CN113900280B - Polarization-independent optical switch - Google Patents

Abstract

The invention discloses a polarization-independent optical switch, which relates to the field of semiconductors, and is of a bilateral symmetry structure and comprises: the lithium niobate waveguide layer is integrated above the lower cladding layer, the silicon nitride layer is above the lithium niobate waveguide layer, and the upper cladding layer is filled between the lithium niobate waveguide layer and the silicon nitride layer; etching the upper half parts of the uniform light splitting multimode interference coupler and the upper and lower interlayer coupling structures in the silicon nitride layer in sequence along the light propagation direction; etching the lower half part of the upper and lower interlayer coupling structures and the polarization-independent modulation waveguide on the lithium niobate waveguide layer; and manufacturing metal electrodes on two sides of the polarization-independent modulation waveguide of the lithium niobate waveguide layer, and adjusting the positions of the metal electrodes according to the modulation characteristics of the lithium niobate waveguide layer. The optical switch utilizes the silicon nitride material to realize polarization independence of basic passive devices, and simultaneously utilizes the electro-optical characteristic of the lithium niobate material to realize low-loss high-speed modulation characteristic.

Description

Polarization-independent optical switch

Technical Field

The invention relates to the field of semiconductor technology, and in particular to a polarization-independent optical switch.

Background technique

In the information age, the emergence of cloud and data-intensive computing has led to an increase in data communication volume and a continuous increase in bandwidth requirements. In data exchange networks, the development of electrical switches has gradually encountered bottlenecks and is gradually unable to meet current performance requirements such as large bandwidth, low power consumption, and low latency. Optical communication technology has the advantages of large bandwidth, low latency, and low power consumption, providing a feasible technical solution for current communication networks.

As a key component in optical communication networks, optical switches are mainly used in large data exchange centers and supercomputers. They are usually required to have characteristics such as large bandwidth, low loss and polarization independence. In addition, in data packet switching, the switching speed of optical switches needs to reach nanoseconds to avoid data packet loss. Therefore, achieving large bandwidth, low loss, high switching speed and polarization independence at the same time is an urgent problem that optical switches need to solve.

Optical switches can be divided into two types according to their working principles: wavelength routing switching and path switching. Wavelength routing switches the optical path by changing the carrier wavelength. Although its communication bandwidth is narrow and the channel data carrying capacity is limited, its principle is simple and easy to implement. The data communication capacity can be increased by adding channels. Therefore, wavelength routing is still the main application in practice. Path switching arrays can be divided into space type and waveguide type. The space type is mainly MEMS, which is very popular in commercial use due to its low cost. The waveguide type can be divided into different material types such as silicon dioxide, silicon, III-V materials, etc. Compared with the first two, the performance is superior, but it is still in the research stage.

Silicon-based photonics is an emerging discipline based on silicon and silicon-based substrate materials, using the current advanced complementary metal oxide (CMOS) process to develop and integrate optical devices, and has been widely studied. Therefore, the current waveguide optical switches are mainly made of silicon materials, with a few using new materials such as lithium niobate and new phase change materials. The former uses the silicon-based plasma dispersion effect, through the form of ridge waveguide, process ion implantation to form a PIN structure, and achieve nanosecond switching speeds. The latter uses some electro-optical effects of the material, and can also achieve nanosecond or even faster switching speeds.

However, whether it is a silicon-based optical switch based on a traditional silicon-based plasma dispersion effect PIN structure or an optical switch using new materials, it is difficult to achieve polarization independence and low loss and high switching speed performance at the same time. In practical applications, optical switches, as data exchange nodes in communication networks, are usually required to have characteristics such as large bandwidth, polarization independence, low loss, and high switching speed.

Existing optical switches have the following disadvantages:

(1) In silicon-based high-speed optical switches that utilize the silicon-based plasma dispersion effect, carrier injection introduces additional light absorption losses, which results in relatively large losses in actual use.

