A tunable optical frequency comb generation system and method based on a magnesium fluoride crystal microcavity

Abstract

This invention discloses a tunable optical frequency comb generation system and method based on a magnesium fluoride crystal microcavity. Using magnesium fluoride crystal as the microcavity material, it possesses extremely high quality factor and low absorption loss optical properties, enabling the system to achieve efficient optical frequency comb generation at relatively low pump power, thus improving the generation efficiency and stability of the optical frequency comb. The structure incorporates piezoelectric ceramic elements to achieve tunability of the optical frequency comb, and the application of a piezoelectric fine-tuning module makes stress tuning of the microcavity more precise and flexible. By finely adjusting the axial stress applied to the microcavity, rapid tuning of the optical frequency comb teeth position can be achieved, simplifying the system structure and operation process and improving system usability. Achieving tunability of the optical frequency comb without changing the pump laser frequency greatly enhances the system's stability and anti-interference capability. Simultaneously, the stable mode field inside the microcavity provides high signal quality, contributing to improved output stability of the optical frequency comb and enhancing the system's application value.

Description

Technical Field

[0001] This invention belongs to the field of optical frequency comb technology. Specifically, this invention relates to an adjustable optical frequency comb generation system and method based on a magnesium fluoride crystal microcavity. Background Technology

[0002] An optical frequency comb (OFC) is a light source with a fixed comb tooth spacing in the frequency domain. It possesses high bandwidth and precise optical frequency characteristics, and is widely used in many cutting-edge technology fields, including precision frequency measurement, optical clocks, coherent optical communication, spectral analysis, space exploration, precision ranging, and microwave photonics. Miniature optical frequency combs based on the Kerr effect in optical microcavities (i.e., Kerr optical frequency combs, hereinafter referred to as optical frequency combs) have attracted widespread attention due to their ultra-high quality factor and extremely small mode volume. The generation of a Kerr optical frequency comb originates from a four-wave mixing process within an optical microcavity, where two pump photons annihilate to generate a pair of signal and idler beams, thereby achieving parametric oscillation. This process not only improves the generation efficiency of the optical frequency comb but also provides the foundation for its flexibility and adaptability in various applications.

[0003] In recent years, various methods have been employed to produce tunable optical frequency combs. Tunability can be understood from two dimensions: the tunability of the comb tooth position and the tunability of the tooth frequency spacing. A Chinese invention patent, titled "Tunable Optical Frequency Comb Based on Lithium Niobate Microcavity and Its Preparation Method," application number 201911171685.5, publication date 2020.0508, utilizes integrated electrodes to regulate the optical frequency comb generated by a lithium niobate microcavity. Lithium niobate material is relatively expensive, and the preparation process of the lithium niobate microcavity is relatively complex. Furthermore, lithium niobate material is sensitive to temperature changes, and its optical performance may be affected by environmental changes, thus affecting the stability of the optical frequency comb. Another Chinese invention patent, titled "A Communication Device," application number 202211525913.6, publication date 2024.05.13, uses a laser optical feedback device combined with frequency locking to generate a laser optical frequency comb with an adjustable repetition frequency. This system is relatively complex to implement.

[0004] Among the many methods for generating tunable optical frequency combs, very few tuning schemes involve tuning the position of the comb teeth. The position of the comb teeth is primarily determined by the diameter of the optical microcavity and the refractive index of the material. By applying a voltage to a piezoelectric ceramic (PZT) element, tension or pressure can be applied to the microcavity. During the stress application process, the size of the optical microcavity changes accordingly, and the refractive index also changes due to the photoelastic effect, leading to a change in the wavelength of the optical mode. This change provides the possibility for achieving tunability of the optical frequency comb. In summary, the tunable optical frequency comb generation system and method based on magnesium fluoride crystal microcavities combines advanced optical frequency comb technology with the unique properties of magnesium fluoride crystals, and has broad application prospects. Researching the tunability of optical frequency combs is of great significance for promoting their application in high-tech fields. This research further explores how to fabricate high-quality crystal optical microcavities and optimize the tuning mechanism to achieve more flexible and precise optical frequency comb generation. Such research will not only help improve the performance of optical frequency combs but also provide new impetus for the innovation of next-generation optical technologies. Summary of the Invention

