An electro-optic polarization modulator based on thin film lithium niobate

Abstract

The application discloses a thin-film lithium niobate-based electro-optic polarization modulator, characterized by comprising a first single-mode optical fiber, an input optical coupler, an electro-optic modulation phase shift region, an output double-waveguide vertical direct receiving coupler and a second single-mode optical fiber connected in sequence, wherein the modulated light enters from the first single-mode optical fiber, is coupled into the electro-optic modulation phase shift region through the input optical coupler, is coupled into the second single-mode optical fiber through the output double-waveguide vertical direct receiving coupler after being modulated in the electro-optic modulation phase shift region, and exits from the second single-mode optical fiber. The application is based on X-cut or Y-cut thin-film lithium niobate, the electric field of the electro-optic modulation region is along the Z-axis direction, the excellent electro-optic effect of lithium niobate material is effectively utilized, the electro-optic polarization modulator has the advantages of fast response speed, low driving voltage and low power consumption. Compared with the polarization multiplexing device with a length of hundreds of microns, the compactness of the structure is greatly improved, and the miniaturization of the device is facilitated.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of optoelectronics and optical communication, in particular to an electro-optic polarization modulator based on thin-film lithium niobate. BACKGROUND

[0002] As one of the basic properties of electromagnetic field, polarization state can be used to calculate, process and store information in light, and has a large number of application prospects in optical perception and manipulation. Effective modulation of optical polarization state plays a remarkable ability in a wide range of scientific and industrial applications, and the polarization modulator is an important optical device, which plays a very important role in optical fiber communication and optical fiber sensing system. However, the traditional polarization manipulation method often relies on bulky optical elements, such as wave plate type and fiber ring type polarization controller, so it is difficult to drive quickly and control poorly. In contrast, it is of great significance to further expand the technology and evolution in various fields to realize effective manipulation of polarization state on a micro scale and form a polarization manipulation device with compactness, lightness and integration.

[0003] In recent years, the development of thin-film lithium niobate has attracted widespread attention from academia and industry. Benefiting from the strong light confinement brought by the high refractive index contrast of thin-film structure and the strong electro-optic effect of lithium niobate material, thin-film lithium niobate shows great potential in realizing high-density integration and ultra-high-speed response. These characteristics make it one of the key candidates for the next generation of photonic integration platform, and it has broad prospects in the fields of optical communication and photonic integration technology. Typical photonic devices are bulky, have limited bandwidth, high driving voltage and pJ / bit level or even more power consumption. In contrast, electro-optic modulators based on thin-film lithium niobate have been proved to achieve energy consumption of several tens of fJ / bit or even lower. However, due to the birefringence effect, the strong polarization dependence of the devices based on thin-film lithium niobate directly restricts the performance of the optical system. In view of the superiority of thin-film lithium niobate material and the necessity of polarization state regulation, it is urgent to realize the polarization modulation device based on thin-film lithium niobate. Most of the current polarization control devices based on thin-film lithium niobate are highly dependent on the performance of polarization multiplexing devices, which requires careful design of structural parameters and high-precision control of preparation technology, and aggravates the difficulty of actual process manufacturing; the polarization multiplexing device is several hundred microns long, which is also very unfavorable to realize compact on-chip regulation device.

[0004] Therefore, the skilled in the art is committed to developing an electro-optic polarization modulator based on thin-film lithium niobate, which is expected to be free of dependence on polarization multiplexing devices. SUMMARY

[0005] To achieve the above object, the application provides a thin-film lithium niobate-based electro-optic polarization modulator, which comprises a first single-mode optical fiber, an input optical coupler, an electro-optic modulation phase shift region, an output double-waveguide vertical direct receiving coupler and a second single-mode optical fiber connected in sequence, wherein the light to be modulated enters the first single-mode optical fiber, is coupled into the electro-optic modulation phase shift region through the input optical coupler, is coupled into the second single-mode optical fiber through the output double-waveguide vertical direct receiving coupler after being modulated in the electro-optic modulation phase shift region, and exits from the second single-mode optical fiber.

