Polarization-independent directional coupler based on subwavelength grating structure

Abstract

The present invention discloses a polarization-independent directional coupler based on a subwavelength grating structure. The coupler comprises two mirror-image waveguides, each of which includes an input straight waveguide, an input tapered waveguide, a coupling straight waveguide, an output tapered waveguide, and an output straight waveguide connected in sequence. The width of the input straight waveguide is the same as that of the output straight waveguide, and the width of the input straight waveguide is greater than that of the coupling straight waveguide. The two sides of the input tapered waveguide, the coupling straight waveguide, and the output tapered waveguide are respectively connected to a first grating waveguide and a second grating waveguide, both of which are subwavelength gratings. The duty cycle of the first grating waveguide is greater than that of the second grating waveguide. This directional coupler fully utilizes the characteristics of the subwavelength grating in regulating the refractive index of the mode and enhancing evanescent coupling. While achieving polarization-independent coupling, it can further reduce the size of the device, providing a key component for future large-scale, high-density integrated polarization-independent systems.

Description

Technical Field

[0001] The present invention belongs to the technical field of integrated optics and relates to a polarization-independent directional coupler, in particular to a polarization-independent directional coupler based on a sub-wavelength grating structure. Background Art

[0002] Since the 1960s, building on the development trajectory of integrated circuits accurately predicted by Moore's Law, the minimum linewidth of CMOS has now reached the nanometer scale, approaching its physical limits. The channel length of MOS transistors is difficult to shorten further, and currently, higher-density integration can only be achieved by modifying the MOS channel structure. Furthermore, traditional integrated circuits are affected by electromagnetic noise and RC delay, which are difficult to address with interconnection methods. Driven by a range of industries such as the digital economy, big data, artificial intelligence, and cloud computing, the amount of data in data centers has continued to grow rapidly. Existing integrated circuits are also unable to meet the hardware requirements of this growing data volume. The number of data centers in my country continues to grow rapidly, but compared to this explosive growth in data volume, the establishment of new data centers still cannot meet the data processing needs, and building data centers is not the best solution. Therefore, to increase transmission bandwidth and speed while avoiding the high power consumption and heat dissipation costs associated with electrical interconnects, major data centers are turning to optical interconnect solutions with greater development prospects, higher speeds, and greater reliability.

[0003] Light waves are extremely high-frequency electromagnetic waves. Compared to electrons, photons have zero rest mass and zero charge, which means that using light as a carrier for signal transmission results in very low energy loss. Furthermore, light, as an electromagnetic wave, possesses abundant bandwidth resources and can serve as a carrier for high-capacity communications. Optical carriers can be modulated in multiple dimensions, such as polarization, mode, and wavelength, further increasing communication capacity, multiplying it.

[0004] Polarization manipulation is often necessary during signal transmission. Due to cross-sectional asymmetry, the mode fields of the transverse electric (TE) and transverse magnetic (TM) modes are distributed differently within the waveguide. These differing mode field distributions and refractive index differences can lead to a series of polarization-dependent issues during TE and TM mode transmission. Two approaches have been proposed to address these polarization-dependent issues. The first involves polarization diversity, which typically utilizes polarization beam splitters and polarization rotators to convert one polarization state into another (typically, converting the TM0 mode into the TE0 mode). This approach then combines this with existing single-polarization-state devices to achieve polarization independence. However, the additional use of polarization beam splitters and polarization rotators increases system complexity and size. The second approach requires more careful design of device structure and parameters to ensure polarization independence. This approach does not increase system size or complexity, but requires additional considerations and a more complex design.

[0005] The various polarization-independent devices with different functions in the prior art include polarization-independent filters, polarization-independent 3dB power splitters, and polarization-independent directional couplers. Among them, the polarization-independent directional coupler is a key component for building a polarization-independent system, and can achieve optical power output with arbitrary power distribution ratios based on this device. In 2016, Lu Liu et al. proposed in the article "Subwavelength-grating-assisted broadband polarization-independent directional coupler" that a polarization-independent directional coupler can be achieved by introducing a subwavelength grating inside a waveguide. The directional coupler is designed on a silicon-on-insulator platform. By changing the duty cycle and grating tooth width of the grating, the effective refractive index corresponding to the two polarization states is changed, so that both polarization states meet the coupling conditions and achieve polarization-independent directional coupling. Because the evanescent coupling on the lithium niobate silicon nitride platform is weaker than that on the silicon-on-insulator platform, the conventional device structure design on the lithium niobate silicon nitride platform in the prior art will result in a larger device size. Summary of the Invention

[0006] The purpose of the present invention is to provide a polarization-independent directional coupler based on a subwavelength grating structure. By introducing a subwavelength grating structure outside the waveguide, the device size is reduced and the system compactness on the lithium niobate silicon nitride platform is further improved.

