Composite low-loss edge coupling structure based on trident waveguide and sub-wavelength grating

Abstract

The invention discloses a low-loss edge coupling structure based on compounding of a trigeminal waveguide and a sub-wavelength grating, and belongs to the field of optoelectronic integration. The trident waveguide is composed of a middle waveguide in the middle and side waveguides on the two sides, the outer side of each side waveguide is provided with a section of sub-wavelength grating, the central axes of the sub-wavelength gratings and the side waveguides incline towards the center by a certain angle, and the two side waveguides and the two sub-wavelength gratings are symmetrically arranged on the two sides of the middle waveguide. According to the structure, the unique trident layout is combined with the sub-wavelength grating, and an external large-spot light field can be effectively converted into a waveguide-supported single-mode field in an adiabatic manner, so that high coupling efficiency, large alignment tolerance and good process compatibility are realized.

Description

Technical Field

[0001] This invention belongs to the field of optoelectronic integration, specifically designing an edge coupler structure that uses a trident structure to achieve low-loss coupling between a single-mode SOI waveguide and a large-spot-scale edge-emitting laser. Background Technology

[0002] Due to the advantages of wide bandwidth, low latency, low power consumption, and low crosstalk of optical signals, on-chip silicon photonic interconnects based on silicon-based photonic chips have become a promising candidate for overcoming the bandwidth bottleneck of large-scale integrated circuits. However, due to the significant mode mismatch between the emitted light spot of the edge-emitting semiconductor laser (edge-emitting semiconductor optical amplifier / modulator and edge-incident photodetector) and the light spot transmitted between it and the on-chip integrated silicon waveguide, achieving high-performance coupling between them and the chip is crucial. Compared to vertical coupling schemes with small operating bandwidth and extreme sensitivity to polarization, edge coupling schemes based on mode converters can achieve efficient edge-emitting semiconductor laser-chip coupling with a larger operating bandwidth.

[0003] The most common approach to coupling side-emitting semiconductor lasers (side-emitting semiconductor optical amplifiers / modulators and side-incident photodetectors) to silicon photonic chips is to use a linear inverted cone as an edge coupler. However, the coupling efficiency of the edge coupler is closely related to the cone tip width. Due to the large aspect ratio of the inverted cone, it is difficult to achieve a single-mode edge coupling scheme with strong characteristics using a 2-3 μm thick silicon waveguide. Therefore, a series of two-port edge couplers that require connection to the bus waveguide, such as Y-couplers, directional couplers, or MMI couplers, have been designed in the past. These schemes not only increase the complexity of the device itself and the tolerance of fabrication, but also introduce additional polarization and wavelength effects on the structure. Other schemes, such as cantilever couplers and subwavelength inverted cone coupler structures, require more complex manufacturing processes, materials, and precision requirements for standard CMOS processes.

[0004] Meanwhile, the industry typically requires edge-emitting semiconductor lasers (edge-emitting semiconductor optical amplifiers / modulators and edge-incident photodetectors) to have large emission beams for spatial optical coupling with the outside world. However, edge-emitting semiconductor lasers (edge-emitting semiconductor optical amplifiers / modulators and edge-incident photodetectors) usually have large beam sizes and asymmetric characteristics. Therefore, when integrated on a SOI platform, special structures are required to achieve low-loss coupling. Currently, the losses of mainstream edge coupler structures are usually higher than 3dB. Existing on-chip edge couplers are mostly based on silicon nitride platforms. Compared to SOI platforms, silicon nitride is more difficult to process and cannot be integrated with active materials (such as Ge). Furthermore, edge couplers on SOI platforms often use single-cone structures. Single-cone structures are mostly suitable for coupling with optical fibers. When coupled with the beam of an edge-emitting semiconductor laser (edge-emitting semiconductor optical amplifier / modulator and edge-incident photodetector) with asymmetric characteristics, the inherent symmetric beam characteristics of the structure make it difficult to further optimize for large beam sizes.

