Mzi type reversible fundamental mode-vertical high order mode converter and preparation method thereof

Abstract

The application discloses a kind of based on MZI type reversible fundamental mode-vertical high-order mode converter and preparation method, the converter includes: substrate layer;Three layers of cladding layer, first cladding layer is arranged above substrate layer, and is covered around first waveguide core layer;Second cladding layer is arranged above first cladding layer and first waveguide core layer, and is covered around second waveguide core layer;Third cladding layer is arranged above second cladding layer and second waveguide core layer, and is covered two sides and above third waveguide core layer;Three layers of waveguide core layer, each layer of waveguide core layer includes successively connected input straight waveguide, at least one Y branch waveguide, intermediate waveguide and output straight waveguide in optical transmission direction;In vertical direction, the input straight waveguide of each layer waveguide core layer is arranged in upper and lower coincidence, and the output straight waveguide of each layer waveguide core layer is arranged in upper and lower coincidence;Electrode structure is located above third cladding layer, and is arranged directly above the intermediate waveguide of third waveguide core layer.

Description

Technical Field

[0001] This invention belongs to the field of device technology, specifically relating to an MZI-based reversible fundamental-to-vertical higher-order mode converter and its fabrication method. Background Technology

[0002] The rapid development of information technology has placed higher expectations on faster data processing and more efficient resource utilization. Against this backdrop, on-chip photonic interconnect technology for large-scale parallel computing, specifically mode division multiplexing (MDM) technology based on an integrated optical platform, can excite and control spatial modes of different orders, achieving multi-channel parallel transmission at a single wavelength. This offers advantages in spatial resource utilization. Furthermore, combining it with wavelength division multiplexing (WDM) technology can construct a multi-dimensional multiplexed communication architecture, effectively improving spectrum utilization. A complete on-chip MDM system includes a mode converter, mode multiplexer / demultiplexer, multimode bent waveguide, and reconfigurable mode switch. Among these, the mode converter, as the core component for efficient conversion between the fundamental mode and higher-order modes, directly determines the system's mode multiplexing capability and transmission quality.

[0003] Current mode converters are primarily based on silicon-based photonic platforms, and their integration with CMOS processes has formed a mature technological path. The high refractive index difference between silicon and silicon dioxide cladding is beneficial for device miniaturization; however, high-refractive-index waveguides are susceptible to dimensional deviations and sidewall roughness, increasing mode crosstalk during mode conversion. Furthermore, heterogeneous integration of silicon is challenging, exhibiting significant lattice constant and thermal expansion coefficient mismatches with III-V group luminescent materials, strong electro-optic materials (such as lithium niobate), or magneto-optic materials. The fabrication of multilayer waveguide structures through vertical stacking is difficult, hindering breakthroughs in two-dimensional planar photonic integration architectures and restricting the development of 3D photonics technology. Among other material systems, photonic platforms such as silicon nitride, lithium niobate, and indium phosphide have complex fabrication processes, high costs, limited refractive index adjustment ranges, and poor compatibility with other functional materials, making it difficult to efficiently integrate multiple functional devices onto a single chip, thus limiting their integration density and functional expansion.

[0004] Currently proposed waveguide structures for fundamental mode to higher-order mode conversion include asymmetric directional coupling structures, Y-branch structures, micro-ring resonators, and Mach-Zehnder interferometers (MZI) structures. Asymmetric directional couplers use two phase-matched asymmetric waveguides to couple light energy from the fundamental mode of one waveguide to the higher-order mode of the other. However, the mode conversion efficiency of this structure is easily affected by the operating wavelength and manufacturing deviations (such as edge roughness and uneven etching depth), resulting in small process tolerances. Y-branch structures have a simple design method, dividing a single waveguide into two or more branch waveguides and introducing a path difference to generate a phase difference, thereby synthesizing a higher-order mode at the output. However, the branch gap needs to reach a deep subwavelength scale, placing high demands on the etching process. Furthermore, directional coupling structures and Y-branch structures are functionally limited, only enabling unidirectional transmission. After completing the conversion from the fundamental mode to a higher-order mode, it is difficult to achieve the reverse conversion from the higher-order mode to a lower-order mode (e.g., invention patent CN119738923B). Microring resonator (MZI) mode converters utilize resonance effects to achieve selective coupling and conversion between modes at specific wavelengths. However, these devices have narrow operating bandwidths, making them unsuitable for wavelength division multiplexing-mode division multiplexing hybrid systems requiring wide spectral resources. MZI structures achieve precise control of the output field distribution through constructive and destructive interference, offering design flexibility. However, the mode conversion designs of these traditional structures are primarily limited to mode manipulation in the lateral dimension, achieving transitions from the fundamental mode to higher-order lateral modes (such as E0). 10 E 20 The limitations of traditional structures restrict their application in multidimensional multiplexing systems. To overcome these limitations, surface plasmon resonance (SPR) or metasurface transducers have been proposed in recent years. These transducers utilize metals or subwavelength nanostructures to manipulate the optical field, achieving extremely strong optical field confinement and flexible wavefront modulation. However, metal structures suffer from inherent ohmic losses, resulting in excessive signal transmission losses. Metasurface-based transducers, with their complex three-dimensional nanostructures, also face challenges such as poor compatibility with existing CMOS processes, high fabrication difficulty, and difficulty in achieving on-chip monolithic integration.

[0005] Therefore, existing CMOS-based fabrication structures suffer from limitations such as small process tolerance, limited functionality, and dimensional constraints. Controllable modes are primarily limited to lateral modes, and inverse mode conversion is difficult. Currently, there is a lack of on-chip mode converter solutions that can simultaneously achieve low loss, low cost, high-dimensional mode conversion, large process tolerance, wide operating bandwidth, and compatibility with mainstream CMOS processes. This has become a key research bottleneck driving the development of high-dimensional optical communication and optical computing. Summary of the Invention

[0006] In view of this, the main objective of the present invention is to provide a reversible fundamental mode-vertical higher-order mode converter based on MZI and its fabrication method.

