A 90° optical mixer and its manufacturing method

Abstract

The present invention provides a 90° optical mixer and a manufacturing method thereof. The 90° optical mixer includes a Si3N4 MMI coupler. The Si3N4 MMI coupler uses silicon nitride as the core layer material of the waveguide and silicon dioxide as the cladding layer material of the waveguide. The manufacturing process of the Si3N4 MMI coupler made of silicon nitride material is simple and compatible with integrated devices and CMOS processes. The optical powers output by the four output waveguides of the Si3N4 MMI coupler are kept consistent, and the phase error of the light beams output by the four output waveguides is in the range of -5° to 5°. When the total transmittance of the four output waveguides is greater than 0.9, the corresponding bandwidth is 68 nm, covering the C band with a wavelength range between 1530 nm and 1565 nm, and has high practical value.

Description

Technical Field

[0001] The invention belongs to the field of optical communications, and in particular relates to a 90° optical mixer and a manufacturing method thereof. Background Art

[0002] Compared with traditional transmission systems, coherent optical transmission systems have higher spectral efficiency and receiver sensitivity. The 90° optical mixer is an important component of coherent receivers in optical communications. The function of the 90° optical mixer is to obtain the intermediate signal by coherently mixing the signal light (signal) and the local polarized light (local), and then combine the signal beam and the local oscillator beam and decompose them into four beams with relative phase differences of 0°, 90°, 180°, and 270° respectively.

[0003] Most existing 90° optical mixers use an optical device structure composed of a polarizer, a beam splitter, a λ / 2 wave plate, a λ / 4 wave plate, etc. This structure occupies a large area, is difficult to process and assemble, and has poor compatibility with semiconductor materials.

[0004] Therefore, it is necessary to provide a 90° optical mixer and a manufacturing method thereof. Summary of the invention

[0005] The object of the present invention is to provide a 90° optical mixer and a manufacturing method thereof, so as to solve the problems in the prior art that the 90° optical mixer occupies a large area, is difficult to process and assemble, and has poor compatibility with semiconductor materials.

[0006] To achieve the above object, the present invention provides a 90° optical mixer, wherein the 90° optical mixer comprises a Si3N4 MMI coupler, and the Si3N4 MMI coupler comprises:

[0007] Silicon substrate;

[0008] A silicon dioxide lower cladding layer is formed on the silicon substrate;

[0009] a silicon nitride waveguide formed on the silicon dioxide lower cladding;

[0010] A silicon dioxide upper cladding layer covering the silicon nitride waveguide and the silicon dioxide lower cladding layer;

[0011] The silicon nitride waveguide includes an input waveguide, a multimode waveguide and an output waveguide connected in sequence, the input waveguide includes a first input waveguide and a second input waveguide, the output waveguide includes a first output waveguide, a second output waveguide, a third output waveguide and a fourth output waveguide, signal light is input through the first input waveguide, local polarized light is input through the second input waveguide, and the signal light and the local polarized light form a light beam with the same power and different phases through the multimode waveguide, and the relative phase differences of the light beam output through the output waveguide are 0°, 90°, 180° and 270° respectively.

[0012] Optionally, the Si3N4 MMI coupler comprises a 4×4 Si3N4 MMI coupler.

[0013] Optionally, the multimode waveguide is a rectangular waveguide.

[0014] Specifically, within the wavelength range of 1525 nm to 1565 nm, the phase error of the light beam output through the output waveguide is in the range of -5° to 5°.

[0015] Optionally, the refractive index of the silicon nitride waveguide is in the range of 1.9 to 2.1.

[0016] The present invention provides a method for manufacturing a 90° optical mixer, the manufacturing method comprising the following steps:

[0017] S1: providing a silicon substrate, and depositing a silicon dioxide lower cladding layer on the surface of the silicon substrate;

[0018] S2: depositing a silicon nitride layer on the surface of the silicon dioxide lower cladding layer;

[0019] S3: forming a hard mask layer on the surface of the silicon nitride layer;

[0020] S4: forming a photoresist layer on the surface of the hard mask layer;

[0021] S5: obtaining silicon nitride waveguide by patterning;

[0022] S6: forming a silicon dioxide upper cladding layer covering the silicon dioxide lower cladding layer and the silicon nitride waveguide;

[0023] The silicon nitride waveguide includes an input waveguide, a multimode waveguide and an output waveguide connected in sequence, the input waveguide includes a first input waveguide and a second input waveguide, the output waveguide includes a first output waveguide, a second output waveguide, a third output waveguide and a fourth output waveguide, signal light is input through the first input waveguide, local polarized light is input through the second input waveguide, and the signal light and the local polarized light form a light beam with the same power and different phases through the multimode waveguide, and the relative phase differences of the light beam output through the output waveguide are 0°, 90°, 180° and 270° respectively.

