Optical fiber coupler and method of manufacturing

Abstract

The present invention provides an optical waveguide coupler (1) comprising a crystalline substrate (2) of a (100) type having a top surface (2a) and a (100) crystallographic plane (2100), wherein the crystalline substrate (2) comprises: a V-groove (3) extending in a Z-direction (Z) in a projection onto the top surface (2a), wherein the Z-direction (Z) lies within a 〈110〉 crystal plane (2110) of the crystalline substrate (2), wherein the top surface (2a) and a 〈100〉 crystal plane (2100) of the crystalline substrate (2) form a first angle (α1) in an X / Y plane (XY) perpendicular to the Z-direction (Z), and an optical waveguide arranged on the top surface (2a) of the crystalline substrate (2). (4), wherein the optical waveguide (4) extends along the Z-direction (Z) in a projection to the top side, wherein the V-groove (3) and the optical waveguide (4) are designed such that,When an optical fiber (5) having a predetermined size and a fiber core (51) and a fiber tip (52) is inserted into the V-groove (3) such that a distance between an outer end (42) of the waveguide core (4) and the fiber tip (52) is less than a predetermined distance (d), the fiber core (51) is aligned with the waveguide core (41) such that a displacement of the fiber core (51) and the waveguide core (41) is less than a predetermined displacement. Furthermore, the present invention provides a method for manufacturing an optical fiber coupler (1).

Description

TECHNICAL FIELD OF THE INVENTION

[0001] The invention relates to an optical fiber coupler and to a method for producing an optical fiber coupler. TECHNICAL BACKGROUND

[0002] The recent manufacturing process of photonic integrated circuits (PICs) relies on the strategic use of anisotropic wet etching, such as TMAH, to create V-grooves that serve as entry points for the optical fibers within the PIC. However, precise alignment between the direction of the V-groove and the optical waveguide in the PIC remains a major challenge.

[0003] One obstacle arises from a misalignment between the crystalline plane (110) and a processing direction, indicated or aligned, for example, by a notch in the wafer. Consequently, the lithography and etching processes for SiN waveguides can be carefully aligned to the plane perpendicular to the processing direction. The misalignment between the crystalline plane (110) and the wafer notch can lead to uncertainties in the alignment of the V-grooves and affect their alignment with the waveguide direction. A secondary factor for alignment is the tilted orientation of the wafer surface with respect to the crystalline plane (100) of Si. This orientation mismatch leads to a misalignment between the center of the optical fiber and the optical waveguide.This misalignment can be determined by an angle orthogonal to the direction of the V-groove or by another angle in a plane parallel to the V-groove and orthogonal to the top surface, depending on the direction of the inclined wafer surface with respect to the (100) Si crystal plane.

[0004] Therefore, it is an object of the invention to provide a V-groove and an optical waveguide such that a core of an optical fiber of known dimension is well aligned with the optical waveguide.

[0005] DE 694 26 178 T2 describes a manufacturing method of an integrated structure with an optical waveguide and a corresponding monolithic optical coupling carrier with an optical fiber. The coupling carrier has a V-shaped recess with respect to a plane perpendicular to a surface of a substrate and to a direction of the optical waveguide formed on the surface. The surface of the substrate has a crystalline orientation <1,0,0>. The waveguide is parallel to the crystalline orientation <0,1,-1> of the substrate, and surfaces of the V-shaped recess are arranged in <2,1,1> planes of the substrate. These <2,1,1> planes are etch stop planes during an etching process of the manufacturing method.

[0006] DE 41 34 940 A1 describes an integrated optical component with an integrated optical waveguide whose coupling end face terminates at a positioning trench with a V-shaped cross-section, etched anisotropically into a silicon substrate and aligned with the optical waveguide, for accommodating a glass fiber. The positioning trench has a constant cross-section up to the coupling end face, with lateral ribs on the substrate surface supporting the optical waveguide. The silicon substrate has a {100} surface orientation, and the edges of the positioning trench are aligned along the 〈110〉 direction. SUMMARY OF THE INVENTION

[0007] According to the invention, this problem is in any case solved by the subject-matter of the independent claims.

[0008] According to a first aspect of the invention, an optical waveguide coupler is provided. The optical waveguide coupler comprises a (100)-type crystalline substrate having a top surface and a (100) crystallographic plane, the crystalline substrate comprising: a V-groove extending in a Z direction in a projection toward the top surface, the Z direction lying within a (110) crystallographic plane of the crystalline substrate, the top surface and a (100) crystal plane of the crystalline substrate forming a first angle in an X / Y plane perpendicular to the Z direction, the V-groove having a first inclined surface and a second inclined surface opposite the first inclined surface, and an optical waveguide disposed on the top surface of the crystalline substrate, the optical waveguide having a waveguide core extending along the Z direction in a projection toward the top surface.wherein the V-groove and the optical waveguide are configured such that when an optical fiber of a predetermined size and having a fiber core and a fiber tip is inserted into the V-groove such that a distance between an outer end of the waveguide core and the fiber tip is less than a predetermined distance, the fiber core is aligned with the waveguide core such that a displacement of the fiber core and the waveguide core is less than a predetermined displacement.

