Micron-sized crystal film electro-optical modulator and preparation method thereof

Abstract

The invention discloses a micron-sized crystal film electro-optical modulator and a preparation method thereof, and relates to the technical field of optics. The micron-sized crystal thin film electro-optical modulator comprises a crystal thin film layer, a silicon dioxide buffer layer and a substrate layer which are sequentially stacked from top to bottom, a ridge waveguide is arranged on the crystal thin film layer, and the micron-sized crystal thin film electro-optical modulator is characterized in that the crystal thin film layer is formed by splicing crystal thin films on two x-cut insulators in an anti-parallel mode. The optical axis anti-parallel x-tangent lithium niobate is spliced to realize electro-optic coefficient inversion, so that the modulation speed higher than that of a single tangential lithium niobate modulator can be realized through quasi-speed matching, the modulation efficiency during high-speed modulation can be improved on the basis of an original coplanar planar electrode, the upper limit of the modulation rate can be improved, and the modulation efficiency can be improved. The micron waveguide prepared by the method is natural and equivalent to an optical fiber in size, small in mode mismatch, high in coupling efficiency, low in requirements on equipment (ultraviolet exposure) and process, easy to package and good in practical and commercial value.

Description

Technical Field

[0001] This invention relates to the field of optical technology, specifically to a micron-scale crystal thin-film electro-optic modulator and its fabrication method. Background Technology

[0002] Electro-optic modulators are modulators made using the electro-optic effect of electro-optic crystals. That is, by applying a modulation voltage to these crystals, a change in the crystal's refractive index occurs, causing a change in the phase of the transmitted light, thus modulating the phase of the optical signal. Among these, lithium niobate crystals are particularly valuable due to their large transparency window (0.4 - 5.0 μm) and high refractive index (n). o =2.21, n e =2.14 @1550 nm), large electro-optic coefficient (γ 33 With advantages such as 32 pm / V and stable physicochemical properties, it is widely used in electro-optic modulation.

[0003] Lithium niobate electro-optic modulators are core devices that utilize the electro-optic effect of lithium niobate material to achieve high-speed electro-optic modulation. By applying a microwave electric field, light undergoes a phase shift in the lithium niobate waveguide due to the linear electro-optic effect, enabling high-speed phase modulators and intensity modulators. Traditional proton-exchange or titanium-diffused lithium niobate has a low refractive index contrast (Δn ~ 0.01), resulting in weakly bound waveguides with bending radii on the order of millimeters or even centimeters, and device lengths typically ranging from 5 to 10 cm. While weakly bound waveguides have relatively large cross-sections (5-10 μm in diameter) and low insertion loss when directly coupled to standard optical fibers, they also result in larger microwave electrode spacing, leading to higher half-wave voltages. Furthermore, the device length needs to be increased to reduce the modulator drive voltage. However, the refractive index mismatch between microwaves and optical wavegroups is more severe in longer devices, significantly limiting the modulator bandwidth. Currently, bulk lithium niobate modulators typically have a drive voltage of around 3.5 V, a modulation efficiency of 10-20 V·cm, an insertion loss of around 3 dB, and a maximum electro-optic bandwidth of 40 GHz.

[0004] The lithium niobate-on-insulator (LNI) technology, which has developed in recent years, is revolutionary for lithium niobate-based integrated optics. LNI ridge waveguides have a high refractive index contrast (Δn ~ 0.7), enabling the realization of micro / nano-level strongly confined waveguides. Currently, the performance of various LNI devices far exceeds the limits of traditional waveguides or bulk devices. The modes in nanowaveguides are very small, enabling highly efficient electro-optic modulation, resulting in correspondingly excellent device performance. Currently, the modulation bandwidth of nano-thin-film LNI modulators exceeds 110 GHz, with modulation efficiency around 2 V·cm. The fabrication methods for LNI nanowaveguides include electron beam lithography or deep ultraviolet lithography combined with dry etching. However, the coupling efficiency of nanowaveguides to standard single-mode fiber is limited due to severe mode mismatch; the insertion loss of nano-thin-film LNI waveguide devices is 10-20 dB. Low-loss fiber input / output requires mode-spot converters, although in principle and in research reports, mode-spot converters can reduce the insertion loss to as low as about 2 dB. However, it requires multiple electron beam exposure, etching and deposition processes, which are generally costly and challenging for quantitative fabrication.

