Frequency conversion device based on lithium niobate ridge waveguide on insulator and preparation method

Abstract

The embodiment of the invention provides a frequency conversion device based on a lithium niobate ridge waveguide on an insulator and a preparation method, and belongs to the field of integrated optical, nonlinear optical and optoelectronic devices. The device comprises a silicon substrate layer, a silicon dioxide buffer layer, an x-cut lithium niobate film layer and a silicon dioxide cladding from bottom to top, a ridge waveguide with width periodically changing is arranged on the x-cut lithium niobate layer, and light in the waveguide is propagated in the y-axis direction. The waveguide with the width periodically changing is prepared on the lithium niobate film, quasi-phase matching is achieved, and therefore the second harmonic conversion efficiency is greatly improved. By changing the period of the waveguide, second-order nonlinear phase matching conditions under different wavelengths, such as phase matching conditions of second harmonic generation, sum frequency generation and difference frequency generation, can be met. Correspondingly, according to the preparation method, the waveguide structure with the periodically changed width can be formed through a single etching process, and a complex ferroelectric domain inversion step is not needed. The preparation method has the advantages of simple process, controllable morphology and flexible design.

Description

Technical Field

[0001] This application relates to the fields of integrated optics, nonlinear optics and optoelectronic devices, and in particular to a frequency conversion device based on a lithium niobate ridge waveguide on an insulator and its fabrication method. Background Technology

[0002] Nonlinear frequency conversion (such as second harmonic generation, sum-frequency generation, and difference-frequency generation) is a core technology for extending laser wavelengths, realizing all-optical signal processing, and generating quantum light sources. Lithium niobate crystals, due to their extremely large second-order nonlinear coefficient and excellent electro-optic properties, are considered an ideal platform for realizing efficient nonlinear processes.

[0003] In recent years, the "Lithium Niobate on Insulator" (LNOI) technology has been used to fabricate lithium niobate into a thin film with a submicron thickness and combine it with a low refractive index substrate (such as silicon dioxide). This strongly confines the light field in the waveguide, greatly enhancing the interaction between light and matter and laying the foundation for realizing high-efficiency, miniaturized integrated nonlinear devices.

[0004] However, achieving efficient nonlinear frequency conversion in waveguides requires meeting strict phase-matching conditions. Due to material dispersion and waveguide dispersion, the fundamental frequency light and the converted light (such as frequency-doubled light) typically propagate at different phase velocities, resulting in phase mismatch. This causes the conversion efficiency to oscillate periodically during propagation and prevents it from continuously increasing.

[0005] On the thin-film lithium niobate platform, the mainstream technology for achieving quasi-phase matching is periodically polarized lithium niobate. This technology periodically reverses the ferroelectric domains of lithium niobate by applying an external electric field, forming a periodically distributed nonlinear coefficient to compensate for phase mismatch. However, this approach has significant drawbacks: 1) The process is complex, requiring electrode deposition and high-voltage polarization, resulting in a cumbersome and costly process; 2) The domain reversal morphology is difficult to control precisely, easily leading to problems such as sidewall roughness and uneven duty cycle, affecting device performance and yield; 3) Limited by the precision of the polarization electric field, it is difficult to fabricate structures with periods smaller than 4 micrometers, limiting its application in short-wavelength conversion and other scenarios.

[0006] Therefore, there is an urgent need in this field for a lithium niobate thin film nonlinear frequency conversion device and its preparation method that is simpler in process, lower in cost, and can achieve high-precision phase matching. Summary of the Invention

[0007] The main objective of this application is to propose a frequency conversion device and its fabrication method based on a lithium niobate ridge waveguide on an insulator. This aims to overcome the problems of complex processes, high costs, low yields, difficulty in controlling domain structures, and limitations of polarizable material systems in existing nonlinear frequency conversion devices based on periodically polarized lithium niobate.