[0009] According to a second aspect of the invention, a method for manufacturing an optical fiber coupler is provided. The method for manufacturing an optical fiber coupler comprises etching a reference V-groove into a top surface of a (100)-type crystalline substrate by anisotropic etching, wherein the anisotropic etching is performed such that the reference V-groove extends along a Z direction in a projection toward the top surface, wherein the Z direction lies within a (110) crystal plane of the crystalline substrate; etching a primary mark extending along a reference direction into the top surface; determining a reference angle between the Z direction and the reference direction;Determining a first angle between the top surface and a (100) crystal plane of the crystalline substrate in a projection toward an X / Y plane perpendicular to the Z direction, and / or determining a second angle between the top surface and the (100) crystal plane in a Y / Z plane perpendicular to the X direction; fabricating an optical waveguide on the top surface of the crystalline substrate based on the reference angle such that a fiber core of an optical fiber with predetermined dimensions inserted into a second V-groove is aligned with a waveguide core of the optical waveguide, and etching the second V-groove into the top surface by anisotropic etching, wherein a width of the second V-groove at the top surface in an X direction and a depth of the second V-groove in the Y direction from the top surface are adjusted based on the determined first angle and / or the second angle.

[0010] A basic idea of ​​the present invention is to adapt the manufacturing of the V-groove of an optical fiber coupler so that a fiber placed in the V-groove is reliably aligned with the waveguide core of the waveguide on the crystalline substrate within the tolerances of the crystallographic axes in a provided (100)-type crystalline substrate. A critical part of this alignment is the X / Y plane, which is substantially perpendicular to the V-groove and the optical fiber inserted into the V-groove.

[0011] A basic concept of the invention is to determine a misalignment between the (100) crystal plane and the top surface of a (100)-type crystalline substrate, particularly in the direction perpendicular to the fiber and the V-groove. A (100)-type crystalline substrate refers to a crystalline substrate, e.g., made of silicon, whose top surface is intended to be a (100) crystallographic plane. However, due to manufacturing tolerances, the top surface is often not exactly a (100) crystallographic plane. To determine such misalignment, a primary mark extending in a reference direction is etched into the substrate. The reference direction can be, for example, an initial operating direction. The initial direction can be completely arbitrary, but it can be indicated by a wafer notch, which typically indicates the (110) crystal plane of the (100)-type crystalline substrate.

[0012] Before or after the primary mark is formed, a reference V-groove is created by anisotropic etching into the top surface. By such etching, e.g., using TMAH or KOH as the etchant, a V-groove can be created, wherein the V-groove, i.e., the trench axis at the bottom of the V-groove, extends between the two inclined surfaces along a direction in the (110) crystal plane of the crystalline substrate. The reference V-groove can be etched at any other location on the substrate, preferably near the primary mark, to facilitate the determination of the angular displacement of the reference direction to the V-groove, i.e., the reference angle. The Z-direction is thus defined as the direction of the reference V-groove in a projection onto the surface. The direction perpendicular to the top surface is defined as the Y-direction, which is orthogonal to the Z-direction. The X direction extends along the top and is orthogonal to the Y and Z directions.

[0013] These two structures determine the reference angle between the reference direction and the Z direction, which can be viewed from a top view.

[0014] This reference angle can be measured by observation, for example with a microscope such as a critical dimension scanning electron microscope (CDSEM).

[0015] A first angle, indicating the misalignment of the (100) crystal plane to the top surface in a plane perpendicular to the Z direction, i.e., X / Y plane, is determined by observing the reference V-groove. In case of misalignment of the top surface to the (100) crystal plane, an asymmetric V-groove is etched. This means that the width of a first inclined surface of the reference V-groove in a projection onto the top surface differs from the width of a second inclined surface of the reference V-groove opposite the first inclined surface in a projection onto the top surface. By measuring these widths and / or their difference, the misalignment between the top surface and the (100) crystal plane perpendicular to the Z direction can be derived. This angle is called the first angle.

[0016] Furthermore, depending on the direction of the misalignment of the (100) crystal plane relative to the top surface of the wafer, there is a second angle between the top surface and the (100) crystal plane in a Y / Z plane along the V-groove perpendicular to the X direction. This second angle results in a misalignment of the fiber core relative to the waveguide core in the Y direction orthogonal to the top surface of the substrate. Etching the second V-groove must compensate for this second angle, so the width of the second V-groove gradually varies based on the determined second angle for optimal alignment.

[0017] By knowing these angles, the resulting manufacturing step can be adjusted based on this angle. When manufacturing the optical waveguide on the substrate, the reference angle is taken into account to achieve correct alignment of the optical waveguide to the optical fiber in a second V-groove. By inserting the optical fiber into the V-groove, a cladding of the fiber comes into contact with the first and second inclined surfaces. This second V-groove is machined by etching to adjust or correct the depth and position of the V-groove taking the first angle into account. If no adjustment were performed, the V-groove would generally be too deep due to the described misalignment for the optical waveguide manufactured at a given height above the top to match a fiber core of an optical fiber of a given size or dimensions.By adjusting the amount of etchant, the depth of the groove is reduced to compensate for the misalignment of the planes.

[0018] A fiber with a predetermined size means that the dimensions of the fiber are known. For example, the diameter of the fiber core and cladding is assumed to be that of a standard single-mode fiber, such as an SMF-28 (e.g., SMF-28 ULL from Corning) with a core diameter of approximately 8 µm and a cladding diameter of 125 µm. As will be explained in more detail below, the first and second inclined surfaces may preferably be located in a (111) crystal plane of the crystalline substrate.

[0019] The depth is the distance from the top surface to the groove axis in the Y direction, which is a vertical direction when the top surface is a horizontal plane. Methods known in the field of lithography can be used to fabricate the optical waveguide on the crystalline substrate. Such fabrication typically involves several manufacturing steps, beginning with a deposition, e.g., of an oxide, followed by lithography and etching.

