Spot size converter for adjusting the diameter and / or shape of a mode field of an optical component and method for producing a spot size converter

Abstract

The invention relates to a spot size converter for adjusting the diameter and / or shape of a mode field of an optical component, comprising a substrate (10); an optical waveguide (3) with at least one core layer (31) and at least a first and a second cladding layer (32, 33), wherein the first cladding layer (32) is located between the substrate (10) and the core layer (31), while the second cladding layer (33) is arranged on a side of the core layer (31) facing away from the first cladding layer (32); a first section (100) in which the core layer (31) has a first thickness; and a second section (200) in which the core layer (31) has a second thickness that is less than the first thickness. According to the invention, a ramp region (400) is located between the first and second sections (100, 200) and in which the thickness of the core layer (31) decreases continuously.The invention also relates to a method for producing a spot size converter.

Description

[0001] The invention relates to a spot size converter for adapting the diameter and / or the shape of a mode field of an optical component according to claim 1 and to a method for producing a spot size converter according to claim 13.

[0002] In thin-film photonic integrated circuits (TF-PICs), optical waveguides have a buried oxide cladding layer (BOX) and a thin-film waveguide core layer, which results in strong vertical mode guiding. The refractive index of a BOX material typically ranges between 1.5 and the refractive index of the thin-film waveguide core layer can be greater than 2. Typical thicknesses of the thin-film waveguide core layer are below 1 µm. Furthermore, mode guiding in the horizontal direction (i.e., parallel to a substrate of the TF-PIC) is provided by lateral patterning of the TF-WL layer. For example, mode guiding in the horizontal direction is usually achieved by forming a waveguide ridge (e.g., using an etching process), with the ridge having a width of approximately 2 µm, thus ensuring single-mode waveguiding.However, this waveguide configuration complicates the coupling between a TF-PIC and another optical device, such as a fiber optic cable. The article by Lingyan He et al., "Low-loss fiber-to-chip interface for lithium niobate photonic integrated circuits," Opt. Lett. 44, 2314-2317 (2019), describes a spot size converter that provides an interface between a TF-PIC and an optical fiber. The spot size converter is used to adapt the optical mode field output by the PIC to the optical properties, such as the core diameter, of the optical fiber. However, the disclosed spot size converter may require tight manufacturing tolerances.

[0003] It is an object of the invention to facilitate the manufacture of a spot size converter.

[0004] According to the invention, a spot size converter for adjusting the diameter and / or shape of a mode field of an optical component is provided, comprising - a substrate; - an optical waveguide with at least one core layer and at least a first and a second cladding layer, wherein - the first cladding layer is located between the substrate and the core layer and the second cladding layer is arranged on a side of the core layer facing away from the first cladding layer; - a first section in which the core layer has a first thickness; - a second portion in which the core layer has a second thickness which is less than the first thickness; and - a ramp region located between the first and second sections in which the thickness of the core layer continuously decreases.

[0005] The spot size converter according to the invention can be used as an interface between a TF-PIC or a planar lightwave circuit (PLC) and another optical device such as an optical fiber, a lens, or another PIC. In particular, the optical mode of the TF-PIC can be converted with the aid of the spot size converter to a mode with a larger spot size that corresponds to the size of an optical mode supported by the optical fiber. The continuous decrease in the core layer thickness can lead to a continuous reduction in the vertical confinement of a mode guided by the waveguide, thus broadening the mode size (spot size). The "thickness" of the core layer refers to the extension of the core layer in the vertical direction, i.e., perpendicular to the substrate. Furthermore, it should be noted that the ramp region does not necessarily include a linear ramp.However, the invention is not limited to a particular shape of the ramp area. The invention can allow for less tight manufacturing tolerances and thus the use of lower-resolution processes (e.g., the use of i-line steppers instead of deep-UV steppers or electron beam processes).

[0006] The core layer of the optical waveguide may comprise a guiding structure configured to guide the light laterally, i.e., the guiding structures provide confinement in the horizontal direction (parallel to the substrate).

[0007] The conductive structure comprises or consists of, for example, a rib. The rib may be a structure with lateral sidewalls, wherein a different material than the material forming the core layer is present adjacent to the lateral sidewalls (e.g., adjacent to the material of the second cladding layer).