[0005] In many user-facing applications, such as fiber optic access networks, the performance requirements for lithium niobate electro-optic modulators are certain but not extremely high. The primary requirements are direct-drive voltage below 2 V, modulator bandwidth of 1–10 GHz, insertion loss of 3 dB, and further cost reduction for easy mass deployment. These requirements are higher than those for traditional proton-exchange lithium niobate modulators but not quite at the level of thin-film lithium niobate modulators. Often, the focus is on considering the overall performance-to-price ratio and employing lower-cost, scalable fabrication processes.

[0006] There is currently no effective method for low-cost fabrication of traditional proton-exchange lithium niobate electro-optic modulators, mainly due to the limitation of lithium niobate waveguide size; while nano-thin film lithium niobate modulators offer superior performance but are costly.

[0007] Therefore, those skilled in the art are dedicated to developing a micron-scale lithium niobate ridge waveguide and its fabrication method, and to using it to achieve low-cost, optimized electro-optic modulation technology. Summary of the Invention

[0008] In view of the above-mentioned deficiencies of the prior art, the technical problem to be solved by the present invention is to provide a micron-scale lithium niobate ridge waveguide and its fabrication method, and to use it to achieve low-cost, optimized electro-optic modulation technology.

[0009] To achieve the above-mentioned technical objectives, the present invention mainly adopts the following technical solutions: This invention discloses a micron-scale crystal thin film electro-optic modulator, comprising a crystal thin film layer, a silicon dioxide buffer layer and a substrate layer stacked sequentially from top to bottom. A ridge waveguide is disposed on the crystal thin film layer, and the crystal thin film layer is formed by antiparallel splicing of two X-cut insulator crystal thin films.

[0010] In a preferred embodiment of the present invention, the thickness of the crystal thin film layer is 2-3 μm, and the thickness of the silicon dioxide buffer layer is 2-5 μm.

[0011] In a preferred embodiment of the present invention, the ridge width of the ridge waveguide is equal to the thickness of the crystal thin film layer, and the height of the ridge waveguide is 0.5-0.6 times that of the crystal thin film layer.

[0012] In a preferred embodiment of the present invention, the angle between the ridge waveguide sidewall and the crystal thin film layer is 65 degrees.

[0013] In a preferred embodiment of the present invention, the cross-sectional height and width of the modulator ridge waveguide are equal, and are similar in size to the focusing spot of the lens fiber or the core of the high numerical aperture fiber of the measuring device.

[0014] In a preferred embodiment of the present invention, the material of the crystal thin film is lithium niobate or lithium tantalate.

[0015] In a preferred embodiment of the present invention, the substrate material is silicon, lithium niobate, or lithium tantalate.

[0016] Preferably, the substrate material of the crystal thin film layer is the same as the crystal thin film substrate material.

[0017] This invention also discloses a method for fabricating a micron-scale crystal thin-film electro-optic modulator as described above, comprising the following steps: Step 1: Deposit a silicon dioxide buffer layer on the surface of the substrate layer; Step 2: Optically polish the side of the X-cut insulator crystal film, then splice the two X-cut insulator crystal films with antiparallel optical axes together, and place them on the silicon dioxide buffer layer through crystal bonding technology. Step 3: Fabricate a ridge waveguide on the spliced ​​crystal thin film layer using ultraviolet exposure lithography and etching processes.

[0018] In a preferred embodiment of the present invention, the etching process is dry etching.

[0019] Compared with the prior art, the present invention has the following beneficial effects: This invention utilizes micron-thin lithium niobate to replace traditional proton exchange or titanium diffusion waveguides. The waveguide cross-section is smaller (~4μm), and the electrode spacing can be even smaller (5-10 microns), which improves the modulation efficiency of the electro-optic modulator by a factor of 10. At the same time, the ridge waveguide can withstand watt-level laser input.

[0020] This invention utilizes anti-parallel x-cut lithium niobate splicing along the optical axis to achieve electro-optic coefficient reversal, thereby enabling a higher modulation speed than a single tangential lithium niobate modulator through quasi-velocity matching. It can improve the modulation efficiency during high-speed modulation and increase the upper limit of the modulation rate based on the original coplanar electrode.

[0021] Traditional thin-film lithium niobate nanowaveguide modulators require nanometer-level processing precision, placing high demands on equipment (electron beam exposure) and processes. Packaging also requires nanometer precision, and coupling is difficult, necessitating specialized mode converters. This results in long fabrication cycles, high costs, and significant insertion loss. The micrometer-sized waveguide fabricated in this invention is naturally comparable in size to optical fibers, exhibits low mode mismatch, high coupling efficiency, lower equipment (ultraviolet exposure) and process requirements, and is easier to package.

