A fiber laser and method of generating optical pulses

Abstract

The application discloses a kind of optical fiber laser and optical pulse generation method, it is related to optical fiber laser technical field.The specific inclusion nonlinear amplification ring mirror structure, one-way loop structure and first optical coupler;Nonlinear amplification ring mirror structure is connected with one-way loop structure by first optical coupler and constitutes eight-shaped resonator;Thulium-doped optical fiber is arranged in nonlinear amplification ring mirror structure to accumulate nonlinear phase shift;Second polarization controller and polarization maintaining optical fiber are sequentially arranged in one-way loop structure, and polarization maintaining optical fiber and single-mode optical fiber in one-way loop structure form hybrid optical fiber to generate phase delay.The application compensates the nonlinear phase shift accumulated by positive dispersion evolution through the periodic phase matching mechanism formed by hybrid optical fiber, solves the problem that traditional positive dispersion cavity relies on complex cavity dispersion compensation device, realizes the direct stable output of near-chirp birefringence management soliton pulse.

Description

Technical Field

[0001] This application relates to the field of fiber laser technology, and in particular to a fiber laser and a method for generating optical pulses. Background Technology

[0002] Fiber lasers are primarily used to generate and emit ultrashort pulse lasers, such as 1.7-micron ultrashort pulse lasers. These ultrashort pulse lasers fall within the low water absorption and high penetration depth window of biological tissues, and are widely used in biomedical engineering fields such as multiphoton microscopy, optical coherence tomography, and non-destructive testing of deep tissues. In these fields, ultrashort pulse sources capable of directly outputting narrow pulse widths and low chirp are core equipment for ensuring high-resolution imaging and high signal-to-noise ratio detection.

[0003] In existing technologies, a fully normal dispersion cavity structure is commonly used to build 1.7-micron fiber lasers. However, because both silicon-based fibers and thulium-doped gain fibers, commonly used in the 1.7-micron band, exhibit strong normal dispersion characteristics, a large amount of normal dispersion accumulates within the resonant cavity during passive mode-locked operation. To obtain practically applicable extremely short pulses, existing solutions involve extracting the pulse output from the resonant cavity and using a spatial dispersion compensation element composed of a diffraction grating, or connecting it in series with a dispersion compensation fiber. These external components then adjust the phase and compress the pulse width of the output optical pulse.

[0004] However, the aforementioned existing technologies have the following shortcomings: the pulses directly output from the resonant cavity have a huge positive chirp, which requires complex diffraction grating pairs or dispersion compensation fiber groups outside the cavity for pulse compression. The complex diffraction grating pairs or dispersion compensation fiber groups outside the cavity result in a cumbersome optical path and a large size for the entire laser system, making it difficult to meet the miniaturization requirements of modern medical and testing equipment. At the same time, the introduction of additional external spatial optical elements will also increase additional insertion loss, resulting in a reduction in the effective pulse energy acting on the sample. Furthermore, the external grating spatial optical path is highly susceptible to interference from environmental vibrations and thermal drift, causing insufficient reliability and beam pointing stability of the laser system during long-term operation.

[0005] Therefore, this application aims to solve the problem that existing fiber lasers rely on extracavity dispersion compensation, resulting in large size and excessive loss, while also solving the problem that external complex optical paths are easily affected by environmental interference. Summary of the Invention

[0006] The main objective of this application is to provide a fiber laser and a method for generating optical pulses, which aims to optimize the structure of the fiber laser and eliminate the need for external optical components while ensuring near-chill-free output pulses. This not only solves the problems of miniaturization and high insertion loss of the laser, but also avoids the problem of environmental interference during the interaction between the optical path and external optical components.

[0007] To achieve the above objectives, this application proposes a fiber laser, including a nonlinear amplifying ring mirror structure, a unidirectional loop structure, and an optical coupling component. The optical coupling component is connected between the nonlinear magnifying ring mirror structure and the unidirectional loop structure. The nonlinear amplifying ring mirror structure contains a gain fiber, which is used to accumulate a nonlinear phase shift in the optical pulse during transmission. The main optical path of the unidirectional loop structure is composed of single-mode optical fiber, and a unidirectional transmission component, a polarization adjustment component and a polarization-maintaining fiber are arranged sequentially along the beam transmission direction. The polarization adjustment component is used to adjust the polarization state of the beam input to the polarization-maintaining fiber, so as to change the polarization components of the beam on the orthogonal polarization axis. The polarization-maintaining fiber is used to generate phase delay for different polarization components, and the phase delay is used to compensate for the nonlinear phase shift.

