A lithium niobate electro-optic modulator based on interdigital electrodes

Abstract

The application discloses a lithium niobate electro-optic modulator based on interdigital electrodes, comprising a coplanar waveguide electrode structure containing a periodic arrangement of 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. The device uses the interdigital electrode structure to construct a total of six electro-optic modulation sections in the upper and lower groups, and uses the optical waveguide delay line to align the phases of different electro-optic modulation sections to form the upper and lower modulation arms. The optical signal enters the input waveguide from the left coupling grating, is divided into two beams through the multimode interference coupler, is subjected to equal and opposite modulation in the upper and lower modulation arms, is coupled and modulated in intensity at the second multimode interference coupler, and is output through the output waveguide and the coupling grating. The proposed electro-optic modulator can further expand the electrode multiplexing times, and is compatible with other multiplexing technologies.

Description

TECHNICAL FIELD

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

[0002] With the development of social economy, people's requirements for the speed and quality of information transmission are increasing, and traditional information transmission means cannot meet these high-standard demands. High-speed and high-bandwidth information transmission becomes particularly critical. In this context, photonics, as a new alternative technology to electronics, has gradually attracted the attention of many researchers. Using integrated optical circuits to replace traditional electronic circuits, i.e., using "light" to realize the function of "electricity", can more efficiently perform logical operations and device interconnection. The application of optical fiber technology replaces copper wire, which not only greatly improves the data transmission speed and computing power, but also reduces energy consumption. Optical communication technology has become the mainstream means to realize long-distance and large-capacity communication, and the electro-optic modulator is one of the core devices in the optical communication system.

[0003] Traditional lithium niobate electro-optic modulators cannot effectively balance the problems of floor area and working bandwidth, but lithium niobate has attracted much attention in recent years due to its excellent electro-optic, acousto-optic and piezoelectric properties. With the continuous development of process technology, the Lithium Niobate on Insulator (LNOI) platform has emerged, which overcomes the limitations of traditional lithium niobate devices in waveguide fabrication, such as low refractive index difference and large device volume, and realizes higher 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 high refractive index difference, making it easier to integrate multiple unit devices on the 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. Many excellent electro-optic modulators based on the LNOI platform have been reported at home and abroad, such as the thin film lithium niobate electro-optic modulator with CMOS-compatible driving voltage published by Marko Loncar's research group at Harvard University in Nature ("Integrated lithium niobate electro-optic modulators operating at CMOS-compatible voltages" Nature 562(7725), 101-104 (2018)).

[0004] Currently, the electro-optical modulators widely used in optical communication systems are mostly based on Mach-Zehnder interferometer structure, and the bandwidth of mainstream products has gradually moved from 70GHz to 110GHz. However, as the bandwidth continues to increase, the radio frequency loss also increases significantly, which puts higher requirements on the output capability of the driving circuit. Therefore, it is urgent to further reduce the half-wave voltage of the electro-optical modulator and improve its modulation efficiency. The modulation efficiency of the existing electro-optical modulator per unit length is limited by the electrode and waveguide modulation cross-section structure. In order to improve the modulation efficiency, the commonly used method includes extending or folding the electrode to reduce the required driving voltage, but this approach will be limited by microwave loss. In the current mainstream platform, the microwave electrode structure will significantly weaken the microwave signal strength after more than 5cm, resulting in the inability to further improve the modulation efficiency. In addition, this method may also cause refractive index mismatch and reduction of modulation bandwidth. Another method to improve efficiency is to let the light pass through the modulation region 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

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

[0006] To this end, the present application adopts the following technical solutions:

[0007] The lithium niobate electro-optical modulator based on interdigital electrode mainly comprises an electrical part and an optical part. The electrical part comprises a coplanar waveguide metal electrode carrying an electrical signal, which is composed of a signal electrode in the middle, two ground electrode structures on both sides, and two electrode slits formed by the signal electrode and the ground electrode; a metal Pad for connecting an external radio frequency probe, a packaging gold wire and other electrical interfaces; a 'dry' interdigital electrode for guiding the electrical signal to form a plurality of modulation regions; and a hot electrode for thermal biasing. The optical part comprises a first coupling grating and an input waveguide on the right side of the first coupling grating, a 1x2 multimode interference coupler beam splitting structure on the right side of the input waveguide, which comprises an input port, a flat plate coupling region and two output ports, a plurality of modulation arms are connected to the right side of the multimode interference coupler beam splitting structure respectively, 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, 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, a sixth modulation waveguide after the fourth optical waveguide delay line, the upper and lower arms are connected to a second multimode interference coupler beam combining structure composed of two input ports, a flat plate coupling region and an output port, and an output waveguide on the right side of the second multimode interference coupler beam combining structure and a second coupling grating.

