Phase-controlled waveguide structure based on anisotropic materials and their wavelength multiplexer structure

Abstract

Phase-controlled waveguide structure based on anisotropic materials, characterized in that the phase-controlled waveguide structure comprises symmetrically arranged waveguide units on both sides, wherein the waveguide unit comprises a straight waveguide and a curved waveguide connected to each other, and the phase-controlled waveguide structure comprises a straight waveguide A, a curved waveguide A, a curved waveguide B and a straight waveguide B connected sequentially, wherein the straight waveguide A, the curved waveguide A, the curved waveguide B and the straight waveguide B are arranged symmetrically along an axis having an angle of 45° or 135° to the two crystallographic axes of the surface of the anisotropic material.

Description

Technical area The present invention relates to the field of photonic integrated components, in particular to a phase-controlled waveguide structure based on anisotropic materials and its wavelength division multiplexer structure. State of the art In recent years, global communication traffic has increased exponentially, resulting in unprecedented improvements in communication technology. Optical communication technology has become the most important transmission method in modern communications due to its advantages such as large communication capacity, strong anti-interference ability, and low construction cost. In optical communication systems, wavelength division multiplexing (WDM) technology significantly expands communication bandwidth compared with other multiplexing technologies because it can transmit multiple independent signals simultaneously on the same optical fiber. Array waveguide gratings are one of the key devices for realizing wavelength division multiplexing technology on a single chip. They offer comprehensive advantages such as high stability, high resolution, and a large number of channels.Its main structure includes a free propagation area at the input end, an arrayed waveguide, a free propagation area at the output end, and so on. Cascaded MZI (Mach-Zehnder interferometer) is also one of the key components of wavelength division multiplexing technology. Compared with arrayed waveguide gratings, it offers the advantages of simple fabrication, small area, and low insertion loss. Its main structure includes an input waveguide, a 3 dB directional coupler, a phase-locked waveguide, and an output waveguide. Thin-film lithium niobate (TFLN), as a new photonic integrated chip material, not only offers the advantages of low loss and a broad light transmission spectrum, but also possesses excellent electro-optical effects, large nonlinear optical coefficients, and stable chemical properties. Furthermore, compared to conventional bulk lithium niobate materials, thin-film lithium niobate can significantly reduce device size due to its higher refractive index contrast. Thin-film lithium niobate wafers can be divided into X-cut, Y-cut, and Z-cut. X-cut and Z-cut are the most common.To maximize the electro-optic coefficient r33 of the lithium niobate crystal, the waveguide transmission mode is selected as the transverse electric (TE) mode for X-cut wafers, while the waveguide transmission mode is selected as the transverse magnetic (TM) mode for Z-cut wafers. Compared to the TM mode, the TE mode has a higher overlap integral with the electric field, which is why X-cut lithium niobate thin films exhibit better electro-optical properties. The array waveguide grating must be designed such that the optical path difference between the array waveguide paths is constant. However, it is inevitable that there will be curved waveguides at different angles and straight waveguides in different directions. The refractive index of X-cut thin-film lithium niobate depends on the waveguide direction, which makes the design more difficult and requires higher device manufacturing processes. Thus, the array waveguide grating on X-cut thin-film lithium niobate has not been realized anywhere in the world. The cascaded MZI must be designed such that individual single-stage MZIs ensure that the optical path difference between the upper and lower arms is a constant value. Similarly, the design of phased-array waveguides on X-cut thin-film lithium niobate must consider the change in refractive index in different directions. The currently more common design is to use a special structural design to compensate for the curved waveguide structures of the upper and lower arms, and the optical path difference is determined only by the length difference and the refractive index difference of the straight waveguide. Cascaded MZIs designed using this method tend to be larger. Content of the invention The object of the present invention is to provide a phased-array waveguide structure based on anisotropic materials and its wavelength-division multiplexer structure. The main idea is to design the phased-array waveguide part of the wavelength-division multiplexer symmetrically along a specific angle in order to avoid the difficulties caused by the birefringence effects of anisotropic materials in the design of the wavelength-division multiplexer and their negative impact on precise phase control. A particular focus is on a wavelength-division multiplexer based on anisotropic materials. The technical solution for achieving the objective of the present invention is to arrange