An end face coupling structure of a low-loss silicon optical chip

Abstract

This invention relates to an end-face coupling structure for a low-loss silicon photonic chip, comprising a silicon substrate, a laser mounting region on the silicon substrate, a buried oxide layer on one side of the laser mounting region and on the silicon substrate, a shallow etched region on the buried oxide layer, and a beveled end face etched on the cross section formed by the buried oxide layer and the shallow etched region. The beveled end face is used to reduce reflection loss. A ridge waveguide facing the laser mounting region is disposed on the shallow etched region, and the incident end of the ridge waveguide is located at the beveled end face. This invention can achieve high coupling efficiency and alignment tolerance.

Description

Technical Field

[0001] This invention relates to the field of low-loss silicon photonics chip structure design technology, and in particular to an end-face coupling structure for a low-loss silicon photonics chip. Background Technology

[0002] Communication networks form the technological foundation of the entire information society. Today's popular technologies, such as 5G and cloud computing, owe their existence and development to advanced communication technologies. In my country, over the past decade, the "fiber-to-the-home" project has steadily progressed, and optical interconnects have become increasingly important in the field of data communication.

[0003] Silicon-based optoelectronics, as an emerging technology, can achieve large-scale integration of optoelectronic devices using existing mature CMOS processes, and has broad development prospects. Because silicon is an indirect bandgap semiconductor, it is difficult to generate stimulated emission like group III-V semiconductors, making on-chip light sources a challenge in silicon photonic chips. A common approach is to integrate external lasers on the silicon photonic chip. Distributed feedback (DFB) lasers have extremely narrow linewidths and high power, making them suitable for long-distance, high-speed optical communication, and are therefore widely used as light sources. Depending on the silicon photonics process platform and the laser's emission location, hybrid integration methods for light sources can be divided into two types: end-face (horizontal) coupling and grating (vertical) coupling. Among them, end-face coupling structures are naturally compatible with side-emitting DFB lasers and are widely used on silicon-on-insulator (SOI) platforms with a top silicon layer thickness of 3µm.

[0004] However, existing end-face coupling structures generally have high losses, which on the one hand leads to severe attenuation of the optical power coupled into the chip, and on the other hand imposes strict requirements on the integration accuracy of the laser, with very small tolerable alignment errors. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide an end-face coupling structure for a low-loss silicon photonic chip, which can achieve high coupling efficiency and alignment tolerance.

[0006] The technical solution adopted by the present invention to solve its technical problem is: to provide an end-face coupling structure for a low-loss silicon photonic chip, including a silicon substrate, a laser mounting area on the silicon substrate, a buried oxide layer on one side of the laser mounting area and on the silicon substrate, a shallow etched area on the buried oxide layer, and a beveled end face etched on the cross section formed by the buried oxide layer and the shallow etched area. The beveled end face is used to reduce reflection loss. A ridge waveguide facing the laser mounting area is provided on the shallow etched area, and the incident end of the ridge waveguide is located at the beveled end face.

[0007] The bevel angle of the beveled end face is calculated based on the incident energy of the DFB laser, and the bevel angle of the beveled end face satisfies the following condition: the energy loss reflected back to the output surface of the DFB laser through the beveled end face is at least less than -30dB.

[0008] The beveled end face is coated with an anti-reflective layer.

[0009] The thickness of the anti-reflection layer is calculated based on the output wavelength of the DFB laser, using the formula: 2kd=π, where k is the wave vector in the anti-reflection layer and d is the thickness of the anti-reflection layer.

[0010] The anti-reflective layer is a silicon nitride layer.

[0011] The oblique end face is provided with a focusing surface, which is located directly below the input end of the ridge waveguide. The focusing surface is used to compensate for the phase and size of the light spot on the input end face of the ridge waveguide.

[0012] The shape of the focusing surface is calculated based on the positional relationship between the DFB laser and the oblique end face;

[0013] The focused surface satisfies the equation n1sinθ1=n2sinθ2;

[0014] Furthermore, the focused surface also satisfies the equation n1L1=n2L2;

[0015] Where n1 is the refractive index between the end-face coupling structure of the DFB laser and the silicon photonic chip, θ1 is the angle between the ray and the normal between the end-face coupling structure of the DFB laser and the silicon photonic chip, and the normal is perpendicular to the oblique end face; n2 is the refractive index of the ridge waveguide, θ2 is the angle between the ray and the normal in the ridge waveguide, L1 is the propagation distance between the end-face coupling structure of the DFB laser and the silicon photonic chip, and L2 is the propagation distance in the ridge waveguide.

