Thin film lithium niobate fast adiabatic mode converter and method of implementing the same

Abstract

This invention discloses a thin-film lithium niobate fast adiabatic mode converter and its implementation method, belonging to the field of integrated optoelectronics technology. The converter is a lithium niobate layer, a dual waveguide system composed of waveguide a and waveguide b, including an input waveguide, a mode converter waveguide, and an output waveguide connected in sequence; the implementation method includes the following steps: Step 1. Determine the width w2 of waveguide b and the width w of waveguide a. 1max and w 1min Step 1. Waveguide spacing gap1, gap2, and gap3, and waveguide lengths L1 and L3; Step 2. Obtain the taper function y(w1) of the mode converter waveguide in waveguide a using fast adiabatic theory; Step 3. Scan the mode converter waveguide length L2 to obtain the mode conversion efficiency curve; Step 4. Select a suitable L2 according to application requirements; This invention provides a highly efficient and compact fast adiabatic mode converter implementation method, which has the advantages of large bandwidth, low insertion loss, compact structure, and large fabrication tolerance.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of integrated optoelectronic technology and the field of adiabatic mode converter design, and particularly relates to a thin film lithium niobate fast adiabatic mode converter with the advantages of large bandwidth, low insertion loss, large preparation tolerance and compact structure. BACKGROUND

[0002] With the development of the information age and the increasing demand for information transmission and data processing, the field of integrated photonics using light as an information carrier has developed. Lithium niobate material has a high electro-optic coefficient, a high damage threshold and a wide optical transparent window, and is an ideal electro-optic modulation material. With the development of thin film lithium niobate integrated photonics platform, low-loss optical waveguides with strong optical field confinement capability have been realized, and thin film lithium niobate electro-optic modulators with low half-wave voltage and high electro-optic bandwidth have been successfully developed. With the advantages of thin film lithium niobate high-speed electro-optic modulators, it is of great research value to realize large-bandwidth, large-capacity and high-speed on-chip optical interconnection based on thin film lithium niobate integrated photonics platform. Multi-dimensional multiplexing technology can transmit multiple signals in the same channel, which can greatly improve the transmission efficiency and capacity. Mode division multiplexing utilizes the spatial dimension in an optical waveguide to load optical signals into different modes in the same optical waveguide. Since each mode is orthogonal to each other, each mode can independently transmit data, and multiple signals can be transmitted simultaneously in the same optical waveguide. In order to realize high-performance mode multiplexing, a mode converter is needed to complete the conversion from the fundamental mode to the high-order mode. Linear adiabatic mode converters can realize the conversion of optical modes without introducing additional loss and scattering through a slow gradual structure, and have wideband and high efficiency. However, in order to ensure the adiabatic coupling condition, the linear adiabatic mode converter is large in size, which limits its integration advantage. Therefore, a compact, wideband and efficient adiabatic mode converter is a key device in the mode multiplexing system of the thin film lithium niobate photonics integrated platform. SUMMARY

[0003] In order to solve the above problems existing in the prior art, the purpose of the present application is to design a thin film lithium niobate fast adiabatic mode converter with large bandwidth, low insertion loss, small size and large preparation tolerance.

[0004] The purpose of the present application is achieved by the following technical solutions:

[0005] A thin film lithium niobate fast adiabatic mode converter, comprising a substrate layer, a buried oxygen layer, an X-cut lithium niobate layer and a silicon dioxide upper cladding layer which are sequentially stacked from bottom to top; the X-cut lithium niobate layer contains a thin film lithium niobate waveguide formed by etching technology, the thin film lithium niobate waveguide is a double waveguide system composed of waveguide a and waveguide b, and comprises an input end waveguide, a mode converter waveguide and an output end waveguide connected in sequence.

[0006] Furthermore, the input waveguide length is L1, comprising the input waveguide in waveguide a and the input waveguide in waveguide b; the input waveguide in waveguide a is a Bessel curved waveguide, and the spacing between the input waveguides in waveguide a and waveguide b changes from gap1 to gap2, where gap1 is greater than gap2; the width of the input waveguide remains constant, and the width of the input waveguide in waveguide a is w. 1max The width of the input waveguide in waveguide b is w2.

[0007] Furthermore, the mode converter waveguide has a length of L2, comprising a mode converter waveguide in waveguide a and a mode converter waveguide in waveguide b, with a fixed spacing gap2 between the two waveguides; the width of the mode converter waveguide in waveguide a is from w... 1max Change to w 1min The waveguide width of the mode converter in waveguide b is w2.

