Multi-mode interference coupler and forming method thereof

Abstract

The embodiment of the invention provides a multimode interference coupler and a forming method thereof. The multimode interference coupler comprises a substrate and a multimode interference coupling waveguide area located on the surface of the substrate, the multimode interference coupling waveguide region is used for performing interference processing on the received M initial optical signals to generate N target optical signals; m and N are positive integers; wherein the multimode interference coupling waveguide region comprises a plurality of waveguide core layers which are sequentially stacked along a first direction; the two adjacent waveguide core layers are made of different materials, and the effective refractive indexes of the initial optical signals in different polarization states in the multimode interference coupling waveguide area are the same.

Description

Technical Field

[0001] The present disclosure relates to the field of optical communications, and in particular to a multimode interference coupler and a method for forming the same. Background Art

[0002] Multi-mode interference coupler (MMI) is one of the core passive devices in silicon photonics integration technology. It can use the principle of multi-mode interference to achieve power distribution of optical signals and has wide applications in optical communications and optical signal processing.

[0003] In the related art, multimode interference couplers are usually polarization-dependent devices, that is, multimode interference couplers can only transmit optical signals of a specific polarization state. In practice, when optical signals are transmitted in optical fibers, the polarization state of the optical signals is difficult to predict. If multimode interference couplers are only applicable to a single polarization state, their application range will be greatly limited. Summary of the invention

[0004] Embodiments of the present disclosure provide a multi-mode interference coupler and a method for forming the same.

[0005] In a first aspect, an embodiment of the present disclosure provides a multimode interference coupler, comprising: a substrate, and a multimode interference coupling waveguide region located on a surface of the substrate;

[0006] The multi-mode interference coupling waveguide region is used to perform interference processing on the received M initial optical signals to generate N target optical signals; M and N are both positive integers;

[0007] Among them, the multi-mode interference coupling waveguide region includes multiple layers of waveguide core layers stacked in sequence along a first direction; the materials of two adjacent waveguide core layers are different, and the effective refractive index of the initial light signals with different polarization states in the multi-mode interference coupling waveguide region is the same; the first direction is perpendicular to the direction where the substrate is located.

[0008] In some embodiments, the multimode interference coupling waveguide region includes a first waveguide core layer, a second waveguide core layer, and a third waveguide core layer stacked in sequence along the first direction;

[0009] The refractive index of the second waveguide core layer is greater than the refractive indexes of the first waveguide core layer and the third waveguide core layer.

[0010] In some embodiments, the refractive index of the second waveguide core layer ranges from 1.72 to 3.43.

[0011] In some embodiments, the material of the second waveguide core layer is SiNx; wherein the nitridation degree of the second waveguide core layer is inversely proportional to the refractive index of the second waveguide core layer.

[0012] In some embodiments, the refractive index of the first waveguide core layer and the refractive index of the third waveguide core layer are equal.

[0013] In some embodiments, the refractive index of the first waveguide core layer and the refractive index of the third waveguide core layer are both 2.1.

[0014] In some embodiments, the material of the first waveguide core layer and the material of the third waveguide core layer are both LiNbO 3 .

[0015] In some embodiments, the multimode interference coupling waveguide region constitutes M input waveguides, a multimode interference region, and N output waveguides connected along the second direction;

[0016] The M input waveguides are used to receive the M initial optical signals and transmit the M initial optical signals to the multimode interference region; the initial optical signals are generated by an external light source;

[0017] The multimode interference zone is used to perform interference processing on the initial optical signal to generate N target optical signals;

[0018] The N output waveguides are used to receive and output corresponding target optical signals.

[0019] In some embodiments, the multimode interference coupler further comprises:

[0020] A first electrode located on the upper surface of the multi-mode interference region, and a second electrode located on the lower surface of the multi-mode interference region;

[0021] A voltage is applied to the multi-mode interference region through the first electrode and the second electrode, so that the refractive index of the first waveguide core layer and the refractive index of the third waveguide core layer change in the same direction as the voltage changes.

[0022] In a second aspect, an embodiment of the present disclosure provides a method for forming a multimode interference coupler, the method comprising:

[0023] providing a substrate;

[0024] Forming a plurality of initial waveguide core layers stacked in sequence along a first direction on the surface of the substrate; the materials of two adjacent initial waveguide core layers are different;

[0025] Etching the multiple layers of the initial waveguide core layer to form multiple layers of waveguide core layers stacked in sequence along the first direction; the multiple layers of the waveguide core layer constitute a multi-mode interference coupling waveguide region;

[0026] Among them, the multi-mode interference coupling waveguide area is used to perform interference processing on the M received initial optical signals to generate N target optical signals, where M and N are both positive integers; the initial optical signals with different polarization states have the same effective refractive index in the multi-mode interference coupling waveguide area; and the first direction is perpendicular to the direction where the substrate is located.

