Grating coupler

Abstract

The invention provides a grating coupler, and relates to the technical field of semiconductors, and the grating coupler comprises a first grating coupling structure; the second grating coupling structure is completely identical to the first grating coupling structure, and the first grating coupling structure comprises a substrate; the first insulating layer is arranged on the substrate; the first-stage grating is arranged on one side, far away from the substrate, of the first insulating layer; the second-stage grating is arranged on one side, far away from the substrate, of the first-stage grating, the first-stage grating and the second-stage grating have the same grating period, and the duty ratio of the first-stage grating is larger than that of the second-stage grating; the first grating coupling structure is arranged on one side of the substrate, the second grating coupling structure is arranged on one side of the substrate, and the second insulating layer is arranged on one side, far away from the substrate, of the second-stage grating. The orthographic projection of the first grating coupling structure on the substrate is rotated by 180 degrees and then coincides with the orthographic projection of the second grating coupling structure on the substrate.

Description

Technical Field

[0001] The present invention relates to the field of semiconductor technology, and more particularly, to a grating coupler. Background Art

[0002] With the increasing integration of optoelectronic chips, the requirement for interconnection density is also getting higher and higher. Among them, optical interconnection is one of the key technologies. At present, optical interconnection mainly solves the coupling of lasers to optical chips and optical fibers to optical chips in traditional packaging forms. Facing the future demand for high-density optical interconnection, optical interconnection between chips will become increasingly important.

[0003] Currently, the main methods for optical interconnection between chips include lens / 45° mirror, grating coupler, end-face coupling, photon lead polymerization, etc. Among them, the preparation process of the lens / 45° mirror is complex, the stability and reliability of photon lead polymerization in large-scale mass production need to be verified, the end-face coupler has high requirements for process tolerance, and wafer-level testing cannot be achieved. In contrast, the grating coupler has the advantages of small size, easy preparation, flexible design, and can be used for wafer-level testing. However, on the one hand, the insertion loss of the optical interconnection between chips based on the grating structure is more than 2 dB, and it is necessary to improve the efficiency by adding an Au mirror, etc., but the Au material is not compatible with the CMOS process. On the other hand, due to the high wavelength correlation of the grating, the bandwidth is very narrow, which limits the application of the grating coupler in optical interconnection between chips. Summary of the Invention

[0004] In view of this, an embodiment of the present disclosure provides a grating coupler, including: a first grating coupling structure; and a second grating coupling structure identical to the first grating coupling structure, wherein the first grating coupling structure includes: a substrate; a first insulating layer disposed on the substrate, the first insulating layer configured to limit the propagation of optical signals in the first grating coupling structure; a first-stage grating disposed on the first insulating layer on the side away from the substrate, the first-stage grating including periodically arranged first grating units, the first-stage grating being isolated from the substrate by the first insulating layer; a second-stage grating disposed on the first-stage grating on the side away from the substrate, the second-stage grating including periodically arranged second grating units, the first-stage grating and the second-stage grating having the same grating period, and the duty cycle of the first-stage grating being greater than the duty cycle of the second-stage grating; and a second insulating layer disposed on the second-stage grating on the side away from the substrate, the second insulating layer configured to limit the propagation of optical signals in the first grating coupling structure, wherein the first grating coupling structure and the second grating coupling structure are connected in an opposing manner, and the orthographic projection of the first grating coupling structure on the substrate coincides with the orthographic projection of the second grating coupling structure on the substrate after rotating 180°.

[0005] According to an embodiment of the present disclosure, the duty cycle of the first - stage grating is 0.3 to 0.5, and the duty cycle of the second - stage grating is 0.2 to 0.35.

[0006] According to an embodiment of the present disclosure, the duty cycle of the first - stage grating is 0.4, and the duty cycle of the second - stage grating is 0.28.

[0007] According to an embodiment of the present disclosure, wherein the period of the first - stage grating is 640 to 700 nm.

[0008] According to an embodiment of the present disclosure, wherein the period of the first - stage grating is 670 nm.

[0009] According to an embodiment of the present disclosure, wherein the thickness of the first - stage grating is less than that of the second - stage grating

[0010] According to an embodiment of the present disclosure, wherein the thickness of the first - stage grating is 60 nm to 80 nm, and the thickness of the second - stage grating is 175 nm to 205 nm.

