90-degree optical mixer based on double-layer waveguide and preparation method thereof

Abstract

The invention relates to the technical field of optical communication, in particular to a double-layer waveguide-based 90-degree optical mixer and a preparation method thereof, and the double-layer waveguide-based 90-degree optical mixer comprises a substrate, and an oxide layer, a first waveguide layer, a second waveguide layer and a cladding which are arranged on the substrate; the first waveguide layer and the second waveguide layer are both arranged in the cladding, and the second waveguide layer and the first waveguide layer are arranged at an interval in the direction perpendicular to the waveguide transmission plane through the other oxide layer to form an interlayer structure; an interlayer directional coupling part is arranged between the first waveguide layer and the second waveguide layer; and the interlayer directional coupling part is used for providing optical signal coupling between the first waveguide layer and the second waveguide layer, so that signal light and local oscillation light are transmitted between the first waveguide layer and the second waveguide layer, and frequency mixing occurs in respective multimode interference regions. According to the 90-degree optical mixer provided by the invention, through a double-layer waveguide structure, the transmission loss and crosstalk of crossed waveguides are remarkably reduced, and high-performance and high-integration-level optical signal mixing is realized.

Description

Technical Field

[0001] The present invention relates to the field of optical communication technology, and in particular to a 90-degree optical mixer based on a double-layer waveguide and a preparation method thereof. Background Art

[0002] In the context of the rapid development of optical communication and sensing technology, the 90-degree optical mixer, as a key optical component, can convert the phase difference information of two input light beams into optical intensity information, and can indirectly obtain phase information by measuring the output light intensity, so it is widely used in coherent optical communication, coherent optical imaging, optical sensing and other fields. At present, the 90-degree optical mixer is mainly realized by integrated optical technology, which has significant advantages such as compact structure and high stability.

[0003] Planar waveguide 90-degree optical mixers are mainly divided into two categories. The first category is an optical mixer based on a 3dB power divider and a phase shifter, and the second category is an optical mixer based on a four-port multimode interference coupler. The first type of optical mixer has a small phase deviation and stable performance. However, this type of device usually adopts a single-layer waveguide structure. The crossed waveguides in the single-layer waveguide structure are prone to crosstalk between waveguides during transmission, affecting the transmission efficiency of the signal. In order to reduce this crosstalk and loss, it is usually necessary to set a sufficiently large crossing angle, but this also increases the size of the device, which is inconsistent with the development trend of integration and miniaturization, and limits its application in high-density integrated optical modules; on the other hand, the second type of optical mixer based on a four-port multimode interference coupler is relatively simple in structure and has a large process tolerance, but has high requirements for process processing. In order to reduce the phase deviation, a wedge structure is usually used to reduce the number of modes excited at the input end to eliminate the phase error caused by high-order modes. However, this structural design often limits the bandwidth of the device, which limits its performance in high-bandwidth applications. In addition, due to the need to accurately control the mode excitation and interference conditions, this type of device is easily affected by process errors in the actual manufacturing process, resulting in unstable performance.

[0004] In summary, the existing 90-degree optical mixer design, while pursuing high performance, also faces challenges in terms of size, integration and process complexity. Therefore, there is an urgent need for an optical mixer that can maintain excellent phase performance while effectively reducing device size and improving integration to meet the development needs of high-density, high-bandwidth integrated optical modules. Summary of the invention

[0005] In order to solve the above technical problems, the present invention provides a 90-degree optical mixer based on a double-layer waveguide and a preparation method thereof.

[0006] In a first aspect, the present invention provides a 90-degree optical mixer based on a double-layer waveguide, comprising a substrate, an oxide layer, a first waveguide layer, a second waveguide layer and a cladding layer arranged on the substrate; wherein the first waveguide layer and the second waveguide layer are both arranged in the cladding layer, and the second waveguide layer is spaced apart from the first waveguide layer in a direction perpendicular to the waveguide transmission plane through another oxide layer to form an interlayer structure;

[0007] An interlayer directional coupling part is arranged between the first waveguide layer and the second waveguide layer; the interlayer directional coupling part is used to provide optical signal coupling between the first waveguide layer and the second waveguide layer, so that the signal light and the local oscillation light are transmitted between the first waveguide layer and the second waveguide layer, and frequency mixing occurs in their respective multi-mode interference regions.

