Dual-polarization tunable mode converter and mode multiplexing system based on thin-film lithium niobate

Abstract

The invention provides a dual-polarization tunable mode converter and mode multiplexing system based on thin film lithium niobate, and belongs to the technical field of integrated photonic devices. The lithium niobate film sequentially comprises a substrate layer, a silicon dioxide layer, a lithium niobate film layer and a silicon dioxide cladding. The Mach-Zehnder structure manufactured on the lithium niobate thin film layer is composed of an input waveguide, an output waveguide, two Y-branch waveguides and two asymmetric strip-shaped waveguides. The input waveguide and the two asymmetric strip-shaped waveguides are connected through one Y-branch waveguide, and the output waveguide and the two asymmetric strip-shaped waveguides are connected through the other Y-branch waveguide; by adding a section of wide waveguide and tapered waveguides at two ends, TE and TM fundamental modes have a phase difference of pi with the other arm after passing through a phase shift region, and the phase difference of the front and rear arms is regulated and controlled to change between pi and 0, so that the switching of output modes is realized, and meanwhile, an effect irrelevant to polarization is achieved; and the device has the characteristics of high integration level, insensitive polarization, tunability and large bandwidth.

Description

Technical Field

[0001] The invention belongs to the technical field of integrated photonic devices, and in particular relates to a dual-polarization tunable mode converter and a mode multiplexing system based on thin-film lithium niobate. Background Art

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

[0003] In these hybrid multiplexing technologies, mode converters are key components for achieving efficient multimode transmission. In particular, mode converters that can simultaneously operate in dual polarization and ultra-wideband are particularly important for realizing high-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 an MDM switching system, the information capacity will increase several times. Large-bandwidth mode converters can be used to simultaneously route and switch multimode signals of multiple wavelengths, providing a viable option for high-capacity transmission.

[0004] Lithium niobate (LiNbO3, LN) has many advantages as one of the materials in the field of integrated photonics. 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. Secondly, LN has a large refractive index at a wavelength of 1550nm, 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 400nm to 5μm, which is suitable for a variety of optical applications. Most importantly, LN has stable physical and chemical properties, which ensures the stability and reliability of the device.

[0005] Lithium niobate on insulator (LNOI) technology has further expanded the application of lithium niobate. The LNOI platform consists of a submicron-thick LN film covered on a silicon dioxide substrate with a high refractive index contrast. This feature makes the optical waveguide devices manufactured on LNOI highly integrated and small in 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 will not only help improve the performance and stability of photonic devices, but also help to achieve miniaturization, low power consumption and integration of devices, thereby promoting the development of optical communication and optical interconnection technologies. In order to achieve efficient mode multiplexing on the thin-film lithium niobate platform, mode converters are essential multi-mode passive devices.

[0006] Patent CN105158850A provides an electro-optical mode converter of a Mach-Zehnder interferometer structure and an implementation method thereof, including 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 to achieve mutual conversion between the two modes. On the one hand, the above scheme is designed to use a symmetrical MZI structure, and mode conversion cannot be performed without an external power supply. On the other hand, the above scheme does not have the function of simultaneously realizing mode conversion for both TE and TM polarizations, which limits its scope of application.

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

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

[0009] To achieve the above object, the present invention provides a dual polarization mode converter based on thin-film lithium niobate, comprising a lithium niobate thin film and a Mach-Zehnder structure fabricated on the lithium niobate thin film layer, wherein the Mach-Zehnder structure comprises 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 another of the Y-branch waveguides; the two asymmetric strip waveguides have different widths, which are used to adjust the phase difference of the light beams passing through the two asymmetric strip waveguides.

[0011] Furthermore, 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] Furthermore, the width of the wide waveguide is 1.1-1.5 μm, 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 to 35 μm, preferably 29 to 31 μm;

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

[0015] Furthermore, the width of the input waveguide and the output waveguide is 2 to 2.4 μm;

[0016] The width of the branch portion on one side where the two Y-branch waveguides are connected to the wide waveguide is a gradual structure, and the width gradually increases toward one end of the wide waveguide;

[0017] The width of the branch portion on one side where the two Y-branch waveguides are connected to the narrow waveguide is the same as the width of the narrow waveguide.

