CN110618486B - Polarization-independent power divider based on symmetrical three-waveguide and sub-wavelength structure - Google Patents
Polarization-independent power divider based on symmetrical three-waveguide and sub-wavelength structure Download PDFInfo
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- G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
- G02B6/12007—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind forming wavelength selective elements, e.g. multiplexer, demultiplexer
- G02B6/12009—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind forming wavelength selective elements, e.g. multiplexer, demultiplexer comprising arrayed waveguide grating [AWG] devices, i.e. with a phased array of waveguides
- G02B6/12016—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind forming wavelength selective elements, e.g. multiplexer, demultiplexer comprising arrayed waveguide grating [AWG] devices, i.e. with a phased array of waveguides characterised by the input or output waveguides, e.g. tapered waveguide ends, coupled together pairs of output waveguides
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- G02B6/12009—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind forming wavelength selective elements, e.g. multiplexer, demultiplexer comprising arrayed waveguide grating [AWG] devices, i.e. with a phased array of waveguides
- G02B6/12023—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind forming wavelength selective elements, e.g. multiplexer, demultiplexer comprising arrayed waveguide grating [AWG] devices, i.e. with a phased array of waveguides characterised by means for reducing the polarisation dependence, e.g. reduced birefringence
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- G—PHYSICS
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- G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
- G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
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- G02B2006/12154—Power divider
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Abstract
The invention discloses a power divider which is based on a symmetrical three-waveguide and has no relation with polarization of a sub-wavelength structure, the power divider sequentially comprises a silicon-based substrate (8), a buried oxide layer (9), a silicon waveguide layer (11) and an upper cladding (10) from bottom to top, the silicon waveguide layer (11) forms a symmetrical three-waveguide directional coupler structure (7) by a right path through channel (2), a middle path through channel (3) and a left path through channel (4), one side of the symmetrical three-waveguide directional coupler structure (7) is provided with an input channel (1), the other side of the symmetrical three-waveguide directional coupler structure is provided with a right output channel (5) and a left output channel (6), and the surface of each channel is attached with a sub-wavelength grating structure; the directional coupler structure (7) performs 3dB power distribution on input light with different polarizations. The power divider effectively reduces the insertion loss and the reflection loss of the existing power divider, improves the power dividing ratio of a device, and shortens the size of the device.
Description
Technical Field
The invention relates to a polarization-independent power divider based on a symmetrical three-waveguide and sub-wavelength structure, and belongs to the technical field of integrated optics.
Background
Photonic Integrated Circuits (PICs) are considered to be a good choice for developing optical computing and high broadband interconnects in next generation optical networks by providing low cost solutions for optical interconnects and higher transmission rates. In recent years, in order to realize photonic integrated circuits, silicon-on-insulator (SOI) materials have attracted extensive attention and have been practically applied in many optical fields due to their cmos-compatible fabrication processes, high refractive index contrast, low-loss nanowire waveguides and small device footprint. The power divider is an important component of complex optical devices such as a mode multiplexer, an optical phase control array, an optical switch and the like. Because the refractive index contrast between the core layer and the cladding layer is high, the power divider based on the SOI platform has strong birefringence characteristics for transverse electric field modes (TE) and transverse magnetic field modes (TM) with different polarization characteristics; making most SOI-based power splitters generally polarization sensitive, which negatively impacts PIC performance. The traditional polarization-independent power divider realizes the equal coupling length of TE and TM modes by introducing a thicker silicon waveguide layer, a wider waveguide gap, a bent waveguide or a lower waveguide width; but at the same time, it brings higher design complexity and power Splitting Ratio (SR), and also leads to higher insertion loss and reflection loss. In recent years, sub-wavelength grating (SWG) structures have received much attention from researchers due to a variety of effective diffraction suppression capabilities and index processing capabilities; a new degree of freedom is provided for the design of the optical device. Therefore, it is necessary to design a power divider with a compact structure, low insertion loss, large operating bandwidth and uniform power division ratio to realize next-generation PIC.
