Spot size converter for coupling single mode fiber to SOI waveguide

Abstract

A photonic edge coupler or spot size converter (SSC) includes a plurality of juxtaposed auxiliary waveguide layers, and an underlying silicon-on-insulator (SOI) waveguide. In one embodiment, the SSC includes two waveguide layers, each having four auxiliary waveguides, including two inner auxiliary waveguides interposed between two outer auxiliary waveguides. The waveguide layer includes at least two portions in total. In the first portion, each auxiliary waveguide gradually widens in the light propagation direction. In the second portion, each of the external auxiliary waveguides converges with each other. The SOI waveguide partially overlaps the first portion, completely overlaps the second portion, and extends therefrom. The SSC facilitates coupling light at 1550 nm from a flat-split standard single-mode fiber having a mode field diameter of 10.4 [mu] m to an SOI waveguide having low loss and flat band response at C-band and L-band telecommunications wavelengths.

Description

Technical Field

[0001] The present disclosure relates to integrated photonic waveguides, and more particularly, to photonic edge couplers or spot size converters that provide mode conversion between waveguides supporting propagation modes of different sizes and further provide effective adiabatic coupling between the waveguides to achieve a smaller device footprint, efficient coupling, improved mechanical reliability, and simpler manufacturing and packaging. Background Art

[0002] Low-loss coupling of light into and out of photonic chips is highly desirable in reducing the link budget of optical communication links.

[0003] Existing solutions for achieving efficient coupling between silica waveguides and single-mode fibers (SMFs) include surface gratings and spot size converters (SSCs), such as inverse tapering, suspended edge couplers, and multilayer edge couplers. However, existing solutions are not satisfactory.

[0004] Surface gratings have limited optical bandwidth, and they are polarization sensitive, thus limiting their use in wavelength division multiplexing.

[0005] Edge couplers, including reverse tapering, offer wide optical bandwidths because they operate based on the principle of mode overlap. However, modal expansion is limited by leakage from the substrate, due to the proximity of the silicon substrate to the buried oxide layer (BOX) of the silicon-on-insulator (SOI) wafer. While low coupling losses can be achieved for fibers with small mode field diameters (MFDs), they increase significantly for standard single-mode fibers with larger MFDs.

[0006] Another class of edge couplers uses a silica cladding material (hereafter, "silicon oxide" and "oxide" are used interchangeably) to define the coupled waveguides. This is achieved by isolating the silica coupling waveguides from the bulk cladding material through a series of etching steps. The resulting suspended oxide waveguide provides isolation from the cladding material as well as the bulk substrate, preventing substrate leakage. In addition, the width of the suspended oxide waveguide can be tailored to match the mode size of the waveguide to the mode size of the input beam. The coupling interface is further improved by using an index matching oil between the fiber and the cleaved chip end face to mitigate the oxide-air-oxide interface.

[0007] However, the suspended nature of these edge couplers presents risks in terms of mechanical reliability, such as tip damage and collapse, especially during the fiber packaging process. Furthermore, the use of index-matching oil and UV post-curing steps increase packaging complexity. Therefore, a mechanically robust, oil-free spot size converter is needed to facilitate an efficient coupling interface between standard single-mode optical fibers and silicon photonic chips.

[0008] To improve mechanical reliability while maintaining efficient coupling to standard single-mode fibers, another class of edge couplers has emerged. These couplers consist of multiple layers of higher-index waveguides with smaller feature sizes within a lower-index cladding material. These SSCs offer the advantages of traditional edge couplers, including larger optical bandwidth and lower polarization-dependent loss compared to grating couplers, while maintaining mechanical rigidity due to the lack of any suspended structure.

[0009] These devices consist of several layers of higher refractive index waveguides covered with lower refractive index materials, typically on top of routing waveguides. In these SSCs with multilayer waveguides, they are typically equipped with three or more high refractive index waveguide layers made of silicon oxynitride (SiON), silicon nitride (SiN), and a cladding layer of silicon dioxide (SiO2) to couple light from SMFs with various MFDs to various waveguide materials, such as SiN or SOI. Summary of the Invention

[0010] According to one aspect, there is provided a spot size converter comprising: a layered arrangement having a first portion and an adjacent second portion arranged along a light propagation direction, the light propagation direction passing through the first portion and then through the second portion, the layered arrangement having a plurality of upper layers and a lower layer, wherein each upper layer includes a plurality of auxiliary waveguides, the auxiliary waveguides including a plurality of inner auxiliary waveguides and a plurality of outer auxiliary waveguides, wherein the lower layer includes a silicon-on-insulator (SOI) waveguide that partially passes through the first portion and completely passes through the second portion, wherein a width of the SOI waveguide widens in the light propagation direction; and The cladding covering the auxiliary waveguide and SOI waveguide, wherein, within the first portion, the width of each auxiliary waveguide widens in the light propagation direction, wherein within the second portion, the width of each auxiliary waveguide is non-tapered, and the outer auxiliary waveguides are arranged to converge with each other.

