Spot-size converter for coupling single-mode fiber to SOI waveguides.

Abstract

The present disclosure relates to an optical edge coupler or spot-size converter (SSC) that includes multiple juxtaposed layers of auxiliary waveguides with underlying silicon-on-insulator (SOI) waveguides. 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 layers collectively include at least two sections. In the first section, each auxiliary waveguide diverges in the direction of light propagation. In the second section, each outer auxiliary waveguide converges toward each other. The SOI waveguides partially overlap the first section and completely overlap and extend beyond the second section. The SSC facilitates coupling of light from a flat-cleaved standard single-mode fiber with a mode field diameter of 10.4 μm at 1550 nm into the SOI waveguide, which has low loss and flat-band response at C-band and L-band communication wavelengths.

Description

[Technical Field]

[0001] The present disclosure relates to integrated optical waveguides, and more particularly to optical waveguide edge couplers or spot size converters that provide mode conversion between waveguides that support propagation modes of different orders, and further provide efficient adiabatic coupling between the waveguides to achieve smaller device footprints, high-efficiency coupling, improved mechanical reliability, and simpler manufacturing and packaging. [Background technology]

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

[0003] Existing solutions for achieving efficient coupling between silicon waveguides and single-mode fibers (SMFs) include surface gratings and spot-size converters (SSCs), such as inverse tapers, suspended edge couplers, and multilayer edge couplers. However, existing solutions are insufficient. Summary of the Invention [Problem to be solved by the invention]

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

[0005] Edge couplers containing inverse tapers operate based on the mode overlap principle and therefore have wide optical bandwidths. However, mode expansion is limited by substrate leakage due to the proximity of the silicon substrate to the buried oxide layer (BOX) of silicon-on-insulator (SOI) wafers. Low coupling loss can be achieved with fibers with small mode-field diameters (MFDs), but coupling loss increases substantially with standard single-mode fibers with larger MFDs.

[0006] Another class of edge couplers uses silicon oxide cladding material (hereafter, "silicon oxide" and "oxide" are used interchangeably) to form the coupling waveguide. This is achieved by separating the silicon oxide coupling waveguide from the bulk cladding material through a series of etching steps. As a result, the suspended oxide waveguide is isolated from the cladding material and the bulk substrate, preventing substrate leakage. Furthermore, the width of the suspended oxide waveguide can be adjusted to match the mode size of the waveguide to that of the input beam. The coupling interface can be further improved by using index-matching oil between the fiber and the cleaved chip facet by mitigating the oxide-air-oxide interface.

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

[0008] To improve mechanical reliability while maintaining high-efficiency coupling with standard single-mode fiber, another class of edge couplers has emerged, which contain multiple layers of high-index waveguides with small geometries within a low-index cladding material. These SSCs possess the advantages of conventional edge couplers, including large optical bandwidths and low polarization-dependent losses compared to grating couplers, while maintaining mechanical rigidity due to the absence of a suspension structure.

[0009] These devices contain several layers of high-index waveguides coated with low-index materials, usually placed on top of a routing waveguide. In these SSCs with multi-layer waveguides, they usually have three or more high-index waveguide layers, including silicon oxynitride (SiON), silicon nitride (SiN), and silicon dioxide (SiO2) coated materials, to couple light from SMFs with various MFDs into various waveguide materials, such as SiN or SOI. [Means for solving the problem]

[0010] According to one aspect, there is provided a spot size converter comprising: a layered arrangement having a first section and an adjacent second section, the layered arrangement being arranged in a light propagation direction subsequent to the first section and crossing the second section, the layered arrangement having a plurality of upper and lower layers, each upper layer including a plurality of auxiliary waveguides, the plurality of auxiliary waveguides including a plurality of inner auxiliary waveguides and a plurality of outer auxiliary waveguides, the lower layer including a silicon-on-insulator (SOI) waveguide that partially crosses the first section and completely crosses the second section, the width of the SOI waveguides expanding in the light propagation direction; and a cladding covering the auxiliary waveguides and the SOI waveguides, wherein in the first section, the width of each auxiliary waveguide expands in the light propagation direction, and in the second section, the width of each auxiliary waveguide is not tapered, and the outer auxiliary waveguides are arranged to converge to each other.

