High-modulation-efficiency phase modulator based on silicon nitride-lithium niobate waveguide
By optimizing the electrode design through a silicon nitride-lithium niobate waveguide structure and an embedded double-layer capacitor microstructure, the problem of limited modulation efficiency and bandwidth of thin-film lithium niobate phase modulators is solved, achieving efficient reduction of optical loss and optimal overlap between electric and optical fields, making it suitable for a variety of optical applications.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-12-30
- Publication Date
- 2026-04-10
- Estimated Expiration
- Not applicable · inactive patent
AI Technical Summary
Existing thin-film lithium niobate phase modulators have limited modulation efficiency and bandwidth, complex electrode design, high process difficulty, and are incompatible with CMOS processes, resulting in a low optical damage threshold.
A silicon nitride-lithium niobate waveguide structure is adopted, and the traveling wave electrode is optimized into an embedded double-layer capacitor microstructure. Combined with the composite waveguide design, optical loss is reduced and the overlap efficiency of electric and optical fields is improved.
Significantly improves modulation efficiency and reduces optical loss, suitable for high-speed fiber optic communication, coherent optical transmission systems, optical phased arrays and quantum optics experiments, providing a low-power phase modulation solution.
Smart Images
Figure CN121832136A_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The application belongs to the technical field of optoelectronic devices and integrated optics, and particularly relates to a high-modulation-efficiency phase modulator based on a silicon nitride-lithium niobate waveguide. BACKGROUND
[0002] The existing thin-film lithium niobate (TFLN) phase modulator is a new generation of optical modulator developed on the basis of the thin-film technology of lithium niobate material. Compared with the traditional bulk material lithium niobate modulator, the thin-film lithium niobate (TFLN) technology processes lithium niobate into a thin film with a thickness of hundreds of nanometers to microns (typical thickness 300-800 nm) and bonds it to a low refractive index substrate (such as SiO2 / Si), which significantly improves the light field restriction capability. Combined with the ridge waveguide or nanowire waveguide design, the optical mode area can be compressed to below 10 μm², thereby enhancing the interaction efficiency of the optical field and the electric field. Such a device has realized Vπ L (half-wave voltage-length product) as low as 2 V·cm, a modulation bandwidth of 40-100 GHz, and a device size that can be reduced to the millimeter level, showing the potential of compatibility with the silicon-based optoelectronic platform. The core advantage of the thin-film lithium niobate (TFLN) technology is that the strong restriction waveguide makes it easier to match the optical field and the microwave field when designing the modulator, thereby facilitating lower half-wave voltage and greater modulation bandwidth in a shorter length.
[0003] However, the performance of the existing thin-film lithium niobate phase modulator is still restricted by multiple factors. First, the electric field distribution of the horizontal coplanar waveguide (CPW) and the longitudinal overlap efficiency of the optical mode field in the thin-film waveguide are insufficient, resulting in a far lower utilization rate of the electro-optic effect than the theoretical limit, and the actual Vπ value is still high. Second, the reduced electrode spacing to achieve low half-wave voltage significantly increases the electrode capacitance, exacerbating radio frequency loss (such as skin effect and dielectric loss), and the group velocity mismatch between microwave and optical waves and the impedance deviation from the 50 Ω system in the traveling wave electrode design limit the further improvement of the modulation bandwidth. In addition, the manufacturing process of the thin-film lithium niobate device is complex, and the lattice defects generated in the micro-nano processing process also reduce the material's light damage threshold, making the device prone to performance degradation in a high-temperature (>80°C) environment. Moreover, the monolithic LNOI platform is not compatible with the current complementary metal oxide semiconductor (CMOS) process, and the diffusion of lithium in lithium niobate and the residues in the etching process will cause contamination of the material.