[0022] The process technologies used in this invention (ultraviolet lithography, dry etching, electron beam evaporation of metal and packaging of micron-waveguide devices, etc.) are relatively mature, and the final product is a high-performance integrated micron-waveguide lithium niobate electro-optic modulation device with good practical and commercial value.

[0023] The following will further explain the concept, specific structure, and technical effects of the present invention in conjunction with the accompanying drawings, so as to fully understand the purpose, features, and effects of the present invention. Attached Figure Description

[0024] Figure 1 This is a schematic cross-sectional view of the structure of the micron-scale crystal thin-film electro-optic modulator provided by the present invention; Figure 2 A top view of the structure of a conventional micron-scale crystal thin-film electro-optic modulator provided by the present invention; Figure 3 A schematic diagram of the structure of the micron-scale crystal thin-film electro-optic modulator provided by the present invention; Figure 4 This is a schematic diagram of the connection structure between the crystal thin film layer and the silicon dioxide buffer layer provided by the present invention. Detailed Implementation

[0025] The following description, with reference to the accompanying drawings, illustrates several preferred embodiments of the present invention to make its technical content clearer and easier to understand. The present invention can be embodied in many different forms, and the scope of protection of the present invention is not limited to the embodiments mentioned herein.

[0026] 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 the present invention 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.

[0027] Example 1

[0028] The electro-optic modulator proposed in this embodiment is based on lithium niobate on a 3-micrometer-thick insulator. Unlike conventional electro-optic modulators, light in this embodiment is transmitted in a ridge waveguide of lithium niobate on an x-cut micrometer-scale insulator.

[0029] like Figure 1 As shown, the main structure of the electro-optic modulator includes, from top to bottom, a lithium niobate thin film layer 1, a silicon dioxide buffer layer 2, and a substrate layer 3. The upper lithium niobate single-crystal thin film 1 has a thickness of 3 μm. The width of the ridge 1-1 on the ridge waveguide is generally comparable to the thickness of the lithium niobate thin film layer 1, also 3 μm. The height of the ridge waveguide is 0.5-0.6 times that of the crystal thin film layer. It is fabricated using ultraviolet lithography and dry etching methods. Due to etching process limitations, the angle between the sidewall of the ridge waveguide and the lithium niobate thin film layer 1 is generally around 65 degrees, and the minimum width of the ridge 1-1 on the ridge waveguide is also limited by the ultraviolet lithography method.

[0030] Furthermore, in this embodiment, the cross-sectional height and width of the micron-shaped waveguide in the electro-optic modulator are comparable, and similar to the size of the focusing spot 1-5 of the lens fiber or the core of the high numerical aperture fiber in the measurement device. Therefore, direct coupling can be achieved using the lens fiber or the high numerical aperture fiber. Due to the large size of the micron-shaped waveguide, the lithium niobate single-crystal thin film planar portion 1-3 can leak higher-order modes, thus maintaining fundamental mode transmission even in straight waveguides or waveguides with large bending radii. Light in this micron-shaped waveguide still transmits in single mode. In this embodiment, the thickness of the silicon dioxide buffer layer 2 is 2-5 μm, and the substrate layer 3 is generally silicon or lithium niobate.

[0031] A top view of a traditional micron-scale crystal thin-film electro-optic modulator is shown below. Figure 2 As shown, taking a ridge waveguide with a length of 5 cm and a width of 5 mm (the width is not limited in other similar structures) as an example, this parameter is mainly limited based on the modulation efficiency and the direct drive voltage required for the application. The electro-optic intensity modulator is based on a Mach-Zehnder interferometer configuration. After the input light is coupled into the ridge waveguide, it passes through a 50:50 beam splitter 2-2 (Y-waveguide or multimode interferometer MMI). The two arms of the interferometer achieve phase shift avoidance through two opposing electric fields between the GSG electrodes, i.e., 3-1 and 3-2, and 3-3 and 3-2, and the interference at the 50:50 beam splitter 2-2 at the output end forms an intensity change, ultimately achieving intensity modulation.

[0032] In this embodiment, as Figure 3 As shown, the lithium niobate-on-insulator thin-film chip used in this type of modulator is formed by antiparallel splicing of two x-cut lithium niobate-on-insulator sheets (1-6 and 1-7, respectively), with the remaining parts being... Figure 2 similar.