Disclosure of Invention
The technical problem is as follows: the invention aims to provide a power divider which is based on a symmetrical three-waveguide and has no relation with polarization of a sub-wavelength structure, wherein the power divider adopts a symmetrical three-waveguide directional coupler structure consisting of a right path through channel, a middle path through channel and a left path through channel, and carries out power 3dB distribution on input different polarized lights TE (TM) so as to couple the input different polarized lights into an output channel waveguide; the scheme effectively reduces the insertion loss of the power divider, improves the power dividing ratio of the device and shortens the size of the device.
The technical scheme is as follows: the invention provides a power divider which is based on symmetrical three-waveguide and has no relation with polarization of a sub-wavelength structure, the power divider is manufactured by adopting an insulating silicon chip platform, the bottommost layer of the power divider is a silicon-based substrate, the upper surface of the silicon-based substrate is a buried oxide layer, a silicon waveguide layer is distributed on the upper surface of the buried oxide layer, and the silicon waveguide layer is covered with an upper cladding, wherein:
the silicon waveguide layer comprises an input channel, a right through channel, a middle through channel, a left through channel, a right output channel and a left output channel, and all the channels are made of silicon waveguides;
the right path through channel, the middle path through channel and the left path through channel are arranged in parallel and aligned to form a symmetrical three-waveguide directional coupler structure, and the width of the left path through channel is consistent with that of the right path through channel;
an input channel is arranged on one side of the symmetrical three-waveguide directional coupler structure and is connected with one end of the middle-path through channel;
the other side of the symmetrical three-waveguide directional coupler structure is provided with a right output channel and a left output channel, the right output channel is connected with one end of a right through channel, the left output channel is connected with one end of a left through channel, the right side of the right output channel is aligned with the right side of the right through channel, and the left side of the left output channel is aligned with the left side of the left through channel;
the left side surface of a right path straight-through channel, the right side surface of a left path straight-through channel and the left and right side surfaces of a middle path straight-through channel in the symmetrical three-waveguide directional coupler structure are all provided with sub-wavelength grating structures;
the two sides of the input channel are attached with conical sub-wavelength grating structures to form a transition structure, and the sum of the widths of the conical sub-wavelength grating structures and the input channel in the transition structure is equal to the sum of the widths of the intermediate path through channel and the sub-wavelength grating structures attached to the left and the right of the intermediate path through channel;
the left side surface of the right output channel and the right side surface of the left output channel are provided with conical sub-wavelength grating structures, the sum of the widths of the conical sub-wavelength gratings on the right output channel and the left side surface of the right output channel is equal to the sum of the widths of the conical sub-wavelength gratings on the right through channel and the left side surface of the right through channel, and the sum of the widths of the conical sub-wavelength gratings on the left output channel and the right side surface of the left output channel is equal to the sum of the widths of the conical sub-wavelength gratings on the left through channel and the;
the sub-wavelength grating structure and the conical sub-wavelength grating structure are both composed of high-refractive-index material layers and low-refractive-index material layers alternately; and if the length of the high-refractive-index material along the channel direction is a and the period formed by the high-refractive-index material and the low-refractive-index material is lambada, the duty ratio is defined as f ═ a/lambada, and the sub-wavelength grating structure and the conical sub-wavelength grating structure have the same period and duty ratio.
Wherein:
the sum of the widths of the sub-wavelength grating structures on the left side surface of the right through channel and the left through channel and the sum of the widths of the sub-wavelength grating structures on the right side surface of the left through channel and the left through channel are both 490nm to 500nm, wherein the widths of the sub-wavelength grating structures on the left side surface of the right through channel and the sub-wavelength grating structures on the right side surface of the left through channel are both 200nm to 250nm, the periods are both 200nm to 240nm, and the duty ratio is both 45% to 50%; the sum of the widths of the middle-path straight-through channel and the sub-wavelength grating structures on the left side surface and the right side surface of the middle-path straight-through channel is 510 nm-570 nm, wherein the widths of the sub-wavelength grating structures on the left side surface and the right side surface are respectively 130 nm-160 nm, the period is 200 nm-240 nm, and the duty ratio is 45% -50%.
