A lithium niobate thin film electro-optic modulator based on an MZI structure electro-optic polymer loaded strip waveguide and a preparation method thereof

Abstract

The application discloses a lithium niobate thin film electro-optic modulator based on an MZI structure electro-optic polymer loaded strip waveguide and a preparation method thereof, and belongs to the technical field of planar optical waveguide optical modulators and preparation thereof. The lithium niobate thin film electro-optic modulator is composed of a silicon substrate, a silicon dioxide oxide layer, a lithium niobate thin film and an electro-optic polymer loaded strip waveguide from bottom to top. The electro-optic polymer loaded strip waveguide is of an MZI structure, and is composed of an input straight waveguide, an input tapered waveguide, a first curved waveguide and a second curved waveguide, a first modulation arm straight waveguide and a second modulation arm straight waveguide, a third curved waveguide and a fourth curved waveguide, an output tapered waveguide and an output straight waveguide from left to right along an optical input direction. First grounding electrodes, second grounding electrodes and signal electrodes are respectively prepared on the lithium niobate thin films outside and inside the first modulation arm straight waveguide and the second modulation arm straight waveguide. The device is simple in manufacturing process, can effectively reduce the modulation efficiency of the modulator, reduce the power consumption of the modulator and is more conducive to high integration.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of planar optical waveguide light modulators and its preparation technology, and particularly relates to a lithium niobate thin film electro-optic modulator based on an MZI structure electro-optic polymer loaded strip waveguide and a preparation method thereof. BACKGROUND

[0002] With the rapid development of new generation information technologies such as artificial intelligence and Internet of Things, the traditional communication network has been difficult to meet people's demand for information transmission and exchange, and the development of high-speed optical communication network is urgent. As the core device of optical communication system and chip-on-chip optical interconnection, electro-optic modulator is used to complete the loading of information and the regulation of signal; its principle is to use the electro-optic effect of materials to make the refractive index of materials change linearly, so as to regulate the intensity, phase and other information of the light wave transmitted in the medium.

[0003] The materials commonly used in existing electro-optic modulators mainly include the following: lithium niobate (LN), silicon (Si) and III-V compounds and polymers. Lithium niobate has good physical and chemical stability and excellent electro-optic effect, and is a commonly used material for manufacturing electro-optic modulators. The waveguide of the traditional lithium niobate electro-optic modulator is made by titanium diffusion or proton exchange process, and the refractive index difference between the waveguide core layer and the cladding layer is small, and the light binding ability is poor, which leads to the total length of the packaged device usually being 5-10 cm, which is very unfavorable for integration. The electro-optic modulator based on thin film lithium niobate overcomes the shortcomings of the traditional electro-optic modulator, and the refractive index difference between the waveguide core layer and the cladding layer is high, which is beneficial to reduce the overall size of the device.

[0004] According to the difference of waveguide type, thin film lithium niobate waveguide can be divided into etched type and loaded type waveguide. Due to the chemical inertness of lithium niobate material, physical dry etching is a common method for preparing etched type lithium niobate waveguide. The method can accurately control the etching depth and waveguide shape. However, in different etching equipment, the dry etching process shows low selectivity and reproducibility, and the inclined sidewall formed by etching will increase the transmission loss of the waveguide. The loaded type waveguide is to directly deposit or spin-coat a second material (SiN, a-Si, TiO2, Ti2O5, Polymer, etc.) on lithium niobate to form a waveguide structure, which avoids the dry etching process of lithium niobate and is conducive to realizing low-loss transmission of the waveguide. Silicon nitride is a CMOS process compatible material, which has a mature processing technology and is convenient to make, and has always been a common material for loaded strip waveguide. However, the refractive index of silicon nitride is similar to that of lithium niobate, and the lithium niobate waveguide with silicon nitride as the loaded strip has part of the mode field distributed in the silicon nitride loaded strip; when performing electro-optical modulation, the silicon nitride does not have electro-optical effect, and the mode field in the loaded strip is not utilized, which greatly reduces the modulation efficiency of the modulator.

[0005] At present, the electro-optical modulator based on lithium niobate thin film has been widely studied and rapidly developed, but with the upgrading of optical network to ultra-high speed and ultra-long distance transmission, it is necessary to further improve the modulation efficiency to meet the growing demand of optical communication network. SUMMARY

[0006] In order to further improve the modulation efficiency of lithium niobate electro-optical modulator, the application provides a lithium niobate thin film electro-optical modulator based on MZI structure electro-optical polymer loaded strip waveguide and a preparation method thereof.

[0007] The application uses silicon with a silicon dioxide buffer layer as a waveguide substrate, lithium niobate thin film as a flat plate layer of loaded type waveguide, and an electro-optical polymer with a high dielectric constant as a loaded strip. The application selects an electro-optical polymer with multiple types, small refractive index, large electro-optical coefficient, and large refractive index difference between the lithium niobate thin film. The preparation process adopted by the application is simple, can be well compatible with semiconductor process, easy to integrate, meets the requirements of large-scale production, and has certain practical value.