(2) Silicon-based high-speed optical switches that utilize silicon-based plasma dispersion effects are difficult to implement due to the harsh polarization-independent waveguide conditions of thin-film silicon materials.

(3) High-speed optical switches are realized using new materials such as lithium niobate and phase change materials. The etching process of lithium niobate is difficult, while the loss of phase change materials is large.

In summary, how to provide a low-loss, high-speed, polarization-independent optical switch structure has become one of the technical problems that need to be solved urgently.

Summary of the invention

The present invention aims to solve one of the technical problems in the related art at least to a certain extent.

To this end, the purpose of the present invention is to provide a polarization-independent optical switch, which solves the technical problem of polarization-independent characteristics that are difficult to achieve with silicon waveguides or other materials, and at the same time solves the technical problem of introducing large additional losses during electro-optical modulation.

To achieve the above object, an embodiment of the present invention provides a polarization-independent optical switch, which has a bilaterally symmetrical structure and includes:

A lower cladding layer, a lithium niobate waveguide layer, an upper cladding layer and a silicon nitride layer, wherein the lithium niobate waveguide layer is integrated above the lower cladding layer, the silicon nitride layer is above the lithium niobate waveguide layer, and the upper cladding layer is filled between the lithium niobate waveguide layer and the silicon nitride layer;

Sequentially etching the upper part of the uniform light splitting multimode interference coupler and the upper and lower interlayer coupling structure on the silicon nitride layer along the light propagation direction, wherein the upper part is connected to the output of the uniform light splitting multimode interference coupler;

Etching the lower half of the upper and lower interlayer coupling structure and the polarization-independent modulation waveguide on the lithium niobate waveguide layer, wherein the lower half is connected to the polarization-independent modulation waveguide; and

Metal electrodes are fabricated on both sides of the polarization-independent modulation waveguide of the lithium niobate waveguide layer, and positions of the metal electrodes are adjusted according to the modulation characteristics of the lithium niobate waveguide layer.

The polarization-independent optical switch of the embodiment of the present invention provides a novel optical switch heterogeneously integrated with silicon nitride material and lithium niobate material, wherein a lithium niobate wafer on an insulator is used to etch a modulation waveguide of a modulation arm of the optical switch, and a silicon nitride film is grown on the lithium niobate to etch basic passive components such as a multimode interference coupler. The optical switch structure utilizes silicon nitride material to achieve polarization independence of basic passive components, and utilizes the electro-optical properties of lithium niobate material to achieve low-loss and high-speed modulation characteristics.

In addition, the polarization-independent optical switch according to the above embodiment of the present invention may also have the following additional technical features:

Furthermore, in one embodiment of the present invention, a hole is opened above the metal electrode to be exposed to the air.

Furthermore, in one embodiment of the present invention, light is input from a port of the uniformly split multimode interference coupler, coupled to the polarization-independent modulation waveguide through the upper and lower interlayer coupling structure, an external voltage is applied to the metal electrode to change the phase of the light so that the optical path is changed, and then light is output from a port of the uniformly split multimode interference coupler through the upper and lower interlayer coupling structure.

Furthermore, in one embodiment of the present invention, the upper cladding layer and the lower cladding layer are made of silicon dioxide material.

Furthermore, in one embodiment of the present invention, the silicon nitride layer has a structure with a gradually narrowing width.

Furthermore, in one embodiment of the present invention, the length of the gradually narrowing structure of the silicon nitride layer and the interlayer spacing between the lithium niobate waveguide layer and the silicon nitride layer are changed to achieve low-loss coupling between the transverse electric mode and the transverse magnetic mode.

Furthermore, in one embodiment of the present invention, the uniform light-splitting multimode interference coupler is a polarization-independent 2×2 uniform light-splitting multimode interference coupler, including four input and output curved waveguides and an intermediate multimode waveguide.

Furthermore, in one embodiment of the present invention, in the process of adjusting the position of the metal electrode according to the modulation characteristics of the lithium niobate waveguide layer, the distribution of the electric field lines in the waveguide is changed to make the modulation efficiency of the transverse electric mode and the transverse magnetic mode the same.