[0008] According to a further embodiment of the invention, the spot size converter may comprise a region in which the width of the conductive structure decreases (e.g., continuously). The “width” refers to the extent of the conductive structure (e.g., the rib) in a direction parallel to the substrate. For example, the region in which the width of the conductive structure decreases overlaps at least partially with the ramp region. In other words, there may be a part of the spot size converter in which both the thickness (height) of the core layer and the width of the conductive structure decrease. However, it is also possible for the ramp region (e.g., the vertically tapered region) and the region in which the width of the conductive structure decreases (the laterally tapered region) to be arranged one behind the other. For example, the ramp region and the region in which the width of the conductive structure decreases are adjacent to one another.

[0009] The width of the guide structure can be at least substantially constant within the ramp region (i.e., within the vertically tapered region), while decreasing in a region of the spot size converter outside the ramp region. The width of the guide structure can be constant over the entire ramp region or only over a part of it. For example, the width of the guide structure within the ramp region is at least substantially constant, while the width of the guide structure decreases in a region between the ramp region and a first facet that provides an interface to an optical device, e.g., an optical fiber. However, this does not necessarily mean that the guide structure extends to the facet. Rather, the guide structure (e.g., the rib) must not extend to the facet, as described below. A region with decreasing width of the guide structure (i.e.,The ramp region (e.g., a laterally tapered region) may alternatively or additionally be arranged on a side of the ramp region facing away from the first facet, e.g., between the ramp region (with constant width) and another side of the spot size converter (e.g., a second facet) or between the ramp region (with constant width) and an optical component coupled to the spot size converter. The optical component may be an integrated component (e.g., a PIC) and / or a component integrally connected to the spot size converter.

[0010] The generation of the vertically tapered region (i.e. the ramp region) can be independent of the generation of the conductive structure (e.g. a rib) and, for example, independent of the generation of the laterally tapered region. In particular, the process of forming the ramp region can be at least partially separate and independent of the process used to form the conductive structure. For example, the ramp region is generated before the (e.g. laterally tapered) conductive structure, or vice versa. It is also possible for the ramp region and the conductive structure to be produced at least partially simultaneously - but using separate and independent processes (e.g. separate etching processes). The ramp region is produced, for example, using a first lithography process (e.g. grayscale lithography). After the first lithography process, the ramp region can be generated using an etching process, e.g. a dry etching process.To create the guide structure, a second lithography process is performed, which differs from the one used to create the ramp region. After the second lithography process, the guide structure is formed, for example, using an etching process. The formation of the guide structure can be performed before or after the ramp region fabrication process. Since both processes rely on two independent lithography processes, the inclination of the ramp region can be changed at any position along the spot size converter, regardless of the width of the guide structure.

[0011] Furthermore, the shape of the guide structure in the first section may differ from the shape of the guide structure in the second section. The "shape" may refer to a contour in a cross-section perpendicular to a main extension direction of the optical waveguide. For example, the guide structure may have a substantially rectangular shape in one of the sections and a non-rectangular shape in the other section.

[0012] The waveguide core layer can comprise an upper and a lower part, wherein the upper part has the guiding structure formed therein and the lower part extends laterally beyond the lateral light guiding structure. The lower part forms, for example, a layer or slab waveguide structure. The upper part can consist of the guiding structure. Furthermore, the upper and a lower part of the core layer can comprise the same material or be made of the same material. The height of the guiding structure (e.g., the height of a rib) can be at least one-third of the total thickness of the core layer, i.e., the sum of the thicknesses of the upper and lower parts. It is also possible for the guiding structure to be formed by completely removing (e.g., etching) parts of the core layer adjacent to the guiding structure, so that the height of the guiding structure corresponds to the total thickness of the core layer.In this case, the core layer—at least in one area of ​​the spot size converter, e.g., between the ramp area and the facet—consists of the guiding structure. If the guiding structure is a rib, for example, the core layer adjacent to the rib may have been completely removed, so that the thickness of the core layer corresponds to the height of the rib.

[0013] The total thickness of the core layer can be less than 1 µm. The width of the guide structure (e.g., the width of the ridge) can be approximately 2 µm or less (at least outside the region where the width of the guide structure decreases).