[0008] As shown in Figure 1 and Figure 3 , a lithium niobate thin film electro-optical modulator based on MZI structure electro-optical polymer loaded strip waveguide is composed of a silicon substrate (1), a silicon dioxide oxidation layer (2) prepared on the silicon substrate (1), a lithium niobate thin film (3) prepared on the silicon dioxide oxidation layer (2), and an electro-optical polymer loaded strip waveguide (4) prepared on the lithium niobate thin film (3) from bottom to top.

[0009] AsFigure 2 As shown, the electro-optical polymer loaded strip waveguide (4) is in MZI structure, and is composed of input straight waveguide (8), input tapered waveguide (9), first curved waveguide (10') and second curved waveguide (10) with the same structure, first modulation arm straight waveguide (11) and second modulation arm straight waveguide (12) with the same structure and parallel to each other, third curved waveguide (15') and fourth curved waveguide (15) with the same structure, output tapered waveguide (16), and output straight waveguide (17) from left to right along the light input direction. A first ground electrode (13') and a second ground electrode (13) with long strip structure are respectively prepared symmetrically on the outside of the lithium niobate film (3) of the first modulation arm straight waveguide (11) and the second modulation arm straight waveguide (12), and a signal electrode (14) with rectangular structure is prepared on the inside of the lithium niobate film (3) of the first modulation arm straight waveguide (11) and the second modulation arm straight waveguide (12). The first curved waveguide (10') and the third curved waveguide (15') are symmetrically arranged about the first modulation arm straight waveguide (11), the second curved waveguide (10) and the fourth curved waveguide (15) are symmetrically arranged about the second modulation arm straight waveguide (12), and the first ground electrode (13') and the second ground electrode (13) are symmetrically arranged about the signal electrode (14). The input straight waveguide (8), the input tapered waveguide (9), the first curved waveguide (10') and the second curved waveguide (10) constitute a 3-dB Y splitter, and the third curved waveguide (15') and the fourth curved waveguide (15), the output tapered waveguide (16), and the output straight waveguide (17) constitute a 3-dB Y combiner.

[0010] Further, the length a1 and a1' of the input straight waveguide (8) and the output straight waveguide (17) are equal to 10-500 μm, the width w1 and w1' are equal to 1-5 μm, the length a2 and a2' of the first curved waveguide (10'), the second curved waveguide (10), the third curved waveguide (15') and the fourth curved waveguide (15) parallel to the projection of the input light direction in the input straight waveguide (8) are equal to 15-300 μm, the width w2', w2, w5' and w5 are equal to 1-5 μm; the length a3' and a3 of the first modulation arm straight waveguide (11) and the second modulation arm straight waveguide (12) are equal to 3.5-9 mm, the width w3' and w3 are equal to 1-5 μm; the length a4 and a4' of the input tapered waveguide (9) and the output tapered waveguide (16) parallel to the projection of the input light direction in the input straight waveguide (8) are equal to 5-100 μm, the width w4 and w4' of the connection of the input tapered waveguide (9) with the first curved waveguide (10') and the second curved waveguide (10) and the output tapered waveguide (16) with the third curved waveguide (15') and the fourth curved waveguide (15) are equal to 1-30 μm; the length a5' and a5 of the first ground electrode (13') and the second ground electrode (13) are equal to 4-8 mm, the width b1' and b1 are equal to 80-150 μm; the length a6 of the signal electrode (14) is 4-8 mm, the width b2 is 10-20 μm; the distance gap' and gap between the signal electrode (14) and the first ground electrode (13') and the second ground electrode (13) are equal to 2-20 μm; the distance between the first modulation arm straight waveguide (11) and the second modulation arm straight waveguide (12) is 10-60 μm.

[0011] The thickness h1 of the silicon substrate (1) is 480-520 μm, the thickness h2 of the silicon dioxide oxide layer (2) is 1-5 μm, the thickness h3 of the lithium niobate thin film (3) is 100-800 nm, the thickness h4' and h4 of the electro-optic polymer loaded strip waveguide (4', 4 corresponding to the first modulation arm waveguide 11 and the second modulation arm waveguide 12) is 0.5-5 μm, the thickness h5, h6 and h6' of the signal electrode (14), the first ground electrode (13') and the second ground electrode (13) are equal to 0.5-6 μm. The signal electrode (14), the first ground electrode (13') and the second ground electrode (13) are some commonly used metal materials including Al and Au.

