A mode-insensitive mode spot converter based on a two-dimensional grating waveguide

Abstract

A mode-insensitive spot size converter based on a two-dimensional grating waveguide, belonging to the technical field of planar optical waveguide devices. It is composed of a silicon wafer substrate, a silica lower cladding, a core straight waveguide, and a polymer upper cladding. The core straight waveguide is completely covered by the polymer upper cladding. Along the light transmission direction, the core straight waveguide is composed of a polymer outer core straight waveguide and an inner core straight waveguide covered by the polymer outer core straight waveguide. The inner core straight waveguide is composed of a two-dimensional grating waveguide and an output straight waveguide connected in sequence. The length of the inner core straight waveguide is less than that of the polymer outer core straight waveguide, and the output end faces of the output straight waveguide and the polymer outer core straight waveguide are located in the same plane. The device structure of the present invention gives full play to the advantages of the large transparent window and high thermal stability of the silicon nitride material, as well as the mature and compatible processing technologies of the polymer material and the silicon nitride material, providing a new idea for solving the coupling method between a few-mode waveguide-based photonic integrated chip and a few-mode optical fiber.

Description

Technical Field

[0001] The present invention belongs to the technical field of planar optical waveguide devices, and particularly relates to a mode-insensitive mode spot converter based on a two-dimensional grating waveguide, which uses a silicon wafer as a substrate, silicon dioxide as a lower cladding layer, silicon nitride as a waveguide core layer, an organic polymer as an outer waveguide core layer, and an organic polymer as an upper waveguide cladding layer. Background Art

[0002] In recent years, with the rapid development of new technologies and services such as 5G, big data, Internet of Things, and cloud computing, people have put forward higher requirements for the capacity and transmission rate of optical communication systems. In order to improve the communication capacity, technologies such as wavelength division multiplexing (WDM), time division multiplexing (TDM), and polarization division multiplexing (PDM) have been successively proposed and gradually matured. However, due to the influence of the nonlinear effect of optical fibers and the Shannon limit, traditional single-mode optical fibers still have limitations. In order to overcome the bottleneck of single-mode fiber communication, researchers have proposed mode division multiplexing (MDM) technology, that is, using multiple mutually orthogonal spatial modes to transmit information simultaneously, doubling the transmission capacity of the communication system. In order to further meet the requirements of large-capacity optical communication systems, on-chip MDM systems have also developed rapidly to flexibly process the optical signals of MDM systems.

[0003] Currently, the waveguide materials used for developing on-chip MDM systems mainly include lithium niobate, silicon nitride, SOI, and organic polymer materials, etc. Among them, silicon nitride has developed rapidly and received extensive attention in planar waveguide devices and on-chip MDM systems in recent years due to its advantages such as low transmission loss, small device size, wide transparent window, and compatibility with CMOS processes. Although silicon nitride has certain advantages in realizing device miniaturization and integration, due to its small size and large refractive index difference, it will lead to high coupling loss, which poses a challenge to the efficient coupling between on-chip MDM systems and few-mode optical fibers. In optical communication networks, mode spot converters are often used for inter-board optical interconnection, inter-module optical interconnection, and interconnection between chips and optical fibers, etc., and their coupling efficiency will directly affect the performance of various photonic integrated devices and systems. Although researchers have proposed mode spot converters based on structures such as three-dimensional tapered couplers, suspended tapered couplers, inverted tapered array waveguides, and stepped structures, there are still problems such as high manufacturing cost, large process difficulty, and narrow bandwidth, and most devices can only achieve mode spot conversion of fundamental mode signals, and it is a certain challenge to achieve efficient mode spot conversion between few-mode waveguides and few-mode optical fibers.

[0004] The present invention uses a hybrid structure of two-dimensional grating waveguide and polymer waveguide to design a mode spot converter, and utilizes the advantages of low manufacturing cost, easy integration, and high coupling efficiency with optical fibers of polymer waveguides to achieve high-efficiency mode spot conversion of different modes, and the mode field shape does not change after passing through the mode spot converter, which has important application value in MDM systems. Summary of the Invention

[0005] In order to overcome the deficiencies of the prior art, the purpose of the present invention is to provide a mode-insensitive mode spot converter based on a two-dimensional grating waveguide. The device structure connects a polymer waveguide and a silicon nitride waveguide, realizing low-loss mode spot conversion for multiple modes, and solving the problems of few supported modes, polarization sensitivity, and narrow bandwidth in existing devices.

