Lithium niobate electro-optical modulator based on interdigital electrode

Abstract

The invention discloses a lithium niobate electro-optical modulator based on interdigital electrodes. The lithium niobate electro-optical modulator comprises a coplanar waveguide electrode structure containing periodically arranged interdigital electrodes, two groups of six modulation waveguides, two groups of four optical waveguide delay lines, two groups of multimode interference couplers, an input waveguide, an output waveguide and a pair of coupling gratings. According to the device, an interdigital electrode structure is used for constructing six electro-optical modulation sections in an upper group and a lower group, and optical waveguide delay lines are used for aligning phases of different electro-optical modulation sections to form an upper group of modulation arms and a lower group of modulation arms. An optical signal enters the input waveguide from the coupling grating on the left side, is divided into two beams through the multi-mode interference coupler, is subjected to equal-size and opposite modulation effects on the upper and lower groups of modulation arms, is combined and coupled in the second multi-mode interference coupler to generate an intensity modulation effect, and is output through the output waveguide and the coupling grating. The electro-optical modulator provided by the invention can be further expanded on the number of times of electrode multiplexing, and is compatible with other multiplexing technologies at the same time.

Description

Technical Field

[0001] The invention belongs to the technical field of optical communication and relates to a lithium niobate electro-optic modulator based on interdigital electrodes. Background Art

[0002] With the development of social economy, people's requirements for the speed and quality of information transmission are increasing, and traditional means of information transmission can no longer meet these high standards. High-speed, high-bandwidth information transmission has become particularly critical. In this context, photonics, as an emerging alternative technology to electronics, has gradually attracted the attention of many researchers. Using integrated optical circuits to replace traditional electronic circuits, that is, using "light" to realize the function of "electricity", can more efficiently perform logical operations and device interconnection. The application of optical fiber technology has replaced copper wires, which not only greatly improves data transmission speed and computing power, but also reduces energy consumption. Optical communication technology has become the mainstream means to achieve long-distance, large-capacity communication, and electro-optical modulators are one of the core components in optical communication systems.

[0003] Traditional lithium niobate electro-optic modulators cannot effectively balance the issues of footprint and working bandwidth, but lithium niobate has attracted much attention in recent years due to its excellent electro-optical, acousto-optical, and piezoelectric properties. With the continuous development of process technology, the lithium niobate on insulator (LNOI) platform came into being. It overcomes the limitations of traditional lithium niobate devices in waveguide manufacturing, such as low refractive index difference and large device volume, and achieves a higher degree of integration. The thin-film lithium niobate of the LNOI platform not only inherits the excellent properties of the material itself, but also has the advantage of a high refractive index difference, making it easier to integrate multiple unit devices on LNOI. At present, high-performance electro-optic modulators based on the LNOI platform have been widely studied, which marks a new breakthrough and great potential in the field of optoelectronics. At present, many excellent electro-optic modulators based on the LNOI platform have been reported at home and abroad. For example, the research group of Marko Lončar at Harvard University published a thin-film lithium niobate electro-optic modulator with CMOS-compatible driving voltage in Nature ("Integrated lithium niobate electro-optic modulators operating at CMOS-compatible voltages" Nature 562(7725), 101-104(2018)).

[0004] At present, most of the electro-optic modulators widely used in optical communication systems are based on the Mach-Zehnder interferometer structure, and the bandwidth of their mainstream products has gradually moved from 70GHz to 110GHz. However, as the bandwidth continues to increase, the RF loss also increases significantly, which puts higher requirements on the output capacity of the driving circuit. Therefore, there is an urgent need to further reduce the half-wave voltage of the electro-optic modulator and improve its modulation efficiency. The modulation efficiency of existing electro-optic modulators per unit length is limited by the modulation cross-sectional structure of the electrode and waveguide. In order to improve the modulation efficiency, common methods include extending or folding the electrode to reduce the required driving voltage, but this approach is limited by microwave loss. In the current mainstream platform, the microwave signal intensity of the microwave electrode structure will be significantly weakened after a distance of more than 5cm, resulting in the inability to further improve the modulation efficiency. In addition, this method may also lead to refractive index mismatch and a reduction in modulation bandwidth. Another way to improve efficiency is to let the light pass through the modulation area multiple times, but this requires the use of multimode waveguides and multiple mode conversions, which will increase the crosstalk and noise of the modulator. Summary of the invention

[0005] The present invention provides a lithium niobate electro-optic modulator based on interdigital electrodes, aiming to improve the modulation efficiency without significantly increasing the device area and crosstalk, and to provide a broader application prospect for the application of the electro-optic modulator.

