An asymmetric directional coupler based on non-equal-height straight waveguide and a design method thereof

Abstract

The application relates to an asymmetric directional coupler based on non-equal-height straight waveguides and a design method thereof, the coupling part of the coupler comprising a first straight waveguide and a second straight waveguide arranged in parallel on a substrate; the first straight waveguide supports a single mode in the vertical direction and supports multiple modes in the horizontal direction; the second straight waveguide supports a single mode in the vertical and horizontal directions; the thickness of the first straight waveguide is greater than that of the second straight waveguide; the effective refractive index of a high-order mode TE n supported by the first straight waveguide is equal to that of a basic mode TE0 supported by the second straight waveguide, and the derivative of the effective refractive index of the high-order mode TE n with respect to the width thereof is equal to that of the basic mode TE0 with respect to the width thereof; by synchronously changing the offset of the two different mode effective refractive indexes with the waveguide line width, the problem of low coupling efficiency of the asymmetric directional coupler caused by the error of the waveguide width in the exposure process is solved.

Description

Technical Field

[0001] This invention relates to the field of optical communication, and in particular to an asymmetric directional coupler based on a non-uniform-height straight waveguide and its design method. Background Technology

[0002] In recent years, with the rise of big data technology, people have placed higher demands on data transmission bandwidth. To achieve high bandwidth in optical communication devices, mode division multiplexing (MDM) systems have been proposed. Mode multiplexers and mode demultiplexers play extremely important roles in MDM systems.

[0003] Asymmetric directional couplers, capable of multiplexing multiple modes, hold significant promise for extended mode multiplexing applications. However, they require extremely stringent phase-matching conditions and impose stringent requirements on the fabrication process. In the fabrication process, the exposure process determines the waveguide width variation, and changes in linewidth have a substantial impact on the effective refractive index of the fundamental mode and higher-order modes. In traditional asymmetric directional couplers, the high dependence of the fundamental mode TE0 in a single-mode waveguide on the waveguide width means that even an error of tens of nanometers in the waveguide width can have an immeasurable impact on the device's mode conversion efficiency. Therefore, asymmetric directional couplers are severely limited in practical applications.

[0004] For the current improved asymmetric directional coupler design, the waveplate-etched directional coupler can maintain good performance even with high errors (±40nm). However, the analysis of device process errors only studies the performance under individual errors of each device parameter. It does not provide in-depth analysis of the situation where multiple device parameter errors occur simultaneously, thus making it difficult to demonstrate good performance in practical applications.

[0005] Similarly, people have conducted in-depth research and analysis on the impact of process errors on waveplate-type asymmetric directional couplers. Currently, the research on TE0-TE1 only supports an error range of ±20nm, which is still not conducive to practical production applications. Summary of the Invention

[0006] The primary objective of this invention is to provide an asymmetric directional coupler and its design method that significantly reduces sensitivity to process errors and greatly improves mode conversion efficiency. In this asymmetric directional coupler, the first waveguide supports the higher-order mode TE. n The effective refractive index is equal to the effective refractive index of the fundamental mode TE0 supported by the second straight waveguide, while the higher-order mode TE... nThe derivative of the effective refractive index with respect to its corresponding cross-sectional width is equal to the derivative of the effective refractive index of the fundamental mode TE0 with respect to its corresponding cross-sectional width. By selecting two different modes to synchronously change the effective refractive index with the offset of the waveguide width, the low coupling efficiency problem of asymmetric directional couplers caused by waveguide width errors during the exposure process is significantly solved. Based on this objective, the present invention provides at least the following technical solutions.

[0007] This invention provides an asymmetric directional coupler based on a non-uniform-height straight waveguide, comprising a substrate, a waveguide core layer on the substrate, and a cover layer enclosing the waveguide core layer. The refractive index of the waveguide core layer is higher than that of the substrate and the cover layer. The waveguide core layer includes an input terminal, a coupling portion, and an output terminal connected in sequence. The coupling portion includes a first straight waveguide and a second straight waveguide. The first and second straight waveguides are arranged parallel to each other on the substrate at a certain spacing. The straight waveguides have thickness and width in their cross-sectional direction.

[0008] The first straight waveguide is a multimode waveguide. The first straight waveguide supports a single mode in the vertical direction and multiple modes in the horizontal direction. The thickness of the first straight waveguide is h1.