[0004] In view of the above problems, the prior art has proposed various improvement schemes, but the actual effect still has significant limitations. Therefore, an innovative design scheme is urgently needed to reconfigure the electrode-waveguide interaction mechanism, realize multi-dimensional collaborative optimization in the aspects of materials, structure and process, thereby breaking through the bottleneck of the prior art, and providing a thin-film lithium niobate phase modulator with high modulation efficiency, ultra-high frequency response, high process tolerance and heterogeneous integration compatibility for the next generation of high-performance optoelectronic systems. SUMMARY
[0005] In view of the problems of mutual restriction between modulation efficiency and bandwidth and great difficulty in etching process of the existing thin-film lithium niobate phase modulator, the present application proposes a high-modulation-efficiency phase modulator based on a silicon nitride-lithium niobate waveguide. The modulator solves the problem of mutual restriction between performance indicators of the thin-film lithium niobate phase modulator by optimizing the traveling wave electrode structure, and reduces the manufacturing difficulty and cost by using a loaded hybrid waveguide.
[0006] To achieve the above-mentioned purposes, the present application adopts the following technical solutions: A high-modulation-efficiency phase modulator based on a silicon nitride-lithium niobate waveguide, the modulator comprising an optical transmission unit and an electrical transmission unit. The optical transmission unit comprises a quartz substrate layer, a silicon dioxide buffer layer arranged above the quartz substrate layer, a composite waveguide arranged above the middle position of the silicon dioxide buffer layer, and a silicon dioxide cladding layer covering the outside of the composite waveguide. The electrical transmission unit comprises a metal traveling wave electrode, which is a embedded double-layer capacitive microstructure loaded traveling wave electrode.
[0007] As a further improvement of the above technical solution, the composite waveguide comprises a lithium niobate flat plate layer and a silicon nitride waveguide arranged on the top of the lithium niobate flat plate layer.
[0008] As a further improvement of the above technical solution, the embedded double-layer capacitive microstructure loaded traveling wave electrode comprises a main electrode and a plurality of segment portions extending from the edge of the main electrode, each segment portion being uniformly distributed along the length direction of the main electrode.
[0009] As a further improvement of the above technical solution, the non-modulation area of the lithium niobate flat plate layer is completely etched.
[0010] As a further improvement of the above technical solution, the silicon nitride waveguide is loaded on the surface of the modulation area of the lithium niobate flat plate layer, and the two form a composite waveguide.
[0011] As a further improvement of the above technical solution, the main electrode is a rectangular channel electrode region formed by two spaced-apart planar electrodes; and the segment portions extend transversely from the edge of the main electrode close to the composite waveguide towards the composite waveguide.
[0012] As a further improvement of the above technical solution, the lower part of the main electrode is embedded in the silica cladding, and the upper part is exposed to the surface of the silica cladding, forming an embedded structure; the embedded structure forms a double-layer capacitor structure, which includes a bottom capacitor and a top capacitor, the bottom capacitor is composed of the lower part of the main electrode embedded in the silica cladding, and the top capacitor is composed of the upper part of the main electrode exposed to the surface of the silica cladding.
[0013] As a further improvement of the above technical solution, the main electrode and the segment part are in a metal one-piece structure.
[0014] As a further improvement of the above technical solution, the plan view of the embedded double-layer capacitor microstructure loaded with a traveling wave electrode is a periodic I-shaped structure, and the side view is a T-shaped structure.
