A double-layer low-loss lithium niobate optical grating coupler and a preparation method thereof

Abstract

The application relates to a double-layer low-loss lithium niobate optical grating coupler and a preparation method thereof. The coupler comprises, from bottom to top, a substrate (1), a buried oxygen layer (2), a silicon nitride layer (3), a lithium niobate layer (4) and an upper cladding layer (5). The application adds another grating layer under the single-layer lithium niobate grating through a heterogeneous integration mode, so that the coupling efficiency similar to that of the prior art is achieved, and the coupler has the advantages of low loss, simple process flow and large manufacturing process tolerance.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of semiconductor devices, and particularly relates to a double-layer low-loss lithium niobate grating coupler and a preparation method thereof. BACKGROUND

[0002] In recent years, with the development of technologies such as big data, artificial intelligence and cloud computing, the demand for chips has greatly increased. Lithium niobate crystal (LiNbO3, LN) is a multifunctional optoelectronic material with many excellent physical properties, such as electro-optic, acousto-optic, photorefractive, nonlinear optics, piezoelectric, dielectric, ferroelectric, pyroelectric, etc. It also has good mechanical stability, large intrinsic bandwidth and wide wavelength transmission range. Lithium niobate is expected to become the next generation of integrated photonic chip substrate material in the field of integrated optics. In the field of optical communication, the coupling between signals in optical fibers and chips is an important research direction. Currently, the coupling scheme between optical fibers and chips can be mainly divided into horizontal coupling and vertical coupling. Compared with horizontal coupling, vertical coupling has the advantages that the layout position is flexible and can be arranged at any position on the photonic chip, while the horizontal coupling scheme needs to be arranged at the end face position, which limits the overall layout of the device; the alignment tolerance is large, and the mode spot size of the coupled light field is large, much larger than that of the horizontal coupling scheme, so the requirement for the alignment of the optical fiber is relaxed, and the coupling efficiency can be ensured stable even if there is disturbance.

[0003] In the vertical coupling scheme, the coupling loss of the grating mainly comes from the substrate leakage and the mode mismatch between the optical fiber and the grating coupler, so the existing inventions generally use the methods of increasing the substrate metal reflection layer and changing the period and duty cycle of the grating to obtain a chirped grating to improve the coupling efficiency of the grating coupler. However, these two methods will increase the process difficulty of the grating coupler in the actual manufacturing process. SUMMARY

[0004] The technical problem to be solved by the present application is to provide a double-layer low-loss lithium niobate grating coupler and a preparation method thereof, which has the advantages of low loss, simple process flow and large process tolerance.

[0005] The present application provides a double-layer low-loss lithium niobate grating coupler, which comprises, from bottom to top, a substrate, a buried oxygen layer, a silicon nitride layer, a lithium niobate layer and an upper cladding layer; the duty cycle of the silicon nitride layer and the lithium niobate layer changes linearly in the first half period and the degree of linear change is different, and the duty cycle is the same in the second half period; and the upper cladding layer covers the silicon nitride layer and the lithium niobate layer.

[0006] Preferably, the material of the substrate is silicon.

[0007] Preferably, the materials of the buried oxygen layer and the upper cladding layer are silicon dioxide.

[0008] The application further provides a preparation method of the double-layer low-loss lithium niobate optical grating coupler.

[0009] (1) cleaning the substrate and performing thermal oxidation to form a buried oxygen layer;

[0010] (2) performing chemical mechanical polishing on the buried oxygen layer;

[0011] (3) depositing silicon nitride and performing photolithography and etching to form a silicon nitride layer;

[0012] (4) depositing silicon oxide and performing chemical mechanical polishing on the silicon oxide layer;

[0013] (5) bonding with a lithium niobate wafer, forming a lithium niobate layer and etching the lithium niobate layer to form a corresponding pattern;

[0014] (6) finally depositing an upper cladding layer to obtain the double-layer low-loss lithium niobate optical grating coupler.

[0015] Advantages

[0016] The application can achieve a coupling efficiency similar to that of the prior art by adding another grating layer under the single-layer lithium niobate grating in a heterogeneous integration manner, and has the advantages of low loss, simple process flow and large process tolerance. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 Fig. 1 is a structural schematic diagram of the coupler of the application.

[0018] Figure 2 Fig. 2 is a structural schematic diagram of the coupler of Example 1.

[0019] Figure 3 Fig. 3 is a relationship diagram of the coupling loss of the lithium niobate double-layer grating with the incident wavelength.

[0020] Figure 4 Fig. 4 is a normalized electric field distribution diagram of the chirped grating.

[0021] Figure 5 Fig. 5 is a preparation flowchart of the coupler of Example 1. DETAILED DESCRIPTION

[0022] The application will be further described below in conjunction with specific examples. It should be understood that these examples are only used to illustrate the application and not used to limit the scope of the application. In addition, it should be understood that those skilled in the art can make various modifications or changes to the application after reading the content of the application, and these equivalent forms also fall within the scope of the appended claims of the application.