[0005] Based on the above factors, an edge coupler structure combining a trident and a subwavelength grating waveguide is proposed. This structure can achieve the advantages of high coupling performance, large operating bandwidth, polarization insensitivity, and large process tolerance while being compatible with traditional thin-film waveguide silicon photonics fabrication processes. Summary of the Invention

[0006] The objective of this invention is to provide a low-loss edge-coupled structure based on a trident waveguide and subwavelength grating composite. This structure, through a unique trident layout combined with a subwavelength grating, can effectively and adiabatically convert an external large-mode optical field into a waveguide-supported single-mode field, thereby achieving high coupling efficiency, large alignment tolerance, and good process compatibility.

[0007] The technical solution of the present invention: A low-loss edge coupling structure based on a trident waveguide and a subwavelength grating is disclosed. The trident waveguide consists of a middle waveguide and two side waveguides. Each side waveguide has a subwavelength grating on its outer side. The central axes of the subwavelength gratings and the side waveguides are tilted towards the center at a certain angle. The two side waveguides and the two subwavelength gratings are symmetrically arranged on both sides of the middle waveguide.

[0008] The middle waveguide consists of a first straight waveguide segment, a gradient segment, and a second straight waveguide segment. The width of the side waveguides gradually increases and then gradually decreases, and the width of the subwavelength grating gradually decreases.

[0009] The width of the first straight waveguide segment is d1. The transition segment is divided into two segments. The width of the first transition segment d1 increases to d2 with a small rate of change k1. The width of the second transition segment d2 increases to d3 with a larger rate of change k2. The width of the second straight waveguide segment is d3.

[0010] The width of the side waveguide gradually increases from d1 to d4 with a rate of change k, and then gradually decreases from d4 to d1 with a rate of change -k.

[0011] The initial width of the subwavelength grating is greater than d1, and the end width is less than d1.

[0012] The length of the first straight waveguide segment is L1, the length of the first gradient segment is L2, the length of the second gradient segment is L3, the projection length of the side waveguide on the central axis is L4, and the projection length of the subwavelength grating on the central axis is L5. (L1+L2+L3)>L4>L5.

[0013] The minimum spacing between the subwavelength grating and the side waveguide is greater than 110 nm, and the minimum spacing between the side waveguide and the middle waveguide is greater than 110 nm.

[0014] The subwavelength grating, side waveguide, and middle waveguide have the same height.

[0015] The middle waveguide, two side waveguides, and two subwavelength gratings are all located on the upper surface of the buried oxide layer and encapsulated with a cladding.

[0016] The cladding material is silicon dioxide, and the middle waveguide, two side waveguides, and two subwavelength gratings are passive silicon waveguides.

[0017] The beneficial effects of this invention are: 1. Low coupling loss: Five cones arranged laterally in a subwavelength grating, side waveguide, and middle waveguide are used to initially receive and match a large incident light spot, utilizing its tunable equivalent refractive index to improve end-face coupling efficiency. Subsequently, a trident structure composed of symmetrically tilted side waveguides and middle waveguides is used to adiabatically focus the light energy and convert it into a single-mode waveguide mode, achieving low-loss mode-field conversion throughout the entire link.

[0018] 2. High process and alignment tolerances: The unique composite structure and optimized dimensional design (such as specific width variation rates and length relationships) make the device insensitive to manufacturing errors. Meanwhile, the tilted side waveguides and gratings expand the effective coupling area, allowing for greater horizontal, angular, and spot size deviations during alignment, reducing packaging difficulty and cost.

[0019] 3. Compact structure and process compatibility: All components can be fabricated on a standard SOI wafer through a single photolithography and etching process, without the need for complex steps or special materials. The small device size is beneficial for the realization of highly integrated photonic chips.

[0020] 4. Superior performance: This structure exhibits high coupling efficiency, wide bandwidth and low polarization dependence in communication windows such as the O-band, and its overall performance is superior to that of traditional single-cone or symmetrical couplers. Attached Figure Description

[0021] Figure 1 This is a cross-sectional view of the trident and subwavelength grating composite coupling structure.

[0022] Figure 2 This is a top view of the coupled structure.

[0023] Figure 3 This is a side view of the coupled structure.

[0024] Figure 4 The diagram shows the coupling efficiency of the coupled structure in the O-band.

