A method for preparing a ridge waveguide for quasi-phase matching and the ridge waveguide obtained thereby

Abstract

The application provides a preparation method of a ridge waveguide achieving quasi-phase matching and the obtained ridge waveguide, wherein the ridge waveguide is asymmetric in configuration, polarization is carried out along the straight line direction of the center of the waveguide, the polarization intensity on the left side of the ridge waveguide is opposite to that on the right side, and quasi-phase matching is achieved through cross integration of TEM00 mode and TEM10 mode. The method of the application avoids high-difficulty ridge waveguide waveguide period polarization process, reduces the number of processes and operation difficulty in electrode patterning, high-voltage polarization and other steps, helps to improve the overall process efficiency, shorten the production cycle and reduce the manufacturing cost.

Description

Technical Field

[0001] This invention belongs to the field of optical design, specifically relating to a method for fabricating a ridge waveguide that achieves quasi-phase matching and the resulting ridge waveguide. Background Technology

[0002] Quasi-phase matching refers to the process in nonlinear optics where the natural phases of two light sources are mismatched through periodic polarization, achieving frequency doubling by designing periodic polarization parameters. Ridge waveguides on lithium niobate are the preferred structure for nonlinear frequency conversion. Ridge waveguides are formed by ion-implanted and stripped thin films of lithium niobate, followed by etching using microfabrication techniques. Because this fabrication process can achieve micrometer-scale ridge waveguides with extremely strong light field confinement capabilities, high power densities can be achieved, making efficient nonlinear processes possible.

[0003] However, traditional periodic polarization processes typically involve complex electrode pattern design and high-voltage application, resulting in significant challenges in process control and limitations on device consistency. Currently, most quasi-phase-matched crystals employ electric field polarization for domain inversion, with domain widths ranging from sub-micrometers to tens of micrometers depending on the wavelength required for compensation. According to the paper "Efficient photon-pairgeneration in layer-poled lithium niobate nanophotonic waveguides" (Shi, Xiaodong, et al. Light: Science & Applications 13.1 (2024): 282.), ridge waveguides can achieve frequency doubling quasi-phase matching and improved conversion efficiency through layered polarization. While this polarization method can achieve effective quasi-phase matching, the vertical polarization layering makes polarization very difficult.

[0004] Therefore, there is an urgent need for a new ridge waveguide that can achieve quasi-phase matching, making polarization easier. Summary of the Invention

[0005] Purpose of the invention: To address the problem of polarization difficulties in existing ridge waveguides that achieve quasi-phase matching, this invention proposes a new ridge waveguide that reduces polarization difficulty, achieves quasi-phase matching, and improves the conversion efficiency of frequency doubling.

[0006] Technical solution:

[0007] A method for fabricating a ridge waveguide achieving quasi-phase matching, comprising:

[0008] Set the fundamental wavelength λ, initialize the width and height of the protruding portion of the ridge waveguide, and the initialized cross-section of the protruding portion is rectangular;

[0009] Cross-integration calculations are performed on the TEM00 and TEM10 modes inside the initialized ridge waveguide to obtain the frequency doubling efficiency and nonlinear coupling coefficient. Based on the calculation results, the updated height and width are selected. The above steps are repeated until the updated height and width converge to obtain the optimal width W and optimal height H of the protrusion.

[0010] Electrode patterns are fabricated on the initial raised portion using a metal coating etching method. A high-voltage pulsed electric field is applied to cause polarization reversal in the ferroelectric material, forming positive and negative domains with opposite polarization directions.

[0011] The cross-sectional shape of the waveguide is selected, and the ridge portion of the ridge waveguide is etched according to the optimal width W and optimal height H of the ridge portion to obtain an asymmetric ridge waveguide.

[0012] Furthermore, let the polarization direction of the laser propagating in the ridge waveguide be the Z-axis, the domain polarization of the positive domains be along the +Z-axis, the domain polarization of the negative domains be along the -Z-axis, the X-axis be perpendicular to the Z-axis and both be in the same cross-sectional plane, and the Y-axis be the extension direction of the ridge waveguide.

