A stepped grating coupler for efficient coupling of optical waveguides and optical fibers and its fabrication method.

Abstract

This invention provides a stepped grating coupler for efficient coupling between optical waveguides and optical fibers. The coupler includes a substrate, a confinement layer, a waveguide, a grating layer, and a protective layer. Its grating region is composed of fixed-period units, each containing a stepped structure formed by adjacent first and second grooves of different depths. This structure breaks the top-to-bottom symmetry of the device, modulating the continuous-domain bound state (BIC) into a unidirectional guided resonance (UGR) mode, thus physically suppressing downward radiation. During fabrication, the stepped structure can be formed through only two photolithography and two etching processes. This invention achieves coupling efficiency exceeding 80% in the 50nm wavelength range of 1530-1580nm, and has the advantages of large process tolerance and compatibility with standard CMOS processes.

Description

Technical Field

[0001] This invention relates to the field of optical communication and integrated photonic device technology, specifically to a stepped grating coupler for efficient coupling of optical waveguides and optical fibers and its fabrication method. Background Technology

[0002] In today's fiber optic communication systems, optical transmitting and receiving modules, as well as various optical communication devices required for signal processing, are typically integrated onto the same optical chip. However, the core diameter of single-mode optical fiber used for communication is approximately 10 μm, much larger than the size of the optical waveguide within the optical chip. Direct end-to-end coupling between the fiber and the waveguide would result in significant coupling loss, severely impacting signal transmission quality. Therefore, achieving low-loss coupling between the sub-micron-sized optical waveguide on the chip and the external single-mode optical fiber with a core diameter of approximately 10 μm is a key technical challenge. Currently, the mainstream coupling schemes are divided into two categories: horizontal coupling and vertical coupling.

[0003] Horizontal coupling requires alignment with the fiber end face, therefore it must be placed at the end face of the optical chip; hence, it is also called an edge-coupled device. The most common horizontal coupling scheme uses an inverted tapered waveguide structure, such as... Figure 11 As shown, as the waveguide length increases, the waveguide width gradually decreases from 500 nm, forming an inverted conical structure. At this point, due to the reduced waveguide width, the transmitted modes can no longer be confined, causing the modes to diverge into the silicon dioxide cladding. Typically, a mode field diameter of about 3 µm can be obtained. Combined with structures such as lenses and optical fibers, horizontal coupling can be achieved. Although the structure is linear and easy to manufacture, its size is relatively large, and it must be placed on the chip end face, limiting integration density and layout flexibility.

[0004] In vertical coupling schemes, the optical waveguide and optical fiber are not in the same horizontal plane, but in the same vertical plane, and the optical signal in the waveguide is vertically coupled into the optical fiber. To achieve this vertical coupling, a grating coupler is required. A grating coupler is a coupling device etched into the SOI (Optically Integrated Circuit) chip device layer. Its main structural parameters include the grating period, duty cycle, etching depth, and etching angle. Grating couplers can therefore be classified into uniform grating couplers, non-uniform grating couplers, and blazed grating couplers. Compared to horizontal coupling schemes, vertical coupling schemes using grating couplers have advantages such as larger alignment tolerance, flexible layout, simple manufacturing, and compatibility with CMOS processes. In 1970, Dakss et al. first designed a uniform grating coupler for vertical coupling between optical waveguides and optical fibers, such as... Figure 12As shown, a coupling efficiency of approximately 40% was achieved. Researchers, through optimizing structural parameters and changing device materials, could only increase the coupling efficiency of the uniform grating coupler to around 50%. This is mainly because the uniform grating coupler is a vertically symmetrical structure; when coupling light upwards into the optical fiber, a considerable portion of the optical power inevitably leaks downwards, generating coupling loss and making it difficult to further improve the coupling efficiency of the uniform grating coupler.