[0020] Please note that the finished optical fiber coupler may also include the primary mark and the reference V-groove. The optical waveguide may also be fabricated on the primary mark and / or the reference V-groove. Alternatively, the primary mark and the reference V-groove may be fabricated at a location remote from the substrate and cut off from the optical fiber coupler.

[0021] By positioning the optical fiber at a predetermined distance, e.g., less than 3 µm, preferably between 1.5 µm and 2.5 µm, from the end of the waveguide, the fiber core is aligned with the waveguide core by the optical fiber coupler manufactured as described above. This means that a plane orthogonal to the fiber core and / or the waveguide core has a displacement of less than a predetermined displacement, which can be less than 0.2 µm or 0.2 µm, preferably 0.05 µm.

[0022] A particular advantage of the solution according to one aspect of the invention is that the method provides a reliable optical fiber coupler with reliable efficiency, independent of the orientation of the crystallographic planes in the (100)-type crystalline substrate (at least within the typical tolerances of such a substrate). In this way, a reliable optical fiber coupler can be provided for coupling light from an optical fiber to an optical waveguide as part of a photonic integrated circuit, or vice versa.

[0023] The crystallographic planes (100), (110), and (111), as previously designated, correspond to the Miller indices commonly used in this field. In particular, indices in angle brackets such as (100) denote a family of directions that are equivalent due to symmetry operations, such as

[100] ,

[010] ,

[001] , or the negative of one of these directions. Furthermore, a first angle, a second angle, or any other angle with respect to such crystallographic planes means that the angle in question is greater than 0, preferably greater than 0.05°, unless otherwise stated.

[0024] According to some further aspects of the optical fiber coupler according to the invention, the top surface and a (100) crystal plane of the crystalline substrate form a second angle in a Y / Z plane. The Y / Z plane is formed by a normal to the top surface and the Z direction, wherein the width of the V-groove at the top surface varies along its length according to the second angle. The variation along its length is in particular gradual, so that the groove axis at the bottom of the V-groove follows the second angle with respect to the top surface. In this way, misalignment of the (100) crystal plane to the top surface toward the V-groove can be compensated for in all directions for optimal alignment.

[0025] According to some further aspects of the optical waveguide coupler according to the invention, the V-groove has a first inclined surface and a second inclined surface opposite the first inclined surface, wherein the groove axis of the second V-groove, where the first and second inclined surfaces meet, lies in a (100) crystal plane of the crystalline substrate. According to this aspect, the waveguide core can extend parallel to the top surface at a predetermined height. Furthermore, the waveguide core can also extend along a (100) crystal plane and thus run parallel to the groove axis of the V-groove.

[0026] According to some further aspects of the method according to the invention, the reference V-groove comprises a first inclined surface and a second inclined surface opposite the first inclined surface, wherein the anisotropic etching is performed such that the first and second inclined surfaces are located in a (111) crystal plane of the crystalline substrate. In this way, a fixed direction of the second V-groove can be achieved, which can be reliably manufactured.

[0027] According to some embodiments of the inventive method, etching a second V-groove comprises calculating a shift in the depth of the second V-groove based on the determined first angle. Using this information, an amount of etchant can be adjusted based on the calculated depth shift. The adjusted amount of etchant can then be applied when etching the second V-groove. In this way, the second V-groove can be manufactured to compensate for the misalignment of the top surface to the crystallographic (100) plane of the crystalline substrate.

[0028] According to some embodiments of the method according to the invention, manufacturing the optical waveguide comprises adjusting an orientation of an etching mask on the top surface based on the determined reference angle and manufacturing the optical waveguide such that a waveguide core of the optical waveguide extends along the Z direction in a projection toward the top surface. In this way, the optical waveguide can be aligned with the V-groove in the (110) crystal plane, which proves to be highly reliable during manufacturing.

[0029] According to some embodiments of the method according to the invention, manufacturing the optical waveguide comprises calculating a first X-shift based on the determined reference angle and manufacturing the optical waveguide such that a waveguide core of the optical waveguide is positioned based on the first X-shift. This improves the alignment accuracy of the fiber core to the waveguide core.

[0030] According to some embodiments of the method according to the invention, manufacturing the optical waveguide comprises calculating a second X-shift based on the corrected depth and manufacturing the optical waveguide such that the waveguide core of the optical waveguide is positioned based on the second X-shift. Due to the depth compensation, a further shift in the X-direction occurs due to the asymmetric shape of the V-groove. This further step compensates for this additional shift in the X-direction, thereby increasing the alignment accuracy and reducing the coupling losses of the light coupling from the fiber core to the waveguide core or vice versa.

[0031] According to some embodiments of the method according to the invention, manufacturing the optical waveguide comprises manufacturing the optical waveguide such that the waveguide core is arranged along the Z direction in a projection toward the top side. As described above, lithographic methods can be used for this manufacturing step. The waveguide core can extend parallel to the top side at a predetermined height. This would be the simplest way to manufacture the optical waveguide. Alternatively or additionally, the waveguide core can extend at least partially, in particular near the end of the waveguide in the direction of the second V-groove, along a (100) crystal plane, such that it runs parallel to the groove axis of the V-groove and an optical fiber in the second groove. This improves the light coupling efficiency of the light propagating from the fiber core into the waveguide core, or vice versa.

[0032] According to some embodiments of the method according to the invention, etching the second V-groove comprises etching the second V-groove such that a trench axis, where the first and second inclined surfaces of the second V-groove meet, lies in a (100) crystal plane of the crystalline substrate. In this case, the trench axis of the second V-groove also lies in the (110) crystal plane. In this way, a fixed direction of the second V-groove can be achieved, which can be reliably manufactured.

