Preform for a speckle-free output optical fiber having a structured silica segment, method for making such a preform, and improved speckle-free output optical fiber

Abstract

Disclosed are the fabrication of preforms and novel preforms which, after being drawn, produce novel optical fibers, improved speckle-free output fibers. Useful fibers are made from the preforms described herein to provide speckle-free smooth output with flat-top transmission of light from Gaussian or few-mode sources. Also disclosed are the unique fabrication of these improved preforms. The preforms and the resulting fibers having core sizes varying from about 100 μm to over 1000 μm are based on structured silica segments in the mode mixing region adjacent to the inner core, or in the case of non-circular cores, within the core. The plasma vapor deposition process is modified to achieve the structured segments in a well-controlled manner. The structured segments are composed of multiple paired layers in which thin down-doped layers alternate with much thicker layers of core material. The ratio of the thickness of the core layer to the thickness of the down-doped layer 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 segments depends on the details of the structured silica segments and the individual down-doped layers. Both circular core examples and non-circular core examples are possible and discussed herein.

Description

Technical Field

[0001] For many applications employing lasers and fiber lasers, it is preferred to have a speckle-free output at the distal output end of the fiber system, rather than the typical Gaussian distribution obtainable from a laser source. Such outputs are often referred to as Top Hat or Flat Top distributions. Typically, they require an effective mode mixing fiber section to work well in most systems / applications. In other applications, speckle-free output is required to provide speckle-free emission, such as for laser cleaning or spatially sensitive sensors. We disclose herein a manufacturing method and preform structure that can be drawn into a desired optical fiber having a speckle-free output on its output surface. Background Art

[0002] Laser welding or joining has become a very big business for a variety of applications. In many cases, to gain the full benefits of these processes, it is critical to have very clean and very smooth surfaces, down to the atomic / molecular size. Laser cleaning has become the method of choice when preparing surfaces for precision and long-lasting welds, as well as for repainting, etc.

[0003] The need and therefore pursuit for speckle-free output fiber optic output has been around for some time, especially since the beginning of the use of laser and fiber laser sources in many applications. In addition to the need for ultra-clean surfaces for improved welding and joining, the continued miniaturization of various opto-optical devices and the push for single-mode or few-mode sources with high-density power have led to the need for speckle-free beams in laser processing systems for large and small components. Whether it is laser welding, laser cleaning, or laser joining / sealing procedures, they can all have a negative impact on non-speckle-free beams. For example, ripples are produced in the surface after cleaning using a laser with speckle output, achieving a regular joint, but not as ideally tight, continuous or defect-free as laser joining / welding can provide. In addition, in high-power (CW or pulsed) sources, local power peaks can cause damage to the fiber itself. Avoiding this situation is very desirable, especially in high-power applications.

[0004] Optical fiber is often used to transport and distribute laser radiation to areas far from the laser source. This may be beneficial in protecting the source, providing a larger operating beam, and / or more flexibility in reaching different surface targets. Typically, these benefits are due to the use of multimode optical fiber with a large core count, which has multiple modes of laser energy delivery.

[0005] Optical fibers are typically drawn from a preform. The cross-sectional structure of the preform determines the cross-sectional structure of the drawn optical fiber. The production of the preform can be accomplished by a variety of processes, but one described herein is plasma outside vapor deposition (POVD). Here, the preform is formed by depositing successive layers of material on a core rod to provide a cladding and a glass sheath. Sometimes after the cladding deposition is complete, a pure silica tube is fused to the preform to bring the outer diameter to the desired thickness. In this process, the core, cladding, and outer pure silica are all coaxial with each other. A polygonal core preform can be made by starting with a non-circular core and depositing cladding and sheath layers in a manner similar to a circular core. Depending on the shape of the starting material preform, standard-sized optical fibers can be drawn from such a preform with a circular or non-circular core.