[0005] To improve coupling efficiency, researchers have proposed various non-uniform gratings, such as tilted gratings, shallow and deep etched gratings, and chirped gratings. These structures suppress downward radiation by breaking symmetry, but they often introduce complex features such as variable period and variable angle, leading to complex manufacturing processes, small tolerances, and extremely high requirements for processing precision, making large-scale, low-cost applications difficult. Furthermore, some solutions require the fabrication of complex distributed Bragg reflectors (DBRs) below the grating to reflect downward-leaking light, further increasing the manufacturing difficulty and cost.

[0006] Therefore, there is an urgent need for a grating coupler technology solution that is simple in structure, highly compatible in manufacturing processes, and capable of achieving efficient unidirectional coupling. Summary of the Invention

[0007] This invention aims to overcome the shortcomings of existing technologies and provides a stepped grating coupler for efficient coupling of optical waveguides and optical fibers, comprising: Substrate layer; A confinement layer, located above the substrate layer; The waveguide and grating layer is located above the confinement layer. The waveguide and grating layer includes an input waveguide, an output waveguide, and a diffraction grating region located between the two. The diffraction grating region includes at least one periodic unit, which has a central region and a stepped structure formed therein. The stepped structure includes an adjacent first groove and a second groove, the depth of which is different from that of the second groove.

[0008] Furthermore, the waveguide and grating layers have a total thickness, and the heights of the input waveguide, output waveguide, and diffraction grating regions are all equal to the total thickness.

[0009] Furthermore, the period of the diffraction grating region is a fixed period.

[0010] Furthermore, the number of periodic units does not exceed 30.

[0011] Furthermore, the first groove is a first rectangular groove, and the second groove is a second rectangular groove; the depth of the second groove is greater than the depth of the first groove, and the bottom boundary of the second groove is located within the bottom boundary of the first groove, forming a stepped morphology.

[0012] Furthermore, the total thickness is 340nm, and the fixed period is 528nm. The depth of the first groove is 209nm and the width is 158nm; the depth of the second groove is 297nm and the width is 200nm.

[0013] The design principle of the stepped grating coupler for efficient coupling between optical waveguides and optical fibers described in this invention is as follows: D1. Based on the property that the grating structure has a continuous bound state (BIC) in momentum space, construct the initial geometric configuration of the grating and model the uniform grating coupler.

[0014] D2. Preferably, the coupling efficiency of this coupler is calculated based on the fundamental mode optical field that can be transmitted in a standard single-mode fiber. A pair of ports (port1 and port2) are used as the transmitting port and receiving port of the light, respectively.

[0015] D3. Preferably, port2 is located within an optical waveguide with a thickness of 340nm, and the fundamental mode transmitted in the optical waveguide is selected as the optical wave mode of the transmitting end in the mode calculator of this port.

[0016] D4. Preferably, port1 is arranged in the plane of the single-mode fiber parallel to the grating section, and the fundamental mode transmitted in the single-mode fiber is selected as the optical wave mode of the receiving end in its mode calculator. The default mode is TM mode.

[0017] D5. In the "Analysis" module of the FDTD simulation model, write a script to extract the T_out value from the port1 calculation results. This value represents the transmittance of the fundamental mode in the optical fiber, that is, the actual coupling efficiency of the fundamental mode in a single-mode fiber.

[0018] D6. Design the waveguide structures for the input and output sides of the grating coupler separately. Set the height of both the input and output waveguides to a uniform 340nm, which is the same as the waveguide height in the grating region, facilitating actual manufacturing.

[0019] D7. Design the grating region of a stepped grating coupler. The topological grating configuration is achieved by etching two adjacent rectangular structures at the center of each period to break the symmetry. The key design parameters for this stepped unit structure are the width and height of the two empty rectangles.

[0020] D8. Set the total height of the grating region of the stepped grating coupler to 340nm and the initial period to 528nm. The actual width and height of the two rectangles are obtained by the program through proportional conversion, with initial widths of 158nm and 200nm, and initial depths of 100nm and 162nm, respectively.

[0021] D9. Using the particle swarm optimization function of FDTD software, global optimization is performed on parameters such as the width and height of two adjacent rectangles, the period of the raster region, and the total height. The optimization target is set to the maximum value of T_out.

