A thin film lithium niobate based dual polarization tunable mode converter and mode multiplexing system

Abstract

The application provides a kind of based on thin film lithium niobate dual polarization tunable mode converter and mode multiplexing system, belong to integrated photonic device technical field.It successively includes substrate layer, silicon dioxide layer, lithium niobate film layer and silicon dioxide cladding.Mach-Zehnder structure made on lithium niobate film layer is composed of input waveguide, output waveguide, two Y branch waveguides and two asymmetric strip waveguides;input waveguide and two asymmetric strip waveguides are connected by a Y branch waveguide, and output waveguide and two asymmetric strip waveguides are connected by another Y branch waveguide;the application makes TE and TM base mode have phase difference of π with another arm after passing through phase shift region by adding a wide waveguide and tapered waveguide at both ends, and the phase difference of front and rear arms is changed between π and 0, so as to realize the switching of output mode, and the effect of polarization independence is achieved, with the characteristics of high integration, polarization insensitivity, tunable and large bandwidth.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of integrated photonics, and particularly relates to a dual-polarization tunable mode converter and a mode multiplexing system based on thin-film lithium niobate. BACKGROUND

[0002] With the rapid growth of global data demand, optical communication and optical interconnection technology has become the core pillar of the modern information society. Traditional single-mode fiber communication systems gradually fail to meet the increasing demand for data transmission due to their limited capacity. Therefore, how to effectively improve the capacity of optical communication systems has become a problem that scientists and engineers need to solve. In this context, multiplexing technologies, including wavelength division multiplexing (WDM), polarization multiplexing (PDM) and mode division multiplexing (MDM), have received extensive attention and research. In addition, in order to further improve the communication capacity, a new research trend is to simultaneously utilize these independent degrees of freedom to form a kind of multi-dimensional hybrid multiplexing technology, which has been widely reported. Hybrid multiplexing technology is an attractive solution to improve communication capacity by combining multiple wavelengths, multiple modes and dual polarization, but there is still a lack of dual-polarization mode converters with large bandwidth for hybrid multiplexing.

[0003] Among these hybrid multiplexing technologies, mode converters are key devices for efficient multimode transmission. In particular, mode converters capable of operating simultaneously in dual polarization and ultra-wideband are particularly important for realizing large-capacity, multi-degree-of-freedom optical communication systems. In a flexible MDM switching network, mode converters are responsible for switching or routing data transmitted on different mode channels. If PDM and WDM can be introduced into the MDM switching system, the information capacity will increase several times. A wideband mode converter can be used to simultaneously route and switch multiple wavelength multimode signals, providing a viable option for large-capacity transmission.

[0004] Lithium niobate (LiNbO3, LN) is one of the materials in the field of integrated photonics, which has many advantages. Compared with traditional material platforms such as silicon, silicon nitride and indium phosphide, LN exhibits more outstanding characteristics. First, LN has a large electro-optic coefficient and nonlinear optical coefficient, which makes it perform well in electro-optic modulation and nonlinear optical applications. Second, LN has a large refractive index at 1550 nm wavelength, which enables it to effectively guide and control light in optical waveguide devices. In addition, LN has a wide transparent window, covering a wavelength range from 400 nm to 5 μm, suitable for various optical applications. Most importantly, LN has stable physical and chemical properties, ensuring the stability and reliability of the device.

[0005] Lithium niobate on insulator (LNOI) technology further expands the application field of lithium niobate. The LNOI platform is covered by a sub-micron thick LN film on a silicon dioxide substrate, which has a high refractive index contrast. This feature makes the optical waveguide devices manufactured on the LNOI have the characteristics of high integration and small size, thereby greatly reducing the size and power consumption of the device. In addition, the optical devices on the LNOI platform are compatible with existing CMOS processes, which means that large-scale photonic integrated circuits (PICs) can be realized using existing semiconductor manufacturing processes, thereby promoting the development and application of photonic integration technology. Therefore, choosing LNOI as a material platform not only helps to improve the performance and stability of photonic devices, but also is conducive to the miniaturization, low power consumption and integration of devices, thereby promoting the development of optical communication and optical interconnection technology. In order to realize efficient mode multiplexing on the thin film lithium niobate platform, mode converters are essential passive devices for multi-mode.