[0018] Furthermore, the thickness of the lithium niobate film is 200-400 nm, and the thickness of the input waveguide, the output waveguide, the two symmetrical Y-branch waveguides and the two asymmetrical strip waveguides are 200-400 nm.

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

[0020] Furthermore, the lithium niobate film, input waveguide, output waveguide, two symmetrical Y-branch waveguides and two asymmetric strip waveguides are all covered with a silicon dioxide cladding, and a metal heater is provided above the silicon dioxide cladding, and the metal heater is located above the wider waveguide of the two asymmetric strip waveguides to achieve thermal modulation; or a metal electrode structure is provided on both sides of the two asymmetric strip waveguides to achieve electro-optical modulation of lithium niobate. Before and after regulation, the phase difference of the TE and TM modes in the two arms varies between π and 0.

[0021] Furthermore, a silicon dioxide cladding layer is provided below the lithium niobate film, and the silicon dioxide cladding layer is located on a silicon substrate.

[0022] The present invention proposes for the first time a high-bandwidth, dual-polarization tunable mode converter on a thin-film lithium niobate (TFLN) platform, which is used to achieve efficient mode conversion on a thin-film lithium niobate platform. This mode converter has a simple structure and is easy to integrate, while achieving polarization multiplexing and mode multiplexing. The device bandwidth is greater than 100nm, and the device length is about 100μm.

[0023] A mode multiplexing system comprises a mode demultiplexer, a mode converter as described in any one of the above items, four input grating couplers and four output grating couplers, wherein the four input grating couplers are respectively connected to the mode multiplexer, the mode multiplexer is connected to an input waveguide of the mode converter, the output waveguide of the mode converter is connected to the mode demultiplexer, and the four output grating couplers are respectively connected to the mode demultiplexer.

[0024] In general, the above technical solution conceived by the present invention has the following technical advantages compared with the prior art:

[0025] 1. The present invention adopts for the first time a Mach-Zehnder interferometer (MZI) structure based on thin-film lithium niobate to construct a dual-polarization mode converter, and utilizes the differentiated width design of the two branch waveguides to make the propagation constants and phase velocities of the TE and TM light beams in the two branches different, thereby generating a phase difference. It has the function of simultaneously realizing mode conversion for both TE and TM polarizations, realizing polarization-insensitive mode conversion, and has the characteristics of high integration, polarization insensitivity, tunability and large bandwidth.

[0026] 2. The present invention 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 π with the other arm after passing through the phase shift zone, thereby realizing polarization-insensitive mode conversion; at the same time, by adding thermo-optical or electro-optical modulation, the phase difference can be regulated between π and 0. Therefore, the present invention can not only improve device integration, but also realize the switching of output modes, and finally realize dual-polarization large-bandwidth multi-mode conversion.

[0027] 3. The present invention can simultaneously perform mode conversion on TE and TM modes. Without thermo-optical modulation, the TE0-TE1, TE1-TE0, TM0-TM1, and TM1-TM0 mode conversion processes can be realized simultaneously, and the conversion efficiency is relatively high. In the wavelength range of 1500 to 1600 nm, the conversion efficiency of each output is higher than -0.37 dB, and the total crosstalk is less than -15 dB, achieving the effects of low insertion loss and large bandwidth, and has great application prospects in mode multiplexing optical communication systems.

[0028] 4. The present invention reduces the loss caused by the sudden change of the waveguide width in the phase shift region by designing the width of the Y branch to be a tapered gradient and an inverse gradient waveguide connected to the input and output waveguides respectively.