[0006] Laser bonding; high demand for speckle-free, clean surfaces in electronics and high-tech miniaturization. Mode-mixing fibers do not always result in a true top-hat output, creating or leaving ripples on the surface at a molecular scale, the effects of which can actually damage / impair the performance of the device or high-tech application (such as supersonic jets, high-value aircraft parts, space applications, etc.). The smaller the device, the more high speckle-free properties are required for the output surface area of ​​the beam output across the delivery fiber. Otherwise, severe negative effects can be produced on the processed workpiece surface. For example, the requirements for mode mixing required for efficient cladding-pumped fiber lasers are relatively low compared to the needs of laser processing of micro to ultra-micro electronic devices. Asymmetric cores or non-circular cores alone are insufficient for the level of mixing required to produce a true top-hat output that is truly speckle-free over a wide range of input sources.

[0007] As a result, optical fibers with excellent mode mixing are needed for many critical applications in laser cleaning, laser bonding, and laser welding to achieve speckle-free output. Ideally, such optical fibers are simply drawn from a properly constructed preform having all the necessary features of the final optical fiber in proportion to the draw ratio of the desired size of the optical fiber.

[0008] In the prior art, several approaches have been taken to produce asymmetric core cross-sections by using asymmetric cores, non-circular cores, claddings containing disruptive changes in refractive index by adding local sections of new material or air, and by changing the cross-section at different points along the long axis of the optical fiber by modifying the drawing parameters during the drawing process.

[0009] Content and purpose of the invention

[0010] Our technical advances in preform structure have increased the likelihood of better and more speckle-free performance of remote process-required optical fibers drawn from the preforms described herein, as well as new manufacturing techniques to achieve and provide the desired speckle-free output characteristics in optical fibers drawn from these preforms. The primary objective is to design and prepare optical preforms whose structure is inherently better (ideally) adapted to allow drawing of optical fibers of various sizes that are speckle-free in transmission from Gaussian output sources or other sources that do not have a speckle-free cross-sectional output.

[0011] Another object is to provide a preform for a speckle-free output optical fiber which can be drawn into a speckle-free output optical fiber using a standard drawing process, thereby allowing no losses in the output of the drawing process and maintaining additional costs only in the preform making process.

[0012] Another object is to provide a manufacturing process for a preform that can be used to produce a speckle-free output optical fiber.

[0013] Other objectives are to provide speckle-free output fibers for various laser processing of materials, including laser cleaning, laser processing, and laser welding. A circular core preform structure that meets these objectives is the object of the current patent. The fabrication and processing of non-circular core preform structures that can also be successfully drawn into speckle-free output fibers is also one of the objects of this patent application.

[0014] In summary, we describe novel circular core; and non-circular core preforms for drawing speckle-free output fibers of equivalent cross-section; and methods for making the same. These preforms are designed to make better speckle-free output fibers. Fibers of different sizes (core sizes ranging from 100pm to 1000pm and above) can effectively convert Gaussian or low-mode light source output to a speckle-free working surface output, such as a flat-top output. The new and improved speckle-free output fiber products made from these preforms are well suited for use in laser processing applications (including laser cleaning of surfaces and laser welding of critical surfaces) and other applications that benefit from top-hat type output. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 The basic structure of the initial preform is shown, having a core and surrounded by a structured silica layer, as can be seen in Figure 1A.

[0016] Figure 2 The refractive index profile of a cross section of the initial preform is shown with more detail in Figures 2A and 2B.

[0017] Figure 3 An intermediate preform is shown where the shaded asymmetric area has been ground away. The inner core is made asymmetric with the resulting preform, as shown in FIG. Figure 4 shown.

[0018] Figure 4 Shows Figure 3 An inner intermediate preform surrounded by a reflective layer in FIG. 1 is ready for drawing a speckle-free output optical fiber.

[0019] Figure 5 shows the preliminary stages of an intermediate preform for a non-circular core optical fiber, with Figure 1 Related to the initial prefab in .

[0020] Figure 6 Shown by Figure 5 One of two preforms made from the intermediate preform shown is ready to be drawn into a non-circular core, speckle-free output fiber.