The width of the middle straight-through channel is the same as the width of the contact ends of the input channel and the middle straight-through channel, and is 245-255 nm; the width of the right through channel is the same as the width of a contact end of the output channel and the right through channel, and is 250 nm-300 nm; the width of the left through channel is the same as that of a contact end of the output channel and the left through channel and is 250 nm-300 nm; the width of the right through channel is kept the same as that of the left through channel.
The width of the input channel is transited from 245-255 nm to 490-500 nm from the contact end with the middle path through channel to the far end, the sum of the width of the input channel and the width of the tapered sub-wavelength grating structure in the transition structures attached to the left side and the right side is 510-570 nm, the period of the grating is 200-240 nm, and the duty ratio is 45-50%.
The width sum of the right output channel (and the width sum of the tapered sub-wavelength grating structure on the left side surface of the right output channel, and the width sum of the tapered sub-wavelength grating structure on the left output channel and the right side surface of the left output channel are 490 nm-500 nm, wherein the right output channel and the left output channel are transited from 250 nm-300 nm to 490 nm-500 nm from the contact end of the directional coupler structure to the far end, the period of the tapered sub-wavelength grating structure attached to the surface of the right output channel is 200 nm-240 nm, the width of the tapered sub-wavelength grating structure is linearly transited from 200 nm-250 nm to 50 nm-70 nm from the contact end of the directional coupler structure to the far end within 3 mu m-4 mu m, and the duty ratio is.
In the parallel and aligned arrangement of the right path through channel, the middle path through channel and the left path through channel, the distance between the structure formed by the middle path through channel and the sub-wavelength gratings on the left and right side surfaces of the middle path through channel is equal to the distance between the structure formed by the right path through channel and the sub-wavelength gratings on the left side surface of the middle path through channel and the structure formed by the left path through channel and the sub-wavelength gratings on the right side surface of the left path through channel, and the distances between the structures are all 100-150 nm.
The sub-wavelength grating structure and the conical sub-wavelength grating structure have uniform period and duty ratio.
The material of the high-refractive-index material layer is silicon, and the material of the low-refractive-index material layer is silicon dioxide.
The sizes of the right through channel, the middle through channel and the left through channel meet the following conditions:
1) the effective refractive indexes of TE and TM modes supported by the right through channel and the left through channel are equal;
2) the difference of effective refractive indexes of TE modes supported by the middle-path through channel and the right-path through channel and the left-path through channel distributed on the two sides is less than 0.1, and the phases are matched;
3) the difference between the effective refractive indexes of TM modes supported by the middle through channel and the right through channel and the left through channel distributed on the two sides is less than 0.1, and the phases are matched;
4) lowest order even mode TE supported by symmetrical three-waveguide directional coupler structure0And TE2Effective refractive index difference of (1) and TM supported thereby0And TM2Are equal.
The coupling length L of the symmetrical three-waveguide directional coupler structureCSatisfies the following formula:
in the formula: lambda is the wavelength of operation and,
representing the effective refractive index of the 0 th order TM or TE mode supported by the symmetric three-waveguide directional coupler structure,
representing the effective refractive index of the 2 nd order TM or TE mode supported by the symmetric three-waveguide directional coupler structure.
The silicon-based substrate is a silicon wafer with standard size, the thickness of the buried oxide layer is 1-3 mu m, and the buried oxide layer is made of silicon dioxide material; the upper cladding material is silicon dioxide, polymethyl methacrylate or air.
Has the advantages that: compared with the prior art, the technical scheme of the invention has the following beneficial effects:
1. the insertion loss is low: the most important condition met by the polarization-independent 3dB dual-mode power divider is that both TE and TM modes meet the phase matching condition; the TE polarized light and the TM polarized light are subjected to phase matching, nearly 50% of power is coupled to the through channels on two sides, and therefore insertion loss is low; if the structural parameters are further adjusted, the coupling distance of the two polarized lights is reduced, the size of the device is reduced, and the insertion loss is further reduced.
