Spot-size converting structures and photonic devices

Abstract

Spot size conversion structure (100) and photonic device (1). The spot size conversion structure (100) includes a substrate (101), an insulating layer (102), and a waveguide layer (103) arranged in sequence. The waveguide layer (103) includes N sub-waveguide layers (1030) arranged in sequence along a direction away from the substrate (101), each of the sub-waveguide layers having a protruding shape, N being a natural number, N≧3. The spot size conversion structure (100) has a first end face (100a) configured to be coupled to an integrated optical waveguide, and a second end face (100b) opposite the first end face (100a) and configured to be coupled to an optical fiber. In a direction away from the substrate (101), the orthogonal protrusion of the (n+1)th sub-waveguide layer (1030) on the substrate (101) is within the range of the orthogonal protrusion of the nth sub-waveguide layer (1030) on the substrate (101), where n is a natural number and 1≦n≦N-1. In a direction away from the first end face (100a), the width of the orthogonal protrusion of each sub-waveguide layer (1030) on the substrate (101) gradually narrows. The wide end of each sub-waveguide layer (1030) extends to the first end face (100a), the narrow end of the first sub-waveguide layer (1030) extends to or is a fixed distance from the second end face (100b), and the narrow end of the (n+1)th sub-waveguide layer (1030) is a fixed distance from the narrow end of the nth sub-waveguide layer (1030).

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to Patent Application No. 202111158077.8, filed September 28, 2021, entitled "SPOT-SIZE CONVERSION STRUCTURE AND PHOTONIC DEVICE," the disclosure of which is hereby incorporated by reference in its entirety.

[0002] FIELD OF THE DISCLOSURE The present disclosure relates to the field of photonic technology, and in particular to spot size conversion structures and photonic devices. [Background technology]

[0003] An optical waveguide is a dielectric device that guides the propagation of light waves, also called a dielectric optical waveguide. There are two main types of optical waveguides: one type is an integrated optical waveguide, which includes planar dielectric optical waveguides and slab dielectric optical waveguides, which generally function as part of an optoelectronic integrated device and are therefore also called integrated optical waveguides; the other type is a cylindrical optical waveguide, which is usually called an optical fiber.

[0004] The technique of coupling integrated optical waveguides to optical fibers has extremely wide and important applications in the fields of optical communications, microwave photonics, laser beam deflection, wavefront conditioning, etc. Edge coupling is a common method used to couple integrated optical waveguides to optical fibers.

[0005] However, due to the large difference in spot size between integrated optical waveguides and optical fibers, how to improve the coupling efficiency between integrated optical waveguides and optical fibers has always been an important topic under research by those skilled in the art. Summary of the Invention

[0006] According to one aspect of the present disclosure, there is provided a spot size conversion structure including a substrate, an insulating layer, and a waveguide layer arranged in sequence, wherein the waveguide layer includes N sub-waveguide layers arranged in sequence along a direction away from the substrate, each of the sub-waveguide layers having a protruding shape, N being a natural number and N≧3, the spot size conversion structure having a first end face configured to be coupled to an integrated optical waveguide and a second end face opposite to the first end face and configured to be coupled to an optical fiber, and a (n+1)th sub-waveguide layer on the substrate in the direction away from the substrate. The orthogonal protrusion of a sub-waveguide layer is within the range of the orthogonal protrusion of an n-th sub-waveguide layer on the substrate, where n is a natural number and 1≦n≦N−1; the width of each orthogonal protrusion of the sub-waveguide layer on the substrate gradually narrows in a direction away from the first end face; the wide end of each sub-waveguide layer extends to the first end face, the narrow end of the first sub-waveguide layer extends to or is a certain distance from the second end face, and the narrow end of the (n+1)-th sub-waveguide layer is a certain distance from the narrow end of the n-th sub-waveguide layer.

[0007] In some embodiments, the thickness c of at least one sub-waveguide layer other than the Nth sub-waveguide layer and the (N-1)th sub-waveguide layer satisfies c≦200 nm.