[0023] Example 1

[0024] As Figure 1As shown, the embodiment provides a double-layer low-loss lithium niobate grating coupler, which comprises, from bottom to top, a substrate 1, a buried oxygen layer 2, a silicon nitride layer 3, a lithium niobate layer 4 and an upper cladding layer 5; the silicon nitride layer 3 and the lithium niobate layer (4) have linearly changed duty cycles in the first half period and different linearly changed duty cycles in the second half period; and the upper cladding layer 5 covers the silicon nitride layer 3 and the lithium niobate layer 4.

[0025] The embodiment also provides a specific implementation of a double-layer low-loss lithium niobate grating coupler, as shown in Figure 2

[0026] The structure is composed of two layers of regularly arranged strip waveguides, as shown in Figure 2 The upper layer is a 400-nm-thick thin-film lithium niobate, the lower layer is a 400-nm-thick silicon nitride, the distance between the two layers is 200 nm, the thickness of the buried oxygen layer is 2.9 μm, and the etching depth of the two layers of gratings is uniformly 400 nm, and the etching is performed with the same period. The entire grating is provided with 18 periods, the first nine grating periods are designed to be chirped, and the duty cycles of the first nine periods are linearly changed. The particle swarm optimization algorithm is used to optimize the grating period, the duty cycles of the first and tenth periods of the silicon nitride grating, and the relative position between the two layers of gratings. When the fiber incidence angle is 10 degrees, the coupling loss at 1546 nm is the lowest, about 1.04 dB, and the 3-dB bandwidth is about 87 nm. The relationship between the coupling loss and the wavelength is shown in Figure 3 .

[0027] The basic principle of the double-layer grating is based on the Bragg diffraction principle. The grating structure will only be diffracted upward when the Bragg diffraction condition is met, which is related to the grating period, the duty cycle and the equivalent refractive index of each period. The Bragg condition is also called the phase matching condition. When light is transmitted in the waveguide, the phase changes by an integer multiple of 2π after passing through a grating period. At this time, the light waves are coherently superimposed due to phase matching, thereby achieving coupling. The parameters that can be designed and optimized for a traditional grating generally include the period, the duty cycle and the etching depth, and the design freedom is relatively small. The double-layer grating increases the design freedom to achieve the purpose of improving the coupling efficiency and increasing the bandwidth. At the same time, the grating is designed to be chirped, which effectively improves the directionality of grating diffraction, as shown in Figure 4 .

[0028] As shown in Figure 5 , the embodiment also provides a preparation method of a double-layer low-loss lithium niobate grating coupler, which comprises the following steps:

[0029] (1) cleaning the substrate 1 and performing thermal oxidation to form a buried oxygen layer 2;

[0030] (2) performing chemical mechanical polishing on the buried oxygen layer 2 to control the thickness to about 2.9 μm; ​

[0031] (3) depositing silicon nitride and performing photoetching and etching to form a silicon nitride layer 3;

[0032] (4) depositing silicon oxide and performing chemical mechanical polishing on the silicon oxide layer;

[0033] (5) bonding with a lithium niobate wafer to form a lithium niobate layer 4 and etching the lithium niobate layer 4 to form a corresponding pattern;

[0034] (6) finally depositing an upper cladding layer 5 to obtain a double-layer low-loss lithium niobate grating coupler.

[0035] Compared with other designs of low-loss lithium niobate gratings, the manufacturing process of the lithium niobate grating proposed in the application is simpler and compatible with other device manufacturing processes.

Claims

1. A double layer low loss lithium niobate grating coupler characterized by: The coupler comprises, from bottom to top, a substrate (1), a buried oxygen layer (2), a silicon nitride layer (3), a lithium niobate layer (4) and an upper cladding layer (5); the silicon nitride layer (3) and the lithium niobate layer (4) change linearly in duty cycle in the first half period and change linearly in different degrees, and the duty cycle is the same in the second half period; the upper cladding layer (5) covers the silicon nitride layer (3) and the lithium niobate layer (4).

2. The dual-layer low-loss lithium niobate optical grating coupler of claim 1, wherein: The material of the substrate (1) is silicon.

3. The dual-layer low-loss lithium niobate optical grating coupler of claim 1, wherein: The material of the buried oxygen layer (2) and the upper cladding layer (5) is silicon dioxide.

4. A preparation method of the double-layer low-loss lithium niobate optical grating coupler according to claim 1, comprising the following steps: (1) cleaning the substrate (1) and performing thermal oxidation to form a buried oxygen layer (2); (2) performing chemical mechanical polishing on the buried oxygen layer (2); (3) depositing silicon nitride and performing photolithography and etching to form a silicon nitride layer (3); (4) depositing silicon oxide and performing chemical mechanical polishing on the silicon oxide layer; (5) bonding with a lithium niobate wafer to form a lithium niobate layer (4) and etching the lithium niobate layer (4) to form a corresponding pattern; (6) finally depositing an upper cladding layer (5) to obtain the double-layer low-loss lithium niobate optical grating coupler.

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