An end coupler based on coupling of optical fiber and lithium niobate waveguide and its preparation method

Abstract

This invention relates to the field of optical device technology, and proposes an end-face coupler based on the coupling of optical fiber and lithium niobate waveguide, comprising a substrate, an insulating layer, a first planar layer, a first waveguide core layer, and a second waveguide core layer. The first waveguide core layer includes a first waveguide and a first tapered waveguide, with one side of the first waveguide connected to one side of the first tapered waveguide, and the widths of the connected sides of the first and second waveguides being the same. The second waveguide core layer includes a second waveguide and a second tapered waveguide, with one side of the second waveguide connected to one side of the second tapered waveguide, and the widths of the connected sides of the second and second waveguides being the same. The first and second tapered waveguides coincide with each other in opposite directions; the side of the second waveguide away from the second tapered waveguide is mode-matched to an external optical fiber. The first planar layer and the first waveguide core layer are made of thin-film lithium niobate material, and the second waveguide core layer is a material with a refractive index higher than that of the insulating layer or a material with a periodic distribution of high and low refractive indices.

Description

Technical Field

[0001] This invention relates to the field of optical device technology, and more specifically, to an end-face coupler based on the coupling of optical fiber and lithium niobate waveguide and its fabrication method. Background Technology

[0002] Electro-optic modulators based on thin-film lithium niobate have seen rapid development, exhibiting significant advantages in modulator bandwidth, on-chip insertion loss, linearity, and modulation efficiency. However, in implementing chip-to-optical network connections, the high refractive index of lithium niobate thin films can lead to severe mode mismatch issues, resulting in significant insertion loss. End-face couplers, also known as mode converters, are a common solution. Essentially, this approach adjusts the refractive index, gradually reducing the refractive index of the larger modes in the waveguide through a specific structure until it matches the spatial refractive index outside the chip. This achieves mode mismatch between the chip and the external environment, thereby reducing coupling loss.

[0003] Currently, there are two common methods for mode matching: one is to make the tapered waveguide sufficiently thin, distributing the light energy around the lithium niobate waveguide. This method can couple with small mode field beams, but the lithium niobate waveguide must be sufficiently thin, so even a small change in its geometry can affect the coupling efficiency with the optical fiber. Therefore, it suffers from drawbacks such as small tolerance, high fabrication precision, and low yield. The other method is to use a low-refractive-index material transition to amplify the mode light in the lithium niobate waveguide and thus match it with the mode in the optical fiber. This material is generally a low-refractive-index polymer or silicon oxynitride. However, due to the relatively high refractive index of lithium niobate waveguides, two or more stages of cascaded lithium niobate tapered waveguides are generally used for mode transition. From a design perspective, the tip of the lithium niobate tapered waveguide needs to be sufficiently thin to avoid a significant abrupt change in refractive index at the splicing point, which could cause mode reflection within the waveguide. However, in actual fabrication, considering the resolution limitations of the equipment, tapered waveguide tips smaller than 200nm can only be fabricated using high-resolution electron beam lithography or high-precision deep ultraviolet lithography with a process technology of less than 100nm. Therefore, the fabrication cost is high, hindering the advancement of mass production and commercialization. Summary of the Invention

[0004] To overcome the shortcomings of the prior art, such as small tolerance and high manufacturing cost of the end-face coupler, this invention provides an end-face coupler based on the coupling of optical fiber and lithium niobate waveguide and its manufacturing method.

[0005] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows:

[0006] An end-face coupler based on optical fiber and lithium niobate waveguide coupling includes a substrate, an insulating layer, a first flat plate layer, a first waveguide core layer and a second waveguide core layer arranged sequentially from bottom to top.

[0007] The first waveguide core layer includes a first waveguide and a first tapered waveguide, with one side of the first waveguide connected to one side of the first tapered waveguide, and the widths of the connected sides of the first and second waveguides are the same. The second waveguide core layer includes a second waveguide and a second tapered waveguide, with one side of the second waveguide connected to one side of the second tapered waveguide, and the widths of the connected sides of the second and second waveguides are the same. The first and second tapered waveguides are arranged in opposite directions and coincidentally. The side of the second waveguide away from the second tapered waveguide is mode-matched with an external optical fiber.