[0008] In some embodiments, each of the sub-waveguide layers includes a plurality of waveguide portions, the plurality of waveguide portions including reduced-width waveguide portions, the widths of the orthogonal protrusions of the reduced-width waveguide portions on the substrate gradually decreasing in a direction away from the first end face, the plurality of waveguide portions of the Nth sub-waveguide layer further including equal-width waveguide portions extending to the first end face, the widths of the orthogonal protrusions of the equal-width waveguide portions on the substrate being equal at all positions.

[0009] In some embodiments, each of the sub-waveguide layers includes a plurality of waveguide portions, the plurality of waveguide portions including reduced-width waveguide portions and equal-width waveguide portions, the widths of the orthogonal protrusions of the reduced-width waveguide portions on the substrate gradually decrease in a direction away from the first end face, the widths of the orthogonal protrusions of the equal-width waveguide portions on the substrate are equal at all positions, the waveguide portion of the Nth sub-waveguide layer extending to the first end face is an equal-width waveguide portion, and the waveguide portion of each sub-waveguide layer closest to the second end face is an equal-width waveguide portion.

[0010] In some embodiments, the waveguide portion of each of the sub-waveguide layers closest to the second end face is an equal width waveguide portion having a triangular or trapezoidal cross section.

[0011] In some embodiments, the spot size conversion structure further includes a first side surface intersecting the first end face and the second end face, and a second side surface, and a waveguide portion of at least one sub-waveguide layer other than the Nth sub-waveguide layer extending to the first end face further extends to the first side surface and / or the second side surface.

[0012] In some embodiments, each of the orthogonal projections of the N sub-waveguide layers on the substrate has the same surface against mirror surface It has a symmetrical structure.

[0013] In some embodiments, the spot size converting structure has a plurality of slots extending toward and exposing the substrate, and a communication structure provided in the substrate and communicating with the plurality of slots, wherein orthogonal projections of the plurality of slots on the substrate are distributed on both sides of the orthogonal projections of the waveguide layer on the substrate. In some embodiments, at least one slot is exposed to the second end face.

[0014] According to one aspect of the present disclosure, there is provided a photonic device including a spot size converting structure according to any one of the above-mentioned embodiments.

[0015] It should be understood that the contents of this section are not intended to identify critical or important features of the embodiments of the present disclosure, and are not intended to limit the scope of the present disclosure. Other features of the present disclosure will be readily apparent from the following description.

[0016] More details, features and advantages of the present disclosure are disclosed in the following description of exemplary embodiments with reference to the accompanying drawings. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a top view of a spot size converting structure according to some example embodiments of the present disclosure. [Figure 2A] FIG. 2A is a schematic cross-sectional view of FIG. 1 at AA according to some exemplary embodiments of the present disclosure. [Figure 2B] FIG. 2B is a schematic cross-sectional view at BB in FIG. 1 according to some exemplary embodiments of the present disclosure. [Figure 2C] FIG. 2C is a schematic cross-sectional view at CC in FIG. 1 according to some exemplary embodiments of the present disclosure. [Figure 2D] FIG. 2D is a schematic cross-sectional view at DD in FIG. 1 according to some exemplary embodiments of the present disclosure. [Figure 2E] FIG. 2E is a schematic cross-sectional view at EE of FIG. 1 according to some exemplary embodiments of the present disclosure. [Figure 3] FIG. 3 is a top view of a spot size converting structure according to some other example embodiments of the present disclosure. [Figure 4] FIG. 4 is a structural block diagram of a photonic device according to some example embodiments of the present disclosure. DETAILED DESCRIPTION OF THE INVENTION

[0018] Only some exemplary embodiments are briefly described below. As will be understood by those skilled in the art, the described embodiments may be modified in various ways without departing from the spirit and scope of the present disclosure. Accordingly, the accompanying drawings and descriptions are to be regarded as illustrative in nature, and not as restrictive.