[0027] An embodiment of the present disclosure provides a multimode interference coupler and a method for forming the same, wherein the multimode interference coupler comprises: a substrate, and a multimode interference coupling waveguide region located on the surface of the substrate; the multimode interference coupling waveguide region is used to perform interference processing on M received initial optical signals to generate N target optical signals; M and N are both positive integers; wherein the multimode interference coupling waveguide region comprises a plurality of waveguide core layers stacked in sequence along a first direction; the materials of two adjacent waveguide core layers are different, and the effective refractive index of the initial optical signals in different polarization states in the multimode interference coupling waveguide region is the same. Here, since the materials of two adjacent waveguide core layers are different, that is, the refractive indexes of the two adjacent waveguide core layers are different, the electric field (E field) of the two adjacent waveguide core layers is discontinuous at the interface, so that the initial optical signals of the transverse magnetic mode (TM mode) and the transverse electric mode (TE mode) can propagate in different waveguide core layers; further, since the effective refractive indexes of the initial optical signals of different polarization modes in the multi-mode interference coupling waveguide region are the same, the propagation speeds and phase changes of the initial optical signals of the TM mode and the TE mode in the multi-mode interference coupling waveguide region are similar, so that no matter what the polarization state of the initial optical signal is, its propagation characteristics will remain consistent, thereby realizing that the multi-mode interference coupler is a polarization-independent device, so that the multi-mode interference coupler can be applicable to optical signals of different polarization states, thereby expanding the application range of the multi-mode interference coupler. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In the accompanying drawings (which are not necessarily drawn to scale), like reference numerals may describe similar components in different views. Like reference numerals with different letter suffixes may represent different examples of similar components. The accompanying drawings generally illustrate various embodiments discussed herein by way of example and not limitation.

[0029] Figure 1 A schematic diagram of a multimode interference coupler provided in an embodiment of the present disclosure Figure 1 ;

[0030] Figure 2 A field distribution of a TE mode in a multimode interference coupler provided in an embodiment of the present disclosure;

[0031] Figure 3 A field distribution of a TM mode in a multimode interference coupler provided in an embodiment of the present disclosure;

[0032] Figure 4 A schematic diagram of a multimode interference coupler provided in an embodiment of the present disclosure Figure 2 ;

[0033] Figure 5 A schematic diagram of a multimode interference coupler provided in an embodiment of the present disclosure Figure 3 ;

[0034] Figure 6 A schematic diagram of a multimode interference coupler provided in an embodiment of the present disclosure Figure 4 ;

[0035] Figure 7 A schematic diagram of a process for forming a multimode interference coupler provided in an embodiment of the present disclosure;

[0036] Figure 8 and Fig. 9 A structural schematic diagram of the multi-mode interference coupler formation process provided in an embodiment of the present disclosure. DETAILED DESCRIPTION

[0037] The exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although the exemplary embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the specific embodiments set forth herein. On the contrary, these embodiments are provided in order to enable a more thorough understanding of the present disclosure and to fully convey the scope of the present disclosure to those skilled in the art.

[0038] In the following description, a large number of details are given to provide a more thorough understanding of the present disclosure. However, it is obvious to those skilled in the art that the present disclosure can be implemented without one or more of these details. In other examples, in order to avoid confusion with the present disclosure, some technical features known in the art are not described; that is, all features of actual embodiments are not described here, and well-known functions and structures are not described in detail.

[0039] In the drawings, the sizes of layers, regions, elements and their relative sizes may be exaggerated for clarity. Like reference numerals denote like elements throughout.

[0040] It should be understood that when an element or layer is referred to as "on ...", "adjacent to ...", "connected to" or "coupled to" other elements or layers, it can be directly on, adjacent to, connected to or coupled to other elements or layers, or there can be intervening elements or layers. On the contrary, when an element is referred to as "directly on ...", "directly adjacent to ...", "directly connected to" or "directly coupled to" other elements or layers, there is no intervening element or layer. It should be understood that although the terms first, second, third, etc. can be used to describe various elements, components, regions, layers and / or parts, these elements, components, regions, layers and / or parts should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or part from another element, component, region, layer or part. Therefore, without departing from the teachings of the present disclosure, the first element, component, region, layer or part discussed below can be represented as the second element, component, region, layer or part. And when the second element, component, region, layer or part is discussed, it does not indicate that the present disclosure necessarily has the first element, component, region, layer or part.

[0041] The purpose of the terms used herein is only to describe specific embodiments and is not intended to be a limitation of the present disclosure. When used herein, the singular forms "one", "an" and "said / the" are also intended to include plural forms, unless the context clearly indicates otherwise. It should also be understood that the terms "consisting of" and / or "comprising", when used in this specification, determine the presence of the features, integers, steps, operations, elements and / or parts, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, parts and / or groups. When used herein, the term "and / or" includes any and all combinations of the relevant listed items.

[0042] The multimode interference coupler and the method for forming the same in the embodiments of the present disclosure are described in detail below with reference to the accompanying drawings.

