Magnesium oxide doped film lithium niobate microcavity optical comb generation method

Abstract

The invention belongs to the technical field of integrated photonics and nonlinear optics, and particularly relates to a magnesium oxide doped film lithium niobate microcavity optical comb generation method. Firstly, a film lithium niobate material doped with 5 mol.% of magnesium oxide is adopted, the optical damage threshold of the film lithium niobate material is 130 times that of pure lithium niobate, and the high pumping power bearing capacity and the long-term operation stability of the microcavity are remarkably improved. Secondly, the Kerr soliton optical comb is generated in the two microcavity structures of the air cladding and the silicon dioxide in combination with the third-order nonlinearity of the material and dispersion optimization of the microcavity structures. The silicon dioxide cladding microcavity provides structural support for integration of functional units such as a subsequent thermal tuning electrode. Besides, the invention also verifies that the micro-cavity based on the material platform has a weak photorefractive effect and has the capability of generating soliton optical combs under low pumping power, which indicates that the micro-cavity has good engineering realizability and application potential.

Description

Technical Field

[0001] This invention belongs to the fields of integrated photonics and nonlinear optics, specifically relating to a method for generating a magnesium oxide-doped thin-film lithium niobate microcavity optical comb. Background Technology

[0002] Optical frequency combs (or simply "optical combs") are coherent light sources that exhibit discrete, equally spaced comb-shaped spectral lines in the frequency domain. They have become a key tool for high-precision optical frequency measurement and have wide applications in precision metrology, optical atomic clocks, quantum information processing, and low-noise microwave synthesis. Kerr soliton optical combs based on optical microcavities have become one of the most promising light sources in integrated photonics due to their high repetition rate, compact size, and ease of integration. Currently, soliton microcavity optical combs have been realized on various material platforms, including silicon nitride (Si3N4), aluminum nitride (AlN), thin-film lithium niobate (LN), thin-film lithium tantalate (LT), silicon carbide (SiC), and gallium nitride (GaN). Among these materials, thin-film lithium niobate shows great potential for constructing high-performance, multifunctional integrated optical comb generation platforms due to its excellent electro-optic properties, second- and third-order nonlinear characteristics, and broadband transparent window.

[0003] However, pure thin-film lithium niobate materials still face significant challenges in practical applications. First, the low photodamage threshold of pure LN materials makes it difficult for microcavities to maintain long-term operational stability under high-power pumping. Second, existing methods for generating thin-film lithium niobate microcavities based on silica (SiO2) cladding are limited by interfacial absorption losses, leading to… Q The values ​​are usually low, making it difficult to meet the low nonlinear threshold conditions required to generate soliton microcavity optical combs. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a method for generating a magnesium oxide-doped thin-film lithium niobate microcavity optical comb.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows: This invention provides a method for generating a magnesium oxide-doped thin-film lithium niobate microcavity optical comb, comprising: optimizing the cross-sectional geometry of the microcavity to exhibit anomalous dispersion characteristics within a selected pump wavelength range; fabricating the microcavity on a 5 mol.% magnesium oxide-doped thin-film lithium niobate substrate according to the determined cross-sectional geometry, to provide a nonlinear gain environment for exciting a Kerr soliton optical comb; performing frequency sweeping operations on the pump laser in different scanning directions, monitoring the transmission spectral changes of the microcavity in real time until the soliton step signal is captured; and finely tuning the wavelength of the pump laser to guide the pump wavelength to stably enter the effective width range of the soliton step, thereby exciting and generating a stable microcavity soliton optical comb.

[0006] The magnesium oxide-doped lithium niobate thin film substrate with a doping concentration of 5 mol.% described in this invention consists of, from bottom to top, a silicon substrate with a thickness of 525 μm, a silicon dioxide layer with a thickness of 2 μm, and a 5 mol.% magnesium oxide-doped z-cut lithium niobate thin film with a thickness of 800 nm.

[0007] Furthermore, the microcavity is a first microcavity or a second microcavity; the surface of the first microcavity has an air cladding structure; the surface of the second microcavity has a silicon dioxide cladding structure.

