A device and method for generating cavity solitons and optical frequency combs

Abstract

The application provides a cavity soliton and optical frequency comb generating device and method, comprising a pump source, a polarization beam splitter, an acousto-optic modulator, an optical coupler I, an optical coupler II, a mode selection coupler I and a mode selection coupler II; the pump source, the polarization beam splitter, the acousto-optic modulator, the optical coupler I and the mode selection coupler I are sequentially connected through a single-mode optical fiber; the polarization beam splitter, the optical coupler II and the mode selection coupler I are sequentially connected through a single-mode optical fiber; the mode selection coupler I and the mode selection coupler II are connected through a few-mode optical fiber; the mode selection coupler II and the optical coupler I and the optical coupler II are connected through single-mode optical fibers respectively. The cavity soliton and optical frequency comb generating device of the application circulates and utilizes the fundamental mode and high-order modes in turn based on a few-mode optical fiber resonant cavity, the obtained optical frequency comb bandwidth is much larger than the traditional cavity soliton bandwidth, and the spectral flatness is better.

Description

Technical Field

[0001] This application belongs to the field of passive resonant cavity soliton optical frequency comb technology, and more specifically, it relates to a cavity soliton and optical frequency comb generation device and method. Background Technology

[0002] Kerr resonators are among the simplest systems supporting complex nonlinear optical phenomena, and have attracted widespread attention in recent years due to their practical value in generating time-cavity solitons and frequency combs with ultrashort pulse widths. Ultrashort laser pulses exhibit femtosecond laser pulses in the time domain, providing femtosecond or even attosecond time resolution, while possessing broadband spectral characteristics in the frequency domain. Broadband optical frequency combs show promising applications in precision metrology, coherent communication, and lidar.

[0003] Cavity solitons in Kerr resonators exhibit many significant time-frequency characteristics. The formation mechanism of traditional soliton optical frequency combs stems from the balance between anomalous dispersion and nonlinearity within the cavity, resulting in hyperbolic secant shapes in both the time and frequency domains. This hyperbolic secant envelope significantly limits spectral flatness and bandwidth, and also results in relatively high peak-to-average power, making them susceptible to nonlinear effects during transmission. This limitation generally necessitates more complex filtering processes to achieve spectral shaping, which reduces the overall signal-to-noise ratio during data transmission.

[0004] Previous all-optical soliton operations only considered a single polarization scalar model (i.e., single-mode excitation). However, due to the increasingly high requirements for high-speed and high-capacity signal processing, it is difficult to meet the requirements of high-speed and high-capacity signal processing by only considering single polarization characteristics with a single spatial distribution. Summary of the Invention

[0005] The purpose of this application is to provide a cavity soliton and optical frequency comb generation device and method to solve the technical problems in the prior art, such as poor spectral flatness of soliton optical frequency combs, limited spectral bandwidth, and difficulty in meeting the requirements of high-speed and high-capacity signal processing by single-mode excitation.

[0006] To achieve the above objectives, the technical solution adopted in this application is as follows:

[0007] In a first aspect, this application provides a cavity soliton and optical frequency comb generating device, including a pump source, a polarization beam splitter, an acousto-optic modulator, an optical coupler I, an optical coupler II, a mode selection coupler I, and a mode selection coupler II;

[0008] The pump source, the polarization beam splitter, the acousto-optic modulator, the optical coupler I, and the mode selection coupler I are connected in sequence via single-mode optical fiber;

[0009] The polarization beam splitter, the optical coupler II, and the mode selection coupler I are connected sequentially via single-mode optical fiber;

[0010] The mode selection coupler I and the mode selection coupler II are connected by a few-mode fiber; a few-mode fiber resonant cavity is formed between the mode selection coupler I and the mode selection coupler II;

[0011] The mode selection coupler II and the optical coupler I are connected by single-mode optical fibers.