[0011] In some embodiments, in the first section, the spacing between the inner auxiliary waveguides tapers in the light propagation direction, and in the second section, the spacing between the inner auxiliary waveguides does not taper.

[0012] In some embodiments, within the first section and the second section, the 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, within the first section, 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, and within the second section, the spacing between the one of the inner auxiliary waveguides and the adjacent one of the outer auxiliary waveguides does not taper.

[0014] In some embodiments, within the first section, 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, and within at least a portion of the second section, the spacing between one of the inner auxiliary waveguides and the adjacent one of the outer auxiliary waveguides is zero.

[0015] In some embodiments, the upper layer includes up to two auxiliary waveguide layers.

[0016] In some embodiments, each upper auxiliary waveguide layer includes up to four auxiliary waveguide layers.

[0017] In some embodiments, the layered arrangement has an initial section adjacent to the first section and distal from the second section, within which the width of each auxiliary waveguide layer does not taper.

[0018] In some embodiments, within the second section, the outer auxiliary waveguide layer is formed to converge towards an axial plane of the SOI waveguide layer.

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

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

[0021] In some embodiments, each auxiliary waveguide comprises silicon nitride, silicon oxynitride, or aluminum nitride, and the cladding material comprises silicon oxide or silicon dioxide. [Brief explanation of the drawings]

[0022] The drawings are for illustrative purposes only and are not intended to limit the invention.

[0023] [Figure 1] FIG. 1 is a top perspective view of an embodiment of a spot size converter. [Figure 2A] FIG. 2A is a cross-sectional view of the spot size converter taken along line AA' shown in FIG. [Figure 2B] FIG. 2B is a cross-sectional view of the spot size converter taken along line BB' shown in FIG. [Figure 2C] FIG. 2C is a cross-sectional view of the spot size converter taken along line CC' shown in FIG. [Figure 3] FIG. 3 shows the simulated optical mode shape at the front face of the spot-size converter at 1550 nm for transverse electric (TE) mode excitation. [Figure 4A] FIG. 4A shows the measured optical loss spectra of the spot-size converter in the telecommunications C-band and L-band obtained from five fabricated samples. [Figure 4B] FIG. 4B shows the corresponding 1-dB deviation tolerance window along the horizontal axis, ie, the direction perpendicular to the substrate surface normal. [Figure 5A]5A-5F are isometric views of various multi-layer edge coupler schemes, with FIG. 5A showing a conventional multi-layer edge coupler. [Figure 5B] FIG. 5B shows a conventional multilayer edge coupler. [Figure 5C] FIG. 5C shows a conventional multilayer edge coupler. [Figure 5D] FIG. 5D shows a conventional multilayer edge coupler. [Figure 5E] FIG. 5E shows a conventional multilayer edge coupler. [Figure 5F] FIG. 5F illustrates a spot size converter according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0024] In the following description, numerous specific details are set forth in order to provide a thorough understanding of various exemplary and non-limiting embodiments. However, it will be understood by those skilled in the art that embodiments of the present invention may be practiced without some or all of these specific details. It is understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of the present invention. In the drawings, like reference labels or numerals refer to the same or similar functionality or features throughout the several views.

[0025] An embodiment described in relation to one of the devices or methods is equally valid for the other devices or methods, and similarly, an embodiment described in relation to the device is equally valid for the method, and vice versa.

[0026] Features described in relation to one embodiment may be applicable to the same or similar features in other embodiments. Features described in relation to one embodiment may be applicable to other embodiments even if not explicitly described in those other embodiments. Furthermore, additions and / or combinations and / or alternatives to features described in relation to one embodiment may be applicable to the same or similar features in the other embodiments.

[0027] The articles "a," "an," and "the" when used in reference to a feature or element should be understood to include a reference to one or more features or elements. The term "and / or" includes any combination of one or more associated features or elements. As used in the description and claims, the terms "comprising," "including," "having," and any of their related terms are intended to be open-ended, meaning that there may be additional features or elements other than those listed. 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 construed as imposing limitations on relative position or time sequence between them. The term "to" can include references to "configured to," "adapted to," and "constructed and arranged to," which may be used interchangeably. Additionally, as used herein, the terms "top," "bottom," "upper," "lower," "under," "over," "on," and their related terms are merely for ease of description and may refer to the orientation 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.