[0006] The patent CN105158850A provides an electro-optical mode converter with a Mach-Zehnder interferometer structure and an implementation method, which includes a Mach-Zehnder interferometer structure and an electro-optical phase modulation electrode. By controlling the voltage applied to the electro-optical phase modulation electrode, the phase relationship of the light waves on the two interference arms is changed, and the mutual conversion between the two modes is realized. On the one hand, the above-mentioned scheme uses a symmetric MZI structure, which cannot perform mode conversion without an external power supply. On the other hand, the above-mentioned scheme does not have the function of simultaneously realizing mode conversion for TE and TM polarizations, which limits its application range.

[0007] Therefore, in the mode multiplexing optical communication system, it is necessary to provide a large bandwidth, dual-polarization mode converter to solve the above problems. SUMMARY

[0008] The purpose of the present application is to provide a dual-polarization tunable mode converter based on thin film lithium niobate and a mode multiplexing system to realize large bandwidth, dual-polarization mode conversion and provide a basis for efficient mode multiplexing.

[0009] To achieve the above-mentioned purpose, the present application provides a dual-polarization mode converter based on thin film lithium niobate, which includes a lithium niobate film and a Mach-Zehnder structure fabricated on the lithium niobate film layer, the Mach-Zehnder structure includes an input waveguide, an output waveguide, two Y-branch waveguides and two asymmetric strip waveguides.

[0010] The input waveguide and the two asymmetric strip waveguides are connected through one of the Y-branch waveguides, and the output waveguide and the two asymmetric strip waveguides are connected through the other Y-branch waveguide; the two asymmetric strip waveguides have different widths for adjusting the phase difference of the light beams passing through the two asymmetric strip waveguides.

[0011] Further, one of the two asymmetric strip waveguides is a narrow waveguide without a phase shift region, and the other is a wide waveguide with a phase shift region.

[0012] Further, the width of the wide waveguide is 1.1-1.5 μm, and the width of the narrow waveguide is 0.8-1.0 μm, and the phase difference of the light beams passing through the two asymmetric strip waveguides is preferably π.

[0013] Further, the length of the wide waveguide is 25-35 μm, preferably 29-31 μm.

[0014] The distance between the wide waveguide and the narrow waveguide is 3-5 μm.

[0015] Further, the width of the input waveguide and the output waveguide is 2-2.4 μm.

[0016] The width of the one side branch part of the two Y branch waveguides connected to the wide waveguide is a gradual change structure, and gradually increases towards the one end of the wide waveguide.

[0017] The width of the one side branch part of the two Y branch waveguides connected to the narrow waveguide is the same as the width of the narrow waveguide.

[0018] Further, the thickness of the lithium niobate thin film is 200-400 nm, and the thickness of the input waveguide, the output waveguide, the two symmetric Y branch waveguides and the two asymmetric strip waveguides is 200-400 nm.

[0019] Further, the lithium niobate thin film is an x-cut lithium niobate thin film; and the input waveguide, the output waveguide, the two symmetric Y branch waveguides and the two asymmetric strip waveguides are formed by etching on a lithium niobate thin film with a thickness of 500-700 nm.

[0020] Further, the top of the lithium niobate thin film, the input waveguide, the output waveguide, the two symmetric Y branch waveguides and the two asymmetric strip waveguides is covered with a silicon dioxide cladding layer, and a metal heater is arranged above the silicon dioxide cladding layer, the metal heater is above the strip waveguide with a wider width among the two asymmetric strip waveguides, and is used for thermal tuning; or a metal electrode structure is arranged on both sides of the two asymmetric strip waveguides to realize electro-optic modulation of lithium niobate. The phase difference of the TE and TM modes in the two arms before and after the modulation is between π and 0.