[0024] Optionally, the step S5 specifically includes:

[0025] S5-1: patterning the photoresist layer by a photolithography process to form a photoresist coverage area;

[0026] S5-2: removing the exposed hard mask layer through a chemical etching process, and patterning the silicon nitride layer;

[0027] S5-3: removing the hard mask and the photoresist covering area by a chemical etching process to form a silicon nitride waveguide.

[0028] Optionally, the Si3N4 MMI coupler formed comprises a 4×4 Si3N4 MMI coupler.

[0029] Specifically, within the wavelength range of 1525 nm to 1565 nm, the phase error of the light beam output through the output waveguide is in the range of -5° to 5°.

[0030] Optionally, in step S2, the silicon nitride layer is deposited by a PECVD process, and the reactants for depositing the silicon nitride layer include ammonia and silane. The refractive index of the silicon nitride layer is changed by controlling the flow ratio of the ammonia to the silane, and the refractive index of the silicon nitride layer is in the range of 1.9 to 2.1.

[0031] As described above, the present invention provides a 90° optical mixer and a manufacturing method, wherein the 90° optical mixer includes a Si3N4MMI coupler, wherein the Si3N4MMI coupler uses silicon nitride material as the core material of the waveguide and silicon dioxide as the cladding material of the waveguide. The Si3N4MMI coupler made of silicon-based silicon nitride material has a simple manufacturing process and is compatible with integrated devices and CMOS processes. When the silicon nitride layer is deposited by PECVD, the refractive index of the silicon nitride waveguide layer can be controlled by controlling the flow ratio of ammonia to silane, thereby reducing polarization sensitivity and reducing loss. The optical power output by the four output waveguides of the Si3N4MMI coupler remains consistent, and the relative phase differences between the light beams output by the four output waveguides are 0°, 90°, 180°, and 270°, respectively. When the total transmittance of the four output waveguides is greater than 0.9, the corresponding bandwidth is 68nm, covering the C band with a wavelength range between 1530nm-1565nm. In addition, within the wavelength band of 1525nm to 1565nm, the phase error between the light beams output by the four output waveguides is within the range of -5° to 5°, which meets commercial standards, and the total loss corresponding to the light beams output by the four output waveguides is less than 0.5dB, showing low loss and high practical value. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 Shown is a schematic diagram of the three-dimensional structure of the Si3N4 MMI coupler in an embodiment of the present invention.

[0033] Figure 2 It is a schematic diagram of the planar structure of the Si3N4 MMI coupler in an embodiment of the present invention.

[0034] Figure 3-10 It is a schematic diagram of the cross-sectional structure presented in each step of manufacturing the Si3N4 MMI coupler in an embodiment of the present invention.

[0035] Fig.11 It is a schematic diagram showing the light intensity distribution of a 90° optical mixer in an embodiment of the present invention.

[0036] Fig.12 The diagram shows the corresponding relationship between the optical loss and wavelength of the output waveguide of the 90° optical mixer in the embodiment of the present invention.

[0037] Fig.13 The diagram shows the corresponding relationship between the phase and wavelength of the output waveguide of the 90° optical mixer in the embodiment of the present invention.

[0038] Fig.14 The graph shows the corresponding relationship between the total transmittance and wavelength of the 90° optical mixer output waveguide in an embodiment of the present invention.

[0039] Fig.15The diagram shows the corresponding relationship between the output waveguide phase error and the wavelength corresponding to the 90° optical mixer signal light in an embodiment of the present invention.

[0040] Fig.16 The diagram shows the corresponding relationship between the output waveguide phase error and the wavelength corresponding to the local polarized light of the 90° optical mixer in an embodiment of the present invention.

