LNOI edge coupler compatible with SMF28 optical fiber and preparation method thereof

Abstract

The present invention relates to an LNOI edge coupler compatible with SMF28 optical fiber, a preparation method and an application thereof, belonging to the technical field of silicon photonic integrated circuits, and solving the problem in the prior art that the LNOI edge coupler cannot be directly coupled with the SMF28 single-mode optical fiber. The LNOI edge coupler comprises a silicon substrate, an oxide-buried layer, an LNOI layer and a SiO2 cladding arranged in sequence from bottom to top, and a cavity connected to the outside air is arranged inside the silicon substrate, and the cavity extends downward from the upper surface of the silicon substrate. The LNOI edge coupler can match the mode field diameter in SMF28, can achieve efficient coupling with the SMF‑28 single-mode optical fiber, and greatly increase the coupling tolerance.

Description

Technical Field

[0001] The present invention relates to the technical field of silicon photonic integrated circuits, and in particular to a LNOI edge coupler compatible with SMF28 optical fiber, a preparation method and an application thereof. Background Art

[0002] Silicon photonics has attracted increasing attention due to its advantages such as ultra-low cost, high refractive index, high-density integration, and compatibility with existing semiconductor manufacturing processes. It is a promising key technology for future daily applications. Optical interconnection is an important issue in silicon photonic integrated circuits for optical transmission, and fiber-to-chip optical interconnection is crucial in application scenarios such as data centers and optical transmission systems.

[0003] The currently published results on LNOI edge couplers are all based on lens fibers or small-core fibers, and there is a lack of edge couplers that can directly couple with SMF28 single-mode fibers, which is precisely the most commonly used fiber in optical links. Summary of the invention

[0004] In view of the above analysis, the embodiments of the present invention aim to provide a LNOI edge coupler compatible with SMF28 optical fiber, and a preparation method and application thereof, so as to solve the problem that the existing LNOI edge coupler cannot be directly coupled with the SMF28 single-mode optical fiber.

[0005] On the one hand, the present invention provides an LNOI edge coupler compatible with SMF28 optical fiber, the LNOI edge coupler comprising a silicon substrate, a buried oxide layer, an LNOI layer and a SiO2 cladding arranged in sequence from bottom to top, a cavity connected to the outside air is arranged inside the silicon substrate, and the cavity extends downward from the upper surface of the silicon substrate.

[0006] Preferably, the thickness h1 of the SiO2 cladding layer is 6-7 μm.

[0007] Preferably, the thickness h1 of the SiO2 cladding layer is 6.5 μm.

[0008] Preferably, the width w1 of the SiO2 cladding layer is 13-15 μm.

[0009] Preferably, the width w1 of the SiO2 cladding layer is 14 μm.

[0010] Preferably, the depth h2 of the cavity is 20-30 μm.

[0011] Preferably, the buried oxide layer is provided with a plurality of through holes located on both sides of the LNOI layer and symmetrically arranged with respect to the LNOI layer (3), and the cavity is connected with the outside air through the through holes.

[0012] Preferably, the distance w2 between each pair of through holes symmetrically arranged on both sides of the LNOI layer is 16-20 μm.

[0013] In a second aspect, the present invention provides a method for preparing a LNOI edge coupler compatible with SMF28 optical fiber, which is used to prepare the above-mentioned LNOI edge coupler, and the preparation method comprises:

[0014] (a) preparing an LNOI wafer, wherein the LNOI wafer comprises a silicon substrate, a buried oxide layer, and an LNOI layer stacked in sequence from bottom to top;

[0015] (b) removing part of the LNOI layer in the LNOI wafer;

[0016] (c) forming a layer of SiO2 on the LNOI layer;

[0017] (d) etching the SiO2 layer to form a SiO2 cladding layer;

[0018] (e) forming a through hole in the buried oxide layer;

[0019] (f) The silicon substrate is hollowed out to form a cavity connected to the outside air.

[0020] In a third aspect, the present invention further provides the use of the above-mentioned LNOI edge coupler or the LNOI edge coupler prepared by the above-mentioned preparation method in a lithium niobate modulator.