2. The power division ratio is uniform: the polarization-independent 3dB dual-mode power divider adopts a centrosymmetric design, and polarized light in TE and TM modes is input from a middle-path through channel and symmetrically coupled to two side through channels through a symmetric three-waveguide directional coupler; the power available from the left and right output channels can be highly approximately equal, resulting in an output power ratio of approximately 1.
3. The reflection loss is low: the 3dB dual-mode power divider adopts a symmetrical design, and also introduces a sub-wavelength grating structure and a three-waveguide directional coupler structure; the comprehensive use of the two structures ensures that the device performance is more stable; meanwhile, the use of a bent output waveguide can be omitted, so that the reflection loss is reduced, and the coupling efficiency is higher.
Drawings
FIG. 1 is a schematic view of a silicon waveguide layer structure in embodiment 1 of the present invention;
fig. 2 is a schematic cross-sectional structure diagram of a symmetric three-waveguide directional coupler structure in embodiment 1 of the present invention;
fig. 3 is a diagram showing the relationship between the coupling length of TE and TM modes at 1.55 μm and the change in the channel pitch in the conventional symmetric three-waveguide directional coupler structure in embodiment 1 of the present invention;
fig. 4 is a diagram showing the relationship between the coupling length corresponding to TE and TM modes at 1.55 μm and the width change of the intermediate-path through channel in the conventional symmetric three-waveguide directional coupler structure in embodiment 1 of the present invention;
fig. 5 is a diagram showing the relationship between the coupling lengths of TE and TM modes at 1.55 μm and the SWG width variation in the mid-path waveguide in the symmetric three-waveguide directional coupler structure after the SWG structure is added in embodiment 1 of the present invention;
fig. 6 is a diagram showing the relationship between the coupling lengths of TE and TM modes at 1.55 μm and the SWG period variation of a symmetric three-waveguide directional coupler structure after the SWG structure is added in embodiment 1 of the present invention;
FIG. 7 shows TE at an operating wavelength of 1.55 μm in example 1 of the present invention0Mode and TM0Transmission diagram of main components of the mode in the power divider;
the figure shows that: the silicon waveguide directional coupler comprises an input channel 1, a right through channel 2, a middle through channel 3, a left through channel 4, a right output channel 5, a left output channel 6, a symmetrical three-waveguide directional coupler structure 7, a silicon substrate 8, a buried oxide layer 9, an upper cladding layer 10 and a silicon waveguide layer 11.
Detailed Description
The invention is further explained below with reference to the drawings.
Example 1
A silicon-based power divider (as shown in fig. 1 and 2) based on symmetrical triple waveguides and independent of polarization of sub-wavelength structures is manufactured by adopting an insulated silicon wafer platform, the bottommost layer of the power divider is a silicon-based substrate 8 composed of a standard 6-inch silicon wafer, a buried oxide layer 9 with the thickness of 2 microns and thermally grown on the upper surface of the silicon-based substrate 8 is made of silicon dioxide materials, a silicon waveguide layer 11 is distributed on the upper surface of the buried oxide layer 9, and the silicon waveguide layer 11 is covered with an upper cladding layer 10 composed of silicon dioxide, wherein:
the silicon waveguide layer 11 comprises an input channel 1, a right path through channel 2, a middle path through channel 3, a left path through channel 4, a right output channel 5, a left output channel 6 and a symmetrical three-waveguide directional coupler structure 7, and all the channels are made of silicon waveguides;
the right through channel 2, the middle through channel 3 and the left through channel 4 are arranged in parallel and aligned to form a symmetrical three-waveguide directional coupler structure 7, and the widths of the left through channel 4 and the right through channel 2 are kept consistent;
an input channel 1 is arranged on one side of the symmetrical three-waveguide directional coupler structure 7, and the input channel 1 is connected with one end of the middle-path through channel 3;