[0008] In this invention, the method for preparing the microcavity is as follows: First, using electron beam lithography and ARN7520.17 negative photoresist, the geometry of the microcavity was precisely defined on the surface of a 5 mol.% magnesium oxide-doped lithium niobate thin film substrate. Then, using argon ion beam etching, the photoresist microcavity pattern was transferred to the 5 mol.% magnesium oxide-doped lithium niobate thin film layer to form a microcavity waveguide structure. After etching, the chip underwent wet processing to remove residual photoresist and re-deposited products. The resulting microcavity waveguide structure without cladding is the first microcavity with an air cladding structure. Furthermore, using plasma-enhanced chemical vapor deposition, a 1 μm thick silicon dioxide cladding was uniformly deposited on the surface of the microcavity waveguide structure, forming a second microcavity with a silicon dioxide cladding structure. Finally, the prepared first and second microcavity chips were annealed at 500°C for 3 hours to repair etching damage and reduce scattering loss. After annealing, the chip underwent cleaving to form a flat waveguide coupling end face.

[0009] This invention optimizes the geometric parameters of the microcavity cross-section, specifically by coordinating and optimizing the width and radius of the microcavity waveguide cross-section. Experimental testing and verification were conducted in the C-band (1530nm~1565nm) to determine the intrinsic properties of the first microcavity. Q The value is 5.5 × 10 6 The free spectral range (FSR) is approximately 203 GHz, and the second-order dispersion parameter D² / (2π) is approximately 1.18 MHz; the intrinsic characteristics of the second microcavity... Q The value is 1.4 × 10 6 The FSR is approximately 339 GHz, and D² / (2π) is approximately 3.67 MHz. Both structures exhibit significant anomalous dispersion characteristics, satisfying the conditions for microcavity soliton optical comb generation.

[0010] Furthermore, the cross-sectional geometric parameters of the first microcavity are: waveguide width of 2.8 μm and microcavity radius of 100 μm.

[0011] Furthermore, the cross-sectional geometric parameters of the second microcavity are: waveguide width of 1.7 μm and microcavity radius of 60 μm.

[0012] Furthermore, performing frequency sweeping operations on the pump laser in different scanning directions specifically includes: First, gradually increasing the pump power and scanning the pump laser frequency in both the forward and reverse directions near the microcavity resonant wavelength until a soliton step is observed in the transmission power curve; Second, at a fixed pump power corresponding to the soliton step, finely tuning the wavelength of the pump laser to guide the pump wavelength into the detuning range corresponding to the soliton step, so as to excite and generate a microcavity soliton optical comb; Finally, reducing the pump power to a preset value, tuning the pump wavelength, and exciting a soliton crystal optical comb with specific repetition frequency distribution characteristics.

[0013] Furthermore, the fixed pump power corresponding to the soliton step satisfies the following condition: ;in, P in Pump power; c The speed of light; A eff The effective model area; β 2 represents group velocity dispersion; η The coupling efficiency from the waveguide to the microcavity; n 2 represents the third-order nonlinear coefficient; τ The pulse width; Q To load the quality factor; D 1 represents the angular frequency interval corresponding to the free spectral range; β The formula for calculating 2 is: ; η The calculation formula is: ; D The formula for calculating 1 is: ;in, n The refractive index of the material; D 2 represents the second-order dispersion coefficient; Q 0 represents the intrinsic quality factor; FSR It is the free spectral range.

[0014] Furthermore, during the bidirectional frequency sweep of the first microcavity with increasing or decreasing frequency, a single soliton optical frequency comb is generated at an on-chip power of 35mW; the on-chip pump power is reduced to 18mW, the pump wavelength is tuned, and a soliton crystal optical comb with specific repetition frequency distribution characteristics is excited.

[0015] Furthermore, during the unidirectional frequency sweep process of the second microcavity with increasing frequency, a single soliton optical frequency comb is generated at an on-chip power of 79mW; the on-chip pump power is reduced to 20mW, the pump wavelength is tuned, and a soliton crystal optical comb with specific repetition frequency distribution characteristics is excited.

[0016] This invention enables the generation of microcavity soliton optical combs under high pump power and the generation of soliton crystals under low pump power. Experimental results demonstrate the significant advantages of the technical solution provided by this invention in low-power soliton generation, requiring only ≤20mW of on-chip pump power to generate soliton crystals, which facilitates more compact system integration.