[0007] To achieve the above objectives, the technical solution of the present invention is implemented as follows: A reversible fundamental mode-vertical higher-order mode converter based on MZI type is characterized in that the converter includes: a substrate layer, a three-layer waveguide core layer, a three-layer cladding layer and an electrode structure; The three cladding layers, from bottom to top, include a first cladding layer, a second cladding layer, and a third cladding layer, wherein: The first cladding layer is disposed above the substrate layer and covers the area around the first waveguide core layer; The second cladding layer is disposed above the first cladding layer and the first waveguide core layer, and covers the area around the second waveguide core layer; The third cladding layer is disposed above the second cladding layer and the second waveguide core layer, and covers both sides and the top of the third waveguide core layer; The three waveguide core layers are, from bottom to top, the first waveguide core layer, the second waveguide core layer, and the third waveguide core layer; each waveguide core layer includes, along the optical transmission direction, a connected input straight waveguide, at least one Y-branch waveguide, an intermediate waveguide, and an output straight waveguide. In the vertical direction, the input straight waveguides of each waveguide core layer are arranged to overlap vertically, and the output straight waveguides of each waveguide core layer are arranged to overlap vertically. The electrode structure is located above the third cladding layer and is positioned directly above the intermediate waveguide of the third waveguide core layer.

[0008] Furthermore, the first waveguide core layer includes a first input straight waveguide, a first Y-branch waveguide, a first intermediate waveguide, a second Y-branch waveguide, and a first output straight waveguide connected in sequence.

[0009] Furthermore, the intermediate waveguide of the second waveguide core layer is the first interferometer waveguide; the second waveguide core layer includes a second input straight waveguide, a third Y-branch waveguide, a first connecting waveguide, a fourth Y-branch waveguide, a first interferometer waveguide, a fifth Y-branch waveguide, a second connecting waveguide, a sixth Y-branch waveguide, and a second output straight waveguide connected in sequence.

[0010] Furthermore, the intermediate waveguide of the third waveguide core layer is the second interferometer waveguide; the third waveguide core layer includes a third input straight waveguide, a seventh Y-branch waveguide, a third connecting waveguide, an eighth Y-branch waveguide, a second interferometer waveguide, a ninth Y-branch waveguide, a fourth connecting waveguide, a tenth Y-branch waveguide and a third output straight waveguide connected in sequence.

[0011] Furthermore, each Y-branch waveguide in the third waveguide core layer structure is symmetrically distributed with each Y-branch waveguide in the second waveguide core layer structure along the optical transmission direction; in the vertical direction, the third connecting waveguide and the fourth connecting waveguide of the third waveguide core layer are respectively located directly above the first connecting waveguide and the second connecting waveguide of the second waveguide core layer.

[0012] Furthermore, the materials of the substrate layer and the three-layer cladding are selected from polymethyl methacrylate (PMMA) and its derivatives, polydimethylsiloxane (PDMS), polyimide (PI), polycarbonate (PC), and EPOClad photoresist; the materials of the three-layer waveguide core layer are selected from SU8, NOA, EPOCore photoresist, sol-gel, and resin materials; and the materials of the electrode structure are selected from gold, silver, copper, aluminum, and their alloys.

[0013] Furthermore, the first waveguide core layer has a thickness of 2–5 μm and a width of 2–8 μm; the second waveguide core layer has a thickness of 3–10 μm and a width of 3–10 μm; and the third waveguide core layer has a thickness of 3–10 μm and a width of 3–10 μm.

[0014] This invention also provides a method for fabricating an MZI-based reversible fundamental-to-vertical higher-order mode converter as described above, the method comprising the following steps: S1. Preparation of substrate layer: A polymer solution is spin-coated onto the surface of a glass substrate and then heated and cured to form the substrate layer; S2. Preparation of the first waveguide core layer: Photoresist is spin-coated on the substrate layer, and then subjected to pre-baking, ultraviolet exposure based on the digital micromirror device (DMD) lithography system, post-baking and development processes to form the first waveguide core layer. S3. Preparation of the first cladding: Spin-coating a polymer material onto a sample with the first waveguide core layer and heating to cure it to form the first cladding layer covering the first waveguide core layer. S4. Fabrication of the second waveguide core layer and the second cladding: Following the same process as steps S2 and S3, the second waveguide core layer and the second cladding layer surrounding the first waveguide core layer are sequentially fabricated above the first waveguide core layer and the first cladding layer. S5. Fabrication of the third waveguide core layer and the third cladding: Following the same process as steps S2 and S3, the third waveguide core layer and the third cladding covering both sides and the top of the third waveguide core layer are sequentially fabricated on top of the second waveguide core layer and the second cladding. S6. Electrode structure preparation: A metal layer is deposited above the third cladding layer, followed by spin coating of positive photoresist on the surface of the metal layer and pre-baking; UV exposure, post-baking and development are performed using the DMD lithography system to remove some of the photoresist, followed by cleaning to remove residual photoresist, and the electrode structure pattern is obtained directly above the intermediate waveguide of the third waveguide core layer.

[0015] Furthermore, in steps S2, S4 and S5, the wavelength of the light source for ultraviolet exposure is 365nm; before performing the ultraviolet exposure, a focusing calibration pattern is loaded based on the DMD lithography system, and the focus is adjusted by observing the clarity of the projected image by adjusting the Z-axis height of the stage, so that the focal plane corresponds to the upper surface of the sample.

[0016] Furthermore, in step S6, the metal layer is deposited using an electron beam evaporation process, and the vacuum level of the vacuum chamber during deposition is [missing information]. Pa to Pa; The positive photoresist is BP212 photoresist. After development, the development reaction is terminated with deionized water, and the surface is soaked in anhydrous ethanol solution to remove residual photoresist.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention provides a reversible fundamental mode-to-vertical higher-order mode converter based on an MZI-type structure. By stacking three waveguide core layers in a three-dimensional manner in the vertical direction, it breaks through the dimensional limitations of traditional two-dimensional planar mode converters, effectively saving chip space and improving device integration and functional density. Structurally, it employs an asymmetric Mach-Zehnder interferometer and a cascaded branched optical path design, and configures an electrode structure on the top layer to effectively control the refractive index using the high thermo-optical coefficient of the polymer material. This allows the converter to directly complete the reversible cross-order conversion from the fundamental mode to the vertical higher-order mode with a single device, without needing to undergo E... 01 The transition to intermediate modes not only greatly simplifies the processing flow of higher-order modes, but also takes into account the advantages of a wide operating wavelength range and low loss, thereby significantly improving the integration, information transmission capacity and flexibility of dense wavelength division multiplexing optical communication systems. Attached Figure Description

[0018] The accompanying drawings, which are included to provide a further understanding of the invention and form part of this invention, illustrate exemplary embodiments of the invention and, together with their descriptions, serve to explain the invention and do not constitute an undue limitation thereof. In the drawings:

[0019] Figure 1(a) is a three-dimensional structural diagram of the MZI-type reversible fundamental mode-vertical higher-order mode converter structure; Figure 1(b) is a cross-sectional view of the interference region based on the MZI-type reversible fundamental mode-vertical higher-order mode converter structure; Figure 1(c) is a cross-sectional view of the first waveguide core layer YZ based on the MZI-type reversible fundamental mode-vertical higher-order mode converter structure; Figure 1(d) is a cross-sectional view of the YZ section of the second waveguide core layer based on the MZI-type reversible fundamental mode-vertical higher-order mode converter structure; Figure 1(e) is a cross-sectional view of the third waveguide core layer YZ based on the MZI-type reversible fundamental mode-vertical higher-order mode converter structure.