[0025] Figure 5 The diagram shows the coupling efficiency of the coupled structure under different horizontal alignment deviations.

[0026] Figure 6 The diagram shows the coupling efficiency of the coupled structure under different incident angle deviations.

[0027] Figure 7 The figure shows the coupling efficiency of the coupling structure under different incident light spot sizes.

[0028] Figure 8 This is a simulation diagram of the optical field of the coupled structure.

[0029] Figure reference numerals: 001 - Silica cladding; 002 - Passive trident and subwavelength grating silicon waveguide; 003 - Buried oxide layer. Detailed Implementation

[0030] Example 1: A low-loss edge-coupled structure based on a trident waveguide and subwavelength grating composite is disclosed. The trident waveguide consists of a central waveguide and two side waveguides symmetrically distributed on either side. Its innovation lies in the addition of a subwavelength grating on the outer side of each side waveguide. The central axes of both the subwavelength gratings and the side waveguides are tilted at a certain angle towards the structure's centerline (i.e., the axis of symmetry of the central waveguide). The two side waveguides and two subwavelength gratings are arranged symmetrically around the central waveguide. Adding subwavelength gratings to the outer side of the trident structure results in five laterally arranged cones that make the end-face fundamental mode elliptical, similar to the laser's output beam shape, thus reducing end-face coupling loss. Simultaneously, the outer subwavelength grating has a lower effective refractive index, while the inner solid silicon waveguide has a higher effective refractive index. This refractive index distribution allows light to converge rapidly from the outside to the center.

[0031] Furthermore, the middle waveguide is formed by connecting a first straight waveguide segment, a gradient segment, and a second straight waveguide segment sequentially along the optical transmission direction. The width of the side waveguides is designed to gradually increase first and then gradually decrease. The width of the subwavelength grating is designed to gradually decrease along the optical transmission direction.

[0032] Furthermore, the first straight waveguide segment of the middle waveguide has a constant width d1. Its gradient segment is divided into two parts: the width of the first gradient segment starts from d1 and gradually increases to width d2 at a first rate of change k1; the width of the second gradient segment starts from d2 and increases more rapidly to width d3 at a second rate of change k2, which is greater than k1. The second straight waveguide segment of the middle waveguide maintains a constant width d3. The smaller slope of the first gradient segment allows for adiabatic coupling of optical energy. The larger slope of the second gradient segment allows for rapid coupling of optical energy into a 500nm wide standard single-mode silicon waveguide fundamental mode. This multi-segment gradient design helps to reduce device size and improve design flexibility.

[0033] The width variation pattern of the side waveguide is as follows: starting from its initial end, the width gradually increases linearly from d1 to the maximum width d4 at a rate of change k; after reaching d4, its width then gradually decreases linearly from d4 back to the final width d1 at a rate of change -k (i.e., the same absolute value of the slope in the increasing stage, but with the opposite sign). The initial linear increase at the beginning leads to a gradual increase in effective refractive index, allowing light from the outer subwavelength grating to couple into the side waveguide. The subsequent linear decrease at the rear end leads to a gradual decrease in effective refractive index, allowing light to couple into the middle waveguide, and finally converting it into the fundamental mode of a standard single-mode silicon waveguide.

[0034] The width of the subwavelength grating is designed such that its initial width at the beginning is greater than d1, while its width at the end is less than d1, exhibiting an overall linear decreasing trend. This linear decreasing trend causes the effective refractive index of the subwavelength grating to gradually decrease, allowing light to couple rapidly into the inner solid silicon waveguide.

[0035] Regarding the length relationships, let L1 be the length of the first straight waveguide segment, L2 be the length of the first gradient segment, L3 be the length of the second gradient segment, L4 be the projection length of the side waveguide onto the central axis of the middle waveguide, and L5 be the projection length of the subwavelength grating onto the central axis of the middle waveguide. They satisfy the following relationship: (L1+L2+L3)>L4>L5. This means that the effective operating area of ​​the middle waveguide is the longest, followed by the side waveguide, and the subwavelength grating is the shortest.