[0017] The beneficial effects of this invention are: 1. This invention provides a method for generating a magnesium oxide-doped thin-film lithium niobate microcavity optical comb. Utilizing the third-order nonlinear characteristics of 5 mol.% magnesium oxide-doped thin-film lithium niobate material, combined with optimized microcavity structure design and fabrication process, anomalous dispersion and high quality factor are achieved, and the stable generation of Kerr soliton optical combs is realized within the microcavity structure.

[0018] 2. This invention utilizes 5 mol.% magnesium oxide-doped thin-film lithium niobate material to significantly improve the optical damage threshold of the microcavity, enabling the microcavity to maintain long-term stable operation under high pump power and improving the practicality of the device in precision measurement and other fields.

[0019] 3. This invention utilizes 5 mol.% magnesium oxide-doped thin-film lithium niobate material to not only excite and obtain soliton microcavity optical combs at higher pump power, but also to generate soliton crystals at lower pump power, which is beneficial for reducing system power consumption and promoting more compact system-level integration.

[0020] 4. This invention achieves stable generation of Kerr soliton optical combs in both air-clad and silica microcavity structures. The silica-clad microcavity will provide structural support for the subsequent integration of functional units such as thermally tunable electrodes. Attached Figure Description

[0021] Figure 1 The image shows the soliton optical comb spectrum of the silicon nitride microring resonator in the existing scheme 1. A, B, C, and D are the soliton optical comb spectra at repetition frequencies of 50 GHz, 100 GHz, 150 GHz, and 200 GHz, respectively. f rep Indicates the repetition frequency.

[0022] Figure 2 The test results of photorefractive effect in the pure lithium niobate microring resonator in the existing scheme 2 are presented, mainly showing and comparing the resonance curves obtained under different incident powers and different scanning directions.

[0023] Figure 3This is a schematic diagram of the soliton optical comb generation device based on a gallium nitride microring resonator in the existing scheme 3. Wherein, TLS1: tunable laser source 1; TLS2: tunable laser source 2; PC: polarization controller; EDFA1: erbium-doped fiber amplifier 1; EDFA2: erbium-doped fiber amplifier 2; GaN MRR: gallium nitride microring resonator; SW: optical switch; ILP: online polarizer; OSA: spectrometer; OSC: oscilloscope.

[0024] Figure 4 This is a schematic diagram of soliton optical comb generation in a 5 mol.% magnesium oxide-doped thin film lithium niobate microcavity as described in an embodiment of the present invention.

[0025] Figure 5 This is a schematic cross-sectional view of the air-clad microcavity (a) and the silica-clad microcavity (b) described in the embodiments of the present invention, along with simulation results of the transverse electrical substrate.

[0026] Figure 6 The results are the calculation results of the integrated dispersion curves of the air-clad microcavity (a) and the silica-clad microcavity (b) described in the embodiments of the present invention.

[0027] Figure 7 This is the fabrication result of the 5 mol.% magnesium oxide-doped thin film lithium niobate microcavity described in the embodiments of the present invention. Among them, (a) is a microscope image of the fabricated device; (b) is a scanning electron microscope image of the waveguide end face of the silicon dioxide clad microcavity.

[0028] Figure 8 These are the air-clad microcavities (a) and silica-clad microcavities (b) described in the embodiments of the present invention. Q Value test results.

[0029] Figure 9 These are the integrated dispersion curve test results of the air-clad microcavity (a) and the silica-clad microcavity (b) described in the embodiments of the present invention in the C-band communication.

[0030] Figure 10 These are the photorefractive effect test results of the air-clad microcavity (a) and the silica-clad microcavity (b) described in the embodiments of the present invention.

[0031] Figure 11 These are the thermo-optical effect test results of the air-clad microcavity (a) and the silica-clad microcavity (b) described in the embodiments of the present invention.

[0032] Figure 12 This is a schematic diagram of the system for generating a 5 mol% magnesium oxide-doped thin-film lithium niobate microcavity optical comb as described in an embodiment of the present invention.

[0033] Figure 13These are the soliton steps (a) and the single soliton state microcavity optical comb spectrum (b) of the air-clad microcavity described in the embodiments of the present invention.

[0034] Figure 14 These are the soliton steps (a) and the single soliton state microcavity optical comb spectrum (b) of the silica cladding microcavity described in the embodiments of the present invention.

