A high-dispersion, chirped, high-beam-quality all-fiber laser

Abstract

The application provides a large-dispersion-chirped high-beam-quality all-fiber laser, which comprises a single-mode fiber mode-locked oscillator for generating wide-spectrum high-frequency ultrashort pulse seed light, a single-mode fiber pulse stretching and frequency reduction system and a fiber power amplifier; wherein the single-mode fiber pulse stretching and frequency reduction system comprises a fiber beam splitter, a fiber ring, a tunable fiber stretcher, a fiber isolator, a fiber acousto-optic modulator, a high-speed photoelectric detector and a pulse selection driver; and the fiber power amplifier is used for power amplification of the signal light. The application can provide a very large group delay dispersion by cascading multiple large-dispersion-chirped fiber gratings, is easy to integrate and control, and has dynamic tunability, so that a linear chirped pulse with a large and adjustable stretching amount can be provided to match a rear-end compressor; the pulse generated by the mode-locked oscillator is stretched to generate a linear chirped pulse after the stretching system, and the linear chirped pulse is more easily power amplified after the pulse selection and frequency reduction.

Description

Technical Field

[0001] This invention relates to the field of laser technology, and in particular to a high-dispersion chirped pulsed all-fiber laser with high beam quality. Background Technology

[0002] In laser pulse energy amplification technology, chirped pulse amplification is the mainstream method. Its system typically includes a laser oscillator, a pulse stretcher, a laser amplifier, and a pulse compressor. The principle is as follows: the oscillator generates a mode-locked pulse. While maintaining the spectral width, frequency chirping is introduced through a dispersive device to broaden the pulse, reducing the peak laser power, avoiding damage to the amplifier system, and mitigating the nonlinear effects caused by high peak power. This greatly improves the pulse energy amplification effect. After energy amplification, the dispersion is compensated by a dispersion compensation device, i.e., the compressor, which compresses the pulse and outputs a high-energy ultrashort pulse laser.

[0003] Currently, the generation of high-energy ultrashort pulse lasers mainly relies on solid-state laser technology. However, the output stability of solid-state lasers is highly susceptible to the characteristics of the front-end laser. To improve pulse amplification efficiency and reduce the burden on the back-end solid-state amplifier, the front-end light source needs to maintain a wide spectral width while pursuing a wider pulse width, lower repetition frequency, and higher output pulse energy. However, current fiber lasers mainly employ a master oscillator power amplification structure, and chirped pulse amplification systems require a stretching system after the oscillator stage. The stretching system is typically a long-distance passive fiber, but during the stretching process of long-distance fibers, even lower pulse energies will accumulate significant nonlinearity due to the long medium.

[0004] To address this issue, fiber chirped gratings offer unique advantages such as small size, low insertion loss, low nonlinearity, and direct fusion splicing with optical fibers. They also allow for adjustable dispersion when external conditions like temperature stress cause refractive index changes. In pulse stretching systems, they can effectively stretch pulses, reduce peak power, prevent damage to amplifier systems, and mitigate nonlinear effects. However, their dispersion is limited, making it difficult to stretch pulses from hundreds of femtoseconds to the nanosecond level. Summary of the Invention

[0005] This invention provides a high-dispersion chirped pulsed all-fiber laser with high beam quality, which solves the problem of limited dispersion in related fiber lasers.

[0006] To achieve the above objectives, this application adopts the following technical solution:

[0007] In a first aspect, a high-dispersion chirped pulsed all-fiber laser with high beam quality is provided, comprising:

[0008] Single-mode fiber mode-locked oscillator, single-mode fiber pulse broadening and down-frequency reduction system, and fiber power amplifier;

[0009] The single-mode fiber mode-locked oscillator is used to generate broadband high-frequency ultrashort pulse seed light.

[0010] The single-mode fiber pulse stretching and down-frequency reduction system includes: a fiber beam splitter, a fiber circulator, a tunable fiber stretcher, a fiber isolator, a fiber acousto-optic modulator, a high-speed photodetector, and a pulse selection driver.

[0011] The main optical path end of the fiber optic beam splitter is connected to the input end of the fiber optic circulator, the reflective end of the fiber optic circulator is connected to the tunable fiber stretcher, the output end of the fiber optic circulator is connected to the fiber optic isolator, and the output end of the fiber optic isolator is connected to the optical input end of the fiber optic acousto-optic modulator.

[0012] One end of the fiber optic beam splitter for outputting reference light is connected to the high-speed photodetector. The high-speed photodetector is electrically connected to the pulse selection driver. The pulse selection driver is connected to the synchronization signal end of the fiber optic acousto-optic modulator to achieve pulse selection. The pulse selection ratio of the pulse selection driver is adjusted to control the repetition frequency. The optical output end of the fiber optic acousto-optic modulator outputs the broadened and down-frequency signal light. The tunable fiber stretcher is configured as a stretcher based on a second chirped fiber grating.

[0013] The fiber optic power amplifier is used to amplify the power of the signal light.

