A light cathode driven laser based on switchable cluster emission mode

Abstract

The application provides a light cathode driving laser based on a switchable cluster mode, and relates to the technical field of lasers.The light cathode driving laser comprises a reference microwave source, a master control module, a mode-locked oscillator, a multi-stage polarization maintaining fiber amplification module, a cluster mode switching module, a frequency doubling module, a wavelength switching module and a frequency quadrupling module.The master control module is electrically connected with the reference microwave source, the mode-locked oscillator and the cluster mode switching module.The mode-locked oscillator is connected with the multi-stage polarization maintaining fiber amplification module.The multi-stage polarization maintaining fiber amplification module is connected with the cluster mode switching module.The cluster mode switching module is connected with the frequency doubling module.The cluster mode switching module is switched according to a cluster mode control signal, and outputs cluster pulses according to a high-voltage electro-optical driving signal in the cluster mode.The frequency doubling module is connected with the wavelength switching module.The wavelength switching module is connected with the frequency quadrupling module.The frequency quadrupling module outputs frequency quadrupling pulse laser in the deep ultraviolet band.

Description

Technical Field

[0001] This invention relates to the field of laser technology, and more particularly to a photocathode-driven laser based on a switchable cluster emission mode. Background Technology

[0002] The photocathode-driven laser is the core component of the photocathode electron gun, and its performance determines the quality of the output electron beam of the electron accelerator. Photocathode-based electron accelerators feature high emittance, low energy dispersion, and flexible adjustable beam parameters, and are widely used in free-electron lasers, energy-recovery linear accelerators, ultrafast electron diffraction, synchrotron radiation sources, and high-energy industrial non-destructive testing. Different photocathode materials have different characteristics, and their driving laser wavelength mainly depends on the material's bandgap. For example, materials such as NEA-GaAs, AlGaAs, and K₂CsSb are typically excited in the visible light band (532 nm, etc.), while materials such as Cs-Te are typically excited in the deep ultraviolet band (266 nm, etc.). Therefore, when using different cathode materials, it is often necessary to change the driving laser.

[0003] In existing technologies, the switching between cluster and continuous emission modes in photocathode-driven lasers typically employs an acousto-optic modulator (AOM). However, photocathode-driven lasers are limited by the bandwidth of the AOM, resulting in macropulse edges in cluster mode that are generally longer than 10 ns. This cannot meet the fast pulse edge requirements of some electron accelerators. Furthermore, photocathode-driven lasers generally adjust their output power by regulating the amplifier's pump drive current. However, adjusting the pump current causes changes in the thermal lensing parameters of isolators and gain crystals, which may lead to variations in the laser beam at the cathode target, resulting in instability in the beam waist position. Therefore, it is essential to provide a photocathode-driven laser based on a switchable cluster emission mode to improve the stability of the beam waist position and effectively shorten the pulse edge. Summary of the Invention

[0004] In view of this, the present invention proposes a photocathode-driven laser based on a switchable cluster emission mode, which helps to improve the stability of the beam waist position and effectively shorten the pulse edge, and can realize the switching of the output wavelength.

[0005] This invention provides a photocathode-driven laser based on switchable clustering modes, comprising a reference microwave source, a main control module, a mode-locked oscillator, a multi-stage polarization-maintaining fiber amplification module, a clustering mode switching module, a second harmonic module, a wavelength switching module, and a fourth harmonic module, wherein... The reference microwave source is used to output a reference microwave signal; The main control module is electrically connected to the reference microwave source, the mode-locked oscillator, and the cluster mode switching module, respectively. The main control module receives the reference microwave signal, the sampling signal of the mode-locked oscillator, and the external trigger control signal to lock the frequency of the mode-locked oscillator and the reference microwave source, and outputs the high-voltage electro-optic drive signal and the cluster mode control signal. The mode-locked oscillator is connected to the multi-stage polarization-maintaining fiber amplifier module to generate the first fundamental frequency light; The multi-stage polarization-maintaining fiber amplification module is connected to the cluster mode switching module and is used to perform multi-stage polarization-maintaining amplification on the first fundamental frequency light and output the first fundamental frequency light after power amplification. The cluster emission mode switching module is connected to the second harmonic module. The cluster emission mode switching module performs mode switching according to the cluster emission mode control signal, so that the cluster emission mode switching module switches between continuous mode and cluster emission mode. In cluster emission mode, the cluster emission pulse is output according to the repetition rate and pulse width of the high voltage electro-optic drive signal. In different modes, the second fundamental frequency light and the third fundamental frequency light with mutually orthogonal polarization states are output. The third fundamental frequency light forms a cluster emission pulse in cluster emission mode. The frequency doubling module is connected to the wavelength switching module. The frequency doubling module performs frequency doubling conversion on the third fundamental frequency light to output a frequency doubling pulsed laser. The wavelength switching module is connected to the fourth harmonic module. The wavelength switching module outputs the second harmonic pulse laser in the first working state and transmits the second harmonic pulse laser to the fourth harmonic module in the second working state. The output power can be continuously adjusted by changing the polarization state in different working states. The frequency-doubled module is used to perform a fourth-harmonic conversion on the second-harmonic pulsed laser to output a fourth-harmonic pulsed laser in the deep ultraviolet band.

[0006] Based on the above technical solutions, preferably, the multi-stage polarization-maintaining fiber amplification module includes a first fiber amplifier, a second fiber amplifier, a third fiber amplifier, a fiber collimator, and a spatial isolator cascaded in sequence. The first fundamental frequency light is amplified by passing through the first fiber amplifier, the second fiber amplifier, and the third fiber amplifier in sequence. The first fiber amplifier, the second fiber amplifier, and the third fiber amplifier are all polarization-maintaining structures so that the polarization direction of the amplified first fundamental frequency light remains unchanged.

[0007] Based on the above technical solutions, preferably, the first fiber amplifier is a polarization-maintaining single-mode fiber core pump amplifier, the second fiber amplifier is a polarization-maintaining double-clad ytterbium-doped fiber amplifier, and the third fiber amplifier is any one of a polarization-maintaining double-clad ytterbium-doped fiber amplifier, a photonic crystal fiber amplifier, and a tapered fiber amplifier, and the core mode area of ​​the third fiber amplifier is greater than or equal to the core mode area of ​​the second fiber amplifier.