[0011] In some embodiments, within the first portion, the spacing between the inner auxiliary waveguides tapers in the light propagation direction, wherein within the second portion, the spacing distance between the inner auxiliary waveguides is non-tapered.

[0012] In some embodiments, within the first portion and the second portion, a spacing between one of the inner auxiliary waveguides and an adjacent one of the outer auxiliary waveguides tapers in the light propagation direction.

[0013] In some embodiments, in a first portion, the spacing between one of the inner auxiliary waveguides and an adjacent one of the outer auxiliary waveguides tapers in the direction of light propagation, wherein in a second portion, the spacing between one of the inner auxiliary waveguides and an adjacent one of the outer auxiliary waveguides does not taper.

[0014] In some embodiments, within the first portion, the spacing distance between one of the internal auxiliary waveguides and an adjacent one of the external auxiliary waveguides gradually becomes thinner in the direction of light propagation, wherein within at least a portion of the second portion, the spacing distance between one of the internal auxiliary waveguides and an adjacent one of the external auxiliary waveguides is zero.

[0015] In some embodiments, the upper layer includes at most two layers of auxiliary waveguides.

[0016] In some embodiments, each upper layer of auxiliary waveguides includes at most four auxiliary waveguides.

[0017] In some embodiments, the layered arrangement has an initial portion adjacent to the first portion and distal to the second portion, wherein within the initial portion, the width of each auxiliary waveguide is non-tapered.

[0018] In some embodiments, within the second portion, an external auxiliary waveguide is formed to converge toward an axial plane of the SOI waveguide.

[0019] In some embodiments, within the first portion, the width of each auxiliary waveguide linearly widens in the light propagation direction.

[0020] In some embodiments, the device footprint of the spot size converter is at most 345 μm.

[0021] In some embodiments, each auxiliary waveguide comprises silicon nitride, silicon oxynitride, or aluminum nitride, and wherein the cladding material comprises silicon oxide or silicon dioxide. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The accompanying drawings are for illustration only and do not limit the present invention, in which:

[0023] Figure 1 is a perspective view taken from the top of one embodiment of a spot size converter;

[0024] Figures 2A to 2C Along Figure 1 Cross-sectional views of the spot size converter taken along lines A-A', B-B', and C-C';

[0025] Figure 3 shows the simulated optical mode distribution in front of the spot size converter excited with the transverse electric (TE) mode at 1550 nm;

[0026] Figure 4A shows the measured optical loss spectra of telecom C-band and L-band spot size converters obtained from five fabricated samples; Figure 4B The corresponding 1 dB misalignment tolerance window along the horizontal axis (i.e., perpendicular to the substrate surface normal) is shown;

[0027] Figures 5A to 5F Isometric views of various multilayer edge coupler schemes are shown, where Figures 5A to 5E shows a conventional multilayer edge coupler, while Figure 5F A spot size converter according to an embodiment of the present invention is shown. DETAILED DESCRIPTION

[0028] In the following description, numerous specific details are set forth in order to provide a thorough understanding of various illustrative and non-limiting embodiments. However, those skilled in the art will appreciate that embodiments of the present invention may be practiced without some or all of these specific details. It should be understood that the terminology used herein is intended only to describe specific embodiments and is not intended to limit the scope of the present invention. In the accompanying drawings, the same reference labels or numbers refer to the same or similar functions or features in multiple views.

[0029] Embodiments described in the context of one device or method are also applicable to the other device or method. Similarly, embodiments described in the context of a device are also applicable to the method, and vice versa.

[0030] Features described in the context of an embodiment may be applied accordingly to the same or similar features in other embodiments. Features described in the context of an embodiment may be applied accordingly to other embodiments even if not explicitly described in these other embodiments. Furthermore, additions, combinations, and / or substitutions of features described in the context of an embodiment may be applied accordingly to the same or similar features in other embodiments.