[0012] Furthermore, the few-mode fiber includes a cladding and an intermediate core located within the cladding, multiple germanium rods, and two boron-doped stress rods;

[0013] The fiber core has an elliptical structure;

[0014] Multiple germanium rods are arranged in multiple hexagonal rings around the fiber core, and the germanium doping concentration of the fiber core is different from the germanium doping concentration of the germanium rods; the diameter of the germanium rods in the innermost ring is smaller than the diameter of the germanium rods in the outer ring;

[0015] Two boron-doped stress bars are symmetrically arranged within the cladding and are respectively located on both sides of the array structure formed by the fiber core and the plurality of germanium bars.

[0016] Furthermore, the major axis and minor axis of the fiber core are m = 8 μm and n = 6 μm, respectively, and the cladding diameter is 125 μm.

[0017] Furthermore, the germanium rods are arranged in a three-ring hexagonal pattern around the fiber core, with the diameter of the germanium rod in the innermost hexagonal ring being d1 = 1.9 μm and the distance between the centers of two adjacent germanium rods being P0 = 6 μm.

[0018] Furthermore, the diameter of the germanium rods located in the hexagonal arrays in the middle and outermost circles is d2 = 2.9 μm, and the distance between the centers of two adjacent germanium rods is P = 5.5 μm.

[0019] Furthermore, the distance between the center of the boron-doped stress bar and the fiber core is 42 μm, and the radius of the boron-doped stress bar is 15 μm.

[0020] Furthermore, the germanium doping concentration of the fiber core is 10.39%, the germanium doping concentration of the germanium rod is 4.3%, and the boron doping concentration of the boron-doped stress rod is 15%.

[0021] Furthermore, the pump source is a monochromatic continuous-wave laser with a power of 0.5W and a wavelength of 1570nm, and the linear polarization angle of the pump field is 0.25π; the polarization beam splitter has a splitting ratio of 50:50; the coupling coefficients of optical coupler I and optical coupler II are both 2 / 15, and the total cavity length of the few-mode fiber resonator is 34m.

[0022] Furthermore, the second-order dispersions of the fundamental mode LP01 and the higher-order mode LP11 in the few-mode fiber at 1570 nm are -2.5 × 10⁻⁶. -26 s 2 / m and 2.5×10 -26 s 2 / m, and the third-order dispersions are 1.3×10 -40 s 3 / m and -2.3×10 -40 s 3 / m; the self-phase nonlinearities of the fundamental mode and higher-order modes of the few-mode fiber are 0.0032W, respectively. -1 m -1 and 0.0024W -1 m -1 The cross-phase nonlinearity coefficient between the two modes is 0.0017W. -1 m -1 .

[0023] Secondly, this application also provides a method for generating a cavity soliton and an optical frequency comb using the cavity soliton and optical frequency comb generating device described above, comprising the following steps:

[0024] When the pump source is turned on, the optical pulse is split into two optical paths in the fundamental mode state after passing through the polarization beam splitter. One optical path is converted into a higher-order mode state after passing through the acousto-optic modulator, optical coupler I, and mode selection coupler I; the other optical path remains in the fundamental mode state after passing through optical coupler II and mode selection coupler I.

[0025] The optical paths of the fundamental mode and higher-order modes propagate in the few-mode fiber resonator, then pass through mode selection coupler II. The output optical field then passes through optical coupler I and optical coupler II respectively. The above process is repeated, and the optical field circulates once more in the few-mode fiber resonator. After the fundamental mode and higher-order modes circulate twice in the few-mode fiber resonator, it is considered a complete cycle. The first optical field and the second optical field are observed from the output end of mode selection coupler II, respectively, thus obtaining the flat cavity soliton and the optical frequency comb.

[0026] Compared with the prior art, this application has the following technical effects:

[0027] This application discloses a cavity soliton and optical frequency comb generation device that can simultaneously output optical paths in both fundamental and higher-order modes, breaking the barrier of single-mode excitation. By providing another controllable dimension, namely multi-mode simultaneous excitation, it can meet the requirements for high-speed and high-capacity signal processing. The device of this application has a simple structure, low cost, and is easy to operate. It uses a mode selection coupler to realize the conversion between the fundamental mode and higher-order modes, thereby realizing the coupled transmission of multi-mode optical fields in the cavity. It can be used to study the nonlinear coupling dynamics between modes in a few-mode resonant cavity.

