Electrooptical modulator based on artificial surface plasmon and design method

Abstract

The invention discloses an electro-optical modulator based on artificial surface plasmon and a design method.The electro-optical modulator comprises a substrate layer, a functional layer and a protective layer which are sequentially stacked from bottom to top, the functional layer comprises an optical waveguide, a beam splitter arranged in the optical waveguide, a ground electrode and a signal electrode, and the ground electrode and the signal electrode are arranged on the optical waveguide; the beam splitter is used for splitting a light path; the ground electrode and the signal electrode are used for applying an electric field; the ground electrode and the signal electrode are both periodic artificial surface plasmon structures, each periodic artificial surface plasmon structure comprises a plurality of metal teeth which are linearly arranged, and the adjacent metal teeth are connected through grooves; a convex ridge is arranged in the middle of the optical waveguide and located between the ground electrode and the signal electrode. By setting the size of the convex ridge and the size of the periodic artificial surface plasmon structure, the microwave phase velocity of the electro-optical modulator is matched with the light wave group velocity, so that the structural design of high-efficiency modulation and miniaturization integration is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of electro-optic modulation, and in particular to an electro-optic modulator based on artificial surface plasmon and a design method thereof. Background Art

[0002] Electro-optical modulators, which encode and convert high-speed electrical signals into optical signals, are a core component of modern optical communication links. Their performance directly determines the system's transmission rate, bandwidth capacity, and energy efficiency. Especially in high-capacity, high-speed optical communication systems, modulators are required to be compact, have low loss, low drive voltage, and high-speed response.

[0003] In an electro-optic modulator, the electrode structure is a key component for achieving effective conversion of electrical signals into optical signals. By applying an external voltage across the electrodes, an electrostatic field can be generated in the electrode gap region. This electric field acts on the electro-optic material, causing a change in the material's refractive index, thereby modulating the phase or intensity of the light wave. In the design of a high-efficiency electro-optic modulator, the following difficulties are encountered: (1) Matching problem between electrodes and waveguides: In traditional microstrip or coplanar structures, the electric field generated by the electrodes tends to diffuse vertically, mainly concentrating on the electrode surface and edges, making it difficult to effectively cover the core area of the waveguide, resulting in a limited modulation area and reduced electro-optical coupling efficiency; (2) The contradiction between miniaturization and loss: While pursuing device compactness to meet the needs of optoelectronic integration, the reduction of electrode spacing and length may lead to greater transmission loss, electric field leakage, and uneven coupling, thereby affecting the modulation depth and energy utilization.

[0004] Currently, electro-optic modulators typically employ electrode structures such as microstrip lines or coplanar waveguides and have been widely used in fields such as optical communications. However, these structures still have significant deficiencies in terms of electric field confinement, electro-optical coupling efficiency, and bandwidth, which impacts electro-optical modulation efficiency. Summary of the Invention

[0005] Purpose of the invention: In view of the above shortcomings, the present invention provides an electro-optic modulator based on artificial surface plasmons, which can achieve high-efficiency electro-optic modulation and design method on the basis of miniaturization.

[0006] Technical solution: To solve the above problems, the present invention adopts an electro-optic modulator based on artificial surface plasmons, including a substrate layer, a functional layer, and a protective layer stacked in sequence from bottom to top. The functional layer includes an optical waveguide, a beam splitter arranged in the optical waveguide, a ground electrode and a signal electrode arranged on the optical waveguide. The beam splitter is used to split the light path, and the ground electrode and the signal electrode are used to apply an electric field; the ground electrode and the signal electrode are both periodic artificial surface plasmon structures, and the periodic artificial surface plasmon structure includes a plurality of linearly arranged metal teeth, and adjacent metal teeth are connected by grooves; a ridge is provided in the middle of the optical waveguide, and the ridge is located between the ground electrode and the signal electrode; by setting the size of the ridge and the size of the periodic artificial surface plasmon structure, the microwave phase velocity of the electro-optic modulator is matched with the group velocity of the light wave.

[0007] Furthermore, two rows of ground electrodes and one row of signal electrodes are provided on the optical waveguide. The signal electrode is located between the two rows of ground electrodes. A row of ridges is provided between the ground electrodes and the signal electrodes. The beam splitter includes two interference arms, which are coaxially arranged with the ridges.

