Polarization-maintaining multi-core optical fiber

Abstract

The invention relates to an optical fiber having a plurality of light-conducting core regions (1), which extend at a distance from one another in the longitudinal extension direction of the optical fiber and all lie in an adjoining signal region (2), seen in cross-section of the optical fiber, which signal region is completely surrounded by a cladding region (3), the optical fiber comprising a stress element (6), according to the invention, the stress elements (6) are designed to generate a mechanical stress field in the optical fiber, each core region (1) is subjected by the stress elements (6) to mechanical stresses causing birefringence and further causing polarization-maintaining behavior, a main polarization axis is assigned to each of the core regions (1), and all the stress elements (6) are located outside the signal region (2). The purpose of the present invention is to provide a multi-core optical fiber having improved polarization maintaining characteristics compared to the prior art. To achieve this, the stress elements (6) are distributed in a plurality of groups, each group comprising two or more stress elements (6) across the cross-section of the optical fiber, where the stress elements (6) have an asymmetrical (i.e. Non-rotationally symmetrical) arrangement, viewed from the cross-section of the optical fiber, which causes the principal polarization axes in all the core regions (1) to point in the same direction. Furthermore, the invention relates to a laser system using such an optical fiber.

Description

Technical Field

[0001] The present invention relates to an optical fiber having a plurality of light-guiding core regions that extend spaced apart from each other along the longitudinal direction of the optical fiber and are all located within an adjacent signal region in view of the cross-section of the optical fiber, the signal region being completely surrounded by a cladding region, wherein the optical fiber includes stress elements designed to generate a mechanical stress field in the optical fiber, wherein each core region is subjected to mechanical stress by the stress elements that causes birefringence and thus polarization-maintaining behavior, wherein a principal polarization axis is assigned to each core region.

[0002] The present invention also relates to a laser system using such optical fibers. Background Technology

[0003] Optical fibers with various configurations are known from existing technologies. Optical fiber is a line used to transmit light. The most common type of optical fiber is a dielectric fiber composed of concentric layers. At the center of the cross-section is the light-guiding core region, which is surrounded by a cladding region with a lower refractive index than the core region. In commercial optical fiber cables, a plastic protective layer is also provided around the cladding region. Depending on the application, the core region has a diameter ranging from a few micrometers to over one millimeter. Optical fibers are distinguished by the number of electromagnetic radiation modes that can propagate (which is limited by the core diameter) (single-mode fiber / multimode fiber).

[0004] Recent advancements in fiber lasers have enabled the use of optical fibers as active media (with an active laser core region), providing a reliable solution for high-power lasers. The development of lasers from low-energy to high-power for industrial applications is based on the ability of optical fibers to handle high power. The excellent surface area to effective volume ratio allows for efficient heat dissipation, a key prerequisite for high-power operation. However, confining the optical signal to the fiber's core region results in high optical intensity and interactions between the fiber material and the optical signal. In particular, this leads to uncontrollable nonlinear effects, which severely compromise signal quality.

[0005] Therefore, optical fibers should be designed in a way that reduces nonlinear effects and interactions with fiber materials.

[0006] An optical fiber having multiple guiding core regions is known from WO 2016 / 050898 A1. These guiding core regions are spaced apart from each other along the length of the fiber and are surrounded by a common cladding region when viewed from the cross-section of the fiber. Previously known optical fibers have been used as optical amplifiers in laser systems where a laser beam from a laser source is split into at least two spatially separated partial beams by a beam-splitting element. The partial beams then propagate in the fiber, with each core region guiding its own partial beam. A combining element is provided that coherently superimposes the partial beams after propagation through the fiber to form an output beam. The individual partial beams are amplified in parallel within the fiber; for this purpose, the core regions are doped with rare-earth ions. These are optically pumped by pump radiation guided in the common cladding region. Previously known methods are based on reducing nonlinear effects and interactions with the fiber material by amplifying the individual partial beams at correspondingly reduced intensities. The desired overall performance is achieved only through the coherent superposition of the partial beams in the output beam.