[0016] The ridge waveguide is equipped with a mode-filtering structure to reduce the transmission of higher-order modes.

[0017] The buried oxide layer is made of silicon dioxide.

[0018] Beneficial effects

[0019] Due to the adoption of the above-mentioned technical solutions, this invention has the following advantages and positive effects compared with the prior art: This invention introduces a specific angled end-face bevel structure on the silicon photonic chip, reducing anti-reflection loss while maintaining high-efficiency coupling. A silicon nitride layer is deposited on the waveguide end face to further reduce reflection, and a filter structure ensures single-mode transmission. This invention forms a focusing surface with a specific shape at the edge of the silicon photonic chip through shallow etching, compensating for the phase and size of the incident beam at the end face, making it more closely matched with the fundamental mode of the ridge waveguide, thereby achieving higher coupling efficiency. This invention uses an end-face bevel layout, making the distance between the DFB laser and the waveguide end face sufficiently small to reduce the incident beam spot and ensure high-efficiency coupling. The key parameters of the entire structure of this invention are designed to achieve a large alignment tolerance. Attached Figure Description

[0020] Figure 1 This is a three-dimensional schematic diagram of the end-face coupling structure of the low-loss silicon photonic chip according to Embodiment 1 of the present invention.

[0021] Figure 2 This is a top view of the end-face coupling structure of the low-loss silicon photonic chip according to Embodiment 1 of the present invention.

[0022] Figure 3 This is a schematic cross-sectional view of the end-face coupling structure of the low-loss silicon photonic chip according to Embodiment 1 of the present invention.

[0023] Figure 4 This is a graph showing the relationship between reflection loss and end face oblique angle in Embodiment 1 of the present invention;

[0024] Figure 5 The alignment tolerance of the end-face coupling structure of the low-loss silicon photonic chip in Embodiment 1 of the present invention is in three dimensions.

[0025] Figure 6 This is a three-dimensional schematic diagram of the end-face coupling structure of the low-loss silicon photonic chip according to Embodiment 2 of the present invention.

[0026] Figure 7 This is an experimental diagram comparing the coupling efficiency and alignment tolerance of Embodiment 2 of the present invention with that of a conventional end-face coupling structure. Detailed Implementation

[0027] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0028] The embodiments of the present invention relate to an end-face coupling structure for a low-loss silicon photonic chip, which can achieve high coupling efficiency and alignment tolerance.

[0029] Implementation Method 1:

[0030] Please see Figures 1-3 The system includes a silicon substrate 1, on which a DFB flip-chip region (i.e., a laser mounting region) is provided. A DFB laser is mounted on the DFB flip-chip region. A buried oxide layer 2 is provided on one side of the DFB flip-chip region and on the silicon substrate 1. A shallow etched region 3 is provided on the buried oxide layer 2. The cross section formed by the buried oxide layer 2 and the shallow etched region 3 is etched with a beveled end face 5. The beveled end face 5 is used to reduce reflection loss. A ridge waveguide 4 facing the DFB flip-chip region is provided on the shallow etched region 3. The incident end of the ridge waveguide 4 is located at the beveled end face 5.

[0031] Furthermore, the bevel angle of the beveled end face 5 is calculated based on the incident energy of the DFB laser, and the bevel angle of the beveled end face 5 satisfies the following condition: the energy loss reflected back to the output surface of the DFB laser through the beveled end face 5 is at least less than -30dB.

[0032] Furthermore, the beveled end face 5 is coated with an anti-reflection layer, which is a silicon nitride layer. The thickness of the anti-reflection layer is calculated based on the output wavelength of the DFB laser, using the formula: 2kd=π, where k is the wave vector in the anti-reflection layer and d is the thickness of the anti-reflection layer.

[0033] Furthermore, the buried oxide layer 2 is made of silicon dioxide.

