Polarization rotator based on sub-wavelength grating
By setting a polarization rotator with a sub-wavelength grating above the progressively widened second waveguide segment in the silicon oxide cladding, the vertical symmetry of the silicon waveguide is broken, and the optical signal conversion from TM0 mode to TE1 mode is realized, solving the problem of modal signal conversion in the silicon waveguide while having multiple advantages.
Patent Information
- Application Number
- CN202510181439.7
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-02-19
- Publication Date
- 2025-05-02
AI Technical Summary
In silicon waveguides in silicon oxide cladding, it is difficult to convert a modal signal, especially from TM0 mode to TE1 mode.
A polarization rotator based on a sub-wavelength grating is designed. By setting a sub-wavelength grating above the progressively widened second waveguide segment in the silicon oxide cladding, the vertical symmetry of the silicon waveguide is broken, thereby realizing the disturbance of light in the optical waveguide and converting the TMO mode signal into the TE1 mode.
The effective conversion of optical modes in silicon waveguides is achieved, and the problem of difficulty in achieving modal signal conversion in silicon oxide cladding is solved. It also has the advantages of simple processing, large manufacturing tolerance, low loss, and high integration.
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Figure CN119916529A_ABST
Abstract
Description
Technical Field
[0001] The invention relates to the technical field of integrated photonics, and in particular to a polarization rotator based on a sub-wavelength grating. Background Art
[0002] In recent years, silicon-based optoelectronics has been regarded as one of the most promising integrated technologies due to its high refractive index contrast and compatibility with complementary metal oxide semiconductor (CMOS) technology. The top silicon (TopSi) and buried oxide (B0X) on silicon on insulator (SOI) have a large refractive index difference, which can well confine the light field in the silicon waveguide. At the same time, the high refractive index contrast is also accompanied by a strong birefringence effect, resulting in a strong polarization sensitivity of the silicon waveguide. The birefringence effect has both advantages and disadvantages for photonic integrated circuits (PICs): on the one hand, it will increase the difficulty of coupling the chip and the optical fiber, because the polarization state changes randomly in ordinary optical fibers, and the polarization state in the optical fiber needs to be controlled before the light is coupled from the optical fiber to the chip; on the other hand, polarization sensitivity can be used to increase the transmission dimension of the optical chip, such as multiplexing the TE0 and TM0 modes, which can greatly improve the transmission capacity of the PIC.
[0003] In order to facilitate the transmission and modulation of the light field in the chip, it is usually necessary to convert the TM0 mode to the TE0 mode, and before converting to the TE0 mode, it is generally necessary to convert the TM0 mode to the TE1 mode. The process of converting the TM0 mode to the TE1 mode requires breaking the mode symmetry of the waveguide. If the cladding is air, the silicon strip waveguide can achieve mode conversion through a directional coupler; if the cladding is silicon oxide, the refractive index of the cladding and the buried oxide layer is similar, and the silicon waveguide can maintain good mode symmetry, making it difficult to achieve mode rotation.
[0004] Therefore, in the case that the silicon waveguide in the silicon oxide cladding has good polarization maintaining properties, how to achieve the conversion of modal signals becomes an urgent problem to be solved. Summary of the invention
[0005] The purpose of the present invention is to provide a polarization rotator based on a sub-wavelength grating to solve the problem in the prior art that silicon waveguides in silicon oxide claddings are difficult to achieve modal signal conversion.
[0006] In particular, the present invention provides a polarization rotator based on a sub-wavelength grating, comprising:
[0007] A silicon substrate, wherein the upper surface of the silicon substrate is provided with a silicon oxide buried oxide layer and a silicon oxide cladding layer in order from bottom to top;
[0008] An optical waveguide, wherein the optical waveguide is embedded in the silicon oxide cladding, and the optical waveguide comprises a first waveguide segment, a second waveguide segment, and a third waveguide segment connected in sequence;
[0009] Wherein, the width of the first waveguide segment between the first end and the second end along the first direction is constant;
[0010] The first end of the second waveguide segment continues the second end of the first waveguide segment, the width of the second waveguide segment gradually increases linearly from the first end to the second end along the first direction, and a sub-wavelength grating periodically arranged along the first direction is further provided above the second waveguide segment in the silicon oxide cladding;
[0011] The first end of the third waveguide segment is continuous with the second end of the second waveguide segment, and the width of the third waveguide segment between the first end and the second end along the first direction is constant.
