A full etched polarization independent subwavelength grating coupler

Abstract

The application discloses a full-etching polarization-independent subwavelength grating coupler, which comprises a top silicon structure, an upper cladding layer, a lower cladding layer and a silicon substrate, wherein the top silicon structure comprises a subwavelength grating structure optimized by using more free parameters and a reverse design method, two mutually perpendicular output / input ports, a single-mode overwaveguide and a polarization combiner / polarization splitter designed by using a reverse method based on an adjoint method; the polarization combiner / polarization splitter is designed by using the reverse method. The application can be manufactured on a standard silicon photonics platform, the introduced subwavelength metamaterial structure and the reverse design method can effectively reduce the coupling loss and the size of the device, and the application has a higher working bandwidth; the application can simultaneously allow light in a transverse electric mode or a transverse magnetic mode in an optical fiber to be coupled into a waveguide; the subwavelength grating structure of the application has a simple manufacturing process, a larger manufacturing tolerance and a smaller influence of a manufacturing process level.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optical communication and optical interconnection, and particularly relates to a full-etching polarization-independent subwavelength grating coupler. BACKGROUND

[0002] The dense integration of silicon photonic devices is a solution to provide low-cost and high-performance photonic devices and systems, and the complementary metal-oxide-semiconductor process (CMOS) compatible with silicon-on-insulator (SOI) has become one of the most important platforms for the next generation of on-chip optical interconnection technology, and high-performance optical elements such as on-chip light sources and high-speed modulators are designed and implemented. However, the challenge faced by silicon photonic chips is how to effectively couple light to and from optical fibers. The standard optical fiber for data communication is a single-mode fiber (SMF) with a mode field diameter (MFD) of about 10 μm at 1310 nm, while the width of the waveguide is only a few hundred nanometers. Due to the mismatch in modal size, efficient coupling between the fiber and the chip is challenging. This problem is usually solved using two solutions: edge couplers and vertical grating couplers.

[0003] Edge couplers can achieve high coupling efficiency (CE), large bandwidth (BW), and low polarization-dependent loss (PDL), but require cutting and polishing with strict smoothness requirements, and because their footprint is relatively large and must be placed at the edge, the manufacturing cost is increased and the flexibility is reduced. In addition, edge couplers have low alignment tolerance and are not convenient for wafer-level testing, which will further increase the chip cost and require higher testing precision and packaging.

[0004] Vertical grating couplers are more flexible in terms of arbitrariness and are relatively simple in chip packaging, but gratings are sensitive to both wavelength and polarization. Some existing vertical grating couplers can simultaneously achieve high coupling efficiency and polarization independence, but they require multiple steps in the process, such as overlaying or growing materials such as silicon nitride. Given that CMOS tape-out often takes a long time and is not suitable for testing of discrete devices, and for laboratory conditions, it is difficult to implement additional processes, a polarization-independent grating coupler with a simpler process is needed. SUMMARY

[0005] The present application aims at the deficiencies of the prior art, and provides a full-etching polarization-independent subwavelength grating coupler. By simultaneously adjusting the duty cycle and the period length in the x direction and the y direction, the present application adjusts the refractive index of each period of the grating coupler to achieve polarization-independent coupling between the single-mode fiber and the waveguide.

[0006] The purpose of the present application is achieved by the following technical solutions: a full-etching polarization-independent subwavelength grating coupler, which simultaneously allows light in a transverse electric mode or a transverse magnetic mode in an optical fiber to be coupled into a waveguide, the grating coupler comprises, from top to bottom, an upper cladding, a top silicon structure, a lower cladding, and a silicon substrate, the top silicon structure comprises a first subwavelength grating structure and a second subwavelength grating structure optimized using a free parameter and a reverse design method, a first output port and a second output port perpendicular to each other, a first input port and a second input port perpendicular to each other, a first single-mode transition waveguide, a second single-mode transition waveguide, a polarization beam combiner, and a polarization beam splitter.

[0007] The first subwavelength grating structure and the second subwavelength grating structure are designed by the following method: transverse electric mode light propagates along the x-axis, and is output from the second output port or input from the second input port, transverse magnetic mode light propagates along the y-axis, and is output from the first output port or input from the first input port, the duty cycle of each unit subwavelength structure in the x direction and the y direction is different, and the period length in the x direction and the y direction is related to the equivalent refractive index and the duty cycle of the unit subwavelength structure corresponding to the transmission mode in the corresponding direction; the unit subwavelength structures are arranged in sequence in the x direction and the y direction, and are called unit subwavelength structure queues; the unit subwavelength structure parameters need to be determined through multiple iterations; the polarization independence of the grating is realized by adjusting the unit subwavelength structure parameters; and the parameters are optimized using a reverse design method.

[0008] Optionally, the polarization beam combiner and the polarization beam splitter are designed using a reverse method based on the adjoint method, the polarization beam combiner is used to combine the transverse electric mode light and the transverse magnetic mode light coupled into the waveguide, and the polarization beam splitter is used to separate the transverse electric mode light and the transverse magnetic mode light coupled into the optical fiber.

[0009] Optionally, the first output port comprises a first tapered waveguide and a first single-mode waveguide using an adiabatic structure; the second output port comprises a second tapered waveguide and a second single-mode waveguide using an adiabatic structure; the first input port comprises a third tapered waveguide and a third single-mode waveguide using an adiabatic structure; the second input port comprises a fourth tapered waveguide and a fourth single-mode waveguide using an adiabatic structure; and the first single-mode waveguide, the second single-mode waveguide, the third single-mode waveguide, and the fourth single-mode waveguide are used to transition single-mode light from the grating to the waveguide or from the waveguide to the grating.