[0009] The second straight waveguide is a single-mode waveguide. The second straight waveguide supports a single mode in the vertical direction and a single mode in the horizontal direction. The thickness of the second straight waveguide is h2, where h1 > h2.

[0010] The first straight waveguide supports higher-order TE modes n The effective refractive index is equal to the effective refractive index of the fundamental mode TE0 supported by the second straight waveguide, and the higher-order mode TE0 supported by the first straight waveguide is... n The derivative of the effective refractive index of the second straight waveguide with respect to its width is equal to the derivative of the effective refractive index of the fundamental mode TE0 supported by the second straight waveguide with respect to its width, and n≥1.

[0011] Furthermore, the first straight waveguide and the second straight waveguide are made of the same material.

[0012] Furthermore, the first straight waveguide is selected from at least one of Si, Si3N4, GaAs, and InP; the second straight waveguide is selected from at least one of Si, Si3N4, GaAs, and InP; and the capping layer is selected from at least one of air, SiO2, and polymer.

[0013] Furthermore, the thickness h2 of the second straight waveguide is 0.5 to 0.85 times the thickness h1 of the first straight waveguide.

[0014] Furthermore, the input terminal has a cross-section perpendicular to the optical transmission direction with a thickness and a width, the thickness being equal to the thickness of the corresponding straight waveguide and the width being equal to the width of the corresponding straight waveguide.

[0015] Furthermore, the output terminal has a cross-section perpendicular to the optical transmission direction with a thickness and a width, the thickness being equal to the thickness of the corresponding straight waveguide and the width being equal to the width of the corresponding straight waveguide.

[0016] Another aspect of the present invention provides a design method for an asymmetric directional coupler based on a non-uniform height straight waveguide, comprising the following steps:

[0017] S1. Determine the fixed cross-sectional thickness h1 of the first straight waveguide along the beam propagation direction, and calculate the higher-order modes TE of the first straight waveguide under different cross-sectional widths along the beam propagation direction. n The effective refractive index is n≥1, and the first direct waveguide is a multimode waveguide;

[0018] S2. Calculate the higher-order modes TE in the first direct waveguide. n The derivative of the effective refractive index with respect to the width of the corresponding cross section;

[0019] S3. Select a fixed cross-sectional thickness h2 along the beam propagation direction of the second straight waveguide, and calculate the effective refractive index of the fundamental mode TE0 under different cross-sectional widths along the beam propagation direction of the second straight waveguide. The second straight waveguide is a single-mode waveguide, where h1 > h2.

[0020] S4. Calculate the derivative of the effective refractive index of the fundamental mode TE0 in the second straight waveguide with respect to the width of the corresponding cross section;

[0021] S5. Select the higher-order mode TE of the first straight waveguide at a certain cross-sectional width. n The effective refractive index is equal to the effective refractive index of the fundamental mode TE0 of the second straight waveguide at a certain cross-sectional width. At the same time, the derivatives of the effective refractive indices of the first and second straight waveguides with respect to the corresponding cross-sectional widths are equal. In this case, the corresponding cross-sectional width is the width of the straight waveguide.

[0022] Furthermore, the first straight waveguide and the second straight waveguide are made of the same material.

[0023] Furthermore, the first straight waveguide is selected from at least one of Si, Si3N4, GaAs, and InP; the second straight waveguide is selected from at least one of Si, Si3N4, GaAs, and InP; and the capping layer is selected from at least one of air, SiO2, and polymer.

[0024] Furthermore, the thickness h2 of the second straight waveguide is 0.5 to 0.85 times the thickness h1 of the first straight waveguide.

[0025] The present invention has at least the following beneficial effects:

[0026] At a center wavelength of 1550nm, with an exposure error within ±50nm, the asymmetric directional coupler provided by this invention achieves a coupling efficiency of over 90% between the fundamental mode TE0 and the higher-order mode TE1, significantly reducing the sensitivity to process errors and markedly improving mode conversion efficiency.

[0027] In addition, the present invention can achieve the thickness of the second straight waveguide by combining the secondary exposure required in the fabrication process of the grating coupler, thereby controlling the thickness of the second straight waveguide, reducing unnecessary process steps, and making the fabrication process simple and feasible. Attached Figure Description

[0028] Figure 1 This is a three-dimensional structural schematic diagram of the coupling portion in an asymmetric directional coupler according to an embodiment of the present invention.

[0029] Figure 2 This is a graph showing the relationship between the effective refractive index and width of the higher-order mode TE1 of the first straight waveguide in one embodiment of the present invention under a fixed thickness of 0.22 μm.