[0008] Furthermore, the waveguide length of the output waveguide is L3, encompassing the output waveguides in waveguide a and waveguide b; the output waveguide in waveguide a is a Bessel curved waveguide, and the spacing between the output waveguides in waveguide a and waveguide b changes from gap2 to gap3, where gap2 is smaller than gap3; the width of the output waveguide remains constant, and the width of waveguide a is w. 1min The width of waveguide b is w2.

[0009] Furthermore, the wider end of the mode converter waveguide in waveguide a is connected to the input waveguide in waveguide a, and the narrower end is connected to the output waveguide in waveguide a. One end of the mode converter waveguide in waveguide b is connected to the input waveguide in waveguide b, and the other end is connected to the output waveguide in waveguide b.

[0010] This invention also discloses a method for implementing a thin-film lithium niobate fast adiabatic mode converter, wherein the thin-film lithium niobate fast adiabatic mode converter is the aforementioned thin-film lithium niobate fast adiabatic mode converter, and the implementation method includes the following steps:

[0011] Step 1: Determine the width w2 of waveguide b and the width w of waveguide a. 1max The width w of waveguide a 1min The values ​​of parameters such as waveguide spacing gap1, waveguide spacing gap2, waveguide spacing gap3, input waveguide length L1, and output waveguide length L3;

[0012] Step 2: Using rapid adiabatic theory, obtain the taper function y(w1) and adiabatic parameter c(y) of the mode converter waveguide in waveguide a;

[0013] Step 3: Scan the length L2 of the mode converter waveguide by Lumerical Mode Solutions numerical simulator to obtain a mode conversion efficiency curve;

[0014] Step 4: Select the length L2 of the mode converter waveguide according to application requirements.

[0015] Specifically, the specific implementation method of step 1 is as follows:

[0016] The mode purity of the input port and the output port of the mode converter waveguide at different widths w2, widths w 1max , widths w 1min , and waveguide spacing gap2 is calculated by Lumerical Mode Solutions numerical simulator, and the appropriate widths w2, widths w 1max , widths w 1min , and waveguide spacing gap2 are selected by the mode purity, and the optimization principle of the mode purity is that the energy of the supermode in waveguide a excited by the TE0 mode input by the input waveguide in waveguide a is greater than 90%, and the energy of the supermode in waveguide b output by the mode converter waveguide is also greater than 90%, the supermode refers to a mode of a double waveguide system, and 90% refers to 90% of the total energy of the double waveguide system in waveguide a or waveguide b.

[0017] The loss of the input waveguide at different waveguide spacings gap1 and waveguide lengths L1 is calculated by Lumerical FDTD Solutions numerical simulator, and the waveguide spacing gap1 and waveguide length L1 with a waveguide loss less than 0.044 dB are selected; the output waveguide spacing gap3 and waveguide length L3 are the same parameter values as the input waveguide.

[0018] Specifically, the specific implementation method of step 2 is as follows:

[0019] The adiabatic parameter c lin (y) of the linear adiabatic mode converter is calculated. lin The expression of c

[0020]

[0021] wherein n is a supermode excited by the input light input into the mode converter waveguide, m is another supermode of the double waveguide system, the supermode n and the supermode m are even mode and odd mode of the double waveguide system respectively, E mt is the electric field of the supermode m, H nt is the magnetic field of the supermode n, β m , and β nThe propagation constants of the supermodes m and n respectively, y is the direction of light propagation, is the unit vector of y direction, is the conjugate of the magnetic field of the supermode n;

[0022] The expression of the taper function of the fast adiabatic mode converter is shown in the second equation,

[0023]

[0024] The expression of the adiabatic parameter of the fast adiabatic mode converter is shown in the third equation,

[0025]

[0026] By using the Lumerical Mode Solutions numerical simulator, the supermode n excited by the incident light input into the mode converter waveguide region and another supermode m in the mode converter waveguide region are obtained, and the mode conversion of the supermode m and the supermode n in the waveguide a is calculated, in which the mode converter waveguide width changes from w 1max to w 1min , the effective refractive index n eff , the electric field E mt , the magnetic field H nt , the propagation constant β m and the propagation constant β n , are substituted into the first equation to obtain the adiabatic parameter c lin (y) of the linear adiabatic mode converter waveguide per unit length.