[0041] Component number description

[0042] 10 Silicon substrate

[0043] 20 Silica lower cladding

[0044] 30 Silicon nitride layer

[0045] 31 Silicon Nitride Waveguide

[0046] 40 Hard mask layer

[0047] 41 Hard Mask

[0048] 50 Photoresist layer

[0049] 51 Photoresist coverage area

[0050] 60 Silica upper cladding

[0051] 301 Input waveguide

[0052] 302 Multimode Waveguide

[0053] 303 Output waveguide

[0054] 3011 First input waveguide

[0055] 3012 Second input waveguide

[0056] 3013 Third input waveguide

[0057] 3014 Fourth input waveguide

[0058] 3031 First output waveguide

[0059] 3032 Second output waveguide

[0060] 3033 Third output waveguide

[0061] 3034 Fourth output waveguide

[0062] port1 first output waveguide

[0063] port2 second output waveguide

[0064] port3 The third output waveguide

[0065] port4 fourth output waveguide DETAILED DESCRIPTION

[0066] The following describes the embodiments of the present invention through specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention.

[0067] It should be noted that the diagrams provided in this embodiment only illustrate the basic concept of the present invention in a schematic manner. Although the diagrams only show the components related to the present invention rather than the number, shape and size of the components in actual implementation, the form, quantity, positional relationship and proportion of each component in actual implementation can be changed at will under the premise of realizing the technical solution of this party, and the layout form of the components may also be more complex. Therefore, it can be foreseen that the shape in the schematic diagram may change, for example, due to manufacturing technology and / or tolerance. Therefore, the exemplary embodiment should not be considered to be limited to the specific shape of the area shown in the figure, but may also include shape deviations caused by, for example, the manufacturing process.

[0068] The present invention provides a 90° optical mixer, wherein the 90° optical mixer is prepared based on a Si3N4 MMI coupler. Figure 1 The three-dimensional structure of the Si3N4 MMI coupler is shown in Figure 2. Figure 1 The silicon dioxide upper cladding layer 60 is not shown. Figure 2 The figure shows the planar structure diagram of the Si3N4MMI coupler. Fig.10 The figure shows a schematic diagram of the cross-sectional structure of a Si3N4 MMI coupler. The Si3N4 MMI coupler includes:

[0069] Silicon substrate 10;

[0070] A silicon dioxide lower cladding layer 20 formed on the silicon substrate 10;

[0071] A silicon nitride waveguide 31 is formed on the silicon dioxide lower cladding layer 20;

[0072] A silicon dioxide upper cladding layer 60, covering the silicon nitride waveguide 31 and the silicon dioxide lower cladding layer 20;

[0073] The silicon nitride waveguide 31 includes an input waveguide 301, a multimode waveguide 302 and an output waveguide 303 connected in sequence, the input waveguide 301 includes a first input waveguide 3011 and a second input waveguide 3012, the first input waveguide 3011 is used to input signal light (Signal), and the second input waveguide 3012 is used to input local polarized light (Local); the multimode waveguide 302 is used to mix the signal light and the local polarized light and demodulate them into light beams with the same power and different phases; the output waveguide 303 includes four, namely, a first output waveguide 3031, a second output waveguide 3032, a third output waveguide 3033 and a fourth output waveguide 3034, and the phase differences of the light beams output through the output waveguides 303 are 0°, 90°, 180° and 270° respectively.

[0074] In this embodiment, the Si3N4 MMI coupler is used as the core element of the 90° optical mixer, and the principle used is: the self-image principle of light in the MMI device, that is, light propagates in a multimode waveguide, and in a specific length of the waveguide, the light entering the multimode waveguide will excite multiple guided modes, which interfere with each other, and along the waveguide direction, at different periodic intervals, one or more replicated images will be output, and an output port is set at its position, so that light with the same power but different phases can be obtained at the port. Therefore, the 90° optical mixer made based on the Si3N4 MMI coupler in this embodiment has many attractive advantages, such as compact footprint, low cost and good compatibility with semiconductor materials such as silicon, SOI platform, etc.