[0006] Further, the electro-optic modulation phase shift region comprises an input optical waveguide, a beam splitter, a modulation signal input region, a traveling wave electrode and an electro-optic modulation waveguide arm.

[0007] The electro-modulation signal enters the traveling wave electrode through the modulation signal input region, and the light to be modulated is electro-optically modulated in the electro-optic modulation phase shift region.

[0008] Further, the traveling wave electrode comprises one metal signal electrode and two metal ground electrodes, the metal signal electrode and the metal ground electrodes are arranged in parallel, and the metal ground electrodes are arranged on both sides of the metal signal electrode.

[0009] Further, two electro-optic modulation phase shift regions, i.e., a first electro-optic modulation phase shift region and a second electro-optic modulation phase shift region, are included, and the first electro-optic modulation phase shift region and the second electro-optic modulation phase shift region are connected through a 2x2 multimode interferometer.

[0010] Further, the input optical coupler has polarization sensitivity and is a waveguide that only allows TE polarization or TM polarization to be coupled into light.

[0011] Further, the input optical waveguide and the electro-optic modulation waveguide arm are single-mode waveguides that only allow TE0 mode or TM0 mode transmission.

[0012] Further, the beam splitter is a multimode interferometer that can divide the input light into two beams of equal intensity and the same polarization.

[0013] Further, the electro-optic polarization modulator is based on X-cut or Y-cut thin-film lithium niobate, and the electric field generated in the electro-optic modulation phase shift region is along the direction of the optical axis of lithium niobate.

[0014] Further, it comprises, from bottom to top, a substrate layer, an optical isolation layer, a lithium niobate single crystal thin film substrate, a lithium niobate optical waveguide layer, a low refractive index upper cladding layer and a traveling wave electrode, wherein

[0015] The input optical coupler, the electro-optic modulation phase shift region and the output double-waveguide vertical direct receiving coupler are all located in the lithium niobate optical waveguide layer.

[0016] Further,

[0017] The material of the substrate layer is silicon or lithium niobate crystal, and the thickness is 0.1mm to 2mm;

[0018] The optical isolation layer is a silicon dioxide film, which is arranged between the substrate layer and the lithium niobate single crystal thin film substrate, and the thickness is 2μm to 5μm;

[0019] The lithium niobate single crystal thin film substrate is an etching residual part of the lithium niobate single crystal thin film, and the thickness is 0 to 0.4μm;

[0020] The lithium niobate optical waveguide layer is an unetched part of the lithium niobate single crystal thin film, and the structure is a ridge waveguide with an etching angle of 60 degrees to 75 degrees, and the thickness is 0.2μm to 0.6μm;

[0021] The low refractive index upper cladding layer is a silicon dioxide protective layer, and the thickness is 0 to 2μm.

[0022] Compared with the prior art, the technical scheme of the present application has at least the following beneficial technical effects:

[0023] Firstly, the present application is based on X-cut or Y-cut thin film lithium niobate, and the electric field of the electro-optical modulation region is along the Z-axis direction, so that the excellent electro-optical effect of lithium niobate material is effectively utilized, and the electro-optical polarization modulator has the advantages of fast response speed, low driving voltage and low power consumption.

[0024] Secondly, the input light coupler and the output double waveguide vertical direct receiving coupler used in the present application have sizes of 10×15μm 2 , 15×15μm 2 , and the beam splitter is a multimode interference beam splitter with a size of 2.8×5.3μm 2 , compared with the polarization multiplexing device with a length of hundreds of microns, the compactness of the structure is greatly improved, which is beneficial to the miniaturization of the device.

[0025] Thirdly, the multimode interference beam splitter used in the present application is calculated to have a transmittance of >96.8% within the range of length ±2μm and width ±0.4μm under the premise of ensuring performance, which greatly improves the process tolerance compared with the polarization multiplexing device.

[0026] Fourthly, the two lithium niobate optical waveguides included in the electro-optical modulation phase shifter form a Mach-Zehnder structure, so that a "push-pull" modulation design can be adopted, and the modulation length is short and the driving voltage is low.

