Fully etched polarization-independent compact subwavelength grating structure and its coupler

Abstract

The present invention discloses a fully etched polarization-independent compact subwavelength grating structure and its coupler. The subwavelength grating structure is fabricated by the following method: a rotated subwavelength structure is employed and the topology is optimized using an inverse design method, so that both transverse electric mode light and transverse magnetic mode light propagate along the x-axis. In the y-axis direction, the period length py and duty cycle fy of the unit subwavelength structure in each period are equal, forming a unit subwavelength array structure. In the x-axis direction, the period length px and duty cycle fx of the unit subwavelength array structure in each period are different, and there are different rotation angles, so that the equivalent refractive index of the unit subwavelength array structure under transverse electric mode light and transverse magnetic mode light is approximately equal, thereby achieving polarization independence of the grating coupling. The original rectangular subwavelength grating structure is converted into a fan-shaped structure using a coordinate transformation method. The present invention effectively reduces the coupling loss and size of the grating device and can achieve polarization independence of the grating coupler.

Description

Technical Field

[0001] The present invention relates to the fields of optical communication and optical interconnection, and in particular to a fully etched polarization-independent compact sub-wavelength grating structure and a coupler thereof. Background Art

[0002] The mode field diameter of an ordinary single-mode optical fiber differs greatly from the width of a silicon waveguide, and edge couplers or vertical grating coupler modes are usually used to solve the size mismatch problem. Edge couplers have high coupling efficiency, large bandwidth and are related to low polarization, but the manufacturing process is complex and the cost is high. Grating couplers are relatively simple in chip packaging, but are more sensitive to both wavelength and polarization. Although the design of existing grating couplers can achieve high coupling efficiency and polarization-independent functions at the same time, multiple steps are required in the process. There are difficulties in process manufacturing and polarization sensitivity problems in the existing technology. Therefore, the present invention proposes a single-ended coupled compact grating coupler composed of a rotated subwavelength structure that only requires one-step etching on the SOI wafer, which can simultaneously allow transverse electric mode (TE) and transverse magnetic mode (TM) light to be coupled from the optical fiber into the waveguide, or from the waveguide into the optical fiber. Summary of the Invention

[0003] The present invention aims to address the shortcomings of the prior art and provide a fully etched polarization-independent compact sub-wavelength grating structure and its coupler. The present invention has a simple manufacturing process and solves the problem of polarization sensitivity.

[0004] The objectives of the present invention are achieved through the following technical solutions: First, the present invention provides a fully etched polarization-independent compact subwavelength grating structure, which is designed and obtained by the following method: a rotated subwavelength structure is adopted, and the topological structure is optimized by an inverse design method, and both the transverse electric mode light and the transverse magnetic mode light propagate along the x-axis; in the y-axis direction, the first period length py and the first duty cycle fy of the unit subwavelength structure of each period are equal, forming a unit subwavelength array structure; in the x-axis direction, the second period length px and the second duty cycle fx of the unit subwavelength array structure of each period are different, and there are different rotation angles α, so that the equivalent refractive index of the unit subwavelength array structure under the transverse electric mode light and the transverse magnetic mode light differs by -10% to 10%, so as to realize the polarization independence of the grating coupling; and the original rectangular subwavelength grating structure is converted into a fan-shaped structure by adopting a coordinate transformation method.

[0005] Optionally, the number of the rotated subwavelength structures is referred to as the period number in the x-axis direction and the period number in the y-axis direction, which is determined by the mode spot radius, the first period length py and the second period length px of the optical fiber.

[0006] Optionally, the parameters of the unit subwavelength array structure include a second period length px in the x-direction, a second duty cycle fx, a rotation angle α, a first period length py in the y-direction, and a first duty cycle fy; wherein, the second duty cycle fx in the x-direction and the first period length py in the y-direction are determined by the first period length py that meets the requirements of the 0th-order diffraction order formula; the second period length px in the x-direction, the rotation angle α, and the first duty cycle fy in the y-direction are determined by the theoretical calculated value and simulation value of the equivalent refractive index and the theoretical calculated value and simulation value of the leakage factor.