[0075] Among them, this embodiment uses silicon nitride (Si3N4) as the core material of the Si3N4 MMI coupler, and the lower cladding uses a silicon dioxide upper cladding 60, and the lower cladding uses a silicon dioxide lower cladding 20, so as to provide a core layer and a cladding with a large refractive index difference, so as to improve the optical transmission efficiency of the MMI coupler and reduce the loss.

[0076] Furthermore, since the refractive index of silicon nitride can be adjusted by the process parameters during deposition, for example, when the silicon nitride layer 30 is deposited by PECVD, the main reactants used for deposition are ammonia (NH3) and silane (SiH4). By controlling the flow ratio of ammonia and silane, the refractive index of the silicon nitride material can be changed to adapt to incident light of different wavelengths to minimize the loss. In this embodiment, the refractive index of the silicon nitride layer 30 is in the range of 1.9-2.1, preferably 2.0, but not limited thereto.

[0077] As an example, the multimode waveguide 302 may be a rectangular waveguide.

[0078] As an example, the Si3N4 MMI coupler is a 4×4 Si3N4 MMI coupler.

[0079] Specifically, Figure 1 and Figure 2 In this embodiment, the input waveguide 301 includes the first input waveguide 3011, the second input waveguide 3012, the third input waveguide 3013 and the fourth input waveguide 3014. The first input waveguide 3011 and the second input waveguide 3012 are used to input TE polarized light, and the third input waveguide 3013 and the fourth input waveguide 3014 are used to input TM polarized light. The first input waveguide 3011 is used to input signal light (Signal) in TE polarization state, the second input waveguide 3012 is used to input local polarized light (Local) in TE polarization state, the third input waveguide 3013 is used to input signal light (Signal) in TM polarization state, and the fourth input waveguide 3014 is used to input local polarized light (Local) in TM polarization state. However, the structure of the Si3N4 MMI coupler is not limited to this, for example, only a 2×4 Si3N4 MMI coupler can be used, that is, only two input waveguides are used to input a light beam in a certain polarization state.

[0080] In this embodiment, by utilizing the self-imaging principle of light and designing specific dimensions of the multimode waveguide 302, such as the refractive index, width, length, thickness and other parameters of the multimode waveguide 302, the powers of the light beams outputted from the first output waveguide 3031, the second output waveguide 3032, the third output waveguide 3033 and the fourth output waveguide 3034 can be made the same, and the relative phase differences between the light beams outputted from the four output waveguides are 0°, 90°, 180° and 270° respectively. The features of the specific design and dimensions of the input waveguide 301, the multimode waveguide 302 and the output waveguide 303 are not described in detail here, and reference may be made to the prior art.

[0081] As an example, the 90° optical mixer, within the wavelength range of 1525nm to 1565nm, has a phase error of the light beam output through the output waveguide 303 ranging from -5° to 5°.

[0082] Specifically, the relative phase difference between the light beams output by the four output waveguides 302 is not limited to the 0°, 90°, 180°, and 270°. According to actual applications, when the phase error is within a certain range, it can also be used as a 90° optical mixer, thereby expanding the application range of the 90° optical mixer and reducing the difficulty of the manufacturing process. In this embodiment, within the 1525nm to 1565nm band, the phase error of the light beam output by the output waveguide 303 ranges from -5° to 5°, such as -5°, -2°, 0°, 2°, 5°, etc.

[0083] The performance of the 90° optical mixer in this embodiment is characterized by a specific experiment as follows, which specifically includes:

[0084] See also Fig.11 , shows a schematic diagram of the light intensity distribution of the Si3N4 MMI coupler in this embodiment. The figure shows that the light beam input from the input waveguide in the upper left corner is divided into four light beams and output to four output waveguides. It can be seen from the figure that the light intensity output from the four output waveguides of the Si3N4 MMI coupler remains consistent.

[0085] See also Fig.12 , illustrates the corresponding relationship between the optical loss and wavelength of the four output waveguides of the Si3N4 MMI coupler in this embodiment, wherein port1, port2, port3, and port4 shown in the figure represent the corresponding relationship curves of the first output waveguide 3031, the second output waveguide 3032, the third output waveguide 3033, and the fourth output waveguide 3034, respectively. It can be seen from the figure that the four output waveguides all show low optical loss.