[0027] The value of c lin (y) and L2=1 are substituted into the second equation to obtain the taper function y(w1) of the fast adiabatic mode converter waveguide per unit length; the first equation and the second equation are substituted into the third equation to obtain the adiabatic parameter c(y) of the fast adiabatic mode converter waveguide.

[0028] The beneficial effects of the present application are as follows:

[0029] (1) The present application proposes a thin film lithium niobate mode converter optimization design method based on the fast adiabatic theory, which efficiently realizes the conversion from TE0 mode to TE1 mode, and has the advantages of small size, small insertion loss, large bandwidth and large preparation tolerance.

[0030] (2) The mode purity optimization method of the thin film lithium niobate mode converter region proposed by the present application can quickly determine the waveguide width and waveguide spacing of the mode converter waveguide region, which is beneficial to efficiently complete the device design.

[0031] (3) The application proposes a taper function optimization formula of a thin film lithium niobate fast adiabatic mode converter, through which the optimal waveguide shape of the fast adiabatic mode converter can be directly obtained, and the designed device structure is simple and easy to process. BRIEF DESCRIPTION OF DRAWINGS

[0032] Figure 1 It is a structure schematic diagram of a top view of a thin film lithium niobate waveguide layer of the application.

[0033] Figure 2 It is a structure schematic diagram of a mode converter cross section in the embodiment of the application.

[0034] Figure 3 The upper part is a structure schematic diagram of a mode converter waveguide region in the embodiment of the application and a mode field intensity distribution diagram at an input end cross section, an intersection point avoiding cross section and an output end cross section, and the lower part is an effective refractive index schematic diagram of the embodiment mode converter at different widths.

[0035] Figure 4 It is a comparison diagram of a taper function of a narrow waveguide of a fast adiabatic mode converter waveguide region and a taper function of a narrow waveguide of a linear adiabatic mode converter waveguide region in the embodiment of the application.

[0036] Figure 5 It is a comparison diagram of an adiabatic parameter of a fast adiabatic mode converter and an adiabatic parameter of a linear adiabatic mode converter in the embodiment of the application.

[0037] Figure 6 It is a comparison diagram of a power conversion efficiency of a fast adiabatic mode converter and a power conversion efficiency of a linear adiabatic mode converter in the embodiment of the application.

[0038] Figure 7 It is a comparison diagram of a transmission spectrum of a fast adiabatic mode converter and a transmission spectrum of a linear adiabatic mode converter in the embodiment of the application.

[0039] Figure 8 It is a simulation electric field intensity propagation diagram of a fast adiabatic mode converter in the embodiment of the application.

[0040] Figure 9 It is a tolerance analysis diagram of a fast adiabatic mode converter in the embodiment of the application. DETAILED DESCRIPTION

[0041] In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the following will be combined with the accompanying drawings of the embodiments of the present application to make a clear and complete description of the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. The components of the embodiments of the present application described and shown in the accompanying drawings can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present application provided in the accompanying drawings is not intended to limit the scope of the claimed present application, but only represents selected embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present application.

[0042] Referring to Figures 1-9 , the specific embodiments of the present application are as follows.

[0043]

Embodiment 1

[0044] In order to better illustrate the present embodiment, the components in the accompanying drawings can be enlarged or reduced, and do not represent the actual device size. The waveguide spacing and waveguide width both refer to the size of the upper surface of the waveguide.

[0045] A thin-film lithium niobate fast adiabatic mode converter, as shown in Figure 2 , comprises a substrate layer, a buried oxygen layer, an X-cut lithium niobate layer and a silicon dioxide upper cladding layer which are sequentially stacked from bottom to top; as shown in Figure 1 , the X-cut lithium niobate layer contains a thin-film lithium niobate waveguide formed by etching technology, the thin-film lithium niobate waveguide is a double waveguide system composed of waveguide a and waveguide b, and includes sequentially connected input end waveguide, mode converter waveguide and output end waveguide; waveguide a is a single-mode waveguide, and the waveguide width w1 is in the range of 0.5-1.4 μm; waveguide b is a multi-mode waveguide, and the waveguide width w2 is greater than 1.8 μm.

[0046] The input end waveguide length is L1, and contains the input end waveguide in waveguide a and the input end waveguide in waveguide b; the input end waveguide in waveguide a is a Bessel curved waveguide, and the spacing between the input end waveguide in waveguide a and the input end waveguide in waveguide b changes from gap1 to gap2, and gap1 is greater than gap2; the input end waveguide width is constant, the width of the input end waveguide in waveguide a is w 1max , and the width of the input end waveguide in waveguide b is w2.