[0031] It should be understood that the articles "a," "an," and "the," when referring to features or elements, include reference to one or more features or elements. The term "and / or" includes any and all combinations of one or more related features or elements. The terms "including," "comprising," "having," and any related terms used in the specification and claims are open-ended, meaning that additional features or elements may be present in addition to the listed features or elements. Identifiers such as "first," "second," and "third" are used merely as labels and are not intended to impose numerical requirements on their objects, nor are they to be interpreted in a manner that imposes any relative position or chronological order between limitations. The term "to" may include reference to "configured to," "adapted to," and "constructed and arranged to," which may be used interchangeably. In addition, terms such as "top," "bottom," "upper," "lower," "below," "above," and "upper," and their related terms, as used herein, are merely for convenience of description and may refer to orientations of features or elements as shown in the figures. It should be understood that any orientation of the features described herein is within the scope of the present invention.

[0032] The term "coupled" may be used to include references to operational meanings and may include, but is not limited to, direct or indirect physical, optical, and / or electrical connections or couplings. Thus, for example, two devices may be directly coupled or indirectly coupled through one or more intermediate devices. Based on this disclosure, one of ordinary skill in the art will understand that there are various ways of coupling according to the above definition.

[0033] The term "length" and its related terms refer to the dimension of an element in the direction of light propagation and along the longitudinal axis, which is shown as the x-axis in the figures. The term "width" and its related terms refer to the dimension of an element perpendicular to the propagation of light. The term "height" and its related terms refer to the dimension of an element perpendicular to the length and width. In some cases, the term "height" can also be replaced by the term "thickness". The term "vertical" refers to the direction along the "height" (or "thickness") direction. Similarly, the term "horizontal" refers to the direction lying in a plane perpendicular to the vertical direction, including the "width" and "length" directions.

[0034] The term "layer" may include a combination of two or more layers or sub-layers. The term "covering" may refer to partial or full covering.

[0035] In view of the above and other problems with existing solutions, there is a need to reduce coupling losses between silicon-on-insulator (SOI) photonic chips and standard single-mode optical fibers without reducing the mechanical robustness of the edge coupler.

[0036] In addition, existing solutions utilize multi-layer auxiliary waveguides to facilitate spot size conversion of light from single-mode fiber (SMF) to waveguides with sub-micron feature sizes. However, existing solutions still lack a device that can facilitate efficient optical power coupling from standard SMF (e.g., SMF28e) with a mode field diameter (MFD) of 10.4 μm at 1550 nm to single-mode SOI waveguides, with a small footprint and low cost, such as requiring fewer mask settings.

[0037] Embodiments of the present invention provide a spot size converter or device comprising an upper layer and at least one lower layer. The upper layer includes a plurality of clad tapered auxiliary waveguides, while the lower layer includes a clad SOI waveguide. In each upper layer, the auxiliary waveguides include inner and outer auxiliary waveguides. The layered arrangement comprises at least two sections arranged continuously along the direction of light propagation. In the first section, the auxiliary waveguides are configured to convert the spot size from a larger MFD supported by a standard SMF to a smaller spot size supported by the lower SOI waveguide. This is achieved by increasing or widening the width of each auxiliary waveguide in the direction of light propagation. After a certain propagation length, an SOI waveguide is introduced into this first section to promote adiabatic transition of the mode into the SOI waveguide. In the second section, the outer auxiliary waveguides converge. This converging arrangement helps maintain adiabatic coupling of the modes from the auxiliary waveguides to the lower SOI waveguide. As a result, adiabatic coupling from the auxiliary waveguides to the SOI waveguide is more efficient, enabling a smaller device footprint.

[0038] Figure 1 is a perspective view taken from the top of one embodiment of the spot size converter 100 . Figures 2A to 2C are along Figure 1 FIG. 1 is a cross-sectional view of the spot size converter 100 taken along lines AA′, BB′, and CC′.

[0039] In general, the layered arrangement has a longitudinal direction (x-direction), which is defined as the direction of light propagation (see Figure 1 The layered arrangement generally includes at least one first portion and one adjacent second portion, which are arranged consecutively in the longitudinal direction. The light propagation direction 10 first passes through the first portion and then through the second portion. Generally speaking, the layered arrangement may also include an initial portion that precedes the first portion. In other words, the first portion is located between the initial portion and the second portion. Thus, the light propagation direction 10 first passes through the initial portion and then passes through the first and second portions in sequence.

[0040] The layered arrangement includes multiple identical upper layers covering at least one lower layer. Each upper layer includes a lateral array of multiple auxiliary waveguides, wherein the auxiliary waveguides include multiple internal auxiliary waveguides and multiple external auxiliary waveguides. In this embodiment, four auxiliary waveguides are provided, which include two internal auxiliary waveguides 106, 116, 108, 118 placed between two external auxiliary waveguides 102, 112, 104, 114. The internal auxiliary waveguides 106, 116, 108, 118 can be different from the external auxiliary waveguides 102, 112, 104, 114. The internal auxiliary waveguides 106, 116, 108, 118 can be identical to each other. The external auxiliary waveguides 102, 112, 104, 114 can be identical to each other or different from each other. The layout of the auxiliary waveguides in each upper layer is identical. Therefore, the auxiliary waveguides of adjacent upper layers overlap or superimpose each other.