[0021] Further, a silicon dioxide cladding layer is arranged below the lithium niobate thin film, and the silicon dioxide cladding layer is on a silicon substrate.

[0022] The application first proposes a large-bandwidth, dual-polarization tunable mode converter on a thin-film lithium niobate (TFLN) platform, which is used to realize efficient mode conversion on a thin-film lithium niobate platform. The mode converter is simple in structure and easy to integrate, and simultaneously realizes polarization multiplexing and mode multiplexing. The device has a bandwidth of more than 100 nm and a length of about 100 mu m.

[0023] A mode multiplexing system comprises a mode demultiplexer, the mode converter of any one of the above, four input grating couplers and four output grating couplers, the four input grating couplers are connected with the mode multiplexer respectively, the mode multiplexer is connected with the input waveguide of the mode converter, the output waveguide of the mode converter is connected with the mode demultiplexer, and the four output grating couplers are connected with the mode demultiplexer respectively.

[0024] Overall, compared with the prior art, the above technical solutions conceived by the application mainly have the following technical advantages:

[0025] 1. The application first adopts a Mach-Zehnder interferometer (MZI) structure based on thin-film lithium niobate to construct a dual-polarization mode converter, and utilizes the width difference of two branch waveguides to make the propagation constant and phase velocity of TE and TM beams different in the two branches, so as to generate a phase difference, thereby realizing mode conversion for both TE and TM polarizations, achieving polarization-insensitive mode conversion, and having the characteristics of high integration, polarization insensitivity, tunability and large bandwidth.

[0026] 2. The application further optimizes the width and length of one of the branch waveguides, so that the TE and TM fundamental modes have a phase difference of pi after passing through the phase-shifting region, and polarization-insensitive mode conversion is realized. In addition, with thermal or electro-optic modulation, the phase difference can be controlled between pi and 0, so that the application can improve device integration and realize output mode switching, and finally realize dual-polarization large-bandwidth multimode conversion.

[0027] 3. The application can simultaneously convert TE and TM modes. Without thermal modulation, the application can simultaneously realize TE0-TE1, TE1-TE0, TM0-TM1 and TM1-TM0 mode conversion processes, and has high conversion efficiency. In the wavelength range of 1500-1600 nm, the conversion efficiency of each output is higher than -0.37 dB, the total crosstalk is less than -15 dB, the insertion loss is low, the bandwidth is large, and the application has great application prospect in a mode multiplexing optical communication system.

[0028] 4. The application connects the Y branch with the input and output waveguides through tapering and reverse tapering waveguides respectively, so as to reduce the loss caused by the width mutation of the phase-shifting region waveguide.

[0029] 5、The application further adds a thermal tuning or electric tuning structure to the phase shifter region, and uses thermal tuning or electric tuning to change the phase difference of the phase shifter, so as to control whether the output mode is the fundamental mode or the first-order mode, and achieve the purpose of tunability. In the case of thermal light modulation, the mode of the output port depends on the phase shift caused by the phase shifter. When the phase shifter introduces 0 phase difference, the input mode is transferred to the output without any change; when the phase shifter introduces π phase difference, the input mode will be mode order converted, the input fundamental mode is converted into the first-order mode, and the input first-order mode will be converted into the fundamental mode.

[0030] 6、The application adopts the MZI structure, which is relatively simple compared with the previous structure, and the structure of the phase shifter is difficult to manufacture, which further reduces the process difficulty and reduces the overall length of the device. By designing the overall structure of the double-polarization tunable mode converter to be mirror-symmetric about the middle position of the phase shift region, and the structure of the phase shift region is relatively simple, there is a larger process tolerance. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 The structure diagram of the double-polarization tunable mode converter based on thin film lithium niobate obtained by the application.

[0032] Figure 2 The cross-sectional view of the lithium niobate thin film ridge waveguide in the mode converter.