[0008] In a further embodiment, the first waveguide layer includes a local oscillator light input waveguide, a first multimode interference coupler, a phase shifter, a fourth multimode interference coupler, and two first output waveguides; the second waveguide layer includes a signal light input waveguide, a third multimode interference coupler, a second multimode interference coupler, and two second output waveguides;

[0009] The first waveguide layer is used to divide the input local oscillator light into two optical signals with equal intensity and phase through the first multimode interference coupler, one of which is coupled to the second multimode interference coupler through the interlayer directional coupling part after the phase is changed by the phase shifter; the other optical signal directly enters the fourth multimode interference coupler;

[0010] The second waveguide layer is used to divide the input signal light into two optical signals of equal intensity and phase through the third multimode interference coupler, one of which is coupled to the fourth multimode interference coupler through the interlayer directional coupling part; the other optical signal directly enters the second multimode interference coupler.

[0011] In a further embodiment, the local oscillator light input waveguide and the signal light input waveguide are single-mode straight waveguides of equal length and symmetrically distributed in a direction parallel to the input signal light, and are used to transmit the local oscillator light and the signal light, respectively.

[0012] In a further embodiment, the phase difference between the two optical signals outputted by the two first output waveguides is 180 degrees;

[0013] The phase difference between the two optical signals outputted by the two second output waveguides is 180 degrees, and the phase difference between the two optical signals outputted by the two first output waveguides and the two optical signals outputted by the two second output waveguides is 90 degrees.

[0014] In a further embodiment, the first multimode interference coupler is a 1×2 multimode interference coupler;

[0015] The second multimode interference coupler is a 2×2 multimode interference coupler;

[0016] The third multimode interference coupler is a 1×2 multimode interference coupler;

[0017] The fourth multimode interference coupler is a 2×2 multimode interference coupler.

[0018] In a further embodiment, the length of the multimode interference region of the 1×2 multimode interference coupler is:

[0019]

[0020] In the formula, is the length of the multimode interference region of the 1×2 multimode interference coupler; L π is the beat length; N is the number of output ports of the multimode interferometer coupler; β0 and β1 are the propagation constants of the fundamental mode and the first-order mode, respectively.

[0021] In a further embodiment, the length of the multimode interference region of the 2×2 multimode interference coupler is:

[0022]

[0023] In the formula, is the length of the multimode interference zone of the 2×2 multimode interference coupler.

[0024] In a further embodiment, the length calculation formula of the interlayer directional coupling part is:

[0025]

[0026] Where, L x is the length of the interlayer directional coupling part; λ is the working wavelength of the input light; Δn is the effective refractive index difference between the symmetric mode and the antisymmetric mode of the coupling waveguide in the interlayer directional coupling part.

[0027] In a further embodiment, the refractive index of the first waveguide layer and the second waveguide layer are both greater than the refractive index of the further oxide layer.

[0028] In a second aspect, the present invention provides a method for preparing a 90-degree optical mixer based on a double-layer waveguide, the preparation method comprising the following steps:

[0029] Providing wafers;

[0030] Coating photoresist on the waveguide layer of the wafer, performing waveguide etching after photolithography, and completing patterning of the first waveguide layer;

[0031] Depositing an oxide layer on the first waveguide layer, polishing the deposited oxide layer and bonding it to another inverted wafer, and removing the substrate of the inverted wafer to form a second waveguide layer;

[0032] Coating photoresist on the second waveguide layer again, performing waveguide etching after photolithography, and completing patterning of the second waveguide layer;

[0033] A cladding layer is deposited on the second waveguide layer so that both the first waveguide layer and the second waveguide layer are enclosed in the cladding layer.