[0041] Each auxiliary waveguide passes through the initial part (if applicable), the first part and the second part. Therefore, each auxiliary waveguide has a length defined in the longitudinal direction, a width defined in the transverse direction and a thickness defined in the height direction. The length of each auxiliary waveguide includes its length through the initial part (if applicable), which can be represented by d1, its length through the first part, which can be represented by d3, and its length through the second part, which can be represented by d5. The width of each auxiliary waveguide varies as described in the following paragraphs. The thickness of each auxiliary waveguide can remain unchanged or not be thinner. Each auxiliary waveguide can include silicon nitride (SiN), silicon oxynitride (SiON), aluminum nitride (AlN), a material with a refractive index similar to SiN, SiON or AlN, or other suitable materials.

[0042] Within the initial section, the width of each auxiliary waveguide is constant or not tapered. Figure 1 In an initial portion having a length d1, each auxiliary waveguide has a constant width w1 as it extends along the length d1 in the light propagation direction 10, while the spacing between adjacent auxiliary waveguides has a constant spacing s1. The initial portion includes two opposing edges in the light propagation direction 10, wherein one of the edges not adjacent to or further from the first portion is an interface edge configured to interface with the SMF. Therefore, each auxiliary waveguide is introduced at the interface edge.

[0043] In the first portion, the width of each auxiliary waveguide 102, 104, 106, 108 gradually becomes thinner. Figure 1In the first section of length d3, the width of each auxiliary waveguide 102, 104, 106, 108 along the length d3 in the light propagation direction 10 increases from w1 to w2, i.e., w1 < w2. In other words, within the first section, the width of each auxiliary waveguide 102, 104, 106, 108 gradually widens or tapers in the light propagation direction 10. The spacing between adjacent auxiliary waveguides 102, 104, 106, 108 along the length d3 in the light propagation direction 10 decreases or narrows from s1 to s2, i.e., s1 > s2. In other words, within the first section, the spacing between adjacent auxiliary waveguides 102, 104, 106, 108 gradually tapers or narrows in the light propagation direction 10.

[0044] In the second portion, the width of each auxiliary waveguide 112, 114, 116, 118 is constant or not tapered. Figure 1 In the second portion of length d5, each auxiliary waveguide 112, 114, 116, 118 has a constant width w2 along the length d5 ​​in the light propagation direction 10. The inner auxiliary waveguides 116, 118 are arranged parallel to each other. Therefore, the spacing between the inner auxiliary waveguides 116, 118 is constant or not tapered at s2. The outer auxiliary waveguides 112, 114 are not arranged parallel to each other, for example, they converge toward each other, for example, toward the axial plane of the SOI waveguide 110. The axial plane is at Figure 1 It is shown by the white dashed line passing through the SOI waveguide 110. Therefore, the spacing between each outer auxiliary waveguide 112, 114 and its adjacent inner auxiliary waveguide 116, 118 gradually tapers from s2 to s3 along the length d5 ​​in the light propagation direction 10, that is, s2>s3.

[0045] It will be appreciated that the portions of the auxiliary waveguide passing through the first and second portions are integrally formed or connected to provide an integral structure, i.e. portions 102 and 112 provide a single auxiliary waveguide; the same applies to portions 104 and 114, portions 106 and 116, portions 108 and 118.

[0046] The lower layer includes an SOI structure including a silicon substrate 122, silicon oxide 120 (buried oxide) disposed on the silicon substrate 122, and silicon nanopyramids 110 disposed on an insulator, the silicon nanopyramids providing an SOI or routing waveguide. The SOI waveguide has a length defined in the longitudinal direction, a width defined in the transverse direction, and a thickness defined in the height direction. Figure 1, SOI waveguide 110 has a length d4, a tapering width, and a non-tapering or constant thickness. SOI waveguide 110 does not extend through the initial portion; it partially extends through the first portion and completely extends through the second portion. SOI waveguide 110 is introduced into the first portion after a propagation length d2 from the start of the first portion, e.g., at a location within the first portion that promotes efficient adiabatic mode conversion between modes supported by auxiliary waveguides 102, 112, 104, 114, 106, 116, 108, 118 and modes jointly supported by the underlying SOI waveguide 110. At the introduction location of length d4, SOI waveguide 110 has a width w3, which increases or widens to w4 at the termination location of length t4, i.e., w3 < w4. In other words, the width of SOI waveguide 110 gradually widens or tapers in the direction of light propagation 10.