[0033] Figure 3 The cross-sectional mode field distribution diagram of the MZI waveguide region in the mode converter.

[0034] Figure 4 The schematic diagram of a four-channel mode multiplexing and mode demultiplexing system based on thin film lithium niobate.

[0035] Figure 5 The overall field distribution diagram of the device corresponding to the input TE0 of the double-polarization mode converter in Example 1.

[0036] Figure 6 The overall field distribution diagram of the device corresponding to the input TE1 of the double-polarization mode converter in Example 1.

[0037] Figure 7 The overall field distribution diagram of the device corresponding to the input TM0 of the double-polarization mode converter in Example 1.

[0038] Figure 8 The overall field distribution diagram of the device corresponding to the input TM1 of the double-polarization mode converter in Example 1.

[0039] Figure 9 The transmission spectrum of the device corresponding to the input TE0 of the double-polarization mode converter in Example 1.

[0040] Figure 10 Transmission spectrum in the device after inputting TE1 into the dual-polarization mode converter of Example 1.

[0041] Figure 11 Transmission spectrum in the device after inputting TM0 into the dual-polarization mode converter of Example 1.

[0042] Figure 12 Transmission spectrum in the device after inputting TM1 into the dual-polarization mode converter of Example 1.

[0043] Figure 13 Corresponding device conversion efficiency spectrum after inputting TE0 into the dual-polarization mode converter of Example 2.

[0044] Figure 14 Corresponding device conversion efficiency spectrum after inputting TE1 into the dual-polarization mode converter of Example 2.

[0045] Figure 15 Corresponding device conversion efficiency spectrum after inputting TM0 into the dual-polarization mode converter of Example 2.

[0046] Figure 16 Corresponding device conversion efficiency spectrum after inputting TM1 into the dual-polarization mode converter of Example 2. DETAILED DESCRIPTION

[0047] In order to make the objectives, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to examples. It should be understood that the specific examples described herein are only used to explain the present application and should not be used to limit the present application. In addition, the technical features involved in each embodiment of the present application described below can be combined with each other as long as they do not conflict with each other.

[0048] As Figure 1 The three-dimensional (3D) schematic diagram of the mode converter and the cross-sectional view of the input waveguide are shown, wherein the cross-section at the α inclination angle corresponds to the input end in the figure. In an embodiment of the present application, a thin-film lithium niobate-based dual-polarization tunable mode converter is provided, which includes a lithium niobate thin film and a Mach-Zehnder structure fabricated on the lithium niobate thin film, the Mach-Zehnder structure including an input waveguide, an output waveguide, two Y-branch waveguides and two asymmetric strip waveguides;

[0049] The input waveguide and the two asymmetric strip waveguides are connected through one of the Y-branch waveguides, and the output waveguide and the two asymmetric strip waveguides are connected through the other Y-branch waveguide; the two asymmetric strip waveguides have different widths for adjusting the phase difference of the light beams passing through the two asymmetric strip waveguides.

[0050] Thus, by adjusting the width and length of the wide side of the strip waveguide, the propagation constant and phase velocity are different, so that the phase shift of the light beam passing through the waveguide is adjusted, and then the light beam and the light beam in another waveguide produce a phase difference, so that the mode conversion of TE and TM can be realized at the same time. The present application differentiates the width and length of one branch waveguide based on the MZI (Mach-Zehnder interferometer) structure, so that the phase difference can be adjusted without external optical modulator, which can improve the integration and realize the multi-mode conversion of double polarization and large bandwidth.

