An MZI type electro-optic modulator based on electro-optic polymer / lithium niobate thin film hetero-integrated waveguide and a preparation method thereof

Abstract

The application discloses an MZI type electro-optic modulator based on an electro-optic polymer / lithium niobate thin film hetero-integrated waveguide and a preparation method thereof, and belongs to the technical field of planar optical waveguide light modulators and preparation thereof.The MZI type electro-optic modulator comprises, from bottom to top, a silicon substrate, a silicon dioxide oxide layer, a lithium niobate flat plate layer, a ridge structure lithium niobate core layer and an electro-optic polymer upper cladding layer; the ridge structure lithium niobate core layer 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; and a ground electrode and a signal electrode are respectively prepared on the lithium niobate flat plate layer outside and inside the first modulation arm straight waveguide and the second modulation arm straight waveguide.The application effectively reduces the modulation efficiency of the modulator, reduces the size of the device, is compatible with a semiconductor process, can realize large-scale integration and production, and has important practical application value.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of planar optical waveguide light modulator and its preparation technology, and particularly relates to a high-efficiency Mach-Zehnder interferometer (MZI) type electro-optic modulator based on electro-optic polymer / lithium niobate thin film hetero-integrated waveguide and a preparation method thereof, which takes an etched lithium niobate thin film as a waveguide core layer and an electro-optic polymer with a smaller refractive index than the lithium niobate thin film as an upper cladding layer. BACKGROUND

[0002] With the rapid development of information technology, data traffic is growing explosively, and the requirements of human production and life for data transmission speed and data capacity are getting higher and higher. Data exchange equipment based on electrical interconnection is facing severe challenges in terms of bandwidth and power consumption. As the main transmission means of communication networks in the twenty-first century, optical fiber communication systems have developed rapidly and have the advantages of large bandwidth, low loss and fast transmission speed, meeting the needs of people in production and life for information transmission rate and transmission capacity. Among them, the electro-optic modulator is the core device of the optical communication system and the chip-on-chip optical interconnection chip, mainly used for completing information loading and signal regulation. Its principle is to use the electro-optic effect to change the refractive index of the material by applying an external voltage, thereby realizing the regulation of signal light amplitude, phase and intensity.

[0003] According to different material platforms, the current electro-optic modulator is mainly based on silicon platform, indium phosphide (InP) platform, lithium niobate platform and electro-optic polymer platform. The electro-optic modulator based on silicon platform and InP platform has been developed for many years in the field of optical communication, and has been matured day by day, and has put a batch of products on the market. However, the silicon modulator and the InP modulator both have the problems of slow modulation speed and large power consumption, which cannot meet the requirements of optical fiber communication network for high-speed information processing. Lithium niobate has excellent electro-optic effect and is an ideal material for preparing high-speed electro-optic modulators in recent years. The traditional lithium niobate electro-optic modulator is mainly based on a diffused lithium niobate waveguide, which is doped by titanium diffusion or proton exchange process to change the local refractive index of bulk lithium niobate to form a waveguide structure. However, the refractive index difference of the lithium niobate optical waveguide formed based on titanium diffusion is usually small, and the light limiting ability is poor. In order to reduce the power consumption and the driving voltage of the modulator, a very long modulation length is usually designed, and the entire device after packaging is mostly 5-10 cm, which is difficult to meet the needs of device miniaturization and integration.

[0004] To solve the above problems, thin film lithium niobate (LNOI) emerges as the times require. By means of ion slicing, lithium niobate thin film is peeled off from bulk lithium niobate and bonded to a silicon substrate with a silicon dioxide buffer layer to form thin film lithium niobate. Then, the directly etched lithium niobate substrate (the cross section after etching is usually ridge type) is used as a waveguide core layer, and a layer of silicon oxide is covered on the lithium niobate as an upper cladding layer through plasma enhanced chemical vapor deposition. The refractive index difference between the thin film lithium niobate optical waveguide core layer and the cladding layer constructed in this way is increased by several tens of times compared with the diffusion type lithium niobate waveguide, and the light binding capacity is significantly enhanced. The electro-optic modulator made of the above structure has higher modulation efficiency, smaller device size and larger modulation bandwidth, and is more conducive to realizing large-scale integration, and is a very promising development direction.

[0005] At present, the electro-optic modulator based on lithium niobate thin film has been widely studied and rapidly developed, but with the upgrading of optical networks 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. Therefore, the present application proposes an electro-optic modulator based on electro-optic polymer / lithium niobate thin film hetero-integrated waveguide to significantly improve the modulation efficiency of the device. SUMMARY

[0006] In order to further improve the modulation efficiency of the modulator, the present application proposes an MZI type electro-optic modulator based on electro-optic polymer / lithium niobate thin film hetero-integrated waveguide and a preparation method thereof.

[0007] The present application uses silicon with a silicon dioxide buffer layer as a waveguide substrate, etched lithium niobate as a waveguide core layer, and electro-optic polymer with a high dielectric constant as a waveguide upper cladding layer. The present application makes full use of the advantages of electro-optic polymer, such as large electro-optic coefficient, easy processing and variety. In addition, the preparation process of the present application is simple and compatible with semiconductor process, which can realize large-scale integration and production, and thus has important practical application value.

