Trapezoid Subwavelength Grating Waveguide Based Transitionless Mode Multiplexer / Demultiplexer

Abstract

The present invention discloses a mode multiplexer / demultiplexer based on a trapezoidal subwavelength grating waveguide. The main structure thereof is two trapezoidal subwavelength gratings with relatively wide bottoms facing each other. By utilizing the characteristic of the trapezoidal subwavelength grating for mode field regulation, a mode multiplexer / demultiplexer with a constant bus waveguide width is realized. At the same time, the function of extracting the fundamental mode signal in a multimode waveguide is also realized, greatly improving the flexibility of mode signal processing and avoiding the transition taper required by a conventional mode multiplexer / demultiplexer. It has good application prospects in the fields of optical mode switching, optical interconnection, optical communication, optical computing, etc.

Description

Technical Field

[0001] The present invention belongs to the field of optoelectronics technology, and relates to an on-chip optical signal processing device for different modes that has high application value in the fields of optical communication, optical computing, optical interconnection, etc. in the future, and specifically relates to a transition zone-free mode multiplexer / demultiplexer based on a trapezoidal subwavelength grating. Background Art

[0002] Semiconductor technology has brought great convenience to people's lives, promoted the modernization of management, production automation, modernization of scientific and technological means, and modernization of national defense technology, and also promoted the automation of intelligence information. The information highway marked by the global Internet is shortening the distance of human communication. As the basic technology in semiconductor technology, electronic chips have faced many problems since their development, including on-chip metal interconnection, single-tube power consumption and heat dissipation caused by the high integration of chips, making it impossible for Moore's Law to continue to develop along its trajectory.

[0003] As a common carrier, light has a series of advantages over electricity, such as large bandwidth, high speed, and low latency. At the same time, as an electromagnetic wave, light has multi-dimensional characteristics that electricity does not have, namely wavelength, mode, polarization, etc. This feature makes it possible for light to be used in multiple dimensions as an information carrier, and thus has advantages that electricity does not have in the fields of information transmission and computing. At the same time, the rapidly growing demand for data transmission and processing has made light, as an information carrier, greatly demonstrate its greater information processing capabilities than electricity.

[0004] As a traditional optical multiplexing technology, wavelength division multiplexing technology has achieved relatively mature applications, while mode, as another dimension of light, has the ability to further expand channel capacity.

[0005] A prominent disadvantage of wavelength division multiplexing / demultiplexing technology is that it requires multiple laser sources to generate light of different wavelengths for processing. Laser sources have always been a problem that needs to be overcome on silicon-based optoelectronic platforms. However, for modes, multiplexing technology can be used to perform a similar conversion on light of the same wavelength, so that light carrying different signals at one wavelength can be converted into different modes, which can be transmitted and processed in the same waveguide or optical fiber. This greatly reduces the demand for on-chip light sources and expands the available dimensions of light. If there are m wavelength channels and n mode channels, then for an optical device, its channel capacity can become m×n, which is a huge improvement.

[0006] For mode multiplexing / demultiplexing, common structures for implementing this function include: asymmetric directional coupler structure, multimode interference coupler structure, and Y-branch splitter structure. In 2014, in the article "WDM-compatible mode-division multiplexing on a silicon chip" published by Dai Daoxin et al. in Nature communication, the principle of mode multiplexing / demultiplexing based on an asymmetric directional coupler was elaborated. This is also the most common type among the current multiplexers / demultiplexers used. Compared with the other two structures, the asymmetric directional coupler structure has strong mode scalability and simple matching conditions (the refractive indices of the modes to be matched in the two waveguides are equal, i.e., Neff1 = Neff2). These advantages enable the vast majority of solutions in the field of multiplexing / demultiplexing to adopt the solution based on the asymmetric directional coupler. Based on the simplest asymmetric waveguide type directional coupler, different solutions have been proposed to optimize this multiplexer / demultiplexer. For example, in 2018, in the article "Silicon High-Order Mode (De)Multiplexer on Single Polarization" published by Su Yikai et al. in JOURNAL OF LIGHTWAVE TECHNOLOGY, a multiplexer / demultiplexer with 11 modes with high manufacturing tolerance was realized through an asymmetric directional coupler with subwavelength gratings. Recently, Wang Jian et al. also published relevant work in Optics Express using a double-tapered coupling region. However, the multiplexers / demultiplexers in the prior art are large in size and cannot extract low-order mode information from a multimode waveguide in a short length. Summary of the Invention

