Adjustable optical frequency comb generation system and method based on magnesium fluoride crystal micro bottle cavity

Abstract

The invention discloses an adjustable optical frequency comb system and method based on a magnesium fluoride crystal micro-bottle cavity, magnesium fluoride crystals are used as a micro-bottle cavity material, and the system has the optical performance of extremely high quality factor and low absorption loss, so that the system can realize efficient optical frequency comb generation under low pumping power, and the generation efficiency and stability of the optical frequency comb are improved. The piezoelectric ceramic element is introduced into the structure to realize the tuning of the optical frequency comb, and the application of the piezoelectric fine tuning module enables the stress tuning of the micro-bottle cavity to be more accurate and flexible. Through fine adjustment of the axial stress applied to the micro-bottle cavity, rapid tuning of the positions of the comb teeth of the optical frequency comb can be realized, the system structure and the operation process are simplified, and the usability of the system is improved. The adjustability of the optical frequency comb is realized under the condition that the optical frequency of the pump laser is not changed, and the stability and the anti-interference capability of the system are greatly enhanced. Meanwhile, the stable mode field in the micro-bottle cavity provides high signal quality, the output stability of the optical frequency comb is improved, and the application value of the system is enhanced.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical frequency combs, and in particular, relates to an adjustable optical frequency comb generation system and method based on a magnesium fluoride crystal micro-bottle cavity. Background Art

[0002] An optical frequency comb (OFC) is a light source with a fixed comb tooth spacing in the frequency domain. It has high bandwidth and precise optical frequency characteristics and is widely used in many cutting-edge scientific and technological fields, including precision frequency measurement, optical clocks, coherent optical communications, spectral analysis, cosmic exploration, precision ranging, and microwave photonics. The micro-optical frequency comb based on the Kerr effect in an optical microcavity (i.e., the Kerr optical frequency comb, hereinafter referred to as the optical frequency comb) has attracted widespread attention due to its ultra-high quality factor and extremely small mode volume. The generation of the Kerr optical frequency comb originates from the four-wave mixing process in the optical microcavity, in which two pump photons generate a pair of signal light and idler light when annihilated, thereby realizing parametric oscillation. This process not only improves the generation efficiency of the optical frequency comb, but also provides a basis for its flexibility and adaptability in various applications.

[0003] In recent years, people have used a variety of methods to produce tunable optical frequency combs. Tunability can be understood from two dimensions: one is the tunability of the position of the comb teeth of the optical frequency comb, and the other is the tunability of the frequency spacing of the comb teeth. Chinese invention patent, patent name: Tunable optical frequency comb based on lithium niobate microcavity and its preparation method, application number: 201911171685.5, publication date: 2020.0508 patent uses integrated electrodes to regulate the optical frequency comb generated by the lithium niobate microcavity. The cost of lithium niobate materials is relatively high, and the process of preparing lithium niobate microcavities is relatively complex. In addition, lithium niobate materials are more sensitive to temperature changes, and their optical properties may be affected by environmental changes, thereby affecting the stability of the optical frequency comb. Chinese invention patent, patent name: A communication device, application number: 202211525913.6, publication date: 2024.05.13 patent uses a laser optical feedback device combined with a frequency locking effect to produce a laser optical frequency comb with adjustable repetition frequency. The system is relatively complex to implement.

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

[0005] The present invention provides a tunable optical frequency comb generation system and method based on a magnesium fluoride crystal micro-bottle cavity, so as to solve the problems of insufficient frequency stability and tunability of the optical frequency comb in the above-mentioned background technology, high material cost and complex system implementation.