[0012] The preparation method of the lithium niobate thin film electro-optic modulator based on the MZI structure electro-optic polymer loaded strip waveguide is shown in the attached Figure 4 , and the specific steps are as follows:

[0013] A: Cleaning of lithium niobate wafer (lithium niobate wafer was purchased from Shanghai Xin Silicon Polymer Semiconductor Co., Ltd., and from top to bottom, it was composed of Si substrate (1), SiO2 oxide layer (2), and lithium niobate thin film layer (3))

[0014] First, the surface of the lithium niobate thin film layer (3) was cleaned by using acetone, methanol, and isopropanol solvents for 2-3 times in sequence;

[0015] B: Preparation of polymer loading strip

[0016] The electro-optic polymer (25, which is an electro-optic polymer doped with a chromophore molecule, including a series of organic polymer materials with good transparency, such as polymethyl methacrylate (PMMA), SU-8 2002, and SU-8 2005, and the refractive index of the electro-optic polymer is lower than that of lithium niobate) was coated on the surface of the cleaned lithium niobate thin film layer (3) by spin coating process at a speed of 2000-8000 rpm, and the thickness of the electro-optic polymer (25) thin film was 200 nm-2000 nm. The device was baked at 50-500°C for 10-40 minutes for curing, and then cooled to room temperature. A layer of aluminum metal with a thickness of 1-4 μm was evaporated on the electro-optic polymer (25) thin film by vacuum evaporation process to serve as an aluminum mask (26). A layer of positive photoresist BP 212 (27) with a thickness of 0.5-6 μm was coated on the aluminum mask (26) by spin coating process at a speed of 1000-6000 rpm. The device coated with photoresist BP 212 was heated at a temperature of 70-300°C for 10-30 minutes, and then cooled to room temperature. The core mask (30) was used as a mask (the light-shielding part of the core mask was the same as the structure of the electro-optic polymer loading strip waveguide (4) in Figure 2 The structure of the electro-optic polymer loading strip waveguide (4) in Figure 2The structure of the electro-optic polymer loaded strip waveguide (4) is the same; the core layer mask (30) is removed, and the aluminum mask (26) is etched by inductive coupled plasma (ICP) with the photoresist BP 212 (27') pattern as a mask to obtain the same aluminum layer pattern (26') as the structure of the electro-optic polymer loaded strip waveguide (4); then the photoresist BP 212 (27') is removed, that is, the device is soaked in an ethanol solution for 10-500 s, repeatedly washed with deionized water along the waveguide direction, dried with nitrogen, and heated for 5-50 minutes; then the electro-optic polymer (25) is etched by ICP with the aluminum layer pattern (26') as a mask, and the remaining electro-optic polymer pattern is the same as the structure of the electro-optic polymer loaded strip waveguide (4); the aluminum layer pattern (26') is removed, that is, the device is soaked in a NaOH solution with a mass concentration of 2-5 ‰ for 10-200 s, repeatedly washed with deionized water along the waveguide direction, dried with nitrogen, and heated for 5-50 minutes, and thus the electro-optic polymer loaded strip waveguide (4) is prepared;

[0017] C: Preparation of modulator electrodes

[0018] An Al electrode layer (28) with a thickness of 1-4 μm is deposited on the electro-optic polymer loaded strip waveguide (4) and lithium niobate film (3) by vacuum evaporation process, and a positive photoresist BP 212 (29) is coated on the surface of the Al electrode layer (28) by spin coating process at a speed of 1000-8000 rpm to obtain a thickness of 2-10 μm; the device with the spin-coated photoresist BP 212 is heated, that is, baked at a temperature of 30-300 °C for 10-50 minutes and then cooled to room temperature; the electrode mask (31) is used as a mask (the light-shielding part of the electrode mask is the same as the structure of the ground electrode and signal electrode to be prepared), and the photoresist BP 212 is exposed to ultraviolet light with a wavelength of 300-500 nm for 10-50 s to remove the photoresist outside the area where the ground electrode and signal electrode are to be prepared; the device is placed in a NaOH solution with a mass concentration of 2-5 ‰ for 5-300 s to remove the exposed photoresist, and then repeatedly washed with deionized water along the waveguide direction, dried with nitrogen, and heated for 5-50 minutes; the electrode mask (31) is removed, and then the unexposed BP 212 (29') (that is, the photoresist on the first ground electrode 13', the second ground electrode 13, and the signal electrode 14) is removed, that is, the device is soaked in an ethanol solution for 10-500 s, repeatedly washed with deionized water along the waveguide direction, dried with nitrogen, and heated for 5-50 minutes, and thus the preparation of the modulator electrodes is completed, thereby obtaining the lithium niobate film electro-optic modulator based on the MZI structure electro-optic polymer loaded strip waveguide.

