Thin film lithium niobate waveguide polarizer

Abstract

The invention discloses a thin film lithium niobate waveguide polarizer. The overall structure adopts the two mixed bent waveguides, insertion loss caused by mode field mismatch at the joint of the straight waveguide and the bent waveguide is reduced, high-order mode excitation is reduced, the positions of the input and output waveguides are staggered through the two mixed bent waveguides which are centrosymmetric and have the same bending angle, the polarization extinction ratio of the chip is increased, and the polarization effect of the chip is improved. Waveguide modes except the quasi TE00 mode are filtered out, and polarization with the high polarization extinction ratio is achieved.

Description

Technical Field

[0001] The invention relates to the technical field of waveguide polarizers, and in particular to a thin-film lithium niobate waveguide polarizer. Background Art

[0002] Thin-film lithium niobate has the advantages of strong electro-optic coefficient, wide transparent window and strong light confinement, and is an excellent photonic integration platform. Photonic integration platforms based on thin-film lithium niobate have realized many high-performance photonic devices, including modulators, nonlinear optical devices, etc. However, these photonic devices are sensitive to polarization due to their asymmetric ridge waveguide shape and birefringence caused by the anisotropy of lithium niobate. Therefore, polarization management devices such as polarization rotators, polarization beam splitters and polarizers based on thin-film lithium niobate photonic integration platforms are very necessary. The polarizer is the simplest device to control polarization.

[0003] At present, the polarizers of the photonic integration platform based on thin-film lithium niobate are mainly mode leakage type polarizers. However, the mode leakage type polarizer ignores the leakage light of the straight waveguide passing through the flat layer and being received by the output end, which leads to the limitation of the extinction ratio, greatly limiting the application and promotion of the mode leakage type polarizer. Summary of the invention

[0004] The present invention provides a thin-film lithium niobate waveguide polarizer, which solves the technical problem in the prior art that the mode leakage type polarizer ignores the leakage light of the straight waveguide being received by the output end through the flat layer, thereby resulting in a limited extinction ratio, and achieves the technical effect of high polarization extinction ratio polarization.

[0005] The invention provides a thin-film lithium niobate waveguide polarizer, comprising: a first optical fiber-lithium niobate waveguide mode converter, a first polarization straight waveguide, a second polarization straight waveguide, a polarization bent waveguide and a second optical fiber-lithium niobate waveguide mode converter; the first end of the first optical fiber-lithium niobate waveguide mode converter is used to be connected to an external light source, the second end of the first optical fiber-lithium niobate waveguide mode converter is connected to the first end of the first polarization straight waveguide; the second end of the first polarization straight waveguide is connected to the first end of the polarization bent waveguide; the second end of the polarization bent waveguide is connected to the first end of the second polarization straight waveguide, the second end of the second polarization straight waveguide is connected to the first end of the second optical fiber-lithium niobate waveguide mode converter; the second end of the second optical fiber-lithium niobate waveguide mode converter is used to be connected to an external detector; the polarization bent waveguide is composed of two sections of curved waveguides that are centrally symmetrical and have the same bending angle, and is used to connect two sections of the first polarization straight waveguide and the second polarization straight waveguide that are parallel to each other and offset in lateral position.

[0006] Specifically, the first polarization straightening waveguide, the second polarization straightening waveguide, and the polarization curved waveguide are all composed of shallowly etched and / or narrow width lithium niobate waveguides, and the single-mode single-polarization operation of the lithium niobate waveguide is achieved by utilizing the mode leakage design of the waveguide to form a waveguide polarizer with a high polarization extinction ratio; the width of the first polarization straightening waveguide, the second polarization straightening waveguide, and the polarization curved waveguide is <2.5μm, the etching depth is <0.4μm, and the operating wavelength range is 1210-1410nm.

[0007] Specifically, the first polarization straight waveguide, the second polarization straight waveguide, and the polarization bent waveguide are respectively an air layer, a low refractive index cover layer, a lithium niobate ridge waveguide, a SiO2 substrate layer, and a Si substrate layer from top to bottom. The ridge waveguide is formed by etching away part of the lithium niobate layer outside the waveguide; the refractive index of the low refractive index cover layer is 1-1.7.

[0008] Specifically, the deflecting curved waveguide is a hybrid curved waveguide, which is composed of a linear variable curvature curved waveguide and a constant curvature curved waveguide.

[0009] Specifically, the width of the polarizing curved waveguide is less than 2.5 μm, the angle of the linear variable curvature curved waveguide accounts for less than 95% of the total bending angle of the polarizing curved waveguide, and the effective radius of the polarizing curved waveguide is greater than 80 μm.

[0010] Specifically, the first optical fiber-lithium niobate waveguide mode converter and the second optical fiber-lithium niobate waveguide mode converter both adopt an inverted taper structure to achieve mode field matching between the lithium niobate waveguide at the end face and the tapered optical fiber.