[0006] To this end, the present invention adopts the following technical solutions: A lithium niobate electro-optic modulator based on interdigital electrodes mainly includes an electrical part and an optical part. The electrical part includes a coplanar waveguide metal electrode that carries electrical signals, which is composed of a signal electrode in the middle, a ground electrode structure on both sides, and two electrode slits formed by the signal electrode and the ground electrode; a metal pad for connecting electrical interfaces such as external radio frequency probes and packaging gold wires; a "stem"-shaped interdigital electrode for guiding electrical signals to construct multiple groups of modulation zones; and a hot electrode for thermal biasing. The optical part comprises: a first coupling grating and an input waveguide on the right side thereof, a 1x2 multimode interference coupler beam splitting structure on the right side of the input waveguide, which comprises an input port, a plate coupling region and two output ports, the multimode interference coupler beam splitting structure is respectively connected to upper and lower groups of modulation arms on the right side, the upper side comprises a first modulation waveguide, a first optical waveguide delay line on the right side of the first modulation waveguide, a second modulation waveguide on the left side of the first optical waveguide delay line, a second optical waveguide delay line on the left side of the second modulation waveguide, and a third modulation waveguide after the second optical waveguide delay line, the lower side comprises a fourth modulation waveguide, a third optical waveguide delay line on the right side of the fourth modulation waveguide, a fifth modulation waveguide on the left side of the third optical waveguide delay line, a fourth optical waveguide delay line on the left side of the fifth modulation waveguide, and a sixth modulation waveguide of the fourth optical waveguide delay line, the upper and lower arms are connected to a second multimode interference coupler beam combining structure consisting of two input ports, a plate coupling region and an output port, the output waveguide on the right side of the second multimode interference coupler beam combining structure and a second coupling grating.

[0007] The two groups of modulation waveguides respectively constitute the upper and lower modulation arms of the Mach-Zehnder interferometer in the electro-optic modulator. The present invention uses a coplanar waveguide electrode structure to form two modulation arms with opposite modulation directions, that is, the electrode configuration is a push-pull structure. The present invention uses a heating electrode to change the working point of the Mach-Zehnder interferometer. When used for intensity modulation, the working point of the modulator is biased at the orthogonal working point. The present invention uses a coplanar waveguide electrode structure based on the interdigital electrode design, and constructs multiple electro-optical action areas in the two metal electrode slits of the coplanar waveguide electrode. Phase modulation is performed between different electro-optical action areas through an optical waveguide delay line to ensure that the modulation effect can be superimposed.

[0008] The waveguide in the electro-optic modulator of the present invention is a thin-film lithium niobate waveguide loaded with silicon nitride. Since silicon nitride has a transparent window similar to that of lithium niobate material and the refractive index of silicon nitride is slightly lower than that of lithium niobate material, the mode field of the silicon nitride-lithium niobate composite waveguide can be mostly in the lithium niobate layer and its working bandwidth is not affected. Most importantly, silicon nitride material has a mature processing technology. Compared with the traditional direct-etched lithium niobate waveguide, there is no waveguide inclination angle. It can easily process structures with ultra-small line width such as sub-wavelength waveguide gratings, and is conducive to large-scale integration of devices.

[0009] The electro-optic modulator of the present invention creates multiple modulation regions without significantly increasing the device area by constructing a special interdigitated electrode structure in the traveling wave electrode, so that crosstalk caused by mode conversion can be avoided when light enters the modulation region multiple times. At the same time, this design allows a larger electrode gap, which helps to reduce microwave loss and improve the modulation bandwidth.

[0010] The beneficial effects of the present invention are: 1. Based on the thin-film lithium niobate platform, the excellent electro-optical modulation coefficient and high optical transparency window of lithium niobate material can be used to achieve ultra-wideband optical working bandwidth from visible light to 4μm band. The device can be highly integrated on chip, with the advantages of small size, low power consumption and low driving voltage, which is conducive to combining with the driving circuit to realize a compact driving module.

[0011] 2. The device achieves low modulation voltage and high modulation efficiency based on the multiplexing of the modulation area, avoiding the problem of excessive mismatch between the microwave refractive index and the optical refractive index and microwave loss caused by the introduction of an overly long traveling wave electrode structure, and avoiding the device's working bandwidth being electrically limited and causing excessive echo reflection noise to reduce the quality of the optical modulation signal.