[0028] The term "coupled" can be used to include reference to an operative sense and can include, but is not necessarily limited to, physical, optical, and / or electrical connections or couplings, which can be direct or indirect. Thus, for example, two devices can be coupled directly or indirectly via one or more intermediary devices. Based on this disclosure, one of ordinary skill in the art will recognize various ways in which coupling exists in accordance with the foregoing definition.

[0029] The term "length" and its related terms refer to the dimension of an element in the direction of light propagation and along the vertical axis, shown in the figures as the x-axis. 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 both the length and width. In some cases, the term "height" may be alternatively referred to by the term "thickness." The term "vertical" refers to a direction along the "height" (or "thickness") direction. Similarly, the term "horizontal" refers to a direction that lies in a plane perpendicular to the vertical direction, and includes the "width" and "length" directions.

[0030] The term "layer" can include a combination of multiple layers or sub-layers. The term "cover" can refer to partial or complete covering.

[0031] Considering these and other issues with existing solutions, there is a need to reduce coupling losses between silicon-on-insulator (SOI) optical chips and standard single-mode fibers without degrading the mechanical robustness of the edge coupler.

[0032] Furthermore, existing solutions utilize multiple layers of auxiliary waveguides to facilitate spot-size conversion of light from single-mode fiber (SMF) to waveguides with submicron geometries. 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 short device footprint, e.g., requiring fewer mask sets and at lower cost.

[0033] An embodiment of the present invention provides a spot size converter or device including a layered arrangement having an upper layer and at least one lower layer. The upper layer includes multiple clad, tapered auxiliary waveguides, and the lower layer includes a clad SOI waveguide. In each upper layer, the auxiliary waveguides include an inner auxiliary waveguide and an outer auxiliary waveguide. The layered arrangement includes at least two sections arranged adjacently along the optical propagation direction. 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 mode with 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 optical propagation direction. The SOI waveguide is introduced within this first section after a certain propagation length to facilitate adiabatic transition of the mode to the SOI waveguide. In the second section, the outer auxiliary waveguides converge toward each other. This converging arrangement helps maintain adiabatic coupling of the mode from the auxiliary waveguide to the lower SOI waveguide. Therefore, the adiabatic coupling from the auxiliary waveguide to the SOI waveguide is more efficient and therefore the device footprint can be smaller.

[0034] Figure 1 is a top perspective view of one embodiment of a spot size converter 100. Figures 2A-2C are cross-sectional views of the spot size converter 100 taken along lines A-A', B-B', and C-C', respectively, shown in Figure 1.

[0035] The layered array collectively has a longitudinal direction (x-direction) defined in the light propagation direction (see arrow 10 in FIG. 1 ), a transverse direction (y-direction) transverse to the longitudinal direction or light propagation direction 10, and a height direction (z-direction) transverse to the longitudinal and transverse directions. The layered array collectively includes at least a first section and an adjacent second section arranged consecutively in the longitudinal direction. The light propagation direction 10 is conveyed to first traverse the first section and then traverse the second section. The layered array collectively may further include an initial section preceding the first section. In other words, the first section is interposed between the initial section and the second section. Thus, the light propagation direction 10 is conveyed to first traverse the initial section and then traverse the first section and the second section in that order.

[0036] The layered arrangement includes multiple identical upper layers overlying at least one lower layer. Each upper layer includes a lateral array of auxiliary waveguides, including multiple inner auxiliary waveguides and multiple outer auxiliary waveguides. In this embodiment, four auxiliary waveguides are provided, including two inner auxiliary waveguides 106, 116, 108, and 118 sandwiched between two outer auxiliary waveguides 102, 112, 104, and 114. The inner auxiliary waveguides 106, 116, 108, and 118 may be identical to each other. The outer auxiliary waveguides 102, 112, 104, and 114 may or may not be identical to each other. The layout of the auxiliary waveguides in each upper layer is identical. Thus, the auxiliary waveguides in adjacent upper layers overlap each other.