[0034] The present invention provides a 90-degree optical mixer based on a double-layer waveguide, the 90-degree optical mixer comprises a substrate and an oxide layer, a first waveguide layer, a second waveguide layer and a cladding layer arranged on the substrate; wherein the first waveguide layer and the second waveguide layer are both arranged in the cladding layer, and the second waveguide layer is arranged in a direction perpendicular to the waveguide transmission plane with the first waveguide layer through another oxide layer to form an interlayer structure; an interlayer directional coupling part is arranged between the first waveguide layer and the second waveguide layer; the interlayer directional coupling part is used to provide optical signal coupling between the first waveguide layer and the second waveguide layer, so that the signal light and the local oscillation light can be transmitted between the first waveguide layer and the second waveguide layer, and mixing occurs in their respective multi-mode interference regions. Compared with the prior art, the 90-degree optical mixer proposed by the present invention significantly reduces the transmission loss and crosstalk of the cross waveguide through the double-layer waveguide structure, and realizes high-performance and high-integration optical signal mixing. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 Schematic diagram of the structure of a 90-degree optical mixer based on a double-layer waveguide provided in an embodiment of the present invention;

[0036] Figure 2 Schematic diagram of a cross-sectional waveguide structure of a 90-degree optical mixer based on a double-layer waveguide provided in an embodiment of the present invention;

[0037] Figure 3 is a schematic diagram of waveguide transmission loss curves at different crossing angles provided by an embodiment of the present invention;

[0038] Figure 4 is a schematic diagram of light field distribution respectively input from input ports of the first waveguide layer and the second waveguide layer provided by an embodiment of the present invention;

[0039] Figure 5 Schematic diagram of light field distribution when the input light beam phase differences are 0°, 90°, 180° and 270° respectively, provided by an embodiment of the present invention;

[0040] Figure 6 is a schematic diagram of a common mode suppression ratio curve at different wavelengths when inputted from an input port of a first waveguide layer provided by an embodiment of the present invention;

[0041] Figure 7 is a schematic diagram of the imbalance degree at different wavelengths when input from the input port of the first waveguide layer provided by an embodiment of the present invention;

[0042] Figure 8 is a schematic diagram of a common mode suppression ratio curve at different wavelengths when inputted from an input port of a second waveguide layer provided by an embodiment of the present invention;

[0043] Fig. 9 is a schematic diagram of the imbalance at different wavelengths when input from the input port of the second waveguide layer provided by an embodiment of the present invention;

[0044] Fig.10 is a schematic diagram of phase error provided by an embodiment of the present invention;

[0045] Fig.11 is a schematic diagram of a method for preparing a 90-degree optical mixer based on a double-layer waveguide provided in an embodiment of the present invention;

[0046] Fig.12 It is a schematic diagram of the process flow of a 90-degree optical mixer based on a double-layer waveguide provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0047] The following specifically illustrates the implementation mode of the present invention in conjunction with the accompanying drawings. The embodiments are provided for illustrative purposes only and cannot be understood as limiting the present invention. The accompanying drawings are provided for reference and illustration only and do not constitute a limitation on the scope of patent protection of the present invention, because many changes can be made to the present invention without departing from the spirit and scope of the present invention.

[0048] like Figure 1 , Figure 2 As shown, an embodiment of the present invention provides a 90-degree optical mixer based on a double-layer waveguide, the 90-degree optical mixer based on a double-layer waveguide comprises a substrate 11 and an oxide layer 12, a first waveguide layer 13, a second waveguide layer 14 and a cladding layer 15 arranged on the substrate 11; wherein the first waveguide layer 13 and the second waveguide layer 14 are both arranged in the cladding layer 15, and the second waveguide layer 14 is spaced apart from the first waveguide layer 13 in a direction perpendicular to the waveguide transmission plane through another oxide layer to form an interlayer structure. In this embodiment, the refractive indexes of the first waveguide layer and the second waveguide layer are both greater than the refractive index of the another oxide layer.