Additional aspects and advantages of the present invention will be given in part in the following description and in part will be obvious from the following description, or will be learned through practice of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and/or additional aspects and advantages of the present invention will become apparent and easily understood from the following description of the embodiments in conjunction with the accompanying drawings, in which:

FIG1 is a schematic diagram of the structure of a polarization-independent optical switch according to an embodiment of the present invention;

FIG2 is a diagram showing simulation results of single-mode conditions of silicon nitride material according to an embodiment of the present invention;

FIG3 is a top view of a silicon nitride layer in an optical switch according to an embodiment of the present invention;

FIG. 4 is a schematic cross-sectional view of a lithium niobate waveguide layer in an optical switch according to an embodiment of the present invention.

Figure numerals: lower cladding layer-1; lithium niobate waveguide layer-2; silicon nitride layer-3; uniform light-splitting multimode interference coupler-4, 9; upper and lower layer coupling structure-5, 8; polarization-independent modulation waveguide-6; metal electrode-7.

Detailed ways

Embodiments of the present invention are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present invention, and should not be construed as limiting the present invention.

Currently, common high-speed optical switches are usually implemented using silicon-based plasma dispersion effects. Ion doping is introduced into silicon waveguides, and the effective refractive index of the waveguide mode is changed by applying electrical signals to achieve the purpose of phase shift. The injection of carriers not only causes additional phase shift, but also brings additional losses. At the same time, the polarization-independent conditions of thin-film silicon materials are harsh. High-speed optical switches implemented using silicon-based plasma dispersion effects are difficult to achieve polarization-independence, which is a technical problem existing in current silicon-based high-speed optical switches.

In addition to the silicon-based plasma dispersion effect, the common use of phase change materials to achieve high-speed optical switches also has the problems of high loss, polarization dependence and difficulty in heterogeneous integration. The embodiment of the present invention proposes a polarization-independent optical switch structure with low loss and high switching speed. Through the heterogeneous integration of silicon nitride materials and lithium niobate materials, the technical problem of polarization-independent characteristics that are difficult to achieve with silicon waveguides or other materials is solved, and the technical problem of introducing large additional losses during electro-optical modulation is also solved.

The polarization-insensitive optical switch proposed according to the embodiments of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 is a schematic structural diagram of a polarization-independent optical switch according to an embodiment of the present invention.

As shown in FIG. 1 , the polarization-independent optical switch has a bilaterally symmetrical structure.

As shown in FIG. 1 , the polarization-independent optical switch includes a lower cladding layer 1 , a lithium niobate waveguide layer 2 , an upper cladding layer (not shown in the figure), and a silicon nitride layer 3 .

The lower cladding layer 1 is at the bottom layer, a lithium niobate waveguide layer 2 is integrated above the lower cladding layer 1, a silicon nitride layer 3 is integrated above the lithium niobate waveguide layer 2, and an upper cladding layer is filled between the lithium niobate waveguide layer and the silicon nitride layer.

In one embodiment of the present invention, both the upper cladding layer and the lower cladding layer are made of silicon dioxide material.

Along the light propagation direction (from left to right in FIG1 ), a uniformly splitting multimode interference coupler 4 (MMI, Multimode Interference), an upper and lower interlayer coupling structure 5, a polarization-independent modulation waveguide 6, a metal electrode 7, a uniformly splitting multimode interference coupler 9 and an upper and lower interlayer coupling structure 8 are etched in sequence.

Specifically, the uniform light splitting multimode interference coupler MMI is etched on the silicon nitride layer and is a polarization-independent 2×2 uniform light splitting multimode interference coupler, including 4 input and output bending waveguides and an intermediate multimode waveguide.

The upper half of the upper and lower interlayer coupling structure 5 is etched in the silicon nitride layer, and the lower half is etched in the lithium niobate waveguide layer. The upper half is connected to the output of the uniform light splitting multimode interference coupler 4 (2×2MMI), and the lower half is connected to the polarization-independent modulation waveguide 6. Thus, light coupling between different layers is achieved.