[0007] The object of the present invention is to provide a mode multiplexer / demultiplexer based on a trapezoidal subwavelength grating waveguide, which removes the transition region in the coupling region and realizes the mode multiplexing / demultiplexing of equal-width bus waveguides.

[0008] To achieve the above object, the technical solution adopted by the present invention is: a mode multiplexer / demultiplexer based on a trapezoidal subwavelength grating waveguide, which is a mode multiplexer / demultiplexer based on an asymmetric directional coupler, and includes a bus waveguide and a plurality of U-shaped coupling members with the same number as the number of optical wave modes in the optical signal. The plurality of coupling members are arranged side by side in sequence along the axis direction of the bus waveguide, and all the coupling members are located above the bus waveguide. The coupling member and the part of the bus waveguide directly below the coupling member form a mode coupling region;

[0009] The bus waveguide is a trapezoidal subwavelength grating waveguide, and the side with a larger duty cycle in the bus waveguide faces the coupling member, and more modes can be supported by changing the width of the first trapezoidal subwavelength grating;

[0010] The coupling component includes a first bent grating, a coupling waveguide, and a second bent grating that are connected in sequence. The first bent grating, the coupling waveguide, and the second bent grating form a U-shaped structure. The first bent grating is embedded with a first bent waveguide, the coupling waveguide is embedded with a bridging waveguide, and the second bent grating is embedded with a second bent waveguide; the first bent waveguide, the bridging waveguide, and the second bent waveguide are connected in sequence;

[0011] The widths of the first bent grating, the coupling waveguide, and the second bent grating are the same, and the duty cycles of the first bent grating, the coupling waveguide, and the second bent grating are the same; the widths of the first bent waveguide, the bridging waveguide, and the second bent waveguide are the same; the radii of the first bent grating, the first bent waveguide, the second bent grating, and the second bent waveguide are the same; the first bent grating and the first bent waveguide are concentric; the second bent grating and the second bent waveguide are concentric;

[0012] The coupling waveguide is a trapezoidal subwavelength grating waveguide. The coupling waveguides in different coupling components are all waveguides that support the transmission of the fundamental mode, and the coupling waveguides in different coupling components couple different modes; the side with a larger duty cycle in the coupling waveguide faces the bus waveguide; the width of the coupling waveguide is determined by the phase matching condition after coupling with the width of the bus waveguide.

[0013] The mode multiplexer / demultiplexer without a transition region of the present invention tunes the mode fields of different modes in the waveguide through a trapezoidal subwavelength grating. Taking the waveguide that supports the propagation of the fourth-order mode as an example, the multiplexing / demultiplexing of the fundamental mode - fundamental mode is realized (higher-order mode propagation can be supported by changing the width of the first grating, and this function can also be realized by the same principle). At the same time, by avoiding the transition region commonly used in the multiplexer / demultiplexer in the prior art, the mode multiplexing / demultiplexing based on waveguides with equal widths is realized. The multiplexer / demultiplexer of the present invention can extract low-order modes from a multimode waveguide on a relatively short scale, and has high application value in the future in the fields of optical communication, optical computing, electro-optic modulation, etc. Compared with the prior art, the present invention realizes the extraction of low-order mode information in a multimode waveguide through a trapezoidal subwavelength grating, and at the same time can realize mode multiplexing / demultiplexing without a transition region and a shorter coupling length, reducing the size of the mode multiplexer / demultiplexer. Moreover, it also has the advantages of lower crosstalk and higher coupling efficiency.