[0022] D10. While keeping the period, the width of the two rectangles, and the total height of the grating unchanged, further optimize the height of the two rectangles separately.

[0023] D11. Preferably, the depths of the two rectangles are simultaneously optimized to maximize upward coupling efficiency. This results in a first step depth of 209 nm and a second step depth of 297 nm, thus determining the final stepped grating coupler structure that guarantees the highest unidirectional coupling efficiency. The present invention also provides a method for fabricating any of the above-mentioned stepped grating couplers, comprising: A substrate is provided, and a confinement layer and a top semiconductor layer are formed on the substrate; The first photolithography and the first etching are performed on the top semiconductor layer to form a first groove with a first depth within a periodic unit of a predetermined grating region; After removing the photoresist from the first photolithography, a second photolithography and a second etching are performed to further etch the bottom of the first groove, forming a second groove adjacent to the first groove and with a depth greater than the first groove. The first groove and the second groove together form a stepped structure. Remove the photoresist and mask from the second photolithography step; A protective layer is formed on the top semiconductor layer.

[0024] Compared with the prior art, the present invention has the following beneficial effects: By introducing a stepped asymmetric structure, the vertical symmetry of the grating is physically broken, enabling the evolution from the continuous domain bound state (BIC) to the unidirectional guided resonance (UGR) mode, suppressing downward radiation, and thus achieving a simulated coupling efficiency of up to 81.99% at the 1550nm communication wavelength, while maintaining a coupling efficiency of over 80% in a wide band from 1530nm to 1580nm.

[0025] Furthermore, the grating period of this invention is fixed, avoiding complex gradient or chirped structures. The entire waveguide layer is highly uniform, eliminating the need for secondary epitaxial growth. Most importantly, the asymmetric stepped morphology can be achieved with only two standard photolithography and vertical etching steps, significantly reducing manufacturing difficulty and cost, and ensuring high compatibility with CMOS large-scale manufacturing processes. Simultaneously, the total number of periodic cells in the entire grating coupler is controlled to within 30, simplifying the manufacturing process.

[0026] Finally, since this invention utilizes the UGR mechanism to fundamentally suppress downward radiation, it eliminates the need to design complex DBR reflectors in the substrate, further simplifying the chip structure. Attached Figure Description

[0027] Figure 1 This is a schematic diagram illustrating the charge splitting principle of the present invention. Figure 2 This is a schematic diagram illustrating the charge evolution principle of this invention. Figure 3 This is a schematic diagram illustrating the charge merging principle of the present invention. Figure 4 This is a schematic diagram of the overall structure of the stepped grating coupler for efficient coupling of optical waveguides and optical fibers according to the present invention. Figure 5 This is a schematic diagram of the unit period and height of the present invention, taking four units as an example; Figure 6 This is a structural diagram of a single unit structure model of the present invention; Figure 7 This is a diagram showing the final optimized parameter selection for this invention; Figure 8 The diagram shows the global coupling efficiency of the stepped grating coupler used for high-efficiency coupling between optical waveguides and optical fibers in this invention, when the commonly used single wavelength of 1550nm in communication is taken. Figure 9 This is a diagram showing the global coupling efficiency of the present invention in the 1530-1580nm communication band; Figure 10 This is a flowchart illustrating the process implementation of the present invention; Figure 11 An inverted conical waveguide structure for a horizontal coupling scheme; Figure 12 This is a schematic diagram of a uniform grating coupler structure. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with embodiments. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0029] This invention's coupler, based on a unidirectional guided resonant (UGR) mechanism, achieves high-efficiency coupling of upward-radiated light to the grating. The topological definition of a BIC (Bound States in the Continuum) is as follows: In a mirror-symmetric photonic crystal slab, BICs are topological defects in momentum space (k-space), whose principal polarization axes form a closed loop around the BIC position, and the rotation angle of the principal polarization axes for one revolution is... The corresponding integer topological charge q=1. The number of topological charges carried by BIC in k-space is (1) Where φ(k) is the angle between the principal polarization axis and the x-axis, C is the closed path around the BIC, and k is the in-plane wave vector. By splitting the integer topological charge of the BIC and controlling the evolution of the half-integer charge, unilateral radiation suppression is achieved while breaking the upper and lower mirror symmetry.