[0021] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0022] 1. The present invention sets a cavity connected to the outside air in the silicon substrate, and directly sets a SiO2 cladding on the lithium niobate waveguide. On the one hand, the refractive index of the SiO2 cladding is close to the refractive index of the core and cladding in SMF28. On the other hand, the SiO2 cladding can increase the mode field area of ​​the optical fiber coupling section to match the mode field diameter in SMF28, thereby achieving efficient coupling with the SMF-28 single-mode optical fiber and greatly increasing the coupling tolerance.

[0023] 2. The SiO2 cladding layer is directly disposed on the lithium niobate waveguide of the LNOI edge coupler of the present invention without covering with SiON or polymer materials, thereby avoiding the introduction of other materials during the manufacturing process and simplifying the manufacturing process of the coupler.

[0024] In the present invention, the above-mentioned technical solutions can also be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages can become obvious from the description, or can be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

[0026] Figure 1 A three-dimensional structural diagram of the LNOI edge coupler of the present invention;

[0027] Figure 2 for Figure 1 The front view of the LNOI edge coupler cut along A-A'.

[0028] Figure 3 for Figure 1 Side view of the LNOI edge coupler.

[0029] Figure 4 for Figure 1 Top view of the LNOI edge coupler.

[0030] Figure 5 The preparation process of the LNOI edge coupler of the present invention;

[0031] Figure 6 A coupling tolerance test diagram of the LNOI edge coupler of the present invention and the LNOI edge coupler of the prior art;

[0032] Figure 7 The effect of the thickness and width of the SiO2 cladding of the LNOI edge coupler of the present invention on the coupling efficiency;

[0033] Figure 8 FIG. 4 is a structural diagram of a LNOI edge coupler in the prior art.

[0034] Reference numerals:

[0035] 1-silicon substrate; 101-cavity; 2-buried oxide layer; 201-through hole; 3-LNOI layer; 4-SiO2 cladding; 5-SiNO cladding; h1-thickness of SiO2 cladding; w1-width of SiO2 cladding; h2-depth of cavity; w2-distance between each pair of through holes symmetrically arranged on both sides of the LNOI layer; h3-thickness of buried oxide layer. DETAILED DESCRIPTION

[0036] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not used to limit the scope of the present invention.

[0037] A large number of literatures on LNOI edge couplers have been reported. Similar to SOI edge couplers, researchers have also thought of using a double-layer inverted cone structure to adiabatically convert light in the LNOI ridge waveguide and edge-couple it with the optical fiber. In 2019, Lingyan He et al. from Harvard University first realized a double-layer inverted cone structure on the LNOI platform, and achieved a coupling loss of less than 1.7dB / facet by coupling with a lens fiber (MFD~2μm); in 2020, ChangRan Hu et al. from Huazhong University of Science and Technology covered a layer of SiON on the LNOI double-layer taper to increase the mode field area of ​​the fiber coupling end face and overcome the influence of the LNOI taper tip width on the coupling efficiency. By coupling with UHNA7 optical fiber (MFD~3.5μm), a coupling loss of 0.5dB / facet was achieved. In the same year, Pan Ying et al. from Sun Yat-sen University first combined the LNOI modulator with the LNOI edge coupler and achieved a coupling loss of 0.5dB / facet. However, the above-mentioned LNOI edge couplers disclosed all have the following problems: they cannot be coupled with single-mode optical fibers; the mode field diameter (MFD) of the coupler is small, and the coupling tolerance is poor when coupled with SMF28 single-mode optical fibers (MFD is 9.92 μm); the polymer and SiON covering the end of the coupler increase the process complexity and device manufacturing cost.

[0038] Based on this, the present invention provides a LNOI edge coupler that is compatible with SMF28 optical fiber, such as Figure 1-Figure 4 As shown, the LNOI edge coupler includes a silicon substrate 1, a buried oxide layer 2, an LNOI layer 3 and a SiO2 cladding layer 4 arranged in sequence from bottom to top, and a cavity 101 connected to the outside air is arranged inside the silicon substrate 1, and the cavity 101 extends downward from the upper surface of the silicon substrate 1.