[0019] Although existing spot size converters can functionally realize coupling between integrated optical waveguides and optical fibers, the coupling efficiency between the integrated optical waveguide and the optical fiber is not high due to the large difference in spot size between them, resulting in a large loss of optical energy. For example, the spot size of an integrated optical waveguide is generally on the order of hundreds of nanometers, while the spot size of an optical fiber, such as a flat-ended optical fiber, is on the order of tens of microns. The lost optical energy can lead to severe heating of the coupling end face, which can affect the reliability and service life of the device. Coupling efficiency can be understood as the ratio of the optical power emitted by the integrated optical waveguide to the optical power received by the optical fiber, or the ratio of the optical power emitted by the optical fiber to the optical power received by the integrated optical waveguide.

[0020] SUMMARY OF THE INVENTION Embodiments of the present disclosure provide spot-size converting structures and photonic devices that improve coupling efficiency between integrated optical waveguides and optical fibers and reduce optical losses during spot-size conversion.

[0021] 1, 2A, 2B, 2C, 2D, and 2E, a spot size converting structure 100 provided in some embodiments of the present disclosure includes a substrate 101, an insulating layer 102, and a waveguide layer 103, which are arranged in sequence. The waveguide layer 103 includes N sub-waveguide layers 1030 (e.g., four as shown in the drawings) arranged in sequence along a direction away from the substrate 101, each of the sub-waveguide layers having a protruding shape, where N is a natural number and N≧3. The spot size converting structure 100 has a first end face 100a configured to be coupled to an integrated optical waveguide and a second end face 100b opposite the first end face 100a and configured to be coupled to an optical fiber. In a direction away from the substrate 101, the orthogonal protrusion of the (n+1)th sub-waveguide layer 1030 on the substrate 101 is within the range of the orthogonal protrusion of the nth sub-waveguide layer 1030 on the substrate 101, where n is a natural number and 1≦n≦N−1. The width of the orthogonal protrusion of each sub-waveguide layer 1030 on the substrate 101 gradually narrows in a direction away from the first end face 100a. The wide end of each sub-waveguide layer 1030 extends to the first end face 100a, the narrow end of the first sub-waveguide layer 1030 extends to or is a certain distance from the second end face 100b, and the narrow end of the (n+1)th sub-waveguide layer 1030 is a certain distance from the narrow end of the nth sub-waveguide layer 1030.

[0022] In a direction away from the substrate 101, the N sub-waveguide layers 1030 are the first sub-waveguide layer, the second sub-waveguide layer, the third sub-waveguide layer, ..., the (N-1)th sub-waveguide layer, and the Nth sub-waveguide layer, respectively. In one embodiment shown in Figure 1, N = 4, and in one embodiment shown in Figure 3, N = 3. Additionally, N may be other numbers, such as N = 5 or N = 6.

[0023] In an embodiment of the present disclosure, the length direction is defined as the direction in which light propagates within the waveguide layer 103, and the width direction is defined as being parallel to the substrate 101 and perpendicular to the direction in which light propagates within the waveguide layer 103. The first end face 100a and the second end face 100b are aligned substantially along the direction in which light propagates within the waveguide layer 103. In addition to the first end face 100a and the second end face 100b, the spot size converting structure 100 further includes a first side face 100c and a second side face 100d that intersect with the first end face 100a and the second end face 100b. In one embodiment, the first end face 100a is parallel to the second end face 100b, and the first side face 100c is parallel to the second side face 100d.

[0024] As shown in FIGS. 2A, 2B, 2C, 2D, and 2E, in some embodiments of the present disclosure, the spot size conversion structure 100 further includes a cover layer 104 on one side of the waveguide layer 103 away from the substrate 101. The refractive index n1 of the waveguide layer 103, the refractive index n2 of the cover layer 104, the refractive index n3 of the insulating layer 102, and the refractive index n4 of the substrate 101 satisfy n1 > n2 and n1 > n3. The refractive indices of the insulating layer 102 and the cover layer 104 are smaller than those of the waveguide layer 103, so that light may be primarily confined to propagate within the waveguide layer 103. After the waveguide layer 103 narrows and disappears completely, the insulating layer 102 and a portion of the cover layer 104 near the second end face 100b form a waveguide structure used for light transmission. In some other embodiments of the present disclosure, the spot size conversion structure 100 may also not include the cover layer 104.

