CN109709641B - Multimode cross structure based on periodic dielectric waveguide and design method thereof - Google Patents
Multimode cross structure based on periodic dielectric waveguide and design method thereof Download PDFInfo
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Abstract
The invention relates to a multimode cross structure based on periodic dielectric waveguides and a design method thereof, the multimode cross structure comprises linear transmission waveguides which are arranged in a cross shape, the linear transmission waveguides comprise a straight waveguide, an inverted cone-shaped waveguide and a semi-cylindrical waveguide positioned at the tip of the inverted cone-shaped waveguide, and a cylindrical waveguide array spaced apart from the semi-cylindrical waveguide by a predetermined distance, the cylindrical waveguide array including a plurality of gradually increasing pitches, and the radius of the cylindrical waveguide is gradually reduced, the cylindrical waveguide is arranged along the central axis of the inverted conical waveguide, the structure adopts a coupling resonant optical waveguide CROWs with a periodic structure to construct a multimode cross unit, the cross function of low loss and low crosstalk can be simultaneously realized by a plurality of longitudinal propagation modes on one cross structure, the device structure is very compact, and the problem that the traditional waveguide cross scheme can not realize simultaneous and efficient cross of more than two longitudinal modes by using a single cross structure is solved.
Description
Technical Field
The invention relates to the field of optical communication, in particular to a multimode cross structure based on periodic dielectric waveguides and a design method thereof.
Background
The development of modern photon technology has higher and higher requirements on the integration of devices, the density, the functions, the performance and the like of devices on a chip, so that the crossing times of optical waveguides on a single chip are greatly increased. Meanwhile, an SOI (silicon On isolator) material has good light guiding property as a hotspot material of optical integration research, but the spatial divergence angle of a guided mode of the SOI is large due to large core-cladding refractive index difference of the SOI, so that light can generate remarkable scattering at the crossed part of the waveguide. A single direct crossover of the SOI optical waveguide will result in severe crosstalk and multimode excitation, and the loss and crosstalk generated by a large number of crossovers will be unacceptable for a single chip.
For an optical waveguide cross-over cell, the scattered power is proportional to the refractive index difference of the waveguide material. The problems of loss and crosstalk due to optical waveguide crossings are particularly acute in high index-contrast materials. Currently, methods for solving the problem include sub-wavelength diffraction gratings, impedance-matched "metamaterials", multimode interference, bridge structures, and the like. However, these structures have problems that it is difficult to meet the intersection of a plurality of modes or the process, the manufacturing difficulty is large, and the like.
The periodic dielectric waveguide has a photonic band gap, and electromagnetic waves of any frequency in the band gap range cannot pass through the periodic dielectric waveguide, namely, the propagation characteristic of the periodic dielectric waveguide is related to the wavelength, and only electromagnetic waves of specific wavelengths can be transmitted in the periodic dielectric waveguide. The periodic dielectric waveguide is divided into one-dimensional, two-dimensional and three-dimensional, and the one-dimensional periodic dielectric waveguide is adopted conventionally because the two-dimensional and three-dimensional structures are difficult to manufacture. Common one-dimensional periodic dielectric waveguides are gratings, Coupled Resonant Optical Waveguides (CROWs), and the like. Because CROWs are not spatially continuous in their waveguide material, light is concentrated in the high index regions and propagates along the CROWs by jumping between each two adjacent pillars. When two CROWs form an optical cross structure, the central cross is made of a low refractive index material (such as air), and light rays in two orthogonal CROWs are difficult to couple, so that low crosstalk is realized.
Disclosure of Invention
Aiming at the defects of the prior art, the primary object of the present invention is to provide a multimode cross structure based on periodic dielectric waveguides and a design method thereof, and based on the purpose, the present invention at least provides the following technical solutions:
a multimode cross structure based on periodic dielectric waveguides comprises linear transmission waveguides which are arranged in a cross mode, wherein each linear transmission waveguide comprises a straight waveguide, an inverted cone-shaped waveguide, a semi-cylindrical waveguide located at the tip end of the inverted cone-shaped waveguide and a cylindrical waveguide array which is at a certain distance from the semi-cylindrical waveguide, the cylindrical waveguide array comprises a plurality of cylindrical waveguides with gradually increased intervals and gradually decreased radiuses, and the cylindrical waveguides are arranged along the central axis of the inverted cone-shaped waveguide.