[0039] Compared with the prior art, the LNOI edge coupler provided by the present invention sets a cavity connected to the outside air in the silicon substrate, and directly sets a SiO2 cladding on the lithium niobate waveguide. On the one hand, the refractive index of the SiO2 cladding is similar to the refractive index of the core and cladding in SMF28. On the other hand, the SiO2 cladding can increase the mode field area of ​​the optical fiber coupling section to match the mode field diameter in SMF28, and can achieve efficient coupling with the SMF-28 single-mode optical fiber, greatly increasing the coupling tolerance. In addition, the SiO2 cladding is directly set on the lithium niobate waveguide of the LNOI edge coupler of the present invention, without covering SiON or polymer materials, avoiding the introduction of other materials in the manufacturing process, and simplifying the manufacturing process of the coupler.

[0040] Specifically, Figure 8In the prior art LNOI edge coupler shown, a SiNO cladding layer 5 and a SiO2 cladding layer 4 are sequentially arranged on the LNOI layer 3, and the silicon substrate 1 is solid, that is, the LNOI layer 3 in the prior art LNOI edge coupler is surrounded by SiNO, SiO2 and silicon materials; while in the LNOI edge coupler of the present invention, only a SiO2 cladding layer 4 is arranged on the LNOI layer 3, and the silicon substrate 1 is provided with a cavity 101 connected to the outside air, that is, the LNOI layer 3 of the present invention is surrounded by SiO2, air and silicon materials. The refractive index of light in lithium niobate is 2.3, the refractive index of light in SiNO material is 1.56, the refractive index of light in SiO2 material is 1.44, the refractive index of light in silicon is 3.42, and the refractive index of light in air is 1; obviously, compared with the prior art, the refractive index of the surrounding materials of the LNOI layer 3 is smaller than the refractive index of the LNOI layer 3, and in the LNOI edge coupler of the present invention, the difference between the refractive index of light in the LNOI layer 3 and the refractive index of light in its surrounding materials is large, and the larger the refractive index difference, the more conducive it is to confine the light to the LNOI material, thereby increasing the mode field diameter to match the mode field diameter in SMF28, achieving efficient coupling with the SMF-28 single-mode optical fiber, and greatly increasing the coupling tolerance.

[0041] It should be noted that if Figure 1 As shown, the LNOI layer 3 of the present invention is a double-layer inverted tapered waveguide, including a first inverted tapered waveguide and a second inverted tapered waveguide arranged on the first inverted tapered waveguide. Along the direction of light propagation, the first inverted tapered waveguide includes a flat plate portion and an inverted tapered portion. The second inverted tapered waveguide extends from the side of the flat plate portion away from the inverted tapered portion to the inverted tapered portion. The length of the second inverted tapered waveguide is greater than the extension width of the flat plate portion along the direction of light propagation, and is less than the length of the first inverted tapered waveguide. The cavity 101 is arranged below the inverted tapered portion of the first inverted tapered waveguide, and the SiO2 cladding 4 wraps the first inverted tapered waveguide and the second inverted tapered waveguide.

[0042] In the present invention, the thickness and width of the SiO2 cladding layer 4 will directly affect the coupling efficiency of the coupler. Too large or too small thickness and width will cause the coupling efficiency to decrease.

[0043] Exemplarily, the thickness h1 of the SiO2 cladding layer 4 is 6-7 μm, for example, 6.1 μm, 6.2 μm, 6.3 μm, 6.4 μm, 6.5 μm, 6.6 μm, 6.7 μm, 6.8 μm, 6.9 μm, etc.

[0044] Further preferably, the thickness h1 of the SiO2 cladding layer 4 is 6.5 μm.

[0045] Exemplarily, the width w1 of the SiO2 cladding layer 4 is 13-15 μm, for example, 13.2 μm, 13.4 μm, 13.6 μm, 13.8 μm, 14 μm, 14.2 μm, 14.4 μm, 14.6 μm, 14.8 μm, etc.

