CN110618487A - Multimode interference type polarization insensitive power divider based on sub-wavelength grating structure - Google Patents
Multimode interference type polarization insensitive power divider based on sub-wavelength grating structure Download PDFInfo
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- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
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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
- G02B6/12023—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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- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- G02B6/24—Coupling light guides
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- G02B6/28—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals
- G02B6/2804—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers
- G02B6/2808—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers using a mixing element which evenly distributes an input signal over a number of outputs
- G02B6/2813—Optical coupling means having data bus means, i.e. plural waveguides interconnected and providing an inherently bidirectional system by mixing and splitting signals forming multipart couplers without wavelength selective elements, e.g. "T" couplers, star couplers using a mixing element which evenly distributes an input signal over a number of outputs based on multimode interference effect, i.e. self-imaging
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- G—PHYSICS
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- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- 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
- G02B2006/12083—Constructional arrangements
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- G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
- 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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- 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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Abstract
The invention discloses a multimode interference type polarization insensitive power divider based on a sub-wavelength grating structure, and belongs to the field of silicon-based photonics. The power divider component comprises an input waveguide (1), an input tapered waveguide (2), a multi-mode interference coupling area (3), a first output tapered waveguide (4), a second output tapered waveguide (5), a first output waveguide (6), a second output waveguide (7), an input end tapered sub-wavelength grating structure (8) and a sub-wavelength grating structure (9). The invention combines the sub-wavelength grating structure and the multi-mode interference coupler, and realizes the polarization insensitivity of the power divider by utilizing the refractive index adjustability and the birefringence characteristic of the sub-wavelength grating structure. The power divider has the advantages of low insertion loss, low reflection loss, large working bandwidth, compact structure, convenient manufacture and the like, and is convenient to integrate with other devices by adopting a silicon-on-insulator system.
Description
Technical Field
The invention relates to a multimode interference type polarization insensitive power divider based on a sub-wavelength grating structure, and belongs to the technical field of integrated optics.
Background
The passive device based on the power divider has important significance in the technical field of integrated optics, and is widely applied to devices such as optical modulators, optical switches and optical sensors. Generally, the function of the power divider is realized by mainly utilizing a Y-shaped branch waveguide, a directional coupler, a multi-mode interference coupler, a photonic crystal and the like. The multimode interference coupler has received much attention because of its advantages such as simple and compact structure, large process tolerance, small insertion loss, good uniformity, low sensitivity to polarization and wavelength, etc. In order to meet the requirement of integration development and achieve the goal of integrating active devices and passive devices, the silicon material has a high refractive index contrast and a complementary metal oxide semiconductor compatible characteristic, so that the silicon material becomes a research focus of people. In addition, the insertion loss, compactness and bandwidth of the power divider need to be further improved and optimized in order not to affect the operation performance of other devices cascaded with the power divider. The provision of the sub-wavelength grating structure increases the degree of freedom in the design of the photonic device, and the effective refractive index of the sub-wavelength grating can be changed by changing the duty ratio of the grating, thereby being beneficial to reducing the size of the device and greatly contributing to improving the process tolerance and reducing the reflection loss.
Disclosure of Invention
The technical problem is as follows: in order to solve the defects of the prior art, the invention aims to provide a multimode interference type polarization insensitive power divider based on a sub-wavelength grating structure, which reduces the coupling length by embedding the sub-wavelength grating structure in a central line region of a multimode interference coupling region, realizes the polarization insensitive function, reduces the insertion loss and the reflection loss, and improves the manufacturing tolerance and the bandwidth of a device.
