Waveguide polarizer

Abstract

The utility model discloses a 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 utility model relates to the technical field of waveguide polarizers, and in particular to a waveguide polarizer. Background Art

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

[0003] Currently, the polarizer based on the photon integration platform of thin-film lithium niobate is mainly a mode leakage type polarizer. However, the mode leakage type polarizer ignores that the leaked light of the straight waveguide is received by the output end through the flat layer, resulting in a limited extinction ratio, which greatly limits the application and promotion of the mode leakage type polarizer. Summary of the Utility Model

[0004] By providing a waveguide polarizer, the utility model solves the technical problem in the prior art that the mode leakage type polarizer ignores that the leaked light of the straight waveguide is received by the output end through the flat layer, resulting in a limited extinction ratio, and realizes the technical effect of a polarizer with a high polarization extinction ratio.

[0005] The utility model provides a waveguide polarizer, including: a first fiber-waveguide mode converter, a first polarizing straight waveguide, a second polarizing straight waveguide, a polarizing bent waveguide, and a second fiber-waveguide mode converter; the first end of the first fiber-waveguide mode converter is connected to a light source through a first fiber, and the second end of the first fiber-waveguide mode converter is connected to the first end of the first polarizing straight waveguide; the second end of the first polarizing straight waveguide is connected to the first end of the polarizing bent waveguide; the second end of the polarizing bent waveguide is connected to the first end of the second polarizing straight waveguide, and the second end of the second polarizing straight waveguide is connected to the first end of the second fiber-waveguide mode converter; the second end of the second fiber-waveguide mode converter is connected to a detector through a second fiber; the polarizing bent waveguide is composed of two bent waveguides that are centrosymmetric and have the same bending angle, and is used to connect the two mutually parallel and laterally offset first polarizing straight waveguide and the second polarizing straight waveguide.

[0006] Specifically, the first polarization-maintaining straight waveguide, the second polarization-maintaining straight waveguide, and the polarization-maintaining bent waveguide are all composed of lithium niobate waveguides with shallow etching and / or narrow widths. By using the mode leakage design of the waveguide, single-mode and single-polarization operation of the lithium niobate waveguide is achieved, forming a waveguide polarizer with a high polarization extinction ratio. The widths of the first polarization-maintaining straight waveguide, the second polarization-maintaining straight waveguide, and the polarization-maintaining bent waveguide are < 3 μm, and the etching depth is < 0.5 μm. The operating wavelength range is 1.2 - 1.5 μm.

[0007] Specifically, from top to bottom, the first polarization-maintaining straight waveguide, the second polarization-maintaining straight waveguide, and the polarization-maintaining bent waveguide are an air layer, a low-refractive-index capping layer, a lithium niobate ridge waveguide, an SiO2 substrate layer, and an Si substrate layer respectively. 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 capping layer is 1 - 1.7.

[0008] Specifically, the polarization-maintaining bent waveguide is a hybrid bent waveguide, which is jointly composed of a linearly varying curvature bent waveguide and a constant curvature bent waveguide.

[0009] Specifically, the width of the polarization-maintaining bent waveguide is < 2.5 μm. The angle of the linearly varying curvature bent waveguide accounts for < 95% of the total bending angle of the polarization-maintaining bent waveguide. The effective radius of the polarization-maintaining bent waveguide is > 80 μm.

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

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

[0012] 1. The overall structure adopts two hybrid bent waveguides, which reduces the insertion loss caused by the mode field mismatch between the straight waveguide and the bent waveguide at the connection, and reduces the excitation of high-order modes. Moreover, by staggering the positions of the input and output waveguides through two centrally symmetric hybrid bent waveguides with the same bending angle, the leakage light received by the output end through the flat layer is reduced, the polarization extinction ratio of the chip is increased, and the waveguide modes other than the quasi-TE mode are further filtered, thereby achieving polarization with a high polarization extinction ratio. 00 mode, thus realizing polarization with a high polarization extinction ratio.