[0029] 5. The present invention further adds a thermal or electrical adjustment structure to the waveguide in the phase shift region, and uses thermal adjustment or electrical modulation to change the phase difference of the phase shifter, thereby controlling whether the output mode is the fundamental mode or the first-order mode, thereby achieving the purpose of tunability. In the case of thermo-optical modulation, the mode of the output port depends on the phase shift caused by the phase shifter. When the phase shifter introduces a 0 phase difference, the input mode is transferred to the output end without any change; when the phase shifter introduces a π phase difference, the input mode will undergo a mode-order conversion, the input fundamental mode is converted to the first-order mode, and the input first-order mode will be converted to the fundamental mode.

[0030] 6. The present invention adopts the MZI structure, which is simpler than the existing structure, and the phase shifter structure is easy to manufacture, which further reduces the process difficulty and reduces the overall length of the device. The overall structure of the dual-polarization tunable mode converter is designed to be mirror-symmetrical about the middle position of the phase shift region, and the structure of the phase shift region is relatively simple, with a relatively large process tolerance. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0032] Figure 2 Schematic diagram of the cross-section of the lithium niobate thin film ridge waveguide in the mode converter.

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

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

[0035] Figure 5 This is the overall field distribution diagram of the device corresponding to the dual polarization mode converter of Example 1 after TE0 is input.

[0036] Figure 6 This is the overall field distribution diagram of the device corresponding to the dual polarization mode converter of Example 1 after TE1 is input.

[0037] Figure 7 This is the overall field distribution diagram of the device corresponding to the dual polarization mode converter of Example 1 after TM0 is input.

[0038] Figure 8 This is the overall field distribution diagram of the device corresponding to the dual polarization mode converter of Example 1 after TM1 is input.

[0039] Fig. 9 This is the transmission spectrum of the device after TE0 is input into the dual polarization mode converter of Example 1.

[0040] Fig.10 This is the transmission spectrum of the device after TE1 is input into the dual polarization mode converter of Example 1.

[0041] Fig.11 This is the transmission spectrum of the device after TM0 is input into the dual polarization mode converter of Example 1.

[0042] Fig.12 This is the transmission spectrum of the device after TM1 is input into the dual polarization mode converter of Example 1.

[0043] Fig.13 This is the device conversion efficiency spectrum corresponding to the dual polarization mode converter of Example 2 after TE0 is input.

[0044] Fig.14 This is the device conversion efficiency spectrum corresponding to the dual polarization mode converter of Example 2 after TE1 is input.

[0045] Fig.15 This is the device conversion efficiency spectrum corresponding to the dual polarization mode converter of Example 2 after TM0 is input.

[0046] Fig.16 This is the device conversion efficiency spectrum corresponding to the dual polarization mode converter of Example 2 after TM1 is input. DETAILED DESCRIPTION

[0047] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0048] like Figure 1 The figure shows a schematic diagram of the three-dimensional (3D) principle structure of the mode converter and a cross-sectional view of the input waveguide, wherein the angle α corresponds to the cross section of the input end in the figure. In an embodiment of the present invention, a dual-polarization tunable mode converter based on thin-film lithium niobate is provided, comprising a lithium niobate thin film and a Mach-Zehnder structure made on the lithium niobate thin film, wherein the Mach-Zehnder structure comprises 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 via one of the Y-branch waveguides, and the output waveguide and the two asymmetric strip waveguides are connected via another of the Y-branch waveguides; the two asymmetric strip waveguides have different widths, which are used to adjust the phase difference of the light beams passing through the two asymmetric strip waveguides.

[0050] With such a configuration, it is only necessary to adjust the width and length of the wider strip waveguide to make the propagation constant and phase velocity different, thereby achieving the control of the phase shift of the light beam passing through the waveguide, and then causing the light beam to have a phase difference with the light beam in the other waveguide, so that the TE and TM mode conversions can be achieved simultaneously. Based on the MZI (Mach-Zehnder Interferometer) structure, the present invention makes a differentiated design for the width and length of one of the branch waveguides, so that the phase difference can be controlled without an external optical modulator, which can not only improve the integration, but also achieve dual-polarization large-bandwidth multi-mode conversion.