The technical scheme is as follows: the invention provides a multi-mode interference type polarization insensitive power divider based on a sub-wavelength grating structure, which sequentially comprises a buried oxide layer, a power divider component and an upper cladding layer from bottom to top, wherein the upper cladding layer covers the upper surface of the buried oxide layer, and the power divider component horizontally grows on the upper surface of the buried oxide layer and is covered by the upper cladding layer;
the power divider component comprises an input waveguide, an input tapered waveguide, a multi-mode interference coupling area, a first output tapered waveguide, a second output tapered waveguide, a first output waveguide, a second output waveguide, an input end tapered sub-wavelength grating structure and a sub-wavelength grating structure; a sub-wavelength grating structure is etched in the multimode interference coupling region from left to right on a central axis, the left end of the multimode interference coupling region is connected with the wide end of the input tapered waveguide, the narrow end of the input tapered waveguide is connected with the input waveguide, the right end of the multimode interference coupling region is respectively connected with the wide end of the first output tapered waveguide and the wide end of the second output tapered waveguide, the first output tapered waveguide and the second output tapered waveguide are symmetrically arranged with the central axis of the multimode interference coupling region as a symmetry axis, the narrow end of the first output tapered waveguide is connected with the first output waveguide, and the narrow end of the second output tapered waveguide is connected with the second output waveguide;
the tapered sub-wavelength grating structure is etched in the input tapered waveguide, and the wide end of the tapered sub-wavelength grating structure is connected with the sub-wavelength grating structure.
Wherein:
the multimode interference coupling area is connected with one end of the input tapered waveguide, a first isosceles right triangle etching area and a second isosceles right triangle etching area are symmetrically etched on two sides of the end face by a central axis, the etching depth is consistent with the thickness of other parts of the power divider, and the width of the end face after etching is the same as that of the end face of the connected input tapered waveguide.
The input waveguide, the first output waveguide and the second output waveguide are all single-mode waveguides with the same width, and the multimode interference coupling area is a multimode waveguide.
The maximum width of the first output tapered waveguide and the second output tapered waveguide is smaller than that of the input tapered waveguide.
The length of the multimode interference coupling region is equal to the length of the sub-wavelength grating structure; the length of the tapered sub-wavelength grating structure is smaller than that of the input tapered waveguide; the maximum width of the conical sub-wavelength grating is 0.05-0.1 μm as the width of the sub-wavelength grating structure, and the width of the sub-wavelength grating structure is less than 0.05 times of the width of the multimode interference coupling region.
The multimode interference coupling region has the first second ghost of the input light field at the position close to the output tapered waveguide to realize the power distribution function, namely the length of the multimode interference coupling region needs to be 3/8 with beat lengthLong LΠThe calculation formula is as follows:
in the formula beta0And beta2The propagation constants of the two modes of the lowest order excited in the multimode interference coupling region, i.e., the fundamental mode and the second order mode, respectively.
The thickness of each part contained in the power divider part is the same and is 0.18-0.34 μm, and the etching depth of the structures of the tapered sub-wavelength grating and the sub-wavelength grating is the same as the thickness of each part.
The tapered sub-wavelength grating structure and the sub-wavelength grating structure are characterized in that the sub-wavelength grating structure is formed by alternately and periodically arranging silicon with high refractive index and silicon dioxide with low refractive index, the arrangement period and the duty ratio of the tapered sub-wavelength grating structure and the sub-wavelength grating structure are consistent, the duty ratio is that the ratio of the length of a high refractive index area to the period is 0.3-0.4, the period lambada is 0.17-0.18 mu m, and the period lambada satisfies the following formula:
where λ is the operating wavelength, nBIs the effective refractive index of the Bragg grating, nGHIs the refractive index of the high refractive index region in the sub-wavelength grating structure, nGLIs the refractive index of the low refractive index region in the sub-wavelength grating structure, a is the length of the high refractive index region in one period, ΛBraggIs the bragg period.
The buried oxide layer and the upper cladding are made of silicon dioxide, and the power divider component is made of silicon.
The thickness of the buried oxide layer is 2-3 mu m, and the thickness of the upper cladding is larger than that of the power divider component.