[0008] Furthermore, the nonlinear amplifying ring mirror structure and the unidirectional loop structure are interconnected through the optical coupling component to jointly form a figure-eight all-fiber resonant cavity.

[0009] Furthermore, both the input and output ends of the polarization-maintaining fiber are connected to the single-mode fiber to form a hybrid fiber.

[0010] Furthermore, the optical coupling component is a first optical coupler; The two ends of the nonlinear amplifying ring mirror structure are respectively connected to the two loop ports of the first optical coupler to form a bidirectional ring optical path that allows beams to propagate in opposite directions. The bidirectional ring optical path is sequentially connected in series with a bandpass filter, a dispersion-compensating fiber, a wavelength division multiplexer, the gain fiber, and a first polarization controller; wherein... The dispersion-compensating fiber is used to provide positive dispersion. The wavelength division multiplexer is used to inject pump light into the gain fiber; The bandpass filter is used to lock the center wavelength of the opposing beams; The first polarization controller is used to adjust the nonlinear phase shift difference between the opposing propagating beams in order to establish passive mode-locking.

[0011] Furthermore, the unidirectional loop structure is also provided with a second optical coupler; The second optical coupler is used to extract the optical signal of part of the unidirectional loop structure and output it outside the cavity.

[0012] Furthermore, it also includes a third polarization controller and a polarization beam splitter connected in sequence; The input terminal of the third polarization controller is connected to the output terminal of the second optical coupler to adjust the polarization state of the optical signal; The polarization beam splitter is used to split the output optical signal into two orthogonal polarization components.

[0013] Furthermore, the gain fiber is a thulium-doped fiber; and / or the dispersion compensation fiber is an ultra-high numerical aperture fiber.

[0014] Furthermore, the unidirectional transmission component is a polarization-independent optical isolator, and the polarization adjustment component is a second polarization controller; The polarization-independent optical isolator, the second optical coupler, the second polarization controller, and the polarization-maintaining fiber are connected in series along the beam transmission direction of the unidirectional loop structure.

[0015] Furthermore, the operating parameters of the laser and the fiber length satisfy the following ranges: The center wavelength of the bandpass filter is in the range of 1700nm to 1750nm, and its bandwidth is 15nm to 35nm. The length of the gain fiber is 1.0m to 3.0m; The polarization-maintaining fiber has a length of 2.0m to 6.0m; The length of the dispersion compensation fiber is 15.0m to 20.0m.

[0016] This application also discloses a method for generating optical pulses, applied to the aforementioned fiber laser, comprising the following steps: Pump light is injected into the nonlinear amplifying ring mirror structure through a pump source, and the pump power is gradually increased until it exceeds the mode-locking threshold. Adjusting the polarization state within the nonlinear amplifying ring mirror structure changes the nonlinear phase shift accumulation between opposing propagating beams, thereby establishing a passive mode-locked state under positive dispersion. While maintaining a stable mode-locked state, the polarization adjustment component within the unidirectional loop structure is adjusted to change the polarization component of the beam entering the polarization-maintaining fiber. The polarization-maintaining fiber is used to generate phase delays for different polarization components to compensate for the nonlinear phase shift of the accumulated optical pulse, so as to output a near-chirp-free birefringent managed soliton pulse.

[0017] The above technical solution has the following advantages: This application discloses a nonlinear amplifying ring mirror structure, a unidirectional loop structure, and an optical coupling component. The nonlinear amplifying ring mirror structure contains a gain fiber, and the main optical route of the unidirectional loop structure is composed of a single-mode fiber. A unidirectional transmission component, a polarization adjustment component, and a polarization-maintaining fiber are sequentially arranged along the beam transmission direction. When the laser operates in the positive dispersion region, the high birefringence phase delay generated by the polarization-maintaining fiber and the nonlinear phase shift accumulated in the pulse transmission form a periodic phase matching mechanism. This effectively solves the defects of traditional 1.7-micron band positive dispersion fiber lasers, such as huge pulse chirp, large size, and high loss due to reliance on external spatial diffraction gratings for compensation. It realizes the direct and stable output of near-chirp-free birefringence managed soliton pulses in the cavity, greatly improving the compactness and anti-environment interference capability of the whole machine. Attached Figure Description

[0018] The present application will now be described in detail with reference to specific embodiments and accompanying drawings, wherein: Figure 1 This is a structural block diagram of the fiber laser of this application; Figure 2 This is a schematic diagram of the fiber laser of this application; Figure 3 This is a flowchart of the optical pulse generation method of this application.