[0007] Optionally, the second period length px in the x-direction, the rotation angle α and the first duty cycle fy in the y-direction are determined by the theoretical calculated value and simulation value of the equivalent refractive index and the theoretical calculated value and simulation value of the leakage factor, specifically: first, the equivalent refractive index and leakage factor simulation values ​​corresponding to the first duty cycle fy and the rotation angle α are calculated based on the formula of the theoretical calculated value of the equivalent refractive index and the simulation results of the unit subwavelength structure with different first duty cycles fy and different rotation angles α, and nonlinear fitting is performed on the first duty cycle fy-equivalent refractive index, the rotation angle α-equivalent refractive index and the first duty cycle fy-leakage factor simulation value to obtain the fitting relationship between the first duty cycle fy and the rotation angle α with respect to the equivalent refractive index and the fitting relationship between the first duty cycle fy and the rotation angle α with respect to the leakage factor simulation value, and then the first duty cycle fy equal to the theoretical value of the leakage factor is successively searched for, and the second period length fx in the x-direction is calculated according to the Bragg diffraction formula.

[0008] The present invention secondly provides a fully etched polarization-independent compact sub-wavelength grating coupler, comprising:

[0009] Silicon substrate;

[0010] A lower cladding layer is provided above the silicon substrate;

[0011] a top silicon structure disposed above the lower cladding layer; and

[0012] an upper cladding layer, disposed above the top silicon structure;

[0013] The top silicon structure includes a silicon waveguide structure, a tapered waveguide structure and the above-mentioned sub-wavelength grating structure, the silicon waveguide structure and the tapered waveguide structure are connected, the tapered waveguide structure and the sub-wavelength grating structure are connected, and the silicon waveguide structure and the sub-wavelength grating structure are not connected.

[0014] Optionally, the angle of the tapered waveguide structure is greater than or equal to 10° and less than or equal to 90°.

[0015] Optionally, the angle of the tapered waveguide structure is 60°.

[0016] Optionally, the length of the tapered waveguide structure is determined by the angle and mode spot radius of the tapered waveguide structure.

[0017] Optionally, the tapered waveguide structure and the transformed sub-wavelength grating structure are simultaneously optimized in terms of topology using an inverse design method.

[0018] Optionally, the light transmitted in the sub-wavelength grating structure is a fundamental mode.

[0019] The beneficial effects of the present invention are that the present invention improves the coupling efficiency of the fully etched grating coupler and reduces the polarization sensitivity of the coupler; simplifies the manufacturing process, requiring only one etching step, while achieving a compact structure of the grating coupler and reducing the size of the grating coupler; based on a subwavelength grating structure and a reverse design optimization method, the present invention achieves polarization-independent coupling between a single-mode optical fiber and a grating coupler, and its preparation process is compatible with both electron beam and CMOS processes. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Schematic diagram of the main structure of the grating coupler in an embodiment of the present invention;

[0021] Figure 2 Schematic diagram of the top view of the top silicon structure in an embodiment of the present invention;

[0022] Figure 3 is a partial enlarged view of the tapered waveguide structure in an embodiment of the present invention;

[0023] Figure 4 is a relationship diagram between the first duty cycle fy in the y direction and the equivalent refractive index of the unit subwavelength structure;

[0024] Figure 5 Schematic diagram of the structure during the preparation method; among them, (a) is the SOI wafer, (b) is a schematic diagram of the silicon waveguide structure, tapered waveguide structure and subwavelength grating structure formed after etching, and (c) is a schematic diagram of the structure after the upper cladding layer is generated.

[0025] In the figure: top silicon structure 1, silicon waveguide structure 100, tapered waveguide structure 101, subwavelength grating structure 102, upper cladding 2, lower cladding 3, silicon substrate 4, single-mode optical fiber 5. DETAILED DESCRIPTION

[0026] Exemplary embodiments will be described in detail herein, examples of which are illustrated in the accompanying drawings. In the following description, when referring to the drawings, like numbers in different figures represent like or similar elements unless otherwise indicated. The embodiments described in the following exemplary embodiments are not intended to represent all possible embodiments consistent with the present invention. Rather, they are merely examples of apparatus and methods consistent with certain aspects of the present invention, as detailed in the appended claims.