[0008] The insulating layer is a low-refractive-index non-metallic oxide material or polymer, the first flat plate layer and the first waveguide core layer are thin-film lithium niobate materials, and the second waveguide core layer is a material with a refractive index higher than that of the insulating layer or a material with a periodic distribution of high and low refractive indices.

[0009] As a preferred embodiment, the thickness of the first planar layer is less than or equal to 250 nm; the thickness of the first waveguide core layer is equal to the thickness of the first planar layer, or differs from the thickness of the first planar layer by ±100 nm; and the thickness of the second waveguide core layer is less than or equal to 8 μm.

[0010] As a preferred embodiment, the tip width of the first tapered waveguide is less than or equal to 350 nm, and the width of the first waveguide is less than 5 μm; the tip width of the second tapered waveguide is less than or equal to 2 μm; and the width of the second waveguide is less than or equal to 10 μm.

[0011] As a preferred embodiment, the lengths of the first tapered waveguide and the second tapered waveguide are equal and greater than or equal to 80 μm, or the absolute value of the difference between the lengths of the first tapered waveguide and the second tapered waveguide is less than or equal to 50 μm.

[0012] As a preferred embodiment, the second waveguide core layer is a ridge waveguide structure, and its etching depth is greater than or equal to 70% of the thickness of the second waveguide layer.

[0013] As a preferred embodiment, an etching stop layer made of a low refractive index medium is provided between the first waveguide core layer and the second waveguide core layer; the refractive index of the etching stop layer is lower than that of the thin film lithium niobate.

[0014] As a preferred embodiment, the upper surface of the second waveguide core layer is covered with a cladding made of a low-refractive-index dielectric material or a polymer material; the upper surface of the first waveguide is covered with a cladding made of a low-refractive-index dielectric material or a polymer material.

[0015] As a preferred embodiment, the side of the first waveguide away from the first tapered waveguide is covered with a polymer or low-refractive-index dielectric layer.

[0016] Furthermore, this invention also proposes a method for fabricating an end-face coupler based on the coupling of optical fiber and lithium niobate waveguide, used to fabricate the end-face coupler proposed in any of the above-mentioned technical solutions. This includes the following steps:

[0017] S1: The first waveguide and the first tapered waveguide are fabricated on a thin-film lithium niobate wafer using photolithography and lithium niobate etching techniques;

[0018] S2: In the sample prepared in step S1, a low refractive index dielectric material is prepared as an etching stop layer using a deposition process.

[0019] S3: A second waveguide core layer is prepared by spin-coating a high-refractive-index polymer or depositing a high-refractive-index medium material or a medium material with periodic distribution of high and low refractive indices on the sample prepared in step S2.

[0020] S4: The second waveguide core layer is etched on the sample prepared in step S3 using photolithography and etching processes to prepare the second waveguide and the second tapered waveguide.

[0021] S5: The sample prepared in step S4 is used to prepare a low refractive index dielectric material by deposition or a polymer material by spin coating as the cladding of the second waveguide core layer.

[0022] S6: After cleaving or slicing the sample prepared in step S5, polish it to complete the preparation of the end face coupler.

[0023] As a preferred embodiment, in step S2, the low refractive index medium includes silicon oxide, silicon oxynitride, and polymers.

[0024] When silicon oxide and silicon oxynitride are used as low-refractive-index media to prepare the etching stop layer, they are prepared by PECVD or ICP-CVD deposition process; when polymers are used as low-refractive-index media to prepare the etching stop layer, they are prepared by spin coating or spray coating process, and then thermally cured or UV cured.