[0019] Compared with the existing device structure and technology, the present application has the following advantages:

[0020] The present application uses electro-optic polymer as the loading strip of the loaded lithium niobate waveguide, which has a larger electro-optic coefficient and a lower refractive index compared with silicon nitride. The lower the refractive index of the loading strip, the greater the refractive index difference between the loading strip and the flat plate layer, and the larger the mode field area in lithium niobate. When an external electric field is applied for modulation, the mode field area in lithium niobate increases, which increases the overlap integral factor of the electric field and the mode field. In addition, the light mainly distributed in the lithium niobate waveguide and the light in the small amount of polymer loading strip undergo the same phase change during transmission, so the phase change of the modulator under the same external voltage is larger. In summary, the electro-optic polymer is selected as the loading strip of the lithium niobate loaded waveguide, which can effectively reduce the modulation efficiency of the modulator, reduce the power consumption of the modulator, and is more conducive to the high integration of the modulator. In addition, the device manufacturing process is simple, only some common equipment and conventional preparation process are needed, without expensive process equipment and high-difficulty preparation technology, with low production cost and high efficiency. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 : The schematic diagram of the passive area cross section of the lithium niobate thin film electro-optic modulator prepared by the present application is shown in Fig. 1(a-a'); Figure 2 : The schematic diagram of the passive area cross section of the lithium niobate thin film electro-optic modulator prepared by the present application is shown in Fig. 1(a-a');

[0022] Figure 2 : The schematic diagram of the structure of the electro-optic polymer loading strip waveguide of the lithium niobate thin film electro-optic modulator prepared by the present application is shown in Fig. 2(a-a');

[0023] Figure 3 : The schematic diagram of the active area cross section of the lithium niobate thin film electro-optic modulator prepared by the present application is shown in Fig. 3(b-b'); Figure 2 : The schematic diagram of the active area cross section of the lithium niobate thin film electro-optic modulator prepared by the present application is shown in Fig. 3(b-b');

[0024] Figure 4 : The flow chart of the preparation process of the lithium niobate thin film electro-optic modulator prepared by the present application is shown in Fig. 4;

[0025] Figure 5 :: The schematic diagram of the light field distribution in the cross section waveguide in Fig. 1(a-a') is shown in Fig. 5(a-a'); Figure 2

[0026] : : The schematic diagram of the electric field distribution in the cross section waveguide in Fig. 3(b-b') is shown in Fig. 5(b-b'); Figure 6 Figure 2 : The microwave impedance-frequency curve of the electro-optic modulator obtained by the HFSS software simulation is shown in Fig. 7(a), in which the abscissa is frequency and the ordinate is microwave impedance, which is directly obtained by the HFSS software;

[0027] : The microwave refractive index-frequency curve of the electro-optic modulator obtained by the HFSS software simulation is shown in Fig. 7(b), in which the abscissa is frequency and the ordinate is microwave refractive index, which is obtained by the formula

[0028] : The microwave refractive index-frequency curve of the electro-optic modulator obtained by the HFSS software simulation is shown in Fig. 7(b), in which the abscissa is frequency and the ordinate is microwave refractive index, which is obtained by the formula (Where c is the speed of light, f is the frequency, γ is the propagation constant, and Im(γ) represents the imaginary part of the propagation constant γ. All parameters are obtained from HFSS software.) The microwave refractive index is calculated as follows:

[0029] Figure 7(c): The transmission coefficient of the electro-optic modulator as a function of frequency, obtained by simulation using HFSS software. The horizontal axis represents frequency and the vertical axis represents transmission coefficient. This data was obtained directly using HFSS software.

[0030] Figure 7(d): The reflection coefficient of the electro-optic modulator as a function of frequency, obtained by simulation using HFSS software. The horizontal axis represents frequency and the vertical axis represents reflection coefficient. This was obtained directly using HFSS software.

[0031] like Figure 1 As shown, the names of each part are: silicon substrate (1), silicon dioxide oxide layer (2), lithium niobate thin film layer (3), electro-optic polymer loading strip (4, corresponding to the input straight waveguide (8)).

[0032] like Figure 2 As shown, the names of each part are: input straight waveguide (8), input tapered waveguide (9), first curved waveguide (10'), second curved waveguide (10), first modulation arm straight waveguide (11), second modulation arm straight waveguide (12), third curved waveguide (15'), fourth curved waveguide (15), output tapered waveguide (16), output straight waveguide (17), first ground electrode (13'), second ground electrode (13), and signal electrode (14).

[0033] like Figure 3 As shown, the names of each part are: silicon substrate (1), silicon dioxide oxide layer (2), lithium niobate thin film layer (3), electro-optic polymer loading strips (4' and 4, corresponding to the first modulation arm straight waveguide (11) and the second modulation arm straight waveguide (12)), first ground electrode (13'), second ground electrode (13), and signal electrode (14).

[0034] like Figure 5 As shown in the figure, most of the mode field is confined within the lithium niobate plate. While the light is transmitted effectively, the overlap area between the mode field and the electric field increases, thereby improving the modulation efficiency of the modulator.