[0012] 3. The modulation electrode and working waveguide of this device are isolated by a silicon dioxide buffer layer. For the optical transverse electric (TE) mode required for modulation, the propagation mode generation of the composite waveguide composed of the traveling wave electrode and the substrate can be effectively reduced, thereby greatly reducing the absorption loss of the optical electric field by the metal material of the traveling wave electrode, and improving the modulation quality and signal-to-noise ratio of the optical modulation signal.

[0013] 4. This device greatly increases the metal spacing of the coplanar waveguide electrode structure through the interdigital structure, reduces the leakage loss caused by the metal spacing and the resulting drift of the working bias point of the Mach-Zehnder interferometer, and ultimately reduces the modulator microwave loss and driving voltage.

[0014] 5. The interdigitated structure can construct multiple modulation zones without significantly increasing the device area. Compared with the traditional folded electrode, it avoids introducing multiple sets of coplanar waveguide electrode structures to increase the area.

[0015] 6. This device is compatible with waveguide multiplexing technology, and allows the modulation efficiency of the electro-optic modulator to be further expanded by combining it with mode-based multiplexing technology at the expense of some crosstalk and loss. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 is a schematic diagram of an electro-optic modulator of the present invention; Figure 2 It is the optical part structure of the electro-optic modulator of the present invention; Figure 3 It is the electrical part structure of the electro-optic modulator of the present invention; Figure 4 Schematic diagram of the electrical interdigital electrode structure of the electro-optic modulator of the present invention; Figure 5 It is the electric field modulation cross section simulation of the electro-optic modulator of the present invention, including the optical field energy distribution and the electric field energy distribution; Figure 6 It is the electric field simulation of the present invention, including the electric field distribution of three groups of modulation cross sections; Figure 7 It is a schematic diagram of the layering of the waveguide and electrode structure materials of the present invention.

[0017] In the figure: 1. electrical part, 2. optical part, 3. input coupling grating, 4. input waveguide, 5. first multimode interference coupler, 6. first modulation waveguide, 7. second modulation waveguide, 8. third modulation waveguide, 9. first optical waveguide delay line, 10. second optical waveguide delay line; 11. fourth modulation waveguide, 12. fifth modulation waveguide, 13. sixth modulation waveguide, 14. third optical waveguide delay line, 15. fourth optical waveguide delay line; 16. second multimode interference coupler, 17. output waveguide, 18. output coupling grating; 19. hot electrode, 20. upper ground electrode, 21. signal electrode, 22. lower ground electrode, 23. left electrode pad, 24. right electrode pad, 25. interdigital electrode, 26. trunk, 27. first branch, 28. second branch; 61 silicon substrate, 62 silicon dioxide buried layer, 63 lithium niobate film, 64 silicon nitride loaded waveguide, 65 silicon dioxide cladding, 66 gold electrode. DETAILED DESCRIPTION

[0018] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0019] like Figure 1 As shown, the electro-optical modulation device of the present invention includes an electrical part 1 and an optical part 2.

[0020] like Figure 2 As shown, the optical part 2 in the electro-optical modulation device of the present invention includes an input coupling grating 3 and an output coupling grating 18. The input coupling grating 3 is connected to the first multimode interference coupler 5 through the input waveguide 4, and the output coupling grating 18 is connected to the second multimode interference coupler 16 through the output waveguide 17. The first multimode interference coupler 5 is connected to the second multimode interference coupler 16 through two modulation arms.

[0021] One of the modulation arms includes a first modulation waveguide 6, a first optical waveguide delay line 9, a second modulation waveguide 7, a second optical waveguide delay line 10 and a third modulation waveguide 8 which are connected in sequence. The head end of the first modulation waveguide 6 is connected to the first multimode interference coupler 5, and the tail end of the third modulation waveguide 8 is connected to the second multimode interference coupler 16.

[0022] The first optical waveguide delay line 9 and the second optical waveguide delay line 10 are both door frame-shaped, and the second optical waveguide delay line 10 is located inside the first optical waveguide delay line 9. The first optical waveguide delay line 9 and the second optical waveguide delay line 10 are both located above the first modulation waveguide 6, the end of the first optical waveguide delay line 9 facing the second multimode interference coupler 16 is connected to one end of the first modulation waveguide 6, the end of the first optical waveguide delay line 9 away from the second multimode interference coupler 16 is connected to one end of the second modulation waveguide 7; the other end of the second modulation waveguide 7 is connected to the end of the second optical waveguide delay line 10 facing the second multimode interference coupler 16, the end of the second optical waveguide delay line 10 away from the second multimode interference coupler 16 is connected to one end of the third modulation waveguide 8, and the other end of the third modulation waveguide 8 is connected to the second multimode interference coupler 16.