[0011] According to an embodiment of the present disclosure, wherein the thickness of the first - stage grating is 70 nm, and the thickness of the second - stage grating is 190 nm.

[0012] According to an embodiment of the present disclosure, wherein the projection of the second grating unit on the substrate is located inside the projection of the first grating unit on the substrate, and there is an offset between the projection of the second grating unit on the substrate and the projection of the first grating unit on the substrate in the direction away from the incident light.

[0013] According to an embodiment of the present disclosure, the offset between the projection of the second grating unit on the substrate and the projection of the first - stage grating on the substrate in the direction away from the incident light is 30% to 50% of the width of the second grating unit.

[0014] According to an embodiment of the present disclosure, wherein the vertical distance between the second - stage grating in the first grating coupling structure and the second - stage grating in the second grating coupling structure is proportional to the offset.

[0015] According to an embodiment of the present disclosure, wherein the first grating coupling structure and the second grating coupling structure are connected in an opposing manner by direct bonding.

[0016] According to an embodiment of the present disclosure, wherein the materials of the first - stage grating and the second - stage grating are different.

[0017] According to an embodiment of the present disclosure, wherein the material of the first - stage grating is single - crystal silicon, and the material of the second - stage grating is poly - silicon or silicon nitride.

[0018] The grating coupler provided by the embodiment of the present disclosure comprises two identical grating coupling structures connected face to face, and uses two levels of gratings with the same period and different duty cycles arranged in sequence to optimize the coupling efficiency and bandwidth of the gratings. One of the grating coupling structures can diffract the light in the planar waveguide to a vertical direction, and the light then enters another grating coupler to couple the light field into another planar waveguide to complete the optical interconnection between different optical chips. The grating coupler can achieve high coupling efficiency and high bandwidth inter-chip optical interconnection. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The above and other purposes, features and advantages of the present disclosure will become more apparent through the following description of the embodiments of the present disclosure with reference to the accompanying drawings. Obviously, the drawings described below are some embodiments of the present disclosure, and for those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative work. In the drawings:

[0020] Figure 1 A cross-sectional view of a grating coupler according to some embodiments of the present disclosure is exemplarily shown.

[0021] Figure 2 A cross-sectional view of a grating coupler according to other embodiments of the present disclosure is exemplarily shown.

[0022] Figure 3 The flowchart of the direct bonding method according to some embodiments of the present disclosure is exemplarily shown.

[0023] Figure 4 A simulation diagram showing the coupling loss and 1 dB bandwidth of a grating coupler in the C-band according to an example of the present disclosure is shown.

[0024] Figure numerals: 01 - grating coupler, 10 - first grating coupling structure, 20 - second grating coupling structure, 30 - first optical signal, 31 - second optical signal, 32 - third optical signal, 101 - substrate, 102 - first insulating layer, 103 - first-order grating, 104 - second-order grating, 105 - second insulating layer, d - offset. DETAILED DESCRIPTION

[0025] To make the above-mentioned objects, features, and advantages of the disclosure more apparent and understandable, the following provides a detailed description of the specific embodiments of the disclosure in conjunction with the accompanying drawings. However, it should be understood that these descriptions are merely exemplary and do not limit the scope of the disclosure. In the following detailed description, for the sake of explanation, numerous specific details are set forth to provide a comprehensive understanding of the embodiments of the disclosure. However, obviously, one or more embodiments can also be implemented without these specific details. In addition, in the following description, descriptions of well-known structures and technologies are omitted to avoid unnecessarily confusing the concepts of the disclosure.

[0026] The terms used herein are merely for the purpose of describing specific embodiments and are not intended to limit the disclosure. The terms "including", "comprising", etc. used herein indicate the presence of the described features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0027] All terms used herein (including technical and scientific terms) have the meanings commonly understood by those of ordinary skill in the art, unless otherwise defined. It should be noted that the terms used herein should be interpreted as having a meaning consistent with the context of this specification and should not be interpreted in an idealized or overly rigid manner.

[0028] In cases where expressions similar to "at least one of A, B, and C, etc." are used, generally, it should be interpreted according to the meaning commonly understood by those of ordinary skill in the art (for example, "having at least one of A, B, and C" should include, but not be limited to, having only A, having only B, having only C, having A and B, having A and C, having B and C, and / or having A, B, and C, etc.). The terms "first" and "second" are used only for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, features defined with "first" and "second" may explicitly or implicitly include one or more of the described features.