[0014] The mode multiplexer / demultiplexer without a transition region of the present invention has the following advantages:

[0015] 1. The excellent properties of light can be utilized to expand the mode dimension, thereby multiplicatively expanding the number of channels for optical information processing. Moreover, since the manufacturing process of the device can highly conform to CMOS technology, the device can be highly integrated on-chip, featuring small size and low power consumption, which is conducive to integration with electrical components for performing relevant mode information processing on-chip, such as on-chip optical computing, optical mode networks, optical mode switches, and so on.

[0016] 2. The coupling regions of different modes are independent of each other. Therefore, these coupling regions can work independently or in parallel, enabling the present invention to be used as a mode multiplexing device to convert the fundamental mode into different high-order modes in different coupling regions on-chip. As a demultiplexing device, it can work alone to extract any mode in the multimode waveguide, or work simultaneously to achieve the demultiplexing of multiple different modes.

[0017] 3. The trapezoidal subwavelength grating waveguide is used to enhance the evanescent field, thereby reducing the coupling length and shrinking the device size. Secondly, any mode in the multimode waveguide can be coupled / extracted, enabling the device to eliminate the tapered transition region and further reduce the device size.

[0018] 4. In the prior art, most mode multiplexing / demultiplexing devices based on asymmetric directional couplers require a tapered transition region to reduce the waveguide width, resulting in the waveguide with a reduced width not supporting the transmission of high-order modes. This causes the high-order mode signals to be unable to directly pass through the bus waveguide and instead need to be demultiplexed and then multiplexed back into the bus waveguide. In contrast, the multiplexing / demultiplexing device of the present invention does not require a transition region, greatly improving the flexibility of mode signal processing and having excellent application prospects in fields such as optical mode networks and optical add-drop multiplexers.

[0019] 5. The multiplexing / demultiplexing device of the present invention has good mode expandability characteristics. The number of modes supported by this mode multiplexing / demultiplexing device can be increased by increasing the width of the bus waveguide. Additionally, different modes can be coupled in the coupling region by changing the waveguide width of the coupling region through a phase matching scheme to achieve the expansion of the number of modes.

[0020] 6. In the prior art, it is relatively difficult to extract the fundamental mode signal in a multimode bus waveguide. However, the multiplexing / demultiplexing device of the present invention realizes the direct extraction of the fundamental mode signal in the multimode waveguide by adding a bridging waveguide and changing the mode dispersion curve, while the crosstalk of the remaining high-order modes is relatively small.

[0021] 7. The trapezoidal subwavelength grating waveguide is used to adjust the refractive index distribution, that is, the refractive index is larger on the side closer to the coupling region, thereby realizing the tuning of the internal mode field of the waveguide. This enables the fundamental mode to be more easily coupled out in a wide waveguide supporting high-order modes, thus expanding the flexibility of mode processing. Description of the Drawings

[0022] Figure 1 It is a schematic structural diagram of the mode multiplexer / demultiplexer without a transition region of the present invention.

[0023] Figure 2 It is a schematic diagram of the third-order mode coupling region of four mode multiplexers / demultiplexers in the mode multiplexer / demultiplexer without a transition region of the present invention.

[0024] Figure 3 It is Figure 2 A schematic diagram of the third-order mode coupling region shown as a multiplexer.

[0025] Figure 4 It is Figure 2 A schematic diagram of the third-order mode coupling region shown as a demultiplexer.

[0026] Figure 5 It is a mode dispersion diagram of the mode coupling principle realized by the mode multiplexer / demultiplexer without a transition region of the present invention.

[0027] Figure 6 It is a schematic structural diagram of the mode multiplexer / demultiplexer without a transition region of the present invention taking SOI material as an example.

[0028] Figure 7 It is an implementation manner of the present invention taking four-mode multiplexing / demultiplexing as an example.

[0029] In the figure: 1. First grating, 2. Third-order mode coupling region, 3. Second-order mode coupling region, 4. First-order mode coupling region, 5. Fundamental mode coupling region, 6. First curved waveguide, 7. Second grating, 8. Bridging waveguide, 9. Third grating, 10. Second curved waveguide, 11. Fourth grating. Specific implementation manner

[0030] The present invention will be described in detail below with reference to the accompanying drawings and specific implementation manners.