the phased waveguide portion in the wavelength division multiplexer along an axis that is at an angle of 45° or 135° to the two crystallographic axes of the surface of the anisotropic material. Specifically, in the array waveguide grating, the input free propagation region, the array waveguide, the output free propagation region, and other structures are arranged symmetrically with respect to the 45° or 135° axis; in the cascaded MZI, the individual single-stage MZI portions are arranged symmetrically with respect to the 45° or 135° axis. The present invention is achieved by the following technical solutions: The present invention discloses a phased waveguide structure. The phased waveguide structure comprises symmetrically arranged waveguide units on both sides.The waveguide unit comprises a straight waveguide and a curved waveguide connected to each other. The phased waveguide structure comprises a straight waveguide A, a curved waveguide A, a curved waveguide B, and a straight waveguide B connected in sequence. The straight waveguide A, the curved waveguide A, the curved waveguide B, and the straight waveguide B are arranged symmetrically along an axis at an angle of 45° or 135° to the two crystallographic axes of the surface of an anisotropic material. In a further development, the lengths of the straight waveguide A and the straight waveguide B according to the present invention are the same and the bending radii and bending angles of the curved waveguide A and the curved waveguide B are the same. In a further development, the cross section of the phased waveguide structure according to the present invention comprises, from bottom to top, a buried oxide layer, a waveguide layer and an upper silicon dioxide cladding layer, and the anisotropic material consists of X-cut thin film lithium niobate. In a further development, the radius of the curved waveguides according to the present invention is greater than or equal to 50 µm, and the waveguide layer is a ridge waveguide or a strip waveguide. The present invention also discloses a wavelength division multiplexer structure based on anisotropic materials, comprising a phased waveguide structure based on an arrayed waveguide grating. The structure comprises an input waveguide, a free propagation region at the input end, a tapered waveguide at the input end, an arrayed waveguide, a tapered waveguide at the output end, a free propagation region at the output end, and an output waveguide, which are sequentially connected. In a further development, the free propagation region at the input end and the free propagation region at the output end, the conical waveguide at the input end and the conical waveguide at the output end as well as the array waveguide itself according to the present invention are arranged symmetrically with respect to an axis which has an angle of 45° or 135° to the two crystallographic axes of the surface of the anisotropic material. In a further development, the conical waveguide at the input end and the conical waveguide at the output end according to the present invention serve to reduce coupling losses between the modes. The input and output waveguides respectively serve for the input and output of optical signals. The present invention also discloses a wavelength division multiplexer structure based on anisotropic materials, comprising a phased-array waveguide structure and based on a cascaded Mach-Zehnder interferometer. The structure is constructed as a binary tree, with single-stage MZI units connected by 3 dB directional couplers. One of the single-stage MZI units comprises an upper and a lower arm of the phased-array waveguide structure, wherein the upper and lower arms are arranged symmetrically with respect to an axis that has an angle of 45° or 135° to the two crystallographic axes of the surface of the anisotropic material. In a further development, the structure according to the present invention comprises an input waveguide, single-stage MZI units, 3 dB directional couplers for beam focusing and beam splitting connecting the individual single-stage MZI units, and an output waveguide. As a further improvement, the individual single-stage MZI units according to the present invention each comprise two interference arms, namely the upper and the lower arm, which are formed after beam splitting, wherein the upper and the lower arm have a length difference. Compared with the prior art, the present invention has the following advantageous effects: 1) A wavelength division multiplexer based on an arrayed waveguide grating structure was designed on anisotropic materials, which overcomes the difficulties in precise phase control caused by the birefringence effect. 2) The method for designing cascaded MZI on anisotropic materials was optimized, so that the phased waveguide is more efficient and compact. 