Thin film lithium niobate modulator based on double-capacitor cross electrode structure

Abstract

The invention discloses a thin-film lithium niobate modulator based on a double-capacitor cross-shaped electrode structure, and belongs to the field of integrated photonic devices. The modulator sequentially comprises a silicon substrate layer, a silicon dioxide lower cladding, a lithium niobate waveguide layer, a ridge-shaped optical waveguide layer formed by etching the upper surface of the lithium niobate layer, a silicon dioxide thin cover layer, a thin double-capacitor cross-shaped electrode structure and a thick ground-signal-ground traveling wave electrode, and a silicon dioxide upper cladding covers the upper surface. The ground-signal-ground traveling wave electrode is composed of a first ground transmission electrode, a signal transmission electrode and a second ground transmission electrode. The double-capacitor cross-shaped electrode structure is periodically arranged, the overlooking structure is a cross-shaped metal electrode, and the side-looking structure is a stepped metal electrode. The novel cross-shaped electrode structure provided by the invention can optimize refractive index matching while realizing good impedance matching, so that the electro-optical modulation bandwidth is remarkably improved.

Description

Technical Field

[0001] The present invention provides a thin-film lithium niobate modulator based on a double-capacitance cross-shaped electrode structure, belonging to the field of electro-optic modulation technology. Background Art

[0002] Electro-optic modulators modulate optical signals, enabling information to be transmitted quickly and efficiently in the form of light, providing a solid foundation for the excellent performance of modern communication networks. Among them, thin-film lithium niobate electro-optic modulators have become one of the highly regarded directions due to their unique material properties and device performance advantages.

[0003] In practice, the bandwidth and voltage performance are limited by many factors. Among them, we need the effective refractive index of microwaves and the group refractive index of light waves to be as close as possible, which means that microwaves can propagate along the transmission line at the same speed as light waves. Compared with the traditional rectangular electrode design, the microwave speed of segmented electrodes is significantly reduced, known as the slow-wave effect. This is because the capacitance per unit length of the segmented electrodes increases during each cycle. The slow-wave effect is used to precisely match the microwave phase velocity and the optical group velocity.

[0004] However, the slow-wave effect caused by the T-shaped electrode structure and the silicon substrate introduces additional capacitance. These two points together result in the microwave group velocity being less than the optical group velocity. At this time, the slow-wave effect will cause the recurrence of velocity mismatch.

[0005] Therefore, in view of the above problems, further improvements are made. Summary of the Invention

[0006] To solve the problems in the background art, the present invention provides a lithium niobate thin-film electro-optic modulator with a double-capacitance cross-shaped electrode structure, which is suitable for mass production, achieves perfect group velocity matching, and breaks the voltage-bandwidth trade-off limitation of thin-film lithium niobate electro-optic Mach-Zehnder modulators.

[0007] To achieve the above object, the present invention provides a thin-film lithium niobate modulator based on a double-capacitance cross-shaped electrode structure, including a substrate layer, a lower cladding layer, a lithium niobate thin film layer, a ridge waveguide layer with a Mach-Zehnder structure, a thin capping layer, a metal traveling-wave electrode, and an upper cladding layer; the silicon dioxide lower cladding layer (2) is located above the silicon substrate layer (1), the lithium niobate waveguide layer (3) is located above the silicon dioxide lower cladding layer (2), the ridge waveguide layer (301, 302) with a Mach-Zehnder structure is located on the lithium niobate waveguide layer (3) for optical signal transmission, the silicon dioxide thin capping layer (4) covers the lithium niobate waveguide layer (3), and the metal traveling-wave electrode is located between the silicon dioxide thin capping layer (4) and the silicon dioxide upper cladding layer (6), including a thin double-capacitance cross-shaped electrode structure (501) and a thick ground-signal-ground traveling-wave electrode (502, 503, 504), and the ground-signal-ground traveling-wave electrode is composed of a first ground transmission electrode, a signal transmission electrode, and a second ground transmission electrode.