[0014] In a first possible implementation, the second chirped fiber grating is configured as a high-dispersion chirped fiber grating, and its reflection wavelength and dispersion characteristics are dynamically tunable by temperature adjustment or mechanical stretching; alternatively, chirped fiber gratings with the same dispersion but different but continuous reflection spectra can be connected in series at the output end of the second chirped fiber grating to increase the reflection bandwidth.

[0015] Based on any of the above possible implementations, in the second possible implementation, the single-mode fiber pulse stretching and down-conversion system includes: at least one fiber circulator and a tunable fiber stretcher used in a one-to-one correspondence with it, and different combinations of the fiber circulator and the tunable fiber stretcher are cascaded.

[0016] Based on the second possible implementation, in the third possible implementation, the single-mode fiber pulse stretching and down-frequency reduction system specifically includes: a first fiber circulator, a second fiber circulator, a first tunable fiber stretcher, and a second tunable fiber stretcher.

[0017] The main optical path end of the fiber optic beam splitter is connected to the input end of the first fiber optic circulator, the reflective end of the first fiber optic circulator is connected to the input end of the second chirped fiber grating, the output end of the first fiber optic circulator is connected to the input end of the second fiber optic circulator, and the output end of the second fiber optic circulator is connected to the fiber optic isolator.

[0018] Based on the third possible implementation, in the fourth possible implementation, the single-mode fiber pulse broadening and down-frequency reduction system further includes: a wavelength division multiplexer and a second gain fiber;

[0019] The output of the second fiber circulator is connected to the signal light transmission end of the wavelength division multiplexer, while the first semiconductor pump laser is connected to the pump input reflection end of the wavelength division multiplexer. The common end of the wavelength division multiplexer is connected to the input end of the second gain fiber, and the output end of the second gain fiber is connected to the input end of the fiber isolator, forming an amplification stage for power compensation and pre-amplification; wherein, the second gain fiber is configured as a single-mode polarization-maintaining gain fiber.

[0020] Based on the fourth possible implementation, in the fifth possible implementation, the single-mode fiber pulse broadening and down-conversion system may be optionally configured to include at least one set of the amplification stages.

[0021] Based on any of the above possible implementations, in the sixth possible implementation, the high-speed photodetector and the pulse selection driver are integrated into a synchronization signal circuit placed in the fiber optic acousto-optic modulator.

[0022] Based on any of the above possible implementations, in the seventh possible implementation, the single-mode fiber mode-locked oscillator includes: a resonant cavity formed by a first semiconductor pump laser, a semiconductor saturable absorber integrated wavelength division multiplexer, a first gain fiber and a dispersion management reflector.

[0023] The first semiconductor pump laser is connected to the pump input of the semiconductor saturable absorber integrated wavelength division multiplexer. The saturable absorber and the wavelength division multiplexer are integrated as a resonant cavity input. The signal light end of the saturable absorber integrated wavelength division multiplexer is connected to the first gain fiber input end, and the first gain fiber output end is connected to the dispersion management reflector.

[0024] The dispersion management reflector is formed based on a first chirped fiber grating and is used to provide negative dispersion and serve as the output mirror of the resonant cavity. By controlling the total length of the silicon fiber in the cavity, the dispersion is controlled to match the customized dispersion of the chirped fiber grating, thereby achieving zero-dispersion oscillation in the resonant cavity.

[0025] Based on any of the above possible implementations, in the eighth possible implementation, the optical fiber power amplifier includes: a first-stage optical fiber amplifier, a second-stage optical fiber amplifier, and an output system;

[0026] The first-stage fiber amplifier includes: a second semiconductor pump laser, a first fiber combiner, a third gain fiber, and a first high-power fiber isolator;

[0027] The output of the fiber optic acousto-optic modulator is connected to the signal light input of the first fiber optic combiner, the second semiconductor pump laser is connected to the pump light input of the first fiber optic combiner, the signal light output of the first fiber optic combiner is connected to the third gain fiber, and the output of the third gain fiber is connected to the input of the first high-power fiber optic isolator.

[0028] The second-stage fiber amplifier includes: a third semiconductor pump laser, a second fiber combiner, and a chiral coupled fiber core gain fiber;

[0029] The output of the first high-power fiber isolator is connected to the signal light input of the second fiber combiner, the third semiconductor pump laser is connected to the pump light input of the second fiber combiner, and the signal light output of the second fiber combiner is connected to the chiral coupled fiber core gain fiber.

[0030] The output system includes a pump light stripper and an output cap. The output end of the chiral coupled fiber core gain fiber is connected to the pump light stripper and then to the output cap.

[0031] Based on the eighth possible implementation, in the ninth possible implementation, the signal light input end of the first fiber combiner is a single-mode polarization-maintaining fiber, the pump light input end is a multimode fiber with a core diameter of 105μm; and the signal light output end is a double-clad fiber with a core diameter of 14μm.