[0011] One or more technical solutions provided in the present invention have at least the following technical effects or advantages:

[0012] 1. The overall structure adopts two hybrid curved waveguides, which reduces the insertion loss caused by the mode field mismatch between the straight waveguide and the curved waveguide at the junction, and reduces the excitation of high-order modes. In addition, the positions of the input and output waveguides are staggered by two centrally symmetrical hybrid curved waveguides with the same bending angle, which reduces the leakage light received by the output end through the flat layer, increases the polarization extinction ratio of the chip, and further filters out the quasi-TE 00 mode, thereby achieving polarization with a high polarization extinction ratio.

[0013] 2. The present invention adopts a lithium niobate waveguide with shallow etching and / or narrow width, and utilizes the mode leakage design of the waveguide to achieve single-mode single-polarization operation of the lithium niobate waveguide, forming a waveguide polarizer with a high polarization extinction ratio. A suitable waveguide structure will enable the waveguide polarizer to have a large width and thickness tolerance and broadband characteristics.

[0014] In summary, the present invention effectively improves the stability, reliability and integration while ensuring the polarization degree of the waveguide polarizer, and also reduces the cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 is a schematic structural diagram of a thin-film lithium niobate waveguide polarizer provided by an embodiment of the present invention;

[0016] Figure 2 The polarizing straight waveguide in the thin-film lithium niobate waveguide polarizer provided by the embodiment of the present invention is a quasi-TM based on the x-cut thin-film lithium niobate wafer xz plane straight waveguide at a wavelength of 1310nm 00 Leakage Mode Loss (dB / cm) and Quasi-TE 10 How the presence of modes varies with waveguide width and etch depth;

[0017] Figure 3 The polarizing straight waveguide in the thin-film lithium niobate waveguide polarizer provided by the embodiment of the present invention is based on the quasi-TM of the x-cut thin-film lithium niobate wafer xz plane waveguide with a waveguide width of 1 μm and an etching depth of 0.06 μm. 00 Variation of leakage mode loss (dB / cm) with wavelength;

[0018] Figure 4 is a schematic cross-sectional view of a polarizing waveguide in a thin-film lithium niobate waveguide polarizer provided by an embodiment of the present invention;

[0019] Figure 5 53.13° hybrid curved waveguide used in an embodiment of the present invention; (a) is a schematic diagram of the top view of the hybrid curved waveguide; (b) is a diagram showing the variation of the curvature of the hybrid curved waveguide with the length of the waveguide path;

[0020] Among them, 1 is a first optical fiber-lithium niobate waveguide mode converter, 2 is a first polarized straight waveguide, 3 is a polarized bent waveguide, 4 is a second polarized straight waveguide, and 5 is a second optical fiber-lithium niobate waveguide mode converter. DETAILED DESCRIPTION

[0021] The embodiment of the present invention provides a thin-film lithium niobate waveguide polarizer, which solves the technical problem in the prior art that the mode leakage type polarizer ignores the leakage light of the straight waveguide being received by the output end through the flat layer, thereby resulting in a limited extinction ratio, and achieves the technical effect of high polarization extinction ratio polarization.

[0022] In order to better understand the above technical solution, the above technical solution will be described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0023] like Figure 1As shown, the thin-film lithium niobate waveguide polarizer provided by the embodiment of the present invention comprises: a first optical fiber-lithium niobate waveguide mode converter 1, a first polarization straightening waveguide 2, a second polarization straightening waveguide 4, a polarization bending waveguide 3 and a second optical fiber-lithium niobate waveguide mode converter 5; the first end of the first optical fiber-lithium niobate waveguide mode converter 1 is used to be connected to an external light source, and the second end of the first optical fiber-lithium niobate waveguide mode converter 1 is connected to the first end of the first polarization straightening waveguide 2; the second end of the first polarization straightening waveguide 2 is connected to the first end of the polarization bending waveguide 3; the second end of the polarization bending waveguide 3 is connected to the first end of the second polarization straightening waveguide 4, and the second end of the second polarization straightening waveguide 4 is connected to the first end of the second optical fiber-lithium niobate waveguide mode converter 5; the second end of the second optical fiber-lithium niobate waveguide mode converter 5 is used to be connected to an external detector; the polarization bending waveguide 3 is composed of two sections of curved waveguides that are centrally symmetrical and have the same bending angle, and is used to connect two sections of the first polarization straightening waveguide 2 and the second polarization straightening waveguide 4 that are parallel to each other and offset in lateral position.