[0030] The specific process consists of three steps: A. Charge Splitting: (Integer Charge → A Pair of Half-Integer Charges) When the sidewalls of the photonic crystal plate are tilted from vertical (θ=90°) to non-vertical, the vertical mirror symmetry is broken, and the BIC disappears. At this time, the original integer topological charge q=1 of the BIC will split into two half-integer topological charges q=±1 / 2, corresponding to left-hand circular polarization (LCP) and right-hand circular polarization (RCP) resonances, respectively. These two half-integer charges have opposite helicalities (LCP and RCP) due to the mirror symmetry of the structure's y-axis, and each corresponds to a non-zero vertical radiation attenuation rate (...). and (None of them are 0), and there is no unilateral radiation characteristic. See attached... Figure 1 As shown.

[0031] B. Charge Evolution: (Movement of Half-Integer Charges in Momentum Space) As the sidewall angle further decreases (e.g., from 81° to 75°), two half-integer charges will move along specific trajectories in momentum space ( As the LCP charge (red trajectory) moves, the RCP charge (green trajectory) gradually approaches the LCP charge. During this process, the upper and lower radiation attenuation rates remain non-zero, but the ratio between the two (asymmetry ratio) changes. The area gradually increased, showing a trend towards unilateral radiation. (See attached image.) Figure 2 As shown.

[0032] C. Charge Combining: (Half-integer charge → Integer charge, forming UGR) When the sidewall tilt angle decreases to 75°, the two half-integer charges in momentum space... They meet and merge on the axis, reverting to an integer topological charge q=1. At this point, downward radiation requires both LCP and RCP polarization to be satisfied (physically impossible), leading to a decrease in the downward radiation attenuation rate. Since the vertical mirror symmetry has been broken, the upward radiation attenuation rate... It remains limited, eventually forming a UGR that radiates only upwards and not downwards. (See attached diagram) Figure 3 As shown.

[0033] From a topological perspective, the UGR is the merging point of half-integer charges, and the topological constraints of its polarization field directly determine its unilateral radiation characteristics.

[0034] The present invention provides a stepped grating coupler for efficient coupling of optical waveguide and optical fiber signals. It employs a silicon-on-insulator (SOI) platform, consisting of a silicon substrate layer, a silicon dioxide confinement layer, a silicon waveguide and grating layer, and a silicon dioxide protective layer, arranged sequentially from bottom to top. The input optical signal enters a fixed-period diffraction grating region via the input waveguide, is diffracted upwards by the stepped structure, and efficiently coupled into the optical fiber placed vertically or nearly vertically above the grating. The key feature is that each periodic unit of the grating does not contain a single etched groove, but rather a stepped groove with varying depths and widths formed by two etching processes.

[0035] The overall process of its fabrication method is as follows: prepare the substrate and form the top silicon layer → first photolithography and etching to define the first step → remove the photoresist → second precise alignment photolithography and etching to define the second step → remove all sacrificial layers → deposit a protective layer.

[0036] Specifically, the stepped grating coupler of the present invention for efficient coupling of optical waveguide and optical fiber signals mainly includes the following structure: Substrate: Made of silicon, it provides mechanical support for the device.

[0037] Confinement layer: Made of silicon dioxide, located above the substrate layer, it serves as the lower cladding layer of the waveguide and also reflects downward-leaking light to a certain extent.

[0038] Waveguide and grating layer: Made of semiconductor material, preferably silicon (such as monocrystalline or polycrystalline silicon). The total thickness of this layer is 340 nm. This layer is etched to form three continuous sections: the input waveguide, the diffraction grating region, and the output waveguide. The diffraction grating region consists of no more than 30 periodically arranged units, with a fixed period of 528 nm.