[0008] The two groups of modulation waveguides respectively constitute the upper and lower modulation arms of the Mach-Zehnder interferometer in the electro-optical modulator, the coplanar waveguide electrode structure is used in the present application, two modulation arms with opposite modulation directions can be formed, that is, the electrode is configured in a push-pull structure, the working point of the Mach-Zehnder interferometer is changed by using a heating electrode, and when used for intensity modulation, the working point of the modulator is biased at a quadrature working point. The coplanar waveguide electrode structure based on the interdigital electrode design is used in the present application, a plurality of electro-optical action regions are respectively constructed in the two metal electrode slits of the coplanar waveguide electrode, and the modulation effects can be superimposed by phase modulation between different electro-optical action regions through optical waveguide delay lines.

[0009] The waveguide in the electro-optical modulator is a silicon nitride-loaded thin film lithium niobate waveguide. Since silicon nitride has a similar transparent window as lithium niobate material and the refractive index of silicon nitride is slightly lower than that of lithium niobate material, most of the mode field of the silicon nitride-lithium niobate composite waveguide is in the lithium niobate layer and the working bandwidth is not affected. Most importantly, the silicon nitride material has a mature processing technology, compared with the traditional straight-lithography lithium niobate waveguide, there is no waveguide inclination, subwavelength waveguide gratings and other structures with ultra-small line width can be easily processed, and large-scale integration of the device can be realized.

[0010] The electro-optical modulator of the present application creates multiple modulation regions without significantly increasing the device area by configuring a special interdigital electrode structure in the traveling wave electrode, so that the light can avoid crosstalk caused by mode conversion when entering the modulation region multiple times, and at the same time, this design allows a larger electrode gap, which helps to reduce microwave loss and improve modulation bandwidth.

[0011] The present application has the advantages of:

[0012] 1. Based on the thin film lithium niobate platform, the excellent electro-optical modulation coefficient and high optical transparent window of lithium niobate material can be utilized to realize ultra-wideband optical working bandwidth from visible light to 4μm waveband, the device can realize high integration on chip, has the advantages of small volume, low power consumption and low driving voltage, is conducive to combination with driving circuit to realize compact driving module.

[0013] 2. The device realizes low modulation voltage and high modulation efficiency based on the multiplexing of the modulation region, avoids the problems of excessive mismatch between microwave refractive index and optical refractive index and microwave loss caused by the introduction of too long traveling wave electrode structure, avoids the working bandwidth of the device being limited by electricity and brings too much echo reflection noise to reduce the quality of optical modulation signal.

[0014] 3. The modulation electrode and working waveguide of the present 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.

[0015] 4. The present device greatly increases the metal-to-metal spacing of the coplanar waveguide electrode structure through the interdigital structure, reduces the leakage loss caused by the metal-to-metal spacing and the resulting drift of the working bias point of the Mach-Zehnder interferometer, and finally reduces the microwave loss and driving voltage of the modulator.

[0016] 5. The interdigital structure can construct multiple modulation regions without significantly increasing the device area, compared with the traditional folded electrode, which avoids introducing multiple sets of coplanar waveguide electrode structures to increase the area.

[0017] 6. The present device is compatible with waveguide multiplexing technology, and allows further expansion of the modulation efficiency of the electro-optical modulator by combining with mode-based multiplexing technology at the cost of part of the crosstalk and loss. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 is a schematic diagram of the electro-optical modulator of the present application;

[0019] Figure 2 is the optical part structure of the electro-optical modulator of the present application;

[0020] Figure 3 is the electrical part structure of the electro-optical modulator of the present application;

[0021] Figure 4 is the schematic diagram of the electrical interdigital electrode structure of the electro-optical modulator of the present application;

[0022] Figure 5 is the cross-section simulation of the electrical field modulation of the electro-optical modulator of the present application, including the light field energy distribution and the electrical field energy distribution;

[0023] Figure 6 is the electrical field simulation of the present application, including the electrical field distribution of three groups of modulation cross-sections;

[0024] Figure 7 is the schematic diagram of the waveguide and electrode structure material layering of the present application.