[0051] Specifically, the mode converter designed by the present application adopts a 600nm thick x-cut lithium niobate film (i.e. the total thickness of the light blue lithium niobate film in the figure, h is 300nm), and the waveguide is arranged along the y direction. The bottom of the x-cut lithium niobate film is a silicon dioxide cladding and a silicon substrate in turn. The device adopts a typical MZI (Mach-Zehnder interferometer) structure, which mainly consists of an input waveguide, an output waveguide, two identical 3dB 1x2 Y branch waveguides, a waveguide without phase shift region and a waveguide with phase shift region. The input waveguide is connected with one end of the Y branch, and then divided into two branch waveguides, i.e. the two branches of the MZI structure. One arm of the mode converter based on MZI is a strip waveguide, and the other arm is a phase shift region composed of a wide waveguide, wherein the width of the wide waveguide is w1 and the width of the narrow waveguide is w2. Then the two branches are connected through another Y branch and connected with the output waveguide, thus forming the MZI structure.

[0052] Because the waveguide widths of the upper and lower arms of the MZI are different, the propagation constant and phase velocity are also different, so that a phase difference can be generated; and without thermal light modulation, the mode converter can simultaneously realize the TE0-TE1, TE1-TE0, TM0-TM1 and TM1-TM0 mode conversion processes, and the mode profile at the output end has very high mode purity.

[0053] In order to reduce the loss caused by the sudden change of the width of the phase shift region waveguide, the two Y branches are designed as tapered and inverse tapered waveguides connected with the input and output waveguides respectively. That is, the width of the connection section of the Y branch and the two branches is designed as a tapered structure, and the width gradually increases from the input end and the output end to the branch end.

[0054] By optimizing the structure and length of the phase shifter, higher conversion efficiency and lower crosstalk value can be obtained, and efficient dual polarization mode conversion can be realized.

[0055] As Figure 2As shown, in order to make the mode output by the mode converter tunable, a silicon dioxide cladding layer (existing on the entire lithium niobate film) and a metal heater (only existing above the phase shifter) can be deposited above the phase shifter, the phase difference of the phase shifter is changed through thermal modulation, so as to control whether the output mode is the fundamental mode or the first-order mode. Or a metal electrode structure is arranged on both sides of the two asymmetric strip waveguides, and electro-optic modulation of lithium niobate is realized, that is, a metal electrode structure is additionally arranged along the side of the width of the waveguide (phase shifter area). For example, the input TE0 mode, through thermal-optic modulation (by heating to change the effective refractive index of the material, thereby changing the phase, so that the phase difference is switched between 0 and Π), the output mode can be switched between TE0 and TE1. Specifically, in the case of thermal-optic modulation, the mode of the output port depends on the phase shift caused by the phase shifter. In the case that the phase shifter introduces a 0 phase difference, the input mode is transferred to the output without any change, and in the case that the phase shifter introduces a π phase difference, the input mode will undergo mode order conversion, the input fundamental mode is converted into a first-order mode, and the input first-order mode will be converted into a fundamental mode. Therefore, this tunable mode converter can simultaneously perform mode conversion on the four input modes of TE0, TE1, TM0 and TM1, and can output the original mode or perform mode order conversion on the input signal.

[0056] In an embodiment of the present application, a model of a lithium niobate thin film ridge waveguide and an MZI structure is established by using lumerical FDTD software, and the specific implementation is as follows:

[0057] Taking the input TE0 as an example, the interface mode field distribution corresponding to each position of the mode converter is as shown in the figure, Figure 3 wherein the dark blue wide waveguide region is the phase shifter area, and the phase shift amount is mainly determined by the width and length of the wide waveguide of the phase shifter area. The input TE0 mode is divided into two beams of equal power and the same phase, and in the transmission process, the phase shifter introduces a phase difference of π between the two beams of light. Finally, the two anti-phase TE0 modes recombine to produce a TE1 mode. Similarly, the input TM mode also has the same conversion process.

[0058] The symmetric Y-branch junction is commonly used in various photonic integrated circuits. The branches of the Y-junction only support the fundamental mode, and the trunk (i.e. the two branch strip waveguides) simultaneously supports the fundamental mode and the first-order mode. The working mechanism of the symmetric Y-branch can be described as the evolution of the odd and even supermodes (S0 and S1) from the trunk to the output branch. The input TE0 mode at the Y-junction will first evolve into the S0 mode, and as the waveguide gap increases, two in-phase TE0 mode parts will be excited at the output branch. Similarly, the TE1 mode in the trunk waveguide first evolves into the S1 mode, and then the S1 mode is equally divided into two TE0 modes with a phase difference of π.

