CN118605044A - A lithium niobate thin film electro-optic modulator based on MZI structure electro-optic polymer loaded strip waveguide and its preparation method - Google Patents

Abstract

A lithium niobate thin film electro-optical modulator based on MZI structure electro-optical polymer loading strip waveguide and a preparation method thereof belong to the technical field of planar optical waveguide light modulators and preparation thereof. The electro-optic polymer loading strip waveguide consists of a silicon substrate, a silicon dioxide oxide layer, a lithium niobate thin film and an electro-optic polymer loading strip waveguide from bottom to top. The electro-optic polymer loading strip waveguide is of an MZI structure and consists of an input straight waveguide, an input conical waveguide, a first bending waveguide, a second bending waveguide, a first modulation arm straight waveguide, a second modulation arm straight waveguide, a third bending waveguide, a fourth bending waveguide, an output conical waveguide and an output straight waveguide along the light input direction from left to right; a first ground electrode, a second ground electrode and a signal electrode 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 has simple manufacturing process, can effectively reduce the modulation efficiency of the modulator, reduces the power consumption of the modulator and is more beneficial to the high integration of the modulator.

Description

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

Technical Field

The invention belongs to the technical field of planar optical waveguide optical modulators and preparation thereof, and in particular 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 Art

With the rapid development of new generation information technologies such as artificial intelligence and the Internet of Things, traditional communication networks can no longer meet people's needs for information transmission and exchange, and the development of high-speed optical communication networks is urgent. Electro-optical modulators, as core components of optical communication systems and on-chip optical interconnect chips, are used to load information and regulate signals. The principle is to use the electro-optical effect of materials to make the refractive index of the materials change linearly, thereby regulating the intensity, phase and other information of the light waves transmitted in the medium.

The materials commonly used in existing electro-optic modulators are mainly the following: lithium niobate (LN), silicon (Si), 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. The refractive index difference between the core layer and the cladding of this waveguide is very small, and the light binding ability is poor, resulting in the total length of the device after packaging is usually 5 to 10 cm, which is very unfavorable for integration. The electro-optic modulator based on thin-film lithium niobate overcomes the shortcomings of traditional electro-optic modulators. The refractive index difference between the core layer and the cladding of the waveguide is higher, which is conducive to reducing the overall size of the device.

According to the different waveguide types, thin-film lithium niobate waveguides can be divided into etched (Dry-etched) and loaded (Rib-loaded) waveguides. Due to the chemical inertness of lithium niobate materials, physical dry etching is a common method for preparing etched lithium niobate waveguides. This 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 sidewalls formed by etching will increase the transmission loss of the waveguide. Loaded waveguides are formed by directly depositing or spin-coating a second material (SiN, a-Si, TiO ₂ , Ti ₂ O ₅ , Polymer, etc.) on lithium niobate to form a waveguide structure, which avoids the lithium niobate dry etching process and is conducive to achieving low-loss transmission of the waveguide. Silicon nitride is a material compatible with CMOS technology, has a mature processing technology and is easy to manufacture, and has always been a common material for loaded strip waveguides. However, the refractive index of silicon nitride is similar to that of lithium niobate. In a lithium niobate waveguide using silicon nitride as a loading bar, part of the mode field is distributed in the silicon nitride loading bar. When performing electro-optic modulation, the mode field in the loading bar is not utilized because silicon nitride has no electro-optic effect, which greatly reduces the modulation efficiency of the modulator.

Currently, electro-optic modulators based on lithium niobate films have been widely studied and rapidly developed. However, as optical networks upgrade towards ultra-high speed and ultra-long distance transmission, their modulation efficiency needs to be further improved to meet the growing demand for optical communication networks.

Summary of the invention

In order to further improve the modulation efficiency of the lithium niobate electro-optic modulator, the present invention provides a lithium niobate thin film electro-optic modulator based on an MZI structure electro-optic polymer loaded strip waveguide and a preparation method thereof.