[0007] Coherent combination of partial beams is a highly effective way to increase the output power of fiber laser systems because it overcomes the limitations of single-channel systems. Integrating optical amplifiers for each partial beam within a single fiber with multiple core regions (also known as a multi-core fiber) allows for a very large number of "channels"—optical amplifiers allocated to individual beams. This is because the complexity, size, and cost of a laser system are not proportional to the number of channels. In fact, a laser system with a multi-core fiber having, for example, a 2×2 core region array is virtually indistinguishable from a system with a 10×10 core region array in terms of space, complexity, and cost. This characteristic makes multi-core fibers very attractive for power enhancement in high-power fiber laser systems.

[0008] The coherent combination of partial beams in laser systems of the aforementioned types requires them to have well-defined polarization states relative to each other. For example, when combining two partial beams, it may be desirable that they have the same or mutually orthogonal linear polarizations. If this condition is not met, the power in the output beam (i.e., the efficiency of coherent combination) will decrease. In the context of multi-core fibers, these requirements lead to the use of polarization-maintaining structures. Multi-core fibers typically exhibit non-uniform birefringence modes across the core region due to the inherent mechanical stress field induced by the arrangement of the core regions. This non-uniform birefringence mode can significantly degrade combination performance after only a few tens of centimeters of propagation distance. To prevent this, polarization-maintaining structures should be integrated into the fiber to ensure a uniform birefringence distribution across the core region.

[0009] However, the need for polarization-maintaining (PM) multicore fibers is not limited to the described coherent beam combination, as other applications (such as multibeam frequency conversion) also require well-defined polarization of portions of the beam. The use of polarization-maintaining multicore fibers is also useful for the simple propagation of portions of the beam through any optical system with polarization-sensitive elements (such as isolators, compressors, etc.).

[0010] As known from EP 3 163 339 A1, polarization-maintaining multicore fibers are based on the integration of stress elements in the space between core regions, such that the stress elements act on each core region and expose them to a mechanical stress field that produces birefringence. This is problematic for high-power fiber laser systems because the arrangement of stress elements leads to pump absorption losses. Furthermore, the stress elements in this arrangement are significantly larger than the cores. This also results in damage to pump absorption and is therefore undesirable. The stress elements can be made of, for example, a material different in its coefficient of thermal expansion from the material of the fiber surrounding the stress elements. This generates the desired mechanical stress when the fiber is cooled after drawing.

[0011] In principle, polarization maintenance can also be achieved using a core with a non-circular cross-section (shape birefringence). However, this is undesirable for high-power fiber lasers due to the asymmetry of the emitted beam.

[0012] WO 2014 / 132793 A1 describes a multi-core optical fiber having a cladding and a plurality of light-guiding core elements disposed within the cladding. Each core element has an inner cladding surrounding an actual core. An outer cladding is provided, surrounding the inner cladding and having an average refractive index lower than the average refractive index of the cladding and the inner cladding. A plurality of stress elements are provided within the cladding. The stress elements are arranged such that the effective refractive index for polarized waves of the same LP mode is reduced in light propagating through the core elements. Summary of the Invention

[0013] Against this backdrop, the objective of this invention is to provide an improved multi-core optical fiber with polarization-maintaining properties.

[0014] This invention solves this problem based on the type of optical fiber described above, wherein all stress elements are located outside the signal range, and the mechanical stress field generated by the arrangement of the stress elements causes the main polarization axis to point in the same direction in all core regions.

[0015] When this specification refers to the arrangement and / or shape of different components of an optical fiber, unless otherwise expressly stated, it always refers to a cross-sectional view of the optical fiber.

[0016] This invention proposes a method for realizing polarization-maintaining multi-core optical fibers, i.e., optical fibers with any number and arrangement of light-guiding core regions. A key aspect is that the stress elements are not located between the core regions, but rather outside the common signal region where the core regions reside. The signal region is the adjacent area containing all core regions (and all imaginary connections between adjacent core regions). In other words, the stress elements do not act on individual core regions, but rather on the overall arrangement of the core regions "as a whole." This provides high flexibility when designing the arrangement and shape of the core regions and minimizes adverse factors in terms of pump absorption.