3) The present invention uses X-cut thin-film lithium niobate as an anisotropic material to design a wavelength division multiplexer. Compared with conventional materials such as silicon, it offers advantages such as a broad light transmission spectrum and low loss, as well as excellent nonlinear and electro-optical properties and the potential to realize a wider range of functions. Brief description of the drawings Fig. 1 is a schematic representation of the principle of the design method and the arrangement of the phased waveguides. Reference numeral 1 represents a straight waveguide A, 2 represents a curved waveguide A, 3 represents a curved waveguide B, and 4 represents a straight waveguide B. The structure is arranged symmetrically with respect to an axis that has an angle of 45° or 135° to both the crystallographic Y and Z axes. Fig. 2 is a schematic plan view of a wavelength division multiplexer based on an anisotropic material according to the invention, based on an array waveguide grating structure.Here, 5 is an input waveguide, 6 is a free propagation region at the input end, 7 is a tapered waveguide at the input end, 8 is an array waveguide, 9 is a tapered waveguide at the output end, 10 is a free propagation region at the output end, and 11 is an output waveguide. When the anisotropic material is X-cut thin-film lithium niobate, the Z direction is a direction of its crystallographic Z axis. Fig. 3 is a plan view of a wavelength division multiplexer based on an anisotropic material according to the invention, which is based on a cascaded MZI structure. Here, 12 is an upper arm, 13 is a lower arm, 14 is an input waveguide, 15 is a 3 dB directional coupler, and 16 is an output waveguide. When the anisotropic material is X-cut thin-film lithium niobate, the Z direction is a direction of its crystallographic Z axis. Fig.4 is a schematic representation of the cross-sectional structure of the waveguide when the anisotropic material is an X-cut thin film lithium niobate according to the present invention. Where 17 is an upper silicon dioxide cladding layer, 18 is a waveguide layer, and 19 is a buried oxide layer; the X-direction is a direction of the crystallographic X-axis of the thin-film lithium niobate. Detailed description of embodiments The present invention provides a phased-array waveguide structure and its wavelength-division multiplexer structure. It is a design method and structures for implementing the wavelength-division multiplexer on anisotropic materials, including a phased-array waveguide structure, a wavelength-division multiplexer structure based on an arrayed waveguide grating, and a wavelength-division multiplexer structure based on a cascaded Mach-Zehnder interferometer. It is explained using X-cut thin-film lithium niobate as an anisotropic material as an example. The anisotropic material can also be barium titanate. Fig. 1 is a schematic representation of a partial path of a phased-array waveguide according to the present invention. The phased-array waveguide structure comprises symmetrically arranged waveguide units on both sides. The waveguide unit comprises a straight waveguide and a curved waveguide connected to each other. It consists of a straight waveguide A1, a straight waveguide B4, and a curved waveguide A2 and B3, with the four waveguides connected one after the other. The structure is arranged symmetrically with respect to an axis that has an angle of 45° to both the crystallographic Y and Z axes. That is, the lengths of the two straight waveguides A1 and B4 are equal, and the bending radii and bending angles of the two curved waveguides A2 and B3 are also equal.The straight waveguide A1, the curved waveguide A2, the curved waveguide B3, and the straight waveguide B4 are arranged symmetrically along an axis at an angle of 45° or 135° to the two crystallographic axes of the surface of the anisotropic material. The lengths of the straight waveguide A1 and the straight waveguide B4 are equal, and the bending radii and angles of the curved waveguide A2 and the curved waveguide B3 are equal. Fig. 2 shows a plan view of a wavelength division multiplexer based on X-cut thin-film lithium niobate and based on an arrayed waveguide grating structure. This is a wavelength division multiplexer structure comprising a phased waveguide structure and based on an arrayed waveguide grating. The arrayed waveguide path is the same as in Fig. 1 and also consists of straight waveguides and curved waveguides. The structure includes an input waveguide 5, a free propagation region 6 at the input end, a tapered waveguide 7 at the input end, an arrayed waveguide 8, a tapered waveguide 9 at the output end, a free propagation region 10 at the output end, and an output waveguide 11, which are sequentially connected.The free propagation region 6 at the input end itself, the free propagation region 10 at the output end itself, the conical waveguide 7 at the input end and the conical waveguide 9 at the output end, as well as the array waveguide 8 itself, are arranged symmetrically with respect to an axis that has an angle of 45° to both the crystallographic Y and Z axes. The conical waveguide 7 at the input end and the conical waveguide 9 at the output end serve to reduce coupling losses between the modes. The input waveguide 5 and the output waveguide 11 serve for the input and output of optical signals, respectively. Fig. 3 shows a top view of a wavelength division multiplexer based on X-cut thin-film lithium niobate and based on a cascaded MZI structure. This wavelength division multiplexer structure includes a phased-array waveguide structure and is based on a cascaded Mach-Zehnder interferometer. The phased-array waveguide path is the same as in Fig. 1 and also consists of straight waveguides and curved waveguides. The structure is constructed as a binary tree, with single-stage MZI units connected by 3 dB directional couplers 15. One of the single-stage MZI units comprises an upper arm 12 and a lower arm 13 of the phased waveguide structure, wherein the upper arm 12 and the lower arm 13 are arranged symmetrically