The present invention uses silicon with a silicon dioxide buffer layer as a waveguide substrate, a lithium niobate film as a planar layer of a loaded waveguide, and an electro-optic polymer with a relatively high dielectric constant as a loading strip. The electro-optic polymer selected in the present invention has many types, a small refractive index, a large electro-optic coefficient, and a large refractive index difference with the lithium niobate film. The preparation process used in the present invention is simple, can be well compatible with semiconductor processes, is easy to integrate, meets the requirements of large-scale production, and has certain practical value.

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

As shown in FIG2 , the electro-optic polymer loaded strip waveguide (4) is an MZI structure, and is composed of an input straight waveguide (8), an input tapered waveguide (9), a first curved waveguide (10′) and a second curved waveguide (10) having the same structure, a first modulation arm straight waveguide (11) and a second modulation arm straight waveguide (12) having the same structure and being parallel to each other, a third curved waveguide (15′) and a fourth curved waveguide (15) having the same structure, an output tapered waveguide (16), and an output straight waveguide (17) from left to right along the light input direction; first ground electrodes (13′) having a long strip structure are symmetrically prepared on the lithium niobate film (3) outside the first modulation arm straight waveguide (11) and the second modulation arm straight waveguide (12). and a second grounding electrode (13); a signal electrode (10) with a rectangular structure is prepared on the lithium niobate film (3) inside 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 with respect to the first modulation arm straight waveguide (11); the second curved waveguide (10) and the fourth curved waveguide (15) are symmetrically arranged with respect to the second modulation arm straight waveguide (12); the first grounding electrode (13') and the second grounding electrode (13) are symmetrically arranged with respect to the signal electrode (10); 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 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.

Furthermore, the lengths a1 and a1′ of the input straight waveguide (8) and the output straight waveguide (17) are equal to 10 to 500 μm, the widths w1 and w1′ are equal to 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 to 15 to 300 μm, and the widths w2′, w2, w5′ and w5 are equal to 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 to 3.5 to 9 mm, and the widths w3′ and w3 are equal to 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. The widths w4 and w4' of the connection points between the input tapered waveguide (9) and the first curved waveguide (10) and the second curved waveguide (10'), and between the output tapered waveguide (16) and the third curved waveguide (15) and the fourth curved waveguide (15') are equal to 1 to 30 μm; the lengths a5' and a5 of the first grounding electrode (13') and the second grounding electrode (13) are equal to 4 to 8 mm, and the widths b1' and b1 are equal to 80 to 150 μm; the length a6 of the signal electrode (14) is 4 to 8 mm, and the width b2 is 10 to 20 μm; the spacings gap' and gap between the signal electrode (14) and the first grounding electrode (13') and the second grounding electrode (13) are equal to 2 to 20 μm; and the distance between the first modulation arm straight waveguide (11) and the second modulation arm straight waveguide (12) is 10 to 60 μm.

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 film (3) is 100-800 nm, the thickness h4' and h4 of the electro-optic polymer loading strip waveguide (4', 4 respectively corresponding to the first modulation arm waveguide 11 and the second modulation arm waveguide 12) are 0.5-5 μm, and the thickness h5, h6 and h6' of the signal electrode (14), the first grounding electrode (13') and the second grounding electrode (13) are equal to 0.5-6 μm. The signal electrode (14), the first grounding electrode (13') and the second grounding electrode (13) are some commonly used metal materials including Al and Au.