[0086] See also Fig.13 The corresponding relationship diagram between the phase angle and wavelength of the four output waveguides of the Si3N4 MMI coupler in this embodiment is illustrated, wherein port1, port2, port3, and port4 shown in the figure represent the corresponding relationship curves of the first output waveguide 3031, the second output waveguide 3032, the third output waveguide 3033, and the fourth output waveguide 3034, respectively. It can be seen from the figure that the relative phase differences between the light beams output by the four output waveguides are 0°, 90°, 180°, and 270°, respectively, which meets the requirements of a 90° optical mixer.

[0087] See also Fig.14, which illustrates the corresponding relationship between the total transmittance and wavelength of the four output waveguides of the Si3N4 MMI coupler in this embodiment. It can be seen from the figure that when the total transmittance is greater than 0.9, the corresponding bandwidth is 68nm, covering the C band, which meets the requirements of commercial optical communication technology. The C band ranges from 1530nm to 1565nm and represents the conventional band. Generally speaking, optical fiber exhibits the lowest loss in the C band, has a great advantage in long-distance transmission systems, and is usually used in many fields combined with WDM.

[0088] See also Fig.15 , which illustrates the corresponding relationship between the output waveguide phase error and the wavelength corresponding to the signal light of the Si3N4 MMI coupler in this embodiment, Fig.16 The corresponding relationship between the output waveguide phase error and the wavelength corresponding to the local polarized light of the Si3N4 MMI coupler in this embodiment is shown. It can be seen from the figure that in the wavelength range of 1525nm to 1565nm, the phase error (phase error) between the light beams output by the first output waveguide 3031, the second output waveguide 3032, the third output waveguide 3033 and the fourth output waveguide 3034 is in the range of -5° to 5°. The phase error is a measure of signal purity. An MMI coupler with a phase error less than ±5° can be commercially used, indicating that the Si3N4 MMI coupler in this embodiment has good performance.

[0089] In addition, within the wavelength band of 1525nm to 1565nm where the phase error is -5° to 5°, the total transmittance of the light beams output by the four output waveguides 303 is greater than 0.89, that is, the corresponding loss is less than 0.5dB. The Si3N4 MMI coupler has a low loss of less than 0.5dB while maintaining the ± phase error. The Si3N4 MMI coupler in this embodiment exhibits a lower loss, which means that it has a higher practical value.

[0090] This embodiment also provides a method for manufacturing a 90° optical mixer. Figure 3-10 The figure shows the structural schematic diagram of each step in preparing the Si3N4 MMI coupler in the 90° optical mixer. The process specifically includes the following steps:

[0091] S1: providing a silicon substrate 10, and depositing a silicon dioxide lower cladding layer 20 on the surface of the silicon substrate 10 in sequence, such as Figure 3 As shown;

[0092] S2: Depositing a silicon nitride layer 30 on the surface of the silicon dioxide lower cladding layer 20, such as Figure 4 As shown;

[0093] S3: forming a hard mask layer 40 on the surface of the silicon nitride layer 30, such as Figure 5 As shown;

[0094] S4: forming a photoresist layer 50 on the surface of the hard mask layer 40, such as Figure 6 As shown;

[0095] S5: Obtain the silicon nitride waveguide 31 by patterning, such as Fig. 9 As shown;

[0096] S6: forming a silicon dioxide upper cladding layer 60 covering the silicon dioxide lower cladding layer 20 and the silicon nitride waveguide 31, such as Fig.10 As shown;

[0097] The silicon nitride waveguide 31 includes an input waveguide 301, a multimode waveguide 302 and an output waveguide 303 connected in sequence, the input waveguide 301 includes a first input waveguide 3011 and a second input waveguide 3012, the first input waveguide 3011 is used to input signal light (Signal), and the second input waveguide 3012 is used to input local polarized light (Local); the multimode waveguide 302 is used to mix the signal light and the local polarized light and demodulate them into light beams with the same power and different phases; the output waveguide 303 includes four, namely, a first output waveguide 3031, a second output waveguide 3032, a third output waveguide 3033 and a fourth output waveguide 3034, and the phase differences of the light beams output through the output waveguides 303 are 0°, 90°, 180° and 270° respectively.