[0027] The concept, specific structure and technical effects of the present application will be further described below with reference to the accompanying drawings, so as to fully understand the purpose, features and effects of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 This is a top view structural diagram of one embodiment of this application;

[0029] Figure 2 This is a cross-sectional view of an embodiment of the present application.

[0030] Figure 3 This is a top view structural diagram of another embodiment of this application. Detailed Implementation

[0031] The following description, with reference to the accompanying drawings, illustrates several preferred embodiments of this application to make its technical content clearer and easier to understand. This application can be embodied in many different forms, and the scope of protection of this application is not limited to the embodiments mentioned herein.

[0032] In the accompanying drawings, components with the same structure are indicated by the same numerical designation, and components with similar structures or functions are indicated by similar numerical designations. The dimensions and thicknesses of each component shown in the drawings are arbitrary, and this application does not limit the dimensions and thicknesses of each component. To make the illustrations clearer, the thickness of some components has been appropriately exaggerated in the drawings.

[0033] like Figure 1 As shown, the electro-optic polarization modulator based on thin-film lithium niobate described in this embodiment includes a first single-mode fiber 1, an input optical coupler 2, an electro-optic modulation phase-shifting region 3, an output dual-waveguide vertical receiving coupler 4, and a second single-mode fiber 5 connected in sequence. Specifically, the electro-optic modulation phase-shifting region 3 includes an input optical waveguide 31, a beam splitter 32, a modulation signal input region 33, a traveling wave electrode 34, and an electro-optic modulation waveguide arm 35. The light to be modulated enters from the first single-mode fiber 1, is coupled into the electro-optic modulation phase-shifting region 3 through the input optical coupler 2, and after being modulated in the electro-optic modulation phase-shifting region 3, is coupled into the second single-mode fiber 5 through the output dual-waveguide vertical receiving coupler 4, and then leaves the electro-optic polarization modulator from the second single-mode fiber 5. The electro-modulation signal enters the traveling wave electrode 34 through the modulation signal input region 33, and electro-optically modulates the light to be modulated in the electro-optic modulation phase-shifting region 3. Preferably, the traveling wave electrode 34 includes a metal signal electrode 342 and two metal ground electrodes 341 and 343, which are arranged in parallel, and the two metal ground electrodes 341 and 343 are arranged on both sides of the metal signal electrode 342.

[0034] Preferably, the input optical coupler 2 is polarization sensitive and is a waveguide that only allows TE polarization or TM polarization to couple into the light.

[0035] Preferably, the input optical waveguide 31 and the electro-optic modulation waveguide arm 35 are single-mode waveguides, allowing only TEO or TMO mode transmission.

[0036] Preferably, the beam splitter 32 is a multimode interferometer, which can split the input light into two beams of light with equal intensity and the same polarization.

[0037] The following is a detailed description of the working principle of the electro-optic polarization modulator described in this embodiment.

[0038] After the signal light is coupled into the input optical coupler from the first single-mode fiber 1, only one type of linearly polarized light is retained in the optical waveguide. It is then split into two beams of equal amplitude by the beam splitter 32. These two components propagate in the optical waveguide and are modulated in the electro-optic modulation phase-shifting region to generate an arbitrary phase difference. After being bent by two electro-optic modulation waveguide arms 35, they are transmitted as two orthogonal components and collected by the output dual-waveguide vertical receiving coupler 4. Finally, they are coupled to the second single-mode fiber 5. Under the action of the electro-optic effect, the phase difference between the two branches changes. By adjusting the voltage, the polarization state of the electrical signal can be modulated. When the second single-mode fiber 5 is collinear with the axis of symmetry of the output dual-waveguide vertical receiver coupler 4, the power ratio of the two components coupled into the second single-mode fiber is 1:1. At this time, when the phase difference is 0 and π respectively, 45° linearly polarized light and 135° linearly polarized light can be obtained; when the phase difference is π / 2 and 3π / 2, right-hand circularly polarized light and left-hand circularly polarized light can be obtained; when the phase difference is other values ​​in the range of 0 to 2π, elliptically polarized light can be obtained.

