Polarization-maintaining multi-core optical waveguide

Abstract

The present invention relates to an optical waveguide having a plurality of optical waveguide core regions (1) extending spaced apart from one another along the longitudinal extension of the optical waveguide, the optical waveguide core regions (1) all being located within a continuous signal region (2) completely surrounded by a cladding region (3) on the side of the optical waveguide core regions (1) in a cross-section of the optical waveguide, the optical waveguide including a stress element (6) designed to generate a mechanical stress field within the optical waveguide, each core region (1) being subjected by the stress element (6) to a mechanical stress that results in birefringence and thus polarization-maintaining behavior, each core region (1) being associated with one principal polarization axis, and the stress element (6) being entirely located outside the signal region (2). It is an object of the present invention to provide an improved multi-core optical waveguide having polarization-maintaining properties compared to the prior art. For this purpose, the present invention proposes that the stress elements (6) are distributed over the cross section of the optical waveguide in groups of two or more stress elements (6), and that the stress elements (6) have an asymmetric, i.e., non-rotationally symmetric, arrangement in the cross section of the optical waveguide, such that the main polarization axes point in the same direction in all core regions (1). Furthermore, the present invention also relates to a laser system using such an optical waveguide.

Description

[Technical Field]

[0001] The present invention relates to an optical waveguide having a plurality of optical waveguide core regions extending spaced apart from one another along the longitudinal extension of the optical waveguide, which optical waveguide core regions are all located within a continuous signal region, which is completely surrounded by a cladding region on the side of the optical waveguide core region, as viewed in a cross section of the optical waveguide, and which optical waveguide includes a stress element designed to generate a mechanical stress field within the optical waveguide, and each core region is exposed by the stress element to a mechanical stress that results in birefringence and thus polarization-maintaining behavior, and each core region is associated with one main polarization axis.

[0002] Furthermore, the present invention relates to laser systems that use such optical waveguides. [Background technology]

[0003] Optical waveguides are known in various configurations from the prior art. Optical waveguides are lines for transmitting light. The most well-known optical waveguides are dielectric waveguides, which are made up of several concentrically arranged layers. The central cross-section contains a core region for guiding light, which is surrounded by a cladding region with a lower refractive index than the core region. Commercially available optical waveguides also contain a protective plastic layer surrounding the cladding region. Depending on the application, the core region has a diameter ranging from a few microns to over 1 mm. Optical waveguides are distinguished, inter alia, by the number of propagating modes of optical electromagnetic radiation (single-mode fiber / multimode fiber), which is limited by the core diameter.

[0004] Recent developments in fiber lasers have led to the concept of reliable high-power lasers using optical waveguides as the active medium (with the laser-active core region). The evolution from low-energy lasers to high-power lasers for industrial applications is based on the ability of optical waveguides to handle high powers. The extremely favorable ratio of surface area to active volume allows for efficient heat dissipation, which is an excellent prerequisite for high-power operation. However, confining the optical signal to the core region of the optical waveguide results in high optical intensities and further interactions between the optical waveguide material and the optical signal. This also leads to nonlinear effects that are particularly difficult to control. This significantly impairs the signal quality.

[0005] Therefore, it is desirable that the optical waveguide be designed to reduce nonlinear effects and interactions with the fiber material.

[0006] Patent Document 1 discloses an optical waveguide having a plurality of optical waveguide core regions extending spaced apart from one another along the longitudinal extension of the optical waveguide, the optical waveguide core regions being surrounded by a common cladding region in a cross-section of the optical waveguide. The 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 a splitting element. Each partial beam propagates through the optical waveguide, with each core region guiding one partial beam. A combining element is provided, configured to coherently superimpose the partial beams after propagation through the optical waveguide to form an output beam. The individual partial beams are amplified in parallel within the optical waveguide; for this purpose, the core regions are doped with rare-earth ions. The core regions are optically pumped by pump radiation guided in the common cladding region. Known approaches are based on the fact that the amplification of the individual partial beams is carried out at a correspondingly reduced intensity, thereby reducing nonlinear effects and interactions with the fiber material, and the desired total power is only achieved in the output beam by coherently superimposing the partial beams.