[0098] As an example, step S5 may specifically include the following steps:

[0099] S5-1: removing the photoresist in the redundant area of ​​the photoresist layer 50 by photolithography to form a photoresist covering area 51, such as Figure 7 As shown;

[0100] S5-2: Remove the hard mask layer 40 and the silicon nitride layer 30 not covered by the photoresist covering area 51 by chemical etching process, leaving the silicon nitride waveguide 31, the hard mask 41 and the photoresist covering area 51, as shown in FIG. Figure 8 As shown;

[0101] S5-3: Remove the hard mask 41 and the photoresist covering area 51 by chemical etching process, leaving the silicon nitride waveguide 31, as shown in FIG. Fig. 9 As shown;

[0102] As an example, step S5-3 may further include performing chemical mechanical polishing on the silicon nitride waveguide (31) so that the thickness of the silicon nitride waveguide 31 meets the design requirements.

[0103] As an example, the Si3N4 MMI coupler formed may include a 4×4 Si3N4 MMI coupler.

[0104] As an example, within the wavelength range of 1525 nm to 1565 nm, the phase error of the light beam output through the output waveguide 303 is in the range of -5° to 5°.

[0105] As an example, in step S2, the silicon nitride layer 30 may be deposited by a PECVD process, and the reactants for depositing the silicon nitride layer 30 include ammonia and silane. The refractive index of the silicon nitride layer 30 is changed by controlling the flow ratio of the ammonia and the silane, and the refractive index of the silicon nitride layer 30 is in the range of 1.9 to 2.1.

[0106] The silicon nitride layer 30 is used as the core material of the Si3N4 MMI coupler, and silicon dioxide (SiO2) is used as the cladding material of the Si3N4 MMI coupler, so as to provide a core layer and a cladding layer with a large refractive index difference, so as to improve the optical transmission efficiency of the MMI coupler and reduce the loss.

[0107] Furthermore, since the refractive index of silicon nitride can be adjusted by the process parameters during deposition, for example, when the silicon nitride layer 30 is deposited by PECVD, the main reactants used for deposition are ammonia (NH3) and silane (SiH4). By controlling the flow ratio of ammonia and silane, the refractive index of the silicon nitride material can be changed to adapt to incident light of different wavelengths to minimize the loss. In this embodiment, the refractive index of the silicon nitride layer 30 is in the range of 1.9-2.1, preferably 2.0, but is not limited thereto.

[0108] In summary, the 90° optical mixer and manufacturing method of the present invention include a Si3N4 MMI coupler, which uses silicon nitride material as the core material of the waveguide and silicon dioxide as the cladding material of the waveguide. The Si3N4 MMI coupler made of silicon-based silicon nitride material has a simple manufacturing process and is compatible with integrated devices and CMOS processes. When the silicon nitride layer is deposited by PECVD, the refractive index of the silicon nitride waveguide layer can be controlled by controlling the flow ratio of ammonia to silane, thereby reducing polarization sensitivity and reducing losses. The optical power output by the four output waveguides of the Si3N4 MMI coupler remains consistent, and the relative phase differences between the light beams output by the four output waveguides are 0°, 90°, 180°, and 270°, respectively. When the total transmittance of the four output waveguides is greater than 0.9, the corresponding bandwidth is 68nm, covering the C band with a wavelength range of 1530nm-1565nm. In addition, within the wavelength band of 1525nm to 1565nm, the phase error between the light beams output by the four output waveguides is within the range of -5° to 5°, which meets commercial standards, and the total loss corresponding to the light beams output by the four output waveguides is less than 0.5dB, showing low loss and high practical value.

[0109] The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Anyone familiar with the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by a person of ordinary skill in the art without departing from the spirit and technical concept disclosed by the present invention shall still be covered by the claims of the present invention.