[0047] The mode converter waveguide length is L2, the value range of L2 is 0-800 μm, including the mode converter waveguide in waveguide a and the mode converter waveguide in waveguide b, the interval gap2 between the two waveguides is a fixed value, according to the preparation requirement, the value range of gap2 is 0.4-0.8 μm; the mode converter waveguide width in waveguide a changes from w 1max to w 1min according to the taper function y(w1), and the mode converter waveguide width in waveguide b is w2.

[0048] The waveguide length of the output end waveguide is L3, including the output end waveguide in waveguide a and the output end waveguide in waveguide b; the output end waveguide in waveguide a is a Bessel curved waveguide, the interval between the output end waveguide in waveguide a and the output end waveguide in waveguide b changes from gap2 to gap3, and gap2 is less than gap3; the output end waveguide width is constant, the width of waveguide a is w 1min , and the width of waveguide b is w2.

[0049] The wider end of the mode converter waveguide in waveguide a is connected with the input end waveguide in waveguide a, and the narrower end is connected with the output end waveguide in waveguide a; one end of the mode converter waveguide in waveguide b is connected with the input end waveguide in waveguide b, and the other end is connected with the output end waveguide in waveguide b. As shown in FIG. 2, when TE0 mode is input from the input end waveguide in waveguide a, the mode energy is mainly concentrated in the narrower waveguide a at first, and is quickly adiabatically converted into TE1 mode through the mode converter waveguide, and the mode field energy is distributed in the wide waveguide b, and finally is output through the output end waveguide in waveguide b. Figure 3

[0050] The implementation method of the above-mentioned thin film lithium niobate rapid adiabatic mode converter includes the following steps:

[0051] Step 1: determine the values of the width w2 of waveguide b, the width w 1max of waveguide a, the width w 1min of waveguide a, the waveguide interval gap1, the waveguide interval gap2, the waveguide interval gap3, and the input end waveguide length L1 and the output end waveguide length L3; specifically, the mode purity of the input port and the output port of the mode converter waveguide at different widths w2, widths w 1max , widths w 1min , and waveguide interval gap2 is calculated by Lumerical Mode Solutions numerical simulator, and the appropriate width w2, width w 1max , and width w 1min ​, the waveguide gap gap2, the principle of the mode purity optimization is that the energy of the supermode in the waveguide a which is excited by the TE0 mode inputted by the input waveguide in the waveguide a is greater than 90%, and the energy of the supermode in the waveguide b which is outputted by the mode converter waveguide is also greater than 90%, the supermode refers to a mode of the double waveguide system, and 90% refers to 90% of the total energy of the double waveguide system in the waveguide a or the waveguide b.

[0052] The loss of the input waveguide under different waveguide gaps gap1 and waveguide lengths L1 is calculated by the Lumerical FDTD Solutions numerical simulator, and the waveguide gap gap1 and the waveguide length L1 with the waveguide loss less than 0.044 dB are selected; the output waveguide gap gap3 and the waveguide length L3 are equal to the parameter values of the input waveguide.

[0053] Step 2: In order to avoid the cross effect in the strong coupling area, the taper function y(w1) and the adiabatic parameter c(y) of the mode converter waveguide in the waveguide a are obtained by the fast adiabatic theory; specifically, the adiabatic parameter c lin (y) of the linear adiabatic mode converter is calculated. lin The expression of the taper function of the fast adiabatic mode converter is shown in the first equation,

[0054]

[0055] wherein n is a supermode excited by the input light inputted into the mode converter waveguide, m is another supermode of the double waveguide system, the supermode n and the supermode m are even mode and odd mode of the double waveguide system respectively, E mt is the electric field of the supermode m, H nt is the magnetic field of the supermode n, β m and β n are the propagation constants of the supermode m and the supermode n respectively, y is the propagation direction of the light, is the unit vector in the y direction, is the conjugate of the magnetic field of the supermode n.