[0007] Coherent combination of partial beams is a highly efficient method for scaling the output energy of fiber laser systems because it overcomes the limitations of single-channel systems. By integrating optical amplifiers for individual partial beams in a single optical waveguide with multiple core regions (also called a multicore fiber), a very large number of “channels,” i.e., optical amplifiers associated with each partial beam, can be realized. This is because the complexity, size, and cost of the laser system do not scale with the number of channels. In fact, there is almost no significant difference in the space requirements, complexity, and cost between a laser system with a multicore fiber having, for example, a 2 × 2 core region arrangement and a system having a 10 × 10 core region arrangement. These properties make multicore fibers very attractive for power scaling of high-power fiber laser systems.

[0008] Coherent combination of partial beams in laser systems of the above-mentioned type requires that these partial beams have well-defined polarization states relative to one another. When combining two partial beams, it may be desirable for them to have identical or mutually orthogonal linear polarizations, for example. If this condition is not met, the power in the output beam, i.e., the efficiency of the coherent combination, will decrease. In the context of multicore fibers, this requirement leads to the use of polarization-maintaining structures. Multicore fibers generally have a non-uniform birefringence pattern across the core region, which is due to intrinsic mechanical stress fields induced by the layout of the core region. Such a non-uniform birefringence pattern can significantly impair the performance of the combination even after a propagation distance of several tens of centimeters. To avoid this, polarization-maintaining structures should be integrated into the optical waveguide to ensure a uniform birefringence profile across the core region.

[0009] It should be noted that the demand for polarization-maintaining (PM) multicore fibers is not limited to the coherent beam combining described above, since other applications, such as multibeam frequency conversion, also require well-defined polarizations of the partial beams. The use of PM multicore fibers is also beneficial for the simple propagation of partial beams through any optical system containing polarization-sensitive elements (isolators, compressors, etc.).

[0010] Patent Document 2 discloses a PM multicore fiber based on a stress element integrated into the intermediate space between core regions, which acts on the individual core regions and exposes them to a mechanical stress field that generates birefringence. This is problematic for high-power fiber laser systems because the placement of the stress element results in losses in pump absorption. Furthermore, such a placement results in the stress element being significantly larger than the core. This also leads to impaired pump absorption and is therefore undesirable. The stress element may be made of a material that differs, for example, in terms of its thermal expansion coefficient, from the material of the optical waveguide surrounding it. This allows the desired mechanical stress to be generated during cooling of the optical waveguide after drawing.

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

[0012] Patent Document 3 describes a multicore fiber having a cladding and multiple optical waveguide core elements disposed within the cladding. Each core element has an inner cladding layer surrounding the original core. The outer cladding layer surrounds the inner cladding layer and is configured to have an average refractive index lower than the average refractive index of the cladding and the average refractive index of the inner cladding layer. Multiple stress elements are disposed within the cladding. The stress elements are arranged so that the effective refractive index of polarized waves of the same LP mode is reduced in light propagating through the core elements. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] International Publication No. 2016 / 050898 [Patent Document 2] European Patent Application Publication No. 3163339 [Patent Document 3] International Publication No. 2014 / 132793 Summary of the Invention

[0014] SUMMARY OF THE INVENTION In view of this background, it is an object of the present invention to provide an improved multi-core optical waveguide with polarization-maintaining properties.

[0015] The above problem is solved according to the invention, starting from the type of optical waveguide mentioned at the beginning, by the fact that the entire stress element is located outside the signal region and the mechanical stress field formed by the arrangement of the stress element causes the main polarization axis to point in the same direction in all core regions.

[0016] References made herein to the arrangement and / or shape of various elements of an optical waveguide will be made with respect to the cross section of the optical waveguide unless expressly stated otherwise.