[0047] The SOI waveguide 110 includes an axial plane extending in the height direction along the central axis of the SOI waveguide 110. Figure 1 1 is shown by a dotted line passing through the SOI waveguide 110 .

[0048] The cladding 120 is arranged to cover or surround the auxiliary waveguides 102, 112, 104, 114, 106, 116, 108, 118 and the SOI waveguide 110 at least in the longitudinal direction. In particular, the cladding 120 is placed between adjacent layers of the layered arrangement and between the auxiliary waveguides 102, 112, 104, 114, 106, 116, 108, 118. In other words, the auxiliary waveguides 102, 112, 104, 114, 106, 116, 108, 118 and the SOI waveguide 110 are embedded in the cladding 120. The cladding 120 has a length defined in the longitudinal direction, a width defined in the transverse direction, and a thickness defined in the height direction. The cladding 120 may include silicon oxide, silicon dioxide, or other suitable materials.

[0049] refer to Figures 2A to 2C , a cross-sectional view shows two upper-layer auxiliary waveguides 102, 112, 104, 114, 106, 116, 108, 118 arranged on a lower layer of SOI waveguide 110. In the upper layer, the thickness t1 or t2 of the auxiliary waveguides 102, 112, 104, 114, 106, 116, 108, 118 is constant or not tapered throughout the first and second portions, where t1 and t2 can be equal, i.e., t1=t2. The thickness t3 of the SOI waveguide 110 can be different from t1 and t2.

[0050] The height distance between the silicon substrate 122 and the SOI waveguide 110 is i4. The height distance between the SOI waveguide 110 and the auxiliary waveguide 102, 112, 104, 114, 106, 116, 108, 118 in the upper layer that is closest to the SOI waveguide 110 is i1. The height distance between the auxiliary waveguides 102, 112, 104, 114, 106, 116, 108, 118 in the adjacent upper layer is i2. The height distance between the auxiliary waveguides 102, 112, 104, 114, 106, 116, 108, 118 in the layer farthest from the SOI waveguide 110 and the top edge of the layered arrangement is i3.

[0051] Within the above-mentioned intervals i1 to i4 taken in the height direction and the intervals s1 to s3 taken in the lateral direction, a cladding layer 120 is provided to cover or surround the auxiliary waveguides 102, 112, 104, 114, 106, 116, 108, 118 and the SOI waveguide 110. Therefore, the intervals i1 to i4 may alternatively be referred to as cladding thicknesses i1 to i4, respectively; and the intervals s1 to s3 may alternatively be referred to as cladding widths s1 to s3, respectively.

[0052] It will be appreciated that modifications may be made to the embodiments shown.

[0053] In the illustrated embodiment, within a first portion, the width of each auxiliary waveguide widens linearly (or inversely tapers) in the direction of light propagation. However, in alternative embodiments, the width of each auxiliary waveguide may widen nonlinearly (or inversely taper). The nonlinear inverse taper may be an inverse parabolic inverse taper. In another alternative embodiment, the width of each auxiliary waveguide may widen linearly along a portion of its length and nonlinearly along another portion of its length.

[0054] In the illustrated embodiment, the width of the SOI waveguide widens linearly (or inversely tapers) in the direction of light propagation. However, in another embodiment, the width of the SOI waveguide may widen nonlinearly (or inversely taper). In another alternative embodiment, the width of the SOI waveguide may widen linearly along a portion of its length and nonlinearly along another portion of its length.

[0055] In the illustrated embodiment, the lateral spacing between adjacent auxiliary waveguides in the first portion of each upper layer tapers linearly in the direction of light propagation. However, in another embodiment, the spacing between adjacent auxiliary waveguides can taper nonlinearly. The nonlinear taper can be parabolic. In another alternative embodiment, the spacing can widen linearly along one portion of the length and nonlinearly along another portion of the length.

[0056] In the illustrated embodiment, the spacing between the internal auxiliary waveguides in the lateral direction within the second portion of each upper layer is constant or does not taper. For example, the spacing between the auxiliary waveguides 116 and 118 is s2 throughout the second portion. However, in alternative embodiments, the spacing between the auxiliary waveguides 116 and 118 may taper or narrow over the light propagation distance.