When the multi-mode interference type polarization insensitive power divider based on the sub-wavelength grating structure is used, a TE/TM polarized light signal enters a multi-mode interference coupling area through an input waveguide and an input conical waveguide, the input waveguide is located at the center of the multi-mode interference coupling area, the multi-mode interference coupling area only excites a fundamental mode and a second-order mode, the two modes interfere with each other, and a double image of an input light field appears at the position of 3/8 beat length, so that the beam splitting effect is realized. The introduction of the sub-wavelength grating structure changes the propagation constants of the excited fundamental mode and the second order mode in the multi-mode region, thereby shortening the coupling length.
Has the advantages that: compared with the prior art, the invention has the following beneficial effects:
1. the coupling region is short in length and compact in structure. Compared with the traditional multimode interference coupler, the multimode interference coupling region is embedded with the sub-wavelength grating structure, and the beat length of TE polarization is adjusted by adjusting the effective refractive index of the sub-wavelength grating structure, so that the beat length of the TE polarization is consistent with that of the TM polarization under the condition of ensuring that the coupling region is shorter, the compactness of a device is improved, and the integrated design is facilitated;
2. low insertion loss and reflection loss and large working bandwidth. The input waveguide, the output waveguide and the multimode interference coupling region are connected by the tapered waveguide for transition, and the introduction of the tapered waveguide reduces the proportion of the input field coupled into a radiation mode and reduces the loss; embedding a tapered sub-wavelength grating structure in the input end tapered waveguide to prevent reflection caused by different refractive indexes of multimode interference regions of the input end and the embedded sub-wavelength grating structure; meanwhile, triangular areas are etched on two sides of one side, close to the input end, of the multimode interference coupling area, so that reflection loss can be reduced; the multi-mode interference coupler has the characteristic of large working bandwidth, so that the device can keep high performance in the whole C wave band.
3. The process is simple and easy to integrate. The etching depths of the sub-wavelength grating structure and the triangular areas at two sides of the multimode interference coupling area are consistent with the thicknesses of other parts of the device, so that the device can be realized only by single-step photoetching, and the manufacturing difficulty is reduced; the device is manufactured on a silicon-on-chip material platform and is easy to integrate with other traditional photonic devices.
Drawings
Fig. 1 is a schematic structural diagram of a polarization insensitive power divider according to the present invention;
FIG. 2 is a cross-sectional view of a multimode interference coupling region in the polarization insensitive power splitter according to the present invention;
FIG. 3 is a TE polarization transmission mode field distribution diagram according to the present invention; wherein the abscissa represents the dimension in the direction of device transport and the ordinate represents the dimension in the lateral direction of the device.
FIG. 4 is a diagram of the distribution of the TM polarization transmission mode field in the present invention;
FIG. 5 is a graph showing the relationship between the insertion loss and the operating wavelength of the polarization insensitive power divider according to the present invention;
the figure shows that: the waveguide structure comprises an input waveguide 1, an input tapered waveguide 2, a multi-mode interference coupling region 3, a first output tapered waveguide 4, a second output tapered waveguide 5, a first output waveguide 6, a second output waveguide 7, an input end tapered sub-wavelength grating structure 8, a sub-wavelength grating structure 9, a first isosceles right triangle etching region 10, a second isosceles right triangle etching region 11, a buried oxide layer 12 and an upper cladding layer 13.
Detailed Description
The invention combines the sub-wavelength grating structure and the multi-mode interference coupler, utilizes the refractive index adjustability and the birefringence characteristic of the sub-wavelength grating structure to realize the polarization insensitivity of the power divider, has the advantages of low insertion loss, low reflection loss, large working bandwidth, compact structure, convenient manufacture and the like, and adopts the silicon-on-insulator system to facilitate the integration with other devices. Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the same or similar elements or elements having the same or similar function throughout. The embodiments described below with reference to the accompanying drawings are illustrative only for the purpose of explaining the present invention, and are not to be construed as limiting the present invention.