[0017] The present invention proposes an approach to realize polarization-maintaining multicore fibers, i.e., optical waveguides with any number and arrangement of optical waveguide core regions. An important aspect is that the stress elements are not located between the core regions, but are located outside the common signal region in which the core regions are located. The signal region is a continuous region in which the entire core regions (and all of the virtual connecting lines between adjacent core regions) are located. In other words, the stress elements do not act on individual core regions, but "globally" on the entire arrangement of core regions. This provides high flexibility in the arrangement and shape of the core regions and minimizes defects in pump absorption.

[0018] Each core region of the optical waveguide is subjected to mechanical stress by a stress element, which induces birefringence. The overall arrangement of stress elements outside the signal region acts simultaneously on the entire core region, as mentioned above. Due to the birefringence, each core region or optical wave guided in each core region has polarization-maintaining behavior. Each core region is associated with one main polarization axis.

[0019] The arrangement of the stress elements is in particular non-rotationally symmetric. This means that it is not possible to map the arrangement of the stress elements itself with respect to any rotation around the axis of the optical waveguide. In other words, the arrangement is in particular non-cylindrically symmetric. Optionally, the stress elements can have axial symmetry when viewed in the cross section of the optical waveguide. It is important that the distribution of the stress elements across the cross section of the optical waveguide results in optical anisotropy. The stress field has a single dominant direction, i.e., the main direction of the effective stress, in (at least) the entire signal region, in particular so that the main polarization axes of all core regions point in the same direction.

[0020] In other words, the non-uniform arrangement of the stress elements defines a directional distribution of stress in the stress field across the cross section of the optical waveguide, particularly in the signal region, where the stress elements are arranged such that the direction of maximum stress (the main direction of the stress field) and therefore the direction of the main polarization axis is (substantially) the same over the entire core region.

[0021] The stress elements are optionally distributed in groups across the cross section of the optical waveguide. The distance between stress elements in one group is smaller than the distance between stress elements in different groups. The combination of several small stress elements in one group has the advantage that the effect of individual large stress elements can be achieved with respect to the generated mechanical stress field. For example, the groups can be arranged on two opposite sides of the signal region, with the connecting lines between the groups defining the dominant axes of the generated mechanical stress field and correspondingly setting the orientation of the optical anisotropy in the channel region. The groups can also be formed by arranging the stress elements 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 region due to the group arrangement.

[0022] In one possible configuration, the stress element is located in the cladding region, which surrounds the signal region. The stress element can be advantageously positioned at this location so that it directly acts on the entire core region. The material of the stress element can have a lower refractive index than the material of the optical waveguide in the cladding region. In this way, the stress element can perform an additional function in guiding the pump light in the cladding region. In this case, the stress element can be advantageously distributed across the cross section of the optical waveguide so as to surround the signal region on all sides. This allows for good overlap of the pump light guided in the cladding region with the core region, thereby achieving high pump absorption.

[0023] In one alternative configuration, the stress element is located outside the cladding region (which guides the pumping light), i.e., in a separate region of the optical waveguide that surrounds the cladding region. This has the advantage that the pumping absorption by the stress element is not impaired at all, and a larger material choice for the stress element is possible. The refractive index of the stress element does not need to be lower than the refractive index of the material (e.g., SiO2) in the cladding region.

[0024] In another possible configuration, the signal region is a rectangular region with a centroid of its surface coinciding with the central longitudinal axis of the optical waveguide. Alternatively, the signal region may be a circular or toric region whose centroid of its surface also coincides with the central longitudinal axis of the optical waveguide. Finally, any arbitrary shape of the signal region and any arbitrary arrangement of the core regions is possible. For example, the core regions can be arranged front to back and above each other (at the intersections of an imaginary rectangular grid) in a two-dimensional array. Similarly, the core regions may be arranged circumferentially. Irregular arrangements of the core regions are also possible.

