Polarisation-maintaining multi-core optical fiber

Abstract

The invention relates to an optical fiber having a plurality of light-guiding core regions (1) which extend, mutually spaced, along the longitudinal extension of the optical fiber and which are all located, viewed in the cross-section of the optical fiber, inside a contiguous signal region (2) which is completely enclosed by a cladding region (3), wherein: the optical fiber contains stress elements (6) which are designed to produce a mechanical stress field in the optical fiber; each core region (1) is subjected, by the stress elements (6), to a double refraction and therefore to mechanical stress causing polarisation-maintaining behavior; one main polarisation axis is associated with each of the core regions (1); and all the stress elements (6) are located outside of the signal region (2). The aim of the invention is to provide a multi-core optical fiber having polarisation-maintaining properties which is improved in comparison to the prior art. To achieve this aim, the stress elements (6) are distributed in groups each composed of two or more stress elements (6) across the cross-section of the optical fiber, wherein the stress elements (6), viewed in the cross-section of the optical fiber, are arranged asymmetrically, specifically non-rotationally-symmetrically, which causes the main polarisation axes in all core regions (1) to point in the same direction. In addition, the invention relates to a laser system which uses an optical fiber of this kind.

Description

[0001] Multi-core optical fiber with polarization maintenance

[0002] The invention relates to an optical waveguide having a plurality of light-guiding core regions which extend at a distance from one another along the longitudinal extent of the optical waveguide and which, viewed in the cross-section of the optical waveguide, are all located within a contiguous signal region which, in turn, is completely enclosed by a cladding region, wherein the optical waveguide contains stress elements which are designed to generate a mechanical stress field in the optical waveguide, wherein each core region is exposed to a mechanical stress which causes birefringence and thus polarization-maintaining behavior by the stress elements, wherein the core regions are each assigned a main polarization axis.

[0003] Furthermore, the invention relates to a laser system using such an optical waveguide.

[0004] Optical fibers are known in various designs from the prior art. Optical fibers are cables for transmitting light. The most common optical fibers are dielectric waveguides constructed from concentric layers. At the center of the cross-section lies a light-guiding core region, enclosed by a cladding region with a lower refractive index than the core region. Commercial optical fibers also feature protective plastic layers surrounding the cladding region. Depending on the application, the core region has a diameter of a few micrometers to over a millimeter. Optical fibers are differentiated, among other things, by the number of propagable modes of electromagnetic radiation, which is limited by the core diameter (single-mode versus multi-mode fibers).

[0005] Recent advancements in fiber lasers have resulted in the use of optical fibers as the active medium (with a laser-active core region) as a reliable concept for high-power lasers. The development of low-energy lasers into high-power lasers for industrial applications is based on the ability of optical fibers to handle high power. The excellent surface-to-active-volume ratio allows for efficient heat dissipation. This is an excellent prerequisite for high-power operation. However, confining the light signal to the core region of the optical fiber leads to high light intensity and interactions between the optical fiber material and the light signal. This creates nonlinear effects that are particularly difficult to control. This severely impairs signal quality.

[0006] Optical fibers should therefore be designed in such a way that nonlinear effects and interactions with the fiber material are reduced.

[0007] WO 2016 / 050898 A1 discloses an optical waveguide comprising a plurality of light-guiding core regions spaced apart from one another along the longitudinal extent of the optical waveguide, which, viewed in the cross-section of the optical waveguide, are enclosed by a common cladding region. The previously known optical waveguide is used as an optical amplifier in a laser system in which a laser beam from a laser source is split into at least two spatially separated partial beams by means of a splitting element. The partial beams then propagate through the optical waveguide, with the core regions each guiding a partial beam. A combining element is provided that coherently superimposes the partial beams to form an output beam after propagation through the optical waveguide. The individual partial beams are amplified in parallel in the optical waveguide, for which purpose the core regions are doped with rare earth ions.These are optically pumped by pump radiation guided through the common cladding region. This previously known approach is based on reducing nonlinear effects and interactions with the fiber material by amplifying the individual beams at correspondingly reduced intensities. The desired overall power is only achieved through the coherent superposition of the beams in the output beam.