[0046] Further preferably, the width w1 of the SiO2 cladding layer 4 is 14 μm.

[0047] In the present invention, the cavity 101 is communicated with the outside, so the cavity 101 is filled with air.

[0048] In order to prevent the pattern spot from leaking onto the silicon substrate 1, preferably, the depth h2 of the cavity 101 is 20-30 μm. When the depth of the cavity 101 is less than 20 μm, the light coupled to the lithium niobate inverted cone waveguide is easily leaked onto the underlying silicon substrate 1; the depth of the cavity 101 is also limited by the release process of the silicon substrate 1, and a cavity depth greater than 30 μm will reduce the stability of the structure.

[0049] It should be noted that the depth h2 refers to the maximum distance extending vertically downward from the lower surface of the buried oxide layer 2 to the silicon substrate 1 .

[0050] There is no special requirement for the lateral width of the cavity 101 and the shape of the cavity 101 , which can be determined by the preparation process of the cavity 101 .

[0051] In one embodiment, the buried oxide layer 2 is provided with a plurality of through holes 201 located on both sides of the LNOI layer 3 and symmetrically arranged about the LNOI layer 3, and the cavity 101 is connected to the outside air through the through holes 201. In addition, the function of the through holes 201 also includes: during the preparation process, gas is passed to the silicon substrate 1 through the through holes 201 to form the cavity 101.

[0052] For example, the distance w2 between each pair of through holes 201 symmetrically arranged on both sides of the LNOI layer 3 is 16-20 μm. This is because when hollowing out the silicon substrate 1, if the distance between the through holes 201 on both sides is too large, it will be subject to process limitations, while if the distance between the through holes 201 on both sides is too small, it will affect the efficiency of coupling with the optical fiber. The distance w2 refers to the width of the buried oxide layer 2 between the two symmetrical through holes 201.

[0053] In the present invention, the buried oxide layer 2 may be made of SiO2 material.

[0054] Exemplarily, the thickness h3 of the buried oxide layer 2 is 4-5 μm, such as 4.1 μm, 4.2 μm, 4.3 μm, 4.4 μm, 4.5 μm, 4.6 μm, 4.7 μm, 4.8 μm, 4.9 μm, etc.

[0055] More preferably, the thickness h3 of the buried oxide layer 2 is 4.7 μm.

[0056] In a second aspect, the present invention also provides a method for preparing a LNOI edge coupler compatible with SMF28 optical fiber, which is used to prepare the above-mentioned LNOI edge coupler, such as Figure 5 As shown, the preparation method comprises:

[0057] (a) preparing an LNOI wafer, wherein the LNOI wafer comprises a silicon substrate, a buried oxide layer, and an LNOI layer stacked in sequence from bottom to top;

[0058] (b) removing part of the LNOI layer in the LNOI wafer;

[0059] (c) forming a layer of SiO2 on the LNOI layer;

[0060] (d) etching the SiO2 layer to form a SiO2 cladding layer;

[0061] (e) forming a through hole in the buried oxide layer;

[0062] (f) The silicon substrate is hollowed out to form a cavity connected to the outside air.

[0063] Specifically, in step (b), a portion of the LNOI layer with a thickness of 200 nm is removed by using an inductively coupled plasma reactive ion etching (ICP-RIE) technique to form a double-layer inverted tapered waveguide.

[0064] In step (c), silicon oxide is grown by PECVD.

[0065] In step (d), the SiO2 cladding layer is etched out using dry etching technology.

[0066] In step (e), the buried oxide layer is etched by HF dry etching to form a through hole.

[0067] In step (f), XeF2 gas is passed through the through hole to the silicon substrate to hollow out the silicon substrate to form a cavity.

[0068] In a third aspect, the present invention also provides the use of the above LNOI edge coupler or the LNOI edge coupler prepared by the above preparation method in a lithium niobate modulator. The LNOI edge coupler can match the mode field diameter in SMF28, achieve efficient coupling with SMF-28 single-mode optical fiber, and greatly increase the coupling tolerance.

[0069] The LNOI edge coupler compatible with SMF28 optical fiber and the preparation method of the present invention are further described below through specific embodiments.