[0035] Figure 15 This is the soliton crystal optical comb spectrum of the air-clad microcavity described in this embodiment of the invention under low-power pumping. Wherein, (a) to (c) represent the repetition frequencies, respectively. f rep The soliton crystal optical comb spectra are equal to 12FSR, 9FSR, and 6FSR.

[0036] Figure 16 This is the soliton crystal optical comb spectrum of the silica-clad microcavity described in this embodiment of the invention under low-power pumping. Wherein, (a) to (c) represent the repetition frequencies, respectively. f rep The soliton crystal optical comb spectra are equal to 8FSR, 4FSR, and 2FSR. Detailed Implementation

[0037] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0038] Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0039] The following are the specific implementation processes of several existing solutions.

[0040] Existing Scheme 1: A soliton optical comb generation method based on a silicon nitride microring resonator. This scheme employs a standard semiconductor fabrication process: first, a silicon nitride thin film is grown on a silicon substrate via low-pressure chemical vapor deposition (LPCVD), followed by electron beam lithography and reactive ion etching (RIE) to define the microring resonator pattern. During the microring resonator design phase, anomalous dispersion is achieved by precisely controlling the waveguide width and thickness. In the experimental system, a tunable laser is used as the light source, coupled with an arbitrary function generator for frequency scanning. After power amplification via an erbium-doped fiber amplifier, the light is coupled to the microcavity using a tapered fiber. When the pump power reaches the excitation threshold of the soliton optical comb, a soliton step phenomenon can be observed. The soliton optical comb spectrum of the silicon nitride microring resonator in Existing Scheme 1 is as follows: Figure 1As shown. The main advantage of this scheme is that silicon nitride material has low transmission loss and mature processing technology, but it lacks inherent second-order nonlinear characteristics, which limits its application in electro-optic tuning and second harmonic generation.

[0041] Existing Scheme 2: A soliton optical comb generation method based on a pure lithium niobate thin-film microring resonator. This scheme involves defining a microring resonator pattern on a pure lithium niobate thin-film substrate using electron beam lithography, followed by argon ion beam etching to form the microring resonator structure. During testing, a tunable laser is used as the light source, and an erbium-doped fiber amplifier is used for power amplification. An arbitrary function generator is then used for frequency scanning, and a tapered fiber is used to couple the light to the microcavity. When the pump power reaches the excitation threshold of the soliton optical comb, soliton steps can be observed. By finely tuning the pump wavelength, Kerr soliton optical comb generation is achieved. However, Existing Scheme 2 has two significant problems in practical applications: 1) Under high-power pumping, the microring resonator exhibits a significant photorefractive effect, such as... Figure 2 As shown, this leads to a drift in the resonant frequency, which in turn affects the long-term stability of the soliton optical comb. 2) When the pump power exceeds 1W, the pure lithium niobate thin film microring resonator may suffer irreversible optical damage, which in turn affects the long-term reliability of the device and significantly limits its use in high-power scenarios.

[0042] Existing Solution 3: A soliton optical comb generation method based on gallium nitride microring resonators. This solution involves growing a gallium nitride thin film on a sapphire substrate via metal-organic chemical vapor deposition, and then fabricating the gallium nitride microring resonator using electron beam lithography and dry etching. Figure 3 This is a schematic diagram of the soliton optical comb generation device based on a gallium nitride microring resonator in the existing scheme 3. In the experimental system, TLS1 acts as the main pump laser, outputting vertically polarized light, while TLS2 acts as the auxiliary pump laser, outputting horizontally polarized light for microcavity thermal stabilization control. When the power of the main pump laser is high enough, soliton optical comb generation can be achieved. The drawback of this scheme is that high-quality gallium nitride material remains difficult to obtain commercially, leading to increased manufacturing costs. Furthermore, the electro-optic coefficient of gallium nitride material is relatively low, making it difficult to achieve a microcavity soliton optical comb with efficient electro-optic control.