[0027] The terms used in this invention are for the purpose of describing specific embodiments only and are not intended to limit the invention. The singular forms "a," "the," and "the" used in this invention and the appended claims are also intended to include plural forms unless the context clearly indicates otherwise. It should also be understood that the term "and / or" as used herein refers to and includes any or all possible combinations of one or more of the associated listed items.

[0028] It should be understood that although the terms "first," "second," "third," etc. may be used in the present invention to describe various information, such information should not be limited to these terms. These terms are merely used to distinguish information of the same type from one another. For example, first information may also be referred to as second information, and similarly, second information may also be referred to as first information, without departing from the scope of the present invention. Depending on the context, the term "if" as used herein may be interpreted as "when," "when," or "in response to determining."

[0029] The present invention will be described in detail below with reference to the accompanying drawings. Unless there is any conflict, the features of the following embodiments and implementations may be combined with each other.

[0030] The fully etched polarization-independent compact subwavelength grating structure in the present invention is as follows: Figure 2 As shown, the sub-wavelength grating structure 102 is prepared by the following method:

[0031] A rotated subwavelength structure is adopted, and the topological structure is optimized by an inverse design method. Both transverse electric mode light (TE mode light) and transverse magnetic mode light (TM mode light) propagate along the x-axis. In the y-axis direction, the first period length py and the first duty cycle fy of the unit subwavelength structure of each period are equal, forming a unit subwavelength array structure. In the x-axis direction, the second period length px and the second duty cycle fx of the unit subwavelength array structure of each period are different, and there are different rotation angles α, so that the equivalent refractive index of the unit subwavelength array structure under transverse electric mode light and transverse magnetic mode light differs by -10% to 10%, so as to realize polarization independence of grating coupling. The original rectangular subwavelength grating structure 102 is converted into a fan-shaped structure by a coordinate transformation method.

[0032] It should be understood that, in the x-axis direction, there are multiple periods, and the second period length px and second duty cycle fx of the unit subwavelength array structure in each period are different, but the period length and duty cycle within the same period are equal. Furthermore, the equivalent refractive index of the unit subwavelength array structure under transverse electric mode light and transverse magnetic mode light differs by -10% to 10%, which can be understood as the equivalent refractive index under transverse electric mode light and transverse magnetic mode light being approximately equal.

[0033] For example, in this embodiment, the original rectangular sub-wavelength grating structure 102 may be transformed into a fan-shaped structure by using a rectangular coordinate-polar coordinate transformation.

[0034] In this embodiment, the number of rotated subwavelength structures is referred to as the period number in the x-axis direction and the period number in the y-axis direction, which is determined by the mode spot radius, the first period length py, and the second period length px of the optical fiber.

[0035] Specifically, the number of periods in the x-axis direction = twice the fiber spot radius / the second period length px, and the number of periods in the y-axis direction = twice the fiber spot radius / the first period length py.

[0036] In this embodiment, the inverse design method optimizes the topology structure as follows: in conventional design (i.e., forward design), given the input x n (n is a natural number), and output y n With input x n There exists a constraint F n , given the design area parameter p in the design n , and get the output x n When optimizing the output results, the design area parameter p is changed multiple times. n (such as particle swarm optimization algorithm) to obtain the optimal solution based on the initial parameters.

[0037] In reverse design, the design model is composed of the equations F n (x n ,p n )=y n Indicates that, given the input x n And the desired output y n , for F n (x n ,p n )=y n Solve and get the design area parameter p n When optimizing the output results, the output y is compared multiple times. n and the desired output y n The difference between them changes the design area parameter p n , so that the output result is close to the output y n , get the design area parameter p n The optimal solution of .

[0038] Among them, the design area parameter p nIt is capable of completing the selection of the first period length py and the first duty cycle fy of the unit subwavelength structure of each period, the second period length px and the second duty cycle fx of the unit subwavelength array structure of each period, and the rotation angle α, and is capable of obtaining a pixelated discrete image of the subwavelength grating structure 102 of the fan-shaped structure obtained by adopting a coordinate transformation method.