[0024] The lithium niobate waveguide in the embodiment of the present invention is a single-mode single-polarization waveguide. After the incident light enters the waveguide through the first optical fiber-lithium niobate waveguide mode converter 1, it enters the first polarization straightening waveguide 2. 00 Other modes leak out and then enter the two centrally symmetrical polarizing waveguides 3 with the same bending angle for misalignment. 00 The other modes leak to both sides again, and the light leaked in the polarization straightening waveguide enters the output waveguide through the substrate due to less misalignment of the second polarization straightening waveguide 4, further increasing the polarization extinction ratio. Finally, only the quasi-TE 00 The light passes through the second optical fiber-lithium niobate waveguide mode converter 5 and enters the external detector.

[0025] The structures of the first polarization straightening waveguide 2, the second polarization straightening waveguide 4, and the polarization curved waveguide 3 are described in detail. The first polarization straightening waveguide 2, the second polarization straightening waveguide 4, and the polarization curved waveguide 3 are all composed of shallowly etched and / or narrow-width lithium niobate waveguides, and the single-mode single-polarization operation of the lithium niobate waveguide is achieved by utilizing the mode leakage design of the waveguide to form a waveguide polarizer with a high polarization extinction ratio; the width of the first polarization straightening waveguide 2, the second polarization straightening waveguide 4, and the polarization curved waveguide 3 is less than 2.5μm, the etching depth is less than 0.4μm, and the operating wavelength range is 1210-1410nm.

[0026] The design goal of the embodiment of the present invention is to optimize the waveguide structure so that it has as large a quasi-TM 00 Mode loss and TE polarization only has quasi-TE 00 The quasi-TM mode based on the x-cut thin-film lithium niobate wafer xz plane straight waveguide at a wavelength of 1310nm 00Leakage Mode Loss (dB / cm) and Quasi-TE 10 The existence of the mode varies with the waveguide width and etching depth as shown in Figure 2 As shown. Figure 2 As shown in the results, the waveguide size in the embodiment of the present invention can be selected as a waveguide width of 1 μm and an etching depth of 0.06 μm. At this time, the quasi-TM 00 The transmission loss of the mode due to lateral leakage is about 332dB / cm, TE 10 The mode does not exist because it is in the cutoff region, so at this time there is only TE in the waveguide 00 It can transmit stably, and the calculated transmission loss is 2.6×10 - 10 dB / cm, the waveguide is a single-mode single-polarization straight waveguide.

[0027] The design target of the embodiment of the present invention is a broadband TE polarizer. Figure 3 The polarized straight waveguide in the waveguide polarizer of the embodiment of the present invention is a quasi-TM based on the x-cut thin film lithium niobate wafer xz plane waveguide with a waveguide width of 1 μm and an etching depth of 0.06 μm. 00 Leakage mode loss (dB / cm) versus wavelength. Figure 3 The results are shown in FIG. 1 . In the embodiment of the present invention, the waveguide size is selected as a waveguide width of 1 μm and an etching depth of 0.06 μm. At this time, the quasi-TM 00 The transmission loss of the mode due to lateral leakage is always greater than 100dB / cm in the wavelength range of 1.21μm to 1.36μm, and the polarizer is a broadband polarizer.

[0028] like Figure 4 As shown, the first polarization straight waveguide 2, the second polarization straight waveguide 4, and the polarization bending waveguide 3 are respectively an air layer, a low refractive index cap layer, a lithium niobate ridge waveguide, a SiO2 substrate layer, and a Si substrate layer from top to bottom. The ridge waveguide is formed by etching away part of the lithium niobate layer outside the waveguide; the total thickness of the lithium niobate is 300nm. The refractive index of the low refractive index cap layer is 1-1.7.

[0029] In order to reduce the insertion loss caused by the mode field mismatch between the straight waveguide and the curved waveguide at the junction, and reduce the excitation of high-order modes, the polarizing curved waveguide 3 is a hybrid curved waveguide composed of a linear variable curvature curved waveguide and a constant curvature curved waveguide.

[0030] Specifically, the width of the polarizing curved waveguide 3 is less than 2.5 μm, the angle of the linear variable curvature curved waveguide accounts for less than 95% of the total bending angle of the polarizing curved waveguide 3 , and the effective radius of the polarizing curved waveguide 3 is greater than 80 μm.

[0031] In this embodiment, in order to stagger the input and output waveguides by 400 μm in the vertical waveguide direction, two hybrid curved waveguides with a partially continuously variable curvature of 53.13° are used. Figure 5 (a) is a schematic diagram of the top view of the structure of a 53.13° partially continuously variable curvature hybrid bending waveguide, in which the linear variable curvature angle of the bending waveguide accounts for 30% of the total bending angle. The starting point and end point of the bending waveguide are consistent with those of a fixed curvature bending waveguide with a radius of 500 μm at the same angle. The effective radius is 500 μm, and the minimum radius Rmin is 390.25 μm. Figure 5 (b) is the variation of the curvature of the curved waveguide with the length of the waveguide path. When the curved waveguide is in the angle range of [0°, 7.97°], the curvature changes linearly in the range of [0, 1 / Rmin]. When it is in the angle range of [7.97°, 45.16°], the curvature is 1 / Rmin. When it is in the angle range of [45.16°, 53.13°], the curvature changes linearly in the range of [1 / Rmin, 0]. The maximum curvature 1 / Rmin is 2562.46 m. -1 The linearly changing curvature can reduce the mode field mismatch at the junction of the straight waveguide and the curved waveguide.