[0005] This invention provides a tunable optical frequency comb generation system and method based on a magnesium fluoride crystal microcavity, to solve the problems of insufficient frequency stability, insufficient tunability, high material cost, and complex system implementation of optical frequency combs in the above-mentioned background art.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a tunable optical frequency comb generation system based on a magnesium fluoride crystal microcavity, comprising a laser, a wavelength division multiplexer, a high-concentration erbium-doped fiber, a polarization controller, a circulator, a coupling system consisting of a tapered fiber and a microcavity, a piezoelectric ceramic element consisting of a piezoelectric fine-tuning module and a clamping coarse-tuning module, an optical fiber beam splitter, a photodetector, an oscilloscope, and a spectrometer;

[0007] The laser is connected to a wavelength division multiplexer (WDM), which is connected to a high-concentration erbium-doped fiber (HBD), which is connected to a polarization controller, which is connected to a circulator. The circulator's output is split into two paths. The back-side output of the circulator is connected to the WDM, forming a backscattering loop system. The forward output light from the circulator enters a coupling system consisting of a tapered fiber waveguide and a magnesium fluoride crystal microcavity. The output light from this coupling system enters a fiber beam splitter and is split into two paths. One path is connected to a photodetector, converting the optical signal into an electrical signal for transmission to an oscilloscope. The other path is connected to a spectrometer to read and store the optical frequency comb, forming a single-mode laser lasing system. By controlling the coupling position between the tapered fiber and the microcavity, optimizing the fiber polarization state, and tuning the pump power of the 980nm laser, a stable optical frequency comb can be output through four-wave mixing, using the single-mode laser as pump. Furthermore, stress is applied to the microcavity using a piezoelectric fine-tuning module, changing the optical mode wavelength of the microcavity and achieving tunability of the optical frequency comb, thus forming a stress-tuned system.

[0008] The laser is used to generate a continuously tunable laser in the 980nm band as pump light, which enters a high-concentration erbium-doped fiber through a wavelength division multiplexer to excite erbium ions in the high-concentration erbium-doped fiber to undergo energy level transitions, thereby providing gain to the system.

[0009] The wavelength division multiplexer (WDM) described is a 980 / 1550nm WDM, a key optical component responsible for effectively combining and separating optical signals of different wavelengths. This multiplexer can simultaneously process optical signals in both the 980nm and 1550nm bands, allowing the pump light and signal light generated by the laser to transmit in the same optical fiber without interference. Through WDM technology, the system can achieve higher optical signal transmission efficiency and better spectral utilization. Furthermore, the high selectivity and low insertion loss characteristics of the WDM ensure high fidelity of the optical signal during transmission, significantly improving the overall system performance.

[0010] The primary function of a polarization controller is to precisely control the polarization state of the input light. By adjusting the polarization direction and intensity of the light, the polarization controller ensures that the optical signal in the system has the required polarization characteristics. Optimizing the polarization state can also enhance the output quality and stability of the optical frequency comb.

[0011] Circulators are used to form a loop system, allowing backscattered signals to be fed back to the wavelength division multiplexer, while also protecting the laser.

[0012] The coupling system consists of a tapered optical fiber and a micro-cavity. For the micro-cavity, it is necessary to couple light into the cavity's resonant modes efficiently using an evanescent field. The tapered optical fiber utilizes the evanescent field generated by its tapered section to achieve optical field coupling, resulting in extremely high coupling efficiency. Furthermore, the phase-matching condition between the tapered optical fiber and the micro-cavity is adjustable, allowing for a coupling efficiency approaching 100%. This design not only improves system performance but also provides a more reliable foundation for the generation of optical frequency combs.

[0013] The micro-bottle cavity is fixed on a piezoelectric ceramic element, which mainly consists of a piezoelectric fine adjustment module and a clamping coarse adjustment module. The piezoelectric fine adjustment module is used to apply minute stress to the micro-bottle cavity, and the clamping coarse adjustment module is used to fix the micro-bottle cavity and coarsely adjust its position.

[0014] A photodetector is used to receive optical signals from a coupling system and convert them into electrical signals. By detecting optical signals with high sensitivity, the photodetector can capture changes in weak optical signals, which are then transmitted to an oscilloscope for further analysis and display.

[0015] A spectrometer is used to read and store the spectral characteristics of laser signals. It can monitor and acquire data from the optical frequency comb generated by the system in real time, and provide important information about the comb's output characteristics, including comb tooth range, tooth spacing, and signal-to-noise ratio.

