CN112269224A

CN112269224A - Silicon-silicon nitride integrated polarization beam splitter based on vertical coupling structure

Info

Publication number: CN112269224A
Application number: CN202010932820.XA
Authority: CN
Inventors: 王嘉源
Original assignee: Nanjing University of Posts and Telecommunications
Current assignee: Nanjing University of Posts and Telecommunications
Priority date: 2020-09-08
Filing date: 2020-09-08
Publication date: 2021-01-26
Anticipated expiration: 2040-09-08
Also published as: CN112269224B

Abstract

The invention discloses a silicon-silicon nitride integrated polarization beam splitter based on a vertical coupling structure, which comprises an input waveguide, a polarization beam splitting waveguide, a first output waveguide and a second output waveguide, wherein the input waveguide is connected with one end of the polarization beam splitting waveguide, the other end of the polarization beam splitting waveguide is respectively connected with the first output waveguide and the second output waveguide, and the polarization beam splitting waveguide is a three-layer structure formed by silicon-silicon nitride-silicon from bottom to top. The invention has compact structure, shorter transverse size and longitudinal length, and improves the integration level of devices; the polarization extinction ratio is higher; the method has the advantages of large manufacturing tolerance, relatively low precision requirement on the photoetching process and greatly reduced process cost.

Description

Silicon-silicon nitride integrated polarization beam splitter based on vertical coupling structure

Technical Field

The invention relates to a polarization beam splitter, in particular to a silicon-silicon nitride integrated polarization beam splitter based on a vertical coupling structure.

Background

With the increasing demand for information exchange, optical communication systems require larger and larger transmission capacity and faster processing speed, and the requirements for circuit and optical circuit integration are higher and higher. The step-by-step application of 5G technology and data center high-speed optical interconnects have made large-scale photonic integration the direction of future development. In recent years, a photonic integrated circuit based on a silicon-on-insulator (SOI) platform has attracted wide attention and applications, and mainly benefits from the characteristic of ultra-high refractive index difference between silicon dioxide and silicon, so that an optical field can be well limited in a core layer, and thus a submicron-sized device cross section and a small-radius waveguide bend can be realized. However, due to the extremely high structural birefringence, SOI-based photonic integrated systems are sensitive to the polarization state of light, i.e., they respond differently to the Transverse Electric (TE) and Transverse Magnetic (TM) modes of the transmitted light, and thus control and management of the polarization state of light is important. A polarizing beam splitter is indispensable in an SOI integrated optical system as an optical device capable of effectively separating TE and TM modes. For this reason, various polarization beam splitter device schemes have been proposed, including a directional coupler type, a multimode interference type, a photonic crystal type, a surface plasmon type, and the like, in which the directional coupler type polarization beam splitter becomes a main constituent structure of the polarization beam splitter due to its simple structure and excellent performance.

However, the existing directional coupler type polarization beam splitter has the following drawbacks: firstly, the directional coupler generally has mode coupling by two parallel waveguides, and the size of the two waveguides is required to just meet the phase matching condition of the mode, so the general directional coupler has smaller manufacturing tolerance and higher requirement on manufacturing precision; secondly, the directional coupler is mainly a horizontal coupling structure, and two waveguides are required to be transversely arranged, so that the horizontal size of the device is large, and the device is unfavorable for large-scale photonic integration.

Disclosure of Invention

The purpose of the invention is as follows: in view of the above problems, it is an object of the present invention to provide a silicon-silicon nitride integrated polarization beam splitter based on a vertical coupling structure with large manufacturing tolerances and a compact structure.

The technical scheme is as follows: the silicon-silicon nitride integrated polarization beam splitter based on the vertical coupling structure comprises an input waveguide for inputting optical signals, a polarization beam splitting waveguide, a first output waveguide and a second output waveguide for outputting the optical signals, wherein the input waveguide is connected with one end of the polarization beam splitting waveguide, the other end of the polarization beam splitting waveguide is respectively connected with the first output waveguide and the second output waveguide, the polarization beam splitting waveguide is of a three-layer structure formed by silicon-silicon nitride-silicon which is distributed from bottom to top in sequence, and TE polarization of the input optical signals is vertically coupled between an upper silicon layer and a lower silicon layer of the polarization beam splitter.

The input waveguide and the first output waveguide are of a two-layer structure formed by silicon-silicon nitride which is distributed from bottom to top in sequence.

The second output waveguide is of a single-layer silicon structure and is in an S-shaped bent shape.

The polarization beam splitter also comprises a cladding which is coated on the outer sides of the input waveguide, the polarization beam splitting waveguide, the first output waveguide and the second output waveguide.