[0043] Before introducing the embodiments of the present disclosure, the following three directions that may be used to describe the three-dimensional structure are defined. Taking the Cartesian coordinate system as an example, the direction perpendicular to the plane where the substrate is located is defined as the first direction; in the direction of the plane where the substrate is located, two directions that intersect each other (for example, perpendicular to each other) are defined as the second direction and the third direction. In the embodiments of the present disclosure, the first direction is the Z-axis direction, the second direction is the X-axis direction, and the third direction is the Y-axis direction.

[0044] In one embodiment of the present disclosure, see Figure 1, a multimode interference coupler 100 provided in an embodiment of the present disclosure, includes: a substrate 110, and a multimode interference coupling waveguide area 120 located on the surface of the substrate 110; the multimode interference coupling waveguide area 120 is used to perform interference processing on M received initial optical signals to generate N target optical signals; M and N are both positive integers; wherein the multimode interference coupling waveguide area 120 includes a plurality of waveguide core layers stacked in sequence along the Z-axis direction; the materials of two adjacent waveguide core layers are different, and the effective refractive index of the initial optical signals of different polarization modes in the multimode interference coupling waveguide area 120 is the same.

[0045] It should be noted that the effective refractive index of the TM mode optical signal (i.e., the initial optical signal) in the multimode interference coupling waveguide region 120 is: the proportion of the TM mode optical signal in each layer of the waveguide core layer*the refractive index of the corresponding material. The effective refractive index of the TE mode optical signal in the multimode interference coupling waveguide region 120 is: the proportion of the TE mode optical signal in each layer of the waveguide core layer*the refractive index of the corresponding material.

[0046] In this disclosure, please continue to refer to Figure 1 The base 110 may include a substrate; wherein the substrate may be a silicon substrate, a germanium substrate, a silicon germanium substrate, a silicon-on-insulator (SOI) substrate or a germanium on insulator (GOI) substrate, etc.; the substrate may also include other element semiconductors or compound semiconductors, such as gallium arsenide, indium phosphide or silicon carbide, etc.

[0047] In addition, the base 110 may also include a lower waveguide cladding located on the surface of the substrate, the refractive index of the lower waveguide cladding is smaller than the refractive index of the multimode interference coupling waveguide region 120, so that the optical signal can be confined to the core layer for transmission; wherein the material of the lower waveguide cladding may be silicon oxide.

[0048] In the disclosed embodiment, the multimode interference coupler 100 further includes: an upper waveguide cladding (not shown) covering the multimode interference coupling waveguide region 120, wherein the refractive index of the upper waveguide cladding is smaller than the refractive index of the multimode interference coupling waveguide region 120, so that the optical signal can be confined to be transmitted within the multimode interference coupling waveguide region 120; wherein the material of the upper waveguide cladding can be silicon oxide.

[0049] In the embodiment of the present disclosure, the multi-mode interference coupling waveguide region 120 includes multiple waveguide core layers stacked in sequence along the Z-axis direction; here, the number of layers of the waveguide core layer can be set according to actual needs; for example, Figure 1 The multi-mode interference coupling waveguide region 120 shown includes three waveguide core layers stacked in sequence along the Z-axis direction, namely a first waveguide core layer 121 , a second waveguide core layer 122 and a third waveguide core layer 123 .

[0050] In the embodiment of the present disclosure, since the materials of two adjacent waveguide core layers are different, the refractive indexes of the two adjacent waveguide core layers are different, so that the electric field of the optical signal (i.e., the initial optical signal) at the interface of the two adjacent waveguide core layers is discontinuous, and thus the TE mode optical signal is more inclined to be transmitted in the waveguide core layer with a higher refractive index, and the TM mode optical signal is affected by the electric field discontinuity at the interface and will be reflected to the waveguide core layer with a lower refractive index for transmission. In this way, each waveguide core layer can transmit an optical signal of a polarization state; for example, when the refractive index of the second waveguide core layer 122 is greater than that of the first waveguide core layer 121 and the third waveguide core layer 123, the TE mode optical signal is mainly propagated in the second waveguide core layer 122 (e.g., Figure 2 As shown), the TM mode optical signal mainly propagates in the first waveguide core layer 121 and the third waveguide core layer 123 (as shown Figure 3 shown).

[0051] Furthermore, by adjusting the refractive index of each waveguide core layer, the proportion of optical signals of different polarization states in each waveguide core layer can be adjusted, so that the effective refractive index of optical signals of different polarization states in the multimode interference coupling waveguide region 120 is the same. In this way, the propagation speed and phase change of the TM mode and TE mode optical signals in the multimode interference coupling waveguide region are similar, so that regardless of the polarization state of the optical signal, its propagation characteristics will remain consistent, thereby realizing the multimode interference coupler as a polarization-independent device, so that the multimode interference coupler can be applied to optical signals of different polarization states, and expanding the application range of the multimode interference coupler.

[0052] In some embodiments, please refer to Figure 1 The multi-mode interference coupling waveguide region 120 includes a first waveguide core layer 121, a second waveguide core layer 122 and a third waveguide core layer 123 stacked in sequence along the Z-axis direction; wherein the refractive index of the second waveguide core layer 122 is greater than the refractive indexes of the first waveguide core layer 121 and the third waveguide core layer 123 at the same time.