[0025] Compared with existing technologies, the beneficial effects of the present invention are as follows: The widths of the first and second tapered waveguides in the present invention gradually transition from the tip towards the direction of connection with other waveguides, and gradually widen. That is, the tip of the first tapered waveguide and the second tapered waveguide have large dimensions, are simple to fabricate, have large tolerances, and are conducive to large-scale mass production. The core layer of the first waveguide uses thin-film lithium niobate material, and the core layer of the second waveguide uses a material with a high refractive index or a material with a periodic distribution of high and low refractive indices. The thin-film lithium niobate only needs to be etched once to achieve high-efficiency coupling, and the process flow is simple. In addition, the present invention can achieve stable and efficient coupling between lithium niobate waveguides and small-mode-field optical fibers, covering the wavelength range from near-visible light to near-infrared light, and the coupling efficiency is greater than 85%. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the end-face coupler based on the coupling of optical fiber and lithium niobate waveguide of the present invention.

[0027] Figure 2 This is a top view of the end-face coupler of the present invention.

[0028] Figure 3 This is a side view of the end-face coupler of the present invention.

[0029] Figure 4 This is a cross-sectional view of the end-face coupler of the present invention.

[0030] Figure 5 The figure shows the coupling efficiency between the lithium niobate waveguide and the small-mode fiber of the present invention.

[0031] Figure 6 This diagram shows the coupling efficiency between the lithium niobate waveguide layer and the small-mode fiber of the present invention.

[0032] Figure 7 This is a flowchart illustrating the fabrication method of the end-face coupler based on optical fiber and lithium niobate waveguide coupling according to the present invention. Wherein, 1-substrate, 2-insulating layer, 3-first flat plate layer, 4-first waveguide core layer, 41-first waveguide, 42-first tapered waveguide, 5-second waveguide core layer, 51-second waveguide, 52-second tapered waveguide. Detailed Implementation

[0033] The accompanying drawings are for illustrative purposes only and should not be construed as limiting the scope of this patent.

[0034] To better illustrate this embodiment, some parts in the accompanying drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions;

[0035] It will be understood by those skilled in the art that certain well-known structures and their descriptions may be omitted in the accompanying drawings.

[0036] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0037] Example 1

[0038] This embodiment proposes an end-face coupler based on the coupling of optical fiber and lithium niobate waveguide, such as... Figures 1-4 The diagram shown is a structural schematic of the end-face coupler in this embodiment.

[0039] The end-face coupler based on optical fiber and lithium niobate waveguide coupling proposed in this embodiment includes a substrate 1, an insulating layer 2, a first flat plate layer 3, a first waveguide core layer 4, and a second waveguide core layer 5 arranged sequentially from bottom to top.

[0040] The first waveguide core layer 4 includes a first waveguide 41 and a first tapered waveguide 42. One side of the first waveguide 41 is connected to one side of the first tapered waveguide 42, and the widths of the connected sides of the first tapered waveguide 42 and the first waveguide 41 are the same.

[0041] The second waveguide core layer 5 includes a second waveguide 51 and a second tapered waveguide 52. One side of the second waveguide 51 is connected to one side of the second tapered waveguide 52, and the width of the side where the second tapered waveguide 52 and the second waveguide 51 are connected is the same.

[0042] The first tapered waveguide 42 and the second tapered waveguide 52 are arranged in opposite directions and coincidentally; the side of the second waveguide 51 away from the second tapered waveguide 52 is mode-matched with the external optical fiber. The tip direction of the first waveguide core layer 4 is the direction of coupling with the optical fiber.

[0043] The insulating layer 2 is a low-refractive-index non-metallic oxide material or polymer, the first flat plate layer 3 and the first waveguide core layer 4 are thin-film lithium niobate materials, and the second waveguide core layer 5 is a material with a refractive index higher than that of the insulating layer 2 or a material with a periodic distribution of high and low refractive indices.

[0044] Preferably, the refractive index of the second waveguide core layer 5 is between 1.5 and 2.1.

[0045] In an optional embodiment, the thickness of the first planar layer 3 is less than or equal to 250 nm; the thickness of the first waveguide core layer 4 is equal to or differs from the thickness of the first planar layer 3 by ±100 nm; and the thickness of the second waveguide core layer 5 is less than or equal to 8 μm.

[0046] Furthermore, the tip width of the first tapered waveguide 42 is less than or equal to 350 nm, and the width of the first waveguide 41 is less than 5 μm.