The present invention discloses a method for preparing a lithium niobate thin film electro-optic modulator based on an MZI structure electro-optic polymer loaded strip waveguide. The manufacturing process is shown in FIG4 . The specific steps are as follows:

A: Lithium niobate wafer cleaning (Lithium niobate wafer purchased from Shanghai New Silicon Polymer Semiconductor Co., Ltd., from top to bottom consists of three parts: Si substrate (1), _SiO2 oxide layer (2) and lithium niobate film layer (3))

Firstly, the surface of the lithium niobate film layer (3) is cleaned 2 to 3 times using acetone, methanol, and isopropanol solvents in sequence to ensure that the surface of the lithium niobate is clean;

B: Polymer loading strip preparation

The electro-optic polymer (25, which is an electro-optic polymer doped with chromophore molecules, 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 the refractive index of lithium niobate) is coated on the surface of the cleaned lithium niobate film layer (3) at a rotation speed of 2000 to 8000 revolutions per minute to obtain an electro-optic polymer (25) film with a thickness of 200 nm to 2000 nm. The device is baked at 50 to 500° C. for 10 to 40 minutes for curing, and then cooled to room temperature. A layer of aluminum metal with a thickness of 1 to 4 μm is evaporated on the electro-optic polymer (25) film by a vacuum evaporation process as an aluminum mask (26). A layer of positive photoresist BP with a thickness of 0.5 to 6 μm is coated on the aluminum mask (26) by a spin coating process. 212 (27), the rotation speed is 1000-6000 rpm; the device spin-coated with photoresist BP 212 is heated, that is, heated at a temperature of 70-300° C. for 10-30 minutes, and then cooled to room temperature after heating; using the core layer mask (30) as a mask (the light-shielding part of the core layer mask is the same as the structure of the electro-optic polymer loading strip waveguide (4) in FIG. 2, and does not include the signal electrode and the ground electrode), the photoresist BP 212 (27) is exposed to an ultraviolet lamp with a wavelength of 300-500 nm for 5-50 seconds, the exposed device is placed in a NaOH solution with a mass concentration of 2-5‰ for 10-200 seconds, and then repeatedly rinsed with deionized water along the waveguide direction, and dried with nitrogen and heated for 5-50 minutes to remove the exposed photoresist BP 212, leaving the unexposed photoresist BP 212 (27') pattern, which has the same structure as the electro-optic polymer loaded strip waveguide (4) in FIG. 2; removing the core layer mask (30), and then using the photoresist BP 212 (27') pattern as a mask, inductively coupled plasma (ICP) etching is performed on the aluminum mask (26) to obtain an aluminum layer pattern (26') having the same structure as the electro-optic polymer loaded strip waveguide (4); then removing the photoresist BP 212 (27'), that is, immersing the device in an ethanol solution for 10 to 500 seconds, repeatedly rinsing it with deionized water along the waveguide direction, blowing it dry with nitrogen and heating it for 5 to 50 minutes; then using the aluminum layer pattern (26') as a mask, ICP etching the electro-optic polymer (25), and the remaining electro-optic polymer pattern has the same structure as the electro-optic polymer loaded strip waveguide (4); removing the aluminum layer pattern (26'), that is, immersing the device in a NaOH solution with a mass concentration of 2 to 5‰ for 10 to 200 seconds, repeatedly rinsing it with deionized water along the waveguide direction, blowing it dry with nitrogen and heating it for 5 to 50 minutes, so that the preparation of the electro-optic polymer loaded strip waveguide (4) is completed;