[0019] In the picture: 10. Nonlinear amplifying ring mirror structure; 11. Wavelength division multiplexer; 12. Gain fiber; 13. First polarization controller; 14. Bandpass filter; 15. Dispersion compensation fiber; 16. Bidirectional ring optical path; 20. Optical coupling assembly; 21. First optical coupler; 30. Unidirectional loop structure; 31. Unidirectional transmission assembly; 32. Second optical coupler; 33. Polarization adjustment assembly; 34. Polarization-maintaining fiber; 40. Third polarization controller; 50. Polarization beam splitter; 60. Pump source; 70. Single-mode fiber; 80. Hybrid fiber. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the following specific embodiments are merely illustrative of this application and do not constitute a limitation thereof.

[0021] In the field of fiber laser technology, especially in the 1.7-micron biological tissue imaging window, existing fully normal dispersive cavity lasers typically produce pulses with large positive chirps during operation, resulting in severe pulse broadening in the time domain. To achieve narrow pulse widths in end-use applications, existing solutions require the introduction of complex grating pairs or spatial dispersion compensation fibers 15 outside the cavity for pulse compression. The root cause of this phenomenon is that the silicon-based single-mode fiber 70 and gain medium commonly used in the 1.7-micron band exhibit strong normal dispersion characteristics, making it impossible to spontaneously cancel the accumulated nonlinear phase shift within the resonant cavity. Furthermore, conventional external compensation elements have rigid spatial alignment requirements and also suffer from high insertion loss.

[0022] Based on the analysis of the problems in the prior art, the core design idea of ​​this application is to directly introduce a birefringence-managed hybrid fiber 80 inside the resonant cavity, so as to use the periodic phase delay induced by the hybrid fiber 80 to offset the positive chirp of pulse accumulation.

[0023] Based on this, this embodiment provides a fiber laser and a method for generating optical pulses thereon, which are described in detail below with reference to specific claims: like Figure 1 and Figure 2 As shown, this application discloses a fiber laser, including a nonlinear amplifying ring mirror structure 10, a unidirectional loop structure 30, and an optical coupling component 20; the optical coupling component 20 is connected between the nonlinear amplifying ring mirror structure 10 and the unidirectional loop structure 30; the nonlinear amplifying ring mirror structure 10 is provided with a gain fiber 12 for accumulating nonlinear phase shift during optical pulse transmission; the main optical route of the unidirectional loop structure 30 is composed of a single-mode fiber 70, and a unidirectional transmission component 31, a polarization adjustment component 33, and a polarization-maintaining fiber 34 are sequentially arranged along the beam transmission direction; the polarization adjustment component 33 is used to adjust the polarization state of the beam input to the polarization-maintaining fiber 34 to change the polarization components of the beam on the orthogonal polarization axes; the polarization-maintaining fiber 34 is used to generate a phase delay for different polarization components, and the phase delay is used to compensate for the nonlinear phase shift.

[0024] In some embodiments, the optical coupling component 20 is disposed between the nonlinear amplifying ring mirror structure 10 and the unidirectional loop structure 30, serving as a bridge and energy distribution mechanism. In specific engineering implementations, the optical coupling component 20 can be a fiber fused biconical tapered coupler; it can also be replaced with a spatial beam splitter assembly. This embodiment employs the optical coupling component 20, enabling stable and quantitative interference and energy transfer of the light beam between two independent ring paths.

[0025] The nonlinear amplifying ring mirror structure 10 contains a gain fiber 12, which constitutes a Sagnac interferometer with gain capability. Regarding material selection, the gain fiber 12 can be made of thulium-doped silica fiber; alternatively, to obtain a wider gain spectrum, it can be replaced with thulium / holmium co-doped silica fiber, without specific limitations. This embodiment, by setting up a nonlinear amplifying ring mirror structure 10 with a gain medium, allows the opposing beams to accumulate a significant nonlinear phase shift after undergoing asymmetric power amplification, thereby inducing the formation of ultrashort pulses through a saturable absorption-like effect, avoiding the risk of thermal damage to saturable absorbers.