[0007] The technical solution adopted by the present invention is: a polarization-independent directional coupler based on a subwavelength grating structure, comprising two waveguides arranged in a mirror image, each waveguide comprising an input straight waveguide, an input tapered waveguide, a coupling straight waveguide, an output tapered waveguide, and an output straight waveguide connected in sequence, the width of the input straight waveguide being the same as that of the output straight waveguide, and the width of the input straight waveguide being greater than that of the coupling straight waveguide; in one waveguide, a side of the input tapered waveguide facing the other waveguide, a side of the coupling straight waveguide facing the other waveguide, and a side of the output tapered waveguide facing the other waveguide are all connected to a first grating waveguide; in one waveguide, a side of the input tapered waveguide facing away from the other waveguide, a side of the coupling straight waveguide facing away from the other waveguide, and a side of the output tapered waveguide facing away from the other waveguide are all connected to a second grating waveguide, and both the first grating waveguide and the second grating waveguide are subwavelength gratings; and the duty cycle of the first grating waveguide is greater than that of the second grating waveguide.

[0008] The polarization-independent directional coupler of the present invention has the following advantages:

[0009] 1) Not only is a subwavelength grating introduced inside the waveguide to control the refractive index and achieve consistency in the beat lengths of the two polarization states, but a subwavelength grating is also introduced outside the waveguide, fully leveraging the subwavelength grating's ability to control the mode refractive index and enhance evanescent coupling. This allows for further reduction in device size based on achieving polarization-independent coupling (the length of the polarization-independent directional coupler of the present invention can be reduced to 36.3 μm on a lithium niobate silicon nitride platform), providing a key component for future large-scale, high-density integrated polarization-independent systems.

[0010] 2) A subwavelength grating structure is introduced into the waveguide structure of the symmetric directional coupler, which can smooth the dispersion curve of the TE0 and TM0 effective refractive index without increasing the loss, giving the device a larger manufacturing tolerance and operating bandwidth.

[0011] 3) Based on this device, the coupling length of the directional coupler can be adjusted and changed to achieve polarization-independent optical power output with arbitrary power distribution ratio. The output optical power can be distributed in different proportions according to the actual application scenario, making the device more functional and more practical. It has extremely high application value in future optical communications, all-optical networks and other fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 Schematic diagram of the polarization-independent directional coupler of the present invention.

[0013] Figure 2 It is a schematic diagram of the upper waveguide in the polarization-independent directional coupler of the present invention.

[0014] Figure 3This is a curve showing how the TM0 mode transmission efficiency changes with the waveguide width.

[0015] Figure 4 This is a graph showing how the difference between the refractive index of odd and even supermodes changes with the equivalent refractive index of the subwavelength grating outside the waveguide.

[0016] Figure 5 This is a graph showing the relationship between the TE0 mode beat length and the TM0 mode beat length and the equivalent refractive index of the subwavelength grating outside the waveguide.

[0017] Figure 6 This is a graph showing how the difference between the refractive index of odd and even supermodes changes with the equivalent refractive index of the subwavelength grating inside the waveguide.

[0018] Figure 7 This is a graph showing the relationship between the TE0 mode beat length and the TM0 mode beat length and the equivalent refractive index of the subwavelength grating inside the waveguide.

[0019] Figure 8 This is a graph showing the relationship between the TE0 mode beat length and the TM0 mode beat length and the equivalent refractive index of the subwavelength grating inside the waveguide when the equivalent refractive index of the subwavelength grating outside the waveguide is 1.4.

[0020] Figure 9 This is a schematic diagram of the finite-difference time-domain (FDTD) simulation structure with sub-wavelength gratings added inside and outside the waveguide (top view).

[0021] Figure 10 It is the transmission spectrum diagram of TE0 mode and TM0 mode.

