emission for spatial detection sensors. Disclosed herein are fabrication methods and preform structures that can be drawn into the desired optical fiber with a speckle-free output across the entire output face. [Background technology]

[0002] Laser welding or joining has become very big business in a variety of applications. In many cases, to reap the full benefits of these processes, it is important that the surfaces are very clean and very smooth, down to the atomic / molecular level. In addition to surface preparation for repainting etc., laser cleaning has become the method of choice for surface preparation for precision welding and long-life welds.

[0003] The need for, and therefore the pursuit of, speckle-free fiber output has existed for some time, especially since laser and fiber laser sources began to be used in many applications. The need for ultra-clean surfaces for improved welding and joining, along with the demand for single-mode or few-mode sources for high-density output, and the increasing miniaturization of various imaging optics, are driving the need for speckle-free beams in laser processing systems for both large and small components. Whether it's laser welding, laser cleaning, or laser joining / sealing procedures, each can be adversely affected by a beam that is not speckle-free. For example, surface waviness after cleaning with a speckle-containing laser output can result in a normal joint, but not the ideal tight, continuous, or defect-free joint that laser joining / welding can achieve. Furthermore, with high-power (CW or pulsed) sources, localized power peaks can lead to destruction of the fiber itself. Avoiding this is highly desirable, especially in high-power applications.

[0004] Optical fibers are often used to transmit and distribute laser radiation to areas remote from the laser source. This may be to protect the source, achieve a larger working beam, and / or be more flexible and reach a variety of surface targets. Generally, these advantages are due to the use of multimode optical fibers, which have a large core and many modes for the transmission of laser energy.

[0005] Optical fiber is typically drawn from a preform. The cross-sectional structure of the preform determines the cross-sectional structure of the drawn fiber. Preform fabrication can be accomplished by several processes, but the process described herein is Plasma Outside Vapor Deposition (POVD). Here, the preform is constructed from a core rod by depositing successive layers of material to achieve the cladding and glass jacket. After cladding deposition is complete, a pure silica tube may be fused over the preform to achieve the desired outer diameter. In such a process, the core, cladding, and outer pure silica are all coaxial with one another. Starting with a noncircular core, polygonal core preforms can be fabricated in a similar manner by depositing cladding and jacket layers in the same manner as for circular cores. Optical fiber of standard dimensions can be drawn from such preforms with either circular or noncircular cores, depending on the shape of the starting preform.

[0006] Laser bonding: There is a high demand for speckle-free, clean surfaces in electronics and high-tech miniaturization. Mode-mixing fibers do not always result in truly flat-top output; they can create or leave behind surface ripples on a molecular scale that can actually damage or degrade device operation or high-tech applications such as supersonic jets, expensive aircraft components, and space applications. The smaller the device, the more desirable it is for the beam output to be highly speckle-free across the entire output surface area of ​​the delivery fiber. Otherwise, serious adverse effects can occur across the entire surface of the processed component. The mode mixing required to obtain an effective cladding-pumped fiber laser is relatively undemanding compared to the needs of laser processing, for example, in small to ultra-small electronic devices. Simply asymmetric or noncircular cores are not sufficient for the level of mixing required to achieve true top-hat output, i.e., truly speckle-free output, across a wide range of input sources.

[0007] As a result, many important applications in laser cleaning, laser joining, and laser welding require optical fibers with excellent mode mixing to achieve speckle-free output. Ideally, such optical fibers are simply drawn from a properly constructed preform that has all the required characteristics of the final optical fiber proportional to the drawdown ratio of the required size optical fiber.

[0008] In the prior art, there have been several approaches taken to creating asymmetric core cross sections by using asymmetric cores, non-circular cores, claddings that contain decoupled changes in refractive index by adding local sections of new material or air, and varying the cross section at different spots along the longitudinal axis of the optical fiber over its length by adjusting the drawing parameters during the drawing process. Summary of the Invention [Problem to be solved by the invention]

[0009] Our technological advances in preform structure improve the potential for better, more speckle-free performance in remote processes involving optical fiber drawn from the preforms described herein, as well as refine new manufacturing techniques to achieve and provide the required speckle-free output performance in fiber drawn from these preforms. The primary objective is to design and fabricate optical preforms with structures that are inherently better (ideally) suited to enabling the drawing of optical fibers of various sizes with speckle-free output in the transmission of a Gaussian output source or other source lacking a speckle-free cross-sectional output.