[0033] The above embodiments and further developments can be combined with one another as desired. In particular, all features of the optical fiber coupler are transferable to the method for producing the optical fiber coupler, and vice versa. Further possible aspects, further developments, and refinements of the invention also include combinations of features of the invention described above or below with respect to the exemplary embodiments that are not expressly mentioned. In particular, those skilled in the art will also add individual aspects as improvements or additions to the respective basic form of the present invention.

[0034] Advantageous embodiments and further developments emerge from the description with reference to the illustrations. BRIEF SUMMARY OF THE DRAWINGS

[0035] The present invention is explained in more detail below with reference to the exemplary embodiments shown in the schematic figures, wherein: Fig. 1 shows a schematic representation of a V-groove for an optical fiber coupler according to an embodiment of the invention; Fig. 2 shows a schematic representation of a reference V-groove for an optical fiber coupler according to an embodiment of the invention; Fig. 3 shows a cross section of a V-groove for an optical fiber coupler according to an embodiment of the invention; Fig. 4a-c show a top view and cross sections of an optical fiber coupler according to an embodiment of the invention; Fig. 5 shows a flowchart for a method of manufacturing an optical fiber coupler according to an embodiment of the invention; Fig. 6a-d show cross sections of a V-groove of an optical fiber coupler according to another embodiment of the invention; Fig. 7 shows a schematic diagram of an exemplary optical fiber coupler for illustration purposes; Fig. 8 shows a schematic representation of an optical fiber coupler according to another embodiment of the invention; Fig. 9 shows a plan view of an exemplary reference V-groove for illustration purposes; and Fig. 10 shows a plan view of a V-groove of an optical fiber coupler according to another embodiment of the invention.

[0036] The accompanying drawings are intended to provide a further understanding of embodiments of the invention. They illustrate the embodiments and are used in conjunction with the description to explain the principles and concepts of the invention. Other embodiments and many of the noted advantages will be apparent from the drawings. Elements of the drawings are not necessarily drawn to scale. Directional terms such as "top", "bottom", "left", "right", "upper", "down", "horizontal", "vertical", "front", "rear" and similar terms are for purposes of explanation only and do not limit generality to particular embodiments shown in the figures.

[0037] In the figures of the drawing, the same elements, features and components that have the same function and the same effect are each provided with the same reference numerals - unless otherwise stated. DESCRIPTION OF THE EMBODIMENTS

[0038] Fig. 1 shows a schematic representation of a V-groove 3 for an optical fiber coupler 1 according to an embodiment of the invention.

[0039] Fig. 1 shows a crystalline substrate 2 of type (100) with a top surface 2a and a crystallographic (100) plane 2100. A V-groove 3 is formed in the crystalline substrate, which in this case extends in a Z-direction Z. The Z-direction Z lies within a (110) crystal plane 2110 of the crystalline substrate 2. The V-groove 3 has a first inclined surface 3a and a second inclined surface 3b opposite the first inclined surface 3a.

[0040] In Fig. 1 shows that the top surface 2a and a (100) crystal plane 2100 of the crystalline substrate 2 form a first angle α1 in an X / Y plane XY normal to the Z direction Z. This may be due to the manufacturing tolerance of the substrate 2 or the corresponding wafer, which may be in the range of more than 0.1° and up to 1°, 1.5° or more.

[0041] Fig. 2 shows a schematic representation of a reference V-groove 6 for an optical fiber coupler 1 according to an embodiment of the invention.

[0042] Fig. Figure 2 shows another problem that must be considered when manufacturing a V-groove in a crystalline substrate 2 of type (100). With a reference direction R, which can be arbitrary or indicated, for example, by a notch 20 of a wafer of the crystalline substrate 2, a reference V-groove 6 can be manufactured by anisotropic etching. Such anisotropic etching, e.g., using TMAH or KOH, is performed such that the direction of a trench axis 6c, at which the first and second inclined surfaces 6a, 6b meet, of the reference V-groove 6 runs along a Z-direction Z, at least in a projection toward the top side 2a of the substrate. Due to the nature of this anisotropic etching, the Z direction extends in a (110) crystal plane 2110 of the crystalline substrate 2. The Z direction of the groove thus produced and the reference direction R are generally different from each other, so adjustments are required, e.g.to arrange the optical waveguide 4 in the correct orientation, as explained later.

[0043] To get an indication of the misalignment between the reference direction and the Z-direction, a primary mark 7 (in Fig. 2 not shown), extending along a reference direction R, is etched into the top surface 2a, as will be described in detail later.

[0044] Fig. 3 shows a cross section of a V-groove for an optical fiber coupler according to an embodiment of the invention.

[0045] Fig. Figure 3 shows such an asymmetric V-groove 3 due to the existing first angle α1. In this cross-section in the X / Y plane XY, the V-groove 3 has a shorter first inclined surface 3a compared to a second inclined surface 3b. The depth D from the top side 2a to the trench axis 3c is deeper than at a first angle α1 of zero. It follows that the width K1 of a first inclined surface 3a of the V-groove 3 in a projection onto the top side 2a differs from the width K2 of a second inclined surface 3b of the reference V-groove 3 in a projection onto the top side 2a. In the Fig. In the example shown in Figure 3, the width K1 of a first inclined surface 3a in a projection onto the top surface 2a is greater than the width K2 of the second inclined surface 3b in a projection onto the top surface 2a. By measuring these widths K1, LK2 and / or their difference, it is possible to derive the first angle α1, i.e., the misalignment between the top surface and the (100) crystal plane perpendicular to the Z direction. These widths K1, K2 can be measured by observation, e.g., with a microscope such as a critical dimension scanning electron microscope (CDSEM).