[0016] Furthermore, the micro-cavity is fabricated from magnesium fluoride crystal material, possessing a complex three-dimensional structure and abundant axial modes. Axial modes within the micro-cavity can eliminate the limitations imposed by azimuth modes on dispersion control, enhancing the flexibility of dispersion control. The micro-cavity itself exhibits strong surface characteristics, supporting both radial and angular mode families. When the radius of the micro-cavity varies along the long axis, axial modes of different orders are excited, thus demonstrating excellent dispersion management potential. This dispersion characteristic plays a crucial role in the generation of the optical frequency comb, as it allows for precise control of phase-matching conditions over a wide bandwidth. Light coupled into the micro-cavity through a tapered fiber propagates around the cavity axis several times before rotating back to the initial incident point at a turning point, thereby forming a stable mode field inside the micro-cavity. By selecting appropriate axial coupling point locations, different whispering-gallery modes can be effectively excited, further enhancing the system's tuning capability and the tunability of the frequency comb teeth.

[0017] The microcavity uses magnesium fluoride crystal material, which has extremely high stability and is insensitive to ambient humidity, allowing the fabricated microcavity to be stored for a long time. Its high quality factor reduces the threshold power for generating the optical frequency comb. Magnesium fluoride crystal material exhibits weak anomalous dispersion near 1550nm, improving the four-wave mixing phase matching efficiency during optical frequency comb generation, thereby increasing the broadband range of the optical frequency comb. Its low absorption coefficient, high nonlinear coefficient, high purity, and few defects provide significant advantages for the generation and tuning of the microcavity optical frequency comb.

[0018] Furthermore, the magnesium fluoride crystal micro-cavity is prepared by ultra-precision turning and ultra-precision polishing. The prepared micro-cavity has two end faces. One end is bonded to a millimeter-sized aluminum rod before preparation. The other end provides favorable conditions for effective contact with the piezoelectric ceramic element.

[0019] Furthermore, by adjusting the polarization controller to optimize the polarization state of the optical fiber and adjusting the coupling position between the micro-cavity and the tapered optical fiber, the generated single-mode laser further pumps the micro-cavity to generate a parametric oscillation signal. Part of the generated oscillation signal is reflected into the optical fiber loop and subjected to the cyclic amplification effect of the high-concentration erbium-doped fiber, ultimately outputting a stable optical frequency comb.

[0020] The piezoelectric fine-tuning module uses a piezoelectric ceramic stacking method. By changing the voltage of the piezoelectric ceramic element, an adjustable axial force can be applied to the micro-cavity. Under stress, the resonant peak mode spectrum of the micro-cavity will also drift.

[0021] Furthermore, the relationship between the resonant wavelength shift and the change in the equatorial radius ΔR of the micro-cavity is as follows: In the formula, λ is the resonant wavelength, Δλ is the change in resonant wavelength, R is the equatorial radius of the micro-cavity (7), and ΔR is the change in the equatorial radius of the micro-cavity (7);

[0022] The relationship between the change in the equatorial radius ΔR of the micro-bottle cavity and the applied axial stress is as follows: In the formula, v is Poisson's ratio, E is Young's modulus of the material, and F is the magnitude of the axial stress applied to the micro-cavity.

[0023] Finally, the relationship between the resonant wavelength shift and the change in the equatorial radius ΔR of the micro-cavity can be further simplified to:

[0024] The tunability of the optical frequency comb refers to the tunability of the comb tooth position, which is mainly determined by the micro-cavity diameter and the refractive index of the material. Stress is applied to the micro-cavity through a piezoelectric fine-tuning module, causing deformation. Simultaneously, the refractive index of the micro-cavity changes due to the photoelastic effect, resulting in a change in the resonant wavelength of the micro-cavity with the voltage across the piezoelectric ceramic element. Ultimately, this changes the position of the optical frequency comb teeth, achieving tunability. Furthermore, the optical frequency of the 980nm pump laser remains constant during the tuning process.

[0025] In summary, the tunable optical frequency comb generation system based on a magnesium fluoride crystal microcavity proposed in this application has the characteristics of strong anti-interference, high comb tooth stability, fast tuning response, and simple tuning method.

[0026] The beneficial effects of adopting the above technical solutions are:

[0027] 1. The present invention provides an adjustable optical frequency comb generation system based on a magnesium fluoride crystal microcavity. The magnesium fluoride crystal is used as the microcavity material, which has an extremely high quality factor and low absorption loss optical properties. This enables the system to achieve efficient generation of optical frequency combs at a lower pump power, thereby improving the generation efficiency and stability of the optical frequency comb.