[0056] The expression of the taper function of the fast adiabatic mode converter is shown in the second equation,

[0057]

[0058] The expression of the adiabatic parameter of the fast adiabatic mode converter is shown in the third equation,

[0059]

[0060] Using the Lumerical Mode Solutions numerical simulator, we obtain a supermode n excited by incident light input to the mode converter waveguide region and another supermode m in the same waveguide region. We then calculate the mode converter waveguide widths of supermode m and supermode n in waveguide a, respectively, from w. 1max Linear transformation to w 1min Effective refractive index n during the process eff Electric field E mt Magnetic field H nt Propagation constant β m and propagation constant β n Substituting into the first equation, we obtain the adiabatic parameter c of the linear adiabatic mode converter waveguide per unit length. lin (y);

[0061] c lin Substituting the value of (y) and L2=1 into the second equation, we obtain the taper function y(w1) of the waveguide of the fast adiabatic mode converter per unit length; substituting the first and second equations into the third equation, we obtain the adiabatic parameter c(y) of the waveguide of the fast adiabatic mode converter.

[0062] Step 3: Using the Lumerical Mode Solutions numerical simulator, scan the waveguide length L2 of the mode converter to obtain the mode conversion efficiency curve;

[0063] Step 4: Select the waveguide length L2 of the mode converter according to the application requirements.

[0064]

Example 2

[0065] To better illustrate this embodiment, the components in the accompanying drawings may be enlarged or reduced, and do not represent the actual device dimensions. This embodiment will use specific numerical values ​​to describe the invention in detail.

[0066] like Figure 2 As shown, the cross-section of the fast adiabatic mode converter is a ridge waveguide, which is stacked from bottom to top as a silicon substrate, a silicon dioxide buried oxide layer, a thin film lithium niobate layer, and a silicon dioxide top cladding. The light propagation direction is along the Y direction of the lithium niobate crystal. The ridge height is 180 nm, the plate thickness is 180 nm, the angle between the waveguide sidewall and the horizontal plane is 60°, the thickness of the silicon dioxide buried oxide layer is 2 μm, and its refractive index is 1.457. The thickness of the top cladding is 0.7 μm, and its refractive index is 1.455.

[0067] To minimize the losses in the input and output waveguides, the input waveguide spacing gap1 and the output waveguide spacing gap3 are 5.2 μm, and the input waveguide length L1 and the output waveguide length L3 are 38 μm.

[0068] The mode purity of the input and output ports of the mode converter region at various widths and waveguide separations is calculated by the Lumerical Mode Solutions numerical simulator, and the appropriate waveguide width and waveguide separation are selected by the mode purity. The mode converter waveguide width w1 in waveguide a 1min The smaller the mode converter waveguide width w2 in waveguide b and the waveguide separation gap2 are, the more supermode energy of the output mode converter waveguide region is in waveguide b, but it is necessary to ensure that both supermodes are transverse electric modes. The mode converter waveguide width w1 in waveguide a 1max The larger the mode converter waveguide width w2 in waveguide b and the waveguide separation gap2 are, the more supermode energy of the input mode converter waveguide region is in waveguide a, but it is necessary to ensure that waveguide a is a single-mode waveguide. Considering that the wider the waveguide width and the larger the waveguide separation, the larger the device size will be, w1 = 1.4 μm, w2 = 0.5 μm, and gap = 0.7 μm are selected here. 1max 1min = 2.5 μm, and gap = 0.7 μm.

[0069] The supermode (SP2) excited by the input mode converter waveguide region of the incident light and another supermode (SP3) in the mode converter waveguide region are found by the Lumerical Mode Solutions numerical simulator, and the effective refractive index (n eff ), electric field, magnetic field, and propagation constant of SP2 and SP3 in the mode converter waveguide width of waveguide a are calculated from 1.4 μm to 0.5 μm linearly. As shown in Figure 3 , the strong coupling region is located near w1 = 0.975 μm; the three gray-scale diagrams above the device represent the mode field distribution diagrams of the SP2 input mode converter waveguide, the strong coupling region, and the output mode converter waveguide, respectively, and the mode conversion from TE0 to TE1 is achieved; the three gray-scale diagrams below the device represent the mode field distribution diagrams of the SP3 input mode converter waveguide, the strong coupling region, and the output mode converter waveguide, respectively, and the strong coupling of SP2 and SP3 of the linear adiabatic mode converter occurs in the strong coupling region.

