Ultra-compact and ultra-wideband waveguide mode converter based on longitudinal refractive index modulation

Abstract

The present invention discloses an ultra-compact and ultra-wideband waveguide mode converter based on longitudinal refractive index modulation. The waveguide mode converter includes an input waveguide and an output waveguide with different widths, and a refractive index perturbation modulation structure with a longitudinal refractive index modulation function, wherein: the input waveguide and the output waveguide are connected in a non-central axisymmetric manner; a refractive index perturbation modulation structure is introduced into the cross-section of the output waveguide, and the refractive index perturbation modulation structure is located on the splicing boundary of the input waveguide and the output waveguide and extends along the positive direction of waveguide transmission. By designing a binary refractive index modulation structure with a certain length in the longitudinal direction in the cross-section of the waveguide and reverse designing and optimizing the length and material distribution of the longitudinal refractive index region, the present invention realizes an ultra-compact and ultra-wideband mode conversion, and this method can be extended to the conversion of any order mode. The present invention provides a new method for improving the integration degree and performance of devices in an on-chip integrated system.

Description

Technical Field

[0001] The present invention relates to a micro-nano photonics integrated device, and particularly to an ultra-compact and ultra-wideband waveguide mode converter based on longitudinal refractive index modulation. Background Art

[0002] With the rapid development of the information society, the demand for the capacity of information and data transmission is getting higher and higher. At present, integrated circuit electronic devices are the main body of information transmission, data transmission and calculation in the big data era. However, with the continuous increase in the number and continuous reduction in the size of electronic devices in integrated circuits, the performance of integrated circuit systems will be severely limited, especially bandwidth and power consumption. Different from this, light, as a carrier of information transmission, has the advantages of high speed, low energy consumption and parallel transmission. Therefore, an on-chip optical information transmission and optical interconnection processing system mainly based on integrated optics has become an effective way to address the problem of the demand for information data transmission capacity in the big data era. Such a system is often composed of a large number of optical device units combined. Therefore, it is of great importance to design and manufacture high-performance and highly integrated on-chip optical devices. The on-chip mode multiplexing / demultiplexing system has great advantages in improving the information transmission capacity and speed because it uses multiple modes supported by waveguides for parallel transmission and processing of information. And the mode converter plays a key role in this system. Researchers have conducted some studies on it. Most traditional mode converters are realized by gradually changing waveguide coupling structures, such as directional couplers, multimode interference couplers, etc. Although such photon devices based on coupling structures have good performance, the coupling length is mostly dozens of micrometers, and some are even as high as hundreds of micrometers, with relatively large sizes, which is not conducive to improving the integration of on-chip systems. Metamaterials and metasurfaces are a class of structures composed of artificially designed sub-wavelength structural units to achieve specific functions. They have great flexibility and can arbitrarily control the optical field at the micro-nano scale through customized design. Introducing them into the waveguide structure to achieve mode conversion can make the size of the mode conversion device smaller and improve the integration. However, most of the currently realized mode converters based on metasurfaces introduce transverse refractive index modulation perturbation structural units into the waveguide. Although the conversion length for realizing mode conversion is relatively short, there are problems of limited bandwidth and large loss. Therefore, it is of great significance to propose a reasonable method and a suitable structure to achieve ultra-compact and high-performance mode conversion. Summary of the Invention

[0003] To solve the problem that existing waveguide mode conversion devices are difficult to simultaneously consider conversion size and conversion performance, and the problem that existing mode conversion based on metasurface structures introduces large losses and limited bandwidth due to transverse refractive index modulation, the present invention provides a super-compact and ultra-wideband waveguide mode converter based on longitudinal refractive index modulation. By designing a longitudinal binary refractive index modulation structure of a certain length in the cross-section of the waveguide and reverse-designing and optimizing the length and material distribution of the longitudinal refractive index region, a super-compact and ultra-wideband mode conversion is achieved, and this method can be extended to the conversion of any order mode. The present invention provides a new method for improving the integration and performance of devices in on-chip integrated systems.