[0028] The cavity soliton and optical frequency comb generation device of this application utilizes the fundamental mode and higher-order modes alternately and cyclically based on a few-mode fiber resonant cavity, resulting in an optical frequency comb bandwidth that is much greater than that of a traditional cavity soliton, and also has better spectral flatness. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 A schematic diagram of a cavity soliton and optical frequency comb generating device provided in an embodiment of this application;

[0031] Figure 2 A schematic diagram of the cross-sectional structure of a few-mode optical fiber provided in an embodiment of this application;

[0032] Figure 3 A graph showing the variation of few-mode fiber dispersion with wavelength provided in the embodiments of this application;

[0033] Figure 4 (a) The time-domain waveform of the flat soliton frequency comb (without considering third-order dispersion) provided in the application embodiment;

[0034] Figure 4 (b) Spectrum diagrams of the flat soliton frequency comb (without considering third-order dispersion) and the conventional cavity soliton (i.e., standard CS) provided for the application embodiments;

[0035] Figure 5 (a) Time-domain waveform of a flat soliton frequency comb (considering third-order dispersion) provided in the application embodiment;

[0036] Figure 5 (b) Spectrum diagrams of a flat soliton frequency comb (considering third-order dispersion) and a conventional cavity soliton (i.e., standard CS) provided for the embodiments of the application;

[0037] Figure 5 (c) A time-domain pulse evolution diagram of the flat soliton frequency comb 1 (considering third-order dispersion) provided in the application embodiment;

[0038] Figure 5 (d) is the time-domain pulse evolution diagram of the flat soliton frequency comb 2 (considering third-order dispersion) provided in the application embodiment.

[0039] The following are the labeling elements in the figure:

[0040] 1. Pump source, 2. Polarization beam splitter, 3. Acousto-optic modulator, 4. Optical coupler I, 5. Optical coupler II, 6. Mode selection coupler I, 7. Mode selection coupler II. Detailed Implementation

[0041] To make the technical problems, technical solutions, and beneficial effects to be solved by this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and are not intended to limit the scope of this application.

[0042] It should be noted that when a component is referred to as being "fixed to" or "set on" another component, it can be directly on or indirectly on that other component. When a component is referred to as being "connected to" another component, it can be directly connected to or indirectly connected to that other component.

[0043] It should be understood that the terms "length", "upper", "lower", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application.

[0044] Furthermore, the terms "first," "second," "third," "fourth," and "fifth" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined with "first," "second," "third," "fourth," or "fifth" may explicitly or implicitly include one or more of that feature. In the description of this application, "multiple" means two or more, unless otherwise explicitly specified.

[0045] Please read Figure 1 The present application will now describe a cavity soliton and optical frequency comb generating device provided in the embodiments of this application.

[0046] In one embodiment of this application, a cavity soliton and optical frequency comb generating device includes a pump source 1, a polarization beam splitter 2, an acousto-optic modulator 3, an optical coupler I 4, an optical coupler II 5, a mode selection coupler I 6, and a mode selection coupler II 7.

[0047] Among them, pump source 1, polarization beam splitter 2, acousto-optic modulator 3, optical coupler I4 and mode selection coupler I6 are connected in sequence via single-mode fiber; polarization beam splitter 2, optical coupler II5 and mode selection coupler I6 are connected in sequence via single-mode fiber; mode selection coupler I6 and mode selection coupler II7 are connected via few-mode fiber; a few-mode fiber resonant cavity is formed between mode selection coupler I6 and mode selection coupler II7; mode selection coupler II7 is connected to optical coupler I4 and optical coupler II5 respectively via single-mode fiber.