Further, the cylindrical waveguide with the smallest radius is arranged at the cross.
Further, the structures of the linear transmission waveguides arranged in the cross shape in the x direction and the y direction are completely the same.
Furthermore, at least two inverted cone-shaped waveguides are arranged along the x direction or the y direction of the crisscross arrangement.
Further, in the inverted tapered waveguide, the output waveguide and the input waveguide in the latter half in the x-axis direction are completely symmetrical with respect to the yz plane.
Further, the linear transmission waveguide has silicon as a waveguide layer, and the waveguide layer is coated with silicon dioxide.
A design method of multimode cross structure based on periodic dielectric waveguide is characterized by comprising the following steps:
s1: designing a linear transmission waveguide structure of the multi-mode dielectric waveguide, determining the width and the height of a straight waveguide in the linear transmission waveguide according to the refractive index of a material, so that the linear transmission waveguide structure can support transmission of at least two modes in the longitudinal direction, namely the height direction vertical to the surface of a chip and has the characteristic of low bending radius;
s2: designing a compact multimode cross unit, wherein the compact multimode cross unit comprises an inverted cone waveguide in a linear transmission waveguide, a semi-cylindrical waveguide positioned at the tip of the inverted cone waveguide and a cylindrical waveguide array at a certain distance from the semi-cylindrical waveguide, and determining a parameter scheme with the highest transmission efficiency by changing the width and the length of the tip of the inverted cone and the radius and the number of cylindrical dielectric waveguides so as to meet all performance requirements which can be met by the device.
Further, step S2 specifically includes:
s2.1: determining the height and width of a straight waveguide to ensure that light can efficiently transmit at least two longitudinal modes, and setting the width as the initial width of an input inverted conical waveguide and the final width of an output conical waveguide;
s2.2: determining the widths of the tips of the inverted conical waveguide and the conical waveguide, and taking the widths as the diameters of the semi-cylindrical waveguide at the tips;
s2.3: determining the radius of the first cylindrical dielectric waveguide and the distance between the first cylindrical dielectric waveguide and the tip of the inverted conical waveguide by referring to the band gap of the one-dimensional photonic crystal under a specific period parameter and the linear variation trend of the inverted conical waveguide;
s2.4: determining parameters of the next cylindrical waveguide according to the method for determining the radius and the distance of the cylindrical dielectric waveguides in S2.3, finely adjusting the radius and the distance of the cylindrical waveguides by observing the transmittance curve of the cylindrical waveguide, and repeating the steps until the number and the radius of the cylindrical waveguides reaching the highest light passing efficiency and the distance between two adjacent cylinders are determined, and finally determining all the parameters of the input waveguide, wherein the output waveguide and the input waveguide are symmetrical about a yz plane;
s2.5: determining the distance between two cylindrical waveguides at the intersection, calculating the transmittance and crosstalk of the multimode intersection structure when the distances at a series of intersections have different values through three-dimensional electromagnetic wave simulation software, drawing a curve, and determining an optimal distance scheme;
s2.6: designing a compact low-loss low-crosstalk crossing structure according to the parameters, and vertically placing a second path of input waveguide, a periodic cylindrical waveguide and an output waveguide with the same structural parameters at 90 degrees with the first waveguide channel to ensure that the symmetrical centers of the two paths of waveguides are overlapped and form a cross at the symmetrical center.
Compared with the prior art, the invention has at least the following beneficial effects:
the multimode cross structure provided by the invention aims at the requirement of high integration level of modern photon technology, avoids the generation of larger transmission loss and crosstalk caused by the crossing of a large number of single-mode optical waveguides, and adopts the coupled resonant optical waveguides CROWs with periodic structures to construct the multimode cross unit, so that a plurality of longitudinal propagation modes can simultaneously realize the cross function of low loss and low crosstalk on one cross structure, and the device structure is very compact. The problem that the traditional waveguide crossing scheme cannot realize simultaneous high-efficiency crossing of more than two longitudinal modes by using a single crossing structure is solved.
Drawings
FIG. 1 is a schematic diagram of a cylindrical waveguide in a crossover structure of the present invention.
Fig. 2 is an inverted cone structure of a cylindrical tip used in the crossing structure of the present invention.
Fig. 3 is a top view of a cross structure unit of the present invention.