[0046] Fig. 4a-c show a top view and cross sections of an optical fiber coupler 1 according to an embodiment of the invention.

[0047] Fig. 4a is a plan view in an X / Z plane XZ of an optical fiber coupler 1 fabricated on a crystalline substrate 2 configured as a wafer. Fig. Figure 4b shows a cross-section in the X / Y plane XY near a fiber tip 52 of the fiber 5 and an end 42 of the optical waveguide 4. Fig. Figure 4c shows a cross-section of the optical fiber coupler in the Y / YZ plane.

[0048] The optical waveguide coupler 1 comprises the crystalline substrate 2 of type (100) with a top surface 2a and a crystallographic (100) plane 2100. The substrate may be any suitable crystalline substrate, preferably made of silicon, Si, but may also consist of other semiconductor materials such as GaAs or InP.

[0049] The crystalline substrate 2 comprises a V-groove 3 extending in a Z-direction Z in a projection toward the top side 2a. The Z-direction Z lies within a (110) crystal plane 2110 of the crystalline substrate 2. The top side 2a and a (100) crystal plane 2110 of the crystalline substrate 2 form a first angle α1 in an X / Y plane XY perpendicular to the Z-direction Z.

[0050] In addition, an optical waveguide 4 is arranged on the top surface 2a of the crystalline substrate 2 next to the V-groove, the optical waveguide 4 extending along the Z-direction Z.

[0051] The V-groove 3 and the optical waveguide 4 are designed such that an optical fiber 5 of a predetermined size and having a fiber core 51 and a fiber tip 52 can be inserted into the V-groove 3 such that the distance between an outer end 42 of the waveguide core 4 and the fiber tip 52 is smaller than a predetermined distance. In some embodiments, the predetermined distance may be 3 µm or between 1.5 mm and 3 mm. Fig. 4a, it can be seen that the fiber core 51 and the waveguide core 41 are aligned such that optical loss is generally minimized. This means that a displacement in a projection in the direction of the X / Y plane XY perpendicular to the Z direction Z is, for example, less than 0.1 µm, e.g., 0.05 µm. In some embodiments, the displacement is even less than 0.01 µm. Fig. Figure 4b, which shows a cross-section in the X / Y plane XY, shows that there is virtually no displacement between the fiber core 51 and the waveguide core 41. This results in optimal light coupling between these two elements.

[0052] In addition, Fig. 4a shows a reference groove 6 extending in the Z-direction Z and a primary mark 7 extending in a reference direction R. As described above, the reference direction R can be an initial processing direction of the manufacturing apparatus. To obtain the second V-groove 3 and the Z-direction-extending optical waveguide 4 with its Z-direction-extending core, an etching mask is aligned on the top surface based on the determined reference angle αR. Although the reference groove 6 and the primary mark 7 are adjacent to the essential elements (V-groove 3, optical waveguide 4) and the optical waveguide coupler 1, in further embodiments, the optical waveguide 4 can be fabricated on the top surface 2a such that it at least partially covers at least one of the V-grooves 3 and the optical waveguide 4.

[0053] In Fig. 4a shows that the optical waveguide 4 is arranged such that the waveguide core 41 is arranged along the Z-direction Z in a projection onto the top side 2a, which in this case runs parallel to the top side at a predetermined height H. To fabricate the optical waveguide 4 on the crystalline substrate 2, several manufacturing steps are typically used, starting with a deposition, e.g., of an oxide, followed by lithography and subsequent etching. Such steps are typically known in the field of lithography. These steps usually comprise a deposition of oxides, lithography, and subsequent etching using a customized mask and etchant.

[0054] Although the embodiment in Fig. 4a-c with a substrate having a misalignment between the top side 2a and a (100) crystal plane 2110 in an X / Y plane XY normal to the Z direction Z by a first angle α1, in further embodiments the substrate 2 further has a misalignment between the top side 2a and a (100) crystal plane 2110 in a Y / Z plane YZ perpendicular to the X direction X by a second angle α2, as described below with reference to the Fig. 7 and Fig. 8 is described.

[0055] Fig. 5 shows a flowchart for a method for manufacturing an optical fiber coupler 1 according to an embodiment of the invention.

[0056] The method for manufacturing an optical fiber coupler 1 comprises the step of etching S1 a reference V-groove 6 into a top surface 2a of a (100) type crystalline substrate 2 by anisotropic etching, wherein the anisotropic etching is performed such that the reference V-groove 6 extends along a Z-direction Z in a projection onto the top surface 2a, wherein the Z-direction lies within a 〈110〉 crystal plane 2110 of the crystalline substrate 2. The method further comprises the step of etching S2 a primary mark 7 extending along a reference direction R into the top surface 2a.

[0057] After completing these steps, the reference angle αR between the Z direction Z and the reference direction R is now determined S3. Furthermore, a first angle α1 between the top side 2a and a (100) crystal plane 2100 of the crystalline substrate 2 is determined in a projection onto an X / Y plane XY perpendicular to the Z direction Z S4a. Alternatively or in addition to the previous step, a second angle α2 between the top side and the (100) crystal plane 2100 in a Y / Z plane YZ perpendicular to the X direction X is determined S4b.

[0058] After this determination step, an optical waveguide 4 is fabricated S5 on the top surface 2a of the crystalline substrate 2 based on the reference angle αR such that a fiber core 51 of an optical fiber 5 with predetermined dimensions, which is inserted into the second V-groove 3, is aligned with a waveguide core 41 of the optical waveguide 4. The position of the optical waveguide 4 is determined based on the position of a second V-groove V, and the orientation of a second V-groove V is determined based on the determined first angle α1 and the determined second angle α2. This manufacturing step typically comprises several manufacturing steps, starting with a deposition, e.g., of an oxide, followed by lithography and then etching, which are well known in the art.