[0022] Figure 1 Middle: 1. Input region, 2. Input transition region, 3. Coupler, 4. Output transition region, 5. Output region, 6. Upper waveguide, 7. Lower waveguide, 8. Input straight waveguide, 9. Input tapered waveguide, 10. Coupled straight waveguide, 11. First grating waveguide, 12. Output tapered waveguide, 13. Output straight waveguide, 14. Second grating waveguide. DETAILED DESCRIPTION

[0023] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0024] like Figure 1 As shown, the polarization-independent directional coupler of the present invention includes an upper waveguide 6 and a lower waveguide 7 arranged in a mirror image. The above waveguide 6 is used as an example for description:

[0025] The upper waveguide 6 includes an input straight waveguide 8, an input tapered waveguide 9, a coupled straight waveguide 10, an output tapered waveguide 12, and an output straight waveguide 13, which are connected in sequence. The width of the input straight waveguide 8 is the same as that of the output straight waveguide 13, and the width of the input straight waveguide 8 is greater than that of the coupled straight waveguide 10. The side of the input tapered waveguide 9 facing the lower waveguide 7, the side of the coupled straight waveguide 10 facing the lower waveguide 7, and the side of the output tapered waveguide 12 facing the lower waveguide 7 are all connected to a first grating waveguide 11. The side of the input tapered waveguide 9 facing away from the lower waveguide 7, the side of the coupled straight waveguide 10 facing away from the lower waveguide 7, and the side of the output tapered waveguide 12 facing away from the lower waveguide 7 are all connected to a second grating waveguide 14. Both the first grating waveguide 11 and the second grating waveguide 14 are subwavelength gratings.

[0026] The input straight waveguide 8 in the upper waveguide 6 and the input straight waveguide 8 in the lower waveguide constitute the input region 1; the input tapered waveguide 9 in the upper waveguide 6 and the input tapered waveguide 9 in the lower waveguide constitute the input transition region 2; the coupling straight waveguide 10 in the upper waveguide 6 and the coupling straight waveguide 10 in the lower waveguide constitute the coupler 3; the output tapered waveguide 12 in the upper waveguide 6 and the output tapered waveguide 12 in the lower waveguide constitute the output transition region 4; the output straight waveguide 13 in the upper waveguide 6 and the output straight waveguide 13 in the lower waveguide constitute the output region 5.

[0027] The width of the input straight waveguide 8 and the output straight waveguide 13 are both 1.3 μm.

[0028] The effective refractive index of the first grating waveguide 11 is 1.9; the equivalent refractive index of the second grating waveguide 14 is 1.4. The duty cycle of the first grating waveguide 11 is greater than the duty cycle of the second grating waveguide 14. The grating period of the first grating waveguide 11 and the grating period of the second grating waveguide (14) must meet the subwavelength condition to support low-loss transmission of light in the grating waveguide.

[0029] On the inner side of the waveguide, by introducing a sub-wavelength grating waveguide with a larger duty cycle (the first grating waveguide 11), the beat lengths of the TE0 mode and the TM0 mode can be made consistent; on the outer side of the waveguide, by introducing a sub-wavelength grating waveguide with a smaller duty cycle (the second grating waveguide 14), the evanescent coupling between the waveguides can be enhanced, thereby reducing the size of the device.