[0008] The technical scheme adopted by the present application to improve the modulation efficiency is as follows:

[0009] As shown in Figure 1 and Figure 3 , an MZI type electro-optic modulator based on electro-optic polymer / lithium niobate thin film hetero-integrated waveguide comprises, from bottom to top, a silicon substrate (1), a silicon dioxide oxide layer (2), a lithium niobate flat plate layer (3), a ridge type structure lithium niobate core layer (4) and an electro-optic polymer upper cladding layer (5); the electro-optic polymer upper cladding layer (5) is located above the lithium niobate flat plate layer (3) and the ridge type structure lithium niobate core layer (4), and the ridge type structure lithium niobate core layer (4) is completely covered in the electro-optic polymer upper cladding layer (5).

[0010] As shown in Figure 2As shown, the ridge-shaped structure lithium niobate core layer (4) is composed of input straight waveguide (14), input tapered waveguide (12), first curved waveguide (15) and second curved waveguide (15'), first modulation arm straight waveguide (8) and second modulation arm straight waveguide (9), third curved waveguide (16) and fourth curved waveguide (16') from left to right along the light input direction; the first modulation arm straight waveguide (8) and the second modulation arm straight waveguide (9) are symmetrical on the lithium niobate flat plate layer (3) outside which the first ground electrode (11) and the second ground electrode (11') are respectively prepared in long strip structure, and the signal electrode (10) is prepared in rectangular structure on the lithium niobate flat plate layer (3) inside which; the first curved waveguide (15) and the third curved waveguide (16) are symmetrically arranged about the first modulation arm straight waveguide (8), the second curved waveguide (15') and the fourth curved waveguide (16') are symmetrically arranged about the second modulation arm straight waveguide (9), and the first ground electrode (11) and the second ground electrode (11') are symmetrically arranged about the signal electrode (10); the input straight waveguide (14), the input tapered waveguide (12), the first curved waveguide (15) and the second curved waveguide (15') constitute a 3-dB Y splitter, and the third curved waveguide (16) and the fourth curved waveguide (16'), the output tapered waveguide (13) and the output straight waveguide (17) constitute a 3-dB Y combiner.

[0011] Further, the lengths a1 and a1' of the input straight waveguide (14) and the output straight waveguide (17) are equal to 10-500 μm, and the widths w1 and w1' are equal to 1-5 μm; the lengths a2 and a2' of the first curved waveguide (15), the second curved waveguide (15'), the third curved waveguide (16) and the fourth curved waveguide (16') are equal to 15-300 μm, and the widths w2, w2', w5 and w5' are equal to 1-5 μm; the lengths a3 and a3' of the first modulation arm straight waveguide (8) and the second modulation arm straight waveguide (9) are equal to 3.5-9 mm, and the widths w3 and w3' are equal to 1-5 μm; the lengths a4 and a4' of the input tapered waveguide (12) and the output tapered waveguide (13) are equal to 5-100 μm, and the widths w4 and w4' at the connection of the input tapered waveguide (12) with the first curved waveguide (15) and the second curved waveguide (15') and the output tapered waveguide (13) with the third curved waveguide (16) and the fourth curved waveguide (16') are equal to 1-30 μm; the lengths a5 and a5' of the first ground electrode (11) and the second ground electrode (11') 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 (10) is 4-8 mm, and the width b2 is 10-20 μm; the distances gap and gap' between the signal electrode (10) and the first ground electrode (11) and the second ground electrode (11') are equal to 2-20 μm; and the distance between the first modulation arm straight waveguide (8) and the second modulation arm straight waveguide (9) is 10-60 μm. The widths mentioned above are the upper bottom widths w of the ridge-shaped lithium niobate core layer 4, and the lower bottom widths w' follow the formula w'=w+2xh4xtanθ, wherein h4 is the thickness of the ridge-shaped lithium niobate core layer 4, and θ is the inclination angle of the ridge-shaped structure relative to the vertical direction.

[0012] The preparation method of the MZI-type electro-optic modulator based on the electro-optic polymer / lithium niobate thin film hetero-integrated waveguide is shown in the following process flow Figure 4 , and the specific steps are as follows:

[0013] A: cleaning of the lithium niobate wafer (purchased from Shanghai Xin Silicon Polymer Semiconductor Co., Ltd., the lithium niobate wafer is composed of a Si substrate (1), a SiO2 oxide layer (2) and a lithium niobate thin film layer (18) from bottom to top)

[0014] The surface of the lithium niobate thin film layer (18) is cleaned by using acetone, methanol and isopropyl alcohol for 2-3 times in sequence;