[0057] In the illustrated embodiment, within the first and second portions of each upper layer, the lateral spacing between an inner auxiliary waveguide and an adjacent outer auxiliary waveguide tapers in the direction of light propagation. Figure 1 In the first portion, the spacing between the inner auxiliary waveguide 106 and the outer auxiliary waveguide 102 narrows from s1 to s2 in the light propagation direction 10; in the second portion, the spacing between the same inner auxiliary waveguide 116 and the outer auxiliary waveguide 112 further narrows from s2 to s3 in the same direction, where s1>s2>s3. This tapering or narrowing spacing in the second portion is configured to confine the mode to the axial plane of the SOI waveguide 110. However, in another embodiment, although the spacing between the inner auxiliary waveguide 106 and the outer auxiliary waveguide 102 narrows from s1 to s2 in the light propagation direction 10, the spacing between the same inner auxiliary waveguide 116 and the outer auxiliary waveguide 112 may remain unchanged or non-tapered in the second portion. This non-tapering spacing in the second portion is not configured to confine the mode to the axial plane of the SOI waveguide. However, in another alternative embodiment, if mode coupling with the underlying SOI waveguide is achieved, the spacing between the inner and outer auxiliary waveguides 116, 112 can be tapered to zero within at least a portion of the second portion, or maintained at zero throughout the second portion. In the absence of such spacing, the inner auxiliary waveguide 116 and the outer auxiliary waveguide 112 can be adjacent to or physically contact each other. The description and modifications in this paragraph also apply to the other auxiliary waveguides 106, 102 in the first portion and the other auxiliary waveguides 114, 118 in the second portion.

[0058] In the embodiment shown, the upper layer includes two layers of auxiliary waveguides. In a non-limiting example, the upper layer includes at most two layers of auxiliary waveguides. However, in some alternative embodiments, the upper layer may include at least two layers of auxiliary waveguides, for example, two, three, four, or more layers.

[0059] In the illustrated embodiment, each upper layer of auxiliary waveguides includes four auxiliary waveguides. In a non-limiting example, each upper layer of auxiliary waveguides includes at most four auxiliary waveguides. However, in some alternative embodiments, each upper layer of auxiliary waveguides may include at least two auxiliary waveguides, for example, two, four, six, or a higher even number of auxiliary waveguides.

[0060] In the embodiment shown, the layered arrangement comprises an initial portion adjacent to the first portion and distal to the second portion, wherein within the initial portion, the width of each auxiliary waveguide is non-tapered. In an alternative embodiment, the layered arrangement comprises at most the first portion and the second portion, i.e., the layered arrangement has no additional portions.

[0061] In the non-limiting example of the embodiment shown, the device footprint of the spot size converter is at most 345 μm. The device footprint is the length between the leftmost edge and the rightmost edge of the device, e.g. Figure 1 The sum of d1, d2 and d4 is shown. In this example, d1 is 5 μm, d3 is 200 μm, d5 is 100 μm, the extension of the SOI waveguide 110 beyond the second portion is 40 μm, and the total length of the spot size converter is 345 μm.

[0062] In the embodiment shown, at least in the first portion, the auxiliary waveguide has a symmetrical tapering profile, for example symmetrical about the longitudinal direction. In another example, the auxiliary waveguide may have an asymmetrical tapering profile.

[0063] It will be appreciated that two or more of the above-described alternative embodiments may be combined as appropriate.

[0064] Figure 3 The simulated optical mode distribution of the disclosed embodiment of the present invention at 1550 nm with TE mode excitation at the edge of the spot size converter device is shown.

[0065] Figure 4A shows the measured coupling loss spectrum, Figure 4B The figure shows the lateral misalignment tolerance measured along the transverse axis (i.e., perpendicular to the substrate surface normal and the light propagation direction 10). The device has a total coupling loss of 2.01 ± 0.08 dB / plane, including mode coupling loss with the underlying SOI waveguide, with flat-band response in the C- and L-bands, and excess coupling loss is kept within 1 dB within a 5.0 μm alignment window.

[0066] Figures 5A to 5F Isometric views of various multilayer edge coupler schemes are shown. In particular, Figures 5A to 5E shows a conventional multilayer edge coupler, while Figure 5F A spot size converter according to an embodiment is shown.

[0067] Figure 5A Structure 510 is shown, comprising three juxtaposed SiN waveguide layers. In the top layer 511, the waveguide gradually widens in the direction of light propagation 10. In the middle layer 512 and the bottom layer 513, the waveguide gradually tapers in the direction of light propagation 10. In operation, light is coupled from an SMF with an MFD of 10.4 μm into the waveguide of the top layer 511.