[0008] Furthermore, the longitudinal section of the ridge is an isosceles trapezoid, the height of the isosceles trapezoid is 0.8 μm, the width of the upper base is 0.5 μm, and the width of the lower base is 1 μm.

[0009] Furthermore, the metal teeth are in a rectangular parallelepiped shape, the gap between adjacent metal teeth is 100 μm, and the length of the metal teeth is 100 μm, the width is 12 μm, and the height is 1.2 μm.

[0010] Furthermore, the substrate layer is made of silicon, the optical waveguide is made of lithium niobate, the ground electrode and the signal electrode are both made of gold, and the protective layer is made of silicon dioxide.

[0011] Furthermore, the optical waveguide uses an x-cut lithium niobate wafer, and the wafer surface is perpendicular to the x-axis of the crystal.

[0012] The present invention also provides a design method for the above-mentioned electro-optical modulator based on artificial surface plasmons, comprising the following steps:

[0013] Step 1: Set the size of the ridge structure;

[0014] Step 2: Obtaining the light group velocity according to the set ridge structure size;

[0015] Step 3: Set the microwave phase velocity to be equal to the light group velocity, and obtain the corresponding periodic artificial surface plasmon structure size according to the microwave phase velocity value.

[0016] Furthermore, step 2 is specifically as follows: obtaining a light wave dispersion curve diagram under a light wave mode corresponding to a set ridge structure size through simulation software, and calculating the light wave group velocity at the optical communication standard frequency of 193.5 THz based on the light wave dispersion curve diagram.

[0017] Furthermore, step 3 is specifically as follows: select a microwave frequency value as a velocity matching point, the microwave phase velocity of the velocity matching point is equal to the light wave group velocity obtained in step 2, simulate and obtain a standard microwave dispersion curve under the microwave mode containing the velocity matching point, continuously adjust the size of the periodic artificial surface plasmon structure and test the microwave dispersion curve under the size until a standard microwave dispersion curve is obtained. Figure 1 The corresponding size of the figure is the size of the desired periodic artificial surface plasmon structure.

[0018] Furthermore, step 3 is specifically as follows: the microwave frequency value corresponding to the velocity matching point is located in the band edge region, and the frequency value range is 210-220 GHz.

[0019] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: (1) High modulation efficiency: By introducing periodic artificial surface plasmon units into the electrode structure, the present invention regulates the dispersion characteristics of the microwave mode, achieves the matching of the microwave phase velocity and the light wave group velocity, and simultaneously excites a significant slow light effect at the edge of the photonic band gap, so that the microwave field is locally enhanced in the subwavelength metal tooth gap, significantly improving the mutual coupling and overlapping volume between microwaves and light waves in the electro-optical modulator, effectively enhancing the electro-optical interaction in the modulation area, and thus improving the overall electro-optical modulation efficiency. (2) Miniaturization and integration: By utilizing the subwavelength field localization effect of the artificial surface plasmon structure, the electric field leakage is effectively suppressed, and the electric field confinement capability between electrodes is significantly improved, thereby allowing for a smaller electrode spacing and device size, and a more compact structure compared to traditional electro-optical modulators. (3) Low loss and large bandwidth: Since the microwave mode field is effectively localized and the energy is transmitted in a concentrated manner, the loss during transmission is reduced, further reducing the total insertion loss of the modulator. At the same time, by optimizing the structural design, the present invention significantly improves the electro-optical response bandwidth while maintaining a low driving voltage, making it suitable for high-speed optical communication applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Schematic diagram of the overall structure of the electro-optic modulator of the present invention;

[0021] Figure 2 is a top view of the electro-optical modulator of the present invention;

[0022] Figure 3 is a partial longitudinal cross-sectional view of the electro-optic modulator of the present invention;

[0023] Figure 4Schematic diagram of the periodic artificial surface plasmon structure of the present invention;

[0024] Figure 5 Schematic diagram of the optical mode field distribution in the electro-optic modulation core region of the electro-optic modulator of the present invention;

[0025] Figure 6 This is a dispersion curve diagram of the electro-optic modulator in light wave mode according to the second embodiment of the present invention.