[0029] As the integration degree of optoelectronic chips is getting higher and higher, the requirement for interconnection density is also getting higher and higher. Among them, optical interconnection is one of the key technologies. Optical interconnection is a form of optical communication that uses optical fibers or other optical transmission media to exchange data among various components or subsystems within a computer. Currently, optical interconnection mainly addresses the coupling of lasers to optical chips and optical fibers to optical chips under traditional packaging forms. Facing the future demand for high-density optical interconnection, optical interconnection between chips will become increasingly important.

[0030] Currently, the main methods for inter-chip optical interconnection include lens / 45° mirror, grating coupler, end-face coupling, photon lead polymerization, etc. Among them, the preparation process of the lens / 45° mirror is complex, the stability and reliability of photon lead polymerization in large-scale mass production remain to be verified, the end-face coupler has high requirements for process tolerance and cannot achieve wafer-level testing. In contrast, the grating coupler has the advantages of small size, easy preparation, flexible design, and can be used for wafer-level testing. The structure of the grating coupler usually consists of two parallel waveguides, and there is a grating between the two parallel waveguides. The grating is composed of a series of parallel protrusions and depressions. Their spacing is usually a multiple of the wavelength. When the optical signal is transmitted from one waveguide to another, it is divided by the grating into diffracted waves propagating in different directions. There are still the following problems in using the grating coupler: on the one hand, the insertion loss of the inter-chip optical interconnection based on the grating structure is above 2 dB, and it is necessary to improve the efficiency by adding an Au mirror, etc., but the Au material is not compatible with the CMOS process. On the other hand, due to the high wavelength correlation of the grating, the bandwidth is very narrow, which limits the application of the grating coupler in inter-chip optical interconnection.

[0031] In an exemplary embodiment of the present disclosure, a grating coupler is provided.

[0032] Figure 1 It is a cross-sectional view of the grating coupler shown in the embodiment according to the present disclosure.

[0033] As Figure 1 shown, the grating coupler 01 of this embodiment includes a first grating coupling structure 10 and a second grating coupling structure 20 that is exactly the same as the first grating coupling structure 10. Among them, the first grating coupling structure 10 includes: a substrate 101. A first insulating layer 102 disposed on the substrate. The first insulating layer 102 is configured to limit the propagation of the optical signal in the first grating coupling structure 10. Specifically, the first insulating layer 102 can be a waveguide confinement layer, such as a box layer, which can isolate the first-stage grating 103 from the substrate 101. The first-stage grating 103 is disposed on the side of the first insulating layer 102 away from the substrate 101. The first-stage grating 103 can be prepared by etching technology. The first-stage grating 103 includes periodically arranged first grating units, and each first grating unit includes a protrusion and a depression arranged adjacent to each other. In order to further improve the efficiency of grating coupling, in the first grating coupling structure 10, a second-stage grating 104 is further included, which is disposed on the side of the first-stage grating 103 away from the substrate, and the second-stage grating 104 includes periodically arranged second grating units. From Figure 1As can be seen, the first - order grating 103 and the second - order grating 104 have the same grating period. Moreover, the duty cycle of the first - order grating 103 is greater than that of the second - order grating 104 to further improve the diffraction efficiency of the grating. In the first grating coupling structure 10, a second insulating layer 105 is further included, which is disposed on the side of the second - order grating 104 away from the substrate 101. The second insulating layer 105 is used to limit the propagation of the optical signal in the first grating coupling structure 10 to ensure the stable transmission of the optical signal. For example, the second insulating layer 105 can be an upper cladding layer, such as a cladding layer.

[0034] In the grating coupler according to the embodiment of the present disclosure, as Figure 1 shown by way of example, the first grating coupling structure 10 is connected to the second grating coupling structure 20 in an opposing manner. The orthographic projection of the first grating coupling structure 10 on the substrate coincides with the orthographic projection of the second grating coupling structure 20 on the substrate 101 after rotating 180°. By using the grating coupler 01 according to the embodiment of the present disclosure, the first optical signal 30 in the planar waveguide can be diffracted by the first grating structure 10 in the direction of the second grating coupling structure 20 to the propagation direction of the second optical signal 31, and then the second optical signal 31 enters the second grating coupling structure 20. The second grating coupling structure 20 couples the optical field into another planar waveguide and emits it in the direction of the third optical signal 32, thus completing the inter - chip optical interconnection between different optical chips. Therefore, using two completely identical grating coupling structures connected in an opposing and facing manner can meet the mode - field matching. Further, since the second - order grating 104 and the first - order grating 103 are stacked and have the same grating period, and the duty cycle of the second - order grating 104 is less than that of the first - order grating 103, the directivity of grating diffraction and mode - field matching can be further improved, and the coupling efficiency and bandwidth can be increased.