[0059] In addition, the second V-groove 3, which extends along the Z-direction Z, is etched S5 into the top side 2a by anisotropic etching. Thus, similar to the reference V-groove 6, the second V-groove 3 also extends along the Z-direction in a projection onto the top side 2a. The width W of the second V-groove 3 at the top side 3 in the X-direction X and the depth D of the second V-groove 3 in the Y-direction Y from the top side 2a are determined based on the determined first angle α1. In this embodiment, before etching the V-groove, the oxide parts that are not needed and were deposited in the previous manufacturing step for placing the optical waveguide at the correct height H on the substrate 2 are etched based on the alignment with the reference angle αR, preferably also with the first and / or second angles α1, α2.

[0060] It is noted that the order of the first two steps S1, S2, the third and fourth steps S3, S4a and / or S4b and the fifth and sixth steps S5, S6 is arbitrary and may be opposite to each other, as in Fig. 7 shown.

[0061] The Fig. show cross sections of a V-groove of an optical fiber coupler 1 according to another embodiment of the invention.

[0062] Fig. Figure 6a illustrates the ideal case where the top surface 2a of the crystalline substrate is a (100) crystal plane. In this case, the cross-section in the X / Y plane XY, which in this case is orthogonal to a length direction of the V-groove, shows a symmetrical V-groove 3. As can be seen, the V-groove 3 includes a first inclined surface 3a and a second inclined surface 3b opposite the first inclined surface 6a. In such a symmetrical V-groove 3, the first inclined surface 3a is the same size as the second inclined surface 3 of the V-groove 3 in the X / Y plane XY. Furthermore, the anisotropic etching of the V-groove 3 is typically performed such that the first and second inclined surfaces 3a, 3b lie in a (111) crystal plane 2111 of the crystalline substrate 2. A trench axis 3a, at which the first and second inclined surfaces 3a, 3b meet, extends in a (110) crystal plane 2110 that is orthogonal to the top surface 2a.

[0063] In Fig. 6b shows an asymmetric cross-section of the V-groove 3 due to an existing first angle α1. As in Fig. As shown in Figure 6b, a displacement ΔX1 in the X direction is generated, which results from the determined reference angle αR and must be calculated. The optical waveguide 4 can then be arranged such that a waveguide core 41 of the optical waveguide 4 is positioned at a first corrected X position based on the first X displacement ΔX1, i.e., by compensating for the first displacement ΔX1. Note that in this scenario, the (110) crystal plane 2110 is no longer orthogonal to the top surface 2a.

[0064] In Fig. 6c shows that a depth displacement ΔY1 occurs due to the existing first angle α1. Therefore, a corrected depth D is calculated, which represents the depth displacement ΔY1 of the second V-groove 3 based on the determined first angle α1 and the measurement, as shown in Fig. 3, and the V-groove 3 is etched accordingly, e.g. by adjusting the amount of etchant accordingly.

[0065] In Fig. 6d shows that even after correcting the depth to the depth shift ΔY1, a second shift ΔX2 to the lower corrected depth D' with respect to the original depth D occurs. To correct the second X-shift ΔX2, a second corrected X-position is calculated based on the corrected depth D' using geometric relationships of the V-groove. In particular, the first and second inclined surfaces 3a, 3b of the V-groove 3 lie in Fig. 6a-d in the (111) crystal plane, so that an angle θ of an inclined surface 3a, 3b with respect to the (100) crystal plane in the X / Y plane XY is approximately 54.7°. With this information, the second displacement ΔX2 can be derived using the known first angle α1. The optical waveguide 4 is arranged such that the waveguide core 41 is positioned based on the second X displacement ΔX2.

[0066] It is noted that an alignment of an etching mask for the optical waveguide 4 on the top surface 2a is based on the determined reference angle αR. The optical waveguide 4 is arranged such that the waveguide core 41 of the optical waveguide 4 extends along the Z direction Z in a projection toward the top surface 2a.

[0067] Fig. 7 shows a schematic representation of an exemplary optical fiber coupler for illustration purposes.

[0068] In the Fig. In the embodiment shown in Figure 7, the top surface 2a and a (100) crystal plane 2100 of the crystalline substrate 2 open a second angle α2 in a Y / Z plane YZ. The Y / Z plane YZ is defined by an opening of a normal to the top surface 2a and the Z direction Z. As a result, the groove axis 3c of the V-groove 3, where the first and second inclined surfaces 3a, 3b meet, is no longer parallel to the Z direction. Strictly speaking, the groove axis 3c and the Z direction form the same second angle α2.

[0069] As in Fig. 7, the illustrated misalignment leads to a relatively significant displacement ΔY2 along the Y-direction of the fiber core 51 of an optical fiber 5 (in Fig. 8 not shown) relative to the waveguide core 41 of the optical waveguide 4. This displacement ΔY2 is mainly related to the depth D of the V-groove 3, which varies along the edges of the V-groove 3. For example, at a second angle α2 of 1 degree, the displacement along the Y direction Y can easily be up to 35 µm. The remarkable displacement ΔY2 is mainly due to the length of the V-groove 3, which can be on the order of 2 mm.

[0070] Fig. 8 shows a schematic representation of an optical fiber coupler according to another embodiment of the invention.