[0032] The third gain fiber is a double-clad gain fiber with a core diameter of 14 μm;

[0033] The third semiconductor pump laser is configured as a high-power semiconductor laser with multimode fiber-coupled output;

[0034] The signal light output end of the second fiber combiner has a core diameter of 30 μm; it is connected to the chiral coupled fiber core gain fiber through a double-clad passive fiber with a core diameter of 30 μm.

[0035] The core diameter of the chiral coupled fiber core gain fiber is 32 μm.

[0036] Beneficial effects:

[0037] This application utilizes cascaded multiple high-dispersion chirped fiber gratings to provide a very large group delay dispersion, which is easy to integrate and control, and dynamically tunable. It can provide linearly chirped pulses with large and adjustable broadening to match the back-end compressor. The broadband pulse generated by the mode-locked oscillator is broadened into a linearly chirped pulse by the high-dispersion broadening system. After pulse selection and frequency reduction, the repetition frequency is lowered, making power amplification easier. The fully polarization-maintaining fiber structure before the main amplification stage makes the system less susceptible to external interference, supporting long-term stable operation of the entire system. The chiral coupled fiber itself has the advantage of a large core diameter and unique excellent characteristics such as filtering out higher-order modes. The high-dispersion linearly chirped pulse output from the main amplification stage has high energy and excellent beam quality. Attached Figure Description

[0038] Figure 1 A schematic diagram of the structure of a high-dispersion chirped pulsed all-fiber laser with high beam quality provided for an embodiment of this application;

[0039] Figure 2 This is a schematic diagram of the structure of a single-mode fiber mode-locked oscillator provided in an embodiment of this application;

[0040] Figure 3 A schematic diagram of a single-mode fiber pulse broadening and down-frequency reduction system provided in this application embodiment;

[0041] Figure 4 This is a schematic diagram of the structure of an optical fiber power amplifier provided in an embodiment of this application.

[0042] Figure label:

[0043] 100 Single-mode fiber mode-locked oscillator; 200 Single-mode fiber pulse broadening and down-conversion system; 300 Fiber power amplifier; 1 First semiconductor pumped laser; 23 Pump protector; 2 Semiconductor saturable absorber integrated wavelength division multiplexer; 3 First gain fiber; 4 Dispersion management reflector; 5 Fiber beam splitter; 61 First fiber circulator; 62 Second fiber circulator; 71 First tunable fiber broadener; 72 Second tunable fiber broadener; 8 Wavelength division multiplexer; 9 Second gain fiber; 10 Fiber isolator; 11 Fiber acousto-optic modulator; 21 High-speed photodetector; 22 Pulse selection driver; 12 Second semiconductor pumped laser; 13 First fiber combiner; 14 Third gain fiber; 15 First high-power fiber isolator; 16 Third semiconductor pumped laser; 17 Second fiber combiner; 18 Chiral coupled fiber core gain fiber; 19 Pump light stripper; 20 Output cap. Detailed Implementation

[0044] To further illustrate the technical means and effects adopted by the present invention to achieve its intended purpose, the technical solutions in the embodiments of this application are clearly described. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art are within the scope of protection of this application.

[0045] The terms "first," "second," etc., used in the specification and claims of this application are used to distinguish similar objects and not to describe a specific order or sequence. It should be understood that such terms can be used interchangeably where appropriate so that embodiments of this application can be implemented in orders other than those illustrated or described herein, and the objects distinguished by "first," "second," etc., are generally of the same class and the number of objects is not limited; for example, a first object can be one or more. Furthermore, in the specification and claims, "and / or" indicates at least one of the connected objects, and the character " / " generally indicates that the preceding and following objects are in an "or" relationship.

[0046] The steps described in the specification and the flowcharts in the accompanying drawings of this invention are not necessarily strictly executed according to the step numbers; the execution order of the method steps can be changed. Furthermore, certain steps can be omitted, multiple steps can be combined into one step, and / or one step can be broken down into multiple steps.

[0047] The following detailed description, in conjunction with the accompanying drawings and preferred embodiments, describes a high-dispersion chirped pulsed high-beam-quality all-fiber laser provided in this application.

[0048] First, the application scenarios of a high-dispersion chirped pulse high-beam-quality all-fiber laser according to an embodiment of this application will be described in detail.

[0049] In related technologies, for the front-end of chirped pulse amplification systems, all-fiber lasers offer advantages over all-solid-state lasers, including stable output power, stable output pulses, good beam quality, flexible propagation, and compact size, making them a superior choice for high-energy ultrashort pulse solid-state laser front-ends. The pulse oscillator in a chirped pulse amplification system generates pulses that significantly affect pulse characteristics. To compress and obtain ultrashort pulses and avoid pulse distortion in the amplifier, the pulse seed source needs to have a smooth, broad spectrum and output stable ultrashort pulses.

[0050] For pulse broadening systems in chirped pulse amplification systems, the mainstream pulse broadening systems currently are long-distance passive optical fibers, spatial diffraction gratings, and fiber chirped Bragg gratings.