[0051] Specifically, the mode converter designed by the present invention 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), the waveguide is arranged along the y direction, and the bottom of the x-cut lithium niobate film is a silicon dioxide cladding and a silicon substrate in sequence. The device adopts a typical MZI (Mach-Zehnder Interferometer) structure, which is mainly composed of an input waveguide, an output waveguide, two identical 3dB1×2Y branch waveguides, a waveguide without a phase shift region, and a waveguide with a phase shift region. The input waveguide is connected to one end of the Y branch, and then divided into two waveguides, i.e., two arms of the MZI structure. One arm of the MZI-based mode converter 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 arms are connected to the output waveguide after merging through another Y branch, thus forming an MZI structure.

[0052] Since the waveguide widths of the upper and lower arms of the MZI are different, the propagation constant and phase velocity will also be different, which can produce a phase difference; and without thermo-optical modulation, the mode converter can simultaneously realize TE0-TE1, TE1-TE0, TM0-TM1, and TM1-TM0 mode conversion processes, and the mode profile at the output end has a very high mode purity.

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

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

[0055] like Figure 2As shown, in order to make the mode output by the mode converter tunable, a silicon dioxide cladding layer (the silicon dioxide cladding layer exists above the entire lithium niobate film) and a metal heater (existing only above the phase shifter) can be deposited above the phase shifter, and the phase difference of the phase shifter can be changed by thermal modulation, thereby controlling whether the output mode is the fundamental mode or the first-order mode. Alternatively, metal electrode structures are set on both sides of two asymmetric strip waveguides to realize electro-optic modulation of lithium niobate, that is, metal electrode structures are added to the sides along the length direction of the wide waveguide (phase shift zone). For example, the TE0 mode is input, and the output mode can be switched between TE0 and TE1 through thermo-optical modulation (the effective refractive index of the material is changed by heating, thereby changing the phase, so that the phase difference is switched between 0 and π). Specifically, in the case of thermo-optical modulation, the mode of the output port depends on the phase shift caused by the phase shifter. When the phase shifter introduces a phase difference of 0, the input mode is transferred to the output end without any change. When the phase shifter introduces a phase difference of π, the input mode will undergo mode-order conversion, the input fundamental mode is converted to the first-order mode, and the input first-order mode will be converted to the fundamental mode. Therefore, the 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 as well as perform mode-order conversion on the input signal.

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

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

[0058] Symmetrical Y-branch junctions are 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) supports both the fundamental mode and the first-order mode. The working mechanism of the symmetric Y branch can be described as the evolution of 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 π phase difference.

[0059] The thermo-optical phase shifter is composed of a wide waveguide. Since the waveguide widths of the upper and lower arms of the MZI are different, the propagation constant and phase velocity will also be different, which can produce a phase difference. In order to make the mode converter suitable for both TE and TM modes, it is necessary to perform π phase shift on TE0 and TM0 at the design wavelength of 1550nm. By adjusting the structure of the Y branch and the phase shifter, the effective refractive index of the TE and TM modes can be simultaneously adjusted so that the two beams have equal power and a specific phase difference. Therefore, the present invention selects the appropriate length and width of the phase shift zone according to the formula.

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

[0061]

[0062] The above is the design and simulation work of the mode converter. Finally, we need to further design the test structure, that is, the mode multiplexing system, such as Figure 4 As shown in FIG. 1 , this mode multiplexing system consists 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 to the mode multiplexer, the mode multiplexer is connected to the input waveguide of the mode converter, the output waveguide of the mode converter is connected to the mode demultiplexer, and the four output grating couplers are connected to 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 passes through the TE / TM polarization grating coupler respectively. The input TE0 or TM0 mode will be 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 to the corresponding output port through the DEMUX, thereby achieving efficient multi-mode transmission.