[0013] 2. The present utility model adopts lithium niobate waveguides with shallow etching and / or narrow widths. By using the mode leakage design of the waveguide, single-mode and single-polarization operation of the lithium niobate waveguide is achieved, forming a waveguide polarizer with a high polarization extinction ratio. A suitable waveguide structure will endow the waveguide polarizer with large width and thickness tolerances and broadband characteristics.

[0014] In summary, while ensuring the polarization degree of the waveguide polarizer, the present utility model effectively improves the stability, reliability and integration degree, and at the same time reduces the cost. Description of the Drawings

[0015] Figure 1 is a schematic structural diagram of the waveguide polarizer provided by the embodiment of the present utility model;

[0016] Figure 2 is the quasi-TM 00 leakage mode loss (dB / cm) and quasi-TE 10 existence of the mode varying with the waveguide width and etching depth for the polarization straight waveguide in the waveguide polarizer provided by the embodiment of the present utility model;

[0017] Figure 3 is the quasi-TM 00 leakage mode loss (dB / cm) varying with the wavelength for the polarization straight waveguide in the waveguide polarizer provided by the embodiment of the present utility model when the waveguide width is 1 μm and the etching depth is 0.06 μm;

[0018] Figure 4 is a schematic cross-sectional view of the polarization waveguide in the waveguide polarizer provided by the embodiment of the present utility model;

[0019] Figure 5 is the 53.13° hybrid bending waveguide used in the embodiment of the present utility model; (a) is a schematic top view structure diagram of the hybrid bending waveguide; (b) is the variation of the curvature of the hybrid bending waveguide with the waveguide path length;

[0020] wherein, 1 - first optical fiber - waveguide mode converter, 2 - first polarization straight waveguide, 3 - polarization bending waveguide, 4 - second polarization straight waveguide, 5 - second optical fiber - waveguide mode converter. Detailed Embodiment

[0021] The embodiment of the present utility model provides a waveguide polarizer, which solves the technical problem in the prior art that the mode leakage type polarizer ignores the fact that the leaked light of the straight waveguide is received by the output end through the flat layer, resulting in limited extinction ratio, and realizes 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 specification drawings and specific embodiments.

[0023] As Figure 1As shown in the figure, the waveguide polarizer provided by the embodiment of the present invention includes: a light source, a first optical fiber, a first optical fiber-waveguide mode converter 1, a first polarization straight waveguide 2, a second polarization straight waveguide 4, a polarization bending waveguide 3, a second optical fiber-waveguide mode converter 5, a second optical fiber and a detector; the first end of the first optical fiber-waveguide mode converter 1 is connected to the light source through the first optical fiber, and the second end of the first optical fiber-waveguide mode converter 1 is connected to the first end of the first polarization straight waveguide 2; the second end of the first polarization straight 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 straight waveguide 4, and the second end of the second polarization straight waveguide 4 is connected to the first end of the second optical fiber-waveguide mode converter 5; the second end of the second optical fiber-waveguide mode converter 5 is connected to the detector through the second optical fiber; the polarization bending waveguide 3 is composed of two bending waveguides with central symmetry and the same bending angle, and is used to connect the first polarization straight waveguide 2 and the second polarization straight waveguide 4 which are parallel to each other and laterally offset.

[0024] The 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-waveguide mode converter 1 and enters the first polarization straight waveguide 2, at this time, except for the quasi-TE 00 mode, other modes leak. Then it enters the polarization bending waveguide 3 with two central symmetries and the same bending angle for misalignment. At this time, except for the quasi-TE 00 mode, other modes leak to both sides again, and the light leaking in the polarization straight waveguide enters the output waveguide through the substrate with less misalignment in the second polarization straight waveguide 4, further increasing the polarization extinction ratio. Finally, only the quasi-TE 00 mode enters the external detector through the second optical fiber-waveguide mode converter 5.