[0008] The coherent combination of the sub-beams is a very efficient way to scale the output power of fiber laser systems, as it makes it possible to overcome the limitations of single-channel systems. The integration of the optical amplifiers for the individual sub-beams in a single optical fiber with a plurality of core regions (also called multi-core fiber) enables the realization of a very high number of "channels," i.e., optical amplifiers assigned to the sub-beams. The reason for this is that the complexity, size, and cost of the laser system do not scale with the number of channels. In fact, there is hardly any relevant difference in terms of space requirements, complexity, and cost between a laser system with a multi-core fiber with an arrangement of, for example, 2x2 core regions and a system with an arrangement of 10x10 core regions.This property makes multi-core fibers very attractive for power scaling of high-power fiber laser systems.

[0009] The coherent combination of the partial beams in laser systems of the type described above requires that they have a well-defined polarization state with respect to one another. When combining two partial beams, for example, it may be desirable for them to have either identical or mutually orthogonal linear polarization. If this condition is not met, the power in the output beam, i.e., the efficiency of the coherent combination, decreases. In the context of a multi-core fiber, these requirements result in the use of polarization-maintaining structures. A multi-core fiber typically exhibits an inhomogeneous birefringence pattern across the core regions, which is due to the intrinsic mechanical stress field induced by the arrangement of the core regions. This inhomogeneous birefringence pattern can significantly impair the combination performance after just a few tens of centimeters of propagation.To prevent this, a polarization-maintaining structure should be integrated into the optical fiber in such a way that it ensures a homogeneous birefringence profile across the core regions.

[0010] 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 the sub-beams. The use of PM multicore fibers is also beneficial for the simple propagation of sub-beams through any optical system with polarization-sensitive elements (such as isolators, compressors, etc.).

[0011] EP 3 163 339 A1 discloses a PM multi-core fiber based on the integration of stress elements in the spaces between the core regions, such that the stress elements act on individual core regions and subject them to a mechanical stress field that generates birefringence. This is problematic for high-power fiber laser systems because the arrangement of the stress elements leads to a loss of pump absorption. Furthermore, in this arrangement, the stress elements are significantly larger than the cores. This also impairs pump absorption and is therefore undesirable. The stress elements can, for example, be made of a material whose thermal expansion coefficient differs from the material of the optical fiber surrounding the stress elements. This creates the desired mechanical stresses when the optical fiber cools after drawing.

[0012] In principle, polarization maintenance can also be achieved with cores that are not circular in cross-section (shape birefringence). However, this is also undesirable for high-power fiber lasers due to the asymmetry of the emitted beam.

[0013] WO 2014 / 132793 A1 describes a multi-core fiber with a cladding and a plurality of light-guiding core elements provided within the cladding. Each core element has an inner cladding layer surrounding the core itself. An outer cladding layer is provided surrounding the inner cladding layer and has an average refractive index lower than that of the cladding and the inner cladding layer. 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 the light propagating through the core elements.

[0014] Against this background, the object of the invention is to provide an improved multi-core optical waveguide with polarization-maintaining properties.

[0015] The invention achieves this object starting from an optical waveguide of the type specified at the outset in that all voltage elements are located outside the signal area, wherein the mechanical stress field generated by the arrangement of the voltage elements causes the main polarization axes to point in the same direction in all core areas.

[0016] Whenever this description refers to the arrangement and / or shape of the various elements of the optical fiber, this always refers to the cross-sectional view of the optical fiber, unless expressly stated otherwise.