[0015] Compared with the prior art, the application has the following advantages: The phase modulator provided by the application has the advantages of high modulation efficiency, wideband response, low optical loss, and the like, and is suitable for application scenarios sensitive to phase modulation efficiency and power consumption, such as high-speed optical fiber communication, coherent optical transmission systems, optical phased arrays, quantum optical experiments, and the like. The technical core of the phase modulator is to balance electro-optical efficiency, impedance matching, and optical mode constraint, and the phase modulator provides a key device solution for low-power phase modulation in an integrated photonics chip. BRIEF DESCRIPTION OF DRAWINGS
[0016] Figure 1 is a comparison structure diagram of the silicon nitride-lithium niobate composite waveguide in the application and the traditional thin film lithium niobate waveguide, wherein the left side is the silicon nitride-lithium niobate composite waveguide in the application, and the right side is the traditional thin film lithium niobate waveguide; Figure 2 is a top view of the embedded double-layer capacitor microstructure loaded with a traveling wave electrode; Figure 3 is a cross-sectional view of the phase modulator in the application; Figure 4 is an optical field energy distribution diagram of the phase modulator in the application; Figure 5 is a structure diagram of the embedded double-layer capacitor microstructure loaded with a traveling wave electrode structure in the application; Figure 6 is a side view of the phase modulator in the application at positions A, B, and C in Figure 5 ; Figure 7 is an electric field energy distribution diagram of the phase modulator in the application. DETAILED DESCRIPTION
[0017] The application is further described below with reference to the accompanying drawings: The application provides a high-modulation-efficiency phase modulator based on a silicon nitride-lithium niobate waveguide, which comprises an input end spot size conversion zone (SSC, Figure 5 left end), a modulation zone and an output end spot size conversion zone (SSC, Figure 5 right end). The cross-sectional structure of the modulation zone is as shown in Figure 3 The modulation zone comprises an optical transmission unit and an electrical transmission unit. The optical transmission unit is implemented by an optical waveguide, and the electrical transmission unit is implemented by a traveling wave electrode structure. The optical transmission unit implements optical signal transmission, and the electrical transmission unit implements electrical signal transmission and acts on the optical field in the optical transmission part through an electric field to complete phase modulation.
[0018] As a further improvement of the above technical solution, the optical transmission unit comprises a quartz substrate layer, a silicon dioxide buffer layer arranged above the quartz substrate layer, a composite waveguide arranged above the middle position of the silicon dioxide buffer layer, and a silicon dioxide cladding layer covering the outside of the composite waveguide. The composite waveguide comprises a lithium niobate flat plate layer and a silicon nitride waveguide arranged on the top of the lithium niobate flat plate layer. The non-modulation zone of the lithium niobate flat plate layer is completely etched; the silicon nitride waveguide is loaded on the surface of the modulation zone of the lithium niobate flat plate layer, and the two form a composite waveguide. The silicon dioxide cladding layer covers the outside of the composite waveguide.
[0019] To optimize the composite waveguide structure, the thickness of the lithium niobate flat plate layer, the width and thickness of the silicon nitride waveguide and other key parameters are parameterized and scanned by a finite element analysis software, the effective refractive index, transmission loss, optical field restriction factor and group refractive index of the waveguide are calculated, and finally most of the optical field energy (more than 90% of the optical energy in the composite waveguide) can be restricted in the lithium niobate flat plate layer with electro-optic effect. The specific simulation results are as shown in Figure 4 The simulation and actual measurement results in Figure 4 show that only the optical signal in the lithium niobate flat plate layer can be modulated by the electric field, and the optical field energy is efficiently constrained in the lithium niobate flat plate layer, which fully verifies the rationality and effectiveness of the composite waveguide structure design of the application.
[0020] The core of the design of the optical transmission unit is to realize low-loss optical signal transmission and efficient electro-optic modulation through structure optimization, which is as follows: first, the non-modulation zone of the lithium niobate flat plate layer is completely etched, and only the effective structure of the modulation zone is reserved, so as to reduce the contact area between the metal and the TFLN waveguide and minimize unnecessary optical absorption loss from the source. At the same time, a silicon nitride (Si3N4) ridge waveguide is loaded on the surface of the lithium niobate flat plate layer in the modulation zone to form a silicon nitride-lithium niobate composite waveguide, and a silicon dioxide cladding layer is introduced outside the composite waveguide.
[0021] To further optimize the performance of the composite waveguide, the performance parameters of the waveguide (including effective refractive index, loss, light field restriction factor, and group refractive index) are simulated and calculated by using a finite element analysis software through reasonable design of the waveguide structure size. On a lithium niobate thin film wafer of a fixed specification, the width and height of the silicon nitride ridge waveguide are scanned by parameterization, the waveguide structure size is accurately adjusted, and the performance of the composite waveguide is optimized, so that most of the energy can be confined in the lithium niobate thin film (TFLN) layer of the composite waveguide (the dark blue area in FIG. 8) Figure 1 Compared with a lithium niobate (LN) ridge waveguide manufactured by a traditional etching process, the edge wall roughness of the silicon nitride (Si3N4) strip waveguide has a smaller influence on the transmission loss, effectively reduces the overall theoretical minimum value of the transmission loss in the optical device, and enables the light transmission unit to have three core advantages of ultralow propagation loss, high optical power carrying capacity, and CMOS process compatibility.