[0026] The primary function of the unidirectional transmission component 31 is to restrict the transmission direction of the light beam. In practical applications, the unidirectional transmission component 31 can be a magneto-optical isolator based on the Faraday rotation effect. In this embodiment, the unidirectional transmission component 31 forces the light beam to travel in a single direction within the unidirectional loop structure 30, effectively suppressing the standing wave effect and spatial hole burning phenomenon, and ensuring the stability of the system operation.

[0027] The polarization adjustment component 33 is used to change the polarization injection angle of the beam. The polarization adjustment component 33 can be selected as an optical fiber polarization controller. In this embodiment, the polarization adjustment component 33 can be used to precisely control the ratio of fast axis and slow axis energy entering the subsequent optical fiber (such as polarization-maintaining fiber 34).

[0028] The polarization-maintaining fiber 34 possesses high birefringence characteristics, causing a refractive index difference in polarized light propagating along the orthogonal axis. Specifically, the polarization-maintaining fiber 34 can be a panda-type polarization-maintaining fiber 34; a bowtie-type polarization-maintaining fiber 34 can also be used. Of course, specific fibers can be selected according to needs, as long as it is ensured that the selected fiber can generate a phase delay. In this embodiment, through the structure of the polarization-maintaining fiber 34, a phase delay is generated between different polarization components, and the phase delay and the nonlinear phase shift accumulated during the positive dispersion evolution of the light pulse mutually compensate for each other, thereby achieving effective cancellation of the pulse nonlinear chirp.

[0029] In this embodiment, the nonlinear amplifying ring mirror structure 10 is coupled with the unidirectional loop structure 30. By utilizing the high birefringence characteristics of the polarization-maintaining fiber 34 and the coupling with the nonlinear evolution process, a periodic phase-matching mechanism is spontaneously formed inside the resonant cavity. This directly and dynamically compensates for the accumulated nonlinear phase shift and positive chirp in each cycle of optical pulse transmission. Through the periodic phase-matching mechanism, this embodiment eliminates the dependence of traditional all-positive dispersion lasers on external complex diffraction grating pairs or external dispersion compensation fibers 15. This not only significantly reduces the size and insertion loss of the laser system, but also directly achieves stable output of near-chirp-free, high-quality ultrashort pulses from the source.

[0030] like Figure 2As shown, in one embodiment of this application, the nonlinear amplifying ring mirror structure 10 and the unidirectional loop structure 30 are interconnected through the optical coupling component 20 to jointly form a figure-eight all-fiber resonant cavity.

[0031] In some embodiments, the figure-eight resonant cavity can be directly arc-fused with the same type of single-mode fiber 70 (e.g., SMF-28e type) to form an all-fiber structure. Direct arc-fusion effectively reduces the additional loss at the splice point, ensuring low-loss transmission of optical signals within the fiber laser. In certain test conditions requiring rapid insertion and removal, mechanical splicing can also be performed using high-return-loss APC fiber flanges. This embodiment, by limiting the resonant cavity to a figure-eight shape, utilizes the nonlinear phase accumulation of the structure itself to achieve mode-locked interference, eliminating mode-locking devices such as carbon nanotubes or semiconductor saturable absorber mirrors, thereby improving the damage resistance threshold and operational lifespan under prolonged high-power pumping.

[0032] like Figure 2 As shown, in one embodiment of this application, both the input and output ends of the polarization-maintaining fiber 34 are connected to the single-mode fiber 70 to form a hybrid fiber 80.

[0033] In some embodiments, this embodiment specifies the detailed description of the medium along the beam propagation path. The single-mode fiber 70 has weak birefringence characteristics. In actual fusion splicing, the splice point between the polarization-maintaining fiber 34 and the single-mode fiber 70 can be spliced ​​using a conventional 0-degree alignment method; or, in order to introduce a specific initial polarization state, a staggered fusion process with a specific angular offset, such as 45 degrees, can also be used. In this embodiment, by constructing a hybrid fiber 80 of single-mode fiber 70, polarization-maintaining fiber 34, and single-mode fiber 70 within the unidirectional loop structure 30, and utilizing the periodic alternation of the birefringence-containing polarization-maintaining fiber 34, a spatial periodic phase matching mechanism is spontaneously formed within the resonant cavity, greatly enhancing the compensation capability for positive dispersion accumulated chirp.