a right output channel 5 and a left output channel 6 are arranged on the other side of the symmetrical three-waveguide directional coupler structure 7, the right output channel 5 is connected with one end of the right through channel 2, the left output channel 6 is connected with one end of the left through channel 4, the right side surface of the right output channel 5 is aligned with the right side surface of the right through channel 2, and the left side surface of the left output channel 6 is aligned with the left side surface of the left through channel 4;
the left side face of the right through channel 2, the right side face of the left through channel 4 and the left and right side faces of the middle through channel 3 in the symmetrical three-waveguide directional coupler structure 7 are respectively attached with a sub-wavelength grating structure, wherein the width of the sub-wavelength grating structures on the left side face of the right through channel 2 and the right side face of the left through channel 4 is 220nm, the period is 220nm, the duty ratio is 50%, the width of the sub-wavelength grating structures on the left and right side faces of the middle through channel 3 is 160nm, the period is 220nm and the duty ratio is 50%;
the width of the middle through channel 3 is the same as the width of the contact end of the input channel 1 and the middle through channel 3, and is 250 nm; the width of the right through channel 2 is the same as that of the contact end of the right output channel 5 and the right through channel 2, and is 280 nm; the width of the left through channel 4 is the same as that of the contact end of the left output channel 6 and the left through channel 4, and is 280 nm; the width of the input channel 1 is transited from 250nm to 500nm from the end, which is connected with the middle through channel 3, to the far end; the left output channel 6 and the right output channel 5 are transited from 280nm to 500nm within 4 mu m from the contact end with the symmetrical directional coupler structure 7 to the far end; the width of the SWG structure etched on the right side of the left output channel 6 and the width of the SWG structure etched on the left side of the right output channel 5 are changed from 220nm to 50nm under the conditions that the period is 220nm and the duty ratio is 50%; the left conical blank part at the back is not etched with the SWG structure;
the two sides of the input channel 1 are attached with conical sub-wavelength grating structures to form a transition structure, the sum of the widths of the conical sub-wavelength grating structures and the input channel 1 in the transition structure is 570nm, the period is 220nm, and the duty ratio is 50%;
the sub-wavelength grating structure and the conical sub-wavelength grating structure are both composed of high-refractive-index material layers and low-refractive-index material layers alternately; one high refractive index material layer and one low refractive index material layer form a period; if the length of the high-refractive-index material along the channel direction is a, the period formed by the high-refractive-index material and the high-refractive-index material is inverted V, and the duty ratio is defined as f ═ a/\\;
the sub-wavelength grating structure and the conical sub-wavelength grating structure have the same period and duty ratio.
Wherein:
in the parallel and aligned arrangement of the right through channel 2, the middle through channel 3 and the left through channel 4, the distance between the structure formed by the middle through channel 3 and the sub-wavelength gratings on the left and right sides thereof and the structure formed by the right through channel 2 and the sub-wavelength gratings on the left side thereof and the structure formed by the left through channel 4 and the sub-wavelength gratings on the right side thereof is 200 nm.
As shown in fig. 2, the sub-wavelength grating structure and the tapered sub-wavelength grating structure have a uniform period and duty cycle.
The sizes of the right through channel 2, the middle through channel 3 and the left through channel 4 meet the following conditions:
1) the effective refractive indexes of TE and TM modes supported by the right through channel 2 and the left through channel 4 are equal;
2) the difference between the effective refractive indexes of TE modes supported by the middle through channel 3 and the right through channel 2 and the left through channel 4 distributed on the two sides is less than 0.1, and the phases are matched;
3) the difference between the effective refractive indexes of TM modes supported by the middle through channel 3 and the right through channel 2 and the left through channel 4 distributed on the two sides is less than 0.1, and the phases are matched;
4) lowest order even mode TE supported by symmetric three-waveguide directional coupler structure 70And TE2Effective refractive index difference of (1) and TM supported thereby0And TM2Are equal.