[0037] Each auxiliary waveguide traverses the initial section (if applicable), the first section, and the second section. Thus, each auxiliary waveguide has a length defined vertically, a width defined horizontally, and a thickness defined vertically. The length of each auxiliary waveguide includes the length across the initial section (if applicable), denoted d1, the length across the first section, denoted d3, and the length across the second section, denoted d5. The width of each auxiliary waveguide varies as described in the following paragraphs. The thickness of each auxiliary waveguide may remain constant or may not taper. Each auxiliary waveguide may comprise silicon nitride (SiN), silicon oxynitride (SiON), aluminum nitride (AlN), a material having a refractive index similar to SiN, SiON, or AlN, or other suitable material.

[0038] Within the initial section, the width of each auxiliary waveguide is constant or non-tapered. Referring to FIG. 1 , in the initial section having a length d1, each auxiliary waveguide has a constant width w1 as it extends along the length d1 in the optical propagation direction 10, and the spacing between adjacent auxiliary waveguides is a constant spacing distance s1. The initial section includes two edges facing the optical propagation direction 10, and one of the edges not adjacent to or more distal from the first section is an interface edge configured to mate with an SMF. Thus, each auxiliary waveguide is introduced at the interface edge.

[0039] Within the first section, the widths of the auxiliary waveguides 102, 104, 106, 108 taper. Referring to FIG. 1, in the first section having a length d3, each of the auxiliary waveguides 102, 104, 106, 108 has a width that increases from w1 to w2 along the length d3 of the optical propagation direction 10, that is, w1 < w2. In other words, within the first section, the widths of the auxiliary waveguides 102, 104, 106, 108 widen along the optical propagation direction 10 or taper in the reverse direction. The adjacent auxiliary waveguides 102, 104, 106, 108 collectively have a spacing distance that decreases or tapers from s1 to s2, that is, s1 > s2, along the length d3 of the optical propagation direction 10. In other words, within the first section, the spacing between adjacent auxiliary waveguides 102, 104, 106, 108 tapers or becomes narrower along the optical propagation direction 10.

[0040] Within the second section, the widths of the auxiliary waveguides 112, 114, 116, 118 are constant or do not taper. Referring to FIG. 1, in the second section having a length d5, each of the auxiliary waveguides 112, 114, 116, 118 has a constant width w2 along the length d5 of the optical propagation direction 10. The inner auxiliary waveguides 116, 118 are arranged parallel to each other. In this way, the spacing distance between the inner auxiliary waveguides 116, 118 is constant at s2 or does not taper. The outer auxiliary waveguides 112, 114 are arranged so as not to be parallel to each other, for example, converge toward the axial plane of the SOI waveguide 110 or the like. This axial plane is shown by the white dashed line crossing the SOI waveguide 110 in FIG. 1. In this way, the spacing distance between each outer auxiliary waveguide 112, 114 and the adjacent inner auxiliary waveguide 116, 118 tapers from s2 to s3, that is, s2 > s3, along the length d5 of the optical propagation direction 10.

[0041] It should be understood that the portions of the auxiliary waveguides traversing the first section and the second section are integrally formed or joined to provide a single structure, i.e., sections 102 and 112 provide a single auxiliary waveguide. The same applies to sections 104 and 114, sections 106 and 116, and sections 108 and 118.

[0042] The lower layer includes a SOI structure including a silicon substrate 122, silicon oxide 120 (buried oxide) disposed on the silicon substrate 122, and a silicon nanotaper 110 providing a SOI or routing waveguide disposed on an insulator. 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. Referring to FIG. 1, the SOI waveguide 110 has a length d4, a tapered width, and a non-tapered or constant thickness. The SOI waveguide 110 does not traverse the initial section. The SOI waveguide 110 partially traverses the first section and completely traverses a second portion. The SOI waveguide 110 is introduced into the first section at a position that facilitates an efficient adiabatic mode transition between the mode supported by the auxiliary waveguides 102, 112, 104, 114, 106, 116, 108, 118, for example, within the first section, and the mode collectively supported by the lower SOI waveguide 110, after a propagation length d2 from the starting point of the first section. At the introduction position of the length d4, the SOI waveguide 110 has a width w3, and at the end position of the length d4, it increases or expands to a width w4, i.e., w3 < w4. In other words, the width of the SOI waveguide 110 expands along the optical propagation direction 10 or tapers in the reverse direction.