[0051] Among them, long-distance optical fiber and diffraction grating have the advantages of low cost and low nonlinearity, respectively. However, during the widening process of long-distance optical fiber, even low pulse energy will accumulate large nonlinearity due to the long medium. Diffraction grating requires a lot of physical space and is difficult to achieve efficient and stable coupling with the current mature and stable optical fiber front-end system.

[0052] Fiber chirped gratings (FCGs) possess unique advantages such as small size, low insertion loss, low nonlinearity, and direct fusion splicing with optical fibers. They also allow for adjustable dispersion even when external conditions such as temperature stress cause changes in refractive index. However, current broadening systems using CCGs in a single pass have limited dispersion, making it difficult to broaden hundreds of femtosecond pulses to the nanosecond level. Group delay dispersion broadening pulses can relatively reduce light intensity over time, while increasing the fiber mode field area can relatively reduce light intensity spatially. Fiber amplifier stages using large-mode-field fibers can effectively increase pulse energy within the fiber. However, mode control is difficult in large-mode-field fibers with core diameters exceeding 25 micrometers. Increasing the mode field area cannot guarantee single-mode transmission conditions, leading to higher-order modes appearing during transmission through ordinary large-core fibers, thus reducing beam quality.

[0053] Chiral-coupled core fiber, as a large-mode-field fiber, overcomes the limitations of traditional single-mode fiber on core diameter, achieving stable single-mode output with core diameters greater than 30 micrometers. It can also be directly fused with fibers of matching core diameter. Its surrounding side core filters out higher-order modes generated during transmission, ensuring that beam quality does not deteriorate during amplification, providing a new approach to achieving high-beam-quality, high-energy fiber amplifiers. Applying chiral-coupled core fiber to the main amplification stage of high-dispersion chirped pulses enables all-fiber, high-chirp, high-beam-quality short-pulse laser output.

[0054] Therefore, considering the need for solid-state amplification stages in chirped pulse amplification systems to improve the energy and beam quality of broadband linear chirped pulse front-ends while reducing peak power, this application provides a high-dispersion chirped pulse high-beam-quality all-fiber laser. Multiple high-dispersion chirped fiber gratings are cascaded to provide group delay dispersion, which is then down-frequency selected by pulse selection for power amplification. Chiral-coupled fiber cores are used in the signal optical path via fiber fusion splicing as the main amplification stage. A polarization-maintaining fiber structure precedes the main amplification stage, improving the linear chirped dispersion and output beam quality of the front-end while effectively increasing pulse energy.

[0055] See Figure 1 This application provides a high-dispersion chirped pulsed all-fiber laser with high beam quality, such as... Figure 1 As shown, the laser in this embodiment includes a single-mode fiber mode-locked oscillator 100, a single-mode fiber pulse broadening and down-conversion system 200, and a fiber power amplifier 300 connected in sequence. Specifically:

[0056] A single-mode fiber mode-locked oscillator 100, used to generate broadband high-frequency ultrashort pulse seed light, includes a first semiconductor pump laser 1, a pump protector 23, a semiconductor saturable absorber mirror integrated wavelength division multiplexer 2, a first gain fiber 3, and a dispersion management reflector 4 formed based on a first chirped fiber grating, which are sequentially connected to form a linear resonant cavity. The single-mode fiber semiconductor laser is connected to the pump input of the wavelength division multiplexer. The semiconductor saturable absorber mirror serves as a key passive mode-locking device, the highly doped single-mode gain fiber provides gain, and the chirped fiber grating cancels the dispersion generated by the silicon fiber while also serving as an output cavity mirror to output the mode-locked pulse.

[0057] Referring to the figure, in the specific implementation process, the semiconductor pump laser 1 is connected to the pump protection isolator 23 via single-mode fiber coupling output, and then connected to the pump input end of the semiconductor saturable absorber integrated wavelength division multiplexer 2. The saturable absorber and the wavelength division multiplexer are integrated as the input end of the resonant cavity. The signal light can act on the semiconductor saturable absorber. The saturable absorber can be a semiconductor saturable absorber with a recovery time in the femtosecond range. The signal light end of the saturable absorber integrated wavelength division multiplexer 2 is connected to the first gain fiber 3. The first gain fiber 3 is a highly doped single-mode polarization-maintaining gain fiber. The output end of the first gain fiber 3 is connected to the dispersion management reflector 4 formed based on the first chirped fiber grating. The dispersion management reflector 4 provides negative dispersion and serves as the output mirror of the resonant cavity. By controlling the total length of the silicon fiber in the cavity, the dispersion is controlled to match the customized dispersion of the chirped fiber grating, thereby achieving zero-dispersion oscillation in the resonant cavity.

[0058] For example, the semiconductor-pumped laser 1 can be a laser diode, employing a 976nm semiconductor laser with a pump power of 75mW; the semiconductor saturable absorber mirror can be one or more of SESAM, reflective MoS2, and low-dimensional carbon nanomaterial saturable absorber mirrors. Preferably, in this embodiment, it is configured as a SESAM slice package with a recovery time of 500fs; the gain fiber is a standard single-mode ytterbium-doped polarization-maintaining fiber, and the second-order dispersion of the chirped fiber grating dispersion-managed reflector 4 is -0.1407ps. 2 .

