Abstract
The invention relates to an optical waveguide with at least one core region (1) extending along the longitudinal extent of the optical waveguide, and with a first jacket (2) which, viewed in the cross section of the optical waveguide, surrounds the core region (1). The invention further relates to an optical arrangement with such an optical waveguide, and to a method for producing the optical waveguide. The object of the invention is to make available an optical waveguide for high-performance operation, which is improved in relation to the prior art in terms of mode instability. The invention achieves this object by virtue of the fact that the optical waveguide consists of crystalline material at least in the core region (1).
Claims
1. An optical waveguide with at least one core region extending along the longitudinal extent of the optical waveguide, and with a first jacket which, viewed in the cross section of the optical waveguide, surrounds the core region, wherein optical waveguide consists of crystalline material at least in the core region.
2. The optical waveguide as claimed in claim 1, wherein the optical waveguide has a lower refractive index in the region forming the first jacket than it does in the core region.
3. The optical waveguide as claimed in claim 1, wherein the core region is formed by a plurality of channels which extend along the longitudinal extent of the optical waveguide and which, viewed in the cross section of the optical waveguide, are arranged around the core region, wherein the optical waveguide, in the regions forming the channels, has a refractive index that is lower compared to the core region.
4. The optical waveguide as claimed one in claim 1, wherein the crystalline material is yttrium aluminum garnet, sapphire or diamond.
5. The optical waveguide as claimed in claim 1, wherein the optical waveguide, viewed in cross section, has at least one region which is doped preferably with rare earth ions and which preferably at least partially overlaps the core region.
6. The optical waveguide as claimed in claim 1, further comprising a second jacket which, viewed in the cross section of the optical waveguide, surrounds the first jacket, wherein the optical waveguide, in the region forming the second jacket, has a refractive index that is lower than in the region of the first jacket.
7. The optical waveguide as claimed in claim 1, wherein the optical waveguide, in the region of at least one of the first and second jacket, consists of a crystalline or ceramic material or of a glass.
8. The optical waveguide as claimed in claim 1, wherein the core region and at least one of the first and the second jacket each have a rectangular or square cross section.
9. The optical waveguide as claimed in claim 8, wherein the optical waveguide as a whole has a rectangular or square cross section.
10. The optical waveguide as claimed in claim 1, further comprising a plurality of core regions which, viewed in the cross section of the optical waveguide, are spaced apart from one another and arranged at least alongside and over one another.
11. The optical waveguide as claimed in claim 10, wherein the core regions are surrounded by a common first jacket.
12. The optical waveguide as claimed in claim 10, wherein each core region is surrounded by a first jacket assigned only to this core region.
13. The optical waveguide as claimed in claim 1, further comprising an insulation region which, viewed in the cross section of the optical waveguide, is arranged between at least two core regions, wherein the optical waveguide has a reduced thermal conductivity in the insulation region compared to the other regions.
14. The optical waveguide as claimed in claim 1, further comprising at least one cooling element bearing on the surface of the first or second jacket.
15. The optical waveguide as claimed in claim 1, wherein the length of the optical waveguide amounts to at least ten times the Rayleigh length of the light propagating in the optical waveguide.
16. An optical arrangement with a splitting element, which splits an input beam (E) into at least two spatially separate partial beams, at least one optical waveguide with at least two core regions as claimed in claim 1, through which the partial beams propagate, wherein each core region guides a respective partial beam, and at least one combining element, which spatially superposes the partial beams in one output beam (A).
17. The optical arrangement as claimed in claim 16, wherein at least one of the splitting element and the combining element each have a partially reflective element which reflects the radiation of the input beam (E) or output beam (A), respectively, two or more times, wherein the partially reflective element has zones of different reflectivity.
18. A method for producing an optical waveguide as claimed in claim 1, having the following method steps: introduction of at least one depression into a first substrate piece, epitaxial growth of a crystalline material, forming the core region, on the first substrate piece, removal of the crystalline material from the surface of the first substrate piece, such that the crystalline material remains only in the region of the depression, and application of a second substrate piece to the surface of the first substrate piece, and connection of the two substrate pieces such that they together form the first jacket.