The thickness of the silicon layer in the input waveguide, the polarization beam splitting waveguide, the first output waveguide and the second output waveguide is 80-180 micrometers, and the thickness of the silicon nitride layer is 300-600 micrometers.

Has the advantages that: compared with the prior art, the invention has the following remarkable advantages:

1. the TE mode in the input optical signal is mainly vertically coupled between the lower silicon layer and the upper silicon layer of the polarization beam splitting waveguide, and two waveguides are not required to be transversely arranged, so the transverse size of the polarization beam splitting waveguide is small; by adjusting the thickness of each layer of the silicon-silicon nitride-silicon structure in the polarization beam splitting waveguide, the odd mode and the even mode of the TE mode have larger effective refractive index difference, the coupling length of the TE mode is greatly reduced, the longitudinal length of a shorter device is realized, the structure is compact, and the integration level of the device is greatly improved;

2. in the polarization beam splitting waveguide, the TE modes are distributed on the upper silicon layer and the lower silicon layer, and the TM modes are distributed on the middle silicon nitride layer, so that the two modes are transmitted separately, and after the S-shaped bent waveguide of the second output waveguide is output, the TM modes are rarely existed when the TE modes are separated, and the polarization extinction ratio is greatly improved;

3. according to the vertical coupling structure, the thickness of the silicon layer in the silicon optical process is easily and accurately controlled by adjusting the deposition parameters, the waveguide width error caused by the photoetching process has little influence on the performance of the device, the manufacturing tolerance is larger, the requirement on the precision of the photoetching process is relatively lower, and the process cost is greatly reduced.

4. The invention realizes monolithic integration on a silicon-based chip by utilizing a mature CMOS processing technology, and has important significance for the development of silicon photonics.

Drawings

FIG. 1 is a top view of the present embodiment;

FIG. 2 is a side view of the present embodiment;

FIG. 3 is a TE mode electric field distribution diagram in the input waveguide according to the present embodiment;

FIG. 4 is a diagram showing an electric field distribution of a TM mode in the input waveguide according to the present embodiment;

FIG. 5 is a diagram illustrating the distribution of the TE mode electric field in the polarization splitting waveguide according to this embodiment;

FIG. 6 is a diagram illustrating an electric field distribution of a TM mode in the polarization beam splitting waveguide according to the present embodiment;

fig. 7 is a graph of the coupling length of the TE mode of this embodiment as a function of the thickness of the upper and lower silicon layers.

Detailed Description

As shown in fig. 1, the silicon-silicon nitride integrated polarization beam splitter based on the vertical coupling structure according to this embodiment includes an input waveguide 1, a polarization beam splitting waveguide 2, a first output waveguide 3, a second output waveguide 4, and a cladding 5 coated on the outermost side of the polarization beam splitter, where the input waveguide 1 is connected to one end of the polarization beam splitting waveguide 2, the other end of the polarization beam splitting waveguide 2 is connected to the first output waveguide 3 and the second output waveguide 4, and the polarization beam splitting waveguide 2 is a three-layer structure formed by silicon-silicon nitride-silicon sequentially distributed from bottom to top. The polarizing beam splitter in this embodiment is fabricated on a silicon dioxide substrate 6.

The input waveguide 1 and the first output waveguide 3 are of a two-layer structure formed by silicon-silicon nitride distributed from bottom to top in sequence. The second output waveguide 4 is a single-layer silicon structure, and has an S-shaped curved shape. The widths of the input waveguide 1, the polarization beam splitting waveguide 2, the first output waveguide 3 and the second output waveguide 4 are equal and are all 600 nm. The silicon layers in the input waveguide 1 and the first output waveguide 3, the upper and lower silicon layers of the polarization beam splitting waveguide 2, and the second output waveguide 4 have the same height, which are all 120 nm. The thickness of the silicon nitride layer in the input waveguide 1, the polarization splitting waveguide 2 and the first output waveguide 3 was 340 nm. The lateral distance between the first output waveguide 3 and the second output waveguide 4 is 2.5 μm.