[0006] The present invention uses a two-dimensional grating waveguide as the basic structure of the device. Due to its mature fabrication process and stable device performance, it has crucial value in photonic integrated chips and optical communication networks. Based on the diffraction principle of light, the light in the large-size polymer waveguide is coupled to the silicon nitride two-dimensional grating waveguide part. After the light is coupled in the two-dimensional grating waveguide region, it is transmitted to the silicon nitride waveguide, thus solving the limitation that traditional mode spot converters can only transmit the fundamental mode.

[0007] The present invention uses a silicon wafer as the device substrate, silicon dioxide as the lower cladding material of the waveguide, silicon nitride as the inner core layer of the waveguide, an organic polymer as the outer core layer of the waveguide, and a polymer material different from the outer core layer polymer as the upper cladding of the waveguide. The present invention gives full play to the advantages of low transmission loss, wide transparent window, and good thermal stability of the silicon nitride waveguide, as well as the advantages of a variety of polymer materials and simple fabrication processes, and the characteristics that these two materials have mature processes and are mutually compatible, suitable for large-scale preparation and production, and have important application prospects.

[0008] As Figure 1 shown, a mode-insensitive mode spot converter based on a two-dimensional grating waveguide is composed of a silicon wafer substrate 31 from bottom to top, a silicon dioxide lower cladding 32 prepared on the silicon wafer substrate 31, a core layer straight waveguide prepared on the silicon dioxide lower cladding 32, and a polymer upper cladding 35 prepared on the silicon dioxide lower cladding 32 and the core layer straight waveguide. The core layer straight waveguide is completely covered by the polymer upper cladding 35; along the light transmission direction, the core layer straight waveguide is composed of a polymer outer core layer straight waveguide 1 and a silicon nitride inner core layer straight waveguide covered by the polymer outer core layer straight waveguide 1. The silicon nitride inner core layer straight waveguide is composed of a two-dimensional grating waveguide 2 and an output straight waveguide 3 connected in sequence; the length (X-axis direction) of the silicon nitride inner core layer straight waveguide is less than the length of the polymer outer core layer straight waveguide 1, and the output end faces of the output straight waveguide 3 and the polymer outer core layer straight waveguide 1 are located in the same plane.

[0009] As Figure 2 shown, the length L of the polymer outer core layer straight waveguide 1 1is 1.6 to 3 cm; the two-dimensional grating waveguide 2 is a series of groove arrays with the same size etched in the silicon nitride inner core layer straight waveguide in a plane parallel to the upper surface of the silicon wafer substrate 31. The grooves are uniformly arranged in both the direction parallel to the light transmission direction (X-axis direction) and the direction perpendicular to it (Y-axis direction). The interval distance between adjacent grooves parallel to the light transmission direction is the same as the groove length, and the interval distance between adjacent grooves perpendicular to the light transmission direction is the same as the groove width. The length L of the two-dimensional grating waveguide 2 2 is 8 to 30 μm, and the length L of the output straight waveguide 3 3 is 0.8 to 1.5 cm.

[0010] As Figure 3 shown, it is a schematic diagram of the silicon nitride inner core layer structure. The interval distance between adjacent grooves parallel to the light transmission direction in the two-dimensional grating waveguide 2 is the same as the groove length W 1 which is 0.5 to 1 μm, and the interval distance between adjacent grooves perpendicular to the light transmission direction is the same as the groove width W 2 which is 0.08 to 0.15 μm, and adjacent grooves are staggered along the light propagation direction and do not overlap with each other; the number of rows N of the grooves parallel to the light transmission direction x is 16 to 30, and the number of columns N of the grooves perpendicular to the light transmission direction y is 40 to 60;

[0011] As Figure 1 shown, Figure 1 (a), Figure 1 (b) and Figure 1 (c) are respectively the waveguide cross-sectional views at the positions of A-A', B-B', and C-C' in Figure 2 . The width W of the polymer outer core layer straight waveguide 1 3 is 3.5 to 10 μm; the width W of the silicon nitride inner core layer straight waveguide 4 is 3.2 to 9 μm; the thickness (Z-axis direction) of the silicon wafer substrate 31 is 0.6 to 1 mm, the thickness (Z-axis direction) of the silicon dioxide lower cladding 32 is 3 to 5 μm, the thickness (Z-axis direction) of the silicon nitride inner core layer straight waveguide is 0.2 to 0.6 μm, the thickness (Z-axis direction) of the polymer outer core layer straight waveguide 1 located above the silicon dioxide lower cladding 32 is 2 to 3 μm, and the thickness (Z-axis direction) of the polymer upper cladding 35 is 3 to 5 μm.