with respect to an axis having an angle of 45° or 135° to the two crystallographic axes of the surface of an anisotropic material. The phased-array waveguide structure of each MZI stage is arranged symmetrically with respect to an axis at an angle of 135° to both the crystallographic Y and Z axes. The upper arm 12 of the single-stage MZI and the lower arm 13 of the single-stage MZI generate the optical path difference. The 3 dB directional coupler 15 splits and focuses the beams of the MZI. The input waveguide 14 and the output waveguide 16 respectively serve for the input and output of optical signals. The structure comprises an input waveguide 14, single-stage MZI units, 3 dB directional couplers 15 for beam focusing and beam splitting connecting the individual single-stage MZI units, and an output waveguide 16. The individual single-stage MZI units each comprise two interference arms, namely the upper arm 12 and the lower arm 13, which are formed after beam splitting, wherein the upper arm 12 and the lower arm 13 have a length difference. As shown in Figure 4, the cross-section of the phased waveguide structure, from bottom to top, includes a buried oxide layer 19, a waveguide layer 18, and an upper silicon dioxide cladding layer 17. The anisotropic material is X-cut thin-film lithium niobate. The radius of the curved waveguides is greater than or equal to 50 μm, and the waveguide layer is a ridge waveguide or a stripe waveguide. In array waveguide gratings and cascaded MZIs, the refractive index of the waveguide layer 18 is higher than that of the upper silicon dioxide cladding layer 17 and the buried oxide layer 19, and the light field is confined and stably transmitted in the waveguide layer 19. To reduce bending losses and avoid mode hybridization, the bending radius of the curved waveguides in Fig. 2 and Fig. 3 is not less than 50 µm. The cross-sectional structure of the thin-film lithium niobate waveguide in Fig. 4 is a ridge waveguide or a strip waveguide. When the array waveguide 8 is used, multi-wavelength optical signals from an optical fiber enter the free propagation region 6 at the input end through the input waveguide 5, diffraction occurs, and the light energy is distributed in the form of a Gaussian distribution into the conical waveguide 7 at the input end at the entrance port of the single array waveguide 8. The array waveguide 8 is designed so that the optical path difference between adjacent waveguides is constant. When the multi-channel optical signals output via the array waveguide 8 enter the free propagation region 10 at the output end, the optical signals of different wavelengths interfere and superpose due to interference due to different phase differences in the free propagation region 10 at the output end.As a result, they focus at different positions and are coupled out via the output waveguide 11, thereby achieving wavelength separation. When using a cascaded MZI, multi-wavelength optical signals are input from the optical fiber through the input waveguide 14. After passing through the 3 dB directional coupler 15, the light is split into two coherent beams of equal intensity. These two light beams are transmitted in the two interference arms of the MZI. Since the lengths of the upper arm 12 and the lower arm 13 are different in the single-stage MZI, a certain optical path difference is generated. In the 3 dB directional coupler 15, the light beams are superimposed and interfere. Light of a specific wavelength is amplified by interference at a corresponding port and output through different output waveguides 16, thereby achieving wavelength separation. Furthermore, by cascading multiple MZI structures, the filter curve can be optimized, and the binary tree structure can multiplex more light wavelengths. To better understand the subject matter, technical solutions, and advantages of the present invention, the present invention will be described in more detail below in conjunction with the accompanying drawings and exemplary embodiments. The specific exemplary embodiments described here are used only to illustrate the present invention and are not intended to limit the present invention. The design method and wavelength division multiplexer structure are explained using X-cut thin-film lithium niobate material as an example. Example 1 This embodiment of the present invention provides a configuration of an X-cut thin-film lithium niobate wavelength division multiplexer structure based on a novel array waveguide grating. As shown in Figs. 2 and 4, this wavelength division multiplexer comprises an input waveguide 5, a free propagation region 6 at the input end, a tapered waveguide 7 at the input end at the entrance port of an array waveguide, the array waveguide 8, a tapered waveguide 9 at the output end, a free propagation region 10 at the output end, and an output waveguide 11 of Fig. 2, which are sequentially connected. The cross-section of the above waveguide structures is shown in Fig. 4 and includes a thin-film lithium niobate waveguide layer 19, an upper silicon dioxide cladding layer 17, and a buried oxide layer 19. In order for light of different wavelengths to form interference and superposition at corresponding output waveguides, a design must ensure that an equal phase difference occurs between adjacent array waveguides. The X-cut lithium niobate thin film material exhibits birefringence effects, and the effective refractive index and group refractive