[0006] In order to achieve the above-mentioned purpose, the technical scheme adopted by the present invention is: an adjustable optical frequency comb generation system based on a magnesium fluoride crystal micro-bottle cavity, including a laser, a wavelength division multiplexer, a high-concentration erbium-doped optical fiber, a polarization controller, a circulator, a coupling system consisting of a tapered optical fiber and a micro-bottle cavity, a piezoelectric ceramic element consisting of a piezoelectric fine adjustment module and a clamping coarse adjustment module, an optical fiber beam splitter, a photodetector, an oscilloscope and a spectrometer;

[0007] The laser is connected to a wavelength division multiplexer, the wavelength division multiplexer is connected to a high-concentration erbium-doped fiber, the high-concentration erbium-doped fiber is connected to a polarization controller, and the polarization controller is connected to a circulator; the output of the circulator is divided into two paths, the back output end of the circulator is connected to the wavelength division multiplexer, forming a backscattering circulation system; the forward output light of the circulator enters a coupling system composed of a tapered optical fiber waveguide and a magnesium fluoride crystal microbottle cavity, and the output light of the coupling system enters a fiber beam splitter and is divided into two paths, one of which is connected to a photodetector to convert the optical signal into an electrical signal and transmit it to an oscilloscope; the other is connected to a spectrometer to read and store an optical frequency comb, forming a single-mode laser lasing system. By controlling the coupling position between the tapered optical fiber and the microbottle cavity, optimizing the polarization state of the optical fiber and tuning the pump power of the 980nm laser, the single-mode laser can be used as a pump to output a stable optical frequency comb through four-wave mixing. Further, stress is applied to the microbottle cavity through a piezoelectric fine-tuning module to change the optical mode wavelength of the microbottle cavity, thereby realizing the adjustability of the optical frequency comb and forming a stress tuning system.

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

[0009] The wavelength division multiplexer is a 980 / 1550nm wavelength division multiplexer, which is a key optical device responsible for effectively combining and separating optical signals of different wavelengths. The multiplexer can simultaneously process optical signals of two bands, 980nm and 1550nm, so that the pump light and signal light generated by the laser can be transmitted in the same optical fiber without interfering with each other. Through wavelength division multiplexing technology, the system can achieve higher optical signal transmission efficiency and better spectrum utilization. In addition, the high selectivity and low insertion loss characteristics of the wavelength division multiplexer ensure the high fidelity of the optical signal during transmission, greatly improving the performance of the overall system.

[0010] The main function of the polarization controller is to accurately control the polarization state of the input light. By adjusting the polarization direction and intensity of the light, the polarization controller can ensure 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] The circulator is used to form a circulation system to feed back the backscattered signal into the wavelength division multiplexer and also protect the laser.

[0012] The coupling system consists of a tapered optical fiber and a micro-bottle cavity. For the micro-bottle cavity, it is necessary to couple the light into the micro-bottle cavity through the evanescent field to efficiently excite the resonant mode, while the tapered optical fiber uses the evanescent field generated by the thin cone part to achieve light field coupling, which has extremely high coupling efficiency. In addition, the phase matching condition between the tapered optical fiber and the micro-bottle cavity is adjustable, and the coupling efficiency of the two can be close to 100%. This design not only improves the performance of the system, 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 is mainly composed of a piezoelectric fine-tuning module and a clamping coarse-tuning module; the piezoelectric fine-tuning module is used to apply a small stress to the micro-bottle cavity; the clamping coarse-tuning module is used to fix the micro-bottle cavity and coarsely adjust the position of the micro-bottle cavity.

[0014] Photodetectors are used to receive optical signals from the coupling system and convert them into electrical signals. Through high sensitivity detection of optical signals, photodetectors are able to capture changes in weak optical signals, which are then transmitted to an oscilloscope for further analysis and display.

[0015] The spectrometer is used to read and store the spectral characteristics of the laser signal. The spectrometer can monitor and collect data on 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 the comb range, comb spacing, and signal-to-noise ratio.