A multimode interference type polarization insensitive power divider (as shown in figures 1 and 2) based on a sub-wavelength grating structure comprises a buried oxide layer 12, a power divider component and an upper cladding 13 from bottom to top in sequence, wherein the upper cladding 13 covers the upper surface of the buried oxide layer 12, the power divider component is horizontally grown on the upper surface of the buried oxide layer 12 and is covered by the upper cladding 13, the buried oxide layer 12 and the upper cladding 13 are made of silicon dioxide, the power divider component is made of silicon, the buried oxide layer 12 is 2 microns in thickness, the power divider component is 0.18 microns-0.34 microns in thickness, and the upper cladding 13 is larger than the power divider component in thickness.
The power divider component comprises an input waveguide 1, an input tapered waveguide 2, a multi-mode interference coupling region 3, a first output tapered waveguide 4, a second output tapered waveguide 5, a first output waveguide 6, a second output waveguide 7, an input end tapered sub-wavelength grating structure 8 and a sub-wavelength grating structure 9; a sub-wavelength grating structure 9 is etched in the multimode interference coupling region 3 from left to right on a central axis, the length of the multimode interference coupling region 3 is equal to that of the sub-wavelength grating structure 9, the left end of the multimode interference coupling region 3 is connected with the wide end of the input tapered waveguide 2, the narrow end of the input tapered waveguide 2 is connected with the input waveguide 1, the right end of the multimode interference coupling region 3 is respectively connected with the wide end of the first output tapered waveguide 4 and the wide end of the second output tapered waveguide 5, the first output tapered waveguide 4 and the second output tapered waveguide 5 are symmetrically arranged with the central axis of the multimode interference coupling region 3 as a symmetry axis, the narrow end of the first output tapered waveguide 4 is connected with the first output waveguide 6, and the narrow end of the second output tapered waveguide 5 is connected with the second output waveguide 7;
a tapered sub-wavelength grating structure 8 is etched in the input tapered waveguide 2, the wide end of the tapered sub-wavelength grating structure 8 is connected with the sub-wavelength grating structure 9, the length of the tapered sub-wavelength grating structure 8 is smaller than that of the input tapered waveguide, the tapered sub-wavelength grating structure 8 which is linearly widened and has the duty ratio consistent with that of the sub-wavelength grating structure 9 is embedded in the input tapered waveguide 2, and reflection loss caused by inconsistent refractive indexes of the input tapered waveguide 2 and the multimode interference coupling region 3 etched with the sub-wavelength grating structure 9 is reduced.
The multimode interference coupling region 3 is connected with the input tapered waveguide 2, a first isosceles right triangle etching region 10 and a second isosceles right triangle etching region 11 are symmetrically etched on two sides of the end face by a central axis, the etching depth is consistent with the thickness of other parts of the power divider, and the width of the end face after etching is the same as that of the end face of the connected input tapered waveguide 2, so that the effect of reducing reflection loss is achieved.
The input waveguide 1 is connected with the multimode interference coupling area 3 by the input tapered waveguide 2, so that the radiation loss is reduced.
The tapered sub-wavelength grating structure 8 with linearly increased width and consistent duty ratio and the sub-wavelength grating structure 9 is embedded in the input tapered waveguide 2, so that the reflection loss caused by inconsistent refractive index of the input tapered waveguide 2 and the multimode interference coupling region 3 etched with the sub-wavelength grating structure 9 is reduced.
The first output waveguide 6 and the second output waveguide 7 are respectively connected with the multimode interference coupling region 3 through the first output tapered waveguide 4 and the second output tapered waveguide 5, and in order to avoid interference influence of two output ends, the maximum width of the first output tapered waveguide 4 and the second output tapered waveguide 5 is slightly smaller than that of the input end tapered waveguide 2.
The tapered sub-wavelength grating 8 and the sub-wavelength grating structure 9 are etched to the same depth as the thickness of each component.
The input waveguide 1, the first output waveguide 6 and the second output waveguide 7 are all single-mode waveguides with the same width, and the multi-mode interference coupling area 3 is a multi-mode waveguide.