[0048] The cavity soliton and optical frequency comb generation device of this application embodiment can simultaneously output optical paths in both fundamental and higher-order modes, breaking the barrier of single-mode excitation. By providing another controllable dimension, namely multi-mode simultaneous excitation, it can meet the requirements of high-speed and high-capacity signal processing. The device of this application embodiment has a simple structure, low cost, and is easy to operate. It uses mode selection coupler I6 and mode selection coupler II7 to realize the conversion between fundamental and higher-order modes, thereby realizing the coupling transmission of multi-mode optical fields in the cavity. It can be used to study the nonlinear coupling dynamics between modes in a few-mode resonant cavity.

[0049] An embodiment of this application provides a cavity soliton and optical frequency comb generation device that utilizes the fundamental mode and higher-order modes alternately in a few-mode fiber resonant cavity, resulting in an optical frequency comb bandwidth that is much greater than that of a traditional cavity soliton, and also exhibits better spectral flatness.

[0050] Furthermore, the few-mode fiber of this embodiment includes a cladding and an intermediate core within the cladding, multiple germanium rods, and two boron-doped stress rods; the core has an elliptical structure; the multiple germanium rods are arranged in multiple hexagonal rings around the core, and the germanium doping concentration of the core is different from that of the germanium rods; the diameter of the innermost germanium rod is smaller than the diameter of the outermost germanium rod; the two boron-doped stress rods are symmetrically disposed within the cladding and are located on both sides of the array structure formed by the core and the multiple germanium rods. The cross-sectional structure of the few-mode fiber of this embodiment is as follows: Figure 2 As shown, the few-mode fiber in this embodiment is a hybrid of a step-index structure and a microstructure. Two symmetrical boron-doped stress bars are introduced into the cladding of this novel step-microstructure hybrid fiber, which can simultaneously adjust dispersion and birefringence. A few-mode fiber with equal dispersion and opposite signs at a certain wavelength has been designed and optimized. This few-mode fiber has a simple structure and supports different modes as multiple channels for information transmission. Applying it to a few-mode fiber resonator can generate a cavity soliton optical frequency comb with a flatter spectrum.

[0051] Specifically, the few-mode fiber used in this embodiment has an elliptical germanium-doped core, surrounded by an array of microstructured germanium rods of varying concentrations, and contains two boron-doped stress rods in the cladding. The major and minor axes of the core are m = 8 μm and n = 6 μm, respectively, and the cladding diameter is 125 μm. Multiple germanium rods form a three-ring hexagonal arrangement around the core. The diameter of the germanium rods in the innermost hexagonal arrangement is d1 = 1.9 μm, and the distance between the centers of two adjacent germanium rods is P0 = 6 μm. The diameter of the germanium rods in the middle and outermost hexagonal arrangements is d2 = 2.9 μm, and the distance between the centers of two adjacent germanium rods is P = 5.5 μm. The center of the boron-doped stress rod is 42 μm from the core, and its radius is 15 μm. It should be noted that the structural parameters of the few-mode fiber in the embodiments of this application are not limited to the specific values ​​listed above. Any few-mode fiber with step refractive index structure and microstructure hybridization can be applied to this application and generate cavity solitons and optical frequency combs that are the same as or equivalent to those in the embodiments of this application.

[0052] In this embodiment, the germanium doping concentration of the fiber core is 10.39%, the germanium doping concentration of the germanium rod is 4.3%, and the boron doping concentration of the boron-doped stress rod is 15%. The refractive index of the boron-doped stress rod is calculated to be 1.426 according to the mixed Sellmeier equation. The main materials of the fiber core, germanium rod, and boron-doped stress rod are all silicon dioxide.

[0053] In this embodiment, the pump source 1 is a monochromatic continuous-wave laser with a power of 0.5W and a wavelength of 1570nm. The linear polarization angle of the pump field is 0.25π. The polarization beam splitter 2 has a splitting ratio of 50:50. The coupling coefficients of optical coupler I 4 and optical coupler II 5 are both 2 / 15. The total cavity length of the few-mode fiber resonator is 34m.