FIG. 4 shows the distribution of the longitudinal fundamental mode, first, second and third order electric fields of the light source of the present invention.
FIG. 5 is a diagram of a simulation result of the cross-bar structure of the present invention.
Detailed Description
The drawings are for illustrative purposes only and are not to be construed as limiting the patent;
for the purpose of better illustrating the embodiments, certain features of the drawings may be omitted, enlarged or reduced, and do not represent the size of an actual product; it will be understood by those skilled in the art that certain well-known structures in the drawings and descriptions thereof may be omitted.
The technical solution of the present invention is further described below with reference to examples.
Through continuous optimization and adjustment, one set of parameters which can meet the target requirements is given in the following content to realize the performances of low loss, low crosstalk and the like of the cross unit.
The multimode cross structure based on the periodic dielectric waveguide is shown in figures 1-3, and comprises linear transmission waveguides which are arranged in a cross shape, wherein the linear transmission waveguides comprise a straight waveguide, an inverted cone-shaped waveguide, a semi-cylindrical waveguide positioned at the tip end of the inverted cone-shaped waveguide and a cylindrical waveguide array which is at a certain distance from the semi-cylindrical waveguide, the cylindrical waveguide array comprises a plurality of cylindrical waveguides with gradually increased intervals and gradually decreased radiuses, and the cylindrical waveguides are arranged along the central axis of the inverted cone-shaped waveguide. The cylindrical waveguide with the smallest radius is arranged at the cross, the x-direction and y-direction structures of the inverted cone-shaped waveguides arranged in a cross manner are completely the same, and at least two inverted cone-shaped waveguides are arranged along the x or y direction. In the inverted tapered waveguide, the output waveguide and the input waveguide in the latter half in the x direction are completely symmetrical with respect to the yz plane. In this embodiment, the linear transmission waveguide further comprises a straight waveguide, and one end of the straight waveguide is connected with one end of the inverted conical waveguide, which is wider than the tip.
In this embodiment, the inverted tapered waveguide has silicon as a waveguide layer, which is coated with silicon dioxide. The thickness of the silicon waveguide layer is 1.8um, and the refractive index of the silicon dioxide is 1.46.
Specifically, in order to make the waveguide support more than two modes in the longitudinal direction, the width W1 and the height of the straight waveguide are selected to be 0.5 micrometer and 1.8 micrometer respectively, and the field distributions of the four lowest longitudinal modes in the TM mode are shown in fig. 4, where a is the light source electric field distribution of the fundamental mode, b is the light source electric field distribution of the first order, c is the light source electric field distribution of the second order, and d is the light source electric field distribution of the third order.
The input end of the first half part of the structure along the x direction is an inverted cone waveguide with the length of 6um and the width changed from 0.5um to 0.3um behind the straight waveguide, the tip of the inverted cone waveguide is a semi-cylindrical waveguide with the radius of 0.15um, a first cylindrical waveguide is added at the position 0.01um away from the tip of the inverted cone waveguide, and the radius of the first cylindrical waveguide is 0.13 um. The rear is connected with 7 cylindrical dielectric waveguides, the radiuses of the cylindrical dielectric waveguides are gradually reduced, the radiuses of the cylindrical dielectric waveguides are sequentially 0.11um, 0.1um, 0.09um, 0.085um, 0.083um, 0.08um and 0.078um, the gaps between the cylindrical dielectric waveguides adjacent to the cylindrical dielectric waveguides are gradually increased, and the radiuses of the cylindrical dielectric waveguides are sequentially 0.02um, 0.04um, 0.06um, 0.08um, 0.09um and 0.09 um. The main purpose of the structure that the radius of the cylindrical dielectric waveguide is gradually reduced and the distance is gradually increased is to solve the following two technical problems. First, the distance between adjacent cylindrical waveguides at the center intersection should be increased as much as possible, so as to reduce the crosstalk of the intersection structure. Secondly, at the coupling position of the straight waveguide and the cylindrical waveguide, in order to improve the coupling efficiency, a cylinder with a larger radius size is adopted, and the larger radius means stronger optical beam binding, so that the distance between two adjacent cylindrical waveguides cannot be too large to avoid increasing the transmission loss. In order to make the final adjacent cylinders have a larger pitch, it is necessary to gradually decrease the radius of each cylindrical waveguide from the input waveguide end and to increase the pitch of the adjacent cylinders accordingly, without directly pulling up the radius-decreasing pitch to a target value at a time to avoid a larger transmission loss. The output waveguide and the input waveguide in the latter half of the x direction are completely symmetrical about the yz plane, and finally the interstitial space between the input waveguide part and the output waveguide part (namely the space gap of two cylindrical dielectric waveguides at the central intersection) is determined to be 0.15 um. The structure has the same structure in the y direction and the x direction, so that the transmission efficiency in the x direction and the transmission efficiency in the y direction can be the same. All the structures in the x direction are rotated by 90 degrees by taking the left-right symmetry center as a rotation center to obtain the structures in the y direction. Whereby the two input and output waveguides in the x-direction and the y-direction form a cross at the centre of rotation. The top view is shown in fig. 3.