[0008] More preferably, the clustering mode switching module includes a first electrically controlled half-wave plate, an electro-optic Pockels cell, a first polarization beam splitter, and a first waste light bin, wherein, The first electrically controlled half-wave plate, the electro-optic Pockel cell, and the first polarization beam splitter are cascaded in sequence. The first electrically controlled half-wave plate receives the first fundamental frequency light and adjusts the optical axis angle of the first electrically controlled half-wave plate through the cluster emission mode control signal to adjust the polarization state of the first fundamental frequency light, so that the first electrically controlled half-wave plate outputs the first fundamental frequency light and the second fundamental frequency light at different angles respectively. In continuous mode, the electro-optic Pockel cell is paused. At this time, the optical axis of the first electrically controlled half-wave plate is consistent with the polarization direction of the first fundamental frequency light, so that the first fundamental frequency light passes through the first polarization beam splitter and is incident on the second harmonic module. In cluster mode, the electro-optic Pockel cell, driven by the high-voltage electro-optic driving signal, performs rapid polarization rotation on the second fundamental frequency light to convert the second fundamental frequency light into the third fundamental frequency light, wherein the first fundamental frequency light and the third fundamental frequency light are in the same polarization state; The first polarization beam splitter has a transmission port and a reflection port that are orthogonal to each other. The first waste light bin is disposed at the transmission port of the first polarization beam splitter so that the first polarization beam splitter guides the incident second fundamental frequency light to the first waste light bin. The first polarization beam splitter guides the incident third fundamental frequency light to the second harmonic module through the transmission port.

[0009] More preferably, the frequency doubling module includes a first temperature-controlled furnace, a first lens located on the same optical axis, an LBO crystal, a second lens, a first beam splitter, and a second waste light bin, wherein, The first lens focuses the third fundamental frequency light from the cluster mode switching module so that the spot size and convergence angle incident on the nonlinear crystal satisfy the second harmonic condition. The LBO crystal is installed in the first temperature-controlled furnace, and the LBO crystal converts the third fundamental frequency light into a second harmonic pulsed laser under temperature phase matching conditions. The second lens collimates or shapes the second harmonic pulsed laser, and the first beam splitter spatially separates the third fundamental frequency light from the second harmonic pulsed laser so that the third fundamental frequency light is input into the second waste light bin, and the second harmonic pulsed laser is input into the wavelength switching module.

[0010] More preferably, the wavelength switching module includes a second electrically controlled half-wave plate, a second polarization beam splitter, a first optical shutter, and a first window plate, wherein, The second electrically controlled half-wave plate is connected to the second polarization beam splitter and the second frequency harmonic module respectively. The second electrically controlled half-wave plate receives the second frequency harmonic pulse laser output by the second frequency harmonic module, and adjusts the polarization state of the second frequency harmonic pulse laser by adjusting the optical axis angle of the second electrically controlled half-wave plate. The second polarization beam splitter has mutually orthogonal transmission ports and reflection ports. The first optical shutter and the first window are located on the light output path of the reflection port of the second polarization beam splitter, and the fourth frequency doubling module is located on the light output path of the transmission port of the second polarization beam splitter.

[0011] More preferably, the fourth frequency doubling module includes a second temperature control furnace, a second optical shutter, a second window, a third lens located on the same optical axis, a BBO crystal, a fourth lens, a second beam splitter, and a third waste light bin, wherein, The third lens pre-shapes and focuses the second-harmonic pulsed laser so that the spot size and convergence angle of the second-harmonic pulsed laser in the BBO crystal meet the fourth-harmonic condition. The BBO crystal is installed in the second temperature-controlled furnace, and the BBO crystal converts the second-harmonic pulse laser into a fourth-harmonic pulse laser under phase-matching conditions. The fourth lens shapes and collimates the fourth harmonic pulse laser, and the second beam splitter spatially separates the second harmonic pulse laser from the fourth harmonic pulse laser, so that the second harmonic pulse laser is input into the third waste light barrel, and the fourth harmonic pulse laser is output sequentially through the second optical shutter and the second window.

[0012] More preferably, the photocathode-driven laser further includes a grating reflection module, which is disposed between the mode-locked oscillator and the multi-stage polarization-maintaining fiber amplification module. The grating reflection module includes a circulator and a chirped grating. The first port of the circulator is connected to the mode-locked oscillator to receive the fundamental frequency pulse output by the mode-locked oscillator. The second port of the circulator is connected to the chirped grating, and the third port of the circulator is connected to the multi-stage polarization-maintaining fiber amplification module.

[0013] More preferably, the first beam splitter and the second beam splitter are any one of a dichroic mirror, a dispersive prism, and a diffractive optical element.

[0014] More preferably, the mode-locked oscillator is a high repetition rate adjustable mode-locked oscillator, the output center wavelength of the mode-locked oscillator is located in the 1 μm band, the pulse width of the mode-locked oscillator is 500 fs to 20 ps, ​​and the repetition frequency of the mode-locked oscillator is greater than 150 MHz.

[0015] The photocathode-driven laser based on switchable cluster emission mode provided by this invention has the following advantages over the prior art: (1) Under the control of the high-voltage electro-optic drive signal output by the main control module, the cluster mode switching module can quickly switch between continuous output mode and cluster mode to meet the different requirements of the photocathode for pulse structure in different working scenarios, improve the adaptability and flexibility of the laser, and thus effectively shorten the pulse edge. Furthermore, through the cascading of the second and fourth harmonic modules, the fundamental frequency light is converted to second and fourth harmonics, and finally a fourth harmonic pulsed laser in the deep ultraviolet band is obtained, so that the wavelength and pulse characteristics can meet the requirements of photocathode drive for high quantum efficiency, high time resolution and high peak power. The requirement for high efficiency can effectively improve the stability of the beam waist position. At the same time, the wavelength switching module can directly output second-harmonic pulsed laser in the first working state, and send the second-harmonic pulsed laser to the fourth-harmonic module for conversion and output in the second working state. This allows the same device to freely switch between second-harmonic and fourth-harmonic modes, improve the functional integration of the laser, and reduce the need for multiple independent laser systems. The use of multi-stage polarization-maintaining fiber amplification modules can amplify the first fundamental frequency light step by step to output high-power fundamental frequency light, which not only improves the output power, but also maintains good polarization state and beam quality.