[0025] 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;

[0026] 11. fourth modulation waveguide, 12. fifth modulation waveguide, 13. sixth modulation waveguide, 14. third optical waveguide delay line, 15. fourth optical waveguide delay line;

[0027] 16. second multimode interference coupler, 17. output waveguide, 18. output coupling grating;

[0028] 19. thermal electrode, 20. upper ground electrode, 21. signal electrode, 22. lower ground electrode, 23. left electrode pad, 24. right electrode pad, 25. interdigital electrode, 26. main stem, 27. first branch, 28. second branch;

[0029] 61. silicon substrate, 62. buried layer of silicon dioxide, 63. lithium niobate thin film, 64. silicon nitride load waveguide, 65. silicon dioxide cladding, 66. gold electrode. DETAILED DESCRIPTION

[0030] The present application will be described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0031] As shown in the figure, the electro-optical modulation device of the present application includes an electrical part 1 and an optical part 2. Figure 1

[0032] As shown in the figure, the electro-optical modulation device of the present application includes an electrical part 1 and an optical part 2. Figure 2 ​As shown, the optical part 2 in the electro-optical modulation device of the present application comprises an input coupling grating 3 and an output coupling grating 18, the input coupling grating 3 is connected with a first multimode interference coupler 5 through an input waveguide 4, the output coupling grating 18 is connected with a second multimode interference coupler 16 through an output waveguide 17, the first multimode interference coupler 5 is connected with the second multimode interference coupler 16 through two modulation arms.

[0033] One of the modulation arms 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 connected in sequence, the head end of the first modulation waveguide 6 is connected with the first multimode interference coupler 5, the tail end of the third modulation waveguide 8 is connected with the second multimode interference coupler 16.

[0034] 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 in 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, one end of the first optical waveguide delay line 9 is connected with one end of the first modulation waveguide 6 towards the side of the second multimode interference coupler 16, and the other end of the first optical waveguide delay line 9 is connected with one end of the second modulation waveguide 7 away from the side of the second multimode interference coupler 16; the other end of the second modulation waveguide 7 is connected with the end of the second optical waveguide delay line 10 towards the side of the second multimode interference coupler 16, and the end of the second optical waveguide delay line 10 away from the side of the second multimode interference coupler 16 is connected with one end of the third modulation waveguide 8, and the other end of the third modulation waveguide 8 is connected with the second multimode interference coupler 16.

[0035] The second modulation waveguide 7 is located between the first modulation waveguide 6 and the third modulation waveguide 8.

[0036] The third modulation waveguide 8 intersects with the first optical waveguide delay line 9 and the second optical waveguide delay line 10.

[0037] The other 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 connected in sequence, the head end of the fourth modulation waveguide 11 is connected with the first multimode interference coupler 5, and the tail end of the sixth modulation waveguide 13 is connected with the second multimode interference coupler 16.

[0038] 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 in 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, one end of the third optical waveguide delay line 14 connected with one end of the fourth modulation waveguide 11 towards the side of the second multimode interference coupler 16, and the other end of the third optical waveguide delay line 14 connected with one end of the fifth modulation waveguide 12 away from the side of the second multimode interference coupler 16; the other end of the fifth modulation waveguide 12 connected with the end of the fourth optical waveguide delay line 15 towards the side of the second multimode interference coupler 16, and the other end of the fourth optical waveguide delay line 15 connected with one end of the sixth modulation waveguide 13 away from the side of the second multimode interference coupler 16, and the other end of the sixth modulation waveguide 13 connected with the second multimode interference coupler 16.

[0039] The fifth modulation waveguide 12 is located between the fourth modulation waveguide 11 and the sixth modulation waveguide 13.