[0030] Figure 3 According to one embodiment of the present invention Figure 2 The resulting graph shows the relationship between the effective refractive index of the higher-order mode TE1 of the second straight waveguide and the derivative of the effective refractive index with respect to the width.

[0031] Figure 4 This is a graph showing the relationship between the effective refractive index and width of the fundamental mode TE0 of a single-mode waveguide in one embodiment of the present invention under a fixed thickness of 0.16 μm.

[0032] Figure 5 According to one embodiment of the present invention Figure 4 The resulting graph shows the relationship between the effective refractive index of the fundamental mode TE0 of the second straight waveguide and the derivative of the effective refractive index with respect to the width.

[0033] Figure 6 This is a graph showing the relationship between the effective refractive index and the derivative of the effective refractive index with respect to the width for the second straight waveguide TE0 mode and the first straight waveguide TE1 mode in one embodiment of the present invention.

[0034] Figure 7 This is a three-dimensional structural schematic diagram of an asymmetric directional coupler according to an embodiment of the present invention.

[0035] Figure 8 This is a coupling efficiency curve of the fundamental mode TE0 to the higher-order mode TE1 when the waveguide width exposure error is in the range of -50nm to +50nm in one embodiment of the present invention. Detailed Implementation

[0036] The technical solutions of the embodiments of the present invention will now be clearly and completely described with reference to the accompanying drawings. The described embodiments are merely some, not all, of the embodiments of the present invention. Other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are all within the scope of protection of the present invention.

[0037] This specification uses spatially relative terms such as “below,” “under,” “down,” “above,” “above,” and “upper” to explain the positioning of one component relative to a second component. These terms are intended to cover different orientations of the components, except those different from those shown in the figures.

[0038] Furthermore, the use of terms such as "first" and "second" to describe various elements, layers, regions, and segments is not intended to be limiting. The use of terms such as "having," "containing," "including," and "comprising" are open-ended terms, indicating the presence of the stated elements or features but not excluding additional elements or features, unless the context clearly indicates otherwise. In the embodiments of the present invention, the thickness and width of the waveguide refer to the thickness and width in a cross-section perpendicular to the optical transmission direction, such as... Figure 1 In the cross-section, the thickness of the straight waveguide is h1 and h2, and its width is w1 and w2. The horizontal direction refers to the direction along the x-axis, and the vertical direction refers to the direction along the y-axis.

[0039] The present invention will now be described in further detail. The present invention provides a non-directional coupler based on a non-uniform-height straight waveguide and its design method, such as... Figure 1 and Figure 7 As shown, the non-directional coupler includes a substrate, a waveguide core layer located on the substrate, and a capping layer surrounding the waveguide core layer. The refractive index of the waveguide core layer is higher than the refractive index of the substrate and the refractive index of the capping layer. Figure 7 As shown, the waveguide core layer includes an input end, a coupling part, and an output end. One end of the input end is connected to one end of the coupling part, and the other end of the coupling part is connected to the output end. The waveguide core layer has a thickness and a width along a cross section perpendicular to the optical transmission direction. The thickness of the input end, the coupling end, and the output end is consistent, and similarly, their widths are also consistent.

[0040] like Figure 1 As shown, the coupling portion includes a first straight waveguide A and a second straight waveguide B. The first straight waveguide A and the second straight waveguide B are disposed on the substrate at a certain distance apart, and a capping layer covers the first straight waveguide A and the second straight waveguide B. Figure 1 The first and second straight waveguides shown are selected from at least one of Si, Si3N4, GaAs, and InP, and the cladding layer is selected from at least one of air, SiO2, and polymers. The refractive index of the first and second straight waveguides is higher than that of the cladding layer.

[0041] The first straight waveguide A has a thickness h1 and a width w1. This waveguide supports a single mode in the vertical direction, and its thickness is preferably 0.22–0.34 μm. The lateral width of the first straight waveguide A depends not only on supporting two or more modes in the horizontal direction, but also on the thickness and width of the designed second straight waveguide B. The second straight waveguide B has a thickness h2 and a width w2. It supports a single mode in the vertical direction, and its thickness is preferably 0.12–0.25 μm. The specific value of the width of the second straight waveguide B should be related to the derivative of the effective refractive index of the higher-order modes supported by the first straight waveguide A with respect to the width.