[0015] B: preparation of the lithium niobate core layer

[0016] An electron beam evaporator (EBE) was used to deposit a 50 nm to 5 μm chromium (Cr) layer (19) on the surface of the niobium lithium oxide thin film (18) as a mask for etching lithium niobate. Then, a positive photoresist BP 212 (20) was applied onto the chromium layer (19) using a spin-coating process at a speed of 2000 to 7000 rpm. The film was baked at 50 to 500°C for 10 to 40 minutes, and then cooled to room temperature. A core layer mask (21) was used as the mask (the light-blocking portion of the core layer mask is...). Figure 2 The ridge-type lithium niobate core layer (4) has the same structure, but does not include the signal electrode (10), the first ground electrode (11), and the second ground electrode (11'). The photoresist BP 212 is exposed to ultraviolet light with a wavelength of 300-500nm for 5-50s. The exposed device is placed in a NaOH solution with a mass concentration of 2-5‰ for 10-200s, then rinsed repeatedly with deionized water along the waveguide direction, dried with nitrogen, and heated for 5-50 minutes to remove the exposed BP 212. The unexposed BP 212 (20') is used as a mask, and its structure is the same as that of the BP 212 core layer (4). Figure 2 The ridge-type lithium niobate core layer (4) has the same structure; then, using the BP 212 (20') pattern as a mask, the chromium layer (19) is subjected to inductively coupled plasma (ICP) dry etching to obtain the same structure as the BP 212 (20') pattern. Figure 2 The chromium layer pattern (19') with the same structure as the ridge-shaped lithium niobate core layer (4) is then removed; the photoresist BP 212 (20') is then removed, i.e., the device is immersed in an ethanol solution for 10-500s, rinsed repeatedly with deionized water along the waveguide direction, dried with nitrogen, and heated for 5-50 minutes; then, using the chromium layer pattern (19') as a mask, the lithium niobate thin film layer (18) is subjected to ICP dry etching for 5-40s and a etching depth of 200-400nm, to obtain the lithium niobate planar layer (3) and the ridge-shaped lithium niobate core layer (4). Due to the effect of ICP on niobate... The limitations of lithium niobate thin film etching are that the sidewalls of the etched niobate thin film layer have a certain tilt angle θ, and the cross section is an isosceles trapezoid, forming a ridge structure. The device is immersed in chromium removal solution for 5-50 seconds to etch away the chromium layer pattern (19'). Then, the device surface is cleaned with acetone, methanol and isopropanol in sequence 2-3 times and dried with nitrogen. The device is then heated for 5-50 minutes and then cooled to room temperature. Thus, the lithium niobate plate layer (3) and the ridge structure lithium niobate core layer (4) are completed.

[0017] C: Modulator electrode fabrication

[0018] The spin coating process is used to coat photoresist BP 212 (22) on the surface of the lithium niobate flat plate layer (3) and the ridge-shaped structure lithium niobate core layer (4) at a rotation speed of 1000-8000 rpm to obtain a BP 212 layer with a thickness of 2-10 μm, and the device coated with the photoresist BP 212 is heated and baked at a temperature of 30-300 °C for 10-50 min and then cooled to room temperature; the photoresist BP 212 (22) is exposed to ultraviolet light with a wavelength of 300-500 nm for 10-50 s using an electrode mask (25) as a mask (the light-shielding part of the electrode mask (25) is complementary to the structure of the ground electrode and the signal electrode to be prepared), so that the photoresist in the region where the ground electrode and the signal electrode are to be prepared is exposed; the device is placed in a NaOH solution with a mass concentration of 2-5 ‰ for 5-300 s to remove the exposed photoresist BP 212, leaving the photoresist BP 212 (22') under the coverage of the electrode mask (25) and exposing the lithium niobate flat plate layer (3) where the ground electrode and the signal electrode are to be prepared; the electrode mask (25) is removed, and a 1.3-5 μm thick metal electrode layer (the metal can be Al, Au, or some commonly used electrode materials) is evaporated on the photoresist BP 212 (22') and the lithium niobate flat plate layer (3) by vacuum evaporation, followed by peeling off the photoresist BP 212 (22') and the metal electrode layer (23 and 24) thereon, i.e. immersing the device in a stripping solution for 10-150 s, then rinsing with deionized water and blowing dry with nitrogen, baking the device at 80-300 °C for 5-40 min and then cooling to room temperature, to obtain the ground electrode and the signal electrode with the desired structure on the lithium niobate flat plate layer (3);

[0019] D. Preparation of polymer upper cladding layer

[0020] The spin coating process is used to coat a polymer material (the polymer upper cladding layer is an electro-optic polymer doped with a chromophore molecule, including a series of materials such as polyimide (PI), polymethyl methacrylate (PMMA), SU-8 2002, and SU-8 2005, with a refractive index lower than that of the lithium niobate core layer) on the surface of the device obtained in step C at a rotation speed of 1000-6000 rpm, and baking at 100-500 °C for 20-50 min and then cooling to room temperature to prepare a polymer upper cladding layer (5) with a thickness of 0.5-5 μm on the lithium niobate flat plate layer (3), thereby obtaining the MZI-type electro-optic modulator based on the electro-optic polymer / lithium niobate thin film hetero-integrated waveguide according to the present application.