[0036] To ensure effective transmission of the optical field via evanescent wave coupling and improve process compatibility, the minimum spacing between the subwavelength grating and the adjacent side waveguide, as well as the minimum spacing between the side waveguide and the middle waveguide, must both be greater than 110 nm.

[0037] Preferably, the subwavelength grating, side waveguide, and middle waveguide are etched from the same material layer (such as silicon), so they have the same height.

[0038] The entire coupling structure is fabricated on the buried oxide layer. The middle waveguide, two side waveguides and two subwavelength gratings are all located on the upper surface of the buried oxide layer and are completely covered and encapsulated by the upper cladding material (such as silicon dioxide).

[0039] The input light is captured and initially constrained by five cones arranged laterally in a subwavelength grating, side waveguide, and middle waveguide. Subsequently, the optical energy is transferred to the inclined side waveguide via evanescent wave coupling. As the waveguide width changes, the optical field is further guided and gradually converges towards the center. Finally, the optical field is efficiently converged into the middle waveguide through evanescent coupling between the side waveguide and the middle waveguide, and after adiabatic transformation, forms a stable single-mode transmission in the second straight waveguide section. Figure 8 The simulated light field evolution diagram visually illustrates the energy convergence process described above.

[0040] Example 2: like Figure 1 As shown, we designed a novel edge-coupled structure with symmetrical structure. Light propagates in a passive trident and subwavelength grating silicon waveguide 002, which consists of a silicon dioxide cladding 001, a passive trident and subwavelength grating silicon waveguide 002, and a buried oxide layer 003. Figure 3 As shown, according to the process requirements provided by the wafer fab, the silicon waveguides involved in this invention are all strip waveguides, and the photolithography method uses a 220nm deep etching, that is, the total height of the passive silicon waveguide 002 is 220nm, the etched area is 220nm, and the thickness of the upper deposited silicon dioxide layer is 4μm. Figure 2 As shown, the passive trident silicon waveguide consists of a middle waveguide, two inverted conical side waveguides on both sides, and two outermost subwavelength grating side waveguides. At the end face of the coupling structure, the distance between the side waveguide and the middle waveguide is 0.8 μm, and the distance between the middle waveguide and the subwavelength grating side waveguide is 0.6 μm.

[0041] The waveguide is divided into three segments along the light propagation direction (from left to right). The first straight waveguide segment has a constant width of d1 = 100 nm and a length of L1 = 10 μm. Then comes the gradient segment, the first part of which (the first gradient segment) gradually increases the width from 100 nm to d2 = 221 nm at a small rate of change k1, with a length of L2 = 15 μm; the second part (the second gradient segment) rapidly increases the width from 221 nm to d3 = 500 nm at a larger rate of change k2, with a length of L3 = 10 μm. Finally, the second straight waveguide segment maintains a constant width of d3 = 500 nm and is used to output stable single-mode light.

[0042] Two side waveguides are symmetrically and obliquely arranged on both sides of the middle waveguide 1, with their axes forming an angle of approximately 3° with the centerline. Each side waveguide has an initial width of d1 = 100 nm, which increases linearly at a rate of k = 1.67 μm⁻¹ to a maximum width of d4 = 150 nm, and then decreases back to 100 nm at the same slope. Its total projected length along the central axis of the middle waveguide is L4 = 30 μm. The location of the side waveguide at its maximum width is 550 nm perpendicular to the first straight waveguide segment of the middle waveguide.