[0025] The first end face 100a of the spot size converting structure 100 can function as a light input end face of the spot size converting structure 100, and the second end face 100b can function as a light output end face of the spot size converting structure 100. Specifically, light enters the spot size converting structure 100 from the wide ends of the N sub-waveguide layers 1030 and is output from the second end face 100b of the spot size converting structure 100. The first end face 100a of the spot size converting structure 100 can function as a light output end face of the spot size converting structure 100, and the second end face 100b can also function as a light input end face of the spot size converting structure 100. Specifically, light enters the spot size converting structure 100 from the second end face 100b of the spot size converting structure 100 and is output from the wide ends of the N sub-waveguide layers 1030. The spot size of the waveguide structure formed by the insulating layer 102 and the portion of the cover layer 104 near the second end face 100b substantially matches the spot size of the optical fiber, so that the spot size converting structure 100 may be more optimally coupled to the optical fiber at the second end face 100b.

[0026] Some spot size conversion structures in the related art have a waveguide layer including two sub-waveguide layers: a plate-shaped plate layer and a ridge layer protruding from the plate layer. The width of the orthogonal protrusions of the two sub-waveguide layers on the substrate can also gradually narrow in the direction away from the first end face due to the size limitations of the end face on one side that needs to be coupled to the integrated optical waveguide. However, the thickness of the two sub-waveguide layers, especially the plate layer, cannot be designed to be smaller than this. For example, the thickness of the plate layer is usually within the range of about 300 nanometers. When the plate layer is thick, it is difficult to further narrow the width of the narrow end of the plate layer due to limitations in the manufacturing process.

[0027] In the embodiment of the present disclosure, the waveguide layer 103 may have greater freedom in shape design and may be designed to be smoother (e.g., the first sub-waveguide layer in the drawing may have more waveguide portions, resulting in a more gradual narrowing of the width), resulting in a smoother spot transition from the first end face to the second end face, or from the second end face to the first end face. On the other hand, provided that the design requirement for the coupling size of the first end face 100a is satisfied (i.e., the thicknesses of the Nth and (N-1)th sub-waveguide layers can satisfy the design requirement for the coupling size of the first end face 100a), the thicknesses of the first sub-waveguide layer to the (N-2)th sub-waveguide layer may be designed to be as small as possible in accordance with this requirement, and the widths of the narrow ends of the first sub-waveguide layer to the (N-2)th sub-waveguide layer may be free from limitations in the manufacturing process. The thickness may be designed to be as small as possible according to this requirement, so that more light is forced to overflow the waveguide layer 103 and then output from the second end face 100b (in this case, the first end face 100a is the light input end face and the second end face 100b is the light output end face), or more light is forced to enter the waveguide layer 103 (in this case, the second end face 100b is the light input end face and the first end face 100a is the light output end face).

[0028] Therefore, the solution in the embodiments of the present disclosure may improve the freedom in designing the thickness and narrow end size of some sub-waveguide layers, thereby minimizing the loss of light transmission and improving the coupling efficiency of the spot size conversion structure.

[0029] In some embodiments of the present disclosure, based on the improvements described above, the thickness c of at least one of the sub-waveguide layers other than the Nth and (N-1)th sub-waveguide layers satisfies c≦200 nm. In one example of a waveguide layer including four sub-waveguide layers, the thickness of at least one of the first and second sub-waveguide layers may be designed to be 200 nm or less. In one example of a waveguide layer including three sub-waveguide layers, the thickness of the first sub-waveguide layer may be designed to be 200 nm or less.