[0004] The object of the present invention is achieved through the following technical solutions:

[0005] A super-compact and ultra-wideband waveguide mode converter based on longitudinal refractive index modulation, including an input waveguide (width w1) and an output waveguide (width w2) with different widths, and a refractive index perturbation modulation structure with longitudinal refractive index modulation function, wherein:

[0006] The values of the width w1 and the width w2 are required to ensure the existence and stable transmission of the corresponding input and output modes supported by the waveguide;

[0007] The input waveguide and the output waveguide are connected in a non-central axisymmetric manner, and the central offset between the input waveguide and the output waveguide is P y , and the size of the central offset is adjusted to ensure the energy transmittance of the device structure;

[0008] A refractive index perturbation modulation structure is introduced into the cross-section of the output waveguide. The refractive index perturbation modulation structure is located on the splicing boundary of the input waveguide and the output waveguide and extends along the positive direction of waveguide transmission. In order to achieve efficient conversion in a super-compact size, the extension length L x is obtained by optimization. The optimization range is selected near the wavelength and the value is taken according to the optimization result, generally taking the smaller size under high-performance conversion;

[0009] The refractive index perturbation modulation structure is a binary refractive index perturbation modulation structure composed of multiple rectangular structural units. The filling material of each rectangular unit structure is silicon or air, and the material distribution of the refractive index perturbation modulation structure for realizing the required mode conversion function is obtained by reverse design optimization.

[0010] A reverse design optimization method for the above waveguide mode converter includes the following steps:

[0011] Step 1: Modulate the waveguide center offset P between the input waveguide and the output waveguide without introducing a refractive index perturbation modulation structure yTo maximize the energy transmittance, the offset of the waveguide center affects the energy coupling between the input waveguide and the output waveguide (i.e., the energy transmittance). As the offset increases, the energy transmittance first increases to the maximum value and then decreases. Therefore, the waveguide center offset is selected as the corresponding value when the energy transmittance reaches the maximum value;

[0012] Step 2. After determining the waveguide center offset P y Then, a longitudinal binary refractive index perturbation modulation structure with an extension length of L x is designed in the cross-section of the output waveguide. For light propagating in the x direction, the coupling coefficient κ mn between the two modes of the output waveguide with refractive index perturbation modulation is expressed by the following formula:

[0013]

[0014] where the integration surface is the cross-section of the output waveguide, E m (y, z) is the cross-sectional field distribution of the guided mode m, Δε(x, y, z) represents the dielectric perturbation distribution introduced in the output waveguide, and E n (y, z) is the cross-sectional field distribution of the guided mode n;

[0015] Step 3. Optimize the refractive index modulation structure: Discretize the cross-section of the output waveguide into multiple rectangular structural units. The filling material of each unit structure is silicon or air. Use the direct binary search algorithm to optimize the material distribution of the cross-section of the output waveguide with different extension lengths L x . The objective function for optimization is the mode conversion efficiency, and a longitudinal refractive index modulation structure for achieving high conversion efficiency is obtained. Among them, the definition of the mode conversion efficiency is:

[0016]

[0017] where P out (TE n ) is the power of the target mode n at the output end, and P in (TE m ) is the power of the input mode m at the input end.

[0018] Compared with the prior art, the present invention has the following advantages:

[0019] By introducing an optimized longitudinal refractive index modulation structure into the waveguide, the present invention realizes high-performance large-bandwidth conversion with a compact conversion size by transforming and controlling the optical field. Specifically, for the TE0-TE1 conversion, under the condition of a conversion length of 1.5 μm (comparable to the wavelength size), the conversion efficiency is as high as 93.5%, and the crosstalk is less than -10 dB within the wavelength range of 1310 - 1720 nm. Under the condition of a conversion length of 3 μm, the conversion efficiency is as high as 96.3%, and the crosstalk is less than -10 dB within the wavelength range of 1320 - 1800 nm. In short, this is a mode converter with a small size, high performance, and large bandwidth. This structure and method are also easy to extend to the design of other integrated photon function structures. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 FIG. is a schematic diagram of the waveguide mode conversion structure with longitudinal refractive index modulation provided by the present invention.