[0054] Furthermore, in the few-mode fiber of this application embodiment, the second-order dispersions of the fundamental mode LP01 and the higher-order mode LP11 at 1570 nm are -2.5 × 10⁻⁶. -26 s 2 / m and 2.5×10 -26 s 2 / m, and the third-order dispersions are 1.3×10 -40 s 3 / m and -2.3×10 - 40 s 3 / m; the self-phase nonlinearities of the fundamental mode and higher-order modes of the few-mode fiber are 0.0032W, respectively. -1 m -1 and 0.0024W -1 m -1 The cross-phase nonlinearity coefficient between the two modes is 0.0017W. -1 m -1.

[0055] Secondly, embodiments of this application also provide a method for generating a cavity solito and an optical frequency comb using the aforementioned cavity solito and optical frequency comb generating device, comprising the following steps:

[0056] When pump source 1 is turned on, the optical pulse is split into two optical paths in the fundamental mode state after passing through polarization beam splitter 2. Both optical paths are in the fundamental mode state. One optical path is pumped and frequency-shifted by acousto-optic modulator 3 to ensure that the pump frequencies of both paths are within the bistable range of their respective resonant peaks. The fundamental mode optical field of one path is converted into a higher-order mode state after passing through optical coupler I4 and mode selection coupler I6. The other fundamental mode optical field is kept in the fundamental mode state after passing through optical coupler II5 and mode selection coupler I6.

[0057] The optical paths of the fundamental mode and higher-order modes propagate in the few-mode fiber resonator, then pass through mode selection coupler II7. The output optical field then passes through optical coupler I4 and optical coupler II5 respectively. The above process is repeated, and the optical field circulates once more in the few-mode fiber resonator. Thus, the output of one mode is coupled to the input of another mode. This cycle continues, and the fundamental mode and higher-order modes circulate twice in the few-mode fiber resonator, which is considered one complete cycle. The first optical field (i.e., optical field 1) and the second optical field (i.e., optical field 2) are observed from the output of mode selection coupler II7, and the flat cavity soliton and optical frequency comb are obtained.

[0058] Specifically, the optical path of the output light field 1 is as follows: pump source 1 is split into two paths after passing through polarization beam splitter 2, denoted as a (fundamental mode) and b (fundamental mode) respectively; the light field of path a passes through optical coupler I4, and then through mode selection coupler I6, in... Figure 1 As shown in the diagram, the light field a, which enters the few-mode fiber from the left of mode selection coupler I6, is marked as the aa (fundamental mode) light field. That is, after circulating once in the few-mode fiber in its fundamental mode state, the original aa (fundamental mode) light field is transformed to the position marked aaa (fundamental mode) in the diagram. The aaa light field then passes through mode selection coupler II7 and combines with the b light field after being split by polarization beam splitter 2. It then passes through optical coupler II5 and then through mode selection coupler I6, becoming the bb (higher-order mode) light field marked as shown in the diagram. After circulating once in the few-mode fiber resonant cavity, the bb light field in its higher-order mode state is transmitted to… Figure 1 The marked position of bbb, and finally, the bbb optical path is output as optical field 1 after passing through the mode selection coupler II 7; in summary, the output optical field 1 is actually: the optical pulse optical field first goes through a cycle of the fundamental mode state and then couples to the higher-order mode state and then cycles again to form a complete cycle.

[0059] The optical path of the output light field 2 is as follows: Pump source 1 is split into two paths after passing through polarization beam splitter 2, denoted as a (fundamental mode) and b (fundamental mode); the b path light field passes through optical coupler II 5, and then through mode selection coupler I 6. Figure 1 As shown in the diagram, the optical field b, after entering the few-mode fiber via mode selection coupler I6, is marked as bb (higher-order mode). That is, after circulating once in the few-mode fiber in a higher-order mode state, the original bb (higher-order mode) optical field is transformed to the position marked bbb (higher-order mode) in the diagram. The bbb optical field then passes through mode selection coupler II7 and combines with the a optical field after being split by polarization beam splitter 2. It then passes through optical coupler I4 and mode selection coupler I6, becoming the aa (fundamental mode) optical field as shown in the diagram. The aa optical field in its fundamental mode state circulates once within the few-mode fiber resonant cavity before being transmitted to… Figure 1 The position marked 'aaa' in the middle is the 'aaa' optical path. Finally, after passing through the mode selection coupler Ⅱ7, the 'aaa' optical path is output as optical field 2. In summary, the output optical field 2 is essentially: the optical pulse optical field first goes through a higher-order mode state and then couples to the fundamental mode state and then cycles through it again to form a complete cycle.