[0010] Optionally, the structure of the first tapered waveguide, the second tapered waveguide, the third tapered waveguide, and the fourth tapered waveguide is one of linear, exponential, and parabolic.

[0011] Optionally, the first single-mode transition waveguide adopts a smooth circular arc to connect the first single-mode waveguide and the transverse magnetic mode light input port of the polarization beam combiner, so as to ensure that single-mode light is transmitted in the waveguide; the second single-mode transition waveguide adopts a smooth circular arc to connect the third single-mode waveguide and the transverse magnetic mode light output port of the polarization beam splitter, so as to ensure that single-mode light is transmitted in the waveguide.

[0012] Optionally, the radius of the smooth circular arc is 30 μm.

[0013] Optionally, the transverse electric mode light and transverse magnetic mode light mixed input / output ports of the polarization beam combiner and the polarization beam splitter are located on one side and below the device, the transverse electric mode light input / output port and the transverse magnetic mode light input / output port are located on the other side of the device, and the transverse electric mode light input / output port is located below the device and the transverse magnetic mode light input / output port is located above the device.

[0014] Optionally, the materials of the first sub-wavelength grating structure, the second sub-wavelength grating structure, the first output port, the second output port, the first input port, the second input port, the polarization beam combiner, the polarization beam splitter, the first single-mode transition waveguide, and the second single-mode transition waveguide include one or more of silicon, silicon dioxide, silicon nitride, and air.

[0015] Optionally, the parameters of the unit sub-wavelength structure include a first duty cycle in the x direction, a second duty cycle in the y direction, a first period length in the x direction, and a second period length in the y direction.

[0016] Optionally, the parameters of the unit sub-wavelength structure queue include the parameters of the unit sub-wavelength structure constituting the queue, a first number of periods in the x direction, and a second number of periods in the y direction; wherein the first number of periods and the second number of periods are determined by the mode spot radius of the optical fiber and the mode spot radius of the grating, and the expression is: the first number of periods = two times the mode spot radius of the optical fiber / first period length, and the second number of periods = two times the mode spot radius of the optical fiber / second period length.

[0017] The beneficial effects of the present application are that by simultaneously adjusting the duty cycles and period lengths in the x direction and the y direction, the refractive index of each period of the grating coupler is adjusted, so that polarization-independent coupling between the single-mode optical fiber and the waveguide can be realized; the present application can simultaneously allow transverse electric mode light and transverse magnetic mode light to be coupled from the optical fiber into the waveguide or from the waveguide into the optical fiber; the sub-wavelength grating structure of the present application has a simple manufacturing process, a large manufacturing tolerance, and is less affected by the manufacturing process level. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 FIG. 2 is a two-dimensional structural schematic diagram of the grating coupler in the embodiment of the present application;

[0019] Figure 2 Figure 1 is a schematic diagram of the distribution of optical fiber and grating positions in an embodiment of the present application;

[0020] Figure 3 Figure 2 is a schematic diagram of a partial grating structure in an embodiment of the present application;

[0021] Figure 4 Figure 3 is a schematic diagram of a polarization beam combiner / polarization beam splitter in an embodiment of the present application;

[0022] Figure 5 Figure 4 is a schematic diagram of a partial fitting function; (a) is a theoretical value function of the leakage factor with respect to the x-direction coordinate position, (b) is a fitting function of the simulated value of the leakage factor with respect to the second duty cycle fy, and (c) is a fitting function of the simulated result of the power with respect to the x-direction coordinate position and the calculated leakage factor;

[0023] Figure 6 Figure 5 is a schematic diagram of a structure in the preparation process; (a) is an SOI wafer, (b) is a schematic diagram of a silicon waveguide structure, a tapered waveguide structure, and a subwavelength grating structure formed after etching, and (c) is a schematic diagram of a structure after generating an upper cladding layer.

[0024] In the figure: upper cladding layer 1, top silicon structure 2, first subwavelength grating structure 100, first output port 101, first tapered waveguide 1011, first single-mode waveguide 1012, second output port 102, second tapered waveguide 1021, second single-mode waveguide 1022, polarization beam combiner 103, first single-mode transition waveguide 104, first single-mode optical fiber 105, second subwavelength grating structure 200, first input port 201, third tapered waveguide 2011, third single-mode waveguide 2012, second input port 202, fourth tapered waveguide 2021, fourth single-mode waveguide 2022, polarization beam splitter 203, second single-mode transition waveguide 204, first single-mode optical fiber 205, lower cladding layer 3, silicon substrate 4. DETAILED DESCRIPTION

[0025] The exemplary embodiments will be described in detail herein with reference to the attached drawings. The description of the exemplary embodiments is intended to apply to all alternative embodiments, as would be understood by persons skilled in the art. To the extent that they do not particularize to the application, the following description of exemplary embodiments is not intended to be limiting of the application. In other words, it is contemplated that the application will apply to all alternative embodiments, even though they did not result from the present teachings, or indeed, might not even be known by persons skilled in the art at the time of filing. It is intended that the description of exemplary embodiments is not to limit the scope of the application, but rather is provided as illustrative examples of how the application might be practiced or implemented. It is contemplated that the application will apply to other embodiments or implementations after the present teachings have been made available.