[0016] (2) By rapidly rotating and reversing the polarization state of the second fundamental frequency light under the action of the high voltage electro-optic driving signal, compared with the scheme using an acousto-optic modulator, the modulation bandwidth is higher and the response time is shorter. It can realize the rapid switching of macro pulses and the high-speed switching of the cluster mode. Furthermore, the first electrically controlled half-wave plate changes the polarization state of the first fundamental frequency light by adjusting the optical axis angle, thereby controlling the energy distribution ratio between the second fundamental frequency light entering the electro-optic Pockel cell and the bypass first fundamental frequency light. This enables flexible adjustment of parameters such as different macro pulse energies and duty cycles in the cluster mode, improving the matching capability of the electron accelerator operating point. The first polarization beam splitter guides the light of different polarization states to the transmission port and the reflection port respectively. The unconverted second fundamental frequency light is stably introduced into the first waste light barrel, effectively removing unwanted residual light. The third fundamental frequency light converted by the Pockel cell is then guided separately to the second harmonic module. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 A schematic diagram of the framework of a photocathode-driven laser based on a switchable cluster emission mode is provided for this invention; Figure 2 This is a schematic diagram of the structure of the photocathode-driven laser provided by the present invention; Figure 3 A schematic diagram of the 178.5MHz pulse output in continuous mode provided by the present invention; Figure 4 This is a schematic diagram of the 178.5MHz pulse output in cluster mode provided by the present invention.

[0019] Figure labeling: 1. Reference microwave source; 2. Main control module; 3. Mode-locked oscillator; 4. Multi-stage polarization-maintaining fiber amplifier module; 41. First fiber amplifier; 42. Second fiber amplifier; 43. Third fiber amplifier; 44. Fiber collimator; 45. Spatial isolator; 5. Cluster mode switching module; 51. First electrically controlled half-wave plate; 52. Electro-optic Pockels cell; 53. First polarization beam splitter; 54. First waste light bin; 6. Frequency doubling module; 61. First lens; 62. LBO crystal; 63. First temperature... 64. Control furnace; 65. Second lens; 66. First beam splitter; 67. Second waste light bin; 78. Wavelength switching module; 79. Second electrically controlled half-wave plate; 70. Second polarization beam splitter; 71. First optical shutter; 72. First window plate; 83. Fourth frequency harmonic module; 84. Third lens; 85. BBO crystal; 86. Second temperature-controlled furnace; 87. Fourth lens; 88. Second beam splitter; 89. Second optical shutter; 80. Second window plate; 81. Third waste light bin; 92. Grating reflection module; 93. Circulator; 94. Chirped grating. Detailed Implementation

[0020] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0021] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "first," "second," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Similarly, the terms "an" or "a" and similar terms do not indicate a quantity limitation, but rather indicate the presence of at least one. The terms "connected" or "linked" and similar terms are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. "Up," "down," "left," "right," etc., are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship also changes accordingly.

[0022] like Figure 1 As shown, this invention provides a photocathode-driven laser based on switchable cluster emission modes, including a reference microwave source 1, a main control module 2, a mode-locked oscillator 3, a multi-stage polarization-maintaining fiber amplification module 4, a cluster emission mode switching module 5, a second harmonic module 6, a wavelength switching module 7, and a fourth harmonic module 8, wherein... Reference microwave source 1 is used to output a reference microwave signal; The main control module 2 is electrically connected to the reference microwave source 1, the mode-locked oscillator 3, and the cluster mode switching module 5 respectively. The main control module 2 receives the reference microwave signal, the sampling signal of the mode-locked oscillator 3, and the external trigger control signal to lock the frequency of the mode-locked oscillator 3 and the reference microwave source 1, and outputs the high-voltage electro-optic drive signal and the cluster mode control signal. The mode-locked oscillator 3 is connected to the multi-stage polarization-maintaining fiber amplifier module 4 to generate the first fundamental frequency light; The multi-stage polarization-maintaining fiber amplification module 4 is connected to the cluster mode switching module 5, and is used to perform multi-stage polarization-maintaining amplification on the first fundamental frequency light and output the first fundamental frequency light after power amplification. The cluster mode switching module 5 is connected to the second harmonic module 6. The cluster mode switching module 5 switches modes according to the cluster mode control signal so that the cluster mode switching module 5 can switch between continuous mode and cluster mode. In the cluster mode, the cluster pulse is output according to the repetition frequency and pulse width of the high voltage electro-optic drive signal. In different modes, the second fundamental frequency light and the third fundamental frequency light with mutually orthogonal polarization states are output. Among them, the third fundamental frequency light forms a cluster pulse in the cluster mode. The frequency doubling module 6 is connected to the wavelength switching module 7. The frequency doubling module 6 performs frequency doubling conversion on the third fundamental frequency light to output a frequency doubling pulsed laser. The wavelength switching module 7 is connected to the fourth harmonic module 8. In the first working state, the wavelength switching module 7 outputs second harmonic pulsed laser, and in the second working state, it transmits the second harmonic pulsed laser to the fourth harmonic module 8. The output power can be continuously adjusted by changing the polarization state in different working states. The fourth-harmonic generation module 8 is used to perform fourth-harmonic conversion on the second-harmonic pulsed laser to output a fourth-harmonic pulsed laser in the deep ultraviolet band.

[0023] In this embodiment, the cluster mode switching module 5, under the control of the high-voltage electro-optic drive signal output by the main control module 2, can quickly switch between continuous output mode and cluster mode, meeting the different pulse structure requirements of the photocathode in different working scenarios, improving the adaptability and flexibility of the laser, thereby effectively shortening the pulse edge. Furthermore, through the cascading of the second harmonic module 6 and the fourth harmonic module 8, the fundamental frequency light is second- and fourth-harmonic converted, ultimately obtaining a fourth-harmonic pulsed laser in the deep ultraviolet band, so that the wavelength and pulse characteristics can meet the photocathode drive requirements for high quantum efficiency, high time resolution, and high... The peak power requirement effectively improves the stability of the beam waist position. Meanwhile, the wavelength switching module 7 can directly output second-harmonic pulsed laser in the first working state, and in the second working state, the second-harmonic pulsed laser is sent to the fourth-harmonic module 8 for conversion and output. This allows the same device to freely switch between second-harmonic and fourth-harmonic modes, improves the functional integration of the laser, and reduces the need for multiple independent laser systems. The multi-stage polarization-maintaining fiber amplification module 4 amplifies the first fundamental frequency light step by step to output high-power fundamental frequency light, which not only improves the output power but also maintains good polarization state and beam quality.