[0053] Thus, on the one hand, according to the different transmission characteristics of the TE mode and the TM mode in the waveguide, the optical signal of the TE mode can be propagated in the second waveguide core layer 122 (such as Figure 2 As shown), the TM mode optical signal propagates in the first waveguide core layer 121 and the third waveguide core layer 123 (as shown Figure 3 On the other hand, since the refractive index of the second waveguide core layer 122 (i.e., the middle layer) is higher, the second waveguide core layer 122 has a stronger limiting effect on the light field, which helps to maintain the stability of the light field, reduce the scattering and loss of the light field, and improve the efficiency and stability of optical signal transmission.

[0054] It should be noted that, in other embodiments, when the refractive index of the second waveguide core layer 122 in the multi-mode interference coupling waveguide region 120 is the lowest, although the TE mode and TM mode optical signals can also be transmitted through the multi-mode interference coupling waveguide region 120, due to this refractive index distribution, the light field may not be effectively confined, so that the light may be reflected and refracted multiple times between different media, causing the distribution of the light field to become complicated and difficult to control.

[0055] In some embodiments, please refer to Figure 1 The refractive index of the second waveguide core layer 122 ranges from 1.72 to 3.43.

[0056] In this way, since the refractive index of the second waveguide core layer 122 has a wider selection range, the material of the second waveguide core layer 122 can be selected from a wider range, thereby reducing material costs.

[0057] In some embodiments, please refer to Figure 1 , the material of the second waveguide core layer 122 is SiNx; wherein the nitridation degree of the second waveguide core layer 122 is inversely proportional to the refractive index of the second waveguide core layer 122 .

[0058] In the disclosed embodiment, the refractive index of the second waveguide core layer 122 can be controlled by controlling the N (nitrogen) component during the growth of the second waveguide core layer 122; wherein, the higher the N component, the higher the degree of nitridation of SiNx and the lower the refractive index.

[0059] In this way, the refractive index of the second waveguide core layer 122 can be designed according to specific application requirements to improve the optical performance of the waveguide core layer (such as reducing the transmission loss of light in the waveguide, improving the transmission efficiency of the optical signal, etc.), and the design flexibility is high. In addition, SiNx, as a common semiconductor material, has good compatibility with existing semiconductor manufacturing processes, which helps to reduce manufacturing costs and improve manufacturing reliability and consistency; SiNx materials also have good chemical stability and thermal stability, so that the multimode interference coupler 100 can still maintain stable performance in harsh environments.

[0060] In some embodiments, the refractive index of the first waveguide core layer 121 and the refractive index of the third waveguide core layer 123 are equal.

[0061] In this way, on the one hand, the possibility of reflection and scattering at the interface of the two waveguide core layers during the transmission of the optical signal is reduced, which helps to reduce the loss of the optical signal during the transmission process and improve the transmission efficiency of the optical signal. On the other hand, the distribution of the optical field can be evenly controlled, making the transmission of the optical signal in the waveguide more stable and reducing the performance degradation caused by uneven distribution of the optical field.

[0062] In some embodiments, the refractive index of the first waveguide core layer 121 and the refractive index of the third waveguide core layer 123 are both 2.1.

[0063] At this time, the refractive index of the second waveguide core layer 122 can be designed to be 2.4; in this way, the effective refractive index of the TM mode and TE mode optical signals propagating in the multi-mode interference coupling waveguide region 120 can be the same (or close), thereby making the beat lengths of the TM mode and TE mode optical signals equal, that is, the propagation speed and phase change in the multi-mode interference coupling waveguide region are similar, so that no matter what the polarization state of the initial optical signal is, its propagation characteristics will remain consistent, thereby realizing the multi-mode interference coupler as a polarization-independent device.

[0064] Specifically, the beat length of the initial optical signal can be calculated according to the following formula (1):

[0065] Lπ= 4nrWeff 2 / 3λ Formula (1)

[0066] Where L is the interference period length between two adjacent modes in the multimode interference coupler, nr is the effective refractive index, λ is the wavelength, and Weff is the effective width of the waveguide in the multimode interference coupler.

[0067] It can be seen from the above formula that Lπ(λ, TE)=Lπ(λ, TM), that is, the beat lengths of the initial optical signals of the TM mode and the TE mode are equal.

[0068] In other embodiments, the refractive index of the second waveguide core layer 122 can also be designed to be other achievable values, for example, 3.0; in this case, the refractive indices of the first waveguide core layer 121 and the third waveguide core layer 123 can be adjusted by applying voltage to the first waveguide core layer 121 and the third waveguide core layer 123, so that the effective refractive indices of the TM mode optical signal and the TE mode optical signal propagating in the multi-mode interference coupling waveguide region 120 are the same.

[0069] In some embodiments, the material of the first waveguide core layer 121 and the material of the third waveguide core layer 123 are both LiNbO 3 .