[0023] The second modulation waveguide 7 is located between the first modulation waveguide 6 and the third modulation waveguide 8 .

[0024] The third modulation waveguide 8 intersects the first optical waveguide delay line 9 and the second optical waveguide delay line 10 .

[0025] Another modulation arm includes a fourth modulation waveguide 11, a third optical waveguide delay line 14, a fifth modulation waveguide 12, a fourth optical waveguide delay line 15 and a sixth modulation waveguide 13 connected in sequence, the head end of the fourth modulation waveguide 11 is connected to the first multimode interference coupler 5, and the tail end of the sixth modulation waveguide 13 is connected to the second multimode interference coupler 16.

[0026] The third optical waveguide delay line 14 and the fourth optical waveguide delay line 15 are both inverted door frame shapes, and the fourth optical waveguide delay line 15 is located inside the third optical waveguide delay line 14. The third optical waveguide delay line 14 and the fourth optical waveguide delay line 15 are both located below the fourth modulation waveguide 11, and the end of the third optical waveguide delay line 14 facing the second multimode interference coupler 16 is connected to one end of the fourth modulation waveguide 11, and the end of the third optical waveguide delay line 14 away from the second multimode interference coupler 16 is connected to one end of the fifth modulation waveguide 12; the other end of the fifth modulation waveguide 12 is connected to the end of the fourth optical waveguide delay line 15 facing the second multimode interference coupler 16, and the end of the fourth optical waveguide delay line 15 away from the second multimode interference coupler 16 is connected to one end of the sixth modulation waveguide 13, and the other end of the sixth modulation waveguide 13 is connected to the second multimode interference coupler 16.

[0027] The fifth modulation waveguide 12 is located between the fourth modulation waveguide 11 and the sixth modulation waveguide 13 .

[0028] The sixth modulation waveguide 13 intersects the third optical waveguide delay line 14 and the fourth optical waveguide delay line 15 .

[0029] The first multimode interference coupler 5 and the second multimode interference coupler 16 are both 1×2 multimode interference coupler beam splitting structures. The first multimode interference coupler 5 has an input port, a multimode coupling region and two output ports; the input port of the first multimode interference coupler 5 is connected to the tail end of the input waveguide 4, and the two output ports of the first multimode interference coupler 5 are respectively connected to the head end of the second modulation waveguide 7 and the fourth modulation waveguide 11. The second multimode interference coupler 16 has two input ports, a multimode coupling region and an output port, the two input ports of the second multimode interference coupler 16 are respectively connected to the tail end of the third modulation waveguide 8 and the sixth modulation waveguide 13, and the output port of the second multimode interference coupler 16 is connected to the head end of the output waveguide 17.

[0030] The electrical part 1 of the electro-optical modulation device of the present invention adopts a coplanar waveguide electrode structure, such as Figure 3 As shown. It includes a hot electrode 19 and an upper ground electrode 20, a signal electrode 21 and a lower ground electrode 22 arranged in sequence from top to bottom, and the ends of the upper ground electrode 20, the signal electrode 21 and the lower ground electrode 22 close to the first multi-mode interference coupler 5 are all connected to a left electrode pad 23; the ends of the upper ground electrode 20, the signal electrode 21 and the lower ground electrode 22 close to the second multi-mode interference coupler 16 are all connected to a right electrode pad 24.

[0031] The upper ground electrode 20 and the left electrode pad 23 and the right electrode pad 24 connected thereto constitute a first electrode, and the first electrode is located in a space surrounded by the third modulation waveguide 8 and the second optical waveguide delay line 10 .

[0032] The signal electrode 21 and the left electrode pad 23 and the right electrode pad 24 connected thereto constitute a second electrode, and the second electrode is located in a region between the first modulation waveguide 6 and the fourth modulation waveguide 11 .

[0033] The lower ground electrode 22 and the left electrode pad 23 and the right electrode pad 24 connected thereto constitute a third electrode, and the third electrode is located in a space surrounded by the sixth modulation waveguide 13 and the fourth optical waveguide delay line 15 .

[0034] The lengths of the upper ground electrode 20 , the signal electrode 21 and the lower ground electrode 22 are all equal and close to the length of the space region surrounded by the third modulation waveguide 8 and the second optical waveguide delay line 10 .

[0035] A portion of the sixth modulation waveguide 13 located between the third optical waveguide delay line 14 and the second multimode interference coupler 16 is provided with a thermal electrode 19 for biasing on the side facing the second multimode interference coupler 16, and the thermal electrode 19 is not in contact with the sixth modulation waveguide 13, the third optical waveguide delay line 14 and the second multimode interference coupler 16.