[0024] Among them, mode-locked oscillator 3 is a high repetition rate adjustable mode-locked oscillator 3. The output center wavelength of mode-locked oscillator 3 is located in the 1 μm band, the pulse width of mode-locked oscillator 3 is 500 fs to 20 ps, ​​and the repetition rate of mode-locked oscillator 3 is greater than 150 MHz. The output frequency of mode-locked oscillator 3 is aligned with the operating frequency of electron accelerator, which is generally an integer division of 1.3 GHz or 2.856 GHz, such as 178.5 MHz, 325 MHz, 357 MHz, 714 MHz, etc., and typical wavelengths include 1030 nm and 1064 nm.

[0025] The mode-locked oscillator 3 can be a linear short-cavity scheme based on a semiconductor saturable absorber mirror (SESAM) or a figure-9 cavity scheme based on a nonlinear amplifying ring mirror (NALM). This mode-locked oscillator 3 needs to have at least two stages of repetition rate adjustment mechanisms, including: a mechanical adjustment with an adjustment stroke > 5 mm and a repetition rate adjustment range generally > ±1 MHz, which can be electric or manual to precisely align with the accelerator's operating frequency; and a piezoelectric ceramic (PZT) adjustment with an adjustment stroke > 5 μm and a PZT resonant frequency > 40 kHz, serving as an actuator for frequency locking with the accelerator. In addition to the above adjustment methods, precision temperature control and other means can also be included.

[0026] A pulse from mode-locked oscillator 3 is sampled by a photodetector and converted into a clock signal, which is then input to main control module 2. In main control module 2, the mode-locked frequency signal undergoes filtering, amplification, and mixing with an external reference microwave signal. Through a closed-loop feedback circuit, it drives the PZT in mode-locked oscillator 3, achieving frequency locking between the output repetition rate and the reference microwave. After locking, the relative time jitter is <300 fs. Simultaneously, the sampled pulse signal also serves as the clock signal for high-voltage electro-optical drive, ensuring time synchronization between the pulse cluster and the mode-locked pulse.

[0027] Furthermore, the multi-stage polarization-maintaining fiber amplification module 4 includes a first fiber amplifier 41, a second fiber amplifier 42, a third fiber amplifier 43, a fiber collimator 44, and a spatial isolator 45 cascaded in sequence. The first fundamental frequency light is amplified by passing through the first fiber amplifier 41, the second fiber amplifier 42, and the third fiber amplifier 43 in sequence. The first fiber amplifier 41, the second fiber amplifier 42, and the third fiber amplifier 43 are all polarization-maintaining structures so that the polarization direction of the first fundamental frequency light after power amplification remains unchanged.

[0028] The first fiber amplifier 41 is a polarization-maintaining single-mode fiber core pump amplifier, the second fiber amplifier 42 is a polarization-maintaining double-clad ytterbium-doped fiber amplifier, and the third fiber amplifier 43 is any one of a polarization-maintaining double-clad ytterbium-doped fiber amplifier, a photonic crystal fiber amplifier, and a tapered fiber amplifier, and the core mode area of ​​the third fiber amplifier 43 is greater than or equal to the core mode area of ​​the second fiber amplifier 42.

[0029] In one example, the high repetition rate pulse output by mode-locked oscillator 3 is amplified by at least two stages of polarization-maintaining fiber amplifiers. The amplifiers use 20 μm or 25 μm polarization-maintaining double-clad ytterbium-doped fiber, and the output fundamental frequency optical pulse energy is >100 nJ.

[0030] Furthermore, the clustering mode switching module 5 includes a first electrically controlled half-wave plate 51, an electro-optic Pockels cell 52, a first polarization beam splitter 53, and a first waste light bin 54, wherein, The first electrically controlled half-wave plate 51, the electro-optic Pockel cell 52, and the first polarization beam splitter 53 are cascaded in sequence. The first electrically controlled half-wave plate 51 receives the first fundamental frequency light and adjusts the optical axis angle of the first electrically controlled half-wave plate 51 through the cluster mode control signal to adjust the polarization state of the first fundamental frequency light so that the first electrically controlled half-wave plate 51 outputs the first fundamental frequency light and the second fundamental frequency light at different angles respectively. In continuous mode, the electro-optic Pockel cell 52 stops working. At this time, the optical axis of the first electronically controlled half-wave plate 51 is consistent with the polarization direction of the first fundamental frequency light, so that the first fundamental frequency light passes through the first polarization beam splitter 53 and is incident on the second harmonic module 6. In cluster mode, the electro-optic Pockel cell 52 performs rapid polarization rotation on the second fundamental frequency light under the drive of the high-voltage electro-optic drive signal, so as to convert the second fundamental frequency light into the third fundamental frequency light, wherein the first fundamental frequency light and the third fundamental frequency light are in the same polarization state; The first polarization beam splitter 53 has a transmission port and a reflection port that are orthogonal to each other. The first waste light bin 54 is disposed at the transmission port of the first polarization beam splitter 53 so that the first polarization beam splitter 53 guides the incident second fundamental frequency light to the first waste light bin 54. The first polarization beam splitter 53 guides the incident third fundamental frequency light to the second harmonic module 6 through the transmission port.

[0031] Understandably, the amplified first fundamental frequency light is coupled into the spatial optical path via fiber collimator 44, and a spatial isolator 45 is connected after fiber collimator 44 to prevent backlighting. It first passes through the first electrically controlled half-wave plate 51, the electro-optic Pockel cell 52, and the first polarization beam splitter 53 (PBS). The electro-optic Pockel cell 52 is controlled by a high-voltage electro-optic drive, and the control timing is synchronized with the mode-locking pulse via the main control module 2. Simultaneously, macropulse output can be controlled by an external trigger signal with a frequency <10 kHz. The cluster emission mode is switched by controlling the rotation angle of the first electrically controlled half-wave plate 51, as follows:

[0032] Continuous mode: The polarization direction of the first fundamental frequency light output by the fiber collimator 44 is p-polarized, the optical axis direction of the first electrically controlled half-wave plate 51 is consistent with the laser polarization direction, the electro-optic Pockel cell 52 is not working, and the first fundamental frequency light is incident on the rear optical path through the first polarization beam splitter 53.