According to the above design structure, three-dimensional simulation is performed using FDTD Solutions to obtain accurate transmission results of the structure. The polarization of the periodic dielectric waveguide and the different energy band structures are related in Transverse Electric (TE) polarization mode and Transverse Magnetic (TM) polarization mode. In most cases, however, there will be only one polarization state in an actual structure or system, and so the effect in the TM mode will be simulated in this example, where the electric field direction coincides with the axial direction of the cylindrical dielectric waveguide. As shown in fig. 4, a is the distribution of the guided mode electric field of the fundamental mode, b is the distribution of the guided mode electric field of the first order, c is the distribution of the guided mode electric field of the second order, and d is the distribution of the guided mode electric field of the third order. Then, the above 4 cases will be simulated separately and the insertion loss and crosstalk of the device will be monitored therein. Only the fundamental mode and the first order simulation results are provided here.
Several monitors are placed in the propagation direction of the light, in the vertical direction and on the central plane, wherein the situation of monitoring at the straight waveguide 24.5um after the taper in the output waveguide is taken as the result of the transmittance, and likewise the situation of monitoring at the straight waveguide 24.5um perpendicular to the propagation direction of the light is taken as the result of the crosstalk. The result is shown in fig. 5, wherein in fig. 5, a1 is the output-side field distribution when the input field is the fundamental mode, a2 is the electric field distribution of the entire cross structure unit when the input field is the fundamental mode, a3 is the transmittance curve when the input field is the fundamental mode, a4 is the crosstalk curve when the input field is the fundamental mode, b1 is the output-side field distribution when the input field is the first-order mode, b2 is the electric field distribution of the entire cross structure unit when the input field is the first-order mode, b3 is the transmittance curve when the input field is the first-order mode, and b4 is the crosstalk curve when the input field is the first-order mode. As can be seen from FIG. 5, the fundamental mode and the first-order mode both have smaller insertion loss and crosstalk, the insertion loss of the fundamental mode is as low as-0.2 dB, the crosstalk is-21 dB, the insertion loss of the first-order mode is-0.96 dB, and the crosstalk is close to-20 dB. The crosstalk performance of each mode of the cross unit is obviously improved compared with the crosstalk performance of the straight waveguide cross unit. From the analysis of results, the device has higher transmission efficiency, the transmission efficiency of the fundamental mode and the first-order mode reaches more than 85%, and the transmission efficiency of the fundamental mode reaches more than 95%. Through the analysis of optimization results of different parameters in the structure, the number of the cylindrical dielectric waveguides is changed to prolong the taper structure, the change rate of the radius of the cylinder is slowed down, and the transmittance and crosstalk of the waveguide structure are influenced by increasing or reducing the distance between the cylinders.
The above example is only one result of using silicon as the waveguide layer, and the analysis above can further increase the number of columns, increase the pitch of the intersections, and continue to improve the device performance. By the design, the purposes of high integration level, low loss and low crosstalk of the waveguide cross unit under the longitudinal multi-mode can be achieved.
The same or similar reference numerals are used for the same or similar parts, and the positional relationship shown in the drawings is for illustrative purposes only and should not be construed as limiting the present patent, and it is apparent that the above-described embodiments of the present invention are merely illustrative for clearly explaining the present invention and are not limitative to the embodiments of the present invention. Other variations and modifications will be apparent to persons skilled in the art in light of the above description. And are neither required nor exhaustive of all embodiments. Any modification, equivalent replacement, and improvement made within the spirit and principle of the present invention should be included in the protection scope of the claims of the present invention.