[0030] In order to ensure that the TE0 mode and TM0 mode in the waveguide are transmitted and coupled with low loss, the finite-difference time-domain method (FDTD) module is used for simulation to explore the most suitable waveguide width, such as Figure 3 As shown. Considering that in the same waveguide, the effective refractive index of the TE0 mode is larger and can be better confined in the waveguide, the simulation part focuses on the change of the TM0 mode transmission efficiency with the waveguide width. Figure 3It can be seen that the transmission efficiency of the TM0 mode increases as the waveguide width increases. However, when the waveguide width is greater than 1.3μm, the transmission efficiency no longer increases significantly, and the loss is relatively low. Therefore, it is more appropriate to use 1.3μm as the width of the input straight waveguide 8 and the output straight waveguide 13. Next, the simulation verifies the effect of adding a subwavelength grating waveguide on the beat length of the TE0 mode and the TM0 mode. Through the simulation of the Mode module, it can be obtained that the difference between the odd supermode and the even supermode changes with the equivalent refractive index (n) of the subwavelength grating. k ) changes with the change of the relationship curve, such as Figure 4 Then, through the equivalent relationship between the beat length and the difference between the odd and even supermode refractive index, the relationship between the TE mode and TM mode beat length and the change of the equivalent refractive index of the subwavelength grating outside the waveguide can be obtained, see Figure 5 .Depend on Figure 4 and Figure 5 It can be observed that the beat lengths of the two modes increase with the increase of the equivalent refractive index of the subwavelength grating, so the equivalent refractive index of the outer subwavelength grating should be minimized. However, adding an outer subwavelength grating has no obvious effect on reducing the difference in the beat lengths of the two polarization modes. Therefore, this method ultimately only reduces the beat lengths of the TE0 mode and the TM0 mode, and does not solve the problem of inconsistent beat lengths of the two modes. Therefore, the simulation is continued to verify the effect of adding a subwavelength grating waveguide to the inside of the waveguide on the beat lengths of the TE0 mode and the TM0 mode. Similarly, through the simulation of the Mode module, a curve showing the relationship between the difference between the odd supermode and the even supermode and the change in the equivalent refractive index of the subwavelength grating can be obtained, as shown in the figure. Figure 6 Then, through the equivalent relationship between the beat length and the difference between the odd and even supermode refractive index, the relationship between the beat length of the TE0 mode and the TM0 mode and the change of the equivalent refractive index of the subwavelength grating inside the waveguide can be obtained, as shown in Figure 2. Figure 7 As shown. Figure 6 and Figure 7 It can be seen that the beat length of the TE0 mode and the TM0 mode decreases with the increase of the equivalent refractive index, and the difference between the beat lengths of the two modes also decreases. When the effective refractive index of the sub-wavelength grating on the inner side of the waveguide is 1.9, the beat lengths are basically consistent. Taking the above parameters into consideration, since the smaller the equivalent refractive index of the sub-wavelength grating on the outer side of the waveguide, the better, it also means that the smaller the duty cycle of the sub-wavelength grating on the outer side of the waveguide, the better. This puts more stringent requirements on the manufacturing process. Therefore, under the condition that the process allows, an outer sub-wavelength grating with an equivalent refractive index of 1.4 is used. The final simulation results show the relationship between the beat lengths of the TE0 mode and the TM0 mode and the change of the equivalent refractive index of the sub-wavelength grating on the inner side of the waveguide, as shown in the figure. Figure 8As shown. It can be seen that when the equivalent refractive index of the outer subwavelength grating is 1.9, the beat lengths of the two modes reach the same value of 36.3μm. Finally, the device with designed parameters is simulated using the finite difference time domain method (FDTD) module. The structural diagram shown in the finite difference time domain method (FDTD) module is as follows: Figure 8 As shown in the figure, it can be seen that the sub-wavelength grating on the outside of the waveguide has a relatively small duty cycle to reduce the beat length of the two modes, while the sub-wavelength grating on the inside of the waveguide has a relatively large duty cycle to reduce the difference in beat length between the two modes. After further scanning and optimizing the parameters, the final transmission spectra of the two modes are as follows: Figure 10 As shown, it can be seen that within the bandwidth range of 1500-1600nm (100nm), at the output end of the cross waveguide, Figure 1 The measured loss of the TE0 mode in the lower waveguide 7 is less than 0.13dB, the loss of the TM0 mode is less than 0.5dB, and the mode crosstalk is less than -37dB, showing extremely low insertion loss and channel crosstalk, which is conducive to the construction of polarization-independent systems on the lithium niobate platform in the future.

[0031] In the polarization-independent directional coupler of the present invention, the subwavelength gratings added to the outer and inner sides of the directional coupler's waveguide are crucial for device functionality. The subwavelength gratings on either side of the waveguide have different duty cycles and therefore perform different functions. The subwavelength gratings are added to the outer side of the waveguide to simultaneously reduce the beat lengths of both the TE0 and TM0 modes in the waveguide, thereby reducing device size and enabling high integration. The subwavelength gratings are added to the inner side of the waveguide to reduce the difference in beat length between the TE0 and TM0 modes, allowing both the TE0 and TM0 modes input at the input end to be coupled to the crossover end simultaneously with maximum coupling efficiency and output.

[0032] In the polarization-independent directional coupler of the present invention:

[0033] Input 1: An unetched strip waveguide couples light from the optical fiber into the on-chip system through a grating coupler.

[0034] Input transition region 2: The connection area between the strip waveguide and the subwavelength grating waveguide needs to be assisted by a tapered waveguide so that the mode in the strip waveguide can be slowly converted into the subwavelength mode in the subwavelength grating waveguide, reducing the radiation loss caused by mode mutation in the mode field.