[0008] To achieve the above objectives, one aspect of this application proposes a frequency conversion device based on a lithium niobate-on-insulator ridge waveguide, comprising: Substrate layer; A silicon dioxide buffer layer located on the substrate layer; An x-cut lithium niobate thin film layer is located on the silicon dioxide buffer layer; A ridge waveguide located on the lithium niobate thin film layer, the width of which varies periodically along the direction of light propagation; The periodic variation of the ridge waveguide width is used to compensate for phase mismatch during nonlinear frequency conversion, thereby achieving quasi-phase matching.

[0009] In some embodiments, the width of the ridge waveguide varies periodically along its length as a sinusoidal, rectangular, or sawtooth pattern.

[0010] In some embodiments, the width of the ridge waveguide varies sinusoidally along its length, and the coordinates of its upper surface boundary satisfy the following relationship:

[0011]

[0012] in, The reference width of the waveguide. The magnitude of the width change. For the period of change, The coordinates are along the direction of light propagation.

[0013] In some embodiments, the The value ranges from 1 μm to 10 μm and is used to compensate for the phase mismatch between the fundamental frequency light and the second-harmonic frequency light.

[0014] In some embodiments, the nonlinear frequency conversion process includes second harmonic generation, sum frequency generation, or difference frequency generation.

[0015] In some embodiments, the total thickness of the lithium niobate thin film layer is 300-600 nm, and the etching depth of the ridge waveguide is 180-300 nm.

[0016] Commonly used LN film thicknesses and etching depths are: ① Film thickness 360 nm, etching depth 180 nm; ② Film thickness 400 nm, etching depth 250 nm; ③ Film thickness 600 nm, etching depth 300 nm.

[0017] In some embodiments, mode field converters are connected to both ends of the ridge waveguide for coupling input light and output light.

[0018] In some embodiments, the mode field converter is an inverted conical end face coupler.

[0019] In some embodiments, a silicon dioxide cladding covering the ridge waveguide is also included.

[0020] To achieve the above objectives, another aspect of this application provides a method for manufacturing the aforementioned frequency conversion device, the method comprising the following steps: A lithium niobate on insulator thin film substrate is provided, comprising a substrate layer, a silicon dioxide buffer layer, and an x-cut lithium niobate thin film layer; A patterned mask is formed on the lithium niobate thin film layer, and the pattern of the mask defines a ridge waveguide structure whose width varies periodically along a preset direction; Using the mask as an etching barrier layer, the exposed lithium niobate film is etched to form the ridge waveguide; Remove the mask.

[0021] In some embodiments, after removing the mask, the method further includes a step of depositing a silicon dioxide cladding layer on the surface of the device.

[0022] In some embodiments, the etching is dry etching, and the ridge waveguide is formed to its full depth in a single etching process.

[0023] In some embodiments, the mask is formed by electron beam lithography.

[0024] Compared with the prior art, this application has the following advantages and beneficial effects: 1) Simple process and low cost: This application abandons the complex domain inversion process and only requires a single etching to form the core structure for quasi-phase matching—a waveguide with periodically varying width. This process is compatible with mature micro-nano fabrication technologies, has a simplified process, high yield, and is very suitable for mass production.

[0025] 2) Precise and controllable structural morphology: The waveguide shape is directly defined by photolithography and etching, avoiding the morphological irregularities (such as sidewall wrinkles and duty cycle fluctuations) generated during domain inversion. The waveguide size and period accuracy are high, and the device performance consistency is good.

[0026] 3) Achieves smaller feature periods: Thanks to high-precision photolithography (such as electron beam lithography), this application can fabricate waveguide structures with periods much smaller than 4 micrometers, which provides the possibility for nonlinear frequency conversion in the visible light band or larger wave vector mismatch scenarios.