[0021] Compared with the prior art device structure and technology, the present application has the advantages that: the present application uses electro-optic polymer as the upper cladding layer of the etched lithium niobate waveguide, and the electro-optic polymer has a larger electro-optic coefficient and dielectric constant than silica. When an external electric field is applied for modulation, the electric field is uniformly distributed between the ground electrode and the signal electrode, the effective mode field area is increased by using the electro-optic polymer as the upper cladding layer, and the overlap integral factor of the electric field and the mode field is increased. The light distributed in the lithium niobate core layer and the electro-optic polymer cladding layer undergoes a phase change in the same direction during transmission, so the phase change of the modulator with the electro-optic polymer as the upper cladding layer is larger under the same applied voltage. In addition, the larger the dielectric constant of the upper cladding layer, the larger the electric field acting on the lithium niobate, and the applied voltage for the same phase change is reduced. In summary, the electro-optic modulator with the electro-optic polymer as the upper cladding layer of the lithium niobate waveguide effectively reduces the modulation efficiency of the modulator and reduces the size of the device. In addition, the device manufacturing process is simple, only some common equipment and conventional preparation process are needed, without the need for expensive process equipment and high-difficulty preparation technology, and the production cost is low and the efficiency is high. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 : The cross section of the passive region of the electro-optic modulator prepared in the present application is shown in Fig. 1(a-a’); Figure 2

[0023] Figure 2 : The structure diagram of the ridge-type lithium niobate core layer (4) of the electro-optic modulator prepared in the present application is shown in Fig. 2;

[0024] Figure 3 : The cross section of the active region of the electro-optic modulator prepared in the present application is shown in Fig. 3(b-b’); Figure 2

[0025] Figure 4 : The preparation process flow chart of the electro-optic modulator prepared in the present application is shown in Fig. 4;

[0026] Figure 5 : Figure 2 : The simulation diagram of the light field distribution in the cross section waveguide in Fig. 1(a-a’) is shown in Fig. 5;

[0027] Figure 6 : Figure 2 : The simulation diagram of the electric field distribution in the cross section waveguide in Fig. 3(b-b’) is shown in Fig. 6;

[0028] Fig. 7(a): The diagram of the transmission coefficient of the electro-optic modulator varying with frequency, the abscissa is frequency, and the ordinate is transmission coefficient, which is directly obtained by using the HFSS software;

[0029] ​​Figure 7(b) is a graph of the characteristic impedance of the electro-optic modulator as a function of frequency, obtained by simulation using HFSS software, with the horizontal axis representing frequency and the vertical axis representing characteristic impedance, obtained directly from the HFSS software;

[0030] Figure 7(c) is a graph of the reflection coefficient of the electro-optic modulator as a function of frequency, obtained by simulation using HFSS software, with the horizontal axis representing frequency and the vertical axis representing reflection coefficient, obtained directly from the HFSS software;

[0031] Figure 7(d) is a graph of the microwave refractive index as a function of frequency, obtained by simulation using HFSS software, with the horizontal axis representing frequency and the vertical axis representing microwave refractive index, calculated 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 of which are obtained from the HFSS software) to calculate the microwave refractive index;

[0032] As shown in Figure 6, the names of the various parts are: silicon substrate (1), silicon dioxide oxide layer (2), lithium niobate flat plate layer (3), ridge structure lithium niobate core layer (4, which corresponds to the input straight waveguide 14 here), and polymer upper cladding layer (5); Figure 1

[0033] As shown in Figure 6, the names of the various parts are: input straight waveguide (14), input tapered waveguide (12), first curved waveguide (15) and second curved waveguide (15'), first modulation arm straight waveguide (8) and second modulation arm straight waveguide (9), third curved waveguide (16) and fourth curved waveguide (16'), output tapered waveguide (13), output straight waveguide (17), first ground electrode (11) and second ground electrode (11'), signal electrode (10); Figure 2

[0034] Figure 3 ​​​As shown, the names of each part are: silicon substrate (1), silicon dioxide oxide layer (2), lithium niobate plate layer (3), ridge-structured lithium niobate core layer (4) and (4') (corresponding to the first modulation arm straight waveguide (8) and the second modulation arm straight waveguide (9) respectively), polymer cladding layer (5), signal electrode (10), first ground electrode (11), and second ground electrode (11'); 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, and the thickness h2 of the lithium niobate plate layer (3) and (4') is 1-5 μm. The total thickness of the ridge-structured lithium niobate core layer (4) is 600 nm. The thickness of the lithium niobate plate layer (3) is h3, which is 200-400 nm. The thickness of the ridge-structured lithium niobate core layer (4) is h4, which is 200-400 nm. The thicknesses of the signal electrode (10), the first ground electrode (11), and the second ground electrode (11') are h5, h6, and h6', which are 1-10 μm. The thickness of the polymer overlay (5) above the lithium niobate plate layer (3) is h7, which is 0.5-5 μm. The tilt angle θ is 50°-80°.

[0035] like Figure 5 As shown in the figure, the optical field is mostly distributed in the lithium niobate ridge waveguide and the upper cladding, which ensures effective light transmission while increasing the overlap area of ​​the optical and electric fields and improving modulation efficiency.

[0036] 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 the modulation efficiency.

[0037] As shown in Figure 7(a), the transmission coefficient S can be seen. 12 S 21 The two curves completely overlap, with a lower value in the high-frequency range and a higher value in the low-frequency range, and a larger value overall, above -0.5dB.

[0038] As shown in Figure 7(b), it can be seen that the characteristic impedance is low in the high-frequency region and high in the low-frequency region, which is close to the target impedance value of 50Ω.

[0039] As shown in Figure 7(c), the reflection parameter S can be seen. 11 S 22 The values ​​are relatively small, all below -35dB.