[0031] The present invention provides a mode multiplexer / demultiplexer without a transition region, which is a mode multiplexer / demultiplexer based on an asymmetric directional coupler, including a bus waveguide and a plurality of U-shaped coupling members with the same number as the number of optical wave modes in the optical signal. The plurality of coupling members are arranged side by side in sequence along the axis direction of the bus waveguide, and all the coupling members are located above the bus waveguide. The coupling member and the part of the bus waveguide directly below the coupling member form a mode coupling region;

[0032] The bus waveguide is a grating waveguide with one side wider and the other side narrower. The side with a larger duty cycle in the bus waveguide faces the coupling member, that is, the wider side of the grating waveguide faces the coupling member. The bus waveguide supports the transmission of four modes, and more modes can be supported by changing the width of the grating waveguide;

[0033] The grating waveguide with one side wider and the other side narrower adopts a trapezoidal sub-wavelength grating, an elliptical sub-wavelength grating or a similar structure.

[0034] The coupler includes a first bent grating, a coupling waveguide and a second bent grating connected in sequence. The first bent grating, the coupling waveguide and the second bent grating form a U-shaped structure. The first bent grating is embedded with a first bent waveguide, the coupling waveguide is embedded with a bridging waveguide, and the second bent grating is embedded with a second bent waveguide; the first bent waveguide, the bridging waveguide and the second bent waveguide are connected in sequence;

[0035] The widths of the first bent grating, the coupling waveguide and the second bent grating are the same, and the duty ratios of the first bent grating, the coupling waveguide and the second bent grating are the same. The widths of the first bent waveguide, the bridging waveguide and the second bent waveguide are the same. The radii of the first bent grating, the first bent waveguide, the second bent grating and the second bent waveguide are the same. The first bent grating and the first bent waveguide are concentric. The second bent grating and the second bent waveguide are concentric.

[0036] The coupling waveguide is a trapezoidal sub-wavelength grating waveguide. The coupling waveguides in different couplers are all waveguides that support the transmission of the fundamental mode, but the coupling waveguides in different couplers couple different modes; the side with a larger duty ratio in the coupling waveguide faces the bus waveguide, that is, the wider side of the trapezoidal grating of the coupling waveguide faces the bus waveguide. The width of the coupling waveguide is determined by the phase matching condition after coupling with the width of the bus waveguide.

[0037] Taking Figure 1 the mode multiplexer / demultiplexer of the four modes shown as an example, the mode multiplexer / demultiplexer without a transition region of the present invention is described.

[0038] Figure 1 The mode multiplexer / demultiplexer of the four modes shown includes a first grating 1 and four U-shaped couplers arranged side by side. The four couplers are all located above the first grating 1. Each coupler and the part of the first grating 1 directly below the coupler form a mode coupling region. The four mode coupling regions are a third-order mode coupling region 2, a second-order mode coupling region 3, a first-order mode coupling region 4 and a fundamental mode coupling region 5 in sequence.

[0039] The first grating 1 is a bus waveguide and is also a waveguide that supports the transmission of four modes. The first grating 1 is a trapezoidal sub-wavelength grating, and the wider side of the trapezoidal sub-wavelength grating faces the coupler, that is, the side with a larger duty ratio of the first grating 1 faces the coupler.

[0040] The structures of the third-order mode coupling region 2, the second-order mode coupling region 3, the first-order mode coupling region 4 and the fundamental mode coupling region 5 are basically the same, but the specific parameters are different. The third-order mode coupling region 2 is taken as an example for description below.

