A mode spot converter based on a silicon waveguide

Abstract

The present invention discloses a mode spot converter based on a silicon waveguide, belonging to the technical field of silicon-based photonic chips; it includes an oxide cladding, a substrate and a silicon waveguide structure, wherein the silicon waveguide structure is within the silica cladding and adopts a trident structure. The input end of the trident-structured silicon waveguide is a single waveguide, which is connected to the input optical waveguide, and the output end is a dual waveguide, which is butt-jointed with an optical fiber. By adopting such a trident structure at the output end of the mode spot converter, the coupling efficiency between the waveguide and the optical fiber can be improved. The transmission part of the trident silicon waveguide adopts a tapered structure. Through the gradual reduction of the width of the silicon waveguide, the confinement effect of the waveguide on light is weakened. When the width of the waveguide is reduced to a certain extent, light cannot be confined in the waveguide and will overflow from the waveguide, and the spot size will increase accordingly, realizing the efficient coupling of two mode spots between the waveguide and the optical fiber.

Description

Technical Field

[0001] The present invention relates to the technical field of silicon-based photonic chips, and more precisely, to a mode spot converter based on a silicon waveguide. Background Art

[0002] With the increasing demand for the rate and capacity of optical communication technologies, the research and development of photonic integrated devices have received extensive attention in society. Currently, there are many materials for photonic devices, among which the more widely used ones mainly include silicon-on-insulator, InP, lithium niobate, and compound semiconductors. Among them, silicon has a large refractive index, and the silicon-on-insulator technology (SOI) has a high refractive index contrast between the silicon and the silicon dioxide layer, which can reduce the size of nano-photonic devices to a few hundred nanometers. Since silicon is transparent in the communication band, the optical transmission loss of waveguide devices with silicon as the core layer can reach a relatively small value. In addition, the process of nano-photonic devices based on silicon waveguides is compatible with the complementary metal oxide semiconductor (CMOS) process, which has attracted more and more attention to silicon photonic integrated devices.

[0003] A key technology affecting the integration of silicon photonic devices is to achieve efficient coupling between optical waveguides and optical fibers. The size of optical waveguide devices with silicon as the core layer is on the order of hundreds of nanometers, while the size of single-mode fibers (SMFs) is on the order of micrometers, with a difference of two orders of magnitude. Directly docking a single-mode fiber with an optical waveguide will result in a large coupling loss. The mode spot converter based on a silicon waveguide can connect the optical waveguide in a silicon photonic integrated device with an external optical fiber, achieving a large coupling loss and a low associated loss, so it has been widely studied. Currently, there are many studies on silicon-based photonic mode spot converters at home and abroad, including various structural types such as tapered waveguides, cantilever waveguides, and grating couplers. Among them, tapered waveguides have the advantages of simple design, large bandwidth, and high coupling efficiency. However, most current tapered waveguide mode spot converters adopt a single waveguide structure. To achieve high coupling efficiency with an optical fiber, the end face size at the connection between the mode spot converter and the optical fiber needs to be very small, which increases the processing difficulty. Summary of the Invention

[0004] In view of this, the main object of the present invention is to provide a mode spot converter based on a silicon waveguide, including an oxide cladding, a substrate, and a silicon waveguide structure. The silicon waveguide structure is within the silicon dioxide cladding and adopts a trident-shaped tapered structure. By gradually reducing the width of the silicon waveguide, the confinement effect of the waveguide on light weakens. When the waveguide width is reduced to a certain extent, light cannot be confined in the waveguide and will overflow from the waveguide, and the spot size will increase accordingly, achieving efficient coupling of two mode spots between the waveguide and the optical fiber.

[0005] To achieve the above object, the present invention provides a mode spot converter based on a silicon waveguide, which includes an oxide cladding, a substrate, and a silicon waveguide structure. The substrate is at the bottom, the oxide cladding is located above the substrate, and the silicon waveguide structure is located in the oxide cladding.

[0006] Preferably, the silicon waveguide structure is composed of three waveguide conductors arranged in sequence at a certain distance to form a trident structure, including a middle waveguide conductor and two side waveguide conductors.

[0007] Preferably, the structures of the two side waveguide conductors are the same, and the distances from the two side waveguide conductors to the middle waveguide conductor are the same.