[0043] The SOI waveguide 110 includes an axial plane extending in the height direction along the central axis of the SOI waveguide 110. This axial plane is shown in FIG. 1 as a dashed line crossing the SOI waveguide 110.

[0044] The cladding 120 is disposed to cover or surround the auxiliary waveguides 102, 112, 104, 114, 106, 116, 108, and 118 and the SOI waveguide 110, at least along a longitudinal direction. In particular, the cladding 120 is interposed between adjacent layers of the layered arrangement and between the auxiliary waveguides 102, 112, 104, 114, 106, 116, 108, and 118. In other words, the auxiliary waveguides 102, 112, 104, 114, 106, 116, 108, and 118 and the SOI waveguide 110 are embedded in the cladding 120. The cladding 120 has a length defined in a longitudinal direction, a width defined in a lateral direction, and a thickness defined in a height direction. The cladding 120 may include silicon oxide, silicon dioxide, or other suitable material.

[0045] 2A-2C, cross-sectional views show two upper layers of auxiliary waveguides 102, 112, 104, 114, 106, 116, 108, and 118 disposed below an SOI waveguide 110. In the upper layers, the auxiliary waveguides 102, 112, 104, 114, 106, 116, 108, and 118 have a constant or non-tapering thickness t1 or t2 throughout the first and second sections, and t1 and t2 may be equal, i.e., t1=t2. The thickness t3 of the SOI waveguide 110 may not be equal to t1 or t2.

[0046] The spacing in the height direction between the silicon substrate 122 and the SOI waveguide 110 is i4. The spacing in the height direction between the SOI waveguide 110 in the upper layer closest to the SOI waveguide 110 and the auxiliary waveguides 102, 112, 104, 114, 106, 116, 108, and 118 is i1. The spacing in the height direction between the auxiliary waveguides 102, 112, 104, 114, 106, 116, 108, and 118 in the adjacent upper layer is i2. The spacing in the height direction between the auxiliary waveguides 102, 112, 104, 114, 106, 116, 108, and 118 in the layer farthest from the SOI waveguide 110 and the top end of the layered arrangement is i3.

[0047] Of the above-mentioned height intervals i1 to i4 and lateral intervals s1 to s3, a coating 120 is provided to cover the auxiliary waveguides 102, 112, 104, 114, 106, 116, 108, and 118 and the SOI waveguide 110. Therefore, the intervals i1 to i4 can also be the thicknesses i1 to i4 of the coating, respectively. Furthermore, the intervals s1 to s3 can also be the widths s1 to s3 of the coating, respectively.

[0048] It should be noted that modifications may be made to the illustrated embodiment.

[0049] In the illustrated embodiment, the width of each auxiliary waveguide increases (or decreases) linearly in the direction of light propagation within the first section. However, in other embodiments, the width of each auxiliary waveguide increases (or decreases) nonlinearly. The nonlinear decrease may be an inverse parabolic decrease. In other embodiments, the width of each auxiliary waveguide may increase linearly along part of its length and increase nonlinearly along other parts of its length.

[0050] In the illustrated embodiment, the width of the SOI waveguide increases linearly (or tapers in the opposite direction) in the direction of light propagation. However, in other embodiments, the width of the SOI waveguide increases nonlinearly (or tapers in the opposite direction). In other embodiments, the width of the SOI waveguide increases linearly along part of its length and nonlinearly along other parts of its length.

[0051] In the illustrated embodiment, the lateral spacing between adjacent auxiliary waveguides within the first section of each top layer tapers linearly in the direction of light propagation. However, in other embodiments, the spacing distance between adjacent auxiliary waveguides tapers nonlinearly. The nonlinear taper may be a parabolic taper. In another alternative embodiment, the spacing distance may be linear along part of the length and nonlinear along another part of the length.

[0052] In the illustrated embodiment, within the second section of each upper layer, the lateral spacing between the inner auxiliary waveguides is constant or does not taper, e.g., the spacing between auxiliary waveguides 116 and 118 is s throughout the second section, but in other alternative embodiments, this spacing between auxiliary waveguides 116 and 118 may taper or narrow in light propagation distance.