[0014] Furthermore, the reduction in the thickness of the core layer can affect primarily or only the lower layer, while the thickness (height) of the guiding structure (e.g., the rib) can be kept constant. In particular, the thickness of the lower part can be reduced essentially to zero, so that at the end of the ramp region, the core layer consists only of the guiding structure. In general, however, the thickness of the upper and / or lower part of the core layer can decrease over the ramp region.

[0015] According to a further embodiment of the invention, the spot size converter comprises a facet through which light can be coupled into an optical device. The optical device can be an optical fiber, a lens, or another PIC, as already explained above. The facet can be formed by a surface of the spot size converter that extends perpendicular to the substrate and the optical waveguide. The facet can be provided with an anti-reflection coating.

[0016] For example, the guiding structure (e.g., the rib) does not extend to the facet. Thus, the lateral boundary provided by the optical waveguide can end at a distance from the facet. However, it is also possible for the guiding structure to extend to the facet. For example, the region in which the width of the guiding structure decreases extends to the facet. In a further embodiment, the guiding structure has a part located between the ramp region and the facet, in which its width is substantially constant. In particular, this part can be located between the facet and the region in which the width of the guiding structure decreases.

[0017] The first and second cladding layers each have a refractive index that is lower than the refractive index of the core layer. For example, the refractive indices of the first and second cladding layers are substantially identical (e.g., when the cladding layers are formed of the same material) or at least similar (within a deviation of, for example, +-10% from each other). Alternatively, the refractive index of the second cladding layer may be in the range of -20% of the refractive index of the first cladding layer and -10% of the refractive index of the core layer. The first cladding layer may consist of a buried oxide layer (BOX). The buried oxide layer comprises, for example, or consists of silicon oxide. The second cladding layer also comprises or consists of silicon oxide. The core layer may comprise or consist of lithium niobate or silicon nitride. The substrate may comprise or consist of silicon or silicon oxide.

[0018] The second cladding layer can at least partially embed the core layer. For example, the second cladding layer embeds the conductive structure, e.g., at least in the first section, the second section, and / or the ramp region.

[0019] The region where the thickness of the core layer continuously decreases can have a minimum length (along the optical fiber) of 5 µm or 10 µm and / or a maximum length of 200 µm, 300 µm or 400 µm.

[0020] Furthermore, the ramp region may have a first end (near or adjacent to the first portion) and a second end (near or adjacent to the second portion), wherein the thickness of the core layer at the first end is greater than the thickness of the core layer at the second end. For example, the thickness of the core layer at a first end of the ramp region is the first thickness, while the thickness of the core layer at the second end of the ramp region is the second thickness. For example, the thickness of the core layer at the second end of the ramp region may be between 150 and 300 nm.

[0021] The invention also relates to an optical device (such as a PIC) with a spot size converter as described above, wherein an optical waveguide of the optical device is connected (e.g. integrally) to the optical waveguide of the spot size converter.

[0022] Furthermore, the invention relates to a method for producing a spot size converter, in particular as described above, comprising the following steps: - Providing a substrate; - Forming an optical waveguide with at least one core layer and at least a first and a second cladding layer, so that - the first cladding layer is located between the substrate and the core layer and the second cladding layer is arranged on a side of the core layer facing away from the first cladding layer, - the core layer has a first thickness in a first portion of the spot size converter, and - the core layer in a second section of the spot size converter has a second thickness which is less than the first thickness, and - forming a ramp region located between the first and second sections in which the thickness of the core layer continuously decreases; - Forming a facet through which light can be coupled into an optical device, wherein - a guiding structure is formed in the core layer, wherein the guiding structure is configured to guide light laterally, wherein the guiding structure does not extend to the facet, and wherein - the core layer comprises lithium niobate or silicon nitride.

[0023] The features described above in connection with the spot size converter according to the invention can of course also be used to implement embodiments of the above method.