[0034] like Figure 1 and Figure 2As shown, in one embodiment of this application, the optical coupling component 20 is a first optical coupler 21; the two ends of the nonlinear amplifying ring mirror structure 10 are respectively connected to the two loop ports of the first optical coupler 21 to form a bidirectional ring optical path 16 that allows beams to propagate in opposite directions; the bidirectional ring optical path 16 is connected in series with a bandpass filter 14, a dispersion compensation fiber 15, a wavelength division multiplexer 11, the gain fiber 12, and a first polarization controller 13; wherein, the dispersion compensation fiber 15 is used to provide positive dispersion; the wavelength division multiplexer 11 is used to inject pump light into the gain fiber 12; the bandpass filter 14 is used to lock the center wavelength of the beams propagating in opposite directions; the first polarization controller 13 is used to adjust the nonlinear phase shift difference between the beams propagating in opposite directions to establish passive mode-locking.

[0035] In some embodiments, the bidirectional ring optical path 16 is as follows Figure 2 As indicated by the arrow, it represents the optical path trajectory of the nonlinear amplifying ring mirror structure 10. It can be understood that the two ends of the bidirectional ring optical path 16 are disconnected. After connecting the two disconnected ends to the two loop ports of the first optical coupler 21, the optical path can be transmitted in opposite directions.

[0036] The bandpass filter 14 can be a free-space interference filter assembly based on multilayer dielectric films. In this embodiment, the wavelength selection function of the bandpass filter 14 can limit the spectrum to the 1.7-micrometer band, suppressing out-of-band amplified spontaneous emission noise. Of course, limiting the bandpass filter 14 to the 1.7-micrometer band is only the band required by this application, and other bands can be adjusted for different band requirements. There is no specific limitation here.

[0037] The dispersion compensation fiber 15 can be a small-core silica fiber with a high germanium doping concentration. In this embodiment, by adding the dispersion compensation fiber 15, which is an optical fiber with strong positive dispersion characteristics, the total net dispersion of the resonant cavity is shifted and stabilized in the positive dispersion range, thus ensuring the stable evolution of the mode-locked state.

[0038] The wavelength division multiplexer 11 can be a fused biconical tapered WDM or a micro-optical filter WDM. It also includes a pump source 60, which is connected to the wavelength division multiplexer 11, allowing the power of the pump source 60 to be adjusted and outputting pump light to the wavelength division multiplexer 11. This effectively achieves efficient coupling and collinear transmission of the high-frequency pump light and the 1.7-micron signal light within the same single-mode fiber 70.

[0039] The first polarization controller 13 introduces a static phase bias by adjusting the polarization evolution trajectory of the bidirectional transmission beam, which greatly reduces the oscillation threshold of passive mode-locking, enabling the system to quickly start mode-locking at low pump power.

[0040] like Figure 2 As shown, in one embodiment of this application, the unidirectional loop structure 30 is further provided with a second optical coupler 32; the second optical coupler 32 is used to extract part of the optical signal of the unidirectional loop structure 30 and output it to the outside of the cavity.

[0041] In some embodiments, the splitting ratio of the second optical coupler 32 in this embodiment is preferably 30:70; or, for scenarios requiring higher intracavity feedback energy, it can be replaced with an asymmetric coupler of 10:90; where 30:70 means 30% output and 70% feedback, similarly, 10:90 means 10% output and 90% feedback; by setting the second optical coupler 32, this embodiment not only achieves stable output of high-frequency pulse sequences to external workloads, but also ensures sufficient energy to return into the cavity to maintain cyclic interference, thus achieving a balance between energy output and mode locking.

[0042] like Figure 2 As shown, in one embodiment of this application, this embodiment further includes a third polarization controller 40 and a polarization beam splitter 50 connected in sequence; the input terminal of the third polarization controller 40 is connected to the output terminal of the second optical coupler 32 to adjust the polarization state of the optical signal; the polarization beam splitter 50 is used to split the output optical signal into two orthogonal polarization components.

[0043] In some embodiments, the third polarization controller 40 can also be a three-ring fiber controller or a stress birefringence extruder; the polarization beam splitter 50 can be a spatial polarization beam splitter using a Wollaston prism. This embodiment, by introducing a cascaded structure of the third polarization controller 40 and the polarization beam splitter 50, separates the originally mixed output beam into two highly linearly polarized orthogonal sub-beams, meeting the requirements of multi-channel high-precision applications such as polarization-sensitive imaging and polarization-state orthogonal multiplexing communication.