[0016] Furthermore, the micro-bottle cavity is a micro-bottle cavity made of magnesium fluoride crystal material, which has a complex three-dimensional structure and rich axial. The axial mode in the micro-bottle cavity can eliminate the limitation of the azimuthal mode on dispersion control and improve the flexibility of dispersion control. The micro-bottle cavity itself has a strong surface characteristic and supports radial and angular mode families at the same time. When the radius of the micro-bottle cavity changes along the long axis of the bottle, axial modes of different orders are excited, thereby showing excellent dispersion management potential. This dispersion characteristic plays a vital role in the generation of optical frequency combs because it allows precise control of phase matching conditions within a wide band. The light entering the micro-bottle cavity through tapered fiber coupling rotates back to the initial incident point through the turning point after propagating several times around the cavity axis, thereby forming a stable mode field inside the micro-bottle cavity. By selecting the appropriate axial coupling point position, different whispering gallery modes can be effectively excited, further enhancing the tuning ability of the system and the adjustability of the frequency comb teeth.

[0017] The micro-bottle cavity uses magnesium fluoride crystal material, which has ultra-high stability and is insensitive to ambient humidity. The prepared micro-bottle cavity can be stored for a long time; the ultra-high quality factor reduces the threshold power of the micro-bottle cavity to generate an optical frequency comb; the magnesium fluoride crystal material has weak anomalous dispersion near the 1550nm band, which improves the four-wave mixing phase matching efficiency in the optical frequency comb generation process, thereby increasing the broadband range of the optical frequency comb; it has a small absorption coefficient, a large nonlinear coefficient, a high purity and few defects, and these characteristics have great advantages for the generation and tuning of the micro-bottle cavity optical frequency comb.

[0018] Furthermore, the magnesium fluoride crystal micro-bottle cavity is prepared by ultra-precision turning and ultra-precision polishing; the prepared micro-bottle cavity has two end faces, one end of which is adhered to a millimeter-level metal aluminum rod before preparation; and the other end provides favorable conditions for effective contact with a piezoelectric ceramic element.

[0019] Furthermore, by adjusting the polarization controller to optimize the polarization state of the optical fiber and adjusting the coupling position of the micro-bottle cavity and the tapered optical fiber, the generated single-mode laser is used to further pump the micro-bottle cavity to generate a parametric oscillation signal. A part of the generated oscillation signal is reflected into the optical fiber loop and amplified by the high-concentration erbium-doped fiber, and finally a stable optical frequency comb is output.

[0020] Among them, the piezoelectric fine-tuning module 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; under the effect of stress, the resonance peak mode spectrum of the micro-bottle cavity will also drift.

[0021] Furthermore, the relationship between the resonant wavelength shift and the change in the equatorial radius of the microbottle cavity ΔR is:

[0022] The relationship between the change in the equatorial radius of the microbottle cavity ΔR and the axial applied stress is: Where v is Poisson's ratio, E is the Young's modulus of the material, and F is the magnitude of the axial stress applied to the microbottle cavity;

[0023] Finally, the relationship between the resonant wavelength shift and the change in the equatorial radius of the microbottle cavity ΔR can be further simplified as

[0024] The tunability of the optical frequency comb is the tunability of the comb tooth position of the optical frequency comb, and the comb tooth position of the optical frequency comb is mainly determined by the diameter of the micro-bottle cavity and the refractive index of the material. The micro-bottle cavity is deformed by applying stress to the micro-bottle cavity through the piezoelectric fine tuning module, and the refractive index of the micro-bottle cavity also changes due to the influence of the elastic-optical effect, causing the resonant wavelength of the micro-bottle cavity to change with the change of the voltage on the piezoelectric ceramic element, and finally the comb tooth position of the optical frequency comb changes, achieving the purpose of tunable comb tooth position of the optical frequency comb, and the optical frequency of the 980nm pump laser remains unchanged during the tuning process.

[0025] In summary, the tunable optical frequency comb generation system based on the magnesium fluoride crystal micro-bottle cavity 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 solution are:

[0027] 1. The present invention provides an adjustable optical frequency comb generation system based on a magnesium fluoride crystal micro-bottle cavity, which uses magnesium fluoride crystal as the micro-bottle cavity material and has optical properties of extremely high quality factor and low absorption loss, so that the system can achieve efficient optical frequency comb generation at a lower pump power, thereby improving the generation efficiency and stability of the optical frequency comb.