[0036] At least two interdigitated electrodes 25 are connected to the side of the upper ground electrode 20 facing the signal electrode 21, and at least two interdigitated electrodes 25 are connected to the side of the lower ground electrode 22 facing the signal electrode 21. The number of interdigitated electrodes 25 connected to the upper ground electrode 20 is the same as the number of interdigitated electrodes 25 connected to the lower ground electrode 22; at least two interdigitated electrodes 25 are connected to the side of the signal electrode 21 facing the upper ground electrode 20 and the side of the signal electrode 21 facing the lower ground electrode 22, and the number of interdigitated electrodes 25 connected on both sides of the signal electrode 21 is the same as the number of interdigitated electrodes 25 connected to the upper ground electrode 20.

[0037] The first electrode and all the interdigital electrodes 25 connected thereto, the second electrode and all the interdigital electrodes 25 connected thereto, and the third electrode and all the interdigital electrodes 25 connected thereto all constitute a coplanar waveguide traveling-wave electrode.

[0038] The left electrode pad 23 and the right electrode pad 24 are used to introduce radio frequency signals and radio frequency loads when in use.

[0039] like Figure 4 As shown, the interdigitated electrode 25 in the electro-optical modulation device of the present invention includes a trunk 26, on which a first branch 27 and a second branch 28 are provided in parallel; the second branch 28 is located at the end of the trunk 26, and the other end of the trunk 26 is a connecting end, and the trunk 26, the first branch 27 and the second branch 28 form a "trunk" shape.

[0040] The connecting end of the trunk 26 of the interdigital electrode 25 connected to each electrode is connected to the electrode. The distance between the trunks 26 of two adjacent interdigital electrodes 25 in the plurality of interdigital electrodes 25 connected to the same side of the same electrode is 50 μm.

[0041] The interdigital electrodes 25 connected to the side of the signal electrode 21 toward the upper ground electrode 20 are spaced apart from the interdigital electrodes 25 connected to the side of the upper ground electrode 20 toward the signal electrode 21 and do not contact each other. The distance between the trunks 26 of two adjacent interdigital electrodes 25 between the signal electrode 21 and the upper ground electrode 20 is 25 μm.

[0042] The interdigital electrodes 25 connected to the side of the signal electrode 21 toward the lower ground electrode 22 are spaced apart from the interdigital electrodes 25 connected to the side of the lower ground electrode 22 toward the signal electrode 21 and do not contact each other. The distance between the trunks 26 in two adjacent interdigital electrodes 25 between the signal electrode 21 and the lower ground electrode 22 is 25 μm.

[0043] The electro-optical modulation device of the present invention is based on the interdigital electrode structure, and the electric field is rearranged in the slit between the signal electrode 21 and the ground electrode. That is, an electric field distribution with the same electric field direction is formed between the second branch 28 of the interdigital electrode 25 connected to the upper ground electrode 20, between the first branch 27 of all interdigital electrodes 25 between the upper ground electrode 20 and the signal electrode 21, and between the signal electrode and the second branch of the interdigital structure on the upper ground electrode side. By placing the first modulation waveguide between the signal electrode and the second branch of the interdigital structure on the upper ground electrode side, the second modulation waveguide between the first branch of the interdigital structure on the upper ground electrode side and the first branch of the interdigital structure on the signal electrode side, and the third modulation waveguide between the upper ground electrode and the second branch of the interdigital structure on the signal electrode side, a total of three groups of modulation sections are constructed. A mirror image arrangement is also made between the lower ground electrode 22 and the signal electrode 21.

[0044] The modulation cross section involved in the present invention is as follows Figure 5 As shown, light propagates along the light waveguide in the paper direction, and the microwave electric signal propagates along the slit between the signal electrode and the ground electrode in the same direction as the light. The microwave signal propagates in a quasi-TEM mode, and the electric field gradually transitions from the signal electrode to the ground electrode. Under the action of the electric field, the light field in the lithium niobate composite waveguide is affected by the electro-optical effect of the lithium niobate material, and the optical path of the light signal changes, achieving the purpose of modulation. The interdigitated electrode structure introduced in the present invention can establish multiple modulation sections in the single-side electrode slit, such as Figure 6 As shown, the modulation voltage can be greatly reduced and the modulation efficiency can be improved by connecting the modulation cross sections in series.