[0017] The invention proposes an approach for realizing a polarization-maintaining multi-core fiber, i.e. an optical waveguide with any number and arrangement of light-guiding core regions. An essential aspect is that the voltage elements are not located between the core regions, but outside the common signal region in which the core regions are located. The signal region is the contiguous region in which all core regions (and also all imaginary connecting lines between adjacent core regions) are located. In other words: the voltage elements do not act on individual core regions, but "globally" on the entire arrangement of the core regions. This offers a high degree of flexibility in designing the arrangement and shape of the core regions and minimizes disadvantages in pump absorption.Each core region of the optical fiber is subjected to a mechanical stress caused by the stress elements, resulting in birefringence. As mentioned above, the overall arrangement of the stress elements outside the signal region affects all core regions simultaneously. Due to the birefringence, each core region, or rather the light guide of each core region, exhibits polarization-maintaining behavior. Each core region is assigned a principal polarization axis.

[0018] In particular, the arrangement of the stress elements is not rotationally symmetric. This means that the arrangement of the stress elements cannot be mapped onto itself for arbitrary rotations around the axis of the optical fiber. In particular, the arrangement is not cylindrically symmetric. Optionally, the stress elements can exhibit axial symmetry when viewed in the cross-section of the optical fiber. The important thing is that optical anisotropy is generated by the distribution of the stress elements across the cross-section of the optical fiber. The stress field should have a single, distinct direction (at least) across the entire signal range, i.e., a 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 uneven arrangement of the stress elements defines the directional distribution of the stress in the stress field across the cross-section of the optical fiber, particularly in the signal region. The stress elements are arranged such that the direction of the maximum stress (the main direction of the stress field) and thus the direction of the main polarization axis is (essentially) the same for all core regions.

[0020] The stress elements can optionally be arranged in groups distributed across the cross-section of the optical fiber. The distances between the stress elements in a group are smaller than the distances between the stress elements assigned to different groups. Combining several smaller stress elements into a group has the advantage of achieving the effect of a single larger stress element with regard to the generated mechanical stress field. For example, the groups can be arranged on two opposite sides of the signal region; the connecting line between the groups defines a distinct axis of the generated mechanical stress field and accordingly specifies the orientation of the optical anisotropy in the channel regions.Each group can also be formed by an arrangement of stress elements arranged along at least one straight line or along at least one circular arc segment. All of these configurations have manufacturing advantages and are well suited to achieving a suitable mechanical stress field and thus a well-defined birefringence in the core regions through the arrangement of the groups.

[0021] In one possible embodiment, the voltage elements are located in the cladding region. The cladding region encloses the signal region. The voltage elements can be expediently arranged there to act directly on the entire core regions. The material of the voltage elements can have a lower refractive index than the material of the optical fiber in the cladding region. In this way, the voltage elements can fulfill an additional function in guiding pump light in the cladding region. The voltage elements can advantageously be distributed across the cross-section of the optical fiber so that they surround the signal region on all sides. This allows good overlap of the pump light guided in the cladding region with the core regions and thus high pump absorption to be achieved.

[0022] In an alternative design, the stress elements are located outside the cladding region (which carries the pump light), i.e. in other regions of the optical fiber that externally surround the cladding region. This has the advantages that the pump absorption is not impaired at all by the stress elements and a wider selection of materials can be used for the stress elements. The refractive index of the stress elements does not need to be lower than the refractive index of the material (e.g. SiO2) in the cladding region. In another possible design, the signal region is a rectangular region whose centroid coincides with the longitudinal center axis of the optical fiber. Alternatively, the signal region can be a circular or annular region whose centroid again coincides with the longitudinal center axis of the optical fiber.Ultimately, any shape of the signal region and any arrangement of the core regions within it are conceivable. For example, the core regions can be arranged side by side or one above the other as a two-dimensional array (at the intersection points of an imaginary rectangular grid). Likewise, the core regions can be arranged on a circumference. A random arrangement of the core regions is also conceivable.

[0023] The core regions, like the voltage elements, advantageously have a circular cross-section. This circular cross-section enables simple production of the optical waveguide. In the core regions, a circular (e.g., Gaussian) beam profile of the light propagating along the core regions is advantageously achieved.

[0024] In one possible design, at least two of the stress elements differ in cross-sectional size. Thus, stress elements of different sizes can be used to generate the desired mechanical stress field.