[0020] Figure 2 This is a schematic diagram of the fabrication process of a high-order mode converter based on a photolithography system using a digital micromirror device (DMD).

[0021] Figure 3(a) is an optical micrograph of the Y branch of the MZI-based reversible fundamental mode-vertical higher-order mode converter.

[0022] Figure 3(b) is an optical micrograph of the interference region based on the MZI-type reversible fundamental mode-vertical higher-order mode converter.

[0023] Figure 4 This is a schematic diagram of the electrode structure fabrication process based on the MZI-type reversible fundamental mode-vertical higher-order mode converter.

[0024] Figure 5 This is a diagram showing the E00 mode optical field distribution at the input end of a three-layer MZI waveguide.

[0025] Figure 6 The optical field distribution of the E01 mode at the input end of a three-layer MZI waveguide. Figure 7 E-mode optical field distribution at the input end of a three-layer MZI waveguide. Figure 8 This is a schematic diagram showing the changes in the transmission path of the mode converter as the input optical mode (E00, E01, E) and electrode state (on / off) change.

[0026] Figure 9 When the input is E00 mode, the effect of the waveguide thickness at the first input end of the mode converter on the normalized output optical power of E00, E01 and E, respectively.

[0027] Figure 10 When the input is E00 mode, the effect of the waveguide thickness at the second input end of the mode converter on the normalized output optical power of E00, E01 and E, respectively.

[0028] Figure 11 When the input is E00 mode, the effect of the waveguide width at the input of the mode converter on the normalized output optical power of E00, E01 and E, respectively.

[0029] Figure 12The effect of wavelength on the normalized output optical power of E00, E01 and E when E00 mode input is applied after voltage is applied.

[0030] Figure 13 The effect of wavelength on the normalized output optical power of E00, E01 and E when E01 mode input is applied after voltage is applied.

[0031] Figure 14 The effect of wavelength on the normalized output optical power of E00, E01 and E when E01 mode input is applied after voltage is applied.

[0032] The markings in the diagram have the following meanings: 100 represents the substrate layer; 200 is the first waveguide core layer, of which 201 is the first input straight waveguide, 202 is the first Y-branch waveguide, 203 is the first intermediate waveguide, 204 is the second Y-branch waveguide, and 205 is the first output straight waveguide; 300 is the second waveguide core layer, of which 301 is the second input straight waveguide, 302 is the third Y-branch waveguide, 303 is the first connecting waveguide, 304 is the fourth Y-branch waveguide, 305 is the first interferometer waveguide, 306 is the fifth Y-branch waveguide, 307 is the second connecting waveguide, 308 is the sixth Y-branch waveguide, and 309 is the second output straight waveguide; 400 is the third waveguide core layer, of which 401 is the third input straight waveguide, 402 is the seventh Y-branch waveguide, 403 is the third connecting waveguide, 404 is the eighth Y-branch waveguide, 405 is the second interferometer waveguide, 406 is the ninth Y-branch waveguide, 407 is the fourth connecting waveguide, 408 is the tenth Y-branch waveguide, and 409 is the third output straight waveguide; 501 is the first cladding layer, 502 is the second cladding layer, and 503 is the third cladding layer; 600 is the electrode structure. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0034] In the accompanying drawings of this embodiment, the same or similar reference numerals correspond to the same or similar components. In the description of this invention, it should be understood that the terms "up," "down," "left," "right," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the terms used to describe positional relationships in the accompanying drawings are only for illustrative purposes and should not be construed as limiting this patent. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0035] It should be noted that, in this document, the terms include, encompass, or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, article, or apparatus that includes a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, article, or apparatus. Without further limitation, an element defined by the phrase "including a…" does not exclude the presence of other identical elements in the process, article, or apparatus that includes that element.

[0036] This invention provides an MZI-based reversible fundamental mode-vertical higher-order mode converter, as shown in Figures 1-14. The converter includes: a substrate layer 100, a three-layer waveguide core layer, a three-layer cladding layer, and an electrode structure 600. The three cladding layers, from bottom to top, include a first cladding layer 501, a second cladding layer 502, and a third cladding layer 503, wherein: The first cladding layer 501 is disposed above the substrate layer 100 and covers the area around the first waveguide core layer 200; The second cladding layer 502 is disposed above the first cladding layer 501 and the first waveguide core layer 200, and covers the area around the second waveguide core layer 300; The third cladding layer 503 is disposed above the second cladding layer 502 and the second waveguide core layer 300, and covers both sides and the top of the third waveguide core layer 400; The three waveguide core layers are, from bottom to top, a first waveguide core layer 200, a second waveguide core layer 300, and a third waveguide core layer 400; each waveguide core layer includes, along the optical transmission direction, a connected input straight waveguide, at least one Y-branch waveguide, an intermediate waveguide, and an output straight waveguide. In the vertical direction, the input straight waveguides of each waveguide core layer are arranged to overlap vertically, and the output straight waveguides of each waveguide core layer are arranged to overlap vertically. The electrode structure 600 is located above the third cladding 503 and is positioned directly above the intermediate waveguide of the third waveguide core layer 400.

[0037] In this embodiment, the converter breaks through the limitations of the traditional two-dimensional plane in terms of spatial structure, and adopts a vertically stacked three-dimensional photonic integrated architecture.

[0038] Specifically, the converter is mainly composed of a substrate layer 100, three waveguide core layers, three cladding layers, and an electrode structure 600 at the top, from bottom to top. In terms of layering, the three cladding layers, from bottom to top, include a first cladding layer 501, a second cladding layer 502, and a third cladding layer 503. The first cladding layer 501 is directly formed on top of the flat surface of the substrate layer 100, completely covering the periphery of the first waveguide core layer 200. Subsequently, the second cladding layer 502 covers the surfaces of the first waveguide core layer 200 and the first cladding layer 501, similarly tightly covering the periphery of the second waveguide core layer 300. The topmost third cladding layer 503 covers the second waveguide core layer 300 and the second cladding layer 502, providing insulation and optical confinement to the sides and top of the third waveguide core layer 400. This invention, with its alternating growth of cladding and core layers, provides a highly stable physical carrier for the vertical transmission and interlayer coupling of optical signals. In the specific optical path design of the waveguide core layer, the three waveguide core layers are arranged sequentially from bottom to top as the first waveguide core layer 200, the second waveguide core layer 300, and the third waveguide core layer 400. To construct the cascaded Mach-Zehnder interferometer (MZI) optical path, each waveguide core layer includes, along the optical transmission direction (i.e., the Z-axis direction), a connected input straight waveguide, at least one Y-branch waveguide for splitting or combining light, an intermediate waveguide for interference, and an output straight waveguide.