The maximum width of the tapered sub-wavelength grating 8 is the same as the width of the sub-wavelength grating structure 9.
The sub-wavelength grating structures in the tapered sub-wavelength grating structure 8 and the sub-wavelength grating structure 9 are formed by alternately and periodically arranging silicon layers with high refractive indexes and silicon dioxide layers with low refractive indexes, the arrangement period and the duty ratio of the grating structures of the tapered sub-wavelength grating structure and the sub-wavelength grating structure are consistent, the duty ratio, namely the ratio of the length of a high refractive index area to the period is 0.3-0.4, the width of the grating is 0.05-0.1 mu m, the period lambda is 0.17-0.18 mu m, and simultaneously the period lambda meets the following formula:
where λ is the operating wavelength, nBIs the effective refractive index of the Bragg grating, nGHRefractive index of high refractive index region of sub-wavelength grating, nGLIs the refractive index of the low refractive index region of the sub-wavelength grating, a is the length of the high refractive index region in one period, ΛBraggIs the bragg period.
In the invention, an optical field is input at the symmetrical center of a multimode interference coupling area 3, imaging in the multimode interference coupling area 3 belongs to centrosymmetric imaging, the multimode interference coupling area 3 has a first second double image of the input optical field at the position close to an output conical waveguide 4 and an output conical waveguide 5 to realize a power distribution function, namely the length of the multimode interference coupling area 3 needs to be set as a beat length LΠ3/8, beat length L of multimode interference coupling region 3ΠCan be driven byIs calculated to obtain beta in the formula0And beta2The propagation constants of the two modes of the lowest order excited in the multimode waveguide of the multimode interference coupling zone 3, respectively. Only a basic mode and a second-order mode in the multimode waveguide under central excitation are excited, so that two modes at the lowest order are the basic mode and the second-order mode, and the sub-wavelength grating structure is embedded in the middle, so that the refractive index of a symmetrically distributed even mode is mainly influenced, and the influence on an odd mode with the middle intensity of 0 is small, and therefore the length of the multimode interference coupler can be effectively changed by the sub-wavelength grating structure. When a TE polarized optical signal enters a waveguide with a sub-wavelength grating structure, power is mainly bound in the sub-wavelength grating structure, and energy in the sub-wavelength grating structure is small when TM polarized light enters, so that the sub-wavelength grating structure has a large influence on beat length when TE polarized light enters and has a small influence on beat length when TM polarized light enters. In the previous design, the modulation of beat length is realized by embedding an incompletely etched nano-groove structure in an axial line region in a multimode interference coupling region, which has higher requirements on the manufacturing process, and a sub-wavelength grating structure is used for replacing the nano-groove structure, the etching depth and other parts of a power dividerThe sub-thicknesses are consistent, only single-step etching is needed, the manufacturing difficulty is reduced, the refractive index can be adjusted by only changing the period and the duty ratio of the sub-wavelength grating, and then the beat length of TE polarization incidence is mainly designed, so that the beat length of the TE polarization incidence and the TM polarization incidence is consistent, and the effect equivalent to the groove waveguide is achieved.
The width of the sub-wavelength grating structure 9 is required to be less than 0.05 times of the width of the multimode interference coupling region.
Example 1
A multimode interference type polarization insensitive power divider (as shown in figures 1 and 2) based on a sub-wavelength grating structure comprises a buried oxide layer 12, a power divider component and an upper cladding 13 from bottom to top in sequence, wherein the upper cladding 13 covers the upper surface of the buried oxide layer 12, the power divider component is horizontally grown on the upper surface of the buried oxide layer 12 and is covered by the upper cladding 13, the buried oxide layer 12 and the upper cladding 13 are made of silicon dioxide, the power divider component is made of silicon, the buried oxide layer 12 is 2 microns in thickness, the power divider component is 0.25 microns in thickness, and the upper cladding 13 is larger than the power divider component in thickness.