[0013] Furthermore, the calculation of the frequency doubling efficiency includes: setting the TEM00 mode as the fundamental light in the frequency doubling process, and the TEM10 mode as the frequency-doubled light in the frequency doubling process, and calculating the frequency doubling efficiency. for:

[0014]

[0015] in, The intensity of the frequency-doubled optical polarization wave. The intensity of the fundamental polarization wave. The waveguide length is... Let be the vacuum permittivity, and c be the speed of light in vacuum. The refractive index of the fundamental wave, The refractive index of the frequency-doubled light. The fundamental wavelength, For effective nonlinear coefficients, is the nonlinear coupling coefficient.

[0016] Furthermore, the calculation of the nonlinear coupling coefficient includes:

[0017]

[0018] In this context, the subscript LN of the integral sign indicates integration only over the waveguide region. It is a polarization symbol related to coordinates. It is the conjugate of the complex electric field of the fundamental wave light. It is the complex electric field of the frequency-doubled light. The subscript "all" in the integral sign refers to the integration over all intensities in the waveguide region and the substrate. It is the complex electric field of the fundamental wave light.

[0019] Furthermore, one side of the electrode pattern is a semi-metallic electrode with a straight edge, and the other side is a liquid electrode. The protruding part of the ferroelectric material is polarized using a room temperature electric field polarization method, so that half of the protruding part is a positive domain and the other half is a negative domain with the center line as the boundary. After polarization, the boundary and junction of the positive and negative domains are photolithographically etched.

[0020] Furthermore, the metal electrode is removed by wet etching after etching is completed.

[0021] Furthermore, the etching of the protruding portion of the ridge waveguide is performed using an inclined electron beam etching method.

[0022] A ridge waveguide prepared by the method described above includes a waveguide region and a substrate;

[0023] The waveguide region is located above the substrate and has a protrusion that is higher than the surrounding region.

[0024] The protruding portion is divided into positive domains and negative domains, wherein the domain polarization directions of the positive domains and negative domains are opposite;

[0025] The positive domain has a rectangular cross-section, and the negative domain has a different cross-sectional shape than the positive domain, with the height and area of ​​the negative domain's cross-section not exceeding those of the positive domain.

[0026] Furthermore, the waveguide region is made of ferroelectric material, and the substrate material includes silicon, silicon dioxide, sapphire, and silicon nitride.

[0027] Furthermore, the shape of the negative domain cross section includes a triangle with a base equal to the width of the positive domain cross section, a quarter circle with a radius equal to the width of the positive domain cross section, and a chamfered rectangle with a width and height equal to the positive domain cross section.

[0028] Beneficial effects: The ridge waveguide and its fabrication method proposed in this invention have the following advantages compared with the prior art:

[0029] Reduced process complexity and cost: The method of this invention avoids the difficult ridge waveguide periodic polarization process, reduces the number of steps and the difficulty of operation in electrode patterning, high voltage polarization and other steps, which helps to improve the overall process efficiency, shorten the production cycle and reduce manufacturing costs.

[0030] Improving the quality stability of polarization domains: This invention significantly reduces defects such as distorted domains and domain merging by asymmetric design of the ridge waveguide configuration, pre-calculating the optimal geometric parameters, and optimizing control, thereby improving the periodicity and boundary clarity of the domain structure.

[0031] Facilitating large-scale integration and application expansion: On the integrated optical chip platform, the simplified polarization process is more compatible with other micro-nano photonic structures, providing a feasible basis for the application of nonlinear optical devices in mass production and system integration. Attached Figure Description

[0032] Figure 1 This is a cross-sectional view of the ridge waveguide structure of the present invention;

[0033] Figure 2 This is a three-dimensional view of the ridge waveguide of the present invention;

[0034] Figure 3 The modes of the ridge waveguide of the present invention are shown, where a represents the TEM00 mode and b represents the TEM10 mode.

[0035] Figure 4 The integral result of the mode calculation in the embodiment of the present invention. Detailed Implementation

[0036] The present invention will be further explained below with reference to the accompanying drawings and specific embodiments.

[0037] The ridge waveguide of the present invention has a left-right asymmetrical structure. The asymmetrical structure includes left-right asymmetry in height and left-right asymmetry in shape. The asymmetrical shape includes: a right half with a rectangular cross-section and a left half with a triangular cross-section; a right half with a rectangular cross-section and a quarter-circle cross-section; and a right half with a rectangular cross-section and a chamfered rectangle cross-section. In the embodiments of the present invention, the preferred shape is as follows: Figure 1 The shape shown.