[0026] The terminology used in the present application is for the purpose of describing particular embodiments only and is not intended to be limiting of the present application. As used in the present application and the following claims, the singular forms "a," "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "and / or," as used herein, refers to and encompasses any and all possible combinations of one or more of the associated listed items.

[0027] It is to be understood that, although the terms first, second, third, etc. can be used herein to describe various information, the information should not be limited to these terms. These terms are only used to distinguish one piece of information from another. For example, a first information can also be termed a second information, and, similarly, a second information can also be termed a first information, without departing from the scope of the present application. The word "if' as used herein means "when" or "upon" or "in response to the determination" depending on the context.

[0028] The present application will be described in detail below with reference to the attached drawings. The features of the embodiments and implementation forms described below can be combined with each other, without conflict.

[0029] The full-etching polarization-independent subwavelength grating coupler in the present application can simultaneously allow light in a transverse electric mode or a transverse magnetic mode in an optical fiber to be coupled into a waveguide, as shown in Figure 1 The grating coupler comprises, from top to bottom, an upper cladding layer 1, a top silicon structure 2, a lower cladding layer 3 and a silicon substrate 4.

[0030] Referring to Figure 1 , the silicon substrate 4 is located at the lowest layer of the grating coupler; the lower cladding layer 3 is arranged above the silicon substrate 4; the top silicon structure 2 is arranged above the lower cladding layer 3; and the upper cladding layer 1 is arranged above the top silicon structure 2.

[0031] In the present embodiment, the top silicon structure 2 comprises a first subwavelength grating structure 100 and a second subwavelength grating structure 200 optimized using more free parameters and a reverse design method, a first output port 101 and a second output port 102 perpendicular to each other, a first input port 201 and a second input port 202 perpendicular to each other, a first single-mode transition waveguide 104, a second single-mode transition waveguide 204, a polarization combiner 103 and a polarization splitter 203 designed using a reverse method based on the adjoint method, as shown in Figure 2 .

[0032] The first subwavelength grating structure 100 and the second subwavelength grating structure 200 are designed by the following method:

[0033] Transverse electric mode light (TE mode light) propagates along the x-axis and is output by the second output port 102 of the grating or input by the second input port 202 of the grating, and transverse magnetic mode light (TM mode light) propagates along the y-axis and is output by the first output port 101 of the grating or input by the first input port 201 of the grating, the duty cycles of each unit sub-wavelength structure in the x direction and the y direction are different, and the period lengths in the x direction and the y direction are related to the effective refractive index and the duty cycle of the unit sub-wavelength structure corresponding to the transmission mode in the corresponding direction; the unit sub-wavelength structures are arranged in sequence in the x direction and the y direction, and are referred to as a unit sub-wavelength structure queue; the parameters of the unit sub-wavelength structure need to be determined through multiple iterations; the polarization independence of the grating is realized by adjusting the parameters of the unit sub-wavelength structure; and the parameters are optimized by using a reverse design method to reduce the coupling loss.

[0034] In the embodiment, the parameters of the unit sub-wavelength structure include a first duty cycle in the x direction, a second duty cycle in the y direction, a first period length in the x direction, and a second period length in the y direction.

[0035] It should be noted that, since the sub-wavelength grating has high tolerance to errors, the first duty cycle, the second duty cycle, the first period length, and the second period length can be adjusted within a certain range (for example, duty cycle ± 0.2, period length ± 50 nm), and the design requirements of polarization independence are met by adjusting the four parameters. In this way, the four parameters can be selected, thereby improving the degree of freedom of parameter selection. In addition, to solve the problem of increased coupling loss, the parameters are optimized by using a reverse design method, thereby reducing the coupling loss.

[0036] In the embodiment, the parameters of the unit sub-wavelength structure queue include the parameters of the unit sub-wavelength structure, and the number of unit sub-wavelength structures constituting the queue in the x direction and the y direction, i.e., the first period number in the x direction and the second period number in the y direction. The number is determined by the mode spot radius of the optical fiber and the mode spot radius of the grating, and the expression is: the first period number in the x direction = two times the mode spot radius of the optical fiber / the first period length in the x direction, and the second period number in the y direction = two times the mode spot radius of the optical fiber / the second period length in the y direction.

[0037] In the embodiment, the parameters of the unit sub-wavelength structure need to be determined through multiple iterations, and the iteration process includes the following steps:

[0038] (1) The TE mode effective refractive index n swg_TE Points are taken at uniform intervals in the [upper cladding 1 refractive index, silicon refractive index] interval range, and the interval of the taken points is denoted as i1, and the number of the taken points is denoted as n1; when the TE effective refractive index n swg_TEWhen the first duty cycle fx takes a certain value, points are taken in the interval range [0, 1] of the first duty cycle fx, and the interval of the taken points is i2, and the number of the taken points is n2. In this embodiment, in order to reduce the simulation time, i2=0.05, n2=21, that is, 21 points are taken at an interval of 0.05; when the TE equivalent refractive index n swg_TE When the first duty cycle fx takes a certain value, the first period length px in the x direction can be calculated by the Bragg diffraction formula and the equivalent medium theory formula.

[0039] In this embodiment, the Bragg diffraction formula is:

[0040]

[0041] Wherein, p is the period length of the unit subwavelength structure, λ is the central wavelength, n hole is the refractive index of the upper cladding layer 1, θ is the included angle of the single-mode optical fiber 5 and the vertical surface of the upper cladding layer 1, θ=0, n eff is the equivalent refractive index of a period of the unit subwavelength structure queue.