[0033] Clustering mode: The polarization direction of the first fundamental frequency light output by the fiber collimator 44 is p-polarized. The optical axis of the first electrically controlled half-wave plate 51 makes an angle of 45° with the polarization direction of the first fundamental frequency light. After passing through the first electrically controlled half-wave plate 51, the p-polarized first fundamental frequency light is converted into s-polarized second fundamental frequency light. At this time, when the Pockels cell is not working, the second fundamental frequency light is reflected at the first polarization beam splitter 53, and the subsequent optical path is blocked.

[0034] When a high-voltage driving pulse is applied to the Pockel cell, the s-polarized second fundamental frequency light is converted into p-polarized third fundamental frequency light. The third fundamental frequency light passes through the first polarization beam splitter 53 according to the waveform of the applied high-voltage driving pulse, forming a pulse cluster. The rise and fall times of the pulse cluster depend on the rise and fall times of the driving electrical pulse, and can be as fast as <5 ns. The repetition rate and pulse width of the pulse cluster can be flexibly edited, with a typical repetition rate editing range of 0.1 Hz to 200 kHz, a typical pulse width editing range of 10 ns to 100 μs, and a typical pulse duty cycle of <20%.

[0035] In this embodiment, by rapidly rotating and reversing the polarization state of the second fundamental frequency light under the action of a high-voltage electro-optic driving signal, compared with the scheme using an acousto-optic modulator, the modulation bandwidth is higher and the response time is shorter. This enables rapid switching of macropulses and high-speed switching of the clustering structure. Furthermore, the first electrically controlled half-wave plate 51 changes the polarization state of the first fundamental frequency light by adjusting the optical axis angle, thereby controlling the energy distribution ratio between the second fundamental frequency light entering the electro-optic Pockel cell 52 and the bypassed first fundamental frequency light. This allows for flexible adjustment of parameters such as different macropulse energies and duty cycles in the clustering mode, improving the matching capability to the operating point of the electron accelerator. The first polarization beam splitter 53 guides light with different polarization states to the transmission port and reflection port respectively. The unconverted second fundamental frequency light is stably guided into the first waste light bin 54, effectively removing unwanted residual light. The third fundamental frequency light converted by the Pockel cell is then guided separately to the second harmonic module 6. This significantly improves the contrast between the clustered pulse and the background light, and prevents excess beams from entering the frequency doubling crystal or subsequent optical path, reducing interference and thermal load, and improving the long-term stability of the system. After being processed by the electro-optic Pockel cell 52, the third fundamental frequency light and the first fundamental frequency light are adjusted to the same polarization state, thereby ensuring that the working beam entering the second harmonic module 6 has a fixed and optimized polarization direction, which is beneficial to satisfying the phase matching condition of the nonlinear crystal, improving the second harmonic conversion efficiency and maintaining good beam quality.

[0036] Furthermore, the frequency doubling module 6 includes a first temperature-controlled furnace 63, a first lens 61 located on the same optical axis, an LBO crystal 62, a second lens 64, a first beam splitter 65, and a second waste light bin 66, wherein, The first lens 61 focuses the third fundamental frequency light from the cluster mode switching module 5 so that the spot size and convergence angle incident on the nonlinear crystal satisfy the second harmonic condition. The LBO crystal 62 is installed in the first temperature-controlled furnace 63. Under temperature phase matching conditions, the LBO crystal 62 converts the third fundamental frequency light into a second harmonic pulsed laser. The second lens 64 collimates or shapes the second-harmonic pulsed laser, and the first beam splitter 65 spatially separates the third fundamental frequency light from the second-harmonic pulsed laser so that the third fundamental frequency light is input into the second waste light bin 66, and the second-harmonic pulsed laser is input into the wavelength switching module 7.

[0037] The third fundamental frequency light output by the cluster mode switching module 5 is converted into a visible light pulse, i.e., a second-harmonic pulsed laser, through the second harmonic module 6. The typical wavelengths are 515 nm and 532 nm, and the energy is >5 nJ. After the output of the second harmonic module 6, the third fundamental frequency light and the second harmonic pulsed laser are separated by the first beam splitter 65. The second harmonic pulsed laser output by the second harmonic module 6 is p-polarized.

[0038] Furthermore, the wavelength switching module 7 includes a second electrically controlled half-wave plate 71, a second polarization beam splitter 72, a first optical shutter 73, and a first window 74, wherein, The second electrically controlled half-wave plate 71 is connected to the second polarization beam splitter 72 and the frequency-doubled module 6 respectively. The second electrically controlled half-wave plate 71 receives the frequency-doubled pulse laser output by the frequency-doubled module 6, and adjusts the polarization state of the frequency-doubled pulse laser by adjusting the optical axis angle of the second electrically controlled half-wave plate 71. The second polarization beam splitter 72 has a transmission port and a reflection port that are orthogonal to each other. The first optical shutter 73 and the first window 74 are located on the light output path of the reflection port of the second polarization beam splitter 72, and the quadruple frequency module 8 is located on the light output path of the transmission port of the second polarization beam splitter 72.

[0039] The frequency-doubled pulsed laser reflected by the first beam splitter 65 passes sequentially through the second electrically controlled half-wave plate 71 and the second polarization beam splitter 72. The applicable wavelengths of the second electrically controlled half-wave plate 71 and the second polarization beam splitter 72 are visible light wavelengths. When the optical axis of the second electrically controlled half-wave plate 71 is aligned with the input frequency-doubled pulsed laser, the frequency-doubled pulsed laser with p-polarization is transmitted at the second polarization beam splitter 72 and output after passing through the first window plate 74.

[0040] The optical axis of the second electrically controlled half-wave plate 71 is controlled to form an angle of 45° with the polarization direction of the second polarization beam splitter 72, so that the second-harmonic pulsed laser is converted from p-polarized light to s-polarized light, and is reflected at the second polarization beam splitter 72 and then enters the fourth-harmonic module 8 to generate deep ultraviolet light with a typical wavelength of 257 nm or 266 nm and energy > 0.5 nJ.