[0070] In the embodiment of the present disclosure, since LiNbO3 is a nonlinear optical material, an electro-optic effect (or Pockels effect) can be generated, that is, when an external electric field is applied, the refractive index of LiNbO3 increases as the voltage increases. In this way, the refractive index of the first waveguide core layer 121 and the third waveguide core layer 123 can be changed by applying a voltage to the first waveguide core layer 121 and the third waveguide core layer 123.

[0071] In addition, when the material of the second waveguide core layer 122 is SiNx, since SiNx is not an electro-optical material, its refractive index is not sensitive to the change of voltage. Therefore, a voltage can be directly applied to the multimode interference coupler to change the refractive index of the first waveguide core layer 121 and the third waveguide core layer 123, so that the effective refractive index of the TM mode optical signal and the TE mode optical signal propagating in the multimode interference coupling waveguide region 120 is the same, thereby realizing that the multimode interference coupler is a polarization-independent device, so that the multimode interference coupler can be applied to optical signals of different polarization states, thereby expanding the application range of the multimode interference coupler.

[0072] In summary, the refractive index of the second waveguide core layer 122 can be designed by designing the degree of nitridation of the second waveguide core layer 122. Even if the refractive index of the second waveguide core layer 122 is not accurately adjusted, the refractive indices of the first waveguide core layer 121 and the third waveguide core layer 123 can be adjusted by applying voltage to the multimode interference coupler, so that the effective refractive indices of the TM mode optical signal and the TE mode optical signal in the multimode interference coupling waveguide region 120 are equal, which makes the design more flexible and adjustable and reduces the manufacturing difficulty of the process.

[0073] Next, please refer to Figure 4 and Figure 5 , the multimode interference coupler 100 is introduced in detail.

[0074] In some embodiments, please refer to Figure 4 and Figure 5 The multimode interference coupling waveguide region 120 constitutes M input waveguides 120a, a multimode interference region 120b and N output waveguides 120c extending along the X-axis direction; the multimode interference region 120b is connected between the M input waveguides 120a and the N output waveguides 120c.

[0075] Among them, M input waveguides 120a are used to receive M initial optical signals and transmit the M initial optical signals to the multimode interference area 120b; the initial optical signal is generated by an external light source; the multimode interference area 120b is used to perform interference processing on the initial optical signal to generate N target optical signals; and N output waveguides 120c are used to receive and output corresponding target optical signals.

[0076] In this disclosure, please continue to refer to Figure 4 and Figure 5 , M is 2 and N is 2 as an example for explanation; in other embodiments, M and N can be other achievable numbers, for example, M can be 1 and N can be 2; for another example, M can be 2 and N can be 4, and so on.

[0077] In addition, please continue to refer to Figure 4 and Figure 5, the input waveguide 120a is responsible for introducing M (2 in the figure) initial optical signals generated by the external light source into the device, ensuring that the initial optical signals can be efficiently and losslessly coupled into the multimode interference region 120b. Further, the external light source can be any device or system that can emit an optical signal, for example, a laser (such as a semiconductor laser, a fiber laser, etc.), a light emitting diode (LED), or an optical fiber that transmits the initial optical signal to the input waveguide 120a.

[0078] Please continue to refer to Figure 4 and Figure 5 , the multi-mode interference zone 120b realizes the transmission and processing of the initial optical signal by supporting multiple high-order waveguide modes and utilizing the interference effect and self-imaging effect between these modes, thereby generating N (2 in the figure) target optical signals. Specifically, when M initial optical signals enter the multi-mode interference zone 120b, the optical field will be decomposed according to the intrinsic mode of the multi-mode interference zone 120b, and multiple guided modes will be excited; due to the different propagation constants of guided modes of different orders, they will accumulate different phase differences during the propagation process, so that these guided modes maintain the same amplitude during the propagation process, but the phase will change; when the initial optical signal propagates to a certain distance, multi-mode interference will occur between these guided modes, causing the optical field to re-present the optical field distribution consistent with the input field at the self-imaging position (i.e., the self-imaging effect), so that N target optical signals can be generated at the output end of the multi-mode interference zone 120b.

[0079] Furthermore, according to the incident position and mode excitation of the initial light signal (i.e., the incident light field), multimode interference can be divided into general interference and restricted interference. In general interference, the incident position of the initial light signal is arbitrary, and all orders of guided modes can be excited. Restricted interference includes paired interference and symmetric interference. The latter can selectively excite certain orders of guided modes and suppress other orders of guided modes when incident at a specific position. Therefore, the multimode interference coupler 100 can be divided into general imaging type, paired interference type, etc. according to the incident position and mode excitation of the initial light signal, and different types of multimode interference couplers 100 can be selected according to actual applications.

[0080] Please continue to refer to Figure 4 and Figure 5 The N output waveguides 120c are responsible for exporting the N target optical signals obtained after being processed by the multi-mode interference region 120b to the device.