[0059] The single-mode fiber mode-locked oscillator 100 has an output power of 15mW, a full width at half maximum (FWHM) of 10nm, a repetition frequency of 36MHz, and a pulse width of less than 1ps. The light output to the acousto-optic modulator as a synchronization signal can be used for monitoring at any time.

[0060] Referring to the figure, the single-mode fiber pulse broadening and down-frequency reduction system 200 is used to broaden and down-frequency the broadband high-frequency ultrashort pulse seed light of the single-mode fiber mode-locked oscillator 100 while maintaining the spectral width. It includes a fiber beam splitter 5, a fiber circulator group, a tunable fiber broadener group, a wavelength division multiplexer 8, a second gain fiber 9, a fiber isolator 10, a fiber acousto-optic modulator 11, a high-speed photodetector 21, and a pulse selection driver 22. The fiber circulator group may include multiple fiber circulators, and the tunable fiber broadener group may include multiple tunable fiber broadeners. One fiber circulator is used in conjunction with one tunable fiber broadener, and different combinations of fiber circulators and tunable fiber broadeners are cascaded. In this embodiment, the fiber circulator group includes a first fiber circulator 61 and a second fiber circulator 62; the tunable fiber broadener group includes a first tunable fiber broadener 71 and a second tunable fiber broadener 72.

[0061] The main optical path end of the fiber optic beam splitter 5 is connected to the input end of the first fiber optic circulator 61. The reflective end of the first fiber optic circulator 61 is connected to the input end of the second chirped fiber grating. A stretcher is formed based on the second chirped fiber grating. In this embodiment, it is a tunable fiber stretcher 7. The second chirped fiber grating is a high second-order group delay dispersion chirped fiber Bragg grating with a temperature control system. Each one can provide a chirp of more than 500 ps / nm and can achieve adjustable dispersion. The output end of the first fiber optic circulator 61 is connected to the input end of the next-stage fiber optic circulator (that is, the second fiber optic circulator 62). Several identical tunable fiber stretchers 7 formed based on the second chirped fiber grating are cascaded in this way to provide a multiple of high group delay dispersion and provide high dispersion to stretch the pulse, i.e., a stretching system. The output of the last fiber optic circulator (in this embodiment, the second fiber optic circulator 62) is connected to the signal light transmission end of the wavelength division multiplexer 8. Simultaneously, the first semiconductor pump laser 1 is connected to the pump protector 23, which is then connected to the pump output reflection end of the wavelength division multiplexer 8. The common end of the wavelength division multiplexer 8 is connected to the input end of the second gain fiber 9. The second gain fiber 9 is a single-mode polarization-maintaining gain fiber, unlike those in the resonant cavity. This gain fiber can be lengthened to fully absorb the pump light. The first semiconductor pump laser 1, the wavelength division multiplexer 8, and the second gain fiber 9 are sequentially connected to form an amplification stage to compensate and pre-amplify the power. The output of the second gain fiber 9 is connected to the input end of the fiber optic isolator 10. After being stretched and compensated by the stretching system, it is connected to the single-mode polarization-maintaining fiber isolator to prevent backlighting. The output of the fiber optic isolator 10 is connected to the optical input of the fiber optic acousto-optic modulator 11. The fiber optic acousto-optic modulator selects high repetition frequency pulses and outputs low repetition frequency broadened pulses. At the same time, the reference light output by the seed light through the fiber optic beam splitter 5 is converted into an electrical signal by the high-speed photodetector 21 and input to the pulse selection driver 22. The pulse selection driver 22 is the driver of the acousto-optic modulator. The pulse selection driver 22 is connected to the synchronization signal terminal of the fiber optic acousto-optic modulator 11 to realize pulse selection. Adjusting the pulse selection ratio of the pulse selection driver 22 can control the repetition frequency. The optical output of the fiber optic acousto-optic modulator 11 outputs the broadened and down-frequency signal light.

[0062] Furthermore, in practical implementation, the reflection constant and dispersion characteristics of a high-dispersion chirped fiber grating can be dynamically tunable through temperature adjustment or mechanical stretching. Chirped fiber gratings with the same dispersion but different but continuous reflection spectra can be connected in series at the output end of the chirped fiber grating to increase the reflection bandwidth.

[0063] In some possible implementations, fiber optic circulators and tunable fiber stretchers (i.e., fiber optic circulators and high-dispersion chirped fiber gratings) can be cascaded in at least one set. When multiple sets are cascaded, multiple sets can be cascaded simultaneously for compensation amplification. Both high-dispersion stretchers and circulators have losses. Due to the limited reflectivity of chirped gratings, the stretching loss per stage can reach up to 40%. Therefore, when multiple sets are cascaded, optical amplification is required to compensate for the power loss and avoid small-signal amplification in subsequent amplification stages, which could lead to adverse effects such as gain narrowing. A semiconductor-pumped laser, a wavelength division multiplexer, and a gain fiber are sequentially connected to form an amplification stage for power compensation amplification.