[0039] In particular, to ensure precise alignment of mode evolution between different levels, the input straight waveguides of each waveguide core layer are strictly aligned vertically in the spatial projection, while the output straight waveguides of each waveguide core layer are also aligned vertically. The design of strictly aligning the input and output ends and generating an asymmetric path difference through the Y branch is the key geometric feature for realizing cross-layer high-order mode synthesis.

[0040] Furthermore, to achieve dynamic reconfigurability and reversible conversion between modes, the device is equipped with an electrode structure 600 at its top layer. This electrode structure 600 is precisely fabricated on the top surface of the third cladding layer 503. At its vertical projection position, by applying an external driving voltage to the electrode structure 600, the intermediate waveguide region of the third waveguide core layer 400 can be thermo-optically modulated independently, changing its effective refractive index and thus breaking the original phase balance between the interferometer arms. This allows for highly efficient bidirectional switching between the fundamental mode and higher-order vertical modes at the output end without altering the physical structure.

[0041] In some embodiments, the first waveguide core layer 200 includes a first input straight waveguide 201, a first Y-branch waveguide 202, a first intermediate waveguide 203, a second Y-branch waveguide 204, and a first output straight waveguide 205 connected in sequence.

[0042] Specifically, during the optical path travel, the externally input fundamental mode optical signal is first coupled into the first input straight waveguide 201 with low loss. Subsequently, the optical signal undergoes lateral path shift or mode broadening through the first Y-branch waveguide 202 and enters the longer first intermediate waveguide 203 for stable transmission. This first intermediate waveguide 203 essentially constitutes the bottom main interference arm of the entire three-dimensional asymmetric interferometer. After interference and transmission are completed, the optical signal passes through the second Y-branch waveguide 204 for wave combining and convergence, and is finally smoothly exited from the device by the first output straight waveguide 205. This not only ensures the low-loss characteristics of the bottom optical signal transmission but also provides a highly robust optical reference surface for the entire three-dimensional photonic chip. The curvature of the first Y-branch waveguide 202 and the second Y-branch waveguide 204 structurally corresponds to the interlayer coupling nodes of the second waveguide core layer 300 and the third waveguide core layer 400 in the vertical direction. This design enables precise phase matching of the light field during interlayer energy exchange and three-dimensional spatial interference, thereby significantly improving the device's tolerance to processing errors and the purity of high-order mode conversion.

[0043] In some embodiments, the intermediate waveguide of the second waveguide core layer 300 is a first interferometer waveguide 305; the second waveguide core layer 300 includes a second input straight waveguide 301, a third Y-branch waveguide 302, a first connecting waveguide 303, a fourth Y-branch waveguide 304, a first interferometer waveguide 305, a fifth Y-branch waveguide 306, a second connecting waveguide 307, a sixth Y-branch waveguide 308, and a second output straight waveguide 309 connected in sequence.

[0044] Specifically, the second waveguide core layer 300 acts as a crucial bridge in the device, laying a precise spatial wiring foundation for reversible cross-mode conversion in the vertical direction. During the actual transmission of the optical signal, after the optical field is introduced by the second input straight waveguide 301, it first enters a cascaded beam splitting and spatial broadening stage. The optical signal undergoes an initial lateral path shift through the third Y-branch waveguide 302, followed by a smooth optical transition and mode stabilization via the first connecting waveguide 303. Next, the optical signal is further guided by the fourth Y-branch waveguide 304 and precisely injected into the core first interferometer waveguide 305 for independent transmission. This not only effectively reduces light scattering loss caused by abrupt changes in the waveguide structure but also creates superior optical conditions for the long-distance stable travel and phase accumulation of the optical field within the first interferometer waveguide 305. After transmission through the first interferometer waveguide 305 and carrying specific phase information, the optical signal then enters the beam combining and mode reconstruction stage. The optical field passes sequentially through the fifth Y-branch waveguide 306 and the second connecting waveguide 307, which serves as a buffer and alignment mechanism. At the sixth Y-branch waveguide 308, the initial physical convergence of spatial energy is completed, and finally, it is smoothly coupled into the second output straight waveguide 309 and exported outward. The invention's double-cascaded broadening and convergence structure, consisting of four sets of Y-branch waveguides and two sets of connecting waveguides, greatly enhances the degree of freedom of energy distribution within the mode cross-section. This ensures that the optical field can achieve optimal phase matching when coupled in three-dimensional space with the bottom and top waveguides in the vertical dimension, thereby significantly improving the overall converter's operating bandwidth and cross-order conversion efficiency.

[0045] In some embodiments, the intermediate waveguide of the third waveguide core layer 400 is the second interferometer waveguide 405; the third waveguide core layer 400 includes a third input straight waveguide 401, a seventh Y-branch waveguide 402, a third connecting waveguide 403, an eighth Y-branch waveguide 404, a second interferometer waveguide 405, a ninth Y-branch waveguide 406, a fourth connecting waveguide 407, a tenth Y-branch waveguide 408 and a third output straight waveguide 409 connected in sequence.

[0046] Specifically, during the first half of the optical signal's transmission along the top layer, the optical field coupled into this layer is first constrained to its fundamental mode in the third input straight waveguide 401. Subsequently, the optical signal passes through a cascaded beam-splitting broadening structure consisting of the seventh Y-branch waveguide 402, the third connecting waveguide 403, and the eighth Y-branch waveguide 404, which are connected in series. This allows the beam to achieve extremely smooth expansion and mode evolution in both the lateral and vertical spaces, effectively suppressing radiation losses caused by abrupt changes in waveguide size. After a multi-stage smooth transition, the optical signal is precisely and with low loss guided to the core second interferometer waveguide 405 for stable transmission. Within this path, it receives refractive index perturbations applied by the upper electrodes to accumulate the critical dynamic phase difference required for mode transitions.

[0047] After the optical signal carries specific phase information and completes long-distance transmission within the second interferometer waveguide 405, it enters the latter half of the mode interference convergence and combined output stage. The phase-modulated optical field passes sequentially through the ninth Y-branch waveguide 406, the fourth connecting waveguide 407 which acts as a buffer and alignment, and completes the physical convergence and reconstruction of spatial optical energy at the tenth Y-branch waveguide 408. This allows the top-layer optical field with a specific phase to undergo high-precision three-dimensional spatial interference with the optical field in the lower waveguide core layer in the vertical dimension. Finally, the specific order mode optical field synthesized by interference is seamlessly injected and stably output outward from the third output straight waveguide 409.