[0047] The width of the tip of the second tapered waveguide 52 is less than or equal to 2 μm, and the width of the second waveguide is less than or equal to 10 μm.

[0048] Furthermore, the lengths of the first tapered waveguide 42 and the second tapered waveguide 52 are equal and greater than or equal to 80 μm, or the absolute value of the difference between the lengths of the first tapered waveguide 42 and the second tapered waveguide 52 is less than or equal to 50 μm.

[0049] In another alternative embodiment, the second waveguide core layer 5 is a ridge waveguide structure, and its etching depth is greater than or equal to 70% of the thickness of the second waveguide 51 layer.

[0050] Alternatively, the second waveguide core layer 5 may be a strip waveguide structure.

[0051] In this embodiment, the lengths of the second tapered waveguide 52 and the first tapered waveguide 42 are key parameters for achieving mode matching. Figures 2-4 As can be seen, in the end-face coupler of this embodiment, the widths of the first tapered waveguide 42 and the second tapered waveguide 52 gradually transition from the tip towards the direction of connection with other waveguides, and gradually widen. Figure 4 The four sections A, B, C, and D cut sequentially by the dividing lines A'-A", B'-B", C'-C", and D'-D" are as follows: Figure 4 (a) Figure 4 (b) Figure 4 (c) Figure 4 As shown in (d), the mode spot gradually decreases from the A section which matches the optical fiber, and the mode spot at the D section is the smallest. The mode spot at the D section matches the lithium niobate mode spot in the first waveguide 41, thereby greatly improving the mode spot conversion efficiency and realizing the coupling between the optical fiber and the lithium niobate waveguide.

[0052] Furthermore, this embodiment uses a polymer or silicon nitride with a high refractive index as the mode conversion material. The thin-film lithium niobate only needs to be etched once to achieve high-efficiency coupling, and the process is simple. The first tapered waveguide 42 tip and the second tapered waveguide 52 have large dimensions, are simple to fabricate, have large tolerances, and are conducive to large-scale mass production; the wavelength range can cover the near-visible to near-infrared band.

[0053] like Figure 5 The figure shows the coupling efficiency of TE mode light between a lithium niobate waveguide and a small-mode-field fiber at wavelengths of 1310 nm and 1550 nm. As can be seen from the figure, when TE light at wavelengths of 1310 nm or 1550 nm is coupled, a coupling efficiency greater than 85% can be achieved with a coupling distance greater than 80 μm.

[0054] This embodiment enables coupling between lithium niobate waveguides and small-mode field optical fibers, such as... Figure 6The figure shows the coupling efficiency between the lithium niobate waveguide layer and the small-mode fiber at wavelengths of 900-1700 nm. The end-face coupler fabricated in this embodiment has coupling efficiencies of 1.3 dB / facet and 1.5 dB / facet in the 1310 and 1550 nm bands, respectively, indicating that the end-face coupler can achieve stable and efficient coupling in the near-visible to near-infrared band (900 nm-1700 nm).

[0055] Example 2

[0056] This embodiment is an improvement on the end-face coupler based on the coupling of optical fiber and lithium niobate waveguide proposed in Embodiment 1.

[0057] The end-face coupler based on optical fiber and lithium niobate waveguide coupling proposed in this embodiment includes a substrate 1, an insulating layer 2, a first flat plate layer 3, a first waveguide core layer 4, and a second waveguide core layer 5 arranged sequentially from bottom to top.

[0058] The first waveguide core layer 4 includes a first waveguide 41 and a first tapered waveguide 42. One side of the first waveguide 41 is connected to one side of the first tapered waveguide 42, and the widths of the connected sides of the first tapered waveguide 41 and the first waveguide 42 are the same. The second waveguide core layer 5 includes a second waveguide 51 and a second tapered waveguide 52. One side of the second waveguide 51 is connected to one side of the second tapered waveguide 52, and the widths of the connected sides of the second tapered waveguide 52 and the second waveguide 51 are the same.

[0059] The first tapered waveguide 42 and the second tapered waveguide 52 are arranged in opposite directions and coincidentally; the side of the second waveguide 51 away from the second tapered waveguide 52 is mode-matched with the external optical fiber.