[0064] In some possible implementations, the high-speed photodetector and pulse selection drive can be integrated into the synchronization signal circuit placed in the acousto-optic modulator.

[0065] For example, the second-order group delay dispersion of the second chirped fiber grating stretcher is 281 ps. 2 The pump power of the semiconductor laser is increased to 800mW, the pulse rise / fall time of the fiber acousto-optic modulator is 6ns, and the pulse selection drive frequency division range is 2-1024.

[0066] The single-mode fiber pulse broadening and down-frequency reduction system 200 outputs a broadened pulse width greater than 1ns. In this embodiment, the repetition frequency is set to 100kHz, and the pulse selection ratio of the acousto-optic modulator is adjusted so that the repetition frequency is adjustable between 36kHz and 36MHz.

[0067] Referring to the figure, the fiber optic power amplifier 300 includes two stages of fiber optic amplifiers and an output system. The first stage fiber optic amplifier is a double-clad fiber amplification stage, and the second stage fiber optic amplifier is a chiral-coupled fiber amplification stage. The first stage fiber optic amplifier specifically includes: a second semiconductor pump laser 12, a first fiber combiner 13, a third gain fiber 14, and a first high-power fiber isolator 15; the second stage fiber optic amplifier specifically includes: a third semiconductor pump laser 16, a second fiber combiner 17, and a chiral-coupled fiber-core gain fiber 18; the output system includes a pump light stripper 19 and an output cap 20.

[0068] The output of the fiber optic acousto-optic modulator 11 is connected to the signal light input of the first fiber optic combiner 13. The signal light input of the first fiber optic combiner 13 is a single-mode polarization-maintaining fiber, the pump light input is a multimode fiber (105μm core diameter), and the signal light output is a double-clad fiber (14μm core diameter). The second semiconductor pump laser 12 is a multimode fiber-coupled output semiconductor laser connected to the pump light input of the first fiber optic combiner 13. The signal light output of the first fiber optic combiner 13 is connected to the third gain fiber 14. The third gain fiber 14 is a double-clad gain fiber (14μm core diameter), and the output of the third gain fiber 14 is connected to the input of the first high-power fiber optic isolator 15. The output of the first high-power fiber isolator 15 is connected to the signal light input of the second fiber combiner 17. The third semiconductor pump laser 16 is configured as a high-power semiconductor laser with multimode fiber coupling output. The third semiconductor pump laser 16 is connected to the pump light input of the second fiber combiner 17. The signal light output of the second fiber combiner 17 (core diameter 30μm) is connected to the chiral coupled fiber core gain fiber 18 (core diameter 32μm) for efficient pulse amplification. The output of the chiral coupled fiber core gain fiber 18 is connected to the pump light stripper (cladding light stripper) 19 and then to the output cap 20.

[0069] In the specific implementation process, the third semiconductor pump laser 16 serves as the pump source for the main amplification stage of the large-core fiber. Multiple multimode fiber semiconductor lasers in the tens of watt range are selected and injected into a chiral-coupled gain fiber with a side core via a combiner to amplify the signal light. Because the chiral-coupled fiber has a spirally wound side core, higher-order modes are filtered out during transmission, allowing single-mode transmission within the fiber even with a core diameter exceeding the single-mode cutoff condition. The signal light output end of the second fiber combiner 17 uses a common double-clad passive fiber with a core diameter / cladding ratio of 30 / 250 μm, directly fused with a chiral-coupled fiber with a core diameter / cladding ratio of 32 / 250 μm, and the fusion point is cooled. The pump light stripper (cladding light stripper) 19 removes the pump light and other cladding light that are transmitted and remain in the outer cladding. The output cap 20 maintains beam quality, achieves high-energy output, and prevents back reflection.

[0070] In some possible implementations, the second semiconductor pump laser 12 may be a single-mode fiber-coupled high-power laser diode or a multimode fiber-coupled high-power laser diode.

[0071] In some possible implementations, the signal light output end of the second fiber combiner 17 can be directly connected to the chiral coupled fiber core 18, instead of using a double-clad fiber for connection.

[0072] In some possible implementations, the pump light stripper (cladding light stripper) 19 may be fabricated by coating the inner cladding with high-refractive-index UV adhesive, fabricating microstructures on the inner cladding surface, using a cladding fiber grating, or by using a cladding V-groove.

[0073] For example, the second and third semiconductor pump lasers have a center wavelength of 976nm and pump powers of 9W and 27W, respectively. The first and second fiber combiners are both polarization-maintaining 2+1*1 structures. The third gain fiber 14 uses highly doped double-clad ytterbium-doped fiber. The chiral coupled core gain fiber 18 has a core diameter of 32μm and a numerical aperture NA = 0.065, surrounding a side core with a core diameter of 9μm and a numerical aperture NA = 0.09. The output beam quality M2 factor is less than 1.2, and the pulse energy is greater than 10uJ.