[0041] Furthermore, the quadruple frequency harmonic module 8 includes a second temperature control furnace 83, a second optical shutter 86, a second window 87, a third lens 81 located on the same optical axis, a BBO crystal 82, a fourth lens 84, a second beam splitter 85, and a third waste light bin 88, wherein, The third lens 81 pre-shapes and focuses the second-harmonic pulsed laser so that the spot size and convergence angle of the second-harmonic pulsed laser in the BBO crystal 82 meet the fourth-harmonic condition. The BBO crystal 82 is installed in the second temperature control furnace 83. Under phase matching conditions, the BBO crystal 82 converts the second harmonic pulse laser into the fourth harmonic pulse laser. The fourth lens 84 shapes and collimates the fourth harmonic pulse laser, and the second beam splitter 85 spatially separates the second harmonic pulse laser and the fourth harmonic pulse laser so that the second harmonic pulse laser is input into the third waste light bin 88, and the fourth harmonic pulse laser is output through the second shutter 86 and the second window 87 in sequence.

[0042] In one example, the first beam splitter 65 and the second beam splitter 85 are either dichroic mirrors, dispersive prisms, or diffractive optical elements.

[0043] Furthermore, such as Figure 2 As shown, the photocathode-driven laser also includes a grating reflection module 9, which is disposed between the mode-locked oscillator 3 and the multi-stage polarization-maintaining fiber amplification module 4. The grating reflection module 9 includes a circulator 91 and a chirped grating 92. The first port of the circulator 91 is connected to the mode-locked oscillator 3 to receive the fundamental frequency pulse output by the mode-locked oscillator 3. The second port of the circulator 91 is connected to the chirped grating 92, and the third port of the circulator 91 is connected to the multi-stage polarization-maintaining fiber amplification module 4.

[0044] Please continue reading. Figure 2 The mode-locked oscillator 3 is an integrated figure-9 cavity oscillator with a wavelength of 1064 nm, an output pulse repetition rate of 325 MHz, a pulse width of 2 ps, and an output power greater than 50 mW. The cavity length can be adjusted by 6 mm via a mechanical structure, corresponding to a repetition rate adjustment range of ±1 MHz. The PZT adjustment stroke is 10 μm, and the resonant frequency is greater than 50 kHz.

[0045] The output laser passes through a reflector consisting of a circulator 91 and a chirped grating 92. The reflected spectral bandwidth is compressed from 2 nm to 0.7 nm to meet the back-end frequency doubling requirements, and the pulse width is broadened from 2 ps to 6 ps.

[0046] The mode-locked pulse output from mode-locked oscillator 3 is sampled by a high-speed photodetector, and the resulting 325 MHz electrical pulse signal is output to main control module 2. Main control module 2 filters and amplifies the electrical pulse signal, mixes its fourth harmonic with an external 1.3 GHz reference microwave signal, extracts the error signal, and feeds it back to control the PZT in mode-locked oscillator 3. The relative time jitter after locking is <100 fs.

[0047] The fundamental frequency light output from mode-locked oscillator 3 passes through three stages of polarization-maintaining fiber amplifiers: a 6 μm single-mode fiber core pump amplifier, a 10 / 125 double-clad ytterbium-doped fiber amplifier, and a 25 / 250 double-clad ytterbium-doped fiber amplifier. The average power of the output fundamental frequency light is 33 W, and the single-pulse energy is 100 nJ.

[0048] The amplified fundamental frequency light is coupled into free space through an optical fiber collimator 44, at which point the fundamental frequency light is horizontally polarized (p-polarized). The fundamental frequency light first passes through an isolator to prevent backlighting, and then is injected into the cluster mode switching module 5. The cluster mode switching module 5 consists of a first electrically controlled half-wave plate 51, an electro-optic Pockels cell 52, and a first polarization beam splitter 53, all of which are suitable for a wavelength of 1064 nm.

[0049] When the optical axis of the first electrically controlled half-wave plate 51 is horizontal, it is in continuous output mode. At this time, the high-voltage electro-optic drive has no output, and the fundamental frequency light passes through the first polarization beam splitter 53.

[0050] When the optical axis of the first electrically controlled half-wave plate 51 is rotated to 45°, it is in cluster output mode. The polarization state of the fundamental frequency light is rotated to the vertical direction (s-polarization) after passing through the half-wave plate. When there is no output from the electro-optic drive, the fundamental frequency light is reflected at the first polarization beam splitter 53 and enters the waste light bin. When the electro-optic drive outputs a high-voltage electric pulse, the fundamental frequency light passes through the first polarization beam splitter 53 according to the waveform of the high-voltage electric pulse, forming a pulse cluster. The driving voltage of the Pockel cell is approximately 1.6 kV, the pulse width of the pulse cluster is adjustable from 50 to 200 ns, and the pulse repetition rate is adjustable from 1 Hz to 1 kHz.

[0051] The fundamental frequency light is focused by the first lens 61 and then enters the LBO crystal 62 for frequency doubling, generating 532 nm green light. This light is then collimated by the second lens 64 and subsequently separated from the frequency-doubled light by the first beam splitter 65. The first beam splitter 65 is a dichroic mirror, and its surface is coated with a 1064 nm high-transmittance film and a 532 nm high-reflection film. The residual 1064 nm fundamental frequency light is collected by the second waste light bin 66. The LBO crystal 62 is temperature-phase matched, and its temperature is controlled by a temperature-controlled furnace. In this embodiment, the LBO crystal 62 is 70 mm long, has a frequency doubling efficiency >50%, and outputs 532 nm laser energy >50 nJ.

[0052] The generated 532 nm frequency-doubled light has a horizontal polarization state (p-polarization). It passes through a wavelength switching module 7, which consists of a second electrically controlled half-wave plate 71 and a second polarization beam splitter 72. Both the second electrically controlled half-wave plate 71 and the second polarization beam splitter 72 are suitable for a 532 nm wavelength. When the optical axis of the second electrically controlled half-wave plate 71 is horizontal, the 532 nm frequency-doubled light is completely transmitted through the second polarization beam splitter 72 and enters the subsequent fourth-frequency harmonic module 8. When the optical axis of the second electrically controlled half-wave plate 71 is rotated 45°, the 532 nm frequency-doubled light is reflected at the second polarization beam splitter 72 and output after passing through a first optical shutter 73 and a first window 74.