[0049] This embodiment takes a silicon-on-insulator (SOI) platform as an example to illustrate a 90-degree optical mixer based on a double-layer waveguide structure. The structure comprises, from bottom to top, a silicon substrate, a silicon oxide layer, a first Si waveguide layer, another silicon oxide layer, a second Si waveguide layer, and a silicon dioxide cladding layer. The first Si waveguide layer is located on the silicon oxide layer, and the second Si waveguide layer is located on another silicon oxide layer on the first Si waveguide layer. The spacing distance between the first waveguide layer and the second waveguide layer in a direction perpendicular to the surface of the waveguide layer is L2, that is, the thickness of the silicon oxide layer between the first waveguide layer and the second waveguide layer is L2. In this embodiment, the refractive index of the waveguide layer is higher than that of the silicon oxide layer, so that light can be transmitted in the waveguide layer by total internal reflection. In terms of structure, this embodiment preferably selects a silicon waveguide layer with a thickness of 220 nanometers, a silicon oxide layer thickness L2 between the first Si waveguide layer and the second Si waveguide layer of 500 nanometers, a single-mode waveguide width of 500 nanometers, a silicon waveguide layer with a refractive index of 3.44, and a silicon oxide layer with a refractive index of 1.44.

[0050] like Figure 1 As shown, the first waveguide layer includes a local oscillator light input waveguide 101, a first multimode interference coupler (MMI) 102, a phase shifter 103, a fourth multimode interference coupler 203 and two first output waveguides; the second waveguide layer includes a signal light input waveguide 201, a third multimode interference coupler 202, a second multimode interference coupler 104 and two second output waveguides; in this embodiment, an interlayer directional coupling part is arranged between the first waveguide layer and the second waveguide layer, and the first waveguide layer and the second waveguide layer interact with each other through the interlayer directional coupling part (as shown in 301 and 302); the interlayer directional coupling part is used to provide optical signal coupling between the first waveguide layer and the second waveguide layer, so that the signal light and the local oscillator light can be transmitted between the first waveguide layer and the second waveguide layer, and frequency mixing occurs in their respective multimode interference regions.

[0051] In this embodiment, the first waveguide layer is used to divide the input local oscillator light into two optical signals of equal intensity and phase through the first multimode interference coupler 102, one of which is coupled to the second multimode interference coupler 104 through the interlayer directional coupling part after the phase is changed by the phase shifter 103; the other optical signal directly enters the fourth multimode interference coupler 203; in some embodiments, the first multimode interference coupler is a 1×2 multimode interference coupler; the second multimode interference coupler is a 2×2 multimode interference coupler.

[0052] The second waveguide layer is used to divide the input signal light into two optical signals of equal intensity and phase through the third multimode interference coupler 202, one of which is coupled to the fourth multimode interference coupler 203 through the interlayer directional coupling part; the other optical signal directly enters the second multimode interference coupler 104; in some embodiments, the third multimode interference coupler is a 1×2 multimode interference coupler; the fourth multimode interference coupler is a 2×2 multimode interference coupler.

[0053] In some embodiments, the local oscillation light input waveguide and the signal light input waveguide are single-mode straight waveguides of equal length and symmetrically distributed in a direction parallel to the input signal light, and are used to transmit the local oscillation light and the signal light respectively. In specific operation, in the first waveguide layer, the local oscillation light enters from the input port 10 (I1 input) of the first waveguide layer, passes through the single-mode straight waveguide 101, and is divided by the 1×2MMI 102 into two optical signals with equal intensity and the same phase, one of which is transmitted to the second waveguide layer through the directional coupling part 301 after the phase is changed by the phase shifter 301, and enters the 2×2MMI 104 of the second waveguide layer; the other optical signal directly enters the 2×2MMI 203 of the first waveguide layer; at the same time, in the second waveguide layer, the signal light enters from the input port 20 (I2 input) of the second waveguide layer, passes through the single-mode straight waveguide 201, and is divided by the 1×2MMI 202 divides it into two optical signals with equal intensity and phase, one optical signal is transmitted to the first waveguide layer through the interlayer directional coupling part 302 and enters the 2×2MMI 203 of the first waveguide layer; the other optical signal directly enters the 2×2MMI 104 of the second waveguide layer.