[0074] In this embodiment, all optical components are connected by a fully fiber optic network. Except for the chiral coupled fiber core, the signal optical path adopts a fully polarization-maintaining fiber structure.

[0075] The principle and effect of the laser in the embodiments of this application:

[0076] A single-mode fiber mode-locked oscillator generates ultrashort pulses based on passive mode-locking using a saturable absorber mirror, outputting femtosecond or picosecond mode-locked pulses with repetition frequencies of tens of megahertz and spectral widths of several nanometers. These pulses are broadened by a cascaded large-dispersion tunable chirped fiber grating and down-frequencyed by an acousto-optic modulator, reducing the repetition frequency to the kilohertz range. The pulses are then pre-amplified in a double-clad fiber and finally amplified and mode-selected in a chiral-coupled fiber core. This results in an all-fiber laser system that outputs tunable linear chirped pulses with chirps higher than 1 ns / nm, achieving high beam quality and high-energy pulse output while ensuring single-mode transmission in the fiber.

[0077] This application has the following advantages:

[0078] First, this application provides an ideal front-end light source required for solid-state laser amplifiers that generate higher energies in chirped pulse amplification systems.

[0079] Second, by cascading multiple large dispersion chirped fiber gratings, a very large group delay dispersion can be provided, which is easy to integrate and control, and has dynamic tunability. It can provide linear chirped pulses with large and adjustable broadening to match the back-end compressor.

[0080] Third, the chiral-coupled fiber-core main amplifier stage can be compactly coiled, resulting in a compact system structure that is easy to integrate and maintain. Fiber-optic fusion splicing simplifies the manufacturing process, increases mechanical stability, and reduces system complexity.

[0081] Fourth, the broadband pulses generated by the mode-locked oscillator are broadened into linearly chirped pulses by a high-dispersion broadening system. After pulse selection, the frequency is reduced for easier power amplification. The fully polarization-maintaining fiber structure before the main amplification stage makes the system less susceptible to external interference, supporting long-term stable operation of the entire system. The chiral coupled fiber itself has the advantage of a large core diameter and unique excellent characteristics such as filtering out higher-order modes. As the main amplification stage, it outputs high-dispersion linearly chirped pulses with high energy and excellent beam quality.

[0082] It should be noted that, in this document, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes that element. Furthermore, it should be noted that the scope of the methods and apparatuses in the embodiments of this application is not limited to performing functions in the order shown or discussed, but may also include performing functions substantially simultaneously or in the reverse order, depending on the functions involved. For example, the described methods may be performed in a different order than described, and various steps may be added, omitted, or combined. Additionally, features described with reference to certain examples may be combined in other examples.

[0083] The embodiments of this application have been described above with reference to the accompanying drawings. However, this application is not limited to the specific embodiments described above. The specific embodiments described above are merely illustrative and not restrictive. Those skilled in the art can make many other forms under the guidance of this application without departing from the spirit and scope of the claims, and all of these forms are within the protection scope of this application.

Claims

1. A large dispersion-chirped pulse high beam quality all-fiber laser, characterized in that, It comprises: a single-mode fiber mode-locked oscillator, a single-mode fiber pulse stretching and frequency reduction system, and a fiber power amplifier; the single-mode fiber mode-locked oscillator is used to generate a wide-spectrum high-frequency ultrashort pulse seed light; the single-mode fiber pulse stretching and frequency reduction system comprises a fiber coupler, a fiber circulator, a tunable fiber stretcher, a fiber isolator, a fiber acousto-optic modulator, a high-speed photodetector, and a pulse selection driver; the main light path end of the fiber coupler is connected to the input end of the fiber circulator, the reflection end of the fiber circulator is connected to the tunable fiber stretcher, the output end of the fiber circulator is connected to the fiber isolator, and the output end of the fiber isolator is connected to the light input end of the fiber acousto-optic modulator; one end of the fiber coupler for outputting reference light is connected to the high-speed photodetector, the high-speed photodetector is electrically connected to the pulse selection driver, the pulse selection driver is connected to the synchronization signal end of the fiber acousto-optic modulator to realize pulse selection, the pulse selection ratio of the pulse selection driver is adjusted to control the repetition frequency, and the light output end of the fiber acousto-optic modulator outputs the signal light after stretching and frequency reduction; wherein the tunable fiber stretcher is configured as a stretcher formed based on a second chirped fiber grating; the fiber power amplifier is used for power amplification of the signal light.

2. The large-dispersion-chirped-pulse high-beam-quality all-fiber laser according to claim 1, wherein the second chirped fiber grating is configured as a high-dispersion chirped fiber grating, and dynamic tunability of the reflection wavelength and the dispersion characteristic of the second chirped fiber grating is realized through temperature adjustment or mechanical stretching; optionally, a chirped fiber grating with different but continuous reflection spectrum and same dispersion quantity is connected in series at the output end of the second chirped fiber grating to increase the reflection bandwidth.