[0012] When light is input from the polymer outer core layer straight waveguide 1 and passes through the two-dimensional grating waveguide 2 of the silicon nitride inner core layer, the light is diffracted from the polymer outer core layer straight waveguide 1 to the two-dimensional grating waveguide 2. The two-dimensional grating can achieve high-efficiency coupling of two different polarization modes of light. The optimized two-dimensional grating structure can make the light of different modes in the diffraction process maintain the mode unchanged in the inner core layer waveguide after coupling, and then transmit the light to the output straight waveguide 3. The mode spot of the light is converted before and after coupling without changing the mode, thereby realizing a mode-insensitive mode spot conversion function.

[0013] The process flow of manufacturing a mode-insensitive pattern spot converter according to the present invention is shown in Figure 4 , specifically described as follows:

[0014] A: Cleaning treatment of the surface of the lower silica cladding

[0015] Use a cotton ball soaked in acetone to clean the surface of the silicon dioxide substrate (composed of a silicon wafer substrate 31 and a silicon dioxide lower cladding layer 32 located on the silicon wafer substrate 31), then use a cotton ball soaked in ethanol to clean the surface of the silicon dioxide substrate, finally rinse the substrate with deionized water to make the substrate surface clean, then blow dry with nitrogen, and seal it in a clean culture dish;

[0016] B: Silicon nitride film preparation

[0017] Using the LPCVD method, a stoichiometric ratio of Si is deposited on the silicon dioxide lower cladding layer 32 at 750-850°C. 3 N 4 The silicon nitride film deposited has a thickness of 0.2 to 0.6 μm;

[0018] C: Fabrication of straight waveguide with silicon nitride inner core

[0019] The positive photoresist BP218 is spin-coated on the silicon nitride film by a spin coating process. The parameters of the coating machine are first set to 300-600rpm, the acceleration time is 2-5 seconds, and the uniform speed time is 10-15 seconds; then the rotation speed is set to 1000-2000rpm, the acceleration time is 5-10 seconds, and the uniform speed time is 10-30 seconds; then the time for deceleration to 0 is set to 10-30 seconds; after the spin coating is completed, the substrate is placed on a heating table for pre-baking (i.e., heating at 60-80°C for 1-2 minutes by a step-by-step heating method, and then heating at 110-130°C for 2-3 minutes, and after the heating is completed, it is placed at room temperature for natural cooling for 1-2 hours); the photoresist film is subjected to plate alignment photolithography. In the present invention, a contact photolithography machine is used for exposure, the working wavelength is ultraviolet light of 350-400nm, the exposure time is set to 10-25 seconds, and the mask is a silicon nitride inner core layer straight waveguide to be prepared (such as Figure 3The structure shown) is such that the area outside the silicon nitride inner core layer straight waveguide is fully exposed; after lithography, the substrate is removed and post-baking is carried out (that is, heating is carried out in a stepwise manner at 60 - 80 °C for 1 - 2 minutes, then at 120 - 140 °C for 2 - 3 minutes, and after the heating is completed, it is placed at room temperature to cool naturally for 1 - 2 hours); after cooling, development is carried out. The substrate is placed in a BP218 photoresist developer solution for wet etching for 20 - 30 seconds to remove the photoresist in the exposed part. After development, the substrate must be taken out immediately and rinsed several times with deionized water (rinsing should be carried out along the waveguide direction to prevent damage to the waveguide), washing away impurities such as the remaining developer solution on the substrate, and drying the remaining deionized water on the substrate with nitrogen; finally, the cleaned substrate is placed on a drying table for drying operation. This step is called hard baking, which enhances the adhesion of the photoresist and at the same time improves the stability of the remaining part of the photoresist in subsequent processing steps (has stronger corrosion resistance), and will also make the photoresist close to the molten state with clear edge contours. The hard baking time is set to 2 - 4 minutes, and the hard baking temperature is set to 100 - 140 °C. After heating is completed, it is cooled naturally at room temperature for 1 - 2 hours;

[0020] After hard baking is completed, RIE etching (reactive ion etching, which has the advantages of strong anisotropy and selectivity at the same time) is carried out to process the substrate, exposing the upper surface of the silica lower cladding 32 outside the silicon nitride inner core layer straight waveguide structure; trifluoromethane (CHF 3 ) is selected as the etching gas, the gas flow rate is 80 - 130 sccm, the etching power is 110 - 160 W, the chamber pressure is 2.4 Pa, and the etching time is 12 - 15 minutes; after etching is completed, the substrate is subjected to a degumming operation, that is, the substrate is immersed in an organic solvent for 2 - 3 minutes while gently shaking the substrate, and then the substrate is cleaned with deionized water and the remaining deionized water on the substrate is dried with nitrogen; then, the residual photoresist on the silicon nitride inner core layer straight waveguide is bombarded by oxygen plasma under the acceleration of the electric field to further remove the photoresist. The gas flow rate is 60 - 75 sccm, the etching power is 75 - 90 W, the chamber pressure is 8 - 12 Pa, and the etching time is 9 - 13 minutes; after oxygen plasma etching, the substrate needs to be cleaned again with deionized water and the remaining deionized water on the substrate is blown clean. In this way, the silicon nitride inner core layer straight waveguide 33 of the target structure is prepared on the silica lower cladding 32, which consists of two parts: a two-dimensional grating waveguide 2 and an output straight waveguide 3;