[0025] By appropriate arrangement and / or size of the stress elements, a mechanical stress field can be specifically generated, through which a birefringence pattern for a specific “exotic” polarization state (e.g. azimuthal or radial polarization) of the guided light can be achieved.

[0026] The optical waveguide 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 of the core regions is advantageously doped with rare earth ions.

[0027] The invention also relates to a laser system comprising at least one laser source emitting a laser beam, a splitting element splitting the laser beam into at least two spatially separated partial beams, at least one optical waveguide of the type described above through which the partial beams propagate, with the core regions of the optical waveguide each guiding one of the partial beams, and at least one combining element coherently superimposing the partial beams after propagation through the optical waveguide. The polarization-maintaining optical waveguide of the invention advantageously allows a high-performance laser system based on the principle of coherent combination of partial beams to be realized, without the disadvantages known from the prior art with regard to pump absorption.

[0028] Embodiments of the invention are explained in more detail below with reference to the drawings. They show:

[0029] Fig. 1 : schematic cross-sectional view of a first variant of an optical waveguide according to the invention;

[0030] Fig. 2: schematic cross-sectional view of a second variant of an optical waveguide according to the invention;

[0031] Fig. 3: schematic cross-sectional view of a third variant of an optical waveguide according to the invention;

[0032] Fig. 4: schematic cross-sectional view of a fourth variant of an optical waveguide according to the invention;

[0033] Fig. 5: schematic cross-sectional view of a fifth variant of an optical waveguide according to the invention;

[0034] Fig. 6: schematic cross-sectional view of a sixth variant of an optical waveguide according to the invention; Fig. 7: schematic cross-sectional view of a seventh variant of an optical waveguide according to the invention;

[0035] Fig. 8: schematic representation of a laser system according to the invention as a block diagram.

[0036] The invention is primarily aimed at optical waveguides, i.e. multi-core fibers for high-power laser systems. The cross-sectional structure of these multi-core fibers can be divided into different regions, as the figures show. First, several core regions 1 are provided in which the amplification and / or guidance of the light takes place. These are contained in a signal region 2, which is defined as the region that completely encloses all core regions as well as all (imaginary) lines (not shown) that connect the centers of immediately adjacent core regions. In the figures, the signal region 2 is shown separated from the other regions by a boundary line 5. In particular, the boundary line 5 does not intersect any of the aforementioned (imaginary) lines that connect the centers of immediately adjacent core regions.One could also say that the boundary line 5 is the shortest possible curve that completely delimits all core regions 2. Within the signal region 2, the core regions 1 can be arranged arbitrarily. The light can be guided by any means (through step index, refractive index gradient, photonic crystals, band gaps, grazing incidence, etc.). In other words: the realization of the optical waveguide according to the invention is independent of the design of the structures within the signal region 2. The signal region 2 and thus the core regions 1 are located within a cladding region 3. In an active multi-core fiber, this is the region in which the pump radiation is guided. Finally, the cladding region 3 can be surrounded by one or more outer layers 4, which can serve, for example, to keep the pump light within the cladding 3 or to impart mechanical stability to the fiber.

[0037] It should be noted that the material surrounding the core regions 1 of the multi-core fiber within the signal region 2 does not have to differ from the material in the cladding region 3 (outside the stress elements 6). The distinction between signal region 2 and cladding region 3 serves to explain the structure, particularly with regard to the arrangement of the stress elements 6 outside the signal region 2, and not to demarcate regions of the multi-core fiber with different material or structural properties. For the purpose of light guidance, the core regions 1 can be embedded in the same cladding material (e.g., SiO2) of the multi-core fiber as the cladding region 3.