[0059] The thermo-optic phase shifter is composed of a wide waveguide, and due to the difference in waveguide width between the upper and lower arms of the MZI, the propagation constant and phase velocity are also different, so that a phase difference can be generated. In order to make the mode converter suitable for TE and TM modes at the same time, it is necessary to simultaneously perform π phase shift on TE0 and TM0 at the design wavelength 1550 nm. By adjusting the structure of the Y branch and the phase shifter, the effective refractive indices of the TE and TM modes can be simultaneously controlled, so that the two beams of light have equal power and a specific phase difference. Therefore, the length and width of the phase shift region are selected according to the formula.

[0060] The effective refractive indices of the TE mode and the TM mode of the narrow waveguide in the MZI structure are n eff1_TE0 and n eff1_TM0 respectively, the effective refractive indices of the TE mode and the TM mode of the wide waveguide in the phase shift region are n eff2_TE0 and n eff2_TM0 respectively, the difference between the effective refractive indices of the TE0 mode and the TM0 mode between the phase shift region and the narrow waveguide is Δn eff_TE (the difference between n eff1_TE0 and n eff2_TE0 ), Δn eff_TM (the difference between n eff1_TM0 and n eff2_TM0 ), the length of the phase shift region is L, the center wavelength is λ=1550 nm, and the formula is as follows:

[0061]

[0062] The above is the design and simulation work of the mode converter, and finally a test structure needs to be further designed, that is, a mode multiplexing system, as shown in Figure 4 The mode multiplexing system is composed of a mode multiplexer (MUX), a mode demultiplexer (DEMUX), a mode converter (Mode-order conventer) and four input and four output grating couplers. The four input grating couplers are connected with the mode multiplexer, the mode multiplexer is connected with the input waveguide of the mode converter, the output waveguide of the mode converter is connected with the mode demultiplexer, and the four output grating couplers are connected with the mode demultiplexer. The fundamental mode of light propagating in the optical fiber is coupled with the input port to generate TE0 or TM0 mode, which is coupled through the TE / TM polarization grating coupler. The input TE0 or TM0 mode is multiplexed into the bus waveguide by the multiplexer, and then converted into the corresponding mode by the mode converter. Finally, the converted mode is demultiplexed into the corresponding output port by the DEMUX, thereby realizing efficient multi-mode transmission.

[0063] Embodiment one

[0064] A kind of double polarization mode converter based on thin film lithium niobate, as Figure 1 and 2The specific parameters are shown as follows: the thickness of the lithium niobate film is 600 nm, the etching depth is 300 nm, the cover layer is air (n0=1), the input and output waveguide width w=2.2 μm, the narrow waveguide width of the MZI arm is 0.9 μm, the wide waveguide width of the MZI arm is 1.4 μm, and the waveguide side wall angle α=70°. In order to make the device as compact as possible, the length L y (the length of the phase shift region) is optimized and reduced, the distance between the two branches is D y (the distance between the phase shift region and the narrow waveguide) is preferably 4 μm. In order to achieve a π phase shift while reducing the device size, the length L of the phase shift region is controlled to be about 30 μm.

[0065] The following tests are carried out based on the thin-film lithium niobate-based dual-polarization mode converter provided in Embodiment One above:

[0066] The structure is input into the simulation software lumerical FDTD and simulated, and the distribution of the simulated electric field at 1550 nm when the dual-polarization mode converter is input with four different modes can be obtained. Figure 5 The device overall field distribution diagram corresponding to the input TE0 of the dual-polarization mode converter, the input TE0 mode is divided into two beams of equal power and same phase, and a π phase difference is introduced between the two beams of light by the phase shifter in the transmission process. Finally, the two anti-phase TE0 modes are recombined to generate a TE1 mode. Similarly, Figure 6 The device overall field distribution diagram corresponding to the input TE1 of the dual-polarization mode converter; Figure 7 The device overall field distribution diagram corresponding to the input TM0 of the dual-polarization mode converter; Figure 8 The device overall field distribution diagram corresponding to the input TM1 of the dual-polarization mode converter. It can be seen that the device can simultaneously realize the TE0-TE1, TE1-TE0, TM0-TM1 and TM1-TM0 mode conversion processes, and the mode profile at the output end has very high mode purity.