[0026] Figure 7 This is a dispersion curve diagram of the electro-optic modulator in microwave mode according to the second embodiment of the present invention. DETAILED DESCRIPTION

[0027] Example 1

[0028] like Figure 1 and Figure 2 As shown, an electro-optic modulator based on artificial surface plasmons in this embodiment includes a substrate layer 1, a functional layer 2, and a protective layer 3 stacked in sequence from bottom to top. The functional layer 2 includes an optical waveguide 4, a beam splitter 7 disposed within the optical waveguide 4, a ground electrode 5 and a signal electrode 6 disposed on the optical waveguide 4. The substrate layer 1 is made of silicon, the optical waveguide 4 is a lithium niobate optical waveguide, the ground electrode 5 and the signal electrode 6 are both made of gold, and the protective layer 3 is made of silicon dioxide.

[0029] Lithium niobate crystal is a typical anisotropic electro-optical crystal, whose refractive index changes with the change of the external electric field. In engineering applications, lithium niobate wafers are usually divided into three cutting directions: x-cut, y-cut and z-cut, among which x-cut and z-cut are the most commonly used types. When the direction of the external electric field and the polarization direction of the crystal are both in the z-axis direction, the lithium niobate crystal directly utilizes the maximum electro-optic coefficient r 33 , resulting in the most significant refractive index modulation effect. The present invention utilizes an x-cut lithium niobate wafer, with the wafer surface perpendicular to the crystal's x-axis. The optical waveguide is fabricated along the y-axis, and electrodes are designed to be located on both sides of the waveguide. When a voltage is applied, the electric field is along the z-axis. Furthermore, the transmission mode in the optical waveguide is a transverse electric field mode, resulting in polarization along the z-axis, consistent with the direction of the applied electric field.

[0030] like Figure 3As shown, a ridge 41 is provided in the middle of the optical waveguide 4, located between the ground electrode 5 and the signal electrode 6. The longitudinal cross-section of the ridge 41 is an isosceles trapezoid with a height of 0.8 μm, an upper base width of 0.5 μm, and a lower base width of 1 μm. The sidewalls of the ridge 41 form a physical boundary, which, combined with the refractive index difference between the core and the protective layer, significantly enhances the lateral light field confinement capability. Furthermore, the thickness variation of the ridge-shaped raised portion further enhances the vertical light field confinement, reduces the interaction between the light wave and the surrounding material, and reduces light field leakage loss, thereby optimizing the propagation efficiency of the optical signal in the waveguide. The height of the optical waveguide ridge is an important factor affecting the electric field distribution. As the ridge height increases, the waveguide sidewalls better confine the mode, the size of the mode spot decreases, and the electric field attenuation generated by the applied voltage is greater. Therefore, determining a reasonable optical waveguide ridge height helps achieve higher modulation efficiency.

[0031] like Figure 4 As shown, both the ground electrode 5 and the signal electrode 6 are periodic artificial surface plasmon structures, comprising a plurality of linearly arranged metal teeth 8, with adjacent metal teeth 8 connected by grooves 9. The ground electrode 5 and the signal electrode 6 are used to apply an electric field to alter the optical properties of the functional material of the optical waveguide 4. The period, width, height, and gaps between adjacent metal teeth of the periodic artificial surface plasmon structure can be flexibly controlled to achieve matching of the microwave phase velocity and the light group velocity within the modulation region. Because the metal electrode unit is composed of alternating metal teeth and groove regions, it can be equivalently considered a periodically arranged structure composed of materials with different effective refractive indices, resulting in a periodic structure with photonic crystal properties. The artificial surface plasmon structure exhibits a significant slow-light effect in the band-edge region of its dispersion curve, enabling localized enhancement of the microwave field within the subwavelength metal periodic electrode unit, significantly increasing the mode overlap volume and coupling efficiency between the microwave field and the light field, effectively enhancing the electro-optical interaction in the modulation region, and thus improving the overall electro-optical modulation efficiency.

[0032] In order to further reduce the modulator half-wave voltage (V πThe electro-optic modulator of the present invention employs a push-pull Mach-Zehnder modulation structure. In this structure, incident light enters the input port and is split into two optical paths at the beam splitter 7, each entering the two interference arms of the modulator. Applying a voltage to one or both interference arms can change the refractive index of the light wave in the waveguide, thereby changing the phase and achieving electro-optical modulation. This Mach-Zehnder interference structure can further employ, but is not limited to, a push-pull drive method, where equal and opposite modulation voltages are applied to the two interference arms to double the phase difference. Under the condition of the same modulation voltage, this method can effectively shorten the device length; while maintaining the same device size, the modulation voltage can be halved, further improving modulation efficiency and reducing power consumption. At the output port, the two phase-modulated light waves superimpose to form the output light, modulating the light intensity through the interference effect, thereby completing the effective modulation process of the input optical signal. This structure can double the required phase difference at the same drive voltage, thereby effectively reducing the modulation voltage and improving modulation efficiency.