[0010] Another object is to provide a speckle-free output optical fiber preform that can be drawn into a speckle-free output fiber using standard drawing processes, thus avoiding losses in output from the drawing process and keeping additional costs only within the preform manufacturing process.

[0011] Another object is to provide a manufacturing process for a preform that can be used to make a speckle-free output optical fiber.

[0012] A further object is to provide a speckle-free output optical fiber for laser processing of various materials, including laser cleaning, laser machining, and laser welding. A circular core preform structure that meets these objectives is an object of this patent. The manufacture and processing of non-circular core preform structures that can also be successfully drawn into speckle-free output optical fibers is also an object of this patent application. [Means for solving the problem]

[0013] In summary, a new class of circular-core, noncircular-core preforms for drawing speckle-free output optical fiber with equivalent cross-sections and their manufacturing methods are described. These preforms are designed to yield better speckle-free output optical fiber. Fibers of various dimensions, with core sizes ranging from 100 μm to over 1000 μm, effectively convert Gaussian or low-mode source output into a speckle-free work surface output, such as a flat-top output. The new and improved speckle-free output optical fiber products made from these preforms are excellent for use in laser processing applications, including laser cleaning of surfaces and laser welding of critical surfaces, in addition to other applications that benefit from a top-hat output. [Brief explanation of the drawings]

[0014] [Figure 1]As seen in Figure 1(A), the basic structure of the initial preform has a core surrounded by a structured silica layer. [Figure 2] 2(A) and 2(B) show the refractive index profile in cross section of an initial preform, with further details shown. [Figure 3] 4 shows an intermediate preform from which the asymmetric region indicated by the diagonal lines has been removed, resulting in an asymmetric inner core for the resulting preform, as shown in FIG. [Figure 4] FIG. 4 shows the inner intermediate preform of FIG. 3 surrounded by a reflective layer and ready for drawing a speckle-free output optical fiber. [Figure 5] 2 is a diagram showing a preliminary stage of an intermediate preform of a non-circular core optical fiber in relation to the initial preform of FIG. 1; [Figure 6] FIG. 6 shows one of two preforms produced from the intermediate preform shown in FIG. 5, ready to be drawn into a non-circular core speckle-free output optical fiber. [Figure 7] FIG. 1 shows basically a preform with a flat surface and two sets of cutting lines for making the cores of four drawing preforms with non-circular cores. [Figure 8] FIG. 8 shows one of four preforms produced from the initial preform of FIG. 7, ready for drawing into a non-circular core speckle-free output optical fiber. [Figure 9] 1 is a cross-sectional view of plasma external vapor deposition (POVD). [Figure 10] On the right side, a near-field image and plot of a circular core optical fiber of the present invention having a core diameter of 300 μm, and on the left side, a near-field image and plot of a standard circular core optical fiber of the prior art having a core diameter of 300 μm. [Figure 11]On the right side, a near-field image and plot of a circular core optical fiber of the present invention having a core diameter of 600 μm, and on the left side, a near-field image and plot of a standard circular core optical fiber of the prior art having a core diameter of 600 μm. [Figure 12] On the right is a diagram with near-field images and plots of a non-circular core optical fiber of the present invention with core dimensions of 100 μm×100 μm. DETAILED DESCRIPTION OF THE INVENTION