[0021] Figure 7 The preform is shown basically with flat surfaces and two sets of cut lines to create the core for 4 drawn preforms with non-circular cores.

[0022] Figure 8 Shown by Figure 7 One of four preforms made from the initial preform in , which is ready to be drawn into a non-circular core, speckle-free output fiber.

[0023] Fig. 9 A cross-sectional view of plasma outside vapor deposition (POVD) is shown.

[0024] Fig.10 The right side of FIG. 1 is a near-field image and a curve of a round core optical fiber having a core diameter of 300 μm of the present invention; and the left side of FIG. 1 is a near-field image and a curve of a standard round core optical fiber having a core diameter of 300 μm of the prior art.

[0025] Fig.11 The right side of FIG. 1 is a near-field image and a curve of a round core optical fiber having a core diameter of 600 μm of the present invention; and the left side of FIG. 1 is a near-field image and a curve of a standard round core optical fiber having a core diameter of 600 μm of the prior art.

[0026] Fig.12 The right side is a near-field image and curve of a non-circular core optical fiber of the present invention having a core size of 100 μm×100 μm. DETAILED DESCRIPTION

[0027] In the following description, for Figures 1 to 8Features shown in the drawings, with reference numerals having the same last two digits are similar items, e.g., 101, 201, 301, 401, etc. are pure silica cores that include an inner core, and each of 103, 203, 303, 403, 503, etc. is a structured silica mode intermixing region comprised of down-doped silica and pure silica deposits, as described below, which in each case surround the inner core in the drawings. While most of the deposition described herein uses a plasma outside vapor deposition (POVD) process, a plasma chemical vapor deposition (PCVD) process may also be used in the various deposition steps described herein if desired for general reasons. References to plasma deposition herein may refer to either process unless specifically stated. The pure silica core rod 101 has been placed in a POVD chamber to add a series of layers that alternate between down-doped layers 123 and pure silica layers 121, resulting in a. Figure 1 The difference between the diameter of the pure silica core 102 and the diameter of the structured silica segment 104 defines the total thickness of the mode mixed structured silica segment 103. Within the segment 103, there are a number of layered pairs 120, which may vary for different cases, typically in the range of 8 to 30 pairs. Within each layered pair 120, the pure silica layer 121 is typically much thicker than the down doped silica layer 123. The ratio of the two thicknesses is typically in the range of about 1 to 20. This is in the range of about 1 to 20. Figure 1 and Figure 1 Particularly useful ranges for these two parameters are when the thickness ratio within a dyad is 7-13 and the number of dyads is 12-20.

[0028] Of course, starting with a silica core of appropriate size, the inner core 101, 201 can be made from a thinner silica rod, onto which pure silica is deposited, in some cases, by additional plasma deposition of pure silica, to achieve the desired core diameter.

[0029] Figure 2 The refractive index (RI) profile of the preform 100 is shown in cross-section. Figure 2 Figures 2A and 2B show how the RI varies in cross section. These lines represent the refractive index drop of the down-doped silica layer between the refractive index of the core material. The sharpness of the RI change indicates the sharp change of the material during deposition, and the speckle-free bottom determines the speckle-free dopant level in each down-doped layer. In a series of examples, Δn = 5×10 -3 .

[0030] In adopting Figure 1 After the preliminary form in , it is deposited together with additional pure silicon dioxide 305 to form a preform having a diameter 325, as Figure 3 In the next step, a preform with an asymmetric core is produced 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 structured silica segments 303 centered around the core 301 that are offset from the new ground shape.

[0031] Figure 3 Asymmetric removal of outer material 307 is depicted, wherein inner core 301 is off-center within outer core 305. Core 301 is concentrically surrounded by structured silica region 303, with the diameter 302 of the inner core and the diameter 304 of the structured silica defining the total thickness of the structured silica region.

[0032] Figure 4 A cross-sectional view of a finished preform ready to be drawn into an optical fiber with speckle-free output is shown. An inner core 401 is concentrically surrounded by structured silica 403, the thickness of which is defined by the difference in diameter of structured silica 404 and inner core 402. An 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 in the outer core 405 by a difference 411. In one example, 411 is 4 mm.