[0021] Figure 2 FIG. is a non-centrally axisymmetric waveguide structure diagram without refractive index modulation provided by the present invention.

[0022] Figure 3 FIG. is a curve diagram of the influence of the non-central axis offset without refractive index modulation on the energy transmission efficiency provided by the present invention.

[0023] Figure 4 FIG. is a curve diagram of the change of the optimization function under different refractive index modulation conversion lengths provided by the present invention.

[0024] Figure 5 FIG. is a cross-sectional view of the optimized refractive index modulation structure with conversion lengths of 1.5 μm and 3 μm respectively provided by the present invention.

[0025] Figure 6 FIG. is a field distribution result diagram when the optimized device with conversion lengths of 1.5 μm and 3 μm respectively provided by the present invention realizes mode conversion.

[0026] Figure 7 FIG. is a curve diagram of the change of the performance parameters of the mode converter with an optimized conversion length of 1.5 μm provided by the present invention with respect to the wavelength.

[0027] Figure 8 FIG. is a curve diagram of the change of the performance parameters of the mode converter with an optimized conversion length of 3 μm provided by the present invention with respect to the wavelength. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0028] The technical solutions of the present invention will be further described below in conjunction with the accompanying drawings, but are not limited thereto. Any modification or equivalent replacement of the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention shall be covered by the protection scope of the present invention.

[0029] The present invention provides an ultra-compact and ultra-wideband waveguide mode converter based on longitudinal refractive index modulation. By introducing an optimized binary refractive index perturbation modulation structure in the waveguide in the longitudinal direction, the conversion between different order modes is realized. This longitudinal refractive index perturbation modulation structure has the characteristics of low loss, compact conversion size, and large bandwidth when realizing mode conversion. The waveguide adopted by the waveguide mode converter is an SOI waveguide structure, the core layer material is silicon with a thickness of h, and the upper and lower cladding materials are air and silica respectively. The mode converter is composed of an input waveguide, an output waveguide, and a refractive index perturbation modulation structure with longitudinal refractive index modulation function.

[0030] According to the mode coupling principle, when a refractive index perturbation modulation structure is introduced into the waveguide, the modes in the waveguide will couple with each other. The coupling equation of the waveguide modes along the propagation direction is:

[0031]

[0032]

[0033] Among them, A m and A n are the amplitudes of the waveguide modes m and n respectively, β m and β n are the propagation constants of the waveguide modes m and n respectively, and κ mn and κ nm are the coupling coefficients between the two modes respectively. According to the perturbation theory, the coupling coefficient between the two modes can be expressed by the following formula:

[0034]

[0035] Among them, the integration surface is the cross-section of the waveguide, E m (y,z) is the cross-sectional field distribution of the waveguide mode m, and Δε(x,y,z) represents the introduced dielectric perturbation distribution in the waveguide. From this, it can be analyzed that by selecting a suitable refractive index perturbation modulation structure, mode conversion can be realized.

[0036] For the conversion of TE0-TE1 (with different mode parities), first, the input waveguide and the output waveguide are connected in a non-central axisymmetric manner, and the center offset of the two waveguides without the refractive index perturbation modulation structure is adjusted to ensure the energy transmission efficiency.

[0037] Under the condition of ensuring the energy transmission efficiency, a longitudinal refractive index perturbation modulation structure is introduced into the waveguide. The refractive index distribution under different conversion region lengths (extension lengths) is optimized with the goal of maximizing the mode conversion efficiency, so as to obtain an optimized longitudinal refractive index perturbation modulation structure for realizing mode conversion. The optimized refractive index perturbation modulation structure only contains a few simple refractive index control channels, which can achieve high-performance mode conversion under a small conversion length. This method has fewer scattering interfaces, low loss, and large bandwidth. This method is also applicable to the conversion between any other modes.

[0038] Embodiment:

[0039] This embodiment mainly elaborates on the conversion of TE0-TE1 (with different mode parities), and the conversion between other modes can also be obtained by using a similar method.