[0044] like Figure 2 As shown, the gain fiber 12 is a thulium-doped fiber; and / or the dispersion compensation fiber 15 is an ultra-high numerical aperture fiber.

[0045] In some embodiments, the dispersion compensation fiber 15 is preferably confined to an ultra-high numerical aperture fiber because ultra-high numerical aperture fibers (such as the commercial UHNA4 model) provide not only extremely high nonlinear coefficients but also a large positive dispersion value in the 1.7-micron wavelength band due to their extremely small mode field diameter. This embodiment, with its material limitations, allows for rapid accumulation of the target positive dispersion within the resonant cavity within a limited spatial length, effectively reducing the overall size of the laser.

[0046] In one embodiment of this application, the unidirectional transmission component 31 is a polarization-independent optical isolator, and the polarization adjustment component 33 is a second polarization controller; the polarization-independent optical isolator, the second optical coupler 32, the second polarization controller, and the polarization-maintaining fiber 34 are connected in series along the beam transmission direction of the unidirectional loop structure 30.

[0047] In some embodiments, this embodiment specifies that the unidirectional transmission component 31 uses a polarization-independent optical isolator (PI-ISO) instead of a polarization-dependent isolator, enabling complete freedom in the evolution of the polarization state within the loop. The beam first passes through the polarization-independent optical isolator to block backscattering, then the signal is split by the second optical coupler 32. The unsplittered signal then enters the second polarization controller for polarization reconstruction, and is finally injected into the fast and slow axes of the polarization-maintaining fiber 34. This embodiment employs a specific series sequence to ensure that the optical signal entering the polarization-maintaining fiber 34 for birefringence phase delay compensation is in an optimal state that has undergone energy screening and has a pure and controllable polarization state, reducing crosstalk losses between devices.

[0048] like Figure 2 As shown, in one embodiment of this application, the operating parameters of the laser and the length of the optical fiber satisfy the following ranges: the center wavelength of the bandpass filter 14 is in the range of 1700nm to 1750nm, and its bandwidth is 15nm to 35nm; the length of the gain fiber 12 is 1.0m to 3.0m; the length of the polarization-maintaining fiber 34 is 2.0m to 6.0m; and the length of the dispersion compensation fiber 15 is 15.0m to 20.0m.

[0049] In some embodiments, the length of the gain fiber 12 is limited to 1.0m to 3.0m to prevent excessively long gain media from causing parasitic oscillations in the 1.9-micron band. Preferably, the length of the gain fiber 12 is 1.8m. The length of the polarization-maintaining fiber 34 is set between 2.0m and 6.0m. If it is less than 2m, the phase delay is insufficient to fully compensate for the nonlinear chirp; if it is more than 6m, it is prone to causing excessive pulse splitting. The dispersion compensation fiber 15 is in the range of 15.0m to 20.0m, which can accurately lock the net dispersion of the resonant cavity in the weak positive dispersion region of approximately 0.41 ps². In terms of parameter selection, 1720nm is preferably the center wavelength of the bandpass filter 14. This embodiment, through the selection of data from the above experiments, forms a phase diagram trajectory with excellent tolerance and capable of stably inducing near-chirp-free pulse generation.

[0050] like Figure 3 As shown, this application also discloses a method for generating optical pulses, applied to the aforementioned fiber laser, comprising the following steps: Step S1: Pump light is injected into the nonlinear amplifying ring mirror structure 10 through the pump source 60, and the pump power is gradually increased until it exceeds the mode-locking threshold; Step S2: Adjust the polarization state within the nonlinear amplifying ring mirror structure 10 to change the nonlinear phase shift accumulation between the opposing propagating beams, so as to establish a passive mode-locked state under positive dispersion. Step S3: While maintaining a stable mode-locked state, adjust the polarization adjustment component 33 within the unidirectional loop structure 30 to change the polarization component of the beam entering the polarization-maintaining fiber 34. Step S4: The polarization-maintaining fiber 34 is used to generate phase delay for different polarization components to compensate for the nonlinear phase shift of the accumulated optical pulse, so as to output a near-chirp-free birefringent managed soliton pulse.