3. The large-dispersion-chirped-pulse high-beam-quality all-fiber laser according to any one of claims 1-2, wherein the single-mode fiber pulse stretching and frequency reduction system comprises at least one fiber circulator and a tunable fiber stretcher used in one-to-one correspondence with the fiber circulator, and different combinations of the fiber circulator and the tunable fiber stretcher are connected in cascade.

4. The large-dispersion-chirped-pulse high-beam-quality all-fiber laser according to claim 3, wherein the single-mode fiber pulse stretching and frequency reduction system specifically comprises a first fiber circulator, a second fiber circulator, a first tunable fiber stretcher, and a second tunable fiber stretcher.

5. The large-dispersion-chirped-pulse high-beam-quality all-fiber laser according to claim 4, wherein the single-mode fiber pulse stretching and frequency reduction system further comprises a wavelength division multiplexer and a second gain fiber. ​ ​ ​ ​ ​ The output of the second fiber circulator is connected to the signal light transmission end of the wavelength division multiplexer, while the first semiconductor pump laser is connected to the pump input reflection end of the wavelength division multiplexer. The common end of the wavelength division multiplexer is connected to the input end of the second gain fiber, and the output end of the second gain fiber is connected to the input end of the fiber isolator, forming an amplification stage for power compensation and pre-amplification; wherein, the second gain fiber is configured as a single-mode polarization-maintaining gain fiber.

6. The high dispersion chirped pulsed high beam quality all-fiber laser according to claim 5, characterized in that, The single-mode fiber pulse broadening and down-conversion system may be optionally configured to include at least one set of the aforementioned amplification stages.

7. The high dispersion chirped pulsed high beam quality all-fiber laser according to claim 1, characterized in that, The high-speed photodetector and the pulse selection driver are integrated in a synchronization signal circuit placed in the fiber optic acousto-optic modulator.

8. The high dispersion chirped pulsed high beam quality all-fiber laser according to claim 1, characterized in that, The single-mode fiber mode-locked oscillator includes: a resonant cavity formed by a first semiconductor pump laser, a semiconductor saturable absorber integrated wavelength division multiplexer, a first gain fiber, and a dispersion management reflector; The first semiconductor pump laser is connected to the pump input of the semiconductor saturable absorber integrated wavelength division multiplexer. The saturable absorber and the wavelength division multiplexer are integrated as a resonant cavity input. The signal light end of the saturable absorber integrated wavelength division multiplexer is connected to the first gain fiber input end, and the first gain fiber output end is connected to the dispersion management reflector. The dispersion management reflector is formed based on a first chirped fiber grating and is used to provide negative dispersion and serve as the output mirror of the resonant cavity. By controlling the total length of the silicon fiber in the cavity, the dispersion is controlled to match the customized dispersion of the chirped fiber grating, thereby achieving zero-dispersion oscillation in the resonant cavity.

9. The high dispersion chirped pulsed high beam quality all-fiber laser according to claim 1, characterized in that, The fiber optic power amplifier includes: a first-stage fiber optic amplifier, a second-stage fiber optic amplifier, and an output system; The first-stage fiber amplifier includes: a second semiconductor pump laser, a first fiber combiner, a third gain fiber, and a first high-power fiber isolator; The output of the fiber optic acousto-optic modulator is connected to the signal light input of the first fiber optic combiner, the second semiconductor pump laser is connected to the pump light input of the first fiber optic combiner, the signal light output of the first fiber optic combiner is connected to the third gain fiber, and the output of the third gain fiber is connected to the input of the first high-power fiber optic isolator. The second-stage fiber amplifier includes: a third semiconductor pump laser, a second fiber combiner, and a chiral coupled fiber core gain fiber; The output of the first high-power fiber isolator is connected to the signal light input of the second fiber combiner, the third semiconductor pump laser is connected to the pump light input of the second fiber combiner, and the signal light output of the second fiber combiner is connected to the chiral coupled fiber core gain fiber. The output system includes a pump light stripper and an output end cap, the chiral coupled core gain fiber output end is connected to the pump light stripper and then connected to the output end cap.

10. The high beam quality all-fiber laser with large dispersion-chirped pulse according to claim 9, wherein, The signal light input end of the first fiber combiner is a single-mode polarization maintaining fiber, the pump light input end is a multi-mode fiber with a core diameter of 105 μm, and the signal light output end is a double-clad fiber with a core diameter of 14 μm; The third gain fiber is a double-clad gain fiber with a core diameter of 14 μm; The third semiconductor pump laser is configured as a multi-mode fiber coupled high-power semiconductor laser; The signal light output end of the second fiber combiner has a core diameter of 30 μm, and is connected to the chiral coupled core gain fiber through a double-clad passive fiber with a core diameter of 30 μm; The chiral coupled core gain fiber has a core diameter of 32 μm.

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