[0022] As a further improvement of the above technical solutions, the electrical transmission unit includes a metal traveling wave electrode, which is a double-layer embedded capacitive microstructure loaded traveling wave electrode. For the electrical transmission unit, in order to improve the modulation efficiency while maintaining low optical absorption loss, a new double-layer embedded capacitive microstructure loaded traveling wave electrode as shown in FIG. 9 is proposed. Figure 2
[0023] In existing thin film lithium niobate (TFLN) modulators, the electrode gap is designed to be small, and the metal itself has a limited resistivity, so the Ohmic conductor loss generated thereby is the main source of loss in existing thin film lithium niobate (TFLN) modulators. Specifically, a narrow metal gap of several microns will cause a large capacitance to be formed between the electrodes, and this capacitive effect will cause the current to be concentrated near the gap, thereby reducing the effective conductor area of the electrode and ultimately significantly increasing the loss during the transmission of the radio frequency signal.
[0024] Specifically, the double-layer embedded capacitive microstructure loaded traveling wave electrode includes a main electrode and a plurality of segment portions extending from the edge of the main electrode, and each segment portion is uniformly distributed along the length direction of the main electrode. The main electrode is a rectangular channel electrode region formed by two spaced-apart planar electrodes, and its design is the same as that of a conventional coplanar waveguide (CPW). The segment portions extend transversely from the edge of the main electrode close to the composite waveguide to the direction of the composite waveguide.
[0025] Furthermore, the lower part of the main electrode is embedded within the silicon dioxide cladding, while the upper part is exposed on the surface of the silicon dioxide cladding, forming an embedded structure. This embedded structure forms a double-layer capacitor structure, comprising a bottom capacitor and a top capacitor. The bottom capacitor is formed by the lower part of the main electrode embedded in the silicon dioxide cladding, and the top capacitor is formed by the upper part of the main electrode exposed on the surface of the silicon dioxide cladding.
[0026] Furthermore, the main electrode and the segment portion are integrally formed metal structures. The top view of the embedded double-layer capacitor microstructure load traveling wave electrode is a periodic I-shape, and the side view is a T-shape.
[0027] In the traveling wave electrode structure proposed in this invention, the segmented portion can block current from flowing into adjacent gap regions, while guiding the current to be more uniformly distributed within the wide channel region (i.e., the region where the main electrode is located). This increases the effective conductor size without increasing the electrode gap, thereby reducing ohmic losses in the electrodes. Simultaneously, the electrode structure composed of two electrode layers allows the metal layer to form bottom and top capacitors around the composite waveguide. Both capacitor layers can generate horizontal components of the electric field; their superposition not only increases the electric field density but also improves the overlap factor between the electric field and the transmitted optical field. Ultimately, this significantly enhances the electric field strength within the lithium niobate planar layer without generating additional RF losses, thus improving modulation efficiency.
[0028] In this embodiment, the electrical transmission unit employs a metal traveling wave electrode, such as... Figure 5 and Figure 6 As shown, the electrode is an embedded double-layer capacitor microstructure load traveling wave electrode. Its top view structure is periodic I-shaped, and its side view structure is T-shaped, forming an embedded double-layer capacitor microstructure load traveling wave electrode structure.
[0029] To achieve precise optimization of the electrode structure, electromagnetic simulation software and finite element analysis software were used for electrode structure design. The modulation region of the modulator was modeled in the software, with the modeling direction aligned with the crystal orientation of the lithium niobate waveguide. The optimization process employed a controlled variable method, keeping other parameters constant, and parameterizing parameters such as electrode embedding depth, electrode spacing, the period and duty cycle of the I-shaped structure, as well as electrode width and thickness, one by one, to ultimately determine the optimal structural parameters. The optimized electrode structure significantly increases the electric field density of the modulator structure, synchronously enhancing the electric field strength at the waveguide. This concentrates the energy of both the optical and electric fields in a unified region (the lithium niobate planar layer), achieving optimal overlap between the optical and electric fields. The electric field distribution is as follows: Figure 7 As shown.