[0039] In this embodiment, the parameters of the unit subwavelength array structure include the second period length px in the x-direction, the second duty cycle fx, the rotation angle α, the first period length py in the y-direction, and the first duty cycle fy, wherein the second duty cycle fx in the x-direction and the first period length py in the y-direction are determined by the manufacturing limitations of the actual process and the first period length py that meets the requirements of the 0th-order diffraction order formula; the second period length px in the x-direction, the rotation angle α, and the first duty cycle fy in the y-direction are determined by the theoretical calculated value and simulation value of the equivalent refractive index and the theoretical calculated value and simulation value of the leakage factor.

[0040] In this embodiment, the expression of the 0th order diffraction order formula is:

[0041] or

[0042] Where λ is the center wavelength, n eff_TE is the equivalent refractive index of the transverse electric mode optical slab waveguide, n eff_TM is the equivalent refractive index of the transverse magnetic mode optical slab waveguide.

[0043] For example, the manufacturing limitations of the actual process include the minimum line width and line spacing of lithography.

[0044] Exemplarily, the etched width of the grating, i.e., py×fy or px×fx, must be greater than the minimum line width requirement of the lithography, and the width of the non-etched part of the grating, i.e., py×(1-fy) or px×(1-fx), must be greater than the minimum line spacing requirement of the lithography.

[0045] In this embodiment, first, based on the formula of the theoretical calculation value of the equivalent refractive index and the simulation results of the unit subwavelength structure with different first duty cycles fy and different rotation angles α, the equivalent refractive index and leakage factor simulation values ​​corresponding to the first duty cycle fy and the rotation angle α are calculated, and nonlinear fitting is performed on the first duty cycle fy-equivalent refractive index, the rotation angle α-equivalent refractive index and the first duty cycle fy-leakage factor simulation values, and the fitting function is g(x)=ax 5 +bx 4 +cx 3 +dx 2+ex+f, where a, b, c, d, e, and f are constants calculated by fitting. In the fitting of the first duty cycle fy-equivalent refractive index, x represents the first duty cycle fy, and g(x) represents the equivalent refractive index. In the fitting of the rotation angle α-equivalent refractive index, x represents the rotation angle α, and g(x) represents the equivalent refractive index. In the fitting of the first duty cycle fy-leakage factor simulation value, x represents the first duty cycle fy, and g(x) represents the leakage factor simulation value. It should be understood that the values ​​of a, b, c, d, e, and f are different in different fittings. The fitting relationship between the first duty cycle fy and the rotation angle α with respect to the equivalent refractive index and the fitting relationship between the first duty cycle fy and the leakage factor simulation value can be calculated. Then, the first duty cycle fy equal to the theoretical value of the leakage factor is successively found, and the second period length fx in the x direction is calculated according to the Bragg diffraction formula.

[0046] In this embodiment, the theoretical calculated value of the equivalent refractive index can be obtained by calculation according to the following equivalent medium theory (EMT) formula:

[0047]

[0048]

[0049]

[0050]

[0051] Among them, n swg_TE_2 is the second-order approximate solution of the equivalent refractive index under transverse electric mode light, that is, the theoretical approximate value (theoretical calculated value) of the equivalent refractive index under transverse electric mode light, n swg_TM_2 is the second-order approximate solution of the equivalent refractive index under transverse magnetic mode light, that is, the theoretical approximate value (theoretical calculated value) of the equivalent refractive index under transverse magnetic mode light, n swg_TE_1 is the first-order approximate solution of the equivalent refractive index under transverse electric mode light, n swg_TM_1 is the first-order approximate solution of the equivalent refractive index under transverse magnetic mode light, f is the duty cycle of the unit subwavelength array structure, and n si is the refractive index of silicon, n hole is the refractive index of the upper cladding 2, p is the period length of the unit subwavelength structure, λ is the central wavelength, n eff_TE is the equivalent refractive index of the transverse electric mode optical slab waveguide, n eff_TM is the equivalent refractive index of the transverse magnetic mode optical slab waveguide.