[0033] We can also use Figure 4 First, a cross section of an optical fiber drawn from a preform as described above is described. In this case, the reflective layer 409 can be applied when the optical fiber is drawn, and thus the reflective layer can be selected from silicone, hard plastic cladding, other polymer cladding materials. The reflective layer 409 of the speckle-free output optical fiber can also be composite, that is, the optical fiber can be drawn with a reflective layer on the preform and the reflective layer added during the drawing process.

[0034] Another point should be added. Although silica glass optical fibers are very strong when drawn, over time the glass surface is susceptible to damage from various application conditions, which may compromise the outermost glass layer. Therefore, as is well known, optical fibers used in an open environment, as found in most industrial or medical applications, usually have one or more protective outer coatings (jackets) not described herein. These outer coatings are usually applied during the drawing process, but they can be applied in further downstream processing.

[0035] Figures 5 to 8 Various aspects of producing preforms and optical fibers with non-circular cores for speckle-free output are illustrated. First, Figure 1The initial preform shown in is enlarged with additional core material to produce a larger preform having an inner core 501, a structured silica region 503, and a second core surrounding the structured silica region, which has a diameter 525. The second core can be made entirely by a plasma deposition process, or alternatively by sleeved with a pure silica tube (whose internal dimensions closely match the diameter of the initial preform) and then bonding the two into a larger preform without bubbles and having the desired diameter 525. The larger preform is ground to remove material 507 until its width 515 is reached, with a specific height related to its width. The larger preform is ground so that a portion of the second core material remains on all of the structured silica regions 503. For most examples, both the inner core 501 and the outer (second) core 505 are pure silica materials. The ground preform is cut along the cut line 513 to generate two non-circular cores of two new preforms; each of which can be drawn into a speckle-free output fiber.

[0036] exist Figure 6 middle, Figure 5 Each composite core in is set in a plasma deposition apparatus and a reflective coating 609 is deposited onto the composite non-circular core after rounding its corners 619. The core materials 601 and 605 are generally the same and the core has a semi-circular region 603 of structured silica therein. Its width 615 is shown. In this particular example, the width and height are substantially equal in length and the shape of the non-circular core is square. Other shapes are also possible, such as rectangular, triangular, trapezoidal, hexagonal, octagonal, etc.

[0037] The optical fiber drawn from this preform will have an equivalent cross section, with actual dimensions proportional to the preform. In one preform example, the diameter of the inner core 501 is 15 mm. The diameter of the structured silica 503 is 17 mm, making the thickness of the structured silica 503, 603 2 mm. The width and height are equal, 18.5 mm. And the preform diameter 525 is 51 mm.

[0038] Figure 7 and Figure 8 Depicts the division of a milled initial preform with a non-circular core into 4 equivalent square cores and the generation of 4 Figure 8 New preform with the cross section shown. Figure 7 , an inner core 701 is surrounded by structured silica 703 and then by additional core material 705. The initial preform has a diameter 725. After the initial deposition, the preform is ground into a rectangular composite core having a side dimension 735 by removing material 707 and then cutting the resulting rectangular core along cut lines 713 into four non-circular core pieces. The corners of these pieces are then rounded and a reflective layer 709 is deposited, thereby making four similar preforms, such as Figure 8 As previously mentioned, the inner core 701 and the second core 705 are usually the same material, most likely pure silicon dioxide.

[0039] like Figure 8 As shown, the final preform has an arcuate structured silica 803 sandwiched between core material 801 and core material 805 within a square core, which for this example has rounded corners 819, and a reflective material 809 is deposited or otherwise added around the core to form the final preform. The width of the non-circular core 835 is equal to the height of the core because the core is square in this example. Other possible shapes for non-circular cores are mentioned above. The relative areas in the drawn fiber will be the same as Figure 8 The areas of the preform shown are to scale because the shape of the fiber cross section is the same as the shape of the preform.