[0042] Further, for transverse electric mode light, n eff =f×n swg +(1-f)×n si , wherein f is the duty cycle of the unit subwavelength structure, n si is the refractive index of silicon at the corresponding central wavelength, n swg is the equivalent refractive index of the unit subwavelength structure.

[0043] Further, for transverse magnetic mode light, , wherein f is the duty cycle of the unit subwavelength structure, n si is the refractive index of silicon at the corresponding central wavelength, n swg is the equivalent refractive index of the unit subwavelength structure.

[0044] In this embodiment, the theoretical calculation value of the equivalent refractive index can be calculated according to the following equivalent medium theory (EMT) formula:

[0045]

[0046]

[0047]

[0048]

[0049] Wherein, n swg_TE_2 is the second-order approximate solution of the equivalent refractive index under the transverse electric mode light, that is, the theoretical approximate value of the equivalent refractive index under the transverse electric mode light, n swg_TM_2n is the second order approximate solution of the effective refractive index under transverse magnetic mode light, i.e. the theoretical approximation of the effective refractive index under transverse magnetic mode light, n swg_TE_1 n is the first order approximate solution of the effective refractive index under transverse electric mode light, n swg_TM_1 n is the first order approximate solution of the effective refractive index under transverse magnetic mode light, f is the duty cycle of the unit sub-wavelength structure, n si n is the refractive index of silicon under the corresponding central wavelength, n hole n is the refractive index of the upper cladding layer 1, p is the period length of the unit sub-wavelength structure, λ is the central wavelength, n swg_TE n is the effective refractive index of the transverse electric mode light flat waveguide, n eff_TM n is the effective refractive index of the transverse magnetic mode light flat waveguide.

[0050] Exemplarily, when the central wavelength is 1310 nm, for the TE mode, the effective refractive index n swg_TE is taken uniformly at intervals of 0.05 within the interval range of [1.45, 3.5]; when n swg_TE is taken as 2.40, the first duty cycle fx is taken uniformly at intervals of 0.05 within the interval range of [0, 1]; when n swg_TE is taken as 2.40, the first period length p x of the x direction is calculated as 444.07 nm by the Bragg diffraction formula and the effective medium theory formula when the first duty cycle fx is taken as 0.5. Similarly, for the TM mode, the effective refractive index n swg_TM is taken uniformly at intervals of 0.05 within the interval range of [1.45, 3.5]; when n swg_TM is taken as 2.40, the first duty cycle fx is taken uniformly at intervals of 0.05 within the interval range of [0, 1]; when n swg_TM is taken as 2.40, the first period length p x of the x direction is calculated as 460.06 nm by the Bragg diffraction formula and the effective medium theory formula when the first duty cycle fx is taken as 0.5.

[0051] (2) In FDTD, the values of the effective refractive index n swg_TE and the first duty cycle fx are simulated to obtain the simulation values of the leakage factor of the corresponding first duty cycle fx and the first period length p x under n groups of different effective refractive index n swg_TE of the TE mode. The fitting function is expressed as g(x) = ax 5 + bx 4 + cx 3 + dx 2+ ex + f, where a, b, c, d, e, f are constants calculated by fitting, x represents fx, and g(x) represents the simulation value of the leakage factor in the fitting of the first duty cycle fx-the simulation value of the leakage factor; x represents px, and g(x) represents the simulation value of the leakage factor in the fitting of the first period length px-the simulation value of the leakage factor. The fitting is performed on the x-direction coordinate position and the simulation value of the leakage factor to obtain n sets of fitting functions of the leakage factor with respect to the first duty cycle fxunder different TE mode equivalent refractive indexes n swg_TE The fitting function is expressed as:

[0052] Power(x) = P0 x e -2εx .

[0053] where ε is the simulation value of the leakage factor, P0 is the optical field power of the input light source, x is the length of the grating in the x-direction, and Power(x) represents the optical field power.

[0054] Exemplarily, for the TE mode, the unit subwavelength structure queue equivalent refractive index n swg_TE is taken as 2.40, the first duty cycle fxis taken as 0.5, and the first period length pxis taken as 444.07 nm, the simulation value of the leakage factor is calculated as 0.455. When fxis taken as a sufficient number of values, the fitting functions of the first duty cycle fx-the simulation value of the leakage factor and the first period length px-the simulation value of the leakage factor can be obtained. swg_TE is taken as 2.40, the first duty cycle fxis taken as 0.5, and the first period length pxis taken as 444.07 nm, the simulation value of the leakage factor is calculated as 0.455. When fxis taken as a sufficient number of values, the fitting functions of the first duty cycle fx-the simulation value of the leakage factor and the first period length px-the simulation value of the leakage factor can be obtained. swg_TE is taken as 2.40, the first duty cycle fxis taken as 0.5, and the first period length pxis taken as 444.07 nm, the simulation value of the leakage factor is calculated as 0.455. When fxis taken as a sufficient number of values, the fitting functions of the first duty cycle fx-the simulation value of the leakage factor and the first period length px-the simulation value of the leakage factor can be obtained. Figure 5 (c), where the dashed line represents the curve corresponding to the simulation result, and the solid line represents the curve corresponding to the fitting function. For the fitting function of the leakage factor with respect to the first duty cycle fx, the maximum value point in the simulation obtained power and the x-direction coordinate position corresponding thereto are taken, and the fitting is performed according to the fitting function Power(x) = P0 x e -2εx , which can obtain the fitting function of the first duty cycle fx-the simulation value of the leakage factor.