[0035] It should be understood that each part included in the first grating coupling structure in the embodiment of the present disclosure, including but not limited to the substrate, the first insulating layer, the first - order grating, the second - order grating, the second insulating layer, etc., also exist in the second grating coupling structure. Details will not be described hereinafter.

[0036] In some embodiments, the duty cycle of the first - order grating 103 is 0.3 - 0.5, and the duty cycle of the second - order grating 104 is 0.2 - 0.35.

[0037] In an exemplary embodiment, the duty cycle of the first - order grating 103 is 0.4, and the duty cycle of the second - order grating 104 is 0.28.

[0038] According to the embodiment of the present disclosure, the period of the first - order grating 103 is 640 - 700 nm. Correspondingly, the period of the second - order grating 104 is also 640 - 700 nm.

[0039] According to an embodiment of the present disclosure, the period of the first - order grating 103 is 670 nm. Correspondingly, the period of the second - order grating 104 is also 670 nm.

[0040] In an embodiment of the present disclosure, the first - order grating 103 can be a shallow - etched grating, and the thickness of the second - order grating 104 is greater than that of the first - order grating 103. By optimizing the thicknesses of the first - order grating 103 and the second - order grating 104, the diffraction efficiency of the grating can be further improved. In an embodiment of the present disclosure, preferably, the thickness of the first - order grating 103 is 60 nm - 80 nm, and the thickness of the second - order grating 104 is 175 nm - 205 nm. More preferably, the thickness of the first - order grating 103 is 70 nm, and the thickness of the second - order grating 104 is 190 nm.

[0041] By optimizing parameters such as the grating period, duty cycle, and thickness, the matching degree between the grating diffraction directivity and the mode field is further improved, enhancing the coupling efficiency and bandwidth.

[0042] Figure 2 An exemplary cross - sectional view of a grating coupler according to some other embodiments of the present disclosure is shown. Referring to Figure 2 , according to some embodiments of the present disclosure, the first - order grating 103 and the second - order grating 104 are stacked non - directly opposite, but there is an offset d. Thus, the projection of the second grating unit on the substrate 101 is located inside the projection of the first grating unit on the substrate 101, and there is an offset between the projection of the second grating unit on the substrate 101 and the projection of the first grating unit on the substrate 101. Specifically, the offset exists in the direction away from the incident light. More specifically, the projection of the second grating unit on the substrate 101 has a first central axis. The projection of the first grating unit on the substrate 101 has a second central axis. The two central axes do not coincide. Moreover, the distance of the first central axis in the direction away from the first optical signal 30 has an offset from the second central axis. Due to the existence of the offset, the directivity and mode - field matching of the grating can be further improved, so that the diffraction intensity of the light in the direction of the second optical signal 31 is enhanced, and the coupling efficiency and bandwidth are further improved.

[0043] In some embodiments, the offset d between the projection of the second grating unit on the substrate 101 and the projection of the first grating unit on the substrate 101 in the direction away from the incident light is 30% - 50% of the width of the second grating unit. For example, the offset d between the first central axis 11 and the second central axis 12 is 30% - 50% of the width of the second grating unit.

[0044] Referring to Figures 1 to 2, according to an embodiment of the present disclosure, since the grating coupler is provided to achieve inter-chip optical interconnection. The first grating coupling structure 10 may be disposed on the first wafer, and the second grating coupling structure 20 may be disposed on the second wafer. The first wafer and the second wafer are connected in an opposing manner, such that the first grating coupling structure 10 and the second grating coupling structure 20 are connected face to face. Among them, the second insulating layer 105 of the first grating coupling structure 10 may be the surface layer of the first wafer, and the second insulating layer of the second grating coupling structure 20 may be the surface layer of the second wafer that is connected in an opposing manner to the surface layer of the first wafer. There is a vertical distance between the second-stage grating 104 of the first grating coupling structure 10 and the second-stage grating in the second grating coupling structure 20. It should be understood that the vertical distance is the distance from the top of the second-stage grating 104 of the first grating coupling structure 10 to the top of the second-stage grating in the second grating coupling structure 20. Among them, the top is the highest point of the second-stage grating 104 in the direction perpendicular to the substrate 101 and away from the substrate 101. To further improve the bandwidth of the coupling efficiency, the vertical distance between the second-stage grating 104 in the first grating coupling structure 10 and the second-stage grating in the second grating coupling structure 20 is proportional to the offset d.