[0060] In single-mode fiber, the optical field of a laser is always in the form of the fundamental mode, while in few-mode fiber, the fundamental mode and higher-order modes are coupled and transmitted.

[0061] Figure 3 This is a dispersion curve of a few-mode fiber according to an embodiment of this application. The parameters of the few-mode fiber are as follows: the second-order dispersions of the fundamental mode LP01 and the higher-order mode LP11 at 1570 nm are -2.5 × 10⁻⁶. -26 s 2 / m and 2.5×10 -26 s 2 / m, and the third-order dispersions are 1.3×10 -40 s 3 / m and -2.3×10 -40 s 3 / m; the self-phase nonlinearities of the fundamental mode and higher-order modes of the few-mode fiber are 0.0032W, respectively. -1 m -1 and 0.0024W -1 m -1 The cross-phase nonlinearity coefficient between the two modes is 0.0017W. -1 m -1 .

[0062] The intracavity optical field evolution of the fundamental mode and higher-order modes in this embodiment can be described by the nonlinear Schrödinger equation:

[0063]

[0064]

[0065] Where α is the fiber loss, z is the propagation distance of the optical field in the cavity, τ is the fast time, and E1 and E2 represent the electric field envelopes of two orthogonal polarization states, respectively; β 2(1) and β 2(2) These are the second-order dispersions of the two modes; β 3(1) and β3(2) These are the third-order dispersions of the two modes. The boundary conditions for the Ikeda map connecting the electric fields of the two modes are:

[0066]

[0067]

[0068] in, Let represent the light field at the beginning of the (m+1)th cycle of the fundamental mode. Represents the optical field at the end of the m-th cycle of the higher-order mode; θ is the power transfer coefficient of the optical coupler, 2 / 15; E in The pump electric field strength is represented by χ = 0.25π; the linear polarization angle of the injection pump is 0.25π; L represents the length of the few-mode fiber resonator, which is 34m; δ1 and δ2 represent the linear detuning of the fundamental mode and higher-order modes, respectively.

[0069] In this embodiment, a pump light is first injected into the resonant cavity using a laser pump source 1. The laser beam is then split into two paths by a polarization beam splitter 2, used for the LP01 fundamental mode and the LP11 higher-order mode within the cavity, respectively. One path undergoes pump frequency shifting via an acousto-optic modulator 3, ensuring that the pump frequencies of both paths are within the bistable range of their respective resonant peaks. The device design in this embodiment aims to ensure that the optical field undergoes a mode substitution before being input to each optical coupler (optical coupler I 4 and optical coupler II 5), so that the two modes have the same optical path and the same frequency detuning.

[0070] One embodiment of this application describes a method for generating cavity solitons and optical frequency combs. Based on a few-mode fiber resonator, the fundamental mode and higher-order modes are used alternately and cyclically. The resulting optical frequency comb has a bandwidth much greater than that of a traditional cavity soliton, and its spectral flatness is also better.

[0071] Simulation was performed using a cavity soliton and optical frequency comb generation device and method according to an embodiment of this application. The specific simulation process and results are as follows:

[0072] In Matlab simulation software, relevant parameters were set to simulate the evolution of the optical signal in a few-mode fiber resonator. The center wavelength of pump source 1 was set to around 1570nm, the power to 0.5W, and the splitting ratio was set to divide the energy into two equal parts for input. These parts then entered the few-mode fiber ring resonator configured with the above parameters. The simulation results are as follows. Figure 4 and Figure 5 As shown.