[0039] Electro-optic polarization modulators are based on X-cut Z-transmission or Y-cut Z-transmission thin-film lithium niobate to effectively utilize the high electro-optic coefficient (γ33 = 32.2 pm / V) of lithium niobate crystals. A schematic diagram of the cross-sectional structure of the electro-optic modulation phase-shifting region using a coplanar waveguide GSG dual-drive structure is shown below. Figure 2 As shown, the structure includes, from bottom to top, the following layers: a substrate layer 6, an optical isolation layer 7, a lithium niobate single-crystal thin film 8, a low-refractive-index overcoating layer 9, and a traveling-wave electrode 34. Specifically, the lithium niobate single-crystal thin film includes a lithium niobate single-crystal thin film substrate 81 and a lithium niobate optical waveguide layer 82. The traveling-wave electrode 34 includes a metal signal electrode 342 and two metal ground electrodes 341 and 343. The metal signal electrode 342 is located between the two arms of the electro-optic modulator phase shifter, and the metal ground electrodes 341 and 343 are located on both sides of the metal signal electrode 342.

[0040] Furthermore, the input optical coupler 3 and the output dual-waveguide vertical receiver coupler 5 included in this application have dimensions of 10×15μm, respectively. 2 15×15μm 2The included beamsplitter 32 is a multimode interference beamsplitter with dimensions of 2.8 × 5.3 μm². Compared to polarization multiplexing devices with lengths of hundreds of micrometers, it significantly improves structural compactness and facilitates device miniaturization. Calculations show that, while ensuring performance, it can achieve a transmittance of >96.8% within a length range of ±2 μm and a width range of ±0.4 μm, greatly improving process tolerance compared to polarization multiplexing devices.

[0041] Therefore, the electro-optic modulation device based on thin-film lithium niobate described in this application can obtain linearly polarized, circularly polarized, and elliptically polarized light, while avoiding the introduction of polarization multiplexing devices, reducing device size, and effectively improving high-precision process requirements and compactness issues.

[0042] Example 2

[0043] like Figure 3 As shown, this second embodiment, based on the first embodiment, introduces a splitting ratio control function based on the electro-optic effect. This embodiment includes two electro-optic modulation phase shift regions, namely the first electro-optic modulation phase shift region 3 and the second electro-optic modulation phase shift region 11. The first electro-optic modulation phase shift region 3 and the second electro-optic modulation phase shift region 11 are connected by a 2×2 multimode interferometer 10.

[0044] Specifically, this embodiment includes a first single-mode fiber 1, an input optical coupler 2, a first electro-optic modulation phase-shifting region 3, a 2×2 multimode interferometer 10, a second electro-optic modulation phase-shifting region 11, an output dual-waveguide vertical receiving coupler 4, and a second single-mode fiber 5 connected in sequence. The first electro-optic modulation phase-shifting region 3 includes an input optical waveguide 31, a beam splitter 32, a first modulation signal input region 33, a first modulation region traveling wave electrode 34, and a first electro-optic modulation waveguide arm 35. The second electro-optic modulation phase-shifting region 11 includes a second modulation signal input region 111, a second modulation region traveling wave electrode 112, and a second electro-optic modulation waveguide arm 113. After the signal light is coupled into the input optical coupler from the first single-mode fiber 1, only one type of linearly polarized light is retained in the optical waveguide. It is then split into two beams of equal amplitude by the beam splitter 32. Utilizing the electro-optic effect, electro-optic modulation is performed in the first electro-optic modulation phase-shifting region 3 to generate an arbitrary phase difference between the two components. This allows the two input signals of the 2×2 multimode interferometer 10 to achieve an arbitrary phase difference, thereby controlling the output splitting ratio of the 2×2 multimode interferometer 10. Next, the two components with adjustable power ratios generate an arbitrary phase difference via the second electro-optic modulation phase-shifting region 11. After being bent by two second electro-optic modulation waveguide arms 113, they are transmitted as two orthogonal components, which are then collected by the output dual-waveguide vertical receiving coupler 4 and finally coupled to the second single-mode fiber 5. Using this embodiment, arbitrary linearly polarized, circularly polarized, and elliptically polarized light can be obtained.