[0040] As shown in Figure 7(d), it can be seen that the simulated value of microwave refractive index is high in the low frequency region and decreases rapidly, while it tends to be stable in the high frequency region, reaching 2.2856 at 30 GHz, which matches the group refractive index of 2.2856. The refractive index difference in the high frequency region is less than 5‰. Detailed Implementation

[0041] Example 1

[0042] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0043] The ridge-shaped lithium niobate core layer (4) structure of the electro-optic modulator of the present invention is as follows: Figure 2 As shown, the lengths a1 and a1' of the input straight waveguide (14) and the output straight waveguide (17) are equal to 20 μm, and the widths w1 and w1' are equal to 1 μm. The first curved waveguide (15), the second curved waveguide (15'), the third curved waveguide (16), and the fourth curved waveguide (16') projected parallel to the input light direction in the input straight waveguide (14) have lengths a2 and a2' of 45 μm, and widths w2, w2', w5, and w5' of 1.5 μm. The lengths a3 and a3' of the first modulation arm straight waveguide (8) and the second modulation arm straight waveguide (9) are equal to 4.5 mm, and the widths w3 and w3' are equal to 1.5 μm. The lengths a4 and a4' of the input tapered waveguide (12) and the output tapered waveguide (13) projected parallel to the input light direction in the input straight waveguide (14) are equal to 4.5 μm. The widths w4 and w4' at the connection points of the input tapered waveguide (12) with the first curved waveguide (15) and the second curved waveguide (15'), and the output tapered waveguide (13) with the third curved waveguide (16) and the fourth curved waveguide (16') are both 3 μm; the lengths a5 and a5' of the first ground electrode (11) and the second ground electrode (11') are both 4 mm, and the widths b1 and b1' are both 135 μm; the length a6 of the signal electrode (10) is 4 mm, and the width b2 is 15.8 μm; the gaps gap and gap' between the signal electrode (10) and the ground electrodes of the first ground electrode (11) and the second ground electrode (11') are both 7.5 μm; the distance between the first modulation arm straight waveguide (8) and the second modulation arm straight waveguide (9) is 23 μm.

[0044] As attached Figure 1 and Figure 3 As shown, the active and passive regions of the electro-optic modulator of the present invention have cross-sectional structures. The thickness h1 of the silicon substrate (1) is 520 μm, the thickness h2 of the silicon dioxide oxide layer (2) is 2 μm, the thickness h3 of the lithium niobate plate layer (3) is 300 nm, the thickness h4 and h4' of the ridge-shaped lithium niobate core layer (4 and 4') are equal to 300 nm, the thickness h5, h6 and h6' of the signal electrode (10), the first ground electrode (11) and the second ground electrode (11') are equal to 1.3 μm, the thickness h7 of the polymer overlay (5) above the lithium niobate plate layer (3) is 0.8 μm, and the tilt angle θ is 60°.

[0045] Under the above parameter conditions, the modulation efficiency of the single-arm modulation of the modulator is 2.218 V·cm, and the modulation efficiency of the double-arm modulation is 1.109 V·cm, which are calculated by the simulation software Comsol Multiphysics. The RF condition of the modulator is simulated by using the HFSS software, and the impedance normalization is set to 50 Ω to simulate the high-frequency performance of the modulator under the 50 Ω external system. Among them, the transmission parameters (S 12 and S 21 ) are large, more than -0.5 dB, and the reflection parameters (S 11 and S 22 ) are small, less than -35 dB. The results show that under the system of 50 Ω and in a wide frequency range of 0-60 GHz, the traveling wave electrode of this structure has a small reflectivity and a large transmissivity. The microwave refractive index of the modulator is obtained by canceling the normalization setting. The microwave refractive index and the characteristic impedance simulation values of this structure are shown in the attached Figure 7(d) , 7(b) . 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, reaching 2.285 at the frequency position of 30 GHz, which matches the optical group refractive index of 2.285. 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, reaching about 47 Ω at the frequency position of 40 GHz, close to the target impedance value of 50 Ω.

[0046] Example 2: The preparation method of the lithium niobate electro-optical modulator based on the MZI structure and having high modulation efficiency according to the present application includes the following steps:

[0047] Step A: cleaning the lithium niobate wafer

[0048] The lithium niobate wafer is composed of a Si substrate (1), a SiO2 oxide layer (2), and a lithium niobate film layer (18) from bottom to top. The lithium niobate film layer (18) surface is cleaned by using acetone, methanol, and isopropanol in sequence for 2 times, and the surface of the lithium niobate is determined to be clean.