[0027] 4) The material system has strong universality: The principle of this application is based on waveguide geometry modulation rather than ferroelectric domain inversion. Therefore, it is not only applicable to lithium niobate, but can also be extended to other high-performance nonlinear optical material platforms such as silicon carbide, gallium nitride, and AlGaAs that do not have ferroelectricity or are not easy to perform domain inversion, and has broad application prospects. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the structure and principle of the frequency conversion device in the embodiments of this application, wherein (a) is a schematic diagram of the structure of the frequency conversion device, (b) is a cross-section of the lithium niobate waveguide on the insulator, (c) is a mathematical expression for the top width of the frequency conversion device, and (d) is a top view of the frequency conversion device and the lens fiber for input and output.

[0029] Figure 2 This is a simulation comparison of the second harmonic generation efficiency as a function of propagation distance in a conventional constant-width ridge waveguide and a ridge waveguide with periodically varying width according to this application.

[0030] Figure 3 This is a schematic diagram of the equivalent refractive index of the fundamental frequency light and the second-harmonic frequency light obtained by simulation in the embodiments of this application.

[0031] Figure 4 This is a graph showing the relationship between the normalized second harmonic conversion efficiency and the waveguide period, obtained from simulations in this application embodiment. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of this application and are not intended to limit it. In the following description, when referring to the accompanying drawings, unless otherwise indicated, the same numbers in different drawings represent the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with those of this application; they are merely examples of apparatuses and methods consistent with some aspects of the embodiments of this application as detailed in the appended claims.

[0033] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing embodiments of this application only and is not intended to limit this application.

[0034] In fields such as integrated photonics and optical communication, achieving efficient nonlinear frequency conversion is crucial for expanding the applications of on-chip photonic systems. Utilizing second-order nonlinear effects, such as second harmonic generation, difference frequency generation, and parametric amplification, functions such as laser frequency conversion, quantum light sources, all-optical signal processing, sensing, and imaging can be realized, possessing significant research value and application prospects. Limited by the operating wavelength and bandwidth of the gain medium, the wavelength range that lasers can directly output is very limited; therefore, nonlinear frequency conversion technology is an important means of obtaining lasers of different wavelengths. In the field of optical communication, the development of big data and artificial intelligence has led to a surge in communication demand, and the capacity of the most commonly used C-band channel is nearing saturation. To meet the ever-increasing data transmission demands, new bands need to be opened. However, redesigning transmitters, receivers, and other devices in new bands is very complex and costly. Using nonlinear frequency conversion technology, C-band signals can be converted to other bands for transmission, while mature C-band devices can still be used at the transmitting and receiving ends, eliminating the need to redesign a new communication system. This has great application potential in expanding channel capacity.

[0035] Among numerous optical materials, lithium niobate has long been considered an ideal platform for realizing the aforementioned functions due to its excellent second-order nonlinear coefficient, wide transparency window, and powerful electro-optic effect. However, traditional bulk lithium niobate crystals suffer from problems such as large optical mode field area and limited light-matter interaction strength, resulting in low nonlinear conversion efficiency and typically requiring high pump power or long interaction distance. This severely restricts its application in miniaturized and integrated photonic devices. In recent years, with breakthroughs in the fabrication and processing technology of "lithium niobate on insulator" thin films, high-performance integrated lithium niobate optical waveguides have become possible. Integrated waveguides confine the optical field area within the subwavelength scale, significantly improving the optical power density, thereby greatly enhancing the light-matter interaction strength and creating conditions for achieving efficient nonlinear processes at low power consumption and centimeter-scale or even millimeter-scale dimensions.

[0036] However, due to material dispersion and waveguide dispersion, the fundamental and harmonic light propagate at different speeds in the waveguide, leading to phase mismatch. Ideally, the fundamental and harmonic light are in phase, and the newly generated harmonic light coherently superimposes with the previously generated harmonic light, resulting in a continuous increase in frequency conversion efficiency with increasing propagation distance. In reality, however, the newly generated harmonic light has a fixed phase difference with the previously generated harmonic light, preventing them from superimposing and causing interference. This weakens the harmonic light, severely limiting the research and application of nonlinear photonics.