[0045] The preferred embodiments of this application have been described in detail above. It should be understood that those skilled in the art can make numerous modifications and variations based on the concept of this application without inventive effort. Therefore, any technical solution that can be obtained by those skilled in the art based on the concept of this application through logical analysis, reasoning, or limited experimentation on the basis of existing technology should be within the scope of protection defined by the claims.

Claims

1. An electro-optic polarization modulator based on thin-film lithium niobate, characterized in that, The device includes a first single-mode fiber, an input optical coupler, an electro-optic modulation phase-shifting region, an output dual-waveguide vertical receiving coupler, and a second single-mode fiber connected in sequence. The light to be modulated enters from the first single-mode fiber, is coupled into the electro-optic modulation phase-shifting region through the input optical coupler, is modulated in the electro-optic modulation phase-shifting region, is coupled into the second single-mode fiber through the output dual-waveguide vertical receiving coupler, and then exits from the second single-mode fiber. The electro-optic modulation phase-shifting region includes an input optical waveguide, a beam splitter, a modulation signal input region, a traveling wave electrode, and an electro-optic modulation waveguide arm. The beam splitter is a multimode interferometer, which can split the input light into two beams of light with equal intensity and the same polarization. The electro-modulation signal enters the traveling wave electrode through the modulation signal input area, and the light to be modulated is electro-optically modulated in the electro-optic modulation phase-shifting area. Two input optical components are transmitted in the optical waveguide and modulated in the electro-optic modulation phase-shifting region to generate an arbitrary phase difference. After being bent by two electro-optic modulation waveguide arms, they are transmitted as two orthogonal components. The traveling wave electrode includes a metal signal electrode and two metal ground electrodes. The metal signal electrode and the metal ground electrodes are arranged in parallel. The metal signal electrode is located between the two arms of the electro-optic modulator phase shifter, and the metal ground electrodes are arranged on both sides of the metal signal electrode. The input optical coupler is polarization sensitive and is a waveguide that only allows TE polarization or TM polarization to couple into the light; the input optical waveguide and the electro-optic modulation waveguide arm are single-mode waveguides that only allow TE0 mode or TM0 mode transmission. The electro-optic polarization modulator is based on X-cut Z-transmission or Y-cut Z-transmission thin-film lithium niobate. The electric field generated in the electro-optic modulation phase-shifting region is along the optical axis of the lithium niobate and includes, from bottom to top, the following components arranged sequentially: a substrate layer, an optical isolation layer, a lithium niobate single-crystal thin-film substrate, a lithium niobate optical waveguide layer, a low-refractive-index upper cladding layer, and a traveling-wave electrode. The input optical coupler, the electro-optic modulation phase-shifting region, and the output dual-waveguide vertical receiving coupler are all located in the lithium niobate optical waveguide layer. The substrate layer is made of silicon or lithium niobate crystal and has a thickness of 0.1 mm to 2 mm. The optical isolation layer is a silicon dioxide thin film, disposed between the substrate layer and the lithium niobate single crystal thin film substrate, and its thickness is 2μm to 5μm; The lithium niobate single crystal thin film substrate is the etched residue of the lithium niobate single crystal thin film, and its thickness is 0 to 0.4 μm. The lithium niobate optical waveguide layer is the unetched portion of a lithium niobate single-crystal thin film. Its structure is a ridge waveguide with an etching tilt angle of 60 to 75 degrees and a thickness of 0.2 μm to 0.6 μm. The low-refractive-index overcoating is a silicon dioxide protective layer with a thickness of 0 to 2 μm.

2. The electro-optic polarization modulator based on thin-film lithium niobate as described in claim 1, characterized in that, It includes two electro-optic modulation phase shift regions, namely a first electro-optic modulation phase shift region and a second electro-optic modulation phase shift region, which are connected by a 2×2 multimode interferometer.

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