[0040] The sixth modulation waveguide 13 intersects with the third optical waveguide delay line 14 and the fourth optical waveguide delay line 15.

[0041] The first multimode interference coupler 5 and the second multimode interference coupler 16 are both 1x2 multimode interference coupler beam splitting structures. The first multimode interference coupler 5 has one input port, one multimode coupling region and two output ports; the input port of the first multimode interference coupler 5 is connected with the tail end of the input waveguide 4, and the two output ports of the first multimode interference coupler 5 are respectively connected with the head ends of the second modulation waveguide 7 and the fourth modulation waveguide 11. The second multimode interference coupler 16 has two input ports, one multimode coupling region and one output port, the two input ports of the second multimode interference coupler 16 are respectively connected with the tail ends 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 with the head end of the output waveguide 17.

[0042] The electrical part 1 in the electro-optical modulation device adopts a coplanar waveguide electrode structure, as shown in the figure. Figure 3 It includes a hot electrode 19 and an upper ground electrode 20, a signal electrode 21 and a lower ground electrode 22 arranged in turn from top to bottom, and the upper ground electrode 20, the signal electrode 21 and the lower ground electrode 22 are all connected with left electrode pads 23 at one end close to the first multimode interference coupler 5; the upper ground electrode 20, the signal electrode 21 and the lower ground electrode 22 are all connected with right electrode pads 24 at one end close to the second multimode interference coupler 16.

[0043] The upper ground electrode 20 and the left electrode pads 23 and the right electrode pads 24 connected therewith constitute a first electrode, and the first electrode is located in the space surrounded by the third modulation waveguide 8 and the second optical waveguide delay line 10.

[0044] The signal electrode 21 and the left and right electrode pads 23 and 24 connected thereto constitute a second electrode, which is located in the region between the first modulation waveguide 6 and the fourth modulation waveguide 11.

[0045] The lower ground electrode 22 and the left and right electrode pads 23 and 24 connected thereto constitute a third electrode, which is located in the space surrounded by the sixth modulation waveguide 13 and the fourth optical waveguide delay line 15.

[0046] The upper ground electrode 20, the signal electrode 21 and the lower ground electrode 22 are all equal in length and close to the length of the space region surrounded by the third modulation waveguide 8 and the second optical waveguide delay line 10.

[0047] The sixth modulation waveguide 13 is provided with a hot electrode 19 for biasing on the side thereof facing the second multimode interference coupler 16, and 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.

[0048] The upper ground electrode 20 is connected with at least two interdigital electrodes 25 on the side thereof facing the signal electrode 21, and the lower ground electrode 22 is connected with at least two interdigital electrodes 25 on the side thereof facing the signal electrode 21, and the number of the interdigital electrodes 25 connected with the upper ground electrode 20 is the same as the number of the interdigital electrodes 25 connected with the lower ground electrode 22; the signal electrode 21 is connected with at least two interdigital electrodes 25 on the side thereof facing the upper ground electrode 20 and on the side thereof facing the lower ground electrode 22, and the number of the interdigital electrodes 25 connected with the signal electrode 21 on both sides is the same as the number of the interdigital electrodes 25 connected with the upper ground electrode 20.

[0049] The first electrode and all the interdigital electrodes 25 connected therewith, the second electrode and all the interdigital electrodes 25 connected therewith, and the third electrode and all the interdigital electrodes 25 connected therewith all constitute coplanar waveguide traveling wave electrodes.

[0050] The left and right electrode pads 23 and 24 are used to introduce radio frequency signals and radio frequency loads in use.

[0051] As shown in Figure 4 Fig. 4, the interdigital electrode 25 in the electro-optical modulation device comprises a main stem 26, a first branch 27 and a second branch 28 are arranged in parallel on the main stem 26; the second branch 28 is located at the end of the main stem 26, and the other end of the main stem 26 is a connecting end, and the main stem 26, the first branch 27 and the second branch 28 form a "gan" shape.

[0052] The connecting end of the stem 26 of the interdigital electrode 25 connected to each electrode is connected to the electrode. The distance between the stems 26 of two adjacent interdigital electrodes 25 connected to the same electrode on the same side is 50 μm.