[0054] In this embodiment, the phase difference between the two optical signals outputted from the two first output waveguides is 180 degrees; the phase difference between the two optical signals outputted from the two second output waveguides is 180 degrees, and the phase difference between the two optical signals outputted from the two first output waveguides and the two optical signals outputted from the two second output waveguides is 90 degrees. Specifically, in the multimode interference region, the signal light with phase information is mixed with the local oscillator light. Due to the self-imaging effect of the MMI, the 2×2 MMI 203 of the first waveguide layer outputs two optical paths 30 and 40 with a phase difference of 180 degrees, while the 2×2 MMI 203 of the second waveguide layer outputs two optical paths 30 and 40 with a phase difference of 180 degrees. 104 outputs two paths of light 50 and 60 with a phase difference of 180 degrees. Due to the presence of the phase shifter in the first waveguide layer, the phases of the two paths of output light of the first waveguide layer and the two paths of output light of the second waveguide layer differ by 90 degrees, respectively. Finally, four output ports are obtained, and the relative phase differences of the four output ports are 0 degrees, 90 degrees, 180 degrees and 270 degrees, respectively. Among them, the branches at the 0 degree and 180 degree output ends after passing through the balanced detector are called orthogonal Q branches, and the branches at the 90 degree and 270 degree output ends after passing through the balanced detector are called in-phase I branches.

[0055] In this embodiment, the 1×2 multimode interference coupler and the 2×2 multimode interference coupler of the first waveguide layer and the second waveguide layer have the same structure size, wherein the multimode interference region width of the 1×2 multimode interference coupler is 6 microns and the length is 32.5 microns; the multimode interference region width of the 2×2 multimode interference coupler is 6 microns and the length is 130.3 microns. The multimode interference region length of the 1×2 multimode interference coupler is:

[0056]

[0057] In the formula, is the length of the multimode interference region of the 1×2 multimode interference coupler; L π is the beat length (the difference in propagation length between the two lowest-order modes); N is the number of output ports of the multimode interferometer coupler; β0 and β1 are the propagation constants of the fundamental mode and the first-order mode, respectively.

[0058] The length of the multimode interference region of the 2×2 multimode interference coupler is:

[0059]

[0060] In the formula, is the length of the multimode interference zone of the 2×2 multimode interference coupler.

[0061] In addition, this embodiment also considers the directional coupling between waveguides. The optical mixer utilizes the symmetric mode and antisymmetric mode characteristics of the coupled waveguides. When the two modes are in phase, the optical power is mainly concentrated in the first waveguide; when there is a π phase difference between the two modes, the optical power is concentrated in the second waveguide. The distance difference that produces this change is called the coupling length L. x Through calculation, the length L1 of the interlayer directional coupling portion between the first waveguide layer and the second waveguide layer can be obtained as 83 microns in this embodiment. In this embodiment, L1=L x , the length calculation formula of the interlayer directional coupling part is:

[0062]

[0063] From this we can get,

[0064]

[0065] Where, L x is the length of the interlayer directional coupling part; λ is the working wavelength of the input light; Δn is the effective refractive index difference between the symmetric mode and the antisymmetric mode of the coupled waveguide in the interlayer directional coupling part; n1 and n2 are the effective refractive indices of the first two eigenmodes of the coupled waveguide, respectively.

[0066] In order to evaluate the impact of the crossed waveguides on the device performance, this embodiment introduces the transmission loss (IL) parameter and simulates the crossed waveguides at different angles using the finite difference time domain method. Figure 3 As shown, the simulation results show that compared with the single-layer waveguide structure, the 90-degree optical mixer with the double-layer waveguide structure provided in this embodiment has lower transmission loss in the crossing portion. Therefore, this embodiment can select a smaller crossing angle to further reduce the size of the device.

[0067] At the same time, this embodiment verifies the performance of the designed 90-degree optical mixer based on the double-layer waveguide through the beam propagation method simulation. Figure 4-Figure 10 As shown, the simulation results of the beam propagation method show that the average phase error of the device in the 1545-1580nm band is better than 5°, the common mode rejection ratio in the 1520-1600nm band is greater than 20dB, and the imbalance is less than 0.74dB. In summary, the device has low phase error, high common mode rejection ratio and low imbalance, and can well realize the function of a 90-degree optical mixer. According to the mode coupling theory, the optical modes in the intersection area of ​​the optical waveguides will be coupled. In this embodiment, since the crossing waveguides are located in waveguide layers at different depths and the crossing angle is small, the overlapping area between the waveguides is greatly reduced, thereby reducing mode coupling, transmission loss and crosstalk. The simulation results also verify this point. The double-layer waveguide structure has lower transmission loss and crosstalk than the single-layer waveguide structure at the same crossing angle.