[0015] In the following description, in the configurations illustrated in Figures 1 through 8, numbers with the same last two digits refer to similar items, e.g., 101, 201, 301, 401, etc., are pure silica cores that make up the inner core, and 103, 203, 303, 403, 503, etc., are structured silica mode-mixing regions constructed from down-doped silica and pure silica depositions, which surround the inner core in each case throughout the figures, as described below. While most of the depositions described herein use plasma-enhanced external vapor deposition (POVD) processes, plasma-enhanced chemical vapor deposition (PCVD) processes can also be used in the different deposition steps described herein, if desired for conventional reasons. References to plasma deposition herein can refer to either process unless otherwise specified. A pure silica core rod 101 is placed in a POVD chamber, and a series of alternating layers of down-doped silica 123 and pure silica 121 are added to achieve the structured section 103 shown in FIG. 1 . The difference between the diameter 102 of the pure silica core and the diameter 104 of the structured silica section defines the overall thickness of the mode-mixing structured silica section 103. Within the section 103, there are a number of layered pairs 120, which can vary but typically range from 8 to 30 pairs. Within each layered pair 120, the pure silica layer 121 is often much thicker than the down-doped silica layer 123. The ratio of the two thicknesses typically ranges from about 1 to 20. This is summarized in FIGS. 1 and 1(A). Particularly useful ranges for these two parameters are 7 to 13 for the thickness ratio within a pair and 12 to 20 for the number of pairs.

[0016] Of course, to start with an appropriately sized silica core, the inner core 101, 201 may in some cases be made from a thinner silica rod onto which pure silica is deposited by plasma deposition of additional pure silica to achieve the desired core diameter.

[0017] FIG. 2 shows the refractive index (RI) profile of the preform 100 in cross section. FIGS. 2(A) and 2(B) show how the RI varies across the cross section. The lines represent the drop in refractive index of the downdoped silica layer relative to the refractive index of the core material. The abruptness of the RI change indicates the abrupt change in material over the course of deposition, and the speckle-free bottom attests to the speckle-free dopant levels in each downdoped layer. In one set of examples, Δn=5×10 -3 is.

[0018] After the preliminary form of Figure 1 is taken, additional pure silica 305 is deposited thereon to create a preform having a diameter 325, as illustrated in Figure 3. In a next step, a preform with an asymmetric inner core is created by grinding away a portion 307 of the initial preform diameter 325, preferably to one side of the preform, so that the new preform shape has a structured silica section 303 that is offset from the center of the newly ground shape and surrounds the inner core 301.

[0019] 3 illustrates the asymmetric removal of outer material 307, where inner core 301 is off-center within outer core 305. Core 301 is concentrically surrounded by structured silica region 303, with inner core diameter 302 and structured silica diameter 304 defining the overall thickness of the latter region.

[0020] Figure 4 shows a cross-sectional view of a completed preform ready to be drawn into a speckle-free output optical fiber. The inner core 401 is concentrically surrounded by structured silica 403, whose thickness is determined by the difference between the diameter of the structured silica 404 and the diameter of the inner core 402. The outermost core 405 is surrounded by a reflective layer 409, such as down-doped silica deposited by POVD / PCVD. Note that the center of the inner core 401 is offset within the outer core 405 by a distance 411. In one example, 411 was 4 mm.

[0021] 4 can also be used to first show a cross section of an optical fiber drawn from the preform described above. In this case, the reflective layer 409 can be added as the optical fiber is drawn and can be selected from silicone, hard plastic cladding, and other polymer cladding materials. The reflective layer 409 of the speckle-free output optical fiber can also be composite, i.e., the reflective layer can be added during the drawing process while the reflective layer is on the preform and the fiber is drawn.

[0022] One more point must be made: although silica glass fiber is very strong when drawn, over time the glass surface is susceptible to damage from various conditions in the application, which can result in the loss of the outermost glassy layer. Therefore, it is well known that optical fibers used in open environments, such as those found in most industrial or medical applications, typically have one or more protective jackets not shown here. These jackets are typically added during the drawing process, although they may also be added further downstream.

[0023] 5 through 8 illustrate aspects of fabricating a preform and optical fiber with a noncircular core for speckle-free output. First, an initial preform, such as that shown in FIG. 1, is augmented with additional core material to form a larger preform having an inner core 501, a structured silica region 503, and a second core around the structured silica region, with a diameter 525. The second core may be fabricated entirely through a plasma deposition process, or by sleeving a pure silica tube whose inner diameter closely matches the diameter of the initial preform and then bonding the two together to form a larger preform with a desired diameter 525 that is free of air bubbles. The larger preform is ground to its width 515 and a specific height related to that width, removing material 507. The larger preform is ground so that a portion of the second core material remains over the entire structured silica region 503. In most instances, the inner core 501 and outer (second) core 505 are both pure silica materials. The ground preform is cut along cutting line 513 to create two non-circular cores for two new preforms, each of which can be drawn into a speckle-free output optical fiber.