[0081] It should be noted that the input waveguide 120a, the multimode interference region 120b and the output waveguide 120c in the embodiment of the present disclosure are multimode waveguides composed of a multimode interference coupling waveguide region, that is, they are all composed of a first waveguide core layer, a second waveguide core layer and a third waveguide core layer; for example, Figure 4 and Figure 5 The multimode interference region 120b shown in the figure includes a first waveguide core layer 121b, a second waveguide core layer 122b and a third waveguide core layer 123b. In this way, not only can multiple modes be transmitted simultaneously, thereby improving the transmission capacity of the multimode interference coupler, but also the continuity and stability of the optical signal during the transmission process can be guaranteed. Furthermore, since the materials of the multimode interference coupler 100 are SiNx and LiNbO3, the refractive index difference between the core layer (i.e., the multimode interference coupler 100) and the cladding (i.e., the upper waveguide cladding and the lower waveguide cladding) is relatively large, thereby making the phase error and propagation loss of light small when it is transmitted in the multimode interference coupler 100. Therefore, based on the self-imaging principle, a multimode interference coupler 100 with low loss, small phase deviation and wide spectrum can be designed.

[0082] It should be noted that the input waveguide 120a and the output waveguide 120c in the embodiment of the present disclosure may be straight waveguides (eg Figure 4 As shown), it can also be a tapered waveguide (as Figure 5 ), the present disclosure is not limited to this.

[0083] In some embodiments, please refer to Figure 6 The multimode interference coupler 100 further includes: a first electrode 131 located on the upper surface of the multimode interference region 120b, and a second electrode 132 located on the lower surface of the multimode interference region 120b; a voltage is applied to the multimode interference region 120b by the first electrode 131 and the second electrode 132, so that the refractive index of the first waveguide core layer 121 and the refractive index of the third waveguide core layer 123 change in the same direction as the voltage changes.

[0084] In the embodiment of the present disclosure, since the first waveguide core layer 121 and the third waveguide core layer 123 are made of LiNbO3, and the refractive index of LiNbO3 increases with the increase of voltage, the refractive index of the first waveguide core layer 121 and the third waveguide core layer 123 can be changed by applying voltage.

[0085] For example, when the refractive index of the second waveguide core layer 122 is 3.0, the refractive indexes of the first waveguide core layer 121 and the third waveguide core layer 123 are 2.1. Since the material of the second waveguide core layer 122 is SiNx, and SiNx is not sensitive to voltage changes, a voltage can be directly applied to the multimode interference coupler to increase the refractive indexes of the first waveguide core layer 121 and the third waveguide core layer 123, so that the effective refractive indexes of optical signals of different polarization states in the multimode interference coupling waveguide region 120 are equal, thereby realizing that the multimode interference coupler is a polarization-independent device, and further making the multimode interference coupler applicable to optical signals of different polarization states, thereby expanding the application range of the multimode interference coupler.

[0086] In the disclosed embodiment, the multimode interference coupler 100 may be integrated on a silicon photonic chip, thus meeting the requirements of small size, low power consumption and low maintenance cost of the multimode interference coupler 100.

[0087] In addition, the present disclosure also provides a method for forming a multi-mode interference coupler. Figure 7 A schematic diagram of a process for forming a multimode interference coupler provided in an embodiment of the present disclosure, Figure 8 and Fig. 9 A structural schematic diagram of the multi-mode interference coupler formation process provided in an embodiment of the present disclosure.

[0088] like Figure 7 As shown, the method for forming a multi-mode interference coupler includes: steps S110 to S130.

[0089] Step S110, providing a substrate.

[0090] In the embodiments of this disclosure, please refer to Figure 8 The base 110 may include a substrate; wherein the substrate may be a silicon substrate, a germanium substrate, a silicon germanium substrate, a silicon-on-insulator substrate or a germanium-on-insulator substrate, etc.; the substrate may also include other element semiconductors or compound semiconductors, such as gallium arsenide, indium phosphide or silicon carbide, etc.

[0091] In addition, the base 110 may further include a lower waveguide cladding located on the surface of the substrate, wherein the refractive index of the lower waveguide cladding is less than the refractive index of the multimode interference coupling waveguide region 120, so that the optical signal can be confined to be transmitted within the multimode interference coupler; wherein the material of the lower waveguide cladding may be silicon oxide.

[0092] Step S120, forming a plurality of initial waveguide core layers stacked in sequence along the X-axis direction on the surface of the substrate; the materials of two adjacent initial waveguide core layers are different.

[0093] Next, to form Figure 5 The multi-mode interference coupling waveguide region 120 shown is taken as an example for explanation.

[0094] In this disclosure, please continue to refer to Figure 8 An initial first waveguide core layer 121 ′, an initial second waveguide core layer 122 ′ and an initial third waveguide core layer 123 ′ may be sequentially formed on the surface of the substrate 110 by any suitable deposition process.