[0053] In the illustrated embodiment, in both the first and second sections of each upper layer, the lateral spacing between one of the inner auxiliary waveguides and an adjacent one of the outer auxiliary waveguides may taper in the direction of light propagation. With particular reference to FIG. 1 , in the first section, the spacing between the inner auxiliary waveguide 106, 102 and the outer auxiliary waveguide narrows from s1 to s2 in the direction of light propagation 10, while in the second section, the spacing between the same inner and outer auxiliary waveguides 116, 112 narrows further in the same direction from s2 to s3, such that s1 > s2 > s3. This tapered or narrowed spacing in the second section is configured to confine modes toward the axial plane of the SOI waveguide 110. However, in another embodiment, the spacing between the inner and outer auxiliary waveguides 106, 102 narrows from s1 to s2 in the light propagation direction 10, but the spacing between the same inner and outer auxiliary waveguides 116, 112 may remain constant or not taper within the second section. This non-tapering spacing within the second section is not configured to confine the mode toward the axial plane of the SOI waveguide. However, in yet another alternative embodiment, this spacing between the inner auxiliary waveguide 116, 112 and the outer auxiliary waveguide may taper to zero within at least a portion of the second section, or may remain zero throughout the entire second section if mode coupling to the lower SOI waveguide is complete. In the absence of this spacing, the inner and outer auxiliary waveguides 116, 112 may be positioned adjacent to or in physical contact with one another. The explanations and modifications in this section also apply to the other auxiliary waveguides 106, 102 in the first section and the other auxiliary waveguides 114, 118 in the second section.

[0054] In the illustrated embodiment, the upper layer includes two layers of auxiliary waveguides. In a non-limiting embodiment, the upper layer includes at most two layers of auxiliary waveguides. However, in some alternative embodiments, the upper layer can include at least two layers of auxiliary waveguides, e.g., two, three, four, or more layers.

[0055] In the illustrated embodiment, each upper layer auxiliary waveguide includes four auxiliary waveguides. In a non-limiting embodiment, each upper layer auxiliary waveguide includes at most four auxiliary waveguides. However, in some alternative embodiments, each upper layer auxiliary waveguide can include at least two auxiliary waveguides, for example, two, four, six, or even more auxiliary waveguides.

[0056] In the illustrated embodiment, the layered arrangement includes an initial section adjacent to the first section and distal from the second section, within which the width of each auxiliary waveguide does not taper. In an alternative embodiment, the layered arrangement includes at most the first section and the second section, i.e., the layered arrangement lacks additional portions.

[0057] In a non-limiting example of the illustrated embodiment, the device footprint of the spot size converter is at most 345 μm. The device footprint refers to the length between the left and right edges of the device, e.g., the sum of d1, d2, and d4, as shown in FIG. 1. 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 section is 40 μm, and the total length of the spot size converter is 345 μm.

[0058] In the illustrated embodiment, at least within the first section, the auxiliary waveguide has a symmetrical tapering shape, e.g., longitudinally symmetrical. In another example, the auxiliary waveguide can have an asymmetrical tapering shape.

[0059] It will be appreciated that two or more of the above-described alternative embodiments may be combined as appropriate.

[0060] FIG. 3 shows the simulated optical mode shape for an embodiment of the present disclosure at the end of a spot-size converter device at 1550 nm for TE mode excitation.

[0061] Figure 4A shows the measured coupling loss spectrum, and Figure 4B shows the lateral misalignment tolerance measured along the horizontal axis, i.e., the direction orthogonal to the substrate surface normal and the light propagation direction 10. The device exhibits a total coupling loss of 2.01 ± 0.08 dB / facet, including mode coupling loss into the underlying SOI waveguide, and exhibits a flatband response across the C- and L-bands, with excess coupling loss maintained within 1 dB within a 5.0 μm alignment window.

[0062] Figures 5A-5F show isometric views of various multi-layer edge coupler schemes. In particular, Figures 5A-5E show a conventional multi-layer edge coupler, and Figure 5F shows a spot size converter according to one embodiment.

[0063] 5A shows a structure 510 comprising three juxtaposed layers of SiN waveguides. In the top layer 511, the waveguides extend in the light propagation direction 10. In the middle layer 512 and bottom layer 513, the waveguides taper in the light propagation direction 10. In operation, light is coupled into the waveguides in the top layer 511 from an SMF with an MFD of 10.4 μm.