[0052] In this embodiment, the calculation formula for the simulation value of the equivalent refractive index is:

[0053]

[0054] Among them, neff is the equivalent refractive index, ω is the phase of light, l is the distance the light travels, and λ is the center wavelength.

[0055] In this embodiment, the theoretical calculated value of the leakage factor can be obtained by calculating the following formula:

[0056]

[0057]

[0058] Where G(x) represents the normalized Gaussian distribution, x is the length of the grating along the x-direction, w0 is the mode field radius corresponding to the single-mode fiber, and 2a(x) represents the theoretically calculated value of the leakage factor.

[0059] In this embodiment, the relevant formula for the simulation value of the leakage factor is:

[0060] Power(x)=P0×e -2εx

[0061] Wherein, ε is the simulation value of the leakage factor, P0 is the light field power of the input light source, x is the length of the grating along the x direction, and Power(x) represents the light field power.

[0062] In this embodiment, the Bragg diffraction formula is:

[0063]

[0064] Where p is the period length of the unit subwavelength structure, λ is the central wavelength, and n hole is the refractive index of the upper cladding, θ is the angle between the single-mode optical fiber 5 and the vertical plane of the upper cladding 2, and n eff is the equivalent refractive index of one period of the unit subwavelength array structure.

[0065] Furthermore, for transverse electric mode light, n eff =f×n swg +(1-f)×n si , where f is the duty cycle of the unit subwavelength array structure, n si is the refractive index of silicon, n swg is the equivalent refractive index of the unit subwavelength structure.

[0066] Furthermore, for transverse magnetic mode light, Where f is the duty cycle of the unit subwavelength array structure, n si is the refractive index of silicon, n swg is the equivalent refractive index of the unit subwavelength structure.

[0067] For example, in this embodiment, a central wavelength of 1310 nm is selected, and this embodiment is a single-port input / output. Based on actual manufacturing constraints, the minimum line width and line spacing must be greater than 100 nm. According to the formula for the zero-order diffraction order, the first period length py must be less than 0.445 nm. Therefore, the first period length py is selected to be 0.4 nm, and the second duty cycle is 0.6.

[0068] Under the condition of the same first period length py, FDTD simulation of transverse electric mode light and transverse magnetic mode light is performed on the unit subwavelength structure corresponding to different first duty cycle fx values ​​(uniformly sampled between 0 and 1), and the simulation results of the phase of light and the distance of light transmission and the light field power and the distance of light transmission are recorded; under the condition of the same first period length py and the same first duty cycle fy, FDTD simulation of transverse electric mode light and transverse magnetic mode light is performed on the rotated unit subwavelength structure corresponding to different rotation angles α values ​​(uniformly sampled between 0° and 45°), and the simulation results of the phase of light and the distance of light transmission and the light field power and the distance of light transmission are recorded. It should be understood that FDTD is the finite difference time domain method, and simulation is performed using this method.

[0069] The equivalent refractive index corresponding to the first duty cycle fy and the rotation angle α is calculated according to the obtained simulation results and the calculation formula of the simulation value of the equivalent refractive index; the simulation value of the leakage factor corresponding to the first duty cycle fy and the rotation angle α is calculated according to the relevant formula of the obtained simulation results and the simulation value of the leakage factor, and multiple sets of discrete data of the first duty cycle fy-equivalent refractive index, the rotation angle α-equivalent refractive index and the first duty cycle fy-leakage factor simulation value can be obtained, and nonlinear fitting is performed on them to obtain the fitting relationship between the first duty cycle fy and the rotation angle α on the equivalent refractive index and the fitting relationship between the first duty cycle fy and the leakage factor simulation value. The acquisition of the fitting relationship between the first duty cycle fy and the rotation angle α on the equivalent refractive index and the fitting relationship between the first duty cycle fy and the leakage factor simulation value is similar to the acquisition of the fitting relationship between the first duty cycle fy and the rotation angle α on the equivalent refractive index and the fitting relationship between the first duty cycle fy and the leakage factor simulation value.