[0045] In an embodiment of the present disclosure, the materials of the first-stage grating 103 and the second-stage grating 104 are different. In an exemplary embodiment, the material of the first-stage grating 103 may be single-crystalline silicon. The material of the second-stage grating 104 is polysilicon or silicon nitride. Further, the materials of the first insulating layer 102 and the second insulating layer 105 may be silicon dioxide.

[0046] According to an embodiment of the present disclosure, the first grating coupling structure 10 and the second grating coupling structure 20 are connected in an opposing manner by direct bonding. Since the alignment accuracy of the first grating coupling structure 10 and the second grating coupling structure 20 has a great influence on the coupling efficiency, to meet the requirements of high-precision optical interconnection, the first wafer and the second wafer are bonded together by direct bonding to complete the hermetic packaging of the optical port. In an embodiment of the present disclosure, the method shown in Figure 3 is used to complete the direct bonding.

[0047] Referring to Figure 3 , the direct bonding method of the embodiment of the present disclosure includes operations S310 to S330.

[0048] In operation S310, the surface layer of the first wafer and the surface layer of the second wafer are polished respectively to control the roughness of the bonding surface within a preset range. In some examples, the surface layer of the first wafer and the surface layer of the second wafer are SiO2 layers. Chemical mechanical polishing can be used to make the bonding surface reach the preset flatness. For example, the roughness of the bonding surface can be below 3 nm.

[0049] In operation S320, the polished surface layer is subjected to plasma treatment and cleaning to obtain the surface layer to be bonded. Among them, -OH bonds are formed on the surface of the surface layer to be bonded.

[0050] In operation S330, the alignment and bonding of the grating are carried out on the bonder to complete the hermetic packaging of the optical port.

[0051] By using the direct bonding method of the embodiment of the present disclosure, the alignment accuracy can be controlled within 200 nm @ 3σ. And the vertical distance between the second-order grating 104 in the first grating coupling structure 10 and the second-order grating in the second grating coupling structure 20 is adjusted by adjusting the thicknesses of the first wafer surface layer and the second wafer surface layer. It should be understood that the first wafer surface layer becomes the first insulating layer in the first grating coupling structure 10 after being processed by the Figure 3 exemplary method and completed bonding. Similarly, the second wafer surface layer becomes the first insulating layer in the second grating coupling structure 20 after being processed by the Figure 3 exemplary method and completed bonding.

[0052] According to an example of the present disclosure, a grating coupler is prepared by using a direct bonding process. Among them, the first grating coupling structure and the second grating coupling structure in the grating coupler are optimized. Specifically, the periods, duty cycles, and thicknesses of the first-order grating and the second-order grating are optimized. In the example of the present disclosure, the period of the first-order grating is 670 nm, the duty cycle is 0.4, and the thickness is 70 nm. The period of the second-order grating is 670 nm, the duty cycle is 0.28, and the thickness is 190 nm. By adjusting the vertical distance between the second-order gratings in the face-to-face aligned first and second grating coupling structures, and the offset between the projection of the second grating unit on the substrate in the direction away from the incident light and the projection of the first-order grating on the substrate, the directivity of grating diffraction and the mode field matching are further improved, thereby further improving the coupling efficiency and bandwidth. The optimized grating coupler is modeled, simulated, and optimized, and the coupling loss and 1 dB bandwidth of the C band of the grating coupler are obtained through theoretical calculation as Figure 4 shown. See Figure 4 , the coupling loss in the C band < 0.8 dB. The grating coupler of the example of the present disclosure also greatly improves the 1 dB bandwidth, reaching 90 nm. High coupling efficiency and high-bandwidth inter-chip optical interconnection are achieved.

[0053] Unless otherwise specifically stated, the numerical parameters in this specification and the appended claims are approximate values and can be changed according to the required characteristics obtained through the content of this disclosure. Specifically, all numbers used in the specification and claims to represent the contents of components, reaction conditions, etc. should be understood to be modified by the term "about" in all cases. Generally, the meaning expressed is that it includes a change of ±10% in some embodiments, a change of ±5% in some embodiments, a change of ±1% in some embodiments, and a change of ±0.5% in some embodiments for a specific quantity.