[0033] like Figure 5 The figure shows a schematic diagram of the optical mode field distribution in the core area of the electro-optical modulation of the present invention. It can be observed from the figure that the excited light mode is mainly concentrated in the ridge waveguide area, indicating that the waveguide structure has good mode field confinement capability. The simulation result corresponds to an effective refractive index of 1.969 for the light wave, and since it has no imaginary part, the propagation loss can be ignored, and it has good transmission performance, meeting the basic requirements of the electro-optical modulator for low loss and high efficiency transmission. In the structure constructed by the present invention, the microwave mode and the light wave mode have good mode field overlap characteristics inside the lithium niobate thin film waveguide, and the two are coupled and superimposed in space, thereby achieving efficient phase modulation within a shorter device length, significantly improving the overall electro-optical conversion efficiency.

[0034] The length L and the corresponding half-wave voltage V π The product V π L can be used to characterize the voltage-length “cost” required to achieve effective phase modulation, which is calculated as follows:

[0035]

[0036] Where: λ is the operating wavelength, n e is the refractive index of lithium niobate material, r 33 is the effective electro-optic coefficient, Γ is the electro-optic overlap factor, and its size is closely related to the size of the optical waveguide. In particular, the height and top width of the ridge will directly affect the mode field distribution and the degree of electric field overlap. G is the distance between the electrodes on both sides of a waveguide arm, V is the voltage applied to the signal electrode, and |E o (x,z)| is the electric field mode of the light wave in the LiNbO3 waveguide, E rf,z(x, y, z) is the microwave electric field component along the z direction in the LiNbO3 waveguide. The subscripts o and rf are the types of electromagnetic waves, representing light waves and microwaves respectively. The subscript z represents the component along the z direction. π The calculation formula of L can be obtained: The electro-optic modulator of this embodiment can achieve a half-wave voltage lower than 2.2 V·cm and has a high modulation efficiency.

[0037] Example 2

[0038] The present invention also provides a design method for the above-mentioned electro-optical modulator based on artificial surface plasmons, comprising the following steps:

[0039] Step 1: Set the structural dimensions of the ridge 41. In this embodiment, the longitudinal section of the ridge 41 is an isosceles trapezoid with a height of 0.8 μm, an upper base width of 0.5 μm, and a lower base width of 1 μm.

[0040] Step 2: Obtain the light group velocity according to the set structural dimensions of the ridge 41. First, based on COMSOL The full vector finite element simulation is performed to obtain the light wave dispersion curve under the light wave mode corresponding to the ridge size, as shown in Figure 6 As shown in the figure, the horizontal axis is wavelength, and the vertical axis is the effective refractive index. Matching the light group refractive index with the microwave equivalent refractive index is equivalent to matching the light group velocity with the microwave phase velocity. At the standard optical communication frequency of 193.5 THz (corresponding to a wavelength of 1550 nm), the corresponding effective refractive index at that frequency in the figure is approximately 1.969. Taking into account other structural factors of the electro-optic modulator, the light group refractive index is approximately 2.3 (this value is directly given by the simulation software).

[0041] Step 3, set the microwave phase velocity to be equal to the light group velocity, and obtain the corresponding periodic artificial surface plasmon structure size according to the microwave phase velocity value. Based on the periodic design of the metal photonic crystal traveling wave electrode, the microwave dispersion curve shows strong nonlinearity above 210GHz, and the band edge region (210-220GHz) forms an equivalent refractive index abrupt change region with a large equivalent refractive index. Therefore, a velocity matching point is selected in the band edge region. In this embodiment, 220GHz is selected as the velocity matching point, and the microwave equivalent refractive index corresponding to this point is 2.3. The standard microwave dispersion curve diagram under the microwave mode containing this velocity matching point is obtained by simulation, as shown in FIG. Figure 7 As shown in the figure, the horizontal axis is the normalized wave number and the vertical axis is the frequency. The size of the periodic artificial surface plasmon structure is continuously adjusted and the microwave dispersion curve under the size is tested until a standard microwave dispersion curve is obtained. Figure 1The resulting pattern is the size of the desired periodic artificial surface plasmon structure. The final dimensions obtained are a 100μm gap between adjacent metal teeth 8, a length of 100μm, a width of 12μm, and a height of 1.2μm.