C: Modulator Electrode Preparation

A vacuum evaporation process is used to evaporate a layer of Al electrode layer (28) with a thickness of 1 to 4 μm on the electro-optic polymer loading strip waveguide (4) and the lithium niobate film (3), and a positive photoresist BP 212 (29) is applied to the surface of the Al electrode layer (28) by a spin coating process at a rotation speed of 1000 to 8000 revolutions per minute to obtain a BP 212 thickness of 2 to 10 μm. The device on which the photoresist BP 212 is spin-coated is heated, that is, baked at a temperature of 30 to 300° C. for 10 to 50 minutes and then cooled to room temperature; an electrode mask (31) is used as a mask (the light shielding part of the electrode mask has the same structure as the ground electrode and the signal electrode to be prepared), and the photoresist BP 212 is coated on the surface of the Al electrode layer (28). 212 is exposed to ultraviolet light with a wavelength of 300 to 500 nm for 10 to 50 seconds to expose the photoresist outside the area where the ground electrode and the signal electrode need to be prepared, and the device is placed in a NaOH solution with a mass concentration of 2 to 5‰ for 5 to 300 seconds to remove the exposed photoresist, and the device is repeatedly rinsed with deionized water along the waveguide direction, and blown dry with nitrogen and heated for 5 to 50 minutes; the electrode mask (31) is removed, and then the unexposed BP212 (29') (i.e., 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 immersed in an ethanol solution for 10 to 500 seconds, and repeatedly rinsed with deionized water along the waveguide direction, and blown dry with nitrogen and heated for 5 to 50 minutes. At this point, the modulator electrode is prepared, thereby obtaining the lithium niobate thin film electro-optic modulator based on the MZI structure electro-optic polymer loaded strip waveguide.

Compared with the existing device structure and technology, the beneficial effects of the present invention are:

The present invention uses an electro-optic polymer as a loading bar of a loaded lithium niobate waveguide. Compared with silicon nitride, the electro-optic polymer has a larger electro-optic coefficient and a lower refractive index. The lower the refractive index of the loading bar, the greater the refractive index difference between the loading bar and the flat layer, and the mode field area in the lithium niobate increases. When modulated by an external electric field, the mode field area in the lithium niobate increases, increasing 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 a small amount of polymer loading bars undergo phase changes in the same direction during the transmission process, so the phase change of the modulator is greater under the same external voltage. In summary, the use of an electro-optic polymer as a loading bar for a lithium niobate loaded waveguide can effectively reduce the modulation efficiency of the modulator, reduce the power consumption of the modulator, and is more conducive to its high integration. In addition, the manufacturing process of the device is simple, requiring only some common equipment and conventional preparation processes, and does not require expensive process equipment and difficult preparation technology, with low production cost and high efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a schematic diagram of a cross section of a passive region of a lithium niobate thin film electro-optic modulator prepared by the present invention (a-a' in FIG2 );

FIG2 is a schematic diagram of the structure of the electro-optic polymer loaded strip waveguide of the lithium niobate thin film electro-optic modulator prepared by the present invention;

FIG3 is a schematic diagram of a cross section of an active region of a lithium niobate thin film electro-optic modulator prepared by the present invention (b-b' in FIG2 );

FIG4 is a flow chart of the preparation process of the lithium niobate thin film electro-optic modulator prepared by the present invention;

Figure 5: Simulated diagram of the light field distribution in the waveguide at the cross section a-a’ in Figure 2;

Figure 6: Simulated diagram of the electric field distribution in the waveguide at the b-b’ cross section in Figure 2;

Figure 7(a): The graph of the microwave impedance of the electro-optic modulator simulated with HFSS software, where the horizontal axis is the frequency and the vertical axis is the microwave impedance, which is directly obtained by HFSS software;

Figure 7(b): The microwave refractive index of the electro-optic modulator simulated with HFSS software shows how it varies with frequency. The horizontal axis is the frequency and the vertical axis is the microwave refractive index. (where c is the speed of light, f is the frequency, γ is the propagation constant, Im(γ) represents the imaginary part of the propagation constant γ, and the above parameters are obtained from HFSS software) to calculate the microwave refractive index;

Figure 7(c): Transmission coefficient of the electro-optic modulator simulated with HFSS software versus frequency, where the horizontal axis is frequency and the vertical axis is transmission coefficient, directly obtained with HFSS software;

Figure 7(d) shows the reflection coefficient of the electro-optic modulator simulated with HFSS software. The horizontal axis is the frequency and the vertical axis is the reflection coefficient. It is directly obtained by HFSS software.

As shown in FIG. 1 , the names of the various parts 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)).

As shown in FIG2 , the names of the various parts 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 (10).

As shown in FIG3 , the names of the various parts are: silicon substrate (1), silicon dioxide oxide layer (2), lithium niobate thin film layer (3), electro-optic polymer loading strip (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).