The polarization-independent modulation waveguide 6 and the metal electrode 7 are made on the lithium niobate waveguide layer. The metal electrodes 7 are made on both sides of the polarization-independent modulation waveguide 6. A hole is opened above the metal electrode 7 to expose it to the air. The light phase is changed by applying an external voltage to the metal electrode.

Furthermore, the position of the metal electrode can be adjusted according to the modulation characteristics of the lithium niobate waveguide layer, thereby changing the distribution of the electric field lines in the waveguide, making the modulation efficiency of the transverse electric mode and the transverse magnetic mode the same, and achieving polarization independence.

The optical switch has a bilaterally symmetrical structure, so the structures of the uniformly splitting multimode interference coupler 9 and the upper and lower interlayer coupling structure 8 on the right are the same as those of the uniformly splitting multimode interference coupler 4 and the upper and lower interlayer coupling structure 5 on the left.

It is understandable that in the embodiments of the present invention, polarization-independent passive devices are designed and manufactured by silicon nitride materials, which reduces the difficulty of device processing, and utilizes the anisotropic properties of lithium niobate materials and superior electro-optical properties to achieve low-loss and high-switching speed functions, and polarization-independence is achieved. Therefore, the switch structure of the embodiments of the present invention can be changed in many ways, such as microrings and MZI (Mach-Zehnder interferometer).

In one embodiment of the present invention, light is input from one of the ports of the uniform-splitting multimode interference coupler 4, coupled to the polarization-independent modulation waveguide 6 through the upper and lower interlayer coupling structure 5, and the light phase is changed by applying voltage to the metal electrode 7 to change the optical path, and finally the light is output from one of the ports of the second uniform-splitting multimode interference coupler 9.

As shown in FIG. 2, the simulation results of the single-mode condition of silicon nitride material are shown. It can be seen from the simulation results that the polarization-independent characteristics can be achieved when the height of the silicon nitride material is 900nm and the width is about 700nm. As shown in the figure, the effective refractive index curves of the TE mode (transverse electric mode, the electric field is only polarized along the transverse direction) and the TM mode (transverse magnetic mode, the magnetic field is only polarized along the transverse direction) intersect. This size structure is relatively large and can be easily realized by process manufacturing, reducing the difficulty of process etching. Based on this result, the present invention can realize a polarization-independent MMI basic passive device by design.

As shown in FIG3 , Si3N4taper is a silicon nitride taper structure, and LN Waveguide is a lithium niobate waveguide. The silicon nitride designed in the embodiment of the present invention is a tapered width taper (gradually narrowing) structure. When the light field propagates in the silicon nitride waveguide, it gradually diverges and couples with the lithium niobate waveguide. The light gradually couples from the silicon nitride waveguide to the lithium niobate waveguide. The light path is reversible, and the reverse propagation characteristics are the same. The embodiment of the present invention can achieve low-loss coupling between the transverse electric mode (TE) and the transverse magnetic mode (TM) by designing a reasonable silicon nitride layer taper length and the interlayer spacing between the lithium niobate waveguide layer and the silicon nitride layer. Due to the difficulty of lithium niobate material processing, in the embodiment of the present invention, only a large width change is made to the silicon nitride waveguide, and the change in the width of the lithium niobate waveguide is minimized.

As shown in Figure 4, according to the anisotropic characteristics of lithium niobate material, its refractive index can be written in tensor form:

Wherein, no _is the refractive index of lithium niobate material in the x and y directions, and ne _is the refractive index of lithium niobate material in the z direction. Taking the application of electric field in the z direction as an example, when the direction of the electric field line and the direction of light polarization are the same, the modulation efficiency is the maximum. As the angle between the direction of the electric field line and the direction of light polarization changes, the modulation efficiency gradually changes. When the electric field line is perpendicular to the polarization direction, the modulation efficiency is the lowest, which is about one eighth of the former. According to the modulation characteristics of lithium niobate material, the embodiments of the present invention change the distribution of electric field lines in the waveguide by designing and reasonably controlling the position of metal electrodes, so as to make the modulation efficiency of TE and TM modes the same and achieve polarization independence.