[0028] 2. This invention provides an adjustable optical frequency comb generation system based on a magnesium fluoride crystal microcavity. The structure incorporates piezoelectric ceramic elements to achieve tunability of the optical frequency comb. The application of a piezoelectric fine-tuning module makes the stress tuning of the microcavity more precise and flexible. By finely adjusting the axial stress applied to the microcavity, rapid tuning of the optical frequency comb teeth position can be achieved, simplifying the system structure and operation process and improving the system's usability.

[0029] 3. This invention provides a tunable optical frequency comb generation system based on a magnesium fluoride crystal microcavity. The system design allows for the tunability of the optical frequency comb without changing the pump laser frequency, which greatly enhances the system's stability and anti-interference capability. Simultaneously, the stable mode field inside the microcavity provides high signal quality, contributing to improved output stability of the optical frequency comb and further enhancing the system's application value. Attached Figure Description

[0030] Figure 1 This is a schematic diagram of a tunable optical frequency comb generation system based on a magnesium fluoride crystal microcavity.

[0031] Figure 2 This is a geometric schematic diagram of the micro-bottle cavity and its key parameters;

[0032] Figure 3 This is a schematic diagram of the preparation process of the magnesium fluoride crystal micro-cavity;

[0033] Figure 4 This is a Lorentz fitting plot of the transmission spectrum of a magnesium fluoride crystal micro-cavity test.

[0034] Figure 5 This is a simulation diagram of the axial stress in the cavity of a magnesium fluoride crystal micro-bottle;

[0035] Figure 6 This is a schematic diagram of optical frequency comb tooth drift;

[0036] Figure 7 It is an optical frequency comb generated by the micro-cavity testing of magnesium fluoride crystals;

[0037] in:

[0038] 1. Laser; 2. Wavelength division multiplexer; 3. High-concentration erbium-doped fiber; 4. Polarization controller; 5. Circulator; 6. Tapered fiber; 7. Micro-cavity; 8. Piezoelectric fine-tuning module; 9. Clamping coarse-tuning module; 10. Fiber beam splitter; 11. Photodetector; 12. Oscilloscope; 13. Spectrometer. Detailed Implementation

[0039] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings, in order to help those skilled in the art to have a more complete, accurate and in-depth understanding of the concept and technical solutions of the present invention, and to facilitate its implementation.

[0040] like Figures 1 to 7As shown, this invention discloses a tunable optical frequency comb generation system and method based on a magnesium fluoride crystal microcavity. Using magnesium fluoride crystal as the microcavity material, it possesses extremely high quality factor and low absorption loss optical properties, enabling the system to achieve efficient optical frequency comb generation at relatively low pump power, thus improving the generation efficiency and stability of the optical frequency comb. The structure incorporates piezoelectric ceramic elements to achieve tunability of the optical frequency comb. The application of a piezoelectric fine-tuning module makes stress tuning of the microcavity more precise and flexible. By finely adjusting the axial stress applied to the microcavity, rapid tuning of the optical frequency comb teeth position can be achieved, simplifying the system structure and operation process and improving the system's usability. The system design allows for tunability of the optical frequency comb without changing the pump laser frequency, which greatly enhances the system's stability and anti-interference capability. Simultaneously, the stable mode field inside the microcavity provides high signal quality, contributing to improved output stability of the optical frequency comb and further enhancing the system's application value.

[0041] The specific working method is described below using specific embodiments:

[0042] Example 1:

[0043] A tunable optical frequency comb system based on a magnesium fluoride crystal microcavity includes a laser 1, a wavelength division multiplexer 2, a high-concentration erbium-doped fiber 3, a polarization controller 4, a circulator 5, a coupling system consisting of a tapered fiber 6 and a microcavity 7, a piezoelectric ceramic element consisting of a piezoelectric fine adjustment module 8 and a clamping coarse adjustment module 9, an optical fiber beam splitter 10, a photodetector 11, an oscilloscope 12, and a spectrometer 13.

[0044] like Figure 1 As shown, the laser 1 is connected to a wavelength division multiplexer 2, which is connected to a high-concentration erbium-doped fiber 3. The high-concentration erbium-doped fiber 3 is connected to a polarization controller 4, which is connected to a circulator 5. The output of the circulator 5 is split into two paths. The back-facing output of the circulator 5 is connected to the wavelength division multiplexer 2, forming a backscattering loop system. The forward output light of the circulator 5 enters a coupling system composed of a tapered fiber 6 and a micro-cavity 7. The output light of the coupling system enters a fiber beam splitter 10, which splits the light into two paths. One path is connected to a photodetector 11, which converts the optical signal into an electrical signal and transmits it to an oscilloscope 12. The other path is connected to a spectrometer 13 to read and store the optical frequency comb, forming a single-mode laser lasing system. By controlling the coupling position between the tapered fiber 6 and the micro-cavity 7, optimizing the polarization state of the fiber, and tuning the pump power of the 980nm laser, a stable optical frequency comb can be output through four-wave mixing, using the single-mode laser as a pump. Furthermore, stress is applied to the micro-cavity 7 by the piezoelectric fine-tuning module 8, which changes the optical mode wavelength of the micro-cavity 7, thereby realizing the tunability of the optical frequency comb and forming a stress-tuning system.