index of the waveguide depend on the direction of the waveguide (relative to the angle to the crystallographic Z axis). The array waveguide includes a straight waveguide, a curved waveguide, and a tapered waveguide in all directions. Therefore, it is difficult to ensure an equal phase difference between adjacent array waveguides. According to the simulation results of the finite element simulation software, the effective refractive index and group refractive index of straight waveguides, tapered waveguides, and slab waveguides based on X-cut thin film lithium niobate all satisfy the relationship (where 0 is an angle between the waveguide direction and the crystallographic Z axis): Based on this relationship, in this embodiment of the present invention, the phased waveguide part is designed with a 45° or 135° axis as the symmetry axis. The principle is explained below using Fig. 1: The straight waveguide A1 and the straight waveguide B4 are arranged symmetrically with respect to the 45° axis, and S is the length of the straight waveguide. The optical paths of the straight waveguide A1 and the straight waveguide B4 are as follows: Adding the two gives: It can be seen that the optical path after addition has nothing to do with the straight waveguide path, but only with the length. In the same way, the optical path of the curved waveguides 2 and 3 is calculated by integration: where R is the bending radius of the waveguide, θ is the bending angle of the waveguide. After adding the two, we get: It can be seen that the optical path after addition has nothing to do with the curved waveguide path, but only with the radius and bending angle of the curved waveguide. The above conclusion can be extended to structures such as slab waveguides and tapered waveguides. This means that when the waveguide structure is arranged symmetrically along the 45° or 135° axis, the optical path difference between adjacent array waveguides remains constant, regardless of the path of each waveguide, as long as the total length difference of the waveguide paths is kept constant. This design method cleverly avoids birefringence effects and transforms the anisotropic device design into an isotropic design. Under the condition that the optical path difference between adjacent array waveguides is constant ΔL, a main interference maximum can be formed for a specific wavelength at the corresponding output waveguide: If the energy in each array waveguide is the same (example only), the main interference maximum can be obtained as follows: The output waveguides are placed at different positions on the image plane to obtain light of different wavelengths, thus realizing the wavelength division multiplexing function. Example 2 This embodiment of the present invention provides a wavelength division multiplexer structure based on X-cut thin-film lithium niobate based on a cascaded MZI. As shown in Figs. 3 and 4, the wavelength division multiplexer comprises the upper arm 12 and the lower arm 13 of the single-stage MZI, as well as the input waveguide 14, the 3 dB directional coupler 15, and the output waveguide 16 of Fig. 3. The cross-section of the above waveguide structures is shown in Fig. 4 and includes a thin-film lithium niobate waveguide layer 19, an upper silicon dioxide cladding layer 17, and a buried oxide layer 19. As shown in Figure 1, when the waveguide path is symmetrical along the 45° or 135° axis, the optical path of the waveguide has nothing to do with the waveguide path, but only depends on the length and equivalent refractive index of the waveguide. Based on this conclusion, each group of phased waveguides in the cascaded MZI shown in Example 2 is arranged symmetrically along the 135° axis, and the length of the phased waveguide is designed according to the designed optical path difference. Compared with the conventional MZI structure, the phased waveguide here can be designed according to any path, making the structure more efficient and compact to a certain extent. Light is split into two coherent beams of equal intensity by the 3 dB directional coupler in Fig. 4, and the two beams have a phase difference of π / 2. After passing through the phased waveguide, the lower arm 13 lags behind the upper arm 12 by βΔL. After passing through the second 3 dB directional coupler, the lower arm 13 lags behind the upper arm by π / 2. According to the principle of interference amplification, the wavelengths of the optical signals output by the upper arm 12 and the lower arm 13 respectively satisfy: Based on the single-stage MZI structure, wavelength separation can therefore be easily achieved. The passband characteristics of the single-stage MZI filter are sinusoidal, and the effective bandwidth is too narrow to meet the requirements of practical applications. To achieve a flat spectral distribution of the device, a cascaded MZI structure is generally used in practical applications. At the same time, a binary tree structure can be introduced for multiplexing and demultiplexing optical signals with multiple wavelengths, as shown in Figure 3, which can achieve four-channel multiplexing and demultiplexing. The foregoing is not intended to limit the present invention. It should be understood that a person skilled in the art may make various changes, modifications, additions, or substitutions without departing from the essential scope of the present invention, which should also be considered to be within the scope of the present invention.