[0021] D: Preparation of the polymer outer core layer straight waveguide

[0022] The polymer outer core layer straight waveguide material (a series of ultraviolet negative photoresist materials such as EpoCore, EpoClad, SU-8 2002, SU-8 2005, NOA, etc. that can be wet-etched and have a refractive index higher than that of the polymer upper cladding) is spin-coated on the substrate with the prepared silicon nitride inner core layer straight waveguide by using the spin-coating process, and the polymer outer core layer straight waveguide material is evenly filled into the groove array in the two-dimensional grating waveguide 2; the rotation speed of the spin coater is first set to 1000 - 1500 rpm, the acceleration time is 2 - 5 seconds, and the constant speed time is 10 - 20 seconds; then the rotation speed is set to 2000 - 3500 rpm, the acceleration time is 5 - 10 seconds, and the constant speed time is 20 - 30 seconds; then the time for decelerating to 0 is set to 20 - 30 seconds; the thickness of the formed polymer outer core layer straight waveguide film is 2 - 3 μm; the spin-coated substrate is heated by using a stepwise heating method, that is, heated at 60 - 80 °C for 10 - 15 minutes, and then heated at 100 - 120 °C for 20 - 30 minutes. After the heating is completed, the substrate is placed at room temperature for natural cooling, and the cooling time is 1 - 2 hours; the polymer outer core layer straight waveguide film is lithographed by using contact lithography. The working wavelength of the lithography machine is ultraviolet light with a wavelength of 350 - 400 nm, the exposure time is set to 5 - 10 seconds, and the mask is the polymer outer core layer straight waveguide structure to be prepared, so that the material in the polymer outer core layer straight waveguide structure area is exposed; after the substrate is completed with lithography, it is taken down and post-baked and heated, that is, heated at 60 - 100 °C for 10 - 15 minutes, and then heated at 100 - 130 °C for 20 - 30 minutes. After the heating is completed, the substrate is placed at room temperature for natural cooling, and the cooling time is 1 - 2 hours; after the temperature is reduced, the substrate is developed, and the substrate is placed in the corresponding developer for wet etching. The development time is 15 - 30 seconds, and the non-retained area (the area outside the polymer outer core layer straight waveguide structure) that is not exposed is removed. Then the substrate is put into an isopropyl alcohol solution to wash away the residual outer core layer straight waveguide material and developer on the substrate surface; finally, it is rinsed many times with deionized water along the waveguide direction (when rinsing, it should be rinsed along the waveguide direction to prevent the waveguide from being damaged), and impurities such as isopropyl alcohol on the substrate surface are removed, and then dried with nitrogen; finally, the hardening operation is carried out, that is, heated at 120 - 140 °C for 20 - 40 minutes, and the substrate is placed at room temperature for natural cooling, and the cooling time is 1 - 2 hours. In this way, a polymer outer core layer straight waveguide 1 with a thickness of 2 - 3 μm (the thickness above the silicon dioxide lower cladding 32) and a width of 3.5 - 10 μm is fabricated on the silicon nitride waveguide inner core layer straight waveguide and the silicon dioxide lower cladding 32;

[0023] E: Preparation of the polymer upper cladding

[0024] The polymer upper cladding material (a series of organic polymer materials with good transparency, including polymethyl methacrylate (PMMA), polyethylene (PE), polyester (PET), polycarbonate (PC), polyimide (PI), polystyrene (PS), etc.) is spin-coated on the substrate on which the polymer outer core layer straight waveguide 1 has been prepared by spin coating. The spin coating speed is 2000 - 4000 rpm, and then it is heated at 120 - 150 °C for 20 - 40 minutes to obtain the polymer upper cladding 35. The thickness of the polymer upper cladding 35 located above the polymer lower cladding 32 is 3 - 5 μm.