[0055] (3) The theoretical value function of the leakage factor with respect to the x-direction coordinate position as shown in Figure 5 (a) is corresponded to the fitting function of the leakage factor with respect to the first duty cycle fxunder n sets of different TE mode equivalent refractive indexes n swg_TE , which can obtain the TE mode equivalent refractive index n swg_TE and the first duty cycle fxof each period; for an independent unit subwavelength structure queue, there are multiple TE mode equivalent refractive indexes nswg_TE and the x-direction duty cycle fx satisfies a theoretical value function of the leakage factor, while considering the error of the fitting function of the simulation value of the leakage factor and the first period length px calculated by the equivalent medium theory formula as an approximation value, the TE mode equivalent refractive index n swg_TE and the first duty cycle fx, the equivalent refractive index corresponding to the theoretical value function is denoted as n swg_TE_k_theory , where k represents the kth period.

[0056] In this embodiment, the theoretical calculation value of the leakage factor can be obtained by the following formula:

[0057]

[0058]

[0059] where G(x) represents a 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 optical fiber, and 2a(x) represents the theoretical calculation value of the leakage factor.

[0060] Exemplarily, when calculating the TE mode equivalent refractive index n swg_TE_1_theory and the first duty cycle fx1, the fitting function of the first duty cycle px of the leakage factor obtained in step (3) is equal to or close to the theoretical value function about the x-direction coordinate position, and x is solved, that is, the first period length px1 of the corresponding first period is 0.53 μm; when calculating the TE mode equivalent refractive index n swg_TE_5_theory and the first duty cycle fx5, first, the sum of the lengths of the first four periods is calculated, which is brought into the theoretical value function about the x-direction coordinate position, and the corresponding theoretical value of the leakage factor is solved, so that the fitting function of the first duty cycle px of the leakage factor obtained in step (3) is equal to or close to the theoretical value function, and x is solved, that is, the first period length px5 of the corresponding fifth period is 0.96 μm.

[0061] (4) The TM mode equivalent refractive index n swg_TM of the unit subwavelength structure queue (arranged along the x-axis direction) of the transverse magnetic mode can be calculated from the first period length px of the x direction and the value range of the first duty cycle fx, denoted as [n swg_TM_1 , n swg_TM_2 ], and points are taken at uniform intervals in this interval range, denoted as j1, and the number of points is m1; since [n swg_TM_k1 , n swg_TM_k2 ] is known, the value range of the second duty cycle fy of the unit subwavelength structure queue y direction for the transverse electric mode is solved, denoted as [fy _k1 , fy _k2When the TM mode equivalent refractive index n swg_TM When a certain value is taken, the second duty cycle fy in the y direction is within [fy _k1 fy _k2 Points are taken at uniform intervals within the interval, denoted as j2, and the number of points is m2. In this embodiment, j2 = 0.05 is used for point selection; when the TM mode equivalent refractive index n swg_TM When both the second duty cycle fy and the second duty cycle fy take a certain value, the second period length py in the y direction can be calculated using the Bragg diffraction formula and the equivalent medium theory formula.

[0062] For example, the equivalent refractive index n of the unit subwavelength structure queue in the TM mode is calculated within ±50 nm of the first period length px1 = 0.53 μm of the first period. swg_TM The resulting value range is [2.85, 3.05], with points evenly spaced at intervals of 0.02; when n swg_TM When n is 2.95, points are uniformly selected at intervals of 0.02 within the range of [0.6, 0.8] for the second duty cycle fy; when n swg_TM When the second duty cycle fy is 0.7 and the value is 2.95, the second period length py in the y direction can be calculated as 1200.56 nm using the Bragg diffraction formula and the equivalent medium theory formula.

[0063] (5) For all TM modes, the equivalent refractive index n swg_TM Simulations were performed using the values ​​of the second duty cycle fy to obtain m sets of equivalent refractive indices n for different TM modes. swg_TM The simulated values ​​of the leakage factor corresponding to the second duty cycle fy and the second period length py are used to fit the simulated values ​​of the leakage factor in the y-direction coordinate position, and m sets of equivalent refractive indices n for different TM modes are obtained. swg_TM The leakage factor under the given conditions is fitted to the second duty cycle fy in the y-direction. The fitted function is expressed as g(x) = ax 5 +bx 4 +cx 3 +dx 2 +ex+f, where a, b, c, d, e, and f are constants calculated through fitting. In the fitting of the simulated value of the second duty cycle fy - leakage factor, x represents fy, and g(x) represents the simulated value of the leakage factor, such as Figure 5 As shown in (b), in the fitting of the simulated values ​​of the leakage factor for the second period length py, x represents py, and g(x) represents the simulated value of the leakage factor. It should be understood that the constants a, b, c, d, e, and f of each fitting function are different.

[0064] Exemplarily, when the value of fy in (4) is enough, the fitting function of the simulation value of the corresponding second duty cycle fy-leakage factor and the simulation value of the first period length px-leakage factor can be obtained. For the fitting function of the leakage factor with respect to the second duty cycle fy, the maximum point in the simulation obtained power and the y direction coordinate position corresponding thereto are taken, and the fitting function Power(x) = P0xey is obtained according to the fitting function Power(x) = P0xey -2εx The fitting is performed, and the fitting function of the simulation value of the corresponding second duty cycle fy-leakage factor is obtained.