[0070] Example 1

[0071] This embodiment provides a LNOI edge coupler that is compatible with SMF28 optical fiber.

[0072] like Figure 1-Figure 4 As shown, the LNOI edge coupler includes a silicon substrate 1, a buried oxide layer 2, an LNOI layer 3 and a SiO2 cladding 4 arranged in sequence from bottom to top. A cavity 101 connected to the outside air is arranged inside the silicon substrate 1, and the cavity 101 extends downward from the upper surface of the silicon substrate 1. The LNOI layer 3 includes a first inverted tapered waveguide and a second inverted tapered waveguide arranged on the first inverted tapered waveguide. Along the direction of light propagation, the first inverted tapered waveguide includes a flat plate portion and an inverted tapered portion. The second inverted tapered waveguide extends from the side of the flat plate portion away from the inverted tapered portion to the inverted tapered portion. The length of the second inverted tapered waveguide is greater than the extension width of the flat plate portion along the direction of light propagation, and is less than the length of the first inverted tapered waveguide. The cavity 101 is arranged below the inverted tapered portion of the first inverted tapered waveguide, and the SiO2 cladding 4 wraps the first inverted tapered waveguide and the second inverted tapered waveguide.

[0073] The thickness h1 of the SiO2 cladding 4 is 6.5 μm; the width w1 of the SiO2 cladding 4 is 14 μm; the depth h2 of the cavity 101 is 25 μm; the buried oxide layer 2 is provided with a plurality of through holes 201 located on both sides of the LNOI layer 3 and symmetrically arranged about the LNOI layer 3; the distance w2 between each pair of through holes 201 symmetrically arranged on both sides of the LNOI layer 3 is 18 μm; the buried oxide layer 2 is made of SiO2 material, and the thickness h3 of the buried oxide layer 2 is 4.7 μm.

[0074] The mode field diameter (MDF) of the LNOI edge coupler is 9.05 μm.

[0075] Example 2

[0076] This embodiment is used to illustrate the influence of the thickness and width of the SiO2 cladding layer 4 on the coupling efficiency.

[0077] This embodiment provides a LNOI edge coupler similar to that of Embodiment 1, except that the thickness h1 and width w1 of the SiO 2 cladding layer 4 are different values ​​as shown in Table 1.

[0078] h1 4.5μm 5μm 5.5μm 6μm 6.5μm 7μm - - - w1 8μm 10μm 12μm 13μm 14μm 15μm 16μm 18μm 20μm

[0079] The results of the influence of different thickness h1 and different width w1 of SiO2 cladding 4 on the coupling efficiency are shown in Figure 7 shown.

[0080] Example 3

[0081] This embodiment provides a method for preparing an LNOI edge coupler that is compatible with SMF28 optical fiber, the preparation method comprising:

[0082] (a) A LNOI wafer is prepared, wherein the LNOI wafer includes a silicon substrate, a buried oxide layer, and a LNOI layer stacked in sequence from bottom to top.

[0083] (b) The 200 nm thick LNOI layer is partially removed using inductively coupled plasma reactive ion etching (ICP-RIE) technology to form a double-layer inverted tapered waveguide.

[0084] (c) Silicon oxide is grown on the LNOI layer using PECVD.

[0085] (d) The SiO2 cladding is etched out using dry etching technology.

[0086] (e) The buried oxide layer is etched using HF dry etching to form a through hole.

[0087] (f) XeF2 gas is passed through the through hole to the silicon substrate 1 to hollow out the silicon substrate to form a cavity.

[0088] Comparative Example 1

[0089] This comparative example provides a conventional LNOI edge coupler, the structure of which is as follows: Figure 8 As shown, from bottom to top, it includes a silicon substrate 1, a buried oxide layer 2, a LNOI layer 3, a SiNO cladding layer 5 and a SiO2 cladding layer 4.

[0090] The mode field diameter (MDF) of the LNOI edge coupler is 3.5 μm.