[0017] Each core region of the optical fiber experiences mechanical stress that causes birefringence due to stress elements. As mentioned above, the stress elements, arranged globally outside the signal region, act simultaneously on all core regions. Due to birefringence, each core region, or the optical guide within each core region, exhibits polarization-maintaining behavior. A principal polarization axis is assigned to each core region.

[0018] Specifically, the arrangement of stress elements is not rotationally symmetric. This means that the arrangement of stress elements cannot be mapped onto itself when rotated arbitrarily about the axis of the fiber. In particular, the arrangement is therefore not cylindrically symmetric. Alternatively, the stress elements may have axial symmetry when viewed from the cross-section of the fiber. It is important to generate optical anisotropy through the distribution of stress elements across the cross-section of the fiber. The stress field should (at least) have a single principal direction (excellent direction) across the entire signal range, i.e., the principal direction of the effective stress, such that the principal polarization axes of all core regions point in the same direction.

[0019] In other words, the non-uniform arrangement of stress elements defines the directional distribution of stress in the stress field across the cross-section of the optical fiber (especially in the signal region). The stress elements are arranged such that the direction of the maximum stress (the principal direction of the stress field) and therefore the direction of the principal polarization axis are (essentially) the same in all core regions.

[0020] Stress elements can optionally be arranged in groups along the cross-section of the optical fiber. The distance between stress elements within a group is smaller than the distance between stress elements assigned to different groups. The advantage of grouping several smaller stress elements is that the effect of a single larger stress element can be achieved in terms of the generated mechanical stress field. For example, groups can be arranged on two opposite sides of the signal region, where the connecting line between groups defines the principal axis of the generated mechanical stress field and accordingly specifies the orientation of optical anisotropy in the channel region. Each group can also be formed by an arrangement of stress elements along at least one straight line or along at least one arc segment. All these configurations offer manufacturing advantages and are well-suited for achieving a suitable mechanical stress field in the core region by arranging groups, thereby obtaining well-defined birefringence.

[0021] In one possible design, the stress element is located in the cladding region, which surrounds the signal region. The stress element can be appropriately arranged there to act directly on the entire core region. The refractive index of the stress element material can be lower than that of the material in the cladding region of the fiber. In this way, the stress element can perform the additional function of guiding the pump light in the cladding region. The stress elements can be advantageously distributed across the cross-section of the fiber so that they surround the signal region from all sides. In this way, good overlap between the pump light guided in the cladding region and the core region can be achieved, and thus high pump absorption can be achieved.

[0022] In an alternative embodiment, the stress element is located outside the cladding region (which guides the pump light), i.e., in other regions of the optical fiber surrounding the cladding region. This has the advantages that pump absorption is not impaired by the stress element at all, and a wider range of materials can be selected for the stress element. The refractive index of the stress element does not need to be lower than that of the material in the cladding region (e.g., SiO2).

[0023] In another possible embodiment, the signal region is a rectangular area whose centroid coincides with the longitudinal central axis of the optical fiber. Alternatively, the signal region can be a circular or annular area whose centroid coincides with the longitudinal central axis of the optical fiber. Ultimately, any shape of signal region and arbitrarily arranged core regions within it can be envisioned. For example, the core regions can be arranged in a two-dimensional array, adjacent to each other and on top of each other (at the intersections of an imaginary rectangular grid). The core regions can also be arranged on the circumference of a circle. An unordered arrangement of the core regions is also conceivable.

[0024] Similar to stress elements, the core region preferably has a circular cross-section. A circular cross-section facilitates the fabrication of optical fibers. In the case of the core region, this is advantageous for producing a circular (e.g., Gaussian) beam profile of light propagating along the core region.

[0025] In one possible embodiment, at least two of the stress elements are different from each other in terms of cross-sectional dimensions. This means that stress elements of different sizes can be used to generate the desired mechanical stress field.