[0012] Optionally, the sub-wavelength grating and the second waveguide segment are arranged vertically and spaced apart in the silicon oxide cladding, and the spacing distance is 50-100 nm.
[0013] Optionally, the sub-wavelength grating comprises a plurality of grating bars periodically arranged along the first direction.
[0014] A plurality of the grating strips are arranged in the silicon oxide cladding layer parallel to the second waveguide segment and along a direction perpendicular to the first direction.
[0015] Optionally, a vertical projection area of the second waveguide segment completely falls within a vertical projection area of the sub-wavelength grating.
[0016] Optionally, the symmetry axes of the plurality of grating strips along the first direction coincide with the symmetry axes of the first waveguide segment, the second waveguide segment and the third waveguide segment along the first direction.
[0017] Optionally, the plurality of grating strips are of equal length, and the length of the grating strips is greater than the width of the third waveguide segment.
[0018] Optionally, the lengths of the plurality of grating strips linearly widen along the first direction, and the length of the first grating strip close to the first waveguide segment is greater than the width of the first waveguide segment.
[0019] Optionally, the sum of the width of a single grating along the first direction and the width of the interval between adjacent gratings is less than half the wavelength of the transmitted light.
[0020] An average refractive index of the plurality of grating strips is smaller than a refractive index of the second waveguide segment.
[0021] Optionally, the gate bars are made of polysilicon, and the thickness of the plurality of gate bars is smaller than the thickness of the first waveguide segment, the second waveguide segment, and the third waveguide segment.
[0022] Optionally, the grid bars are made of silicon nitride, and the thickness of the plurality of grid bars is greater than the thickness of the first waveguide segment, the second waveguide segment, and the third waveguide segment.
[0023] The embodiment of the present invention provides a polarization rotator based on a sub-wavelength grating, which breaks the vertical symmetry of the silicon waveguide by arranging a sub-wavelength grating above the gradually widened second waveguide segment in the silicon oxide cladding, and achieves disturbance of the light in the optical waveguide, so that the input TMO mode signal undergoes mode rotation when passing through the second waveguide segment, and is converted into a TE1 mode signal output. The polarization rotator of the sub-wavelength grating of the embodiment of the present invention has the advantages of simple processing, large manufacturing tolerance, low loss, high integration, etc.
[0024] Based on the following detailed description of specific embodiments of the present invention in conjunction with the accompanying drawings, those skilled in the art will become more aware of the above and other objects, advantages and features of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS
[0025] Hereinafter, some specific embodiments of the present invention will be described in detail in an exemplary and non-limiting manner with reference to the accompanying drawings. The same reference numerals in the accompanying drawings indicate the same or similar components or parts. It should be understood by those skilled in the art that these drawings are not necessarily drawn to scale. In the accompanying drawings:
[0026] Figure 1 is a schematic structural diagram of a polarization rotator based on a sub-wavelength grating according to a specific embodiment of the present invention;
[0027] Figure 2 is a three-dimensional structural diagram of an optical waveguide according to a specific embodiment of the present invention;
[0028] Figure 3 is a front view of an optical waveguide according to a specific embodiment of the present invention;
[0029] Figure 4 yes Figure 3 A top view of
[0030] Figure 5 is a schematic structural diagram of a sub-wavelength grating according to a specific embodiment of the present invention;
[0031] Figure 6 is a schematic diagram of mode field propagation in an optical waveguide according to a specific embodiment of the present invention;
[0032] Figure 7 is a TM0 mode field distribution diagram in the first waveguide section according to a specific embodiment of the present invention;
[0033] Figure 8 It is a TE1 mode field distribution diagram in the third waveguide segment according to a specific embodiment of the present invention.