[0060] The insulating layer 2 is a low-refractive-index non-metallic oxide material or polymer, the first flat plate layer 3 and the first waveguide core layer 4 are thin-film lithium niobate materials, and the second waveguide core layer 5 is a material with a refractive index higher than that of the insulating layer 2 or a material with a periodic distribution of high and low refractive indices.

[0061] Furthermore, a polymer is spin-coated or a low-refractive-index dielectric layer is deposited on the side of the first waveguide 41 away from the first tapered waveguide 42.

[0062] An etching stop layer made of a low-refractive-index medium is provided between the first waveguide core layer 4 and the second waveguide core layer 5; the refractive index of the etching stop layer is lower than that of the thin film lithium niobate.

[0063] Preferably, the thickness of the etching stop layer is less than or equal to 100 nm.

[0064] More preferably, the low refractive index medium material used includes silicon oxide or silicon oxynitride.

[0065] In this embodiment, the added etching medium layer is used to effectively protect the devices outside the coupling structure.

[0066] In another alternative embodiment, the upper surface of the second waveguide core layer 5 is covered with a cladding made of a low-refractive-index dielectric material or a polymer material. The upper surface of the first waveguide is covered with a cladding made of a low-refractive-index dielectric material or a polymer material.

[0067] Example 3

[0068] This embodiment proposes a method for fabricating an end-face coupler based on the coupling of optical fiber and lithium niobate waveguide, used to fabricate the end-face couplers based on the coupling of optical fiber and lithium niobate waveguide as described in Examples 1 and 2. Figure 7 The diagram shown is a flowchart of the preparation method in this embodiment.

[0069] The fabrication method of the end-face coupler based on the coupling of optical fiber and lithium niobate waveguide proposed in this embodiment includes the following steps:

[0070] S1: The first waveguide 41 and the first tapered waveguide 42 are fabricated on a thin-film lithium niobate wafer using photolithography and lithium niobate etching techniques;

[0071] S2: In the sample prepared in step S1, a low refractive index dielectric material is prepared as an etching stop layer using a deposition process.

[0072] S3: The second waveguide core layer 5 is prepared by spin-coating a high refractive index polymer or depositing a high refractive index medium material or a medium material with periodic distribution of high and low refractive indices on the sample prepared in step S2.

[0073] S4: The second waveguide core layer 5 is etched on the sample prepared in step S3 using photolithography and etching processes to prepare the second waveguide 51 and the second tapered waveguide 52.

[0074] S5: The sample prepared in step S4 is used to prepare a low refractive index dielectric material by deposition or a polymer material by spin coating as the cladding of the second waveguide core layer.

[0075] S6: After cleaving or slicing the sample prepared in step S5, polish it to complete the preparation of the end face coupler.

[0076] Furthermore, in an optional embodiment, the low refractive index medium includes silicon oxide, silicon oxynitride, and polymers.

[0077] Specifically, when silicon oxide and silicon oxynitride are used as low-refractive-index media to prepare the etching stop layer, they are prepared by PECVD or ICP-CVD deposition process; when polymer is used as low-refractive-index media to prepare the etching stop layer, it is prepared by spin coating or spray coating process, and then thermally cured or UV cured.

[0078] The same or similar labels correspond to the same or similar parts;

[0079] The terms used to describe positional relationships in the accompanying drawings are for illustrative purposes only and should not be construed as limiting this patent.