[0024] For example, the method may comprise creating a region in which the width of the conductive structure decreases. The ramp region may be formed using a first lithography process, and the conductive structure is formed using a second lithography process that differs from the first lithography process. As already explained above, the first lithography process may be a grayscale lithography process. As also explained above, the ramp region may be created using an etching process (e.g., a dry etching process) after the first lithography process (and, for example, the removal of mask material). After the second lithography process, the conductive structure is formed, e.g., using an etching process. The formation of the conductive structure (including performing the second lithography process) may be carried out before or after performing the method for producing the ramp region (e.g.,before the first lithography process or after an etching process to form the ramp area).

[0025] Embodiments of the invention are described below with reference to the figure.

[0026] The figure shows a spot size converter (SSC) 1 with a substrate 10, which can be made of silicon or silicon oxide (e.g., SiO2). Furthermore, the SSC 1 comprises an optical waveguide 3 with a core layer 31 and a first and a second cladding layer 32, 33. The first cladding layer 32 is arranged between the substrate 10 and the core layer 31, while the second cladding layer 33 is located on top of the core layer 31, i.e., it extends on a side of the core layer 31 facing away from the substrate 10.

[0027] Furthermore, the core layer 31 has a lower and an upper part 310, 311, which are made of the same material, such as lithium niobate or silicon nitride. A conductive structure in the form of a rib 4 is formed in the upper part 311, wherein the lower part 310 forms a layer or slab waveguide that extends beyond the lateral sidewalls of the rib 4. The second cladding layer 33 embeds the rib 4, i.e., it extends over an upper surface and the adjacent sidewalls of the rib 4. Although the second cladding 33 has a width that is less than the width of the lower part 310 of the core layer 31, this is only optional. The second cladding layer 33 can have the same width as the lower part 310 (or the first cladding layer 32) or can even be wider.

[0028] The SSC 1 further comprises a first and a second section 100, 200, wherein the first section 100 is located near a first side 150 of the SSC 1 and the second section 200 is located near a second side of the SSC 1, which is formed as an optical facet 250. The first side 150 can be connected (e.g., integrally) to an optical component such as a PIC (the optical component is not shown in the figure). The optical facet 250 forms an output of the SSC 1, via which light can exit the SSC 1 and be coupled into an optical device such as an optical fiber (also not shown in the figure).

[0029] The thickness of the core layer 31 in the first section 100 is greater than the thickness of the core layer 31 in the second section 200, with a ramp region 400 extending between the first and second sections 100, 200. The thickness is measured perpendicular to the main extension direction of the substrate 10 and refers to the total thickness of the core layer 31, i.e., the sum of the thickness of the lower part 310 and the thickness of the upper part 311. The thickness of the core layer 31 decreases continuously across the ramp region 400, with the thickness of the core layer 31 at a first end 401 of the ramp region 400 corresponding to its thickness in the first section 100, while the thickness of the core layer 31 at a second end 402 of the ramp region 400 corresponds to its thickness in the second section 200. The width of the rib 4 is constant across the entire ramp region 400.However, according to an alternative embodiment, the width of the rib 4 is constant only over a part of the ramp area 400.

[0030] The ramp region 400 is essentially formed by a decrease in the thickness of the lower part 310 of the core layer 31, while the height of the rib 4 is kept substantially constant. However, it is also possible for the height of the rib 4 to be reduced across the ramp region 400. At the second end 402 of the ramp region 400, the core layer 31 outside the rib 4 is completely removed. However, the complete removal of the core layer 31 adjacent to the rib 4 is only optional. It is also possible for the thickness of the lower part 310 to be reduced without completely removing it.

[0031] The optical waveguide 3 further comprises a laterally tapered region 500 between the ramp region 400 and the facet 250, in which the width of the rib 4 decreases. It is possible for the region 500 not to extend over the entire section between the ramp region 400 and the facet 250. Rather, the waveguide 3 may comprise a section of constant width, which is arranged, for example, between the ramp region 400 and the laterally tapered region 500 and / or between the facet 250 and the laterally tapered region 500.