[0065] (6) According to the theoretical value function of the leakage factor with respect to the y direction coordinate position, the theoretical value function is matched with the leakage factor of m groups of different TM mode equivalent refractive index n swg_TM under different TM mode equivalent refractive index n swg_TE_k with respect to the second duty cycle fy, and the value range of the second period length py is obtained by corresponding period by period, and the value range of the equivalent refractive index n swg_TE_k_theory of the unit subwavelength structure queue of the transverse electric mode is known, and the point closest to n swg_TE_k_result is selected as the final result n _k_result ; similarly, the final result fy _k_result of the second duty cycle in the y direction, the final result py swg_TM_k_result of the second period length in the y direction, the final result n _k_result of the equivalent refractive index of the unit subwavelength structure queue of the transverse magnetic mode, the final result fx _k_result of the first duty cycle in the x direction, and the final result px n of the first period length in the x direction are obtained in turn.

[0066] (7) The optimization is performed using the inverse design method based on the adjoint method. It should be understood that the optimization of the unit subwavelength structure refers to the optimization of the pixelated discrete image of the topology of the subwavelength unit structure.

[0067] In this embodiment, the inverse design method optimizes the topology, which is specifically: in the conventional design (i.e., forward design), the input x n (n is a natural number) is known, and there is a constraint F n between the output y n and the input x n , the design area parameter p n is given in the design, and the output y n = F n (x n , p ) is obtained. When the output result is optimized, the optimal solution according to the initial parameter is obtained by changing the design area parameter p n multiple times (such as particle swarm optimization algorithm).

[0068] In reverse design, the design model is represented by a set of equations F n (x n ,p n )=y n , given the input x n and the desired output y n , F n (x n ,p n )=y n is solved to obtain the design region parameter p n . When optimizing the output result, by comparing the difference between the output y n and the desired output y n , the design region parameter p n is changed to make the output result close to the output y n , and the optimal solution of the design region parameter p n is obtained.

[0069] Where, for a sub-wavelength structure grating, the design region parameter p n is a pixelated discrete image drawn after selecting the first period length px and the first duty cycle fx, the second period length py and the second duty cycle fy of each unit sub-wavelength structure; for a polarization beam combiner / separater, the design region parameter p n is a pixelated discrete image of a 2x2μm square. In addition, part of the grating structure schematic diagram is shown in Figure 3 .

[0070] In this embodiment, the calculation of the simulation value of the leakage factor ε is by simulating the equivalent refractive index n swg and the duty cycle f of different unit sub-wavelength structure queues, obtaining the distribution curve of the optical power of the grating upward diffraction with respect to the position, and the relationship between the optical power and the position is a logarithmic function according to the theoretical value function of the leakage factor. The extreme value of the simulated optical power and its corresponding position are fitted by a logarithmic function to obtain the simulation value of the leakage factor corresponding to the equivalent refractive index n swg and the duty cycle f.

[0071] In this embodiment, the materials of the first sub-wavelength grating structure 100, the second sub-wavelength grating structure 200, the first output port 101, the second output port 102, the first input port 201, the second input port 202, the polarization beam combiner 103, the polarization beam separator 203, the first single-mode transition waveguide 104, and the second single-mode transition waveguide 204 include one or more of silicon, silicon dioxide, silicon nitride, and air.

[0072] In this embodiment, refer to Figure 2The first output port 101 includes a first tapered waveguide 1011 and a first single-mode waveguide 1012 with thermal insulation structures. The second output port 102 includes a second tapered waveguide 1021 and a second single-mode waveguide 1022 with thermal insulation structures. The first input port 201 includes a third tapered waveguide 2011 and a third single-mode waveguide 2012 with thermal insulation structures. The second input port 202 includes a fourth tapered waveguide 2021 and a fourth single-mode waveguide 2022 with thermal insulation structures. The first single-mode waveguide 1012, the second single-mode waveguide 1022, the third single-mode waveguide 2012, and the fourth single-mode waveguide 2022 transition single-mode light from the grating to the waveguide or from the waveguide to the grating.

[0073] Among them, the first conical waveguide 1011, the second conical waveguide 1021, the third conical waveguide 2011 and the fourth conical waveguide 2021 have a structure that is linear, exponential or parabolic.

[0074] Furthermore, the first single-mode transition waveguide 104 uses a smooth arc to connect the first single-mode waveguide 1012 and the transverse magnetic mode light input port of the polarization combiner 103 to the strip waveguide, ensuring that single-mode light is transmitted in the waveguide; the second single-mode transition waveguide 204 uses a smooth arc to connect the third single-mode waveguide 2012 and the transverse magnetic mode light output port of the polarization separator 203 to the strip waveguide, ensuring that single-mode light is transmitted in the waveguide. It should be understood that the first single-mode transition waveguide 104 connects the first output port 101 and the transverse magnetic mode light input port of the polarization combiner 103, and the second single-mode transition waveguide 204 connects the first input port 201 and the transverse magnetic mode light output port of the polarization separator 203, thus ensuring that single-mode light is transmitted in the waveguide. The radius of the smooth arc is 30 μm.