[0063] Embodiment 1

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

[0065] The following tests were performed on the dual polarization mode converter based on thin-film lithium niobate provided in the first embodiment:

[0066] By inputting the structure into the simulation software Lumerical FDTD and performing simulation, the distribution of the simulated electric field at 1550nm can be obtained when the dual-polarization mode converter inputs four different modes. Figure 5 The overall field distribution diagram of the device after the dual polarization mode converter inputs TE0. The input TE0 mode is divided into two beams with equal power and the same phase. During the transmission process, the phase shifter introduces a phase difference of π between the two beams. Finally, the two anti-phase TE0 modes are recombined to produce a TE1 mode. Similarly, Figure 6 The overall field distribution diagram of the device corresponding to the dual polarization mode converter after inputting TE1; Figure 7 The overall field distribution diagram of the device corresponding to the dual polarization mode converter after inputting TM0; Figure 8 The overall field distribution diagram of the device after the dual polarization mode converter inputs TM1. 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 a very high mode purity.

[0067] By processing and drawing the simulation results, Fig. 9 This is the transmission spectrum of the device after TE0 is input into the dual polarization mode converter. After the input TE0 mode passes through the mode converter, more than 92% of the light is converted into TE1 mode. The output light contains very few TE0, TM0, and TM1 modes. The insertion loss at 1550nm is -0.41dB and the crosstalk is -16dB. Fig.10 This is the transmission spectrum of the device after TE1 is input into the dual polarization mode converter. After the input TE1 mode passes through the mode converter, more than 93% of the light is converted into TE0 mode. The output light contains very few TE1, TM0, and TM1 modes. The insertion loss at 1550nm is -0.39dB and the crosstalk is -15dB. Fig.11 This is the transmission spectrum of the device after TM0 is input into the dual polarization mode converter. After the input TM0 mode passes through the mode converter, more than 95% of the light is converted into TM1 mode. The output light contains very few TE0, TE1, and TM0 modes. The insertion loss at 1550nm is -0.27dB and the crosstalk is -16dB. Fig.12 The transmission spectrum of the device after TM1 is input to the dual polarization mode converter. After the input TM1 mode passes through the mode converter, more than 95% of the light is converted into the TM0 mode. The output light contains very few TE0, TE1, and 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 by the present invention significantly improves the mode conversion efficiency by optimizing the size structure, which is above 92%, and has small crosstalk, so that efficient mode conversion can be achieved. Further optimization of simulation and experiment can obtain higher conversion efficiency and lower crosstalk value, realize efficient dual polarization mode conversion, and achieve the expected effect. It can be seen that through the optimization of structural parameters, the TE and TM modes can achieve π phase shift through the phase shift zone, so that the device can achieve polarization-independent effect.

[0068] Embodiment 2

[0069] The difference from the first embodiment is that the wide waveguide width of the MZI arm is 1.2 μm and the length is 35 μm. Figure 13-16 As shown, it can be seen that the conversion efficiency is above 95%, which is better than that of the first embodiment.

[0070] In summary, the dual-polarization tunable mode converter based on thin-film lithium niobate described in the present invention is carried out under the support of the key process and integrated technology development project of thin-film lithium niobate photonic integration (project number: 2021YFB2800100). Through the typical Mach-Zehnder interferometer (MZI) structure, it can be applied to both TE and TM modes. Without thermo-optical modulation, TE0-TE1, TE1-TE0, TM0-TM1, and TM1-TM0 mode conversion processes can be realized simultaneously, and the conversion efficiency is relatively high. In the wavelength range of 1500 to 1600nm, the conversion efficiency of each output is higher than -0.41dB, and the total crosstalk is less than -15dB. In the case of thermo-optical modulation, the mode of the output port depends on the phase shift caused by the phase shifter. The phase difference of the phase shifter is changed by thermal modulation or electrical modulation, thereby controlling whether the output mode is the fundamental mode or the first-order mode. Moreover, thin-film lithium niobate devices can reduce the manufacturing and maintenance costs of the system while ensuring high performance because of their compatibility with existing manufacturing processes and excellent optical properties. In order to achieve efficient mode multiplexing on the thin-film lithium niobate platform, the mode converter is an indispensable 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] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A dual polarization mode converter based on thin film lithium niobate, characterized in that: It includes a lithium niobate film and a Mach-Zehnder structure made on the lithium niobate film, wherein the Mach-Zehnder structure includes 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 via one Y-branch waveguide, and the output waveguide and the two asymmetric strip waveguides are connected via another Y-branch waveguide; the two asymmetric strip waveguides have different widths.