[0025] Compared with the existing device structures and fabrication technologies, the beneficial effects of the present invention are as follows: The mode-insensitive mode spot converter structure described in the present invention fully exploits the advantages of the silicon nitride material having a large transparent window, high thermal stability, and the mature and compatible processing technologies of the polymer material and the silicon nitride material. Compared with the existing mode spot converters, in the present invention, a two-dimensional grating waveguide is adopted to diffract the light in the polymer waveguide into the two-dimensional grating waveguide of the silicon nitride inner core layer. After coupling, the light is transmitted to the silicon nitride waveguide 3. In addition to enabling high-efficiency mode spot conversion for different modes, it can also ensure that the mode field shape remains unchanged before and after mode spot conversion. Moreover, the process of the present invention is simple, mature, and stable, only requiring conventional processes such as spin coating, photolithography, and etching, without the need to fabricate multilayer waveguides, reducing the fabrication error. In addition, the present invention solves the problems of low optical coupling efficiency, large crosstalk, and complex process for the mode spot converter in the on-chip optical communication system, expands the applications related to the mode of the silicon nitride platform on-chip integrated optical devices, and also provides a new idea for solving the coupling method between the few-mode waveguide-based photonic integrated chip and the few-mode optical fiber. Brief Description of the Drawings

[0026] Figure 1 : Schematic cross-sectional structure diagram of a mode-insensitive mode spot converter described in the present invention;

[0027] Figure 1 (a): Figure 2 Schematic cross-sectional diagram of the waveguide at the A - A' position in ;

[0028] Figure 1 (b): Figure 2 Schematic cross-sectional diagram of the waveguide at the B - B' position in ;

[0029] Figure 1 (c): Figure 2 Schematic cross-sectional diagram of the waveguide at the C - C' position in ;

[0030] Figure 2 : Schematic structure diagram of a mode-insensitive mode spot converter described in the present invention;

[0031] Figure 3: Schematic diagram of the straight waveguide structure of the core layer of a mode-insensitive mode spot converter made of silicon nitride according to the present invention;

[0032] Figure 4 : Process flow chart for the preparation of a mode-insensitive mode spot converter according to the present invention;

[0033] Figure 5 : Simulation diagrams of the optical field distribution of the input waveguide of a mode-insensitive mode spot converter based on a two-dimensional grating waveguide for TE 0 mode light (a), simulation diagram of optical field transmission (b), and simulation diagram of the optical field distribution of the output waveguide (c);

[0034] Figure 6 : Simulation diagrams of the optical field distribution of the input waveguide of a mode-insensitive mode spot converter based on a two-dimensional grating waveguide for TE 1 mode light (a), simulation diagram of optical field transmission (b), and simulation diagram of the optical field distribution of the output waveguide (c);

[0035] Figure 7 : Simulation diagrams of the optical field distribution of the input waveguide of a mode-insensitive mode spot converter based on a two-dimensional grating waveguide for TE 2 mode light (a), simulation diagram of optical field transmission (b), and simulation diagram of the optical field distribution of the output waveguide (c);

[0036] Figure 8 : Simulation diagrams of the optical field distribution of the input waveguide of a mode-insensitive mode spot converter based on a two-dimensional grating waveguide for TM 0 mode light (a), simulation diagram of optical field transmission (b), and simulation diagram of the optical field distribution of the output waveguide (c);

[0037] Figure 9 : Simulation diagrams of the optical field distribution of the input waveguide of a mode-insensitive mode spot converter based on a two-dimensional grating waveguide for TM 1 mode light (a), simulation diagram of optical field transmission (b), and simulation diagram of the optical field distribution of the output waveguide (c);

[0038] Figure 10 : Simulation diagrams of the optical field distribution of the input waveguide of a mode-insensitive mode spot converter based on a two-dimensional grating waveguide for TM 2 mode light (a), simulation diagram of optical field transmission (b), and simulation diagram of the optical field distribution of the output waveguide (c);

[0039] Figure 11 : TE in a mode-insensitive mode spot converter based on a two-dimensional grating waveguide 0 、TE 1 、TE 2 、TM 0 、TM 1 and TM 2Graph of the transmission efficiency of six modes varying with wavelength in the wavelength range of 1500 - 1620 nm. Detailed implementation mode

[0040] Example 1

[0041] The present invention will be further described below in conjunction with the drawings and examples.

[0042] As Figure 1 shown, Figure 1 (a), Figure 1 (b), Figure 1 (c) are respectively cross-sectional schematic diagrams of the waveguides at positions A - A', B - B', and C - C' in Figure 2 . The width W 3 of the polymer outer core layer straight waveguide 1 is 6 μm; the width W 4 of the silicon nitride inner core layer straight waveguide is 5.4 μm. The thickness of the silicon wafer substrate 31 is 1 mm, the thickness of the silicon dioxide lower cladding 32 is 5 μm, the thickness of the silicon nitride waveguide inner core layer 33 is 0.3 μm, the thickness of the SU - 8 strip outer core layer straight waveguide 1 located above the silicon dioxide lower cladding (32) is 2.5 μm, and the thickness of the polymer PMMA upper cladding 35 located above the silicon dioxide lower cladding (32) is 3 μm.