[0075] In this embodiment, as Figure 4 As shown, the transverse electric mode light and transverse magnetic mode light mixing input ports of polarization combiner 103 and polarization separator 203 are located on one side of the device and below it, while the transverse electric mode light input port and the transverse magnetic mode light input port are located on the other side of the device, with the transverse electric mode light input port below the device and the transverse magnetic mode light input port above the device. Similarly, the transverse electric mode light and transverse magnetic mode light mixing output ports of polarization combiner 103 and polarization separator 203 are located on one side of the device and below it, while the transverse electric mode light output ports and the transverse magnetic mode light output ports are located on the other side of the device, with the transverse electric mode light output port below the device and the transverse magnetic mode light output port above the device. Furthermore, the transverse electric mode light input port and the transverse magnetic mode light mixing output port are located on the same side of the device, and both the transverse electric mode light input port and the transverse magnetic mode light output port are located on the same side of the device.

[0076] The polarization beam combiner 103 combines the transverse electric mode light and the transverse magnetic mode light coupled into the waveguide, and the polarization beam splitter 203 separates the transverse electric mode light and the transverse magnetic mode light coupled into the optical fiber. The device size is only 2x2 μm by using the inverse method based on the adjoint method design. 2 The structure of the grating coupler is more compact.

[0077] Further, for the transverse electric mode light and the transverse magnetic mode light with a center wavelength of 1310 nm, the widths of the first single-mode waveguide 1012, the second single-mode waveguide 1022, the third single-mode waveguide 2012, the fourth single-mode waveguide 2022, the first single-mode transition waveguide 104, and the second single-mode transition waveguide 204 are all 500 nm.

[0078] In the embodiment, the grating coupler is prepared by electron beam exposure and inductively coupled plasma etching on an SOI wafer, and then a 2-μm-thick silicon dioxide film is deposited on the chip by plasma-enhanced chemical vapor deposition.

[0079] Specifically, the SOI wafer shown in Figure 6 (a) is placed in a spin coater to spin a photoresist layer, and then baked. The photoresist on the surface of the SOI wafer is exposed by an electron beam exposure process to form a hard mask pattern of the first subwavelength grating structure 100, the first output port 101, the second output port 102, the polarization beam combiner 103, the polarization beam splitter 203, the second input port 202, the first input port 201, and the second subwavelength grating structure 200. Inductively coupled plasma etching is used to form the first subwavelength grating structure 100, the first output port 101, the second output port 102, the polarization beam combiner 103, the polarization beam splitter 203, the second input port 202, the first input port 201, and the second subwavelength grating structure 200, and the photoresist is cleaned. The structure is schematically shown in Figure 6 (b). Only one step of etching is needed on the SOI wafer. Finally, a 2-μm-thick silicon dioxide film is deposited on the chip by plasma-enhanced chemical vapor deposition, as shown in Figure 6 (c). That is, 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 by a silicon dioxide film.

[0080] In some other embodiments, an optical device is also provided, which includes an optical link and a test link, wherein the test link includes the aforementioned grating coupler; the optical link includes a laser, a polarization controller for adjusting the polarization state of the transverse electric mode light or the transverse magnetic mode light in the optical fiber, a first single-mode optical fiber 105 for transmitting the transverse electric mode light or the transverse magnetic mode light, a second single-mode optical fiber 205 for transmitting the transverse electric mode light or the transverse magnetic mode light, and a power meter; the test link is optically connected to the optical link.

[0081] In the present embodiment, the laser, the polarization controller, the first single-mode optical fiber 105, the test link, the second single-mode optical fiber 205, and the power meter are connected in sequence, and the first single-mode optical fiber 105 and the second single-mode optical fiber 205 are arranged above the upper cladding layer 1 and perpendicular to the upper cladding layer 1. The single-mode light is input into the Bragg diffraction input waveguide or the optical fiber, thereby completing the fiber-to-waveguide coupling or the waveguide-to-fiber coupling.

[0082] In the test link, the transverse electric mode light and the transverse magnetic mode light are input into the left first subwavelength grating structure 100 from the left first single-mode optical fiber 105. The transverse magnetic mode light is input into the first tapered waveguide 1011 from the first output port 101, passes through the first single-mode waveguide 1012 and the first single-mode transition waveguide 104, and enters the polarization beam combiner 103. The transverse electric mode light is input into the second tapered waveguide 1021 from the second output port 102, passes through the second single-mode waveguide 1022, and enters the polarization beam combiner 103, thereby completing the polarization beam combination. At this time, the light transmitted in the waveguide is the transverse electric mode light and the transverse magnetic mode light. Then, the light is input into the polarization beam splitter 203 from the polarization beam combiner 103. The transverse magnetic mode light is input into the second single-mode transition waveguide 204 from the transverse magnetic mode light output port located above, passes through the fourth tapered waveguide 2012 and the third tapered waveguide 2011, and enters the first input port 201 of the second subwavelength grating structure 200. At the same time, the transverse electric mode light is input into the second input port 202 from the transverse electric mode light output port located below, and enters the second subwavelength grating structure 200. The transverse electric mode light and the transverse magnetic mode light are output from the second single-mode optical fiber 205 through the second subwavelength grating structure 200.

[0083] In the test process, the laser is turned on, the position of the optical fiber is adjusted, the polarization state of the transverse electric mode light or the transverse magnetic mode light is adjusted using the polarization controller, the positions of the first single-mode optical fiber 105 and the second single-mode optical fiber 205 are further adjusted, and the power meter shows the maximum reading. The transverse electric mode light or the transverse magnetic mode light is transmitted in the test link.