[0071] Fig. For the purpose of comparison and illustration, Figure 8 shows the case of a non-existent second angle α2 between the top surface 2a and the crystallographic plane 2100 (100). In this case, the angle α2 is constant with W of the V-groove 3. A fiber core 51 of an optical fiber 5 (in Fig. 8 not shown) with predetermined dimensions, which is inserted into the V-groove 3, is aligned with the waveguide core 41 in the X / Y plane.

[0072] Fig. 9 shows a plan view of an exemplary reference V-groove 6 for illustration.

[0073] To determine the second angle α2 between the top surface and the (100) crystal plane 2100 in a Y / Z plane YZ perpendicular to the X direction X, the dimensions of the reference V-groove 6 must be determined by microscopy, e.g., CD-SEM, as described elsewhere above.

[0074] As in Fig. 9, the reference V-groove 6 has, in addition to the first and second inclined surfaces 6a, 6b, a third and fourth inclined surface 6d, 6e near the first and second ends 61, 62 of the reference V-groove 6. When a second angle α2 is present, a projection of an end portion A' in the Z direction of, for example, the third inclined surface 6d may be larger than a projection of an ideal end portion A due to the second angle α2, i.e., the reference V-groove is deeper at the first end 61. For the same reason, a projection of a center portion B' of the reference V-groove 6, in which the trench axis 63 is located at the bottom of the reference V-groove 6, is shorter than a projection of an ideal center portion B.

[0075] By measuring the extent of the difference between the projection of the end section A' onto the projection of the ideal end section A and / or the difference between the projection of the middle section B' onto the projection of the ideal middle section B, the second angle α2 can be derived.

[0076] Fig. 10 shows a plan view of a V-groove 3 of an optical fiber coupler 1 according to another embodiment of the invention.

[0077] Fig. Figure 10 shows a V-groove 3 corrected for a second angle α2. This V-groove 3 was created as a result of a calculation that specifies a relationship between the second angle α2 and the width W of the V-groove 3. This calculation results in the determination of the width at each end 31, 32 of the V-groove 3 to avoid the misalignment effect in aligning the center of the optical fiber 5 and the optical waveguide 4. This is achieved by maintaining a constant depth D over the entire length of the V-groove 3.

[0078] It can be seen that the width W of the V-groove 3 on the top surface 2a gradually varies along its length according to the second angle α2. Therefore, the second V-groove 3 is etched such that the width W of the second V-groove 3 gradually varies based on the determined second angle α2.

[0079] More specifically, the second V-groove 3 is etched such that a trench axis 3c, where the first and second inclined surfaces 3a, 3b of the second V-groove 3 meet, lies in a (100) crystal plane 2 of the crystalline substrate 2. In this scenario, the trench axis 3c of the V-groove 3, where the first and second inclined surfaces 3a, 3b meet, lies in a (100) crystal plane 2100 of the crystalline substrate 2.

[0080] Out of Fig. 10, it can be seen that the third inclined surface 3d near the first end 31 of the V-groove 3 is smaller than a fourth inclined surface 3e near the second end 32 of the V-groove 3. The optical waveguide 4 can be manufactured accordingly on the top surface 2a of the substrate 2.

[0081] In the above detailed description, various features have been combined into one or more examples to enhance clarity of illustration. However, it should be understood that the above description is merely illustrative and not restrictive in nature. It is intended to cover all alternatives, modifications, and equivalents of the various features and exemplary embodiments. Many other examples will be readily and immediately apparent to one skilled in the art based on their level of expertise in light of the above description.

[0082] The exemplary embodiments were chosen and described in order to best illustrate the principles underlying the invention and their possible practical applications. This will enable those skilled in the art to best modify and utilize the invention and its various exemplary embodiments for their intended use. In the claims and the description, the terms "including" and "having" are used as neutral language concepts for the corresponding term "comprising." Furthermore, the use of the terms "a," "an," and "one" is not intended to exclude the majority of features and components so described.

[0083] Although at least one exemplary embodiment of the present invention(s) is disclosed herein, it should be understood that modifications, substitutions, and alternatives may be apparent to one skilled in the art and may be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiment(s). Furthermore, throughout this disclosure, the terms "comprise" or "comprising" do not exclude other elements or steps, the terms "a" or "an" do not exclude a plurality, and the term "or" means either or both. Furthermore, the described features or steps may also be used in combination with other features or steps and in any order, unless the disclosure or context indicates otherwise.This disclosure hereby incorporates by reference the complete disclosure of all patents or applications from which it claims benefit or priority. LIST OF REFERENCE SYMBOLS USED 1 fiber optic coupler 2 crystalline substrate 2a top 20 notch 2100 (100) crystallographic plane 2110 (110) crystallographic plane 2111 (111) crystallographic plane 3 (second) V-groove 31 first end of the V-groove 32 second end of the V-groove 3a, 3b, 3d, 3e first, second, third and fourth inclined surface of the V-groove 3c Trench axis of the V-groove 4 optical waveguide 41 Waveguide core 42 End of the optical waveguide 5 optical fibers 51 fiber core 52 fiber tip 6 Reference V-groove 61 first end of the reference V-groove 62 second end of the reference V-groove 6a, 6b, 6d, 6e first, second, third and fourth inclined surfaces of the reference V-groove 6c Trench axis of the reference V-groove 7 Primary Marking A End section of the V-groove B Middle section of the V-groove d specified distance between optical fiber and optical waveguide H Height of waveguide core above top K1, K2 Projection of the first, second inclined surface on top R Reference direction W Width of the V-groove X, Y, Z directions XY, XZ, YZ planes αR reference angle α1 first angle α2 second angle θ Angle of the inclined surface of the V-groove to the top ΔX1, ΔX2 first and second X-shift ΔY1, ΔY2 displacements in Y direction