[0068] Figure 5B Structure 520 is shown, comprising three juxtaposed SiN waveguide layers 521, 522, and 523 and an underlying SOI nanopyramid 524. Each layer includes three waveguides. The waveguides in the three juxtaposed layers are non-tapered, i.e., rod-shaped, and are configured to generate supermodes at the coupling interface with the SMF. In operation, light is coupled from an SMF with an MFD of 6.0 μm into the underlying SOI nanopyramid, which gradually widens in the direction of light propagation 10.

[0069] Figure 5C Structure 530 is shown, comprising three juxtaposed SiN waveguide layers 531, 532, and 533 and an underlying SOI nanopyramid 524. Each layer includes four waveguides. The waveguides in the three juxtaposed layers are non-tapered, i.e., rod-shaped, and are configured to generate supermodes at the coupling interface with the SMF. In operation, light is coupled from an SMF with an MFD of 10.4 μm into the underlying SOI nanopyramid, which gradually widens in the direction of light propagation 10°.

[0070] Figure 5D Structure 540 is shown, comprising two juxtaposed SiN waveguide layers 541 and 542 and a lower layer of SOI nanopyramids 543. In the top layer 541, which has three waveguides, the waveguides are non-tapered, i.e., rod-shaped. In the bottom layer 542, which has one waveguide, the waveguide gradually widens in the direction of light propagation 10. The lower layer of SOI nanopyramids 544 also gradually widens in the direction of light propagation 10. In operation, light is coupled from an SMF with an MFD of 10.4 μm into the bottom SiN tapered waveguide 544.

[0071] Figure 5E Structure 550 is shown, consisting of alternating layers of mesa-shaped SiN waveguides 551 and silicon oxide 552, formed by an etching process. The introduction of thin SiN layers increases the effective refractive index of structure 550 at the coupling interface. The underlying SOI nanopyramids 553 gradually widen in the direction of light propagation 10°. In operation, light is coupled from an SMF with an MFD of 10.4 μm into the SOI waveguide 553 below.

[0072] Figure 5FAn embodiment is shown in which structure 560 comprises two juxtaposed SiN layers 561 and 562 and an underlying SOI nanopyramid 563. Each layer contains four SiN waveguides and is divided into two sections. The first section comprises four SiN nanopyramids that gradually widen in the direction of light propagation. This section is followed by a second section of four SiN waveguides that are non-tapered, but the outer waveguides converge toward one another, for example, the axial center of the underlying SOI waveguide, to improve adiabatic mode conversion. In operation, light is coupled into the underlying SOI nanopyramids from an SMF with an MFD of 10.4 μm.

[0073] Embodiments of the present invention provide various advantages, such as but not limited to the following.

[0074] The spot size converter structure according to embodiments of the present invention facilitates efficient optical coupling from a standard SMF to an underlying SOI single-mode waveguide. The non-suspended nature of this structure is advantageous for fiber packaging of silicon photonic chips because it is more robust and less susceptible to mechanical failure than conventional suspended edge couplers, which are less robust and susceptible to undercut etching, which can create cavities in the silicon substrate, leading to waveguide collapse and tip damage. Furthermore, the spot size converter according to embodiments of the present invention is oil-free, meaning it does not contain any refractive index-matching oil.

[0075] Unlike some conventional edge couplers that use three or more SiN or SiON waveguide layers, at least some embodiments of the present invention employ a dual-layer design, simplifying the manufacturing process while achieving comparable coupling performance. Furthermore, only a single mask set is required to define the auxiliary waveguide pattern in each layer, reducing mask costs. Furthermore, using a dual layer prevents mechanical stress on the wafer that would otherwise be caused by thicker films.

[0076] Unlike some conventional edge couplers where the auxiliary waveguides are arranged in a cross-like pattern, at least some embodiments of the present invention use the same waveguide pattern with a set of masks in each layer.

[0077] Unlike some conventional edge couplers, where light is coupled to one of the waveguides forming an auxiliary waveguide group, such as SiN, at least some embodiments of the present invention couple light to an SOI waveguide, which can be made into electro-optic modulators and photodetectors by ion implantation.

[0078] Unlike some conventional edge couplers where the auxiliary waveguide is made of a slab waveguide that requires an additional etching step to form a mesa structure for mode matching, at least some embodiments of the present invention use embedded waveguides that do not require an etching step when forming the auxiliary waveguide.