[0084] The above examples are only used to illustrate the technical solutions of the present application, and are not intended to limit the present application; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that the technical solutions recorded in the foregoing examples can be modified, or some technical features can be replaced by equivalent features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A fully etched polarization independent subwavelength grating coupler that simultaneously allows coupling of light in a fiber in either transverse electric or transverse magnetic mode into a waveguide, characterized in that, The grating coupler comprises, from top to bottom, an upper cladding layer (1), a top silicon structure (2), a lower cladding layer (3), and a silicon substrate (4), wherein the top silicon structure (2) comprises a first subwavelength grating structure (100) and a second subwavelength grating structure (200) optimized by using a free parameter and a reverse design method, a first output port (101) and a second output port (102) perpendicular to each other, a first input port (201) and a second input port (202) perpendicular to each other, a first single-mode transition waveguide (104), a second single-mode transition waveguide (204), a polarization beam combiner (103), and a polarization beam splitter (203). The first subwavelength grating structure (100) and the second subwavelength grating structure (200) are designed by the following method: the transverse electric mode light propagates along the x-axis, and the light is output from the second output port (102) or input from the second input port (202); the transverse magnetic mode light propagates along the y-axis, and the light is output from the first output port (101) or input from the first input port (201); the duty cycles of each unit subwavelength structure in the x-direction and the y-direction are different; the period lengths in the x-direction and the y-direction are related to the equivalent refractive index and the duty cycle of the unit subwavelength structure in the corresponding direction transmission mode; the unit subwavelength structures are arranged in sequence in the x-direction and the y-direction, and are referred to as a unit subwavelength structure queue; the unit subwavelength structure parameters need to be determined by multiple iterations; the unit subwavelength structure parameters are adjusted to realize polarization independence of the grating; and the parameters are optimized by using a reverse design method.

2. The all-etching polarization-independent subwavelength grating coupler according to claim 1, characterized in that, The polarization beam combiner (103) and the polarization beam splitter (203) are designed by using a reverse method based on the adjoint method, the polarization beam combiner (103) is used for combining the transverse electric mode light and the transverse magnetic mode light coupled into the waveguide, and the polarization beam splitter (203) is used for separating the transverse electric mode light and the transverse magnetic mode light coupled into the optical fiber.

3. The all-etching polarization-independent subwavelength grating coupler according to claim 1, characterized in that, The first output port (101) comprises a first tapered waveguide (1011) and a first single-mode waveguide (1012) adopting an adiabatic structure; the second output port (102) comprises a second tapered waveguide (1021) and a second single-mode waveguide (1022) adopting an adiabatic structure; the first input port (201) comprises a third tapered waveguide (2011) and a third single-mode waveguide (2012) adopting an adiabatic structure; the second input port (202) comprises a fourth tapered waveguide (2021) and a fourth single-mode waveguide (2022) adopting an adiabatic structure; and the first single-mode waveguide (1012), the second single-mode waveguide (1022), the third single-mode waveguide (2012), and the fourth single-mode waveguide (2022) are used for transitioning single-mode light from the grating to the waveguide or from the waveguide to the grating.

4. The all-etching polarization-independent subwavelength grating coupler according to claim 3, characterized in that, The first tapered waveguide (1011), the second tapered waveguide (1021), the third tapered waveguide (2011), and the fourth tapered waveguide (2021) have a linear, exponential, or parabolic structure.

5. The all-etching polarization-independent subwavelength grating coupler according to claim 3, characterized in that, The first single-mode transition waveguide (104) connects the first single-mode waveguide (1012) and the transverse magnetic mode light input port of the polarization beam combiner (103) with a smooth circular arc to ensure that single-mode light is transmitted in the waveguide; the second single-mode transition waveguide (204) connects the third single-mode waveguide (2012) and the transverse magnetic mode light output port of the polarization beam splitter (203) with a smooth circular arc to ensure that single-mode light is transmitted in the waveguide.

6. The all-etching polarization-independent subwavelength grating coupler according to claim 5, characterized in that, The radius of the smooth circular arc is 30 μm.

7. The all-etching polarization-independent subwavelength grating coupler according to claim 1, wherein, The transverse electric mode light and the transverse magnetic mode light mixed input / output ports of the polarization beam combiner (103) and the polarization beam splitter (203) are located on one side and below the device, the transverse electric mode light input / output port and the transverse magnetic mode light input / output port are located on the other side of the device, and the transverse electric mode light input / output port is located below the device and the transverse magnetic mode light input / output port is located above the device.

8. The all-etching polarization-independent subwavelength grating coupler according to claim 1, characterized in that, The materials of the first sub-wavelength grating structure (100), the second sub-wavelength grating structure (200), the first output port (101), the second output port (102), the first input port (201), the second input port (202), the polarization beam combiner (103), the polarization beam splitter (203), the first single-mode transition waveguide (104), and the second single-mode transition waveguide (204) include one or more of silicon, silicon dioxide, silicon nitride, and air.

9. The all-etching polarization-independent subwavelength grating coupler according to claim 1, characterized in that, The parameters of the unit sub-wavelength structure include a first duty cycle in the x direction, a second duty cycle in the y direction, a first period length in the x direction, and a second period length in the y direction.

10. The all-etching polarization-independent subwavelength grating coupler according to claim 9, characterized in that, The parameters of the unit sub-wavelength structure queue include the parameters of the unit sub-wavelength structure constituting the queue, the first period number in the x direction, and the second period number in the y direction; wherein the first period number and the second period number are determined by the mode spot radius of the optical fiber and the mode spot radius of the grating, and the expression is: first period number = two times the mode spot radius of the optical fiber / first period length, second period number = two times the mode spot radius of the optical fiber / second period length.

Images