[0068] Compared with the waveguide type 90 degree optical mixer based on complex structures such as 4×4MMI, 2×4MMI and 2×2MMI cascade, the cross section 401 of the present invention has the following structural features: firstly, the two cross waveguides are single-mode straight waveguides with the same length, and from the top view of the device structure, the cross section is symmetrically distributed in the direction parallel to the input signal light, and the structural design is simple; secondly, the two cross waveguides are respectively located in the first waveguide layer and the second waveguide layer, and the two waveguide layers are separated by a distance of L2 in the direction perpendicular to the waveguide transmission plane. The cross-waveguides are spaced apart, which makes them appear to be crossed in the top view, but in fact there is no overlapping area, thus avoiding the defects of the traditional cross-waveguides; thirdly, the crossing angle is small. The crossing angle of the simulation experiment provided in this embodiment is only 20°, which is much smaller than the vertical angle required by the traditional cross-waveguide. For example, in the existing waveguide layer and its multi-mode interference cross-waveguide solution, the cross-waveguide is a waveguide layer located at the same depth, and there is an actual overlapping area. The crossing angle requirement is large, and in order to reduce transmission loss and crosstalk, the waveguide structure is composed of multiple cascaded tapered structures, and the simulation design difficulty and process implementation difficulty are relatively large.

[0069] In summary, the 90-degree optical mixer proposed in this embodiment adopts a double-layer waveguide structure and integrates a 3dB power divider and a phase shifter. By distributing the cross waveguides originally in the same plane to two different waveguide layers, the crosstalk and loss caused by the cross waveguides can be significantly reduced, so that a smaller cross angle can be selected, thereby effectively reducing the size of the device and improving its integration. Therefore, the 90-degree optical mixer proposed in this embodiment not only has high phase accuracy, but also has good stability. The traditional 90-degree optical mixer based on a 3dB power divider and a phase shifter often has the defects of a cross waveguide structure or a star structure. The cross waveguide structure will introduce large transmission loss and crosstalk, and the star structure is limited by the difficulty in adjusting the output port position and the complex phase tuning structure caused by different transmission paths. That is, in the star structure, the four output ports cannot be located on the same side, and a larger curved waveguide is required to adjust the output port position. In addition, since the transmission path of each light beam is different, an additional phase tuning structure is required.

[0070] In addition, in the layout of large-scale planar optical path devices, optical waveguide crossing often occurs. The double-layer waveguide structure proposed in this embodiment can not only reduce cross-talk without increasing the chip size, but also improve the spatial utilization of the chip, which is consistent with the current concept of 3D heterogeneous integration technology that has attracted much attention, that is, by vertically stacking multiple homogeneous and heterogeneous small chips, more functions can be achieved in a smaller area and unit performance can be improved. Therefore, this embodiment provides a practical solution for high-density, high-bandwidth integrated optical modules. In summary, the present invention adopts a double-layer waveguide structure to design a 90-degree optical mixer, which not only improves the integration level, but also reduces the crosstalk and loss of crossed waveguides while maintaining the technical advantages of 3dB power dividers and phase shifters. Further improves the performance of the device.

[0071] The embodiment of the present invention provides a 90-degree optical mixer based on a double-layer waveguide, the 90-degree optical mixer comprises a substrate and an oxide layer, a first waveguide layer, a second waveguide layer and a cladding layer arranged on the substrate; wherein the first waveguide layer and the second waveguide layer are both arranged in the cladding layer, and the second waveguide layer is arranged in a direction perpendicular to the waveguide transmission plane through another oxide layer to form an interlayer structure; an interlayer directional coupling part is arranged between the first waveguide layer and the second waveguide layer; the interlayer directional coupling part is used to provide optical signal coupling between the first waveguide layer and the second waveguide layer, so that the signal light and the local oscillation light can be transmitted between the first waveguide layer and the second waveguide layer, and mixing occurs in their respective multi-mode interference regions. Compared with the prior art, the 90-degree optical mixer proposed in this embodiment significantly reduces the transmission loss and crosstalk of the cross waveguide through the double-layer waveguide structure, and realizes high-performance and high-integration optical signal mixing.