[0043] As Figure 2 shown, the length L 1 of the polymer outer core layer straight waveguide 1 of the mode-insensitive mode spot converter is 1.8 cm; the length L 2 of the silicon nitride inner core layer two-dimensional grating waveguide 2 is 16.8 μm; the length L 3 of the silicon nitride inner core layer output straight waveguide 3 is 0.8 cm.

[0044] As Figure 3 shown, the groove length in the two-dimensional grating waveguide is equal to the groove interval distance parallel to the light propagation direction, W 1 is 0.7 μm; the groove width is equal to the groove interval distance perpendicular to the light propagation direction in the plane, W 2 is 0.1 μm; in the silicon nitride inner core layer two-dimensional grating waveguide 2, the number of rows N x of the etched grooves parallel to the light propagation direction is 24; the number of columns N y of the etched grooves perpendicular to the light propagation direction in the plane is 54.

[0045] Light is input from the polymer outer core layer straight waveguide 1 and, when passing through the silicon nitride inner core layer two-dimensional grating waveguide 2, diffracts from the polymer into the silicon nitride two-dimensional grating waveguide 2, and the coupled light is transmitted into the silicon nitride output straight waveguide 3.

[0046] As Figure 5As shown, it can be seen from the simulation results that TE 0 mode light can perform high-efficiency mode spot conversion with an efficiency of 89.1% @ 1550 nm.

[0047] As Figure 6 shown, it can be seen from the simulation results that TE 1 mode light can perform high-efficiency mode spot conversion with an efficiency of 89.2% @ 1550 nm.

[0048] As Figure 7 shown, it can be seen from the simulation results that TE 2 mode light can perform high-efficiency mode spot conversion with an efficiency of 87.5% @ 1550 nm.

[0049] As Figure 8 shown, it can be seen from the simulation results that TM 0 mode light can perform high-efficiency mode spot conversion with an efficiency of 94.6% @ 1550 nm.

[0050] As Figure 9 shown, it can be seen from the simulation results that TM 1 mode light can perform high-efficiency mode spot conversion with an efficiency of 95.1% @ 1550 nm.

[0051] As Figure 10 shown, it can be seen from the simulation results that TM 2 mode light can perform high-efficiency mode spot conversion with an efficiency of 94.8% @ 1550 nm.

[0052] As Figure 11 shown, for the mode-insensitive mode spot converter of the present invention when transmitting six modes of light including TE 0 、TE 1 、TE 2 、TM 0 、TM 1 、TM 2 the transmission efficiency is greater than 87% in the wavelength range of 1500 - 1620 nm. It can be seen that the mode-insensitive mode spot converter of the present invention realizes the mode-insensitive, high-efficiency, low-loss, and large-bandwidth mode spot conversion function.

[0053] Example 2

[0054] The present invention will be further described below in conjunction with the figures and examples. Cleaning treatment of the surface of the silica lower cladding: Wipe and clean the silica substrate with a cotton ball dipped in acetone, then wipe and clean the substrate with a cotton ball dipped in ethanol, and finally rinse the substrate with deionized water to make the surface of the substrate clean. Then dry the substrate with nitrogen and place it in a clean petri dish and seal it;

[0055] Fabrication of silicon nitride inner core layer straight waveguide: Deposit a silicon nitride thin film using the LPCVD method. When the temperature reaches 800 °C, deposit a silicon nitride thin film with a stoichiometric ratio of Si 3 N 4 , and the thickness of the deposited silicon nitride thin film is 0.3 μm. Spin coat the positive photoresist BP218 on the silicon nitride thin film using the spin coating process. The parameters of the spin coater are first set to 400 rpm, with an acceleration time of 5 seconds and a constant speed time of 15 seconds; then set the rotation speed to 1200 rpm, with an acceleration time of 10 seconds and a constant speed time of 20 seconds; then set the deceleration time to 0 to be 10 seconds. After spin coating, place the substrate on the heating table for pre-baking. Use a stepwise heating method to heat at 80 °C for 2 minutes, then at 120 °C for 3 minutes. After heating, place it at room temperature and let it cool naturally for 2 hours. Perform alignment lithography on the photoresist film. In the present invention, a contact lithography machine is used for exposure, with ultraviolet light having a working wavelength of 350 - 400 nm, and the exposure time is set to 12 seconds. The mask is the silicon nitride inner core layer straight waveguide to be fabricated (as Figure 3 shown), so that the area outside the silicon nitride inner core layer straight waveguide is fully exposed. After lithography, remove the substrate and perform post-baking. Use a stepwise heating method to heat at 70 °C for 2 minutes, then at 120 °C for 3 minutes. After heating, place it at room temperature and let it cool naturally for 2 hours. After cooling, perform development. Place the substrate in the BP218 photoresist developer for wet etching for 20 seconds to remove the photoresist in the exposed part. Immediately remove the substrate after development and rinse it with deionized water multiple times (when rinsing, rinse along the waveguide direction to prevent damage to the waveguide) to wash away impurities such as the remaining developer on the substrate, and finally dry the residual deionized water on the substrate with nitrogen. Finally, place the cleaned substrate on the drying table for drying operation. This step is called hard baking, which enhances the adhesion of the photoresist and at the same time improves the stability of the remaining photoresist in subsequent processing steps (with stronger corrosion resistance), and also makes the photoresist close to the molten state with clear edge contours. The hard baking time is set to 3 minutes, and the hard baking temperature is set to 130 °C. After heating, let it cool naturally at room temperature for 2 hours;