[0024] In Figure 6, each composite core of Figure 5 is set up in a plasma deposition apparatus after rounding its corners 619, and a reflective coating 609 is deposited on the composite non-circular core. The core materials 601 and 605 are generally identical, and the core has a semi-circular region 603 of structured silica therein. Its width 615 is as shown. In this particular example, the width and height are substantially equal lengths, and the non-circular core is square in shape. Other shapes are possible, such as rectangular, triangular, trapezoidal, hexagonal, octagonal, etc.

[0025] Optical fiber drawn from this preform will have a comparable cross section with actual size proportional to the preform. In one example preform, the inner core 501 diameter was 15 mm, the structured silica 503 diameter was 17 mm, and the structured silica 503, 603 thickness was 2 mm. The width and height were equal, 18.5 mm, and the preform diameter 525 was 51 mm.

[0026] 7 and 8 illustrate dividing a ground initial preform having a noncircular core into four equivalent square cores and creating four new preforms with cross-sections as shown in FIG. 8. Thus, in FIG. 7, inner core 701 is surrounded by structured silica 703, which is surrounded by additional core material 705. The initial preform has a diameter 725. After the initial deposition, the preform is ground into an oval composite core by removing material 707, and the resulting oval core is then cut along cutting lines 713 into four noncircular core pieces having side dimensions 735. These pieces then have their corners rounded and a reflective layer 709 deposited, creating four equivalent preforms as shown in FIG. 8. As before, inner core 701 and second core 705 are generally the same material, most likely pure silica.

[0027] As shown in Figure 8, the final preform has an arc of structured silica 803 sandwiched between core material 801 and core material 805, in this example, inside a square core with rounded corners 819, and reflective material 809 surrounding the core is deposited or otherwise added to create the final preform. The width of the non-circular core 835 is equal to the height of the core, since in this example the core is square. Other possible shapes for non-circular cores are described above. The relative area within the optical fiber when drawn will be proportional to the area of ​​the preform, as shown in Figure 8, since the fiber cross-section will be similar in shape to the cross-section of the preform.

[0028] In one example, the diameter of the pure silica inner core 701 was 15 mm and the diameter of the surrounding structured silica 703 was 17 mm, resulting in a thickness of 2 mm for the structured silica 703, 803. The diameter 725 was 51 mm. The four non-circular cores each had side dimensions 735, 835 of 18.5 mm by 18.5 mm.

[0029] A typical POVD setup is shown in Figure 9, where 901 is a screening box, 902 is a substrate rod, 903 is a glass lathe, 904 is a plasma torch, and 905 is a handle attached to the substrate rod 902. In many instances, the plasma torch 904 operates at 5.28 MHz and a power level of 50 kW. As mentioned above, in different cases, either plasma vapor deposition, i.e., POVD or PCVD, may be used.

[0030] There is a wide range of materials that can be used as the core material within the structured silica section and reflective coating. Pure silica is often selected for the core and sleeve, but up-doped silicon, such as germanium-doped silicon (Ge-Si), or graded-index silica-based cores may also be used. The reflective layer is most often a fluorosilicate, but other low-index silicas, such as borosilicate, may also be used. Reflective / cladding-type coatings, added after fiber drawing, include fluoroacrylates and silicone plastic materials. The choice of core material will affect the viable choices for the mating layer material of the structured silica section. For example, if pure silica is used as the core material, a down-doped (low RI) silica, such as fluorosilica with a selected fluorine dopant level, would be the first layer in the mating layer, and the high-RI second layer could be pure silica, or a more lightly doped fluorosilica, or an up-doped silica, such as Ge-Si, or similar materials, as long as the total refractive index of the structured silica section is lower than that of the core, as desired for optical fiber. If one or more of the layers in either pair is changed to updoped silica, special effects could occur as long as the refractive index of the structured silica section remains lower than the core refractive index.

[0031] The preferred combinations, thickness ratios within a pair of layers, and number of pairs of layers are many and depend on the intended application, available preform equipment and materials, and core requirements. Some of the more useful ranges for the number of layers and thickness ratios between layers within a pair of layers have been described above.