[0035] like Figure 6 As shown in the figure, the electric field distribution is uniform, with larger electric fields at the boundaries between the electrode and the cladding and between the cladding and lithium niobate. This is because the electric field at the dielectric boundary satisfies Maxwell's equations. The cladding with a larger relative permittivity can increase the electric field strength inside the lithium niobate waveguide, reduce the voltage-length product of the modulator, and improve its modulation efficiency.

[0036] As shown in Figure 7(a), it can be seen that the value is lower in the high-frequency region and higher in the low-frequency region, approaching the target impedance value of 50Ω; as shown in Figure 7(b), it can be seen that the simulated value of microwave refractive index is higher in the low-frequency region and decreases rapidly, while it tends to be stable in the high-frequency region, matching the refractive index of the optical group of 2.2856, and the refractive index difference is less than 0.05 in the high-frequency region; as shown in Figure 7(c), it can be seen that the two curves of transmission coefficient S12 and S21 completely overlap, are lower in the high-frequency region and higher in the low-frequency region, and are relatively large overall, above -0.75dB; as shown in Figure 7(d), it can be seen that the reflection parameter S 11 S 22 The values ​​are relatively small, all below -34dB. Detailed Implementation

[0037] Example 1

[0038] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0039] Example structure diagram as follows Figure 2 As shown, the lengths a1 and a1' of the input straight waveguide (8) and the output straight waveguide (17) are equal to 10 μm, and the widths w1 and w1' are equal to 2 μm. The first curved waveguide (10'), the second curved waveguide (10), the third curved waveguide (15'), and the fourth curved waveguide (15) projected parallel to the input light direction in the input straight waveguide (8) have lengths a2 and a2' of 65 μm, and widths w2', w2, w5', and w5 of 2 μm. The lengths a3' and a3 of the first modulation arm straight waveguide (11) and the second modulation arm straight waveguide (12) are equal to 4.5 mm, and the widths w3' and w3 are equal to 2 μm. The input tapered waveguide (9) and the output tapered waveguide (16) projected parallel to the input light direction in the input straight waveguide (8). The lengths a4 and a4' are equal to 10 μm, and the widths w4 and w4' at the connection points with the first curved waveguide (10'), the second curved waveguide (10), the third curved waveguide (15'), and the fourth curved waveguide (15) are equal to 5 μm; the lengths a5' and a5 of the first ground electrode (13') and the second ground electrode (13) are equal to 4 mm, and the widths b1' and b1 are equal to 130 μm; the length a6 of the signal electrode (14) is 4 mm, and the width b2 is 15 μm; the gaps gap' and gap between the signal electrode (14) and the ground electrodes of the first ground electrode (13') and the second ground electrode (13) are equal to 5 μm; the distance between the first modulation arm straight waveguide (11) and the second modulation arm straight waveguide (12) is 20 μm.

[0040] like Figure 2 Appendix Figure 3As shown, the thickness h1 of the silicon substrate (1) is 520 μm, the thickness h2 of the silicon dioxide oxide layer (2) is 4.7 μm, the thickness of the lithium niobate thin film (3) is 300 nm, the thickness h4 of the electro-optic polymer loading strip (4) is 0.5 μm, and the thicknesses h5, h6 and h6' of the signal electrode (14), the first ground electrode (13') and the second ground electrode (13) are equal to 0.8 μm.

[0041] Under the above parameters, the modulation efficiency of the single-arm modulation of the modulator is 2.517 V·cm, and the modulation efficiency of the double-arm modulation is 1.259 V·cm, which are calculated by the simulation software Comsol Multiphysics. The radio frequency of the modulator is simulated by using the HFSS software. The impedance normalization is set to 50 Ω to simulate the high frequency performance of the modulator under a 50 Ω external system. The results show that the transmission parameters (S 12 and S 21 ) are large, more than -0.75 dB, and the reflection parameters (S 11 and S 22 ) are small, less than -35 dB. Under the 50 Ω system and the wide frequency range of 0-100 GHz, the traveling wave electrodes of this structure have small reflectivity and large transmissivity. The microwave refractive index is obtained by canceling the impedance normalization setting. The characteristic impedance simulation value and the microwave refractive index of this structure are shown in Figure 7(a) 、 7(b) As shown, the simulation value of the microwave refractive index is high in the low frequency region and decreases rapidly, and tends to be stable in the high frequency region. The simulation value of the characteristic impedance is also high in the low frequency region and decreases rapidly, and tends to be stable in the high frequency region. The impedance is about 47 Ω in the high frequency region, close to the target impedance value of 50 Ω.

[0042] The preparation method of the lithium niobate electro-optic modulator with high modulation efficiency based on the MZI structure according to the present application comprises the following steps:

[0043] A: cleaning of the lithium niobate wafer

[0044] The lithium niobate wafer is composed of a Si substrate (1), a SiO2 oxide layer (2) and a lithium niobate thin film layer (3) from bottom to top. First, the surface of the lithium niobate thin film layer (3) is cleaned by using acetone, methanol and isopropanol solvents in sequence for 3 times to ensure the cleanliness of the lithium niobate surface.