[0032] Reducing the thickness of the waveguide core layer 31 and the width of the rib 4, i.e., the vertical and lateral tapering of the waveguide core layer 31, weakens the confinement of a mode guided by the waveguide 3. This weakened confinement results in an expanding mode size toward the facet 250, allowing efficient coupling to an optical device via the facet 250, as explained above. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited non-patent literature

[0000] Lingyan He, et al. „Lowloss fiber-to-chip interface for lithium niobate photonic integrated circuits“, Opt. Lett. 44, 2314-2317 (2019

[0002]

Claims

[1] Spot size converter for adjusting the diameter and / or shape of a mode field of an optical component, comprising - a substrate (10); - an optical waveguide (3) comprising at least one core layer (31) and at least one first and one second cladding layer (32, 33), wherein - the first cladding layer (32) is located between the substrate (10) and the core layer (31), while the second cladding layer (33) is arranged on a side of the core layer (31) facing away from the first cladding layer (32); - a first section (100) in which the core layer (31) has a first thickness; and - a second portion (200) in which the core layer (31) has a second thickness which is less than the first thickness; - a ramp region (400) located between the first and second sections (100, 200) and in which the thickness of the core layer (31) continuously decreases; - a facet (250) via which light can be coupled into an optical device, wherein - the core layer (31) comprises a guiding structure (4) configured to guide light laterally, wherein the guiding structure (4) does not extend to the facet (250), and wherein - the core layer (31) comprises lithium niobate or silicon nitride. [2] Spot size converter according to claim 1, wherein the guide structure (4) comprises or consists of a rib. [3] Spot size converter according to claim 1 or 2, further comprising a region (500) in which the width of the conductive structure (4) decreases. [4] Spot size converter according to claim 3, wherein the region (500) in which the width of the conductive structure (4) decreases at least partially overlaps with the ramp region (400). [5] Spot size converter according to one of the preceding claims, wherein the width of the guide structure (4) is constant over at least part of the ramp region (400) or over at least the entire ramp region (400). [6] Spot size converter according to one of the preceding claims, wherein the shape of the conductive structure (4) in the first section (100) is different from the shape of the conductive structure (4) in the second section (200). [7] Spot size converter according to one of the preceding claims, insofar as it relates to claim 3, wherein the region (500) in which the width of the conductive structure (4) decreases extends to the facet (250). [8] Spot size converter according to one of the preceding claims, insofar as it relates to claim 3, wherein the conductive structure (4) has a part in which its width is substantially constant, this part being located between the facet (250) and the region (500) in which the width of the conductive structure (4) decreases. [9] Spot size converter according to one of the preceding claims, wherein the core layer (31) is removed outside the conductive structure (4) at least in a region between the ramp region (400) and the facet (250), so that a height of the conductive structure (4) corresponds to the thickness of the core layer (31). [10] Spot size converter according to one of the preceding claims, wherein the first cladding layer (32) is formed by a buried oxide layer. [11] A spot size converter according to any one of the preceding claims, wherein the first and second cladding layers (32, 33) each have a refractive index lower than the refractive index of the core layer (31). [12] An optical device comprising a spot size converter according to any one of the preceding claims, wherein an optical waveguide of the optical device is connected to the optical waveguide (3) of the spot size converter (1). [13] Method for producing a spot size converter, in particular according to one of claims 1 to 12, comprising the following steps: - providing a substrate (10); - forming an optical waveguide (3) with at least one core layer (31) and at least one first and one second cladding layer (32, 33), so that - the first cladding layer (32) is located between the substrate (10) and the core layer (31) and the second cladding layer (33) is arranged on a side of the core layer (31) facing away from the first cladding layer (32), - the core layer (31) has a first thickness in a first section (100) of the spot size converter (1), and - the core layer (31) in a second section (200) of the spot size converter (1) has a second thickness which is less than the first thickness; - forming a ramp region (400) located between the first and second sections (100, 200) and in which the thickness of the core layer (31) continuously decreases, - forming a facet (250) via which light can be coupled into an optical device, wherein - a guide structure (4) is formed in the core layer (31), wherein the guide structure (4) is configured to guide light laterally, wherein the guide structure (4) does not extend to the facet (250), and wherein - the core layer (31) comprises lithium niobate or silicon nitride. [14] Method according to claim 13, further comprising creating a region (500) in which the width of the conductive structure (4) decreases. [15] The method according to claim 13 or 14, wherein the ramp region (400) is formed using a first lithography process and the conductive structure (4) is formed using a second lithography process that is different from the first lithography process.

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