[0073] Figure 4 (a) is the time-domain waveform of the flat soliton optical frequency comb (without considering third-order dispersion). It can be seen that the time-domain waveforms of the cavity solitons at optical field 1 and optical field 2 are completely symmetrical. The peak power of the time-domain waveforms of the two pulses can reach 2.8W and 1.4W, respectively.

[0074] Figure 4 (b) is a comparison of the spectrum of a flat soliton optical frequency comb (ignoring third-order dispersion) and a conventional cavity soliton (i.e., the standard CS curve in the figure). It can be seen from the figure that the spectra of optical field 1 and optical field 2 are consistent, when the second-order dispersions of the fundamental mode and higher-order modes are -2.5 × 10⁻⁶. -26 s 2 / m and 2.5×10 -26 s 2 At / m, the 3dB and 10dB bandwidths of the optical frequency comb are 8nm and 10nm, respectively. Under equivalent conditions, the 3dB and 10dB bandwidths of the conventional soliton frequency comb generated by the standard single-mode resonant cavity (anomalous dispersion) are 2nm and 4nm, respectively. In comparison, the bandwidth of the optical frequency comb obtained by the device structure of the present application embodiment is much greater than that of the conventional cavity soliton. It can be seen that the soliton optical frequency comb generated by the device structure of the present application embodiment has a larger bandwidth and a flatter spectrum.

[0075] Figure 5 (a) is the time-domain waveform of a flat soliton optical frequency comb (considering third-order dispersion). The time-domain waveform is generally unchanged compared to that without considering third-order dispersion, but a slight asymmetry occurs, and the pulse in the time domain acquires a certain group velocity and drifts.

[0076] Figure 5 (b) is a comparison of the spectra of the obtained flat soliton frequency comb (considering third-order dispersion) and the conventional cavity soliton frequency comb. Note that the third-order dispersion introduced here has opposite signs and relatively close numerical values, i.e., β 3(1) =1.3×10 - 40 s 3 / m, β 3(2) = -2.3 × 10 -40 s 3 / m, theoretically the two can partially cancel each other out, which is also part of the reason why the cavity soliton characteristics do not change much after considering the third-order dispersion.

[0077] Figure 5 (c) and 5(d) respectively plot the time-domain evolution dynamics of the two optical frequency combs after 800 cycles. It can be seen that the pulse waveform basically stabilizes after about 50 cycles, and the stabilized waveform is similar to... Figure 5 The waveforms in (a) match.

[0078] The above embodiments merely illustrate several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of this patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A cavity soliton and optical frequency comb generating device, characterized in that, It includes a pump source, a polarization beam splitter, an acousto-optic modulator, optical coupler I, optical coupler II, mode selection coupler I, and mode selection coupler II; The pump source, the polarization beam splitter, the acousto-optic modulator, the optical coupler I, and the mode selection coupler I are connected in sequence via single-mode optical fiber; The polarization beam splitter, the optical coupler II, and the mode selection coupler I are connected sequentially via single-mode optical fiber; The mode selection coupler I and the mode selection coupler II are connected by a few-mode fiber; a few-mode fiber resonant cavity is formed between the mode selection coupler I and the mode selection coupler II; The mode selection coupler II and the optical coupler I and the optical coupler II are respectively connected by single-mode optical fiber; The optical pulse emitted by the pump source is split into two paths after passing through the polarization beam splitter. One path propagates in the few-mode fiber resonator after passing through the acousto-optic modulator, the optical coupler I, and the mode selection coupler I, serving as the first optical field. The other path propagates in the few-mode fiber resonator after passing through the optical coupler II and the mode selection coupler I, serving as the second optical field. The mode selection coupler II is configured to: transmit the first optical field, after fundamental mode cycling, output from the few-mode fiber to the optical coupler II via a single-mode fiber; and transmit the second optical field, after higher-order mode cycling, output from the few-mode fiber to the optical coupler I via a single-mode fiber.