[0095] In the embodiment of the present disclosure, since the materials of two adjacent initial waveguide core layers are different, the refractive indices of the two adjacent waveguide core layers are different, so that the TE mode optical signal is more inclined to be transmitted in the waveguide core layer with a higher refractive index, and the TM mode optical signal is affected by the electric field discontinuity at the interface and will be reflected to the waveguide core layer with a lower refractive index for transmission. In this way, the multi-mode interference coupling waveguide region 120 can transmit optical signals of different polarization states.

[0096] In the embodiment of the present disclosure, the material of the initial first waveguide core layer 121' and the initial third waveguide core layer 123' may be LiNbO3; the material of the initial second waveguide core layer 122' may be SiNx. Here, the refractive index of the initial second waveguide core layer 122' (i.e., the second waveguide core layer 122 formed subsequently) may be adjusted by adjusting the degree of nitridation of SiNx.

[0097] In the disclosed embodiment, the refractive index of the subsequently formed second waveguide core layer 122 can be controlled by controlling the N (nitrogen) component during the growth of the initial second waveguide core layer 122'; wherein the higher the N component, the higher the degree of nitridation of SiNx and the lower the refractive index.

[0098] In addition, since LiNbO3 is a nonlinear optical material, it can produce an electro-optical effect (or Pockels effect), that is, when an external electric field is applied, the refractive index of LiNbO3 increases as the voltage increases. In this way, the refractive indexes of the first waveguide core layer 121 and the third waveguide core layer 123 formed subsequently can be changed by applying a voltage to the first waveguide core layer 121 and the third waveguide core layer 123.

[0099] Step S130 , etching the multiple initial waveguide core layers to form multiple waveguide core layers stacked in sequence along the X-axis direction; the multiple waveguide core layers constitute the multi-mode interference coupling waveguide region 120 .

[0100] The multimode interference coupling waveguide region 120 is used to perform interference processing on the received M initial optical signals to generate N target optical signals; M and N are both positive integers; and the initial optical signals with different polarization states have the same effective refractive index in the multimode interference coupling waveguide region 120 .

[0101] Here, please refer to Fig. 9First, a patterned mask layer 140 may be formed on the surface of the initial third waveguide core layer 123'; wherein the patterned mask layer 140 may be aligned with the projection of the multimode interference coupling waveguide region 120 along the Z-axis direction (eg, Figure 5 Next, the initial third waveguide core layer 123', the initial second waveguide core layer 122' and the initial first waveguide core layer 121' can be etched by a dry etching process (such as a plasma etching process, a reactive ion etching process or an ion milling process) to form a Figure 5 The third waveguide core layer 123, the second waveguide core layer 122 and the first waveguide core layer 121 (ie, the multi-mode interference coupling waveguide region 120) are shown.

[0102] Finally, please refer to Figure 5 After forming the multi-mode interference coupling waveguide region 120, the method further includes: removing the mask layer 140, and forming an upper waveguide cladding layer (not shown) covering the multi-mode interference coupling waveguide region 120. The material of the upper waveguide cladding layer may be silicon oxide.

[0103] In this way, the upper waveguide cladding and the lower waveguide cladding constitute the cladding, and the multimode interference coupling waveguide region 120 constitutes the core layer; since the refractive index of the core layer material (SiNx and LiNbO3) is higher than that of the cladding (silicon oxide), the optical signal is totally reflected at the interface between the core layer and the cladding, and is thereby confined to propagate within the core layer.

[0104] In the disclosed embodiment, the multimode interference coupling waveguide region 120 comprises M input waveguides 120a, a multimode interference region 120b and N output waveguides 120c; the multimode interference region 120b is connected between the M input waveguides 120a and the N output waveguides 120c.

[0105] In some embodiments, Figure 6 The method for forming the multimode interference coupler shown also includes: forming a first electrode 131 located on the upper surface of the multimode interference region 120b, and forming a second electrode 132 located on the lower surface of the multimode interference region 120b; applying a voltage to the multimode interference region 120b through the first electrode 131 and the second electrode 132, so that the refractive index of the first waveguide core layer 121 and the refractive index of the third waveguide core layer 123 change in the same direction as the voltage changes.

[0106] In the embodiment of the present disclosure, since the first waveguide core layer 121 and the third waveguide core layer 123 are composed of LiNbO3, and the refractive index of LiNbO3 increases with the increase of voltage, the refractive index of the first waveguide core layer 121 and the third waveguide core layer 123 can be changed by applying voltage, so that the effective refractive index of optical signals with different polarization states in the multi-mode interference coupling waveguide region is the same.

[0107] In the disclosed embodiment, the formed multimode interference coupler 100 is a polarization-independent device, that is, the multimode interference coupler can be applied to optical signals of different polarization states, thereby expanding the application scope of the multimode interference coupler.