[0048] In some embodiments, each Y-branch waveguide in the third waveguide core layer 400 structure is symmetrically distributed with each Y-branch waveguide in the second waveguide core layer 300 structure along the optical transmission direction; in the vertical direction, the third connecting waveguide 403 and the fourth connecting waveguide 407 of the third waveguide core layer 400 are respectively located directly above the first connecting waveguide 303 and the second connecting waveguide 307 of the second waveguide core layer 300.

[0049] Specifically, when the optical signal undergoes lateral branching and broadening evolution in the second waveguide core layer 300, the optical signal in the third waveguide core layer 400 above it also undergoes a spatial mirror-like branching direction at the same axial position. This not only ensures the high consistency of the upper and lower optical paths in terms of effective physical transmission length, but also provides an extremely important structural guarantee for the phase synchronization and precise matching of optical signals in the subsequent interference region.

[0050] In the vertical direction (i.e., the Y-axis direction), the third connecting waveguide 403 and the fourth connecting waveguide 407 of the third waveguide core layer 400 are respectively located directly above the first connecting waveguide 303 and the second connecting waveguide 307 of the second waveguide core layer 300. This allows the two waveguides to establish an extremely stable mode field distribution reference in their transition connection region after undergoing initial Y-branch spatial broadening. Through the precise vertical alignment of the upper and lower connecting waveguides, the device can effectively eliminate the additional phase noise caused by the misalignment of multilayer polymer spin coating and photolithography, thereby greatly improving the process tolerance of the device in the three-dimensional micro-nano fabrication process.

[0051] The symmetrical distribution along the transmission direction and the overlapping design of the upper part along the vertical direction allow the second waveguide core layer 300 and the third waveguide core layer 400 to jointly construct a highly synchronized and closely coordinated three-dimensional coupling pair in physical space. This enables the optical field to be distributed and combined in the asymmetric MZI structure at each level, perfectly combining the inherent path difference introduced by the bottom waveguide with the symmetrical evolution mechanism of the middle and upper waveguides. When the top electrode is not in operation, this symmetrical and aligned structure ensures high-fidelity transmission of the original input fundamental mode; while when a driving voltage is applied for thermo-optical phase modulation, the three-dimensional alignment structure ensures that the optical field carrying the new phase information is accurately reconstructed through cross-layer spatial interference at the output end, ultimately achieving reversible vertical high-order mode output with extremely high purity and low crosstalk characteristics.

[0052] In some embodiments, the materials of the substrate layer 100 and the three cladding layers are selected from polymethyl methacrylate (PMMA) and its derivatives, polydimethylsiloxane (PDMS), polyimide (PI), polycarbonate (PC), and EPOClad photoresist; the materials of the three waveguide core layers are selected from SU8, NOA, EPOCore photoresist, sol-gel, and resin materials; and the materials of the electrode structure 600 are selected from gold, silver, copper, aluminum, and their alloys.

[0053] Specifically, the materials of the substrate layer 100 and the three cladding layers are selected from polymethyl methacrylate (PMMA) and its derivatives, polydimethylsiloxane (PDMS), polyimide (PI), polycarbonate (PC), and EPOClad photoresist. When applied to the substrate layer 100 and the cladding structure, a dense optical cladding layer with a low refractive index can be formed around the waveguide core layer. This allows the light field energy to be firmly confined inside the waveguide core layer by relying on the principle of total internal reflection, effectively suppressing interlayer radiation and mode crosstalk. At the same time, it provides a solid and smooth physical substrate for the multilayer vertical stacking photolithography process.

[0054] The three-layer waveguide core is made of a material selected from SU8, NOA, EPOCore photoresist, sol-gel, and resin. As the essential carrier for optical signal transmission within the device, these polymeric core materials have relatively high refractive indices. They not only form a suitable refractive index difference with the cladding materials to enhance mode field confinement, but more importantly, they generally possess a thermo-optic coefficient an order of magnitude higher than that of traditional silicon-based materials. Furthermore, photosensitive polymer materials such as SU8 have excellent process compatibility with digital micromirror device (DMD) lithography systems. Through standardized steps such as spin coating, UV exposure, and development, three-dimensional waveguide cores with micro / nano-scale Y-branching and connecting segments can be fabricated at low cost and with high precision, greatly simplifying the manufacturing process of high-order mode converters.

[0055] The electrode structure 600 is made of a material selected from gold, silver, copper, aluminum, and their alloys. It is deposited onto the top cladding layer using mature processes such as electron beam evaporation, resulting in excellent interfacial adhesion with the polymer surface. When a small control voltage is applied externally, this metal electrode structure rapidly generates Joule heating and utilizes the high thermal response characteristics of the polymer material beneath it to efficiently and quickly change the effective refractive index of the interferometer waveguide directly below in a localized region. This not only significantly reduces the overall drive power consumption of the device but also endows the converter with a dynamic response capability for high-frequency, stable switching between the fundamental mode and vertical higher-order modes.

[0056] In some embodiments, the first waveguide core layer 200 has a thickness of 2-5 μm and a width of 2-8 μm; the second waveguide core layer 300 has a thickness of 3-10 μm and a width of 3-10 μm; and the third waveguide core layer 400 has a thickness of 3-10 μm and a width of 3-10 μm.

[0057] Specifically, the first waveguide core layer 200 has a thickness of 2-5 μm and a width of 2-8 μm. This not only effectively meets the strict transmission conditions of single-mode or quasi-single-mode in the optical communication band and suppresses the scattering of the underlying optical signal and intrinsic transmission loss to the greatest extent, but also provides a compact and high-energy-density basic mode field distribution for the upper optical waveguide layer, thus laying a solid physical size foundation for the precise coupling of subsequent cross-layer high-order modes.

[0058] For the middle layer structure of the device, the second waveguide core layer 300 has a thickness of 3–10 μm and a width of 3–10 μm. Compared to the first waveguide core layer 200, the second waveguide core layer 300 has a moderately relaxed upper limit range for its thickness and width. Defining its size between 3 and 10 micrometers significantly improves the cross-sectional capacity of this waveguide layer to accommodate optical field energy and reduces radiation losses at multi-level Y-branch and transition connection waveguides. Furthermore, this size range provides the waveguide with a high tolerance in polymer multilayer spin coating and photolithography alignment processes, effectively reducing interlayer crosstalk caused by micro-dimensional deviations or sidewall roughness.