[0041] As shown Figure 2 in the figure, the third-order mode coupling region 2 in the mode multiplexer / demultiplexer of the four modes includes a U-shaped coupling member and a portion of the first grating 1 located directly below the coupling member. The coupling member includes a second grating 7, a third grating 9, and a fourth grating 11 connected in sequence. The second grating 7, the third grating 9, and the fourth grating 11 form a U-shaped structure. The second grating 7 is embedded with a first curved waveguide 6, the third grating 9 is embedded with a bridging waveguide 8, and the fourth grating 11 is embedded with a second curved waveguide 10. The first curved waveguide 6, the bridging waveguide 8, and the second curved waveguide 10 are connected in sequence. Both the second grating 7 and the fourth grating 11 are curved trapezoidal subwavelength gratings, the third grating 9 is a trapezoidal subwavelength grating. The wider side of the second grating 7 and the wider side of the fourth grating 11 both face the outside of the U-shaped structure, and the wider side (the side with a larger duty cycle) of the third grating 9 faces the first grating 1, and the wider side (the side with a larger duty cycle) of the first grating 1 faces the third grating 9.

[0042] The function of the bridging waveguide 8 is to adjust the refractive index distribution so that the effective refractive index of the fundamental mode in the waveguide formed by the third grating 9 and the bridging waveguide 8 is the same as the effective refractive index of the third-order mode in the first grating 1, thereby satisfying the phase matching condition required for coupling and achieving coupling. The lengths of the third grating 9 and the bridging waveguide 8 are obtained through simulation using the coupled mode theory and the finite difference time domain method. The second grating 6 and the fourth grating 11 are symmetrically connected to both sides of the third grating 9, and the first curved waveguide 7 and the second curved waveguide 10 are symmetrically connected to both sides of the bridging waveguide 8 to make the coupling occur only between the third grating 9 and the first grating 1 as much as possible, thereby better controlling the coupling length.

[0043] The duty cycles and widths of the second grating 7, the fourth grating 11, and the third grating 9 are all kept consistent. The bending radii of the second grating 7 and the fourth grating 11 are 20 μm to satisfy the low-loss transmission of the fundamental mode therein. The widths of the first curved waveguide 6, the second curved waveguide 10, and the bridging waveguide 8 are the same. In the second grating 11, since the side with a larger duty cycle faces the outside of the circle, larger losses are likely to occur. By embedding the second curved waveguide 10 in the second grating 11, the losses in this part can be optimized to achieve lower transmission losses in the bent part.

[0044] As a multiplexer, light needs to input the fundamental mode from the second curved waveguide 10, so as to output the third-order mode in the first grating 1, as shown Figure 3 . As a demultiplexer, the third-order mode needs to be input from the first grating 1 and the fundamental mode is output in the second curved waveguide 10, as shown Figure 4 in the figure.

[0045] The structures of the third-order mode coupling region 2, the second-order mode coupling region 3, the first-order mode coupling region 4, and the fundamental mode coupling region 5 are basically the same. However, to meet the phase matching conditions for mode coupling in each coupling region, the widths of the trapezoidal sub-wavelength gratings and the widths of the bridging waveguides 8 in each coupling region are different. As shown in Figure 5 the mode dispersion diagram of the trapezoidal sub-wavelength grating shown, among the first four modes ( Figure 5 the curves from top to bottom), the effective refractive index of the third-order mode coupling region 2 is the smallest, while the effective refractive index of the fundamental mode coupling region 5 is the largest. Therefore, among several coupling regions, it is necessary that the effective refractive index of the fundamental mode in the combined waveguide of the third grating 9 and the bridging waveguide 8 in the third-order mode coupling region 2 is the smallest, and the effective refractive index of the fundamental mode in the combined waveguide of the third grating 9 and the bridging waveguide 8 in the fundamental mode coupling region 5 is the largest. Thus, the widths of the trapezoidal sub-wavelength grating waveguides and the widths of the bridging waveguides in the coupled waveguide are adjusted based on this, and the purpose is to achieve the phase matching conditions.

[0046] When the second-order mode coupling region 3, the first-order mode coupling region 4, and the fundamental mode coupling region 5 are used as multiplexers, the fundamental mode is respectively input into the second bending waveguide 10 in each coupling region. Then:

[0047] In the second-order mode coupling region 3, the fundamental mode is coupled to the first grating 1 and converted into the second-order mode;

[0048] In the first-order mode coupling region 4, the fundamental mode is coupled to the first grating 1 and converted into the first-order mode;

[0049] In the fundamental mode coupling region 5, the fundamental mode is coupled to the first grating 1 and coupled into the fundamental mode.