[0091] According to the comparison of the mode field diameters of Example 1 and Comparative Example 1, it can be seen that the mode field diameter of the LNOI edge coupler of the present invention is significantly larger than the mode field diameter of Comparative Example 1, and the present invention increases the bun spot size. The mode field diameter (9.05 μm) of the LNOI edge coupler of the present invention can match the mode field diameter (9.92 μm) of the SMF28 single-mode optical fiber.

[0092] from Figure 7 It can be seen from the results that for transverse electromagnetic waves (TE mode), at 1550nm, when the thickness of the SiO2 cladding is 6-7um and the width is 13-15um, the coupling efficiency can reach 96%; when the thickness of the SiO2 cladding is 6.5um and the width is 14um, the coupling efficiency is the highest at 96.47%. Therefore, the present invention preferably has a SiO2 cladding thickness of 6-7um and a width of 13-15um; the most preferred SiO2 cladding thickness is 6.5um and a width of 14um.

[0093] The LNOI edge couplers of Example 1 and Comparative Example 1 were tested for coupling tolerance, and the results are as follows: Figure 6 As shown, from Figure 6It can be seen that when the coupling efficiency decreases by 1 dB, the offset of the coupling tolerance of Comparative Example 1 is 0.75 um, and the offset of the coupling tolerance of Example 1 is 2 um. It can be seen that the LNOI edge coupler of the present invention improves the coupling tolerance.

[0094] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A LNOI edge coupler compatible with SMF28 optical fiber, characterized in that: The LNOI edge coupler comprises a silicon substrate (1), a buried oxide layer (2), an LNOI layer (3) and a SiO2 cladding layer (4) arranged in sequence from bottom to top, wherein a cavity (101) communicating with the outside air is arranged inside the silicon substrate (1), and the cavity (101) extends downward from the upper surface of the silicon substrate (1); The LNOI layer (3) is a double-layer inverted cone waveguide; The SiO2 cladding layer (4) has a thickness h1 of 6-7 μm and a width w1 of 13-15 μm; The depth h2 of the cavity (101) is 20-30 μm; The buried oxide layer (2) is provided with a plurality of through holes (201) located on both sides of the LNOI layer (3) and symmetrically arranged with respect to the LNOI layer (3); the cavity (101) is connected to the outside air through the through holes (201); and a distance w2 between each pair of through holes (201) symmetrically arranged on both sides of the LNOI layer (3) is 16-20 μm.

2. The LNOI edge coupler according to claim 1, characterized in that The thickness h1 of the SiO2 cladding layer (4) is 6.5 μm.

3. The LNOI edge coupler according to claim 1, characterized in that The width w1 of the SiO2 cladding (4) is 13.2-15 μm.

4. The LNOI edge coupler according to claim 1, characterized in that The width w1 of the SiO2 cladding (4) is 14 μm.

5. The LNOI edge coupler according to claim 1, characterized in that The depth h2 of the cavity (101) is 25-30 μm.

6. The LNOI edge coupler according to claim 1, characterized in that The distance w2 between each pair of through holes (201) symmetrically arranged on both sides of the LNOI layer (3) is 18-20 μm.

7. A method for preparing a LNOI edge coupler compatible with SMF28 optical fiber, used for preparing the LNOI edge coupler according to any one of claims 1 to 6, characterized in that: The preparation method comprises: (a) preparing an LNOI wafer, wherein the LNOI wafer comprises a silicon substrate, a buried oxide layer and an LNOI layer stacked in sequence from bottom to top; (b) Using inductively coupled plasma reactive ion etching technology to remove part of the LNOI layer in the LNOI wafer; (c) A layer of SiO2 is grown on the LNOI layer using PECVD. (d) etching the SiO2 layer to form a SiO2 cladding layer; (e) forming a through hole in the buried oxide layer by HF dry etching; (f) XeF2 gas is passed through the through hole to the silicon substrate 1 to hollow out the silicon substrate, thereby forming a cavity connected to the outside air.

8. Use of the LNOI edge coupler according to any one of claims 1 to 6 or the LNOI edge coupler prepared by the preparation method according to claim 7 in a lithium niobate modulator.

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