19. A method for producing an optical waveguide as claimed in claim 1, having the following method steps: stacking layers of a crystalline material, wherein undoped material and material doped with rare earth ions are arranged alternately in the layered stack, cutting the layered stack transverse to the orientation of the layers, and covering the cut surfaces with undoped crystalline material.
Description
[0038] Illustrative embodiments of the invention are explained in more detail below with reference to the drawings, in which:
[0039] FIG. 1 shows a sectional plan view of an optical waveguide according to the invention with a plurality of cores arranged alongside one another;
[0040] FIG. 2 shows a cross-sectional view of an optical waveguide according to the invention with cores arranged alongside and over one another;
[0041] FIG. 3 shows a cross-sectional view of an optical waveguide according to the invention with an optimized arrangement of the core regions;
[0042] FIG. 4 shows a sectional plan view of an optical waveguide according to the invention with doping arranged outside the cores;
[0043] FIG. 5 shows a sectional plan view of an optical waveguide according to the invention with core regions delimited by channels;
[0044] FIG. 6 shows a cross-sectional view of an optical waveguide according to the invention with cooling elements;
[0045] FIG. 7 shows a cross-sectional view of an optical waveguide according to the invention with thermal insulation regions between the core regions;
[0046] FIG. 8 shows a schematic view of an optical arrangement with an optical waveguide according to the invention.
[0047] FIG. 1 shows an optical waveguide according to the invention in a sectional plan view. Viewed in the cross section, the optical waveguide has four different regions consisting of different materials with different optical properties. Each of the regions extends along the entire longitudinal extent of the optical waveguide.
[0048] The optical waveguide shown in FIG. 1 has four core regions 1 arranged alongside one another. The core regions 1 are surrounded by a common first jacket 2, which has a lower refractive index than the core regions 1. At their center, the core regions 1 each have a region 3 doped with rare earth ions. A second jacket 4, which for its part has a lower refractive index than the first jacket 2, surrounds the arrangement of cores 1 and first jacket 2 and optically shields these from the outside. The length 5 of the optical waveguide is many times greater than the edge lengths of the square or rectangular core regions 1 and jacket regions 2 and 4. The length 5 of the optical waveguide is at least ten times the Rayleigh length of the light propagating in the optical waveguide.
[0049] According to the invention, the core regions 1 consist of crystalline material, for example yttrium aluminum garnet. The jacket regions 2 and 4 also preferably consist of crystalline material. However, this is not a requirement. The jacket regions 2 and 4 can likewise consist of ceramic material or of glass. The second jacket 4 is optional. The optical waveguide can alternatively have an air jacket.
[0050] While FIG. 1 shows a multi-core optical waveguide with a one-dimensional arrangement of the cores 1, FIG. 2 shows a multi-core optical waveguide according to the invention with, viewed in the cross section of the optical waveguide, a two-dimensional arrangement of the cores, in which arrangement the core regions are spaced apart from one another and arranged alongside and over one another.
[0051] In the illustrative embodiments shown in FIGS. 1 and 2, no special measures are taken to thermally shield the core regions 1 from one another. The regions of thermal influence of the cores 1 extend well beyond the doped regions 3, such that thermal coupling between the various core regions 1 cannot be ruled out. The thermal coupling can be influenced in a specific way by the arrangement of the core regions 1 over the cross section of the optical waveguide. For example, in the arrangement in FIG. 2, the two central core regions 1′ are each surrounded by eight other cores. The core regions 1″ arranged at the periphery are each surrounded by three or five other cores. This means that the chance of thermal interaction between the core regions 1′, 1″ is greater in the central core regions 1′ than in the peripheral core regions 1″.
[0052] FIG. 3 shows a modified arrangement in which each core region 1 is surrounded by two further core regions, such that all the core regions 1 have a similar thermal interaction with the other cores. In this way, the performance of the optical waveguide can be improved. In particular, the threshold at which mode instability sets in can be raised further.
[0053] In the illustrative embodiments in FIGS. 1-3, all the core regions 1 are surrounded by a common first jacket region 2. In the illustrative embodiment shown in FIG. 4, each core region 1 is surrounded by a first jacket 2′ assigned only to this core region, in order to better shield the individual channels of the multi-core optical waveguide from one another. The two first jacket regions 2′ are surrounded by a common second jacket region 4. Moreover, in the illustrative embodiment shown in FIG. 4, the doped region 3 lies slightly outside the core region 1. The arrangement of the doped region 3 relative to the light-guiding core region 1 and, if appropriate, the overlap of the doped region 3 with the core region 1 can be specifically chosen to promote or suppress the strengthening of individual modes of the light propagating in the optical waveguide.