[0038] The ridge waveguide of this invention is made of ferroelectric materials, including lithium niobate (LiNbO3), lithium tantalate (LiTaO3), potassium titanium oxyphosphate (KTiOPO4, KTP), etc., with lithium niobate being the preferred material. The substrate material includes silicon, silicon dioxide, sapphire, silicon nitride, etc. In this embodiment, thin-film lithium niobate is used as the main material for fabricating the waveguide, and silicon dioxide is used as the substrate.

[0039] The relationship between the three-dimensional view of the ridge waveguide in this embodiment and the coordinate axes is as follows: Figure 2 As shown. The crystal axis (Z-axis) of the ridge waveguide is perpendicular to the X-axis and lies within the cross-sectional plane. The waveguide extends along the Y-direction. The electric field along the Z-axis can change the domain polarization direction; domains polarized along the +Z-axis are called positive domains, and those polarized along the -Z-axis are called negative domains.

[0040] Quasi-phase matching of the ridge waveguide can be achieved through the aforementioned asymmetric shape and the polarization methods of its positive and negative domains. The laser propagating in the ridge waveguide has a Z-axis polarization direction and multiple modes, including a fundamental mode (TEM00) and a higher-order mode (TEM10), such as... Figure 3 As shown, a represents the TEM00 mode and b represents the TEM10 mode. Due to the asymmetry of the ridge waveguide pattern, the above modes also exhibit asymmetry. This asymmetry, along with the difference in left and right polarization directions, enables the TEM00 and TEM10 modes to achieve effective frequency doubling.

[0041] The TEM00 mode is set as the fundamental light in the frequency doubling process, and the TEM10 mode is set as the frequency-doubled light in the frequency doubling process, with frequency doubling efficiency. :

[0042]

[0043] in, The intensity of the frequency-doubled optical polarization wave. The intensity of the fundamental polarization wave. The waveguide length is... Let be the vacuum permittivity, and c be the speed of light in vacuum. The refractive index of the fundamental wave, The refractive index of the frequency-doubled light. The fundamental wavelength, For effective nonlinear coefficients, is the nonlinear coupling coefficient.

[0044]

[0045] In this context, the subscript LN of the integral sign indicates integration only over the waveguide region. It is a polarization symbol related to coordinates. It is the conjugate of the complex electric field of the fundamental wave light. It is the complex electric field of the frequency-doubled light. The subscript "all" in the integral sign refers to the integration over all intensities in the waveguide region and the substrate. It is the complex electric field of the fundamental wave light.

[0046] Based on the above integral calculation theory, the geometric parameters of the ridge waveguide are determined in the following manner:

[0047] (1) Let the wavelength of the fundamental wave be... The wavelength is 1550nm. The width and height of the ridge waveguide are initialized according to the common parameters of the ridge waveguide. Due to the thickness of the thin lithium niobate film, which is about 0.2μm-10μm, the width and height are initialized to 1μm in this embodiment.

[0048] (2) Determine the cross-sectional shape of the waveguide. The cross-sectional shape in this embodiment is as follows: Figure 1 As shown.

[0049] (3) Based on the wavelength of the fundamental light Using the geometric parameters of the ridge waveguide determined in the preceding steps, the TEM00 and TEM10 modes within the ridge waveguide are calculated.

[0050] (4) Based on the calculation results of step (3), the nonlinear coupling coefficient is integrally calculated to obtain the optimal width W and height H. In this embodiment, H = 1 μm is fixed, and the relationship between the width W and the nonlinear coupling coefficient is calculated as follows: Figure 4 Therefore, the optimal width is W = 1.2 μm.

[0051] (5) Reset W=1.2μm, keep other parameters unchanged, and repeat the above steps until W converges to a certain value.

[0052] In this embodiment, the fabrication process involves creating a semi-metallic electrode with straight edges on a 1 μm thick z-cut lithium niobate film using metal deposition and etching, with a liquid electrode on the other side. The lithium niobate film is then polarized using a room-temperature electric field polarization method, forming a polarization pattern with half positive domains and half negative domains, using the center line of the raised portion as the boundary. The metal electrode is then removed using a wet etching method.

[0053] With the aid of an electron microscope, the above-mentioned thin film lithium niobate was photolithographically etched along the straight line at the boundary between positive and negative domains on both sides of a strip with a width of W=1.1μm and a length of 10mm, with an etching depth of 1μm.