[0008] Among them, a traveling-wave electrode structure with periodic double-capacitance loading is adopted, which is composed of a metal main electrode and a periodic micro-structure electrode. The periodic micro-structure electrode is the double-capacitance cross-shaped electrode structure, and the side view shows a stepped metal electrode. The thick electrode part is the main electrode with a thickness of 1.4 μm, and the thin electrode part is cross-shaped with a thickness of 0.2 μm. By adjusting the parameters of the cross-shaped structure, characteristic impedance matching and velocity matching are achieved.

[0009] Among them, the double-capacitance cross-shaped electrode structure is based on the T-shaped electrode structure, with a strip electrode added to form a "cross" layout. The longest strip electrode is located between the upper and lower short strip electrodes, and the lengths of the two short strip electrodes can be inconsistent.

[0010] Among them, the metal electrodes are regularly distributed in the traveling-wave electrode transmission direction, forming a periodic capacitance between the signal transmission electrode and the ground transmission electrode.

[0011] Among them, the first ground transmission electrode (502), the signal transmission electrode (503), and the second ground transmission electrode (504) are arranged in parallel, and the cross-shaped electrode structure is aligned left and right in space.

[0012] Among them, the thickness of the lithium niobate waveguide layer (3, 301, 302) is 0.5 μm, and the height of the lithium niobate ridge waveguide (301, 302) is 0.25 μm, the upper width is 1.5 μm, and the angle is 67°.

[0013] Among them, the gap between two opposite double-capacitance cross-shaped electrodes is 3 - 5 μm.

[0014] Among them, the thickness of the silicon dioxide lower cladding layer (2) between the silicon substrate layer (1) and the lithium niobate waveguide layer (3) is 4.7 μm.

[0015] Among them, a silicon oxide thin capping layer (4) is covered between the lithium niobate waveguide layer (3, 301, 302) and the ground-signal-ground traveling wave electrode, and the thickness is 0.2 μm.

[0016] Among them, a silicon oxide upper cladding layer is covered above the ground-signal-ground traveling wave electrode, and the thickness is 0.8 μm.

[0017] A thin-film lithium niobate modulator based on a dual-capacitance cross-shaped electrode structure of the present invention has the following remarkable advantages and technical effects:

[0018] The cross-shaped electrode structure is used for the electrode, which can further adjust the electric field distribution, realize the optimal overlap between the optical field and the electric field, and at the same time increase the effective conductor area, reduce the microwave transmission loss while improving the modulation efficiency, and break through the trade-off relationship between the bandwidth and the half-wave voltage.

[0019] Flexibly design various parameters affecting the electromagnetic wave propagation speed such as the period, duty cycle, electrode spacing, and electrode width to achieve a higher modulation bandwidth.

[0020] The combination of the thick main electrodes (502, 503, 504) and the cross-shaped thin electrode (501) structure avoids the current aggregation caused by the skin effect of the high-frequency electromagnetic field, reduces the equivalent resistance, makes the ohmic loss decrease, and thus improves the electro-optic bandwidth. Description of the Drawings

[0021] Figure 1 is a three-dimensional view of a thin-film lithium niobate modulator based on a dual-capacitance cross-shaped electrode structure of the present invention.

[0022] Figure 2 is a side view of a thin-film lithium niobate modulator based on a dual-capacitance cross-shaped electrode structure of the present invention.

[0023] Figure 3 is a top view of a thin-film lithium niobate modulator based on a dual-capacitance cross-shaped electrode structure of the present invention.

[0024] Figure 4 is a comparison diagram of the relationship between the microwave loss and the frequency response of a thin-film lithium niobate modulator based on a dual-capacitance cross-shaped electrode structure of the present invention, a conventional cross-shaped electrode modulator, and a dual-capacitance T-shaped modulator.

[0025] Figure 5 is a comparison diagram of the relationship between the microwave refractive index and the frequency response of a thin-film lithium niobate modulator based on a dual-capacitance cross-shaped electrode structure of the present invention, a conventional cross-shaped electrode modulator, and a dual-capacitance T-shaped modulator.