[0028] 2. The present invention provides an adjustable optical frequency comb generation system based on a magnesium fluoride crystal micro-bottle cavity. The structure introduces piezoelectric ceramic elements to achieve the tunability of the optical frequency comb. The application of the piezoelectric fine-tuning module makes the stress tuning of the micro-bottle cavity more accurate and flexible. By finely adjusting the axial stress applied to the micro-bottle cavity, the position of the optical frequency comb teeth can be quickly tuned, which simplifies the system structure and operation process and improves the ease of use of the system.

[0029] 3. The present invention provides an adjustable optical frequency comb generation system based on a magnesium fluoride crystal microbottle cavity. The system design allows the optical frequency comb to be adjustable without changing the optical frequency of the pump laser. Such a design greatly enhances the stability and anti-interference ability of the system. At the same time, the stable mode field inside the microbottle cavity provides high signal quality, which helps to improve the output stability of the optical frequency comb and further enhances the application value of the system. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0031] Figure 2 It is the geometrical diagram and important parameters of the microbottle cavity;

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

[0033] Figure 4 This is the Lorentz fitting diagram of the transmission spectrum of the magnesium fluoride crystal micro-bottle cavity test;

[0034] Figure 5 This is a simulation diagram of the magnesium fluoride crystal microbottle cavity subjected to axial stress;

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

[0036] Figure 7 It is the optical frequency comb produced by the magnesium fluoride crystal microbottle cavity test;

[0037] in:

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

[0039] The specific implementation methods of the present invention are further explained in detail below by describing the embodiments with reference to the accompanying drawings, with the aim of helping those skilled in the art to have a more complete, accurate and in-depth understanding of the concept and technical solution of the present invention and facilitating its implementation.

[0040] like Figures 1 to 7As shown, the present invention is an adjustable optical frequency comb generation system and method based on magnesium fluoride crystal microbottle cavity. Magnesium fluoride crystal is used as the microbottle cavity material, which has the optical properties of extremely high quality factor and low absorption loss, so that the system can achieve efficient generation of optical frequency comb at a lower pump power, thereby improving the generation efficiency and stability of the optical frequency comb. The structure introduces piezoelectric ceramic elements to achieve the tunability of the optical frequency comb, and the application of the piezoelectric fine tuning module makes the stress tuning of the microbottle cavity more accurate and flexible. By finely adjusting the axial stress applied to the microbottle cavity, the position of the comb teeth of the optical frequency comb can be quickly tuned, which simplifies the system structure and operation process and improves the ease of use of the system. The design of the system allows the adjustability of the optical frequency comb without changing the optical frequency of the pump laser. Such a design greatly enhances the stability and anti-interference ability of the system. At the same time, the stable mode field inside the microbottle cavity provides high signal quality, which helps to improve the output stability of the optical frequency comb and further enhances the application value of the system.

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

[0042] Embodiment 1:

[0043] An adjustable optical frequency comb system based on a magnesium fluoride crystal micro-bottle cavity, comprising a laser 1, a wavelength division multiplexer 2, a high-concentration erbium-doped optical fiber 3, a polarization controller 4, a circulator 5, a coupling system consisting of a tapered optical fiber 6 and a micro-bottle cavity 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, the wavelength division multiplexer 2 is connected to a high-concentration erbium-doped fiber 3, the high-concentration erbium-doped fiber 3 is connected to a polarization controller 4, and the polarization controller 4 is connected to a circulator 5; the output of the circulator 5 is divided into two paths, and the backward output end of the circulator 5 is connected to the wavelength division multiplexer 2 to form a backscattering circulation system; the forward output light of the circulator 5 enters the coupling system composed of the tapered fiber 6 and the micro-bottle cavity 7, and the output light of the coupling system enters the fiber beam splitter 10 to divide the light into two paths, one of which is connected to the photodetector 11 to convert the optical signal into an electrical signal and transmit it to the oscilloscope 12; the other is connected to the 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-bottle cavity 7, optimizing the polarization state of the optical fiber and tuning the pump power of the 980nm laser, the single-mode laser can be used as a pump to output a stable optical frequency comb through four-wave mixing. Further, stress is applied to the micro-bottle cavity 7 by the piezoelectric fine tuning module 8 so that the optical mode wavelength of the micro-bottle cavity 7 changes, thereby achieving the adjustability 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 transition, thereby providing gain for the system.