[0108] In the disclosed embodiment, the multimode interference coupler 100 formed is an optical device based on the interference effect, which uses the self-imaging effect to realize the processing and conversion of optical signals. In optical communications, it is used for multiplexing and demultiplexing of optical signals to improve data transmission performance; in laser radar, it enhances multi-target detection capabilities. Since the multimode interference coupler 100 is formed on a silicon photonic chip, its structure is compact and simple to manufacture; at the same time, the multimode interference coupler 100 has wavelength selectivity and can effectively filter and distribute optical signals. In addition, multimode interference couplers are also used in optical imaging systems as lenses and filters to improve imaging resolution.

[0109] The multimode interference coupler formed by the method for forming a multimode interference coupler provided in the embodiment of the present disclosure is similar to the multimode interference coupler in the above-mentioned embodiment. For the technical features not fully disclosed in the embodiment of the present disclosure, please refer to the above-mentioned embodiment for understanding, and no further details will be given here.

[0110] In the several embodiments provided in the present disclosure, it should be understood that the disclosed structures and methods can be implemented in a non-targeted manner. The structural embodiments described above are only schematic. For example, the division of units is only a logical function division. There may be other division methods in actual implementation, such as: multiple units or components can be combined, or can be integrated into another system, or some features can be ignored or not executed. In addition, the components shown or discussed are coupled or directly coupled to each other.

[0111] The features disclosed in several method or structural embodiments provided in the present disclosure may be arbitrarily combined without conflict to obtain new method embodiments or structural embodiments.

[0112] The above are only some embodiments of the present disclosure, but the protection scope of the present disclosure is not limited thereto. Any technician familiar with the technical field can easily think of changes or substitutions within the technical scope disclosed in the present disclosure, which should be included in the protection scope of the present disclosure. Therefore, the protection scope of the present disclosure should be based on the protection scope of the claims.

Claims

1. A multimode interference coupler, characterized in that: It comprises: a substrate, and a multi-mode interference coupling waveguide region located on the surface of the substrate; The multi-mode interference coupling waveguide region is used to perform interference processing on the M received initial optical signals to generate N target optical signals; M and N are both positive integers; Among them, the multi-mode interference coupling waveguide region includes multiple layers of waveguide core layers stacked in sequence along a first direction; the materials of two adjacent waveguide core layers are different, and the effective refractive index of the initial light signals with different polarization states in the multi-mode interference coupling waveguide region is the same; the first direction is perpendicular to the direction where the substrate is located.

2. The multimode interference coupler according to claim 1, characterized in that: The multi-mode interference coupling waveguide region comprises a first waveguide core layer, a second waveguide core layer and a third waveguide core layer sequentially stacked along the first direction; The refractive index of the second waveguide core layer is greater than the refractive indexes of the first waveguide core layer and the third waveguide core layer.

3. The multimode interference coupler according to claim 2, characterized in that: The refractive index of the second waveguide core layer ranges from 1.72 to 3.

43.

4. The multimode interference coupler according to claim 3, characterized in that: The material of the second waveguide core layer is SiNx; wherein the nitridation degree of the second waveguide core layer is inversely proportional to the refractive index of the second waveguide core layer.

5. The multimode interference coupler according to claim 2, characterized in that: The refractive index of the first waveguide core layer is equal to the refractive index of the third waveguide core layer.

6. The multimode interference coupler according to claim 5, characterized in that: The refractive index of the first waveguide core layer and the refractive index of the third waveguide core layer are both 2.

1.

7. The multimode interference coupler according to claim 6, characterized in that: The material of the first waveguide core layer and the material of the third waveguide core layer are both LiNbO3.

8. The multimode interference coupler according to any one of claims 2 to 7, characterized in that: The multimode interference coupling waveguide region constitutes M input waveguides, a multimode interference region and N output waveguides connected along the second direction; The M input waveguides are used to receive the M initial optical signals and transmit the M initial optical signals to the multimode interference region; the initial optical signals are generated by an external light source; The multimode interference zone is used to perform interference processing on the initial optical signal to generate N target optical signals; The N output waveguides are used to receive and output corresponding target optical signals.

9. The multimode interference coupler according to claim 8, characterized in that: The multimode interference coupler further comprises: A first electrode located on the upper surface of the multi-mode interference region, and a second electrode located on the lower surface of the multi-mode interference region; A voltage is applied to the multi-mode interference region through the first electrode and the second electrode, so that the refractive index of the first waveguide core layer and the refractive index of the third waveguide core layer change in the same direction as the voltage changes.

10. A method for forming a multimode interference coupler, characterized in that: The method comprises: providing a substrate; Forming a plurality of initial waveguide core layers stacked in sequence along a first direction on the surface of the substrate; the materials of two adjacent initial waveguide core layers are different; Etching the multiple layers of the initial waveguide core layer to form multiple layers of waveguide core layers stacked in sequence along the first direction; the multiple layers of the waveguide core layer constitute a multi-mode interference coupling waveguide region; Among them, the multi-mode interference coupling waveguide area is used to perform interference processing on the M received initial optical signals to generate N target optical signals, where M and N are both positive integers; the initial optical signals with different polarization states have the same effective refractive index in the multi-mode interference coupling waveguide area; and the first direction is perpendicular to the direction where the substrate is located.

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