[0059] In the top active modulation region, the thickness of the third waveguide core layer 400 is also set to 3–10 μm, and the width is set to 3–10 μm. This waveguide dimension setting not only maintains a high degree of geometric matching with the middle waveguide, but also fully utilizes the thermo-optical modulation efficiency above it. This waveguide thickness range ensures that the thermal field generated by the upper metal electrodes efficiently and uniformly penetrates into the core region of the entire interferometer waveguide, thereby maximizing the thermo-optical overlap integral factor and achieving rapid phase shift. More importantly, when the phase-modulated optical field and the optical fields of the bottom layers are reconstructed in three dimensions vertically at the output end, this parameter range ensures that the finally synthesized vertical higher-order modes (such as the E-mode) have optimal mode-field matching and extremely low inter-mode crosstalk.

[0060] By combining the three layers of specific dimensions, this converter can achieve efficient and reversible cross-mode conversion across a wide operating band while maintaining high fabrication tolerance.

[0061] This invention also provides a method for fabricating an MZI-type reversible fundamental-to-vertical higher-order mode converter, such as... Figure 2-4 As shown, the method includes the following steps: S1. Preparation of substrate layer 100: A polymer solution is spin-coated onto the surface of a glass substrate and then heated and cured to form the substrate layer 100; Specifically, a glass substrate is selected as the substrate and thoroughly cleaned using ultrasonic cleaning technology. The substrate is then subjected to ultrasonic treatment in acetone, anhydrous ethanol, and ultrapure deionized water for 5–15 minutes each. After cleaning, the glass substrate is removed, and its surface is purged with a high-purity nitrogen gun to completely remove any residual liquid. Subsequently, a prepared polymer solution (such as PMMA polymer solution) is dropped onto the center of the substrate. The spin coater is then started, initially rotating at low speed to achieve uniform solution spreading, then switching to high speed to form a uniform film. Finally, the substrate is transferred to a hot plate and heated at 80–100°C for 15–30 minutes to cure the adhesive film.

[0062] This step thoroughly removes organic contaminants and particulate impurities from the substrate surface. Nitrogen drying prevents water stains from forming spots, thereby significantly enhancing the adhesion between the polymer film and the substrate. The combination of high and low speed spin coating and hot plate curing allows for complete solvent evaporation, constructing a substrate layer 100 with uniform thickness, a smooth surface, and low absorption loss on the substrate, providing an ideal physical substrate for the subsequent stacked growth of three-dimensional optical waveguides.

[0063] S2. Preparation of the first waveguide core layer 200: Photoresist is spin-coated on the substrate layer 100, and then subjected to pre-baking, ultraviolet exposure based on the digital micromirror device (DMD) lithography system, post-baking and development treatment in sequence to form the first waveguide core layer 200. Specifically, the sample with substrate layer 100 is fixed on a spin coater, and waveguide core layer material (such as SU8 photoresist) is added and then spin-coated at high and low speeds. After spin coating, it is pre-baked at 50–80°C for 10–30 minutes. After cooling, it is transferred to a DMD lithography system, and the focusing calibration pattern is loaded. The Z-axis height of the stage is adjusted so that the focal plane precisely corresponds to the upper surface of the sample. Then, the vector pattern of the first waveguide core layer is imported, and exposed using a 365nm wavelength ultraviolet light source for 5–60 seconds. After exposure, it is post-baked at 60–100°C for 10–30 minutes. After cooling to room temperature, it is immersed in photoresist developer for 1–10 minutes, and finally rinsed with deionized water to form the first waveguide core layer pattern.

[0064] Pre-baking slowly and thoroughly evaporates the solvent in the polymer film, preventing contamination of the mask or lithography lens during exposure. The maskless lithography system based on DMD, combined with precise Z-axis focus calibration, overcomes the difficulties of traditional mask alignment, enabling high-precision exposure patterning of complex micron-scale Y-branch and straight waveguide structures. Post-baking promotes full cross-linking and curing of the photoresist in the exposed area, while the development step precisely removes uncross-linked areas, resulting in a high-quality bottom waveguide core structure with steep sidewalls and extremely low loss.

[0065] S3. Preparation of the first cladding layer 501: Spin-coating a polymer material onto a sample with the first waveguide core layer 200 and heating to cure it, forming the first cladding layer 501 covering the first waveguide core layer 200. Specifically: A first cladding material (such as PMMA) is spin-coated onto the surface of a sample where the first waveguide core layer 200 has been developed using a spin coater. The sample is then placed on a heating plate and cured by evaporating the solvent in the polymer film through constant temperature heating.

[0066] The material of the first cladding layer 501 has a lower refractive index than that of the core layer. Its spin-coating and curing can not only perfectly fill the gaps around the first waveguide core layer 200 and tightly wrap it to form an optical confinement layer that meets the total internal reflection condition, effectively reducing the radiation loss of mode transmission; at the same time, the cladding layer can also planarize the device surface, creating a smooth interface for the precise spin-coating and photolithography of the first waveguide core layer 200.

[0067] S4. Prepare the second waveguide core layer 300 and the second cladding layer 502: Following the same process as steps S2 and S3, prepare the second waveguide core layer 300 and the second cladding layer 502 surrounding the first waveguide core layer 300 and the first cladding layer 501 in sequence. Specifically, above the planarized first cladding layer 501, the second waveguide core layer pattern is formed by sequentially performing photoresist spin coating, pre-baking, DMD high-precision alignment exposure, post-baking, and development processes. Then, the second cladding material is spin-coated and heated to cure.

[0068] The use of consistent spin coating and photolithography processes ensures a high degree of matching between the thermal expansion coefficients and refractive indices of the interlayer materials. Through precise interlayer overlay alignment using the DMD photolithography system, the input / output straight waveguides of the second waveguide core layer 300 can be strictly overlapped with the first waveguide core layer 200 in the vertical direction, thereby constructing a high-precision cascaded interference and interlayer mode evolution path in the vertical dimension.

[0069] S5. Preparation of the third waveguide core layer 400 and the third cladding layer 503: Following the same process as steps S2 and S3, the third waveguide core layer 400 and the third cladding layer 503 covering both sides and the top of the third waveguide core layer 400 are sequentially prepared on top of the second waveguide core layer 300 and the second cladding layer 502. Specifically, the above-mentioned core layer and cladding fabrication process is repeated above the second cladding 502 to form a third waveguide core layer 400 with a multi-level branch structure, which is completely covered by a thicker third cladding material.