[0072] In one embodiment, Fig.11 As shown, an embodiment of the present invention provides a method for preparing a 90-degree optical mixer based on a double-layer waveguide, and the preparation method comprises the following steps:

[0073] S1. Provide wafers;

[0074] S2. coating a photoresist on the waveguide layer of the wafer, performing waveguide etching after photolithography, and completing patterning of the first waveguide layer;

[0075] S3. depositing an oxide layer on the first waveguide layer, polishing the deposited oxide layer and bonding it to another inverted wafer, and removing the substrate of the inverted wafer to form a second waveguide layer;

[0076] S4. coating the second waveguide layer with photoresist again, performing waveguide etching after photolithography, and completing patterning of the second waveguide layer;

[0077] S5. Depositing a cladding layer on the second waveguide layer so that both the first waveguide layer and the second waveguide layer are covered in the cladding layer.

[0078] Specifically, Fig.12 As shown, in this embodiment, a SOI wafer is first prepared; then a layer of photoresist is coated on the surface of the wafer, and the photoresist is exposed by using a photolithography device. Then, a waveguide etching technology such as dry etching is used for etching to form a structure of a first waveguide layer. After the first waveguide layer structure is formed, a layer of oxide layer is deposited by using a technology such as chemical vapor deposition (CVD), and polishing is performed to ensure the flatness and smoothness of the surface. The polishing process can use chemical mechanical polishing (CMP) or other polishing technologies. The SOI wafer that has undergone the above treatment is bonded to another inverted SOI wafer. After the bonding is completed, the Si substrate of the inverted wafer is removed to expose the second waveguide layer. Photoresist is coated on the second waveguide layer again, and exposure and etching are performed to form the structure of the second waveguide layer. After the second waveguide layer structure is formed, a layer of SiO2 is deposited as a protective layer to protect the waveguide structure from subsequent processing or environmental factors.

[0079] It should be noted that the size of the serial numbers of the above-mentioned processes does not mean the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0080] For the specific definition of the preparation method of a 90-degree optical mixer based on a double-layer waveguide, please refer to the above-mentioned definition of a 90-degree optical mixer based on a double-layer waveguide, which will not be repeated here. A person of ordinary skill in the art can appreciate that the various modules and steps described in conjunction with the embodiments disclosed in this application can be implemented in hardware, software, or a combination of both. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professional and technical personnel can use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of this application.

[0081] The embodiment of the present invention provides a method for preparing a 90-degree optical mixer based on a double-layer waveguide, the method comprising providing a wafer; coating a photoresist on the waveguide layer of the wafer, performing waveguide etching after photolithography, and completing the patterning of the first waveguide layer; depositing an oxide layer on the first waveguide layer, polishing the deposited oxide layer and bonding it to another inverted wafer, and removing the substrate of the inverted wafer to form a second waveguide layer; coating the second waveguide layer with photoresist again, performing waveguide etching after photolithography, and completing the patterning of the second waveguide layer; depositing a cladding on the second waveguide layer, so that both the first waveguide layer and the second waveguide layer are coated in the cladding. Compared with the prior art, the method for preparing a 90-degree optical mixer proposed in this embodiment significantly reduces the transmission loss and crosstalk of the cross waveguides through the double-layer waveguide structure, and realizes high-performance and high-integration optical signal mixing.

[0082] The above-mentioned embodiments only express several preferred implementation modes of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in the technical field, several improvements and substitutions can be made without departing from the technical principles of the present invention, and these improvements and substitutions should also be regarded as the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be based on the protection scope of the claims.

Claims

1. A 90-degree optical mixer based on a double-layer waveguide, characterized in that: The invention comprises a substrate, an oxide layer, a first waveguide layer, a second waveguide layer and a cladding layer arranged on the substrate; wherein the first waveguide layer and the second waveguide layer are both arranged in the cladding layer, and the second waveguide layer is spaced apart from the first waveguide layer in a direction perpendicular to the waveguide transmission plane through another oxide layer to form an interlayer structure; An interlayer directional coupling part is arranged between the first waveguide layer and the second waveguide layer; the interlayer directional coupling part is used to provide optical signal coupling between the first waveguide layer and the second waveguide layer, so that the signal light and the local oscillation light are transmitted between the first waveguide layer and the second waveguide layer, and frequency mixing occurs in their respective multi-mode interference regions.