[0035] Coupling Zone 3: Utilizing the subwavelength grating's enhanced evanescent mode field coupling and modulated refractive index, this system not only achieves maximum coupling efficiency for both the TE0 and TM0 modes at the input to the crossover output, but also further reduces device size and enables higher integration.

[0036] Output transition region 4: The connection area between the subwavelength grating waveguide and the strip waveguide needs to be assisted by a tapered waveguide so that the subwaveguide mode in the subwavelength grating waveguide can be slowly converted into the mode in the strip waveguide, reducing the radiation loss caused by mode mutation in the mode field.

[0037] Output 5: The unetched strip waveguide couples the light from the on-chip system into the optical fiber through the grating coupler.

[0038] The device of the present invention is a symmetrical directional coupler based on a subwavelength grating structure. The coupling process of the mode can be analyzed using coupled mode theory. The mode module in the optoelectronic simulation software Lumerical is used to simulate and analyze the refractive index of the excited odd and even supermodes. Finally, the finite difference time domain method (FDTD) module in the optoelectronic simulation software Lumerical is used to simulate and optimize the coupling length. The subwavelength grating waveguide added inside and outside the waveguide will affect the size of the odd and even supermode refractive indices, thereby playing a role in regulating the beat length. L π The relationship between the refractive index difference between the odd and even supermodes is shown in the following equation:

[0039] L π = L / [2( n even - n odd )]

[0040] Where L is the center wavelength (1550nm), n even is the effective refractive index of the even supermode, n odd is the effective refractive index of the odd supermode.

[0041] The polarization-independent directional coupler of the present invention has arbitrary energy output. The polarization-independent directional coupler is described by taking the example of energy being completely output from the cross end. The device can achieve arbitrary energy distribution by changing the structural length, and output energy in different proportions at the through end and the cross end.

[0042] The polarization-independent directional coupler of the present invention utilizes a subwavelength grating waveguide to adjust the refractive index distribution, thereby achieving regulation of the beat lengths of different polarization states and providing a key component for the construction of a polarization-independent system.

[0043] The polarization-independent directional coupler of the present invention enhances evanescent coupling by adjusting the grating duty cycle of the second grating waveguide 14 during mode coupling. This solution can also be applied to devices such as mode multiplexing devices and polarization beam splitting devices to achieve smaller-sized coupling.

[0044] In the above content, the polarization-independent directional coupler of the present invention uses a lithium niobate platform as an example to illustrate its working principle. However, based on the same principle, the same function can also be achieved on other material platforms, including but not limited to silicon nitride platforms, silicon platforms, etc.

Claims

1. A polarization-independent directional coupler based on a subwavelength grating structure, characterized in that: The invention relates to a waveguide comprising two mirror-image-arranged waveguides, each waveguide comprising an input straight waveguide (8), an input tapered waveguide (9), a coupled straight waveguide (10), an output tapered waveguide (12) and an output straight waveguide (13) connected in sequence, the width of the input straight waveguide (8) being the same as the width of the output straight waveguide (13), and the width of the input straight waveguide (8) being greater than the width of the coupled straight waveguide (10); in one waveguide, the input tapered waveguide (9) is directed toward one side of the other waveguide, the coupling straight waveguide (10) is directed toward one side of the other waveguide and the output tapered waveguide (12) is directed toward one side of the output straight waveguide (13). The side of the output tapered waveguide (12) facing the other waveguide is connected to the first grating waveguide (11); the side of the input tapered waveguide (9) in one waveguide facing away from the other waveguide, the side of the coupled straight waveguide (10) facing away from the other waveguide, and the side of the output tapered waveguide (12) facing away from the other waveguide are all connected to the second grating waveguide (14); the first grating waveguide (11) and the second grating waveguide (14) are both sub-wavelength gratings; the duty cycle of the first grating waveguide (11) is greater than the duty cycle of the second grating waveguide (14).

2. The polarization-independent directional coupler based on a subwavelength grating structure according to claim 1, wherein: The effective refractive index of the first grating waveguide (11) is 1.9; the equivalent refractive index of the second grating waveguide (14) is 1.

4.

3. The polarization-independent directional coupler based on a subwavelength grating structure according to claim 1, wherein: The width of the input straight waveguide (8) and the output straight waveguide (13) are both 1.3 μm.

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