[0053] The 532 nm frequency-doubled light entering the fourth-harmonic generation module 8 is pre-shaped and focused by the third lens 81, then enters a temperature-controlled BBO crystal 82 to undergo fourth-harmonic generation, producing 266 nm deep ultraviolet laser. After being shaped by the fourth lens 84 and split by the second beam splitter 85, it is output through the second optical shutter 86 and the second window 87. The fourth-harmonic generation efficiency is >10%, and the output 266 nm energy is >4.5 nJ. The third lens 81 and the fourth lens 84 are cylindrical lenses used to compensate for walk-off in the BBO crystal 82 and optimize the output beam quality. The second beam splitter 85 is a dichroic mirror with a 266 nm high-reflectivity coating and a 532 nm high-transmittance coating on its surface. Residual 532 nm laser light is collected by the third waste light bin 88. The first optical shutter 73 and the second optical shutter 86 are motorized shutters; closing the shutter corresponding to another wavelength when switching output wavelengths prevents light leakage due to imperfect link efficiency. The first window 74 is a 532 nm antireflection window with a 1° angle, and the second window 87 is a Brewster angle window.

[0054] The power adjustment of the photocathode-driven laser is achieved by rotating the second polarization beam splitter 72, which can realize 0~100% power adjustment of 532nm or 266nm, with an adjustment step of 0.1%, and the beam waist position will not change due to thermal lensing effect during adjustment.

[0055] like Figure 3 As shown, Figure 3 This is a schematic diagram of the 178.5MHz pulse output in continuous mode. Figure 3 The oscilloscope acquires the fundamental frequency optical intensity signal after multi-stage polarization-maintaining fiber amplification and passing through the cluster mode switching module 5. The intervals between adjacent pulses are basically consistent, and the peak amplitude and pulse width of each pulse remain stable, indicating that the mode-locked pulse has good repeatability and stability within a cluster. This figure reflects the consistency of the time interval and amplitude of each fundamental frequency pulse within a single cluster when forming cluster pulses under high repetition rate continuous output conditions in this example.

[0056] like Figure 4 As shown, Figure 4 This is a schematic diagram of the 178.5MHz pulse output in cluster mode. Figure 4 The curve on the oscilloscope shows the fundamental frequency light intensity signal after being selected by the cluster mode switching module 5. The measurement results show that the repetition frequency of adjacent pulses is approximately 34 MHz. Each pulse is continuously arranged within a macropulse duration, forming a pulse cluster composed of numerous mode-locked pulses, with the pulse amplitudes within the cluster being essentially consistent. This figure further illustrates that this example uses the high-voltage electro-optic drive signal output from the main control module 2 to select and control the electro-optic Pockel cell 52, enabling the stable generation of cluster laser pulse sequences within the set macropulse width and macro repetition frequency.

[0057] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A photocathode-driven laser based on switchable cluster emission mode, characterized in that, The system includes a reference microwave source (1), a main control module (2), a mode-locked oscillator (3), a multi-stage polarization-maintaining fiber amplifier module (4), a cluster mode switching module (5), a second harmonic module (6), a wavelength switching module (7), and a fourth harmonic module (8). The reference microwave source (1) is used to output a reference microwave signal; The main control module (2) is electrically connected to the reference microwave source (1), the mode-locked oscillator (3) and the cluster mode switching module (5) respectively. The main control module (2) receives the reference microwave signal, the sampling signal of the mode-locked oscillator (3) and the external trigger control signal to lock the frequency of the mode-locked oscillator (3) and the reference microwave source (1), and outputs the high-voltage electro-optic drive signal and the cluster mode control signal. The mode-locked oscillator (3) is connected to the multi-stage polarization-maintaining fiber amplifier module (4) and is used to generate the first fundamental frequency light; The multi-stage polarization-maintaining fiber amplification module (4) is connected to the cluster mode switching module (5) and is used to perform multi-stage polarization-maintaining amplification on the first fundamental frequency light and output the first fundamental frequency light after power amplification. The cluster emission mode switching module (5) is connected to the second harmonic module (6). The cluster emission mode switching module (5) switches modes according to the cluster emission mode control signal so that the cluster emission mode switching module (5) switches between continuous mode and cluster emission mode. In the cluster emission mode, the cluster emission pulse is output according to the repetition frequency and pulse width of the high voltage electro-optic drive signal. In different modes, the second fundamental frequency light and the third fundamental frequency light with mutually orthogonal polarization states are output. The third fundamental frequency light forms a cluster emission pulse in the cluster emission mode. The frequency doubling module (6) is connected to the wavelength switching module (7). The frequency doubling module (6) performs frequency doubling conversion on the third fundamental frequency light to output a frequency doubling pulse laser. The wavelength switching module (7) is connected to the fourth harmonic module (8). The wavelength switching module (7) outputs the second harmonic pulse laser in the first working state and transmits the second harmonic pulse laser to the fourth harmonic module (8) in the second working state. The output power can be continuously adjusted by changing the polarization state in different working states. The fourth harmonic module (8) is used to perform fourth harmonic conversion on the second harmonic pulse laser to output a fourth harmonic pulse laser in the deep ultraviolet band.

2. A photocathode-driven laser based on a switchable cluster emission mode as described in claim 1, characterized in that, The multi-stage polarization-maintaining fiber amplifier module (4) includes a first fiber amplifier (41), a second fiber amplifier (42), a third fiber amplifier (43), a fiber collimator (44), and a spatial isolator (45) cascaded in sequence. The first fundamental frequency light is amplified by passing through the first fiber amplifier (41), the second fiber amplifier (42), and the third fiber amplifier (43) in sequence. The first fiber amplifier (41), the second fiber amplifier (42), and the third fiber amplifier (43) are all polarization-maintaining structures so that the polarization direction of the first fundamental frequency light after power amplification remains unchanged.

3. A photocathode-driven laser based on a switchable cluster emission mode as described in claim 2, characterized in that, The first fiber amplifier (41) is a polarization-maintaining single-mode fiber core pump amplifier, the second fiber amplifier (42) is a polarization-maintaining double-clad ytterbium-doped fiber amplifier, and the third fiber amplifier (43) is any one of a polarization-maintaining double-clad ytterbium-doped fiber amplifier, a photonic crystal fiber amplifier, and a tapered fiber amplifier, and the core mode area of ​​the third fiber amplifier (43) is greater than or equal to the core mode area of ​​the second fiber amplifier (42).