[0039] Periodic cell structure: The central region of each periodic cell contains a stepped structure. This structure consists of a first rectangular groove (step) formed by the first etching and a second rectangular groove (step) formed by the second etching, which are adjacent to each other. The second groove is located inside the first groove on one side, and the two grooves have different bottom surfaces but are connected on the sides to form a step.

[0040] Protective layer: Covering the waveguide and grating layer, it is made of silicon dioxide and serves as the upper cladding.

[0041] The method for fabricating a stepped grating coupler for efficient coupling of optical waveguide and optical fiber signals according to the present invention specifically includes the following steps: S1. Substrate preparation: Select a clean silicon substrate and ultrasonically clean it in sequence with acetone, isopropanol, and deionized water, then dry it with nitrogen to remove organic and particulate contaminants from the surface.

[0042] S2. Forming a hard mask / confinement layer: A 2μm thick silicon dioxide layer is deposited on the surface of a silicon substrate using plasma-enhanced chemical vapor deposition. This layer can serve as both a confinement layer and a hard mask for subsequent etching.

[0043] S3. Forming the top silicon layer: Amorphous silicon thin films are deposited by PECVD and then annealed to convert them into polycrystalline silicon with a thickness controlled at 340nm.

[0044] S4. First photolithography: Photoresist is spin-coated onto the top silicon layer. After pre-baking, ultraviolet or electron beam lithography is performed using the first mask. After development, the first rectangular opening area is formed, exposing the top silicon layer to be etched. This opening corresponds to the first step position at the center of the stepped grating period.

[0045] S5. First Etching: Using patterned photoresist as a mask, anisotropic dry etching techniques such as reactive ion etching or inductively coupled plasma etching are employed to etch the top layer of silicon. After etching, the photoresist is removed by oxygen plasma ashing.

[0046] S6. Second photolithography: Spin-coat photoresist again, and use pre-made alignment marks to precisely align (overlay) the second mask with the first groove pattern formed in S5. After exposure and development, a second rectangular opening is exposed in the area inside the first groove and immediately adjacent to one side of it.

[0047] S7. Second Etching: Using the second photoresist as a mask, a second anisotropic etching is performed. Based on the depth already achieved in S5, a certain etching depth and width are added to ensure that the final total depth of the region reaches the target depth. A mixture of SF6 and C4F8 can be used as the etching gas to achieve high selectivity for silicon and vertical sidewall etching.

[0048] S8. Mask Removal: The photoresist is removed by oxygen plasma ashing in sequence, and then the silicon dioxide layer used as a hard mask is removed by using buffered oxide etching solution or diluted hydrofluoric acid (if silicon is etched in S5 and S7, the SiO2 mask can be removed at this time).

[0049] S9. Structural characterization: The sample was washed with deionized water and dried with nitrogen gas. The step depth, sidewall steepness and alignment accuracy were characterized by scanning electron microscopy (SEM) or atomic force microscopy (AFM) to confirm the key dimensions.

[0050] S10. Covering and protective layer: Silicon dioxide is deposited as the upper cladding and protective layer on the entire waveguide and grating structure through methods such as PECVD.

[0051] An example is provided here.

[0052] In this embodiment, a stepped grating coupler for efficient coupling of optical waveguides and optical fiber signals is designed with a uniform height of 340 nm for both the input and output waveguides. This height is the same as the waveguide height of the grating region, facilitating actual manufacturing. The grating region of the stepped grating coupler is formed by etching two adjacent rectangular structures at the center of each period to disrupt symmetry, thereby achieving the topological grating configuration. The key design parameters for this stepped unit structure are the width and height of the two empty rectangles.