[0030] In embodiments of the present disclosure, each sub-waveguide layer may include one or more waveguide sections, and these should include at least a reduced-width waveguide section. A reduced-width waveguide section refers to a waveguide section in which the width of the orthogonal protrusion on the substrate gradually decreases in a direction away from the first end face. The present disclosure does not specifically limit the number or structural shape of the waveguide sections in each sub-waveguide layer. For example, if a sub-waveguide layer includes multiple waveguide sections, the waveguide section of the sub-waveguide layer closest to the second end face may be either an equal-width waveguide section or a reduced-width waveguide section. An equal-width waveguide section is a waveguide section in which the width of the orthogonal protrusion on the substrate is equal at all positions. The following are merely some exemplary embodiments.

[0031] In some embodiments of the present disclosure, each sub-waveguide layer includes multiple waveguide portions, including reduced-width waveguide portions and equal-width waveguide portions. The widths of the orthogonal protrusions of the reduced-width waveguide portions on the substrate 101 are gradually reduced, e.g., gradually reduced linearly or nonlinearly, in a direction away from the first end face 100a, and the widths of the orthogonal protrusions of the equal-width waveguide portions on the substrate 101 are equal at all positions. The waveguide portion of the Nth sub-waveguide layer extending to the first end face 100a is an equal-width waveguide portion, and the waveguide portion of each sub-waveguide layer closest to the second end face 100b is an equal-width waveguide portion. 1, the third sub-waveguide layer includes a first equal-width waveguide portion 301, a first reduced-width waveguide portion 302, and a second equal-width waveguide portion 303, arranged in sequence in a direction away from the first end face 100a. The first and second sub-waveguide layers each include a third equal-width waveguide portion 304, a second reduced-width waveguide portion 305, a fourth equal-width waveguide portion 306, a third reduced-width waveguide portion 307, and a fifth equal-width waveguide portion 308, arranged in sequence in a direction away from the first end face 100a. Of course, the number of waveguide portions in each sub-waveguide layer may be the same or different.

[0032] In some other embodiments of the present disclosure, each sub-waveguide layer includes a plurality of waveguide portions, the plurality of waveguide portions including reduced-width waveguide portions. The widths of the orthogonal protrusions of the reduced-width waveguide portions on the substrate are gradually reduced linearly or nonlinearly in a direction away from the first end face. The plurality of waveguide portions of the Nth sub-waveguide layer further include equal-width waveguide portions extending to the first end face, the widths of the orthogonal protrusions of the equal-width waveguide portions on the substrate are equal at all positions. In these embodiments, sub-waveguide layers other than the Nth sub-waveguide layer may include only reduced-width waveguide portions, or may include both reduced-width and equal-width waveguide portions.

[0033] Each sub-waveguide layer in the embodiment of the present disclosure includes at least one reduced-width waveguide section to achieve the narrowing of the sub-waveguide layer width. Furthermore, the waveguide section of the Nth sub-waveguide layer extending to the first end face 100b must be an equal-width waveguide section. According to design requirements, the waveguide section of at least one sub-waveguide layer closest to the second end face 100b can also be an equal-width waveguide section or a reduced-width waveguide section.

[0034] The waveguide portion of each sub-waveguide layer closest to the second end face 100b is an equal-width waveguide portion with a triangular or trapezoidal cross section, such as an isosceles triangle or an isosceles trapezoid, for example. The design may allow light to pass through the equal-width waveguide portion of the upper sub-waveguide layer closest to the second end face 100b and enter the lower sub-waveguide layer, or pass through the equal-width waveguide portion of the first sub-waveguide layer closest to the second end face 100b and enter the insulating layer 102, thereby contributing to light transmission between the sub-waveguide layers, such as increasing the efficiency of light transmission and further improving the coupling efficiency between the integrated optical waveguide and the optical fiber.

[0035] 1 or 3, the waveguide portion of each sub-waveguide layer (which may be one or several sub-waveguide layers closer to the substrate) extending to the first end face 100a other than the Nth sub-waveguide layer is a waveguide portion of equal width extending to the first end face 100a, the first side face 100c, and the second side face 100d at the same time. In this manner, the spot size converting structure 100 may be adapted to an integrated optical waveguide having a larger spot size, thereby improving the applicability of the spot size converting structure 100.