[0033] By antiparallel splicing of lithium niobate on two X-cut insulators, it is helpful to achieve group velocity matching conditions and avoid the technological challenges of large-area positing of X-cut lithium niobate. The splicing process can be achieved through... Figure 4 The implementation is shown. In order to achieve a comparable coefficient of thermal expansion, the substrate material used should be the same as that of the lithium niobate on insulator (typically silicon or lithium niobate).

[0034] Example 2

[0035] A method for fabricating a micron-scale crystal thin-film electro-optic modulator includes the following steps: Step 1: Deposit a silicon dioxide buffer layer on the surface of the substrate; Step 2: Optically polish the side of the X-cut insulator crystal film, then splice the two X-cut insulator crystal films with antiparallel optical axes together, and place them on the silicon dioxide buffer layer through crystal bonding technology. Step 3: Fabricate a ridge waveguide on the spliced ​​crystal thin film layer using ultraviolet exposure lithography and etching processes.

[0036] In this embodiment, the etching process is preferably dry etching.

[0037] Comparative Example 1

[0038] It is basically the same as Example 1, except that it is made of two completely parallel lithium niobate thin film layers spliced ​​together, or a complete lithium niobate thin film layer is used to replace the anti-parallel splicing structure of the two x-cut insulators on lithium niobate in Example 1.

[0039] In the electro-optic modulator of this invention, during high-speed modulation, the propagation speed of the light wave is greater than that of the microwave. Starting from the left side of the modulator towards the middle, the light wave gradually leads the microwave. Initially, the light wave is at the microwave peak (positive modulation), but gradually it becomes at the microwave trough (negative modulation), and the modulation effects cancel each other out. At this time, due to the reversal of the optical axis of the latter half of the modulator, the electro-optic coefficient also reverses, and the light wave is at the trough. However, the modulation effect reverses from negative modulation to positive modulation, so the light wave is continuously modulated and reaches the end before falling into negative modulation. Overall, the light wave is always positively modulated, and this modulation rate can achieve twice the effect of the modulation in Comparative Example 1.

[0040] The preferred embodiments of the present invention 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 the present invention without creative effort. Therefore, all technical solutions that can be obtained by those skilled in the art based on the concept of the present invention 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. A micrometer-scale crystal thin-film electro-optic modulator, comprising a crystal thin film layer, a silicon dioxide buffer layer, and a substrate layer stacked sequentially from top to bottom, wherein a ridge waveguide is disposed on the crystal thin film layer, characterized in that, The crystal thin film layer is formed by antiparallel splicing of crystal thin films on two x-cut insulators.

2. The micron-scale crystal thin-film electro-optic modulator according to claim 1, characterized in that, The thickness of the crystalline thin film layer is 2-3 μm, and the thickness of the silicon dioxide buffer layer is 2-5 μm.

3. The micron-scale crystal thin-film electro-optic modulator according to claim 1, characterized in that, The ridge width of the ridge waveguide is equal to the thickness of the crystal thin film layer, and the height of the ridge waveguide is 0.5-0.6 times that of the crystal thin film layer.

4. The micron-scale crystal thin-film electro-optic modulator according to claim 1, characterized in that, The angle between the ridge waveguide sidewall and the crystal thin film layer is 65 degrees.

5. The micron-scale crystal thin-film electro-optic modulator according to claim 1, characterized in that, The modulator ridge waveguide has a cross-sectional height and width that are equal and similar in size to the focusing spot of the lens fiber or the core of the high numerical aperture fiber of the measuring device.

6. The micron-scale crystal thin-film electro-optic modulator according to claim 1, characterized in that, The material of the crystalline thin film is lithium niobate or lithium tantalate.

7. The micron-scale crystal thin-film electro-optic modulator according to claim 1, characterized in that, The substrate material is silicon, lithium niobate, or lithium tantalate.

8. The micron-scale crystal thin-film electro-optic modulator according to claim 7, characterized in that, The substrate material of the crystal thin film layer is the same as the crystal thin film substrate material.

9. The method for fabricating a micron-scale crystal thin-film electro-optic modulator as described in claim 1, characterized in that, Includes the following steps: Step 1: Deposit a silicon dioxide buffer layer on the surface of the substrate layer; Step 2: Optically polish the side of the X-cut insulator crystal film, then splice two X-cut insulator crystal films with antiparallel optical axes together and place them on the silicon dioxide buffer layer using crystal bonding technology. Step 3: Fabricate a ridge waveguide on the spliced ​​crystal thin film layer using ultraviolet exposure lithography and etching processes.

10. The method for fabricating a micron-scale crystal thin-film electro-optic modulator according to claim 9, characterized in that, The etching process is dry etching.

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