[0045] The laser 1 is used to generate a continuously tunable laser in the 980nm band as pump light, which enters the high-concentration erbium-doped fiber 3 through the wavelength division multiplexer 2 to excite the erbium ions in the high-concentration erbium-doped fiber 3 to undergo energy level transitions, thereby providing gain to the system.

[0046] The wavelength division multiplexer 2, a 980 / 1550nm multiplexer, is a key optical component responsible for effectively combining and separating optical signals of different wavelengths. This multiplexer can simultaneously process optical signals in both the 980nm and 1550nm bands, allowing the pump light and signal light generated by laser 1 to transmit in the same optical fiber without interference. Through wavelength division multiplexing technology, the system can achieve higher optical signal transmission efficiency and better spectral utilization. Furthermore, the high selectivity and low insertion loss characteristics of the wavelength division multiplexer 2 ensure high fidelity of the optical signal during transmission, greatly improving the overall system performance.

[0047] The primary function of polarization controller 4 is to precisely control the polarization state of the input light. By adjusting the polarization direction and intensity of the light, polarization controller 4 ensures that the optical signal in the system has the required polarization characteristics. Optimizing the polarization state can also enhance the output quality and stability of the optical frequency comb.

[0048] Circulator 5 is used to form a loop system, so that the backscattered signal is fed back to wavelength division multiplexer 2, and at the same time, it protects laser 1.

[0049] The coupling system consists of a tapered optical fiber 6 and a micro-cavity 7. For the micro-cavity 7, it is necessary to couple light into its efficient excitation resonant mode using an evanescent field. The tapered optical fiber 6 utilizes the evanescent field generated by its tapered section to achieve optical field coupling, resulting in extremely high coupling efficiency. Furthermore, the phase-matching condition between the tapered optical fiber 6 and the micro-cavity 7 is adjustable, allowing for a coupling efficiency approaching 100%. This design not only improves system performance but also provides a more reliable foundation for the generation of the optical frequency comb.

[0050] The micro-bottle cavity 7 is fixed on a piezoelectric ceramic element, which mainly consists of a piezoelectric fine adjustment module 8 and a clamping coarse adjustment module 9. The piezoelectric fine adjustment module 8 is used to apply minute stress to the micro-bottle cavity 7. The clamping coarse adjustment module 9 is used to fix the micro-bottle cavity 7 and perform preliminary position adjustment to ensure the stability and alignment accuracy of the micro-bottle cavity 7 in the system.

[0051] The photodetector 11 is used to receive optical signals from the coupling system and convert them into electrical signals. By detecting optical signals with high sensitivity, the photodetector 11 can capture changes in weak optical signals, which are then transmitted to the oscilloscope 12 for further analysis and display.

[0052] The spectrometer 13 is used to read and store the spectral characteristics of the laser signal. The spectrometer 13 can monitor and acquire data from the optical frequency comb generated by the system in real time, and provide important information on the output characteristics of the optical frequency comb, including characteristics such as comb tooth range, comb tooth spacing and signal-to-noise ratio.

[0053] Furthermore, the micro-cavity 7 is a micro-cavity made of magnesium fluoride crystal material, and its structure has axisymmetric characteristics, which can simultaneously support radial and angular mode families, enabling light to propagate in multiple dimensions within the micro-cavity 7 and possessing rich axial modes. For example... Figure 2 As shown, the micro-bottle cavity 7 has a bottle-shaped optical structure, and its main parameters include the diameter L of the cavity bottleneck. b Maximum diameter D b and minor axis diameter D S The profile of the micro-cavity 7 can be fitted by an approximate parabolic function, expressed as D(z) = D b ×[1-1 / 2(Δk·z) 2 In this equation, D(z) represents the cross-sectional radius of the micro-cavity 7 at different axial positions, z represents the length along the center of the cavity axis, and Δk represents the curvature of the micro-cavity 7 profile. Light enters the micro-cavity 7 through near-field coupling via tapered fiber 6 and can propagate in an axial spiral pattern within it. This significantly enhances the intensity of two spatially distant optical fields. Furthermore, the micro-cavity 7 possesses a series of equidistant axial characteristic frequencies. By changing the coupling distance between the tapered fiber 6 and the micro-cavity 7, different transmission spectra can be selectively excited. The rich axial modes eliminate the limitations of azimuth modes on dispersion control, improving the flexibility of dispersion control and providing ideal conditions for the generation of micro-cavity optical frequency combs.