The power divider component comprises an input waveguide 1, an input tapered waveguide 2, a multi-mode interference coupling region 3, a first output tapered waveguide 4, a second output tapered waveguide 5, a first output waveguide 6, a second output waveguide 7, an input end tapered sub-wavelength grating structure 8 and a sub-wavelength grating structure 9; a sub-wavelength grating structure 9 is etched in the multimode interference coupling region 3 from left to right on a central axis, the length of the multimode interference coupling region 3 is equal to that of the sub-wavelength grating structure 9, the left end of the multimode interference coupling region 3 is connected with the wide end of the input tapered waveguide 2, the narrow end of the input tapered waveguide 2 is connected with the input waveguide 1, the right end of the multimode interference coupling region 3 is respectively connected with the wide end of the first output tapered waveguide 4 and the wide end of the second output tapered waveguide 5, the first output tapered waveguide 4 and the second output tapered waveguide 5 are symmetrically arranged along the central axis direction of the multimode interference coupling region 3, the narrow end of the first output tapered waveguide 4 is connected with a first output waveguide 6, and the narrow end of the second output tapered waveguide 5 is connected with a second output waveguide 7;
a tapered sub-wavelength grating structure 8 is etched in the input tapered waveguide 2, the wide end of the tapered sub-wavelength grating structure 8 is connected with a sub-wavelength grating structure 9, and the length of the tapered sub-wavelength grating structure 8 is smaller than that of the input tapered waveguide 2; the tapered sub-wavelength grating structure 8 with linearly increased width and consistent duty ratio and the sub-wavelength grating structure 9 is embedded in the input tapered waveguide 2, so that the reflection loss caused by inconsistent refractive index of the input tapered waveguide 2 and the multimode interference coupling region 3 etched with the sub-wavelength grating structure 9 is reduced.
The maximum width of the first output tapered waveguide 4 and the second output tapered waveguide 5 is 0.9 μm and is less than the maximum width of the input tapered waveguide 2 by 1 μm.
The etch depth of the tapered sub-wavelength grating 8 and sub-wavelength grating structure 9 is the same as the thickness of each feature, 0.25 μm.
The input waveguide 1 is connected with the multimode interference coupling area 3 by the input tapered waveguide 2, so that the radiation loss is reduced.
The first output waveguide 6 and the second output waveguide 7 are respectively connected with the multimode interference coupling region 3 through the first output tapered waveguide 4 and the second output tapered waveguide 5, and in order to avoid interference influence of two output ends, the maximum width of the first output tapered waveguide 4 and the second output tapered waveguide 5 is slightly smaller than that of the input end tapered waveguide 2.
The multimode interference coupling region 3 is connected with the input tapered waveguide 2, a first isosceles right triangle etching region 10 and a second isosceles right triangle etching region 11 are symmetrically etched on two sides of the end face by a central axis, the etching depth is consistent with the thickness of other parts of the power divider, and the width of the end face after etching is the same as that of the end face of the connected input tapered waveguide 2, so that the effect of reducing reflection loss is achieved.
The input waveguide 1, the first output waveguide 6 and the second output waveguide 7 are all single-mode waveguides with the same width, and the multi-mode interference coupling area 3 is a multi-mode waveguide.
The maximum width of the tapered sub-wavelength grating 8 is 0.05 μm as the width of the sub-wavelength grating structure 9, and the width of the multimode interference coupling region 3 is 2.2 μm.
The sub-wavelength grating structures in the conical sub-wavelength grating structures 8 and 9 are formed by alternately and periodically arranging silicon layers with high refractive indexes and silicon dioxide layers with low refractive indexes, the arrangement period lambda is 0.18 mu m, and the proportion of the length of a high refractive index area in the period, namely the duty ratio is 0.31.