[0008] Preferably, each of the two side waveguide conductors and the middle waveguide conductor includes two waveguides arranged in sequence, and the first waveguide is tightly connected to the second waveguide.

[0009] Preferably, the top surfaces of the first waveguides and the second waveguides of the two side waveguide conductors and the middle waveguide conductor are all flat. The heights of the first waveguide and the second waveguide are the same. The width of the first waveguide changes gradually and continuously. The width of the output end of the first waveguide is the smallest, and the output end is connected to an optical fiber or a laser. The width of the input end of the first waveguide is the largest. The second waveguide is a straight waveguide, and the width of the second waveguide is the same as the width at the connection with the first waveguide.

[0010] Preferably, the input end faces of the two side waveguide conductors are in the same plane as the connection surface between the first waveguide and the second waveguide of the middle waveguide.

[0011] Preferably, the two side waveguide conductors and the middle waveguide conductor have the same height.

[0012] A mode spot converter based on a silicon waveguide disclosed by the present invention adopts a trident silicon waveguide structure. The three waveguide conductors used in the trident waveguide structure all adopt a tapered structure. On the one hand, the coupling efficiency between the optical fiber and the optical waveguide is improved. On the other hand, the processing difficulty of the process is reduced while ensuring the coupling efficiency, laying a foundation for the development of future photonic integrated devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 is a side view of a mode spot converter based on a silicon waveguide of the present invention.

[0014] Figure 2 is a top view of the silicon waveguide structure of a mode spot converter based on a silicon waveguide of the present invention.

[0015] Figure 3It is the variation curve of the coupling efficiency between the silicon waveguide width W1 of the fiber connection end face of a mode spot converter based on silicon waveguide of the present invention and the end face of a 3-μm-diameter optical fiber, as well as the variation curve of W1 - transmittance.

[0016] Figure 4 It is the field pattern of the output end face of a mode spot converter based on silicon waveguide of the present invention.

[0017] Figure 5 It is the variation curve of the transmittance with respect to the narrow end face width W2 of the intermediate waveguide taper structure of a mode spot converter based on silicon waveguide of the present invention.

[0018] Figure 6 It is the variation curve of the transmittance with respect to the width W3 of the straight waveguides on both sides of the waveguide of a mode spot converter based on silicon waveguide of the present invention.

[0019] Figure 7 They are the variation curves of the transmittance with respect to the length changes of the two tapered waveguide structures of a mode spot converter based on silicon waveguide of the present invention, respectively.

[0020] Figure 8 It is the field pattern in the horizontal direction of a mode spot converter based on silicon waveguide of the present invention. Specific embodiments

[0021] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0022] As Figure 1 shown, this example provides a mode spot converter based on silicon waveguide, including a substrate 1, an oxide cladding 2, and a silicon waveguide structure 30. The oxide cladding 2 is located above the substrate 1, and the silicon waveguide structure 30 is located in the oxide cladding 2. As Figure 2As shown in the figure, the silicon waveguide structure 30 is a trident structure, which is composed of two side waveguides and a middle waveguide arranged at a certain distance in sequence. The two side waveguides are composed of a tapered waveguide 301, a straight waveguide 302, a tapered waveguide 303, and a straight waveguide 304, and the middle waveguide is composed of a tapered waveguide 305 and a straight waveguide 306. Among them, the tapered waveguide 301 and the tapered waveguide 303 have the same structure and size, the straight waveguide 302 and the straight waveguide 304 have the same structure and size, the distance from the two side waveguides to the middle waveguide is the same, and the height of the entire silicon waveguide structure 30 from the input end to the output end is a fixed value. The part connected to the optical waveguide is the input end of the mode spot converter, and the part connected to the optical fiber is the output end. The input end faces of the straight waveguides 302 and 304 are on the same plane as the output end face of the tapered waveguide 305, and the waveguide widths of the straight waveguides 302, 304, and 306 remain unchanged. The waveguide widths of the tapered waveguide 301, the tapered waveguide 303, and the tapered waveguide 305 are the largest at the input end, and gradually decrease from the input end to the output end.