2. The cavity soliton and optical frequency comb generating device as described in claim 1, characterized in that, The few-mode fiber includes a cladding and an intermediate core located within the cladding, multiple germanium rods, and two boron-doped stress rods; The fiber core has an elliptical structure; Multiple germanium rods are arranged in multiple hexagonal rings around the fiber core, and the germanium doping concentration of the fiber core is different from the germanium doping concentration of the germanium rods; the diameter of the germanium rods in the innermost ring is smaller than the diameter of the germanium rods in the outer ring; Two boron-doped stress bars are symmetrically arranged within the cladding and are respectively located on both sides of the array structure formed by the fiber core and the plurality of germanium bars.

3. The cavity soliton and optical frequency comb generating device as described in claim 2, characterized in that, The major axis and minor axis of the fiber core are m=8 μm and n=6 μm, respectively, and the cladding diameter is 125 μm.

4. The cavity soliton and optical frequency comb generating device as described in claim 2, characterized in that, Multiple germanium rods are arranged in a three-ring regular hexagonal pattern around the fiber core. The diameter of the germanium rod in the innermost regular hexagonal pattern is d1=1.9 μm, and the distance between the centers of two adjacent germanium rods is P0=6 μm.

5. The cavity soliton and optical frequency comb generating device as described in claim 4, characterized in that, The diameter of the germanium rods located in the hexagonal arrays in the middle and outermost circles is d2=2.9 μm, and the distance between the centers of two adjacent germanium rods is P=5.5 μm.

6. The cavity soliton and optical frequency comb generating device as described in claim 2, characterized in that, The distance between the center of the boron-doped stress bar and the fiber core is 42 μm, and the radius of the boron-doped stress bar is 15 μm.

7. The cavity soliton and optical frequency comb generating device as described in claim 2, characterized in that, The germanium doping concentration of the fiber core is 10.39%, the germanium doping concentration of the germanium rod is 4.3%, and the boron doping concentration of the boron-doped stress rod is 15%.

8. The cavity soliton and optical frequency comb generating device as described in claim 1, characterized in that, The pump source is a monochromatic continuous-wave laser with a power of 0.5W and a wavelength of 1570 nm. The linear polarization angle of the pump field is 0.25π. The polarization beam splitter has a splitting ratio of 50:

50. The coupling coefficients of optical coupler I and optical coupler II are both 2 / 15. The total cavity length of the few-mode fiber resonator is 34 m.

9. The cavity soliton and optical frequency comb generating device as described in claim 8, characterized in that, The second-order dispersions of the fundamental mode LP01 and the higher-order mode LP11 in the few-mode fiber at 1570 nm are -2.5 × 10⁻⁶. -26 s 2 / m and 2.5×10 -26 s 2 / m, and the third-order dispersions are 1.3×10 -40 s 3 / m and -2.3×10 -40 s 3 / m; the self-phase nonlinearities of the fundamental mode and higher-order modes of the few-mode fiber are 0.0032 W, respectively. -1 m -1 and 0.0024 W -1 m -1 The cross-phase nonlinearity coefficient between the two modes is 0.0017 W. -1 m -1 .

10. A method for generating a cavity soliton and an optical frequency comb using the cavity soliton and optical frequency comb generating apparatus according to any one of claims 1-9, characterized in that, Includes the following steps: When the pump source is turned on, the optical pulse is split into two optical paths in the fundamental mode state after passing through the polarization beam splitter. One optical path is converted to the fundamental mode state after passing through the acousto-optic modulator, optical coupler I, and mode selection coupler I; the other optical path is kept in the higher-order mode state after passing through optical coupler II and mode selection coupler I. The optical paths of the fundamental mode and higher-order modes propagate in the few-mode fiber resonator, then pass through mode selection coupler II. The output optical field then passes through optical coupler I and optical coupler II respectively. The above process is repeated, and the optical field circulates once more in the few-mode fiber resonator. After the fundamental mode and higher-order modes circulate twice in the few-mode fiber resonator, it is considered a complete cycle. The first optical field and the second optical field are observed from the output end of mode selection coupler II, respectively, thus obtaining the flat cavity soliton and the optical frequency comb.

Images