[0037] To achieve efficient nonlinear conversion, strict phase-matching conditions must be met. Currently, common phase-matching methods include birefringence phase matching, mode phase matching, and quasi-phase matching. Birefringence phase matching utilizes the birefringence property of materials, where light with different polarization directions has different refractive indices. By selecting the polarization states of the fundamental and harmonic light, their equivalent refractive indices are made the same, thus eliminating phase mismatch. However, birefringence matching is very sensitive to temperature and wavelength changes and cannot utilize the maximum nonlinear coefficient (such as d33 in lithium niobate). In integrated optical waveguides, the intermodal dispersion can be adjusted by designing the waveguide geometry, thereby making the equivalent refractive indices of the fundamental and harmonic light equal in a specific mode. However, this method requires high manufacturing precision; even small deviations in waveguide dimensions can lead to phase-matching failure. Furthermore, mode phase matching typically uses higher-order modes. The different mode field distributions of the fundamental and harmonic light reduce the overlap area of ​​the light participating in frequency conversion, limiting the conversion efficiency. Quasi-phase matching, by introducing a periodic structure into a lithium niobate waveguide, can periodically correct phase errors, effectively utilizing the largest second-order nonlinear coefficient and improving second-harmonic generation efficiency. Currently, quasi-phase matching is widely used in frequency conversion devices. The most commonly used method in quasi-phase matching technology is periodically polarized lithium niobate (PPLN). Lithium niobate is a ferroelectric material that undergoes domain inversion under a strong external electric field. Periodic domain inversion can compensate for phase mismatch, thereby improving frequency conversion efficiency. However, the fabrication process of periodically polarized lithium niobate is very complex, currently employing two methods: polarization followed by etching and etching followed by polarization. Polarization followed by etching leads to different material properties in different polarization regions, resulting in selective etching, sidewall wrinkles, and increased waveguide loss. Etching followed by polarization can solve this problem, but it is difficult to fabricate an ideal rectangular domain structure, the duty cycle is difficult to control, and polarization delamination may occur. Furthermore, both methods involve very complex processes, high costs, low yields, and difficulty in achieving high precision, making it challenging to fabricate structures with a cycle length of less than 4-5 μm.

[0038] In contrast, grating-assisted quasi-phase matching (GA-QPM) achieves quasi-phase matching through periodic variations in the waveguide width (i.e., forming a grating), rather than targeting the nonlinear coefficients themselves, thus eliminating the need for domain inversion. The advantages of this structure include a simple fabrication process requiring only a single etching step, mature technology, high repeatability, and the ability to be fabricated on a large scale at low cost. Furthermore, quasi-phase matching at different wavelengths can be achieved by adjusting the waveguide width, period, or temperature, making it highly practical for various frequency conversion scenarios.

[0039] In view of this, this application provides a frequency conversion device and its fabrication method based on a lithium niobate ridge waveguide on an insulator, in order to solve the problems of complex and costly process (requiring overlay, electrode deposition, domain inversion), low yield, unsatisfactory domain morphology, and difficulty in fabricating small periods of periodic polarized lithium niobate nonlinear frequency conversion devices on lithium niobate thin films.

[0040] like Figure 1 As shown, this embodiment provides a frequency conversion device based on a lithium niobate-on-insulator ridge waveguide. The device, from bottom to top, includes a silicon substrate layer, a silicon dioxide buffer layer, an X-cut lithium niobate thin film layer, and a silicon dioxide cladding layer. The X-cut lithium niobate layer has a ridge waveguide with a periodically varying width, the width of which varies sinusoidally. A schematic diagram of the device is shown below. Figure 1 As shown in (a), the thickness of the lithium niobate film and the amplitude of the waveguide width variation have been exaggerated to make the illustration clearer. The cross-section of the waveguide is shown in Figure (a). Figure 1 As shown in (b), the bottom layer is a 500 μm thick silicon substrate, above which is a 2 μm thick silicon dioxide buffer layer. Above the buffer layer is an X-cut lithium niobate film with a total thickness htotal = 400 nm. The height of the ridge waveguide is h = 250 nm, the thickness of the remaining lithium niobate plate is 150 nm, and the tilt angle of the waveguide sidewalls is 70°. The reference width of the waveguide is... Based on this, the width varies periodically along the propagation direction, the waveguide is symmetrical along the y-axis, and the coordinates of the points on the upper surface satisfy:

[0041] Where A is the amplitude of the waveguide width variation. Representing the period of the waveguide, such as Figure 1 As shown in (c). To achieve second harmonic generation at a wavelength of 1550 nm, the waveguide period was calculated to be 3.258 μm. The fundamental frequency light uses the TE mode, with the main electric field component in the x-direction, utilizing the maximum nonlinear coefficient of x-cut lithium niobate. To achieve fiber-to-chip coupling, a lens fiber with a mode field radius of 4.5 μm is used for input and output. Inverted conical end couplers are placed at both ends of the waveguide to match the mode fields of the lens fiber and the waveguide, as shown in (c). Figure 1 As shown in (d), the inverted conical end coupler has a width of 200 nm at one end to match the mode field of the lens fiber, and a width of 1 μm at the other end. Equal to the waveguide, the length of the inverted conical end face coupler is 500 μm.

[0042] This embodiment also provides a preparation method for preparing the above-mentioned frequency conversion device, comprising the following steps: Step 1: Spin-coat a layer of high-resolution photoresist suitable for electron beam lithography onto the surface of a commercially available x-cut lithium niobate thin film (400 nm thick) on an insulator. Then, according to the preset waveguide structure, perform high-precision exposure on the photoresist to define the mask pattern required for etching on the photoresist.

[0043] Step 2: After electron beam exposure, the photoresist layer undergoes development. A developing solution is used to selectively remove the photoresist from the unexposed areas, exposing the underlying lithium niobate film surface and forming a mask pattern that highly matches the design.

[0044] Step 3: Inductively Coupled Plasma Etching (ICP-RIE) is used to transfer the photoresist mask pattern formed in Step 2 to the underlying lithium niobate thin film layer. Process parameters and etching time are precisely controlled to ensure the etching depth reaches the target value of 250 nm, forming a ridge waveguide structure. The LN ridge waveguide typically has a sidewall tilt angle of 60-80°; in this embodiment, the tilt angle is 70°.

[0045] Step 4: After etching is completed, use oxygen plasma cleaning process to thoroughly remove the photoresist remaining on the surface of the lithium niobate waveguide to ensure the cleanliness of the waveguide surface.

[0046] Step 5: Use plasma-enhanced chemical vapor deposition (PECVD) to uniformly deposit a 2 μm thick silicon dioxide cladding layer on the device surface.

[0047] The solutions of the embodiments of this application will be described in detail and explained below with reference to specific application examples.

[0048] This embodiment compensates for phase mismatch during nonlinear frequency conversion by periodically varying the waveguide width, thereby improving frequency conversion efficiency. By changing the waveguide's width, period, thickness, and other geometric parameters, different phase matching conditions can be met, making it widely applicable in various wavelengths and scenarios.

[0049] The grating-assisted quasi-phase-matching method has less stringent manufacturing requirements and is suitable for materials that cannot undergo domain inversion. Compared to the commonly used PPLN structure, the fabrication process of this application requires only one etching step, eliminating the need for periodic polarization electrode fabrication. This simplifies the process, reduces costs, and makes it suitable for large-scale fabrication. Furthermore, the structure used in this application features a periodically varying waveguide width, allowing for a smaller period compared to PPLN, enabling applications in a wider range of scenarios—something currently impossible with PPLN technology. This invention improves the nonlinear frequency conversion efficiency in LNOI, reduces manufacturing costs, and offers highly practical solutions with significant application prospects in multi-wavelength lasers, optical frequency combs, optical communication, quantum light sources, and parametric amplification.