[0023] In this embodiment, the substrate 1 of the mode spot converter and the waveguide structure 30 are made of silicon material, the oxide cladding 2 uses silicon dioxide material, the thickness h of the silicon waveguide structure 30 is 220 nm, the thickness H1 of the oxide cladding above the silicon waveguide structure 30 is 2 μm, the thickness H2 of the cladding below the silicon waveguide structure 30 is 3 μm, and the distance L between the central axes of the two side waveguides along the light transmission direction is 1 μm. By optimizing the width and length of the waveguide, a higher coupling efficiency can be obtained.

[0024] The following describes the parameter design process:

[0025] First, study the influence of the width W1 of the narrowest end face of the tapered waveguide 301 and the tapered waveguide 303 on the coupling efficiency. The influence of the width W1 on the coupling efficiency mainly includes two aspects. On the one hand, it affects the transmittance during the transmission process. On the other hand, since the narrow-width end faces of the tapered waveguide 301 and the tapered waveguide 303 are connected to the light ray or the laser, the width W1 will also affect the coupling efficiency with the optical fiber. Figure 3 is the curve of the coupling efficiency between the mode spot converter and the end face of the 3-μm-diameter optical fiber with respect to the width W1 and the curve of the transmittance with respect to the width W1. According to the curve of the transmittance with respect to the width W1, it can be found that as W1 increases, the transmittance becomes higher and higher. However, the curve of the coupling efficiency between the mode spot converter and the end face of the 3-μm-diameter optical fiber with respect to the width W1 shows that when W1 > 0.15 μm, the coupling efficiency decreases with the increase of the width. Considering both factors, the width W1 = 0.16 μm is selected. At this time, the coupling efficiency between the mode spot converter and the optical fiber is 88%, and the transmittance is 0.984. Figure 4It is the field pattern of the output end face of the mode spot converter connected to the optical fiber when the width W1 = 0.16 μm. According to the field pattern, it can be seen that by using the structure of the bilateral waveguide, compared with the single waveguide structure, it can increase the mode spot size to a certain extent while reducing the difficulty of the processing technology, thereby improving the coupling efficiency.

[0026] Then, study the influence of the width W2 of the widest end face of the tapered waveguide 301 and the tapered waveguide 303 on the coupling efficiency. When fixing the width W1 and changing the width W2, essentially the change rate of the width of the tapered waveguide is changed. When the width of the waveguide W2 gradually increases, the change rate of the width of the tapered waveguide gradually increases. Figure 5 It is the curve of the transmittance change of the mode spot converter for the width W2. In the range of W2 from 0.2 μm to 0.3 μm, the transmittance increases slowly. When the width W2 > 0.3 μm, the transmittance of the device will decrease sharply. When W2 = 0.4 μm, the transmittance decreases to 0.8. When the width W2 = 0.28 μm, the maximum transmittance is 0.984.

[0027] The change in the size of the width W3 of the narrowest end face of the tapered waveguide 305 will also have an impact on the coupling efficiency. Figure 6 It is the curve of the transmittance change of the mode spot converter for the width W3. According to Figure 6 it can be found that the change in the width W2 of the tapered waveguide 305 will affect the transmittance. However, in the range of W3 from 0.05 μm to 0.15 μm, the change amplitude of the transmittance is 0.01, and the change degree of the transmittance is not large. Considering the process problems, when the width is smaller, the processing difficulty in lithography, etching, etc. is greater. Therefore, the width W3 = 0.15 μm is selected, and at this time the transmittance of the mode spot converter is 0.984.

[0028] After the width of the tapered waveguide is determined, study the influence of the change in the length of the tapered waveguide structure on the coupling efficiency of the device. Respectively scan the lengths L1 of the tapered waveguide 301 and the tapered waveguide 303 and the length L2 of the tapered waveguide 305. The relationship curves of L1 and L2 with the transmittance are as Figure 7 shown. According to the relationship curve between the waveguide length of the tapered waveguide and the transmittance, it can be found that when the waveguide length increases, the transmittance increases and the coupling efficiency improves.