[0034] The technical principle of Implementation Method 1 is as follows: By selecting a specific angled end face bevel, perpendicular laser incidence is avoided, thereby reducing the optical power reflected back to the DFB laser's output surface. A silicon nitride layer (i.e., an anti-reflection layer) is deposited on the end face of the silicon photonic chip to further reduce reflectivity and increase coupling efficiency. The key parameters of the coupling interface are designed to match the output spot of the DFB laser to maximize alignment tolerance. The coupling structure also includes a mode filter structure to reduce higher-order modes, achieving pure single-mode transmission.

[0035] In this coupling structure, the ridge waveguide end face used as the input on the silicon photonics chip is beveled, and an anti-reflection layer is deposited on the beveled end face. The bevel angle and waveguide dimensions are optimized based on the incident light spot size. The thickness of the anti-reflection layer can be initially calculated based on the wavelength of the DFB laser, and further verified through simulation after other parameters are determined. Ridge waveguide 4 (straight waveguide) is subsequently formed into a curved waveguide to achieve a mode filtering effect and ensure single-mode transmission.

[0036] Through experiments, the relationship between the reflection loss of Implementation Method 1 and the oblique angle of the end face is shown in [reference]. Figure 4 It can be seen that the larger the bevel angle of the end face, the lower the reflection loss. When the bevel angle of the end face is greater than 23°, the reflection loss is reduced to below -30dB. Figure 5 It refers to the alignment tolerance in the three dimensions of the end-face coupling structure.

[0037] Implementation Method Two:

[0038] like Figure 6 As shown, based on the first embodiment, the oblique end face 5 is also provided with a focusing surface 6. The focusing surface 6 is located directly below the input end of the ridge waveguide 4. The focusing surface 6 is used to compensate for the phase and size of the light spot on the incident end face of the ridge waveguide 4.

[0039] Furthermore, the shape of the focusing surface 6 is calculated based on the positional relationship between the DFB laser and the oblique end face. Given the coordinates of the light source and the end face, for each point on the focusing surface 6, both the law of refraction and the condition of equal optical path length should be satisfied simultaneously. The specific formula is as follows:

[0040] That is, the focused surface 6 satisfies two equations:

[0041] n1sinθ1=n2sinθ2;

[0042] n1L1 = n2L2;

[0043] Where n1 is the refractive index between the end-face coupling structure of the DFB laser and the silicon photonic chip, θ1 is the angle between the ray and the normal between the end-face coupling structure of the DFB laser and the silicon photonic chip, n2 is the refractive index of the ridge waveguide, θ2 is the angle between the ray and the normal in the ridge waveguide, L1 is the propagation distance between the end-face coupling structure of the DFB laser and the silicon photonic chip, and L2 is the propagation distance in the ridge waveguide.

[0044] Points that meet the above two constraints combine to form the shape of the entire focused surface 6.

[0045] Furthermore, the ridge waveguide 4 is provided with a mode-filtering structure to weaken the transmission of higher-order modes.

[0046] The technical principle of Implementation Method Two is as follows: a focusing surface 6 with a specific shape is formed by shallow etching at the edge of the silicon photonic chip. This compensates for the phase and size of the light spot on the incident end face, making it more closely matched with the fundamental mode of the ridge waveguide 4, thereby achieving higher coupling efficiency. This implementation method uses a beveled end face layout, making the distance between the DFB laser and the waveguide end face sufficiently small to reduce the incident light spot and ensure efficient coupling. The focusing surface 6 is coated with an anti-reflection layer to reduce reflection. The key parameters of the entire structure are designed to achieve a large alignment tolerance.

[0047] Specific implementation methods are as follows: Figure 6As shown, this coupling structure, based on the beveled end face, introduces a focusing surface 6 in the shallow etched area 3 and the buried oxide layer 2. This focusing surface 6 is compatible with the double-layer etching process. The shape of the focusing surface 6 can be calculated based on parameters such as the divergence angle and incident angle of the DFB laser, which can compensate for the phase and position distribution of the Gaussian light emitted by the DFB laser in the horizontal direction. Each end face of the focusing surface 6 is coated with an anti-reflection layer, the thickness of which is calculated based on the wavelength of the light source. The parameters of the entire structure have been calculated and verified using the finite-difference time-domain method.