[0054] A corner of the rectangular waveguide is chamfered using a tilted electron beam etching method to form the cross-sectional shape as described above. The aforementioned ridged waveguide is then coupled with light to input... The fundamental wave light is used to obtain a frequency-doubled output.

Claims

1. A method for fabricating a ridge waveguide achieving quasi-phase matching, characterized in that, include: Step 1: Set the fundamental wavelength λ, and initialize the width and height of the ridge waveguide's protrusion. The initialized protrusion has a rectangular cross-section. Step 2: Perform cross-integration calculations on the TEM00 and TEM10 modes inside the ridge waveguide to obtain the frequency doubling efficiency and nonlinear coupling coefficient. Select the updated height and updated width based on the calculation results. Step 3: Repeat step 2 until the updated height and updated width converge to obtain the optimal width W and optimal height H of the protrusion. Step 4: Electrode patterns are fabricated on the initial raised portion using metal coating etching. A high-voltage pulsed electric field is applied to cause polarization reversal in the ferroelectric material, forming positive and negative domains with opposite polarization directions. Step 5: Select the cross-sectional shape of the waveguide. Based on the optimal width W and optimal height H of the protrusion, etch the protrusion of the ridge waveguide to obtain a ridge waveguide with an asymmetrical configuration. The polarization direction of the laser transmitted in the ridge waveguide is the Z-axis. The domain polarization of the positive domains is along the +Z-axis, and the domain polarization of the negative domains is along the -Z-axis. The X-axis is perpendicular to the Z-axis and both are in the same cross-sectional plane. The Y-axis is the extension direction of the ridge waveguide.

2. The preparation method according to claim 1, characterized in that, The calculation of frequency doubling efficiency includes: setting the TEM00 mode as the fundamental light in the frequency doubling process, and the TEM10 mode as the frequency-doubled light in the frequency doubling process, and calculating the frequency doubling efficiency. for: in, The intensity of the frequency-doubled optical polarization wave. The intensity of the fundamental polarization wave. The waveguide length is... Let be the vacuum permittivity, and c be the speed of light in vacuum. The refractive index of the fundamental wave, For frequency-doubled refractive index, The fundamental wavelength, For effective nonlinear coefficients, is the nonlinear coupling coefficient.

3. The preparation method according to claim 2, characterized in that, The calculation of the nonlinear coupling coefficient includes: In this context, the subscript LN of the integral sign indicates integration only over the waveguide region. It is a polarization symbol related to coordinates. It is the conjugate of the complex electric field of the fundamental wave light. It is the complex electric field of the frequency-doubled light. The subscript "all" in the integral sign refers to the integration over all intensities in the waveguide region and the substrate. It is the complex electric field of the fundamental wave light.

4. The preparation method according to claim 3, characterized in that, The electrode pattern has a semi-metallic electrode with a straight edge on one side and a liquid electrode on the other side. The protruding part of the ferroelectric material is polarized by a room temperature electric field polarization method, so that half of the protruding part is a positive domain and the other half is a negative domain with the center line as the boundary. After polarization, the boundary and junction of the positive and negative domains are photolithographically etched.

5. The preparation method according to claim 4, characterized in that, The metal electrode is removed by wet etching after etching is completed.

6. The preparation method according to claim 5, characterized in that, The protruding portion of the ridge waveguide is etched using an inclined electron beam etching method.

7. A ridge waveguide obtained by the fabrication method as described in claim 1, characterized in that, Including the waveguide region and the substrate; The waveguide region is located above the substrate, and a protrusion is provided on the waveguide region; The raised portion is divided into positive domains and negative domains, and the domain polarization directions of the positive domains and negative domains are opposite. The positive domain has a rectangular cross-section, and the negative domain has a different cross-sectional shape than the positive domain, with the height and area of ​​the negative domain's cross-section not exceeding those of the positive domain.

8. The ridge waveguide according to claim 7, characterized in that, The waveguide region is made of ferroelectric material, and the substrate material includes silicon, silicon dioxide, sapphire, and silicon nitride.

9. The ridge waveguide according to claim 8, characterized in that, The shape of the negative domain cross section includes a triangle with a base equal to the width of the positive domain cross section rectangle, a quarter circle with a radius equal to the width of the positive domain cross section rectangle, and a chamfered rectangle with the same width and height as the positive domain cross section rectangle.

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