[0043] On the outer side of each side waveguide, there is a subwavelength grating with an axial tilt angle of 2° to make it more "open" for light reception. The subwavelength grating is composed of periodically arranged silicon pillars, with its equivalent width linearly decreasing from 120 nm at the beginning to 90 nm at the end, and a projected length L5 = 20 μm. The minimum gap between all adjacent components (such as the grating and the side waveguide, and the side waveguide and the middle waveguide) is greater than a critical value of 110 nm to ensure sufficient evanescent coupling and improve process compatibility. The optical signal enters from the silicon waveguide input end, propagates in the side waveguides on both sides, enters the central inverted conical waveguide region through evanescent coupling, and is finally output through the straight waveguide. There is a certain distance between the inverted conical waveguides on both sides and the subwavelength grating side waveguides, forming a trident-subwavelength grating composite structure together with the central inverted conical waveguide and the straight waveguide, effectively achieving mode conversion with low transmission loss. Because the passive silicon waveguide employs a novel trident-and-subwavelength grating composite structure, the size of the optical field transmitted within the passive silicon waveguide is matched to the laser source, allowing for adiabatic transmission of the optical signal within this segment. This design improves the coupling efficiency of the passive silicon waveguide and enables more optical signals to be transmitted through the passive trident-and-subwavelength grating silicon waveguide, effectively coupling the optical signal from the laser into the passive silicon waveguide. Figure 4 The curve shows the coupling efficiency of this coupling structure for input light of different wavelengths. Figure 5 and Figure 6 This indicates that the coupling efficiency remains acceptable within a horizontal alignment deviation of ±1μm or an incident angle deviation of ±5°, demonstrating its excellent alignment tolerance. Figure 7 This demonstrates that the present invention can be applied to different incident spot sizes, proving its excellent input spot size tolerance.

Claims

1. A low-loss edge-coupled structure based on a trident waveguide and subwavelength grating composite, wherein the trident waveguide consists of a middle waveguide and two side waveguides, characterized in that: Each side waveguide has a subwavelength grating on its outer side. The subwavelength grating and the central axis of the side waveguide are tilted towards the center at a certain angle. The two side waveguides and the two subwavelength gratings are symmetrically arranged on both sides of the middle waveguide.

2. The low-loss edge-coupled structure based on a trident waveguide and subwavelength grating composite as described in claim 1, characterized in that: The middle waveguide consists of a first straight waveguide segment, a gradient segment, and a second straight waveguide segment. The width of the side waveguides gradually increases and then gradually decreases, and the width of the subwavelength grating gradually decreases.

3. The low-loss edge-coupled structure based on a trident waveguide and subwavelength grating composite as described in claim 2, characterized in that: The width of the first straight waveguide segment is d1. The transition segment is divided into two segments. The width of the first transition segment d1 increases to d2 with a small rate of change k1. The width of the second transition segment d2 increases to d3 with a larger rate of change k2. The width of the second straight waveguide segment is d3.

4. The low-loss edge-coupled structure based on a trident waveguide and subwavelength grating composite as described in claim 3, characterized in that: The width of the side waveguide gradually increases from d1 to d4 with a rate of change k, and then gradually decreases from d4 to d1 with a rate of change -k.

5. The low-loss edge-coupled structure based on a trident waveguide and subwavelength grating composite as described in claim 3, characterized in that: The initial width of the subwavelength grating is greater than d1, and the end width is less than d1.

6. The low-loss edge-coupled structure based on a trident waveguide and subwavelength grating composite as described in claim 4, characterized in that: The length of the first straight waveguide segment is L1, the length of the first gradient segment is L2, the length of the second gradient segment is L3, the projection length of the side waveguide on the central axis is L4, and the projection length of the subwavelength grating on the central axis is L5. (L1+L2+L3)>L4>L5.

7. The low-loss edge-coupled structure based on a trident waveguide and subwavelength grating composite as described in claim 3, characterized in that: The minimum spacing between the subwavelength grating and the side waveguide is greater than 110 nm, and the minimum spacing between the side waveguide and the middle waveguide is greater than 110 nm.

8. The low-loss edge-coupled structure based on a trident waveguide and subwavelength grating composite as described in claim 3, characterized in that: The subwavelength grating, side waveguide, and middle waveguide have the same height.

9. The low-loss edge-coupled structure based on a trident waveguide and subwavelength grating composite according to any one of claims 1-8, characterized in that: The middle waveguide, two side waveguides, and two subwavelength gratings are all located on the upper surface of the buried oxide layer and encapsulated with a cladding.

10. The low-loss edge-coupled structure based on a trident waveguide and subwavelength grating composite as described in claim 9, characterized in that: The cladding material is silicon dioxide, and the middle waveguide, two side waveguides, and two subwavelength gratings are passive silicon waveguides.

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