Claims

1. A 90° optical mixer, characterized in that: The 90° optical mixer comprises a Si3N4 MMI coupler, and the Si3N4 MMI coupler comprises: Silicon substrate; A silicon dioxide lower cladding layer is formed on the silicon substrate; a silicon nitride waveguide formed on the silicon dioxide lower cladding; A silicon dioxide upper cladding layer covering the silicon nitride waveguide and the silicon dioxide lower cladding layer; wherein the refractive index of the silicon nitride waveguide is in the range of 1.9 to 2.1; The silicon nitride waveguide includes an input waveguide, a multimode waveguide and an output waveguide connected in sequence, the input waveguide includes a first input waveguide, a second input waveguide, a third input waveguide and a fourth input waveguide, the output waveguide includes a first output waveguide, a second output waveguide, a third output waveguide and a fourth output waveguide, a signal light in a TE polarization state is input through the first input waveguide, a local polarization light in a TE polarization state is input through the second input waveguide, a signal light in a TM polarization state is input through the third input waveguide, and a local polarization light in a TM polarization state is input through the fourth input waveguide, and the signal light and the local polarization light form a light beam with the same power and different phases through the multimode waveguide, and the relative phase differences of the light beam output through the output waveguide are 0°, 90°, 180° and 270° respectively; Wherein, the refractive index, width and thickness parameters of the multimode waveguide are set by utilizing the self-imaging principle of light, so as to mix the signal light and the local polarized light and demodulate them into light beams with the same power and different phases; the multimode waveguide is a rectangular waveguide; Among them, when the total transmittance of the output waveguide is greater than 0.9, the corresponding bandwidth is 68nm, covering the range of 1530nm to 1565nm; and within the wavelength range of 1530nm to 1565nm, the phase error of the light beam output through the output waveguide ranges from -5° to 5°.

2. The 90° optical mixer according to claim 1, characterized in that: The Si3N4MMI coupler includes a 4×4Si3N4MMI coupler.

3. A method for manufacturing the 90° optical mixer according to claim 1, characterized in that: The production method comprises the following steps: S1: providing a silicon substrate, and depositing a silicon dioxide lower cladding layer on the surface of the silicon substrate; S2: depositing a silicon nitride layer on the surface of the silicon dioxide lower cladding layer; wherein the silicon nitride layer is deposited by a PECVD process, and the reactants for depositing the silicon nitride layer include ammonia and silane, and the refractive index of the silicon nitride layer is changed by controlling the flow ratio of the ammonia and the silane, and the refractive index of the silicon nitride layer is in the range of 1.9 to 2.1; S3: forming a hard mask layer on the surface of the silicon nitride layer; S4: forming a photoresist layer on the surface of the hard mask layer; S5: Silicon nitride waveguides obtained by patterning; S6: forming a silicon dioxide upper cladding layer covering the silicon dioxide lower cladding layer and the silicon nitride waveguide; The silicon nitride waveguide includes an input waveguide, a multimode waveguide and an output waveguide connected in sequence, the input waveguide includes a first input waveguide, a second input waveguide, a third input waveguide and a fourth input waveguide, the output waveguide includes a first output waveguide, a second output waveguide, a third output waveguide and a fourth output waveguide, a signal light in a TE polarization state is input through the first input waveguide, a local polarization light in a TE polarization state is input through the second input waveguide, a signal light in a TM polarization state is input through the third input waveguide, and a local polarization light in a TM polarization state is input through the fourth input waveguide, and the signal light and the local polarization light form a light beam with the same power and different phases through the multimode waveguide, and the relative phase differences of the light beam output through the output waveguide are 0°, 90°, 180° and 270° respectively; Wherein, the refractive index, width and thickness parameters of the multimode waveguide are set by utilizing the self-imaging principle of light, so as to mix the signal light and the local polarized light and demodulate them into light beams with the same power and different phases; the multimode waveguide is a rectangular waveguide; Among them, when the total transmittance of the output waveguide is greater than 0.9, the corresponding bandwidth is 68nm, covering the range of 1530nm to 1565nm; and within the wavelength range of 1530nm to 1565nm, the phase error of the light beam output through the output waveguide ranges from -5° to 5°.

4. The method for manufacturing a 90° optical mixer according to claim 3, characterized in that: The step of S5 specifically includes: S5-1: patterning the photoresist layer by a photolithography process to form a photoresist coverage area; S5-2: removing the exposed hard mask layer through a chemical etching process, and patterning the silicon nitride layer; S5-3: removing the hard mask and the photoresist covering area by a chemical etching process to form a silicon nitride waveguide.

5. The method for manufacturing a 90° optical mixer according to claim 3, characterized in that: The Si3N4MMI coupler formed includes a 4×4 Si3N4MMI coupler.

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