The coupling length L of the symmetrical three-waveguide directional coupler structure 7CSatisfies the following formula:
in the formula: lambda is the wavelength of operation and,
representing the effective refractive index of the 0 th order TM or TE mode supported by the symmetric three-waveguide directional coupler structure 7,
representing the effective refractive index of the 2 nd order TM or TE mode supported by the symmetric three-waveguide directional coupler structure 7.
The symmetrical three-waveguide directional coupler structure 7 completely and uniformly couples the energy of TM and TE modes into the right through channel 2 and the left through channel 4 on two sides. The symmetrical three-waveguide directional coupler structure 7 selected here effectively avoids the use of bent waveguides, increases the gaps of the waveguides on two sides, reduces insertion loss and reflection loss, and improves the overall performance of the device.
Figure 3 shows the coupling length of TE and TM modes versus channel gap in a conventional three waveguide directional coupler at an operating wavelength of 1.55 μm, where the height of the silicon waveguide is 250 nm. As can be seen from the figure, the larger the channel gap is, the coupling length of the TE mode grows exponentially under the same channel width; for the TM mode, the coupling length is relatively stable and slightly changed; therefore, it is necessary to enhance the coupling capability of the TE mode to achieve the equal coupling length of the dual modes, thereby achieving polarization independence; to achieve a compact power divider, the waveguide gap is chosen to be 200 nm.
FIG. 4 is a graph showing the coupling length of TE and TM modes in a conventional three-waveguide directional coupler as a function of the channel width of a center path at an operating wavelength of 1.55 μm; the width of the waveguide on both sides is 500nm, and the gap between the waveguides is 200 nm. It can be seen from the figure that the coupling capability of the TE mode can be enhanced by changing the width of the waveguide, so that the coupling length of the TE mode becomes shorter, even smaller than that of the TM under the same condition; therefore, the waveguide can be transversely adjusted, and polarization independence can be realized if an SWG structure is added.
Fig. 5 shows a graph of the coupling length of TE and TM modes in a symmetric directional coupler as a function of the maximum width of the mid-channel (corresponding to different widths of the strip waveguides on both sides) after adding SWGs at the gap sides at an operating wavelength of 1.55 μm. As can be seen from the figure, the narrower the width of the strip waveguide, the wider the SWG at both sides, the more favorable the TE mode coupling; the wider the middle SWG, the more beneficial the TE coupling, thereby reducing the coupling length; for the TM mode, the addition and the widening of the SWG structure lead the effective coupling distance of the TM mode to be widened, the coupling length of the TM mode is weakly increased, but the influence on the coupling strength of the TM mode is relatively small; when the total width of the middle waveguide reaches 570nm and the strip waveguides at two sides reach 280nm, the coupling lengths of the TE mode and the TM mode are consistent, and the coupling length is less than 10 mu m, so that the compact polarization-independent power divider is realized.
Fig. 6 shows a graph of TE and TM mode coupling loss versus SWG period in a symmetric directional coupler for 3 different duty cycles after adding SWG on either side of the gap at a 1.55 μm operating wavelength. It can be seen from the figure that when the period is equal to 220nm and the duty cycle is 0.48, the losses of TE and TM coupling are minimal relative to the other cases, and the overall performance is optimal.
FIG. 7 shows TE0Mode EyComponent sum TM0Mode EzThe transmission variation of the components in the power divider. It can be seen that TE0Mode and TM0The modes are gradually and uniformly coupled from the middle waveguide to the two side waveguides in the symmetrical three-waveguide coupler, and almost no loss exists.