[0053] The total height of the grating region in the stepped grating coupler was set to 340 nm, and the initial period was 528 nm. The actual width and height of the two rectangles were obtained by proportional calculation using the program, with initial widths of 158 nm and 200 nm, and initial depths of 100 nm and 162 nm, respectively. The particle swarm optimization function of FDTD software was used to globally optimize parameters such as the width and height of the two adjacent rectangles, the period of the grating region, and the total height, with the optimization target set as maximizing the output light field intensity. While keeping the period, the width of the two rectangles, and the total grating height constant, the height of the two rectangles was further optimized separately. Then, the depth of the two rectangles was simultaneously optimized comprehensively, aiming for the highest upward coupling efficiency. The final depth of the first step was 209 nm, and the depth of the second step was 297 nm, thus determining the final stepped grating coupler structure that guarantees the highest unidirectional coupling efficiency, as shown in the attached figure. Figure 4 , 5 6. See appendix for specific parameters. Figure 7 .

[0054] The final simulation results show that, with a 1550nm light source incident, the unidirectional upward coupling efficiency reaches 81.99% (see attached figure). Figure 8 As shown in the attached figure; when the light source is incident at 1530-1580nm, the upward coupling efficiency is above 80%. Figure 9 As shown.

[0055] It should be understood that the above embodiments are merely exemplary. Those skilled in the art can make various non-substantial improvements and substitutions based on the technical concept of this invention, and all such improvements and substitutions should be considered to fall within the protection scope of this application.

Claims

1. A stepped grating coupler for efficient coupling of optical waveguides and optical fibers, characterized in that, include: Substrate layer; A confinement layer is located above the substrate layer; A waveguide and grating layer is located above the confinement layer, and the waveguide and grating layer includes an input waveguide, an output waveguide, and a diffraction grating region located between the two. The diffraction grating region includes at least one periodic unit, which has a stepped structure. The stepped structure includes an adjacent first groove and a second groove, the depth of which is different from that of the second groove.

2. The stepped grating coupler for high-efficiency coupling of optical waveguides and optical fibers according to claim 1, characterized in that, The waveguide and grating layer have a total thickness, and the heights of the input waveguide, output waveguide, and diffraction grating region are all equal to the total thickness.

3. The stepped grating coupler for high-efficiency coupling of optical waveguides and optical fibers according to claim 2, characterized in that, The diffraction grating region has a fixed period, and the total number of periodic units does not exceed 30.

4. The stepped grating coupler for high-efficiency coupling of optical waveguides and optical fibers according to claim 3, characterized in that, The total thickness is 340nm, and the fixed period is 528nm.

5. The stepped grating coupler for high-efficiency coupling of optical waveguides and optical fibers according to claim 1, characterized in that, The depth of the second groove is greater than the depth of the first groove, and the bottom boundary of the second groove is located within the bottom boundary of the first groove, forming a stepped morphology.

6. The stepped grating coupler for high-efficiency coupling of optical waveguides and optical fibers according to claim 5, characterized in that, The first groove is a first rectangular groove with a width of 158nm and a depth of 209nm; the second groove is a second rectangular groove with a width of 200nm and a depth of 297nm.

7. A method for fabricating a stepped grating coupler for efficient coupling of optical waveguides and optical fibers as described in any one of claims 1-6, characterized in that, Includes the following steps: A substrate is provided, and a confinement layer and a top semiconductor layer are sequentially formed on the substrate; The top semiconductor layer is subjected to a first photolithography and a first etching to form a first groove with a first depth within a periodic unit of a preset grating region; After removing the photoresist from the first photolithography, a second photolithography and a second etching are performed to further etch the bottom of the first groove, forming a second groove adjacent to the first groove and with a depth greater than the first groove. The first groove and the second groove together form a stepped structure. Remove the photoresist used in the second photolithography and the hard mask used for etching; A protective layer is deposited on the formed structure.

8. The preparation method according to claim 7, characterized in that, The second photolithography uses pre-made alignment marks to overlay the pattern formed by the first etching.

9. The preparation method according to claim 7, characterized in that, Both the first and second etching processes are anisotropic etching performed using reactive ion etching or inductively coupled plasma etching.

10. The preparation method according to claim 9, characterized in that, The etching gases for the first and second etching processes are and A mixture of gases.

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