[0029] According to the relationship curve between the waveguide length L1 and the transmittance, in the range of the scanning length from 0 to 60 μm, when L1 < 30 μm, the transmittance of the device changes from 0.75 to about 0.98, and the change in transmittance is relatively obvious; when the waveguide length L2 gradually increases in the range greater than 30 μm, it can be found from the change curve that the increasing rate of the transmittance becomes slower, and the change range of the transmittance does not exceed 0.02 during the process of the length changing by 30 μm. Considering the requirements for the subsequent integration of the device, without affecting the coupling efficiency, a shorter waveguide length is preferably selected. The waveguide lengths L1 of the tapered waveguide 301 and the tapered waveguide 303 are 42 μm, and the transmittance of the device is 0.99 at this time.

[0030] According to the relationship curve between the waveguide length L2 and the transmittance, in the range of the scanning length from 0 to 60 μm, when L1 < 15 μm, the transmittance of the device fluctuates between 0.2 and 0.99, the change range of the transmittance is large, and the transmittance is unstable; when the waveguide length L2 gradually increases in the range greater than 15 μm, it can be found from the L2 - transmittance change curve that the transmittance has basically stabilized, and the transmittance fluctuates around 0.99 during the process of the length changing by 45 μm, and the amplitude change does not exceed 0.01. Similarly, considering the factors of device integration, the waveguide length L2 of the tapered waveguide 305 is selected to be 30 μm, and the transmittance of the mode spot converter is 0.99 at this time.

[0031] Figure 8 It is the field pattern of the TE mode in the horizontal direction of the mode spot converter under the above selected dimensions (W1 = 0.16 μm, W2 = 0.15 μm, W3 = 0.28 μm, L1 = 42 μm, L2 = 30 μm). Figure 8 The transmission process of the TE mode inside the device can be observed. According to the field pattern, a relatively obvious trident shape can also be seen. The TE mode enters the mode spot converter from the input end of the straight waveguide 306 of the middle waveguide body. When it transmits to the input end face positions of the straight waveguides 302 and 304 of the two side waveguide bodies, the light is gradually coupled into the two side waveguide bodies for transmission. During the process of the width of the tapered waveguides of the two side waveguide bodies decreasing, the light can no longer be confined in the waveguide, and the mode spot size also becomes larger, thus realizing efficient coupling with the output end connecting optical fiber.

Claims

1. A mode spot converter based on a silicon waveguide, characterized in that It includes an oxide cladding, a substrate, and a silicon waveguide structure. The substrate is at the bottom, the oxide cladding is located above the substrate, and the silicon waveguide structure is located in the oxide cladding; The silicon waveguide structure is composed of three waveguide conductors arranged in sequence at a certain distance to form a trident structure, including a middle waveguide conductor and waveguide conductors on both sides; The waveguide conductors on both sides and the middle waveguide conductor each include two waveguides arranged in sequence, and the first waveguide is tightly connected to the second waveguide; The top surfaces of the first waveguide and the second waveguide are both flat. The first waveguide and the second waveguide have the same height. The width of the first waveguide changes gradually and continuously. The width of the output end of the first waveguide is the smallest, and the output end is connected to an optical fiber or a laser. The width of the input end of the first waveguide is the largest. The second waveguide is a straight waveguide, and the width of the second waveguide is the same as the width at the connection with the first waveguide; The TE mode enters the mode converter from the input end of the straight waveguide of the middle waveguide conductor. When it reaches the input end face positions of the straight waveguides of the waveguide conductors on both sides, the light is gradually coupled into the waveguide conductors on both sides for transmission. During the process of the decreasing width of the tapered waveguides of the waveguide conductors on both sides, the light can no longer be confined in the waveguide, and the mode size also increases accordingly, thus achieving efficient coupling with the optical fiber at the output end.

2. The mode spot converter based on a silicon waveguide according to claim 1, characterized in that The waveguide conductors on both sides are symmetric up and down, and the distances from the waveguide conductors on both sides to the middle waveguide conductor are the same.

3. A mode spot converter based on a silicon waveguide according to claim 1, characterized in that, The input end faces of the waveguide conductors on both sides are in the same plane as the connection surface between the first waveguide and the second waveguide of the middle waveguide.

4. A mode spot converter based on a silicon waveguide according to claim 1, characterized in that, The waveguide conductors on both sides and the middle waveguide conductor have the same height.

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