[0042] This embodiment takes TE0→TE1 as an example. In order to achieve phase matching and thus perform mode conversion, the effective refractive index N1 of the higher-order mode TE1 in the first straight waveguide A and the effective refractive index N0 of the fundamental mode TE0 in the second straight waveguide B should satisfy the relationship (1);

[0043] N1(w1)=N0(w2) (1)

[0044] When the waveguide width error is Δw (Δw is small), the expressions for the effective refractive index N1 of the higher-order mode TE1 of the first straight waveguide A and the effective refractive index N0 of the fundamental mode TE0 of the second straight waveguide B are shown in (2) and (3).

[0045]

[0046]

[0047] If the effective refractive index of the higher-order mode TE1 of the first straight waveguide A is equal to the derivative of the effective refractive index of the fundamental mode TE0 of the second straight waveguide B with respect to the width, then the relationship (4) holds.

[0048] N1(w1+Δw)=N0(w2+Δw) (4)

[0049] Therefore, when the waveguide width error is Δw, the effective refractive indices of the two straight waveguides change synchronously and remain equal, thus still satisfying the good phase matching condition and achieving high coupling conversion efficiency, thereby improving the tolerance to process errors.

[0050] Figure 2 Let N = f(w1) represent the effective refractive index N of the higher-order mode TE1 as a function of the waveguide width w1 under the condition of a fixed thickness h1 = 0.22 μm for the first straight waveguide A. Figure 2The data allows us to calculate the derivative of the effective refractive index N with respect to the width w1 of the first direct waveguide high-order mode TE1, i.e., dN / dw1. This yields the curve distribution of the effective refractive index derivative dN / dw1 with respect to the width w1, denoted as f'(w1). Combined with... Figure 2 The relationship between the effective refractive index N and the width w1 can be transformed from the curve distribution of the derivative of the effective refractive index dN / dw1 (ordinate) with respect to the width w1 (broad axis) into a curve relationship between the derivative of the effective refractive index dN / dw1 (ordinate) and the effective refractive index N (broad axis), that is, f'(w1) is converted into f'(N), as follows. Figure 3 As shown. Figure 3 This is the relationship between the effective refractive index N of the higher-order mode TE1 and dN / dw1. Similarly, for a fixed thickness, the relationship between the effective refractive index of the fundamental mode TE0 in the second straight waveguide B and the width is calculated. Figure 4 This is a graph showing the relationship between the effective refractive index of the fundamental mode TE0 and the waveguide width for the second straight waveguide B under a fixed thickness h2 = 0.16 μm. Based on the same principle, through... Figure 4 The data can be used to determine the relationship between the effective refractive index N of the second straight waveguide fundamental mode TE0 and its effective refractive index derivative dN / dw2, such as... Figure 5 Combining Figure 3 and Figure 5 ,exist Figure 6 The paper simultaneously plots two curves showing the effective refractive index N versus dN / dw for the fundamental mode TE0 of the second straight waveguide and the higher-order mode TE1 of the first straight waveguide. The intersection point of these two curves is then identified, where both curves exhibit the same effective refractive index (x-axis) and the same effective refractive index derivative (y-axis). This means that, under the condition that the effective refractive indices of the higher-order mode TE1 and the fundamental mode TE0 are equal (in which case phase matching occurs), the higher-order mode TE1 supported by the first straight waveguide is also satisfied. n The derivative of the effective refractive index with respect to width is also equal to the derivative of the effective refractive index with respect to width of the fundamental mode TE0 supported by the second straight waveguide. The effective refractive index corresponding to this intersection point is the basis for determining the widths of the first straight waveguide A and the second straight waveguide B. Substituting this matched effective refractive index into... Figure 2 and Figure 4 The width w1 corresponding to the first straight waveguide A and the width w2 corresponding to the second straight waveguide B can be calculated respectively. This completes the selection of the widths of the first straight waveguide A and the second straight waveguide B. To verify the influence of process errors on this non-directional coupler based on non-uniform height straight waveguides, a design was also made using TE0→TE1 as an example. Figure 1The coupling section of the asymmetric directional coupler is shown. Regarding the selection of waveguide width error, a comparison chart is provided here showing results for +50nm, +40nm, +30nm, +20nm, +10nm, -10nm, -20nm, -30nm, -40nm, and -50nm errors, and results without process error. Figure 8 As shown, the coupling efficiency of this invention can still reach 90% at the optimal wavelength even with an error of ±50nm, while the coupling efficiency of conventional designs typically drops to 90% or even lower with an error of ±10nm. Furthermore, it should be noted that this design method is not only applicable to the TE0→TE1 mode conversion, but also to any TE0→TE... n The mode conversion also applies to the transformation from the fundamental mode TM0 to higher-order modes TM0. n The transformation of the model.