[0043] Despite the significant potential of thin-film lithium niobate in integrated nonlinear photonics, existing microcavity optical comb generation technologies based on pure thin-film lithium niobate materials still face several technical bottlenecks. 1) The low optical damage threshold of pure thin-film lithium niobate significantly limits its application under high-power pumping conditions. When the pump power increases to a certain level, the microcavity of pure thin-film lithium niobate is prone to optical damage, which not only weakens the long-term reliability of the device but also hinders its use in high-power applications. 2) Pure lithium niobate materials also exhibit a strong photorefractive effect, causing the microcavity resonant frequency to drift with changes in illumination conditions, thus affecting the long-term stability of the microcavity optical comb. 3) At the device implementation level, existing silica-clad pure thin-film lithium niobate microcavities, due to the combined factors of fabrication processes and material systems, typically exhibit large transmission losses, resulting in the current inability to generate soliton microcavity optical combs. In summary, it is difficult to achieve Kerr soliton optical combs with high optical damage thresholds, low photorefractive effects, and silica-clad microcavity structures on existing pure thin-film lithium niobate material platforms.

[0044] This invention systematically solves the aforementioned bottleneck problems. First, it employs a 5 mol.% magnesium oxide-doped thin-film lithium niobate material, whose optical damage threshold is 130 times that of pure thin-film lithium niobate, significantly improving the microcavity's ability to withstand high pump power and its long-term operational stability. Second, by combining the third-order nonlinear effect of the 5 mol.% magnesium oxide-doped thin-film lithium niobate material with the dispersion optimization design of the microcavity structure, this invention achieves stable generation of Kerr soliton optical combs in both air-clad and silica-clad microcavity structures. The silica-clad structure will provide structural support for the subsequent integration of functional units such as thermally tuned electrodes. Furthermore, this invention verifies that the 5 mol.% magnesium oxide-doped thin-film lithium niobate microcavity possesses a weak photorefractive effect and the ability to generate soliton optical combs at lower pump power, indicating its good engineering feasibility and application potential.

[0045] This invention achieves the generation of high-performance soliton microcavity optical combs based on magnesium oxide-doped lithium niobate thin films through a combination of material innovation and structural optimization. The core of this invention lies in using a specific concentration of magnesium oxide doping, combined with precise and controllable micro-nano fabrication processes, to prepare a microring resonator with a high quality factor, controllable dispersion characteristics, and stable operation under high pump power.

[0046] This invention provides a method for generating a magnesium oxide-doped thin-film lithium niobate microcavity optical comb, comprising: optimizing the cross-sectional geometry of the microcavity to exhibit anomalous dispersion characteristics within a selected pump wavelength range; fabricating the microcavity on a 5 mol.% magnesium oxide-doped thin-film lithium niobate substrate according to the determined cross-sectional geometry, to provide a nonlinear gain environment for exciting a Kerr soliton optical comb; performing frequency sweeping operations on the pump laser in different scanning directions, monitoring the transmission spectrum changes of the microcavity in real time until the soliton step signal is captured; and finely tuning the wavelength of the pump laser to guide the pump wavelength to stably enter the effective width range of the soliton step, thereby exciting and generating a stable microcavity soliton optical comb. Wherein, as... Figure 4 As shown, the 5 mol.% magnesium oxide-doped thin-film lithium niobate microcavity consists of a silicon substrate, a silicon dioxide layer, and a 5 mol.% magnesium oxide-doped z-cut thin-film lithium niobate (abbreviated as z-cut 5 mol.% MgO:LN) from bottom to top.

[0047] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0048] Perform steps S101 to S105 to design and prepare high-molecular-weight MgO:LN using z-cutting 5 mol.%MgO:LN. Q The microcavity serves as the core device for the next stage of generating microcavity soliton optical combs. For example... Figure 4 As shown, the microcavity consists of a silicon substrate, a silicon dioxide layer, and a z-cut 5mol.%MgO:LN layer from bottom to top.

[0049] Step S101: Optimize the cross-sectional geometric parameters of the microcavity using the finite element method, specifically as follows: Figure 5 As shown. The microcavity is either a first microcavity or a second microcavity. The surface of the first microcavity has an air-clad structure, which will be referred to as an air-clad microcavity for ease of description; the surface of the second microcavity has a silica-clad structure, which will be referred to as a silica-clad microcavity.