[0049] Step B: preparation of the lithium niobate core layer

[0050] A 80nm chromium (Cr) layer (19) is evaporated on the surface of the niobium lithium acetate thin film layer (18) as a mask for etching lithium niobate using an electron beam evaporator (EBE), and then a positive photoresist BP212 (20) is coated on the chromium layer (19) by a spin coating process at a speed of 6500rpm, and baked at 200℃ for 20 minutes, and then cooled to room temperature; the core layer mask (21) is used as a mask, the light shielding part of the core layer mask (21) has the same structure as the ridge-shaped structure niobium lithium core layer (4), the photoresist BP212 is exposed to ultraviolet light with a wavelength of 365nm for 16s, the exposed device is placed in a NaOH solution with a mass concentration of 2‰ for 15s, then repeatedly washed with deionized water along the waveguide direction and dried with nitrogen, and then heated for 35 minutes to remove the exposed BP212, and the remaining unexposed BP212 (20') is used as a mask, which has the same structure as Figure 2 the ridge-shaped structure niobium lithium core layer (4); then the BP212 (20') pattern is used as a mask, and the chromium layer (19) is subjected to inductively coupled plasma dry etching to obtain a chromium layer pattern (19') with the same structure as Figure 2 the ridge-shaped structure niobium lithium core layer (4); then the photoresist BP212 (20') is removed, that is, the device is soaked in an ethanol solution for 30s, then repeatedly washed with deionized water along the waveguide direction and dried with nitrogen, and then heated for 30 minutes; then the chromium layer pattern (19') is used as a mask, and the niobium lithium acetate thin film layer (18) is subjected to ICP dry etching, the etching time is 10s, and the etching depth is 300nm, to obtain a lithium niobate flat layer (3) and a ridge-shaped structure niobium lithium core layer (4); due to the limitation of ICP etching of the niobium lithium thin film, the sidewall of the niobium lithium acetate thin film layer after etching has a certain inclination angle θ, and the cross section is isosceles trapezoidal, forming a ridge-shaped structure; the device is soaked in a chromium removal solution for 9s to corrode the chromium layer pattern (19'), then the device surface is cleaned with acetone, methanol and isopropyl alcohol for 2 times and dried with nitrogen, and then the device is heated for 20 minutes and cooled to room temperature, thus the lithium niobate flat layer (3) and the ridge-shaped structure niobium lithium core layer (4) are prepared;

[0051] Step C: preparation of modulator electrode

[0052] The spin coating process is used to coat photoresist BP 212 (22) on the surface of the lithium niobate flat plate layer (3) and the ridge structure lithium niobate core layer (4) at a rotation speed of 7500 rpm to obtain a BP 212 layer with a thickness of 3 μm. The device coated with photoresist BP 212 is heated and baked at a temperature of 240 °C for 15 minutes and then cooled to room temperature. The electrode mask (25) is used as a mask, the light shielding part of the electrode mask (25) is complementary to the structure of the ground electrode and signal electrode to be prepared, and the photoresist BP 212 (22) is exposed to ultraviolet light with a wavelength of 365 nm for 12 s to expose the photoresist in 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 ‰ for 30 s to remove the exposed photoresist BP 212, leaving the photoresist BP 212 (22') under the coverage of the electrode mask (25) and exposing the lithium niobate flat plate layer (3) where the ground electrode and signal electrode are to be prepared. The electrode mask (25) is removed, and a 1.3 μm thick Al electrode layer is evaporated on the photoresist BP 212 (22') and the lithium niobate flat plate layer (3) by vacuum evaporation. Then, the photoresist BP 212 (22') and the metal electrode layer (23 and 24) thereon are stripped, i.e. the device is soaked in a stripping solution for 80 s, then rinsed with deionized water and dried with nitrogen. The device is baked at 120 °C for 40 minutes and then cooled to room temperature to obtain the ground electrode and signal electrode with the desired structure on the lithium niobate flat plate layer (3).

[0053] Step D: preparation of a polymer upper cladding layer

[0054] The spin coating process is used to coat polymer material (SU-8 2002) on the surface of the device obtained in step C at a rotation speed of 6000 rpm. The device is baked at 130 °C for 30 minutes and then cooled to room temperature to obtain a polymer upper cladding layer (5) with a thickness of 800 nm on the lithium niobate flat plate layer (3), thereby obtaining the MZI type electro-optic modulator based on the electro-optic polymer / lithium niobate thin film heterogeneous integrated waveguide.

[0055] It should be noted that although this application document 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 may be required, but rather as a description of features that may be specific to a particular embodiment of the disclosed technology, and the application can have many variations, such as using barium titanate (BaTiO3), lithium tantalate (LiTaO3), potassium niobate (KNbO3), etc. electro-optic materials; the modulator structure can also use symmetric (asymmetric) directional couplers, multi-mode interferometers, micro-ring resonators, etc. Those skilled in the art, with the explicit disclosure of the present application or without doubt from the written description of the document, are within the scope of protection of the present patent.