[0036] As shown in FIG. 1 or 3, each of the orthogonal projections of the N sub-waveguide layers 1030 on the substrate 101 has the same surface (not shown in the drawing) mirror surfaceThe N sub-waveguide layers may have an asymmetric structure. For example, the N sub-waveguide layers may be gradually narrowed on only one side.

[0037] 2A, 2B, 2C, 2D, and 2E, the spot size converting structure 100 further includes a cover layer 104 on one side of the waveguide layer 103 away from the substrate 101, and the spot size converting structure 100 has a plurality of slots 105 extending from the surface of the cover layer 104 toward the substrate 101 and exposing the substrate 101, and a communication structure 106 provided in the substrate 101 and communicating with the plurality of slots 105. Orthogonal protrusions of the plurality of slots 105 on the substrate 101 are distributed on both sides of the orthogonal protrusions of the waveguide layer 103 on the substrate 101. In addition, at least one slot 105 is exposed to the second end face 100b. In some embodiments, the spot size converting structure may not include a cover layer, and the plurality of slots extend from the surface of the insulating layer toward the substrate and expose the substrate. Since light is mainly transmitted within the medium, such a design may have a limiting effect on the light input or output from the second end face 100b, thereby preventing the diffusion of light entering the substrate 101, thereby reducing the loss of light transmission, achieving more optimal coupling to the optical fiber, and further improving the coupling efficiency between the integrated optical waveguide and the optical fiber.

[0038] 4, an embodiment of the present disclosure further provides a photonic device 1 including the spot size converting structure 100 according to any one of the above-described embodiments. The specific product type of the photonic device 1 is not limited, and for example, it may be an electro-optic modulator, a splitter, a star coupler, a variable optical attenuator (VOA), an optical switch, a frequency comb, an arrayed waveguide grating (AWG), etc.

[0039] The spot-size converting structure 100 may be integrated into the photonic device 1. The spot-size converting structure 100 has a higher coupling efficiency, so that the light loss in the photonic device 1 is smaller and its performance is improved.

[0040] In this description, the orientations, positional relationships, or dimensions indicated by terms such as "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" are orientations, positional relationships, or dimensions shown based on the accompanying drawings, and it should be understood that these terms are used merely for ease of description and do not indicate or imply that the referred-to devices or elements need to have a particular orientation or be constructed or operated in a particular orientation, and therefore should not be construed to limit the scope of protection of the present disclosure.

[0041] Additionally, terms such as "first," "second," and "third" are for descriptive purposes only and should not be construed to indicate or imply the relative importance of the indicated technical features or the number of indicated technical features. Thus, features defined using "first," "second," and "third" may explicitly or implicitly include one or more features. In the description of this disclosure, the term "plurality" means two or more unless otherwise explicitly or specifically defined.

[0042] In this disclosure, unless otherwise expressly stated or defined, terms such as "attach," "connect," "connected," and "secure" should be interpreted broadly, for example, they may be fixed, detachable, or integral connections, may be mechanical or electrical connections or communications, and may be direct connections or indirect connections using intermediate media, or internal communications between two elements, or interactions between two elements. Those skilled in the art may understand the specific meanings of the above terms in this disclosure according to the specific circumstances.

[0043] In this disclosure, unless expressly stated or defined otherwise, a reference to a first feature being "above" or "below" a second feature may include a case where the first feature is in direct contact with the second feature, or a case where the first feature and the second feature are not in direct contact but are in contact via another feature. Furthermore, a reference to a first feature being "above," "above," or "on" a second feature merely indicates that the first feature is directly or diagonally above the second feature, or that the first feature is at a higher level than the second feature. A reference to a first feature being "below," "below," or "below" a second feature merely indicates that the first feature is directly or diagonally below the second feature, or that the first feature is at a lower level than the second feature.

[0044] This description provides many different implementations or examples that can be used to implement the present disclosure. It should be understood that these different implementations or examples are purely illustrative and are not intended to limit the scope of protection of the present disclosure in any way. Based on the disclosure of the description of the present disclosure, those skilled in the art will be able to come up with various modifications or alternatives. All these modifications or alternatives should fall within the scope of protection of the present disclosure. Therefore, the scope of protection of the present disclosure should be subject to the scope of protection of the claims.