[0045] The present invention uses a coplanar waveguide traveling wave electrode as a loading structure for microwave signals. After the microwave signal enters the traveling wave electrode through a probe, a traveling wave effect occurs. The electric field strength of different parts of the traveling wave electrode will change over time. It is necessary to consider the interaction time between the microwave signal and the optical modulation signal so that the optical modulation signal can accurately match the microwave signal. Therefore, the present invention uses a delay line to ensure phase alignment of different modulation areas. The present invention records the phase of the optical modulation signal when entering the first modulation waveguide as the initial phase of the optical modulation signal, and records the phase of the microwave signal at the same time as the initial phase of the microwave signal. At this time, the initial phase of the optical modulation signal is naturally aligned with the initial phase of the microwave signal modulation. After being modulated by the first modulation waveguide, the optical modulation signal and the microwave signal still remain in phase. After leaving the first modulation waveguide, the optical modulation signal enters the first optical waveguide delay line. The delay generated by the optical modulation signal in the first optical waveguide delay line is the same as the integer multiple of the microwave signal period. When the optical modulation signal leaves the first optical waveguide delay line and arrives at the second delay line waveguide, the phase of the optical modulation signal and the microwave signal are aligned again, and the modulation depth of the optical modulation signal is further deepened in the second modulation waveguide. After the optical modulation signal leaves the second modulation waveguide and enters the second optical waveguide delay line, the delay generated by the optical modulation signal in the second optical waveguide delay line is the integer multiple of the microwave signal period, so that the phase of the optical modulation signal when arriving at the third modulation waveguide is aligned with the microwave signal again, and the modulation depth of the optical modulation signal reaches the maximum in the third modulation waveguide. The working principle of the lower modulation arm is similar to that of the upper modulation arm, but the direction of the applied electric field is opposite. The upper and lower modulation arms produce equal and opposite modulation effects, and interfere at the second multi-mode interference coupler, converting phase information into intensity information to achieve the modulation effect.

[0046] The process platform used in the present invention is as follows Figure 7 As shown, the present invention is realized based on a thin-film lithium niobate integrated photonic process platform. The commonly used thin-film lithium niobate wafer structure includes a 500μm thick silicon substrate 61, a 4.7μm thick silicon dioxide buried layer 62, a 300nm thick lithium niobate film 63, a 300nm thick silicon nitride load waveguide 64, a 1μm thick silicon dioxide coating layer 65, and a 500nm thick metal electrode 66 from bottom to top. The wafer size is usually a 4-inch wafer, which can be changed according to the requirements of the foundry unit. The silicon dioxide coating layer used in the present invention can isolate the lithium niobate composite waveguide from the metal electrode, allowing the two to cross on the layout, so that the traveling wave electrode structure based on the interdigital waveguide can cross the waveguide to realize that only a single electrode is required to work in multiple modulation zones at the same time. At the same time, the silicon dioxide coating layer raises the height of the metal electrode, so that the lateral leakage and evanescent field of the optical signal when propagating in the TE mode are less in contact with the metal electrode, reducing the transmission loss of the optical signal.

[0047] The present invention is highly dependent on the material system selected. In the process of selecting materials, the waveguide and interferometer parts of the electro-optic modulator are both selected from the lithium niobate platform deposited on an insulator (Lithium Niobate on Insulator, LNOI). LNOI technology involves carefully depositing a certain thickness of lithium niobate film on a silicon dioxide insulating layer, a process that is compatible with the current standard process of the microelectronics industry. The silicon dioxide material ensures the surface flatness of the lithium niobate film, which facilitates subsequent processing steps. The composite waveguide made of LNOI material has a core composed of lithium niobate (LN) with a refractive index of 2.1, while its cladding is composed of silicon dioxide with a refractive index of 1.44. This significant refractive index contrast greatly enhances the waveguide's confinement effect on the light field. Given that lithium niobate and silicon nitride have significant refractive index differences from silicon dioxide and air in optical properties, the high refractive index characteristics of these two materials provide excellent confinement effects for the propagation of light waves in the waveguide. Therefore, when the light wave enters the silicon nitride-lithium niobate waveguide system, the light wave is effectively confined inside the waveguide, significantly reducing the leakage and loss of light energy. The preparation method of the waveguide is detailed in the literature (“Lowloss CMOS-compatible silicon nitride photonics utilizing reactive sputtered thinfilms,” Opt. Express 27(26), 37795-37805 (2019)).