[0034] Description of reference numerals:
[0035] Silicon substrate - 100; silicon oxide buried oxide layer - 200; silicon oxide cladding layer - 300;
[0036] Optical waveguide 400 ; first waveguide segment 410 ; second waveguide segment 420 ; third waveguide segment 430 ; sub-wavelength grating 440 ; grating strip 441 . DETAILED DESCRIPTION
[0037] In the description of this embodiment, it should be understood that the terms "length", "width", "height", "up", "down", "left", "right", "vertical", "horizontal", "bottom", "inside" and "outside" etc. indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present invention.
[0038] As a specific embodiment of the present invention, Figure 1 and Figure 2 As shown, the present invention provides a polarization rotator based on a sub-wavelength grating, which may include a silicon substrate 100 and an optical waveguide 400. The upper surface of the silicon substrate 100 is provided with a silicon oxide buried oxide layer 200 and a silicon oxide cladding 300 in sequence from bottom to top. The optical waveguide 400 is embedded in the silicon oxide cladding 300, and the optical waveguide 400 includes a first waveguide segment 410, a second waveguide segment 420 and a third waveguide segment 430 connected in sequence. The width of the first waveguide segment 410 between the first end and the second end along the first direction is constant. The first end of the second waveguide segment 420 continues the second end of the first waveguide segment 410, and the width of the second waveguide segment 420 gradually increases linearly from the first end to the second end along the first direction. A sub-wavelength grating 440 periodically arranged along the first direction is also provided above the second waveguide segment 420 in the silicon oxide cladding 300. The first end of the third waveguide segment 430 is continuous with the second end of the second waveguide segment 420 , and the width of the third waveguide segment 430 between the first end and the second end along the first direction is constant.
[0039] Specifically, the polarization rotator of the sub-wavelength grating of the embodiment of the present invention embeds an optical waveguide 400 composed of a first waveguide segment 410, a second waveguide segment 420 and a third waveguide segment 430 connected in sequence in the silicon oxide cladding 300. The first waveguide segment 410 has a constant width along the length direction of the optical waveguide 400, and the first end of the second waveguide segment 420 continues the second end of the first waveguide segment 410 (that is, the first end of the second waveguide segment 420 has the same width as the second end of the first waveguide segment 410). The first waveguide segment 410 serves as an input waveguide, and the TM0 mode signal is input by the first waveguide segment 410 (refer to Figure 7 As shown, it represents the TM0 mode field, and the mode field intensity is mainly concentrated on the upper and lower sides of the waveguide). The width of the second waveguide section 420 gradually increases from the first end to the second end along the length direction of the optical waveguide 400. The second waveguide section 420 serves as a conversion waveguide to convert the TM0 mode signal input by the first waveguide section 410 into a TE1 mode signal. Figure 6 As shown, a sub-wavelength grating 440 periodically arranged along the length direction of the optical waveguide 400 is embedded above the second waveguide segment 420 in the silicon oxide cladding 300, thereby disturbing the light in the optical waveguide 400, breaking the vertical symmetry of the silicon waveguide, so that the TM0 mode can be converted to the TE1 mode. It should be noted that Figure 6 This is a schematic diagram of field propagation. It can be clearly seen that the propagation field is converted from TM0 to TE1, and the light is changed from one concentrated area to two concentrated areas.
[0040] The first end of the third waveguide segment 430 is continuous with the second end of the second waveguide segment 420 (i.e., the first end of the third waveguide segment 430 is equal to the width of the second waveguide segment 420), and the width of the third waveguide segment 430 along the length direction of the optical waveguide 400 is constant. The third waveguide segment 430 is used as an output waveguide to output the TE1 mode signal converted by the second waveguide segment 420 (refer to Figure 8 As shown, it shows the TE1 mode field distribution, the mode field is mainly concentrated in the waveguide, and has two peaks). If the optical signal is input into the first waveguide section 410 in the TE0 mode polarization state, the output light of the third waveguide section 430 is still in the TE0 mode.