[0064] Figure 5B shows a structure 520 including three juxtaposed layers 521, 522, and 523 of SiN waveguides and an underlying SOI nanotaper 524. Each layer contains three waveguides. The waveguides in the three juxtaposed layers are untapered, i.e., rod waveguides, and are configured to generate a supermode at the coupling interface with the SMF. In operation, light is coupled from the SMF, which has an MFD of 6.0 μm, into the underlying SOI nanotaper, which diverges in the light propagation direction 10.

[0065] Figure 5C shows a structure 530 including three juxtaposed layers 531, 532, and 533 of SiN waveguides and a bottom SOI nanotaper 524. Each layer contains four waveguides. The waveguides in the three juxtaposed layers are untapered, i.e., rod waveguides, and are configured to generate a supermode at the coupling interface with the SMF. In operation, light is coupled from the SMF, which has an MFD of 10.4 μm, into the bottom SOI nanotaper, which diverges in the light propagation direction 10.

[0066] 5D shows a structure 540 including two juxtaposed layers 541, 542 of SiN waveguides and an underlying SOI nanotaper 543. In the upper layer 541 with three waveguides, the waveguides are untapered, i.e., rod waveguides. In the lower layer 542 with one waveguide, the waveguide diverges in the light propagation direction 10. The lower SOI nanotaper 544 also diverges in the light propagation direction 10. In operation, light is coupled into the lower SiN tapered waveguide 544 from an SMF with an MFD of 10.4 μm.

[0067] Figure 5E shows a structure 550 containing alternating layers of mesa-shaped SiN waveguides 551 and silicon oxide 552 formed by an etching process. The introduction of a thin SiN layer increases the effective refractive index of the structure 550 at the coupling interface. The lower SOI nanotaper 553 extends in the light propagation direction 10. During operation, light is coupled into the lower SOI waveguide 553 from an SMF with an MFD of 10.4 μm.

[0068] Figure 5F shows an embodiment in which structure 560 includes two juxtaposed layers 561, 562 of SiN and a lower SOI nanotaper 563. Each layer contains four SiN waveguides arranged in two sections. The first section contains four SiN nanotapers extending in the direction of light propagation. This section is followed by a second section of four untapered SiN waveguides, but the outer waveguides converge toward the axial center of the lower SOI waveguide to improve adiabatic mode conversion. In operation, light is coupled into the lower SOI nanotaper from an SMF with an MFD of 10.4 µm.

[0069] Embodiments of the present invention provide a variety of advantages, including, but not limited to:

[0070] 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 the structure is beneficial for optical fiber packaging of silicon optical chips because the structure is more robust, i.e., less susceptible to mechanical failure. Conventional suspended edge couplers are less robust and prone to waveguide collapse and chip damage due to etching of undercuts to form cavities in the silicon substrate. Furthermore, the spot-size converter according to embodiments of the present invention is oil-free, i.e., without index-matching oil.

[0071] Unlike conventional edge couplers that use three or more SiN or SiON waveguide layers, at least some embodiments of the present invention utilize a two-layer design that achieves comparable coupling performance while simplifying the manufacturing process. Furthermore, only one mask set is required to define the auxiliary waveguide pattern for each layer, keeping reticle costs low. Furthermore, the use of two layers prevents mechanical stress across the wafer that would be induced by thicker films.

[0072] Unlike conventional edge couplers, where auxiliary waveguides are arranged in a cross pattern, at least some embodiments of the present invention use the same waveguide pattern for each layer in one mask set.

[0073] Unlike some conventional edge couplers in which light is coupled into one of the waveguides that form a set of auxiliary waveguides, e.g., SiN, at least some embodiments of the present invention couple light into SOI waveguides, which can be used to form electro-optic modulators and photodetectors by ion implantation.

[0074] Unlike some conventional edge couplers in which the auxiliary waveguide is constructed from a slab waveguide, which requires an additional etching step to form a mesa structure for mode matching, at least some embodiments of the present invention use buried waveguides, which do not require an etching step to form the auxiliary waveguide.