[0051] In some embodiments, during the process of increasing the pump power in step S1, the constant current source ramp command can be output by the built-in MCU. When the built-in photodetector detects the mode-locked pulse envelope for 10ms continuously, it is determined that the threshold is exceeded and the pump current is locked. In step S2, the process of adjusting the polarization state can be achieved either by manually applying mechanical stress or by using a D / A converter to output a specific driving voltage array to the electro-optic polarization controller to achieve step-by-step deflection. This establishes a passive mode-locked state under positive dispersion in the nonlinear amplifying ring mirror structure 10. While maintaining the stability of the mode-locked state, the polarization adjustment component 33 inside the unidirectional loop is adjusted, and the polarization injection angle of the beam entering the polarization-maintaining fiber 34 is changed. The birefringence characteristics introduced by the hybrid structure of the polarization-maintaining fiber 34 and the single-mode fiber 70 are used to induce a phase-matching mechanism. By observing the morphological characteristics of the output spectrum, when the two orthogonally polarized spectra exhibit characteristic sidebands, and the time-bandwidth product of the pulse width measured by the autocorrelator and the spectral width measured by the spectrometer is close to 0.315, a near-chirp-free birefringent managed soliton output is achieved. The birefringent managed soliton state can be smoothly switched to a dissipative soliton state with higher single-pulse energy simply by adjusting the angle of the polarization adjustment component 33.

[0052] Working principle: The following values ​​are merely one example used in this application and can be replaced with the values ​​mentioned above, but do not mean that only these values ​​can be used.

[0053] like Figure 2As shown, firstly, a 1570nm high-frequency pump light is output from an external pump source 60. The high-frequency pump light stream is injected into the gain fiber 12 (i.e., thulium-doped fiber) through a wavelength division multiplexer 11, and stimulated emission excites a 1.7-micron band signal light. Subsequently, the signal light is split into two beams of light propagating in opposite directions at the first optical coupler 21 and enters the nonlinear amplification ring mirror structure 10. During bidirectional transmission, due to the parameter asymmetry of the bidirectional ring optical path 16, the beam accumulates and generates a nonlinear phase shift difference, which interferes when returning to the first optical coupler 21. The high-intensity pulse is transmitted into the unidirectional loop, and the low-intensity background light is reflected back to the gain medium. This process completes the passive mode-locking oscillation. Next, the transmitted high-intensity light pulse passes sequentially through the polarization-independent optical isolator (ensuring unidirectional flow) and the second optical coupler 32 (outputting a portion of the pulse to peripheral devices); the remaining signal flow that continues to circulate enters the polarization adjustment component 33 (i.e., the second polarization controller) where the polarization injection angle is reshaped and projected into the fast and slow axes of the polarization-maintaining fiber 34; within the polarization-maintaining fiber 34, the refractive index difference causes a phase delay in the time dimension of the signal flow along the two orthogonal polarization axes; finally, the beam carrying this compensated phase is re-injected into the ordinary single-mode fiber 70, returns to the first optical coupler 21, and begins the next cycle.

[0054] In summary, the technical solution provided in this application compensates for the self-starting interference mode-locking capability of the nonlinear amplifying ring mirror structure 10 with the periodic birefringence effect of the hybrid fiber 80 (i.e., single-mode fiber 70, polarization-maintaining fiber 34, and single-mode fiber 70). In practical applications, due to the phase-matching mechanism inside the hybrid fiber 80, the fiber laser can utilize the phase delay generated by the polarization-maintaining fiber 34 to compensate for and cancel the positive chirp caused by self-phase modulation of the optical pulse in real time and effectively in each intracavity cycle when facing the inherent strong positive dispersion accumulation interference in the 1.7-micron band. Simultaneously, in conjunction with the coordinated action of the second polarization controller, the problem of relying on a large external dispersion compensation grating pair in traditional positive dispersion schemes is eliminated. Therefore, this application has the advantages of a compact structure, no external alignment loss, and the ability to directly output extremely high-quality near-chirp-free birefringence managed solitons.

[0055] The above description is merely a preferred embodiment of this application and does not limit the patent scope of this application. Any equivalent structural transformations made based on the inventive concept of this application and the contents of the specification and drawings of this application, or direct / indirect applications in other related technical fields, are included within the patent protection scope of this application.