[0041] In addition, it should be noted that the first waveguide section 410, the second waveguide section 420 and the third waveguide section 430 in the optical waveguide 400 of the embodiment of the present invention are an integrated structure, and the thickness along the length direction is consistent. They are etched from the top silicon of the SOI wafer. The materials are all single crystal silicon, and the center line structure along the length direction is symmetrical. The thickness of the optical waveguide 400 is determined by the wafer material itself, and the common thickness is 150 to 300nm. The sub-wavelength grating 440 is deposited, grown and etched by the CMOS process. The thickness is determined by the deposition process, and the typical value is 80 to 150nm. The thickness of the silicon oxide buried oxide layer 200 of the SOI in the embodiment of the present invention is determined by the wafer material itself (the typical thickness is 2 microns to 3 microns), and the thickness of the silicon oxide cladding is determined by the deposition growth of the CMOS process.
[0042] That is, the polarization rotator of the sub-wavelength grating of the embodiment of the present invention breaks the vertical symmetry of the silicon waveguide by arranging the sub-wavelength grating 440 above the gradually widened second waveguide segment 420 in the silicon oxide cladding 300, and achieves perturbation of the light in the optical waveguide 400, so that the TMO mode signal undergoes mode rotation when passing through the second waveguide segment 420 and is converted into a TE1 mode signal. In addition, the polarization rotator of the sub-wavelength grating of the embodiment of the present invention has the advantages of simple processing, large manufacturing tolerance, low loss, high integration, etc.
[0043] It should be noted that one condition for the conversion of TM0 and TE1 modes is that the refractive indexes of the two modes are equal, that is, the refractive indexes match. When the narrow waveguide TM0 in the second waveguide segment 420 gradually widens, the refractive indexes of TM0 and TE1 will gradually approach, and finally there is a mixed state that makes the refractive indexes of the two modes equal or approximately equal. The linearly widened second waveguide segment 420 is designed to achieve the equality of the refractive indexes of the two modes without introducing additional losses. If a waveguide structure with a "sudden width change" is designed, the scattering loss of the introduced light at the sudden interface will result.
[0044] As a specific embodiment of the present invention, Figure 1 and Figure 3 As shown, the sub-wavelength grating 440 and the second waveguide segment 420 are arranged vertically and spaced apart in the silicon oxide cladding layer 300, and the spacing distance is 50-100 nm.
[0045] Specifically, the upper and lower spacing between the sub-wavelength grating 440 and the second waveguide segment 420 should not be too large. If it is too large, the effect of the sub-wavelength grating 440 on breaking the mode symmetry of the second waveguide segment 420 will be very small, resulting in a long length of the region required for mode conversion in the second waveguide segment 420 (i.e., the length of the second waveguide segment 420); if it is too small, process control will be difficult, resulting in low overall production efficiency.
[0046] As a specific embodiment of the present invention, Figure 1 and Figure 5 As shown, the sub-wavelength grating 440 includes a plurality of grating strips 441 periodically arranged along a first direction, and the plurality of grating strips 441 are arranged in the silicon oxide cladding layer 300 parallel to the second waveguide segment 420 and perpendicular to the first direction.
[0047] Specifically, the spaced arrangement of the grating strips 441 can reduce the equivalent refractive index of the sub-wavelength grating 440, reduce the binding capacity of the light field, and prevent light from being transmitted from the optical waveguide 400 to the sub-wavelength grating 440, with low loss. It should be noted that another condition for the conversion of the TM0 and TE1 modes is that the mode conversion overlap is not 0. The present invention uses spaced sub-wavelength gratings to break the symmetry and achieve a mode conversion overlap that is not 0, and no additional loss is introduced. In addition, the grating strips 441 are arranged along a length direction perpendicular to the optical waveguide 400, and the effect is better than an inclined setting.