The sub-wavelength grating structures in the tapered sub-wavelength grating structure 8 and the sub-wavelength grating structure 9 are formed by alternately and periodically arranging a silicon layer with a high refractive index and a silicon dioxide layer with a low refractive index, the arrangement period and the duty cycle of the grating structures of the tapered sub-wavelength grating structure and the sub-wavelength grating structure are consistent, the duty cycle, namely the ratio of the length of a high refractive index area to the period, is 0.31, the period lambada is 0.18 mu m, and the period lambada satisfies the following formula:
where λ is the operating wavelength, nBIs the effective refractive index of the Bragg grating, nGHRefractive index of high refractive index region of sub-wavelength grating, nGLIs the refractive index of the low refractive index region of the sub-wavelength grating, a is the length of the high refractive index region in one period, ΛBraggIs the bragg period.
Fig. 3 and 4 are mode field diagrams of the multimode interference type polarization insensitive power divider based on the sub-wavelength grating structure when TE polarization incidence and TM polarization incidence are respectively performed, which are combined to illustrate the polarization insensitive characteristic of the power divider, and it is obvious that a considerable portion of light energy is bound in the sub-wavelength grating structure when TE polarization incidence occurs, so that the insertion loss of TE polarization incidence is slightly higher than that of TM polarization incidence.
FIG. 5 is a graph of insertion loss versus operating wavelength for TE-polarization incident and TM-polarization incident multimode interference-type polarization insensitive power splitters based on sub-wavelength grating structures, where the insertion loss IL (dB) is related to input power and output power and is defined as
In the formula Poutput1And Poutput2The output power, P, of the first output waveguide 6 and the second output waveguide 7, respectivelyinputFor the input power to the waveguide 1, it can be seen that the insertion loss for both TE polarization incidence and TM polarization incidence is kept below 0.3dB in the wavelength range from 1500nm to 1600nm, and at the operating wavelength of 1550nm, the insertion loss for TE polarization incidence is 0.246dB, slightly higher than the insertion loss for TM polarization incidence by 0.11dB, thus demonstrating that the device can operate at higher performance in the C-band.
The above embodiments are only for illustrating the technical idea of the present invention, and the protection scope of the present invention is not limited thereby, and any modifications made on the basis of the technical scheme according to the technical idea of the present invention fall within the protection scope of the present invention.
Claims (10)
1. A multimode interference type polarization insensitive power divider based on a sub-wavelength grating structure is characterized in that: the power divider is integrally provided with a buried oxide layer (12), a power divider component and an upper cladding layer (13) from bottom to top in sequence, the upper cladding layer (13) covers the upper surface of the buried oxide layer (12), and the power divider component horizontally grows on the upper surface of the buried oxide layer (12) and is covered by the upper cladding layer (13);
the power divider component comprises an input waveguide (1), an input tapered waveguide (2), a multi-mode interference coupling region (3), a first output tapered waveguide (4), a second output tapered waveguide (5), a first output waveguide (6), a second output waveguide (7), an input end tapered sub-wavelength grating structure (8) and a sub-wavelength grating structure (9); a sub-wavelength grating structure (9) is etched in the multimode interference coupling region (3) from left to right on the central axis, the left end of the multimode interference coupling region (3) is connected with the wide end of the input tapered waveguide (2), the narrow end of the input tapered waveguide (2) is connected with the input waveguide (1), the right end of the multimode interference coupling region (3) is respectively connected with the wide end of the first output tapered waveguide (4) and the wide end of the second output tapered waveguide (5), the first output tapered waveguide (4) and the second output tapered waveguide (5) are symmetrically arranged with the central axis of the multimode interference coupling region (3) as a symmetry axis, the narrow end of the first output tapered waveguide (4) is connected with a first output waveguide (6), and the narrow end of the second output tapered waveguide (5) is connected with a second output waveguide (7);
a tapered sub-wavelength grating structure (8) is etched in the input tapered waveguide (2), and the wide end of the tapered sub-wavelength grating structure (8) is connected with a sub-wavelength grating structure (9).