[0054] Those skilled in the art can understand that the features recited in the various embodiments and / or claims of this disclosure can be combined or / and combined in various ways, even if such combinations or combinations are not explicitly recited in this disclosure. In particular, without departing from the spirit and teachings of this disclosure, the features recited in the various embodiments and / or claims of this disclosure can be combined and / or combined in various ways. All such combinations and / or combinations fall within the scope of this disclosure.

[0055] The embodiments of the present disclosure have been described above. However, these embodiments are only for illustrative purposes and not for limiting the scope of the present disclosure. Although the embodiments have been described separately above, this does not mean that the measures in each embodiment cannot be used advantageously in combination. The scope of the present disclosure is defined by the appended claims and their equivalents. Without departing from the scope of the present disclosure, those skilled in the art can make various substitutions and modifications, and all such substitutions and modifications should fall within the scope of the present disclosure.

Claims

1. A grating coupler, characterized in that, Including: A first grating coupling structure; And A second grating coupling structure identical to the first grating coupling structure, wherein the first grating coupling structure includes: A substrate; A first insulating layer disposed on the substrate, the first insulating layer configured to limit the propagation of optical signals in the first grating coupling structure; The first-stage grating disposed on the first insulating layer on the side away from the substrate, the first-stage grating including periodically arranged first grating units, the first-stage grating being isolated from the substrate by the first insulating layer; A second-stage grating disposed on the first-stage grating on the side away from the substrate, the second-stage grating including periodically arranged second grating units, the first-stage grating and the second-stage grating having the same grating period, and the duty cycle of the first-stage grating being greater than the duty cycle of the second-stage grating; and A second insulating layer disposed on the second-stage grating on the side away from the substrate, the second insulating layer for limiting the propagation of optical signals in the first grating coupling structure, wherein the first grating coupling structure and the second grating coupling structure are oppositely and connectedly joined, and the orthographic projection of the first grating coupling structure on the substrate coincides with the orthographic projection of the second grating coupling structure on the substrate after being rotated 180°.

2. The grating coupler according to claim 1, wherein, The duty cycle of the first-stage grating is 0.3 to 0.5, and the duty cycle of the second-stage grating is 0.2 to 0.

35.

3. The grating coupler according to claim 2, wherein, The duty cycle of the first-stage grating is 0.4, and the duty cycle of the second-stage grating is 0.

28.

4. The grating coupler according to claim 1, wherein The period of the first-stage grating is 640 to 700 nm.

5. The grating coupler according to claim 4, wherein, The period of the first-stage grating is 670 nm.

6. The grating coupler according to claim 2, wherein, The thickness of the first-stage grating is less than that of the second-stage grating.

7. The grating coupler according to claim 6, wherein, The thickness of the first-stage grating is 60 nm to 80 nm, and the thickness of the second-stage grating is 175 nm to 205 nm.

8. The grating coupler according to claim 7, wherein, The thickness of the first-stage grating is 70 nm, and the thickness of the second-stage grating is 190 nm.

9. The grating coupler according to any one of claims 1 to 8, wherein, The projection of the second grating unit on the substrate is located inside the projection of the first grating unit on the substrate, and there is an offset between the projection of the second grating unit on the substrate and the projection of the first grating unit on the substrate in the direction away from the incident light.

10. The grating coupler according to claim 9, wherein, The offset between the projection of the second grating unit on the substrate and the projection of the first-stage grating on the substrate in the direction away from the incident light is 30% to 50% of the width of the second grating unit.

11. The grating coupler according to claim 9, wherein, The vertical distance between the second-stage grating in the first grating coupling structure and the second-stage grating in the second grating coupling structure is proportional to the offset.

12. The grating coupler according to any one of claims 1 to 8, wherein, The first grating coupling structure and the second grating coupling structure are oppositely and connectedly joined by direct bonding.

13. The grating coupler according to any one of claims 1 to 8, wherein, The materials of the first-stage grating and the second-stage grating are different.

14. The grating coupler according to claim 10, wherein, The material of the first-stage grating is single-crystalline silicon, and the material of the second-stage grating is polycrystalline silicon or silicon nitride.

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