[0026] A mechanical stress field can be specifically generated by the appropriate arrangement and / or size of the stress elements, which can be used to specifically achieve a birefringence distribution for a specific "unconventional" polarization state (e.g., azimuth polarization or radial polarization) of the guide light.

[0027] The optical fiber according to the invention can be used as an optical amplifier or as an active element in a laser resonator. For this purpose, at least one core region (advantageously, all core regions) is suitably doped with rare earth ions.

[0028] The present invention also relates to a laser system comprising: at least one laser source emitting a laser beam; a beam splitter splitting the laser beam into at least two spatially separated partial beams; at least one optical fiber of the type described above, in which the partial beams propagate, each core region of the fiber guiding one of the partial beams; and at least one combining element coherently superimposing the partial beams after they have propagated through the optical fiber. Using the polarization-maintaining fiber of the present invention, a high-power laser system based on the principle of coherent combination of partial beams can be advantageously realized, while avoiding the known drawbacks regarding pump absorption in the prior art. Attached Figure Description

[0029] Examples of embodiments of the invention will now be explained in more detail with reference to the accompanying drawings. They illustrate...

[0030] Figure 1 : A schematic cross-sectional view of a first variant of the optical fiber according to the present invention;

[0031] Figure 2 : A schematic cross-sectional view of a second variant of the optical fiber according to the present invention;

[0032] Figure 3 : A schematic cross-sectional view of a third variant of the optical fiber according to the present invention;

[0033] Figure 4 : A schematic cross-sectional view of a fourth variant of the optical fiber according to the present invention;

[0034] Figure 5 : A schematic cross-sectional view of a fifth variant of the optical fiber according to the present invention;

[0035] Figure 6 : A schematic cross-sectional view of a sixth variant of the optical fiber according to the present invention;

[0036] Figure 7 : A schematic cross-sectional view of the seventh variant of the optical fiber according to the present invention;

[0037] Figure 8 : A block diagram of a laser system according to the present invention. Detailed Implementation

[0038] This invention primarily targets optical fibers, specifically multi-core optical fibers used in high-power laser systems. The cross-sectional structure of these multi-core optical fibers can be divided into different regions, as shown in the figure. First, multiple core regions 1 are provided, in which light amplification and / or guidance are performed. These core regions are contained within a signal region 2, defined as the region completely surrounding all core regions and all (imaginary) lines (not shown) connecting the centers of adjacent core regions. In the figure, the signal region 2 is distinguished from other regions by a boundary line 5. Specifically, the boundary line 5 does not intersect any of the aforementioned (imaginary) lines connecting the centers of adjacent core regions. Alternatively, the boundary line 5 can be described as the shortest possible curve completely surrounding all core regions 1. Core regions 1 can be arranged within the signal region 2 as needed. Light guidance can be performed using any means (through step refractive index, refractive index gradient, photonic crystal, bandgap, grazing incidence, etc.). In other words, the implementation of the optical fiber according to the invention is independent of the design of the structure within the signal region 2. The signal region 2, and therefore the core regions 1, are located within the cladding region 3. In the case of active multicore optical fibers, this is the region that guides the pump radiation. Finally, the cladding region 3 may be surrounded by one or more outer layers 4, for example, the outer layers 4 can be used to retain the pump light within the cladding 3 or to impart mechanical stability to the optical fiber.

[0039] It should be noted that the material of the multi-core optical fiber surrounding the core region 1 within the signal region 2 need not differ from the material of the cladding region 3 (outside the stress element 6). The distinction between the signal region 2 and the cladding region 3 is used to illustrate the structure, particularly the arrangement of the stress element 6 outside the signal region 2, and not to distinguish regions of multi-core optical fibers with different material or structural properties. For optical guidance, the core region 1 can be embedded in the same cladding material (e.g., SiO2) as the multi-core optical fiber, and the cladding region 3 can also be made of the same material.