[0050] Waveguide dimensions were optimized for two microcavity structures: silica-clad and air-clad. The silica-clad microcavity had a waveguide width of 1.7 μm and a cavity radius of 60 μm, while the air-clad microcavity had a waveguide width of 2.8 μm and a cavity radius of 100 μm. Both structures used an 800 nm film thickness, a 425 nm etching depth, and a 70° sidewall tilt angle. The microcavity integration dispersion results calculated based on these optimized parameters are shown below. Figure 6 As shown. By controlling and optimizing the cross-sectional geometric parameters of these two types of microcavities, the intrinsic properties of the microcavities are effectively improved. Q The value also ensures that it possesses the anomalous dispersion characteristics required to generate Kerr soliton optical combs in the pump wavelength range of 1530nm to 1565nm.

[0051] Step S102: Microcavity fabrication process. First, using electron beam lithography and ARN7520.17 photoresist, a microcavity pattern is precisely defined on a z-cut 5 mol.% MgO:LN surface. Then, the microcavity pattern is transferred to the MgO:LN layer using argon ion beam etching. Residual photoresist and etch-redeposited material are removed by wet processing to fabricate the first microcavity with an air cladding structure. Based on this, a 1 μm thick silicon dioxide cladding is deposited on the surface of the microcavity waveguide structure using plasma-enhanced chemical vapor deposition to fabricate the second microcavity with a silicon dioxide cladding structure. Finally, both types of microcavity chips are annealed at 500°C for 3 hours and cleavage is performed to obtain a flat waveguide end face. Figure 7 (a) shows the complete finished device. Figure 7 (b) shows the waveguide end face morphology of the silica-clad microcavity.

[0052] Step S103: The transmission characteristics of the microcavity are experimentally characterized using a rapid wavelength scanning method. This includes using a tunable laser for rapid wavelength scanning, and calibrating the laser wavelength using a non-equilateral Mach-Zehnder interferometer to accurately measure the microcavity's resonance curve. For example... Figure 8 (a) and Figure 8 As shown in (b), the intrinsic properties of the air-clad microcavities and the silica-clad microcavities were measured. Q The values ​​can reach 5.5 × 10⁻⁶. 6 and 1.4×10 6 This provides a guarantee for the subsequent generation of microcavity soliton optical combs.

[0053] Step S104: Experimentally characterize the integrated dispersive properties of the microcavity. For example... Figure 9 The measurement results show that the free spectral range (FSR) of the air-clad microcavity is 203 GHz, and the second-order dispersion parameter D² / (2π) is approximately 1.18 MHz; while the FSR of the silica-clad microcavity is 339 GHz, and D² / (2π) is approximately 3.67 MHz. Both structures exhibit significant anomalous dispersion characteristics in the pump band of 1530 nm to 1565 nm, satisfying the phase-matching conditions required for generating Kerr soliton optical combs.

[0054] Step S105: Photorefractive effect test. The resonant curves of the microcavity under different pump powers are tested using a bidirectional wavelength rapid scan. For example... Figure 10 The experimental results show that the 5 mol.% MgO:LN microcavity with z-cut structure exhibits a weak photorefractive effect in both its air cladding and silica cladding structures.

[0055] Step S106: Thermo-optical effect test. The temperature of the microcavity chip is actively controlled using a Peltier cooler placed at the bottom of the experimental stage, and the change in the microcavity resonant wavelength with temperature is recorded. For example... Figure 11 The test results show that the thermo-optical coefficients of the air cladding and the microcavity silica cladding are 2.7 pm / ℃ and 6.1 pm / ℃, respectively. The silica cladding structure not only exhibits higher thermo-optical response but also facilitates the direct integration of thermal electrodes on the top of the microcavity. This allows for the integration of thermal electrodes without sacrificing device performance. Q Under the premise of a certain value, it becomes possible to lock the resonant frequency of the thin-film lithium niobate microcavity through temperature feedback.

[0056] Perform steps S201 to S205 to construct a complete microcavity optical comb testing system. By monitoring soliton steps and spectral envelope, the stable generation of soliton microcavity optical combs is achieved.

[0057] Step S201: Construct a microcavity optical comb testing system, such as... Figure 12 As shown, the system first uses a triangular wave signal output from an arbitrary waveform generator to drive a tunable continuous laser, achieving precise control of the wavelength scanning process. Then, an erbium-doped fiber amplifier amplifies the pump laser power, and a polarization controller selects and excites the desired transverse electric fundamental mode. A lens fiber is used to achieve efficient optical coupling. At the output end, residual pump light is filtered out using a fiber Bragg grating, and soliton steps are observed using a photodetector and oscilloscope. Simultaneously, a spectrometer records the evolution of the optical comb lines in real time.