Claims

[1] Optical fiber coupler (1), comprising a crystalline substrate (2) of a (100) type having a top surface (2a) and a crystallographic (100) plane (2100), the crystalline substrate (2) comprises: a V-groove (3) extending in a Z-direction (Z) in a projection towards the top side (2a), wherein the Z-direction (Z) lies within a (110) crystal plane (2110) of the crystalline substrate (2), wherein the top side (2a) and a 〈100〉 crystal plane (2100) of the crystalline substrate (2) form a first angle (α1) in an X / Y plane (XY) perpendicular to the Z-direction (Z), and an optical waveguide (4) arranged on the top side (2a) of the crystalline substrate (2), the optical waveguide (4) extending along the Z-direction (Z) in a projection towards the top side (2a), wherein the V-groove (3) and the optical waveguide (4) are designed such that when an optical fiber (5) of a predetermined size and having a fiber core (51) and a fiber tip (52) is inserted into the V-groove (3) such that a distance between an outer end (42) of the waveguide core (4) and the fiber tip (52) is smaller than a predetermined distance (d), the fiber core (51) is aligned with the waveguide core (41) such that a displacement of the fiber core (51) and the waveguide core (41) is smaller than a predetermined displacement. [2] Optical fiber coupler (1) according to claim 1, wherein the top surface (2a) and a (100) crystal plane (2100) of the crystalline substrate (2) form a second angle (α2) in a Y / Z plane (YZ) formed by a normal to the top surface (2a) and the Z direction (Z), wherein a width (W) of the V-groove (3) on the top surface (2a) varies along its length according to the second angle (α2), in particular gradually. [3] Optical fiber coupler (1) according to claim 2, wherein a groove axis (5c) of the V-groove (3) at which the first and second inclined surfaces (3a, 3b) meet lies in a (100) crystal plane (2100) of the crystalline substrate (2). [4] A method for manufacturing an optical waveguide coupler (1), comprising etching (S1) a reference V-groove (6) into a top surface (2a) of a (100)-type crystalline substrate (2) by anisotropic etching, wherein the anisotropic etching is performed such that the reference V-groove (6) extends along a Z-direction (Z) in a projection toward the top surface (2a), the Z-direction lying within a (110) crystal plane (2110) of the crystalline substrate (2); Etching (S2) a primary mark (7) extending along a reference direction (R) into the top side (2a); Determining (S3) a reference angle (αR) between the Z direction (Z) and the reference direction (R); Determining (S4a) a first angle (α1) between the top side (2a) and a (100) crystal plane (2100) of the crystalline substrate (2) in a projection in the direction of an X / Y plane (XY) which is perpendicular to the Z direction (Z), and / or Determining (4b) a second angle (α2) between the top side and the (100) crystal plane (2100) in a Y / Z plane (YZ) perpendicular to the X direction (X); Producing (S5) an optical waveguide (4) on the top side (2a) of the crystalline substrate (2) based on the reference angle (αR) such that a fiber core (51) of an optical fiber (5) with predetermined dimensions inserted into a second V-groove (3) is aligned with a waveguide core (41) of the optical waveguide (4), and Etching (S6) the second V-groove (3) into the top surface (2a) by anisotropic etching, wherein a width (W) of the second V-groove (3) at the top surface (3) in an X direction (X) and a depth (D) of the second V-groove (3) in a Y direction (Y) from the top surface (2a) are adjusted on the basis of the determined first angle (α1) and / or the second angle (α2). [5] The method according to claim 4, wherein the reference V-groove (6) has a first inclined surface (6a) and a second inclined surface (6b) opposite to the first inclined surface (6a), wherein the anisotropic etching is performed such that the first and second inclined surfaces (6a, 6b) lie in a (111) crystal plane (2111) of the crystalline substrate (2). [6] A method according to any one of claims 4 or 5, wherein etching a second V-groove (3) comprises: Calculating a displacement of the depth (ΔD) of the second V-groove (3) based on the determined first angle (α1). [7] A method according to any one of claims 4 to 6, wherein the manufacturing of the optical waveguide (4) comprises: Setting an orientation of an etching mask on the top side based on the determined reference angle (αR); and Producing the optical waveguide (4) such that a waveguide core (41) of the optical waveguide (4) extends along the Z direction (Z) in a projection towards the top side (2a). [8] Method according to one of claims 4 to 7, wherein the manufacturing of the optical waveguide (4) comprises: Calculating a first X-shift (ΔX1) based on the determined reference angle (αR); and Manufacturing the optical waveguide (4) such that the waveguide core (41) of the optical waveguide (4) is positioned based on the first X-shift (ΔX1). [9] A method according to claim 8, wherein the manufacture of the optical waveguide (4) comprises: Calculating a second X-displacement (ΔX2) based on the displacement in depth (ΔD); and Manufacturing the optical waveguide (4) such that the waveguide core (41) is positioned based on the second X-shift (ΔX2). [10] A method according to any one of claims 4 to 9, wherein the manufacturing of the optical waveguide (4) comprises: Producing the optical waveguide (4) such that the waveguide core (41) is arranged along the Z-direction (Z) in a projection towards the top side (2a). [11] A method according to any one of claims 5 to 10, wherein the etching of the second V-groove (3) comprises: Etching the second V-groove (3) such that a trench axis (3c) at which the first and second inclined surfaces (3a, 3b) of the second V-groove (3) meet lies in a (100) crystal plane (2) of the crystalline substrate (2).

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