[0050] When the second-order mode coupling region 3, the first-order mode coupling region 4, and the fundamental mode coupling region 5 are used as demultiplexers, in each coupling region, the corresponding mode in the first grating 1 is demultiplexed into the fundamental mode.

[0051] The present invention has mode scalability. Only taking the four-mode as an example to show the realization of the device function, the number of modes supported in the bus waveguide can be changed by increasing the width of the first grating 1. After increasing the width of the bus waveguide, it will cause the effective refractive index of the modes supported in the bus waveguide to change, which changes the phase matching conditions required for mode coupling. Therefore, the widths of the third gratings 9 coupled in different coupling regions, including the widths of the bridging waveguides 8 embedded in the third gratings 9, need to change to meet the phase matching conditions and realize a higher-order multiplexer / demultiplexer based on trapezoidal sub-wavelength grating waveguides.

[0052] The input of the transition region-free mode multiplexer / demultiplexer of the present invention is a fundamental mode broadband optical signal, and the transmitted signal is an optical signal. Compared with electrical signals, optical signals have the characteristics of fast transmission speed, low loss, and low delay, which can improve its application prospects in the fields of optical switches, optical networks, optical communications, etc.

[0053] The mode multiplexer / demultiplexer without a transition region of the present invention is implemented based on SOI material. By using the CMOS process in the prior art, the device has a small volume, low power consumption, and good scalability, facilitating integration with electrical components to realize an optoelectronic hybrid chip. In the future, it is expected to increase the number of channels for on-chip transmission based on this invention to improve the transmission efficiency.

[0054] The performance advantages of the mode multiplexer / demultiplexer without a transition region of the present invention are closely related to the material properties it uses and the specific parameters of the device. Using different materials will cause changes in the phase matching conditions required for coupling. Therefore, it is necessary to readjust the widths of the coupling waveguide, bridging waveguide, and bus waveguide to re-match the phase matching conditions, but Figure 1 the overall structural layout of the device shown remains unchanged.

[0055] In terms of materials: The waveguides and sub-wavelength gratings of the mode multiplexer / demultiplexer without a transition region of the present invention use silicon-on-insulator (SOI) material on an insulating substrate. SOI refers to growing a single-crystalline silicon thin film with a certain thickness on a SiO2 insulating layer, and its process is compatible with the CMOS process widely used in the current microelectronics field. The waveguides and sub-wavelength grating waveguides made of SOI material have a Si core layer (refractive index 3.45) and a SiO2 cladding layer (refractive index 1.44), as Figure 6 shown; in this way, the refractive index difference between the cladding layer and the core layer is very large, so the waveguide has a strong ability to confine the optical field, and the loss of light during transmission in the sub-wavelength grating is also low.

[0056] The waveguides and sub-wavelength gratings of the mode multiplexer / demultiplexer without a transition region of the present invention include but are not limited to a silicon nitride platform, a lithium niobate platform, etc.

[0057] Taking the four-mode multiplexer / demultiplexer based on a trapezoidal sub-wavelength grating waveguide as an example, the working mode of the device of the present invention will be specifically described. As Figure 7 shown in a, the fundamental mode is injected into the fourth grating 11 in different mode coupling regions at the multiplexing port. After passing through different mode coupling regions, different modes can be obtained in the bus waveguide (the first grating 1). That is, after passing through the fundamental mode coupling region 2, the fundamental mode appears in the first trapezoidal sub-wavelength grating waveguide. After passing through the first-order mode coupling region 3, the first-order mode appears in the first grating 1, and so on. After passing through the third-order mode coupling region 5, four modes exist in the first grating 1 and are transmitted forward, as Figure 7a. At the demultiplexing port, the four modes existing in the first grating 1 are demultiplexed into the fundamental mode through the coupling regions of different modes. That is, through the third-order mode coupling region 5, the third-order mode in the first grating 1 is coupled into the fundamental mode. After passing through the third-order mode coupling region 5, there are three remaining modes in the first grating 1, and so on, until the fundamental mode in the first grating 1 is coupled out in the fundamental mode coupling region 2. At this time, there is no remaining light in the first grating 1, as Figure 7 b; When the light of different modes is demultiplexed from the first grating 1 into the coupling waveguides of each mode coupling region, it is then output through the second grating 7 in each mode coupling region.