[0025] Specifically describe the structures of the first polarization straight waveguide 2, the second polarization straight waveguide 4, and the polarization bending waveguide 3. The first polarization straight waveguide 2, the second polarization straight waveguide 4, and the polarization bending waveguide 3 are all composed of shallowly etched and / or narrow-width lithium niobate waveguides. The single-mode single-polarization operation of the lithium niobate waveguide is realized by using the mode leakage design of the waveguide, and a waveguide polarizer with a high polarization extinction ratio is formed; the widths of the first polarization straight waveguide 2, the second polarization straight waveguide 4, and the polarization bending waveguide 3 are <3μm, the etching depth is <0.5μm, and the working wavelength range is 1.2 - 1.5μm.

[0026] The design goal of the embodiment of the present invention is to optimize the waveguide structure so that it has the largest possible quasi-TM 00 mode loss and only the quasi-TE 00 mode for TE polarization, that is, a TE-pass polarization polarizer. Based on the x-cut thin-film lithium niobate wafer x-z plane straight waveguide at a wavelength of 1310nm, the quasi-TM 00 leakage mode loss (dB / cm) and quasi-TE10 The existence of the mode varies with the waveguide width and etching depth as shown in Figure 2 . According to Figure 2 In the results shown in, in the embodiment of the present invention, the waveguide size 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 mode has a transverse leakage transmission loss of approximately 332 dB / cm, and the TE 10 mode does not exist because it is in the cutoff region. Therefore, only the TE 00 mode can stably transmit at this time, and its calculated transmission loss is 2.6×10 -10 dB / cm. The waveguide is a single-mode single-polarization straight waveguide.

[0027] The design goal of the embodiment of the present invention is a broadband TE polarizer. Figure 3 The polarization straight waveguide in the waveguide polarizer of the embodiment of the present invention is based on the x-z plane waveguide of an x-cut thin-film lithium niobate wafer with a waveguide width of 1 μm and an etching depth of 0.06 μm. The quasi-TM 00 Leakage mode loss (dB / cm) varies with wavelength. According to Figure 3 In the results shown in, 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 mode has a transverse leakage transmission loss > 100 dB / cm in the wavelength range of 1.21 μm to 1.36 μm, and this polarizer is a broadband polarizer.

[0028] As shown in Figure 4 , the first polarization straight waveguide 2, the second polarization straight waveguide 4, and the polarization bending waveguide 3 are, from top to bottom, an air layer, a low refractive index cover layer, a lithium niobate ridge waveguide, a SiO2 substrate layer, and a Si substrate layer. 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 300 nm. The refractive index of the low refractive index cover layer is 1 - 1.7.

[0029] In order to reduce the insertion loss caused by the mode field mismatch at the junction of the straight waveguide and the bending waveguide, and to reduce the excitation of higher-order modes, the polarization bending waveguide 3 is a hybrid bending waveguide, which is composed of a linearly variable curvature bending waveguide and a constant curvature bending waveguide.

[0030] Specifically, the width of the polarization bending waveguide 3 < 2.5 μm, the proportion of the angle of the linearly variable curvature bending waveguide in the total bending angle of the polarization bending waveguide 3 < 95%, and the effective radius of the polarization bending waveguide 3 > 80 μm.

[0031] In this embodiment, in order to stagger the input and output waveguides by 400 μm in the direction perpendicular to the waveguide, 2 hybrid bending waveguides with a 53.13° partially continuous variable curvature are used. Figure 5(a) is a top - view structural schematic diagram of a 53.13° partially continuously variable - curvature hybrid bent waveguide. The angle of the linearly variable curvature of this bent waveguide accounts for 30% of the total bending angle. The starting point and the ending point of this bent waveguide are consistent with those of a fixed - curvature bent waveguide with the same angle and a radius of 500 μm. The effective radius is 500 μm, and the minimum radius Rmin is 390.25 μm. Figure 5 (b) shows the variation of the curvature of this bent waveguide with the waveguide path length. In the angular range of [0°, 7.97°] of the bent waveguide, the linear variation range of the curvature is [0, 1 / Rmin]. In the angular range of [7.97°, 45.16°], the curvature value is 1 / Rmin. In the angular range of [45.16°, 53.13°], the linear variation range of the curvature is [1 / Rmin, 0]. The maximum curvature 1 / Rmin is 2562.46 m -1 The linearly varying curvature can reduce the mode - field mismatch at the junction of the straight waveguide and the bent waveguide.