[0070] The taper function and the adiabatic parameter of the fast adiabatic mode converter are calculated by the fast adiabatic theory, and the results are shown in Figure 4 and Figure 5 . As can be seen from Figure 5 , the adiabatic parameter c lin ​(y) reaches a maximum at w1 = 0.975 μm, if the device length is short, the target mode SP2 and SP3 will be strongly coupled, and the energy in the target mode SP2 will decrease. The waveguide width w1 near the strong coupling region changes slowly, which is beneficial to the adiabaticity of the device, but for the linear adiabatic taper, the change rate of the width w1 in the whole taper region is constant, which makes the device length of the linear adiabatic mode converter long. In order to make the whole adiabatic mode converter adiabatic, unlike the change rate of the width in the whole taper region of the linear adiabatic mode converter, the fast adiabatic mode converter allocates the width in the whole taper region on demand, changes rapidly at the positions where the starting point and the terminal point c(y) are small, and changes slowly at the positions where the strong coupling region c(y) is large, and overall makes c(y) present a stable value, which avoids the cross effect, as shown in Figure 4 and Figure 5 .

[0071] Scan the length of the waveguide of the fast adiabatic mode converter, and obtain the mode conversion efficiency curve, as shown in Figure 6 From the figure, it can be seen that in order to make the whole mode converter adiabatic, the length of the linear adiabatic mode converter is long; after experiencing the same length L2, the fast adiabatic mode converter can keep more energy in the target mode SP2. For example, at the wavelength λ = 1550 nm and L2 = 115 μm, the conversion efficiency of the fast adiabatic mode converter is 98%, while the conversion efficiency of the linear adiabatic mode converter is only 52%, which indicates that more energy is coupled to SP3.

[0072] When a power conversion efficiency of 99% is required, the length L2 of the waveguide of the fast adiabatic mode converter of the present application is 276 μm. Construct a complete fast adiabatic mode converter, and obtain the transmission spectrum diagram of the whole mode converter, as shown in Figure 7 From the figure, it can be seen that in the wavelength range of 1400-1600 nm, the conversion efficiency of the fast adiabatic mode converter is more than 90%, while the linear adiabatic mode converter is very sensitive to the wavelength; under the same device length, the fast adiabatic mode converter has a larger bandwidth and a smaller insertion loss than the linear adiabatic mode converter.

[0073] Figure 8 For the simulation electric field intensity propagation diagram of the fast adiabatic mode converter at the wavelength λ = 1550 nm and L2 = 276 μm, it can be seen that the energy of the TE0 mode input to the fast adiabatic mode converter is basically converted to the TE1 mode in the mode converter waveguide region.

[0074] Figure 9For the tolerance analysis chart of the fast adiabatic mode converter when the etching depth and the waveguide width of the L2=276 μm thin film lithium niobate waveguide change by ±10 nm and ±20 nm respectively, it can be seen that the conversion efficiency of the fast adiabatic mode converter is still above 90% in the wavelength range of 1450-1575 nm, which shows that the preparation tolerance of the fast adiabatic mode converter is large.

[0075] The present application is not limited to the above-mentioned optional embodiments, and anyone can derive other various forms of products under the inspiration of the present application, but regardless of any change in its parameters, any technical solution falling within the scope defined by the claims of the present application falls within the protection scope of the present application.

Claims

1. A method for implementing a thin-film lithium niobate fast adiabatic mode converter, characterized in that, The thin-film lithium niobate fast adiabatic mode converter includes a substrate layer, a buried oxide layer, an X-cut lithium niobate layer, and a silicon dioxide cladding layer stacked sequentially from bottom to top; the X-cut lithium niobate layer contains a thin-film lithium niobate waveguide formed by etching technology; the thin-film lithium niobate waveguide is a dual waveguide system composed of waveguide a and waveguide b, including an input waveguide, a mode converter waveguide, and an output waveguide connected sequentially; The input waveguide has a length of L1 and includes the input waveguide in waveguide a and the input waveguide in waveguide b. The input waveguide in waveguide a is a Bessel curved waveguide. The spacing between the input waveguides in waveguide a and waveguide b changes from gap1 to gap2, where gap1 is larger than gap2. The width of the input waveguide in waveguide b is w2, and the width of the input waveguide in waveguide a is w. 1max； The mode converter waveguide has a length of L2 and includes a mode converter waveguide in waveguide a and a mode converter waveguide in waveguide b. The spacing gap2 between the two waveguides is a fixed value. The width of the mode converter waveguide in waveguide a follows a taper function from w... 1max Change to w 1min The mode converter waveguide width in waveguide b is w. 2； The output waveguide has a length of L3 and includes the output waveguides in waveguide a and waveguide b. The output waveguide in waveguide a is a Bessel curved waveguide. The spacing between the output waveguides in waveguide a and waveguide b changes from gap2 to gap3, where gap2 is smaller than gap3. The width of the output waveguide in waveguide a is w. 1min The width of the output waveguide in waveguide b is w2; The wider end of the mode converter waveguide in waveguide a is connected to the input waveguide in waveguide a, and the narrower end is connected to the output waveguide in waveguide a. One end of the mode converter waveguide in waveguide b is connected to the input waveguide in waveguide b, and the other end is connected to the output waveguide in waveguide b. Its implementation method includes the following steps: Step 1: Determine the width w2 of waveguide b and the width w of waveguide a. 1max The width w of waveguide a 1min The values ​​of parameters such as waveguide spacing gap1, waveguide spacing gap2, waveguide spacing gap3, input waveguide length L1, and output waveguide length L3; Step 2: Using rapid adiabatic theory, obtain the taper function y(w1) and adiabatic parameter c(y) of the mode converter waveguide in waveguide a; Step 3: Using the Lumerical Mode Solutions numerical simulator, scan the waveguide length L2 of the mode converter to obtain the mode conversion efficiency curve; Step 4: Select the waveguide length L2 of the mode converter according to the application requirements.