[0058] Taking the multiplexing and demultiplexing process of the third-order mode as an example, the overall light transmission process is described. That is, light enters the fundamental mode from the fourth grating 11 in the third-order mode coupling region 2, undergoes coupling through the third grating 9, and is coupled into the first grating 1 to become the third-order mode and then propagates forward. Until passing through the third-order mode coupling region 2, demultiplexing occurs. When the third-order mode in the first grating 1 passes through the coupling region, it is coupled into the third grating 9 in the third-order mode coupling region 2 to become the fundamental mode, and then is output in the second grating 6. The transmission of other modes in the multiplexer / demultiplexer of the present invention is the same as that of the third-order mode.

[0059] In the mode multiplexer / demultiplexer without a transition region of the present invention, the width of the bus waveguide (the first grating 1) is constant, and there is no need for a tapered transition region to change the width of the bus waveguide. While achieving the purpose of compactness, the flexibility of mode processing is increased, making this multiplexer / demultiplexer have extremely high application value in the fields of optical communication, optical computing, optical interconnection, etc. in the future.

[0060] Only four modes are taken as an example to illustrate the principle of the multiplexer / demultiplexer of the present invention, but the device can be extended in the number of modes through the same principle to realize a mode multiplexer / demultiplexer with more orders.

Claims

1. A transition - free mode multiplexer / demultiplexer based on a trapezoidal sub - wavelength grating waveguide, characterized in that, the transition - free mode multiplexer / demultiplexer is a mode multiplexer / demultiplexer based on an asymmetric directional coupler, including a bus waveguide and a plurality of U - shaped couplers with the same number as the number of optical wave modes in the optical signal. The plurality of couplers are arranged side by side in sequence along the axial direction of the bus waveguide. All the couplers are located above the bus waveguide. The coupler and the part of the bus waveguide directly below the coupler form a mode coupling region; the bus waveguide is a trapezoidal sub - wavelength grating waveguide. The side with a larger duty cycle in the bus waveguide faces the coupler, and more modes of transmission are supported by changing the width of the first trapezoidal sub - wavelength grating; the coupler includes a first bent grating, a coupling waveguide, and a second bent grating connected in sequence. The first bent grating, the coupling waveguide, and the second bent grating form a U - shaped structure. The first bent grating is embedded with a first bent waveguide, the coupling waveguide is embedded with a bridging waveguide, and the second bent grating is embedded with a second bent waveguide; the first bent waveguide, the bridging waveguide, and the second bent waveguide are connected in sequence; the widths of the first bent grating, the coupling waveguide, and the second bent grating are the same, and the duty cycles of the first bent grating, the coupling waveguide, and the second bent grating are the same; the widths of the first bent waveguide, the bridging waveguide, and the second bent waveguide are the same; the radii of the first bent grating, the first bent waveguide, the second bent grating, and the second bent waveguide are the same; the first bent grating and the first bent waveguide are concentric; the second bent grating and the second bent waveguide are concentric; the coupling waveguide is a trapezoidal sub - wavelength grating waveguide. The coupling waveguides in different couplers are all waveguides supporting the fundamental mode transmission, and the coupling waveguides in different couplers couple different modes; the side with a larger duty cycle in the coupling waveguide faces the bus waveguide; the width of the coupling waveguide is determined by the phase - matching condition after coupling with the width of the bus waveguide.

2. The transition - free mode multiplexer / demultiplexer based on a trapezoidal sub - wavelength grating waveguide according to claim 1, characterized in that, the width of the bus waveguide remains unchanged.

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