The two-layer structure of silicon-silicon nitride as an input waveguide has discontinuity of electric field component perpendicular to the boundary due to high refractive index difference at the material boundary, so that the mode distribution of TE and TM polarization is completely different from that of a common silicon linear waveguide. As shown in fig. 3 and 4, the TE mode is mainly distributed in the silicon layer, and the TM mode is mainly distributed in the silicon nitride layer. Thus, the TE and TM modes are naturally transmitted in different material layers. The polarization beam splitting waveguide 2 is formed by depositing a layer of silicon on the silicon nitride layer, referred to as the upper silicon layer 23, on the basis of the input waveguide 1. The upper silicon layer 23 has a thickness equal to that of the lower silicon layer, forming a silicon-silicon nitride-silicon three-layer structure. The mode distribution characteristics of this structure are similar to those of a silicon-silicon nitride two-layer structure, in which TE modes are mainly distributed in the upper and lower silicon layers and TM modes are mainly distributed in the silicon nitride layer, as shown in fig. 5 and 6. The TE mode distributed in the first lower silicon layer 11 in the input waveguide 1 is only required to be vertically coupled into the upper silicon layer 23 of the polarization beam splitting waveguide 2 and be led out through the S-shaped second output waveguide 4, so that the effective separation from the TE mode mainly distributed in the silicon nitride layer is realized. The two polarization modes are transmitted along different media, and have higher polarization extinction ratio when separated. And the device is a vertical coupling structure, and does not need the transverse arrangement of the coupling waveguide, so the transverse size of the device is compact.

As shown in fig. 2, the transmission characteristics of the optical signal in the polarization beam splitter according to the embodiment are as follows: an input signal containing TE and TM modes is input from the input waveguide 1, the TE modes being mainly distributed in the first lower silicon layer 11 and transmitted into the polarization splitting waveguide 2. In the polarization splitting waveguide 2, the TE mode at the second lower silicon layer 21 is mode field matched with the upper silicon layer 23, resulting in efficient vertical coupling. The TE mode will couple into the upper silicon layer 23 and be transmitted into the S-shaped second output waveguide 4. Since the second output waveguide 4 is gradually separated from the first output waveguide 3, the TE mode in the second output waveguide 4 is not coupled back to the third lower silicon layer 31 and the third silicon nitride layer 32. On the other hand, the TM mode is mainly distributed in the first silicon nitride layer 12 when entering the input waveguide 1, and is mainly distributed in the second silicon nitride layer 22 after entering the polarization beam splitting waveguide 2, and since the mode mismatch does not couple with the upper and lower silicon layers, the TM mode is transmitted through the silicon nitride layer and is finally output through the third silicon nitride layer 32.

Fig. 7 shows the coupling length of the TE mode as a function of the thickness of the upper and lower silicon layers of the polarization splitting waveguide 2. When the thickness of the silicon layer is increased from 80nm to 180nm, the coupling length of the TE mode, i.e., the length of the polarization splitting waveguide 2, is increased from 2.2 μm to 8.6 μm. The coupling length was 3.8 μm when the silicon layer width was taken to be 120 nm. The surface area size of the device is as small as 3 mu m multiplied by 10 mu m by adding the input and output waveguides.

Claims

1. Silicon-silicon nitride integrated polarization beam splitter based on vertical coupling structure, characterized in that, including input waveguide (1), polarization beam splitting waveguide (2), first output waveguide (3) and second output waveguide (4), input waveguide (1) is connected with polarization beam splitting waveguide (2) one end, polarization beam splitting waveguide (2) other end is connected with first output waveguide (3), second output waveguide (4) respectively, polarization beam splitting waveguide (2) is by the lower supreme three-layer construction that silicon-silicon nitride-silicon that distributes in proper order constitutes.

2. The integrated polarization splitter of silicon-silicon nitride based on vertical coupling structure of claim 1, wherein the input waveguide (1) is a two-layer structure composed of silicon-silicon nitride distributed from bottom to top.

3. The silicon-silicon nitride integrated polarization beam splitter based on the vertical coupling structure of claim 1, wherein the first output waveguide (3) is a two-layer structure composed of silicon-silicon nitride distributed from bottom to top.

4. The integrated polarization splitter of silicon-silicon nitride based on vertical coupling structure of claim 1, wherein the second output waveguide (4) is a single layer silicon structure with S-shaped curved shape.

5. The silicon-silicon nitride integrated polarization beam splitter based on the vertical coupling structure of claim 1, further comprising a cladding (5) cladding outside the input waveguide (1), the polarization splitting waveguide (2), the first output waveguide (3) and the second output waveguide (4).

6. The silicon-silicon nitride integrated polarization beam splitter based on the vertical coupling structure of any one of claims 1 to 5, wherein the thickness of the silicon layer in the input waveguide (1), the polarization beam splitting waveguide (2), the first output waveguide (3) and the second output waveguide (4) is 80 to 180 micrometers, and the thickness of the silicon nitride layer is 300 to 600 micrometers.