2. The dual polarization mode converter based on thin film lithium niobate according to claim 1, characterized in that: One of the two asymmetric strip waveguides is a narrow waveguide without a phase shift zone, and the other is a wide waveguide with a phase shift zone. The width of the narrow waveguide is the same as the width of the Y-branch waveguide, and the width of the wide waveguide is greater than the width of the Y-branch waveguide.

3. The dual polarization mode converter based on thin film lithium niobate according to claim 2, characterized in that: 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 phase difference of the light beams passing through the two asymmetric strip waveguides is preferably π.

4. The dual polarization mode converter based on thin film lithium niobate according to claim 2, characterized in that: The length of the wide waveguide is 25 to 35 μm, preferably 29 to 31 μm; the length of the narrow waveguide is the same as the length of the wide waveguide; The distance between the wide waveguide and the narrow waveguide is 3-5 μm.

5. The dual polarization mode converter based on thin film lithium niobate according to claim 2, characterized in that: The width of the input waveguide and the output waveguide is 2 to 2.4 μm; The width of the branch portion on one side where the two Y-branch waveguides are connected to the wide waveguide is a gradual structure, and the width gradually increases toward one end of the wide waveguide; The width of the branch portion on one side where the two Y-branch waveguides are connected to the narrow waveguide is the same as the width of the narrow waveguide.

6. The dual polarization mode converter based on thin film lithium niobate according to claim 1, characterized in that: The thickness of the lithium niobate film is 200-400 nm, and the thickness of the input waveguide, the output waveguide, the two symmetrical Y-branch waveguides and the two asymmetrical strip waveguides are 200-400 nm.

7. The dual polarization mode converter based on thin film lithium niobate according to claim 6, characterized in that: The lithium niobate film is an x-cut lithium niobate film; the input waveguide, output waveguide, two symmetrical Y-branch waveguides, two asymmetrical strip waveguides and a lithium niobate film with a thickness of 200 to 400 nm are obtained by etching the lithium niobate film with a thickness of 500 to 700 nm.

8. The dual polarization mode converter based on thin film lithium niobate according to any one of claims 1 to 7, characterized in that: The lithium niobate film, input waveguide, output waveguide, two symmetrical Y-branch waveguides and two asymmetric strip waveguides are all covered with a silicon dioxide cladding, and a metal heater is provided above the silicon dioxide cladding. The metal heater is located above the wider waveguide of the two asymmetric strip waveguides to achieve thermal adjustment; or metal electrode structures are provided on both sides of the two asymmetric strip waveguides to achieve electro-optical modulation of lithium niobate, and the phase difference of the TE and TM modes in the two arms before and after the adjustment varies between π and 0.

9. The dual polarization mode converter based on thin film lithium niobate according to claim 8, characterized in that: A silicon dioxide cladding layer is provided below the lithium niobate film, and the silicon dioxide cladding layer is located on a silicon substrate.

10. A mode multiplexing system, characterized in that: It comprises a mode demultiplexer, the mode converter according to any one of claims 1 to 9, four input grating couplers and four output grating couplers, wherein the four input grating couplers are respectively connected to the mode multiplexer, the mode multiplexer is connected to the input waveguide of the mode converter, the output waveguide of the mode converter is connected to the mode demultiplexer, and the four output grating couplers are respectively connected to the mode demultiplexer.

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