As shown in FIG. 5 , it can be seen from the figure that most of the mode field is confined in the lithium niobate plate. While the light is effectively transmitted, the overlapping area of the mode field and the electric field is increased, thereby improving the modulation efficiency of the modulator.

As shown in Figure 6, it can be seen that the electric field is evenly distributed, and the electric field is larger at the boundary between the electrode and the cladding and the boundary between the cladding and lithium niobate. This is because the electric field satisfies Maxwell's equations at the dielectric boundary. The cladding with a larger relative dielectric constant can increase the electric field strength inside the lithium niobate waveguide, reduce the voltage-length product of the modulator, and improve its modulation efficiency.

As shown in Figure 7(a), it can be seen that it is lower in the high-frequency region and higher in the low-frequency region, close to the target impedance value of 50Ω; as shown in Figure 7(b), it can be seen that the simulated 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, matching the optical group refractive index of 2.2856, and the refractive index difference in the high-frequency region is less than 0.05; as shown in Figure 7(c), it can be seen that the two curves of the transmission coefficients S12 and S21 completely overlap, are lower in the high-frequency region, higher in the low-frequency region, and are generally larger, above -0.75dB; as shown in Figure 7(d), it can be seen that the reflection parameters _S11 and _S22 are small, both below -34dB.

DETAILED DESCRIPTION

Example 1

The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

The structure of the embodiment is shown in FIG2 . 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 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 to 65 μm, and the widths w2′, w2, w5′ and w5 are equal to 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) are parallel to the input light direction in the input straight waveguide (8). The projection lengths a4 and a4' in the direction are equal to 10 μm, and the widths w4 and w4' at the connection points with the first curved waveguide (10) and 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 grounding electrode (13') and the second grounding 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 spacings gap' and gap between the signal electrode (14) and the first grounding electrode (13') and the second grounding electrode (13) are equal to 5 μm; and the distance between the first modulation arm straight waveguide (11) and the second modulation arm straight waveguide (12) is 20 μm.

As shown in Figure 2 and Figure 3, the thickness h1 of the silicon substrate (1) is 520μm, the thickness h2 of the silicon dioxide oxide layer (2) is 4.7μm, the thickness of the lithium niobate film (3) is 300nm, the thickness h4 of the electro-optic polymer loading strip (4) is 0.5μm, 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 0.8μm.

Under the above parameters, the modulation efficiency of the modulator with single-arm regulation is 2.517V·cm calculated by the simulation software Comsol Multiphysics. The modulation efficiency of the dual-arm regulation is 1.259V·cm. The RF condition of the modulator is simulated using HFSS software. The impedance normalization is set to 50Ω to simulate its high-frequency performance in a 50Ω external system. The results show that the transmission parameters (S ₁₂ and S ₂₁ ) are large, above -0.75dB, and the reflection parameters (S ₁₁ and S ₂₂ ) are small, below -35dB. In a 50Ω system and in a wide frequency range of 0 to 100GHz, the traveling wave electrode of this structure has a small reflectivity and a large transmittance. Cancel the impedance normalization setting to obtain the microwave refractive index. The simulated characteristic impedance value and microwave refractive index of this structure are shown in Figures 7(a) and 7(b). The simulated 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 simulated 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 in the high-frequency region is about 47Ω, close to the target impedance value of 50Ω.