[0048] Lithium niobate material plays a vital role in the design and manufacture of electro-optic modulators due to its excellent electro-optical properties. A notable feature of this material is that when an external electric field is applied, the niobium ions and lithium ions inside it will slightly shift from the equilibrium position originally located at the center of the oxygen octahedron. This movement of ions will cause a slight change in the shape of the material's refractive index ellipsoid, which in turn affects the optical signal passing through the lithium niobate material, causing the optical path to change accordingly with the change of the external electric field. The characteristics of this electro-optical effect are closely related to the crystallographic orientation of lithium niobate. Therefore, in the present invention, an X-cut lithium niobate film is used to ensure that the external electric field can be applied along the Z-axis direction of the lithium niobate crystal. When the optical signal propagates in the waveguide in the transverse electric (TE) mode, the direction of its electric field component is consistent with the direction of the external electric field, achieving efficient optical signal modulation.

[0049] In order to further improve the efficiency and performance of electro-optical modulation, the present invention adopts an innovative interdigitated electrode structure. In this structure, we realize three electro-optical modulation structures with the same orientation in a single electrode slit. This design significantly increases the interaction area between the electrode and the lithium niobate material, thereby ensuring that the electric field can be evenly distributed throughout the material. This design, combined with a specially designed optical waveguide delay line, allows the modulation effects of the light field passing through different electro-optical modulation waveguides to be effectively superimposed, achieving highly efficient electro-optical modulation.

[0050] This highly efficient electro-optical modulation technology provides strong performance support for future optical communication systems and is expected to play a key role in high-speed, large-capacity optical networks. Through this innovative design, the present invention not only promotes the development of electro-optical modulation technology, but also brings new breakthroughs to the field of optoelectronics and provides a broader prospect for the application of electro-optical modulators.

[0051] The optical waveguides used in the present invention are all nanowire waveguides, and the waveguide dimensions are consistent with those of the commonly used fundamental mode waveguides in the prior art.

Claims

1. A lithium niobate electro-optic modulator based on interdigital electrodes, characterized in that: The electro-optic modulator comprises an electrical part (1) and an optical part (2); the electrical part (1) is a traveling wave electrode structure based on interdigital electrodes (25), and the optical part (2) comprises a modulation arm connected to two multi-mode interference couplers, wherein the modulation arm comprises two modulation arms arranged in parallel; The modulation arm includes a modulation waveguide and an optical waveguide delay line.

2. The lithium niobate electro-optic modulator based on interdigital electrodes according to claim 1, characterized in that: Based on the structure of interdigitated interdigitated electrodes (25), multiple electro-optic modulation areas are formed, and the modulation directions of the multiple electro-optic modulation areas are all consistent. When an external electric field is applied, the refractive index variables of the modulation waveguides on the same arm of the electro-optic modulator are in phase and of equal magnitude, allowing for mutual superposition.

3. The lithium niobate electro-optic modulator based on interdigital electrodes according to claim 1, characterized in that: The electro-optical modulation waveguides of the modulation arms are connected by optical waveguide delay lines to ensure that the phases of different electro-optical modulation waveguides are equal and the modulation effects can be superimposed on each other.

4. The lithium niobate electro-optic modulator based on interdigital electrodes according to claim 1, characterized in that: The first modulation waveguide (6), the second modulation waveguide (7) and the third modulation waveguide (8) constitute an upper half arm of the modulation arm, and the fourth modulation waveguide (11), the fifth modulation waveguide (12) and the sixth modulation waveguide (13) constitute a lower half arm of the modulation arm. The modulation directions of the upper half arm modulation and the lower half arm modulation are opposite, forming a push-pull arrangement of the modulator.

5. The lithium niobate electro-optic modulator based on interdigital electrodes according to claim 1, characterized in that: Its waveguide structure is composed of a thin-film lithium niobate waveguide loaded with silicon nitride; during the manufacturing process, the etching step is only performed on the silicon nitride layer, while the lithium niobate layer remains unetched; the etched silicon nitride layer and the unetched lithium niobate layer together form a composite waveguide structure.