[0040] In one example, the diameter of the pure silica inner core 701 is 15 mm, and the diameter of the surrounding structured silica 703 is 17 mm, making the thickness of the structured silica 703, 803 2 mm. The diameter 725 is 51 mm. Each of the 4 non-circular cores has a side dimension 735, 835 of 18.5 mm x 18.5 mm.

[0041] exist Fig. 9 A typical POVD setup is shown in FIG, where 901 is a screen box; 902 is a substrate rod; 903 is a glass processing lathe; 904 is a plasma torch; and 905 is a handle attached to the substrate rod 902. In many examples, the plasma torch 904 operates at 5.28 MHz and a 50 kW power level. Note that plasma vapor deposition, i.e., POVD or PCVD, is performed in different cases.

[0042] Within the structured silica segment and reflective coating, there is a wide range of materials that can be used as core materials. Pure silica is usually selected as the core material and used for the sleeve, but up-doped Si, such as germanium-doped silica (Ge-Si), or a graded index silica-based core can be used. The reflective layer is most commonly fluorosilicate, but other lower refractive index silicas, such as borosilicates, can also be used. In the reflective / cladding type coating, the coating applied after the fiber is drawn includes fluoroacrylates and silicone plastic materials. The choice of core material will affect the feasible choice of materials for the paired layers of the structured silica segment. For example, using pure silica as the core material, the down-doped (lower RI) silica will be the first layer in the paired layer, such as Fluoro-silica with a selected F dopant level, and the second higher RI layer can be selected from: pure silica, or down-doped fluorine silica, or up-doped silica (such as Ge-Si), or similar materials, as long as the total refractive index of the structured silica segment is lower than the core refractive index required for the optical fiber. As long as the refractive index of the structured silica segments remains lower than the core refractive index, some special effects may occur if one or more of either layer in the paired layers becomes up-doped silica.

[0043] Preferred combinations, thickness ratios within a dyad, and number of dyads are numerous, depending on the intended application, available preform equipment and materials, and core requirements.Some of the more useful ranges for the number of layers within a dyad and the thickness ratios therebetween have been described previously.

[0044] Additionally, to make fiber lasers or amplifiers, the rare earth doped innermost core can be incorporated into the structure of silica or other core material, into the preform and thus into the drawn fiber, adding structures such as structured silica. Alternatively, a tubular preform can be made and then sleeved over a rare earth core or clad rare earth core rod.

[0045] Figures 10 to 12 Some representative results for optical fibers made from preforms having structured silica segments contained within their cores are shown. In particular, to the right of each figure is a near-field image and below are corresponding output plots for three sample fibers having a 300 μm circular core, a 600 μm circular core, and a non-circular 100 μm x 100 μm square core. For comparison, Fig.10 and Fig.11 In the figure, the left half shows the corresponding near-field images and curves of standard 300μm and 600μm core fibers, respectively.

[0046] At the time of filing, 300 μm core, 600 μm core or larger core fibers will be preferred examples of the present invention. For non-circular core fibers, preferred non-circular core patterns will be square or rectangular cores with semicircular arc structured silica or with quarter-circular arc segments.

[0047] Another potentially useful configuration would be to have a thin upper doped layer immediately before or after the structured silicon dioxide segments described above; or before and after the structured silicon dioxide segments described earlier in this document. The thickness of this upper doped layer should be as thin as or thinner than the lower RI layer of the paired layer.

Claims

1. A preform for a speckle-free output optical fiber, the speckle-free output optical fiber being drawn from the preform, the cross-sectional structure of the preform comprising: A composite core structure and a reflective layer, the composite core structure comprising a circular inner core and a circular outer core, the circular inner core having a refractive index, the circular outer core having the same refractive index as the circular inner core, the circular inner core being surrounded by a structured circular region, the average refractive index of the structured circular region being lower than the refractive index of the circular inner core and the circular outer core, wherein the circular inner core is offset from the center within the circular outer core, wherein the innermost core, the structured circular region and the outermost core are surrounded by a reflective cladding having a refractive index less than the refractive index of the core and any layer of the structured circular region, Wherein the structured circular region has many paired layers, starting from the circular core, the first layer having a lower refractive index than the material of the circular core, followed by a next layer having a higher refractive index than the material of the first layer, and each layer has a thickness.