[0045] B: preparation of the polymer loading strip

[0046] The electro-optic polymer (25, SU-8 2002) is coated on the surface of the cleaned lithium niobate thin film layer (3) by a spin coating process at a speed of 7500 rpm, and the thickness of the electro-optic polymer (25) film is 800 nm. The device is baked at 95°C for 30 minutes for curing, and then cooled to room temperature. A layer of aluminum metal with a thickness of 2 μm is evaporated on the electro-optic polymer (25) film by a vacuum evaporation process to serve as an aluminum mask (26). A layer of positive photoresist BP 212 (27) with a thickness of 1.5 μm is coated on the aluminum mask (26) by a spin coating process at a speed of 5000 rpm. The device coated with the photoresist BP 212 is heated at a temperature of 120°C for 20 minutes, and then cooled to room temperature. The core layer mask (30) is used as a mask, and the light-shielding part of the core layer mask is the same in structure as the electro-optic polymer-loaded strip waveguide (4) in Figure 2 The photoresist BP 212 (27) is exposed to ultraviolet light with a wavelength of 365 nm for 9 s, and the exposed device is placed in a NaOH solution with a mass concentration of 2‰ for 15 s. Then, the device is repeatedly washed with deionized water along the waveguide direction and dried with nitrogen. After that, the exposed photoresist BP 212 is removed by heating for 35 minutes. The remaining photoresist BP 212 (27') pattern is the same in structure as the electro-optic polymer-loaded strip waveguide (4) in Figure 2 The core layer mask (30) is removed, and the photoresist BP 212 (27') pattern is used as a mask to perform inductively coupled plasma etching on the aluminum mask (26), so as to obtain an aluminum layer pattern (26') which is the same in structure as the electro-optic polymer-loaded strip waveguide (4). Then, the unexposed photoresist BP 212 (27') is removed by immersing the device in an ethanol solution for 30 s, repeatedly washing the device with deionized water along the waveguide direction, and drying the device with nitrogen. After that, the electro-optic polymer (25) is etched by ICP using the aluminum layer pattern (26') as a mask, and the remaining electro-optic polymer pattern is the same in structure as the electro-optic polymer-loaded strip waveguide (4). The aluminum layer pattern (26') is removed by immersing the device in a NaOH solution with a mass concentration of 2‰ for 12 s, repeatedly washing the device with deionized water along the waveguide direction, and drying the device with nitrogen. After that, the electro-optic polymer-loaded strip waveguide (4) is prepared by heating for 30 minutes.

[0047] C: Preparation of modulator electrodes

[0048] A 1.2 μm thick Al electrode layer (28) is deposited on the electro-optic polymer-loaded strip waveguide (4) and the lithium niobate film (3) by vacuum evaporation, and a positive photoresist BP 212 (29) is coated on the surface of the Al electrode layer (28) by spin coating at a speed of 4000 rpm to obtain a thickness of 3 μm, and the device with the spin-coated photoresist BP 212 is heated, i.e. baked at a temperature of 130 °C for 35 min and then cooled to room temperature; the electrode mask (31) is used as a mask, the light-shielding part of the electrode mask (31) has the same structure as the ground electrode and signal electrode to be prepared, the photoresist BP 212 is exposed to a UV lamp with a wavelength of 365 nm for 15 s, the photoresist outside the region where the ground electrode and signal electrode are to be prepared is exposed, the device is placed in a NaOH solution with a mass concentration of 2 ‰ for 20 s to remove the exposed photoresist, the device is repeatedly washed along the waveguide direction with deionized water and dried with nitrogen, and then heated for 30 min; the electrode mask (31) is removed, and then the unexposed BP 212 (29') is removed, i.e. the device is soaked in an ethanol solution for 30 s, repeatedly washed along the waveguide direction with deionized water, dried with nitrogen, and then heated for 30 min, and thus the preparation of the electrodes of the modulator is completed, thereby obtaining the lithium niobate film electro-optic modulator based on the MZI structure electro-optic polymer-loaded strip waveguide.

[0049] It should be noted that although the present application file contains a description of many details, it should not be interpreted as a limitation on the scope or content of any disclosed technology or what can be required, but as a description of features that can be specific to a particular embodiment of the disclosed technology, and the present application can have many variations, such as using barium titanate (BaTiO3), lithium tantalate (LiTaO3), potassium niobate (KNbO3), etc. electro-optic material; the modulator structure can also use symmetric (asymmetric) directional coupling, multi-mode interferometer, micro-ring resonator, etc. structure. Those skilled in the art, who obtain the scope of protection of the present patent based on the explicit disclosure of the present application or the written description of the file without any doubt, all belong to the scope of protection of the present patent.