Claims (4)
1. A multimode cross structure based on periodic dielectric waveguides comprises linear transmission waveguides arranged in a cross mode, wherein the linear transmission waveguides comprise straight waveguides, inverted cone waveguides, semi-cylindrical waveguides located at the tips of the inverted cone waveguides and cylindrical waveguide arrays at a certain distance from the semi-cylindrical waveguides, one end, opposite to the tips of the inverted cone waveguides, of each inverted cone waveguide is connected with the straight waveguides, the width of each straight waveguide is equal to that of one end, opposite to the tips of the inverted cone waveguides, of each inverted cone waveguide, each cylindrical waveguide array comprises a plurality of cylindrical waveguides with gradually increasing intervals and gradually decreasing radiuses, the cylindrical waveguides are arranged along the central axis of the inverted cone waveguides, the cylindrical waveguide with the smallest radius is arranged at the cross position, and the x-direction structure and the y-direction structure of the linear transmission waveguides arranged in the cross mode are completely the same, in the inverted conical waveguide, an output waveguide and an input waveguide along the x-axis direction are completely symmetrical about a yz plane; wherein the linear transmission waveguide structure is capable of supporting transmission of at least two modes in a longitudinal direction, i.e., a height direction perpendicular to a chip surface.
2. The multimode crossbar structure of claim 1 wherein two of said inversely tapered waveguides are disposed along the x or y direction of said crisscrossed arrangement.
3. The multimode crossbar structure of claim 1 wherein the linear transmission waveguides have silicon as a waveguide layer, the waveguide layer being clad with silicon dioxide.
4. A design method of multimode cross structure based on periodic dielectric waveguide is characterized by comprising the following steps:
s1: designing a linear transmission waveguide structure of the multi-mode dielectric waveguide, determining the width and the height of a straight waveguide in the linear transmission waveguide according to the refractive index of a material, so that the linear transmission waveguide structure can support transmission of at least two modes in the longitudinal direction, namely the height direction vertical to the surface of a chip and has the characteristic of low bending radius;
s2: designing a compact multimode cross unit which comprises an inverted cone waveguide in a linear transmission waveguide, a semi-cylindrical waveguide positioned at the tip of the inverted cone waveguide and a cylindrical waveguide array at a certain distance from the semi-cylindrical waveguide, and determining a parameter scheme with the highest transmission efficiency by changing the width and the length of the tip of the inverted cone and the radius and the number of cylindrical dielectric waveguides so as to meet all performance requirements which can be met by the multimode cross structure;
step S2 specifically includes:
s2.1: determining the height and width of a straight waveguide to ensure that light can efficiently transmit at least two longitudinal modes, and setting the width as the initial width of an input inverted conical waveguide and the final width of an output conical waveguide;
s2.2: determining the widths of the tips of the inverted conical waveguide and the conical waveguide, and taking the widths as the diameters of the semi-cylindrical waveguide at the tips;
s2.3: determining the radius of the first cylindrical dielectric waveguide and the distance between the first cylindrical dielectric waveguide and the tip of the inverted conical waveguide by referring to the band gap of the one-dimensional photonic crystal under a specific period parameter and the linear variation trend of the inverted conical waveguide;
s2.4: determining parameters of the next cylindrical waveguide according to the method for determining the radius and the distance of the cylindrical dielectric waveguides in S2.3, finely adjusting the radius and the distance of the cylindrical waveguides by observing the transmittance curve of the cylindrical waveguide, and repeating the steps until the number and the radius of the cylindrical waveguides reaching the highest light passing efficiency and the distance between two adjacent cylinders are determined, and finally determining all the parameters of the input waveguide, wherein the output waveguide and the input waveguide are symmetrical about a yz plane;
s2.5: determining the distance between two cylindrical waveguides at the intersection, calculating the transmittance and crosstalk of the multimode intersection structure when the distances at a series of intersections have different values through three-dimensional electromagnetic wave simulation software, drawing a curve, and determining an optimal distance scheme so as to form a first waveguide passage;
s2.6: designing a compact low-loss low-crosstalk crossing structure according to the parameters, and vertically placing a second path of input waveguide, a periodic cylindrical waveguide and an output waveguide with the same structural parameters at 90 degrees with the first waveguide channel to ensure that the symmetrical centers of the two paths of waveguides are overlapped and form a cross at the symmetrical center.
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