Claims

1. An MZI-type electro-optic modulator based on an electro-optic polymer / lithium niobate thin-film heterogeneous integrated waveguide, characterized in that: From bottom to top, it consists of a silicon substrate (1), a silicon dioxide oxide layer (2), a lithium niobate plate layer (3), a ridge-structured lithium niobate core layer (4), and an electro-optic polymer cladding layer (5). The electro-optic polymer cladding layer (5) is located above the lithium niobate plate layer (3) and the ridge-structured lithium niobate core layer (4), and the ridge-structured lithium niobate core layer (4) is completely encapsulated in the electro-optic polymer cladding layer (5). The ridge-structured lithium niobate core layer (4) consists of, from left to right along the light input direction, an input straight waveguide (14), an input tapered waveguide (12), a first curved waveguide (15) and a second curved waveguide (15') with the same structure, a first modulation arm straight waveguide (8) and a second modulation arm straight waveguide (9) with the same structure and parallel to each other, a third curved waveguide (16) and a fourth curved waveguide (16') with the same structure, an output tapered waveguide (13), and an output straight waveguide (17). A first ground electrode (11) and a second ground electrode (11') with elongated structures are symmetrically fabricated on the lithium niobate plate layer (3) on the outer side of the first modulation arm straight waveguide (8) and the second modulation arm straight waveguide (9), respectively. A signal electrode (10) with a rectangular structure is fabricated on the lithium niobate plate layer (3) on the inner side of the first modulation arm straight waveguide (8) and the second modulation arm straight waveguide (9); a first curved waveguide (15) and a third curved waveguide (16) are symmetrically arranged about the first modulation arm straight waveguide (8), a second curved waveguide (15') and a fourth curved waveguide (16') are symmetrically arranged about the second modulation arm straight waveguide (9), and a first ground electrode (11) and a second ground electrode (11') are symmetrically arranged about the signal electrode 10; an input straight waveguide (14), an input tapered waveguide (12), a first curved waveguide (15), and a second curved waveguide (15') constitute a 3-dB The Y-beam splitter, the third curved waveguide (16) and the fourth curved waveguide (16'), the output tapered waveguide (13), and the output straight waveguide (17) constitute a 3-dB beam splitter. Y-jointer; the polymer cladding (5) is made of an electro-optic polymer doped with chromophore molecules, and its refractive index is lower than that of lithium niobate; wherein, the lengths a1 and a1' of the input straight waveguide (14) and the output straight waveguide (17) are equal to 10-500 μm, and the widths w1 and w1' are equal to 1-5 μm; the projection lengths a2 and a2' of the first curved waveguide (15), the second curved waveguide (15'), the third curved waveguide (16), and the fourth curved waveguide (16') parallel to the direction of the input light in the input straight waveguide (14) are equal to 15-300 μm, and the widths w2, w2', w5, and w5' are equal to 1-5 μm; the lengths a3 and a3' of the first modulation arm straight waveguide (8) and the second modulation arm straight waveguide (9) are equal to 3.5-9 mm, widths w3 and w3' are equal to 1-5 μm; the projection lengths a4 and a4' of the input tapered waveguide (12) and output tapered waveguide (13) parallel to the input light direction in the input straight waveguide (14) are equal to 5-100 μm; the widths w4 and w4' at the connection points between the input tapered waveguide (12) and the first curved waveguide (15) and the second curved waveguide (15'), and between the output tapered waveguide (13) and the third curved waveguide (16) and the fourth curved waveguide (16') are equal to 1-30 μm; the lengths a5 and a5' of the first ground electrode (11) and the second ground electrode (11') are equal to 4-8 mm, and the widths b1 and b1' are equal. The thickness of the signal electrode (10) is 80–150 μm; the length a6 of the signal electrode (10) is 4–8 mm, and the width b2 is 10–20 μm; the spacing gap and gap' between the signal electrode (10) and the first ground electrode (11) and the second ground electrode (11') are equal and are 2–20 μm; the distance between the first modulation arm straight waveguide (8) and the second modulation arm straight waveguide (9) is 10–60 μm; the widths mentioned above refer to the upper bottom width w of the ridge-shaped lithium niobate core layer 4, and the lower bottom width w' follows the formula w′=w+2×h4×tanθ, where h4 is the thickness of the ridge-shaped lithium niobate core layer 4, and θ is the tilt angle of the ridge structure relative to the vertical direction.

2. The MZI-type electro-optic modulator based on an electro-optic polymer / lithium niobate thin-film heterogeneous integrated waveguide as described in claim 1, characterized in that: Electro-optic polymers doped with chromophore molecules are polyimide, polymethyl methacrylate, SU-82002, or SU-8 2005.

3. The MZI-type electro-optic modulator based on an electro-optic polymer / lithium niobate thin-film heterogeneous integrated waveguide as described in claim 1, characterized in that: The tilt angle θ is 50° to 80°.

4. The MZI-type electro-optic modulator based on an electro-optic polymer / lithium niobate thin-film heterogeneous integrated waveguide as described in claim 1, characterized in that: The thickness h1 of the silicon substrate (1) is 480-520 μm, and the thickness h2 of the silicon dioxide oxide layer (2) is 1-5 μm; the total thickness of the lithium niobate plate layer (3) and the ridge-structured lithium niobate core layer (4) is 600 nm, wherein the thickness h3 of the lithium niobate plate layer (3) is 200-400 nm, and the thickness h4 of the ridge-structured lithium niobate core layer (4) is 200-400 nm; the thicknesses h5, h6, and h6' of the signal electrode (10), the first ground electrode (11), and the second ground electrode (11') are equal to 1-10 μm, and the thickness h7 of the polymer overlay layer (5) above the lithium niobate plate layer (3) is 0.5-5 μm.