[0026] Figure 6It is a comparison chart of the relationship between the parameters of a thin-film lithium niobate modulator, a conventional cross-shaped electrode modulator, and a dual-capacitance T-shaped modulator based on a dual-capacitance cross-shaped electrode structure of the present invention with respect to frequency response. Detailed implementation manners

[0027] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings. Any modifications, equivalent replacements, improvements, etc. made without departing from the spirit and scope of the present invention defined by the appended claims shall be included within the protection scope of the present invention.

[0028] The present invention provides a thin-film lithium niobate modulator based on a dual-capacitance cross-shaped electrode structure: including a substrate layer, a lower cladding layer, a lithium niobate thin film layer, a ridge waveguide layer of a Mach-Zehnder structure, a thin capping layer, a metal traveling-wave electrode, and an upper cladding layer; the silicon dioxide lower cladding layer (2) is located above the silicon substrate layer (1), the lithium niobate waveguide layer (3) is located above the silicon dioxide lower cladding layer (2), the ridge waveguide layer (301, 302) of the Mach-Zehnder structure is located on the lithium niobate waveguide layer (3) for optical signal transmission, the silicon dioxide thin capping layer (4) covers the lithium niobate waveguide layer (3), and the metal traveling-wave electrode is located between the silicon dioxide thin capping layer (4) and the silicon dioxide upper cladding layer (6), including a thin dual-capacitance cross-shaped electrode structure (501) and a thick ground-signal-ground traveling-wave electrode (502, 503, 504), and the ground-signal-ground traveling-wave electrode is composed of a first ground transmission electrode, a signal transmission electrode, and a second ground transmission electrode.

[0029] In this embodiment, as Figure 1 、 2As shown in FIGS. 2 and 3, the double-capacitor cross-shaped electrode structure includes a periodic top-view structure of a cross-shaped metal electrode and a side-view structure of a stepped metal electrode. The thick electrodes (502, 503, 504) are the main electrodes with a thickness h4 = 1.4 um, and the thin electrode (501) is partially cross-shaped with a thickness h5 = 0.2 um. The thickness of the lithium niobate waveguide layer (3, 301, 302) is 0.5 um. The height h1 of the lithium niobate ridge waveguide (301, 302) is 0.25 um, the upper width w1 is 1.5 um, and the angle is 67°. The gap GAP between two opposite double-capacitor cross-shaped electrodes is 3 um. The thickness of the silica bottom cladding (2) between the silicon substrate layer (1) and the lithium niobate waveguide layer (3) is 4.7 um. The silica thin capping layer (4) with a thickness h3 = 0.2 um covers between the lithium niobate waveguide layer (3, 301, 302) and the ground-signal-ground traveling-wave electrode. The silica capping layer (6) with a thickness h6 = 0.8 um covers above the ground-signal-ground traveling-wave electrode. In the double-capacitor cross-shaped electrode structure (501), s1 = s3 = s5 = 1 um, the lengths of s2 and s6 can be inconsistent, but in this example, they are set to be the same, s2 = s6 = 30 um, s4 = 50 um. The number of cross-shaped electrodes is multiple and arranged periodically. The distance between two electrodes is s gap = 6 um. The width of the signal electrode is 15 um, and the widths of the left and right ground electrodes are set to 150 um. The length of the device is 1000 um.

[0030] Secondly, the use of the cross-shaped electrode structure for the electrodes can further adjust the electric field distribution, achieve the best overlap between the optical field and the electric field, increase the effective conductor area at the same time, reduce the microwave transmission loss while improving the modulation efficiency, and break through the trade-off relationship between bandwidth and half-wave voltage.

[0031] The combination of the thick main electrodes (502, 503, 504) and the cross-shaped thin electrode (501) structure avoids the current aggregation caused by the skin effect of high-frequency electromagnetic fields, reduces the equivalent resistance, decreases the ohmic loss, and thus improves the electro-optic bandwidth.

[0032] In addition, by flexibly designing various parameters that affect the electromagnetic wave propagation speed, such as the period, duty cycle, electrode spacing, and electrode width, a higher modulation bandwidth can be achieved.