[0046] The wavelength division multiplexer 2 is a 980 / 1550nm wavelength division multiplexer, which is a key optical device responsible for effectively combining and separating optical signals of different wavelengths. The multiplexer can simultaneously process optical signals of two bands, 980nm and 1550nm, so that the pump light and signal light generated by the laser 1 can be transmitted in the same optical fiber without interfering with each other. Through wavelength division multiplexing technology, the system can achieve higher optical signal transmission efficiency and better spectrum utilization. In addition, the high selectivity and low insertion loss characteristics of the wavelength division multiplexer 2 ensure the high fidelity of the optical signal during transmission, greatly improving the performance of the overall system.

[0047] The main function of the polarization controller 4 is to accurately control the polarization state of the input light. By adjusting the polarization direction and intensity of the light, the polarization controller 4 can ensure 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] The circulator 5 is used to form a circulation system to feed back the backscattered signal to the wavelength division multiplexer 2 and also plays a role in protecting the laser 1 .

[0049] The coupling system consists of a tapered optical fiber 6 and a micro-bottle cavity 7. For the micro-bottle cavity 7, it is necessary to couple light into the micro-bottle cavity 7 by means of the evanescent field to efficiently excite the resonance mode, and the tapered optical fiber 6 uses the evanescent field generated by the thin cone part to achieve light field coupling, which has a very high coupling efficiency. In addition, the phase matching condition between the tapered optical fiber 6 and the micro-bottle cavity 7 is adjustable, and the coupling efficiency of the two can be close to 100%. This design not only improves the performance of the system, but also provides a more reliable basis for the generation of optical frequency combs.

[0050] The micro-bottle cavity 7 is fixed on a piezoelectric ceramic element, which is mainly composed of a piezoelectric fine-tuning module 8 and a clamping coarse-tuning module 9; the piezoelectric fine-tuning module 8 is used to apply a small stress to the micro-bottle cavity 7; the clamping coarse-tuning 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 the optical signal from the coupling system and convert it into an electrical signal. Through the high sensitivity detection of the optical signal, the photodetector 11 can capture the change of the weak optical signal, and these electrical signals 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 collect data of 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 the comb tooth range, comb tooth spacing, and signal-to-noise ratio.

[0053] Furthermore, the micro-bottle cavity 7 is a micro-bottle cavity made of magnesium fluoride crystal material, and its structure has an axial symmetry characteristic, which can support radial and angular mode families at the same time, so that light can propagate in multiple dimensions in the micro-bottle cavity 7 and has rich axial modes. Figure 2 As shown, the micro-bottle cavity 7 has a bottle-shaped optical structure, and its main parameters are the cavity bottleneck diameter L b , maximum diameter D b And the short axis diameter D s The contour of the micro-bottle cavity 7 can be fitted by an approximate parabolic function, expressed as D(z)=D b ×[1-1 / 2(Δk·z) 2 ], where D(z) is the cross-sectional radius of the micro-bottle cavity 7 at different axial positions, z is the length along the center of the bottle axis, and Δk is the curvature of the contour of the micro-bottle cavity 7. Light enters the micro-bottle cavity 7 through the near-field coupling of the tapered optical fiber 6, and can propagate in the axial spiral form inside it, which can significantly enhance the intensity of two light fields that are far apart in space. In addition, the micro-bottle cavity 7 has a series of equidistant axial characteristic frequencies. By changing the coupling distance between the tapered optical fiber 6 and the micro-bottle cavity 7, transmission spectra of different forms can be selectively excited. The rich axial modes can eliminate the limitations of the azimuthal modes on dispersion control, improve the flexibility of dispersion control, and provide ideal conditions for the generation of micro-bottle cavity optical frequency combs.