[0058] Step S202: Gradually increase the pump power and perform forward and reverse frequency sweeps on the air-clad microcavity until soliton steps are observed. Test results are as follows: Figure 13 As shown in (a), soliton steps can be observed in both increasing and decreasing frequency sweeps of the air-clad microcavity. With a fixed pump power (35mW on-chip power) corresponding to the soliton steps, the pump wavelength is finely tuned to fall within the wavelength range corresponding to the soliton steps, thereby exciting and generating a single soliton microcavity optical comb. The final result is shown in [image missing]. Figure 13 As shown in (b). This operation can also produce soliton microcavity optical combs under higher power pumping.

[0059] Step S203: Gradually increase the pump power and perform forward and reverse frequency sweeps on the silica-clad microcavity until soliton steps are observed. Test results are as follows: Figure 14 As shown in (a), soliton steps can only be observed in the silica-clad microcavity during a unidirectional frequency sweep with increasing frequency. By finely tuning the pump wavelength to fall within the wavelength range corresponding to the soliton steps at a fixed pump power (79mW on-chip power), a single soliton microcavity optical comb is excited and generated. The final result is shown in [image missing]. Figure 4As shown in (b). This operation can also be used to implement soliton microcavity optical combs with higher power pumps.

[0060] In steps S202 and S203, the fixed pump power corresponding to the soliton step satisfies the following condition: ;in, P in Pump power; c The speed of light; A eff The effective model area; β 2 represents group velocity dispersion; η The coupling efficiency from the waveguide to the microcavity; n 2 represents the third-order nonlinear coefficient; τ The pulse width; Q To load the quality factor; D 1 represents the angular frequency interval corresponding to the free spectral range; β The formula for calculating 2 is: ; η The calculation formula is: ; D The formula for calculating 1 is: ;in, n The refractive index of the material; D 2 represents the second-order dispersion coefficient; Q 0 represents the intrinsic quality factor; FSR It is the free spectral range.

[0061] Step S204: Reduce the pump power to a preset value and excite soliton crystal optical combs with different repetition frequencies by tuning the pump wavelength. In the air-clad microcavity, an on-chip pump power of only 18mW is required to generate a soliton crystal optical comb. Test results are as follows: Figure 15 As shown, the optical comb repetition frequency is greater than 2.4 THz.

[0062] Step S205: Reduce the pump power in the silica-clad microcavity. When the on-chip pump power is 20mW, the following is observed: Figure 16 The soliton crystal optical comb shown has a repetition frequency greater than 2.7 THz.

[0063] The above experimental results demonstrate that the technical solution provided in this invention has advantages in low-power soliton generation. Based on experimental data, the third-order nonlinear refractive index of the 5 mol.% MgO:LN material is estimated to be approximately 3.0 × 10⁻⁶. -19 m 2 / W, higher than the 1.8×10 of pure lithium niobate material. -19 m 2 / W. This result explains, from a physical perspective, why the 5 mol.%MgO:LN platform can generate low-threshold soliton optical combs. The technical solution described in this embodiment not only verifies the feasibility of 5 mol.%MgO:LN in the generation of soliton microcavity optical combs, but also demonstrates its application potential in high-power stable operation and on-chip thermo-optical control, providing a feasible technical path for constructing a high-stability microcavity optical comb system.

[0064] In summary, the embodiments of the present invention, based on a 5 mol.% MgO:LN photonic platform, achieved high intrinsic quality by optimizing the structural parameters and fabrication process of the air-clad and silica-clad microcavities. Q The experiment demonstrated that both microcavity structures exhibited weak photorefractive effects, with the silica-clad microcavity possessing a higher thermo-optic coefficient, providing structural support for the subsequent realization of thermo-optically modulated soliton optical combs. Furthermore, this invention, for the first time, achieved soliton optical comb generation in a silica-clad thin-film lithium niobate microcavity, overcoming the traditional limitation that such structures are difficult to generate soliton microcavity optical combs due to high losses. In addition, this invention achieved soliton crystal optical comb generation with low pump power thresholds in both air-clad and silica-clad microcavities, further demonstrating the good engineering feasibility and application potential of this technical solution.