Claims

1. A fiber laser, characterized in that, It includes a nonlinear amplifying ring mirror structure, a unidirectional loop structure, and optical coupling components; The optical coupling component is connected between the nonlinear magnifying ring mirror structure and the unidirectional loop structure. The nonlinear amplifying ring mirror structure contains a gain fiber, which is used to accumulate a nonlinear phase shift in the optical pulse during transmission. The main optical path of the unidirectional loop structure is composed of single-mode optical fiber, and a unidirectional transmission component, a polarization adjustment component and a polarization-maintaining fiber are arranged sequentially along the beam transmission direction. The polarization adjustment component is used to adjust the polarization state of the beam input to the polarization-maintaining fiber, so as to change the polarization components of the beam on the orthogonal polarization axis. The polarization-maintaining fiber is used to generate phase delay for different polarization components, and the phase delay is used to compensate for the nonlinear phase shift.

2. The fiber laser as described in claim 1, characterized in that, The nonlinear amplifying ring mirror structure and the unidirectional loop structure are interconnected through the optical coupling component, together forming a figure-eight shaped all-fiber resonant cavity.

3. The fiber laser as described in claim 1, characterized in that, Both the input and output ends of the polarization-maintaining fiber are connected to the single-mode fiber to form a hybrid fiber.

4. The fiber laser as described in any one of claims 1 to 3, characterized in that, The optical coupling component is a first optical coupler; The two ends of the nonlinear amplifying ring mirror structure are respectively connected to the two loop ports of the first optical coupler to form a bidirectional ring optical path that allows beams to propagate in opposite directions. The bidirectional ring optical path is sequentially connected in series with a bandpass filter, a dispersion-compensating fiber, a wavelength division multiplexer, the gain fiber, and a first polarization controller; wherein... The dispersion-compensating fiber is used to provide positive dispersion. The wavelength division multiplexer is used to inject pump light into the gain fiber; The bandpass filter is used to lock the center wavelength of the opposing beams; The first polarization controller is used to adjust the nonlinear phase shift difference between the opposing propagating beams in order to establish passive mode-locking.

5. The fiber laser as described in claim 1, characterized in that, The unidirectional loop structure is also provided with a second optical coupler; The second optical coupler is used to extract the optical signal of part of the unidirectional loop structure and output it outside the cavity.

6. The fiber laser as described in claim 5, characterized in that, It also includes a third polarization controller and a polarization beam splitter connected in sequence; The input terminal of the third polarization controller is connected to the output terminal of the second optical coupler to adjust the polarization state of the optical signal; The polarization beam splitter is used to split the output optical signal into two orthogonal polarization components.

7. The fiber laser as described in claim 4, characterized in that, The gain fiber is a thulium-doped fiber; and / or the dispersion compensation fiber is an ultra-high numerical aperture fiber.

8. The fiber laser as described in claim 5, characterized in that, The unidirectional transmission component is a polarization-independent optical isolator, and the polarization adjustment component is a second polarization controller; The polarization-independent optical isolator, the second optical coupler, the second polarization controller, and the polarization-maintaining fiber are connected in series along the beam transmission direction of the unidirectional loop structure.

9. The fiber laser as described in claim 4, characterized in that, The operating parameters of the laser and the length of the optical fiber meet the following ranges: The center wavelength of the bandpass filter is in the range of 1700nm to 1750nm, and its bandwidth is 15nm to 35nm. The length of the gain fiber is 1.0m to 3.0m; The polarization-maintaining fiber has a length of 2.0m to 6.0m; The length of the dispersion compensation fiber is 15.0m to 20.0m.

10. A method for generating optical pulses, applied to a fiber laser as described in any one of claims 1 to 9, characterized in that, Includes the following steps: Pump light is injected into the nonlinear amplifying ring mirror structure through a pump source, and the pump power is gradually increased until it exceeds the mode-locking threshold. Adjusting the polarization state within the nonlinear amplifying ring mirror structure changes the nonlinear phase shift accumulation between opposing propagating beams, thereby establishing a passive mode-locked state under positive dispersion. While maintaining a stable mode-locked state, the polarization adjustment component within the unidirectional loop structure is adjusted to change the polarization component of the beam entering the polarization-maintaining fiber. The polarization-maintaining fiber is used to generate phase delays for different polarization components to compensate for the nonlinear phase shift of the accumulated optical pulse, so as to output a near-chirp-free birefringent managed soliton pulse.

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