[0054] In the illustrative embodiment shown in FIG. 5, the core regions 1 are formed by a plurality of channels 6 which extend along the longitudinal extent of the optical waveguide and which, viewed in the cross section of the optical waveguide as shown, are arranged around each core region 1, wherein the optical waveguide, in the regions forming the channels 6, has a lower refractive index in relation to the respective core region 1 or the jacket 2. Within the regions delimited by the channels 6, the optical waveguide has substantially the same refractive index as in the region of the first jacket 2. The channels 6 guide the light in the core regions 1. The channels 6 can be arranged specifically to influence the guiding of the light and to permit the localization of a Gaussian base mode in the core region and a simultaneous delocalization of higher-order modes, so that their propagation in the optical waveguide is suppressed as far as possible. By this strategy, the performance threshold at which mode instability sets in can be raised further. The channels 6 can consist of crystalline or non-crystalline material, with crystalline material also being preferred here. It is possible to produce the channels 6 from material of low thermal conductivity (for example air), such that the channels 6 at the same time serve as a thermal and optical barrier between the core regions 1. In the embodiment with channels 6 for forming the core regions 1, as shown in FIG. 5, it is also possible, instead of the common first jacket 2, that each core region 1 is assigned just one first jacket 2′, similarly to what is shown in FIG. 4.
[0055] In all of the illustrative embodiments shown in the figures, the optical waveguide has, as has been mentioned, a rectangular cross section. This geometry is advantageous in terms of the production of the optical waveguide from crystalline materials. The rectangular cross section has further advantages, as can be seen from FIG. 6, for example. The optical waveguide shown is designed in the manner shown in FIG. 1. For heat dissipation, plate-shaped cooling elements 7 are arranged on the upper and lower surfaces of the optical waveguide. On account of the flat shape of the optical waveguide, the heat arising in the doped core regions 1 during optical amplification can be dissipated particularly effectively. With regard to avoiding mode instability, it is advantageous that all the core regions 1 are at the same distance from the cooling elements 7, thereby providing for uniform cooling.
[0056] In the illustrative embodiment shown in FIG. 7, insulation regions 8 are arranged between the core regions 1, wherein the optical waveguide has, in the insulation regions 8, a reduced thermal conductivity compared to the other regions. The insulation regions serve to avoid thermal coupling between the core regions 1, in order to avoid mode instability. The insulation regions can consist of air or of other suitable materials.
[0057] The optical arrangement shown in FIG. 8 has a splitting element 9 which splits an input beam E into several spatially separate partial beams. These partial beams propagate through an optical waveguide 10 according to the invention, which is accordingly designed as a multi-core optical waveguide as shown in FIGS. 1 to 7. Each core region 1 of the optical waveguide guides a respective partial beam. Moreover, a combining element 11 is provided which spatially superposes the partial beams in an output beam A. The splitting element 9 comprises two elements, namely a partially reflective element 12 and a reflective element 13. The reflective element 13 is a mirror with the highest possible reflectivity. The partially reflective element 12 reflects a part of the incident radiation E (e.g. from a pulsed laser). The non-reflected part is transmitted and generates a partial beam. The partially reflective element 12 consists of N (N=4 in the illustrative embodiment shown) zones with different reflectivity. The input beam E is reflected back and forth several times between the partially reflective element 12 and the plane-parallel reflective element 13 lying opposite and at a distance from the element 12. The degrees of reflectivity of the zones of the partially reflective element 12 are chosen such that the incident input beam E is divided in a defined ratio into the N partial beams. The generated partial beams are parallel and equidistant here. The combining element 11 has an identical structure with partially reflective element 12′ and reflective element 13′. The combining element 11 superposes the partial beams in an output beam A. It is in this case arranged antisymmetrically with respect to the splitting element 1 in such a way that the resulting differences in path length of the N partial beams cancel each other out exactly. On account of the integration of the 1:N split or combination in a respective individual element 9 or 11, a compact design is possible and simple adjustment is ensured.