The method for preparing the lithium niobate electro-optic modulator with high modulation efficiency based on the MZI structure of the present invention comprises 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); firstly, the surface of the lithium niobate thin film layer (3) is cleaned three times using acetone, methanol and isopropanol solvents in sequence to ensure that the surface of the lithium niobate is clean;

B: Polymer loading strip preparation

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 rotation speed of 7500 rpm to obtain an electro-optic polymer (25) thin film with a thickness of 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) thin film as an aluminum mask (26) by a vacuum evaporation process. 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 rotation speed of 5000 rpm. The spin-coated photoresist BP The device of 212 is heated, that is, heated at 120°C for 20 minutes, and then cooled to room temperature after heating; using the core layer mask (30) as a mask, the light-shielding part of the core layer mask is the same as the structure of the electro-optic polymer loaded strip waveguide (4) in Figure 2, and does not include the signal electrode and the ground electrode, and the photoresist BP 212 (27) is exposed to a UV lamp with a wavelength of 365nm for 9s, and the exposed device is placed in a NaOH solution with a mass concentration of 2‰ for 15s, and then repeatedly rinsed with deionized water along the waveguide direction, and dried with nitrogen and heated for 35 minutes to remove the exposed photoresist BP 212, leaving a pattern of unexposed photoresist BP 212 (27'), which has the same structure as the electro-optic polymer loaded strip waveguide (4) in Figure 2; removing the core layer mask (30), and then using the photoresist BP 212 (27') pattern is used as a mask, and the aluminum mask (26) is subjected to inductively coupled plasma etching to obtain an aluminum layer pattern (26') having the same structure as the electro-optic polymer loaded strip waveguide (4); then, the unexposed photoresist BP 212 (27') is removed, that is, the device is immersed in an ethanol solution for 30 seconds, and is repeatedly rinsed with deionized water along the waveguide direction, and is blown dry with nitrogen and heated for 20 minutes; then, the aluminum layer pattern (26') is used as a mask, and the electro-optic polymer (25) is subjected to ICP etching, and the remaining electro-optic polymer pattern has the same structure as the electro-optic polymer loaded strip waveguide (4); the aluminum layer pattern (26') is removed, that is, the device is immersed in a NaOH solution with a mass concentration of 2‰ for 12 seconds, and is repeatedly rinsed with deionized water along the waveguide direction, and is blown dry with nitrogen and heated for 30 minutes, and the preparation of the electro-optic polymer loaded strip waveguide (4) is completed;

C: Modulator Electrode Preparation

A 1.2 μm thick Al electrode layer (28) is deposited on an electro-optic polymer loading strip waveguide (4) and a lithium niobate film (3) by a vacuum evaporation process, a positive photoresist BP 212 (29) is coated on the surface of the Al electrode layer (28) by a spin coating process at a rotation speed of 4000 rpm, and a BP 212 thickness of 3 μm is obtained. The device on which the photoresist BP 212 is spin-coated is heated, that is, baked at a temperature of 130° C. for 35 minutes and then cooled to room temperature. An electrode mask (31) is used as a mask, and the light shielding part of the electrode mask (31) has the same structure as the ground electrode and the signal electrode to be prepared. 212 is exposed to an ultraviolet lamp with a wavelength of 365nm for 15s to expose the photoresist outside the area where the ground electrode and the signal electrode need to be prepared, and the device is placed in a NaOH solution with a mass concentration of 2‰ for 20s to remove the exposed photoresist, and the device is repeatedly rinsed with deionized water along the waveguide direction, and blown dry with nitrogen and heated for 30 minutes; the electrode mask (31) is removed, and then the unexposed BP212 (29') is removed, that is, the device is immersed in an ethanol solution for 30s, and repeatedly rinsed with deionized water along the waveguide direction, and blown dry with nitrogen and heated for 30 minutes. At this point, the modulator electrode is prepared, thereby obtaining the lithium niobate thin film electro-optical modulator based on the MZI structure electro-optic polymer loaded strip waveguide.

It should be pointed out that although the present application document contains descriptions of many details, they should not be interpreted as limiting the scope of any disclosed technology or the content that may be required, but should be interpreted as a description of features that may be specific to a specific embodiment of the disclosed technology. The present invention may also have many variations, such as using electro-optical materials such as barium titanate (BaTiO ₃ ), lithium tantalate (LiTaO ₃ ), and potassium niobate (KNbO ₃ ); the modulator structure may also use symmetrical (asymmetrical) directional coupling, multi-mode interferometer, micro-ring resonator, etc. The technical personnel in this field, based on the explicit disclosure of the present invention or the undisputed conclusion based on the written description of the document, all belong to the scope to be protected by this patent.