6. The lithium niobate electro-optic modulator based on interdigital electrodes according to claim 4, characterized in that: The upper half arm of the modulation arm comprises a first modulation waveguide (6), a first optical waveguide delay line (9), a second modulation waveguide (7), a second optical waveguide delay line (10) and a third modulation waveguide (8) which are connected in sequence, the head end of the first modulation waveguide (6) is connected to the first multimode interference coupler (5), and the tail end of the third modulation waveguide (8) is connected to the second multimode interference coupler (16); The first optical waveguide delay line (9) and the second optical waveguide delay line (10) are both door frame-shaped, and the second optical waveguide delay line (10) is located inside the first optical waveguide delay line (9); the first optical waveguide delay line (9) and the second optical waveguide delay line (10) are both located above the first modulation waveguide (6); the end of the first optical waveguide delay line (9) facing the second multimode interference coupler (16) is connected to one end of the first modulation waveguide (6), and the end of the first optical waveguide delay line (9) away from the second multimode interference coupler (16) is connected to one end of the second modulation waveguide (7); the other end of the second modulation waveguide (7) is connected to the end of the second optical waveguide delay line (10) facing the second multimode interference coupler (16), and the end of the second optical waveguide delay line (10) away from the second multimode interference coupler (16) is connected to one end of the third modulation waveguide (8), and the other end of the third modulation waveguide (8) is connected to the second multimode interference coupler (16); The second modulation waveguide (7) is located between the first modulation waveguide (6) and the third modulation waveguide (8).

7. The lithium niobate electro-optic modulator based on interdigital electrodes according to claim 6, characterized in that: The lower half arm of the modulation arm comprises a fourth modulation waveguide (11), a third optical waveguide delay line (14), a fifth modulation waveguide (12), a fourth optical waveguide delay line (15) and a sixth modulation waveguide (13) which are connected in sequence, the head end of the fourth modulation waveguide (11) being connected to the first multimode interference coupler (5), and the tail end of the sixth modulation waveguide (13) being connected to the second multimode interference coupler (16); The third optical waveguide delay line (14) and the fourth optical waveguide delay line (15) are both in the shape of an inverted door frame, and the fourth optical waveguide delay line (15) is located inside the third optical waveguide delay line (14); the third optical waveguide delay line (14) and the fourth optical waveguide delay line (15) are both located below the fourth modulation waveguide (11), and the end of the third optical waveguide delay line (14) facing the second multimode interference coupler (16) is connected to one end of the fourth modulation waveguide (11), and the third optical waveguide delay line (14) is far away from the The end of the optical waveguide (15) on the side away from the second multimode interference coupler (16) is connected to one end of the fifth modulation waveguide (12); the other end of the fifth modulation waveguide (12) is connected to the end of the fourth optical waveguide delay line (15) on the side facing the second multimode interference coupler (16); the end of the fourth optical waveguide delay line (15) on the side away from the second multimode interference coupler (16) is connected to one end of the sixth modulation waveguide (13); and the other end of the sixth modulation waveguide (13) is connected to the second multimode interference coupler (16); The fifth modulation waveguide (12) is located between the fourth modulation waveguide (11) and the sixth modulation waveguide (13).

8. The lithium niobate electro-optic modulator based on interdigital electrodes according to claim 7, characterized in that: The electrical part (1) adopts a coplanar waveguide electrode structure, including a hot electrode (19) and an upper ground electrode (20), a signal electrode (21) and a lower ground electrode (22) arranged in sequence from top to bottom, wherein the ends of the upper ground electrode (20), the signal electrode (21) and the lower ground electrode (22) close to the first multimode interference coupler (5) are all connected to a left electrode pad (23); and the ends of the upper ground electrode (20), the signal electrode (21) and the lower ground electrode (22) close to the second multimode interference coupler (16) are all connected to a right electrode pad (24).

9. The lithium niobate electro-optic modulator based on interdigital electrodes according to claim 8, characterized in that: The upper ground electrode (20) and the left electrode pad (23) and the right electrode pad (24) connected thereto constitute a first electrode, and the first electrode is located in a space surrounded by the third modulation waveguide (8) and the second optical waveguide delay line (10); The signal electrode (21) and the left electrode pad (23) and the right electrode pad (24) connected thereto constitute a second electrode, and the second electrode is located in the area between the first modulation waveguide (6) and the fourth modulation waveguide (11); The lower ground electrode (22) and the left electrode pad (23) and the right electrode pad (24) connected thereto constitute a third electrode, and the third electrode is located in a space surrounded by the sixth modulation waveguide (13) and the fourth optical waveguide delay line (15); The lengths of the upper ground electrode (20), the signal electrode (21) and the lower ground electrode (22) are all equal and close to the length of the space region enclosed by the third modulation waveguide (8) and the second optical waveguide delay line (10).

10. The lithium niobate electro-optic modulator based on interdigital electrodes according to claim 9, characterized in that: A portion of the sixth modulation waveguide (13) located between the third optical waveguide delay line (14) and the second multimode interference coupler (16) is provided with a biasing hot electrode (19) on one side facing the second multimode interference coupler (16); the hot electrode (19) is not in contact with the sixth modulation waveguide (13), the third optical waveguide delay line (14) and the second multimode interference coupler (16).

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