[0053] The interdigital electrode 25 connected to the side of the signal electrode 21 facing the upper ground electrode 20 is arranged apart from the interdigital electrode 25 connected to the side of the upper ground electrode 20 facing the signal electrode 21, and the two interdigital electrodes 25 are not in contact with each other. The distance between the stems 26 of two adjacent interdigital electrodes 25 between the signal electrode 21 and the upper ground electrode 20 is 25 μm.

[0054] The interdigital electrode 25 connected to the side of the signal electrode 21 facing the lower ground electrode 22 is arranged apart from the interdigital electrode 25 connected to the side of the lower ground electrode 22 facing the signal electrode 21, and the two interdigital electrodes 25 are not in contact with each other. The distance between the stems 26 of two adjacent interdigital electrodes 25 between the signal electrode 21 and the lower ground electrode 22 is 25 μm.

[0055] The electro-optical modulation device of the present application 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 direction of electric field 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 the interdigital electrodes 25 between the upper ground electrode 20 and the signal electrode 21, and between the second branch of the upper ground electrode side interdigital structure and the signal electrode side interdigital structure. Three sets of modulation sections are constructed by placing the first modulation waveguide between the second branch of the signal electrode side interdigital structure and the upper ground electrode side interdigital structure, the second modulation waveguide between the first branch of the upper ground electrode side interdigital structure and the first branch of the signal electrode side interdigital structure, and the third modulation waveguide between the second branch of the upper ground electrode side interdigital structure and the signal electrode side interdigital structure. The mirror image arrangement is also made between the lower ground electrode 22 and the signal electrode 21.

[0056] The modulation section involved in the present application is shown in Figure 5 As shown in the figure, the light propagates in the direction of the paper along the optical waveguide, and the microwave electrical signal propagates in the same direction as the light along the slit between the signal electrode and the ground electrode. The microwave signal propagates in the 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 optical field in the lithium niobate composite waveguide is affected by the electro-optical effect of lithium niobate material, the optical path of the optical signal changes, and the purpose of modulation is achieved. The interdigital electrode structure introduced in the present application can establish multiple modulation sections in the slit of the single side electrode, as shown in Figure 6 As shown in the figure, the modulation voltage can be greatly reduced and the modulation efficiency can be improved by connecting the modulation sections in series.

[0057] The application uses a coplanar waveguide traveling wave electrode as a loading structure of a microwave signal, and the microwave signal has a traveling wave effect after entering the traveling wave electrode through a probe, the electric field intensity of different parts of the traveling wave electrode changes over time, the interaction time of the microwave signal and the light modulation signal needs to be considered, so that the light modulation signal can be accurately matched with the microwave signal, therefore, the application uses a delay line to ensure the phase alignment of different modulation regions. The phase of the light modulation signal when entering the first modulation waveguide is recorded as the initial phase of the light modulation signal, and the phase of the microwave signal at the same time is recorded as the initial phase of the microwave signal, at this time, the initial phase of the light modulation signal is naturally aligned with the initial phase of the microwave signal, and after modulation by the first modulation waveguide, the light modulation signal and the microwave signal still maintain the same phase, after leaving the first modulation waveguide, the light modulation signal enters the first optical waveguide delay line, the delay generated by the light modulation signal in the first optical waveguide delay line is the same as an integer multiple of the period of the microwave signal, when the light modulation signal leaves the first optical waveguide delay line and reaches the second delay waveguide, the phase of the light modulation signal and the microwave signal is aligned again, the modulation depth of the light modulation signal is further deepened in the second modulation waveguide, after the light modulation signal leaves the second modulation waveguide and enters the second optical waveguide delay line, the delay generated by the light modulation signal in the second optical waveguide delay line is an integer multiple of the period of the microwave signal, so that the phase of the light modulation signal when reaching the third modulation waveguide is aligned with the microwave signal again, and the modulation depth of the light 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 the phase information is converted into intensity information at the second multimode interference coupler to realize the modulation effect.