Claims

1. A spot size conversion structure comprising a substrate, an insulating layer, and a waveguide layer, which are arranged in order, wherein the waveguide layer includes N sub-waveguide layers arranged in order along a direction away from the substrate, each of the sub-waveguide layers having a protruding shape, N being a natural number and N≧3; the spot size converting structure has a first end face configured to be coupled to an integrated optical waveguide and a second end face opposite the first end face configured to be coupled to an optical fiber; In a direction away from the substrate, an orthogonal protrusion of an (n+1)th sub-waveguide layer of the N sub-waveguide layers on the substrate is within a range of an orthogonal protrusion of an nth sub-waveguide layer of the N sub-waveguide layers on the substrate, n being a natural number, 1≦n≦N−1; a width of each of the orthogonal protrusions of the sub-waveguide layer on the substrate gradually narrows in a direction away from the first end face; the wide end of each of the sub-waveguide layers extends to the first end face, the narrow end of a first sub-waveguide layer extends to or is a fixed distance from the second end face, and the narrow end of the (n+1)th sub-waveguide layer is a fixed distance from the narrow end of the nth sub-waveguide layer; a thickness c of at least one sub-waveguide layer among the N sub-waveguide layers other than the N-th sub-waveguide layer and the (N-1)-th sub-waveguide layer satisfies c≦200 nm; the spot size conversion structure has a plurality of slots extending toward the substrate and exposing the substrate, and a communication structure provided in the substrate and communicating with the plurality of slots, the plurality of slots being offset laterally with respect to the waveguide layer, and the slots being distributed on both sides of the waveguide layer in a plan view of the substrate without overlapping with the waveguide layer; Spot size conversion structure.

2. each of the sub-waveguide layers comprises a plurality of waveguide portions, the plurality of waveguide portions comprising reduced-width waveguide portions, and widths of orthogonal protrusions of the reduced-width waveguide portions on the substrate gradually decrease in a direction away from the first end face; 2. The spot size conversion structure of claim 1, wherein the plurality of waveguide portions of an Nth sub-waveguide layer among the N sub-waveguide layers further include equal-width waveguide portions extending to the first end face, and the widths of the orthogonal protrusions of the equal-width waveguide portions on the substrate are equal at all positions.

3. each of the sub-waveguide layers comprises a plurality of waveguide portions, the plurality of waveguide portions comprising reduced-width waveguide portions and equal-width waveguide portions, the widths of orthogonal protrusions of the reduced-width waveguide portions on the substrate gradually decrease in a direction away from the first end face, and the widths of orthogonal protrusions of the equal-width waveguide portions on the substrate are equal at all positions; a waveguide portion of an N-th sub-waveguide layer among the N sub-waveguide layers extending to the first end face is a waveguide portion of equal width; 2. The spot size conversion structure of claim 1, wherein the waveguide portions of each of the sub-waveguide layers closest to the second end face are waveguide portions of equal width.

4. 4. The spot size conversion structure of claim 3, wherein the waveguide portion of each of the sub-waveguide layers closest to the second end face is an equal width waveguide portion having a triangular or trapezoidal cross section.

5. the spot size conversion structure further includes a first side surface and a second side surface intersecting the first end surface and the second end surface; 4. The spot size conversion structure according to claim 3, wherein a waveguide portion of at least one sub-waveguide layer among the N sub-waveguide layers other than the Nth sub-waveguide layer extending to the first end face further extends to at least one of the first side surface or the second side surface.

6. The spot size conversion structure according to claim 1 , wherein each of the orthogonal protrusions of the N sub-waveguide layers on the substrate has a mirror symmetric structure with respect to the same plane.

7. The spot size converting structure of claim 1 , wherein at least one slot of the plurality of slots is exposed to the second end face.

8. A photonic device comprising the spot size converting structure of claim 1.

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