The present invention is not limited to the above preferred embodiments, and any modifications, equivalent replacements, improvements, etc. within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (10)
1. A polarization-independent power divider based on a symmetric three-waveguide and a subwavelength structure is characterized in that: this power distributor adopts the silicon-on-insulator platform to make, and power distributor bottom is silicon-based substrate (8), and silicon-based substrate (8) upper surface is for burying oxide layer (9), has silicon waveguide layer (11) at burying oxide layer (9) upper surface distribution, and silicon waveguide layer (11) cover has upper cladding (10), wherein:
the silicon waveguide layer (11) comprises an input channel (1), a right through channel (2), a middle through channel (3), a left through channel (4), a right output channel (5) and a left output channel (6), and all the channels are made of silicon waveguides;
the right through channel (2), the middle through channel (3) and the left through channel (4) are arranged in parallel and aligned to form a symmetrical three-waveguide directional coupler structure (7), and the widths of the left through channel (4) and the right through channel (2) are kept consistent;
an input channel (1) is arranged on one side of the symmetrical three-waveguide directional coupler structure (7), and the input channel (1) is connected with one end of the middle-path through channel (3);
a right output channel (5) and a left output channel (6) are arranged on the other side of the symmetrical three-waveguide directional coupler structure (7), the right output channel (5) is connected with one end of the right through channel (2), the left output channel (6) is connected with one end of the left through channel (4), the right side surface of the right output channel (5) is aligned with the right side surface of the right through channel (2), and the left side surface of the left output channel (6) is aligned with the left side surface of the left through channel (4);
the left side surface of a right through channel (2), the right side surface of a left through channel (4) and the left and right side surfaces of a middle through channel (3) in the symmetrical three-waveguide directional coupler structure (7) are all provided with sub-wavelength grating structures;
tapered sub-wavelength grating structures are attached to the left side and the right side of the input channel (1) to form a transition structure, and the sum of the widths of the tapered sub-wavelength grating structures and the input channel (1) in the transition structure is equal to the sum of the widths of the intermediate path through channel (3) and the sub-wavelength grating structures attached to the left side and the right side of the intermediate path through channel;
the left side surface of the right output channel (5) and the right side surface of the left output channel (6) are provided with conical sub-wavelength grating structures, the sum of the widths of the conical sub-wavelength gratings on the right output channel (5) and the left side surface of the right output channel is equal to the sum of the widths of the conical sub-wavelength gratings on the right through channel (2) and the left side surface of the right through channel, and the sum of the widths of the conical sub-wavelength gratings on the left output channel (6) and the right side surface of the left output channel is equal to the sum of the widths of the conical sub-wavelength gratings on the left through channel (4) and the;
the sub-wavelength grating structure and the conical sub-wavelength grating structure are composed of high-refractive-index material layers and low-refractive-index material layers in an alternating mode, one high-refractive-index material layer and one low-refractive-index material layer form a period, if the length of the high-refractive-index material in the channel direction is a, the period formed by the high-refractive-index material layer and the low-refractive-index material layer is Λ, the duty ratio is defined as f-a/Λ, and the sub-wavelength grating structure and the conical sub-wavelength grating structure have the same period and duty ratio.
2. The polarization independent power splitter of claim 1 based on a symmetric triple waveguide subwavelength structure, wherein: the sum of the widths of the sub-wavelength grating structures on the left side surface of the right through channel (2) and the left side surface of the left through channel (4) and the right side surface of the left through channel is 490-500 nm, wherein the widths of the sub-wavelength grating structures on the left side surface of the right through channel (2) and the right side surface of the left through channel (4) are 200-250 nm, the periods of the sub-wavelength grating structures are 200-240 nm, and the duty ratio of the sub-wavelength grating structures is 45-50%; the sum of the widths of the intermediate path through channel (3) and the sub-wavelength grating structures on the left side surface and the right side surface of the intermediate path through channel is 510 nm-570 nm, wherein the widths of the sub-wavelength grating structures on the left side surface and the right side surface are respectively 130 nm-160 nm, the period is 200 nm-240 nm, and the duty ratio is 45% -50%.
3. The polarization independent power splitter of claim 1 based on a symmetric triple waveguide subwavelength structure, wherein: the width of the middle-path through channel (3) is the same as the width of the contact end of the input channel (1) and the middle-path through channel (3), and is 245-255 nm; the width of the right through channel (2) is the same as the width of the contact end of the output channel (5) and the right through channel (2), and is 250 nm-300 nm; the width of the left through channel (4) is the same as the width of the contact end of the output channel (6) and the left through channel (4), and is 250 nm-300 nm; the width of the right through channel (2) is the same as that of the left through channel (4).