[0058] The process platform used by the application is as shown in the figure, and the application is realized based on a thin film lithium niobate integrated photon process platform. Figure 7 The commonly used thin film lithium niobate wafer structure includes a 500 mu m thick silicon substrate 61 from bottom to top, a 4.7 mu m thick silicon dioxide buried layer 62, a 300 nm thick lithium niobate film 63, a 300 nm thick silicon nitride load waveguide 64, a 1 mu m thick silicon dioxide coating layer 65, a 500 nm thick metal electrode 66, and a wafer size of 4 inches, which can be changed according to the requirements of the foundry unit. The silicon dioxide coating layer used in the application can isolate the lithium niobate composite waveguide and the metal electrode, allow the two to cross on the layout, enable the traveling wave electrode structure based on the interdigital waveguide to span the waveguide and realize multiple modulation regions working simultaneously under the condition of only a single electrode, and at the same time, the silicon dioxide coating layer raises the height of the metal electrode, so that the transverse leakage and evanescent field of the light signal propagating in the TE mode have less contact with the metal electrode, and the transmission loss of the light signal is reduced.

[0059] The present application has a very high dependence on the material system used. In the process of selecting materials, the waveguide and interferometer part of the electro-optical modulator is selected from the Lithium Niobate on Insulator (LNOI) platform. LNOI technology involves carefully depositing a certain thickness of lithium niobate film on a silicon dioxide insulating layer, which is compatible with the standard process of the current microelectronics industry. The silicon dioxide material ensures the flatness of the lithium niobate film, providing convenience for 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, and a cladding composed of silicon dioxide with a refractive index of 1.44. This significant refractive index contrast greatly enhances the confinement of the waveguide to the optical field. Given the significant refractive index difference between lithium niobate and silicon nitride in optical properties and silicon dioxide and air, the high refractive index characteristics of these two materials provide excellent confinement 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 optical energy. The preparation method of the waveguide is described in detail in the literature (“Lowloss CMOS-compatible silicon nitride photonics utilizing reactive sputtered thinfilms,” Opt. Express 27(26), 37795-37805 (2019)).

[0060] Lithium niobate material plays a crucial role in the design and manufacture of electro-optical modulators due to its excellent electro-optical properties. One of the notable features of this material is that when an external electric field is applied, the internal niobium and lithium ions will slightly shift from their original equilibrium position at the center of the oxygen octahedron. This movement of ions causes 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, resulting in a change in the optical path with the variation of the external electric field. The characteristics of this electro-optical effect are closely related to the crystallographic orientation of lithium niobate, so X-cut lithium niobate film is used in the present application 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, its electric field component direction is consistent with the direction of the external electric field, achieving efficient optical signal modulation.

[0061] In order to further improve the efficiency and performance of electro-optical modulation, the present application adopts an innovative interdigital electrode structure. In this structure, we realize three uniformly oriented electro-optical modulation structures in a single electrode slit. This design significantly increases the interaction area between the electrode and the lithium niobate material, ensuring that the electric field is uniformly distributed throughout the material. This design, combined with a specially designed optical waveguide delay line, allows the modulation effects of the optical field through different electro-optical modulation waveguides to be effectively superimposed, achieving high-efficiency electro-optical modulation.

[0062] This 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, high-capacity optical networks. Through this innovative design, the present application not only promotes the development of electro-optical modulation technology, but also brings new breakthroughs in the field of optoelectronics, providing a wider prospect for the application of electro-optical modulators.

[0063] The optical waveguides used in the present application are all nanowire waveguides, with waveguide sizes consistent with those of commonly used fundamental mode waveguides in the prior art.

Claims

1. An interdigital electrode based lithium niobate electro-optic modulator, characterized in that, The electro-optical modulator comprises an electrical part (1) and an optical part (2); the electrical part (1) is a row wave electrode structure based on interdigital electrodes (25), and the optical part (2) comprises two modulation arms connected to two multimode interference couplers, and the modulation arms comprise two and are arranged side by side; The modulation arm comprises a modulation waveguide and an optical waveguide delay line; 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) connected in sequence; the first optical waveguide delay line (9) and the second optical waveguide delay line (10) are both in the shape of a door frame, and the second optical waveguide delay line (10) is located in 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), and the second modulation waveguide (7) is located between the first modulation waveguide (6) and the third modulation waveguide (8); 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) connected in sequence; 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 in 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 fifth modulation waveguide (12) is located between the fourth modulation waveguide (11) and the sixth modulation waveguide (13); The electrical part (1) adopts a coplanar waveguide electrode structure and comprises a hot electrode (19) and, from top to bottom, an upper ground electrode (20), a signal electrode (21) and a lower ground electrode (22); The upper ground electrode (20) is connected with at least two interdigital electrodes (25) on the side facing the signal electrode (21), the lower ground electrode (22) is connected with at least two interdigital electrodes (25) on the side facing the signal electrode (21), and the signal electrode (21) is connected with at least two interdigital electrodes (25) on the side facing the upper ground electrode (20) and the side facing the lower ground electrode (22).