[0079] Unlike some conventional edge couplers, where the auxiliary waveguide is non-tapered but rod-like, at least some embodiments of the present invention employ a dual-stage or dual-section auxiliary waveguide arrangement, where the first section converts the large mode size at the fiber coupling interface to a smaller mode size supported by the underlying SOI routing waveguide, while maintaining adiabatic coupling of the light modes from the auxiliary waveguide during the second stage, with the second section structurally converging to the axial center of the spot size converter. Consequently, compared to rod-like designs, embodiments of the present invention facilitate more efficient adiabatic coupling from the auxiliary waveguide to the SOI waveguide, thereby enabling a smaller device footprint. Shorter device lengths are achieved by utilizing a dual-stage auxiliary waveguide formation, where the first section converts the mode size to be compatible with the modes supported by the underlying SOI waveguide, while the second section of the auxiliary waveguide structurally converges to the axial center of the device while facilitating the adiabatic transition.

[0080] Therefore, for the illustrated embodiment, which includes a dual-layer auxiliary SiN waveguide with a quad-tapered structure, each embedded in a silicon oxide cladding, to couple light from a standard SMF with an MFD of 10.4 μm at 1550 nm to an SOI waveguide, this embodiment achieves 2 dB / plane coupling loss for TE polarization at 1550 nm within a shorter device length of 345 μm compared to existing literature. Furthermore, the 1 dB lateral misalignment tolerance is superior to that of conventional suspended edge couplers.

[0081] It should be understood that the above embodiments and features should be considered as exemplary, rather than restrictive. By considering the description and practice of the present invention, those skilled in the art will understand many other embodiments. In addition, for clarity of description, certain terms are used without limiting the disclosed embodiments of the present invention.

Claims

1. A spot size converter, comprising: a layered arrangement having a first portion arranged along a light propagation direction and a second portion adjacent to the first portion, the light propagation direction passing through the first portion and then passing through the second portion, the layered arrangement having a plurality of upper layers and a lower layer, wherein each upper layer includes a plurality of auxiliary waveguides, the plurality of auxiliary waveguides including a plurality of inner auxiliary waveguides and a plurality of outer auxiliary waveguides, wherein the lower layer includes a silicon-on-insulator (SOI) waveguide that partially passes through the first portion and completely passes through the second portion, wherein a width of the SOI waveguide gradually widens in the light propagation direction; and a cladding covering the auxiliary waveguide and the SOI waveguide, Wherein, in the first portion, the width of each auxiliary waveguide gradually widens in the light propagation direction, in the second portion, the width of each auxiliary waveguide is non-thinned, and the outer auxiliary waveguides are arranged to converge with each other.

2. The spot size converter according to claim 1, wherein: In the first portion, the spacing between the internal auxiliary waveguides is tapered in the light propagation direction, and in the second portion, the spacing between the internal auxiliary waveguides is non-tapered.

3. The spot size converter according to any one of claims 1 to 2, wherein: In the first portion and the second portion, a spacing between one of the inner auxiliary waveguides and an adjacent one of the outer auxiliary waveguides gradually tapers in the light propagation direction.

4. The spot size converter according to any one of claims 1 to 2, wherein: In the first portion, the spacing between one of the inner auxiliary waveguides and an adjacent one of the outer auxiliary waveguides in the light propagation direction gradually tapers, and in the second portion, the spacing between one of the inner auxiliary waveguides and an adjacent one of the outer auxiliary waveguides is non-tapered.

5. The spot size converter according to any one of claims 1 to 2, wherein: In the first portion, the spacing distance between one of the internal auxiliary waveguides and an adjacent one of the external auxiliary waveguides gradually becomes thinner in the light propagation direction, and in at least a portion of the second portion, the spacing between one of the internal auxiliary waveguides and an adjacent one of the external auxiliary waveguides is zero.

6. The spot size converter according to any one of claims 1 to 5, wherein: The upper layer includes at most two layers of auxiliary waveguides.

7. The spot size converter according to claim 6, wherein: The auxiliary waveguides of each upper layer include at most four auxiliary waveguides.

8. The spot size converter according to any one of claims 1 to 7, the layered arrangement having an initial portion adjacent to the first portion and distal to the second portion, wherein within the initial portion, the width of each auxiliary waveguide is non-tapered.

9. The spot size converter according to any one of claims 1 to 8, wherein: In the second portion, the outer auxiliary waveguide is formed to converge toward an axial plane of the SOI waveguide.

10. The spot size converter according to any one of claims 1 to 9, wherein: Within the first portion, the width of each auxiliary waveguide linearly widens in the light propagation direction.

11. The spot size converter according to any one of claims 1 to 10, wherein: The device space occupied by the spot size converter is at most 345 μm.

12. The spot size converter according to any one of claims 1 to 11, wherein: Each auxiliary waveguide comprises silicon nitride, silicon oxynitride, or aluminum nitride, and wherein the cladding material comprises silicon oxide or silicon dioxide.

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