2. The multi-mode interference type polarization insensitive power divider based on the sub-wavelength grating structure as claimed in claim 1, characterized in that: the multimode interference coupling region (3) is connected with the input tapered waveguide (2), a first isosceles right triangle etching region (10) and a second isosceles right triangle etching region (11) are symmetrically etched on two sides of the end face by a central axis, the etching depth is consistent with the thickness of other parts of the power divider, and the width of the end face after etching is the same as the width of the end face of the connected input tapered waveguide (2).
3. The multi-mode interference type polarization insensitive power divider based on the sub-wavelength grating structure as claimed in claim 1, characterized in that: the input waveguide (1), the first output waveguide (6) and the second output waveguide (7) are all single-mode waveguides with the same width, and the multimode interference coupling region (3) is a multimode waveguide.
4. The multi-mode interference type polarization insensitive power divider based on the sub-wavelength grating structure as claimed in claim 1, characterized in that: the maximum width of the first output tapered waveguide (4) and the second output tapered waveguide (5) is smaller than that of the input tapered waveguide (2).
5. The multi-mode interference type polarization insensitive power divider based on the sub-wavelength grating structure as claimed in claim 1, characterized in that: the length of the multimode interference coupling region (3) is equal to that of the sub-wavelength grating structure (9); the length of the tapered sub-wavelength grating structure (8) is smaller than that of the input tapered waveguide (2); the maximum width of the conical sub-wavelength grating (8) is 0.05-0.1 mu m the same as the width of the sub-wavelength grating structure (9), and the width of the sub-wavelength grating structure (9) is less than 0.05 times of the width of the multimode interference coupling region (3).
6. The multi-mode interference type polarization insensitive power divider based on the sub-wavelength grating structure as claimed in claim 1, characterized in that: the length of the multimode interference coupling region (3) is 3/8 beat length LΠThe calculation formula is as follows:
in the formula beta0And beta2Propagation constants of two modes of the lowest order excited in the multimode interference coupling region (3), namely a fundamental mode and a second-order mode, respectively.
7. The multi-mode interference type polarization insensitive power divider based on the sub-wavelength grating structure as claimed in claim 1, characterized in that: the thicknesses of all parts contained in the power divider part are the same and are all 0.18-0.34 mu m, and the etching depths of the tapered sub-wavelength grating (8) and the sub-wavelength grating structure (9) are the same as the thicknesses of all parts.
8. The multi-mode interference type polarization insensitive power divider based on the sub-wavelength grating structure as claimed in claim 1, characterized in that: the buried oxide layer (12) and the upper cladding layer (13) are made of silicon dioxide, and the power divider component is made of silicon.
9. The multi-mode interference type polarization insensitive power divider based on the sub-wavelength grating structure as claimed in claim 1, characterized in that: the sub-wavelength grating structures in the conical sub-wavelength grating structure (8) and the sub-wavelength grating structure (9) are formed by alternately and periodically arranging silicon with high refractive index and silicon dioxide with low refractive index, the arrangement period and the duty cycle of the grating structures of the conical sub-wavelength grating structure (8) and the sub-wavelength grating structure (9) are consistent, the duty cycle, namely the proportion of the length of a high refractive index area to the period is 0.3-0.4, the period Lambda is 0.17-0.18 mu m, and the period Lambda meets the following formula:
where λ is the operating wavelength, nBIs the effective refractive index of the Bragg grating, nGHIs the refractive index of the high refractive index region in the sub-wavelength grating structure, nGLIs the refractive index of the low refractive index region in the sub-wavelength grating structure, a is the length of the high refractive index region in one period, ΛBraggIs the bragg period.
10. The multi-mode interference type polarization insensitive power divider based on the sub-wavelength grating structure as claimed in claim 1, characterized in that: the thickness of the buried oxide layer (12) is 2-3 mu m, and the thickness of the upper cladding (13) is larger than that of the power divider component.
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