[0070] Since the period length and duty cycle of the same period along the x-direction are equal, the equivalent refractive index of the corresponding unit subwavelength array structure must be equal. From this, the relationship between the equivalent refractive index of the unit subwavelength structure of the transverse electric mode light and the transverse magnetic mode light in the y-direction can be derived: Among them, n TE The equivalent refractive index of the unit subwavelength structure of transverse electric mode light in the y direction, n TMis the equivalent refractive index of the unit subwavelength structure of the transverse magnetic mode light in the y direction. For the unit subwavelength structure distributed along the y direction, when the first period length py and the first duty cycle fy are consistent, the equivalent refractive index of the transverse electric mode light and the transverse magnetic mode light obtained by the EMT formula is different, as shown in Figure 4 As shown, the horizontal dashed line represents the relationship between the first duty cycle fy of the transverse electric mode light in the y direction and the equivalent refractive index of the unit subwavelength structure, the dotted line represents the relationship between the first duty cycle fy of the transverse magnetic mode light in the y direction and the equivalent refractive index of the unit subwavelength structure, and the dotted line represents the relationship between the transverse magnetic mode light and the equivalent refractive index of the unit subwavelength structure in the y direction of the transverse electric mode light. Since the effect of the rotated unit subwavelength structure on the equivalent refractive index of the TE mode light is less than 0.2, and the effect on the equivalent refractive index of the TM mode light is greater than 1, the equivalent refractive indices obtained by decomposing the TE mode light and the TM mode light with the same duty cycle and period length can be made to conform to the above relationship by rotating at different rotation angles α. It should be noted that the same duty cycle and period length include: the period length of the same period along the x direction is equal to the duty cycle, and the first period length py is equal to the first duty cycle fy.

[0071] According to the fitting relationship between the rotation angle α and the equivalent refractive index, find the rotation angle α that makes the transverse electric mode light and the transverse magnetic mode light approximate under the conditions that the first period length py and the first duty cycle fy are equal, and record the rotation angle α and its corresponding equivalent refractive index.

[0072] According to the obtained fitting relationship between the first duty cycle fy and the simulation value of the leakage factor, the first duty cycle fy equal to the theoretical value of the leakage factor is searched period by period.

[0073] The second period length fx in the x direction can be calculated based on the Bragg diffraction formula.

[0074] It is worth mentioning that the present invention also provides a fully etched polarization-independent compact sub-wavelength grating coupler.

[0075] See also Figure 1 The grating coupler includes a silicon substrate 4, a lower cladding layer 3, a top silicon structure 1, and an upper cladding layer 2. The silicon substrate 4 is located at the lowest layer of the grating coupler; the lower cladding layer 3 is disposed above the silicon substrate 4; the top silicon structure 1 is disposed above the lower cladding layer 3; and the upper cladding layer 2 is disposed above the top silicon structure 1.

[0076] In this embodiment, the top silicon structure 1 includes a silicon waveguide structure 100, a tapered waveguide structure 101, and the aforementioned subwavelength grating structure 102. The silicon waveguide structure 100 and the tapered waveguide structure 101 are connected, the tapered waveguide structure 101 and the subwavelength grating structure 102 are connected, and the silicon waveguide structure 100 and the subwavelength grating structure 102 are not connected.

[0077] Furthermore, for transverse electric and transverse magnetic modes with a central wavelength of 1310 nm, the width of the silicon waveguide structure 100 is 410 nm. For transverse electric and transverse magnetic modes with a central wavelength of 1550 nm, the width of the silicon waveguide structure 100 is 500 nm. It should be understood that the width of the silicon waveguide structure 100 may vary depending on the actual situation for different central wavelengths.

[0078] In this embodiment, the angle of the tapered waveguide structure 101 is greater than or equal to 10° and less than or equal to 90°; the length of the tapered waveguide structure 101 can be determined according to the following formula:

[0079]

[0080] Wherein, r represents the length of the tapered waveguide structure 101, β represents the angle of the tapered waveguide structure 101, and w0 is the mode spot radius of the optical fiber.

[0081] Exemplarily, when the angle of the tapered waveguide structure 101 is 60° and the mode spot radius is 5 μm, the length of the tapered waveguide structure 101 is 10 μm.