[0025] The core region, like the stress element, preferably has a circular cross section, which allows for simple manufacturing of the optical waveguide, and the core region accordingly preferably produces a circular (e.g., Gaussian) beam profile of the light propagating along the core region.

[0026] In one possible configuration, at least two of the stress elements differ from one another in terms of cross-sectional dimensions, i.e., stress elements of different dimensions can be used to create the desired mechanical stress field.

[0027] By suitable arrangement and / or dimensions of the stress elements, a tailored mechanical stress field can be created, which results in a tailored birefringence pattern for a particular "exogenous" polarization state of the guided light (e.g., azimuthal or radial polarization).

[0028] The optical waveguide according to the invention can also be used as an optical amplifier or as the active element of a laser resonator, for which at least one of the core regions, preferably all of the core regions, is advantageously doped with rare-earth ions.

[0029] The present invention also relates to a laser system comprising at least one laser source emitting a laser beam, a splitting element configured to split 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, the core regions of which are designed to guide one of the partial beams, and at least one combining element designed to coherently superimpose the partial beams after propagation through the optical waveguide. The polarization-maintaining optical waveguide of the present invention advantageously makes it possible to realize high-power laser systems based on the principle of coherent combination of partial beams, in particular without the disadvantages known from the prior art regarding pump absorption.

[0030] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the accompanying drawings, in which: FIG. [Brief explanation of the drawings]

[0031] [Figure 1] 1 is a schematic cross-sectional view of a first variant of an optical waveguide according to the invention; [Figure 2] 3 is a schematic cross-sectional view of a second variant of the optical waveguide according to the invention; FIG. [Figure 3] 5 is a schematic cross-sectional view of a third variant of an optical waveguide according to the invention; FIG. [Figure 4] 5 is a schematic cross-sectional view of a fourth variant of the optical waveguide according to the invention; FIG. [Figure 5] 10 is a schematic cross-sectional view showing a fifth variant of the optical waveguide according to the invention; FIG. [Figure 6] 10 is a schematic cross-sectional view showing a sixth variant of the optical waveguide according to the invention; FIG. [Figure 7] 10 is a schematic cross-sectional view showing a seventh variant of the optical waveguide according to the invention; FIG. [Figure 8] 1 is a schematic block diagram of a laser system according to the present invention; FIG. DETAILED DESCRIPTION OF THE INVENTION

[0032] The present invention is particularly directed to an optical waveguide, i.e., a multicore fiber, for use in high-power laser systems. The cross-sectional structure of the multicore fiber can be divided into different regions, as shown in the figure. First, multiple core regions 1 are provided in which light is amplified and / or guided. The multiple core regions 1 are contained within a signal region 2, which is defined as a region that completely surrounds all of the core regions and all (imaginary) lines (not shown) connecting the centers of directly adjacent core regions. In the figure, 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 with any of the (imaginary) lines connecting the centers of directly adjacent core regions. It can also be said that the boundary line 5 is the shortest possible curve that completely defines the entire core region 2. The core regions 1 can be positioned anywhere within the signal region 2. Optical guiding can be achieved using any means (e.g., step index, refractive index gradient, photonic crystal, band gap, strip-like injection, etc.). In other words, the realization of the optical waveguide according to the invention is independent of the shape of the structures in the signal region 2. The signal region 2 and thus the core region 1 are located in the cladding region 3. In the case of an active multicore fiber, this is the region in which the pump beam is guided. Finally, the cladding region 3 may be surrounded by one or more outer layers 4, which can be used, for example, to keep the pump light inside the cladding 3 or to provide mechanical stability to the fiber.

[0033] It is noted here that the material surrounding the core region 1 of the multicore fiber inside the signal region 2 may not be different from the material in the cladding region 3 (outside the stress element 6). The distinction between signal region 2 and cladding region 3 is used to describe the structure, in particular with regard to the placement of the stress element 6 outside the signal region 2, and is not used to define between regions of the multicore fiber having different material or structural properties. In an optical guiding sense, the core region 1 may be embedded in the same cladding material (e.g. SiO2) from which the cladding region 3 of the multicore fiber is formed.