[0056] After hard baking, start RIE etching (reactive ion etching, which has the advantages of strong anisotropy and selectivity) to process the substrate. Select trifluoromethane (CHF 3)As the etching gas, the selected gas flow rate is 90 sccm, the etching power is 140 W, the chamber pressure is 2.4 Pa, the etching time is 13 minutes. After the etching is completed, the substrate is subjected to a degumming operation. The substrate is immersed in an organic solvent for 3 minutes, and the substrate is gently shaken at the same time. Then, the substrate is cleaned with deionized water and the residual deionized water on the substrate is blown dry with nitrogen; then, the residual photoresist on the silicon nitride inner core layer straight waveguide is bombarded by oxygen plasma under the acceleration of the electric field to further remove the photoresist. The gas flow rate is 70 sccm, the etching power is 80 W, the chamber pressure is 11 Pa, and the etching time is 11 minutes; after the oxygen plasma etching, the substrate needs to be cleaned again with deionized water, and the residual deionized water on the substrate is blown clean with nitrogen. In this way, the silicon nitride inner core layer straight waveguide with the target structure is prepared on the silicon dioxide lower cladding;

[0057] Preparation of the polymer outer core layer straight waveguide: The polymer core layer material SU-8 is spin-coated on the substrate on which the silicon nitride inner core layer straight waveguide has been prepared by the spin-coating process. The rotation speed of the spin coater is first set to 1300 rpm, the acceleration time is set to 5 seconds, and the uniform speed time is set to 15 seconds; then the rotation speed is set to 2800 rpm, the acceleration time is set to 8 seconds, and the uniform speed time is set to 25 seconds; then the time to decelerate to 0 is set to 25 seconds; the thickness of the formed polymer thin film is 2.5 μm; the spin-coated substrate is heated by a stepwise heating method, heated at 70 °C for 10 minutes, and then heated at 110 °C for 25 minutes. After the heating is completed, the substrate is placed at room temperature for natural cooling treatment, and the cooling time is 2 hours; the polymer thin film is subjected to alignment lithography, and contact lithography is used for processing. The working wavelength of the lithography machine is ultraviolet light with a wavelength of 350-400 nm, the exposure time is set to 6 seconds, and the mask is the polymer outer core layer straight waveguide structure to be prepared, so that the material in the required polymer strip waveguide area is exposed; after the substrate is completed with lithography, it is taken down, and the substrate is post-baked and heated, heated at 80 °C for 15 minutes, and then heated at 120 °C for 25 minutes. After the heating is completed, the substrate is placed at room temperature for natural cooling treatment, and the cooling time is 2 hours; after the temperature is lowered, the substrate is developed, and the substrate is placed in the corresponding developer for wet etching. The development time is 16 seconds, and the non-retained area (the part other than the polymer outer core layer straight waveguide) that is not exposed is removed. Then, the substrate is put into an isopropyl alcohol solution to wash away the residual optical waveguide core layer material and developer on the surface of the substrate; finally, it is rinsed many times with deionized water along the waveguide direction (when rinsing, it should be rinsed along the waveguide direction to prevent the waveguide morphology from being damaged) to remove impurities such as isopropyl alcohol on the surface of the silicon wafer, and then dried with nitrogen; finally, a hardening operation is carried out, heated at 130 °C for 30 minutes, and the substrate is placed at room temperature for natural cooling treatment, and the cooling time is 2 hours. In this way, a 2.5-μm polymer outer core layer strip waveguide is fabricated on the silicon nitride planar waveguide core layer;