[0082] Preferably, the angle of the tapered waveguide structure 101 is 60°, which ensures that the rectangular structure of the grating is transformed into a fan-shaped structure to form a focusing grating structure and reduce back reflection, while avoiding excessive changes in the geometric structure due to coordinate transformation, which greatly reduces the grating coupling efficiency.

[0083] Furthermore, the topology of the transformed fan-shaped subwavelength grating structure 102 and the tapered waveguide structure 101 is optimized using an inverse design method to reduce the overall size. This design can reduce the overall size of the grating by an order of magnitude without significantly reducing the coupling efficiency. For example, before coordinate transformation and optimization, the overall size is usually 250x15μm. 2 to 150x15μm 2 After optimization, the overall size is only 20x15μm 2 Compared with the previous one, the overall size is much smaller, making the structure of the grating coupler more compact.

[0084] For example, the initial structure of the tapered waveguide structure 101 is a fan-shaped structure with an angle of 60° and a length of 10 μm. The inverse design method is used for optimization, and the final structure of the tapered waveguide structure 101 is as follows: Figure 3 shown.

[0085] In this embodiment, the light transmitted in the sub-wavelength grating structure 102 is the fundamental mode. It should be understood that in the grating coupler, the light transmitted in the sub-wavelength grating structure 102 is the fundamental mode.

[0086] In some other embodiments, the grating coupler further includes a single-mode optical fiber 5, such as Figure 1 As shown, the single-mode optical fiber 5 is arranged above the upper cladding 2, and the angle between the single-mode optical fiber 5 and the vertical plane of the upper cladding 2 is θ, wherein θ is greater than or equal to -45° and less than or equal to 45°.

[0087] Preferably, θ is greater than or equal to 10° and less than or equal to 20°.

[0088] For example, in this embodiment, the thickness of the upper cladding 2 is 2 μm, the thickness of the lower cladding 3 is 3 μm, the thickness of the top silicon structure 1 is 220 nm, the silicon substrate 4 is arranged below the lower cladding 3, the distance between the single-mode optical fiber 5 and the upper cladding 2 is 1 μm, and the angle with the vertical plane of the upper cladding 2 is 10°.

[0089] Furthermore, for transverse electric mode light and transverse magnetic mode light with a central wavelength of 1310 nm or 1550 nm, the angle θ is 10°. It should be understood that the angle θ will change for different central wavelengths.

[0090] For example, in this embodiment, Figure 5 The SOI wafer shown in (a) is placed in a spin coater and a photoresist layer is spin-coated. The photoresist is then dried. The photoresist on the surface of the SOI wafer is exposed using an electron beam exposure process to form a hard mask pattern of the silicon waveguide structure 100, the tapered waveguide structure 101, and the sub-wavelength grating structure 102. Inductively coupled plasma etching is used to form the silicon waveguide structure 100, the tapered waveguide structure 101, and the sub-wavelength grating structure 102, and the photoresist is cleaned. The structure is shown in FIG. Figure 5 As shown in (b), only one step of etching is required on the SOI wafer; finally, a 2 μm thick silicon dioxide film is deposited on the chip using plasma enhanced chemical vapor deposition, as shown in FIG. Figure 5 As shown in (c), the upper cladding layer 2 covers the top silicon structure 1. It should be understood that the upper cladding layer 2 is a cladding layer formed of a silicon dioxide film.

[0091] Then, the single-mode optical fiber 5 is placed above the upper cladding 2, and the angle between the single-mode optical fiber 5 and the vertical plane of the upper cladding 2 is θ. For example, θ is set to 10°. It should be understood that other angles can also be set according to actual needs.

[0092] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A fully etched polarization-independent compact subwavelength grating structure, characterized in that: The subwavelength grating structure (102) is designed and obtained by the following method: a rotating subwavelength structure is adopted, and a topological structure is optimized by an inverse design method, wherein both transverse electric mode light and transverse magnetic mode light propagate along the x-axis; in the y-axis direction, the first period length py and the first duty cycle fy of the unit subwavelength structure of each period are equal, thereby forming a unit subwavelength array structure; In the x-axis direction, the second period length px and the second duty cycle fx of each period of the unit subwavelength array structure are different, and there are different rotation angles α, so that the equivalent refractive index of the unit subwavelength array structure under transverse electric mode light and transverse magnetic mode light differs by -10% to 10%, so as to realize polarization independence of grating coupling; and the original rectangular subwavelength grating structure (102) is converted into a fan-shaped structure by adopting a coordinate transformation method.