[0067] The simulation results are processed and plotted, Figure 9 The transmission spectrum in the device when TE0 is input into the dual-polarization mode converter, more than 92% of the input TE0 mode is converted into the TE1 mode through the mode converter, and the output light contains a very small number of TE0, TM0 and TM1 modes, the insertion loss at 1550 nm is -0.41 dB, and the crosstalk is -16 dB; Figure 10 The transmission spectrum in the device when TE1 is input into the dual-polarization mode converter, more than 93% of the input TE1 mode is converted into the TE0 mode through the mode converter, and the output light contains a very small number of TE1, TM0 and TM1 modes, the insertion loss at 1550 nm is -0.39 dB, and the crosstalk is -15 dB.Figure 11 The transmission spectrum of the device after inputting TM0 to the dual-polarization mode converter, the input TM0 mode passes through the mode converter, more than 95% of the light is converted into TM1 mode, the output light contains a small number of TE0, TE1, TM0 modes, the insertion loss at 1550nm is -0.27dB, and the crosstalk is -16dB; Figure 12 The transmission spectrum of the device after inputting TM1 to the dual-polarization mode converter, the input TM1 mode passes through the mode converter, more than 95% of the light is converted into TM0 mode, the output light contains a small number of TE0, TE1, TM1 modes, the insertion loss at 1550nm is -0.27dB, and the crosstalk is -16dB. It can be seen that in the wavelength range of 1500-1600nm, the insertion loss of each mode input of the device is less than -0.41dB, and the crosstalk is less than -15dB. The mode converter designed in the application has a significantly improved mode conversion efficiency through the optimization of the size structure, all of which are more than 92%, and the crosstalk is small, which can realize high-efficiency mode conversion. Further optimization of simulation and experiment can obtain higher conversion efficiency and lower crosstalk value, realize high-efficiency dual-polarization mode conversion, and achieve the expected effect. It can be seen that through the optimization of the structure parameters, the TE and TM modes can realize π phase shift through the phase shift region, so that the device achieves the effect of polarization independence.

[0068] Example two

[0069] The difference from example one is that the width of the wide waveguide of the MZI arm is 1.2μm, and the length is 35μm. The results are shown in Figures 13-16 It can be seen that the conversion efficiency is more than 95%, which is better than example one.

[0070] In summary, the dual-polarization tunable mode converter based on thin-film lithium niobate is supported by the project of Thin-film lithium niobate photonic integration key process and integration technology development (project number: 2021YFB2800100), and through the typical Mach-Zehnder interferometer (MZI) structure, it can be applied to TE and TM modes at the same time, and without thermal-optic modulation, the processes of TE0-TE1, TE1-TE0, TM0-TM1 and TM1-TM0 mode conversion can be realized at the same time, and the conversion efficiency is high, in the wavelength range of 1500-1600 nm, the conversion efficiency of each output is higher than-0.41 dB, and the total crosstalk is less than-15 dB. In the case of thermal-optic modulation, the mode of the output port depends on the phase shift caused by the phase shifter, and by changing the phase difference of the phase shifter through thermal or electrical modulation, the mode of the output is controlled to be the fundamental mode or the first-order mode. Moreover, the thin-film lithium niobate device is compatible with the existing manufacturing process and has excellent optical performance, so that the manufacturing and maintenance costs of the system can be reduced while ensuring high performance. In order to realize efficient mode multiplexing on the thin-film lithium niobate platform, the mode converter is an essential multi-mode passive device. Therefore, the dual-polarization mode converter based on thin-film lithium niobate plays an important role in the field of integrated optics.