5. A method for fabricating an MZI-type electro-optic modulator based on an electro-optic polymer / lithium niobate thin-film heterogeneous integrated waveguide, comprising the following steps: A: Cleaning lithium niobate wafers The lithium niobate wafer consists of a Si substrate (1), a SiO2 oxide layer (2), and a niobate thin film layer (18) from bottom to top. The surface of the niobate thin film layer (18) is cleaned 2 to 3 times with acetone, methanol, and isopropanol in sequence to ensure that the surface of the lithium niobate is clean. B: Fabrication of Lithium Niobate Waveguide Core Layer A 50 nm to 5 μm chromium (Cr) layer (19) was deposited on the surface of the niobium lithium oxide thin film layer (18) using an electron beam evaporation device as a mask for etching lithium niobate. Then, positive photoresist BP 212 (20) was coated onto the chromium layer (19) using a spin coating process at a speed of 2000 to 7000 rpm. The film was baked at 50 to 500°C for 10 to 40 minutes and then cooled to room temperature. Using a core layer mask (21) as a mask, the light-shielding part of the core layer mask (21) has the same structure as the ridge structure lithium niobate core layer (4). The photoresist BP212 was exposed to a UV lamp with a wavelength of 300 to 500 nm for 5 to 50 seconds. The exposed device was placed in a NaOH solution with a mass concentration of 2 to 5‰ for 10 to 200 seconds. Then, it was repeatedly rinsed with deionized water along the waveguide direction, dried with nitrogen, and heated for 5 to 50 minutes to remove the exposed BP. 212, the unexposed BP 212 (20') is used as a mask, and its structure is the same as that of the ridge-shaped lithium niobate core layer (4) in Figure 2; then, using the BP 212 (20') pattern as a mask, the chromium layer (19) is subjected to inductively coupled plasma dry etching to obtain the same chromium layer pattern (19') as the ridge-shaped lithium niobate core layer (4) in Figure 2; the core layer mask (21) is removed, and then the photoresist BP is removed. 212 (20'), the device is immersed in an ethanol solution for 10-500s, rinsed repeatedly with deionized water along the waveguide direction, dried with nitrogen, and heated for 5-50 minutes; then, using the chromium layer pattern (19') as a mask, the niobium lithium oxide thin film layer (18) is etched by ICP dry etching for 5-40s and a depth of 200-400nm to obtain the lithium niobate planar layer (3) and the ridge-structured lithium niobate core layer (4). Due to the effect of ICP on the niobate plate, the device is further etched by the niobate plate. The limitations of etching lithium niobate thin films result in a certain tilt angle θ on the sidewalls of the etched lithium niobate thin film layer, with an isosceles trapezoidal cross section forming a ridge structure. The device is immersed in a chromium removal solution for 5-50 seconds to etch away the chromium layer pattern (19'). Then, the device surface is cleaned 2-3 times with acetone, methanol, and isopropanol in sequence and dried with nitrogen. The device is then heated for 5-50 minutes and cooled to room temperature. Thus, the lithium niobate flat plate layer (3) and the ridge structure lithium niobate core layer (4) are completed. C: Modulator electrode fabrication Using a spin coating process, photoresist BP 212 (22) is coated onto the surface of the lithium niobate planar layer (3) and the ridge-structured lithium niobate core layer (4) at a spin speed of 1000–8000 rpm, resulting in a BP 212 layer thickness of 2–10 μm. The spin-coated device is heated and baked at 30–300 °C for 10–50 minutes, then cooled to room temperature. Using an electrode mask (25) as a mask, with the light-blocking portion of the electrode mask (25) complementary to the structure of the ground electrode and signal electrode to be prepared, the photoresist BP 212 (22) is exposed to a UV lamp with a wavelength of 300–500 nm for 10–50 s, exposing the photoresist within the area where the ground electrode and signal electrode need to be prepared. The device is then immersed in a NaOH solution with a mass concentration of 2–5‰ for 5–300 s to remove the exposed photoresist BP 212. 212, the photoresist BP212 (22') covered by the electrode mask (25) is exposed, exposing the lithium niobate plate layer (3) on which the ground electrode and signal electrode need to be prepared; remove the electrode mask (25), and deposit a metal electrode layer with a thickness of 1.3 to 5 μm on the photoresist BP212 (22') and the lithium niobate plate layer (3) by vacuum evaporation. Then peel off the photoresist BP 212 (22') and the metal electrode layer (23 and 24) on it, immerse the device in the resist remover solution for 10 to 150 s, rinse it with deionized water and blow it dry with nitrogen, bake the device at 80 to 300 °C for 5 to 40 minutes and then cool it to room temperature to obtain the ground electrode and signal electrode with the desired structure on the lithium niobate plate layer (3); D. Preparation of polymer top coating The polymer material is coated onto the device surface obtained in step C using a spin coating process at a speed of 1000-6000 rpm. After baking at 100-500℃ for 20-50 minutes, the coating is cooled to room temperature. A polymer cladding layer (5) with a thickness of 0.5-5 μm is then prepared on the lithium niobate plate layer (3), thereby obtaining the MZI type electro-optic modulator based on the electro-optic polymer / lithium niobate thin film heterogeneous integrated waveguide. The polymer material is an electro-optic polymer doped with chromophore molecules, and its refractive index is lower than that of lithium niobate. The electro-optic polymer doped with chromophore molecules is polyimide, polymethyl methacrylate, SU-8 2002, or SU-8 2005.

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