2. The method for implementing a thin-film lithium niobate fast adiabatic mode converter according to claim 1, characterized in that, The specific implementation method of step 1 is as follows: The mode converter waveguide was calculated using the Lumerical Mode Solutions numerical simulator at different widths w2 and w. 1max Width w 1min The mode purity of the input and output ports under waveguide spacing gap2 is used to select appropriate widths w2 and w based on the mode purity. 1max Width w 1min The waveguide spacing gap2, and the principle of mode purity optimization are as follows: the energy of the supermode excited by the input waveguide of the input end waveguide in the waveguide a should be greater than 90% in waveguide a, and the energy of the supermode output from the waveguide of the mode converter in waveguide b should also be greater than 90%. The supermode refers to a mode of the dual waveguide system, and 90% means that the energy in waveguide a or waveguide b accounts for 90% of the total energy of the dual waveguide system. Using the Lumerical FDTD Solutions numerical simulator, the loss of the input waveguide under different waveguide spacing gap1 and waveguide length L1 was calculated, and waveguide spacing gap1 and waveguide length L1 with waveguide loss less than 0.044 dB were selected; the output waveguide spacing gap3 and waveguide length L3 were the same parameter values ​​as the input waveguide.

3. The method for implementing a thin-film lithium niobate fast adiabatic mode converter according to claim 2, characterized in that, The specific implementation method of step 2 is as follows: Calculate the adiabatic parameter c of the linear adiabatic mode converter. lin (y), c lin The expression for (y) is shown in the first equation. ； Where n is the supermode excited by the waveguide of the mode converter, and m is another supermode of the dual-waveguide system, with supermode n and supermode m being the even and odd modes of the dual-waveguide system, respectively. mt For the electric field of supermode m, H nt For the magnetic field of supermode n, β m and β n Let be the propagation constants of supermode m and supermode n, respectively. For the direction of light propagation, It is the unit vector in the y-direction. For the conjugate of the magnetic field of supermode n; The expression for the taper function of the fast adiabatic mode converter is shown in the second equation. ； The expression for the adiabatic parameters of the fast adiabatic mode converter is shown in the third-party code. ； Using the Lumerical Mode Solutions numerical simulator, we obtain a supermode n excited by incident light input to the mode converter waveguide region and another supermode m in the same waveguide region. We then calculate the mode converter waveguide widths of supermode m and supermode n in waveguide a, respectively, from w. 1max Linear transformation to w 1min Effective refractive index n during the process eff Electric field E mt Magnetic field H nt Propagation constant β m and propagation constant β n Substituting into the first equation, we obtain the adiabatic parameter c of the linear adiabatic mode converter waveguide per unit length. lin (y); c lin Substituting the value of (y) and L2=1 into the second equation, we obtain the taper function y(w1) of the waveguide of the fast adiabatic mode converter per unit length; substituting the first and second equations into the third equation, we obtain the adiabatic parameter c(y) of the waveguide of the fast adiabatic mode converter.

4. A thin-film lithium niobate fast adiabatic mode converter prepared by the implementation method of the thin-film lithium niobate fast adiabatic mode converter according to any one of claims 1-3.

Images