[0038] In order to achieve birefringence and thus polarization-maintaining behavior in the multi-core fiber according to the invention, stress elements 6 are provided which generate a mechanical stress field across the cross-section of the multi-core fiber. As can be seen in the figures, the stress elements 6 are never located within the signal region 2, but rather outside it. The stress elements 6 are never located on one of the aforementioned (imaginary) lines that connect the centers of the adjacent core regions 1. Thus, the stress elements 6 do not act on individual core regions 1, but on the entire signal region 2. This enables a flexible arrangement of the core regions 1 within the signal region 2. The impairment of the pump absorption in the signal region 2 is correspondingly low.

[0039] In the variant of Fig. 1, the tension elements 6 are arranged like the core regions 1. The tension elements 6 have the same size and the same spacing from each other as the core regions 1. In other words, the tension elements 6 are arranged such that they continue the matrix-like arrangement of the core regions 1. The tension elements 6 are arranged in the sheath region 3.

[0040] In the second variant, shown in Fig. 2, the tension elements 6 have a more complex arrangement, while the main parameters of the arrangement of the core regions 1 (ie size and spacing) are retained.

[0041] Fig. 3 illustrates that the arrangement of the voltage elements 6 does not have to maintain the main parameters of the arrangement of the core regions 1, but that they can be freely distributed in the cladding region 3 as long as they are located outside the signal region 2, ie are not placed within the region delimited by the region boundary 5.

[0042] Fig. 4 shows that an asymmetric arrangement of the tension elements 6, which completely surrounds the signal region 2 on all sides, can have a dual benefit by imparting birefringence to the core regions 1 and also causing or at least supporting the guidance of pump light in the cladding region 3. For this purpose, the tension elements 6 have a lower refractive index than the base material of the multi-core fiber in the cladding region 3. In this variant, the arrangement of the tension elements

[0043] 6 the boundary between the cladding region 3 and the outer layers 4. In this variant, the arrangement of the voltage elements 6 does not correspond to the arrangement of the core regions 1, since a denser packing of the voltage elements 6 is required in order to guide the pump light with the lowest possible losses.

[0044] While in Figs. 1 to 4 the tension elements 6 are arranged along straight lines, corresponding to the arrangement of the core regions 1, a completely different arrangement is provided in Fig. 5. There, the tension elements 6 are arranged along two circular segments opposite each other with respect to the center of the multi-core fiber. This allows the homogeneity of the birefringence in the signal region 2 to be improved.

[0045] As shown in Fig. 6, the stress elements 6 can also be arranged completely outside the cladding region 3. They can follow the arrangement parameters of the core regions 1. However, the latter is not absolutely necessary.

[0046] By taking advantage of the additive property of the stress fields, the stress elements 6 can be arranged in groups to simulate the mechanical stress generated by a single larger stress element 6, as shown in Fig.

[0047] 7. In the variants of Figs. 1 to 7, the tension elements 6 are 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 tension elements 6 defines a distinct axis of the tension field generated in the signal region 2. This makes it possible to ensure that the main polarization axes are aligned in the same direction in all core regions 1, i.e., with a maximum deviation of the main polarization axis from core to core of ±10°, more preferably ±5°.

[0048] 1 to 7, the arrangement of the stress elements 6, viewed in the cross section shown in each case, is axially symmetrical with respect to at least one axis. In Figs. 1 to 7, there are two mutually perpendicular (here horizontal and vertical) axes of symmetry, each of which runs through the center of the cross section (where the central axis of the optical fiber is located). As can be seen, the stress elements are spaced further from one axis of symmetry in the direction perpendicular to this axis of symmetry (in Figs. 1 to 6 in the horizontal direction, in Fig. 7 in the vertical direction) than in the direction perpendicular to it (in Figs. 1 to 6 in the vertical direction, in Fig. 7 in the horizontal direction). In each case, a preferred direction of the maximum stress, i.e. an optical anisotropy, is defined in the entire signal region 2, which results in the desired identical orientation of the main polarization axis for all core regions 1.

[0049] It should be noted that the cross-sectional structures shown in Figs. 1 to 7 and also explained elsewhere in this description continue essentially invariably along the longitudinal extent of the optical waveguide or the multi-core fiber.