[0034] In order to achieve birefringence and thus polarization-maintaining behavior in the multicore fiber according to the invention, a stress element 6 is provided which generates a mechanical stress field across the cross section of the multicore fiber. As can be seen from the figure, the stress element 6 is not located within the signal region 2 but outside the signal region 2. The stress element 6 is not located on one of the aforementioned (imaginary) lines connecting the centers of adjacent core regions 1. The stress element 6 therefore acts on the entire signal region 2 rather than on individual core regions 1. This allows for a flexible positioning of the core regions 1 within the signal region 2. Damage to the pump absorption in the signal region 2 is correspondingly minimal.

[0035] 1, the stress elements 6 are arranged in the same manner as the core region 1. The stress elements 6 have the same dimensions and the same mutual spacing as the core region 1. In other words, the stress elements 6 are arranged so as to continue the matrix-like arrangement of the core region 1. The stress elements 6 are arranged within the cladding region 3.

[0036] In a second variant shown in FIG. 2, the stress elements 6 have a more complex arrangement, whereby the main parameters of the arrangement of the core region 1 (i.e. dimensions and spacing) remain maintained.

[0037] Figure 3 shows that the placement of the stress elements 6 does not need to maintain the main parameters of the placement of the core region 1, and they may be freely distributed within the cladding region 3 as long as they are located outside the signal region 2, i.e., not within the region defined by the region boundary 5.

[0038] 4 shows that an asymmetric arrangement of the stress elements 6, in which they completely surround the signal region 2 on all sides, can be used doubly by providing birefringence to the core region 1 and also causing or at least assisting in guiding the pump light in the cladding region 3. For this purpose, the stress elements 6 have a lower refractive index than the base material of the multicore fiber in the cladding region 3. In this variant, the arrangement of the stress elements 6 forms the boundary between the cladding region 3 and the outer layer 4. In this variant, the arrangement of the stress elements 6 does not correspond to that of the core region 1, because the stress elements 6 need to be packed more densely in order to guide the pump light with as few losses as possible.

[0039] 1 to 4, the stress elements 6 are arranged along a straight line corresponding to the arrangement of the core regions 1, whereas a completely different arrangement is defined in Fig. 5. In Fig. 5, the stress elements 6 are arranged along two arc segments that face each other with respect to the center of the multicore fiber. This can improve the uniformity of birefringence within the signal region 2.

[0040] As shown in Figure 6, the stress element 6 can also be placed completely outside the cladding region 3. In this case, the stress element 6 can follow the placement parameters of the core region 1. However, this is not necessary.

[0041] Taking advantage of the additive properties of the stress field, stress elements 6 can be arranged in groups to simulate the mechanical stress created by a single large stress element 6, as shown in Figure 7.

[0042] 1 to 7, the stress elements 6 are not arranged rotationally symmetrically with respect to the center of the cross section, i.e., the central axis of the optical waveguide. The arrangement of the stress elements 6 defines the dominant axis of the stress field formed in the signal region 2. It is therefore possible to achieve that the main polarization axes are oriented in the same direction in all core regions 1, i.e., the maximum deviation of the main polarization axes per core is ±10°, more preferably ±5°.

[0043] 1 to 7, the arrangement of the stress elements 6 is axially symmetrical with respect to at least one axis when viewed in the cross section shown. In FIGS. 1 to 7, these are two mutually perpendicular (here horizontal and vertical) axes of symmetry that extend through the center of the cross section (where the central axis of the optical waveguide is located). As can be seen, the stress elements are spaced farther from one of the axes of symmetry in a direction perpendicular to the axis of symmetry (horizontal in FIGS. 1 to 6, vertical in FIG. 7) than in a direction perpendicular to the axis of symmetry (vertical in FIGS. 1 to 6, horizontal in FIG. 7). In each case, a preferred direction of maximum stress, i.e., an optical anisotropy in the entire signal region 2, is defined, thereby obtaining the same desired orientation of the main polarization axis for all core regions 1.