[0050] Figure 2 Simulation results are provided for the second harmonic generation efficiency as a function of propagation length for a conventional ridge waveguide and a ridge waveguide with periodically varying width. The width of the conventional waveguide and the reference width of the periodically varying width waveguide are also shown. All are 1 μm in diameter, and all have an etching depth of 250 nm. From Figure 2 As can be seen, in a conventional waveguide, due to phase mismatch, the phase difference between the second harmonic generated at the fundamental frequency and the already generated second harmonic accumulates with the propagation distance y. When π is reached, the newly generated second harmonic will be out of phase with the previous second harmonic, causing the energy to no longer interfere constructively but destructively. This results in the second harmonic intensity no longer continuously increasing with the propagation distance y, but instead exhibiting periodic oscillations along the propagation direction. In the ridge waveguide with a periodically varying width proposed in this invention, the periodic structure is equivalent to introducing a reciprocal lattice vector. By selecting an appropriate period, phase mismatch is compensated, thereby greatly improving the nonlinear frequency conversion efficiency.

[0051] To achieve nonlinear frequency conversion efficiency, the distance between the light and the matter needs to be increased. However, in ordinary waveguides, due to phase mismatch, the conversion efficiency does not increase with the propagation distance. This is caused by the phase mismatch between the fundamental and harmonic light. According to Maxwell's equations and the second-order nonlinear coupled-wave equation in the waveguide, for the second harmonic generation process, the field distribution of the harmonic and fundamental light propagating along the y-axis is as follows: (1) (2) in and The field distributions for the frequency-doubled light and the fundamental light are shown respectively. The frequency of the fundamental light. and These are the wave vectors of the frequency-doubled light and the fundamental light, respectively. Let be the second-order nonlinear polarizability tensor of the material. This represents phase mismatch in the waveguide. In a 1 μm wide lithium niobate integrated optical waveguide, the field distributions of the fundamental and harmonic frequencies are as follows: Figure 3 As shown.

[0052] Because the equivalent refractive indices of the fundamental frequency and its harmonics are not the same, phase mismatch occurs, severely limiting the nonlinear frequency conversion efficiency in the waveguide. Figure 2 As shown.

[0053] In a conventional waveguide, the waveguide cross section remains constant along the propagation direction, i.e., in equations (1) and (2). , , All are constant values, and the solution is... It oscillates periodically and its amplitude does not increase with the increase of the propagation distance y. However, in the ridge waveguide with periodically varying width proposed in this application, , , , Both are affected by the waveguide cross-sectional shape, exhibiting periodic changes with the propagation distance y. Since a periodic function can be written as an exponential function... In the form of, The value of is related to the period and the waveguide cross-section shape. Choosing an appropriate period allows for... Compensation for phase mismatch This causes the conversion efficiency to increase with the propagation distance y. The waveguide period, given the fundamental wavelength, can be obtained through parameter scanning.

[0054] like Figure 4 As shown, the normalized conversion efficiency of this application at different wavelengths was simulated for different periods, with a simulated propagation length of 2 mm. The simulation results demonstrate that the method employed in this application can effectively compensate for phase mismatch in the waveguide, and the phase matching conditions at different wavelengths can be satisfied by changing the waveguide period, thereby achieving control over the wavelength of the nonlinear frequency conversion.

[0055] In summary, compared with the prior art, this application has at least the following advantages and beneficial effects: 1) Due to the use of a waveguide structure with a periodically varying width, quasi-phase matching can be performed without domain inversion. The device fabrication process only requires one etching step, resulting in advantages such as simple process and low cost.

[0056] 2) Due to the use of a waveguide structure with a periodically varying width, the morphology of the device is easier to control compared to periodically polarized lithium niobate (PPLN), resulting in a higher yield.