[0051] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. An asymmetric directional coupler based on a non-uniform-height straight waveguide, comprising a substrate, a waveguide core layer located on the substrate, and a capping layer surrounding the waveguide core layer, wherein the refractive index of the waveguide core layer is higher than the refractive index of the substrate and the refractive index of the capping layer, and the waveguide core layer includes an input terminal, a coupling portion, and an output terminal connected sequentially, characterized in that, The coupling portion includes a first straight waveguide and a second straight waveguide made of the same material. The first and second straight waveguides are selected from at least one of Si, Si3N4, GaAs, and InP. The capping layer is selected from at least one of air, SiO2, and a polymer. The first and second straight waveguides are arranged parallel to each other on the substrate at a certain spacing. The first and second straight waveguides have thickness and width in their cross-sectional directions. The input end has thickness and width in its cross-section perpendicular to the light transmission direction. The thickness is equal to the thickness of the corresponding straight waveguide, and the width is equal to the width of the corresponding straight waveguide. The output end has thickness and width in its cross-section perpendicular to the light transmission direction. The thickness is equal to the thickness of the corresponding straight waveguide, and the width is equal to the width of the corresponding straight waveguide. The first straight waveguide is a multimode waveguide. The first straight waveguide supports a single mode in the vertical direction and multiple modes in the horizontal direction. The thickness of the first straight waveguide is h1. The second straight waveguide is a single-mode waveguide that supports a single mode in the vertical direction and a single mode in the horizontal direction. The thickness of the second straight waveguide is h2, which is 0.5 to 0.85 times the thickness of the first straight waveguide h1. The first straight waveguide supports higher-order TE modes n The effective refractive index is equal to the effective refractive index of the fundamental mode TE0 supported by the second straight waveguide, and the higher-order mode TE0 supported by the first straight waveguide is... n The derivative of the effective refractive index of the second straight waveguide with respect to its width is equal to the derivative of the effective refractive index of the fundamental mode TE0 supported by the second straight waveguide with respect to its width, and n≥1.

2. A design method for an asymmetric directional coupler based on a non-uniform-height straight waveguide according to claim 1, wherein the asymmetric directional coupler includes a waveguide core layer, the waveguide core layer including an input terminal, a coupling section, and an output terminal connected in sequence, characterized in that, The coupling section includes a first straight waveguide and a second straight waveguide, both made of the same material and arranged parallel to each other at a certain distance. The first and second straight waveguides have thickness and width in their cross-sectional directions. The input end has thickness and width along its cross-section perpendicular to the light transmission direction; the thickness and width are equal to the thickness and width of the corresponding straight waveguide. The output end also has thickness and width along its cross-section perpendicular to the light transmission direction; the thickness and width are equal to the thickness and width of the corresponding straight waveguide. This design method includes the following steps: S1. Determine the fixed cross-sectional thickness h1 of the first straight waveguide along the beam propagation direction, and calculate the higher-order TE under different cross-sectional widths of the first straight waveguide along the beam propagation direction. n The effective refractive index is n≥1, and the first direct waveguide is a multimode waveguide; S2. Calculate the higher-order modes TE in the first direct waveguide. n The derivative of the effective refractive index with respect to the width of the corresponding cross section; S3. Select a fixed cross-sectional thickness h2 of the second straight waveguide along the beam propagation direction. The thickness h2 of the second straight waveguide is 0.5 to 0.85 times the thickness h1 of the first straight waveguide. Calculate the effective refractive index of the fundamental mode TE0 under different cross-sectional widths of the second straight waveguide along the beam propagation direction. The second straight waveguide is a single-mode waveguide. S4. Calculate the derivative of the effective refractive index of the fundamental mode TE0 in the second straight waveguide with respect to the width of the corresponding cross section; S5. Select the first straight waveguide and its higher-order TE at a certain cross-sectional width. n The effective refractive index is equal to the effective refractive index of the fundamental mode TE0 of the second straight waveguide at a certain cross-sectional width. At the same time, the derivatives of the effective refractive indices of the first and second straight waveguides with respect to the corresponding cross-sectional widths are equal. In this case, the corresponding cross-sectional width is the width of the straight waveguide.

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