[0048] As a specific embodiment of the present invention, Figure 4 As shown, the vertical projection area of the second waveguide segment 420 completely falls within the vertical projection area of the sub-wavelength grating 440. In other words, the entire occupied surface of the second waveguide segment 420 falls below the occupied surface of the sub-wavelength grating 440, thereby ensuring that the light in the second waveguide segment 420 is fully disturbed by the sub-wavelength grating 440, thereby ensuring the reliability of signal mode conversion.
[0049] Preferably, the symmetry axes of the plurality of grating strips 441 along the first direction coincide with the symmetry axes of the first waveguide segment 410 , the second waveguide segment 420 , and the third waveguide segment 430 along the first direction.
[0050] That is, the sub-wavelength grating 440 and the three waveguide segments of the first waveguide segment 410 , the second waveguide segment 420 and the third waveguide segment 430 are symmetrical along the length direction of the optical waveguide 400 , and the processing is simple.
[0051] As a specific embodiment of the present invention, Figure 4 As shown, the plurality of grating strips 441 have the same length, and the length of the grating strips 441 is greater than the width of the third waveguide section 430. Thus, it can be ensured that the plurality of grating strips 441 completely cover the area of the second waveguide section 420.
[0052] As a specific embodiment of the present invention, the lengths of the plurality of grating strips 441 linearly widen along the first direction, and the length of the first grating strip 441 close to the first waveguide segment 410 is greater than the width of the first waveguide segment 410 .
[0053] That is to say, in order to adapt to the width of the second waveguide segment 420, the sub-wavelength grating 440 can also be designed in a form where the grating strips 441 gradually widen along the length direction from the first end to the second end. In this way, while ensuring that the second waveguide segment 420 is fully covered, the material used for the sub-wavelength grating 440 can be reduced, thereby reducing costs.
[0054] Preferably, the sum of the width of a single grating strip along the first direction and the width of the interval between adjacent grating strips is less than half the wavelength of the transmitted light, and the average refractive index of the plurality of grating strips is less than the refractive index of the second waveguide segment.
[0055] As an example, Figure 5 As shown, the refractive index of a single grating 441 is n1, and the width is w1, the refractive index of the interval filling material, i.e., the silicon oxide cladding, is n2, and the width (i.e., the interval distance between adjacent gratings 441) is w2, wherein w1+w2 needs to be less than half of the wavelength of the transmitted light, and the average refractive index of the periodically arranged multiple gratings 441 is n1*w1 / (w1+w2)+n2*w2 / (w1+w2), i.e., n1*w1 / (w1+w2)+n2*w2 / (w1+w2) is less than the refractive index of the second waveguide segment 420. Therefore, the present invention can reduce the equivalent refractive index of the grating through the interval arrangement of the grating, reduce the binding ability of the light field, and avoid the transmission of light from the silicon waveguide to the sub-wavelength grating.
[0056] As a specific embodiment of the present invention, the gate bars 441 are made of polysilicon, and the thickness of the plurality of gate bars 441 is smaller than the thickness of the first waveguide segment 410 , the second waveguide segment 420 , and the third waveguide segment 430 .
[0057] As a specific embodiment of the present invention, the gate strips 441 are made of silicon nitride, and the thickness of the plurality of gate strips 441 is greater than the thickness of the first waveguide segment 410 , the second waveguide segment 420 , and the third waveguide segment 430 .
[0058] Specifically, the refractive index of silicon is about 3.7, the refractive index of silicon nitride is about 1.9, and the refractive index of silicon oxide is about 1.45. In order to break the TM0 symmetry, a certain refractive index effect is required. The refractive index of silicon nitride material itself is small, and the thickness size needs to be increased to increase the effect on the refractive index.