[0054] The micro-bottle cavity 7 uses magnesium fluoride crystal material, which has ultra-high stability and is insensitive to ambient humidity. The prepared micro-bottle cavity 7 can be stored for a long time; the ultra-high quality factor reduces the threshold power of the micro-bottle cavity 7 to generate an optical frequency comb; the magnesium fluoride crystal material has weak anomalous dispersion near the 1550nm band, which improves the four-wave mixing phase matching efficiency in the optical frequency comb generation process, thereby increasing the broadband range of the optical frequency comb; it has a small absorption coefficient, a large nonlinear coefficient, a high purity and few defects, and these characteristics have great advantages for the generation and tuning of the optical frequency comb of the micro-bottle cavity 7.

[0055] Furthermore, the magnesium fluoride crystal micro-bottle cavity 7 is prepared by ultra-precision turning and ultra-precision polishing; Figure 3 As shown, the magnesium fluoride crystal micro-bottle cavity 7 prepared by the four steps of cutting, grinding, rough polishing and fine polishing has an ultra-high quality factor Q value. Figure 4 As shown in the figure, the quality factor Q value after fine polishing reaches 4×10 8, greatly increasing the efficiency of generating optical frequency combs. The prepared micro-bottle cavity 7 has two end faces, one end of which is bonded to a millimeter-level metal aluminum rod before preparation; and the other end provides favorable conditions for effective contact with a 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 of the micro-bottle cavity 7 and the tapered optical fiber 6, the generated single-mode laser is used to further pump the micro-bottle cavity 7 to generate a parametric oscillation signal, and a 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 optical fiber 3, and finally a stable optical frequency comb is output.

[0057] The coupling system is composed of a tapered optical fiber 6 and a micro-bottle cavity 7. For the micro-bottle cavity 7, it is necessary to couple light into the micro-bottle cavity 7 by the evanescent field to efficiently excite the resonance mode, and the tapered optical fiber 6 uses the evanescent field generated by the thin cone part to achieve light field coupling, which has a very high coupling efficiency. In addition, the phase matching condition between the tapered optical fiber 6 and the micro-bottle cavity 7 is adjustable, and the coupling efficiency of the two can be close to 100%. This design not only improves the performance of the system, but also provides a more reliable basis for the generation of optical frequency combs.

[0058] The piezoelectric fine-tuning module 8 adopts a piezoelectric ceramic stacking method, and can apply an axial force of adjustable size to the micro-bottle cavity 7 by changing the voltage of the piezoelectric ceramic element; Figure 5 It is a force distribution diagram obtained by Comsol finite element digital calculation 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 geometric shape of the micro-bottle cavity 7 changes slightly.

[0059] Furthermore, the relationship between the resonant wavelength shift and the change in the equatorial radius ΔR of the microbottle cavity 7 is:

[0060] The relationship between the change in the equatorial radius ΔR of the microbottle cavity 7 and the axial applied stress is: Where 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 microbottle cavity 7;

[0061] Finally, the relationship between the resonant wavelength shift and the change in the equatorial radius ΔR of the microbottle cavity 7 can be further simplified as

[0062] The principle of the tunable optical frequency comb system based on magnesium fluoride crystal micro-bottle cavity is to use the 980nm band laser generated by laser 1 to pump the erbium ions in the erbium-doped fiber, so that it spontaneously radiates and generates new photons in the 1550nm band, providing enough gain for the micro-bottle cavity 7, and then by adjusting the coupling position between the tapered fiber 6 and the micro-bottle cavity 7, a high-performance whispering gallery mode is excited, and finally a stable single-mode laser is emitted. When single-wavelength lasing is achieved, the output is locked to a certain mode of the micro-bottle cavity 7, which meets the key condition that the wavelength matches the pump resonance mode when the optical frequency comb is generated. Subsequently, the newly generated single-mode laser is used to pump the micro-bottle cavity 7 itself to generate a parametric oscillation signal, and finally a stable optical frequency comb output is obtained. The optical frequency comb has extremely high stability, which is also a major advantage of this system. When the optical frequency comb output is stable, the piezoelectric fine adjustment module 8 is used to change the voltage of the piezoelectric ceramic element, and a force is applied to the micro-bottle cavity 7 to change the size of the micro-bottle cavity 7. At the same time, the refractive index of the micro-bottle cavity 7 also changes due to the elastic-photoelectric effect, causing the resonant wavelength of the micro-bottle cavity 7 to change with the change of the voltage of the piezoelectric ceramic element, and finally the comb teeth position of the optical frequency comb is changed, such as Figure 6 As shown, the purpose of tunable comb tooth position of the optical frequency comb is achieved. Figure 7 This is an experimental diagram of the optical frequency comb produced by the system.