[0048] The comparison results of coupling efficiency and alignment tolerance between Implementation Method 2 and the ordinary end-face coupling structure are shown in [link to documentation]. Figure 7 In the figure, curve a represents the comparison result of coupling efficiency and alignment tolerance of the end-face coupling structure in this embodiment, and curve b represents the comparison result of coupling efficiency and alignment tolerance of the ordinary end-face coupling structure. Here, xOffset represents the alignment deviation in the light propagation direction, yOffset represents the alignment deviation in the plane parallel to the chip surface and perpendicular to the light propagation direction, and zOffset represents the alignment deviation perpendicular to the chip surface. It can be seen that the coupling efficiency of this embodiment is improved compared to the ordinary end-face coupling structure, reaching -0.8dB. Furthermore, when yOffset and zOffset are controlled within ±0.5 micrometers, the coupling efficiency is better than -1.5dB.

[0049] The foregoing description of specific exemplary embodiments of the invention is for illustrative and explanatory purposes. These descriptions are not intended to limit the invention to the precise forms disclosed, and it will be apparent that many changes and variations can be made in accordance with the foregoing teachings. The exemplary embodiments were chosen and described in order to explain the specific principles of the invention and its practical application, thereby enabling those skilled in the art to implement and utilize various different exemplary embodiments of the invention, as well as various different choices and variations. The scope of the invention is intended to be defined by the claims and their equivalents.

Claims

1. An end-face coupling structure for a low-loss silicon photonic chip, characterized in that, The system includes a silicon substrate (1), on which a laser mounting area is provided. A buried oxide layer (2) is provided on one side of the laser mounting area and on the silicon substrate (1). A shallow etched area (3) is provided on the buried oxide layer (2). A beveled end face (5) is etched on the cross section formed by the buried oxide layer (2) and the shallow etched area (3). The beveled end face (5) is used to reduce reflection loss. A ridge waveguide (4) facing the laser mounting area is provided on the shallow etched area (3). The incident end of the ridge waveguide (4) is located at the beveled end face (5). A focusing surface (6) is provided on the beveled end face (5). The focusing surface (6) is located directly below the input end of the ridge waveguide (4). The focusing surface (6) is used to compensate for the phase and size of the light spot on the incident end face of the ridge waveguide (4). The shape of the focusing surface (6) is calculated based on the positional relationship between the DFB laser and the beveled end face (5). The focused surface (6) satisfies the equation n1sinθ1=n2sinθ2; Furthermore, the focused surface (6) also satisfies the equation n1L1=n2L2; Where n1 is the refractive index between the end-face coupling structure of the DFB laser and the silicon photonic chip, θ1 is the angle between the ray and the normal between the end-face coupling structure of the DFB laser and the silicon photonic chip, and the normal is perpendicular to the oblique end face; n2 is the refractive index of the ridge waveguide, θ2 is the angle between the ray and the normal in the ridge waveguide, L1 is the propagation distance between the end-face coupling structure of the DFB laser and the silicon photonic chip, and L2 is the propagation distance in the ridge waveguide.

2. The end-face coupling structure of the low-loss silicon photonic chip according to claim 1, characterized in that, The bevel angle of the beveled end face (5) is calculated based on the incident energy of the DFB laser, and the bevel angle of the beveled end face (5) satisfies that the energy loss reflected back to the output surface of the DFB laser through the beveled end face (5) is at least less than -30dB.

3. The end-face coupling structure of the low-loss silicon photonic chip according to claim 1, characterized in that, The beveled end face (5) is coated with an anti-reflective layer.

4. The end-face coupling structure of the low-loss silicon photonic chip according to claim 3, characterized in that, The thickness of the anti-reflection layer is calculated based on the output wavelength of the DFB laser, using the formula: 2kd=π, where k is the wave vector in the anti-reflection layer and d is the thickness of the anti-reflection layer.

5. The end-face coupling structure of the low-loss silicon photonic chip according to claim 3, characterized in that, The anti-reflective layer is a silicon nitride layer.

6. The end-face coupling structure of the low-loss silicon photonic chip according to claim 1, characterized in that, The ridge waveguide (4) is provided with a mode filtering structure to weaken the transmission of higher-order modes.

7. The end-face coupling structure of the low-loss silicon photonic chip according to claim 1, characterized in that, The buried oxide layer (2) is made of silicon dioxide.

Images