[0040] To achieve birefringence and thus polarization-maintaining behavior in the multi-core optical fiber according to the present invention, a stress element 6 is provided that generates a mechanical stress field across the cross-section of the multi-core optical fiber. As shown, the stress element 6 is never located inside the signal region 2, but outside the signal region 2. The stress element 6 is never located on one of the aforementioned (imaginary) lines connecting the centers of adjacent core regions 1. Therefore, the stress element 6 does not act on individual core regions 1, but on the entire signal region 2. This allows for flexible arrangement of the core regions 1 within the signal region 2. The harmful effects of pump absorption in the signal region 2 are correspondingly reduced.

[0041] exist Figure 1 In the variant shown, the stress elements 6 are arranged in the same manner as the core region 1. The stress elements 6 have the same dimensions and spacing as the core region 1. In other words, the arrangement of the stress elements 6 continues the matrix arrangement of the core region 1. The stress elements 6 are arranged in the cladding region 3.

[0042] In such Figure 2 In the second variant shown, the stress element 6 has a more complex arrangement, thereby maintaining the main parameters of the arrangement of the core region 1 (i.e., size and spacing).

[0043] Figure 3 It is shown that the arrangement of stress elements 6 does not have to maintain the main parameters of the arrangement of core region 1. Instead, as long as they are outside signal region 2 (i.e. not placed in the region defined by region boundary 5), they can be freely distributed in cladding region 3.

[0044] Figure 4 The asymmetric arrangement of stress elements 6, which completely surrounds signal region 2 on all sides, is shown to have dual benefits, as it imparts birefringence to core region 1 and also causes or at least supports the guidance of pump light in cladding region 3. For this purpose, stress elements 6 have a lower refractive index than the substrate of the multi-core fiber in cladding region 3. In this variant, the arrangement of stress elements 6 forms the boundary between cladding region 3 and outer layer 4. In this variant, the arrangement of stress elements 6 does not correspond to the arrangement of core region 1 because a denser packing of stress elements 6 is required to guide pump light with minimal possible loss.

[0045] Although Figures 1 to 4 In the middle, the stress elements 6 are arranged along a straight line, corresponding to the arrangement of the core region 1, but in Figure 5 A completely different arrangement is provided here. The stress elements 6 are arranged along two arc segments positioned opposite each other about the center of the multi-core fiber. This improves the uniformity of birefringence in signal region 2.

[0046] like Figure 6 As shown, the stress elements 6 can also be arranged completely outside the cladding region 3. In this case, they can follow the arrangement parameters of the core region 1. However, following the arrangement parameters of the core region 1 is not absolutely necessary.

[0047] Utilizing the cumulative properties of the stress field, stress elements 6 can be arranged in groups to simulate the mechanical stress generated by a single, larger stress element 6, such as... Figure 7 As shown.

[0048] exist Figures 1 to 7 In this variant, the stress element 6 is not arranged rotationally symmetrically with respect to the center of the cross-section (i.e., the central axis of the optical fiber). The arrangement of the stress element 6 defines the principal axis of the stress field generated in the signal region 2. In this way, the principal polarization axes in all core regions 1 can be aligned in the same direction, i.e., the maximum deviation of the principal polarization axes of each core is ±10°, more preferably ±5°.

[0049] exist Figures 1 to 7 In the diagram, viewed from various cross-sections, the arrangement of stress elements 6 is axially symmetric with respect to at least one axis. Figures 1 to 7 In this structure, there are two mutually perpendicular axes of symmetry (horizontal and vertical in this case), each passing through the center of the cross-section (where the central axis of the optical fiber is located). It can be seen that the stress element is located in a direction perpendicular to one of the axes of symmetry (in... Figures 1 to 6 In the horizontal direction, in Figure 7 In the vertical direction, it is more accurate than in the direction that is again perpendicular to the axis of symmetry. Figures 1 to 6 In the vertical direction, in Figure 7 The distance from this axis of symmetry is greater in the horizontal direction. In each case, a preferred direction for maximum stress (i.e., optical anisotropy throughout the signal region 2) is defined, which makes the principal polarization axes of all core regions 1 achieve the desired identical orientation.

[0050] It should be noted that, Figures 1 to 7 The cross-sectional structure shown and otherwise described in this specification generally remains unchanged along the longitudinal range of the optical fiber or multi-core optical fiber.