Claims (6)

1. A lithium niobate thin film electro-optic modulator based on MZI structure electro-optic polymer loading strip waveguide is characterized in that: the electro-optic polymer loading strip waveguide (4) is of an MZI structure and consists of an input straight waveguide (8), an input conical waveguide (9), a first curved waveguide (10 ') and a second curved waveguide (10) which are identical in structure, a first curved waveguide (11) and a second curved waveguide (12) which are identical in structure and parallel to each other, a third curved waveguide (15') and a fourth curved waveguide (15) which are identical in structure, an output conical waveguide (16) and an output straight waveguide (17) from left to right along the light input direction; a first grounding electrode (13') and a second grounding electrode (13) with long strip structures are symmetrically prepared on the lithium niobate thin film (3) outside the first modulation arm straight waveguide (11) and the second modulation arm straight waveguide (12), and a signal electrode (10) with rectangular structures is prepared on the lithium niobate thin film (3) inside 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 (10); the input straight waveguide (8), the input conical waveguide (9), the first curved waveguide (10 ') and the second curved waveguide (10) form a 3-dB Y beam splitter, and the third curved waveguide (15') and the fourth curved waveguide (15), the output conical waveguide (16) and the output straight waveguide (17) form a 3-dB Y beam combiner.

2. A lithium niobate thin film electro-optic modulator based on MZI structure electro-optic polymer loaded strip waveguide as claimed in claim 1, wherein: the material of the electro-optic polymer (25) is an electro-optic polymer doped with chromophore molecules, and the refractive index of the electro-optic polymer is lower than that of lithium niobate; the first grounding electrode (13'), the second grounding electrode (13) and the signal electrode (10) are made of Al or Au.

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

4. A lithium niobate thin film electro-optic modulator based on MZI structure electro-optic polymer loaded strip waveguide as claimed in claim 1, wherein: the lengths a1 and a1' of the input straight waveguide (8) and the output straight waveguide (17) are equal to 10-500 mu m, the widths w1 and w1' are equal to 1-5 mu m, 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 to 15-300 mu m, and the widths w2', w2, w5' and w5 are equal to 1-5 mu m; the lengths 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, and the widths w3' and w3 are equal to 1-5 mu m; 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 to 5-100 mu m, and the widths w4 and w4' of the joints of the input tapered waveguide (9) and the first curved waveguide (10) and the second curved waveguide (10 '), and the joints of the output tapered waveguide (16) and the third curved waveguide (15) and the fourth curved waveguide (15') are equal to 1-30 mu m; the lengths a5' and a5 of the first grounding electrode (13 ') and the second grounding electrode (13) are equal to 4-8 mm, and the widths b1' and b1 are equal to 80-150 mu m; the length a6 of the signal electrode (14) is 4-8 mm, and the width b2 is 10-20 mu m; the distances gap 'and gap between the signal electrode (14) and the first grounding electrode (13') and the second grounding electrode (13) are equal to 2-20 mu m; the distance between the first modulation arm straight waveguide (11) and the second modulation arm straight waveguide (12) is 10-60 mu m.