[0080] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, and are not intended to limit the implementation of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. An end-face coupler based on the coupling of optical fiber and lithium niobate waveguide, characterized in that, It includes a substrate (1), an insulating layer (2), a first flat plate layer (3), a first waveguide core layer (4), and a second waveguide core layer (5) arranged sequentially from bottom to top; The first waveguide core layer (4) includes a first waveguide (41) and a first tapered waveguide (42). One side of the first waveguide (41) is connected to one side of the first tapered waveguide (42), and the widths of the connected sides of the first tapered waveguide (42) and the first waveguide (41) are the same. The second waveguide core layer (5) includes a second waveguide (51) and a second tapered waveguide (52). One side of the second waveguide (51) is connected to one side of the second tapered waveguide (52), and the width of the side connecting the second tapered waveguide (52) and the second waveguide (51) is the same. The first tapered waveguide (42) and the second tapered waveguide (52) are arranged in opposite directions and coincidentally; the side of the second waveguide (51) away from the second tapered waveguide (52) is mode-matched with the external optical fiber; The insulating layer (2) is a low-refractive-index non-metallic oxide material or polymer, the first flat plate layer (3) and the first waveguide core layer (4) are thin-film lithium niobate materials, and the second waveguide core layer (5) is a material with a refractive index higher than that of the insulating layer (2) or a material with a periodic distribution of high and low refractive indices; The thickness of the first plate layer (3) is less than or equal to 250 nm; the thickness of the first waveguide core layer (4) is equal to the thickness of the first plate layer (3), or differs from the thickness of the first plate layer (3) by ±100 nm; the thickness of the second waveguide core layer (5) is less than or equal to 8 μm. The tip width of the first tapered waveguide (42) is less than or equal to 350 nm, and the width of the first waveguide (41) is less than 5 μm; the tip width of the second tapered waveguide (52) is less than or equal to 2 μm; and the width of the second waveguide (51) is less than or equal to 10 μm. The lengths of the first tapered waveguide (42) and the second tapered waveguide (52) are equal and the lengths are greater than or equal to 80 μm, or the absolute value of the difference between the lengths of the first tapered waveguide (42) and the second tapered waveguide (52) is less than or equal to 50 μm; An etching stop layer made of a low refractive index medium is provided between the first waveguide core layer (4) and the second waveguide core layer (5); the refractive index of the etching stop layer is lower than that of thin film lithium niobate; The upper surface of the second waveguide core layer (5) is covered with a cladding made of a low refractive index dielectric material; the upper surface of the first waveguide (41) is covered with a cladding made of a low refractive index dielectric material.

2. The end-face coupler based on optical fiber and lithium niobate waveguide coupling according to claim 1, characterized in that, The second waveguide core layer (5) is a ridge waveguide structure, and its etching depth is greater than or equal to 70% of the thickness of the second waveguide (51) layer.

3. The end-face coupler based on optical fiber and lithium niobate waveguide coupling according to claim 1 or 2, characterized in that, The side of the first waveguide (41) away from the first tapered waveguide (42) is covered with a low refractive index dielectric layer.

4. A method for fabricating an end-face coupler based on the coupling of optical fiber and lithium niobate waveguide as described in any one of claims 1 to 3, characterized in that, Includes the following steps: S1: The first waveguide (41) and the first tapered waveguide (42) are fabricated on a thin-film lithium niobate wafer using photolithography and lithium niobate etching technology. S2: In the sample prepared in step S1, a low refractive index dielectric material is prepared as an etching stop layer using a deposition process. S3: The second waveguide core layer (5) is prepared by spin-coating a high refractive index polymer or depositing a high refractive index medium material or a medium material with periodic distribution of high and low refractive indices on the sample prepared in step S2. S4: The second waveguide core layer (5) is etched on the sample prepared in step S3 using photolithography and etching processes to prepare the second waveguide (51) and the second tapered waveguide (52). S5: The sample prepared in step S4 is used to prepare a low refractive index dielectric material by deposition or a polymer material by spin coating as the cladding of the second waveguide core layer. S6: After cleaving or slicing the sample prepared in step S5, polish it to complete the preparation of the end face coupler.

5. The method for fabricating an end-face coupler based on optical fiber and lithium niobate waveguide coupling according to claim 4, characterized in that, In step S2, the low refractive index medium includes silicon oxide, silicon oxynitride, and polymers; When using silicon oxide and silicon oxynitride as low refractive index media to prepare the etching stop layer, it is prepared by PECVD or ICP-CVD deposition process. When a polymer is used as a low-refractive-index medium to prepare an etching stop layer, it is prepared by spin coating or spray coating process and then subjected to thermal curing or UV curing.

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