Due to the modulation characteristics of lithium niobate materials, the electric field acts directly on lithium niobate, and when the refractive index of lithium niobate changes, no additional light absorption loss is introduced, and it has an extremely fast response speed. According to reported literature, the bandwidth of lithium niobate materials is at least 70GHz, which is much higher than the nanosecond switching speed in current data packet communication methods.

The polarization-independent optical switch proposed in the embodiment of the present invention provides a novel optical switch heterogeneously integrated with silicon nitride material and lithium niobate material, wherein a lithium niobate wafer on an insulator is used to etch a modulation waveguide of a modulation arm of the optical switch, and a silicon nitride film is grown on the lithium niobate to etch basic passive components such as a multimode interference coupler. The optical switch structure utilizes silicon nitride material to achieve polarization independence of basic passive components, and utilizes the electro-optical properties of lithium niobate material to achieve low-loss and high-speed modulation characteristics.

In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In the description of the present invention, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, without contradiction.

Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims (8)

1. A polarization-independent optical switch, characterized in that the optical switch is a bilaterally symmetrical structure, comprising: A lower cladding layer, a lithium niobate waveguide layer, an upper cladding layer and a silicon nitride layer, wherein the lithium niobate waveguide layer is integrated above the lower cladding layer, the silicon nitride layer is above the lithium niobate waveguide layer, and the upper cladding layer is filled between the lithium niobate waveguide layer and the silicon nitride layer; Sequentially etching the upper part of the uniform light splitting multimode interference coupler and the upper and lower interlayer coupling structure on the silicon nitride layer along the light propagation direction, wherein the upper part is connected to the output of the uniform light splitting multimode interference coupler; Etching the lower half of the upper and lower interlayer coupling structure and the polarization-independent modulation waveguide on the lithium niobate waveguide layer, wherein the lower half is connected to the polarization-independent modulation waveguide; and Metal electrodes are fabricated on both sides of the polarization-independent modulation waveguide of the lithium niobate waveguide layer, and positions of the metal electrodes are adjusted according to the modulation characteristics of the lithium niobate waveguide layer. 2. The polarization-independent optical switch according to claim 1, characterized in that: A hole is opened on the metal electrode to be exposed to the air. 3. The polarization-independent optical switch according to claim 2, characterized in that: Light is input from one port of the uniformly split multimode interference coupler, coupled to the polarization-independent modulation waveguide through the upper and lower interlayer coupling structure, an external voltage is applied to the metal electrode to change the light phase so that the light path is changed, and then the light is output from one port of the uniformly split multimode interference coupler through the upper and lower interlayer coupling structure. 4. The polarization-independent optical switch according to claim 1, characterized in that: The upper cladding layer and the lower cladding layer are made of silicon dioxide material. 5. The polarization-independent optical switch according to claim 1, characterized in that: The silicon nitride layer has a structure with a gradually narrowing width. 6. The polarization-independent optical switch according to claim 5, characterized in that: The length of the gradually narrowed structure of the silicon nitride layer and the interlayer spacing between the lithium niobate waveguide layer and the silicon nitride layer are changed to achieve low-loss coupling between the transverse electric mode and the transverse magnetic mode. 7. The polarization-independent optical switch according to claim 1, characterized in that: The uniform light splitting multimode interference coupler is a polarization-independent 2×2 uniform light splitting multimode interference coupler, comprising 4 input and output bending waveguides and an intermediate multimode waveguide. 8. The polarization-independent optical switch according to claim 1 is characterized in that, in the process of adjusting the position of the metal electrode according to the modulation characteristics of the lithium niobate waveguide layer, the distribution of electric field lines in the waveguide is changed so that the modulation efficiency of the transverse electric mode and the transverse magnetic mode is the same.