[0058] Preparation of polymer upper cladding: using a spin coating process to spin-coat the polymer upper cladding material PMMA on the substrate on which the polymer outer core straight waveguide has been prepared, the spin coating speed is 4000rpm, and then heated at 130°C for 30 minutes, the thickness of the polymer upper cladding is 3μm, thereby obtaining a mode-insensitive spot converter according to the present invention;

[0059] It should be pointed out that the specific implementation is only a representative example of the present invention. Obviously, the technical solution of the present invention is not limited to the above-mentioned embodiment, and there are many variations. For example, the inner core layer material adopts waveguide materials such as lithium niobate, titanium dioxide, silicon carbide, and silicon, the polymer outer core layer material adopts EpoCore, SU-8 2002, etc., and the upper cladding material adopts a series of organic polymer materials with good transparency including polycarbonate, polyimide (PI), polyethylene (PE), etc. Those skilled in the art who have obtained the invention clearly disclosed or obtained without objection based on the written description of the document belong to the scope of protection of this patent.

Claims

1. A mode-insensitive spot size converter based on a two-dimensional grating waveguide, characterized in that: from bottom to top, it is composed of a silicon wafer substrate (31), a silica lower cladding (32) prepared on the silicon wafer substrate (31), a core layer straight waveguide prepared on the silica lower cladding (32), and a polymer upper cladding (35) prepared on the silica lower cladding (32) and the core layer straight waveguide. The core layer straight waveguide is completely covered by the polymer upper cladding (35); along the light transmission direction, the core layer straight waveguide is composed of a polymer outer core layer straight waveguide (1) and an inner core layer straight waveguide covered by the polymer outer core layer straight waveguide (1). The inner core layer straight waveguide is composed of a two-dimensional grating waveguide (2) and an output straight waveguide (3) connected in sequence; the length of the inner core layer straight waveguide is less than the length of the polymer outer core layer straight waveguide (1), and the output end surfaces of the output straight waveguide (3) and the polymer outer core layer straight waveguide (1) are located in the same plane; the two-dimensional grating waveguide (2) is a series of grooves with the same size etched in the inner core layer straight waveguide in a plane parallel to the upper surface of the silicon wafer substrate (31). The grooves are uniformly arranged in the direction parallel to and perpendicular to the light transmission direction. The interval distance between adjacent grooves parallel to the light transmission direction is the same as the groove length, and the interval distance between adjacent grooves perpendicular to the light transmission direction is the same as the groove width. And along the light propagation direction, adjacent grooves are staggered with each other and do not overlap.

2. A mode-insensitive spot size converter based on a two-dimensional grating waveguide according to claim 1, characterized in that: The length L of the polymer outer core layer straight waveguide (1) 1 is 1.6 - 3 cm, and the length L of the two-dimensional grating waveguide (2) 2 is 8 - 30 μm, and the length L of the output straight waveguide (3) 3 is 0.8 - 1.5 cm; the interval distance between adjacent grooves parallel to the light transmission direction in the two-dimensional grating waveguide (2) and the groove length W 1 are the same, which is 0.5 - 1 μm, and the interval distance between adjacent grooves perpendicular to the light transmission direction and the groove width are the same W 2 are the same, which is 0.08 - 0.15 μm; the number of rows N of the grooves parallel to the light transmission direction x is 16 - 30, and the number of columns N of the grooves perpendicular to the light transmission direction y is 40 - 60.

3. A mode-insensitive spot size converter based on a two-dimensional grating waveguide according to claim 1, characterized in that: The width W of the polymer outer core layer straight waveguide (1) 3 is 3.5 - 10 μm, and the width W of the inner core layer straight waveguide 4 is 3.2 - 9 μm; the thickness of the silicon wafer substrate (31) is 0.6 - 1 mm, the thickness of the silicon dioxide lower cladding (32) is 3 - 5 μm, the thickness of the inner core layer straight waveguide is 0.2 - 0.6 μm, the thickness of the polymer outer core layer straight waveguide (1) located above the silicon dioxide lower cladding (32) is 2 - 3 μm, and the thickness of the polymer upper cladding (35) located above the silicon dioxide lower cladding (32) is 3 - 5 μm.

4. A mode-insensitive spot size converter based on a two-dimensional grating waveguide according to claim 1, characterized in that: The material of the polymer outer core layer straight waveguide is one of EpoCore, EpoClad, SU-8 2002, SU-8 2005, NOA, and its refractive index is higher than that of the polymer upper cladding material; the material of the polymer upper cladding is one of polymethyl methacrylate, polyethylene, polyester, polycarbonate, polyimide, polystyrene; the material of the inner core layer straight waveguide is one of silicon nitride, lithium niobate, titanium dioxide, silicon carbide, silicon.

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