Claims

1. A lithium niobate thin-film electro-optic modulator based on an MZI structure electro-optic polymer loaded strip waveguide, characterized in that: From bottom to top, it consists of a silicon substrate (1), a silicon dioxide oxide layer (2) prepared on the silicon substrate (1), a lithium niobate film (3) prepared on the silicon dioxide oxide layer (2), and an electro-optic polymer-loaded strip waveguide (4) prepared on the lithium niobate film (3). The electro-optic polymer-loaded strip waveguide (4) is an MZI structure. From left to right along the light input direction, it consists of an input straight waveguide (8), an input tapered waveguide (9), a first curved waveguide (10') and a second curved waveguide (10) with the same structure, a first modulation arm straight waveguide (11) and a second modulation arm straight waveguide (12) with the same structure and parallel to each other, a third curved waveguide (15') and a fourth curved waveguide (15) with the same structure, an output tapered waveguide (16), and an output straight waveguide (17). The first modulation arm straight waveguide (11) and the second modulation arm straight waveguide (17) are arranged in parallel. On the lithium niobate film (3) outside the guide (12), a first ground electrode (13') and a second ground electrode (13) with elongated structures are symmetrically prepared. On the lithium niobate film (3) inside the first modulation arm straight waveguide (11) and the second modulation arm straight waveguide (12), a signal electrode (14) with a rectangular structure is prepared. The first curved waveguide (10') and the third curved waveguide (15') are symmetrically arranged about the first modulation arm straight waveguide (11), the second curved waveguide (10) and the fourth curved waveguide (15) are symmetrically arranged about the second modulation arm straight waveguide (12), and the first ground electrode (13') and the second ground electrode (13) are symmetrically arranged about the signal electrode (14). The input straight waveguide (8), the input tapered waveguide (9), the first curved waveguide (10') and the second curved waveguide (10) constitute a 3-dB The Y-beam splitter, the third curved waveguide (15') and the fourth curved waveguide (15), the output tapered waveguide (16) and the output straight waveguide (17) constitute a 3-dB Y-beam combiner.

2. The lithium niobate thin-film electro-optic modulator based on an MZI structure electro-optic polymer loaded strip waveguide as described in claim 1, characterized in that: The electro-optic polymer (25) is an electro-optic polymer doped with chromophore molecules, and its refractive index is lower than that of lithium niobate; the first ground electrode (13'), the second ground electrode (13) and the signal electrode (14) are made of Al or Au.

3. The lithium niobate thin-film electro-optic modulator based on an MZI structure electro-optic polymer loaded strip waveguide as described in claim 2, characterized in that: The electro-optic polymer (25) is made of polymethyl methacrylate, SU-8 2002 or SU-8 2005.

4. The lithium niobate thin-film electro-optic modulator based on an MZI structure electro-optic polymer loaded strip waveguide as described in claim 1, characterized in that: The lengths a1 and a1' of the input straight waveguide (8) and the output straight waveguide (17) are equal, ranging from 10 to 500 μm, and the widths w1 and w1' are equal, ranging from 1 to 5 μm. The projection lengths a2 and a2' of the first curved waveguide (10'), the second curved waveguide (10), the third curved waveguide (15'), and the fourth curved waveguide (15) parallel to the input light direction in the input straight waveguide (8) are equal, ranging from 15 to 300 μm, and the widths w2', w2, w5', and w5 are equal, ranging from 1 to 5 μm. The lengths a3' and a3 of the first modulation arm straight waveguide (11) and the second modulation arm straight waveguide (12) are equal, ranging from 3.5 to 9 mm, and the widths w3' and w3 are equal, ranging from 1 to 5 μm. The projection lengths a4 and a4' of the input tapered waveguide (9) and the output tapered waveguide (16) parallel to the input light direction in the input straight waveguide (8) are equal, ranging from 5 to 100 μm. μm, the widths w4 and w4' at the connection points of the input tapered waveguide (9) with the first curved waveguide (10') and the second curved waveguide (10), and the output tapered waveguide (16) with the third curved waveguide (15') and the fourth curved waveguide (15) are equal to 1~30 μm; the lengths a5' and a5 of the first ground electrode (13') and the second ground electrode (13) are equal to 4~8 mm, and the widths b1' and b1 are equal to 80~150 μm; the length a6 of the signal electrode (14) is 4~8 mm, and the width b2 is 10~20 μm; the gaps gap' and gap between the signal electrode (14) and the first ground electrode (13') and the second ground electrode (13) are equal to 2~20 μm; The distance between the first modulation arm straight waveguide (11) and the second modulation arm straight waveguide (12) is 10~60 μm.

5. The lithium niobate thin-film electro-optic modulator based on an MZI structure electro-optic polymer loaded strip waveguide as described in claim 1, characterized in that: The thickness h1 of the silicon substrate (1) is 480~520 μm, the thickness h2 of the silicon dioxide oxide layer (2) is 1~5 μm, the thickness h3 of the lithium niobate thin film (3) is 100~800 nm, the thickness h4 of the electro-optic polymer loaded strip waveguide (4) is 0.5~5 μm, and the thicknesses h5, h6 and h6' of the signal electrode (14), the first ground electrode (13'), and the second ground electrode (13) are equal to 0.5~6 μm.