[0070] The physical construction of the top interferometer arm in the three-dimensional asymmetric MZI structure was completed. The thickness of the third cladding layer 503 not only prevents the evanescent wave of the top waveguide optical field from directly contacting the external environment and causing loss, but also provides an insulating and thermally conductive buffer layer for the deposition of the metal electrodes above, ensuring a high degree of symmetry and stability of the optical field distribution.

[0071] S6. Fabrication of electrode structure 600: A metal layer is deposited above the third cladding layer 503, followed by spin coating of positive photoresist on the surface of the metal layer and pre-baking; UV exposure, post-baking and development are performed using the DMD lithography system to remove some of the photoresist, followed by cleaning to remove residual photoresist, and the electrode structure 600 pattern is obtained directly above the intermediate waveguide of the third waveguide core layer 400.

[0072] Specifically, an aluminum layer is deposited on the surface of the third cladding 503 using an electron beam evaporation process, and the vacuum level inside the cavity needs to reach 1~ Pa, deposition thickness 100-300 nm. Then spin-coated BP212 positive photoresist, pre-baked at 50-80°C for 10-30 minutes to evaporate the solvent. The sample was transferred into the DMD lithography system for alignment and exposure for 5-30 seconds, followed by baking (80-95°C, 10-30 minutes) and cooling. After cooling, it was placed in the developer for 1-10 minutes to dissolve the exposed areas, and then cleaned with deionized water and anhydrous ethanol to remove residue.

[0073] Electron beam evaporation under high vacuum can produce high-purity metal films with dense grains and excellent conductivity, reducing the ohmic loss of the electrodes themselves. Using positive photoresist combined with DMD micro-nano exposure, the micro-heated electrode can be precisely positioned above the interferometer arm of the third waveguide core layer with extremely high alignment accuracy. The electrode structure fabricated by this process can efficiently convert applied electrical power into localized Joule heating, and utilize the significant thermo-optical effect of polymers to precisely control the effective refractive index of a single interferometer arm, thereby achieving low-power, high-efficiency reversible mode switching.

[0074] Test Example 1: Verification of Basic Transfer and Dynamic Mode Conversion Characteristics of the Device To quantitatively verify the optical performance of the mode converter described in this invention, a three-dimensional waveguide structure simulation model was constructed using the finite element method in this test example. A fundamental mode matching the 1550nm communication band was applied at the model input. ) and higher-order modes ( , The steady-state transmission characteristics of the device were systematically studied, and the specific test results are as follows:

[0075] 1. Basic Mode Transmission Verification (Electrode Off State): In the electrode-free modulation state, respectively... , , Three modes are input to the waveguide input terminal. For example... Figure 5-7 As shown, the optical field distribution results obtained from finite element simulation indicate that the transmission loss of each mode at a wavelength of 1550 nm is less than 0.1 dB / cm. This result verifies that the three-layer waveguide structure of this invention has excellent independent transmission capability for the above three modes.

[0076] 2. Mode switching path and dynamic response verification: The transmission path under different combinations of input modes and electrode states (on / off) was tested at cross sections of Y=1.5μm, Y=4.25μm, and Y=6.75μm.

[0077] Tests show that: When the electrode is in the off state: Input , , The modes correspond to the outputs respectively. , , The device maintains its original mode and transmits stably.

[0078] When the electrode is in the open state: (e.g.) Figure 8 As shown, the effective refractive index of the waveguide is adjusted by the thermo-optical effect generated by the micro-heating electrode, and the input... , , The modes correspond to the outputs respectively. , , The pattern demonstrates that this invention can be achieved without changing the physical structure. and Efficient and reversible cross-order transformation of the pattern.

[0079] Test Example 2: Verification of Device Process Tolerance and Broadband Transmission Characteristics exist With the fundamental mode input and electrode structure 600 in the off state (i.e., no external voltage applied, and the refractive indices of all interferometer waveguide layers consistent), the system tested the influence of the heights of the first waveguide core layer 200, the second waveguide core layer 300, and the third waveguide core layer 400, as well as the width of each waveguide layer, on the output mode in the three-layer MZI optical waveguide structure. The optimal range of values ​​for each parameter was determined, and the tolerance of the device structure to fabrication errors was verified. Furthermore, for this MZI-based reversible fundamental mode-to-vertical higher-order mode converter, the system tested its effect on the output mode when the input signal wavelength varied within the range of 1.1–2.2 μm. , , The influence of three input modes on output optical power was investigated to verify the broadband transmission characteristics of the device.

[0080] The specific test results are as follows: 1. First waveguide core layer, 200mm height The effects are as follows Figure 9 As shown: The width of each waveguide layer is controlled to be 3μm, and the heights of the second waveguide core layer 300 and the third waveguide core layer 400 are both 2.5μm. Only the height of the first waveguide core layer 200 is adjusted to vary within the range of 2 to 4μm. The output end is then tested. , , Normalized optical power of the mode. Results show that when 2.5 μm < When <3μm, The mode power is stable above 0.95, and , All mode crosstalk was below -20dB.

[0081] 2. The influence of the heights of the second waveguide core layer 300 and the third waveguide core layer 400 is shown in the attached figure. Figure 10 As shown: In this embodiment, the second waveguide core layer 300 and the third waveguide core layer 400 are set to the same height. The height of the first waveguide core layer 200 is fixed at 3μm, and the width of each waveguide layer is 3μm. The heights of the second waveguide core layer 300 and the third waveguide core layer 400 are adjusted to vary within the range of 1 to 4μm. Test results show that when the heights of the second waveguide core layer 300 and the third waveguide core layer 400 are in the range of 2.5 to 3.2μm, the output end... The mode power was consistently maintained above 0.95.

[0082] 3. The effect of waveguide width is shown in the attached figure. Figure 11 As shown: The height of the first waveguide core layer 200 is set to 3μm, and the heights of the second waveguide core layer 300 and the third waveguide core layer 400 are both 2.5μm. In this embodiment, the width of the first waveguide core layer 200 is set to be equal to the widths of the second waveguide core layer 300 and the third waveguide core layer 400, and the waveguide width is adjusted to vary within the range of 2 to 4μm. Test results show that when the waveguide width is between 2.5 and 3.6μm, The normalized power of the model is consistently greater than 0.95.

[0083] 4. Broadband transmission characteristic test: For the three-layer MZI type reversible fundamental-to-vertical higher-order mode converter provided in this invention, as shown in the attached... Figures 12-14 As shown, the system tested the input signal light wavelength in the range of 1.1–2.2 μm, and the results were as follows: , , Normalized output mode optical power curves for each of the three input modes are shown. When the signal light varies in wavelength range of 1.4–1.9 μm, the output optical power can be consistently greater than 0.9 μm, verifying that the device has excellent broadband transmission characteristics.