2. A 90-degree optical mixer based on a double-layer waveguide as claimed in claim 1, characterized in that: The first waveguide layer includes a local oscillator light input waveguide, a first multimode interference coupler, a phase shifter, a fourth multimode interference coupler and two first output waveguides; the second waveguide layer includes a signal light input waveguide, a third multimode interference coupler, a second multimode interference coupler and two second output waveguides; The first waveguide layer is used to divide the input local oscillator light into two optical signals with equal intensity and phase through the first multimode interference coupler, one of which is coupled to the second multimode interference coupler through the interlayer directional coupling part after the phase is changed by the phase shifter; the other optical signal directly enters the fourth multimode interference coupler; The second waveguide layer is used to divide the input signal light into two optical signals of equal intensity and phase through the third multimode interference coupler, one of which is coupled to the fourth multimode interference coupler through the interlayer directional coupling part; the other optical signal directly enters the second multimode interference coupler.

3. A 90-degree optical mixer based on a double-layer waveguide as claimed in claim 2, characterized in that: The local oscillator light input waveguide and the signal light input waveguide are single-mode straight waveguides with equal lengths and symmetrically distributed in a direction parallel to the input signal light, and are used to transmit the local oscillator light and the signal light respectively.

4. A 90-degree optical mixer based on a double-layer waveguide as claimed in claim 2, characterized in that: The phase difference between the two optical signals output by the two first output waveguides is 180 degrees; The phase difference between the two optical signals outputted by the two second output waveguides is 180 degrees, and the phase difference between the two optical signals outputted by the two first output waveguides and the two optical signals outputted by the two second output waveguides is 90 degrees.

5. A 90-degree optical mixer based on a double-layer waveguide as claimed in claim 2, characterized in that: The first multimode interference coupler is a 1×2 multimode interference coupler; The second multimode interference coupler is a 2×2 multimode interference coupler; The third multimode interference coupler is a 1×2 multimode interference coupler; The fourth multimode interference coupler is a 2×2 multimode interference coupler.

6. A 90-degree optical mixer based on a double-layer waveguide as claimed in claim 5, characterized in that: The length of the multimode interference region of the 1×2 multimode interference coupler is: In the formula, is the length of the multimode interference region of the 1×2 multimode interference coupler; L π is the beat length; N is the number of output ports of the multimode interferometer coupler; β0 and β1 are the propagation constants of the fundamental mode and the first-order mode, respectively.

7. A 90-degree optical mixer based on a double-layer waveguide as claimed in claim 6, characterized in that: The length of the multimode interference region of the 2×2 multimode interference coupler is: In the formula, is the length of the multimode interference zone of the 2×2 multimode interference coupler.

8. A 90-degree optical mixer based on a double-layer waveguide as claimed in claim 1, characterized in that: The length calculation formula of the interlayer directional coupling part is: Where, L x is the length of the interlayer directional coupling part; λ is the working wavelength of the input light; Δn is the effective refractive index difference between the symmetric mode and the antisymmetric mode of the coupled waveguide in the interlayer directional coupling part.

9. A 90-degree optical mixer based on a double-layer waveguide as claimed in claim 1, characterized in that: The refractive indexes of the first waveguide layer and the second waveguide layer are both greater than the refractive index of the another oxide layer.

10. A method for preparing a 90-degree optical mixer based on a double-layer waveguide, characterized in that: The preparation method comprises the following steps: Providing wafers; Coating photoresist on the waveguide layer of the wafer, performing waveguide etching after photolithography, and completing patterning of the first waveguide layer; Depositing an oxide layer on the first waveguide layer, polishing the deposited oxide layer and bonding it to another inverted wafer, and removing the substrate of the inverted wafer to form a second waveguide layer; Coating photoresist on the second waveguide layer again, performing waveguide etching after photolithography, and completing patterning of the second waveguide layer; A cladding layer is deposited on the second waveguide layer so that both the first waveguide layer and the second waveguide layer are enclosed in the cladding layer.

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