[0040] The waveguide mode converter structure based on longitudinal refractive index modulation involved in this embodiment is as Figure 1 shown. The waveguide is a typical SOI structure, where the waveguide with width w1 is the input waveguide, the waveguide with width w2 is the output waveguide, the waveguide core layer material is silicon, the upper cladding is air, the lower cladding is silica, and the waveguide thickness h is 220 nm. For the conversion of TE0-TE1, w1 is set to 400 nm, w2 is set to 1000 nm, the centers of the input waveguide and the output waveguide are non-axisymmetric, and a refractive index perturbation modulation structure is introduced into the cross-section of the output waveguide (shown by the dotted line box). The extension length of the refractive index perturbation modulation structure along the x-direction at the splicing boundary of the input waveguide and the output waveguide is L x . According to the mode coupling principle, by reasonably adjusting the distribution of the refractive index perturbation modulation structure, mode conversion can be achieved.

[0041] In order to ensure the energy transmission efficiency between two non-axisymmetric waveguides, first, it is necessary to modulate the influence of the waveguide center offset on the energy transmission efficiency without adding a refractive index perturbation modulation. The top view of the waveguide structure without adding a refractive index perturbation modulation is as Figure 2 shown, and the center offset of the waveguide is P y . The energy transmission efficiency at different offsets calculated using the finite difference time domain method is as Figure 3 shown. It can be seen from Figure 3 that when the center offset P y = 0.233 μm, the maximum energy transmission efficiency is 97.2%. Therefore, in the subsequent design, the center offset of the waveguide is set to 97.2%.

[0042] From the previous analysis, introducing a refractive index perturbation modulation structure into the waveguide can achieve mode conversion, and it is necessary to reasonably modulate the distribution of the refractive index perturbation. This embodiment mainly designs a longitudinal refractive index perturbation modulation structure, that isFigure 1 As shown. For a complex refractive index perturbation modulation structure, it is a good method to adopt the reverse design optimization method to obtain the target refractive index distribution. Therefore, in this embodiment, reverse design will be used to optimize and realize mode conversion at different extension lengths. Specifically, the cross-section of the output waveguide is discretized into 10×11 small rectangular unit structures, the size of each rectangular structure unit is 100nm×20nm, the material of each structure unit is silicon or air, the initial material of all structure units is set to silicon, and TE0 mode light with a central wavelength of 1.55μm is input into the input waveguide, at different extension lengths L x (between 0.5 and 3μm), the material distribution of the structure units is optimized with the mode conversion efficiency as the objective function (FOM), and the optimization algorithm is selected as the DBS (direct binary search) algorithm. The results of the objective function at different conversion lengths are calculated as Figure 4 shown. From Figure 4 it can be obtained that when the extension length L x = L1 = 1.5μm, the conversion efficiency is 93.5%, and when the extension length L x = L2 = 3μm, the conversion efficiency is 96.3%. The extension length is shorter compared with previous studies, thus realizing an ultra-compact mode conversion.

[0043] Figure 5 are the optimized waveguide cross-section refractive index distributions, that is, material distributions, at two different extension lengths. The result at the extension length L x = L1 = 1.5μm is as Figure 5 (a) shown, and the result at the extension length L x = L2 = 3μm is as Figure 5 (b) shown. From Figure 5 it can be found that the optimized structure of the longitudinal refractive index perturbation modulation structure designed in this embodiment is relatively simple, and only a few channels are required to realize mode conversion.

[0044] Figure 6 (a) and (b) are the field distribution results when realizing mode conversion after optimization at extension lengths of 1.5μm and 3μm respectively. From Figure 6 it can be clearly observed that the input TE0 mode is well converted into the TE1 mode, and the symmetry of the output mode field is very good, indicating high mode purity and low crosstalk.