[0065] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for generating a magnesium oxide-doped thin-film lithium niobate microcavity optical comb, characterized in that, include: The cross-sectional geometry of the microcavity is optimized to make it exhibit anomalous dispersion characteristics in the selected pump wavelength range; Based on the determined microcavity cross-sectional geometric parameters, the microcavity was fabricated on a lithium niobate substrate with a magnesium oxide doping concentration of 5 mol.% to provide a nonlinear gain environment for exciting the Kerr soliton optical comb. The pump laser is subjected to frequency sweeping operations in different scanning directions, and the changes in the transmission spectrum of the microcavity are monitored in real time until the soliton step signal is captured. The wavelength of the pump laser is finely tuned to guide the pump wavelength to stably enter the effective width range of the soliton step, thereby exciting and generating a stable microcavity soliton optical comb.

2. The method for generating a magnesium oxide-doped thin-film lithium niobate microcavity optical comb according to claim 1, characterized in that, The microcavity is either a first microcavity or a second microcavity; the surface of the first microcavity has an air cladding structure; the surface of the second microcavity has a silicon dioxide cladding structure.

3. The method for generating a magnesium oxide-doped thin-film lithium niobate microcavity optical comb according to claim 2, characterized in that, The cross-sectional geometric parameters of the first microcavity are: waveguide width of 2.8 μm and microcavity radius of 100 μm.

4. The method for generating a magnesium oxide-doped thin-film lithium niobate microcavity optical comb according to claim 2, characterized in that, The cross-sectional geometric parameters of the second microcavity are: waveguide width of 1.7 μm and microcavity radius of 60 μm.

5. The method for generating a magnesium oxide-doped thin-film lithium niobate microcavity optical comb according to claim 2, characterized in that, The frequency sweep operation of the pump laser in different scanning directions specifically includes: The pump power is gradually increased, and the pump laser frequency is scanned in both the forward and reverse directions near the microcavity resonant wavelength until a soliton step is observed in the transmission power curve. At the fixed pump power corresponding to the soliton step, the wavelength of the pump laser is finely tuned to guide the pump wavelength into the detuning range corresponding to the soliton step, so as to excite and generate a microcavity soliton optical comb. The pump power is then reduced to a preset value, and the pump wavelength is tuned to excite a soliton crystal optical comb with specific repetition frequency distribution characteristics.

6. The method for generating a magnesium oxide-doped thin-film lithium niobate microcavity optical comb according to claim 5, characterized in that, The fixed pump power corresponding to the soliton step satisfies the following condition: ； in, P in Pump power; c The speed of light; A eff The effective model area; β 2 represents group velocity dispersion; η The coupling efficiency from the waveguide to the microcavity; n 2 represents the third-order nonlinear coefficient; τ The pulse width; Q To load the quality factor; D 1 represents the angular frequency interval corresponding to the free spectral range; β The formula for calculating 2 is: ; η The calculation formula is: ; D The formula for calculating 1 is: ;in, n The refractive index of the material; D 2 represents the second-order dispersion coefficient; Q 0 represents the intrinsic quality factor; FSR It is the free spectral range.

7. The method for generating a magnesium oxide-doped thin-film lithium niobate microcavity optical comb according to claim 5, characterized in that, The specific conditions for performing frequency sweeping operations in different scanning directions on the pump laser are as follows: during the bidirectional frequency sweeping process of the first microcavity with increasing or decreasing frequency, a single soliton optical frequency comb is generated at an on-chip power of 35mW; the on-chip pump power is reduced to 18mW, the pump wavelength is tuned, and a soliton crystal optical comb with specific repetition frequency distribution characteristics is excited.

8. The method for generating a magnesium oxide-doped thin-film lithium niobate microcavity optical comb according to claim 5, characterized in that, The specific conditions for performing frequency sweeping operations in different scanning directions on the pump laser are as follows: during the unidirectional frequency sweeping process of the second microcavity with increasing frequency, a single soliton optical comb is generated at an on-chip power of 79mW; the pump power is reduced to 20mW, the pump wavelength is tuned, and a soliton crystal optical comb with specific repetition frequency distribution characteristics is excited.

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