[0030] Figure 7 This is the electric field energy distribution diagram of the phase modulator in this invention, from... Figure 7As can be seen, the embedded double-layer capacitive microstructure load traveling wave electrode structure can significantly enhance the electric field energy of the composite waveguide region, and the enhancement of the electric field energy directly improves the overlap factor of the electric field and the optical field in the lithium niobate flat plate layer, thereby effectively improving the modulation efficiency of the modulator. Therefore, the embedded double-layer capacitive microstructure load traveling wave electrode structure increases the effective conductor area, reduces the microwave transmission loss, and improves the modulation efficiency.
[0031] The above-described embodiments are only used to describe the preferred embodiments of the present application, and are not used to limit the scope of the present application. Without departing from the design spirit of the present application, various modifications and improvements to the technical solutions of the present application made by those skilled in the art shall fall within the protection scope of the present application defined by the claims.
Claims
1. A high-modulation-efficiency phase modulator based on a silicon nitride-lithium niobate waveguide, characterized in that, The modulator includes an optical transmission unit and an electrical transmission unit; The optical transmission unit includes a quartz substrate, a silicon dioxide buffer layer disposed above the quartz substrate, a composite waveguide disposed above the middle position of the silicon dioxide buffer layer, and a silicon dioxide cladding layer covering the outside of the composite waveguide. The electrical transmission unit includes a metal traveling wave electrode, which is an embedded double-layer capacitor microstructure load traveling wave electrode.
2. The high modulation efficiency phase modulator based on silicon nitride-lithium niobate waveguide according to claim 1, characterized in that, The composite waveguide includes a lithium niobate planar layer and a silicon nitride waveguide disposed on top of the lithium niobate planar layer.
3. The high modulation efficiency phase modulator based on silicon nitride-lithium niobate waveguide according to claim 1, characterized in that, The embedded double-layer capacitor microstructure load traveling wave electrode includes a main electrode and multiple segmented portions extending from the edge of the main electrode, with each segmented portion being uniformly distributed along the length direction of the main electrode.
4. The high modulation efficiency phase modulator based on silicon nitride-lithium niobate waveguide according to claim 2, characterized in that, The unmodulated region of the lithium niobate plate layer was completely etched.
5. The high modulation efficiency phase modulator based on silicon nitride-lithium niobate waveguide according to claim 2, characterized in that, The silicon nitride waveguide is loaded onto the modulation region surface of the lithium niobate planar layer, and the two form a composite waveguide.
6. The high modulation efficiency phase modulator based on silicon nitride-lithium niobate waveguide according to claim 3, characterized in that, The main electrode is a rectangular channel electrode region, formed by two spaced-apart planar electrodes.
7. The high modulation efficiency phase modulator based on silicon nitride-lithium niobate waveguide according to claim 3, characterized in that, The segment extends laterally from the edge of the main electrode near the composite waveguide toward the composite waveguide.
8. The high modulation efficiency phase modulator based on silicon nitride-lithium niobate waveguide according to claim 3, characterized in that, The lower part of the main electrode is embedded in the silicon dioxide cladding, and the upper part is exposed on the surface of the silicon dioxide cladding, forming an embedded structure; The embedded structure forms a double-layer capacitor structure, which includes a bottom capacitor and a top capacitor. The bottom capacitor is formed by the lower part of the main electrode embedded in the silicon dioxide cladding, and the top capacitor is formed by the upper part of the main electrode exposed on the surface of the silicon dioxide cladding.
9. The high modulation efficiency phase modulator based on silicon nitride-lithium niobate waveguide according to claim 3, characterized in that, The main electrode and the segment portion are integrally formed metal structures.
10. The high modulation efficiency phase modulator based on silicon nitride-lithium niobate waveguide according to claim 3, characterized in that, The embedded double-layer capacitor microstructure load traveling wave electrode has a periodic I-shaped top view and a T-shaped side view.