[0063] Embodiment 2:

[0064] A method for generating a tunable optical frequency comb based on a magnesium fluoride crystal microbottle cavity, the specific steps 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 the polarization controller 4, and the coupling system is adjusted to generate a whispering gallery mode with an ultra-high quality factor.

[0066] Step S2: Utilize the continuous laser generated by the 980nm laser 1 to pump the erbium ions in the high-concentration erbium-doped fiber, so that it spontaneously radiates and generates new photons in the 1550nm band, and the output light passes through the circulator 5, and the back output of the circulator 5 enters the backscattering circulation system, while the forward output enters the micro-bottle cavity 7 through the tapered optical fiber 6, emitting a single-mode laser with a high signal-to-noise ratio, and then by increasing the power of the laser 1 and optimizing the polarization state of the optical fiber, the single-mode laser further pumps the micro-bottle cavity 7 to generate a parametric oscillation signal. A part of the parametric oscillation signal is reflected into the optical fiber ring and amplified by the cyclic effect of the erbium-doped optical fiber, and finally outputs a stable optical frequency comb.

[0067] Step S3: 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 to change the geometric shape of the micro-bottle cavity 7. Since the equatorial radius of the micro-bottle cavity 7 changes, the resonant wavelength of the micro-bottle cavity 7 changes to a certain extent, so that the output single-mode laser position changes with the voltage change of the piezoelectric ceramic element, and the comb tooth position generated by the single-mode laser changes at the same time, so as to achieve the purpose of tunable comb tooth position of the optical frequency comb, and the optical frequency of the 980nm pump laser remains unchanged during the tuning process.

[0068] In summary, the adjustable optical frequency comb generation system and method based on the magnesium fluoride crystal micro-bottle cavity of the present invention not only has the ability to generate optical frequency combs efficiently and stably, but also provides an innovative solution for the field of optical applications through simple tuning methods and excellent material properties, and has broad market application prospects.

[0069] The present invention is described above by way of example in conjunction with the accompanying drawings. Obviously, the specific implementation of the present invention is not limited to the above-mentioned method. As long as various non-substantial improvements are made using the method concept and technical solution of the present invention; or the above-mentioned concept and technical solution of the present invention are directly applied to other occasions without improvement, they are all within the protection scope of the present invention.

Claims

1. An adjustable optical frequency comb generation system based on magnesium fluoride crystal microbottle cavity, characterized by: It comprises a laser (1), a wavelength division multiplexer (2), a high-concentration erbium-doped optical fiber (3), a polarization controller (4), a circulator (5), a coupling system consisting of a tapered optical fiber (6) and a micro-bottle cavity (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); 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), the wavelength division multiplexer (2) is connected to a high-concentration erbium-doped optical fiber (3), the high-concentration erbium-doped optical fiber (3) is connected to a polarization controller (4), the polarization controller (4) is connected to a circulator (5), the reverse output end of the circulator (5) is connected to the wavelength division multiplexer (2), the forward output end of the circulator (5) is connected to a coupling system, the output light of the coupling system enters an optical fiber beam splitter (10) to split the light into two paths, one of which is connected to a photodetector (11) to convert the optical signal into an electrical signal and transmit it to an oscilloscope (12); the other is connected to a spectrometer (13) to read and store an optical frequency comb.