[0051] Figure 8 The laser system shown includes a laser source 10 that generates a laser beam E. This is fed to a beam splitter 11, which splits the laser beam E into several spatially separated partial beams T. It is also conceivable that the laser source 10 has already generated several laser beams. These partial beams T propagate through an optical fiber 12, which is designed to be a multi-core fiber, such as... Figures 1 to 7 As shown, each core region 1 of the optical fiber carries a portion of the laser beam T. Light P from the pump source 13 is coupled into the cladding region 3 of the optical fiber 12, allowing it to be absorbed in the core region 1. Thus, the laser beam is amplified in each portion of the beam T. Furthermore, a combining element 14 is provided, which coherently or incoherently superimposes the amplified portion beams T into an output beam A. In this way, a high-power laser system can be realized using the optical fiber 12 according to the invention.

Claims

1. An optical fiber having a plurality of light-guiding core regions (1) extending spaced apart from each other along the longitudinal direction of the optical fiber and all located within adjacent signal regions (2) as viewed from a cross-section of the optical fiber, the signal regions (2) being completely surrounded by cladding regions (3), wherein, The optical fiber includes a stress element (6) designed to generate a mechanical stress field in the optical fiber, wherein each core region (1) is subjected to mechanical stress by the stress element (6) that causes birefringence and thus polarization-maintaining behavior, wherein a principal polarization axis is assigned to each core region (1). The feature is that all the stress elements (6) are located outside the signal region (2), wherein the mechanical stress field generated by the arrangement of the stress elements (6) causes the main polarization axis in all core regions (1) to point in the same direction.

2. The optical fiber according to claim 1, wherein, The stress element (6) is located in the cladding region (3).

3. The optical fiber according to claim 2, wherein, The material of the stress element (6) has a lower refractive index than the material of the optical fiber in the cladding region (3).

4. The optical fiber according to claim 1, wherein, The stress element (6) is located outside the cladding region (3).

5. The optical fiber according to any one of claims 1 to 4, wherein, From the cross-section of the optical fiber, the signal region (2) is a rectangular area in which the center of the cross-section coincides with the longitudinal central axis of the optical fiber.

6. The optical fiber according to any one of claims 1 to 4, wherein, From the cross-section of the optical fiber, the signal region (2) is a circular or annular area whose cross-section center coincides with the longitudinal central axis of the optical fiber.

7. The optical fiber according to any one of claims 1 to 6, wherein, The stress elements (6) are arranged in groups of two or more, each stress element (6) being distributed on the cross-section of the optical fiber.

8. The optical fiber according to claim 7, wherein, The groups are arranged on opposite sides of the signal area (2).

9. The optical fiber according to claim 7 or 8, wherein, Each group is formed by arranging stress elements (6) along at least one straight line.

10. The optical fiber according to claim 7 or 8, wherein, Each group is formed by arranging stress elements (6) along at least one arc segment.

11. The optical fiber according to any one of claims 1 to 10, wherein, Each of the stress elements (6) has a circular cross-section.

12. The optical fiber according to any one of claims 1 to 11, wherein, The stress element (6) is distributed on the cross-section of the optical fiber such that the stress element surrounds the signal region (2) on all sides.

13. The optical fiber according to any one of claims 1 to 12, wherein, At least two of the stress elements (6) are different from each other in terms of cross-sectional dimensions.

14. The optical fiber according to any one of claims 1 to 13, wherein, At least one, preferably all, of the core regions (1) are doped with rare earth ions.

15. A laser system, comprising: - A laser source (10) emits a laser beam (E). - Beam splitting element (11), said beam splitting element (11) is designed to split the laser beam (E) into at least two spatially separated partial beams (T), - At least one optical fiber (12) according to any one of claims 1 to 13, through which the partial beam (T) propagates, wherein the core regions (1) of the optical fiber (12) are all designed to guide one of the partial beams (T), and - At least one combining element (14) is designed to coherently or incoherently superimpose the partial beam (T) after it has propagated through the optical fiber (12).

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