[0071] Those skilled in the art will easily understand that the above description is only the preferred embodiment of the present application, and is not intended to limit the present application, and any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A thin film lithium niobate based dual polarization mode converter, characterized in that, The lithium niobate film and the Mach-Zehnder structure fabricated on the lithium niobate film, the Mach-Zehnder structure comprising an input waveguide, an output waveguide, two Y-branch waveguides and two asymmetric strip waveguides; The input waveguide and the two asymmetric strip waveguides are connected through one of the Y-branch waveguides, and the output waveguide and the two asymmetric strip waveguides are connected through the other Y-branch waveguide; the two asymmetric strip waveguides have different widths; One of the two asymmetric strip waveguides is a narrow waveguide without a phase shift region, and the other is a wide waveguide with a phase shift region; the width of the wide waveguide is 1.1-1.5 μm, and the width of the narrow waveguide is 0.8-1.0 μm; the length of the wide waveguide is 25-35 μm; the length of the narrow waveguide is the same as that of the wide waveguide.

2. The thin film lithium niobate based dual polarization mode converter of claim 1, wherein, The phase difference of the light beams passing through the two asymmetric strip waveguides is π.

3. The thin film lithium niobate based dual polarization mode converter of claim 1, wherein, The distance between the wide waveguide and the narrow waveguide is 3-5 μm.

4. The thin film lithium niobate based dual polarization mode converter of claim 3, wherein, The length of the wide waveguide is 29-31 μm.

5. The thin film lithium niobate based dual polarization mode converter of claim 2, wherein, The width of the input waveguide and the output waveguide is 2-2.4 μm; The width of the side branch part of the two Y-branch waveguides connected to the wide waveguide is a gradually changing structure, and gradually increases towards the end of the wide waveguide; The width of the side branch part of the two Y-branch waveguides connected to the narrow waveguide is the same as the width of the narrow waveguide.

6. The thin film lithium niobate based dual polarization mode converter of claim 1, wherein, The thickness of the lithium niobate film is 200-400 nm, and the thickness of the input waveguide, the output waveguide, the two Y-branch waveguides and the two asymmetric strip waveguides is 200-400 nm.

7. The thin film lithium niobate based dual polarization mode converter of claim 6, wherein, The lithium niobate film is an x-cut lithium niobate film; the input waveguide, the output waveguide, the two Y-branch waveguides and the two asymmetric strip waveguides are obtained by etching a lithium niobate film with a thickness of 500-700 nm to obtain a 200-400 nm thick lithium niobate film.

8. Thin film lithium niobate based dual polarization mode converter according to any of claims 1-7, characterized in that, The lithium niobate film, the input waveguide, the output waveguide, the two Y-branch waveguides and the two asymmetric strip waveguides are covered with a silicon dioxide cladding layer, and a metal heater is arranged above the silicon dioxide cladding layer, the metal heater being arranged above the waveguide with a wider width among the two asymmetric strip waveguides for thermal tuning; or metal electrode structures are arranged on both sides of the two asymmetric strip waveguides to realize electro-optic modulation of the lithium niobate, and the phase difference of the TE and TM modes before and after the modulation changes between π and 0.

9. The thin film lithium niobate based dual polarization mode converter of claim 8, wherein, A silicon dioxide cladding layer is arranged below the lithium niobate film, and the silicon dioxide cladding layer is arranged on a silicon substrate.

10. A mode multiplexing system characterized by, The mode demultiplexer, the mode converter of any one of claims 1-9, four input grating couplers and four output grating couplers are provided, the four input grating couplers are connected with a mode multiplexer, the mode multiplexer is connected with the input waveguide of the mode converter, the output waveguide of the mode converter is connected with the mode demultiplexer, and the four output grating couplers are connected with the mode demultiplexer.

Images