[0044] It is further noted that the cross-sectional structures shown in Figures 1 to 7 and otherwise described herein continue substantially unchanged along the longitudinal extension of the optical waveguide or multicore fiber, respectively.

[0045] The laser system shown in FIG. 8 comprises a laser source 10 that generates a laser beam E. The laser beam E is fed to a splitting element 11, which splits the laser beam E into a plurality of spatially distinct partial beams T. It is also possible for the laser source 10 to already generate a plurality of laser beams. These partial beams T propagate through an optical waveguide 12, which is configured as a multicore fiber as shown in FIGS. 1 to 7. In this case, each core region 1 of the optical waveguide guides one partial beam T. Light P from a pumping light source 13 is coupled into the cladding region 3 of the optical waveguide 12, whereby the light P can be absorbed in the core regions 1. In this case, the laser beam is amplified in each partial beam T. Furthermore, a combining element 14 is provided, which coherently or incoherently superimposes the amplified partial beams T in an output beam A. In this way, a high-power laser system can be realized using the optical waveguide 12 according to the invention.

Claims

1. An optical waveguide, The optical waveguide has a plurality of optical waveguide core regions (1) extending along the longitudinal extension of the optical waveguide at a distance from one another, and the optical waveguide core regions (1) are all located within a continuous signal region (2) completely surrounded by a cladding region (3) on the side of the optical waveguide core regions when viewed in cross section of the optical waveguide; the optical waveguide comprises a stress element (6) designed to create a mechanical stress field within the optical waveguide, Each core region (1) is subjected by said stress element (6) to a mechanical stress that results in birefringence and thus polarization-maintaining behavior, Each of the core regions (1) is associated with one main polarization axis. In the optical waveguide, The stress element (6) is entirely located outside the signal region (2), and the mechanical stress field formed by the arrangement of the stress element (6) causes the main polarization axis to point in the same direction in all core regions (1). An optical waveguide comprising:

2. 2. The optical waveguide of claim 1, wherein the stress element (6) is located within the cladding region (3).

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

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

5. 5. The optical waveguide according to claim 1, wherein the signal region (2) is a rectangular region when viewed in cross section of the optical waveguide, and the center of gravity of the region coincides with the longitudinal central axis of the optical waveguide.

6. 5. The optical waveguide according to claim 1, wherein the signal region (2) is a circular or annular region when viewed in cross section of the optical waveguide, and the center of gravity of the region coincides with the longitudinal central axis of the optical waveguide.

7. 7. The optical waveguide according to claim 1, wherein the stress elements (6) are distributed over the cross section of the optical waveguide in groups of two or more stress elements (6).

8. 8. The optical waveguide according to claim 7, wherein the groups are arranged on two mutually opposite sides of the signal region (2).

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

10. 9. An optical waveguide according to claim 7 or 8, wherein each group is formed by an arrangement of stress elements (6) aligned along at least one arc segment.

11. 11. The optical waveguide according to claim 1, wherein the stress elements (6) each have a circular cross section.

12. 12. The optical waveguide according to claim 1, wherein the stress elements (6) are distributed over the cross section of the optical waveguide so as to surround the signal region (2) on all sides.

13. 13. The optical waveguide according to claim 1, wherein at least two of the stress elements (6) differ from one another in terms of cross-sectional dimensions.

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

15. 1. A laser system comprising: a laser source (10) emitting a laser beam (E); a splitting element (11) designed to split the laser beam (E) into at least two spatially separated partial beams (T); at least one optical waveguide (12) according to any one of claims 1 to 13, through which each partial beam (T) propagates, the core region (1) of said optical waveguide (12) being designed to guide each of the partial beams (T); at least one combining element (14) designed to superimpose each partial beam (T) coherently or incoherently after propagation through said optical waveguide (12); A laser system comprising:

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