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

6. A preparation method of a lithium niobate thin film electro-optic modulator based on an electro-optic polymer loading strip waveguide with an MZI structure comprises the following steps:

a: lithium niobate wafer cleaning

The lithium niobate wafer from bottom to top consists of a Si substrate (1), a SiO ₂ oxide layer (2) and a lithium niobate thin film layer (3); firstly, sequentially cleaning the surface of a lithium niobate thin film layer (3) for 2-3 times by using acetone, methanol and isopropanol solvents, and determining that the surface of the lithium niobate is clean;

preparation of Polymer loading Strand

Coating an electro-optic polymer (25) on the surface of the cleaned lithium niobate thin film layer (3) by adopting a spin coating process, wherein the rotating speed is 2000-8000 revolutions per minute, preparing the electro-optic polymer (25) thin film with the thickness of 200-2000 nm, baking the device for 10-40 minutes at 50-500 ℃ for solidification, and cooling to room temperature; evaporating an aluminum metal with the thickness of 1-4 mu m on the electro-optic polymer (25) film by adopting a vacuum evaporation process to be used as an aluminum mask (26); coating a layer of positive photoresist BP 212 (27) with the thickness of 0.5-6 mu m on an aluminum mask (26) by adopting a spin coating process, wherein the rotating speed is 1000-6000 rpm; heating the device with the photoresist BP 212 spin-coated, namely heating at 70-300 ℃ for 10-30 minutes, and cooling to room temperature after heating; taking a core layer mask plate (30) as a mask, exposing a photoresist BP 212 (27) for 5-50 s under an ultraviolet lamp with the wavelength of 300-500 nm, putting the exposed device into a NaOH solution with the mass concentration of 2-5%o for 10-200 s, repeatedly flushing with deionized water along the direction of the waveguide, drying with nitrogen, heating for 5-50 minutes, removing the exposed photoresist BP 212, and leaving an unexposed photoresist BP 212 pattern which is the same as the structure of the electro-optic polymer loading strip waveguide (4) in FIG. 2; removing the core layer mask plate (30), and performing inductively coupled plasma etching on the aluminum mask (26) by taking the photoresist BP 212 pattern as a mask to obtain an aluminum layer pattern (26') with the same structure as the electro-optic polymer loading strip waveguide (4); then removing the unexposed BP 212 (27'), namely soaking the device in ethanol solution for 10-500 s, repeatedly washing the device cleanly along the waveguide direction by deionized water, drying the device by nitrogen, and heating for 5-50 minutes; performing ICP etching on the electro-optic polymer (25) by taking the aluminum layer graph (26') as a mask, wherein the remained electro-optic polymer pattern is the same as the structure of the electro-optic polymer loading strip waveguide (4); removing the aluminum layer graph (26'), namely immersing the device in 2-5%o NaOH solution for 10-200 s, repeatedly flushing the device with deionized water along the direction of the waveguide, drying the device with nitrogen, and heating for 5-50 minutes to finish the preparation of the electro-optic polymer loading strip waveguide (4);

c: modulator electrode preparation

Evaporating an Al electrode layer (28) with the thickness of 1-4 mu m on the electro-optic polymer loading strip waveguide (4) and the lithium niobate film (3) by adopting a vacuum evaporation process, coating positive photoresist BP212 (29) on the surface of the Al electrode layer (28) by adopting a spin coating process, obtaining a BP212 with the thickness of 2-10 mu m at the rotating speed of 1000-8000 r/min, heating a device of spin coating photoresist BP212, namely baking for 10-50 min at the temperature of 30-300 ℃, and cooling to room temperature; taking an electrode mask plate (31) as a mask, exposing the photoresist BP212 for 10-50 s under an ultraviolet lamp with the wavelength of 300-500 nm, exposing the photoresist outside the area where the grounding electrode and the signal electrode are required to be prepared, putting the device into a NaOH solution with the mass concentration of 2-5 per mill for 5-300 s, removing the exposed photoresist, repeatedly flushing the device along the waveguide direction by deionized water, and drying by nitrogen and then heating for 5-50 minutes; and removing the unexposed BP212 (29'), namely soaking the device in ethanol solution for 10-500 s, repeatedly washing the device cleanly along the direction of the waveguide by deionized water, drying the device by nitrogen, and heating the device for 5-50 minutes until the preparation of the modulator electrode is finished, thereby obtaining the lithium niobate thin film electro-optic modulator based on the MZI structure electro-optic polymer loading strip waveguide.