[0075] Unlike some conventional edge couplers in which the auxiliary waveguide is not tapered but is rod-like, at least some embodiments of the present invention employ a two-stage or two-stage split auxiliary waveguide configuration, where the first section converts the large mode size from the fiber-coupling interface to a smaller mode size supported by the underlying SOI routing waveguide, and adiabatic coupling of the optical mode from the auxiliary waveguide is maintained during the second stage, which is structurally focused to the axial center of the spot-size converter. Thus, compared to rod-like designs, embodiments of the present invention facilitate more efficient adiabatic coupling from the auxiliary waveguide to the SOI waveguide, enabling a smaller device footprint. Shorter device lengths are achieved by using a two-stage split auxiliary waveguide configuration. The first section converts the mode size to match the mode supported by the underlying SOI waveguide, and the second section of the auxiliary waveguide facilitates the adiabatic transition while structurally focusing to the axial center of the device.

[0076] Thus, in an exemplary embodiment coupling light from a standard SMF with a 10.4 μm MFD at 1550 nm into an SOI waveguide, comprising a double layer of quadruple-tapered auxiliary SiN waveguides embedded in a silicon oxide cladding, this embodiment achieves a 2 dB / facet coupling loss at 1550 nm for TE polarization in a device length as short as 345 μm compared to prior literature. Furthermore, the 1-dB lateral misalignment tolerance is superior to conventional suspended edge couplers.

[0077] It should be understood that the above-described embodiments and features are to be considered illustrative and not restrictive. Many other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the invention. Moreover, specific terminology has been used for the purpose of clarity of description and is not intended to limit the disclosed embodiments of the invention.

Claims

1. A layered array having a first section and an adjacent second section, arranged in the optical propagation direction following the first section and traversing the second section, having a plurality of upper and lower layers, each upper layer including a plurality of auxiliary waveguides including a plurality of inner auxiliary waveguides and a plurality of outer auxiliary waveguides, and the lower layer including silicon-on-insulator (SOI) waveguides that partially traverse the first section and completely traverse the second section, wherein the width of the SOI waveguides is widened in the optical propagation direction, and A covering that covers the plurality of auxiliary waveguides and the SOI waveguide, wherein in the first section, the width of each auxiliary waveguide is widened in the direction of optical propagation, and in the second section, the width of each auxiliary waveguide is not tapered, and the outer auxiliary waveguides are arranged to converge with each other. Spot size converter.

2. Within the first section, the spacing between the inner auxiliary waveguides tapers in the direction of optical propagation, and within the second section, the spacing between the inner auxiliary waveguides does not taper. The spot size converter according to claim 1.

3. Within the first and second sections, the distance between one of the inner auxiliary waveguides and one of the adjacent outer auxiliary waveguides tapers in the direction of light propagation. The spot size converter according to claim 1 or 2.

4. Within the first section, the distance between one of the inner auxiliary waveguides and one of the adjacent outer auxiliary waveguides tapers in the direction of light propagation. In at least a portion of the second section, the distance between one of the inner auxiliary waveguides and the adjacent one of the outer auxiliary waveguides is zero. The spot size converter according to claim 1 or 2.

5. The aforementioned upper layer includes at most two auxiliary waveguide layers. The spot size converter according to claim 1 or 2.

6. Each of the upper auxiliary waveguide layers includes four auxiliary waveguides. The spot size converter according to claim 5.

7. The layered arrangement has an initial section adjacent to the first section and distal to the second section, Within the aforementioned initial section, the width of each auxiliary waveguide does not taper. The spot size converter according to claim 1 or 2.

8. Within the second section, the outer auxiliary waveguide is formed to converge toward the axial plane of the SOI waveguide. The spot size converter according to claim 1 or 2.

9. Within the first section, the width of each auxiliary waveguide extends linearly in the direction of optical propagation. The spot size converter according to claim 1 or 2.

10. The total length of the aforementioned spot size converter is a maximum of 345 μm. The overall length is defined by the length between the left end and the right end of the spot size converter. The spot size converter according to claim 1 or 2.

11. Each auxiliary waveguide contains silicon nitride, silicon oxynitride, or aluminum nitride. The coating comprises silicon dioxide or silicon oxide. The spot size converter according to claim 1 or 2.

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