2. The fully etched polarization-independent compact sub-wavelength grating structure according to claim 1, characterized in that: The number of the rotated subwavelength structures is called the period number in the x-axis direction and the period number in the y-axis direction, which is determined by the mode spot radius, the first period length py and the second period length px of the optical fiber.

3. The fully etched polarization-independent compact sub-wavelength grating structure according to claim 1, characterized in that: The parameters of the unit subwavelength array structure include the second period length px in the x-direction, the second duty cycle fx, the rotation angle α, the first period length py in the y-direction, and the first duty cycle fy; wherein, the second duty cycle fx in the x-direction and the first period length py in the y-direction are determined by the first period length py that meets the requirements of the 0th-order diffraction order formula; the second period length px in the x-direction, the rotation angle α, and the first duty cycle fy in the y-direction are determined by the theoretical calculated value and simulation value of the equivalent refractive index and the theoretical calculated value and simulation value of the leakage factor.

4. The fully etched polarization-independent compact sub-wavelength grating structure according to claim 3, characterized in that: The second period length px in the x-direction, the rotation angle α and the first duty cycle fy in the y-direction are determined by the theoretical calculated value and simulation value of the equivalent refractive index and the theoretical calculated value and simulation value of the leakage factor. Specifically, the equivalent refractive index and leakage factor simulation values ​​corresponding to the first duty cycle fy and the rotation angle α are calculated based on the formula of the theoretical calculated value of the equivalent refractive index and the simulation results of the unit subwavelength structure with different first duty cycles fy and different rotation angles α. Nonlinear fitting is performed on the first duty cycle fy-equivalent refractive index, the rotation angle α-equivalent refractive index and the first duty cycle fy-leakage factor simulation value to obtain the fitting relationship between the first duty cycle fy and the rotation angle α with respect to the equivalent refractive index and the fitting relationship between the first duty cycle fy and the rotation angle α with respect to the leakage factor simulation value. Then, the first duty cycle fy equal to the theoretical value of the leakage factor is successively searched for, and the second period length fx in the x-direction is calculated according to the Bragg diffraction formula.

5. A fully etched polarization-independent compact sub-wavelength grating coupler, characterized in that: include: Silicon substrate (4); A lower cladding layer (3) is arranged above the silicon substrate (4); A top silicon structure (1) is arranged above the lower cladding layer (3); and an upper cladding layer (2) disposed above the top silicon structure (1); The top silicon structure (1) comprises a silicon waveguide structure (100), a tapered waveguide structure (101), and a sub-wavelength grating structure (102) according to any one of claims 1 to 4, wherein the silicon waveguide structure (100) and the tapered waveguide structure (101) are connected, the tapered waveguide structure (101) and the sub-wavelength grating structure (102) are connected, and the silicon waveguide structure (100) and the sub-wavelength grating structure (102) are not connected.

6. The fully etched polarization-independent compact sub-wavelength grating coupler according to claim 5, characterized in that: The angle of the tapered waveguide structure (101) is greater than or equal to 10° and less than or equal to 90°.

7. The fully etched polarization-independent compact sub-wavelength grating coupler according to claim 6, characterized in that: The angle of the tapered waveguide structure (101) is 60°.

8. The fully etched polarization-independent compact sub-wavelength grating coupler according to claim 5, characterized in that: The length of the tapered waveguide structure (101) is determined by the angle of the tapered waveguide structure (101) and the mode spot radius.

9. The fully etched polarization-independent compact sub-wavelength grating coupler according to claim 5, characterized in that: The tapered waveguide structure (101) and the transformed sub-wavelength grating structure (102) are simultaneously optimized for topological structures using an inverse design method.

10. The fully etched polarization-independent compact sub-wavelength grating coupler according to claim 5, characterized in that: The light transmitted in the sub-wavelength grating structure (102) is a fundamental mode.

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