2. The adjustable optical frequency comb generation system based on magnesium fluoride crystal micro-bottle cavity according to claim 1 is characterized in that: The micro-bottle cavity (7) is a micro-bottle cavity made of magnesium fluoride crystal material, has a three-dimensional structure and supports radial and angular mode families at the same time. The micro-bottle cavity (7) has a rotationally symmetric structure, and light coupled into the micro-bottle cavity (7) via the tapered optical fiber (6) propagates in a spiral form inside the micro-bottle cavity (7), forming an internal stable mode field.

3. The adjustable optical frequency comb generation system based on magnesium fluoride crystal micro-bottle cavity according to claim 2 is characterized in that: The magnesium fluoride crystal micro-bottle cavity (7) is prepared by ultra-precision turning and ultra-precision polishing; the prepared micro-bottle cavity (7) has two end faces, one end of which is adhered to a millimeter-level metal aluminum rod before preparation; and the other end provides favorable conditions for effective contact with a piezoelectric ceramic element.

4. The adjustable optical frequency comb generation system based on magnesium fluoride crystal micro-bottle cavity according to claim 1 is 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-bottle cavity (7) and the tapered optical fiber (6), the generated single-mode laser is used to further pump the micro-bottle cavity to generate a parametric oscillation signal. A part of the generated oscillation signal is reflected into the optical fiber loop and is cyclically amplified by the high-concentration erbium-doped optical fiber (3), and finally a stable optical frequency comb is output.

5. The adjustable optical frequency comb generation system based on magnesium fluoride crystal micro-bottle cavity according to claim 1 is characterized in that: The relationship between the resonant wavelength shift and the change in the equatorial radius ΔR of the microbottle cavity (7) is: The relationship between the change in the equatorial radius ΔR of the microbottle cavity (7) and the axial applied stress is: Where 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 microbottle cavity; Finally, the relationship between the resonant wavelength shift and the change in the equatorial radius ΔR of the microbottle cavity (7) can be further simplified as 6. A method for generating an adjustable optical frequency comb based on a magnesium fluoride crystal micro-bottle cavity according to any one of claims 1 to 5, characterized in that: The specific steps are as follows: Step S1: The continuous laser light generated by the 1550 nm tunable laser (1) enters the coupling system through the optical isolator and the polarization controller (4), and the coupling system is adjusted to generate a whispering gallery mode with an ultra-high quality factor; Step S2: Utilize the continuous laser light generated by the 980nm laser (1) to pump the erbium ions in the high-concentration erbium-doped optical fiber (3), so that the erbium ions spontaneously radiate to generate new photons in the 1550nm band, and the output light passes through the circulator (5), and the back output of the circulator (5) enters the backscattering circulation system, while the forward output enters the microbottle cavity (7) through the tapered optical fiber (6), and emits a single-mode laser with a high signal-to-noise ratio. Then, by increasing the power of the laser (1) and optimizing the polarization state of the optical fiber, the single-mode laser further pumps the microbottle cavity (7) to generate a parametric oscillation signal; a part of the parametric oscillation signal is reflected into the optical fiber ring and is subjected to the cyclic amplification effect of the erbium-doped optical fiber, and finally outputs a stable optical frequency comb; Step S3: The piezoelectric fine-tuning module (8) adopts a piezoelectric ceramic stacking method, and by changing the voltage of the piezoelectric ceramic element, an axial force with adjustable size can be applied to the micro-bottle cavity (7), so that the geometric shape of the micro-bottle cavity (7) changes; As the equatorial radius of the micro-bottle cavity (7) changes, the resonant wavelength of the micro-bottle cavity (7) undergoes a certain displacement change, so that the output single-mode laser position changes with the voltage change of the piezoelectric ceramic element, and the comb tooth position generated by the single-mode laser changes at the same time, so that the comb tooth position of the optical frequency comb can be tuned, and the optical frequency of the 980nm pump laser remains unchanged during the tuning process.

Images