6. A method for fabricating a lithium niobate thin-film electro-optic modulator based on an MZI structure electro-optic polymer loaded strip waveguide as described in any one of claims 1 to 5, comprising the following steps: A: Lithium niobate wafer cleaning The lithium niobate wafer consists of three parts from bottom to top: a Si substrate (1), a SiO2 oxide layer (2), and a lithium niobate thin film layer (3). First, the surface of the lithium niobate thin film layer (3) is cleaned 2-3 times in sequence with acetone, methanol, and isopropanol solvents to ensure that the lithium niobate surface is clean. B: Preparation of Polymer Loading Strips Electro-optic polymer (25) was coated onto the cleaned lithium niobate thin film layer (3) using a spin coating process at a speed of 2000~8000 rpm, and the thickness of the electro-optic polymer (25) thin film was 200 nm~2000 nm. The film was then baked at 50~500 °C for 10~40 minutes for curing, and then cooled to room temperature. A layer of aluminum metal with a thickness of 1~4 μm was deposited on the electro-optic polymer (25) film using a vacuum evaporation process to serve as an aluminum mask (26); then, a layer of positive photoresist BP 212 (27) with a thickness of 0.5~6 μm was coated on the aluminum mask (26) using a spin coating process at a rotation speed of 1000~6000 rpm; Then heat it at 70~300 ℃ for 10~30 minutes, and then cool it to room temperature after heating is complete; Using the core layer mask (30) as a mask, the light-shielding part of the core layer mask (30) has the same structure as the electro-optic polymer loading strip waveguide (4). The photoresist BP 212 (27) is exposed to ultraviolet light with a wavelength of 300~500 nm for 5~50 s, and then placed in a NaOH solution with a mass concentration of 2~5‰ for 10~200 s. Then, it is repeatedly rinsed with deionized water along the waveguide direction, dried with nitrogen, and heated for 5~50 minutes to remove the exposed photoresist BP 212. The unexposed photoresist BP 212 pattern is left, which has the same structure as the electro-optic polymer loading strip waveguide (4). Remove the core layer mask (30), and then use the photoresist BP 212 pattern as a mask to perform inductively coupled plasma etching on the aluminum mask (26) to obtain an aluminum layer pattern (26') that is the same as the structure of the electro-optic polymer loaded strip waveguide (4); then remove the BP 212 (27') that was not exposed, and then soak it in an ethanol solution for 10~500 s, rinse it repeatedly with deionized water along the waveguide direction, and blow it dry with nitrogen and heat it for 5~50 minutes; Using the aluminum layer pattern (26') as a mask, the electro-optic polymer (25) is etched by ICP. The remaining electro-optic polymer pattern is the same as the structure of the electro-optic polymer loaded strip waveguide (4). The aluminum layer pattern (26') is removed by immersing it in a NaOH solution with a mass concentration of 2-5‰ for 10-200 s, rinsing it repeatedly with deionized water along the waveguide direction, drying it with nitrogen gas, and heating it for 5-50 minutes. The electro-optic polymer loaded strip waveguide (4) is thus completed. C: Modulator electrode fabrication A layer of Al electrode (28) with a thickness of 1~4 μm was deposited on the electro-optic polymer loaded strip waveguide (4) and lithium niobate film (3) using vacuum evaporation process. Positive photoresist BP 212 (29) was coated on the surface of Al electrode (28) using spin coating process at a speed of 1000~8000 rpm to obtain a thickness of BP 212 of 2~10 μm. Then, it was heated, that is, baked at a temperature of 30~300 ℃ for 10~50 minutes and then cooled to room temperature. Using an electrode mask (31) as a mask, the light-shielding part of the electrode mask (31) has the same structure as the ground electrode and signal electrode to be prepared. The photoresist BP 212 is exposed to a UV lamp with a wavelength of 300~500 nm for 10~50 s, so that the photoresist outside the area where the ground electrode and signal electrode need to be prepared is exposed. Then, it is placed in a NaOH solution with a mass concentration of 2~5‰ for 5~300 s to remove the exposed photoresist. It is rinsed repeatedly with deionized water along the waveguide direction, dried with nitrogen, and heated for 5~50 minutes. Remove the unexposed BP212 (29'), i.e., soak it in ethanol solution for 10~500 s, rinse it repeatedly with deionized water along the waveguide direction, blow it dry with nitrogen gas and heat it for 5~50 minutes. At this point, the modulator electrode is prepared, thus obtaining the lithium niobate thin film electro-optic modulator based on the MZI structure electro-optic polymer loaded strip waveguide.

Images