[0057] This application improves frequency conversion efficiency by periodically varying the waveguide width to compensate for phase mismatch during nonlinear frequency conversion. By changing the waveguide's width, period, thickness, and other geometric parameters, different phase-matching conditions can be met, making it widely applicable in various wavelengths and scenarios.

[0058] The grating-assisted quasi-phase-matching method leverages the concept and advantages of periodically polarized lithium niobate (LNOI), offering more relaxed manufacturing requirements and applicability to materials where domain inversion is not feasible. Compared to the commonly used PPLN structure, the fabrication process in this application requires only one etching step, eliminating the need for periodic polarization electrode fabrication. This simplifies the process, reduces costs, and makes it suitable for large-scale fabrication. Furthermore, the structure employed in this application features a periodically varying waveguide width, allowing for a smaller period compared to PPLN. This enables phase matching in shorter wavelength applications, a feat currently impossible with PPLN technology. This application improves the nonlinear frequency conversion efficiency in LNOI, reduces manufacturing costs, and demonstrates strong practicality, showing significant application potential in multi-wavelength lasers, optical frequency combs, optical communication, quantum light sources, and parametric amplification.

[0059] The above description is merely a preferred embodiment of this application, but the scope of protection of this application is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the technical scope disclosed in this invention, based on the technical solution and inventive concept of this application, should be included within the scope of protection of this application.

Claims

1. A frequency conversion device based on a lithium niobate-on-insulator ridge waveguide, characterized in that, include: Substrate layer; A silicon dioxide buffer layer located on the substrate layer; An x-cut lithium niobate thin film layer is located on the silicon dioxide buffer layer; A ridge waveguide located on the lithium niobate thin film layer, the width of which varies periodically along the direction of light propagation; The periodic variation of the ridge waveguide width is used to compensate for phase mismatch during nonlinear frequency conversion, thereby achieving quasi-phase matching.

2. The frequency conversion device according to claim 1, characterized in that, The width of the ridge waveguide varies periodically along its length, either sinusoidally, rectangularly, or in a sawtooth pattern.

3. The frequency conversion device according to claim 1 or 2, characterized in that, The width of the ridge waveguide varies sinusoidally along its length, and the coordinates of its upper surface boundary satisfy the following relationship: in, The reference width of the waveguide. The magnitude of the width change. For the period of change, The coordinates are along the direction of light propagation.

4. The frequency conversion device according to claim 3, characterized in that, The The value ranges from 1 μm to 10 μm and is used to compensate for the phase mismatch between the fundamental frequency light and the second-harmonic frequency light.

5. The frequency conversion device according to claim 1, characterized in that, The total thickness of the lithium niobate thin film layer is 300-600 nm, and the etching depth of the ridge waveguide is 180-300 nm.

6. The frequency conversion device according to claim 1, characterized in that, The two ends of the ridge waveguide are connected to mode field converters for coupling input light and output light.

7. The frequency conversion device according to claim 6, characterized in that, The mode field converter is an inverted conical end face coupler.

8. The frequency conversion device according to claim 1, characterized in that, It also includes a silicon dioxide cladding covering the ridge waveguide.

9. A method for manufacturing a frequency conversion device, used to manufacture the frequency conversion device according to any one of claims 1-8, characterized in that, Includes the following steps: A lithium niobate on insulator thin film substrate is provided, comprising a substrate layer, a silicon dioxide buffer layer, and an x-cut lithium niobate thin film layer; A patterned mask is formed on the lithium niobate thin film layer, and the pattern of the mask defines a ridge waveguide structure whose width varies periodically along a preset direction; Using the mask as an etching barrier layer, the exposed lithium niobate film is etched to form the ridge waveguide; Remove the mask.

10. The method according to claim 9, characterized in that, After removing the mask, the process further includes a step of depositing a silicon dioxide cladding layer on the surface of the device.

Images