[0032] Alternatively, to produce a fiber laser or amplifier, the rare-earth doped innermost core can be incorporated into the structure of silica or other core material in the preform and thus in the drawn optical fiber, adding structure such as structured silica, or a tubular preform can be produced and then sleeved over a rare-earth core or clad rare-earth core rod.

[0033] Figures 10 through 12 show some representative results for fibers made from preforms with structured silica sections housed within their cores. Specifically, each figure has near-field images on the right and corresponding output plots below for three sample fibers with 300 μm circular cores, 600 μm circular cores, and 100 μm x 100 μm non-circular square cores, respectively. For comparison, the left halves of Figures 10 and 11 show the corresponding near-field images and plots for standard 300 μm and 600 μm core optical fibers, respectively.

[0034] At the time of filing, 300 μm core, 600 μm core, or larger core fibers would be among the preferred examples of the present invention. For non-circular core fibers, the preferred non-circular core configurations would be either square or rectangular cores with semi-circular arcs of structured silica or quarter-circular arc segments of structured silica.

[0035] A further potentially useful configuration is to have a thin updoped layer preceding or following the structured silica section described above, or to have a thin updoped layer before and after the structured silica section described above, the thickness of which should be as thin as or thinner than the low RI layer of the counterpart layer.

[0036] (Addendum) (Appendix 1) 1. A preform to be drawn into a speckle-free output optical fiber, comprising: the preform cross-sectional structure comprises a circular inner core having a refractive index or refractive index profile surrounded by a structured circular region having an average refractive index lower than the average refractive index of the inner core; The preform can be drawn into a speckle-free optical fiber using standard fiber drawing techniques. Preform.

[0037] (Appendix 2) 2. The preform of claim 1, wherein the structural circular region has a first layer having a lower refractive index (RI) than the material of the core, followed by a next layer having a higher RI than the material of the first layer, and wherein starting from the inner core, the preform has a plurality of pairs of layers, each layer having a thickness.

[0038] (Appendix 3) 3. The preform of claim 2, wherein the low RI layer is a down-doped layer and the subsequent layer is a core layer or an up-doped layer.

[0039] (Appendix 4) 4. The preform of claim 2 or 3, wherein in each of the paired layers, the ratio of the thickness of the core layer to the thickness of the down-doped layer is from about 1 to about 20.

[0040] (Appendix 5) 4. The preform of claim 2 or 3, wherein the number of pairs of layers is from about 8 to about 30.

[0041] (Appendix 6) 6. The preform of any one of claims 1 to 5, wherein a tube of pure silica is folded over the preform without creating gaps or bubbles at the interface between the inner surface of the tube and the second cladding to form a preform for drawing into a speckle-free output optical fiber.

[0042] (Appendix 7) 7. An optical fiber drawn from a preform according to any one of claims 1 to 6, wherein a cross section of the optical fiber is proportional to a cross section of the preform, and wherein the output / transmission of the optical fiber of a high-power low-mode photonic source has reduced speckle.

[0043] (Appendix 8) 8. The optical fiber of claim 7, wherein the structural circular region is as described in any one of claims 2 to 5.

[0044] (Appendix 9) 7. A method for producing a preform according to any one of claims 1 to 6, wherein plasma vapor deposition is used to produce the cross-sectional sections and layers of the preform according to said claim.

[0045] (Appendix 10) A preform from which a speckle-free output optical fiber can be drawn, the preform having a cross-sectional structure comprising: a composite non-circular core surrounded by a reflective cladding-type material, said composite non-circular core comprising: a polygonal section of a core material having a refractive index; an arc segment of a structured silica circular region within the polygonal core section, the arc segment having an average refractive index lower than the average refractive index of the core material; Furthermore, the reflective cladding-type material has a refractive index lower than that of the core material; The preform can be drawn into a speckle-free output optical fiber using standard fiber drawing techniques. Preform.

[0046] (Appendix 11) 11. The preform of claim 10, wherein the structural circular region comprises a down-doped layer followed by a core layer, the structural circular region having pairs of layers starting from the inner core, each layer having a thickness.