[0045] Next, the performance of the waveguide mode conversion device will be analyzed in detail. In this embodiment, the performance results of the device at the optimized extension length L x = L1 = 1.5μm at different wavelengths are as Figure 7 shown. From Figure 7It can be found that the mode conversion efficiency is greater than 90% in the wavelength range of 1370 - 1610 nm, the mode purity is greater than 90% in the wavelength range of 1310 - 1720 nm, the crosstalk is less than -10 dB in the wavelength range of 1310 - 1720 nm, and the total energy loss is less than 1 dB. The optimized extended length L is obtained in the same way. x The performance results of the device with x Figure 8 L2 = 3 μm under a wide wavelength range are as Figure 8 shown. It can be found from

[0046] that the mode conversion efficiency of this device is greater than 90% in the wavelength range of 1410 - 1630 nm, the mode purity is greater than 90% in the wavelength range of 1380 - 1680 nm, the crosstalk is less than -10 dB in the wavelength range of 1320 - 1800 nm, and the total energy loss is less than 0.8 dB. Therefore, it can be seen that the ultra-compact mode converter obtained in this embodiment can maintain good performance within a large bandwidth, and the performance has been improved compared with the previous related research.

[0046] By introducing a longitudinal refractive index perturbation modulation structure into the waveguide, the present invention obtains an ultra-compact and ultra-wideband waveguide mode converter through an optimized method, which is very beneficial for improving the integration and performance of on-chip integrated systems. Although only the conversion of TE0-TE1 is elaborated in detail in the embodiment, the conversion of other order modes can be obtained by using the same structure and method, and only some parameters need to be modified and adjusted.

Claims

1. A super-compact and ultra-wideband waveguide mode converter based on longitudinal refractive index modulation, characterized in that The waveguide mode converter includes an input waveguide and an output waveguide with different widths, and a refractive index perturbation modulation structure with a longitudinal refractive index modulation function, where: The input waveguide and the output waveguide are connected in a non-central axisymmetric manner; A refractive index perturbation modulation structure is introduced into the cross-section of the output waveguide. The refractive index perturbation modulation structure is located on the splicing boundary between the input waveguide and the output waveguide and extends along the positive direction of waveguide transmission. The refractive index distribution under different conversion region lengths is optimized with the goal of maximizing the mode conversion efficiency, so as to obtain an optimized longitudinal refractive index perturbation modulation structure for realizing mode conversion; The refractive index perturbation modulation structure is a binary refractive index perturbation modulation structure composed of multiple rectangular structural units.

2. The super-compact and ultra-wideband waveguide mode converter based on longitudinal refractive index modulation according to claim 1, characterized in that The waveguide adopted by the waveguide mode converter is an SOI waveguide structure, the core layer material is silicon, and the upper and lower cladding materials are air and silicon dioxide respectively.

3. The super-compact and ultra-wideband waveguide mode converter based on longitudinal refractive index modulation according to claim 1, characterized in that The filling material of the rectangular structural unit is silicon or air.

4. An inverse design optimization method for the super-compact and ultra-wideband waveguide mode converter based on longitudinal refractive index modulation according to any one of claims 1-3, characterized in that The method includes the following steps: Step 1: Modulate the waveguide center offset P between the input waveguide and the output waveguide without introducing a refractive index perturbation modulation structure y to achieve the highest energy transmittance; Step 2. After determining the waveguide center offset P y a longitudinal binary refractive index perturbation modulation structure with an extension length of L is designed in the cross-section of the output waveguide x For light propagating in the x direction, the coupling coefficient κ between the two modes of the output waveguide with refractive index perturbation modulation is introduced mn which is expressed by the following formula: where the integration surface is the cross-section of the output waveguide, E m (y,z) is the cross-sectional field distribution of the guided mode m, and Δε(x,y,z) represents the dielectric perturbation distribution introduced in the output waveguide, E n (y,z) is the cross-sectional field distribution of the guided mode n; Step 3. Optimize the refractive index modulation structure: Discretize the cross-section of the output waveguide into multiple rectangular structural units. The filling material of each unit structure is silicon or air. Use the direct binary search algorithm to optimize the material distribution of the output waveguide cross-section at different extension lengths L x to obtain the longitudinal refractive index modulation structure that realizes high conversion efficiency, with the mode conversion efficiency as the objective function for optimization.

5. The inverse design optimization method for the super-compact and ultra-wideband waveguide mode converter based on longitudinal refractive index modulation according to claim 4, characterized in that The definition of the mode conversion efficiency is: Among them, P out (TE n ) is the power of the output end target mode n, and P in (TE m ) is the power of the input end input mode m.

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