[0032] The structures of the first optical fiber-lithium niobate waveguide mode converter 1 and the second optical fiber-lithium niobate waveguide mode converter 5 are specifically described. The first optical fiber-lithium niobate waveguide mode converter 1 and the second optical fiber-lithium niobate waveguide mode converter 5 both adopt an inverted taper structure to achieve mode field matching between the lithium niobate waveguide at the end face and the tapered optical fiber.

[0033] In summary, the embodiment of the present invention utilizes the anisotropy of thin-film lithium niobate and adjusts the structural dimensions of the ridge waveguide to make the ridge waveguide only have quasi-TE 00 The mode can be stably transmitted, and the misalignment of the input and output waveguides is achieved through two mixed linear arcs of variable curvature, so that low-loss TE polarization is achieved on the chip. The embodiment of the present invention ensures the polarization extinction ratio of the polarizer while only using the ridge waveguide, which has a simple structure, convenient design, and is conducive to large-scale production.

[0034] The parts not described in detail in the embodiments of the present invention are all known technologies to those skilled in the art. Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not limiting. Although the present invention is described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solutions of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. A thin film lithium niobate waveguide polarizer, characterized in that: include: A first optical fiber-lithium niobate waveguide mode converter, a first polarization straightening waveguide, a second polarization straightening waveguide, a polarization bending waveguide, and a second optical fiber-lithium niobate waveguide mode converter; The first end of the first optical fiber-lithium niobate waveguide mode converter is used to be connected to an external light source, and the second end of the first optical fiber-lithium niobate waveguide mode converter is connected to the first end of the first polarization straightening waveguide; the second end of the first polarization straightening waveguide is connected to the first end of the polarization bending waveguide; the second end of the polarization bending waveguide is connected to the first end of the second polarization straightening waveguide, and the second end of the second polarization straightening waveguide is connected to the first end of the second optical fiber-lithium niobate waveguide mode converter; the second end of the second optical fiber-lithium niobate waveguide mode converter is used to be connected to an external detector; the polarization bending waveguide is composed of two sections of curved waveguides that are centrally symmetrical and have the same bending angle, and is used to connect two sections of the first polarization straightening waveguide and the second polarization straightening waveguide that are parallel to each other and laterally offset.

2. The thin film lithium niobate waveguide polarizer according to claim 1, characterized in that: The first polarization straightening waveguide, the second polarization straightening waveguide, and the polarization curved waveguide are all composed of shallowly etched and / or narrow-width lithium niobate waveguides, and the single-mode single-polarization operation of the lithium niobate waveguide is achieved by utilizing the mode leakage design of the waveguide to form a waveguide polarizer with a high polarization extinction ratio; the width of the first polarization straightening waveguide, the second polarization straightening waveguide, and the polarization curved waveguide is <2.5μm, the etching depth is <0.4μm, and the operating wavelength range is 1210-1410nm.

3. The thin film lithium niobate waveguide polarizer according to claim 1, characterized in that: The first polarized straight waveguide, the second polarized straight waveguide and the polarized bent waveguide are respectively an air layer, a low refractive index cover layer, a lithium niobate ridge waveguide, a SiO2 substrate layer and a Si substrate layer from top to bottom. The ridge waveguide is formed by etching away part of the lithium niobate layer outside the waveguide; the refractive index of the low refractive index cover layer is 1-1.

7.

4. The thin film lithium niobate waveguide polarizer according to claim 1, characterized in that: The deflecting curved waveguide is a hybrid curved waveguide, which is composed of a linear variable curvature curved waveguide and a constant curvature curved waveguide.

5. The thin film lithium niobate waveguide polarizer according to claim 4, characterized in that: The width of the polarizing curved waveguide is less than 2.5 μm, the angle of the linear variable curvature curved waveguide accounts for less than 95% of the total bending angle of the polarizing curved waveguide, and the effective radius of the polarizing curved waveguide is greater than 80 μm.

6. The thin-film lithium niobate waveguide polarizer according to any one of claims 1 to 5, characterized in that: The first optical fiber-lithium niobate waveguide mode converter and the second optical fiber-lithium niobate waveguide mode converter both adopt an inverted taper structure to achieve mode field matching between the lithium niobate waveguide at the end face and the tapered optical fiber.

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