[0054] The micro-cavity 7 uses magnesium fluoride crystal material, which has extremely high stability and is insensitive to ambient humidity, allowing the prepared micro-cavity 7 to be stored for a long time. Its high quality factor reduces the threshold power for generating the optical frequency comb. Magnesium fluoride crystal material exhibits weak anomalous dispersion near 1550nm, improving the four-wave mixing phase matching efficiency during optical frequency comb generation, thereby increasing the broadband range of the optical frequency comb. Its low absorption coefficient, high nonlinear coefficient, high purity, and few defects provide significant advantages for the generation and tuning of the optical frequency comb in the micro-cavity 7.

[0055] Furthermore, the magnesium fluoride crystal micro-cavity 7 is prepared by ultra-precision turning and ultra-precision polishing; as... Figure 3 As shown, the magnesium fluoride crystal micro-cavity 7, prepared through four steps of cutting, grinding, rough polishing, and fine polishing, possesses an extremely high quality factor Q value. For example... Figure 4 As shown, the quality factor Q value after fine polishing reaches 4×10 8This significantly increases the efficiency of generating the optical frequency comb. The prepared micro-cavity 7 has two end faces: one end is bonded to a millimeter-sized aluminum rod before preparation; the other end provides favorable conditions for effective contact with the piezoelectric ceramic element.

[0056] Furthermore, by adjusting the polarization controller 4 to optimize the polarization state of the optical fiber and adjusting the coupling position between the micro-cavity 7 and the tapered optical fiber 6, the generated single-mode laser further pumps the micro-cavity 7 to generate a parametric oscillation signal. Part of the generated oscillation signal is reflected into the optical fiber loop and subjected to the cyclic amplification effect of the high-concentration erbium-doped fiber 3, ultimately outputting a stable optical frequency comb.

[0057] The coupling system comprises a tapered optical fiber 6 and a micro-cavity 7. For the micro-cavity 7, it is necessary to couple light into its efficient excitation resonant mode using an evanescent field. The tapered optical fiber 6 utilizes the evanescent field generated by its tapered portion to achieve optical field coupling, resulting in extremely high coupling efficiency. Furthermore, the phase matching condition between the tapered optical fiber 6 and the micro-cavity 7 is adjustable, and their coupling efficiency can approach 100%. This design not only improves system performance but also provides a more reliable foundation for the generation of optical frequency combs.

[0058] The piezoelectric fine-tuning module 8 adopts a piezoelectric ceramic stacking method. By changing the voltage of the piezoelectric ceramic element, an adjustable axial force can be applied to the micro-bottle cavity 7. Figure 5 This is a force distribution diagram obtained by Comsol finite element digital simulation when one end of the magnesium fluoride micro-bottle cavity is subjected to axial pressure. When the micro-bottle cavity 7 is subjected to axial pressure, the geometry of the micro-bottle cavity 7 undergoes a slight change.

[0059] Furthermore, the relationship between the resonant wavelength shift and the change in the equatorial radius ΔR of the micro-cavity 7 is as follows: In the formula, λ is the resonant wavelength, Δλ is the change in resonant wavelength, R is the equatorial radius of the micro-cavity (7), and ΔR is the change in the equatorial radius of the micro-cavity (7);

[0060] The relationship between the change in the equatorial radius ΔR of the micro-cavity 7 and the applied axial stress is as follows: In the formula, v is Poisson's ratio, E is Young's modulus of the material, and F is the magnitude of the axial stress applied to the micro-bottle cavity 7;

[0061] Finally, the relationship between the resonant wavelength shift and the change in the equatorial radius ΔR of the micro-cavity 7 is further simplified to:

[0062] The principle of the tunable optical frequency comb system based on a magnesium fluoride crystal microcavity is as follows: A 980nm laser generated by laser 1 pumps erbium ions in an erbium-doped fiber, causing spontaneous emission of new photons in the 1550nm band. This provides sufficient gain to the microcavity 7. By adjusting the coupling position between the tapered fiber 6 and the microcavity 7, a high-performance whispering-gallery mode is excited, ultimately resulting in stable single-mode laser emission. When single-wavelength lasing is achieved, the output is locked to a specific mode in the microcavity 7, satisfying the key condition of matching the wavelength with the pump resonant mode during optical frequency comb generation. Subsequently, the newly generated single-mode laser pumps the microcavity 7 itself to generate a parametric oscillation signal, ultimately obtaining a stable optical frequency comb output. This optical frequency comb exhibits extremely high stability, which is a major advantage of this system. When the optical frequency comb output is stable, the voltage of the piezoelectric ceramic element is changed using the piezoelectric fine-tuning module 8, applying a force to the micro-cavity 7, causing a change in the size of the micro-cavity 7. Simultaneously, the refractive index of the micro-cavity 7 changes due to the photoelastic effect, causing the resonant wavelength of the micro-cavity 7 to change with the voltage of the piezoelectric ceramic element. Ultimately, this results in a change in the position of the comb teeth of the optical frequency comb, such as... Figure 6 As shown, this achieves the goal of adjustable comb tooth position. Figure 7 This is an experimental diagram of the optical frequency comb generated by the system.

[0063] Example 2:

[0064] A method for generating a tunable optical frequency comb based on a magnesium fluoride crystal microcavity, the specific steps of which are as follows:

[0065] Step S1: The continuous laser generated by the 1550nm tunable laser 1 enters the coupling system through the optical isolator and polarization controller 4, and the coupling system is adjusted to generate a whispering gallery mode with an ultra-high quality factor.

[0066] Step S2: A continuous laser generated by a 980nm laser 1 pumps erbium ions in a high-concentration erbium-doped fiber, causing spontaneous emission of new photons in the 1550nm band. The output light passes through a circulator 5. The back-direction output of the circulator 5 enters a backscattering loop system, while the forward output enters a micro-cavity 7 through a tapered fiber 6, lasing out a high signal-to-noise ratio single-mode laser. By increasing the power of laser 1 and optimizing the polarization state of the fiber, the single-mode laser further pumps the micro-cavity 7 to generate a parametric oscillation signal. A portion of the parametric oscillation signal is reflected into the fiber loop and amplified by the erbium-doped fiber, ultimately outputting a stable optical frequency comb.

[0067] Step S3: The piezoelectric fine-tuning module 8 uses a piezoelectric ceramic stacking method. By changing the voltage of the piezoelectric ceramic element, an adjustable axial force is applied to the micro-cavity 7, causing a change in the geometry of the micro-cavity 7. Because the equatorial radius of the micro-cavity 7 changes, the resonant wavelength of the micro-cavity 7 undergoes a certain displacement change. This causes the position of the output single-mode laser to change with the voltage of the piezoelectric ceramic element, and the position of the comb teeth generated by the single-mode laser changes simultaneously, achieving the purpose of tunable comb tooth position. Furthermore, the optical frequency of the 980nm pump laser remains constant during the tuning process.

[0068] In summary, the tunable optical frequency comb generation system and method based on magnesium fluoride crystal microcavity of the present invention not only has efficient and stable optical frequency comb generation capability, but also provides an innovative solution for the field of optical applications through simple tuning method and excellent material properties, and has broad market application prospects.

[0069] The present invention has been described above by way of example with reference to the accompanying drawings. Obviously, the specific implementation of the present invention is not limited to the above-described manner. Any non-substantial improvements made using the inventive concept and technical solution of the present invention, or the direct application of the inventive concept and technical solution of the present invention to other situations without modification, are all within the protection scope of the present invention.

Claims

1. A tunable optical frequency comb generation system based on a magnesium fluoride crystal microcavity, characterized in that: The system includes a laser (1), a wavelength division multiplexer (2), a high-concentration erbium-doped fiber (3), a polarization controller (4), a circulator (5), a coupling system consisting of a tapered fiber (6) and a micro-cavity (7), a piezoelectric ceramic element consisting of a piezoelectric fine-tuning module (8) and a clamping coarse-tuning module (9), an optical fiber beam splitter (10), a photodetector (11), an oscilloscope (12), and a spectrometer (13); The laser (1) is used to generate a continuously tunable laser in the 980nm band as pump light. The laser (1) is connected to a wavelength division multiplexer (2), which is connected to a high-concentration erbium-doped fiber (3). The high-concentration erbium-doped fiber (3) is connected to a polarization controller (4), which is connected to a circulator (5). The back-facing output of the circulator (5) is connected to the wavelength division multiplexer (2), and the forward-facing output of the circulator (5) is connected to a coupling system. The output light from the coupling system enters an optical fiber beam splitter (10) to split the light into two paths. One path is connected to a photodetector (11) to convert the optical signal into an electrical signal and transmit it to an oscilloscope (12). The other path is connected to a spectrometer (13) to read and store the optical frequency comb. The piezoelectric fine-tuning module applies stress to the micro-cavity (7), causing the optical mode wavelength of the micro-cavity (7) to change, thereby achieving the tunability of the optical frequency comb; The micro-cavity (7) is a micro-cavity made of magnesium fluoride crystal material. It has a three-dimensional structure and supports radial and angular mode families at the same time. The micro-cavity (7) has a rotationally symmetric structure. Light coupled into the micro-cavity (7) through the tapered optical fiber (6) propagates in a spiral form inside it to form a stable mode field inside.