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

[0033] In summary, the embodiment of the present utility model utilizes the anisotropy of thin - film lithium niobate. By adjusting the structural dimensions of the ridge waveguide, only the quasi - TE 00 mode can stably propagate. At the same time, the misalignment of the input and output waveguides is realized through two hybrid linearly variable - curvature arcs, achieving low - loss TE polarization on the chip. The embodiment of the present utility model ensures the polarization extinction ratio of the polarizer while only using the ridge waveguide, with a simple structure and convenient design, which is conducive to large - scale production.

[0034] For the parts not detailed in the embodiments of the present utility model, they are all well - known technologies in the technical field. Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present utility model and not to limit them. Although the present utility model has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present utility model can be modified or equivalently replaced without departing from the purpose and scope of the technical solutions of the present utility model, and they should all be covered within the scope of the claims of the present utility model.

Claims

1. A waveguide polarizer, characterized in that, Including: A first fiber - waveguide mode converter, a first polarization - maintaining straight waveguide, a second polarization - maintaining straight waveguide, a polarization - maintaining bent waveguide, and a second fiber - waveguide mode converter; The first end of the first fiber - waveguide mode converter is connected to a light source through a first fiber, and the second end of the first fiber - waveguide mode converter is connected to the first end of the first polarization - maintaining straight waveguide; the second end of the first polarization - maintaining straight waveguide is connected to the first end of the polarization - maintaining bent waveguide; the second end of the polarization - maintaining bent waveguide is connected to the first end of the second polarization - maintaining straight waveguide, and the second end of the second polarization - maintaining straight waveguide is connected to the first end of the second fiber - waveguide mode converter; the second end of the second fiber - waveguide mode converter is connected to a detector through a second fiber; the polarization - maintaining bent waveguide is composed of two bent waveguides with central symmetry and the same bending angle, and is used to connect the first polarization - maintaining straight waveguide and the second polarization - maintaining straight waveguide which are parallel to each other and laterally offset.

2. The waveguide polarizer according to claim 1, characterized in that, The first polarization - maintaining straight waveguide, the second polarization - maintaining straight waveguide, and the polarization - maintaining bent waveguide are all composed of lithium niobate waveguides with shallow etching and / or narrow width. By using the mode leakage design of the waveguide, single - mode and single - polarization operation of the lithium niobate waveguide is achieved, forming a waveguide polarizer with a high polarization extinction ratio; the widths of the first polarization - maintaining straight waveguide, the second polarization - maintaining straight waveguide, and the polarization - maintaining bent waveguide are < 3μm, the etching depth is < 0.5μm, and the operating wavelength range is 1.2 - 1.5μm.

3. The waveguide polarizer according to claim 1, wherein, The first polarization - maintaining straight waveguide, the second polarization - maintaining straight waveguide, and the polarization - maintaining bent waveguide are, from top to bottom, an air layer, a low - refractive - index capping layer, a lithium niobate ridge waveguide, an SiO2 substrate layer, and an Si substrate layer. 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 capping layer is 1 - 1.

7.

4. The waveguide polarizer according to claim 1, characterized in that, The polarization - maintaining bent waveguide is a hybrid bent waveguide, which is jointly composed of a linearly variable curvature bent waveguide and a constant curvature bent waveguide.

5. The waveguide polarizer according to claim 4, wherein The width of the polarization - maintaining bent waveguide is < 2.5μm, the proportion of the angle of the linearly variable curvature bent waveguide in the total bending angle of the polarization - maintaining bent waveguide is < 95%, and the effective radius of the polarization - maintaining bent waveguide is > 80μm.

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

Images