[0050] The laser system shown in Fig. 8 comprises a laser source 10 which generates a laser beam E. This is fed to a splitting element 11 which splits the laser beam E into several spatially separate partial beams T. It would also be conceivable for the laser source 10 to already generate several laser beams. These partial beams T propagate through an optical waveguide 12 which is designed as a multi-core fiber, as shown in Figs. 1 to 7. Each core region 1 of the optical waveguide carries a respective partial beam T. The light P from a pump light source 13 is coupled into the cladding region 3 of the optical waveguide 12 so that it can be absorbed in the core regions 1. The laser radiation is amplified T in each partial beam. Furthermore, a combining element 14 is provided which coherently or incoherently superimposes the amplified partial beams T in an output beam A.

[0051] In this way, a high-power laser system can be realized with the optical waveguide 12 according to the invention.

Claims

Patent claims 1. An optical waveguide comprising a plurality of light-guiding core regions (1) spaced apart from one another along the longitudinal extent of the optical waveguide, which, viewed in cross-section through the optical waveguide, are all located within a coherent signal region (2) which, in turn, is completely enclosed by a cladding region (3), wherein the optical waveguide contains stress elements (6) designed to generate a mechanical stress field in the optical waveguide, wherein each core region (1) is subjected to a mechanical stress by the stress elements (6) causing birefringence and thus polarization-maintaining behavior, wherein a main polarization axis is each assigned to the core regions (1), characterized in that all stress elements (6) are located outside the signal region (2), wherein the mechanical stress field generated by the arrangement of the stress elements (6) causesthat the main polarization axes point in the same direction in all core regions (1 ).

2. Optical waveguide according to claim 1, wherein the Tension elements (6) are located in the jacket area (3).

3. Optical waveguide according to claim 2, wherein the material of the Tension elements (6) have a lower refractive index than the material of the optical waveguide in the cladding region (3).

4. Optical waveguide according to claim 1, wherein the tension elements (6) are located outside the cladding region (3).

5. Optical waveguide according to one of claims 1 to 4, wherein the signal region (2), seen in the cross-section of the optical waveguide, is a rectangular region whose center of gravity coincides with the longitudinal center axis of the optical waveguide.

6. Optical waveguide according to one of claims 1 to 4, wherein the signal region (2), seen in the cross-section of the optical waveguide, is a circular or annular region whose center of gravity coincides with the longitudinal center axis of the optical waveguide.

7. Optical waveguide according to one of claims 1 to 6, wherein the tension elements (6) are arranged in groups of two or more tension elements (6) distributed over the cross section of the optical waveguide.

8. Optical waveguide according to claim 7, wherein the groups are arranged on two opposite sides of the signal area (2).

9. Optical waveguide according to claim 7 or 8, wherein each group is formed by an arrangement of tension elements (6) arranged along at least one straight line.

10. Optical waveguide according to claim 7 or 8, wherein each group is formed by an arrangement of tension elements (6) arranged along at least one circular arc segment.

11. Optical waveguide according to one of claims 1 to 10, wherein the tension elements (6) each have a circular cross-section.

12. Optical waveguide according to one of claims 1 to 11, wherein the voltage elements (6) are distributed over the cross-section of the optical waveguide so that they surround the signal region (2) on all sides.

13. Optical waveguide according to one of claims 1 to 12, wherein at least two of the tension elements (6) differ from one another with regard to cross-sectional size.

14. Optical waveguide according to one of claims 1 to 13, wherein at least one of the core regions (1 ), preferably all core regions (1 ), with is / are doped with rare earth ions.

15. Laser system with a laser source (10) which emits a laser beam (E), a splitting element (11) which is designed to split the laser beam (E) into at least two spatially separated partial beams (T), at least one optical waveguide (12) according to one of claims 1 to 13, through which the partial beams (T) propagate, wherein the core regions (1) of the optical waveguide (12) are each designed to guide one of the partial beams (T), and - at least one combining element (14) which is designed to coherently or incoherently superimpose the partial beams (T) after propagation through the optical waveguide (12).

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