Claims

1. An electro-optic modulator based on artificial surface plasmon, characterized in that: The invention comprises a substrate layer (1), a functional layer (2), and a protective layer (3) which are stacked in sequence from bottom to top. The functional layer (2) comprises an optical waveguide (4), a beam splitter (7) arranged in the optical waveguide (4), a ground electrode (5) and a signal electrode (6) arranged on the optical waveguide (4). The beam splitter (7) is used to split the optical path, and the ground electrode (5) and the signal electrode (6) are used to apply an electric field. The ground electrode (5) and the signal electrode (6) are both periodic artificial surface plasmon structures. The periodic artificial surface plasmon structure comprises a plurality of linearly arranged metal teeth (8), and adjacent metal teeth (8) are connected by grooves (9). A convex ridge (41) is provided in the middle of the optical waveguide (4), and the convex ridge (41) is located between the ground electrode (5) and the signal electrode (6). The size of the convex ridge (41) and the size of the periodic artificial surface plasmon structure are set to match the microwave phase velocity of the electro-optic modulator with the light wave group velocity.

2. The electro-optic modulator based on artificial surface plasmon according to claim 1, wherein: Two rows of ground electrodes (5) and one row of signal electrodes (6) are arranged on the optical waveguide (4); the signal electrode (6) is located between the two rows of ground electrodes (5); a row of ridges (41) is provided between the ground electrodes (5) and the signal electrodes (6); the beam splitter (7) includes two interference arms (71); the interference arms (71) and the ridges (41) are coaxially arranged.

3. The electro-optic modulator based on artificial surface plasmon according to claim 1, wherein: The longitudinal section of the ridge (41) is an isosceles trapezoid, the height of the isosceles trapezoid is 0.8 μm, the width of the upper base is 0.5 μm, and the width of the lower base is 1 μm.

4. The electro-optic modulator based on artificial surface plasmon according to claim 3, characterized in that: The metal teeth (8) are in a rectangular parallelepiped shape, the gap between adjacent metal teeth (8) is 100 μm, and the length of the metal teeth (8) is 100 μm, the width is 12 μm, and the height is 1.2 μm.

5. The electro-optic modulator based on artificial surface plasmon according to claim 1, wherein: The substrate layer (1) is made of silicon, the optical waveguide (4) is made of lithium niobate, the ground electrode (5) and the signal electrode (6) are both made of gold, and the protective layer (3) is made of silicon dioxide.

6. The electro-optic modulator based on artificial surface plasmon according to claim 5, characterized in that: The optical waveguide (4) adopts an x-cut lithium niobate wafer, and the wafer surface is perpendicular to the x-axis of the crystal.

7. A method for designing an electro-optical modulator based on artificial surface plasmon according to any one of claims 1 to 6, characterized in that: The following steps are involved: Step 1, setting the structural dimensions of the ridge (41); Step 2, obtaining the light group velocity according to the set structural dimensions of the ridge (41); Step 3: Set the microwave phase velocity to be equal to the light group velocity, and obtain the corresponding periodic artificial surface plasmon structure size according to the microwave phase velocity value.

8. The design method according to claim 7, wherein: Step 2 specifically comprises: obtaining a light wave dispersion curve diagram under a light wave mode corresponding to a set ridge (41) structural size through simulation software, and calculating the light wave group velocity at an optical communication standard frequency of 193.5 THz based on the light wave dispersion curve diagram.

9. The design method according to claim 8, wherein: Step 3 specifically includes: selecting a microwave frequency value as a velocity matching point, where the microwave phase velocity at this velocity matching point is equal to the light group velocity obtained in step 2, simulating to obtain a standard microwave dispersion curve diagram under the microwave mode containing this velocity matching point, continuously adjusting the size of the periodic artificial surface plasmon structure and testing the microwave dispersion curve diagram under this size until a graph consistent with the standard microwave dispersion curve graph is obtained. The size corresponding to this graph is the desired size of the periodic artificial surface plasmon structure.

10. The design method according to claim 9, wherein: Step 3 is specifically as follows: the microwave frequency value corresponding to the velocity matching point is located in the band edge region, and the frequency value range is 210-220 GHz.

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