4. A photocathode-driven laser based on a switchable cluster emission mode as described in claim 1, characterized in that, The clustering mode switching module (5) includes a first electrically controlled half-wave plate (51), an electro-optic Pockels cell (52), a first polarization beam splitter (53), and a first waste light bin (54), wherein, The first electrically controlled half-wave plate (51), the electro-optic Pockel cell (52), and the first polarization beam splitter (53) are cascaded in sequence. The first electrically controlled half-wave plate (51) receives the first fundamental frequency light and adjusts the optical axis angle of the first electrically controlled half-wave plate (51) through the cluster mode control signal to adjust the polarization state of the first fundamental frequency light, so that the first electrically controlled half-wave plate (51) outputs the first fundamental frequency light and the second fundamental frequency light at different angles respectively. In continuous mode, the electro-optic Pockel cell (52) is paused. At this time, the optical axis of the first electrically controlled half-wave plate (51) is consistent with the polarization direction of the first fundamental frequency light, so that the first fundamental frequency light passes through the first polarization beam splitter (53) and is incident on the second harmonic module (6). In cluster mode, the electro-optic Pockel cell (52) performs rapid polarization rotation on the second fundamental frequency light under the drive of the high-voltage electro-optic driving signal, so as to convert the second fundamental frequency light into the third fundamental frequency light, wherein the first fundamental frequency light and the third fundamental frequency light are in the same polarization state; The first polarization beam splitter (53) has a transmission port and a reflection port that are orthogonal to each other. The first waste light bin (54) is disposed at the transmission port of the first polarization beam splitter (53) so that the first polarization beam splitter (53) guides the incident second fundamental frequency light to the first waste light bin (54). The first polarization beam splitter (53) guides the incident third fundamental frequency light to the second harmonic module (6) through the transmission port.

5. A photocathode-driven laser based on a switchable cluster emission mode as described in claim 1, characterized in that, The frequency doubling module (6) includes a first temperature-controlled furnace (63), a first lens (61) located on the same optical axis, an LBO crystal (62), a second lens (64), a first beam splitter (65), and a second waste light bin (66), wherein, The first lens (61) focuses the third fundamental frequency light from the cluster mode switching module (5) so that the spot size and convergence angle incident on the nonlinear crystal satisfy the second harmonic condition; The LBO crystal (62) is installed inside the first temperature-controlled furnace (63), and the LBO crystal (62) converts the third fundamental frequency light into a second harmonic pulse laser under temperature phase matching conditions; The second lens (64) collimates or shapes the second harmonic pulse laser, and the first beam splitter (65) spatially separates the third fundamental frequency light from the second harmonic pulse laser so that the third fundamental frequency light is input into the second waste light bin (66) and the second harmonic pulse laser is input into the wavelength switching module (7).

6. A photocathode-driven laser based on a switchable cluster emission mode as described in claim 1, characterized in that, The wavelength switching module (7) includes a second electrically controlled half-wave plate (71), a second polarization beam splitter (72), a first optical shutter (73), and a first window (74), wherein, The second electrically controlled half-wave plate (71) is connected to the second polarization beam splitter (72) and the second frequency harmonic module (6) respectively. The second electrically controlled half-wave plate (71) receives the second frequency harmonic pulse laser output by the second frequency harmonic module (6) and adjusts the polarization state of the second frequency harmonic pulse laser by adjusting the optical axis angle of the second electrically controlled half-wave plate (71). The second polarization beam splitter (72) has a transmission port and a reflection port that are orthogonal to each other. The first light shutter (73) and the first window (74) are located on the light output path of the reflection port of the second polarization beam splitter (72). The fourth frequency doubling module (8) is located on the light output path of the transmission port of the second polarization beam splitter (72).

7. A photocathode-driven laser based on a switchable cluster emission mode as described in claim 5, characterized in that, The quadruple frequency module (8) includes a second temperature control furnace (83), a second optical shutter (86), a second window (87), a third lens (81) located on the same optical axis, a BBO crystal (82), a fourth lens (84), a second beam splitter (85), and a third waste light bin (88), wherein, The third lens (81) pre-shapes and focuses the second-harmonic pulsed laser so that the spot size and convergence angle of the second-harmonic pulsed laser in the BBO crystal (82) meet the fourth-harmonic condition; The BBO crystal (82) is installed in the second temperature-controlled furnace (83), and the BBO crystal (82) converts the second-harmonic pulse laser into a fourth-harmonic pulse laser under phase-matching conditions; The fourth lens (84) shapes and collimates the fourth harmonic pulse laser, and the second beam splitter (85) spatially separates the second harmonic pulse laser from the fourth harmonic pulse laser so that the second harmonic pulse laser is input into the third waste light barrel (88), and the fourth harmonic pulse laser is output sequentially through the second shutter (86) and the second window (87).

8. A photocathode-driven laser based on a switchable cluster emission mode as described in claim 1, characterized in that, The photocathode-driven laser also includes a grating reflection module (9), which is disposed between the mode-locked oscillator (3) and the multi-stage polarization-maintaining fiber amplification module (4). The grating reflection module (9) includes a circulator (91) and a chirped grating (92). The first port of the circulator (91) is connected to the mode-locked oscillator (3) to receive the fundamental frequency pulse output by the mode-locked oscillator (3). The second port of the circulator (91) is connected to the chirped grating (92). The third port of the circulator (91) is connected to the multi-stage polarization-maintaining fiber amplification module (4).

9. A photocathode-driven laser based on a switchable cluster emission mode as described in claim 7, characterized in that, The first beam splitter (65) and the second beam splitter (85) are any one of a dichroic mirror, a dispersive prism, and a diffractive optical element.

10. A photocathode-driven laser based on a switchable cluster emission mode as described in claim 1, characterized in that, The mode-locked oscillator (3) is a high repetition rate adjustable mode-locked oscillator (3). The output center wavelength of the mode-locked oscillator (3) is located in the 1 μm band. The pulse width of the mode-locked oscillator (3) is 500 fs to 20 ps. The repetition frequency of the mode-locked oscillator (3) is greater than 150 MHz.

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