2. The tunable optical frequency comb generation system based on a magnesium fluoride crystal microcavity according to claim 1, characterized in that: The magnesium fluoride crystal micro-cavity (7) is prepared by ultra-precision turning and ultra-precision polishing. The prepared micro-cavity (7) has two end faces. One end is bonded to a millimeter-sized aluminum rod before preparation. The other end provides favorable conditions for effective contact with the piezoelectric ceramic element.

3. The tunable optical frequency comb generation system based on a magnesium fluoride crystal microcavity according to claim 1, characterized in that: By adjusting the polarization controller (4) to optimize the polarization state of the optical fiber and adjusting the coupling position between the micro-cavity (7) and the tapered optical fiber (6), the generated single-mode laser is used to further pump the micro-cavity to generate a parametric oscillation signal. Part of the generated oscillation signal is reflected into the optical fiber loop and subjected to the cyclic amplification effect of the high-concentration erbium-doped fiber (3), and finally a stable optical frequency comb is output.

4. The tunable optical frequency comb generation system based on a magnesium fluoride crystal microcavity according to claim 1, characterized in that: The relationship between the resonant wavelength shift and the change ΔR of the equatorial radius of the micro-cavity (7) is as follows: In the formula, λ is the resonant wavelength, Δλ is the change in resonant wavelength, R is the equatorial radius of the micro-cavity (7), and ΔR is the change in the equatorial radius of the micro-cavity (7); The relationship between the change in equatorial radius ΔR of the micro-cavity (7) and the applied axial stress is as follows: In the formula, v is Poisson's ratio, E is Young's modulus of the material, and ΔF is the magnitude of the axial stress applied to the micro-cavity. Finally, the relationship between the resonant wavelength shift and the change in the equatorial radius ΔR of the micro-cavity (7) is further simplified to:

5. A method for generating a tunable optical frequency comb based on a magnesium fluoride crystal microcavity according to any one of claims 1-4, characterized in that: The specific steps are as follows: Step S1: The continuous laser generated by the 1550nm tunable laser (1) enters the coupling system through the optical isolator and polarization controller (4), and the coupling system is adjusted to generate a whispering gallery mode with an ultra-high quality factor. Step S2: The erbium ions in the high-concentration erbium-doped fiber (3) are pumped by the continuous laser generated by the 980nm laser (1), causing it to spontaneously emit new photons in the 1550nm band. The output light passes through the circulator (5), and the back-direction output of the circulator (5) enters the backscattering loop system, while the forward output enters the micro-cavity (7) through the tapered fiber (6), lasing out a single-mode laser with a high signal-to-noise ratio. By increasing the power of the laser (1) and optimizing the polarization state of the fiber, the single-mode laser further pumps the micro-cavity (7) to generate a parametric oscillation signal. A portion of the parametric oscillation signal is reflected into the fiber ring and subjected to the cyclic amplification effect of the erbium-doped fiber, and finally outputs a stable optical frequency comb. Step S3: The piezoelectric fine adjustment module (8) adopts a piezoelectric ceramic stacking method. By changing the voltage of the piezoelectric ceramic element, an adjustable axial force is applied to the micro-bottle cavity (7), so that the geometry of the micro-bottle cavity (7) changes. Because the equatorial radius of the micro-cavity (7) changes, the resonant wavelength of the micro-cavity (7) undergoes a certain displacement change, causing the position of the output single-mode laser to change with the voltage of the piezoelectric ceramic element. The position of the comb teeth generated by the single-mode laser changes simultaneously, achieving the purpose of tunable comb teeth position of the optical frequency comb. Furthermore, the optical frequency of the 980nm pump laser remains unchanged during the tuning process.

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