2. A Lithium Niobate electro-optic modulator based on interdigital electrodes according to claim 1, characterized in that, A plurality of electro-optical modulation regions are formed based on the interdigital electrode (25) structure crossing each other, the modulation directions of the plurality of electro-optical modulation regions are consistent, and when an external electric field is applied, the refractive indexes of the modulation waveguides on the same arm of the electro-optical modulator are equal, allowing mutual superposition.

3. A Lithium Niobate electro-optic modulator based on interdigital electrodes according to claim 1, characterized in that, The electro-optical modulation waveguides of the modulation arm are connected through the optical waveguide delay lines, ensuring that the phases between different electro-optical modulation waveguides are equivalent, and the modulation effects can be superimposed on each other.

4. A Lithium Niobate electro-optic modulator based on interdigital electrodes according to claim 1, characterized in that, The modulation directions of the upper half arm modulation and the lower half arm modulation are opposite, and the push-pull arrangement of the modulator is formed.

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

6. A Lithium Niobate electro-optic modulator based on interdigital electrodes according to claim 4, characterized in that, The head end of the first modulation waveguide (6) is connected with the first multimode interference coupler (5), and the tail end of the third modulation waveguide (8) is connected with the second multimode interference coupler (16); The end of the first optical waveguide delay line (9) on the side of the second multimode interference coupler (16) is connected with one end of the first modulation waveguide (6), and the end of the first optical waveguide delay line (9) away from the side of the second multimode interference coupler (16) is connected with one end of the second modulation waveguide (7); the other end of the second modulation waveguide (7) is connected with the end of the second optical waveguide delay line (10) on the side of the second multimode interference coupler (16), and the end of the second optical waveguide delay line (10) away from the side of the second multimode interference coupler (16) is connected with one end of the third modulation waveguide (8); the other end of the third modulation waveguide (8) is connected with the second multimode interference coupler (16).

7. A Lithium Niobate electro-optic modulator based on interdigital electrodes according to claim 6, characterized in that, The head end of the fourth modulation waveguide (11) is connected with the first multimode interference coupler (5), and the tail end of the sixth modulation waveguide (13) is connected with the second multimode interference coupler (16); The end of the third optical waveguide delay line (14) on the side of the second multimode interference coupler (16) is connected with one end of the fourth modulation waveguide (11), and the end of the third optical waveguide delay line (14) away from the side of the second multimode interference coupler (16) is connected with one end of the fifth modulation waveguide (12); the other end of the fifth modulation waveguide (12) is connected with the end of the fourth optical waveguide delay line (15) on the side of the second multimode interference coupler (16), and the end of the fourth optical waveguide delay line (15) away from the side of the second multimode interference coupler (16) is connected with one end of the sixth modulation waveguide (13); the other end of the sixth modulation waveguide (13) is connected with the second multimode interference coupler (16).

8. A Lithium Niobate electro-optic modulator based on interdigital electrodes according to claim 7, characterized in that, The left electrode pad (23) is connected to one end of the upper ground electrode (20), the signal electrode (21) and the lower ground electrode (22) near the first multimode interference coupler (5); and the right electrode pad (24) is connected to one end of the upper ground electrode (20), the signal electrode (21) and the lower ground electrode (22) near the second multimode interference coupler (16).

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

10. A Lithium Niobate electro-optic modulator based on interdigital electrodes according to claim 9, characterized in that, The sixth modulation waveguide (13) is provided with a heating electrode (19) for biasing on the side facing the second multimode interference coupler (16) in the part between the third optical waveguide delay line (14) and the second multimode interference coupler (16), and the heating 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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