[0047] (Appendix 12) 12. The preform of claim 10 or 11, wherein in each of the paired layers, the ratio of the thickness of the core layer to the thickness of the down-doped layer is from about 1 to about 20.

[0048] (Appendix 13) 12. The preform of claim 10 or 11, wherein the number of paired layers is from about 8 to about 30.

[0049] (Appendix 14) 14. The preform of any one of claims 10 to 13, wherein the polygon is selected from the group consisting of a triangle, a square, a pentagon, a hexagon, a heptagon, an octagon, a decagon, and a dodecagon.

[0050] (Appendix 15) 15. The preform of any one of claims 10 to 14, wherein the polygonal core section is a rectangular / square core for polygons having four sides, or a generally sector-shaped core for all other polygons.

[0051] (Appendix 16) 16. The preform of claim 15, wherein the arc segment of the structured silica circular region is different for different polygonal core shapes and is approximately the portion of the circular region divided by the number of sides in the polygon.

[0052] (Appendix 17) 17. The preform of claim 16, wherein the arc segment of the structured silica circular region within a rectangular / square core has a semicircular shape if the precursor original rectangular core is cut only once, through a long side of the precursor rectangular core, before depositing the reflective cladding layer around the non-circular core.

[0053] (Appendix 18) 18. An optical fiber drawn from a preform according to any one of claims 10 to 17, wherein a cross section of the optical fiber is proportional to a cross section of the preform, and wherein output / transmission of a high power low mode photonic source through the optical fiber is speckle-free.

[0054] (Appendix 19) 19. The optical fiber of claim 18, wherein the structural circular region is as described in any of claims 10 to 17.

[0055] (Appendix 20) 18. A method for manufacturing a preform according to any one of claims 10 to 17, wherein plasma vapor deposition is used to manufacture the cross-sectional sections and layers thereof of the preform according to said claim.

[0056] (Appendix 21) 18. The preform of any one of claims 1 to 6 or 10 to 17, further comprising an innermost core of high refractive index rare earth doped material such that the optical fiber, when drawn, can be used as an optical fiber laser / amplifier or sensing medium.

[0057] (Appendix 22) 20. The optical fiber of any one of claims 7-8 or 18-19, wherein the corresponding preform has an innermost core of rare earth doped material, such that the fiber can function as a fiber laser / amplifier or for sensing purposes.

Claims

1. 1. A preform to be drawn into a speckle-free output optical fiber, comprising: the preform cross-sectional structure comprises a circular inner core having a refractive index or refractive index profile surrounded by a structured circular region having an average refractive index lower than the average refractive index of the inner core; The preform can be drawn into a speckle-free optical fiber using standard fiber drawing techniques; the structured circular region has a first layer having a refractive index (RI) lower than the material of the core, followed by a next layer having a higher RI than the material of the first layer, and starting from the inner core, the structured circular region has a plurality of pairs of layers, each layer having a thickness; Preform.

2. 10. The preform of claim 1, wherein the first layer is a downdoped layer and the next layer is a core layer or an updoped layer.

3. 3. The preform of claim 2, wherein in each of said paired layers, the ratio of the thickness of said core layer to the thickness of said down-doped layer is from about 1 to about 20.

4. 3. The preform of claim 1, wherein the number of pairs of layers is from about 8 to about 30.

5. 5. A method for producing a preform according to claim 1, wherein a tube of pure silica is folded onto an initial preform without creating gaps or bubbles at the interface between the inner surface of the tube and the initial preform to form a preform for drawing into a speckle-free output optical fiber.

6. 5. A method for producing an optical fiber drawn from a preform according to claim 1, wherein the cross-sectional area of ​​the optical fiber is proportional to the cross-sectional area of ​​the preform.

7. 7. The method of claim 6, wherein the structured circular region is as defined in any one of claims 1 to 4.

8. 5. The preform of claim 1 , further comprising an innermost core of high refractive index rare earth doped material such that the optical fiber, when drawn, can be used as an optical fiber laser / amplifier or sensing medium.

9. 8. A method for manufacturing an optical fiber according to claim 6 or 7, wherein the corresponding preform has an innermost core of rare earth doped material, such that the fiber can function as a fiber laser / amplifier or for sensing purposes.

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