[0033] The HFSS software is used to perform three-dimensional modeling and simulation on the thin-film lithium niobate modulator, as Figures 4 - 6As shown in the relationship diagram of the thin-film lithium niobate modulator with a double-capacitance cross-shaped electrode structure, the conventional cross-shaped electrode modulator, and the double-capacitance T-shaped modulator, it can be found that the thin-film lithium niobate modulator with a double-capacitance cross-shaped electrode structure is superior to the other two structures after comparing parameters such as S-parameters, microwave loss, and radio frequency refractive index. By innovating on the T structure to obtain a cross-shaped electrode structure, the group refractive index of microwaves is effectively reduced, solving the problem of the speed mismatch between light waves and microwaves, thereby obtaining a higher modulation bandwidth.

[0034] The above-disclosed is only a preferred embodiment of the present application, and it cannot be used to limit the scope of rights of the present application. Those of ordinary skill in the art can understand all or part of the processes of implementing the above embodiments, and the equivalent changes made according to the claims of the present application still fall within the scope covered by the present application.

Claims

1. A thin-film lithium niobate modulator based on a zigzag electrode structure, characterized in that It includes a silicon substrate layer, a lower cladding layer, a lithium niobate thin film layer, a ridge waveguide layer of a Mach-Zehnder structure, a thin capping layer, metal traveling wave electrodes, and an upper cladding layer; the silicon dioxide lower cladding layer is located above the silicon substrate layer, the lithium niobate waveguide layer is located above the silicon dioxide lower cladding layer, the ridge waveguide layer of the Mach-Zehnder structure is located on the lithium niobate waveguide layer for optical signal transmission, the silicon dioxide thin capping layer covers the lithium niobate waveguide layer, and the metal traveling wave electrodes are located between the silicon dioxide thin capping layer and the silicon dioxide upper cladding layer, including a zigzag electrode structure and a ground-signal-ground traveling wave electrode, and the ground-signal-ground traveling wave electrode is composed of a first ground transmission electrode, a signal transmission electrode, and a second ground transmission electrode.

2. The thin-film lithium niobate modulator based on a zigzag electrode structure according to claim 1, wherein A periodic traveling wave electrode structure is adopted, which is composed of a metal main electrode and a periodic microstructured electrode. The periodic microstructured electrode is the zigzag electrode structure with a thickness of 1.4 μm. By adjusting the parameters of the zigzag structure, characteristic impedance matching and velocity matching are achieved.

3. The thin-film lithium niobate modulator based on a zigzag electrode structure according to claim 1, wherein The zigzag electrode structure is that multiple triangles are distributed regularly in the traveling direction of the traveling wave electrode in a period, forming a periodic capacitance between the signal transmission electrode and the ground transmission electrode. The triangles are not limited to equilateral triangles, isosceles triangles, obtuse or acute triangles.

4. The thin-film lithium niobate modulator based on a zigzag electrode structure according to claim 2, wherein, The first ground transmission electrode, the signal transmission electrode, and the second ground transmission electrode are arranged in parallel, and the zigzag electrode structure is aligned left and right in space.

5. A thin-film lithium niobate modulator based on a zigzag electrode structure according to claim 1, wherein, The thickness of the lithium niobate waveguide layer is 0.5 μm, the height of the lithium niobate ridge waveguide is 0.25 μm, the upper width is 1.5 μm, and the angle is 67°. The extraordinary refractive index of the lithium niobate material is ne = 2.1376, and the ordinary refractive index is no = 2.2111.

6. The thin-film lithium niobate modulator based on a zigzag electrode structure according to claim 1, wherein: The gap between two opposite zigzag electrodes is 3 - 5 μm.

7. A thin-film lithium niobate modulator based on a zigzag electrode structure according to claim 1, wherein The thickness of the silicon dioxide lower cladding layer between the silicon substrate layer and the lithium niobate waveguide layer is 4.7 μm.

8. The thin-film lithium niobate modulator based on a zigzag electrode structure according to claim 1, characterized in that, The silicon dioxide thin capping layer with a thickness of 0.2 μm covers between the lithium niobate waveguide layer and the ground-signal-ground traveling wave electrode.

9. A thin-film lithium niobate modulator based on a zigzag electrode structure according to claim 1, wherein, The silicon dioxide upper cladding layer with a thickness of 0.8 μm covers above the ground-signal-ground traveling wave electrode.

Images