4. The polarization independent power splitter of claim 1 based on a symmetric triple waveguide subwavelength structure, wherein: the width of the input channel (1) is transited from 245-255 nm to 490-500 nm from the contact end with the middle path through channel (3) to the far end, the sum of the width and the width of the tapered sub-wavelength grating structure in the transition structures attached to the left side and the right side is 510-570 nm, the period of the grating is 200-240 nm, and the duty ratio is 45-50%.
5. The polarization independent power splitter of claim 1 based on a symmetric triple waveguide subwavelength structure, wherein: the sum of the widths of the right output channel (5) and the tapered sub-wavelength grating structure on the left side surface of the right output channel, and the sum of the widths of the left output channel (6) and the tapered sub-wavelength grating structure on the right side surface of the left output channel are 490-500 nm, wherein the widths of the right output channel (5) and the left output channel (6) transition from 250 nm-300 nm to 490-500 nm from the contact end with the symmetrical three-waveguide directional coupler structure (7) to the far end, the period of the tapered sub-wavelength grating structure attached to the surface of the right output channel is 200 nm-240 nm, the width of the right output channel and the left output channel is linearly transition from 200 nm-250 nm to 50 nm-70 nm from the contact end with the symmetrical three-waveguide directional coupler structure (7) to the far end within 3-4 mu m, and the duty ratio is 45.
6. The polarization independent power splitter of claim 1 based on a symmetric triple waveguide subwavelength structure, wherein: in the parallel and aligned arrangement of the right through channel (2), the middle through channel (3) and the left through channel (4), the distance between the structure formed by the middle through channel (3) and the sub-wavelength gratings on the left and right side surfaces thereof is equal to the distance between the structure formed by the right through channel (2) and the sub-wavelength gratings on the left side surface thereof, and the distance between the structure formed by the left through channel (4) and the sub-wavelength gratings on the right side surface thereof, and the distances are all 100-150 nm.
7. The polarization independent power splitter of claim 1 based on a symmetric triple waveguide subwavelength structure, wherein: the material of the high-refractive-index material layer is silicon, and the material of the low-refractive-index material layer is silicon dioxide.
8. The polarization independent power splitter of claim 1 based on a symmetric triple waveguide subwavelength structure, wherein: the sizes of the right through channel (2), the middle through channel (3) and the left through channel (4) meet the following conditions:
1) the effective refractive indexes of TE and TM modes supported by the right through channel (2) and the left through channel (4) are equal;
2) the difference between the effective refractive indexes of TE modes supported by the middle-path through channel (3) and the right-path through channel (2) and the left-path through channel (4) distributed on the two sides is less than 0.1, and the phases are matched;
3) the difference between the effective refractive indexes of TM modes supported by the middle-path through channel (3) and the right-path through channel (2) and the left-path through channel (4) distributed on the two sides is less than 0.1, and the phases are matched;
4) lowest order even mode TE supported by a symmetric three-waveguide directional coupler structure (7)0And TE2Effective refractive index difference of (1) and TM supported thereby0And TM2Are equal.
9. The polarization independent power splitter of claim 1 based on a symmetric triple waveguide subwavelength structure, wherein: the coupling length L of the symmetrical three-waveguide directional coupler structure (7)CSatisfies the following formula:
in the formula: lambda is the wavelength of operation and,
represents the effective refractive index of the 0 th order TM or TE mode supported by the symmetrical three-waveguide directional coupler structure (7),
represents the effective refractive index of the 2 nd order TM or TE mode supported by the symmetrical three-waveguide directional coupler structure (7).
10. The polarization independent power splitter of claim 1 based on a symmetric triple waveguide subwavelength structure, wherein: the silicon-based substrate (8) is a silicon wafer with standard size, the thickness of the buried oxide layer (9) is 1-3 mu m, and the buried oxide layer is made of silicon dioxide material; the upper cladding (10) is made of silicon dioxide, polymethyl methacrylate or air.
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