[0059] In addition, the polarization rotator of the embodiment of the present invention can also achieve better process robustness by designing a specific width and period for the sub-wavelength grating 440 of polysilicon material.
[0060] It should be noted that the polarization rotator of the embodiment of the present invention is a reciprocal device, so if the TE1 modal optical signal is incident from the third waveguide segment, it can be efficiently converted into a TM0 modal optical signal through the second waveguide segment and output from the first waveguide segment.
[0061] At this point, those skilled in the art should recognize that, although multiple exemplary embodiments of the present invention have been shown and described in detail herein, many other variations or modifications that conform to the principles of the present invention can still be directly determined or derived based on the content disclosed in the present invention without departing from the spirit and scope of the present invention. Therefore, the scope of the present invention should be understood and recognized as covering all these other variations or modifications.
Claims
1. A polarization rotator based on a sub-wavelength grating, characterized in that: include: A silicon substrate, wherein the upper surface of the silicon substrate is provided with a silicon oxide buried oxide layer and a silicon oxide cladding layer in order from bottom to top; An optical waveguide, wherein the optical waveguide is embedded in the silicon oxide cladding, and the optical waveguide comprises a first waveguide segment, a second waveguide segment, and a third waveguide segment connected in sequence; Wherein, the width of the first waveguide segment between the first end and the second end along the first direction is constant; The first end of the second waveguide segment continues the second end of the first waveguide segment, the width of the second waveguide segment gradually increases linearly from the first end to the second end along the first direction, and a sub-wavelength grating periodically arranged along the first direction is further provided above the second waveguide segment in the silicon oxide cladding; The first end of the third waveguide segment is continuous with the second end of the second waveguide segment, and the width of the third waveguide segment between the first end and the second end along the first direction is constant.
2. The polarization rotator based on sub-wavelength grating according to claim 1, characterized in that: The sub-wavelength grating and the second waveguide segment are arranged vertically and spaced apart in the silicon oxide cladding, and the spacing distance is 50-100 nm.
3. The sub-wavelength grating-based polarization rotator according to claim 2, characterized in that: The sub-wavelength grating comprises a plurality of grating bars periodically arranged along the first direction. A plurality of the grating strips are arranged in the silicon oxide cladding layer parallel to the second waveguide segment and along a direction perpendicular to the first direction.
4. The sub-wavelength grating-based polarization rotator according to claim 3, characterized in that: The vertical projection area of the second waveguide segment completely falls within the vertical projection area of the sub-wavelength grating.
5. The sub-wavelength grating-based polarization rotator according to claim 4, characterized in that: The symmetry axes of the plurality of grating strips along the first direction coincide with the symmetry axes of the first waveguide segment, the second waveguide segment, and the third waveguide segment along the first direction.
6. The sub-wavelength grating-based polarization rotator according to claim 5, characterized in that: The plurality of grid bars are of equal length, and the length of the grid bars is greater than the width of the third waveguide segment.
7. The sub-wavelength grating-based polarization rotator according to claim 5, characterized in that: The lengths of the plurality of grid bars linearly widen along the first direction, The length of the first grating strip close to the first waveguide segment is greater than the width of the first waveguide segment.
8. The sub-wavelength grating-based polarization rotator according to claim 3, characterized in that: The sum of the width of a single grating along the first direction and the width of the interval between adjacent gratings is less than half the wavelength of the transmitted light, An average refractive index of the plurality of grating strips is smaller than a refractive index of the second waveguide segment.
9. The sub-wavelength grating-based polarization rotator according to claim 3, characterized in that: The gate bars are made of polysilicon. The thickness of the plurality of grating strips is smaller than the thickness of the first waveguide segment, the second waveguide segment, and the third waveguide segment.
10. The sub-wavelength grating-based polarization rotator according to claim 3, characterized in that: The grid bars are made of silicon nitride, The thickness of the plurality of grating strips is greater than the thickness of the first waveguide segment, the second waveguide segment, and the third waveguide segment.