2. The preform according to claim 1, in, The first layer having a lower refractive index is a down-doped layer, and the next layer is a layer of the same material as the circular core, or an up-doped layer.

3. The preform according to claim 2, in, In each of the paired layers, a ratio of a thickness of a layer of the same material as that of the circular inner core layer to a thickness of the lower doped layer is 1 to 20.

4. The preform according to claim 1, in, The number of the paired layers is 8 to 30.

5. The preform according to claim 1, in, A tube of pure silica is wrapped onto the preform without creating any gaps or any bubbles at the interface between the inner surface of the tube and the reflective layer to form a drawn preform for the speckle-free output optical fiber.

6. The preform of claim 1, wherein the preform is used to form an optical fiber having a cross-section proportional to the cross-section of the preform, and wherein the output / transmission of a low-mode photon source of the optical fiber has reduced speckle, and the output / transmission of a high-power photon source of the optical fiber has reduced speckle.

7. The preform according to claim 1, The preform also includes an innermost core of a high refractive index, rare earth doped material so that when drawn, the fiber can be used as a fiber laser / amplifier or a sensing medium.

8. A method for manufacturing a preform according to claim 1, wherein: Plasma vapor deposition is used to produce sections of the cross section of the preform as claimed in claim 1 and its layers.

9. A preform from which a speckle-free output optical fiber can be drawn, the cross-sectional structure of the preform comprising: a composite non-circular core surrounded by a reflective cladding type material; Wherein, the composite non-circular core further comprises: a segment of a polygonal core material having a refractive index; and an arc segment of a circular region of structured silica within a segment of the polygonal core material, the arc segment of the circular region of structured silica having an average refractive index lower than an average refractive index of the polygonal core material; The reflective cladding type material has a lower refractive index than the polygonal core material; and The preform can be drawn into a speckle-free output optical fiber using standard optical fiber drawing techniques. wherein the structured silicon dioxide circular region has paired layers, the paired layers starting from an inner core, the paired layers including a lower doped layer and a subsequent core layer; and wherein each layer has a thickness.

10. The preform according to claim 9, in, In each of the paired layers, a ratio of a thickness of the core layer to a thickness of the lower doped layer is 1 to 20.

11. The preform according to claim 9, in, The number of the paired layers is 8 to 30.

12. The preform according to claim 9, in, The polygon is selected from the group consisting of a triangle, a rectangle, a pentagon, a hexagon, a heptagon, an octagon, a decagon, and a dodecagon.

13. The preform according to claim 9, in, Said segments of the polygonal core are rectangular / square cores for 4-sided polygons; or pie-shaped cores for all other polygonal shapes.

14. The preform according to claim 13, in, The arc segment of the circular area of ​​structured silica is different for different polygonal core shapes and is the portion of the circular area divided by the number of sides in the polygon.

15. The preform according to claim 14, in, When the precursor initial rectangular core is cut only once through its long sides before depositing the reflective cladding around the non-circular core, the arc segments of the structured silica circular areas within the rectangular / square core have a semi-circular shape.

16. An optical fiber drawn from the preform according to claim 9, the cross-section of the optical fiber being proportional to the cross-section of the preform, and the output / transmission of the low-mode photon source of the optical fiber being speckle-free output, and the output / transmission of the high-power photon source of the optical fiber being speckle-free output.

17. The optical fiber according to claim 16, in, The structured silicon dioxide circular areas are as defined in claim 9.

18. The optical fiber according to claim 16, in, The associated preform has an innermost core of a rare earth doped material, enabling the optical fiber to function as a fiber laser / amplifier or for sensing purposes.

19. A method for manufacturing a preform according to claim 9, wherein: Plasma vapor deposition is used to produce the cross-sectional sections of the preform as claimed in claim 9 and the layers thereof.

Images