Claims

A phased waveguide structure based on anisotropic materials, characterized in that the phased waveguide structure comprises symmetrically arranged waveguide units on both sides, wherein the waveguide unit comprises a straight waveguide and a curved waveguide which are connected to each other, and the phased waveguide structure comprises a straight waveguide A, a curved waveguide A, a curved waveguide B and a straight waveguide B which are connected one after the other, wherein the straight waveguide A, the curved waveguide A, the curved waveguide B and the straight waveguide B are arranged symmetrically along an axis which has an angle of 45° or 135° to the two crystallographic axes of the surface of the anisotropic material. A phased waveguide structure according to claim 1, characterized in that the lengths of the straight waveguide A and the straight waveguide B are equal and the bending radii and bending angles of the curved waveguide A and the curved waveguide B are equal. A phased waveguide structure according to claim 1 or 2, characterized in that the cross-section of the phased waveguide structure comprises, from bottom to top, a buried oxide layer, a waveguide layer and an upper silicon dioxide cladding layer, and the anisotropic material consists of X-cut thin-film lithium niobate. Phased waveguide structure according to claim 3, characterized in that the radius of the curved waveguides is greater than or equal to 50 µm, and the waveguide layer is a ridge waveguide or a strip waveguide. A wavelength division multiplexer structure based on anisotropic materials, comprising a phased waveguide structure and based on an array waveguide grating, characterized in that the structure comprises an input waveguide, a free propagation region at the input end, a tapered waveguide at the input end, an array waveguide, a tapered waveguide at the output end, a free propagation region at the output end, and an output waveguide, which are connected one after the other. Wavelength division multiplexer structure according to claim 4, comprising a phased waveguide structure and based on an array waveguide grating, characterized in that the free propagation region at the input end and the free propagation region at the output end, the conical waveguide at the input end and the conical waveguide at the output end, as well as the array waveguide itself, are arranged symmetrically with respect to an axis having an angle of 45° or 135° to the two crystallographic axes of the surface of the anisotropic material. A wavelength division multiplexer structure according to claim 5, comprising a phased waveguide structure and based on an array waveguide grating, characterized in that the tapered waveguide at the input end and the tapered waveguide at the output end serve to reduce the coupling losses between the modes, and the input and output waveguides serve respectively for the input and output of optical signals. Wavelength division multiplexer structure based on anisotropic materials, comprising a phased waveguide structure and based on a cascaded Mach-Zehnder interferometer, characterized in that the structure is constructed as a binary tree, with single-stage MZI units connected by 3 dB directional couplers, one of the single-stage MZI units comprising an upper and a lower arm of the phased waveguide structure, the upper and lower arms being arranged symmetrically with respect to an axis having an angle of 45° or 135° to the two crystallographic axes of the surface of the anisotropic material. Wavelength division multiplexer structure according to claim 7, comprising a phased waveguide structure and based on a cascaded Mach-Zehnder interferometer, characterized in that the structure comprises an input waveguide, single-stage MZI units, 3 dB directional couplers for beam focusing and beam splitting connecting the individual single-stage MZI units, and an output waveguide. Wavelength division multiplexer structure according to claim 8, comprising a phased waveguide structure and based on a cascaded Mach-Zehnder interferometer, characterized in that the individual single-stage MZI units each comprise two interference arms, namely the upper and the lower arm, which are formed after beam splitting, the upper and the lower arm having a length difference.

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