[0084] Overall test conclusion: The test results of this embodiment show that when the first waveguide core layer is 200mm high... μm, second waveguide core layer height 300 μm μm, third waveguide core layer 400 height μm, waveguide width of each layer At μm, The output power reaches a peak value of 0.985. Under this optimal parameter combination, the device exhibits output power stability greater than 0.95 in the 1.5–1.8 μm communication wavelength range.

[0085] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of protection of the present invention.

Claims

1. A reversible fundamental-to-vertical higher-order mode converter based on MZI type, characterized in that, The converter includes: a substrate layer, a three-layer waveguide core layer, a three-layer cladding layer, and an electrode structure; The three cladding layers, from bottom to top, include a first cladding layer, a second cladding layer, and a third cladding layer, wherein: The first cladding layer is disposed above the substrate layer and covers the area around the first waveguide core layer; The second cladding layer is disposed above the first cladding layer and the first waveguide core layer, and covers the area around the second waveguide core layer; The third cladding layer is disposed above the second cladding layer and the second waveguide core layer, and covers both sides and the top of the third waveguide core layer; The three waveguide core layers are, from bottom to top, the first waveguide core layer, the second waveguide core layer, and the third waveguide core layer; each waveguide core layer includes, along the optical transmission direction, a connected input straight waveguide, at least one Y-branch waveguide, an intermediate waveguide, and an output straight waveguide. In the vertical direction, the input straight waveguides of each waveguide core layer are arranged to overlap vertically, and the output straight waveguides of each waveguide core layer are arranged to overlap vertically. The electrode structure is located above the third cladding layer and is positioned directly above the intermediate waveguide of the third waveguide core layer.

2. The MZI-based reversible fundamental-to-vertical higher-order mode converter according to claim 1, characterized in that, The first waveguide core layer includes a first input straight waveguide, a first Y-branch waveguide, a first intermediate waveguide, a second Y-branch waveguide, and a first output straight waveguide connected in sequence.

3. The MZI-based reversible fundamental-to-vertical higher-order mode converter according to claim 1 or 2, characterized in that, The middle waveguide of the second waveguide core layer is the first interferometer waveguide; the second waveguide core layer includes a second input straight waveguide, a third Y-branch waveguide, a first connecting waveguide, a fourth Y-branch waveguide, a first interferometer waveguide, a fifth Y-branch waveguide, a second connecting waveguide, a sixth Y-branch waveguide, and a second output straight waveguide connected in sequence.

4. The MZI-based reversible fundamental-to-vertical higher-order mode converter according to claim 3, characterized in that, The middle waveguide of the third waveguide core layer is the second interferometer waveguide; the third waveguide core layer includes the third input straight waveguide, the seventh Y-branch waveguide, the third connecting waveguide, the eighth Y-branch waveguide, the second interferometer waveguide, the ninth Y-branch waveguide, the fourth connecting waveguide, the tenth Y-branch waveguide and the third output straight waveguide connected in sequence.

5. The MZI-based reversible fundamental-to-vertical higher-order mode converter according to claim 4, characterized in that, In the third waveguide core layer structure, each Y-branch waveguide is symmetrically distributed with each Y-branch waveguide in the second waveguide core layer structure along the optical transmission direction; in the vertical direction, the third connecting waveguide and the fourth connecting waveguide of the third waveguide core layer are located directly above the first connecting waveguide and the second connecting waveguide of the second waveguide core layer, respectively.

6. The MZI-based reversible fundamental-to-vertical higher-order mode converter according to claim 5, characterized in that, The substrate and the three-layer cladding are made of polymethyl methacrylate (PMMA) and its derivatives, polydimethylsiloxane (PDMS), polyimide (PI), polycarbonate (PC), and EPOClad photoresist; the three-layer waveguide core is made of SU8, NOA, EPOCore photoresist, sol-gel, and resin; and the electrode structure is made of gold, silver, copper, aluminum, and their alloys.

7. The MZI-based reversible fundamental-to-vertical higher-order mode converter according to claim 6, characterized in that, The first waveguide core layer has a thickness of 2–5 μm and a width of 2–8 μm; the second waveguide core layer has a thickness of 3–10 μm and a width of 3–10 μm; and the third waveguide core layer has a thickness of 3–10 μm and a width of 3–10 μm.

8. A method for fabricating a reversible fundamental-to-vertical higher-order mode converter based on any one of claims 1 to 7, characterized in that, The method includes the following steps: S1. Preparation of substrate layer: A polymer solution is spin-coated onto the surface of a glass substrate and then heated and cured to form the substrate layer; S2. Preparation of the first waveguide core layer: Photoresist is spin-coated on the substrate layer, and then subjected to pre-baking, ultraviolet exposure based on the digital micromirror device (DMD) lithography system, post-baking and development processes to form the first waveguide core layer. S3. Preparation of the first cladding: Spin-coating a polymer material onto a sample with the first waveguide core layer and heating to cure it to form the first cladding layer covering the first waveguide core layer. S4. Fabrication of the second waveguide core layer and the second cladding: Following the same process as steps S2 and S3, the second waveguide core layer and the second cladding layer surrounding the first waveguide core layer are sequentially fabricated above the first waveguide core layer and the first cladding layer. S5. Fabrication of the third waveguide core layer and the third cladding: Following the same process as steps S2 and S3, the third waveguide core layer and the third cladding covering both sides and the top of the third waveguide core layer are sequentially fabricated on top of the second waveguide core layer and the second cladding. S6. Electrode structure preparation: A metal layer is deposited above the third cladding layer, followed by spin coating of positive photoresist on the surface of the metal layer and pre-baking; UV exposure, post-baking and development are performed using the DMD lithography system to remove some of the photoresist, followed by cleaning to remove residual photoresist, and the electrode structure pattern is obtained directly above the intermediate waveguide of the third waveguide core layer.

9. The preparation method according to claim 8, characterized in that, In steps S2, S4 and S5, the wavelength of the light source for ultraviolet exposure is 365nm. Before performing the ultraviolet exposure, a focusing calibration pattern is loaded based on the DMD lithography system, and the focus is adjusted by observing the clarity of the projected image by adjusting the Z-axis height of the stage so that the focal plane corresponds to the upper surface of the sample.

10. The preparation method according to claim 8, characterized in that, In step S6, the metal layer is deposited using an electron beam evaporation process, and the vacuum level of the vacuum chamber during deposition is [missing information]. Pa to Pa; The positive photoresist is BP212 photoresist. After development, the development reaction is terminated with deionized water, and the surface is soaked in anhydrous ethanol solution to remove residual photoresist.

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